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The Journal of Physiology

Impact factor: 4.38 5-Year impact factor: 4.834 Print ISSN: 0022-3751 Online ISSN: 1469-7793 Publisher: Wiley Blackwell (Blackwell Publishing -The Physiological Society)

Subject: Psychology

Most recent papers:

  • Omecamtiv Mercabil and Blebbistatin modulate cardiac contractility by perturbing the regulatory state of the myosin filament.
    Thomas Kampourakis, Xuemeng Zhang, Yin‐Biao Sun, Malcolm Irving.
    The Journal of Physiology. October 20, 2017
    Contraction of heart muscle is triggered by a transient rise in intracellular free calcium concentration linked to a change in the structure of the actin‐containing thin filaments that allows the head or motor domains of myosin from the thick filaments to bind to them and induce filament sliding. It is becoming increasingly clear that cardiac contractility is also regulated through structural changes in the thick filaments, although the molecular mechanisms underlying thick filament regulation are still relatively poorly understood. Here we investigated those mechanisms using small molecules‐ Omecamtiv Mecarbil (OM) and Blebbistatin (BS) ‐ that bind specifically to myosin and respectively activate or inhibit contractility in demembranated cardiac muscle cells. We measured isometric force and ATP utilization at different calcium and small‐molecule concentrations in parallel with in situ structural changes determined using fluorescent probes on the myosin regulatory light chain in the thick filaments and on troponin C in the thin filaments. The results show that BS inhibits contractility and actin‐myosin ATPase by stabilizing the OFF state of the thick filament in which myosin head domains are more parallel to the filament axis. In contrast, OM stabilizes the ON state of the thick filament, but inhibits contractility at high intracellular calcium concentration by disrupting the actin‐myosin ATPase pathway. The effects of BS and OM on the calcium sensitivity of isometric force and filament structural changes suggest that the co‐operativity of calcium activation in physiological conditions is due to positive coupling between the regulatory states of the thin and thick filaments. This article is protected by copyright. All rights reserved
    October 20, 2017   doi: 10.1113/JP275050   open full text
  • Heteromeric α/β glycine receptors regulate excitability in parvalbumin‐expressing dorsal horn neurons through phasic and tonic glycinergic inhibition.
    M. A. Gradwell, K. A. Boyle, R. J. Callister, D. I. Hughes, B. A. Graham.
    The Journal of Physiology. October 19, 2017
    Key points Spinal parvalbumin‐expressing interneurons have been identified as a critical source of inhibition to regulate sensory thresholds by gating mechanical inputs in the dorsal horn. This study assessed the inhibitory regulation of the parvalbumin‐expressing interneurons, showing that synaptic and tonic glycinergic currents dominate, blocking neuronal or glial glycine transporters enhances tonic glycinergic currents, and these manipulations reduce excitability. Synaptically released glycine also enhanced tonic glycinergic currents and resulted in decreased parvalbumin‐expressing interneuron excitability. Analysis of the glycine receptor properties mediating inhibition of parvalbumin neurons, as well as single channel recordings, indicates that heteromeric α/β subunit‐containing receptors underlie both synaptic and tonic glycinergic currents. Our findings indicate that glycinergic inhibition provides critical control of excitability in parvalbumin‐expressing interneurons in the dorsal horn and represents a pharmacological target to manipulate spinal sensory processing. Abstract The dorsal horn (DH) of the spinal cord is an important site for modality‐specific processing of sensory information and is essential for contextually relevant sensory experience. Parvalbumin‐expressing inhibitory interneurons (PV+ INs) have functional properties and connectivity that enables them to segregate tactile and nociceptive information. Here we examine inhibitory drive to PV+ INs using targeted patch‐clamp recording in spinal cord slices from adult transgenic mice that express enhanced green fluorescent protein in PV+ INs. Analysis of inhibitory synaptic currents showed glycinergic transmission is the dominant form of phasic inhibition to PV+ INs. In addition, PV+ INs expressed robust glycine‐mediated tonic currents; however, we found no evidence for tonic GABAergic currents. Manipulation of extracellular glycine by blocking either, or both, the glial and neuronal glycine transporters markedly decreased PV+ IN excitability, as assessed by action potential discharge. This decreased excitability was replicated when tonic glycinergic currents were increased by electrically activating glycinergic synapses. Finally, we show that both phasic and tonic forms of glycinergic inhibition are mediated by heteromeric α/β glycine receptors. This differs from GABAA receptors in the dorsal horn, where different receptor stoichiometries underlie phasic and tonic inhibition. Together these data suggest both phasic and tonic glycinergic inhibition regulate the output of PV+ INs and contribute to the processing and segregation of tactile and nociceptive information. The shared stoichiometry for phasic and tonic glycine receptors suggests pharmacology is unlikely to be able to selectively target each form of inhibition in PV+ INs.
    October 19, 2017   doi: 10.1113/JP274926   open full text
  • Molecular mechanism for muscarinic M1 receptor‐mediated endocytosis of TWIK‐related acid‐sensitive K+ 1 channels in rat adrenal medullary cells.
    Hidetada Matsuoka, Masumi Inoue.
    The Journal of Physiology. October 19, 2017
    Key points The muscarinic acetylcholine receptor (mAChR)‐mediated increase in excitability in rat adrenal medullary cells is at least in part due to inhibition of TWIK (tandem of P domains in a weak inwardly rectifying K+ channel)‐related acid‐sensitive K+ (TASK)1 channels. In this study we focused on the molecular mechanism of mAChR‐mediated inhibition of TASK1 channels. Exposure to muscarine resulted in a clathrin‐dependent endocytosis of TASK1 channels following activation of the muscarinic M1 receptor (M1R). This muscarinic signal for the endocytosis was mediated in sequence by phospholipase C (PLC), protein kinase C (PKC), and then the non‐receptor tyrosine kinase Src with the consequent tyrosine phosphorylation of TASK1. The present results establish that TASK1 channels are tyrosine phosphorylated and internalized in a clathrin‐dependent manner in response to M1R stimulation and this translocation is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells. Abstract Activation of muscarinic receptor (mAChR) in rat adrenal medullary (AM) cells induces depolarization through the inhibition of TWIK‐related acid‐sensitive K+ (TASK)1 channels. Here, pharmacological and immunological approaches were used to elucidate the molecular mechanism for this mAChR‐mediated inhibition. TASK1‐like immunoreactive (IR) material was mainly located at the cell periphery in dissociated rat AM cells, and its majority was internalized in response to muscarine. The muscarine‐induced inward current and translocation of TASK1 were suppressed by dynasore, a dynamin inhibitor. The muscarinic translocation was suppressed by MT7, a specific M1 antagonist, and the dose–response curves for muscarinic agonist‐induced translocation were similar to those for the muscarinic inhibition of TASK1 currents. The muscarine‐induced inward current and/or translocation of TASK1 were suppressed by inhibitors for phospholipase C (PLC), protein kinase C (PKC), and/or Src. TASK1 channels in AM cells and PC12 cells were transiently associated with Src and were tyrosine phosphorylated in response to muscarinic stimulation. After internalization, TASK1 channels were quickly dephosphorylated even while they remained in the cytoplasm. The cytoplasmic TASK1‐like IR material quickly recycled back to the cell periphery after muscarine stimulation for 0.5 min, but not 10 min. We conclude that M1R stimulation results in internalization of TASK1 channels through the PLC–PKC–Src pathway with the consequent phosphorylation of tyrosine and that this M1R‐mediated internalization is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.
    October 19, 2017   doi: 10.1113/JP275039   open full text
  • Endomorphins potentiate ASIC currents and enhance the lactic acid‐mediated increase in arterial blood pressure—effects amplified in hindlimb ischemia.
    Mohamed Farrag, Julie K. Drobish, Henry L. Puhl, Joyce S. Kim, Paul B. Herold, Marc P. Kaufman, Victor Ruiz‐Velasco.
    The Journal of Physiology. October 16, 2017
    Chronic muscle ischemia leads to accumulation of lactic acid and other inflammatory mediators with a subsequent drop in interstitial pH. Acid‐sensing ion channels (ASICs), expressed in thin muscle afferents, sense the decrease in pH and evoke a pressor reflex known to increase mean arterial pressure. The naturally occurring endomorphins are also released by primary afferents under ischemic conditions. We examined whether high affinity mu opioid receptor (MOR) agonists, endomorphin‐1 (E‐1) and ‐2 (E‐2), modulate ASIC currents and the lactic acid‐mediated pressor reflex. In rat dorsal root ganglion (DRG) neurons, exposure to E‐2 in acidic solutions significantly potentiated ASIC currents when compared to acidic solutions alone. The potentiation was significantly greater in DRG neurons isolated from rats whose femoral arteries were ligated for 72 hr. Sustained ASIC current potentiation was also observed in neurons pretreated with pertussis toxin, an uncoupler of G proteins and MOR. The endomorphin‐mediated potentiation was a result of a leftward shift of the activation curve to more basic pH values and a slight shift of the inactivation curve to more acidic pH values. Intra‐arterial co‐administration of lactic acid and E‐2 led to a significantly greater pressor reflex than lactic acid alone in the presence of naloxone. Finally, E‐2 effects were inhibited by pretreatment with the ASIC3 blocker (APETx2) and enhanced by pretreatment with the ASIC1a blocker psalmotoxin‐1. These findings have uncovered a novel role of endomorphins by which the opioids can enhance the lactic acid‐mediated reflex increase in arterial pressure that is MOR stimulation‐independent and APETx2‐sensitive. This article is protected by copyright. All rights reserved
    October 16, 2017   doi: 10.1113/JP275058   open full text
  • Altered NMDA receptor‐evoked intracellular Ca2+ dynamics in magnocellular neurosecretory neurons of hypertensive rats.
    Meng Zhang, Javier E. Stern.
    The Journal of Physiology. October 15, 2017
    A growing body of evidence supports an elevated NMDA receptor‐mediated glutamate excitatory function in the SON and PVN of hypertensive rats that contributes to neurohumoral activation in this disease. Still, the precise mechanisms underlying altered NMDAR signalling in hypertension remains to be elucidated. In this study, we performed simultaneous electrophysiology and fast confocal Ca2+ imaging to determine whether an altered NMDAR‐mediated changes in intracellular Ca2+ levels (NMDAR‐ΔCa2+) occurred in hypothalamic magnocellular neurosecretory cells (MNCs) in renovascular hypertensive (RVH) rats. We found that despite evoking a similar excitatory inward current, activation of NMDARs resulted in a larger and prolonged ΔCa2+ in MNCs from RVH rats. Changes in NMDAR‐ΔCa2+ dynamics were observed both in somatic and dendritic compartments. Inhibition of the ER SERCA pump activity with thapsigargin prolonged NMDAR‐ΔCa2+ responses in MNCs of sham rats, but this effect was occluded in RVH rats, thus equalizing the magnitude and time course of the NMDA‐ΔCa2 responses between the two experimental groups. Taken together, our results support (1) an exacerbated NMDAR‐ΔCa2+ response in somatodendritic compartments of MNCs of RVH rats, and (2) that a blunted ER Ca2+ buffering capacity contributes to the altered NMDAR‐ΔCa2+ dynamics in this condition. Thus, an altered spatiotemporal dynamics of NMDAR‐ΔCa2+ response stands as an underlying mechanisms contributing to neurohumoral activation in neurogenic hypertension. This article is protected by copyright. All rights reserved
    October 15, 2017   doi: 10.1113/JP275169   open full text
  • Asymmetry between ON and OFF α ganglion cells of mouse retina: integration of signal and noise from synaptic inputs.
    Michael A. Freed.
    The Journal of Physiology. October 15, 2017
    Key points Bipolar and amacrine cells presynaptic to the ON sustained α cell of mouse retina provide currents with a higher signal‐to‐noise power ratio (SNR) than those presynaptic to the OFF sustained α cell. Yet the ON cell loses proportionately more SNR from synaptic inputs to spike output than the OFF cell does. The higher SNR of ON bipolar cells at the beginning of the ON pathway compensates for losses incurred by the ON ganglion cell, and improves the processing of positive contrasts. Abstract ON and OFF pathways in the retina include functional pairs of neurons that, at first glance, appear to have symmetrically similar responses to brightening and darkening, respectively. Upon careful examination, however, functional pairs exhibit asymmetries in receptive field size and response kinetics. Until now, descriptions of how light‐adapted retinal circuitry maintains a preponderance of signal over the noise have not distinguished between ON and OFF pathways. Here I present evidence of marked asymmetries between members of a functional pair of sustained α ganglion cells in the mouse retina. The ON cell exhibited a proportionately greater loss of signal‐to‐noise power ratio (SNR) from its presynaptic arrays to its postsynaptic currents. Thus the ON cell combines signal and noise from its presynaptic arrays of bipolar and amacrine cells less efficiently than the OFF cell does. Yet the inefficiency of the ON cell is compensated by its presynaptic arrays providing a higher SNR than the arrays presynaptic to the OFF cell, apparently to improve visual processing of positive contrasts. Dynamic clamp experiments were performed that introduced synaptic conductances into ON and OFF cells. When the amacrine‐modulated conductance was removed, the ON cell's spike train exhibited an increase in SNR. The OFF cell, however, showed the opposite effect of removing amacrine input, which was a decrease in SNR. Thus ON and OFF cells have different modes of synaptic integration with direct effects on the SNR of the spike output.
    October 15, 2017   doi: 10.1113/JP274736   open full text
  • Laminar‐specific encoding of texture elements in rat barrel cortex.
    Benjamin J. Allitt, Dasuni S. Alwis, Ramesh Rajan.
    The Journal of Physiology. October 15, 2017
    Key points For rats texture discrimination is signalled by the large face whiskers by stick‐slip events. Neural encoding of repetitive stick‐slip events will be influenced by intrinsic properties of adaptation. We show that texture coding in the barrel cortex is laminar specific and follows a power function. Our results also show layer 2 codes for novel feature elements via robust firing rates and temporal fidelity. We conclude that texture coding relies on a subtle neural ensemble to provide important object information. Abstract Texture discrimination by rats is exquisitely guided by fine‐grain mechanical stick‐slip motions of the face whiskers as they encounter, stick to and slip past successive texture‐defining surface features such as bumps and grooves. Neural encoding of successive stick‐slip texture events will be shaped by adaptation, common to all sensory systems, whereby receptor and neural responses to a stimulus are affected by responses to preceding stimuli, allowing resetting to signal novel information. Additionally, when a whisker is actively moved to contact and brush over surfaces, that motion itself generates neural responses that could cause adaptation of responses to subsequent stick‐slip events. Nothing is known about encoding in the rat whisker system of stick‐slip events defining textures of different grain or the influence of adaptation from whisker protraction or successive texture‐defining stick‐slip events. Here we recorded responses from halothane‐anaesthetized rats in response to texture‐defining stimuli applied to passive whiskers. We demonstrate that: across the columnar network of the whisker‐recipient barrel cortex, adaptation in response to repetitive stick‐slip events is strongest in uppermost layers and equally lower thereafter; neither whisker protraction speed nor stick‐slip frequency impede encoding of stick‐slip events at rates up to 34.08 Hz; and layer 2 normalizes responses to whisker protraction to resist effects on texture signalling. Thus, within laminar‐specific response patterns, barrel cortex reliably encodes texture‐defining elements even to high frequencies.
    October 15, 2017   doi: 10.1113/JP274865   open full text
  • N1366S mutation of human skeletal muscle sodium channel causes paramyotonia congenita.
    Qing Ke, Jia Ye, Siyang Tang, Jin Wang, Benyan Luo, Fang Ji, Xu Zhang, Ye Yu, Xiaoyang Cheng, Yuezhou Li.
    The Journal of Physiology. October 15, 2017
    Key points Paramyotonia congenita is a hereditary channelopathy caused by missense mutations in the SCN4A gene, which encodes the α subunit of the human skeletal muscle voltage‐gated sodium channel NaV1.4. Affected individuals suffered from myotonia and paralysis of muscles, which were aggravated by exposure to cold. We report a three‐generation Chinese family with patients presenting paramyotonia congenita and identify a novel N1366S mutation of NaV1.4. Whole‐cell electrophysiological recordings of the N1366S channel reveal a gain‐of‐function change of gating in response to cold. Modelling and molecular dynamic simulation data suggest that an arginine‐to‐serine substitution at position 1366 increases the distance from N1366 to R1454 and disrupts the hydrogen bond formed between them at low temperature. We demonstrate that N1366S is a disease‐causing mutation and that the temperature‐sensitive alteration of N1366S channel activity may be responsible for the pronounced paramyotonia congenita symptoms of these patients. Abstract Paramyotonia congenita is an autosomal dominant skeletal muscle channelopathy caused by missense mutations in SCN4A, the gene encoding the α subunit of the human skeletal muscle voltage‐gated sodium channel NaV1.4. We report a three‐generation family in which six members present clinical symptoms of paramyotonia congenita characterized by a marked worsening of myotonia by cold and by the presence of clear episodes of paralysis. We identified a novel mutation in SCN4A (Asn1366Ser, N1366S) in all patients in the family but not in healthy relatives or in 500 normal control subjects. Functional analysis of the channel protein expressed in HEK293 cells by whole‐cell patch clamp recording revealed that the N1366S mutation led to significant alterations in the gating process of the NaV1.4 channel. The N1366S mutant displayed a cold‐induced hyperpolarizing shift in the voltage dependence of activation and a depolarizing shift in fast inactivation, as well as a reduced rate of fast inactivation and accelerated recovery from fast inactivation. In addition, homology modelling and molecular dynamic simulation of N1366S and wild‐type NaV1.4 channels indicated that the arginine‐to‐serine substitution disrupted the hydrogen bond formed between N1366 and R1454. Together, our results suggest that N1366S is a gain‐of‐function mutation of NaV1.4 at low temperature and the mutation may be responsible for the clinical symptoms of paramyotonia congenita in the affected family and constitute a basis for studies into its pathogenesis.
    October 15, 2017   doi: 10.1113/JP274877   open full text
  • Dietary sodium induces a redistribution of the tubular metabolic workload.
    Khalil Udwan, Ahmed Abed, Isabelle Roth, Eva Dizin, Marc Maillard, Carla Bettoni, Johannes Loffing, Carsten A. Wagner, Aurélie Edwards, Eric Feraille.
    The Journal of Physiology. October 15, 2017
    Key points Body Na+ content is tightly controlled by regulated urinary Na+ excretion. The intrarenal mechanisms mediating adaptation to variations in dietary Na+ intake are incompletely characterized. We confirmed and expanded observations in mice that variations in dietary Na+ intake do not alter the glomerular filtration rate but alter the total and cell‐surface expression of major Na+ transporters all along the kidney tubule. Low dietary Na+ intake increased Na+ reabsorption in the proximal tubule and decreased it in more distal kidney tubule segments. High dietary Na+ intake decreased Na+ reabsorption in the proximal tubule and increased it in distal segments with lower energetic efficiency. The abundance of apical transporters and Na+ delivery are the main determinants of Na+ reabsorption along the kidney tubule. Tubular O2 consumption and the efficiency of sodium reabsorption are dependent on sodium diet. Abstract Na+ excretion by the kidney varies according to dietary Na+ intake. We undertook a systematic study of the effects of dietary salt intake on glomerular filtration rate (GFR) and tubular Na+ reabsorption. We examined the renal adaptive response in mice subjected to 7 days of a low sodium diet (LSD) containing 0.01% Na+, a normal sodium diet (NSD) containing 0.18% Na+ and a moderately high sodium diet (HSD) containing 1.25% Na+. As expected, LSD did not alter measured GFR and increased the abundance of total and cell‐surface NHE3, NKCC2, NCC, α‐ENaC and cleaved γ‐ENaC compared to NSD. Mathematical modelling predicted that tubular Na+ reabsorption increased in the proximal tubule but decreased in the distal nephron because of diminished Na+ delivery. This prediction was confirmed by the natriuretic response to diuretics targeting the thick ascending limb, the distal convoluted tubule or the collecting system. On the other hand, HSD did not alter measured GFR but decreased the abundance of the aforementioned transporters compared to NSD. Mathematical modelling predicted that tubular Na+ reabsorption decreased in the proximal tubule but increased in distal segments with lower transport efficiency with respect to O2 consumption. This prediction was confirmed by the natriuretic response to diuretics. The activity of the metabolic sensor adenosine monophosphate‐activated protein kinase (AMPK) was related to the changes in tubular Na+ reabsorption. Our data show that fractional Na+ reabsorption is distributed differently according to dietary Na+ intake and induces changes in tubular O2 consumption and sodium transport efficiency.
    October 15, 2017   doi: 10.1113/JP274927   open full text
  • Task‐dependent output of human parasternal intercostal motor units across spinal levels.
    Anna L. Hudson, Simon C. Gandevia, Jane E. Butler.
    The Journal of Physiology. October 13, 2017
    Key points During breathing, there is differential activity in the human parasternal intercostal muscles and the activity is tightly coupled to the known mechanical advantages for inspiration of the same regions of muscles. It is not known whether differential activity is preserved for the non‐respiratory task of ipsilateral trunk rotation. In the present study, we compared single motor units during resting breathing and axial rotation of the trunk during apnoea. We not only confirmed non‐uniform recruitment of motor units across parasternal intercostal muscles in breathing, but also demonstrated that the same motor units show an altered pattern of recruitment in the non‐respiratory task of trunk rotation. The output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task and these results help us understand the mechanisms that may govern task‐dependent differences in motoneurone output. Abstract During inspiration, there is differential activity in the human parasternal intercostal muscles across interspaces. We investigated whether the earlier recruitment of motor units in the rostral interspaces compared to more caudal spaces during inspiration is preserved for the non‐respiratory task of ipsilateral trunk rotation. Single motor unit activity (SMU) was recorded from the first, second and fourth parasternal interspaces on the right side in five participants in two tasks: resting breathing and ‘isometric’ axial rotation of the trunk during apnoea. Recruitment of the same SMUs was compared between tasks (n = 123). During resting breathing, differential activity was indicated by earlier recruitment of SMUs in the first and second interspaces compared to the fourth space in inspiration (P < 0.01). By contrast, during trunk rotation, the same motor units showed an altered pattern of recruitment because SMUs in the first interspace were recruited later and at a higher rotation torque than those in the second and fourth interspaces (P < 0.05). Tested for a subset of SMUs, the reliability of the breathing and rotation tasks, as well as the SMU recruitment measures, was good–excellent [intraclass correlation (2,1): 0.69–0.91]. Thus, the output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task. Given that the differential inspiratory output of parasternal intercostal muscles is linked to their relative mechanical effectiveness for inspiration and also that this output is altered in trunk rotation, we speculate that a mechanism matching neural drive to muscle mechanics underlies the task‐dependent differences in output of axial motoneurone pools.
    October 13, 2017   doi: 10.1113/JP274866   open full text
  • Intrathecal antibody distribution in the rat brain: surface diffusion, perivascular transport, and osmotic enhancement of delivery.
    Michelle E. Pizzo, Daniel J. Wolak, Niyanta N. Kumar, Eric Brunette, Christina L. Brunnquell, Melanie‐Jane Hannocks, N. Joan Abbott, M. Elizabeth Meyerand, Lydia Sorokin, Danica B. Stanimirovic, Robert G. Thorne.
    The Journal of Physiology. October 12, 2017
    The precise mechanisms governing the central distribution of macromolecules from the cerebrospinal fluid (CSF) to the brain and spinal cord remain poorly understood, despite their importance for physiological processes such as antibody trafficking for central immune surveillance as well as several ongoing intrathecal clinical trials. Here, we clarify how immunoglobulin G (IgG) and smaller single‐domain antibodies (sdAb) distribute throughout the whole brain in a size‐dependent manner after intrathecal infusion in rats using ex vivo fluorescence and in vivo 3D magnetic resonance imaging. Antibody distribution was characterized by diffusion at the brain surface and widespread distribution to deep brain regions along perivascular spaces of all vessel types, with sdAb accessing 4–7 times greater brain area than IgG. Perivascular transport involved blood vessels of all caliber and putative smooth muscle and astroglial basement membrane compartments. Perivascular access to smooth muscle basement membrane compartments also exhibited size‐dependence. Electron microscopy was used to show stomata on leptomeningeal coverings of blood vessels in the subarachnoid space as potential access points for substances in the CSF to enter the perivascular space. Osmolyte co‐infusion significantly enhanced perivascular access of the larger antibody from the CSF, with intrathecal 0.75 m mannitol increasing the number of perivascular profiles per slice area accessed by IgG by approximately 50%. Our results reveal potential distribution mechanisms for endogenous IgG, one of the most abundant proteins in the CSF, as well as provide new insights needed to understand and improve drug delivery of macromolecules to the central nervous system via the intrathecal route. This article is protected by copyright. All rights reserved
    October 12, 2017   doi: 10.1113/JP275105   open full text
  • An investigation of fetal behavioural states during maternal sleep in healthy late gestation pregnancy: an observational study.
    Peter R. Stone, Wendy Burgess, Jordan McIntyre, Alistair J. Gunn, Christopher A. Lear, Laura Bennet, Edwin A Mitchell, John M. D. Thompson,.
    The Journal of Physiology. October 11, 2017
    Background Fetal behavioural states (FBS) are measures of fetal wellbeing. Maternal position affects FBS with supine being associated with an increased likelihood of fetal quiescence consistent with the human fetus adapting to a lower oxygen consuming state. A number of studies now confirm the association of sleep position with risk of late intrauterine death. We designed this study to observe the effects of maternal sleep positions overnight in healthy late gestation pregnancy. Method Twenty nine healthy women had continuous fetal ECG recordings overnight. Two blinded observers, assigned fetal states in 5 minute blocks. Measures of fetal heart rate variability (FHRV) were calculated from ECG beat to beat data. Maternal position was determined from infrared video recording. Results Compared to state 2F (active sleep), 4F (active awake‐high activity) occurred almost exclusively when the mother was in a left or right lateral position. State 1F (quiet sleep) was more common when mother was supine (OR 1.30, 95%CI, 1.11‐1.52) and less common on maternal right side with the left being referent position (OR 0.81, 95%CI, 0.70‐0.93). State 4F was more common between 2100 and 0100 than between 0100 and 0700 (OR 2 2.83, 95%CI, 2.32‐3.47). In each fetal state, maternal position had significant effects on fetal heart rate (FHR) and measures of FHRV. Conclusion In healthy late gestation pregnancy, maternal sleep position affects FBS and heart rate variability. These effects are likely fetal adaptations to positions which may produce a mild hypoxic stress. This article is protected by copyright. All rights reserved
    October 11, 2017   doi: 10.1113/JP275084   open full text
  • Physiological tremor increases when skeletal muscle is shortened: implications for fusimotor control.
    Kian Jalaleddini, Akira Nagamori, Christopher M. Laine, Mahsa A. Golkar, Robert E. Kearney, Francisco J. Valero‐Cuevas.
    The Journal of Physiology. October 11, 2017
    The involuntary force fluctuations associated with physiological (as distinct from pathological) tremor are an unavoidable component of human motor control. While the origins of physiological tremor are known to depend on muscle afferentation, it is possible that the mechanical properties of muscle‐tendon systems also affect its generation, amplification and maintenance. In this paper, we investigated the dependence of physiological tremor on muscle length in healthy individuals. We measured physiological tremor during tonic, isometric plantarflexion torque at 30% of maximum at three ankle angles. The amplitude of physiological tremor increased as calf muscles shortened in contrast to the stretch reflex whose amplitude decreases as muscle shortens. We used a published closed‐loop simulation model of afferented muscle to explore the mechanisms responsible for this behaviour. We demonstrate that changing muscle lengths does not suffice to explain our experimental findings. Rather, the model consistently required the modulation of γ‐static fusimotor drive to produce increases in physiological tremor with muscle shortening—while successfully replicating the concomitant reduction in stretch reflex amplitude. This need to control γ‐static fusimotor drive explicitly as a function of muscle length has important implications. First, it permits the amplitudes of physiological tremor and stretch reflex to be decoupled. Second, it postulates neuromechanical interactions that require length‐dependent γ drive modulation to be independent from α drive to the parent muscle. Lastly, it suggests that physiological tremor can be used as a simple, non‐invasive measure of the afferent mechanisms underlying healthy motor function, and their disruption in neurological conditions. This article is protected by copyright. All rights reserved
    October 11, 2017   doi: 10.1113/JP274899   open full text
  • KLF2 mediates enhanced chemoreflex sensitivity, disordered breathing and autonomic dysregulation in heart failure.
    Noah J. Marcus, Rodrigo Del Rio, Yanfeng Ding, Harold D. Schultz.
    The Journal of Physiology. October 11, 2017
    Key points Enhanced carotid body chemoreflex activity contributes to development of disordered breathing patterns, autonomic dysregulation and increases in incidence of arrhythmia in animal models of reduced ejection fraction heart failure. Chronic reductions in carotid artery blood flow are associated with increased carotid body chemoreceptor activity. Krüppel‐like Factor 2 (KLF2) is a shear stress‐sensitive transcription factor that regulates the expression of enzymes which have previously been shown to play a role in increased chemoreflex sensitivity. We investigated the impact of restoring carotid body KLF2 expression on chemoreflex control of ventilation, sympathetic nerve activity, cardiac sympatho‐vagal balance and arrhythmia incidence in an animal model of heart failure. The results indicate that restoring carotid body KLF2 in chronic heart failure reduces sympathetic nerve activity and arrhythmia incidence, and improves cardiac sympatho‐vagal balance and breathing stability. Therapeutic approaches that increase KLF2 in the carotid bodies may be efficacious in the treatment of respiratory and autonomic dysfunction in heart failure. Abstract Oscillatory breathing and increased sympathetic nerve activity (SNA) are associated with increased arrhythmia incidence and contribute to mortality in chronic heart failure (CHF). Increased carotid body chemoreflex (CBC) sensitivity plays a role in this process and can be precipitated by chronic blood flow reduction. We hypothesized that downregulation of a shear stress‐sensitive transcription factor, Krüppel‐like Factor 2 (KLF2), mediates increased CBC sensitivity in CHF and contributes to associated autonomic, respiratory and cardiac sequelae. Ventilation (Ve), renal SNA (RSNA) and ECG were measured at rest and during CBC activation in sham and CHF rabbits. Oscillatory breathing was quantified as the apnoea–hypopnoea index (AHI) and respiratory rate variability index (RRVI). AHI (control 6 ± 1/h, CHF 25 ± 1/h), RRVI (control 9 ± 3/h, CHF 29 ± 3/h), RSNA (control 22 ± 2% max, CHF 43 ± 5% max) and arrhythmia incidence (control 50 ± 10/h, CHF 300 ± 100/h) were increased in CHF at rest (FIO2 21%), as were CBC responses (Ve, RSNA) to 10% FIO2 (all P < 0.05 vs. control). In vivo adenoviral transfection of KLF2 to the carotid bodies in CHF rabbits restored KLF2 expression, and reduced AHI (7 ± 2/h), RSNA (18 ± 2% max) and arrhythmia incidence (46 ± 13/h) as well as CBC responses to hypoxia (all P < 0.05 vs. CHF empty virus). Conversely, lentiviral KLF2 siRNA in the carotid body decreased KLF2 expression, increased chemoreflex sensitivity, and increased AHI (6 ± 2/h vs. 14 ± 3/h), RRVI (5 ± 3/h vs. 20 ± 3/h) and RSNA (24 ± 4% max vs. 34 ± 5% max) relative to scrambled‐siRNA rabbits. In conclusion, down‐regulation of KLF2 in the carotid body increases CBC sensitivity, oscillatory breathing, RSNA and arrhythmia incidence during CHF.
    October 11, 2017   doi: 10.1113/JP273805   open full text
  • Plasma membrane Ca2+ ATPase 1 is required for maintaining atrial Ca2+ homeostasis and electrophysiological stability in the mouse.
    Yanwen Wang, Claire Wilson, Elizabeth J. Cartwright, Ming Lei.
    The Journal of Physiology. October 10, 2017
    To determine the role of PMCA1 in maintaining Ca2+ homeostasis and electrical stability in the atrium under physiological and stress conditions, mice with a cardiomyocyte‐specific deletion of PMCA1 (PMCA1cko) and their control littermates (PMCA1loxP/loxP) were studied at the organ and cellular levels.   At the organ level, the PMCA1cko hearts became more susceptible to atrial arrhythmias under rapid programmed electrical stimulation (PES) compared with the PMCA1loxP/loxP hearts, and such arrhythmic events became more severe under Ca2+ overload conditions. At the cellular level, the occurrence of irregular‐type APs of PMCA1cko atrial myocytes increased significantly under Ca2+ overload conditions and/or at higher frequency of stimulation. The decay of Na+‐Ca2+ exchanger (NCX) current that followed a stimulation protocol was significantly prolonged in PMCA1cko atrial myocytes under basal conditions, with Ca2+ overload leading to even greater prolongation. In conclusion, PMCA1 is required for maintaining Ca2+ homeostasis and electrical stability in the atrium. This is particularly critical during fast removal of Ca2+ from the cytosol which is required under stress conditions. This article is protected by copyright. All rights reserved
    October 10, 2017   doi: 10.1113/JP274110   open full text
  • Tonic inhibition of brown adipose tissue sympathetic nerve activity via muscarinic acetylcholine receptors in the rostral raphe pallidus.
    Ellen Paula Santos da Conceição, Christopher J. Madden, Shaun F. Morrison.
    The Journal of Physiology. October 10, 2017
    We sought to determine if body temperature and energy expenditure are influenced by a cholinergic input to neurons in the rostral raphe pallidus (rRPa), the site of sympathetic premotor neurons controlling brown adipose tissue (BAT) thermogenesis. Nanoinjections of the muscarinic acetylcholine receptor (mAChR) receptor agonist, oxotremorine, or the cholinesterase inhibitor, neostigmine (NEOS), in the rRPa of anaesthetized rats decreased cold‐evoked BAT sympathetic nerve activity (SNA, nadirs: −72%, and −95%), BAT temperature (TBAT, −0.5°C and −0.6°C), expired CO2 (Exp. CO2, −0.3% and −0.5%), and heart rate (HR, −22 bpm and −41 bpm). NEOS into rRPa reversed the increase in BAT SNA evoked by blockade of GABA receptors in rRPa. Nanoinjections of the mAChR antagonist, scopolamine (SCOP), in the rRPa of warm rats increased BAT SNA (peak: +1087%), TBAT (+1.8°C), Exp. CO2 (+0.7%), core temperature (TCORE, +0.5°C), and HR (+54 bpm). SCOP nanoinjections in rRPa produced similar activations of BAT during cold exposure, following a brain transection caudal to the hypothalamus, and during the blockade of glutamate receptors in rRPa. We conclude that a tonically‐active cholinergic input to the rRPa inhibits BAT SNA via activation of local mAChR. The inhibition of BAT SNA mediated by mAChR in rRPa does not depend on activation of GABA receptors in rRPa. The increase in BAT SNA following mAChR blockade in rRPa does not depend on the activity of neurons in the hypothalamus or on glutamate receptor activation in rRPa. This article is protected by copyright. All rights reserved
    October 10, 2017   doi: 10.1113/JP275299   open full text
  • Calcium and electrical dynamics in lymphatic endothelium.
    Erik J. Behringer, Joshua P. Scallan, Mohammad Jafarnejad, Jorge A. Castorena‐Gonzalez, Scott D. Zawieja, James E. Moore, Michael J. Davis, Steven S. Segal.
    The Journal of Physiology. October 09, 2017
    Subsequent to a rise in intracellular Ca2+ ([Ca2+]i), hyperpolarization of the endothelium coordinates vascular smooth muscle relaxation along resistance arteries during blood flow control. In the lymphatic vasculature, collecting vessels generate rapid contractions coordinated along lymphangions to propel lymph, but the underlying signalling pathways are unknown. We tested the hypothesis that lymphatic endothelial cells (LECs) exhibit Ca2+ and electrical signalling properties that facilitate lymph propulsion. To study electrical and intracellular Ca2+ signalling dynamics in lymphatic endothelium, we excised collecting lymphatic vessels from the popliteal fossa of mice and removed their muscle cells to isolate intact LEC tubes (LECTs). Intracellular recording revealed a resting membrane potential of ∼−70 mV. Acetylcholine (ACh) increased [Ca2+]i with a time course similar to that observed in endothelium of resistance arteries (i.e. rapid initial peak with a sustained “plateau”). In striking contrast to the endothelium‐derived hyperpolarization (EDH) characteristic of arteries, LECs depolarized (>15 mV) to either ACh or TRPV4 channel activation. This depolarization was facilitated by the absence of Ca2+‐activated K+ channels (KCa) as confirmed with PCR, persisted in the absence of extracellular Ca2+, was abolished by LaCl3 and was attenuated ∼70% in LECTs from Trpv4−/− mice. Computational modelling of ion fluxes in LECs indicated that omitting K+ channels supports our experimental results. These findings reveal novel signalling events in LECs, which are devoid of the KCa activity abundant in arterial endothelium. Absence of EDH with effective depolarization of LECs may promote the rapid conduction of contraction waves along lymphatic muscle during lymph propulsion. This article is protected by copyright. All rights reserved
    October 09, 2017   doi: 10.1113/JP274842   open full text
  • Leukoencephalopathy‐causing CLCN2 mutations are associated with impaired Cl− channel function and trafficking.
    Héctor Gaitán‐Peñas, Pirjo M Apaja, Tanit Arnedo, Aida Castellanos, Xabier Elorza‐Vidal, David Soto, Xavier Gasull, Gergely L Lukacs, Raúl Estévez.
    The Journal of Physiology. October 09, 2017
    Key points Characterisation of most mutations found in CLCN2 in patients with CC2L leukodystrophy show that they cause a reduction in function of the chloride channel ClC‐2. GlialCAM, a regulatory subunit of ClC‐2 in glial cells and involved in the leukodystrophy megalencephalic leukoencephalopathy with subcortical cysts (MLC), increases the activity of a ClC‐2 mutant by affecting ClC‐2 gating and by stabilising the mutant at the plasma membrane. The stabilisation of ClC‐2 at the plasma membrane by GlialCAM depends on its localisation at cell–cell junctions. The membrane protein MLC1, which is defective in MLC, also contributes to the stabilisation of ClC‐2 at the plasma membrane, providing further support for the view that GlialCAM, MLC1 and ClC‐2 form a protein complex in glial cells. Abstract Mutations in CLCN2 have been recently identified in patients suffering from a type of leukoencephalopathy involving intramyelinic oedema. Here, we characterised most of these mutations that reduce the function of the chloride channel ClC‐2 and impair its plasma membrane (PM) expression. Detailed biochemical and electrophysiological analyses of the Ala500Val mutation revealed that defective gating and increased cellular and PM turnover contributed to defective A500V‐ClC‐2 functional expression. Co‐expression of the adhesion molecule GlialCAM, which forms a tertiary complex with ClC‐2 and megalencephalic leukoencephalopathy with subcortical cysts 1 (MLC1), rescued the functional expression of the mutant by modifying its gating properties. GlialCAM also restored the PM levels of the channel by impeding its turnover at the PM. This rescue required ClC‐2 localisation to cell–cell junctions, since a GlialCAM mutant with compromised junctional localisation failed to rescue the impaired stability of mutant ClC‐2 at the PM. Wild‐type, but not mutant, ClC‐2 was also stabilised by MLC1 overexpression. We suggest that leukodystrophy‐causing CLCN2 mutations reduce the functional expression of ClC‐2, which is partly counteracted by GlialCAM/MLC1‐mediated increase in the gating and stability of the channel.
    October 09, 2017   doi: 10.1113/JP275087   open full text
  • Sensorimotor control of breathing in the mdx mouse model of Duchenne muscular dystrophy.
    David P. Burns, Arijit Roy, Eric F. Lucking, Fiona B. McDonald, Sam Gray, Richard J. Wilson, Deirdre Edge, Ken D. O'Halloran.
    The Journal of Physiology. October 09, 2017
    Key points Respiratory failure is a leading cause of mortality in Duchenne muscular dystrophy (DMD), but little is known about the control of breathing in DMD and animal models. We show that young (8 weeks of age) mdx mice hypoventilate during basal breathing due to reduced tidal volume. Basal CO2 production is equivalent in wild‐type and mdx mice. We show that carotid bodies from mdx mice have blunted responses to hyperoxia, revealing hypoactivity in normoxia. However, carotid body, ventilatory and metabolic responses to hypoxia are equivalent in wild‐type and mdx mice. Our study revealed profound muscle weakness and muscle fibre remodelling in young mdx diaphragm, suggesting severe mechanical disadvantage in mdx mice at an early age. Our novel finding of potentiated neural motor drive to breathe in mdx mice during maximal chemoactivation suggests compensatory neuroplasticity enhancing respiratory motor output to the diaphragm and probably other accessory muscles. Abstract Patients with Duchenne muscular dystrophy (DMD) hypoventilate with consequential arterial blood gas derangement relevant to disease progression. Whereas deficits in DMD diaphragm are recognized, there is a paucity of knowledge in respect of the neural control of breathing in dystrophinopathies. We sought to perform an analysis of respiratory control in a model of DMD, the mdx mouse. In 8‐week‐old male wild‐type and mdx mice, ventilation and metabolism, carotid body afferent activity, diaphragm muscle force‐generating capacity, and muscle fibre size, distribution and centronucleation were determined. Diaphragm EMG activity and responsiveness to chemostimulation was determined. During normoxia, mdx mice hypoventilated, owing to a reduction in tidal volume. Basal CO2 production was not different between wild‐type and mdx mice. Carotid sinus nerve responses to hyperoxia were blunted in mdx, suggesting hypoactivity. However, carotid body, ventilatory and metabolic responses to hypoxia were equivalent in wild‐type and mdx mice. Diaphragm force was severely depressed in mdx mice, with evidence of fibre remodelling and damage. Diaphragm EMG responses to chemoactivation were enhanced in mdx mice. We conclude that there is evidence of chronic hypoventilation in young mdx mice. Diaphragm dysfunction confers mechanical deficiency in mdx resulting in impaired capacity to generate normal tidal volume at rest and decreased absolute ventilation during chemoactivation. Enhanced mdx diaphragm EMG responsiveness suggests compensatory neuroplasticity facilitating respiratory motor output, which may extend to accessory muscles of breathing. Our results may have relevance to emerging treatments for human DMD aiming to preserve ventilatory capacity.
    October 09, 2017   doi: 10.1113/JP274792   open full text
  • Empowering human cardiac progenitor cells by P2Y14 nucleotide receptor overexpression.
    Farid G. Khalafalla, Waqas Kayani, Arwa Kassab, Kelli Ilves, Megan M. Monsanto, Roberto Alvarez, Monica Chavarria, Benjamin Norman, Walter P. Dembitsky, Mark A. Sussman.
    The Journal of Physiology. October 05, 2017
    Autologous cardiac progenitor cell (hCPC) therapy is a promising alternative approach to current inefficient therapies for heart failure (HF). However, ex vivo expansion and pharmacological/genetic modification of hCPCs are necessary interventions to rejuvenate aged/diseased cells and improve their regenerative capacities. This study was designed to assess the potential of improving hCPC functional capacity by targeting P2Y14 purinergic receptor (P2Y14R), which has been previously reported to induce regenerative and anti‐senescence responses in a variety of experimental models. c‐Kit+ hCPCs were isolated from cardiac biopsies of multiple HF patients undergoing left ventricular assist device (LVAD) implantation surgery. Significant correlations existed between expression of P2Y14R in hCPCs and clinical parameters of HF patients. P2Y14R was downregulated in hCPCs derived from patients with relatively lower ejection fraction and patients diagnosed with diabetes. hCPC lines with lower P2Y14R expression did not respond to P2Y14R agonist UDP‐glucose (UDP‐Glu) while hCPCs with higher P2Y14R expression showed enhanced proliferation in response to UDP‐Glu stimulation. Mechanistically, UDP‐Glu stimulation enhanced activation of canonical growth signalling pathways ERK1/2 and AKT. Restoring P2Y14R expression levels in functionally compromised hCPCs via lentiviral‐mediated overexpression improved proliferation, migration and survival under stress stimuli. Additionally, P2Y14R overexpression reversed senescence‐associated morphology and reduced levels of molecular markers of senescence p16INK4a, p53, p21 and mitochondrial reactive oxygen species (ROS). Findings from this study unveil novel biological roles of the UDP‐sugar receptor P2Y14 in hCPCs and suggest purinergic signalling modulation as a promising strategy to improve phenotypic properties of functionally impaired hCPCs. This article is protected by copyright. All rights reserved
    October 05, 2017   doi: 10.1113/JP274980   open full text
  • Rapid versus slow ascending vasodilatation: intercellular conduction versus flow‐mediated signalling with tetanic versus rhythmic muscle contractions.
    Shenghua Y. Sinkler, Steven S. Segal.
    The Journal of Physiology. October 05, 2017
    In response to exercise, vasodilatation initiated within the microcirculation of skeletal muscle ascends the resistance network into upstream feed arteries (FAs) located external to the tissue. Ascending vasodilatation (AVD) is essential to reducing FA resistance that otherwise restricts blood flow into the microcirculation. We tested the hypothesis that signalling events underlying AVD vary with the intensity and duration of muscle contraction. In the gluteus maximus muscle of anaesthetized male C57BL/6 mice (age, 3‐4 months), brief tetanic contraction (100 Hz, 500 ms) evoked rapid onset vasodilatation (ROV) in FAs that peaked within 4 s. In contrast, during rhythmic twitch contractions (4 Hz), slow onset vasodilatation (SOV) of FAs began after ∼10 s and plateaued within 30 s. Selectively damaging the endothelium with light‐dye treatment midway between a FA and its primary arteriole eliminated ROV in the FA along with conducted vasodilatation of the FA initiated on the arteriole using ACh microiontophoresis. Superfusion of SKCa and IKCa channel blockers UCL 1684 + TRAM 34 attenuated ROV, implicating endothelial hyperpolarization as the underlying signal. Nevertheless, SOV of FAs during rhythmic contractions persisted until superfusion of NO synthase with L‐NAME. Thus, ROV of FAs reflects hyperpolarization of downstream arterioles that conducts along the endothelium into proximal FAs. In contrast, SOV of FAs reflects local production of NO by the endothelium in response to luminal shear stress, which increases secondary to arteriolar dilatation downstream. Thus, AVD ensures increased oxygen delivery to active muscle fibres by reducing upstream resistance via complementary signalling pathways that reflect the intensity and duration of muscle contraction. This article is protected by copyright. All rights reserved
    October 05, 2017   doi: 10.1113/JP275186   open full text
  • Chronic beta2‐adrenoceptor agonist treatment alters muscle proteome and functional adaptations induced by high intensity training in young men.
    Morten Hostrup, Johan Onslev, Glenn Jacobson, Richard Wilson, Jens Bangsbo.
    The Journal of Physiology. October 05, 2017
    Although the effects of training have been studied for decades, data on muscle proteome signature remodelling induced by high intensity training in relation to functional changes in humans remains incomplete. Likewise, β2‐agonists are frequently used to counteract exercise‐induced bronchoconstriction, but the effects β2‐agonist treatment on muscle remodelling and adaptations to training are unknown. In a placebo‐controlled parallel study, we randomized 21 trained men to four weeks of high intensity training with (HIT + β2A) or without (HIT) daily inhalation of β2‐agonist (terbutaline, 4 mg d−1). Of 486 proteins identified by mass‐spectrometry proteomics of muscle biopsies sampled before and after the intervention, 32 and 85 were changing (FDR ≤ 5%) with the intervention in HIT and HIT + β2A. Proteome signature changes were different in HIT and HIT + β2A (P = 0.005), wherein β2‐agonist caused a repression of 25 proteins in HIT + β2A compared to HIT, and an upregulation of 7 proteins compared to HIT. β2‐agonist repressed or even downregulated training‐induced enrichment of pathways related to oxidative phosphorylation and glycogen metabolism, but upregulated pathways related to histone trimethylation and the nucleosome. Muscle contractile phenotype changed differently in HIT and HIT + β2A (P ≤ 0.001), with a fast‐to‐slow twitch transition in HIT and a slow‐to‐fast twitch transition in HIT + β2A. β2‐agonist attenuated training‐induced enhancements in maximal oxygen consumption (P ≤ 0.01) and exercise performance (11.6 vs. 6.1%, P ≤ 0.05) in HIT + β2A compared to HIT. These findings indicate that daily β2‐agonist treatment attenuates the beneficial effects of high intensity training on exercise performance and oxidative capacity, and causes remodelling of muscle proteome signature towards a fast‐twitch phenotype. This article is protected by copyright. All rights reserved
    October 05, 2017   doi: 10.1113/JP274970   open full text
  • Post‐exercise recovery of contractile function and endurance in humans and mice is accelerated by heating and slowed by cooling skeletal muscle.
    Arthur J. Cheng, Sarah J. Willis, Christoph Zinner, Thomas Chaillou, Niklas Ivarsson, Niels Ørtenblad, Johanna T. Lanner, Hans‐Christer Holmberg, Håkan Westerblad.
    The Journal of Physiology. October 04, 2017
    Manipulation of muscle temperature is believed to improve post‐exercise recovery, with cooling being especially popular among athletes. However, it is unclear whether such temperature manipulations actually have positive effects. Accordingly, we studied the effect of muscle temperature on the acute recovery of force and fatigue resistance after endurance exercise. One hour of moderate‐intensity arm cycling exercise in humans was followed by two hours recovery in which the upper arms were either heated to 38°C, not treated (33°C), or cooled to ∼15°C. Fatigue resistance after the recovery period was assessed by performing 3 × 5 min sessions of all‐out arm cycling at physiological temperature for all conditions (i.e. not heated or cooled). Power output during the all‐out exercise was better maintained when muscles were heated during recovery, whereas cooling had the opposite effect. Mechanisms underlying the temperature‐dependent effect on recovery were tested in mouse intact single muscle fibres, which were exposed to ∼12 min of glycogen‐depleting fatiguing stimulation (350 ms tetani given at 10 s interval until force decreased to 30% of the starting force). Fibres were subsequently exposed to the same fatiguing stimulation protocol after 1–2 h of recovery at 16–36°C. Recovery of submaximal force (30 Hz), the tetanic myoplasmic free [Ca2+] (measured with the fluorescent indicator indo‐1), and fatigue resistance were all impaired by cooling (16‐26°C) and improved by heating (36°C). In addition, glycogen resynthesis was faster at 36°C than 26°C in whole FDB muscles. We conclude that recovery from exhaustive endurance exercise is accelerated by raising and slowed by lowering muscle temperature. This article is protected by copyright. All rights reserved
    October 04, 2017   doi: 10.1113/JP274870   open full text
  • Rapid decline in MyHC I(β) mRNA expression in rat soleus during hindlimb unloading is associated with the AMPK dephosphorylation.
    Natalia A. Vilchinskaya, Ekaterina P. Mochalova, Tatiana L. Nemirovskaya, Timur M. Mirzoev, Olga V. Turtikova, Boris S. Shenkman.
    The Journal of Physiology. October 03, 2017
    One of the key events that occurs during skeletal muscle inactivation is a change in myosin phenotype, i.e. increased expression of fast isoforms and decreased expression of slow isoform of myosin heavy chain (MyHC). It is known that calcineurin/NFAT and AMP‐activated protein kinase (AMPK) can regulate the expression of genes encoding MyHC slow isoform. Earlier, we found a significant decrease in phosphorylated AMPK in rat soleus after 24 h of hindlimb unloading (HU). We hypothesized that a decrease in AMPK phosphorylation and subsequent histone deacetylase (HDAC) nuclear translocation can be one of the triggering events leading to a reduced expression of slow MyHC. To test this hypothesis, Wistar rats were treated with AMPK activator (AICAR) for 6 d before HU as well as during 24‐h HU. We discovered that AICAR treatment prevented a decrease in pre‐mRNA and mRNA expression of MyHC I as well as MyHC IIa mRNA expression. 24‐h HS resulted in HDAC4 accumulation in the nuclei of rat soleus but AICAR pretreatment prevented such an accumulation. The results of the study indicate that AMPK dephosphorylation after 24‐h HU had a significant impact on the MyHC I and MyHC IIa mRNA expression in rat soleus. AMPK dephosphorylation also contributed to the HDAC4 translocation to the nuclei of soleus muscle fibres, suggesting an important role of HDAC4 as an epigenetic regulator in the process of myosin phenotype transformation. This article is protected by copyright. All rights reserved
    October 03, 2017   doi: 10.1113/JP275184   open full text
  • δ‐ and β‐cells are electrically coupled and regulate α‐cell activity via somatostatin.
    L. J. B. Briant, T. M. Reinbothe, I. Spiliotis, C. Miranda, B. Rodriguez, P. Rorsman.
    The Journal of Physiology. October 03, 2017
    Glucagon, the body's principal hyperglycaemic hormone, is released from the α‐cells of the pancreatic islet. The secretion of this hormone is dysregulated in type 2 diabetes mellitus but the mechanisms controlling secretion are not well understood. Regulation of glucagon secretion by factors secreted by neighbouring β‐ and δ‐cells (paracrine regulation) have been proposed to be important. In this study, we explored the importance of paracrine regulation by using an optogenetic strategy. Specific light‐induced activation of β‐cells in mouse islets expressing the light‐gated channelrhodopsin‐2 resulted in stimulation of electrical activity in δ‐cells but suppression of α‐cell activity. The activation of the δ‐cells was rapid and sensitive to the gap junction inhibitor carbenoxolone, whereas the effect on electrical activity in α‐cells was blocked by CYN 154806, an antagonist of the somatostatin‐2 receptor. These observations indicate that optogenetic activation of the β‐cells propagates to the δ‐cells via gap junctions, and the consequential stimulation of somatostatin secretion inhibits α‐cell electrical activity by a paracrine mechanism. To explore whether this pathway is important for regulating α‐cell activity and glucagon secretion in human islets, we constructed computational models of human islets. These models had detailed architectures based on human islets and consisted of a collection of >500 α‐, β‐ and δ‐cells. Simulations of these models revealed that this gap junctional/paracrine mechanism accounts for up to 23% of the suppression of glucagon secretion by high glucose. This article is protected by copyright. All rights reserved
    October 03, 2017   doi: 10.1113/JP274581   open full text
  • Maternal exercise modifies body composition and energy substrates handling in high‐fat/high‐sucrose diet fed male offspring.
    Charline Quiclet, Hervé Dubouchaud, Phanélie Berthon, Hervé Sanchez, Guillaume Vial, Farida Siti, Eric Fontaine, Cécile Batandier, Karine Couturier.
    The Journal of Physiology. October 03, 2017
    Maternal exercise during gestation has been reported to modify offspring metabolism and health. Whether these effects are exacerbated when offspring is under high‐fat diet remains rather unclear. Our purpose was to evaluate the effect of maternal exercise before and during gestation on the offspring fed a high‐fat/high‐sucrose diet (HF), by assessing its body composition, pancreatic function and the energy substrates handling by two major glucose‐utilizing tissues: liver and muscle. Fifteen week‐old nulliparous female Wistar rats exercised 4 weeks before as well as during gestation at a constant submaximal intensity (TR) or remained sedentary (CT). At weaning, pups from each group were fed either a standard diet (TRCD or CTCD) or a high‐fat/high‐sucrose diet (TRHF or CTHF) for 10 weeks. Offspring from TR dams gained less weight compared to those from CT dams. Selected fat depots were larger with HF diet compared to CD but significantly smaller in TRHF compared to CTHF. Surprisingly, insulin secretion index was higher in islets from HF offspring compared to CD. TR offspring showed a higher muscle insulin sensitivity estimated by the pPKB/PKB ratio compared with CT offspring (+48%, P < 0.05). With CD, permeabilized isolated muscle fibres from TR rats displayed a lower apparent affinity constant (Km) for pyruvate and palmitoyl Co‐A as substrates compared to the CT group (−46% and −58% respectively, P < 0.05). These results suggest that maternal exercise has positive effects on young adult offspring body composition and on muscle carbohydrate and lipid metabolism depending on the nutritional status. This article is protected by copyright. All rights reserved
    October 03, 2017   doi: 10.1113/JP274739   open full text
  • Visuo‐manual tracking: does intermittent control with aperiodic sampling explain linear power and non‐linear remnant without sensorimotor noise?
    Henrik Gollee, Peter J. Gawthrop, Martin Lakie, Ian D. Loram.
    The Journal of Physiology. October 01, 2017
    Key points A human controlling an external system is described most easily and conventionally as linearly and continuously translating sensory input to motor output, with the inevitable output remnant, non‐linearly related to the input, attributed to sensorimotor noise. Recent experiments show sustained manual tracking involves repeated refractoriness (insensitivity to sensory information for a certain duration), with the temporary 200–500 ms periods of irresponsiveness to sensory input making the control process intrinsically non‐linear. This evidence calls for re‐examination of the extent to which random sensorimotor noise is required to explain the non‐linear remnant. This investigation of manual tracking shows how the full motor output (linear component and remnant) can be explained mechanistically by aperiodic sampling triggered by prediction error thresholds. Whereas broadband physiological noise is general to all processes, aperiodic sampling is associated with sensorimotor decision making within specific frontal, striatal and parietal networks; we conclude that manual tracking utilises such slow serial decision making pathways up to several times per second. Abstract The human operator is described adequately by linear translation of sensory input to motor output. Motor output also always includes a non‐linear remnant resulting from random sensorimotor noise from multiple sources, and non‐linear input transformations, for example thresholds or refractory periods. Recent evidence showed that manual tracking incurs substantial, serial, refractoriness (insensitivity to sensory information of 350 and 550 ms for 1st and 2nd order systems respectively). Our two questions are: (i) What are the comparative merits of explaining the non‐linear remnant using noise or non‐linear transformations? (ii) Can non‐linear transformations represent serial motor decision making within the sensorimotor feedback loop intrinsic to tracking? Twelve participants (instructed to act in three prescribed ways) manually controlled two systems (1st and 2nd order) subject to a periodic multi‐sine disturbance. Joystick power was analysed using three models, continuous‐linear‐control (CC), continuous‐linear‐control with calculated noise spectrum (CCN), and intermittent control with aperiodic sampling triggered by prediction error thresholds (IC). Unlike the linear mechanism, the intermittent control mechanism explained the majority of total power (linear and remnant) (77–87% vs. 8–48%, IC vs. CC). Between conditions, IC used thresholds and distributions of open loop intervals consistent with, respectively, instructions and previous measured, model independent values; whereas CCN required changes in noise spectrum deviating from broadband, signal dependent noise. We conclude that manual tracking uses open loop predictive control with aperiodic sampling. Because aperiodic sampling is inherent to serial decision making within previously identified, specific frontal, striatal and parietal networks we suggest that these structures are intimately involved in visuo‐manual tracking.
    October 01, 2017   doi: 10.1113/JP274288   open full text
  • Inducible satellite cell depletion attenuates skeletal muscle regrowth following a scald‐burn injury.
    Celeste C. Finnerty, Colleen F. McKenna, Lauren A. Cambias, Camille R. Brightwell, Anesh Prasai, Ye Wang, Amina El Ayadi, David N. Herndon, Oscar E. Suman, Christopher S. Fry.
    The Journal of Physiology. October 01, 2017
    Key points Severe burns result in significant skeletal muscle cachexia that impedes recovery. Activity of satellite cells, skeletal muscle stem cells, is altered following a burn injury and likely hinders regrowth of muscle. Severe burn injury induces satellite cell proliferation and fusion into myofibres with greater activity in muscles proximal to the injury site. Conditional depletion of satellite cells attenuates recovery of myofibre area and volume following a scald burn injury in mice. Skeletal muscle regrowth following a burn injury requires satellite cell activity, underscoring the therapeutic potential of satellite cells in the prevention of prolonged frailty in burn survivors. Abstract Severe burns result in profound skeletal muscle atrophy; persistent muscle atrophy and weakness are major complications that hamper recovery from burn injury. Many factors contribute to the erosion of muscle mass following burn trauma, and we have previously shown concurrent activation and apoptosis of muscle satellite cells following a burn injury in paediatric patients. To determine the necessity of satellite cells during muscle recovery following a burn injury, we utilized a genetically modified mouse model (Pax7CreER‐DTA) that allows for the conditional depletion of satellite cells in skeletal muscle. Additionally, mice were provided 5‐ethynyl‐2′‐deoxyuridine to determine satellite cell proliferation, activation and fusion. Juvenile satellite cell‐wild‐type (SC‐WT) and satellite cell‐depleted (SC‐Dep) mice (8 weeks of age) were randomized to sham or burn injury consisting of a dorsal scald burn injury covering 30% of total body surface area. Both hindlimb and dorsal muscles were studied at 7, 14 and 21 days post‐burn. SC‐Dep mice had >93% depletion of satellite cells compared to SC‐WT (P < 0.05). Burn injury induced robust atrophy in muscles located both proximal and distal to the injury site (∼30% decrease in fibre cross‐sectional area, P < 0.05). Additionally, burn injury induced skeletal muscle regeneration, satellite cell proliferation and fusion. Depletion of satellite cells impaired post‐burn recovery of both muscle fibre cross‐sectional area and volume (P < 0.05). These findings support an integral role for satellite cells in the aetiology of lean tissue recovery following a severe burn injury.
    October 01, 2017   doi: 10.1113/JP274841   open full text
  • Defining the neural fulcrum for chronic vagus nerve stimulation: implications for integrated cardiac control.
    Jeffrey L. Ardell, Heath Nier, Matthew Hammer, E. Marie Southerland, Christopher L. Ardell, Eric Beaumont, Bruce H. KenKnight, J. Andrew Armour.
    The Journal of Physiology. September 30, 2017
    Key points The evoked cardiac response to bipolar cervical vagus nerve stimulation (VNS) reflects a dynamic interaction between afferent mediated decreases in central parasympathetic drive and suppressive effects evoked by direct stimulation of parasympathetic efferent axons to the heart. The neural fulcrum is defined as the operating point, based on frequency–amplitude–pulse width, where a null heart rate response is reproducibly evoked during the on‐phase of VNS. Cardiac control, based on the principal of the neural fulcrum, can be elicited from either vagus. Beta‐receptor blockade does not alter the tachycardia phase to low intensity VNS, but can increase the bradycardia to higher intensity VNS. While muscarinic cholinergic blockade prevented the VNS‐induced bradycardia, clinically relevant doses of ACE inhibitors, beta‐blockade and the funny channel blocker ivabradine did not alter the VNS chronotropic response. While there are qualitative differences in VNS heart control between awake and anaesthetized states, the physiological expression of the neural fulcrum is maintained. Abstract Vagus nerve stimulation (VNS) is an emerging therapy for treatment of chronic heart failure and remains a standard of therapy in patients with treatment‐resistant epilepsy. The objective of this work was to characterize heart rate (HR) responses (HRRs) during the active phase of chronic VNS over a wide range of stimulation parameters in order to define optimal protocols for bidirectional bioelectronic control of the heart. In normal canines, bipolar electrodes were chronically implanted on the cervical vagosympathetic trunk bilaterally with anode cephalad to cathode (n = 8, ‘cardiac’ configuration) or with electrode positions reversed (n = 8, ‘epilepsy’ configuration). In awake state, HRRs were determined for each combination of pulse frequency (2–20 Hz), intensity (0–3.5 mA) and pulse widths (130–750 μs) over 14 months. At low intensities and higher frequency VNS, HR increased during the VNS active phase owing to afferent modulation of parasympathetic central drive. When functional effects of afferent and efferent fibre activation were balanced, a null HRR was evoked (defined as ‘neural fulcrum’) during which HRR ≈ 0. As intensity increased further, HR was reduced during the active phase of VNS. While qualitatively similar, VNS delivered in the epilepsy configuration resulted in more pronounced HR acceleration and reduced HR deceleration during VNS. At termination, under anaesthesia, transection of the vagi rostral to the stimulation site eliminated the augmenting response to VNS and enhanced the parasympathetic efferent‐mediated suppressing effect on electrical and mechanical function of the heart. In conclusion, VNS activates central then peripheral aspects of the cardiac nervous system. VNS control over cardiac function is maintained during chronic therapy.
    September 30, 2017   doi: 10.1113/JP274678   open full text
  • Pre‐clinical evaluation of N‐acetylcysteine reveals side effects in the mdx mouse model of Duchenne muscular dystrophy.
    Gavin J. Pinniger, Jessica R. Terrill, Evanna B. Assan, Miranda D. Grounds, Peter G. Arthur.
    The Journal of Physiology. September 30, 2017
    Key points Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disease associated with increased inflammation and oxidative stress. The antioxidant N‐acetylcysteine (NAC) has been proposed as a therapeutic intervention for DMD boys, but potential adverse effects of NAC have not been widely investigated. We used young (6 weeks old) growing mdx mice to investigate the capacity of NAC supplementation (2% in drinking water for 6 weeks) to improve dystrophic muscle function and to explore broader systemic effects of NAC treatment. NAC treatment improved normalised measures of muscle function, and decreased inflammation and oxidative stress, but significantly reduced body weight gain, muscle weight and liver weight. Unexpected significant adverse effects of NAC on body and muscle weights indicate that interpretation of muscle function based on normalised force measures should be made with caution and careful consideration is needed when proposing the use of NAC as a therapeutic treatment for young DMD boys. Abstract Duchenne muscular dystrophy (DMD) is a fatal X‐linked muscle wasting disease characterised by severe muscle weakness, necrosis, inflammation and oxidative stress. The antioxidant N‐acetylcysteine (NAC) has been proposed as a potential therapeutic intervention for DMD boys. We investigated the capacity of NAC to improve dystrophic muscle function in the mdx mouse model of DMD. Young (6 weeks old) mdx and non‐dystrophic C57 mice receiving 2% NAC in drinking water for 6 weeks were compared with untreated mice. Grip strength and body weight were measured weekly, before the 12 week old mice were anaesthetised and extensor digitorum longus (EDL) muscles were excised for functional analysis and tissues were sampled for biochemical analyses. Compared to untreated mice, the mean (SD) normalised grip strength was significantly greater in NAC‐treated mdx [3.13 (0.58) vs 4.87 (0.78) g body weight (bw)−1; P < 0.001] and C57 mice [3.90 (0.32) vs 5.32 (0.60) g bw−1; P < 0.001]. Maximum specific force was significantly greater in NAC‐treated mdx muscles [9.80 (2.27) vs 13.07 (3.37) N cm−2; P = 0.038]. Increased force in mdx mice was associated with reduced thiol oxidation and inflammation in fast muscles, and increased citrate synthase activity in slow muscle. Importantly, NAC significantly impaired body weight gain in both strains of young growing mice, and reduced liver weight in C57 mice and muscle weight in mdx mice. These potentially adverse effects of NAC emphasise the need for caution when interpreting improvements in muscle function based on normalised force measures, and that careful consideration be given to these effects when proposing NAC as a potential treatment for young DMD boys.
    September 30, 2017   doi: 10.1113/JP274229   open full text
  • Interstitial IgG antibody pharmacokinetics assessed by combined in vivo‐ and physiologically‐based pharmacokinetic modelling approaches.
    Miro J. Eigenmann, Tine V. Karlsen, Ben‐Fillippo Krippendorff, Olav Tenstad, Ludivine Fronton, Michael B. Otteneder, Helge Wiig.
    The Journal of Physiology. September 27, 2017
    For most therapeutic antibodies, the interstitium is the target space. Although experimental methods for measuring antibody pharmacokinetics (PK) in this space are not well established, making quantitative assessment difficult, the interstitial antibody concentration is assumed to be low. Here, we combined direct quantification of antibodies in the interstitial fluid with a physiologically‐based PK (PBPK) modelling approach with the goal of better describing the PK of monoclonal antibodies in the interstitial space of different tissues. We isolated interstitial fluid by tissue centrifugation, and conducted an antibody biodistribution study in mice, measuring total tissue‐ and interstitial concentrations in selected tissues. Residual plasma, interstitial volumes and lymph flows, which are important PBPK model parameters, were assessed in vivo. We could thereby refine PBPK modelling of monoclonal antibodies, better interpret antibody biodistribution data and more accurately predict their PK in the different tissue spaces. Our results indicate that in tissues with discontinuous capillaries (liver and spleen), interstitial concentrations are reflected by plasma concentration. In tissues with continuous capillaries (e.g. skin and muscle), ∼50‐60% of plasma concentration is found in the interstitial space. In brain and kidney, on the other hand, antibodies are restricted to the vascular space. Our data may significantly impact the interpretation of biodistribution data of monoclonal antibodies and might be important when relating measured concentrations to a therapeutic effect. Opposing the view that antibodies distribution to the interstitial space is limited, we show by direct measurements and model‐based data interpretation that high antibody interstitial concentrations are reached in most tissues. This article is protected by copyright. All rights reserved
    September 27, 2017   doi: 10.1113/JP274819   open full text
  • The α2A adrenoceptor suppresses excitatory synaptic transmission to both excitatory and inhibitory neurons in layer 4 barrel cortex.
    Minoru Ohshima, Chiaki Itami, Fumitaka Kimura.
    The Journal of Physiology. September 26, 2017
    The mammalian neocortex is widely innervated by noradrenergic (NA) fibres from the locus coeruleus. To determine the effects of NA on vertical synaptic inputs to layer 4 (L4) cells from the ventrobasal (VB) thalamus and layer 2/3 (L2/3), thalamocortical slices were prepared and whole‐cell recordings were made from L4 cells. Excitatory synaptic responses were evoked by electrical stimulation of the thalamus or L2/3 immediately above. Recorded cells were identified as regular spiking (RS), regular spiking non‐pyramidal (RSNP) or fast spiking (FS) cells through their firing patterns in response to current injections. NA suppressed (∼50% of control) excitatory vertical inputs to all cell types in a dose‐dependent manner. The presynaptic site of action of NA was suggested by three independent studies. First, responses caused by iontophoretically applied glutamate were not suppressed by NA. Second, paired pulse ratio was increased during NA suppression. Finally, a CV−2 (CV: coefficient of variation) analysis was performed. The resultant diagonal alignment of the ratio of CV−2 plotted against the ratio of the amplitude of postsynaptic responses suggests a presynaptic mechanism for the suppression. Experiments with phenylephrine (α1 agonist), prazosin (α1 antagonist), yohimbine (α2 antagonist) and propranolol (β antagonist) indicated that suppression was mediated by α2 adrenoceptor. To determine whether the α2A adrenoceptor subtype was involved, α2A adrenoceptor knockout mice were used. NA failed to suppress EPSCs in all cell types, suggesting an involvement of α2A adrenoceptor. Altogether, we concluded that NA suppresses vertical excitatory synaptic connections in L4 excitatory and inhibitory cells through presynaptic α2A adrenoceptor. This article is protected by copyright. All rights reserved
    September 26, 2017   doi: 10.1113/JP275142   open full text
  • Calcium influx through TRPV4 channels modulates the adherens contacts between retinal microvascular endothelial cells.
    Tam T. T. Phuong, Sarah N. Redmon, Oleg Yarishkin, Jacob M. Winter, Dean Y. Li, David Križaj.
    The Journal of Physiology. September 26, 2017
    The identity of microvascular endothelial (MVE) mechanosensors that sense blood flow in response to mechanical and chemical stimuli and regulate vascular permeability in the retina is unknown. Taking advantage of immunohistochemistry, calcium imaging, electrophysiology, impedance measurements and vascular permeability assays, we show that the transient receptor potential isoform 4 (TRPV4) plays a major role in Ca2+/cation signalling, cytoskeletal remodelling and barrier function in retinal microvasculature in vitro and in vivo. Human retinal MVECs (HrMVECs) predominantly expressed Trpv1 and Trpv4 transcripts, and TRPV4 was broadly localized to the plasma membrane of cultured cells and intact blood vessels in the inner retina. Treatment with the selective TRPV4 agonist GSK1016790A (GSK101) activated a nonselective cation current, robustly elevated [Ca2+]i and reversibly increased the permeability of MVEC monolayers. This was associated with disrupted organization of endothelial F‐actin, downregulated expression of occludin and remodelling of adherens contacts consisting of vascular endothelial cadherin (VE‐cadherin) and β−catenin. In vivo, GSK101 increased the permeability of retinal blood vessels in wild type, but not in TRPV4 knockout mice. Agonist‐evoked effects on barrier permeability and cytoskeletal reorganization were antagonized by the selective TRPV4 blocker HC 067047. Human choroidal endothelial cells (CECs) showed lower TRPV4 mRNA/protein levels and less pronounced agonist‐evoked calcium signals compared to MVECs. These findings demonstrate a major role for TRPV4 in Ca2+ homeostasis and barrier function in the human retinal microvascular endothelial barrier and suggest TRPV4 may differentially contribute to the inner vis à vis outer blood‐retinal barrier function. This article is protected by copyright. All rights reserved
    September 26, 2017   doi: 10.1113/JP275052   open full text
  • Cortical thickness is associated with altered autonomic function in cognitively impaired and non‐impaired older adults.
    Feng Lin, Ping Ren, Xixi Wang, Mia Anthony, Duje Tadin, Kathi L. Heffner.
    The Journal of Physiology. September 26, 2017
    Background Parasympathetic nervous system (PNS) is critical for adaptation to environment demands. PNS can reflect an individual's regulatory capacity of frontal brain regions and has been linked to cognitive capacity. Yet, the relationship of PNS function with cognitive decline and abnormal frontal function that characterize preclinical progression toward Alzheimer's disease (AD) is unclear. Here, we aimed to elucidate the relationship between PNS function and AD‐associated neurodegeneration by testing two competing hypotheses involving frontal regions’ activity (neurodegeneration vs. compensation). Methods In 38 older human adults with amnestic mild cognitive impairment (aMCI) or normative cognition, we measured AD‐associated neurodegeneration (AD signature cortical thickness; ADSCT), resting‐state fMRI of frontal regions’ spontaneous activation, and an electrocardiography measure of PNS (high frequency heart rate variability; HF‐HRV). HF‐HRV was assessed at rest and during a cognitive task protocol designed to capture HF‐HRV reactivity. Results Higher HF‐HRV at rest was significantly related to both more severe AD‐associated neurodegeneration (lower ADSCT scores) and worse cognitive ability. Cognitive impairments were also related to greater suppression of HF‐HRV reactivity. High activities of the anterior cingulate cortex significantly mediated relationships between ADSCT and both HF‐HRV at rest and HF‐HRV reactivity. Notably, these relationships were not affected by the clinical phenotype. Conclusions We show that AD‐associated neurodegeneration is associated with altered PNS regulation and that compensatory processes linked to frontal overactivation might be responsible for those alterations. This finding provides the first line of evidence in a new framework for understanding how early‐stage AD‐associated neurodegeneration affects autonomic regulation. This article is protected by copyright. All rights reserved
    September 26, 2017   doi: 10.1113/JP274714   open full text
  • An allosteric gating model recapitulates the biophysical properties of IK,L expressed in mouse vestibular type I hair cells.
    Paolo Spaiardi, Elisa Tavazzani, Marco Manca, Veronica Milesi, Giancarlo Russo, Ivo Prigioni, Walter Marcotti, Jacopo Magistretti, Sergio Masetto.
    The Journal of Physiology. September 24, 2017
    Key points Vestibular type I and type II hair cells and their afferent fibres send information to the brain regarding the position and movement of the head. The characteristic feature of type I hair cells is the expression of a low‐voltage‐activated outward rectifying K+ current, IK,L, whose biophysical properties and molecular identity are still largely unknown. In vitro, the afferent nerve calyx surrounding type I hair cells causes unstable intercellular K+ concentrations, altering the biophysical properties of IK,L. We found that in the absence of the calyx, IK,L in type I hair cells exhibited unique biophysical activation properties, which were faithfully reproduced by an allosteric channel gating scheme. These results form the basis for a molecular and pharmacological identification of IK,L. Abstract Type I and type II hair cells are the sensory receptors of the mammalian vestibular epithelia. Type I hair cells are characterized by their basolateral membrane being enveloped in a single large afferent nerve terminal, named the calyx, and by the expression of a low‐voltage‐activated outward rectifying K+ current, IK,L. The biophysical properties and molecular profile of IK,L are still largely unknown. By using the patch‐clamp whole‐cell technique, we examined the voltage‐ and time‐dependent properties of IK,L in type I hair cells of the mouse semicircular canal. We found that the biophysical properties of IK,L were affected by an unstable K+ equilibrium potential (VeqK+). Both the outward and inward K+ currents shifted VeqK+ consistent with K+ accumulation or depletion, respectively, in the extracellular space, which we attributed to a residual calyx attached to the basolateral membrane of the hair cells. We therefore optimized the hair cell dissociation protocol in order to isolate mature type I hair cells without their calyx. In these cells, the uncontaminated IK,L showed a half‐activation at –79.6 mV and a steep voltage dependence (2.8 mV). IK,L also showed complex activation and deactivation kinetics, which we faithfully reproduced by an allosteric channel gating scheme where the channel is able to open from all (five) closed states. The ‘early’ open states substantially contribute to IK,L activation at negative voltages. This study provides the first complete description of the ‘native’ biophysical properties of IK,L in adult mouse vestibular type I hair cells.
    September 24, 2017   doi: 10.1113/JP274202   open full text
  • Sex differences in the role of phospholipase A2‐dependent arachidonic acid pathway in the perivascular adipose tissue function in pigs.
    Abdulla A. Ahmad, Michael D. Randall, Richard E. Roberts.
    The Journal of Physiology. September 24, 2017
    Key points The fat surrounding blood vessels (perivascular adipose tissue or PVAT) releases vasoactive compounds that regulate vascular smooth muscle tone. There are sex differences in the regulation of vascular tone, but, to date, no study has investigated whether there are sex differences in the regulation of blood vessel tone by PVAT. This study has identified that the cyclooxygenase products thromboxane and PGF2α are released from coronary artery PVAT from pigs. Thromboxane appears to mediate the PVAT‐induced contraction in arteries from females, whereas PGF2α appears to mediate the contraction in arteries from males. These sex differences in the role of these prostanoids in the PVAT‐induced contraction can be explained by a greater release of thromboxane from PVAT from female animals and greater sensitivity to PGF2α in the porcine coronary artery from males. Abstract Previous studies have demonstrated that perivascular adipose tissue (PVAT) causes vasoconstriction. In this present study, we determined the role of cyclooxygenase‐derived prostanoids in this contractile response and determined whether there were any sex differences in the regulation of vascular tone by PVAT. Contractions in isolated segments of coronary arteries were determined using isolated tissue baths and isometric tension recording. Segments were initially cleaned of PVAT, which was then re‐added to the tissue bath and changes in tone measured over 1 h. Levels of PGF2α and thromboxane B2 (TXB2) were quantified by ELISA, and PGF2α (FP) and thromboxane A2 (TP) receptor expression determined by Western blotting. In arteries from both male and female pigs, re‐addition of PVAT caused a contraction, which was partially inhibited by the cyclooxygenase inhibitors indomethacin and flurbiprofen. The FP receptor antagonist AL8810 attenuated the PVAT‐induced contraction in arteries from males, whereas the TP receptor antagonist GR32191B inhibited the PVAT‐induced contraction in arteries from females. Although there was no difference in PGF2α levels in PVAT between females and males, PGF2α produced a larger contraction in arteries from males, correlating with a higher FP receptor expression. In contrast, release of TXB2 from PVAT from females was greater than from males, but there was no difference in the contraction by the TXA2 agonist U46619, or TP receptor expression in arteries from different sexes. These findings demonstrate clear sex differences in PVAT function in which PGF2α and TXA2 antagonists can inhibit the PVAT‐induced vasoconstriction in male and female PCAs, respectively.
    September 24, 2017   doi: 10.1113/JP274831   open full text
  • Do right‐ventricular trabeculae gain energetic advantage from having a greater velocity of shortening?
    Toan Pham, June‐Chiew Han, Andrew Taberner, Denis Loiselle.
    The Journal of Physiology. September 24, 2017
    Key points We designed a study to test whether velocity of shortening in right‐ventricular tissue preparations is greater than that of the left side under conditions mimicking those encountered by the heart in vivo. Our experiments allowed us to explore whether greater velocity of shortening results in any energetic advantage. We found that velocity of shortening was higher in the rat right‐ventricular trabeculae. These results at the tissue level seem paradoxical to the velocity of ventricular ejection at the organ level, and are not always in accord with shortening of unloaded cells. Despite greater velocity of shortening in right‐ventricular trabeculae, they neither gained nor lost advantage with respect to both mechanical efficiency and the heat generated during shortening. Abstract Our study aimed to ascertain whether the interventricular difference of shortening velocity, reported for isolated cardiac tissues in vitro, affects interventricular mechano‐energetic performance when tested under physiological conditions using a shortening protocol designed to mimic those in vivo. We isolated trabeculae from both ventricles of the rat, mounted them in a calorimeter, and performed experiments at 37°C and 5 Hz stimulus frequency to emulate conditions of the rat heart in vivo. Each trabecula was subjected to two experimental protocols: (i) isotonic work‐loop contractions at a variety of afterloads, and (ii) isometric contractions at a variety of preloads. Velocity of shortening was calculated from the former protocol during the isotonic shortening phase of the contraction. Simultaneous measurements of force–length work and heat output allowed calculation of mechanical efficiency. The shortening‐dependent thermal component was quantified from the difference in heat output between the two protocols. Our results show that both extent of shortening and velocity of shortening were higher in trabeculae from the right ventricle. Despite these differences, trabeculae from both ventricles developed the same stress, performed the same work, liberated the same amount of heat, and hence operated at the same mechanical efficiency. Shortening heat was also ventricle independent. The interventricular differences in velocity of shortening and extent of shortening of isolated trabeculae were not manifested in any index of energetics. These collective results underscore the absence of any mechano‐energetic advantage or disadvantage conferred on right‐ventricular trabeculae arising from their superior velocity of shortening.
    September 24, 2017   doi: 10.1113/JP274837   open full text
  • Elevated resting H+ current in the R1239H type 1 hypokalaemic periodic paralysis mutated Ca2+ channel.
    Clarisse Fuster, Jimmy Perrot, Christine Berthier, Vincent Jacquemond, Bruno Allard.
    The Journal of Physiology. September 24, 2017
    Key points Missense mutations in the gene encoding the α1 subunit of the skeletal muscle voltage‐gated Ca2+ channel induce type 1 hypokalaemic periodic paralysis, a poorly understood neuromuscular disease characterized by episodic attacks of paralysis associated with low serum K+. Acute expression of human wild‐type and R1239H HypoPP1 mutant α1 subunits in mature mouse muscles showed that R1239H fibres displayed Ca2+ currents of reduced amplitude and larger resting leak inward current increased by external acidification. External acidification also produced intracellular acidification at a higher rate in R1239H fibres and inhibited inward rectifier K+ currents. These data suggest that the R1239H mutation induces an elevated leak H+ current at rest flowing through a gating pore and could explain why paralytic attacks preferentially occur during the recovery period following muscle exercise. Abstract Missense mutations in the gene encoding the α1 subunit of the skeletal muscle voltage‐gated Ca2+ channel induce type 1 hypokalaemic periodic paralysis, a poorly understood neuromuscular disease characterized by episodic attacks of paralysis associated with low serum K+. The present study aimed at identifying the changes in muscle fibre electrical properties induced by acute expression of the R1239H hypokalaemic periodic paralysis human mutant α1 subunit of Ca2+ channels in a mature muscle environment to better understand the pathophysiological mechanisms involved in this disorder. We transferred genes encoding wild‐type and R1239H mutant human Ca2+ channels into hindlimb mouse muscle by electroporation and combined voltage‐clamp and intracellular pH measurements on enzymatically dissociated single muscle fibres. As compared to fibres expressing wild‐type α1 subunits, R1239H mutant‐expressing fibres displayed Ca2+ currents of reduced amplitude and a higher resting leak inward current that was increased by external acidification. External acidification also produced intracellular acidification at a higher rate in R1239H fibres and inhibited inward rectifier K+ currents. These data indicate that the R1239H mutation induces an elevated leak H+ current at rest flowing through a gating pore created by the mutation and that external acidification favours onset of muscle paralysis by potentiating H+ depolarizing currents and inhibiting resting inward rectifier K+ currents. Our results could thus explain why paralytic attacks preferentially occur during the recovery period following intense muscle exercise.
    September 24, 2017   doi: 10.1113/JP274638   open full text
  • Feto‐ and utero‐placental vascular adaptations to chronic maternal hypoxia in the mouse.
    Lindsay S. Cahill, Monique Y. Rennie, Johnathan Hoggarth, Lisa X. Yu, Anum Rahman, John C. Kingdom, Mike Seed, Christopher K. Macgowan, John G. Sled.
    The Journal of Physiology. September 24, 2017
    Key points Chronic fetal hypoxia is one of the most common complications of pregnancy and is known to cause fetal growth restriction. The structural adaptations of the placental vasculature responsible for growth restriction with chronic hypoxia are not well elucidated. Using a mouse model of chronic maternal hypoxia in combination with micro‐computed tomography and scanning electron microscopy, we found several placental adaptations that were beneficial to fetal growth including capillary expansion, thinning of the interhaemal membrane and increased radial artery diameters, resulting in a large drop in total utero‐placental vascular resistance. One of the mechanisms used to achieve the rapid increase in capillaries was intussusceptive angiogenesis, a strategy used in human placental development to form terminal gas‐exchanging villi. These results contribute to our understanding of the structural mechanisms of the placental vasculature responsible for fetal growth restriction and provide a baseline for understanding adaptive physiological responses of the placenta to chronic hypoxia. Abstract The fetus and the placenta in eutherian mammals have a unique set of compensatory mechanisms to respond to several pregnancy complications including chronic maternal hypoxia. This study examined the structural adaptations of the feto‐ and utero‐placental vasculature in an experimental mouse model of chronic maternal hypoxia (11% O2 from embryonic day (E) 14.5–E17.5). While placental weights were unaffected by exposure to chronic hypoxia, using micro‐computed tomography, we found a 44% decrease in the absolute feto‐placental arterial vascular volume and a 30% decrease in total vessel segments in the chronic hypoxia group compared to control group. Scanning electron microscopy imaging showed significant expansion of the capillary network; consequently, the interhaemal membrane was 11% thinner to facilitate maternal–fetal exchange in the chronic hypoxia placentas. One of the mechanisms for the rapid capillary expansion was intussusceptive angiogenesis. Analysis of the utero‐placental arterial tree showed significant increases (24%) in the diameter of the radial arteries, resulting in a decrease in the total utero‐placental resistance by 2.6‐fold in the mice exposed to chronic maternal hypoxia. Together these adaptations acted to preserve placental weight whereas fetal weight was decreased.
    September 24, 2017   doi: 10.1113/JP274845   open full text
  • Volume loading augments cutaneous vasodilatation and cardiac output of heat stressed older adults.
    Daniel Gagnon, Steven A. Romero, Hai Ngo, Satyam Sarma, William K. Cornwell, Paula Y. S. Poh, Douglas Stoller, Benjamin D. Levine, Craig G. Crandall.
    The Journal of Physiology. September 22, 2017
    Key points Age‐related changes in cutaneous microvascular and cardiac functions limit the extent of cutaneous vasodilatation and the increase in cardiac output that healthy older adults can achieve during passive heat stress. However, it is unclear if these age‐related changes in microvascular and cardiac functions maximally restrain the levels of cutaneous vasodilatation and cardiac output that healthy older adults can achieve during heat stress. We observed that rapid volume loading, performed during passive heat stress, augments both cutaneous vasodilatation and cardiac output in healthy older humans. These findings demonstrate that the microcirculation of healthy aged skin can further dilate during passive heat exposure, despite peripheral limitations to vasodilatation. Furthermore, healthy older humans can augment cardiac output when cardiac pre‐load is increased during heat stress. Abstract Primary ageing markedly attenuates cutaneous vasodilatation and the increase in cardiac output during passive heating. However, it remains unclear if these responses are maximally restrained by age‐related changes in cutaneous microvascular and cardiac functions. We hypothesized that rapid volume loading performed during heat stress would increase cardiac output in older adults without parallel increases in cutaneous vasodilatation. Twelve young (Y: 26 ± 5 years) and ten older (O: 69 ± 3 years) healthy adults were passively heated until core temperature increased by 1.5°C. Cardiac output (thermodilution), forearm vascular conductance (FVC, venous occlusion plethysmography) and cutaneous vascular conductance (CVC, laser‐Doppler) were measured before and after rapid infusion of warmed saline (15 mL kg−1, ∼7 min). While heat stressed, but prior to saline infusion, cardiac output (O: 6.8 ± 0.4 vs. Y: 9.4 ± 0.6 L min−1), FVC (O: 0.08 ± 0.01 vs. Y: 0.17 ± 0.02 mL (100 mL min−1 mmHg−1)−1), and CVC (O: 1.29 ± 0.34 vs. Y: 1.93 ± 0.30 units mmHg−1) were lower in older adults (all P < 0.01). Rapid saline infusion increased cardiac output (O: +1.9 ± 0.3, Y: +1.8 ± 0.7 L min−1), FVC (O: +0.015 ± 0.007, Y: +0.048 ± 0.013 mL (100 mL min−1 mmHg−1)−1), and CVC (O: +0.28 ± 0.10, Y: +0.29 ± 0.16 units mmHg−1) in both groups (all P < 0.01). The absolute increase in cardiac output and CVC were similar between groups, whereas FVC increased to a greater extent in young adults (P < 0.01). These results demonstrate that healthy older adults can achieve greater levels of cutaneous vasodilatation and cardiac output during passive heating.
    September 22, 2017   doi: 10.1113/JP274742   open full text
  • Subunit‐dependent oxidative stress sensitivity of LRRC8 volume‐regulated anion channels.
    Antonella Gradogna, Paola Gavazzo, Anna Boccaccio, Michael Pusch.
    The Journal of Physiology. September 22, 2017
    Key points Swelling‐activated anion currents are modulated by oxidative conditions, but it is unknown if oxidation acts directly on the LRRC8 channel‐forming proteins or on regulatory factors. We found that LRRC8A–LRRC8E heteromeric channels are dramatically activated by oxidation of intracellular cysteines, whereas LRRC8A–LRRC8C and LRRC8A–LRRC8D heteromers are inhibited by oxidation. Volume‐regulated anion currents in Jurkat T lymphocytes were inhibited by oxidation, in agreement with a low expression of the LRRC8E subunit in these cells. Our results show that LRRC8 channel proteins are directly modulated by oxidation in a subunit‐specific manner. Abstract The volume‐regulated anion channel (VRAC) is formed by heteromers of LRRC8 proteins containing the essential LRRC8A subunit and at least one among the LRRC8B–E subunits. Reactive oxygen species (ROS) play physiological and pathophysiological roles and VRAC channels are highly ROS sensitive. However, it is unclear if ROS act directly on the channels or on molecules involved in the activation pathway. We used fluorescently tagged LRRC8 proteins that yield large constitutive currents to test direct effects of oxidation. We found that 8A/8E heteromers are dramatically potentiated (more than 10‐fold) by oxidation of intracellular cysteine residues by chloramine‐T or tert‐butyl hydroperoxide. Oxidation was, however, not necessary for hypotonicity‐induced activation. In contrast, 8A/8C and 8A/8D heteromers were strongly inhibited by oxidation. Endogenous VRAC currents in Jurkat T lymphocytes were similarly inhibited by oxidation, in agreement with the finding that LRRC8C and LRRC8D subunits were more abundantly expressed than LRRC8E in Jurkat cells. Our results show that LRRC8 channels are directly modulated by oxidation in a subunit‐dependent manner.
    September 22, 2017   doi: 10.1113/JP274795   open full text
  • Co‐ordination of the upper and lower limbs for vestibular control of balance.
    Craig P. Smith, Jonathan E. Allsop, Michael Mistry, Raymond F. Reynolds.
    The Journal of Physiology. September 22, 2017
    Key points When standing and holding an earth‐fixed object, galvanic vestibular stimulation (GVS) can evoke upper limb responses to maintain balance. In the present study, we determined how these responses are affected by grip context (no contact, light grip and firm grip), as well as how they are co‐ordinated with the lower limbs to maintain balance. When GVS was applied during firm grip, hand and ground reaction forces were generated. The directions of these force vectors were co‐ordinated such that the overall body sway response was always aligned with the inter‐aural axis (i.e. craniocentric). When GVS was applied during light grip (< 1 N), hand forces were secondary to body movement, suggesting that the arm performed a mostly passive role. These results demonstrate that a minimum level of grip is required before the upper limb becomes active in balance control and also that the upper and lower limbs co‐ordinate for an appropriate whole‐body sway response. Abstract Vestibular stimulation can evoke responses in the arm when it is used for balance. In the present study, we determined how these responses are affected by grip context, as well as how they are co‐ordinated with the rest of the body. Galvanic vestibular stimulation (GVS) was used to evoke balance responses under three conditions of manual contact with an earth‐fixed object: no contact, light grip (< 1 N) (LG) and firm grip (FG). As grip progressed along this continuum, we observed an increase in GVS‐evoked hand force, with a simultaneous reduction in ground reaction force (GRF) through the feet. During LG, hand force was secondary to the GVS‐evoked body sway response, indicating that the arm performed a mostly passive role. By contrast, during FG, the arm became actively involved in driving body sway, as revealed by an early force impulse in the opposite direction to that seen in LG. We then examined how the direction of this active hand vector was co‐ordinated with the lower limbs. Consistent with previous findings on sway anisotropy, FG skewed the direction of the GVS‐evoked GRF vector towards the axis of baseline postural instability. However, this was effectively cancelled by the hand force vector, such that the whole‐body sway response remained aligned with the inter‐aural axis, maintaining the craniocentric principle. These results show that a minimum level of grip is necessary before the upper limb plays an active role in vestibular‐evoked balance responses. Furthermore, they demonstrate that upper and lower‐limb forces are co‐ordinated to produce an appropriate whole‐body sway response.
    September 22, 2017   doi: 10.1113/JP274272   open full text
  • Skeletal muscle protein accretion rates and hindlimb growth are reduced in late gestation intrauterine growth restricted fetal sheep.
    Paul J. Rozance, Laura Zastoupil, Stephanie R. Wesolowski, David A. Goldstrohm, Brittany Strahan, Melanie Cree‐Green, Melinda Sheffield‐Moore, Giacomo Meschia, William W. Hay, Randall B. Wilkening, Laura D. Brown.
    The Journal of Physiology. September 22, 2017
    Reduced skeletal muscle mass in the IUGR fetus persists into adulthood and may contribute to increased metabolic disease risk. To determine how placental insufficiency with reduced oxygen and nutrient supply to the fetus affects hindlimb blood flow, substrate uptake, and protein accretion rates in skeletal muscle, late gestation CON (n = 8) and IUGR (n = 13) fetal sheep were catheterized with aortic and femoral catheters and a flow transducer around the external iliac artery. Muscle protein kinetic rates were measured using isotopic tracers. Hindlimb weight, linear growth rate, muscle protein accretion rate, and fractional synthetic rate were lower in IUGR compared to CON (P < 0.05). Absolute hindlimb blood flow was reduced in IUGR (IUGR: 32.9 ± 5.6, CON: 60.9 ± 6.5 ml·min−1; P < 0.005), although flow normalized to hindlimb weight was similar between groups. Hindlimb oxygen consumption rate was lower in IUGR (IUGR: 10.4 ± 1.4, CON: 14.7 ± 1.3 μmol·min−1·100 g−1; P < 0.05). Hindlimb glucose uptake and lactate output rates were similar between groups, whereas amino acid uptake was lower in IUGR (IUGR: 1.3 ± 0.5, CON: 2.9 ± 0.2 μmol·min−1·100 g−1; P < 0.05). Blood O2 saturation (R2 = 0.80, P < 0.0001) and plasma glucose (R2 = 0.68, P < 0.0001), insulin (R2 = 0.40, P < 0.005), and IGF‐1 (R2 = 0.80, P < 0.0001) were positively associated and norepinephrine (R2 = 0.59, P < 0.0001) was negatively associated with hindlimb weight. Slower hindlimb linear growth and muscle protein synthesis rates match reduced hindlimb blood flow and oxygen consumption rates in the IUGR fetus. Metabolic adaptations to slow hindlimb growth are likely hormonally mediated by mechanisms that include increased fetal norepinephrine and reduced IGF‐1 and insulin. This article is protected by copyright. All rights reserved
    September 22, 2017   doi: 10.1113/JP275230   open full text
  • Kindlin‐2 interacts with endothelial adherens junctions to support vascular barrier integrity.
    Elzbieta Pluskota, Kamila M. Bledzka, Katarzyna Bialkowska, Dorota Szpak, Dmitry A. Soloviev, Sidney V. Jones, Dmitriy Verbovetskiy, Edward F. Plow.
    The Journal of Physiology. September 21, 2017
    Key points A reduction in Kindlin‐2 levels in endothelial cells compromises vascular barrier function. Kindlin‐2 is a previously unrecognized component of endothelial adherens junctions. By interacting directly and simultaneously with β‐ or γ‐catenin and cortical actin filaments, Kindlin‐2 stabilizes adherens junctions. The Kindlin‐2 binding sites for β‐ and γ‐catenin reside within its F1 and F3 subdomains. Although Kindlin‐2 does not associate directly with tight junctions, its downregulation also destabilizes these junctions. Thus, impairment of both adherens and tight junctions may contribute to enhanced leakiness of vasculature in Kindlin‐2+/− mice. Abstract Endothelial cells (EC) establish a physical barrier between the blood and surrounding tissue. Impairment of this barrier can occur during inflammation, ischaemia or sepsis and cause severe organ dysfunction. Kindlin‐2, which is primarily recognized as a focal adhesion protein in EC, was not anticipated to have a role in vascular barrier. We tested the role of Kindlin‐2 in regulating vascular integrity using several different approaches to decrease Kindlin‐2 levels in EC. Reduced levels of Kindlin‐2 in Kindlin‐2+/– mice aortic endothelial cells (MAECs) from these mice, and human umbilical ECs (HUVEC) treated with Kindlin‐2 siRNA showed enhanced basal and platelet‐activating factor (PAF) or lipopolysaccharide‐stimulated vascular leakage compared to wild‐type (WT) counterparts. PAF preferentially disrupted the Kindlin‐2+/− MAECs barrier to BSA and dextran and reduced transendothelial resistance compared to WT cells. Kindlin‐2 co‐localized and co‐immunoprecipitated with vascular endothelial cadherin‐based complexes, including β‐ and γ‐catenin and actin, components of adherens junctions (AJ). Direct interaction of Kindlin‐2 with β‐ and γ‐catenin and actin was demonstrated in co‐immunoprecipitation and surface plasmon resonance experiments. In thrombin‐stimulated HUVECs, Kindlin‐2 and cortical actin dissociated from stable AJs and redistributed to radial actin stress fibres of remodelling focal AJs. The β‐ and γ‐catenin binding site resides within the F1 and F3 subdomains of Kindlin‐2 but not the integrin binding site in F3. These results establish a previously unrecognized and vital role of Kindlin‐2 with respect to maintaining the vascular barrier by linking Vascuar endothelial cadherin‐based complexes to cortical actin and thereby stabilizing AJ.
    September 21, 2017   doi: 10.1113/JP274380   open full text
  • Adenosine and dopamine oppositely modulate a hyperpolarization‐activated current Ih in chemosensory neurons of the rat carotid body in co‐culture.
    Min Zhang, Cathy Vollmer, Colin A. Nurse.
    The Journal of Physiology. September 21, 2017
    Key points Adenosine and dopamine (DA) are neuromodulators in the carotid body (CB) chemoafferent pathway, but their mechanisms of action are incompletely understood. Using functional co‐cultures of rat CB chemoreceptor (type I) cells and sensory petrosal neurons (PNs), we show that adenosine enhanced a hyperpolarization‐activated cation current Ih in chemosensory PNs via A2a receptors, whereas DA had the opposite effect via D2 receptors. Adenosine caused a depolarizing shift in the Ih activation curve and increased firing frequency, whereas DA caused a hyperpolarizing shift in the curve and decreased firing frequency. Acute hypoxia and isohydric hypercapnia depolarized type I cells concomitant with increased excitation of adjacent PNs; the A2a receptor blocker SCH58261 inhibited both type I and PN responses during hypoxia, but only the PN response during isohydric hypercapnia. We propose that adenosine and DA control firing frequency in chemosensory PNs via their opposing actions on Ih. Abstract Adenosine and dopamine (DA) act as neurotransmitters or neuromodulators at the carotid body (CB) chemosensory synapse, but their mechanisms of action are not fully understood. Using a functional co‐culture model of rat CB chemoreceptor (type I) cell clusters and juxtaposed afferent petrosal neurons (PNs), we tested the hypothesis that adenosine and DA act postsynaptically to modulate a hyperpolarization‐activated, cyclic nucleotide‐gated (HCN) cation current (Ih). In whole‐cell recordings from hypoxia‐responsive PNs, cAMP mimetics enhanced Ih whereas the HCN blocker ZD7288 (2 μm) reversibly inhibited Ih. Adenosine caused a potentiation of Ih (EC50 ∼ 35 nm) that was sensitive to the A2a blocker SCH58261 (5 nm), and an ∼16 mV depolarizing shift in V½ for voltage dependence of Ih activation. By contrast, DA (10 μm) caused an inhibition of Ih that was sensitive to the D2 blocker sulpiride (1–10 μm), and an ∼11 mV hyperpolarizing shift in V½. Sulpiride potentiated Ih in neurons adjacent to, but not distant from, type I cell clusters. DA also decreased PN action potential frequency whereas adenosine had the opposite effect. During simultaneous paired recordings, SCH58261 inhibited both the presynaptic hypoxia‐induced receptor potential in type I cells and the postsynaptic PN response. By contrast, SCH58261 inhibited only the postsynaptic PN response induced by isohydric hypercapnia. Confocal immunofluorescence confirmed the localization of HCN4 subunits in tyrosine hydroxylase‐positive chemoafferent neurons in tissue sections of rat petrosal ganglia. These data suggest that adenosine and DA, acting through A2a and D2 receptors respectively, regulate PN excitability via their opposing actions on Ih.
    September 21, 2017   doi: 10.1113/JP274743   open full text
  • Synaptic excitation by climbing fibre collaterals in the cerebellar nuclei of juvenile and adult mice.
    Marion Najac, Indira M. Raman.
    The Journal of Physiology. September 20, 2017
    Key points The inferior olive sends instructive motor signals to the cerebellum via the climbing fibre projection, which sends collaterals directly to large premotor neurons of the mouse cerebellar nuclei (CbN cells). Optogenetic activation of inferior olivary axons in vitro evokes EPSCs in CbN cells of several hundred pA to more than 1 nA. The inputs are three‐fold larger at younger ages, 12 to 14 days old, than at 2 months old, suggesting a strong functional role for this pathway earlier in development. The EPSCs are multipeaked, owing to burst firing in several olivary afferents that fire asynchronously. The convergence of climbing fibre collaterals onto CbN cells decreases from ∼40 to ∼8, which is consistent with the formation of closed‐loop circuits in which each CbN neuron receives input from 4–7 collaterals from inferior olivary neurons as well as from all 30–50 Purkinje cells that are innervated by those olivary neurons. Abstract The inferior olive conveys instructive signals to the cerebellum that drive sensorimotor learning. Inferior olivary neurons transmit their signals via climbing fibres, which powerfully excite Purkinje cells, evoking complex spikes and depressing parallel fibre synapses. Additionally, however, these climbing fibres send collaterals to the cerebellar nuclei (CbN). In vivo and in vitro data suggest that climbing fibre collateral excitation is weak in adult mice, raising the question of whether the primary role of this pathway may be developmental. We therefore examined climbing fibre collateral input to large premotor CbN cells over development by virally expressing channelrhodopsin in the inferior olive. In acute cerebellar slices from postnatal day (P)12–14 mice, light‐evoked EPSCs were large (> 1 nA at −70 mV). The amplitude of these EPSCs decreased over development, reaching a plateau of ∼350 pA at P20–60. Trains of EPSCs (5 Hz) depressed strongly throughout development, whereas convergence estimates indicated that the total number of functional afferents decreased with age. EPSC waveforms consisted of multiple peaks, probably resulting from action potential bursts in single collaterals and variable times to spike threshold in converging afferents. Activating climbing fibre collaterals evoked well‐timed increases in firing probability in CbN neurons, especially in younger mice. The initially strong input, followed by the decrement in synaptic strength coinciding with the pruning of climbing fibres in the cerebellar cortex, implicates the climbing fibre collateral pathway in early postnatal development. Additionally, the persistence of substantial synaptic input at least to P60 suggests that this pathway may function in cerebellar processing into adulthood.
    September 20, 2017   doi: 10.1113/JP274598   open full text
  • Persistent aberrant cortical phase–amplitude coupling following seizure treatment in absence epilepsy models.
    Atul Maheshwari, Abraham Akbar, Mai Wang, Rachel L. Marks, Katherine Yu, Suhyeorn Park, Brett L. Foster, Jeffrey L. Noebels.
    The Journal of Physiology. September 19, 2017
    Key points In two monogenic models of absence epilepsy, interictal beta/gamma power is augmented in homozygous stargazer (stg/stg) but not homozygous tottering (tg/tg) mice. There are distinct gene‐linked patterns of aberrant phase–amplitude coupling in the interictal EEG of both stg/stg and tg/tg mice, compared to +/+ and stg/+ mice. Treatment with ethosuximide significantly blocks seizures in both genotypes, but the abnormal phase–amplitude coupling remains. Seizure‐free stg/+ mice have normal power and phase–amplitude coupling, but beta/gamma power is significantly reduced with NMDA receptor blockade, revealing a latent cortical network phenotype that is separable from, and therefore not a result of, seizures. Altogether, these findings reveal gene‐linked quantitative electrographic biomarkers free from epileptiform activity, and provide a potential network correlate for persistent cognitive deficits in absence epilepsy despite effective treatment. Abstract In childhood absence epilepsy, cortical seizures are brief and intermittent; however there are extended periods without behavioural or electrographic ictal events. This genetic disorder is associated with variable degrees of cognitive dysfunction, but no consistent functional biomarkers that might provide insight into interictal cortical function have been described. Previous work in monogenic mouse models of absence epilepsy have shown that the interictal EEG displays augmented beta/gamma power in homozygous stargazer (stg/stg) mice bearing a presynaptic AMPA receptor defect, but not homozygous tottering (tg/tg) mice with a P/Q type calcium channel mutation. To further evaluate the interictal EEG, we quantified phase–amplitude coupling (PAC) in stg/stg, stg/+, tg/tg and wild‐type (+/+) mice. We found distinct gene‐linked patterns of aberrant PAC in stg/stg and tg/tg mice compared to +/+ and stg/+ mice. Treatment with ethosuximide significantly blocks seizures in both stg/stg and tg/tg, but the abnormal PAC remains. Stg/+ mice are seizure free with normal baseline beta/gamma power and normal theta‐gamma PAC, but like stg/stg mice, beta/gamma power is significantly reduced by NMDA receptor blockade, a treatment that paradoxically enhances seizures in stg/stg mice. Stg/+ mice, therefore, have a latent cortical network phenotype that is veiled by NMDA‐mediated neurotransmission. Altogether, these findings reveal gene‐linked quantitative electrographic biomarkers in the absence of epileptiform activity and provide a potential network correlate for persistent cognitive deficits in absence epilepsy despite effective treatment.
    September 19, 2017   doi: 10.1113/JP274696   open full text
  • Median preoptic glutamatergic neurons promote thermoregulatory heat loss and water consumption in mice.
    Stephen B. G. Abbott, Clifford B. Saper.
    The Journal of Physiology. September 13, 2017
    Key points Glutamatergic neurons in the median preoptic area were stimulated using genetically targeted Channelrhodopsin 2 in transgenic mice. Stimulation of glutamatergic median preoptic area neurons produced a profound hypothermia due to cutaneous vasodilatation. Stimulation also produced drinking behaviour that was inhibited as water was ingested, suggesting pre‐systemic feedback gating of drinking. Anatomical mapping of the stimulation sites showed that sites associated with hypothermia were more anteroventral than those associated with drinking, although there was substantial overlap. Abstract The median preoptic nucleus (MnPO) serves an important role in the integration of water/electrolyte homeostasis and thermoregulation, but we have a limited understanding these functions at a cellular level. Using Cre–Lox genetic targeting of Channelrhodospin 2 in VGluT2 transgenic mice, we examined the effect of glutamatergic MnPO neuron stimulation in freely behaving mice while monitoring drinking behaviour and core temperature. Stimulation produced a strong hypothermic response in 62% (13/21) of mice (core temperature: −4.6 ± 0.5°C, P = 0.001 vs. controls) caused by cutaneous vasodilatation. Stimulating glutamatergic MnPO neurons also produced robust drinking behaviour in 82% (18/22) of mice. Mice that drank during stimulation consumed 912 ± 163 μl (n = 8) during a 20 min trial in the dark phase, but markedly less during the light phase (421 ± 83 μl, P = 0.0025). Also, drinking during stimulation was inhibited as water was ingested, suggesting pre‐systemic feedback gating of drinking. Both hypothermia and drinking during stimulation occurred in 50% of mice tested. Anatomical mapping of the stimulation sites showed that sites associated with hypothermia were more anteroventral than those associated with drinking, although there was substantial overlap. Thus, activation of separate but overlapping populations of glutamatergic MnPO neurons produces effects on drinking and autonomic thermoregulatory mechanisms, providing a structural basis for their frequently being coordinated (e.g. during hyperthermia).
    September 13, 2017   doi: 10.1113/JP274667   open full text
  • Molecular composition and heterogeneity of the LRRC8‐containing swelling‐activated osmolyte channels in primary rat astrocytes.
    Alexandra L. Schober, Corinne S. Wilson, Alexander A. Mongin.
    The Journal of Physiology. September 12, 2017
    Key points The volume‐regulated anion channel (VRAC) is a swelling‐activated chloride channel that is permeable to inorganic anions and a variety of small organic molecules. VRAC is formed via heteromerization of LRRC8 proteins, among which LRRC8A is essential, while LRRC8B/C/D/E serve as exchangeable complementary partners. We used an RNAi approach and radiotracer assays to explore which LRRC8 isoforms contribute to swelling‐activated release of diverse organic osmolytes in rat astrocytes. Efflux of uncharged osmolytes (myo‐inositol and taurine) was suppressed by deletion of LRRC8A or LRRC8D, but not by deletion of LRRC8C+LRRC8E. Conversely, release of charged osmolytes (d‐aspartate) was strongly reduced by deletion of LRRC8A or LRRC8C+LRRC8E, but largely unaffected by downregulation of LRRC8D. Our findings point to the existence of multiple heteromeric VRACs in the same cell type: LRRC8A/D‐containing heteromers appear to dominate release of uncharged osmolytes, while LRRC8A/C/E, with the additional contribution of LRRC8D, creates a conduit for movement of charged molecules. Abstract The volume‐regulated anion channel (VRAC) is the ubiquitously expressed vertebrate Cl−/anion channel that is composed of proteins belonging to the LRRC8 family and activated by cell swelling. In the brain, VRAC contributes to physiological and pathological release of a variety of small organic molecules, including the amino acid neurotransmitters glutamate, aspartate and taurine. In the present work, we explored the role of all five LRRC8 family members in the release of organic osmolytes from primary rat astrocytes. Expression of LRRC8 proteins was modified using an RNAi approach, and amino acid fluxes via VRAC were quantified by radiotracer assays in cells challenged with hypoosmotic medium (30% reduction in osmolarity). Consistent with our prior work, knockdown of LRRC8A potently and equally suppressed the release of radiolabelled d‐[14C]aspartate and [3H]taurine. Among other LRRC8 subunits, downregulation of LRRC8D strongly inhibited release of the uncharged osmolytes [3H]taurine and myo‐[3H]inositol, without major impact on the simultaneously measured efflux of the charged d‐[14C]aspartate. In contrast, the release of d‐[14C]aspartate was preferentially sensitive to deletion of LRRC8C+LRRC8E, but unaffected by downregulation of LRRC8D. Finally, siRNA knockdown of LRRC8C+LRRC8D strongly inhibited the release of all osmolytes. Overall, our findings suggest the existence of at least two distinct heteromeric VRACs in astroglial cells. The LRRC8A/D‐containing permeability pathway appears to dominate the release of uncharged osmolytes, while an alternative channel (or channels) is composed of LRRC8A/C/D/E and responsible for the loss of charged molecules.
    September 12, 2017   doi: 10.1113/JP275053   open full text
  • Engineering defined membrane‐embedded elements of AMPA receptor induces opposing gating modulation by cornichon 3 and stargazin.
    Natalie M. Hawken, Elena I. Zaika, Terunaga Nakagawa.
    The Journal of Physiology. September 12, 2017
    Key points The AMPA‐type ionotropic glutamate receptors (AMPARs) mediate the majority of excitatory synaptic transmission and their function impacts learning, cognition and behaviour. The gating of AMPARs occurs in milliseconds, precisely controlled by a variety of auxiliary subunits that are expressed differentially in the brain, but the difference in mechanisms underlying AMPAR gating modulation by auxiliary subunits remains elusive and is investigated. The elements of the AMPAR that are functionally recruited by auxiliary subunits, stargazin and cornichon 3, are located not only in the extracellular domains but also in the lipid‐accessible surface of the AMPAR. We reveal that the two auxiliary subunits require a shared surface on the transmembrane domain of the AMPAR for their function, but the gating is influenced by this surface in opposing directions for each auxiliary subunit. Our results provide new insights into the mechanistic difference of AMPAR modulation by auxiliary subunits and a conceptual framework for functional engineering of the complex. Abstract During excitatory synaptic transmission, various structurally unrelated transmembrane auxiliary subunits control the function of AMPA receptors (AMPARs), but the underlying mechanisms remain unclear. We identified lipid‐exposed residues in the transmembrane domain (TMD) of the GluA2 subunit of AMPARs that are critical for the function of AMPAR auxiliary subunits, stargazin (Stg) and cornichon 3 (CNIH3). These residues are essential for stabilizing the AMPAR–CNIH3 complex in detergents and overlap with the contacts made between GluA2 TMD and Stg in the cryoEM structures. Mutating these residues had opposite effects on gating modulation and complex stability when Stg‐ and CNIH3‐bound AMPARs were compared. Specifically, in detergent the GluA2‐A793F formed an unstable complex with CNIIH3 but in the membrane the GluA2‐A793F–CNIH3 complex expressed a gain of function. In contrast, the GluA2‐A793F–Stg complex was stable, but had diminished gating modulation. GluA2‐C528L destabilized the AMPAR–CNIH3 complex but stabilized the AMPAR–Stg complex, with overall loss of function in gating modulation. Furthermore, loss‐of‐function mutations in this TMD region cancelled the effects of a gain‐of‐function Stg carrying mutation in its extracellular loop, demonstrating that both the extracellular and the TMD elements contribute independently to gating modulation. The elements of AMPAR functionally recruited by auxiliary subunits are, therefore, located not only in the extracellular domains but also in the lipid accessible surface of the AMPAR. The TMD surface we defined is a potential target for auxiliary subunit‐specific compounds, because engineering of this hotspot induces opposing functional outcomes by Stg and CNIH3. The collection of mutant‐phenotype mapping provides a framework for engineering AMPAR gating using auxiliary subunits.
    September 12, 2017   doi: 10.1113/JP274897   open full text
  • Increased Ca buffering underpins remodelling of Ca2+ handling in old sheep atrial myocytes.
    Jessica D. Clarke, Jessica L. Caldwell, Charles M. Pearman, David A. Eisner, Andrew W. Trafford, Katharine M. Dibb.
    The Journal of Physiology. September 11, 2017
    Key points Ageing is associated with an increased risk of cardiovascular disease and arrhythmias, with the most common arrhythmia being found in the atria of the heart. Little is known about how the normal atria of the heart remodel with age and thus why dysfunction might occur. We report alterations to the atrial systolic Ca2+ transient that have implications for the function of the atrial in the elderly. We describe a novel mechanism by which increased Ca buffering can account for changes to systolic Ca2+ in the old atria. The present study helps us to understand how the processes regulating atrial contraction are remodelled during ageing and provides a basis for future work aiming to understand why dysfunction develops. Abstract Many cardiovascular diseases, including those affecting the atria, are associated with advancing age. Arrhythmias, including those in the atria, can arise as a result of electrical remodelling or alterations in Ca2+ homeostasis. In the atria, age‐associated changes in the action potential have been documented. However, little is known about remodelling of intracellular Ca2+ homeostasis in the healthy aged atria. Using single atrial myocytes from young and old Welsh Mountain sheep, we show the free Ca2+ transient amplitude and rate of decay of systolic Ca2+ decrease with age, whereas sarcoplasmic reticulum (SR) Ca content increases. An increase in intracellular Ca buffering explains both the decrease in Ca2+ transient amplitude and decay kinetics in the absence of any change in sarcoendoplasmic reticulum calcium transport ATPase function. Ageing maintained the integrated Ca2+ influx via ICa‐L but decreased peak ICa‐L. Decreased peak ICa‐L was found to be responsible for the age‐associated increase in SR Ca content but not the decrease in Ca2+ transient amplitude. Instead, decreased peak ICa‐L offsets increased SR load such that Ca2+ release from the SR was maintained during ageing. The results of the present study highlight a novel mechanism by which increased Ca buffering decreases systolic Ca2+ in old atria. Furthermore, for the first time, we have shown that SR Ca content is increased in old atrial myocytes.
    September 11, 2017   doi: 10.1113/JP274053   open full text
  • Pulmonary artery wave propagation and reservoir function in conscious man: impact of pulmonary vascular disease, respiration and dynamic stress tests.
    Junjing Su, Charlotte Manisty, Ulf Simonsen, Luke S. Howard, Kim H. Parker, Alun D. Hughes.
    The Journal of Physiology. September 11, 2017
    Key points Wave travel plays an important role in cardiovascular physiology. However, many aspects of pulmonary arterial wave behaviour remain unclear. Wave intensity and reservoir‐excess pressure analyses were applied in the pulmonary artery in subjects with and without pulmonary hypertension during spontaneous respiration and dynamic stress tests. Arterial wave energy decreased during expiration and Valsalva manoeuvre due to decreased ventricular preload. Wave energy also decreased during handgrip exercise due to increased heart rate. In pulmonary hypertension patients, the asymptotic pressure at which the microvascular flow ceases, the reservoir pressure related to arterial compliance and the excess pressure caused by waves increased. The reservoir and excess pressures decreased during Valsalva manoeuvre but remained unchanged during handgrip exercise. This study provides insights into the influence of pulmonary vascular disease, spontaneous respiration and dynamic stress tests on pulmonary artery wave propagation and reservoir function. Abstract Detailed haemodynamic analysis may provide novel insights into the pulmonary circulation. Therefore, wave intensity and reservoir‐excess pressure analyses were applied in the pulmonary artery to characterize changes in wave propagation and reservoir function during spontaneous respiration and dynamic stress tests. Right heart catheterization was performed using a pressure and Doppler flow sensor tipped guidewire to obtain simultaneous pressure and flow velocity measurements in the pulmonary artery in control subjects and patients with pulmonary arterial hypertension (PAH) at rest. In controls, recordings were also obtained during Valsalva manoeuvre and handgrip exercise. The asymptotic pressure at which the flow through the microcirculation ceases, the reservoir pressure related to arterial compliance and the excess pressure caused by arterial waves increased in PAH patients compared to controls. The systolic and diastolic rate constants also increased, while the diastolic time constant decreased. The forward compression wave energy decreased by ∼8% in controls and ∼6% in PAH patients during expiration compared to inspiration, while the wave speed remained unchanged throughout the respiratory cycle. Wave energy decreased during Valsalva manoeuvre (by ∼45%) and handgrip exercise (by ∼27%) with unaffected wave speed. Moreover, the reservoir and excess pressures decreased during Valsalva manoeuvre but remained unaltered during handgrip exercise. In conclusion, reservoir‐excess pressure analysis applied to the pulmonary artery revealed distinctive differences between controls and PAH patients. Variations in the ventricular preload and afterload influence pulmonary arterial wave propagation as demonstrated by changes in wave energy during spontaneous respiration and dynamic stress tests.
    September 11, 2017   doi: 10.1113/JP274385   open full text
  • Mouse retinal ganglion cell signalling is dynamically modulated through parallel anterograde activation of cannabinoid and vanilloid pathways.
    Andrew O. Jo, Jennifer M. Noel, Monika Lakk, Oleg Yarishkin, Daniel A. Ryskamp, Koji Shibasaki, Maureen A. McCall, David Križaj.
    The Journal of Physiology. September 07, 2017
    Key points Retinal cells use vanilloid transient receptor potential (TRP) channels to integrate light‐evoked signals with ambient mechanical, chemical and temperature information. Localization and function of the polymodal non‐selective cation channel TRPV1 (transient receptor potential vanilloid isoform 1) remains elusive. TRPV1 is expressed in a subset of mouse retinal ganglion cells (RGCs) with peak expression in the mid‐peripheral retina. Endocannabinoids directly activate TRPV1 and inhibit it through cannabinoid type 1 receptors (CB1Rs) and cAMP pathways. Activity‐dependent endocannabinoid release may modulate signal gain in RGCs through simultaneous manipulation of calcium and cAMP signals mediated by TRPV1 and CB1R. Abstract How retinal ganglion cells (RGCs) process and integrate synaptic, mechanical, swelling stimuli with light inputs is an area of intense debate. The nociceptive cation channel TRPV1 (transient receptor potential vanilloid type 1) modulates RGC Ca2+ signals and excitability yet the proportion of RGCs that express it remains unclear. Furthermore, TRPV1's response to endocannabinoids (eCBs), the putative endogenous retinal activators, is unknown, as is the potential modulation by cannabinoid receptors (CBRs). The density of TRPV1‐expressing RGCs in the Ai9:Trpv1 reporter mouse peaked in the mid‐peripheral retina. TRPV1 agonists including capsaicin (CAP) and the eCBs anandamide and N‐arachidonoyl‐dopamine elevated [Ca2+]i in 30–40% of wild‐type RGCs, with effects suppressed by TRPV1 antagonists capsazepine (CPZ) and BCTC ((4‐(3‐chloro‐2‐pyridinyl)‐N‐[4‐(1,1‐dimethylethyl)phenyl]‐1‐piperazinecarboxamide), and lacking in Trpv1−/− cells. The cannabinoid receptor type 1 (CB1R) colocalized with TRPV1:tdTomato expression. Its agonists 2‐arachidonoylglycerol (2‐AG) and WIN55,122 inhibited CAP‐induced [Ca2+]i signals in adult, but not early postnatal, RGCs. The suppressive effect of 2‐AG on TRPV1 activation was emulated by positive modulators of the protein kinase A (PKA) pathway, inhibited by the CB1R antagonist rimonabant and Gi uncoupler pertussis toxin, and absent in Cnr1−/− RGCs. We conclude that TRPV1 is a modulator of Ca2+ homeostasis in a subset of RGCs that show non‐uniform distribution across the mouse retina. Non‐retrograde eCB‐mediated modulation of RGC signalling involves a dynamic push–pull between direct TRPV1 activation and PKA‐dependent regulation of channel inactivation, with potential functions in setting the bandwidth of postsynaptic responses, sensitivity to mechanical/excitotoxic stress and neuroprotection.
    September 07, 2017   doi: 10.1113/JP274562   open full text
  • Chronic morphine reduces the readily releasable pool of GABA, a presynaptic mechanism of opioid tolerance.
    Adrianne R. Wilson‐Poe, Hyo‐Jin Jeong, Christopher W. Vaughan.
    The Journal of Physiology. September 07, 2017
    Key points Chronic treatment with opioids, such as morphine, leads to analgesic tolerance. While postsynaptic opioid tolerance is well documented, the involvement of presynaptic mechanisms remains unclear. We show that chronic morphine reduces the ability of periaqueductal grey (PAG) neurons to maintain GABAergic transmission. This depression of GABAergic transmission was due to a reduction in the effective size of the readily releasable pool. This also led to a reduction in opioid presynaptic inhibition; these presynaptic adaptations need to be considered in the development of strategies to reduce opioid tolerance. Abstract The midbrain periaqueductal grey (PAG) plays a critical role in tolerance to the analgesic actions of opioids such as morphine. While numerous studies have identified the postsynaptic adaptations induced by chronic morphine treatment in this and other brain regions, the presence of presynaptic adaptations remains uncertain. We examined GABAergic synaptic transmission within rat PAG brain slices from animals which underwent a low dose morphine treatment protocol which produces tolerance, but not withdrawal. Evoked GABAergic IPSCs (inhibitory postsynaptic currents) were less in morphine compared to control saline treated animals. Postsynaptic GABAA receptor mediated currents and desensitization, presynaptic release probability (Pr), and inhibition by endogenous neurotransmitters were similar in morphine and saline treated animals. By contrast, the effective size of the readily releasable pool (RRP) was smaller in morphine treated animals. While the μ‐opioid agonist DAMGO produced a reduction in Pr and RRP size in saline treated animals, it only reduced Pr in morphine treated animals. Consequently, DAMGO‐induced inhibition of evoked IPSCs during short burst stimulation was less in morphine, compared to saline treated animals. These results indicate that low dose chronic morphine treatment reduces presynaptic μ‐opioid inhibition by reducing the size of the pool of vesicles available for action potential dependent release. This novel presynaptic adaptation may provide important insights into the development of efficacious pain therapies that can circumvent the development of opioid tolerance.
    September 07, 2017   doi: 10.1113/JP274157   open full text
  • Hypercapnia‐induced active expiration increases in sleep and enhances ventilation in unanaesthetized rats.
    Isabela P. Leirão, Carlos A. Silva, Luciane H. Gargaglioni, Glauber S. F. da Silva.
    The Journal of Physiology. September 02, 2017
    Key points Expiratory muscles (abdominal and thoracic) can be recruited when respiratory drive increases under conditions of increased respiratory demand such as hypercapnia. Studying hypercapnia‐induced active expiration in unanaesthetized rats importantly contributes to the understanding of how the control system is integrated in vivo in freely moving animals. In unanaesthetized rats, hypercapnia‐induced active expiration was not always recruited either in wakefulness or in sleep, suggesting that additional factors influence the recruitment of active expiration. The pattern of abdominal muscle recruitment varied in a state‐dependent manner with active expiration being more predominant in the sleep state than in quiet wakefulness. Pulmonary ventilation was enhanced in periods with active expiration compared to periods without it. Abstract Expiration is passive at rest but becomes active through recruitment of abdominal muscles under increased respiratory drive. Hypercapnia‐induced active expiration has not been well explored in unanaesthetized rats. We hypothesized that (i) CO2‐evoked active expiration is recruited in a state‐dependent manner, i.e. differently in sleep or wakefulness, and (ii) recruitment of active expiration enhances ventilation, hence having an important functional role in meeting metabolic demand. To test these hypotheses, Wistar rats (280–330 g) were implanted with electrodes for EEG and electromyography EMG of the neck, diaphragm (DIA) and abdominal (ABD) muscles. Active expiratory events were considered as rhythmic ABDEMG activity interposed to DIAEMG. Animals were exposed to room air followed by hypercapnia (7% CO2) with EEG, EMG and ventilation (V̇E) recorded throughout the experimental protocol. No active expiration was observed during room air exposure. During hypercapnia, CO2‐evoked active expiration was predominantly recruited during non‐rapid eye movement sleep. Its increased occurrence during sleep was evidenced by the decreased DIA‐to‐ADB ratio (1:1 ratio means that each DIA event is followed by an ABD event, indicating a high occurrence of ABD activity). Moreover, V̇E was also enhanced (P < 0.05) in periods with active expiration. V̇E had a positive correlation (P < 0.05) with the peak amplitude of ABDEMG activity. The data demonstrate strongly that hypercapnia‐induced active expiration increases during sleep and provides an important functional role to support V̇E in conditions of increased respiratory demand.
    September 02, 2017   doi: 10.1113/JP274726   open full text
  • Detection of phasic dopamine by D1 and D2 striatal medium spiny neurons.
    Cedric Yapo, Anu G. Nair, Lorna Clement, Liliana R. Castro, Jeanette Hellgren Kotaleski, Pierre Vincent.
    The Journal of Physiology. September 02, 2017
    Key points Brief dopamine events are critical actors of reward‐mediated learning in the striatum; the intracellular cAMP–protein kinase A (PKA) response of striatal medium spiny neurons to such events was studied dynamically using a combination of biosensor imaging in mouse brain slices and in silico simulations. Both D1 and D2 medium spiny neurons can sense brief dopamine transients in the sub‐micromolar range. While dopamine transients profoundly change cAMP levels in both types of medium spiny neurons, the PKA‐dependent phosphorylation level remains unaffected in D2 neurons. At the level of PKA‐dependent phosphorylation, D2 unresponsiveness depends on protein phosphatase‐1 (PP1) inhibition by DARPP‐32. Simulations suggest that D2 medium spiny neurons could detect transient dips in dopamine level. Abstract The phasic release of dopamine in the striatum determines various aspects of reward and action selection, but the dynamics of the dopamine effect on intracellular signalling remains poorly understood. We used genetically encoded FRET biosensors in striatal brain slices to quantify the effect of transient dopamine on cAMP or PKA‐dependent phosphorylation levels, and computational modelling to further explore the dynamics of this signalling pathway. Medium‐sized spiny neurons (MSNs), which express either D1 or D2 dopamine receptors, responded to dopamine by an increase or a decrease in cAMP, respectively. Transient dopamine showed similar sub‐micromolar efficacies on cAMP in both D1 and D2 MSNs, thus challenging the commonly accepted notion that dopamine efficacy is much higher on D2 than on D1 receptors. However, in D2 MSNs, the large decrease in cAMP level triggered by transient dopamine did not translate to a decrease in PKA‐dependent phosphorylation level, owing to the efficient inhibition of protein phosphatase 1 by DARPP‐32. Simulations further suggested that D2 MSNs can also operate in a ‘tone‐sensing’ mode, allowing them to detect transient dips in basal dopamine. Overall, our results show that D2 MSNs may sense much more complex patterns of dopamine than previously thought.
    September 02, 2017   doi: 10.1113/JP274475   open full text
  • Dissociating external power from intramuscular exercise intensity during intermittent bilateral knee‐extension in humans.
    Matthew J. Davies, Alan P. Benson, Daniel T. Cannon, Simon Marwood, Graham J. Kemp, Harry B. Rossiter, Carrie Ferguson.
    The Journal of Physiology. September 02, 2017
    Key points Continuous high‐intensity constant‐power exercise is unsustainable, with maximal oxygen uptake (V̇O2 max ) and the limit of tolerance attained after only a few minutes. Performing the same power intermittently reduces the O2 cost of exercise and increases tolerance. The extent to which this dissociation is reflected in the intramuscular bioenergetics is unknown. We used pulmonary gas exchange and 31P magnetic resonance spectroscopy to measure whole‐body V̇O2, quadriceps phosphate metabolism and pH during continuous and intermittent exercise of different work:recovery durations. Shortening the work:recovery durations (16:32 s vs. 32:64 s vs. 64:128 s vs. continuous) at a work rate estimated to require 110% peak aerobic power reduced V̇O2, muscle phosphocreatine breakdown and muscle acidification, eliminated the glycolytic‐associated contribution to ATP synthesis, and increased exercise tolerance. Exercise intensity (i.e. magnitude of intramuscular metabolic perturbations) can be dissociated from the external power using intermittent exercise with short work:recovery durations. Abstract Compared with work‐matched high‐intensity continuous exercise, intermittent exercise dissociates pulmonary oxygen uptake (V̇O2) from the accumulated work. The extent to which this reflects differences in O2 storage fluctuations and/or contributions from oxidative and substrate‐level bioenergetics is unknown. Using pulmonary gas‐exchange and intramuscular 31P magnetic resonance spectroscopy, we tested the hypotheses that, at the same power: ATP synthesis rates are similar, whereas peak V̇O2 amplitude is lower in intermittent vs. continuous exercise. Thus, we expected that: intermittent exercise relies less upon anaerobic glycolysis for ATP provision than continuous exercise; shorter intervals would require relatively greater fluctuations in intramuscular bioenergetics than in V̇O2 compared to longer intervals. Six men performed bilateral knee‐extensor exercise (estimated to require 110% peak aerobic power) continuously and with three different intermittent work:recovery durations (16:32, 32:64 and 64:128 s). Target work duration (576 s) was achieved in all intermittent protocols; greater than continuous (252 ± 174 s; P < 0.05). Mean ATP turnover rate was not different between protocols (∼43 mm min−1 on average). However, the intramuscular phosphocreatine (PCr) component of ATP generation was greatest (∼30 mm min−1), and oxidative (∼10 mm min−1) and anaerobic glycolytic (∼1 mm min−1) components were lowest for 16:32 and 32:64 s intermittent protocols, compared to 64:128 s (18 ± 6, 21 ± 10 and 10 ± 4 mm min−1, respectively) and continuous protocols (8 ± 6, 20 ± 9 and 16 ± 14 mm min−1, respectively). As intermittent work duration increased towards continuous exercise, ATP production relied proportionally more upon anaerobic glycolysis and oxidative phosphorylation, and less upon PCr breakdown. However, performing the same high‐intensity power intermittently vs. continuously reduced the amplitude of fluctuations in V̇O2 and intramuscular metabolism, dissociating exercise intensity from the power output and work done.
    September 02, 2017   doi: 10.1113/JP274589   open full text
  • Cortical control of object‐specific grasp relies on adjustments of both activity and effective connectivity: a common marmoset study.
    Banty Tia, Mitsuaki Takemi, Akito Kosugi, Elisa Castagnola, Alberto Ansaldo, Takafumi Nakamura, Davide Ricci, Junichi Ushiba, Luciano Fadiga, Atsushi Iriki.
    The Journal of Physiology. September 02, 2017
    Key points The cortical mechanisms of grasping have been extensively studied in macaques and humans; here, we investigated whether common marmosets could rely on similar mechanisms despite strong differences in hand morphology and grip diversity. We recorded electrocorticographic activity over the sensorimotor cortex of two common marmosets during the execution of different grip types, which allowed us to study cortical activity (power spectrum) and physiologically inferred connectivity (phase‐slope index). Analyses were performed in beta (16–35 Hz) and gamma (75–100 Hz) frequency bands and our results showed that beta power varied depending on grip type, whereas gamma power displayed clear epoch‐related modulation. Strength and direction of inter‐area connectivity varied depending on grip type and epoch. These findings suggest that fundamental control mechanisms are conserved across primates and, in future research, marmosets could represent an adequate model to investigate primate brain mechanisms. Abstract The cortical mechanisms of grasping have been extensively studied in macaques and humans. Here, we investigated whether common marmosets could rely on similar mechanisms despite striking differences in manual dexterity. Two common marmosets were trained to grasp‐and‐pull three objects eliciting different hand configurations: whole‐hand, finger and scissor grips. The animals were then chronically implanted with 64‐channel electrocorticogram arrays positioned over the left premotor, primary motor and somatosensory cortex. Power spectra, reflecting predominantly cortical activity, and phase‐slope index, reflecting the direction of information flux, were studied in beta (16–35 Hz) and gamma (75–100 Hz) bands. Differences related to grip type, epoch (reach, grasp) and cortical area were statistically assessed. Results showed that whole‐hand and scissor grips triggered stronger beta desynchronization than finger grip. Task epochs clearly modulated gamma power, especially for finger and scissor grips. Considering effective connectivity, finger and scissor grips evoked stronger outflow from primary motor to premotor cortex, whereas whole‐hand grip displayed the opposite pattern. These findings suggest that fundamental control mechanisms, relying on adjustments of cortical activity and connectivity, are conserved across primates. Consistently, marmosets could represent a good model to investigate primate brain mechanisms.
    September 02, 2017   doi: 10.1113/JP274629   open full text
  • Rearing‐environment‐dependent hippocampal local field potential differences in wild‐type and inositol trisphosphate receptor type 2 knockout mice.
    Mika Tanaka, Xiaowen Wang, Katsuhiko Mikoshiba, Hajime Hirase, Yoshiaki Shinohara.
    The Journal of Physiology. August 27, 2017
    Key points Mice reared in an enriched environment are demonstrated to have larger hippocampal gamma oscillations than those reared in isolation, thereby confirming previous observations in rats. To test whether astrocytic Ca2+ surges are involved in this experience‐dependent LFP pattern modulation, we used inositol trisphosphate receptor type 2 (IP3R2)‐knockout (KO) mice, in which IP3/Ca2+ signalling in astrocytes is largely diminished. We found that this experience‐dependent gamma power alteration persists in the KO mice. Interestingly, hippocampal ripple events, the synchronized events critical for memory consolidation, are reduced in magnitude and frequency by both isolated rearing and IP3R2 deficiency. Abstract Rearing in an enriched environment (ENR) is known to enhance cognitive and memory abilities in rodents, whereas social isolation (ISO) induces depression‐like behaviour. The hippocampus has been documented to undergo morphological and functional changes depending on these rearing environments. For example, rearing condition during juvenility alters CA1 stratum radiatum gamma oscillation power in rats. In the present study, hippocampal CA1 local field potentials (LFP) were recorded from bilateral CA1 in urethane‐anaesthetized mice that were reared in either an ENR or ISO condition. Similar to previous findings in rats, gamma oscillation power during theta states was higher in the ENR group. Ripple events that occur during non‐theta periods in the CA1 stratum pyramidale also had longer intervals in ISO mice. Because astrocytic Ca2+ elevations play a key role in synaptic plasticity, we next tested whether these changes in LFP are also expressed in inositol trisphosphate receptor type 2 (IP3R2)‐knockout (KO) mice, in which astrocytic Ca2+ elevations are largely diminished. We found that the gamma power was also higher in IP3R2‐KO‐ENR mice compared to IP3R2‐KO‐ISO mice, suggesting that the rearing‐environment‐dependent gamma power alteration does not necessarily require the astrocytic IP3/Ca2+ pathway. By contrast, ripple events showed genotype‐dependent changes, as well as rearing condition‐dependent changes: ISO housing and IP3R2 deficiency both lead to longer inter‐ripple intervals. Moreover, we found that ripple magnitude in the right CA1 tended to be smaller in IP3R2‐KO. Because IP3R2‐KO mice have been reported to have depression phenotypes, our results suggest that ripple events and the mood of animals may be broadly correlated.
    August 27, 2017   doi: 10.1113/JP274573   open full text
  • Physiological vs. pharmacological signalling to myosin phosphorylation in airway smooth muscle.
    Ning Gao, Ming‐Ho Tsai, Audrey N. Chang, Weiqi He, Cai‐Ping Chen, Minsheng Zhu, Kristine E. Kamm, James T. Stull.
    The Journal of Physiology. August 24, 2017
    Key points Smooth muscle myosin regulatory light chain (RLC) is phosphorylated by Ca2+/calmodulin‐dependent myosin light chain kinase and dephosphorylated by myosin light chain phosphatase (MLCP). Tracheal smooth muscle contains significant amounts of myosin binding subunit 85 (MBS85), another myosin phosphatase targeting subunit (MYPT) family member, in addition to MLCP regulatory subunit MYPT1. Concentration/temporal responses to carbachol demonstrated similar sensitivities for bovine tracheal force development and phosphorylation of RLC, MYPT1, MBS85 and paxillin. Electrical field stimulation releases ACh from nerves to increase RLC phosphorylation but not MYPT1 or MBS85 phosphorylation. Thus, nerve‐mediated muscarinic responses in signalling modules acting on RLC phosphorylation are different from pharmacological responses with bath added agonist. The conditional knockout of MYPT1 or the knock‐in mutation T853A in mice had no effect on muscarinic force responses in isolated tracheal tissues. MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction. Abstract Ca2+/calmodulin activation of myosin light chain kinase (MLCK) initiates myosin regulatory light chain (RLC) phosphorylation for smooth muscle contraction with subsequent dephosphorylation for relaxation by myosin light chain phosphatase (MLCP) containing regulatory (MYPT1) and catalytic (PP1cδ) subunits. RLC phosphorylation‐dependent force development is regulated by distinct signalling modules involving protein phosphorylations. We investigated responses to cholinergic agonist treatment vs. neurostimulation by electric field stimulation (EFS) in bovine tracheal smooth muscle. Concentration/temporal responses to carbachol demonstrated tight coupling between force development and RLC phosphorylation but sensitivity differences in MLCK, MYPT1 T853, MYPT1 T696, myosin binding subunit 85 (MBS85), paxillin and CPI‐17 (PKC‐potentiated protein phosphatase 1 inhibitor protein of 17 kDa) phosphorylations. EFS increased force and phosphorylation of RLC, CPI‐17 and MLCK. In the presence of the cholinesterase inhibitor neostigmine, EFS led to an additional increase in phosphorylation of MYPT1 T853, MYPT1 T696, MBS85 and paxillin. Thus, there were distinct pharmacological vs. physiological responses in signalling modules acting on RLC phosphorylation and force responses, probably related to degenerate G protein signalling networks. Studies with genetically modified mice were performed. Expression of another MYPT1 family member, MBS85, was enriched in mouse, as well as bovine tracheal smooth muscle. Carbachol concentration/temporal‐force responses were similar in trachea from MYPT1SM+/+, MYPT1SM‐/− and the knock‐in mutant mice containing nonphosphorylatable MYPT1 T853A with no differences in RLC phosphorylation. Thus, MYPT1 T853 phosphorylation was not necessary for regulation of RLC phosphorylation in tonic airway smooth muscle. Furthermore, MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.
    August 24, 2017   doi: 10.1113/JP274715   open full text
  • Differential effects of late gestation maternal overnutrition on the regulation of surfactant maturation in fetal and postnatal life.
    Mitchell C. Lock, Erin V. McGillick, Sandra Orgeig, I. Caroline McMillen, Beverly S. Mühlhäusler, Song Zhang, Janna L. Morrison.
    The Journal of Physiology. August 24, 2017
    Key points Offspring of overweight and obese women are at greater risk for respiratory complications at birth. We determined the effect of late gestation maternal overnutrition (LGON) in sheep on surfactant maturation, glucose transport and fatty acid metabolism in the lung in fetal and postnatal life. There were significant decreases in surfactant components and numerical density of surfactant producing cells in the alveolar epithelium due to LGON in the fetal lung. However, there were no differences in the levels of these surfactant components between control and LGON lambs at 30 days of age. The reduced capacity for surfactant production in fetuses as a result of LGON may affect the transition to air breathing at birth. There was altered glucose transport and fatty acid metabolism in the lung as a result of LGON in postnatal life. However, there is a normalisation of surfactant components that suggests accelerated maturation in the lungs after birth. Abstract With the increasing incidence of obesity worldwide, the proportion of women entering pregnancy overweight or obese has increased dramatically. The fetus of an overnourished mother experiences numerous metabolic changes that may modulate lung development and hence successful transition to air breathing at birth. We used a sheep model of maternal late gestation overnutrition (LGON; from 115 days’ gestation, term 147 ± 3 days) to determine the effect of exposure to an increased plane of nutrition in late gestation on lung development in the fetus (at 141 days’ gestation) and the lamb (30 days after birth). We found a decrease in the numerical density of surfactant protein positive cells, as well as a reduction in mRNA expression of surfactant proteins (SFTP‐A, ‐B and ‐C), a rate limiting enzyme in surfactant phospholipid synthesis (phosphate cytidylyltransferase 1, choline, α; PCYT1A), and glucose transporters (SLC2A1 and SLC2A4) in the fetal lung. In lambs at 30 days after birth, there were no differences between Control and LGON groups in the surfactant components that were downregulated in the LGON fetuses. However, mRNA expression of SFTP‐A, PCYT1A, peroxisome proliferator activated receptor‐γ, fatty acid synthase and fatty acid transport protein were increased in LGON lambs compared to controls. These results indicate a reduced capacity for surfactant production in late gestation. While these deficits are normalised by 30 days after birth, the lungs of LGON lambs exhibited altered glucose transport and fatty acid metabolism, which is consistent with an enhanced capacity for surfactant synthesis and restoration of surfactant maturity in these animals.
    August 24, 2017   doi: 10.1113/JP274528   open full text
  • Experimental and modelling evidence of shortening heat in cardiac muscle.
    Kenneth Tran, June‐Chiew Han, Edmund John Crampin, Andrew James Taberner, Denis Scott Loiselle.
    The Journal of Physiology. August 22, 2017
    Key points Heat associated with muscle shortening has been repeatedly demonstrated in skeletal muscle, but its existence in cardiac muscle remains contentious after five decades of study. By iterating between experiments and computational modelling, we show compelling evidence for the existence of shortening heat in cardiac muscle and reveal, mechanistically, the source of this excess heat. Our results clarify a long‐standing uncertainty in the field of cardiac muscle energetics. We provide a revised partitioning of cardiac muscle energy expenditure to include this newly revealed thermal component. Abstract When a muscle shortens against an afterload, the heat that it liberates is greater than that produced by the same muscle contracting isometrically at the same level of force. This excess heat is defined as ‘shortening heat’, and has been repeatedly demonstrated in skeletal muscle but not in cardiac muscle. Given the micro‐structural similarities between these two muscle types, and since we imagine that shortening heat is the thermal accompaniment of cross‐bridge cycling, we have re‐examined this issue. Using our flow‐through microcalorimeter, we measured force and heat generated by isolated rat trabeculae undergoing isometric contractions at different muscle lengths and work‐loop (shortening) contractions at different afterloads. We simulated these experimental protocols using a thermodynamically constrained model of cross‐bridge cycling and probed the mechanisms underpinning shortening heat. Predictions generated by the model were subsequently validated by a further set of experiments. Both our experimental and modelling results show convincing evidence for the existence of shortening heat in cardiac muscle. Its magnitude is inversely related to the afterload or, equivalently, directly related to the extent of shortening. Computational simulations reveal that the heat of shortening arises from the cycling of cross‐bridges, and that the rate of ATP hydrolysis is more sensitive to change of muscle length than to change of afterload. Our results clarify a long‐standing uncertainty in the field of cardiac muscle energetics.
    August 22, 2017   doi: 10.1113/JP274680   open full text
  • Differential calcium sensitivity in NaV1.5 mixed syndrome mutants.
    Mena Abdelsayed, Alban‐Elouen Baruteau, Karen Gibbs, Shubhayan Sanatani, Andrew D. Krahn, Vincent Probst, Peter C. Ruben.
    The Journal of Physiology. August 20, 2017
    Key points SCN5a mutations may express gain‐of‐function (Long QT Syndrome‐3), loss‐of‐function (Brugada Syndrome 1) or both (mixed syndromes), depending on the mutation and environmental triggers. One such trigger may be an increase in cytosolic calcium, accompanying exercise. Many mixed syndromes mutants, including ∆KPQ, E1784K, 1795insD and Q1909R, are found in calcium‐sensitive regions. Elevated cytosolic calcium attenuates gain‐of‐function properties in ∆KPQ, 1795insD and Q1909R, but not in E1784K. By contrast, elevated cytosolic calcium further exacerbates gain‐of‐function in E1784K by destabilizing slow inactivation. Action potential modelling, using a modified O'Hara Rudy model, suggests that elevated heart rate rescues action potential duration in ∆KPQ, 1795insD and Q1909R, but not in E1784K. Action potential simulations suggest that E1784K carriers have an increased intracellular sodium‐to‐calcium ratio under bradycardia and tachycardia conditions. Elevated cytosolic calcium, which is common during high heart rates, ameliorates or exacerbates the mixed syndrome phenotype depending on the genetic signature. Abstract Inherited arrhythmias may arise from mutations in the gene for SCN5a, which encodes the cardiac voltage‐gated sodium channel, NaV1.5. Mutants in NaV1.5 result in Brugada Syndrome (BrS1), Long‐QT Syndrome (LQT3) or mixed syndromes (an overlap of BrS1/LQT3). Exercise is a potential arrhythmogenic trigger in mixed syndromes. We aimed to determine the effects of elevated cytosolic calcium, which is common during exercise, in mixed syndrome NaV1.5 mutants. We used whole‐cell patch clamp to assess the biophysical properties of NaV1.5 wild‐type (WT), ∆KPQ, E1784K, 1795insD and Q1909R mutants in human embryonic kidney 293 cells transiently transfected with the NaV1.5 α subunit (WT or mutants), β1 subunit and enhanced green fluorescent protein. Voltage‐dependence and kinetics were measured at cytosolic calcium levels of approximately 0, 500 and 2500 nm. In silico, action potential (AP) model simulations were performed using a modified O'Hara Rudy model. Elevated cytosolic calcium attenuates the late sodium current in ∆KPQ, 1795insD and Q1909R, but not in E1784K. Elevated cytosolic calcium restores steady‐state slow inactivation (SSSI) to the WT‐form in Q1909R, but depolarized SSSI in E1784K. Our AP simulations showed a frequency‐dependent reduction of AP duration in ∆KPQ, 1795insD and Q1909R carriers. In E1784K, AP duration is relatively prolonged at both low and high heart rates, resulting in a sodium overload. Cellular perturbations during exercise may affect BrS1/LQT3 patients differently depending on their individual genetic signature. Thus, exercise may be therapeutic or may be an arrhythmogenic trigger in some SCN5a patients.
    August 20, 2017   doi: 10.1113/JP274536   open full text
  • Paired motor cortex and cervical epidural electrical stimulation timed to converge in the spinal cord promotes lasting increases in motor responses.
    Asht M. Mishra, Ajay Pal, Disha Gupta, Jason B. Carmel.
    The Journal of Physiology. August 20, 2017
    Key points Pairing motor cortex stimulation and spinal cord epidural stimulation produced large augmentation in motor cortex evoked potentials if they were timed to converge in the spinal cord. The modulation of cortical evoked potentials by spinal cord stimulation was largest when the spinal electrodes were placed over the dorsal root entry zone. Repeated pairing of motor cortex and spinal cord stimulation caused lasting increases in evoked potentials from both sites, but only if the time between the stimuli was optimal. Both immediate and lasting effects of paired stimulation are likely mediated by convergence of descending motor circuits and large diameter afferents onto common interneurons in the cervical spinal cord. Abstract Convergent activity in neural circuits can generate changes at their intersection. The rules of paired electrical stimulation are best understood for protocols that stimulate input circuits and their targets. We took a different approach by targeting the interaction of descending motor pathways and large diameter afferents in the spinal cord. We hypothesized that pairing stimulation of motor cortex and cervical spinal cord would strengthen motor responses through their convergence. We placed epidural electrodes over motor cortex and the dorsal cervical spinal cord in rats; motor evoked potentials (MEPs) were measured from biceps. MEPs evoked from motor cortex were robustly augmented with spinal epidural stimulation delivered at an intensity below the threshold for provoking an MEP. Augmentation was critically dependent on the timing and position of spinal stimulation. When the spinal stimulation was timed to coincide with the descending volley from motor cortex stimulation, MEPs were more than doubled. We then tested the effect of repeated pairing of motor cortex and spinal stimulation. Repetitive pairing caused strong augmentation of cortical MEPs and spinal excitability that lasted up to an hour after just 5 min of pairing. Additional physiology experiments support the hypothesis that paired stimulation is mediated by convergence of descending motor circuits and large diameter afferents in the spinal cord. The large effect size of this protocol and the conservation of the circuits being manipulated between rats and humans makes it worth pursuing for recovery of sensorimotor function after injury to the central nervous system.
    August 20, 2017   doi: 10.1113/JP274663   open full text
  • Cortical contributions to sensory gating in the ipsilateral somatosensory cortex during voluntary activity.
    Yuming Lei, Monica A. Perez.
    The Journal of Physiology. August 18, 2017
    Key points It has long been known that the somatosensory cortex gates sensory inputs from the contralateral side of the body. Here, we examined the contribution of the ipsilateral somatosensory cortex (iS1) to sensory gating during index finger voluntary activity. The amplitude of the P25/N33, but not other somatosensory evoked potential (SSEP) components, was reduced during voluntary activity compared with rest. Interhemispheric inhibition between S1s and intracortical inhibition in the S1 modulated the amplitude of the P25/N33. Note that changes in interhemispheric inhibition between S1s correlated with changes in cortical circuits in the ipsilateral motor cortex. Our findings suggest that cortical circuits, probably from somatosensory and motor cortex, contribute to sensory gating in the iS1 during voluntary activity in humans. Abstract An important principle in the organization of the somatosensory cortex is that it processes afferent information from the contralateral side of the body. The role of the ipsilateral somatosensory cortex (iS1) in sensory gating in humans remains largely unknown. Using electroencephalographic (EEG) recordings over the iS1 and electrical stimulation of the ulnar nerve at the wrist, we examined somatosensory evoked potentials (SSEPs; P14/N20, N20/P25 and P25/N33 components) and paired‐pulse SSEPs between S1s (interhemispheric inhibition) and within (intracortical inhibition) the iS1 at rest and during tonic index finger voluntary activity. We found that the amplitude of the P25/N33, but not other SSEP components, was reduced during voluntary activity compared with rest. Interhemispheric inhibition increased the amplitude of the P25/N33 and intracortical inhibition reduced the amplitude of the P25/N33, suggesting a cortical origin for this effect. The P25/N33 receives inputs from the motor cortex, so we also examined the contribution of distinct sets of cortical interneurons by testing the effect of ulnar nerve stimulation on motor‐evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the ipsilateral motor cortex with the coil in the posterior–anterior (PA) and anterior–posterior (AP) orientation. Afferent input attenuated PA, but not AP, MEPs during voluntary activity compared with rest. Notably, changes in interhemispheric inhibition correlated with changes in PA MEPs. Our novel findings suggest that interhemispheric projections between S1s and intracortical circuits, probably from somatosensory and motor cortex, contribute to sensory gating in the iS1 during voluntary activity in humans.
    August 18, 2017   doi: 10.1113/JP274504   open full text
  • The role of T‐type calcium channels in the subiculum: to burst or not to burst?
    Srdjan M. Joksimovic, Pierce Eggan, Yukitoshi Izumi, Sonja Lj. Joksimovic, Vesna Tesic, Robert M. Dietz, James E. Orfila, Michael R. DiGruccio, Paco S. Herson, Vesna Jevtovic‐Todorovic, Charles F. Zorumski, Slobodan M. Todorovic.
    The Journal of Physiology. August 18, 2017
    Key points Pharmacological, molecular and genetic data indicate a prominent role of low‐voltage‐activated T‐type calcium channels (T‐channels) in the firing activity of both pyramidal and inhibitory interneurons in the subiculum. Pharmacological inhibition of T‐channels switched burst firing with lower depolarizing stimuli to regular spiking, and fully abolished hyperpolarization‐induced burst firing. Our molecular studies showed that CaV3.1 is the most abundantly expressed isoform of T‐channels in the rat subiculum. Consistent with this finding, both regular‐spiking and burst firing patterns were profoundly depressed in the mouse with global deletion of CaV3.1 isoform of T‐channels. Selective inhibition of T‐channels and global deletion of CaV3.1 channels completely suppressed development of long‐term potentiation (LTP) in the CA1–subiculum, but not in the CA3–CA1 pathway. Abstract Several studies suggest that voltage‐gated calcium currents are involved in generating high frequency burst firing in the subiculum, but the exact nature of these currents remains unknown. Here, we used selective pharmacology, molecular and genetic approaches to implicate Cav3.1‐containing T‐channels in subicular burst firing, in contrast to several previous reports discounting T‐channels as major contributors to subicular neuron physiology. Furthermore, pharmacological antagonism of T‐channels, as well as global deletion of CaV3.1 isoform, completely suppressed development of long‐term potentiation (LTP) in the CA1–subiculum, but not in the CA3–CA1 pathway. Our results indicate that excitability and synaptic plasticity of subicular neurons relies heavily on T‐channels. Hence, T‐channels may be a promising new drug target for different cognitive deficits.
    August 18, 2017   doi: 10.1113/JP274565   open full text
  • Decline in cellular function of aged mouse c‐kit+ cardiac progenitor cells.
    Alessandra Castaldi, Ramsinh Mansinh Dodia, Amabel M. Orogo, Cristina M. Zambrano, Rita H. Najor, Åsa B. Gustafsson, Joan Heller Brown, Nicole H. Purcell.
    The Journal of Physiology. August 18, 2017
    Key points While autologous stem cell‐based therapies are currently being tested on elderly patients, there are limited data on the function of aged stem cells and in particular c‐kit+ cardiac progenitor cells (CPCs). We isolated c‐kit+ cells from young (3 months) and aged (24 months) C57BL/6 mice to compare their biological properties. Aged CPCs have increased senescence, decreased stemness and reduced capacity to proliferate or to differentiate following dexamethasone (Dex) treatment in vitro, as evidenced by lack of cardiac lineage gene upregulation. Aged CPCs fail to activate mitochondrial biogenesis and increase proteins involved in mitochondrial oxidative phosphorylation in response to Dex. Aged CPCs fail to upregulate paracrine factors that are potentially important for proliferation, survival and angiogenesis in response to Dex. The results highlight marked differences between young and aged CPCs, which may impact future design of autologous stem cell‐based therapies. Abstract Therapeutic use of c‐kit+ cardiac progenitor cells (CPCs) is being evaluated for regenerative therapy in older patients with ischaemic heart failure. Our understanding of the biology of these CPCs has, however, largely come from studies of young cells and animal models. In the present study we examined characteristics of CPCs isolated from young (3 months) and aged (24 months) mice that could underlie the diverse outcomes reported for CPC‐based therapeutics. We observed morphological differences and altered senescence indicated by increased senescence‐associated markers β‐galactosidase and p16 mRNA in aged CPCs. The aged CPCs also proliferated more slowly than their young counterparts and expressed lower levels of the stemness marker LIN28. We subsequently treated the cells with dexamethasone (Dex), routinely used to induce commitment in CPCs, for 7 days and analysed expression of cardiac lineage marker genes. While MEF2C, GATA4, GATA6 and PECAM mRNAs were significantly upregulated in response to Dex treatment in young CPCs, their expression was not increased in aged CPCs. Interestingly, Dex treatment of aged CPCs also failed to increase mitochondrial biogenesis and expression of the mitochondrial proteins Complex III and IV, consistent with a defect in mitochondria complex assembly in the aged CPCs. Dex‐treated aged CPCs also had impaired ability to upregulate expression of paracrine factor genes and the conditioned media from these cells had reduced ability to induce angiogenesis in vitro. These findings could impact the design of future CPC‐based therapeutic approaches for the treatment of older patients suffering from cardiac injury.
    August 18, 2017   doi: 10.1113/JP274775   open full text
  • Calcium/calmodulin‐dependent kinase 2 mediates Epac‐induced spontaneous transient outward currents in rat vascular smooth muscle.
    Edward S. A. Humphries, Tomoko Kamishima, John M. Quayle, Caroline Dart.
    The Journal of Physiology. August 14, 2017
    Key points The Ca2+ and redox‐sensing enzyme Ca2+/calmodulin‐dependent kinase 2 (CaMKII) is a crucial and well‐established signalling molecule in the heart and brain. In vascular smooth muscle, which controls blood flow by contracting and relaxing in response to complex Ca2+ signals and oxidative stress, surprisingly little is known about the role of CaMKII. The vasodilator‐induced second messenger cAMP can relax vascular smooth muscle via its effector, exchange protein directly activated by cAMP (Epac), by activating spontaneous transient outward currents (STOCs) that hyperpolarize the cell membrane and reduce voltage‐dependent Ca2+ influx. How Epac activates STOCs is unknown. In the present study, we map the pathway by which Epac increases STOC activity in contractile vascular smooth muscle and show that a critical step is the activation of CaMKII. To our knowledge, this is the first report of CaMKII activation triggering cellular activity known to induce vasorelaxation. Abstract Activation of the major cAMP effector, exchange protein directly activated by cAMP (Epac), induces vascular smooth muscle relaxation by increasing the activity of ryanodine (RyR)‐sensitive release channels on the peripheral sarcoplasmic reticulum. Resultant Ca2+ sparks activate plasma membrane Ca2+‐activated K+ (BKCa) channels, evoking spontaneous transient outward currents (STOCs) that hyperpolarize the cell and reduce voltage‐dependent Ca2+ entry. In the present study, we investigate the mechanism by which Epac increases STOC activity. We show that the selective Epac activator 8‐(4‐chloro‐phenylthio)‐2′‐O‐methyladenosine‐3′, 5‐cyclic monophosphate‐AM (8‐pCPT‐AM) induces autophosphorylation (activation) of calcium/calmodulin‐dependent kinase 2 (CaMKII) and also that inhibition of CaMKII abolishes 8‐pCPT‐AM‐induced increases in STOC activity. Epac‐induced CaMKII activation is probably initiated by inositol 1,4,5‐trisphosphate (IP3)‐mobilized Ca2+: 8‐pCPT‐AM fails to induce CaMKII activation following intracellular Ca2+ store depletion and inhibition of IP3 receptors blocks both 8‐pCPT‐AM‐mediated CaMKII phosphorylation and STOC activity. 8‐pCPT‐AM does not directly activate BKCa channels, but STOCs cannot be generated by 8‐pCPT‐AM in the presence of ryanodine. Furthermore, exposure to 8‐pCPT‐AM significantly slows the initial rate of [Ca2+]i rise induced by the RyR activator caffeine without significantly affecting the caffeine‐induced Ca2+ transient amplitude, a measure of Ca2+ store content. We conclude that Epac‐mediated STOC activity (i) occurs via activation of CaMKII and (ii) is driven by changes in the underlying behaviour of RyR channels. To our knowledge, this is the first report of CaMKII initiating cellular activity linked to vasorelaxation and suggests novel roles for this Ca2+ and redox‐sensing enzyme in the regulation of vascular tone and blood flow.
    August 14, 2017   doi: 10.1113/JP274754   open full text
  • Post‐translational palmitoylation controls the voltage gating and lipid raft association of the CALHM1 channel.
    Akiyuki Taruno, Hongxin Sun, Koichi Nakajo, Tatsuro Murakami, Yasuyoshi Ohsaki, Mizuho A. Kido, Fumihito Ono, Yoshinori Marunaka.
    The Journal of Physiology. August 14, 2017
    Key points Calcium homeostasis modulator 1 (CALHM1), a new voltage‐gated ATP‐ and Ca2+‐permeable channel, plays important physiological roles in taste perception and memory formation. Regulatory mechanisms of CALHM1 remain unexplored, although the biophysical disparity between CALHM1 gating in vivo and in vitro suggests that there are undiscovered regulatory mechanisms. Here we report that CALHM1 gating and association with lipid microdomains are post‐translationally regulated through the process of protein S‐palmitoylation, a reversible attachment of palmitate to cysteine residues. Our data also establish cysteine residues and enzymes responsible for CALHM1 palmitoylation. CALHM1 regulation by palmitoylation provides new mechanistic insights into fine‐tuning of CALHM1 gating in vivo and suggests a potential layer of regulation in taste and memory. Abstract Emerging roles of CALHM1, a recently discovered voltage‐gated ion channel, include purinergic neurotransmission of tastes in taste buds and memory formation in the brain, highlighting its physiological importance. However, the regulatory mechanisms of the CALHM1 channel remain entirely unexplored, hindering full understanding of its contribution in vivo. The different gating properties of CALHM1 in vivo and in vitro suggest undiscovered regulatory mechanisms. Here, in searching for post‐translational regulatory mechanisms, we discovered the regulation of CALHM1 gating and association with lipid microdomains via protein S‐palmitoylation, the only reversible lipid modification of proteins on cysteine residues. CALHM1 is palmitoylated at two intracellular cysteines located in the juxtamembrane regions of the third and fourth transmembrane domains. Enzymes that catalyse CALHM1 palmitoylation were identified by screening 23 members of the DHHC protein acyltransferase family. Epitope tagging of endogenous CALHM1 proteins in mice revealed that CALHM1 is basally palmitoylated in taste buds in vivo. Functionally, palmitoylation downregulates CALHM1 without effects on its synthesis, degradation and cell surface expression. Mutation of the palmitoylation sites has a profound impact on CALHM1 gating, shifting the conductance–voltage relationship to more negative voltages and accelerating the activation kinetics. The same mutation also reduces CALHM1 association with detergent‐resistant membranes. Our results comprehensively uncover a post‐translational regulation of the voltage‐dependent gating of CALHM1 by palmitoylation.
    August 14, 2017   doi: 10.1113/JP274164   open full text
  • Depletion of Pax7+ satellite cells does not affect diaphragm adaptations to running in young or aged mice.
    Kevin A. Murach, Amy L. Confides, Angel Ho, Janna R. Jackson, Lina S. Ghazala, Charlotte A. Peterson, Esther E. Dupont‐Versteegden.
    The Journal of Physiology. August 14, 2017
    Key points Satellite cell depletion does not affect diaphragm adaptations to voluntary wheel running in young or aged mice. Satellite cell depletion early in life (4 months of age) has minimal effect on diaphragm phenotype by old age (24 months). Prolonged satellite cell depletion in the diaphragm does not result in excessive extracellular matrix accumulation, in contrast to what has been reported in hind limb muscles. Up‐regulation of Pax3 mRNA+ cells after satellite cell depletion in young and aged mice suggests that Pax3+ cells may compensate for a loss of Pax7+ satellite cells in the diaphragm. Future investigations should focus on the role of Pax3+ cells in the diaphragm during adaptation to exercise and ageing. Abstract Satellite cell contribution to unstressed diaphragm is higher compared to hind limb muscles, which is probably attributable to constant activation of this muscle to drive ventilation. Whether satellite cell depletion negatively impacts diaphragm quantitative and qualitative characteristics under stressed conditions in young and aged mice is unknown. We therefore challenged the diaphragm with prolonged running activity in the presence and absence of Pax7+ satellite cells in young and aged mice using an inducible Pax7CreER‐R26RDTA model. Mice were vehicle (Veh, satellite cell‐replete) or tamoxifen (Tam, satellite cell‐depleted) treated at 4 months of age and were then allowed to run voluntarily at 6 months (young) and 22 months (aged). Age‐matched, cage‐dwelling, Veh‐ and Tam‐treated mice without wheel access served as activity controls. Diaphragm muscles were analysed from young (8 months) and aged (24 months) mice. Satellite cell depletion did not alter diaphragm mean fibre cross‐sectional area, fibre type distribution or extracellular matrix content in young or aged mice, regardless of running activity. Resting in vivo diaphragm function was also unaffected by satellite cell depletion. Myonuclear density was maintained in young satellite cell‐depleted mice regardless of running, although it was modestly reduced in aged sedentary (–7%) and running (–19%) mice without satellite cells (P < 0.05). Using fluorescence in situ hybridization, we detected higher Pax3 mRNA+ cell density in both young and aged satellite cell‐depleted diaphragm muscle (P < 0.05), which may compensate for the loss of Pax7+ satellite cells.
    August 14, 2017   doi: 10.1113/JP274611   open full text
  • ATP and astrocytes play a prominent role in the control of the respiratory pattern generator in the lamprey.
    Elenia Cinelli, Ludovica Iovino, Donatella Mutolo.
    The Journal of Physiology. August 08, 2017
    Key points The paratrigeminal respiratory group (pTRG) is responsible for the respiratory pattern generation in the lamprey. The role of ATP and astrocytes, known to control respiratory activity in mammals, was investigated in the lamprey respiratory network. ATP microinjected into the pTRG induces a biphasic response consisting of marked increases in respiratory frequency mediated by P2X receptors followed by a decrease in the respiratory motor output due to the ATP metabolite adenosine. We provide evidence that astrocytes are involved in the genesis of the normal respiratory pattern, ATP‐induced responses and acidification‐induced increases of the respiratory activity. The function of astrocytes in rhythmic networks appears to be phylogenetically conserved. Abstract The role of ATP and astrocytes in respiratory rhythm modulation has been recently investigated in neonatal rodents. However, no information on the role of ATP and astrocytes within the respiratory network of the lamprey is available, particularly within the paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator. To address these issues, the present study was carried out on isolated brainstems of the adult lamprey. Bath application of ATP caused marked increases in respiratory frequency followed by decreases in the respiratory motor output, mediated by the ATP metabolite adenosine at the level of the pTRG. Bath applications and microinjections of agonists and antagonists of purinergic receptors showed that ATP increased respiratory activity through an action on pTRG P2X receptors. To disclose the respiratory role of astrocytes, we used bath application of the gliotoxin aminoadipic acid, which dramatically depressed the respiratory motor output that, however, promptly recovered following glutamine application. Furthermore, the excitatory responses to ATP‐γ‐S (a non‐hydrolysable ATP analogue), but not to substance P, microinjected into the pTRG, were abolished. Finally, we also demonstrated that acidification‐induced increases in respiratory activity were ATP‐independent, but mediated by the astrocytes’ glutamate–glutamine cycle. The results show for the first time that ATP and especially astrocytes strongly contribute to the modulation of the lamprey respiratory pattern. Their role in the modulation or maintenance of rhythmic neuronal activities appears to be phylogenetically conserved.
    August 08, 2017   doi: 10.1113/JP274749   open full text
  • Distinct temporal filters in mitral cells and external tufted cells of the olfactory bulb.
    Christopher E. Vaaga, Gary L. Westbrook.
    The Journal of Physiology. August 08, 2017
    Short‐term synaptic plasticity is a critical regulator of neural circuits, and largely determines how information is temporally processed. In the olfactory bulb, afferent olfactory receptor neurons respond to increasing concentrations of odorants with barrages of action potentials, and their terminals have an extraordinarily high release probability (Sicard, 1986; Murphy et al. 2004). These features suggest that during naturalistic stimuli, afferent input to the olfactory bulb is subject to strong synaptic depression, presumably truncating the postsynaptic response to afferent stimuli. To examine this issue, we used single glomerular stimulation in mouse olfactory bulb slices to measure the synaptic dynamics of afferent‐evoked input at physiological stimulus frequencies. In cell‐attached recordings, mitral cells responded to high frequency stimulation with sustained responses, whereas external tufted cells responded transiently. Consistent with previous reports (Murphy et al. 2004), olfactory nerve terminals onto both cell types had a high release probability (0.7), from a single pool of slowly recycling vesicles, indicating that the distinct responses of mitral and external tufted cells to high frequency stimulation did not originate presyaptically. Rather, distinct temporal response profiles in mitral cells and external tufted cells could be attributed to slow dendrodendritic responses in mitral cells, as blocking this slow current in mitral cells converted mitral cell responses to a transient response profile, typical of external tufted cells. Our results suggest that despite strong axodendritic synaptic depression, the balance of axodendritic and dendrodendritic circuitry in external tufted cells and mitral cells, respectively, tunes the postsynaptic responses to high frequency, naturalistic stimulation. This article is protected by copyright. All rights reserved
    August 08, 2017   doi: 10.1113/JP274608   open full text
  • Gene expression analyses reveal metabolic specifications in acute O2‐sensing chemoreceptor cells.
    Lin Gao, Victoria Bonilla‐Henao, Paula García‐Flores, Ignacio Arias‐Mayenco, Patricia Ortega‐Sáenz, José López‐Barneo.
    The Journal of Physiology. August 08, 2017
    Key points Glomus cells in the carotid body (CB) and chromaffin cells in the adrenal medulla (AM) are essential for reflex cardiorespiratory adaptation to hypoxia. However, the mechanisms whereby these cells detect changes in O2 tension are poorly understood. The metabolic properties of acute O2‐sensing cells have been investigated by comparing the transcriptomes of CB and AM cells, which are O2‐sensitive, with superior cervical ganglion neurons, which are practically O2‐insensitive. In O2‐sensitive cells, we found a characteristic prolyl hydroxylase 3 down‐regulation and hypoxia inducible factor 2α up‐regulation, as well as overexpression of genes coding for three atypical mitochondrial electron transport subunits and pyruvate carboxylase, an enzyme that replenishes tricarboxylic acid cycle intermediates. In agreement with this observation, the inhibition of succinate dehydrogenase impairs CB acute O2 sensing. The responsiveness of peripheral chemoreceptor cells to acute hypoxia depends on a ‘signature metabolic profile’. Abstract Acute O2 sensing is a fundamental property of cells in the peripheral chemoreceptors, e.g. glomus cells in the carotid body (CB) and chromaffin cells in the adrenal medulla (AM), and is necessary for adaptation to hypoxia. These cells contain O2‐sensitive ion channels, which mediate membrane depolarization and transmitter release upon exposure to hypoxia. However, the mechanisms underlying the detection of changes in O2 tension by cells are still poorly understood. Recently, we suggested that CB glomus cells have specific metabolic features that favour the accumulation of reduced quinone and the production of mitochondrial NADH and reactive oxygen species during hypoxia. These signals alter membrane ion channel activity. To investigate the metabolic profile characteristic of acute O2‐sensing cells, we used adult mice to compare the transcriptomes of three cell types derived from common sympathoadrenal progenitors, but exhibiting variable responsiveness to acute hypoxia: CB and AM cells, which are O2‐sensitive (glomus cells > chromaffin cells), and superior cervical ganglion neurons, which are practically O2‐insensitive. In the O2‐sensitive cells, we found a characteristic mRNA expression pattern of prolyl hydroxylase 3/hypoxia inducible factor 2α and up‐regulation of several genes, in particular three atypical mitochondrial electron transport subunits and some ion channels. In addition, we found that pyruvate carboxylase, an enzyme fundamental to tricarboxylic acid cycle anaplerosis, is overexpressed in CB glomus cells. We also observed that the inhibition of succinate dehydrogenase impairs CB acute O2 sensing. Our data suggest that responsiveness to acute hypoxia depends on a ‘signature metabolic profile’ in chemoreceptor cells.
    August 08, 2017   doi: 10.1113/JP274684   open full text
  • NaCl and osmolarity produce different responses in organum vasculosum of the lamina terminalis neurons, sympathetic nerve activity and blood pressure.
    Brian J. Kinsman, Kirsteen N. Browning, Sean D. Stocker.
    The Journal of Physiology. August 02, 2017
    Key points Changes in extracellular osmolarity stimulate thirst and vasopressin secretion through a central osmoreceptor; however, central infusion of hypertonic NaCl produces a greater sympathoexcitatory and pressor response than infusion of hypertonic mannitol/sorbitol. Neurons in the organum vasculosum of the lamina terminalis (OVLT) sense changes in extracellular osmolarity and NaCl. In this study, we discovered that intracerebroventricular infusion or local OVLT injection of hypertonic NaCl increases lumbar sympathetic nerve activity, adrenal sympathetic nerve activity and arterial blood pressure whereas equi‐osmotic mannitol/sorbitol did not alter any variable. In vitro whole‐cell recordings demonstrate the majority of OVLT neurons are responsive to hypertonic NaCl or mannitol. However, hypertonic NaCl stimulates a greater increase in discharge frequency than equi‐osmotic mannitol. Intracarotid or intracerebroventricular infusion of hypertonic NaCl evokes a greater increase in OVLT neuronal discharge frequency than equi‐osmotic sorbitol. Collectively, these novel data suggest that subsets of OVLT neurons respond differently to hypertonic NaCl versus osmolarity and subsequently regulate body fluid homeostasis. These responses probably reflect distinct cellular mechanisms underlying NaCl‐ versus osmo‐sensing. Abstract Systemic or central infusion of hypertonic NaCl and other osmolytes readily stimulate thirst and vasopressin secretion. In contrast, central infusion of hypertonic NaCl produces a greater increase in arterial blood pressure (ABP) than equi‐osmotic mannitol/sorbitol. Although these responses depend on neurons in the organum vasculosum of the lamina terminalis (OVLT), these observations suggest OVLT neurons may sense or respond differently to hypertonic NaCl versus osmolarity. The purpose of this study was to test this hypothesis in Sprague‐Dawley rats. First, intracerebroventricular (icv) infusion (5 μl/10 min) of 1.0 m NaCl produced a significantly greater increase in lumbar sympathetic nerve activity (SNA), adrenal SNA and ABP than equi‐osmotic sorbitol (2.0 osmol l−1). Second, OVLT microinjection (20 nl) of 1.0 m NaCl significantly raised lumbar SNA, adrenal SNA and ABP. Equi‐osmotic sorbitol did not alter any variable. Third, in vitro whole‐cell recordings demonstrate that 50% (18/36) of OVLT neurons display an increased discharge to both hypertonic NaCl (+7.5 mm) and mannitol (+15 mm). Of these neurons, 56% (10/18) displayed a greater discharge response to hypertonic NaCl vs mannitol. Fourth, in vivo single‐unit recordings revealed that intracarotid injection of hypertonic NaCl produced a concentration‐dependent increase in OVLT cell discharge, lumbar SNA and ABP. The responses to equi‐osmotic infusions of hypertonic sorbitol were significantly smaller. Lastly, icv infusion of 0.5 m NaCl produced significantly greater increases in OVLT discharge and ABP than icv infusion of equi‐osmotic sorbitol. Collectively, these findings indicate NaCl and osmotic stimuli produce different responses across OVLT neurons and may represent distinct cellular processes to regulate thirst, vasopressin secretion and autonomic function.
    August 02, 2017   doi: 10.1113/JP274537   open full text
  • A reduction in SK channels contributes to increased activity of hypothalamic magnocellular neurons during heart failure.
    Hildebrando C. Ferreira‐Neto, Vinicia C. Biancardi, Javier E. Stern.
    The Journal of Physiology. August 02, 2017
    Key points Small conductance Ca2+‐activated K+ (SK) channels play an important role in regulating the excitability of magnocellular neurosecretory cells (MNCs). Although an increased SK channel function contributes to adaptive physiological responses, it remains unknown whether changes in SK channel function/expression contribute to exacerbated MNC activity under disease conditions. We show that the input–output function of MNCs in heart failure (HF) rats is enhanced. Moreover, the SK channel blocker apamin enhanced the input–output function in sham, although not in HF rats. We found that both the after‐hyperpolarizing potential magnitude and the underlying apamin‐sensitive IAHP are blunted in MNCs from HF rats. The magnitude of spike‐induced increases in intracellular Ca2+ levels was not affected in MNCs of HF rats. We found a diminished expression of SK2/SK3 channel subunit mRNA expression in the supraoptic nucleus of HF rats. Our studies suggest that a reduction in SK channel expression, but not changes in Ca2+‐mediated activation of SK channels, contributes to exacerbated MNC activity in HF rats. Abstract Small conductance Ca2+‐activated K+ channels (SK) play an important role in regulating the activity of magnocellular neurosecretory cells (MNCs) and hormone release from the posterior pituitary. Moreover, enhanced SK activity contributes to the adaptive responses of MNCs to physiological challenge, such as lactation. Nevertheless, whether changes in SK function/expression contribute to exacerbated MNC activity during diseases such as heart failure (HF) remains unknown. In the present study, we used a combination of patch clamp electrophysiology, confocal Ca2+ imaging and molecular biology in a rat model of ischaemic HF. We found that the input–output function of MNCs was enhanced in HF compared to sham rats. Moreover, although the SK blocker apamin (200 nm) strengthened the input–output function in sham rats, it failed to have an effect in HF rats. The magnitude of the after‐hyperpolarizing potential (AHP) following a train of spikes and the underlying apamin‐sensitive IAHP were blunted in MNCs from HF rats. However, spike‐induced increases in intracellular Ca2+ were not affected in the MNCs of HF rats. Real‐time PCR measurements of SK channel subunits mRNA in supraoptic nucleus punches revealed a diminished expression of SK2/SK3 subunits in HF compared to sham rats. Together, our studies demonstrate that MNCs from HF rats exhibit increased membrane excitability and an enhanced input–output function, and also that a reduction in SK channel‐mediated, apamin‐sensitive AHP is a critical contributing mechanism. Moreover, our results suggest that the reduced AHP is related to a down‐regulation of SK2/SK3 channel subunit expression but not the result of a blunted activity‐dependent intracellular Ca2+ increase following a burst of action potentials.
    August 02, 2017   doi: 10.1113/JP274730   open full text
  • Release of ATP by pre‐Bötzinger complex astrocytes contributes to the hypoxic ventilatory response via a Ca2+‐dependent P2Y1 receptor mechanism.
    Vishaal Rajani, Yong Zhang, Venkatesh Jalubula, Vladimir Rancic, Shahriar SheikhBahaei, Jennifer D. Zwicker, Silvia Pagliardini, Clayton T. Dickson, Klaus Ballanyi, Sergey Kasparov, Alexander V. Gourine, Gregory D. Funk.
    The Journal of Physiology. July 27, 2017
    Key points The ventilatory response to reduced oxygen (hypoxia) is biphasic, comprising an initial increase in ventilation followed by a secondary depression. Our findings indicate that, during hypoxia, astrocytes in the pre‐Bötzinger complex (preBötC), a critical site of inspiratory rhythm generation, release a gliotransmitter that acts via P2Y1 receptors to stimulate ventilation and reduce the secondary depression. In vitro analyses reveal that ATP excitation of the preBötC involves P2Y1 receptor‐mediated release of Ca2+ from intracellular stores. By identifying a role for gliotransmission and the sites, P2 receptor subtype, and signalling mechanisms via which ATP modulates breathing during hypoxia, these data advance our understanding of the mechanisms underlying the hypoxic ventilatory response and highlight the significance of purinergic signalling and gliotransmission in homeostatic control. Clinically, these findings are relevant to conditions in which hypoxia and respiratory depression are implicated, including apnoea of prematurity, sleep disordered breathing and congestive heart failure. Abstract The hypoxic ventilatory response (HVR) is biphasic, consisting of a phase I increase in ventilation followed by a secondary depression (to a steady‐state phase II) that can be life‐threatening in premature infants who suffer from frequent apnoeas and respiratory depression. ATP released in the ventrolateral medulla oblongata during hypoxia attenuates the secondary depression. We explored a working hypothesis that vesicular release of ATP by astrocytes in the pre‐Bötzinger Complex (preBötC) inspiratory rhythm‐generating network acts via P2Y1 receptors to mediate this effect. Blockade of vesicular exocytosis in preBötC astrocytes bilaterally (using an adenoviral vector to specifically express tetanus toxin light chain in astrocytes) reduced the HVR in anaesthetized rats, indicating that exocytotic release of a gliotransmitter within the preBötC contributes to the hypoxia‐induced increases in ventilation. Unilateral blockade of P2Y1 receptors in the preBötC via local antagonist injection enhanced the secondary respiratory depression, suggesting that a significant component of the phase II increase in ventilation is mediated by ATP acting at P2Y1 receptors. In vitro responses of the preBötC inspiratory network, preBötC inspiratory neurons and cultured preBötC glia to purinergic agents demonstrated that the P2Y1 receptor‐mediated increase in fictive inspiratory frequency involves Ca2+ recruitment from intracellular stores leading to increases in intracellular Ca2+ ([Ca2+]i) in inspiratory neurons and glia. These data suggest that ATP is released by preBötC astrocytes during hypoxia and acts via P2Y1 receptors on inspiratory neurons (and/or glia) to evoke Ca2+ release from intracellular stores and an increase in ventilation that counteracts the hypoxic respiratory depression.
    July 27, 2017   doi: 10.1113/JP274727   open full text
  • In situ macrophage phenotypic transition is affected by altered cellular composition prior to acute sterile muscle injury.
    Andreas Patsalos, Attila Pap, Tamas Varga, Gyorgy Trencsenyi, Gerardo Alvarado Contreras, Ildiko Garai, Zoltan Papp, Balazs Dezso, Eva Pintye, Laszlo Nagy.
    The Journal of Physiology. July 17, 2017
    Skeletal muscle regeneration is a complex interplay between various cell types including invading macrophages. Their recruitment to damaged tissues upon acute sterile injuries is necessary for necrotic debris clearance and for coordination of tissue regeneration. This highly dynamic process is characterized by an in‐situ transition of infiltrating monocytes from an inflammatory (Ly6Chigh) to a repair (Ly6Clow) macrophage phenotype. The importance of the macrophage phenotypic shift and the cross‐talk of the local muscle tissue with the infiltrating macrophages during tissue regeneration upon injury are not fully understood and their study lacks adequate methodology. Here, using an acute sterile skeletal muscle injury model combined with irradiation, bone marrow transplantation and in vivo imaging we show that preserved muscle integrity and cell composition prior to the injury is necessary for repair macrophage phenotypic transition and subsequently for proper and complete tissue regeneration. Importantly, by using a model of in vivo ablation of PAX7 positive cells, we show that this radiosensitive skeletal muscle progenitor pool contributes to macrophage phenotypic transition following acute sterile muscle injury. In addition, local muscle tissue radioprotection by lead shielding during irradiation preserves normal macrophage transition dynamics and subsequently muscle tissue regeneration. Taken together, our data suggest the existence of a more extensive and reciprocal cross‐talk between muscle tissue compartments, including satellite cells, and infiltrating myeloid cells upon tissue damage. These interactions are shaping the macrophages in‐situ phenotypic shift, which is indispensable for normal muscle tissue repair dynamics. This article is protected by copyright. All rights reserved
    July 17, 2017   doi: 10.1113/JP274361   open full text
  • Effect of dietary salt intake on Epithelial Na+ Channels (ENaC) in vasopressin magnocellular neurosecretory neurons in the rat supraoptic nucleus.
    Kaustubh Sharma, Masudul Haque, Richard Guidry, Yoichi Ueta, Ryoichi Teruyama.
    The Journal of Physiology. July 17, 2017
    All three epithelial Na+ channel (ENaC) subunits (α, β, and γ) were located in vasopressin (VP) magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular nuclei. Our previous study demonstrated that ENaC mediates a Na+ leak current that affects the steady state membrane potential in VP neurons. In the present study, we evaluated the effect of dietary salt intake on ENaC regulation and activity in VP neurons. High dietary salt intake for 7 days caused an increase in expression of β‐ and γENaC subunits in the SON and the translocation of αENaC immunoreactivity towards the plasma membrane. Patch‐clamp experiments on hypothalamic slices showed that the mean amplitude of the putative ENaC currents was significantly greater in VP neurons from animals that were fed a high‐salt diet compared with controls. The enhanced ENaC current contributed to the more depolarized basal membrane potential observed in VP neurons in the high‐salt diet group. These findings indicate that high dietary NaCl intake enhances the expression and activity of ENaC which augments synaptic drive by depolarizing the basal membrane potential close to the action potential threshold during hormonal demand. However, ENaCs appear to have only a minor role in the regulation of the firing activity of VP neurons in the absence of synaptic inputs as neither the mean intraburst frequency, burst duration, nor interspike interval variability of phasic bursting activity was affected. Moreover, ENaC activity did not affect the initiation, sustention, or termination of the phasic bursting generated in an intrinsic manner without synaptic inputs. This article is protected by copyright. All rights reserved
    July 17, 2017   doi: 10.1113/JP274856   open full text
  • Acetylcholine‐dependent upregulation of TASK‐1 channels in thalamic interneurons by a smooth muscle‐like signalling pathway.
    Michael Leist, Susanne Rinné, Maia Datunashvili, Ania Aissaoui, Hans‐Christian Pape, Niels Decher, Sven G. Meuth, Thomas Budde.
    The Journal of Physiology. July 17, 2017
    The dorsal part of the lateral geniculate nucleus (dLGN) is the main thalamic site for state‐dependent transmission of visual information. Non‐retinal inputs from the ascending arousal system and inhibition provided by γ‐aminobutyric acid (GABA)ergic local circuit interneurons (INs) control neuronal activity within the dLGN. In particular, acetylcholine (ACh) depolarizes thalamocortical relay (TC) neurons by inhibiting two‐pore domain potassium (K2P) channels. Conversely, ACh also hyperpolarizes INs via an as‐yet‐unknown mechanism. By using whole cell patch‐clamp recordings in brain slices and appropriate pharmacological tools we here report that stimulation of type 2 muscarinic ACh receptors (M2AChRs) induces IN hyperpolarization by recruiting the G beta‐gamma complex (Gβγ), class‐1A phosphatidylinositol‐4,5‐bisphosphate 3‐kinase (PI3K), and cellular and sarcoma (c‐Src) tyrosine kinase (TK), leading to activation of two‐pore domain weakly inwardly rectifying K+ channel (TWIK)‐related acid‐sensitive K+ (TASK)‐1 channels. The latter was confirmed by the use of TASK‐1 deficient mice. Furthermore inhibition of phospholipase Cβ (PLCβ) as well as an increase in the intracellular level of phosphatidylinositol‐3,4,5‐trisphosphate (PIP3) facilitated the muscarinic effect. Our results have uncovered a previously unknown role of c‐Src TK in regulating IN function in the brain and identified a novel mechanism by which TASK‐1 channels are activated in neurons. This article is protected by copyright. All rights reserved
    July 17, 2017   doi: 10.1113/JP274527   open full text
  • N‐glycan content modulates kainate receptor functional properties.
    Claire G. Vernon, Bryan A. Copits, Jacob R. Stolz, Yomayra F. Guzmán, Geoffrey T. Swanson.
    The Journal of Physiology. July 17, 2017
    Ionotropic glutamate receptors (iGluRs) are tetrameric proteins with between 4 and 12 consensus sites for N‐glycosylation on each subunit, which potentially allows for a high degree of structural diversity conferred by this post‐translational modification. N‐glycosylation is required for proper folding of iGluRs in mammalian cells, but the impact of oligosaccharides on the function of successfully folded receptors is less clear. Glycan moieties are large, polar, occasionally charged, and mediate many protein‐protein interactions throughout the nervous system. Additionally, they are attached at sites along iGluR subunits that position them for involvement in the structural changes underlying gating. We show here that altering glycan content on kainate receptors (KARs) changes the functional properties of the receptors in a manner dependent on the identity of both the modified sugars and the subunit composition of the receptor to which they are attached. We also report that native KARs carry the complex capping oligosaccharide HNK‐1. Glycosylation patterns likely differ between cell types, across development, or with pathologies, and thus our findings reveal a potential mechanism for context‐specific fine‐tuning of KAR function through diversity in glycan structure. This article is protected by copyright. All rights reserved
    July 17, 2017   doi: 10.1113/JP274790   open full text
  • Ivermectin activates GIRK channels in a PIP2‐dependent, Gβγ‐independent manner and an amino acid residue at the slide helix governs the activation.
    I‐Shan Chen, Michihiro Tateyama, Yuko Fukata, Motonari Uesugi, Yoshihiro Kubo.
    The Journal of Physiology. July 17, 2017
    Ivermectin (IVM) is a widely used antiparasitic drug in humans and pets which activates glutamate‐gated Cl− channel in parasites. It is also known that IVM binds to the transmembrane domains (TMs) of several ligand‐gated channels, such as Cys‐loop receptors and P2X receptors. In this study, we found that the G‐protein‐gated inwardly rectifying K+ (GIRK) channel is activated by IVM directly. By electrophysiological recordings in Xenopus oocytes, we observed that IVM activates GIRK channel in a phosphatidylinositol‐4,5‐biphosphate (PIP2)‐dependent manner, and that the IVM‐mediated GIRK activation is independent of Gβγ. We found that IVM activates GIRK2 more efficiently than GIRK4. In cultured hippocampal neurons, we also observed that IVM activates native GIRK current. By chimeric and mutagenesis analyses, we identified a unique amino acid residue of GIRK2 among GIRK family, Ile82, located in the slide helix between the TM1 and the N‐terminal cytoplasmic tail domain (CTD), which is critical for the activation. The results demonstrate that the TM‐CTD interface in GIRK channel, rather than the TMs, governs IVM‐mediated activation. These findings provide us with novel insights on the action mode of IVM in ion channels, and information toward identification of new pharmacophores which activate GIRK channel. This article is protected by copyright. All rights reserved
    July 17, 2017   doi: 10.1113/JP274871   open full text
  • Muscle carnitine availability plays a central role in regulating fuel metabolism in the rodent.
    Craig Porter, Dumitru Constantin‐Teodosiu, Despina Constantin, Brendan Leighton, Simon M. Poucher, Paul L. Greenhaff.
    The Journal of Physiology. July 16, 2017
    Key points Meldonium inhibits endogenous carnitine synthesis and tissue uptake, and accelerates urinary carnitine excretion, although the impact of meldonium‐mediated muscle carnitine depletion on whole‐body fuel selection, and muscle fuel metabolism and its molecular regulation is under‐investigated. Ten days of oral meldonium administration did not impact on food or fluid intake, physical activity levels or body weight gain in the rat, whereas it depleted muscle carnitine content (all moieties), increased whole‐body carbohydrate oxidation and muscle and liver glycogen utilization, and reduced whole‐body fat oxidation. Meldonium reduced carnitine transporter protein expression across muscles of different contractile and metabolic phenotypes. A TaqMan PCR low‐density array card approach revealed the abundance of 189 mRNAs regulating fuel selection was altered in soleus muscle by meldonium, highlighting the modulation of discrete cellular functions and metabolic pathways. These novel findings strongly support the premise that muscle carnitine availability is a primary regulator of fuel selection in vivo. Abstract The body carnitine pool is primarily confined to skeletal muscle, where it regulates carbohydrate (CHO) and fat usage. Meldonium (3‐(2,2,2‐trimethylhydrazinium)‐propionate) inhibits carnitine synthesis and tissue uptake, although the impact of carnitine depletion on whole‐body fuel selection, muscle fuel metabolism and its molecular regulation is under‐investigated. Male lean Zucker rats received water (control, n = 8) or meldonium‐supplemented water (meldonium, n = 8) for 10 days [1.6 g kg−1 body mass (BM) day−1 days 1–2, 0.8 g kg−1 BM day−1 thereafter]. From days 7–10, animals were housed in indirect calorimetry chambers after which soleus muscle and liver were harvested. Food and fluid intake, weight gain and physical activity levels were similar between groups from days 7 to 10. Compared to control, meldonium depleted muscle total carnitine (P < 0.001) and all carnitine esters. Furthermore, whole‐body fat oxidation was less (P < 0.001) and CHO oxidation was greater (P < 0.05) compared to the control, whereas soleus and liver glycogen contents were less (P < 0.01 and P < 0.01, respectively). In a second study, male Wistar rats received water (n = 8) or meldonium‐supplemented water (n = 8) as above, and kidney, heart and extensor digitorum longus muscle (EDL) and soleus muscles were collected. Compared to control, meldonium depleted total carnitine content (all P < 0.001), reduced carnitine transporter protein and glycogen content, and increased pyruvate dehydrogenase kinase 4 mRNA abundance in the heart, EDL and soleus. In total, 189 mRNAs regulating fuel selection were differentially expressed in soleus in meldonium vs. control, and a number of cellular functions and pathways strongly associated with carnitine depletion were identified. Collectively, these data firmly support the premise that muscle carnitine availability is a primary regulator of fuel selection in vivo.
    July 16, 2017   doi: 10.1113/JP274415   open full text
  • Pronounced limb and fibre type differences in subcellular lipid droplet content and distribution in elite skiers before and after exhaustive exercise.
    Han‐Chow E. Koh, Joachim Nielsen, Bengt Saltin, Hans‐Christer Holmberg, Niels Ørtenblad.
    The Journal of Physiology. July 16, 2017
    Key points Although lipid droplets in skeletal muscle are an important energy source during endurance exercise, our understanding of lipid metabolism in this context remains incomplete. Using transmission electron microscopy, two distinct subcellular pools of lipid droplets can be observed in skeletal muscle – one beneath the sarcolemma and the other between myofibrils. At rest, well‐trained leg muscles of cross‐country skiers contain 4‐ to 6‐fold more lipid droplets than equally well‐trained arm muscles, with a 3‐fold higher content in type 1 than in type 2 fibres. During exhaustive exercise, lipid droplets between the myofibrils but not those beneath the sarcolemma are utilised by both type 1 and 2 fibres. These findings provide insight into compartmentalisation of lipid metabolism within skeletal muscle fibres. Abstract Although the intramyocellular lipid pool is an important energy store during prolonged exercise, our knowledge concerning its metabolism is still incomplete. Here, quantitative electron microscopy was used to examine subcellular distribution of lipid droplets in type 1 and 2 fibres of the arm and leg muscles before and after 1 h of exhaustive exercise. Intermyofibrillar lipid droplets accounted for 85–97% of the total volume fraction, while the subsarcolemmal pool made up 3–15%. Before exercise, the volume fractions of intermyofibrillar and subsarcolemmal lipid droplets were 4‐ to 6‐fold higher in leg than in arm muscles (P < 0.001). Furthermore, the volume fraction of intermyofibrillar lipid droplets was 3‐fold higher in type 1 than in type 2 fibres (P < 0.001), with no fibre type difference in the subsarcolemmal pool. Following exercise, intermyofibrillar lipid droplet volume fraction was 53% lower (P = 0.0082) in both fibre types in arm, but not leg muscles. This reduction was positively associated with the corresponding volume fraction prior to exercise (R2 = 0.84, P < 0.0001). No exercise‐induced change in the subsarcolemmal pool could be detected. These findings indicate clear differences in the subcellular distribution of lipid droplets in the type 1 and 2 fibres of well‐trained arm and leg muscles, as well as preferential utilisation of the intermyofibrillar pool during prolonged exhaustive exercise. Apparently, the metabolism of lipid droplets within a muscle fibre is compartmentalised.
    July 16, 2017   doi: 10.1113/JP274462   open full text
  • Calcium signalling in medial intercalated cell dendrites and spines.
    Cornelia Strobel, Robert K. P. Sullivan, Peter Stratton, Pankaj Sah.
    The Journal of Physiology. July 16, 2017
    Key points Dendritic and spine calcium imaging in combination with electrophysiology in acute slices revealed that in medial intercalated cells of the amygdala: Action potentials back‐propagate into the dendritic tree, but due to the presence of voltage‐dependent potassium channels, probably Kv4.2 channels, attenuate over distance. A mixed population of AMPA receptors with rectifying and linear I–V relations are present at individual spines of a single neuron. Decay kinetics and pharmacology suggest tri‐heteromeric NMDA receptors at basolateral–intercalated cell synapses. NMDA receptors are the main contributors to spine calcium entry in response to synaptic stimulation. Calcium signals in response to low‐ and high‐frequency stimulation, and in combination with spontaneous action potentials are locally restricted to the vicinity of active spines. Together, these data show that calcium signalling in these GABAergic neurons is tightly controlled and acts as a local signal. Abstract The amygdala plays a central role in fear conditioning and extinction. The medial intercalated (mITC) neurons are GABAergic cell clusters interspaced between the basolateral (BLA) and central amygdala (CeA). These neurons are thought to play a key role in fear and extinction, controlling the output of the CeA by feed‐forward inhibition. BLA to mITC cell inputs are thought to undergo synaptic plasticity, a mechanism underlying learning, which is mediated by NMDA receptor‐dependent mechanisms that require changes in cytosolic calcium. Here, we studied the electrical and calcium signalling properties of mITC neurons in GAD67‐eGFP mice using whole‐cell patch clamp recordings and two‐photon calcium imaging. We show that action potentials back‐propagate (bAP) into dendrites, and evoke calcium transients in both the shaft and the dendritic spine. However, bAP‐mediated calcium rises in the dendrites attenuate with distance due to shunting by voltage‐gated potassium channels. Glutamatergic inputs make dual component synapses on spines. At these synapses, postsynaptic AMPA receptors can have linear or rectifying I–V relationships, indicating that some synapses express GluA2‐lacking AMPA receptors. Synaptic NMDA receptors had intermediate decay kinetics, and were only partly blocked by GuN2B selective blockers, indicating these receptors are GluN1/GluN2A/GluN2B trimers. Low‐ or high‐frequency synaptic stimulation raised spine calcium, mediated by calcium influx via NMDA receptors, was locally restricted and did not invade neighbouring spines. Our results show that in mITC neurons, postsynaptic calcium is tightly controlled, and acts as a local signal.
    July 16, 2017   doi: 10.1113/JP274261   open full text
  • Lack of linear correlation between dynamic and steady‐state cerebral autoregulation.
    Daan L. K. Jong, Takashi Tarumi, Jie Liu, Rong Zhang, Jurgen A. H. R. Claassen.
    The Journal of Physiology. July 14, 2017
    Key points For correct application and interpretation of cerebral autoregulation (CA) measurements in research and in clinical care, it is essential to understand differences and similarities between dynamic and steady‐state CA. The present study found no correlation between dynamic and steady‐state CA indices in healthy older adults. There was variability between individuals in all (steady‐state and dynamic) autoregulatory indices, ranging from low (almost absent) to highly efficient CA in this healthy population. These findings challenge the assumption that assessment of a single CA parameter or a single set of parameters can be generalized to overall CA functioning. Therefore, depending on specific research purposes, the choice for either steady‐state or dynamic measures or both should be weighed carefully. Abstract The present study aimed to investigate the relationship between dynamic (dCA) and steady‐state cerebral autoregulation (sCA). In 28 healthy older adults, sCA was quantified by a linear regression slope of proportionate (%) changes in cerebrovascular resistance (CVR) in response to proportionate (%) changes in mean blood pressure (BP) induced by stepwise sodium nitroprusside (SNP) and phenylephrine (PhE) infusion. Cerebral blood flow (CBF) was measured at the internal carotid artery (ICA) and vertebral artery (VA) and CBF velocity at the middle cerebral artery (MCA). With CVR = BP/CBF, Slope‐CVRICA, Slope‐CVRVA and Slope‐CVRiMCA were derived. dCA was assessed (i) in supine rest, analysed with transfer function analysis (gain and phase) and autoregulatory index (ARI) fit from spontaneous oscillations (ARIBaseline), and (ii) with transient changes in BP using a bolus injection of SNP (ARISNP) and PhE (ARIPhE). Comparison of sCA and dCA parameters (using Pearson's r for continuous and Spearman's ρ for ordinal parameters) demonstrated a lack of linear correlations between sCA and dCA measures. However, comparisons of parameters within dCA and within sCA were correlated. For sCA slope‐CVRVA with Slope‐CVRiMCA (r = 0.45, P < 0.03); for dCA ARISNP with ARIPhE (ρ = 0.50, P = 0.03), ARIBaseline (ρ = 0.57, P = 0.03) and PhaseLF (ρ = 0.48, P = 0.03); and for GainVLF with GainLF (r = 0.51, P = 0.01). By contrast to the commonly held assumption based on an earlier study, there were no linear correlations between sCA and dCA. As an additional observation, there was strong inter‐individual variability, both in dCA and sCA, in this healthy group of elderly, in a range from low to high CA efficiency.
    July 14, 2017   doi: 10.1113/JP274304   open full text
  • EEA1 restores homeostatic synaptic plasticity in hippocampal neurons from Rett syndrome mice.
    Xin Xu, Lucas Pozzo‐Miller.
    The Journal of Physiology. July 12, 2017
    Key points Rett syndrome is a neurodevelopmental disorder caused by loss‐of‐function mutations in MECP2, the gene encoding the transcriptional regulator methyl‐CpG‐binding protein 2 (MeCP2). Mecp2 deletion in mice results in an imbalance of excitation and inhibition in hippocampal neurons, which affects ‘Hebbian’ synaptic plasticity. We show that Mecp2‐deficient neurons also lack homeostatic synaptic plasticity, likely due to reduced levels of EEA1, a protein involved in AMPA receptor endocytosis. Expression of EEA1 restored homeostatic synaptic plasticity in Mecp2‐deficient neurons, providing novel targets of intervention in Rett syndrome. Abstract Rett syndrome is a neurodevelopmental disorder caused by loss‐of‐function mutations in MECP2, the gene encoding the transcriptional regulator methyl‐CpG‐binding protein 2 (MeCP2). Deletion of Mecp2 in mice results in an imbalance of synaptic excitation and inhibition in hippocampal pyramidal neurons, which affects ‘Hebbian’ long‐term synaptic plasticity. Since the excitatory–inhibitory balance is maintained by homeostatic mechanisms, we examined the role of MeCP2 in homeostatic synaptic plasticity (HSP) at excitatory synapses. Negative feedback HSP, also known as synaptic scaling, maintains the global synaptic strength of individual neurons in response to sustained alterations in neuronal activity. Hippocampal neurons from Mecp2 knockout (KO) mice do not show the characteristic homeostatic scaling up of the amplitude of miniature excitatory postsynaptic currents (mEPSCs) and of synaptic levels of the GluA1 subunit of AMPA‐type glutamate receptors after 48 h silencing with the Na+ channel blocker tetrodotoxin. This deficit in HSP is bidirectional because Mecp2 KO neurons also failed to scale down mEPSC amplitudes and GluA1 synaptic levels after 48 h blockade of type A GABA receptor (GABAAR)‐mediated inhibition with bicuculline. Consistent with the role of synaptic trafficking of AMPA‐type of glutamate receptors in HSP, Mecp2 KO neurons have lower levels of early endosome antigen 1 (EEA1), a protein involved in AMPA‐type glutamate receptor endocytosis. In addition, expression of EEA1 in Mecp2 KO neurons reduced mEPSC amplitudes to wild‐type levels, and restored synaptic scaling down of mEPSC amplitudes after 48 h blockade of GABAAR‐mediated inhibition with bicuculline. The identification of a molecular deficit in HSP in Mecp2 KO neurons provides potentially novel targets of intervention for improving hippocampal function in Rett syndrome individuals.
    July 12, 2017   doi: 10.1113/JP274450   open full text
  • Heterotypic endosomal fusion as an initial trigger for insulin‐induced glucose transporter 4 (GLUT4) translocation in skeletal muscle.
    Hiroyasu Hatakeyama, Makoto Kanzaki.
    The Journal of Physiology. July 10, 2017
    Key points Comprehensive imaging analyses of glucose transporter 4 (GLUT4) behaviour in mouse skeletal muscle was conducted. Quantum dot‐based single molecule nanometry revealed that GLUT4 molecules in skeletal myofibres are governed by regulatory systems involving ‘static retention’ and ‘stimulus‐dependent liberation’. Vital imaging analyses and super‐resolution microscopy‐based morphometry demonstrated that insulin liberates the GLUT4 molecule from its static state by triggering acute heterotypic endomembrane fusion arising from the very small GLUT4‐containing vesicles in skeletal myofibres. Prior exposure to exercise‐mimetic stimuli potentiated this insulin‐responsive endomembrane fusion event involving GLUT4‐containing vesicles, suggesting that this endomembranous regulation process is a potential site related to the effects of exercise. Abstract Skeletal muscle is the major systemic glucose disposal site. Both insulin and exercise facilitate translocation of the glucose transporter glucose transporter 4 (GLUT4) via distinct signalling pathways and exercise also enhances insulin sensitivity. However, the trafficking mechanisms controlling GLUT4 mobilization in skeletal muscle remain poorly understood as a resuly of technical limitations. In the present study, which employs various imaging techniques on isolated skeletal myofibres, we show that one of the initial triggers of insulin‐induced GLUT4 translocation is heterotypic endomembrane fusion arising from very small static GLUT4‐containing vesicles with a subset of transferrin receptor‐containing endosomes. Importantly, pretreatment with exercise‐mimetic stimuli potentiated the susceptibility to insulin responsiveness, as indicated by these acute endomembranous activities. We also found that AS160 exhibited stripe‐like localization close to sarcomeric α‐actinin and that insulin induced a reduction of the stripe‐like localization accompanying changes in its detergent solubility. The results of the present study thus provide a conceptual framework indicating that GLUT4 protein trafficking via heterotypic fusion is a critical feature of GLUT4 translocation in skeletal muscles and also suggest that the efficacy of the endomembranous fusion process in response to insulin is involved in the benefits of exercise.
    July 10, 2017   doi: 10.1113/JP273985   open full text
  • Mechanisms underlying vestibulo‐cerebellar motor learning in mice depend on movement direction.
    Kai Voges, Bin Wu, Laura Post, Martijn Schonewille, Chris I. Zeeuw.
    The Journal of Physiology. July 10, 2017
    Key points Directionality, inherent to movements, has behavioural and neuronal correlates. Direction of vestibular stimulation determines motor learning efficiency. Vestibulo‐ocular reflex gain–increase correlates with Purkinje cell simple spike potentiation. The locus of neural correlates for vestibulo‐ocular reflex adaptation is paradigm specific. Abstract Compensatory eye movements elicited by head rotation, also known as vestibulo‐ocular reflex (VOR), can be adapted with the use of visual feedback. The cerebellum is essential for this type of movement adaptation, although its neuronal correlates remain to be clarified. In the present study, we show that the direction of vestibular input determines the magnitude of eye movement adaptation induced by mismatched visual input in mice, with larger changes during contraversive head rotation. Moreover, the location of the neural correlate of this changed behaviour depends on the type of paradigm. Gain–increase paradigms induce increased simple spike (SS) activity in ipsilateral cerebellar Purkinje cells (PC), which is in line with eye movements triggered by optogenetic PC activation. By contrast, gain–decrease paradigms do not induce changes in SS activity, indicating that the murine vestibulo‐cerebellar cortical circuitry is optimally designed to enhance ipsiversive eye movements.
    July 10, 2017   doi: 10.1113/JP274346   open full text
  • Ecto‐5′‐nucleotidase (CD73) regulates peripheral chemoreceptor activity and cardiorespiratory responses to hypoxia.
    Andrew P. Holmes, Clare J. Ray, Selina A. Pearson, Andrew M. Coney, Prem Kumar.
    The Journal of Physiology. July 09, 2017
    Key points Carotid body dysfunction is recognized as a cause of hypertension in a number of cardiorespiratory diseases states and has therefore been identified as a potential therapeutic target. Purinergic transmission is an important element of the carotid body chemotransduction pathway. We show that inhibition of ecto‐5′‐nucleotidase (CD73) in vitro reduces carotid body basal discharge and responses to hypoxia and mitochondrial inhibition. Additionally, inhibition of CD73 in vivo decreased the hypoxic ventilatory response, reduced the hypoxia‐induced heart rate elevation and exaggerated the blood pressure decrease in response to hypoxia. Our data show CD73 to be a novel regulator of carotid body sensory function and therefore suggest that this enzyme may offer a new target for reducing carotid body activity in selected cardiovascular diseases. Abstract Augmented sensory neuronal activity from the carotid body (CB) has emerged as a principal cause of hypertension in a number of cardiovascular related pathologies, including obstructive sleep apnoea, heart failure and diabetes. Development of new targets and pharmacological treatment strategies aiming to reduce CB sensory activity may thus improve outcomes in these key patient cohorts. The present study investigated whether ecto‐5′‐nucleotidase (CD73), an enzyme that generates adenosine, is functionally important in modifying CB sensory activity and cardiovascular respiratory responses to hypoxia. Inhibition of CD73 by α,β‐methylene ADP (AOPCP) in the whole CB preparation in vitro reduced basal discharge frequency by 76 ± 5% and reduced sensory activity throughout graded hypoxia. AOPCP also significantly attenuated elevations in sensory activity evoked by mitochondrial inhibition. These effects were mimicked by antagonism of adenosine receptors with 8‐(p‐sulfophenyl) theophylline. Infusion of AOPCP in vivo significantly decreased the hypoxic ventilatory response (ΔV̇E control 74 ± 6%, ΔV̇E AOPCP 64 ± 5%, P < 0.05). AOPCP also modified cardiovascular responses to hypoxia, as indicated by reduced elevations in heart rate and exaggerated changes in femoral vascular conductance and mean arterial blood pressure. Thus we identify CD73 as a novel regulator of CB sensory activity. Future investigations are warranted to clarify whether inhibition of CD73 can effectively reduce CB activity in CB‐mediated cardiovascular pathology.
    July 09, 2017   doi: 10.1113/JP274498   open full text
  • Sympatholytic effect of intravascular ATP is independent of nitric oxide, prostaglandins, Na+/K+‐ATPase and KIR channels in humans.
    Christopher M. Hearon, Jennifer C. Richards, Mathew L. Racine, Gary J. Luckasen, Dennis G. Larson, Michael J. Joyner, Frank A. Dinenno.
    The Journal of Physiology. July 09, 2017
    Key points Intravascular ATP attenuates sympathetic vasoconstriction (sympatholysis) similar to what is observed in contracting skeletal muscle of humans, and may be an important contributor to exercise hyperaemia. Similar to exercise, ATP‐mediated vasodilatation occurs via activation of inwardly rectifying potassium channels (KIR), and synthesis of nitric oxide (NO) and prostaglandins (PG). However, recent evidence suggests that these dilatatory pathways are not obligatory for sympatholysis during exercise; therefore, we tested the hypothesis that the ability of ATP to blunt α1‐adrenergic vasoconstriction in resting skeletal muscle would be independent of KIR, NO, PGs and Na+/K+‐ATPase activity. Blockade of KIR channels alone or in combination with NO, PGs and Na+/K+‐ATPase significantly reduced the vasodilatatory response to ATP, although intravascular ATP maintained the ability to attenuate α1‐adrenergic vasoconstriction. This study highlights similarities in the vascular response to ATP and exercise, and further supports a potential role of intravascular ATP in blood flow regulation during exercise in humans. Abstract Exercise and intravascular ATP elicit vasodilatation that is dependent on activation of inwardly rectifying potassium (KIR) channels, with a modest reliance on nitric oxide (NO) and prostaglandin (PG) synthesis. Both exercise and intravascular ATP attenuate sympathetic α‐adrenergic vasoconstriction (sympatholysis). However, KIR channels, NO, PGs and Na+/K+‐ATPase activity are not obligatory to observe sympatholysis during exercise. To further determine similarities between exercise and intravascular ATP, we tested the hypothesis that inhibition of KIR channels, NO and PG synthesis, and Na+/K+‐ATPase would not alter the ability of ATP to blunt α1‐adrenergic vasoconstriction. In healthy subjects, we measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (FVC) to intra‐arterial infusion of phenylephrine (PE; α1‐agonist) during ATP or control vasodilatator infusion, before and after KIR channel inhibition alone (barium chloride; n = 7; Protocol 1); NO (l‐NMMA) and PG (ketorolac) inhibition alone, or combined NO, PGs, Na+/K+‐ATPase (ouabain) and KIR channel inhibition (n = 6; Protocol 2). ATP attenuated PE‐mediated vasoconstriction relative to adenosine (ADO) and sodium nitroprusside (SNP) (PE‐mediated ΔFVC: ATP: −16 ± 2; ADO: −38 ± 6; SNP: −59 ± 6%; P < 0.05 vs. ADO and SNP). Blockade of KIR channels alone or combined with NO, PGs and Na+/K+‐ATPase, attenuated ATP‐mediated vasodilatation (∼35 and ∼60% respectively; P < 0.05 vs. control). However, ATP maintained the ability to blunt PE‐mediated vasoconstriction (PE‐mediated ΔFVC: KIR blockade alone: −6 ± 5%; combined blockade:−4 ± 14%; P > 0.05 vs. control). These findings demonstrate that intravascular ATP modulates α1‐adrenergic vasoconstriction via pathways independent of KIR channels, NO, PGs and Na+/K+‐ATPase in humans, consistent with a role for endothelium‐derived hyperpolarization in functional sympatholysis.
    July 09, 2017   doi: 10.1113/JP274532   open full text
  • Transcriptomic analysis identifies a role of PI3K/Akt signalling in the responses of skeletal muscle to acute hypoxia in vivo.
    Zhuohui Gan, Frank L. Powell, Alexander C. Zambon, Kyle S. Buchholz, Zhenxing Fu, Karen Ocorr, Rolf Bodmer, Esteban A. Moya, Jennifer C. Stowe, Gabriel G. Haddad, Andrew D. McCulloch.
    The Journal of Physiology. July 08, 2017
    The effects of acute hypoxia have been widely studied, but there are few studies of transcriptional responses to hours of hypoxia in vivo, especially in hypoxia‐tolerant tissues like skeletal muscles. We used RNA‐seq to analyse gene expression in plantaris muscles while monitoring respiration, arterial blood gases, and blood glucose in mice exposed to 8% O2 for 2 or 6 h. Rapid decreases in blood gases and a slower reduction in blood glucose suggest stress, which was accompanied by widespread changes in gene expression. Early down‐regulation of genes associated with the extracellular matrix was followed by a shift to genes associated with the nuclear lumen. Most of the early down‐regulated genes had mRNA half‐lives longer than 2 h, suggesting a role for post‐transcriptional regulation. These transcriptional changes were enriched in signalling pathways in which the PI3K/Akt signalling pathway was identified as a hub. Our analyses indicated that gene targets of PI3K/Akt but not HIF were enriched in early transcriptional responses to hypoxia. Among the PI3K/Akt targets, 75% could be explained by a deactivation of ARE‐binding protein BRF1, a target of PI3K/Akt. Consistent decreases in the phosphorylation of Akt and BRF1 were experimentally confirmed following 2 h of hypoxia. These results suggest that the PI3K/Akt signalling pathway might play a role in responses induced by acute hypoxia in skeletal muscles, partially through the de‐phosphorylation of ARE‐binding protein BRF1. This article is protected by copyright. All rights reserved
    July 08, 2017   doi: 10.1113/JP274556   open full text
  • Sensory feedback from the urethra evokes state‐dependent lower urinary tract reflexes in rat.
    Zachary C. Danziger, Warren M. Grill.
    The Journal of Physiology. July 07, 2017
    Key points The lower urinary tract is regulated by reflexes responsible for maintaining continence and producing efficient voiding. It is unclear how sensory information from the bladder and urethra engages differential, state‐dependent reflexes to either maintain continence or promote voiding. Using a new in vivo experimental approach, we quantified how sensory information from the bladder and urethra are integrated to switch reflex responses to urethral sensory feedback from maintaining continence to producing voiding. The results demonstrate how sensory information regulates state‐dependent reflexes in the lower urinary tract and contribute to our understanding of the pathophysiology of urinary retention and incontinence where sensory feedback may engage these reflexes inappropriately. Abstract Lower urinary tract reflexes are mediated by peripheral afferents from the bladder (primarily in the pelvic nerve) and the urethra (in the pudendal and pelvic nerves) to maintain continence or initiate micturition. If fluid enters the urethra at low bladder volumes, reflexes relax the bladder and evoke external urethral sphincter (EUS) contraction (guarding reflex) to maintain continence. Conversely, urethral flow at high bladder volumes, excites the bladder (micturition reflex) and relaxes the EUS (augmenting reflex). We conducted measurements in a urethane‐anaesthetized in vivo rat preparation to characterize systematically the reflexes evoked by fluid flow through the urethra. We used a novel preparation to manipulate sensory feedback from the bladder and urethra independently by controlling bladder volume and urethral flow. We found a distinct bladder volume threshold (74% of bladder capacity) above which flow‐evoked bladder contractions were 252% larger and evoked phasic EUS activation 2.6 times as often as responses below threshold, clearly demonstrating a discrete transition between continence (guarding) and micturition (augmenting) reflexes. Below this threshold urethral flow evoked tonic EUS activity, indicative of the guarding reflex, that was proportional to the urethral flow rate. These results demonstrate the complementary roles of sensory feedback from the bladder and urethra in regulating reflexes in the lower urinary tract that depend on the state of the bladder. Understanding the neural control of functional reflexes and how they are mediated by sensory information in the bladder and urethra will open new opportunities, especially in neuromodulation, to treat pathologies of the lower urinary tract.
    July 07, 2017   doi: 10.1113/JP274191   open full text
  • Impact of ageing on postsynaptic neuronal nicotinic neurotransmission in auditory thalamus.
    Sarah Y. Sottile, Lynne Ling, Brandon C. Cox, Donald M. Caspary.
    The Journal of Physiology. July 07, 2017
    Key points Neuronal nicotinic acetylcholine receptors (nAChRs) play a fundamental role in the attentional circuitry throughout the mammalian CNS. In the present study, we report a novel finding that ageing negatively impacts nAChR efficacy in auditory thalamus, and this is probably the result of a loss of nAChR density (Bmax) and changes in the subunit composition of nAChRs. Our data support the hypothesis that age‐related maladaptive changes involving nAChRs within thalamocortical circuits partially underpin the difficulty that elderly adults experience with respect to attending to speech and other salient acoustic signals. Abstract The flow of auditory information through the medial geniculate body (MGB) is regulated, in part, by cholinergic projections from the pontomesencephalic tegmentum. The functional significance of these projections is not fully established, although they have been strongly implicated in the allocation of auditory attention. Using in vitro slice recordings, we have analysed postsynaptic function and pharmacology of neuronal nicotinic ACh receptors (nAChRs) in young adult and the aged rat MGB. We find that ACh produces significant excitatory postsynaptic actions on young MGB neurons, probably mediated by β2‐containing heteromeric nAChRs. Radioligand binding studies show a significant age‐related loss of heteromeric nAChR receptor number, which supports patch clamp data showing an age‐related loss in ACh efficacy in evoking postsynaptic responses. Use of the β2‐selective nAChR antagonist, dihydro‐β‐erythroidine, suggests that loss of cholinergic efficacy may also be the result of an age‐related subunit switch from high affinity β2‐containing nAChRs to low affinity β4‐containing nAChRs, in addition to the loss of total nAChR number. This age‐related nAChR dysfunction may partially underpin the attentional deficits that contribute to the loss of speech understanding in the elderly.
    July 07, 2017   doi: 10.1113/JP274467   open full text
  • Promotion of endocytosis efficiency through an ATP‐independent mechanism at rat calyx of Held terminals.
    Hai‐Yuan Yue, Erhard Bieberich, Jianhua Xu.
    The Journal of Physiology. July 05, 2017
    Key points At rat calyx of Held terminals, ATP was required not only for slow endocytosis, but also for rapid phase of compensatory endocytosis. An ATP‐independent form of endocytosis was recruited to accelerate membrane retrieval at increased activity and temperature. ATP‐independent endocytosis primarily involved retrieval of pre‐existing membrane, which depended on Ca2+ and the activity of neutral sphingomyelinase but not clathrin‐coated pit maturation. ATP‐independent endocytosis represents a non‐canonical mechanism that can efficiently retrieve membrane at physiological conditions without competing for the limited ATP at elevated neuronal activity. Abstract Neurotransmission relies on membrane endocytosis to maintain vesicle supply and membrane stability. Endocytosis has been generally recognized as a major ATP‐dependent function, which efficiently retrieves more membrane at elevated neuronal activity when ATP consumption within nerve terminals increases drastically. This paradox raises the interesting question of whether increased activity recruits ATP‐independent mechanism(s) to accelerate endocytosis at the same time as preserving ATP availability for other tasks. To address this issue, we studied ATP requirement in three typical forms of endocytosis at rat calyx of Held terminals by whole‐cell membrane capacitance measurements. At room temperature, blocking ATP hydrolysis effectively abolished slow endocytosis and rapid endocytosis but only partially inhibited excess endocytosis following intense stimulation. The ATP‐independent endocytosis occurred at calyces from postnatal days 8–15, suggesting its existence before and after hearing onset. This endocytosis was not affected by a reduction of exocytosis using the light chain of botulinum toxin C, nor by block of clathrin‐coat maturation. It was abolished by EGTA, which preferentially blocked endocytosis of retrievable membrane pre‐existing at the surface, and was impaired by oxidation of cholesterol and inhibition of neutral sphingomyelinase. ATP‐independent endocytosis became more significant at 34–35°C, and recovered membrane by an amount that, on average, was close to exocytosis. The results of the present study suggest that activity and temperature recruit ATP‐independent endocytosis of pre‐existing membrane (in addition to ATP‐dependent endocytosis) to efficiently retrieve membrane at nerve terminals. This less understood endocytosis represents a non‐canonical mechanism regulated by lipids such as cholesterol and sphingomyelinase.
    July 05, 2017   doi: 10.1113/JP274275   open full text
  • Chronic electromyograms in treadmill running SOD1 mice reveal early changes in muscle activation.
    Katharina A. Quinlan, Elma Kajtaz, Jody D. Ciolino, Rebecca D. Imhoff‐Manuel, Matthew C. Tresch, Charles J. Heckman, Vicki M. Tysseling.
    The Journal of Physiology. July 05, 2017
    Key points The present study demonstrates that electromyograms (EMGs) obtained during locomotor activity in mice were effective for identification of early physiological markers of amyotrophic lateral sclerosis (ALS). These measures could be used to evaluate therapeutic intervention strategies in animal models of ALS. Several parameters of locomotor activity were shifted early in the disease time course in SOD1G93A mice, especially when the treadmill was inclined, including intermuscular phase, burst skew and amplitude of the locomotor bursts. The results of the present study indicate that early compensatory changes may be taking place within the neural network controlling locomotor activity, including spinal interneurons. Locomotor EMGs could have potential use as a clinical diagnostic tool. Abstract To improve our understanding of early disease mechanisms and to identify reliable biomarkers of amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease, we measured electromyogram (EMG) activity in hind limb muscles of SOD1G93A mice. By contrast to clinical diagnostic measures using EMGs, which are performed on quiescent patients, we monitored activity during treadmill running aiming to detect presymptomatic changes in motor patterning. Chronic EMG electrodes were implanted into vastus lateralis, biceps femoris posterior, lateral gastrocnemius and tibialis anterior in mice from postnatal day 55 to 100 and the results obtained were assessed using linear mixed models. We evaluated differences in parameters related to EMG amplitude (peak and area) and timing (phase and skew, a measure of burst shape) when animals ran on level and inclined treadmills. There were significant changes in both the timing of activity and the amplitude of EMG bursts in SOD1G93A mice. Significant differences between wild‐type and SOD1G93A mice were mainly observed when animals locomoted on inclined treadmills. All muscles had significant effects of mutation that were independent of age. These novel results indicate (i) locomotor EMG activity might be an early measure of disease onset; (ii) alterations in locomotor patterning may reflect changes in neuronal drive and compensation at the network level including altered activity of spinal interneurons; and (iii) the increased power output necessary on an inclined treadmill was important in revealing altered activity in SOD1G93A mice.
    July 05, 2017   doi: 10.1113/JP274170   open full text
  • Altered post‐capillary and collecting venular reactivity in skeletal muscle with metabolic syndrome.
    Kent A. Lemaster, Zahra Farid, Robert W. Brock, Carl D. Shrader, Daniel Goldman, Dwayne N. Jackson, Jefferson C. Frisbee.
    The Journal of Physiology. July 05, 2017
    Key points With the development of the metabolic syndrome, both post‐capillary and collecting venular dilator reactivity within the skeletal muscle of obese Zucker rats (OZR) is impaired. The impaired dilator reactivity in OZR reflects a loss in venular nitric oxide and PGI2 bioavailability, associated with the chronic elevation in oxidant stress. Additionally, with the impaired dilator responses, a modest increase in adrenergic constriction combined with an elevated thromboxane A2 production may contribute to impaired functional dilator and hyperaemic responses at the venular level. For the shift in skeletal muscle venular function with development of the metabolic syndrome, issues such as aggregate microvascular perfusion resistance, mass transport and exchange within with capillary networks, and fluid handling across the microcirculation are compelling avenues for future investigation. Abstract While research into vascular outcomes of the metabolic syndrome has focused on arterial/arteriolar and capillary levels, investigation into venular function and how this impacts responses has received little attention. Using the in situ cremaster muscle of obese Zucker rats (OZR; with lean Zucker rats (LZR) as controls), we determined indices of venular function. At ∼17 weeks of age, skeletal muscle post‐capillary venular density was reduced by ∼20% in LZR vs. OZR, although there was no evidence of remodelling of the venular wall. Venular tone at ∼25 μm (post‐capillary) and ∼75 μm (collecting) diameter was elevated in OZR vs. LZR. Venular dilatation to acetylcholine was blunted in OZR vs. LZR due to increased oxidant stress‐based loss of nitric oxide bioavailability (post‐capillary) and increased α1‐ (and α2‐) mediated constrictor tone (collecting). Venular constrictor responses in OZR were comparable to LZR for most stimuli, although constriction to α1‐adrenoreceptor stimulation was elevated. In response to field stimulation of the cremaster muscle (0.5, 1, 3 Hz), venular dilator and hyperaemic responses to lower frequencies were blunted in OZR, but responses at 3 Hz were similar between strains. Venous production of TxA2 was higher in OZR than LZR and significantly higher than PGI2 production in either following arachidonic acid challenge. These results suggest that multi‐faceted alterations to skeletal muscle venular function in OZR may contribute to alterations in upstream capillary pressure profiles and the transcapillary exchange of solutes and water under conditions of metabolic syndrome.
    July 05, 2017   doi: 10.1113/JP274291   open full text
  • Glial EAAT2 regulation of extracellular nTS glutamate critically controls neuronal activity and cardiorespiratory reflexes.
    Michael P. Matott, David D. Kline, Eileen M. Hasser.
    The Journal of Physiology. July 05, 2017
    Glutamatergic signalling is critical in the nucleus tractus solitarii (nTS) for cardiorespiratory homeostasis and initiation of sensory reflexes, including the chemoreflex activated during hypoxia. Maintenance of nTS glutamate concentration occurs in part through astrocytic excitatory amino acid transporters (EAATs). We previously established the importance of EAATs in the nTS by demonstrating their inhibition produced neuronal excitation to alter basal cardiorespiratory function. Since EAAT2 is the most expressed EAAT in the nTS, this study specifically determined EAAT2's role in nTS astrocytes, their influence on neuronal and synaptic properties, and ultimately on basal and reflex cardiorespiratory function. The EAAT2 specific antagonist dihydrokainate (DHK) was microinjected into the anaesthetized rat nTS or applied to rat nTS slices. DHK produced depressor, bradycardic and sympathoinhibitory responses and reduced neural respiration in the intact rat, mimicking responses to glutamate excitation. DHK also enhanced responses to glutamate microinjection. DHK elevated extracellular nTS glutamate concentration, depolarized neurons and enhanced spontaneous EPSCs. EAAT2 block also augmented action potential discharge in chemosensitive nTS neurons. Glial recordings confirmed EAAT2 is functional on nTS astrocytes. Neuronal excitation and cardiorespiratory effects following EAAT2 inhibition were due to activation of putative extrasynaptic AMPA receptors as their antagonism blocked DHK responses in the intact rat nTS and the slice. The DHK‐induced elevation of extracellular glutamate and neuronal excitation augmented chemoreflex‐mediated pressor, sympathoexcitatory and minute neural ventilation responses in the rat. These data shed new light on the important role astrocytic EAAT2 plays on buffering nTS excitation and overall cardiorespiratory function. This article is protected by copyright. All rights reserved
    July 05, 2017   doi: 10.1113/JP274620   open full text
  • Inhibitory modulation of medial prefrontal cortical activation on lateral orbitofrontal cortex‐amygdala information flow.
    Chun‐hui Chang, Ta‐wen Ho.
    The Journal of Physiology. July 05, 2017
    Several neocortical projections converge onto the basolateral complex of the amygdala (BLA), including the lateral orbitofrontal cortex (lOFC) and medial prefrontal cortex (mPFC). Lateral orbitofrontal input to BLA is important for cue‐outcome contingencies, while medial prefrontal input is essential for emotion control. In this study, we examined how mPFC, specifically the infralimbic (IL) division of mPFC, modulates the lOFC‐BLA information flow, using combined in vivo extracellular single‐unit recordings and pharmacological manipulations in anesthetized rats. We found that the majority (over 95%) of BLA neurons that responded to lOFC stimulation also responded to mPFC stimulation. Compared to basal condition, pharmacological (N‐Methyl‐D‐aspartate, NMDA) or electrical activation of the mPFC exerted an inhibitory modulation of the lOFC‐BLA pathway, which was reversed with intra‐amygdala blockade of GABAergic receptors with combined GABAA and GABAB antagonists (bicuculline and saclofen). Moreover, mPFC tetanus potentiated the lOFC‐BLA pathway, but mPFC tetanus or low‐frequency stimulation (LFS) did not alter its inhibitory modulatory gating on the lOFC‐BLA pathway. These results show that the mPFC potently inhibits lOFC drive of BLA neurons in a GABA‐dependent manner. Our result is informative in understanding the normal and potential pathophysiological state of emotion and contingency associations regulating behaviour. This article is protected by copyright. All rights reserved
    July 05, 2017   doi: 10.1113/JP274568   open full text
  • Kir2.1 and K2P1 channels reconstitute two levels of resting membrane potential in cardiomyocytes.
    Dongchuan Zuo, Kuihao Chen, Min Zhou, Zheng Liu, Haijun Chen.
    The Journal of Physiology. July 04, 2017
    Key points Outward and inward background currents across the cell membrane balance, determining resting membrane potential. Inward rectifier K+ channel subfamily 2 (Kir2) channels primarily maintain the resting membrane potential of cardiomyocytes. Human cardiomyocytes exhibit two levels of resting membrane potential at subphysiological extracellular K+ concentrations or pathological hypokalaemia, however, the underlying mechanism is unclear. In the present study, we show that human cardiomyocytes derived from induced pluripotent stem cells with enhanced expression of isoform 1 of Kir2 (Kir2.1) channels and mouse HL‐1 cardiomyocytes with ectopic expression of two pore‐domain K+ channel isoform 1 (K2P1) recapitulate two levels of resting membrane potential, indicating the contributions of Kir2.1 and K2P1 channels to the phenomenon. In Chinese hamster ovary cells that express the channels, Kir2.1 currents non‐linearly counterbalance hypokalaemia‐induced K2P1 leak cation currents, reconstituting two levels of resting membrane potential. These findings support the hypothesis that Kir2 currents non‐linearly counterbalance inward background cation currents, such as K2P1 currents, accounting for two levels of resting membrane potential in human cardiomyocytes and demonstrating a novel mechanism that regulates excitability. Abstract Inward rectifier K+ channel subfamily 2 (Kir2) channels primarily maintain the normal resting membrane potential of cardiomyocytes. At subphysiological extracellular K+ concentrations or pathological hypokalaemia, human cardiomyocytes show both hyperpolarized and depolarized resting membrane potentials; these depolarized potentials cause cardiac arrhythmia; however, the underlying mechanism is unknown. In the present study, we show that inward rectifier K+ channel subfamily 2 isoform 1 (Kir2.1) currents non‐linearly counterbalance hypokalaemia‐induced two pore‐domain K+ channel isoform 1 (K2P1) leak cation currents, reconstituting two levels of resting membrane potential in cardiomyocytes. Under hypokalaemic conditions, both human cardiomyocytes derived from induced pluripotent stem cells with enhanced Kir2.1 expression and mouse HL‐1 cardiomyocytes with ectopic expression of K2P1 channels recapitulate two levels of resting membrane potential. These cardiomyocytes display N‐shaped current–voltage relationships that cross the voltage axis three times and the first and third zero‐current potentials match the two levels of resting membrane potential. Inhibition of K2P1 expression eliminates the phenomenon, indicating contributions of Kir2.1 and K2P1 channels to two levels of resting membrane potential. Second, in Chinese hamster ovary cells that heterologously express the channels, Kir2.1 currents non‐linearly counterbalance hypokalaemia‐induced K2P1 leak cation currents, yielding the N‐shaped current–voltage relationships, causing the resting membrane potential to spontaneously jump from hyperpolarization at the first zero‐current potential to depolarization at the third zero‐current potential, again recapitulating two levels of resting membrane potential. These findings reveal ionic mechanisms of the two levels of resting membrane potential, demonstrating a previously unknown mechanism for the regulation of excitability, and support the hypothesis that Kir2 currents non‐linearly balance inward background cation currents, accounting for two levels of resting membrane potential of human cardiomyocytes.
    July 04, 2017   doi: 10.1113/JP274268   open full text
  • A map of the phosphoproteomic alterations that occur after a bout of maximal‐intensity contractions.
    Gregory K. Potts, Rachel M. McNally, Rocky Blanco, Jae‐Sung You, Alexander S. Hebert, Michael S. Westphall, Joshua J. Coon, Troy A. Hornberger.
    The Journal of Physiology. July 04, 2017
    Key points Mechanical signals play a critical role in the regulation of muscle mass, but the molecules that sense mechanical signals and convert this stimulus into the biochemical events that regulate muscle mass remain ill‐defined. Here we report a mass spectrometry‐based workflow to study the changes in protein phosphorylation that occur in mouse skeletal muscle 1 h after a bout of electrically evoked maximal‐intensity contractions (MICs). Our dataset provides the first comprehensive map of the MIC‐regulated phosphoproteome. Using unbiased bioinformatics approaches, we demonstrate that our dataset leads to the identification of many well‐known MIC‐regulated signalling pathways, as well as to a plethora of novel MIC‐regulated events. We expect that our dataset will serve as a fundamentally important resource for muscle biologists, and help to lay the foundation for entirely new hypotheses in the field. Abstract The maintenance of skeletal muscle mass is essential for health and quality of life. It is well recognized that maximal‐intensity contractions, such as those which occur during resistance exercise, promote an increase in muscle mass. Yet, the molecules that sense the mechanical information and convert it into the signalling events (e.g. phosphorylation) that drive the increase in muscle mass remain undefined. Here we describe a phosphoproteomics workflow to examine the effects of electrically evoked maximal‐intensity contractions (MICs) on protein phosphorylation in mouse skeletal muscle. While a preliminary phosphoproteomics experiment successfully identified a number of MIC‐regulated phosphorylation events, a large proportion of these identifications were present on highly abundant myofibrillar proteins. We subsequently incorporated a centrifugation‐based fractionation step to deplete the highly abundant myofibrillar proteins and performed a second phosphoproteomics experiment. In total, we identified 5983 unique phosphorylation sites of which 663 were found to be regulated by MIC. GO term enrichment, phosphorylation motif analyses, and kinase‐substrate predictions indicated that the MIC‐regulated phosphorylation sites were chiefly modified by mTOR, as well as multiple isoforms of the MAPKs and CAMKs. Moreover, a high proportion of the regulated phosphorylation sites were found on proteins that are associated with the Z‐disc, with over 74% of the Z‐disc proteins experiencing robust changes in phosphorylation. Finally, our analyses revealed that the phosphorylation state of two Z‐disc kinases (striated muscle‐specific serine/threonine protein kinase and obscurin) was dramatically altered by MIC, and we propose ways these kinases could play a fundamental role in skeletal muscle mechanotransduction.
    July 04, 2017   doi: 10.1113/JP273904   open full text
  • VEGF‐A165b protects against proteinuria in a mouse model with progressive depletion of all endogenous VEGF‐A splice isoforms from the kidney.
    Megan Stevens, Christopher R. Neal, Andrew H. J. Salmon, David O. Bates, Steven J. Harper, Sebastian Oltean.
    The Journal of Physiology. July 03, 2017
    Key points Progressive depletion of all vascular endothelial growth factor A (VEGF‐A) splice isoforms from the kidney results in proteinuria and increased glomerular water permeability, which are both rescued by over‐expression of VEGF‐A165b only. VEGF‐A165b rescues the increase in glomerular basement membrane and podocyte slit width, as well as the decrease in sub‐podocyte space coverage, produced by VEGF‐A depletion. VEGF‐A165b restores the expression of platelet endothelial cell adhesion molecule in glomerular endothelial cells and glomerular capillary circumference. VEGF‐A165b has opposite effects to VEGF‐A165 on the expression of genes involved in endothelial cell migration and proliferation. Abstract Chronic kidney disease is strongly associated with a decrease in the expression of vascular endothelial growth factor A (VEGF‐A). However, little is known about the contribution of VEGF‐A splice isoforms to kidney physiology and pathology. Previous studies suggest that the splice isoform VEGF‐A165b (resulting from alternative usage of a 3′ splice site in the terminal exon) is protective for kidney function. In the present study, we show, in a quad‐transgenic model, that over‐expression of VEGF‐A165b alone is sufficient to rescue the increase in proteinuria, as well as glomerular water permeability, in the context of progressive depletion of all VEGF‐A isoforms from the podocytes. Ultrastructural studies show that the glomerular basement membrane is thickened, podocyte slit width is increased and sub‐podocyte space coverage is reduced when VEGF‐A is depleted, all of which are rescued in VEGF‐A165b over‐expressors. VEGF‐A165b restores the expression of platelet endothelial cell adhesion molecule‐1 in glomerular endothelial cells and glomerular capillary circumference. Mechanistically, it increases VEGF receptor 2 expression both in vivo and in vitro and down‐regulates genes involved in migration and proliferation of endothelial cells, otherwise up‐regulated by the canonical isoform VEGF‐A165. The results of the present study indicate that manipulation of VEGF‐A splice isoforms could be a novel therapeutic avenue in chronic glomerular disease.
    July 03, 2017   doi: 10.1113/JP274481   open full text
  • Impaired activity of adherens junctions contributes to endothelial dilator dysfunction in ageing rat arteries.
    Fumin Chang, Sheila Flavahan, Nicholas A. Flavahan.
    The Journal of Physiology. June 30, 2017
    Key points Ageing‐induced endothelial dysfunction contributes to organ dysfunction and progression of cardiovascular disease. VE‐cadherin clustering at adherens junctions promotes protective endothelial functions, including endothelium‐dependent dilatation. Ageing increased internalization and degradation of VE‐cadherin, resulting in impaired activity of adherens junctions. Inhibition of VE‐cadherin clustering at adherens junctions (function‐blocking antibody; FBA) reduced endothelial dilatation in young arteries but did not affect the already impaired dilatation in old arteries. After junctional disruption with the FBA, dilatation was similar in young and old arteries. Src tyrosine kinase activity and tyrosine phosphorylation of VE‐cadherin were increased in old arteries. Src inhibition increased VE‐cadherin at adherens junctions and increased endothelial dilatation in old, but not young, arteries. Src inhibition did not increase dilatation in old arteries treated with the VE‐cadherin FBA. Ageing impairs the activity of adherens junctions, which contributes to endothelial dilator dysfunction. Restoring the activity of adherens junctions could be of therapeutic benefit in vascular ageing. Abstract Endothelial dilator dysfunction contributes to pathological vascular ageing. Experiments assessed whether altered activity of endothelial adherens junctions (AJs) might contribute to this dysfunction. Aortas and tail arteries were isolated from young (3–4 months) and old (22–24 months) F344 rats. VE‐cadherin immunofluorescent staining at endothelial AJs and AJ width were reduced in old compared to young arteries. A 140 kDa VE‐cadherin species was present on the cell surface and in TTX‐insoluble fractions, consistent with junctional localization. Levels of the 140 kDa VE‐cadherin were decreased, whereas levels of a TTX‐soluble 115 kDa VE‐cadherin species were increased in old compared to young arteries. Acetylcholine caused endothelium‐dependent dilatation that was decreased in old compared to young arteries. Disruption of VE‐cadherin clustering at AJs (function‐blocking antibody, FBA) inhibited dilatation to acetylcholine in young, but not old, arteries. After the FBA, there was no longer any difference in dilatation between old and young arteries. Src activity and tyrosine phosphorylation of VE‐cadherin were increased in old compared to young arteries. In old arteries, Src inhibition (saracatinib) increased: (i) 140 kDa VE‐cadherin in the TTX‐insoluble fraction, (ii) VE‐cadherin intensity at AJs, (iii) AJ width, and (iv) acetylcholine dilatation. In old arteries treated with the FBA, saracatinib no longer increased acetylcholine dilatation. Saracatinib did not affect dilatation in young arteries. Therefore, ageing impairs AJ activity, which appears to reflect Src‐induced phosphorylation, internalization and degradation of VE‐cadherin. Moreover, impaired AJ activity can account for the endothelial dilator dysfunction in old arteries. Restoring endothelial AJ activity may be a novel therapeutic approach to vascular ageing.
    June 30, 2017   doi: 10.1113/JP274189   open full text
  • Enhancement of synchronized activity between hippocampal CA1 neurons during initial storage of associative fear memory.
    Yu‐Zhang Liu, Yao Wang, Weida Shen, Zhiru Wang.
    The Journal of Physiology. June 30, 2017
    Key points Learning and memory storage requires neuronal plasticity induced in the hippocampus and other related brain areas, and this process is thought to rely on synchronized activity in neural networks. We used paired whole‐cell recording in vivo to examine the synchronized activity that was induced in hippocampal CA1 neurons by associative fear learning. We found that both membrane potential synchronization and spike synchronization of CA1 neurons could be transiently enhanced after task learning, as observed on day 1 but not day 5. On day 1 after learning, CA1 neurons showed a decrease in firing threshold and rise times of suprathreshold membrane potential changes as well as an increase in spontaneous firing rates, possibly contributing to the enhancement of spike synchronization. The transient enhancement of CA1 neuronal synchronization may play important roles in the induction of neuronal plasticity for initial storage and consolidation of associative memory. Abstract The hippocampus is critical for memory acquisition and consolidation. This function requires activity‐ and experience‐induced neuronal plasticity. It is known that neuronal plasticity is largely dependent on synchronized activity. As has been well characterized, repetitive correlated activity of presynaptic and postsynaptic neurons can lead to long‐term modifications at their synapses. Studies on network activity have also suggested that memory processing in the hippocampus may involve learning‐induced changes of neuronal synchronization, as observed in vivo between hippocampal CA3 and CA1 networks as well as between the rhinal cortex and the hippocampus. However, further investigation of learning‐induced synchronized activity in the hippocampus is needed for a full understanding of hippocampal memory processing. In this study, by performing paired whole‐cell recording in vivo on CA1 pyramidal cells (PCs) in anaesthetized adult rats, we examined CA1 neuronal synchronization before and after associative fear learning. We first found in naive animals that there was a low level of membrane potential (MP) synchronization and spike synchronization of CA1 PCs. In conditioned animals, we found a significant enhancement of both MP synchronization and spike synchronization, as observed on day 1 after learning, and this enhancement was transient and not observed on day 5. Accompanying learning‐induced synchronized activity was a decreased firing threshold and rise time of suprathreshold MP changes as well as an increased spontaneous firing rate, possibly contributing to the enhanced spike synchronization. The transiently enhanced CA1 neuronal synchronization may have important roles in generating neuronal plasticity for hippocampal storage and consolidation of associative memory traces.
    June 30, 2017   doi: 10.1113/JP274212   open full text
  • Threshold position control of anticipation in humans: a possible role of corticospinal influences.
    Lei Zhang, Nicolas A. Turpin, Anatol G. Feldman.
    The Journal of Physiology. June 28, 2017
    Key points Sudden unloading of preloaded wrist muscles elicits motion to a new wrist position. Such motion is prevented if subjects unload muscles using the contralateral arm (self‐unloading). Corticospinal influences originated from the primary motor cortex maintain tonic influences on motoneurons of wrist muscles before sudden unloading but modify these influences prior to the onset and until the end of self‐unloading. Results are interpreted based on the previous finding that intentional actions are caused by central, particularly corticospinal, shifts in the spatial thresholds at which wrist motoneurons are activated, thus predetermining the attractor point at which the neuromuscular periphery achieves mechanical balance with environment forces. By maintaining or shifting the thresholds, descending systems let body segments go to the equilibrium position in the respective unloading tasks without the pre‐programming of kinematics or muscle activation patterns. The study advances the understanding of how motor actions in general, and anticipation in particular, are controlled. Abstract The role of corticospinal (CS) pathways in anticipatory motor actions was evaluated using transcranial magnetic stimulation (TMS) of the primary motor cortex projecting to motoneurons (MNs) of wrist muscles. Preloaded wrist flexors were suddenly unloaded by the experimenter or by the subject using the other hand (self‐unloading). After sudden unloading, the wrist joint involuntarily flexed to a new position. In contrast, during self‐unloading the wrist remained almost motionless, implying that an anticipatory postural adjustment occurred. In the self‐unloading task, anticipation was manifested by a decrease in descending facilitation of pre‐activated flexor MNs starting ∼72 ms before changes in the background EMG activity. Descending facilitation of extensor MNs began to increase ∼61 ms later. Conversely, these influences remained unchanged before sudden unloading, implying the absence of anticipation. We also tested TMS responses during EMG silent periods produced by brief muscle shortening, transiently resulting in similar EMG levels before the onset and after the end of self‐unloading. We found reduced descending facilitation of flexor MNs after self‐unloading. To explain why the wrist excursion was minimized in self‐unloading due to these changes in descending influences, we relied on previous demonstrations that descending systems pre‐set the threshold positions of body segments at which muscles begin to be activated, thus predetermining the equilibrium point to which the system is attracted. Based on this notion, a more consistent explanation of the kinematic, EMG and descending patterns in the two types of unloading is proposed compared to the alternative notion of direct pre‐programming of kinematic and/or EMG patterns.
    June 28, 2017   doi: 10.1113/JP274309   open full text
  • Skeletal myofiber vascular endothelial growth factor is required for the exercise training‐induced increase in dentate gyrus neuronal precursor cells.
    Benjamin Rich, Miriam Scadeng, Masahiro Yamaguchi, Peter D. Wagner, Ellen C. Breen.
    The Journal of Physiology. June 28, 2017
    Key points Peripheral vascular endothelial growth factor (VEGF) is necessary for exercise to stimulate hippocampal neurogenesis. Here we report that skeletal myofiber VEGF directly or indirectly regulates exercise‐signalled proliferation of neuronal precursor cells. Our results found skeletal myofiber VEGF to be necessary for maintaining blood flow through hippocampal regions independent of exercise training state. This study demonstrates that skeletal myofiber VEGF is required for the hippocampal VEGF response to acute exercise. These results help to establish the mechanisms by which exercise, through skeletal myofiber VEGF, affects the hippocampus. Abstract Exercise signals neurogenesis in the dentate gyrus of the hippocampus. This phenomenon requires vascular endothelial growth factor (VEGF) originating from outside the blood–brain barrier, but no cellular source has been identified. Thus, we hypothesized that VEGF produced by skeletal myofibers plays a role in regulating hippocampal neuronal precursor cell proliferation following exercise training. This was tested in adult conditional skeletal myofiber‐specific VEGF gene‐ablated mice (VEGFHSA−/−) by providing VEGFHSA−/− and non‐ablated (VEGFf/f) littermates with running wheels for 14 days. Following this training period, hippocampal cerebral blood flow (CBF) was measured by functional magnetic resonance imaging (fMRI), and neuronal precursor cells (BrdU+/Nestin+) were detected by immunofluorescence. The VEGFf/f trained group showed improvements in both speed and endurance capacity in acute treadmill running tests (P < 0.05). The VEGFHSA−/− group did not. The number of proliferating neuronal precursor cells was increased with training in VEGFf/f (P < 0.05) but not in VEGFHSA−/− mice. Endothelial cell (CD31+) number did not change in this region with exercise training or skeletal myofiber VEGF gene deletion. However, resting blood flow through the hippocampal region was lower in VEGFHSA−/− mice, both untrained and trained, than untrained VEGFf/f mice (P < 0.05). An acute hypoxic challenge decreased CBF (P < 0.05) in untrained VEGFf/f, untrained VEGFHSA−/− and trained VEGFHSA−/− mice, but not trained VEGFf/f mice. VEGFf/f, but not VEGFHSA−/−, mice were able to acutely run on a treadmill at an intensity sufficient to increase hippocampus VEGF levels. These data suggest that VEGF expressed by skeletal myofibers may directly or indirectly regulate both hippocampal blood flow and neurogenesis.
    June 28, 2017   doi: 10.1113/JP273994   open full text
  • Functional severity of CLCNKB mutations correlates with phenotypes in patients with classic Bartter's syndrome.
    Chih‐Jen Cheng, Yi‐Fen Lo, Jen‐Chi Chen, Chou‐Long Huang, Shih‐Hua Lin.
    The Journal of Physiology. June 27, 2017
    Key points The highly variable phenotypes observed in patients with classic Bartter's syndrome (BS) remain unsatisfactorily explained. The wide spectrum of functional severity of CLCNKB mutations may contribute to the phenotypic variability, and the genotype–phenotype association has not been established. Low‐level expression of the human ClC‐Kb channel in mammalian cells impedes the functional study of CLCNKB mutations, and the underlying cause is still unclear. The human ClC‐Kb channel is highly degraded by proteasome in human embryonic kidney cells. The C‐terminal in‐frame green fluorescent protein fusion may slow down the proteasome‐mediated proteolysis. Barttin co‐expression necessarily improves the stability, membrane trafficking and gating of ClC‐Kb. CLCNKB mutations in barttin‐binding sites, dimer interface or selectivity filter often have severe functional consequences. The remaining chloride conductance of the ClC‐Kb mutant channel significantly correlates with the phenotypes, such as age at diagnosis, plasma chloride concentration, and the degree of calciuria in patients with classic BS. Abstract Mutations in the CLCNKB gene encoding the human voltage‐gated chloride ClC‐Kb (hClC‐Kb) channel cause classic Bartter's syndrome (BS). In contrast to antenatal BS, classic BS manifests with highly variable phenotypes. The functional severity of the mutant channel has been proposed to explain this phenomenon. Due to difficulties in the expression of hClC‐Kb in heterologous expression systems, the functional consequences of mutant channels have not been thoroughly examined, and the genotype–phenotype association has not been established. In this study, we found that hClC‐Kb, when expressed in human embryonic kidney (HEK) cells, was unstable due to degradation by proteasome. In‐frame fusion of green fluorescent protein (GFP) to the C‐terminus of the channel may ameliorate proteasome degradation. Co‐expression of barttin increased protein abundance and membrane trafficking of hClC‐Kb and markedly increased functional chloride current. We then functionally characterized 18 missense mutations identified in our classic BS cohort and others using HEK cells expressing hClC‐Kb‐GFP. Most CLCNKB mutations resulted in marked reduction in protein abundance and chloride current, especially those residing at barttin binding sites, dimer interface and selectivity filter. We enrolled classic BS patients carrying homozygous missense mutations with well‐described functional consequences and clinical presentations for genotype–phenotype analysis. We found significant correlations of mutant chloride current with the age at diagnosis, plasma chloride concentration and urine calcium excretion rate. In conclusion, hClC‐Kb expression in HEK cells is susceptible to proteasome degradation, and fusion of GFP to the C‐terminus of hClC‐Kb improves protein expression. The functional severity of the CLCNKB mutation is an important determinant of the phenotype in classic BS.
    June 27, 2017   doi: 10.1113/JP274344   open full text
  • Training alters the distribution of perilipin proteins in muscle following acute free fatty acid exposure.
    S. O. Shepherd, J. A. Strauss, Q. Wang, J. J. Dube, B. Goodpaster, D. G. Mashek, L. S. Chow.
    The Journal of Physiology. June 27, 2017
    Key points The lipid droplet (LD)‐associated perilipin (PLIN) proteins promote intramuscular triglyceride (IMTG) storage, although whether the abundance and association of the PLIN proteins with LDs is related to the diverse lipid storage in muscle between trained and sedentary individuals is unknown. We show that lipid infusion augments IMTG content in type I fibres of both trained and sedentary individuals. Most importantly, despite there being no change in PLIN protein content, lipid infusion did increase the number of LDs connected with PLIN proteins in trained individuals only. We conclude that trained individuals are able to redistribute the pre‐existing pool of PLIN proteins to an expanded LD pool during lipid infusion and, via this adaptation, may support the storage of fatty acids in IMTG. Abstract Because the lipid droplet (LD)‐associated perilipin (PLIN) proteins promote intramuscular triglyceride (IMTG) storage, we investigated the hypothesis that differential protein content of PLINs and their distribution with LDs may be linked to the diverse lipid storage in muscle between trained and sedentary individuals. Trained (n = 11) and sedentary (n = 10) subjects, matched for age, sex and body mass index, received either a 6 h lipid or glycerol infusion in the setting of a concurrent hyperinsulinaemic–euglycaemic clamp. Sequential muscle biopsies (0, 2 and 6 h) were analysed using confocal immunofluorescence microscopy for fibre type‐specific IMTG content and PLIN associations with LDs. In both groups, lipid infusion increased IMTG content in type I fibres (trained: +62%, sedentary: +79%; P < 0.05) but did not affect PLIN protein content. At baseline, PLIN2 (+65%), PLIN3 (+105%) and PLIN5 (+53%; all P < 0.05) protein content was higher in trained compared to sedentary individuals. In trained individuals, lipid infusion increased the number of LDs associated with PLIN2 (+27%), PLIN3 (+73%) and PLIN5 (+40%; all P < 0.05) in type I fibres. By contrast, in sedentary individuals, lipid infusion only increased the number of LDs not associated with PLIN proteins. Acute free fatty acid elevation therefore induces a redistribution of PLIN proteins to an expanded LD pool in trained individuals only and this may be part of the mechanism that enables fatty acids to be stored in IMTG.
    June 27, 2017   doi: 10.1113/JP274374   open full text
  • Aerobic capacity mediates susceptibility for the transition from steatosis to steatohepatitis.
    E. Matthew Morris, Colin S. McCoin, Julie A. Allen, Michelle L. Gastecki, Lauren G. Koch, Steven L. Britton, Justin A. Fletcher, Xiarong Fu, Wen‐Xing Ding, Shawn C. Burgess, R. Scott Rector, John P. Thyfault.
    The Journal of Physiology. June 27, 2017
    Key points Low intrinsic aerobic capacity is associated with increased all‐cause and liver‐related mortality in humans. Low intrinsic aerobic capacity in the low capacity runner (LCR) rat increases susceptibility to acute and chronic high‐fat/high‐sucrose diet‐induced steatosis, without observed increases in liver inflammation. Addition of excess cholesterol to a high‐fat/high‐sucrose diet produced greater steatosis in LCR and high capacity runner (HCR) rats. However, the LCR rat demonstrated greater susceptibility to increased liver inflammatory and apoptotic markers compared to the HCR rat. The progressive non‐alcoholic fatty liver disease observed in the LCR rats following western diet feeding was associated with further declines in liver fatty acid oxidation and mitochondrial respiratory capacity compared to HCR rats. Abstract Low aerobic capacity increases risk for non‐alcoholic fatty liver disease and liver‐related disease mortality, but mechanisms mediating these effects remain unknown. We recently reported that rats bred for low aerobic capacity (low capacity runner; LCR) displayed susceptibility to high fat diet‐induced steatosis in association with reduced hepatic mitochondrial fatty acid oxidation (FAO) and respiratory capacity compared to high aerobic capacity (high capacity runner; HCR) rats. Here we tested the impact of aerobic capacity on susceptibility for progressive liver disease following a 16‐week ‘western diet’ (WD) high in fat (45% kcal), cholesterol (1% w/w) and sucrose (15% kcal). Unlike previously with a diet high in fat and sucrose alone, the inclusion of cholesterol in the WD induced hepatomegaly and steatosis in both HCR and LCR rats, while producing greater cholesterol ester accumulation in LCR compared to HCR rats. Importantly, WD‐fed low‐fitness LCR rats displayed greater inflammatory cell infiltration, serum alanine transaminase, expression of hepatic inflammatory markers (F4/80, MCP‐1, TLR4, TLR2 and IL‐1β) and effector caspase (caspase 3 and 7) activation compared to HCR rats. Further, LCR rats had greater WD‐induced decreases in complete FAO and mitochondrial respiratory capacity. Intrinsic aerobic capacity had no impact on WD‐induced hepatic steatosis; however, rats bred for low aerobic capacity developed greater hepatic inflammation, which was associated with reduced hepatic mitochondrial FAO and respiratory capacity and increased accumulation of cholesterol esters. These results confirm epidemiological reports that aerobic capacity impacts progression of liver disease and suggest that these effects are mediated through alterations in hepatic mitochondrial function.
    June 27, 2017   doi: 10.1113/JP274281   open full text
  • Spike threshold dynamics in spinal motoneurons during scratching and swimming.
    Ramunas Grigonis, Aidas Alaburda.
    The Journal of Physiology. June 27, 2017
    During functional spinal neural network activity motoneurons receive intense synaptic input, and this could modulate the threshold for action potential generation, providing the ability to dynamically adjust the excitability and recruitment order for functional needs. In the present study we investigated the dynamics of action potential threshold during motor network activity. Intracellular recordings from spinal motoneurons in an ex vivo carapace‐spinal cord preparation from adult turtles were performed during two distinct types of motor behaviour – fictive scratching and fictive swimming. We found that the threshold of the first spike in episodes of scratching and swimming was the lowest. The threshold potential depolarizes by about 10 mV within each burst of spikes generated during scratch and swim network activity and recovers between bursts to a slightly depolarized level. Depolarization of the threshold potential results in decreased excitability of motoneurons. Synaptic inputs do not modulate the threshold of the first action potential during episodes of scratching or of swimming. There is no correlation between changes in spike threshold and interspike intervals within bursts. Slow synaptic integration that results in a wave of membrane potential depolarization rather than fast synaptic events preceding each spike is the factor influencing the threshold potential within firing bursts during motor behaviours. This article is protected by copyright. All rights reserved
    June 27, 2017   doi: 10.1113/JP274434   open full text
  • Heterogeneity of Purkinje cell simple spike–complex spike interactions: zebrin‐ and non‐zebrin‐related variations.
    Tianyu Tang, Jianqiang Xiao, Colleen Y. Suh, Amelia Burroughs, Nadia L. Cerminara, Linjia Jia, Sarah P. Marshall, Andrew K. Wise, Richard Apps, Izumi Sugihara, Eric J. Lang.
    The Journal of Physiology. June 26, 2017
    Key points Cerebellar Purkinje cells (PCs) generate two types of action potentials, simple and complex spikes. Although they are generated by distinct mechanisms, interactions between the two spike types exist. Zebrin staining produces alternating positive and negative stripes of PCs across most of the cerebellar cortex. Thus, here we compared simple spike–complex spike interactions both within and across zebrin populations. Simple spike activity undergoes a complex modulation preceding and following a complex spike. The amplitudes of the pre‐ and post‐complex spike modulation phases were correlated across PCs. On average, the modulation was larger for PCs in zebrin positive regions. Correlations between aspects of the complex spike waveform and simple spike activity were found, some of which varied between zebrin positive and negative PCs. The implications of the results are discussed with regard to hypotheses that complex spikes are triggered by rises in simple spike activity for either motor learning or homeostatic functions. Abstract Purkinje cells (PCs) generate two types of action potentials, called simple and complex spikes (SSs and CSs). We first investigated the CS‐associated modulation of SS activity and its relationship to the zebrin status of the PC. The modulation pattern consisted of a pre‐CS rise in SS activity, and then, following the CS, a pause, a rebound, and finally a late inhibition of SS activity for both zebrin positive (Z+) and negative (Z−) cells, though the amplitudes of the phases were larger in Z+ cells. Moreover, the amplitudes of the pre‐CS rise with the late inhibitory phase of the modulation were correlated across PCs. In contrast, correlations between modulation phases across CSs of individual PCs were generally weak. Next, the relationship between CS spikelets and SS activity was investigated. The number of spikelets/CS correlated with the average SS firing rate only for Z+ cells. In contrast, correlations across CSs between spikelet numbers and the amplitudes of the SS modulation phases were generally weak. Division of spikelets into likely axonally propagated and non‐propagated groups (based on their interspikelet interval) showed that the correlation of spikelet number with SS firing rate primarily reflected a relationship with non‐propagated spikelets. In sum, the results show both zebrin‐related and non‐zebrin‐related physiological heterogeneity in SS–CS interactions among PCs, which suggests that the cerebellar cortex is more functionally diverse than is assumed by standard theories of cerebellar function.
    June 26, 2017   doi: 10.1113/JP274252   open full text
  • Coupling of excitation to Ca2+ release is modulated by dysferlin.
    Valeriy Lukyanenko, Joaquin M. Muriel, Robert J. Bloch.
    The Journal of Physiology. June 26, 2017
    Key points Dysferlin, the protein missing in limb girdle muscular dystrophy 2B and Miyoshi myopathy, concentrates in transverse tubules of skeletal muscle, where it stabilizes voltage‐induced Ca2+ transients against loss after osmotic shock injury (OSI). Local expression of dysferlin in dysferlin‐null myofibres increases transient amplitude to control levels and protects them from loss after OSI. Inhibitors of ryanodine receptors (RyR1) and L‐type Ca2+ channels protect voltage‐induced Ca2+ transients from loss; thus both proteins play a role in injury in dysferlin's absence. Effects of Ca2+‐free medium and S107, which inhibits SR Ca2+ leak, suggest the SR as the primary source of Ca2+ responsible for the loss of the Ca2+ transient upon injury. Ca2+ waves were induced by OSI and suppressed by exogenous dysferlin. We conclude that dysferlin prevents injury‐induced SR Ca2+ leak. Abstract Dysferlin concentrates in the transverse tubules of skeletal muscle and stabilizes Ca2+ transients when muscle fibres are subjected to osmotic shock injury (OSI). We show here that voltage‐induced Ca2+ transients elicited in dysferlin‐null A/J myofibres were smaller than control A/WySnJ fibres. Regional expression of Venus‐dysferlin chimeras in A/J fibres restored the full amplitude of the Ca2+ transients and protected against OSI. We also show that drugs that target ryanodine receptors (RyR1: dantrolene, tetracaine, S107) and L‐type Ca2+ channels (LTCCs: nifedipine, verapamil, diltiazem) prevented the decrease in Ca2+ transients in A/J fibres following OSI. Diltiazem specifically increased transients by ∼20% in uninjured A/J fibres, restoring them to control values. The fact that both RyR1s and LTCCs were involved in OSI‐induced damage suggests that damage is mediated by increased Ca2+ leak from the sarcoplasmic reticulum (SR) through the RyR1. Congruent with this, injured A/J fibres produced Ca2+ sparks and Ca2+ waves. S107 (a stabilizer of RyR1–FK506 binding protein coupling that reduces Ca2+ leak) or local expression of Venus‐dysferlin prevented OSI‐induced Ca2+ waves. Our data suggest that dysferlin modulates SR Ca2+ release in skeletal muscle, and that in its absence OSI causes increased RyR1‐mediated Ca2+ leak from the SR into the cytoplasm.
    June 26, 2017   doi: 10.1113/JP274515   open full text
  • Blood flow restricted training leads to myocellular macrophage infiltration and upregulation of heat shock proteins, but no apparent muscle damage.
    Jakob L. Nielsen, Per Aagaard, Tatyana A. Prokhorova, Tobias Nygaard, Rune D. Bech, Charlotte Suetta, Ulrik Frandsen.
    The Journal of Physiology. June 23, 2017
    Key points Muscular contractions performed using a combination of low external loads and partial restriction of limb blood flow appear to induce substantial gains in muscle strength and muscle mass. This exercise regime may initially induce muscular stress and damage; however, the effects of a period of blood flow restricted training on these parameters remain largely unknown. The present study shows that short‐term, high‐frequency, low‐load muscle training performed with partial blood flow restriction does not induce significant muscular damage. However, signs of myocellular stress and inflammation that were observed in the early phase of training and after the training intervention, respectively, may be facilitating the previously reported gains in myogenic satellite cell content and muscle hypertrophy. The present results improve our current knowledge about the physiological effects of low‐load muscular contractions performed under blood flow restriction and may provide important information of relevance for future therapeutic treatment of muscular atrophy. Abstract Previous studies indicate that low‐load muscle contractions performed under local blood flow restriction (BFR) may initially induce muscle damage and stress. However, whether these factors are evoked with longitudinal BFR training remains unexplored at the myocellular level. Two distinct study protocols were conducted, covering 3 weeks (3 wk) or one week (1 wk). Subjects performed BFR exercise (100 mmHg, 20% 1RM) to concentric failure (BFRE) (3 wk/1 wk), while controls performed work‐matched (LLE) (3 wk) or high‐load (HLE; 70% 1RM) (1 wk) free‐flow exercise. Muscle biopsies (3 wk) were obtained at baseline (Pre), 8 days into the intervention (Mid8), and 3 and 10 days after training cessation (Post3, Post10) to examine macrophage (M1/M2) content as well as heat shock protein (HSP27/70) and tenascin‐C expression. Blood samples (1 wk) were collected before and after (0.1–24 h) the first and last training session to examine markers of muscle damage (creatine kinase), oxidative stress (total antibody capacity, glutathione) and inflammation (monocyte chemotactic protein‐1, interleukin‐6, tumour necrosis factor α). M1‐macrophage content increased 108–165% with BFRE and LLE at Post3 (P < 0.05), while M2‐macrophages increased (163%) with BFRE only (P < 0.01). Membrane and intracellular HSP27 expression increased 60–132% at Mid8 with BFRE (P < 0.05–0.01). No or only minor changes were observed in circulating markers of muscle damage, oxidative stress and inflammation. The amplitude, timing and localization of the above changes indicate that only limited muscle damage was evoked with BFRE. This study is the first to show that a period of high‐frequency, low‐load BFR training does not appear to induce general myocellular damage. However, signs of tissue inflammation and focal myocellular membrane stress and/or reorganization were observed that may be involved in the adaptation processes evoked by BFR muscle exercise.
    June 23, 2017   doi: 10.1113/JP273907   open full text
  • Presence of ethanol‐sensitive glycine receptors in medium spiny neurons in the mouse nucleus accumbens.
    B. Förstera, B. Muñoz, M. K. Lobo, R. Chandra, D. M. Lovinger, L. G. Aguayo.
    The Journal of Physiology. June 23, 2017
    Key points The nucleus accumbens (nAc) is involved in addiction‐related behaviour caused by several drugs of abuse, including alcohol. Glycine receptors (GlyRs) are potentiated by ethanol and they have been implicated in the regulation of accumbal dopamine levels. We investigated the presence of GlyR subunits in nAc and their modulation by ethanol in medium spiny neurons (MSNs) of the mouse nAc. We found that the GlyR α1 subunit is preferentially expressed in nAc and is potentiated by ethanol. Our study shows that GlyR α1 in nAc is a new target for development of novel pharmacological tools for behavioural intervention in drug abuse. Abstract Alcohol abuse causes major social, economic and health‐related problems worldwide. Alcohol, like other drugs of abuse, increases levels of dopamine in the nucleus accumbens (nAc), facilitating behavioural reinforcement and substance abuse. Previous studies suggested that glycine receptors (GlyRs) are involved in the regulation of accumbal dopamine levels. Here, we investigated the presence of GlyRs in accumbal dopamine receptor medium spiny neurons (MSNs) of C57BL/6J mice, analysing mRNA expression levels and immunoreactivity of GlyR subunits, as well as ethanol sensitivity. We found that GlyR α1 subunits are expressed at higher levels than α2, α3 and β in the mouse nAc and were located preferentially in dopamine receptor 1 (DRD1)‐positive MSNs. Interestingly, the glycine‐evoked currents in dissociated DRD1‐positive MSNs were potentiated by ethanol. Also, the potentiation of the GlyR‐mediated tonic current by ethanol suggests that they modulate the excitability of DRD1‐positive MSNs in nAc. This study should contribute to understanding the role of GlyR α1 in the reward system and might help to develop novel pharmacological therapies to treat alcoholism and other addiction‐related and compulsive behaviours.
    June 23, 2017   doi: 10.1113/JP273767   open full text
  • Cerebral haemodynamic response to somatosensory stimulation in neonatal lambs.
    Shinji Nakamura, David W. Walker, Flora Y. Wong.
    The Journal of Physiology. June 23, 2017
    The neurovascular coupling response has been defined for the adult brain but in the neonate non‐invasive measurement of local cerebral perfusion using NIRS or BOLD fMRI have yielded variable and inconsistent results, including negative responses suggesting decreased perfusion and localised tissue tissue hypoxia. Also, the impact of permissive hypercapnia (PaCO2 > 50 mmHg) in the management of neonates on cerebrovascular responses to somatosensory input is unknown. Using NIRS to measure changes in cerebral oxy‐ and deoxy‐haemoglobin (ΔoxyHb, ΔdeoxyHb) in 8 anaesthetised newborn lambs, we studied the cerebral haemodynamic functional response to left median nerve stimulation using stimulus trains of 1.8, 4.8 and 7.8 s. Stimulation always produced a somatosensory evoked response, and superficial cortical perfusion measured by Laser Doppler Flowmetry predominantly increased following median nerve stimulation. However, with 1.8 s stimulation, oxyHb responses in the contralateral hemisphere were either positive (i.e. increased oxyHb), negative, or absent; and with 4.8 and 7.8 s stimulations, both positive and negative responses were observed. Hypercapnia increased baseline oxyHb and total Hb consistent with cerebral vasodilatation, and 6 of 7 lambs tested showed increased Δtotal Hb responses after the 7.8 s stimulation; among which 4 lambs also showed increased ΔoxyHb responses. In 2 of 3 lambs, the negative ΔoxyHb response became a positive pattern during hypercapnia. These results show that instead of functional hyperaemia, somatosensory stimulation can evoke negative (decreased oxyHb, total Hb) functional responses in the neonatal brain suggestive of decreased local perfusion and vasoconstriction, and that hypercapnia produces both baseline hyperperfusion and increased functional hyperaemia. This article is protected by copyright. All rights reserved
    June 23, 2017   doi: 10.1113/JP274244   open full text
  • Lack of adaptation during prolonged split‐belt locomotion in the intact and spinal cat.
    Victoria Kuczynski, Alessandro Telonio, Yann Thibaudier, Marie‐France Hurteau, Charline Dambreville, Etienne Desrochers, Adam Doelman, Tayler Ross, Alain Frigon.
    The Journal of Physiology. June 23, 2017
    In humans, gait adapts to prolonged walking on a split‐belt treadmill, where one leg steps faster than the other, by gradually restoring the symmetry of interlimb kinematic variables, such as double support periods and step lengths, and by reducing muscle activity (EMG, electromyography). The adaptation is also characterized by reversing the asymmetry of interlimb variables observed during the early split‐belt period when returning to tied‐belt locomotion, termed an after‐effect. To determine if cats adapt to prolonged split‐belt locomotion and to assess if spinal locomotor circuits participate in the adaptation, we measured interlimb variables and EMG in intact and spinal‐transected cats before, during and after 10 min of split‐belt locomotion. In spinal cats only the hindlimbs performed stepping with the forelimbs stationary. In intact and spinal cats, step lengths and double support periods were, on average, symmetric, during tied‐belt locomotion. They became asymmetric during split‐belt locomotion and remained asymmetric throughout the split‐belt period. Upon returning to tied‐belt locomotion, symmetry was immediately restored. In intact cats, the mean EMG amplitude of hindlimb extensors increased during split‐belt locomotion and remained increased throughout the split‐belt period, whereas in spinal cats, EMG amplitude did not change. Therefore, the results indicate that the locomotor pattern of cats does not adapt to prolonged split‐belt locomotion, suggesting an important physiological difference in the control of locomotion between cats and humans. We propose that restoring left‐right symmetry is not required to maintain balance during prolonged asymmetric locomotion in the cat, a quadruped, as opposed to human bipedal locomotion. This article is protected by copyright. All rights reserved
    June 23, 2017   doi: 10.1113/JP274518   open full text
  • Intraglomerular gap junctions enhance interglomerular synchrony in a sparsely‐connected olfactory bulb network.
    Frederic Pouille, Thomas S. McTavish, Lawrence E. Hunter, Diego Restrepo, Nathan E. Schoppa.
    The Journal of Physiology. June 22, 2017
    A dominant feature of the olfactory bulb response to odour is fast synchronized oscillations at beta (15–40 Hz) or gamma (40–90 Hz) frequencies, thought to be involved in integration of olfactory signals. Mechanistically, the bulb presents an interesting case study for understanding how beta/gamma oscillations arise. Fast oscillatory synchrony in the activity of output mitral cells (MCs) appears to result from interactions with GABAergic granule cells (GCs), yet the incidence of MC‐GC connections is very low, around 4%. Here, we combined computational and experimental approaches to examine how oscillatory synchrony can nevertheless arise, focusing mainly on activity between “non‐sister” MCs affiliated with different glomeruli (interglomerular synchrony). In a sparsely connected model of MCs and GCs, we found first that interglomerular synchrony was generally quite low, but could be increased by a factor of 4 by physiological‐levels of gap junctional coupling between sister MCs at the same glomerulus. This effect was due to enhanced mutually synchronizing interactions between MC and GC populations. The potent role of gap junctions was confirmed in patch‐clamp recordings in bulb slices from wild‐type and connexin 36‐knockout (KO) mice. KO reduced both beta/gamma local field potential oscillations as well as synchrony of inhibitory signals in pairs of non‐sister MCs. These effects were independent of potential KO‐actions on network excitation. Divergent synaptic connections did not contribute directly to the vast majority of synchronized signals. Thus, in a sparsely connected network, gap junctions between a small subset of cells can, through population‐effects, greatly amplify oscillatory synchrony amongst unconnected cells. This article is protected by copyright. All rights reserved
    June 22, 2017   doi: 10.1113/JP274408   open full text
  • Exercise‐induced quadriceps muscle fatigue in men and women: effects of arterial oxygen content and respiratory muscle work.
    Paolo B. Dominelli, Yannick Molgat‐Seon, Donald E. G. Griesdale, Carli M. Peters, Jean‐Sébastien Blouin, Mypinder Sekhon, Giulio S. Dominelli, William R. Henderson, Glen E. Foster, Lee M. Romer, Michael S. Koehle, A. William Sheel.
    The Journal of Physiology. June 19, 2017
    Key points High work of breathing and exercise‐induced arterial hypoxaemia (EIAH) can decrease O2 delivery and exacerbate exercise‐induced quadriceps fatigue in healthy men. Women have a higher work of breathing during exercise, dedicate a greater fraction of whole‐body V̇O2 towards their respiratory muscles and develop EIAH. Despite a greater reduction in men's work of breathing, the attenuation of quadriceps fatigue was similar between the sexes. The degree of EIAH was similar between sexes, and regardless of sex, those who developed the greatest hypoxaemia during exercise demonstrated the most attenuation of quadriceps fatigue. Based on our previous finding that women have a greater relative oxygen cost of breathing, women appear to be especially susceptible to work of breathing‐related changes in quadriceps muscle fatigue. Abstract Reducing the work of breathing or eliminating exercise‐induced arterial hypoxaemia (EIAH) during exercise decreases the severity of quadriceps fatigue in men. Women have a greater work of breathing during exercise, dedicate a greater fraction of whole‐body V̇O2 towards their respiratory muscles, and demonstrate EIAH, suggesting women may be especially susceptible to quadriceps fatigue. Healthy subjects (8 male, 8 female) completed three constant load exercise tests over 4 days. During the first (control) test, subjects exercised at ∼85% of maximum while arterial blood gases and work of breathing were assessed. Subsequent constant load exercise tests were iso‐time and iso‐work rate, but with EIAH prevented by inspiring hyperoxic gas or work of breathing reduced via a proportional assist ventilator (PAV). Quadriceps fatigue was assessed by measuring force in response to femoral nerve stimulation. For both sexes, quadriceps force was equally reduced after the control trial (−27 ± 2% baseline) and was attenuated with hyperoxia and PAV (−18 ± 1 and −17 ± 2% baseline, P < 0.01, respectively), with no sex difference. EIAH was similar between the sexes, and regardless of sex, subjects with the lowest oxyhaemoglobin saturation during the control test had the greatest quadriceps fatigue attenuation with hyperoxia (r2 = 0.79, P < 0.0001). For the PAV trial, despite reducing the work of breathing to a greater degree in men (men: 60 ± 5, women: 75 ± 6% control, P < 0.05), the attenuation of quadriceps fatigue was similar between the sexes (36 ± 4 vs. 37 ± 7%). Owing to a greater relative V̇O2 of the respiratory muscles in women, less of a change in work of breathing is needed to reduce quadriceps fatigue.
    June 19, 2017   doi: 10.1113/JP274068   open full text
  • Functional expression of calcium‐permeable Canonical Transient Receptor Potential 4‐containing channels promotes migration of medulloblastoma cells.
    Wei‐Chun Wei, Wan‐Chen Huang, Yu‐Ping Lin, Esther B. E. Becker, Olaf Ansorge, Veit Flockerzi, Daniele Conti, Giovanna Cenacchi, Maike D. Glitsch.
    The Journal of Physiology. June 19, 2017
    Aberrant intracellular Ca2+ signalling contributes to the formation and progression of a range of distinct pathologies including cancers. Rises in intracellular Ca2+ concentration occur in response to Ca2+ influx through plasma membrane channels and Ca2+ release from intracellular Ca2+ stores, which can be mobilised in response to activation of cell surface receptors. OGR1 (Ovarian cancer G protein coupled Receptor 1, aka GPR68) is a proton‐sensing Gq‐coupled receptor that is most highly expressed in cerebellum. Medulloblastoma (MB) is the most common paediatric brain tumour that arises from cerebellar precursor cells. We find that nine distinct human MB samples all express OGR1. In both normal granule cells and the transformed human cerebellar granule cell line DAOY, OGR1 promotes expression of the proton‐potentiated member of the Canonical Transient Receptor Potential (TRPC) channel family, TRPC4. Consistent with a role for TRPC4 in MB, we find that all MB samples also express TRPC4. In DAOY cells, activation of TRPC4‐containing channels resulted in large Ca2+ influx and enhanced migration, while in normal cerebellar granule (precursor) cells and MB cells not derived from granule precursors, only small levels of Ca2+ influx and no enhanced migration was observed. Our results suggest that OGR1‐dependent increases in TRPC4 expression may favour formation of highly Ca2+‐permeable TRPC4‐containing channels that promote transformed granule cell migration. Increased motility of cancer cells is a prerequisite for cancer invasion and metastasis, and our findings may point towards a key role for TRPC4 in progression of certain types of MB. This article is protected by copyright. All rights reserved
    June 19, 2017   doi: 10.1113/JP274659   open full text
  • Chemosensitive Phox2b‐expressing neurons are crucial for hypercapnic ventilatory response in the nucleus tractus solitarius.
    Congrui Fu, Jinyu Xue, Ri Wang, Jinting Chen, Lan Ma, Yixian Liu, Xuejiao Wang, Fang Guo, Yi Zhang, Xiangjian Zhang, Sheng Wang.
    The Journal of Physiology. June 16, 2017
    Key points Central hypercapnic hypoventilation is highly prevalent in children suffering from congenital central hypoventilation syndrome (CCHS). Mutations of the gene for paired‐like homeobox 2b (Phox2b) are aetiologically associated with CCHS and Phox2b is present in central components of respiratory chemoreflex, such as the nucleus tractus solitarius (NTS). Injection of the neurotoxin substance P‐saporin into NTS destroys Phox2b‐expressing neurons. Impaired hypercapnic ventilatory response caused by this neurotoxin is attributable to a loss of CO2‐sensitive Phox2b‐expressing NTS neurons. A subgroup of Phox2b‐expressing neurons exhibits intrinsic chemosensitivity. A background K+ channel‐like current is partially responsible for such chemosensitivity in Phox2b‐expressing neurons. The present study helps us better understand the mechanism of respiratory deficits in CCHS and potentially locates a brainstem site for development of precise clinical intervention. Abstract The nucleus tractus solitarius (NTS) neurons have been considered to function as central respiratory chemoreceptors. However, the common molecular marker defined for these neurons remains unknown. The present study investigated whether paired‐like homeobox 2b (Phox2b)‐expressing NTS neurons are recruited in hypercapnic ventilatory response (HCVR) and whether these neurons exhibit intrinsic chemosensitivity. HCVR was assessed using whole body plethysmography and neuronal chemosensitivity was examined by patch clamp recordings in brainstem slices or dissociated neurons from Phox2b‐EGFP transgenic mice. Injection of the neurotoxin substance P‐saporin (SSP‐SAP) into NTS destroyed Phox2b‐expressing neurons. Minute ventilation and tidal volume were both reduced by 13% during exposure to 8% CO2 in inspired air when ∼13% of the Phox2b‐expressing neurons were eliminated. However, a loss of ∼18% of these neurons was associated with considerable decreases in minute ventilation by ≥18% and in tidal volume by≥22% when challenged by ≥4% CO2. In both cases, breathing frequency was unaffected. Most CO2‐activated neurons were immunoreactive to Phox2b. In brainstem slices, ∼43% of Phox2b‐expressing neurons from Phox2b‐EGFP mice displayed a sustained or transient increase in firing rate during physiological acidification (pH 7.0 or 8% CO2). Such a response was also present in dissociated neurons in favour of an intrinsic property. In voltage clamp recordings, a background K+ channel‐like current was found in a subgroup of Phox2b‐expressing neurons. Thus, the respiratory deficits caused by injection of SSP‐SAP into the NTS are attributable to proportional lesions of CO2/H+‐sensitive Phox2b‐expressing neurons.
    June 16, 2017   doi: 10.1113/JP274437   open full text
  • An autocrine ATP release mechanism regulates basal ciliary activity in airway epithelium.
    Karla Droguett, Mariana Rios, Daniela V. Carreño, Camilo Navarrete, Christian Fuentes, Manuel Villalón, Nelson P. Barrera.
    The Journal of Physiology. June 15, 2017
    Key points Extracellular ATP, in association with [Ca2+]i regulation, is required to maintain basal ciliary beat frequency. Increasing extracellular ATP levels increases ciliary beating in airway epithelial cells, maintaining a sustained response by inducing the release of additional ATP. Extracellular ATP levels in the millimolar range, previously associated with pathophysiological conditions of the airway epithelium, produce a transient arrest of ciliary activity. The regulation of ciliary beat frequency is dependent on ATP release by hemichannels (connexin/pannexin) and P2X receptor activation, the blockage of which may even stop ciliary movement. The force exerted by cilia, measured by atomic force microscopy, is reduced following extracellular ATP hydrolysis. This result complements the current understanding of the ciliary beating regulatory mechanism, with special relevance to inflammatory diseases of the airway epithelium that affect mucociliary clearance. Abstract Extracellular nucleotides, including ATP, are locally released by the airway epithelium and stimulate ciliary activity in a [Ca2+]i‐dependent manner after mechanical stimulation of ciliated cells. However, it is unclear whether the ATP released is involved in regulating basal ciliary activity and mediating changes in ciliary activity in response to chemical stimulation. In the present study, we evaluated ciliary beat frequency (CBF) and ciliary beating forces in primary cultures from mouse tracheal epithelium, using videomicroscopy and atomic force microscopy (AFM), respectively. Extracellular ATP levels and [Ca2+]i were measured by luminometric and fluorimetric assays, respectively. Uptake of ethidium bromide was measured to evaluate hemichannel functionality. We show that hydrolysis of constitutive extracellular ATP levels with apyrase (50 U ml−1) reduced basal CBF by 45% and ciliary force by 67%. The apyrase effect on CBF was potentiated by carbenoxolone, a hemichannel inhibitor, and oxidized ATP, an antagonist used to block P2X7 receptors, which reduced basal CBF by 85%. Additionally, increasing extracellular ATP levels (0.1–100 μm) increased CBF, maintaining a sustained response that was suppressed in the presence of carbenoxolone. We also show that high levels of ATP (1 mm), associated with inflammatory conditions, lowered basal CBF by reducing [Ca2+]i and hemichannel functionality. In summary, we provide evidence indicating that airway epithelium ATP release is the molecular autocrine mechanism regulating basal ciliary activity and is also the mediator of the ciliary response to chemical stimulation.
    June 15, 2017   doi: 10.1113/JP273996   open full text
  • Systolic [Ca2+]i regulates diastolic levels in rat ventricular myocytes.
    R. Sankaranarayanan, K. Kistamas, D. J. Greensmith, L. A. Venetucci, D. A. Eisner.
    The Journal of Physiology. June 15, 2017
    [Ca2+]i must be low enough in diastole so that the ventricle is relaxed and can refill with blood. Interference with this will impair relaxation. The factors responsible for regulation of diastolic [Ca2+]i, in particular the relative roles of the sarcoplasmic reticulum (SR) and surface membrane are unclear. We investigated the effects on diastolic [Ca2+]i that result from the changes of Ca cycling known to occur in heart failure. Experiments were performed using Fluo‐3 in voltage‐clamped rat ventricular myocytes. Increasing stimulation frequency increased diastolic [Ca2+]i. This increase of [Ca2+]i was larger when SR function was impaired either by making the RyR leaky (with caffeine or ryanodine) or by decreasing SERCA activity with thapsigargin. The increase of diastolic [Ca2+]i produced by interfering with the SR was accompanied by a decrease of the amplitude of the systolic Ca transient such that there was no change of time‐averaged [Ca2+]i. Time‐averaged [Ca2+]i was increased by β‐adrenergic stimulation with isoprenaline and increased in a saturating manner with increased stimulation frequency; average [Ca2+]i was a linear function of Ca entry per unit time. Diastolic and time‐averaged [Ca2+]i were decreased by decreasing the L‐type Ca current (with 50 μm cadmium chloride). We conclude that diastolic [Ca2+]i is controlled by the balance between Ca entry and efflux during systole. Furthermore, manoeuvres which decrease the amplitude of the Ca transient (without decreasing Ca influx) will therefore increase diastolic [Ca2+]i. This identifies a novel mechanism whereby changes of the amplitude of the systolic Ca transient control diastolic [Ca2+]i. This article is protected by copyright. All rights reserved
    June 15, 2017   doi: 10.1113/JP274366   open full text
  • In vivo analysis of synaptic activity in cerebellar nuclei neurons unravels the efficacy of excitatory inputs.
    Yasmin Yarden‐Rabinowitz, Yosef Yarom.
    The Journal of Physiology. June 15, 2017
    It is commonly agreed that the main function of the cerebellar system is to provide well‐timed signals used for the execution of motor commands or prediction of sensory inputs. This function is manifested as a temporal sequence of spiking that should be expressed in the cerebellar nuclei projection neurons (CN). Whether spiking activity is generated by excitation or release from inhibition is still a hotly debated issue. In an attempt to resolve this debate, we recorded intracellularly from CN neurons in anaesthetized mice and performed an analysis of synaptic activity that yielded a number of important observations. First, we demonstrate that CN neurons can be classified into four groups. Second, shape‐index plots of the excitatory events suggest that they are distributed all over the dendritic tree. Third, the rise time of excitatory events is linearly related to the amplitude, suggesting that all excitatory events contribute equally to the generation of action potentials. Fourth, we identified a temporal pattern of spontaneous excitatory events that represent climbing fibre inputs and confirm the results by direct stimulation and analysis on harmaline evoked activity. Finally, we demonstrate that the probability of excitatory inputs to generate an AP is 0.1 yet half of the APs are generated by excitatory events. Moreover, the probability of a presumably spontaneous climbing fibre input to generate an AP is higher, reaching mean population value of 0.15. In view of these results, the mode of synaptic integration at the level of the CN should be re‐considered. This article is protected by copyright. All rights reserved
    June 15, 2017   doi: 10.1113/JP274115   open full text
  • Endothelial mechanotransduction proteins and vascular function are altered by dietary sucrose supplementation in healthy young male subjects.
    Lasse Gliemann, Nicolai Rytter, Mads Lindskrog, Martina H. Lundberg Slingsby, Thorbjörn Åkerström, Lykke Sylow, Erik A. Richter, Ylva Hellsten.
    The Journal of Physiology. June 15, 2017
    Endothelial mechanotransduction is important for vascular function but alterations and activation of vascular mechanosensory proteins have not been investigated in humans. In endothelial cell culture, simple sugars effectively impair mechanosensor proteins. To study mechanosensor‐ and vascular function in humans, twelve young healthy male subjects supplemented their diet with 3 × 75 g sucrose day−1 for 14 days in a randomized cross‐over design. Before and after the intervention period, the hyperemic response to passive lower leg movement and active knee extensor exercise was determined by ultrasound doppler. A muscle biopsy was obtained from the thigh muscle before and after acute passive leg movement, to asses the protein amount and phosphorylation status of mechanosensory proteins and NADPH oxidase. The sucrose intervention led to a reduced flow response to passive movement (by 17 ± 2 %) and to 12 watts of active exercise (by 9 ± 1 %), indicating impaired vascular function. Reduced flow response to passive and active exercise was paralleled by a significant upregulation of Platelet endothelial cell adhesion molecule (PECAM‐1), endothelial nitric oxide synthase, NADPH oxidase and the Rho family GTPase Rac1 protein expression in the muscle tissue as well as an increased basal phosphorylation status of Vascular endothelial growth factor receptor 2 and a reduced phosphorylation status of PECAM‐1. The phosphorylation status was not acutely altered with passive leg movement. These findings indicate that regular intake of high levels of sucrose can impair vascular mechanotransduction, increase the oxidative stress potential and suggest that dietary excessive sugar intake may contribute to the development of vascular disease. This article is protected by copyright. All rights reserved
    June 15, 2017   doi: 10.1113/JP274623   open full text
  • Conservation of alternative splicing in sodium channels reveals evolutionary focus on release from inactivation and structural insights into gating.
    A. Liavas, G. Lignani, S. Schorge.
    The Journal of Physiology. June 15, 2017
    Voltage‐gated sodium channels are critical for neuronal activity, and highly intolerant to variation. Even mutations that cause subtle changes in the activity these channels are sufficient to cause devastating inherited neurological diseases, such as epilepsy and pain. However, these channels do vary in healthy tissue. Alternative splicing modifies sodium channels, but the functional relevance and adaptive significance of this splicing remain poorly understood. Here we use a conserved alternate exon encoding part of the first domain of sodium channels to compare how splicing modifies different channels, and to ask whether the functional consequences of this splicing have been preserved in different genes. Although the splicing event is highly conserved, one splice variant has been selectively removed from Nav1.1 in multiple mammalian species, suggesting that the functional variation in Nav1.1 is less well‐tolerated. We show for three human channels (Nav1.1, Nav1.2 and Nav1.7) splicing modifies the return from inactivated to deactivated states, and the differences between splice variants are occluded by antiepileptic drugs that bind to and stabilize inactivated states. A model based on structural data can replicate these changes, and indicates that splicing may exploit a distinct role of the first domain to change channel availability, and that the first domain of all three sodium channels plays a role in determining the rate at which the inactivation domain dissociates. Taken together, our data suggest that the stability of inactivated states is under tight evolutionary control, but that in Nav1.1 faster recovery from inactivation is associated with negative selection in mammals. This article is protected by copyright. All rights reserved
    June 15, 2017   doi: 10.1113/JP274693   open full text
  • A multiscale computational modelling approach predicts mechanisms of female sex risk in the setting of arousal‐induced arrhythmias.
    Pei‐Chi Yang, Laura L. Perissinotti, Fernando López‐Redondo, Yibo Wang, Kevin R. DeMarco, Mao‐Tsuen Jeng, Igor Vorobyov, Robert D. Harvey, Junko Kurokawa, Sergei Y. Noskov, Colleen E. Clancy.
    The Journal of Physiology. June 14, 2017
    Key points This study represents a first step toward predicting mechanisms of sex‐based arrhythmias that may lead to important developments in risk stratification and may inform future drug design and screening. We undertook simulations to reveal the conditions (i.e. pacing, drugs, sympathetic stimulation) required for triggering and sustaining reentrant arrhythmias. Using the recently solved cryo‐EM structure for the Eag‐family channel as a template, we revealed potential interactions of oestrogen with the pore loop hERG mutation (G604S). Molecular models suggest that oestrogen and dofetilide blockade can concur simultaneously in the hERG channel pore. Abstract Female sex is a risk factor for inherited and acquired long‐QT associated torsade de pointes (TdP) arrhythmias, and sympathetic discharge is a major factor in triggering TdP in female long‐QT syndrome patients. We used a combined experimental and computational approach to predict ‘the perfect storm’ of hormone concentration, IKr block and sympathetic stimulation that induces arrhythmia in females with inherited and acquired long‐QT. More specifically, we developed mathematical models of acquired and inherited long‐QT syndrome in male and female ventricular human myocytes by combining effects of a hormone and a hERG blocker, dofetilide, or hERG mutations. These ‘male’ and ‘female’ model myocytes and tissues then were used to predict how various sex‐based differences underlie arrhythmia risk in the setting of acute sympathetic nervous system discharge. The model predicted increased risk for arrhythmia in females when acute sympathetic nervous system discharge was applied in the settings of both inherited and acquired long‐QT syndrome. Females were predicted to have protection from arrhythmia induction when progesterone is high. Males were protected by the presence of testosterone. Structural modelling points towards two plausible and distinct mechanisms of oestrogen action enhancing torsadogenic effects: oestradiol interaction with hERG mutations in the pore loop containing G604 or with common TdP‐related blockers in the intra‐cavity binding site. Our study presents findings that constitute the first evidence linking structure to function mechanisms underlying female dominance of arousal‐induced arrhythmias.
    June 14, 2017   doi: 10.1113/JP273142   open full text
  • Baroreflex dysfunction and augmented sympathetic nerve responses during mental stress in veterans with post‐traumatic stress disorder.
    Jeanie Park, Paul J. Marvar, Peizhou Liao, Melanie L. Kankam, Seth D. Norrholm, Ryan M. Downey, S. Ashley McCullough, Ngoc‐Anh Le, Barbara O. Rothbaum.
    The Journal of Physiology. June 14, 2017
    Key points Patients with post‐traumatic stress disorder (PTSD) are at a significantly higher risk of developing hypertension and cardiovascular disease. The mechanisms underlying this increased risk are not known. Studies have suggested that PTSD patients have an overactive sympathetic nervous system (SNS) that could contribute to cardiovascular risk; however, sympathetic function has not previously been rigorously evaluated in PTSD patients. Using direct measurements of sympathetic nerve activity and pharmacological manipulation of blood pressure, we show that veterans with PTSD have augmented SNS and haemodynamic reactivity during both combat‐related and non‐combat related mental stress, impaired sympathetic and cardiovagal baroreflex sensitivity, and increased inflammation. Identifying the mechanisms contributing to increased cardiovascular (CV) risk in PTSD will pave the way for developing interventions to improve sympathetic function and reduce CV risk in these patients. Abstract Post‐traumatic stress disorder (PTSD) is associated with increased cardiovascular (CV) risk. We tested the hypothesis that PTSD patients have augmented sympathetic nervous system (SNS) and haemodynamic reactivity during mental stress, as well as impaired arterial baroreflex sensitivity (BRS). Fourteen otherwise healthy Veterans with combat‐related PTSD were compared with 14 matched Controls without PTSD.  Muscle sympathetic nerve activity (MSNA), continuous blood pressure (BP) and electrocardiography were measured at baseline, as well as during two types of mental stress:  combat‐related mental stress using virtual reality combat exposure (VRCE) and non‐combat related stress using mental arithmetic (MA). A cold pressor test (CPT) was administered for comparison. BRS was tested using pharmacological manipulation of BP via the Modified Oxford technique at rest and during VRCE. Blood samples were analysed for inflammatory biomarkers. Baseline characteristics, MSNA and haemodynamics were similar between the groups. In PTSD vs. Controls, MSNA (+8.2 ± 1.0 vs. +1.2 ± 1.3 bursts min–1, P < 0.001) and heart rate responses (+3.2 ± 1.1 vs. −2.3 ± 1.0 beats min–1, P = 0.003) were significantly augmented during VRCE.  Similarly, in PTSD vs. Controls, MSNA (+21.0 ± 2.6 vs. +6.7 ± 1.5 bursts min–1, P < 0.001) and diastolic BP responses (+6.3 ± 1.0 vs. +3.5 ± 1.0 mmHg, P = 0.011) were significantly augmented during MA but not during CPT (P = not significant). In the PTSD group, sympathetic BRS (–1.2 ± 0.2 vs. –2.0 ± 0.3 burst incidence mmHg−1, P = 0.026) and cardiovagal BRS (9.5 ± 1.4 vs. 23.6 ± 4.3 ms mmHg−1, P = 0.008) were significantly blunted at rest. PTSD patients had significantly higher highly sensitive‐C‐reactive protein levels compared to Controls (2.1 ± 0.4 vs. 1.0 ± 0.3 mg L−1, P = 0.047). Augmented SNS and haemodynamic responses to mental stress, blunted BRS and inflammation may contribute to an increased CV risk in PTSD.
    June 14, 2017   doi: 10.1113/JP274269   open full text
  • Facilitation of mossy fibre‐driven spiking in the cerebellar nuclei by the synchrony of inhibition.
    Yeechan Wu, Indira M. Raman.
    The Journal of Physiology. June 11, 2017
    Key points Large premotor neurons of the cerebellar nuclei (CbN cells) integrate synaptic inhibition from Purkinje neurons and synaptic excitation from mossy fibres to generate cerebellar output. We find that mossy fibre inputs to CbN cells generate unitary AMPA receptor EPSCs of ∼1 nS that decay in ∼1 ms and mildly voltage‐dependent NMDA receptor EPSCs of ∼0.6 nS that decay in ∼7 ms. A few hundred mossy fibres active at a few tens of spikes s−1 must converge on CbN cells to generate physiological CbN spike rates (∼60 spikes s−1) during convergent inhibition from spontaneously active Purkinje cells. Dynamic clamp studies in cerebellar slices from weanling mice demonstrate that synaptic excitation from mossy fibres becomes more effective at increasing the rate of CbN cell spiking when the coherence (synchrony) of convergent inhibition is increased. Abstract Large projection neurons of the cerebellar nuclei (CbN cells), whose activity generates movement, are inhibited by Purkinje cells and excited by mossy fibres. The high convergence, firing rates and strength of Purkinje inputs predict powerful suppression of CbN cell spiking, raising the question of what activity patterns favour excitation over inhibition. Recording from CbN cells at near‐physiological temperatures in cerebellar slices from weanling mice, we measured the amplitude, kinetics, voltage dependence and short‐term plasticity of mossy fibre‐mediated EPSCs. Unitary EPSCs were small and brief (AMPA receptor, ∼1 nS, ∼1 ms; NMDA receptor, ∼0.6 nS, ∼7 ms) and depressed moderately. Using these experimentally measured parameters, we applied combinations of excitation and inhibition to CbN cells with dynamic clamp. Because Purkinje cells can fire coincident simple spikes during cerebellar behaviours, we varied the proportion (0–20 of 40) and precision (0–4 ms jitter) of synchrony of inhibitory inputs, along with the rates (0–100 spikes s−1) and number (0–800) of excitatory inputs. Even with inhibition constant, when inhibitory synchrony was higher, excitation increased CbN cell firing rates more effectively. Partial inhibitory synchrony also dictated CbN cell spike timing, even with physiological rates of excitation. These effects were present with ≥10 inhibitory inputs active within 2–4 ms of each other. Conversely, spiking was most effectively suppressed when inhibition was maximally asynchronous. Thus, the rate and relative timing of Purkinje‐mediated inhibition set the rate and timing of cerebellar output. The results suggest that increased coherence of Purkinje cell activity can facilitate mossy fibre‐driven spiking by CbN cells, in turn driving movements.
    June 11, 2017   doi: 10.1113/JP274321   open full text
  • Large‐conductance Ca2+‐activated K+ channel activation by apical P2Y receptor agonists requires hydrocortisone in differentiated airway epithelium.
    Nathan A. Zaidman, Angela Panoskaltsis‐Mortari, Scott M. O'Grady.
    The Journal of Physiology. June 11, 2017
    Key points Hydrocortisone (HC) is required for activation of large‐conductance Ca2+‐activated K+ current (BK) by purinergic receptor agonists. HC reduces insertion of the stress‐regulated exon (STREX) in the KCNMA1 gene, permitting protein kinase C (PKC)‐dependent channel activation. Overlapping and unique purinergic signalling regions exist at the apical border of differentiated surface cells. BK channels localize in the cilia of surface cells. Abstract In the present study we investigated the role of hydrocortisone (HC) on uridine‐5ʹ‐triphosphate (UTP)‐stimulated ion transport in differentiated, pseudostratified epithelia derived from normal human bronchial basal cells. The presence of a UTP‐stimulated, paxilline‐sensitive large‐conductance Ca2+‐activated K+ (BK) current was demonstrated in control epithelia but was not stimulated in epithelia differentiated in the absence of HC (HC0). Addition of the BK channel opener NS11021 directly activated channels in control epithelia; however, under HC0 conditions, activation only occurred when UTP was added after NS11021. The PKC inhibitors GF109203x and Gö6983 blocked BK activation by UTP in control epithelia, suggesting that PKC‐mediated phosphorylation plays a permissive role in purinoceptor‐stimulated BK activation. Moreover, HC0 epithelia expressed significantly more KCNMA1 containing the stress‐regulated exon (STREX), a splice‐variant of the α‐subunit that displays altered channel regulation by phosphorylation, compared to control epithelia. Furthermore, BK channels as well as purinergic receptors were shown to localize in unique and overlapping domains at the apical membrane of ciliated surface cells. These results establish a previously unrecognized role for glucocorticoids in regulation of BK channels in airway epithelial cells.
    June 11, 2017   doi: 10.1113/JP274200   open full text
  • Mammalian target of rapamycin complex 2 regulates muscle glucose uptake during exercise in mice.
    Maximilian Kleinert, Benjamin L. Parker, Andreas M. Fritzen, Jonas R. Knudsen, Thomas E. Jensen, Rasmus Kjøbsted, Lykke Sylow, Markus Ruegg, David E. James, Erik A. Richter.
    The Journal of Physiology. June 11, 2017
    Key points Exercise is a potent physiological stimulus to clear blood glucose from the circulation into skeletal muscle. The mammalian target of rapamycin complex 2 (mTORC2) is an important regulator of muscle glucose uptake in response to insulin stimulation. Here we report for the first time that the activity of mTORC2 in mouse muscle increases during exercise. We further show that glucose uptake during exercise is decreased in mouse muscle that lacks mTORC2 activity. We also provide novel identifications of new mTORC2 substrates during exercise in mouse muscle. Abstract Exercise increases glucose uptake into insulin‐resistant muscle. Thus, elucidating the exercise signalling network in muscle may uncover new therapeutic targets. The mammalian target of rapamycin complex 2 (mTORC2), a regulator of insulin‐controlled glucose uptake, has been reported to interact with ras‐related C3 botulinum toxin substrate 1 (Rac1), which plays a role in exercise‐induced glucose uptake in muscle. Therefore, we tested the hypothesis that mTORC2 activity is necessary for muscle glucose uptake during treadmill exercise. We used mice that specifically lack mTORC2 signalling in muscle by deletion of the obligatory mTORC2 component Rictor (Ric mKO). Running capacity and running‐induced changes in blood glucose, plasma lactate and muscle glycogen levels were similar in wild‐type (Ric WT) and Ric mKO mice. At rest, muscle glucose uptake was normal, but during running muscle glucose uptake was reduced by 40% in Ric mKO mice compared to Ric WT mice. Running increased muscle phosphorylated 5′ AMP‐activated protein kinase (AMPK) similarly in Ric WT and Ric mKO mice, and glucose transporter type 4 (GLUT4) and hexokinase II (HKII) protein expressions were also normal in Ric mKO muscle. The mTORC2 substrate, phosphorylated protein kinase C α (PKCα), and the mTORC2 activity readout, phosphorylated N‐myc downstream regulated 1 (NDRG1) protein increased with running in Ric WT mice, but were not altered by running in Ric mKO muscle. Quantitative phosphoproteomics uncovered several additional potential exercise‐dependent mTORC2 substrates, including contractile proteins, kinases, transcriptional regulators, actin cytoskeleton regulators and ion‐transport proteins. Our study suggests that mTORC2 is a component of the exercise signalling network that regulates muscle glucose uptake and we provide a resource of new potential members of the mTORC2 signalling network.
    June 11, 2017   doi: 10.1113/JP274203   open full text
  • Evolved changes in the intracellular distribution and physiology of muscle mitochondria in high‐altitude native deer mice.
    Sajeni Mahalingam, Grant B. McClelland, Graham R. Scott.
    The Journal of Physiology. June 07, 2017
    Key points Mitochondrial function changes over time at high altitudes, but the potential benefits of these changes for hypoxia resistance remains unclear. We used high‐altitude‐adapted populations of deer mice, which exhibit enhanced aerobic performance in hypoxia, to examine whether changes in mitochondrial physiology or intracellular distribution in the muscle contribute to hypoxia resistance. Permeabilized muscle fibres from the gastrocnemius muscle had higher respiratory capacities in high‐altitude mice than in low‐altitude mice. Highlanders also had higher mitochondrial volume densities, due entirely to an enriched abundance of subsarcolemmal mitochondria, such that more mitochondria were situated near the cell membrane and adjacent to capillaries. There were several effects of hypoxia acclimation on mitochondrial function, some of which were population specific, but they differed from the evolved changes in high‐altitude natives, which probably provide a better indication of adaptive traits that improve performance and hypoxia resistance at high altitudes. Abstract High‐altitude natives that have evolved to live in hypoxic environments provide a compelling system to understand how animals can overcome impairments in oxygen availability. We examined whether these include changes in mitochondrial physiology or intracellular distribution that contribute to hypoxia resistance in high‐altitude deer mice (Peromyscus maniculatus). Mice from populations native to high and low altitudes were born and raised in captivity, and as adults were acclimated to normoxia or hypobaric hypoxia (equivalent to 4300 m elevation). We found that highlanders had higher respiratory capacities in the gastrocnemius (but not soleus) muscle than lowlanders (assessed using permeabilized fibres with single or multiple inputs to the electron transport system), due in large part to higher mitochondrial volume densities in the gastrocnemius. The latter was attributed to an increased abundance of subsarcolemmal (but not intermyofibrillar) mitochondria, such that more mitochondria were situated near the cell membrane and adjacent to capillaries. Hypoxia acclimation had no significant effect on these population differences, but it did increase mitochondrial cristae surface densities of mitochondria in both populations. Hypoxia acclimation also altered the physiology of isolated mitochondria by affecting respiratory capacities and cytochrome c oxidase activities in population‐specific manners. Chronic hypoxia decreased the release of reactive oxygen species by isolated mitochondria in both populations. There were subtle differences in O2 kinetics between populations, with highlanders exhibiting increased mitochondrial O2 affinity or catalytic efficiency in some conditions. Our results suggest that evolved changes in mitochondrial physiology in high‐altitude natives are distinct from the effects of hypoxia acclimation, and probably provide a better indication of adaptive traits that improve performance and hypoxia resistance at high altitudes.
    June 07, 2017   doi: 10.1113/JP274130   open full text
  • Structure and function of human muscle fibres and muscle proteome in physically active older men.
    Lorenza Brocca, Jamie S. McPhee, Emanuela Longa, Monica Canepari, Olivier Seynnes, Giuseppe Vito, Maria Antonietta Pellegrino, Marco Narici, Roberto Bottinelli.
    The Journal of Physiology. June 05, 2017
    Key points Loss of muscle mass and strength in the growing population of elderly people is a major health concern for modern societies. This condition, termed sarcopenia, is a major cause of falls and of the subsequent increase in morbidity and mortality. Despite numerous studies on the impact of ageing on individual muscle fibres, the contribution of single muscle fibre adaptations to ageing‐induced atrophy and functional impairment is still unsettled. The level of physical function and disuse is often associated with ageing. We studied relatively healthy older adults in order to understand the effects of ageing per se without the confounding impact of impaired physical function. We found that in healthy ageing, structural and functional alterations of muscle fibres occur. Protein post‐translational modifications, oxidation and phosphorylation contribute to such alterations more than loss of myosin and other muscle protein content. Abstract Contradictory results have been reported on the impact of ageing on structure and functions of skeletal muscle fibres, likely to be due to a complex interplay between ageing and other phenomena such as disuse and diseases. Here we recruited healthy, physically and socially active young (YO) and elderly (EL) men in order to study ageing per se without the confounding effects of impaired physical function. In vivo analyses of quadriceps and in vitro analyses of vastus lateralis muscle biopsies were performed. In EL subjects, our results show that (i) quadriceps volume, maximum voluntary contraction isometric torque and patellar tendon force were significantly lower; (ii) muscle fibres went through significant atrophy and impairment of specific force (isometric force/cross‐sectional area) and unloaded shortening velocity; (iii) myosin/actin ratio and myosin content in individual muscle fibres were not altered; (iv) the muscle proteome went through quantitative adaptations, namely an up‐regulation of the content of several groups of proteins among which were myofibrillar proteins and antioxidant defence systems; (v) the muscle proteome went through qualitative adaptations, namely phosphorylation of several proteins, including myosin light chain‐2 slow and troponin T and carbonylation of myosin heavy chains. The present results indicate that impairment of individual muscle fibre structure and function is a major feature of ageing per se and that qualitative adaptations of muscle proteome are likely to be more involved than quantitative adaptations in determining such a phenomenon.
    June 05, 2017   doi: 10.1113/JP274148   open full text
  • Convergent ERK1/2, p38 and JNK mitogen activated protein kinases (MAPKs) signalling mediate catecholoestradiol‐induced proliferation of ovine uterine artery endothelial cells.
    Rosalina Villalon Landeros, Sheikh O. Jobe, Gabrielle Aranda‐Pino, Gladys E. Lopez, Jing Zheng, Ronald R. Magness.
    The Journal of Physiology. June 05, 2017
    Key points The catechol metabolites of 17β‐oestradiol (E2β), 2‐hydroxyoestradiol (2‐OHE2) and 4‐hydroxyoestradiol (4‐OHE2), stimulate proliferation of pregnancy‐derived ovine uterine artery endothelial cells (P‐UAECs) through β‐adrenoceptors (β‐ARs) and independently of the classic oestrogen receptors (ERs). Herein we show that activation of ERK1/2, p38 and JNK mitogen activated protein kinases (MAPKs) is necessary for 2‐OHE2‐ and 4‐OHE2‐induced P‐UAEC proliferation, as well as proliferation induced by the parent hormone E2β and other β‐AR signalling hormones (i.e. catecholamines). Conversely, although 2‐OHE2 and 4‐OHE2 rapidly activate phosphatidylinositol 3‐kinase (PI3K), its activation is not involved in catecholoestradiol‐induced P‐UAEC proliferation. We also show for the first time the signalling mechanisms involved in catecholoestradiol‐induced P‐UAEC proliferation; which converge at the level of MAPKs with the signalling mechanisms mediating E2β‐ and catecholamine‐induced proliferation. The present study advances our understanding of the complex signalling mechanisms involved in regulating uterine endothelial cell proliferation during pregnancy. Abstract Previously we demonstrated that the biologically active metabolites of 17β‐oestradiol, 2‐hydroxyoestradiol (2‐OHE2) and 4‐hydroxyoestradiol (4‐OHE2), stimulate pregnancy‐specific proliferation of uterine artery endothelial cells derived from pregnant (P‐UAECs), but not non‐pregnant ewes. However, unlike 17β‐oestradiol, which induces proliferation via oestrogen receptor‐β (ER‐β), the catecholoestradiols mediate P‐UAEC proliferation via β‐adrenoceptors (β‐AR) and independently of classic oestrogen receptors. Herein, we aim to further elucidate the signalling mechanisms involved in proliferation induced by catecholoestradiols in P‐UAECs. P‐UAECs were treated with 2‐OHE2 and 4‐OHE2 for 0, 0.25, 0.5, 1, 2, 4, 12 and 24 h, to analyse activation of mitogen activated protein kinases (MAPKs) and phosphatidylinositol 3‐kinase (PI3K)–AKT. Specific inhibitors for ERK1/2 MAPK (PD98059), p38 MAPK (SB203580), JNK MAPK (SP600125), or PI3K (LY294002) were used to determine the involvement of individual kinases in agonist‐induced P‐UAEC proliferation. 2‐OHE2 and 4‐OHE2 stimulated biphasic phosphorylation of ERK1/2, slow p38 and JNK phosphorylation over time, and rapid monophasic AKT phosphorylation. Furthermore, ERK1/2, p38 and JNK MAPKs, but not PI3K, were individually necessary for catecholoestradiol‐induced proliferation. In addition, when comparing the signalling mechanisms of the catecholoestradiols, to 17β‐oestradiol and catecholamines, we observed that convergent MAPKs signalling pathways facilitate P‐UAEC proliferation induced by all of these hormones. Thus, all three members of the MAPK family mediate the mitogenic effects of catecholoestradiols in the endothelium during pregnancy. Furthermore, the convergent signalling of MAPKs involved in catecholoestradiol‐, 17β‐oestradiol‐ and catecholamine‐induced endothelial cell proliferation may be indicative of unappreciated evolutionary functional redundancy to facilitate angiogenesis and ensure maintenance of uterine blood flow during pregnancy.
    June 05, 2017   doi: 10.1113/JP274119   open full text
  • The angiotensin II receptor type 1b is the primary sensor of intraluminal pressure in cerebral artery smooth muscle cells.
    Paulo W. Pires, Eun‐A. Ko, Harry A.T. Pritchard, Michael Rudokas, Evan Yamasaki, Scott Earley.
    The Journal of Physiology. June 01, 2017
    Key points The angiotensin II receptor type 1b (AT1Rb) is the primary sensor of intraluminal pressure in cerebral arteries. Pressure or membrane‐stretch induced stimulation of AT1Rb activates the TRPM4 channel and results in inward transient cation currents that depolarize smooth muscle cells, leading to vasoconstriction. Activation of either AT1Ra or AT1Rb with angiotensin II stimulates TRPM4 currents in cerebral artery myocytes and vasoconstriction of cerebral arteries. The expression of AT1Rb mRNA is ∼30‐fold higher than AT1Ra in whole cerebral arteries and ∼45‐fold higher in isolated cerebral artery smooth muscle cells. Higher levels of expression are likely to account for the obligatory role of AT1Rb for pressure‐induced vasoconstriction. Abstract Myogenic vasoconstriction, which reflects the intrinsic ability of smooth muscle cells to contract in response to increases in intraluminal pressure, is critically important for the autoregulation of blood flow. In smooth muscle cells from cerebral arteries, increasing intraluminal pressure engages a signalling cascade that stimulates cation influx through transient receptor potential (TRP) melastatin 4 (TRPM4) channels to cause membrane depolarization and vasoconstriction. Substantial evidence indicates that the angiotensin II receptor type 1 (AT1R) is inherently mechanosensitive and initiates this signalling pathway. Rodents express two types of AT1R – AT1Ra and AT1Rb – and conflicting studies provide support for either isoform as the primary sensor of intraluminal pressure in peripheral arteries. We hypothesized that mechanical activation of AT1Ra increases TRPM4 currents to induce myogenic constriction of cerebral arteries. However, we found that development of myogenic tone was greater in arteries from AT1Ra knockout animals compared with controls. In patch‐clamp experiments using native cerebral arterial myocytes, membrane stretch‐induced cation currents were blocked by the TRPM4 inhibitor 9‐phenanthrol in both groups. Further, the AT1R blocker losartan (1 μm) diminished myogenic tone and blocked stretch‐induced cation currents in cerebral arteries from both groups. Activation of AT1R with angiotensin II (30 nm) also increased TRPM4 currents in smooth muscle cells and constricted cerebral arteries from both groups. Expression of AT1Rb mRNA was ∼30‐fold greater than AT1Ra in cerebral arteries, and knockdown of AT1Rb selectively diminished myogenic constriction. We conclude that AT1Rb, acting upstream of TRPM4 channels, is the primary sensor of intraluminal pressure in cerebral artery smooth muscle cells.
    June 01, 2017   doi: 10.1113/JP274310   open full text
  • Does the intercept of the heat–stress relation provide an accurate estimate of cardiac activation heat?
    Toan Pham, Kenneth Tran, Kimberley M Mellor, Anthony Hickey, Amelia Power, Marie‐Louise Ward, Andrew Taberner, June‐Chiew Han, Denis Loiselle.
    The Journal of Physiology. June 01, 2017
    Key points The heat of activation of cardiac muscle reflects the metabolic cost of restoring ionic homeostasis following a contraction. The accuracy of its measurement depends critically on the abolition of crossbridge cycling. We abolished crossbridge activity in isolated rat ventricular trabeculae by use of blebbistatin, an agent that selectively inhibits myosin II ATPase. We found cardiac activation heat to be muscle length independent and to account for 15–20% of total heat production at body temperature. We conclude that it can be accurately estimated at minimal muscle length. Abstract Activation heat arises from two sources during the contraction of striated muscle. It reflects the metabolic expenditure associated with Ca2+ pumping by the sarcoplasmic reticular Ca2+‐ATPase and Ca2+ translocation by the Na+/Ca2+ exchanger coupled to the Na+,K+‐ATPase. In cardiac preparations, investigators are constrained in estimating its magnitude by reducing muscle length to the point where macroscopic twitch force vanishes. But this experimental protocol has been criticised since, at zero force, the observed heat may be contaminated by residual crossbridge cycling activity. To eliminate this concern, the putative thermal contribution from crossbridge cycling activity must be abolished, at least at minimal muscle length. We achieved this using blebbistatin, a selective inhibitor of myosin II ATPase. Using a microcalorimeter, we measured the force production and heat output, as functions of muscle length, of isolated rat trabeculae from both ventricles contracting isometrically at 5 Hz and at 37°C. In the presence of blebbistatin (15 μmol l−1), active force was zero but heat output remained constant, at all muscle lengths. Activation heat measured in the presence of blebbistatin was not different from that estimated from the intercept of the heat–stress relation in its absence. We thus reached two conclusions. First, activation heat is independent of muscle length. Second, residual crossbridge heat is negligible at zero active force; hence, the intercept of the cardiac heat–force relation provides an estimate of activation heat uncontaminated by crossbridge cycling. Both results resolve long‐standing disputes in the literature.
    June 01, 2017   doi: 10.1113/JP274174   open full text
  • Genotype‐specific pathogenic effects in human dilated cardiomyopathy.
    Ilse A. E. Bollen, Maike Schuldt, Magdalena Harakalova, Aryan Vink, Folkert W. Asselbergs, Jose R. Pinto, Martina Krüger, Diederik W. D. Kuster, Jolanda Velden.
    The Journal of Physiology. June 01, 2017
    Key points Mutations in genes encoding cardiac troponin I (TNNI3) and cardiac troponin T (TNNT2) caused altered troponin protein stoichiometry in patients with dilated cardiomyopathy. TNNI3p.98trunc resulted in haploinsufficiency, increased Ca2+‐sensitivity and reduced length‐dependent activation. TNNT2p.K217del caused increased passive tension. A mutation in the gene encoding Lamin A/C (LMNAp.R331Q) led to reduced maximal force development through secondary disease remodelling in patients suffering from dilated cardiomyopathy. Our study shows that different gene mutations induce dilated cardiomyopathy via diverse cellular pathways. Abstract Dilated cardiomyopathy (DCM) can be caused by mutations in sarcomeric and non‐sarcomeric genes. In this study we defined the pathogenic effects of three DCM‐causing mutations: the sarcomeric mutations in genes encoding cardiac troponin I (TNNI3p.98truncation) and cardiac troponin T (TNNT2p.K217deletion; also known as the p.K210del) and the non‐sarcomeric gene mutation encoding lamin A/C (LMNAp.R331Q). We assessed sarcomeric protein expression and phosphorylation and contractile behaviour in single membrane‐permeabilized cardiomyocytes in human left ventricular heart tissue. Exchange with recombinant troponin complex was used to establish the direct pathogenic effects of the mutations in TNNI3 and TNNT2. The TNNI3p.98trunc and TNNT2p.K217del mutation showed reduced expression of troponin I to 39% and 51%, troponin T to 64% and 53%, and troponin C to 73% and 97% of controls, respectively, and altered stoichiometry between the three cardiac troponin subunits. The TNNI3p.98trunc showed pure haploinsufficiency, increased Ca2+‐sensitivity and impaired length‐dependent activation. The TNNT2p.K217del mutation showed a significant increase in passive tension that was not due to changes in titin isoform composition or phosphorylation. Exchange with wild‐type troponin complex corrected troponin protein levels to 83% of controls in the TNNI3p.98trunc sample. Moreover, upon exchange all functional deficits in the TNNI3p.98trunc and TNNT2p.K217del samples were normalized to control values confirming the pathogenic effects of the troponin mutations. The LMNAp.R331Q mutation resulted in reduced maximal force development due to disease remodelling. Our study shows that different gene mutations induce DCM via diverse cellular pathways.
    June 01, 2017   doi: 10.1113/JP274145   open full text
  • Light adaptation and the evolution of vertebrate photoreceptors.
    Ala Morshedian, Gordon L. Fain.
    The Journal of Physiology. June 01, 2017
    Key Points Lamprey are cyclostomes, a group of vertebrates that diverged from lines leading to jawed vertebrates (including mammals) in the late Cambrian, 500 million years ago. It may therefore be possible to infer properties of photoreceptors in early vertebrate progenitors by comparing lamprey to other vertebrates. We show that lamprey rods and cones respond to light much like rods and cones in amphibians and mammals. They operate over a similar range of light intensities and adapt to backgrounds and bleaches nearly identically. These correspondences are pervasive and detailed; they argue for the presence of rods and cones very early in the evolution of vertebrates with properties much like those of rods and cones in existing vertebrate species. Abstract The earliest vertebrates were agnathans – fish‐like organisms without jaws, which first appeared near the end of the Cambrian radiation. One group of agnathans became cyclostomes, which include lamprey and hagfish. Other agnathans gave rise to jawed vertebrates or gnathostomes, the group including all other existing vertebrate species. Because cyclostomes diverged from other vertebrates 500 million years ago, it may be possible to infer some of the properties of the retina of early vertebrate progenitors by comparing lamprey to other vertebrates. We have previously shown that rods and cones in lamprey respond to light much like photoreceptors in other vertebrates and have a similar sensitivity. We now show that these affinities are even closer. Both rods and cones adapt to background light and to bleaches in a manner almost identical to other vertebrate photoreceptors. The operating range in darkness is nearly the same in lamprey and in amphibian or mammalian rods and cones; moreover background light shifts response–intensity curves downward and to the right over a similar range of ambient intensities. Rods show increment saturation at about the same intensity as mammalian rods, and cones never saturate. Bleaches decrease sensitivity in part by loss of quantum catch and in part by opsin activation of transduction. These correspondences are so numerous and pervasive that they are unlikely to result from convergent evolution but argue instead that early vertebrate progenitors of both cyclostomes and mammals had photoreceptors much like our own.
    June 01, 2017   doi: 10.1113/JP274211   open full text
  • Regionalization of the stretch reflex in the human vastus medialis.
    Alessio Gallina, Jean‐Sébastien Blouin, Tanya D. Ivanova, S. Jayne Garland.
    The Journal of Physiology. June 01, 2017
    Key points Regionalization of the stretch reflex, i.e. the notion that the activation of 1a afferents from a muscle region influences only the activation of motor units in the same region, has been demonstrated previously in animals but not in humans. Mechanical stretches applied to regions of vastus medialis as close as 10 mm apart resulted in recruitment of motor units localized topographically with respect to the location of the mechanical stretch. Stretch reflexes are regionalized in the human vastus medialis. The human spinal cord has the neuromuscular circuitry to preferentially activate motoneurones innervating muscle fibres located in different regions of the vastus medialis. Abstract The localization of motor unit territories provides an anatomical basis to suggest that the CNS may have more independence in motor unit recruitment and control strategies than what was previously thought. In this study, we investigated whether the human spinal cord has the neuromuscular circuitry to independently activate motor units located in different regions of the vastus medialis. Mechanical taps were applied to multiple locations in the vastus medialis (VM) in nine healthy individuals. Regional responses within the muscle were observed using a grid of 5 × 13 surface EMG electrodes. The EMG amplitude was quantified for each channel, and a cluster of channels showing the largest activation was identified. The spatial location of the EMG response was quantified as the position of the channels in the cluster. In a subset of three participants, intramuscular recordings were performed simultaneously with the surface EMG recordings. Mechanical taps resulted in localized, discrete responses for each participant. The spatial location of the elicited responses was dependent on the location of the tap (P < 0.001). Recordings with intramuscular electrodes confirmed the regional activation of the VM for different tap locations. Selective stimulation of 1a afferents localized in a region of the VM results in reflex recruitment of motor units in the same region. These findings suggest that the human spinal cord has the neuromuscular circuitry to modulate spatially the motoneuronal output to vastus medialis regions, which is a neuroanatomical prerequisite for regional activation.
    June 01, 2017   doi: 10.1113/JP274458   open full text
  • Early vertebrate origin and diversification of small transmembrane regulators of cellular ion transport.
    Sergej Pirkmajer, Henriette Kirchner, Leonidas S. Lundell, Pavel V. Zelenin, Juleen R. Zierath, Kira S. Makarova, Yuri I. Wolf, Alexander V. Chibalin.
    The Journal of Physiology. May 29, 2017
    Key points Small transmembrane proteins such as FXYDs, which interact with Na+,K+‐ATPase, and the micropeptides that interact with sarco/endoplasmic reticulum Ca2+‐ATPase play fundamental roles in regulation of ion transport in vertebrates. Uncertain evolutionary origins and phylogenetic relationships among these regulators of ion transport have led to inconsistencies in their classification across vertebrate species, thus hampering comparative studies of their functions. We discovered the first FXYD homologue in sea lamprey, a basal jawless vertebrate, which suggests small transmembrane regulators of ion transport emerged early in the vertebrate lineage. We also identified 13 gene subfamilies of FXYDs and propose a revised, phylogeny‐based FXYD classification that is consistent across vertebrate species. These findings provide an improved framework for investigating physiological and pathophysiological functions of small transmembrane regulators of ion transport. Abstract Small transmembrane proteins are important for regulation of cellular ion transport. The most prominent among these are members of the FXYD family (FXYD1–12), which regulate Na+,K+‐ATPase, and phospholamban, sarcolipin, myoregulin and DWORF, which regulate the sarco/endoplasmic reticulum Ca2+‐ATPase (SERCA). FXYDs and regulators of SERCA are present in fishes, as well as terrestrial vertebrates; however, their evolutionary origins and phylogenetic relationships are obscure, thus hampering comparative physiological studies. Here we discovered that sea lamprey (Petromyzon marinus), a representative of extant jawless vertebrates (Cyclostomata), expresses an FXYD homologue, which strongly suggests that FXYDs predate the emergence of fishes and other jawed vertebrates (Gnathostomata). Using a combination of sequence‐based phylogenetic analysis and conservation of local chromosome context, we determined that FXYDs markedly diversified in the lineages leading to cartilaginous fishes (Chondrichthyes) and bony vertebrates (Euteleostomi). Diversification of SERCA regulators was much less extensive, indicating they operate under different evolutionary constraints. Finally, we found that FXYDs in extant vertebrates can be classified into 13 gene subfamilies, which do not always correspond to the established FXYD classification. We therefore propose a revised classification that is based on evolutionary history of FXYDs and that is consistent across vertebrate species. Collectively, our findings provide an improved framework for investigating the function of ion transport in health and disease.
    May 29, 2017   doi: 10.1113/JP274254   open full text
  • Long‐term plasticity of corticostriatal synapses is modulated by pathway‐specific co‐release of opioids through κ‐opioid receptors.
    Sarah L. Hawes, Armando G. Salinas, David M. Lovinger, Kim T. Blackwell.
    The Journal of Physiology. May 26, 2017
    Key points Both endogenous opioids and opiate drugs of abuse modulate learning of habitual and goal‐directed actions, and can also modify long‐term plasticity of corticostriatal synapses. Striatal projection neurons of the direct pathway co‐release the opioid neuropeptide dynorphin which can inhibit dopamine release via κ‐opioid receptors. Theta‐burst stimulation of corticostriatal fibres produces long‐term potentiation (LTP) in striatal projection neurons when measured using whole‐cell patch recording. Optogenetic activation of direct pathway striatal projection neurons inhibits LTP while reducing dopamine release. Because the endogenous release of opioids is activity dependent, this modulation of synaptic plasticity represents a negative feedback mechanism that may limit runaway enhancement of striatal neuron activity in response to drugs of abuse. Abstract Synaptic plasticity in the striatum adjusts behaviour adaptively during skill learning, or maladaptively in the case of addiction. Just as dopamine plays a critical role in synaptic plasticity underlying normal skill learning and addiction, endogenous and exogenous opiates also modulate learning and addiction‐related striatal plasticity. Though the role of opioid receptors in long‐term depression in striatum has been characterized, their effect on long‐term potentiation (LTP) remains unknown. In particular, direct pathway (dopamine D1 receptor‐containing; D1R‐) spiny projection neurons (SPNs) co‐release the opioid neuropeptide dynorphin, which acts at presynaptic κ‐opioid receptors (KORs) on dopaminergic afferents and can negatively regulate dopamine release. Therefore, we evaluated the interaction of co‐released dynorphin and KOR on striatal LTP. We optogenetically facilitate the release of endogenous dynorphin from D1R‐SPNs in brain slice while using whole‐cell patch recording to measure changes in the synaptic response of SPNs following theta‐burst stimulation (TBS) of cortical afferents. Our results demonstrate that TBS evokes corticostriatal LTP, and that optogenetic activation of D1R‐SPNs during induction impairs LTP. Additional experiments demonstrate that optogenetic activation of D1R‐SPNs reduces stimulation‐evoked dopamine release and that bath application of a KOR antagonist provides full rescue of both LTP induction and dopamine release during optogenetic activation of D1R‐SPNs. These results suggest that an increase in the opioid neuropeptide dynorphin is responsible for reduced TBS LTP and illustrate a physiological phenomenon whereby heightened D1R‐SPN activity can regulate corticostriatal plasticity. Our findings have important implications for learning in addictive states marked by elevated direct pathway activation.
    May 26, 2017   doi: 10.1113/JP274190   open full text
  • Role of mucosa in generating spontaneous activity in the guinea pig seminal vesicle.
    Mitsue Takeya, Hikaru Hashitani, Tokumasa Hayashi, Ryuhei Higashi, Kei‐ichiro Nakamura, Makoto Takano.
    The Journal of Physiology. May 25, 2017
    Key points The mucosa may have neuron‐like functions as urinary bladder mucosa releases bioactive substances that modulate sensory nerve activity as well as detrusor muscle contractility. However, such mucosal function in other visceral organs remains to be established. The role of mucosa in generating spontaneous contractions in seminal vesicles (SVs), a paired organ in the male reproductive tract, was investigated. The intact mucosa is essential for the generation of spontaneous phasic contractions of SV smooth muscle arising from electrical slow waves and corresponding increases in intracellular Ca2+. These spontaneous events primarily depend on Ca2+ handling by sarco‐endoplasmic reticulum Ca2+ stores. A population of mucosal cells developed spontaneous rises in intracellular Ca2+ relying on sarco‐endoplasmic reticulum Ca2+ handling. The spontaneously active cells in the SV mucosa appear to drive spontaneous activity in smooth muscle either by sending depolarizing signals and/or by releasing humoral substances. Abstract The role of the mucosa in generating the spontaneous activity of guinea‐pig seminal vesicle (SV) was explored. Changes in contractility, membrane potential and intracellular Ca2+ dynamics of SV smooth muscle cells (SMCs) were recorded using isometric tension recording, intracellular microelectrode recording and epi‐fluorescence Ca2+ imaging, respectively. Mucosa‐intact but not mucosa‐denuded SV preparations generated TTX‐ (1 μm) resistant spontaneous phasic contractions that were abolished by nifedipine (3 μm). Consistently, SMCs developed mucosa‐dependent slow waves (SWs) that triggered action potentials and corresponding Ca2+ flashes. Nifedipine (10 μm) abolished the action potentials and spontaneous contractions, while suppressing the SWs and Ca2+ flashes. Both the residual SWs and spontaneous Ca2+ transients were abolished by cyclopiazonic acid (CPA, 10 μm), a sarco‐endoplasmic reticulum Ca2+‐ATPase (SERCA) inhibitor. DIDS (300 μm) and niflumic acid (100 μm), blockers for Ca2+‐activated Cl− channels (CACCs), or low Cl− solution also slowed or prevented the generation of SWs. In SV mucosal preparations detached from the muscle layer, a population of mucosal cells generated spontaneous Ca2+ transients that were blocked by CPA but not nifedipine. These results suggested that spontaneous contractions and corresponding Ca2+ flashes in SV SMCs arise from action potential generation due to the opening of L‐type voltage‐dependent Ca2+ channels. Spontaneous Ca2+ transients appear to primarily result from Ca2+ release from sarco‐endoplasmic reticulum Ca2+ stores to activate CACCs to develop SWs. The mucosal cells firing spontaneous Ca2+ transients may play a critical role in driving spontaneous activity of SV smooth muscle either by sending depolarizing signals or by releasing humoral substances.
    May 25, 2017   doi: 10.1113/JP273872   open full text
  • α‐Linolenic acid and exercise training independently, and additively, decrease blood pressure and prevent diastolic dysfunction in obese Zucker rats.
    Pierre‐Andre Barbeau, Tanya M. Holloway, Jamie Whitfield, Brittany L. Baechler, Joe Quadrilatero, Luc J. C. Loon, Adrian Chabowski, Graham P. Holloway.
    The Journal of Physiology. May 24, 2017
    Key points α‐linolenic acid (ALA) and exercise training both attenuate hyperlipidaemia‐related cardiovascular derangements, however, there is a paucity of information pertaining to their mechanisms of action when combined. We investigated both the independent and combined effects of exercise training and ALA consumption in obese Zucker rats, aiming to determine the potential for additive improvements in cardiovascular function. ALA and exercise training independently improved cardiac output, end‐diastolic volume, left ventricular fibrosis and mean blood pressure following a 4 week intervention. Combining ALA and endurance exercise yielded greater improvements in these parameters, independent of changes in markers of oxidative stress or endogenous anti‐oxidants. We postulate that divergent mechanisms of action may explain these changes: ALA increases peripheral vasodilation, and exercise training stimulates angiogenesis. Abstract Although α‐linolenic acid (ALA) and endurance exercise training independently attenuate hyperlipidaemia‐related cardiovascular derangements, there is a paucity of information pertaining to their mechanisms of action and efficacy when combined as a preventative therapeutic approach. Therefore, we used obese Zucker rats to investigate the independent and combined effects of these interventions on cardiovascular disease. Specifically, animals were randomly assigned to one of the following groups: control diet‐sedentary, ALA supplemented‐sedentary, control diet‐exercise trained or ALA supplemented‐exercise trained. Following a 4 week intervention, although the independent and combined effects of ALA and exercise reduced (P < 0.05) the serum free/esterified cholesterol ratio, only the ALA supplemented‐exercise trained animals displayed a reduction in the content of both serum free and esterified cholesterol. Moreover, although ALA and endurance training individually increased cardiac output, stroke volume and end‐diastolic volume, as well as reduced left ventricle fibrosis, mean blood pressure and total peripheral resistance, these responses were all greater following the combined intervention (ALA supplemented‐exercise trained). These effects occurred independent of changes in oxidative phosphorylation proteins, markers of oxidative stress or endogenous anti‐oxidant capacity. We propose that the beneficial effects of a combined intervention occur as a result of divergent mechanisms of action elicited by ALA and endurance exercise because only exercise training increased the capillary content in the left ventricle and skeletal muscle, and tended to decrease protein carbonylation in the left ventricle (P = 0.06). Taken together, our data indicate that combining ALA and endurance exercise provides additional improvements in cardiovascular disease risk reduction compared to singular interventions in the obese Zucker rat.
    May 24, 2017   doi: 10.1113/JP274036   open full text
  • Dampened activity of ryanodine receptor channels in mutant skeletal muscle lacking TRIC‐A.
    Sam El‐Ajouz, Elisa Venturi, Katja Witschas, Matthew Beech, Abigail D. Wilson, Chris Lindsay, David Eberhardt, Fiona O'Brien, Tsunaki Iida, Miyuki Nishi, Hiroshi Takeshima, Rebecca Sitsapesan.
    The Journal of Physiology. May 23, 2017
    Key points The role of trimeric intracellular cation (TRIC) channels is not known, although evidence suggests they may regulate ryanodine receptors (RyR) via multiple mechanisms. We therefore investigated whether Tric‐a gene knockout (KO) alters the single‐channel function of skeletal RyR (RyR1). We find that RyR1 from Tric‐a KO mice are more sensitive to inhibition by divalent cations, although they respond normally to cytosolic Ca2+, ATP, caffeine and luminal Ca2+. In the presence of Mg2+, ATP cannot effectively activate RyR1 from Tric‐a KO mice. Additionally, RyR1 from Tric‐a KO mice are not activated by protein kinase A phosphorylation, demonstrating a defect in the ability of β‐adrenergic stimulation to regulate sarcoplasmic reticulum (SR) Ca2+‐release. The defective RyR1 gating that we describe probably contributes significantly to the impaired SR Ca2+‐release observed in skeletal muscle from Tric‐a KO mice, further highlighting the importance of TRIC‐A for normal physiological regulation of SR Ca2+‐release in skeletal muscle. Abstract The type A trimeric intracellular cation channel (TRIC‐A) is a major component of the nuclear and sarcoplasmic reticulum (SR) membranes of cardiac and skeletal muscle, and is localized closely with ryanodine receptor (RyR) channels in the SR terminal cisternae. The skeletal muscle of Tric‐a knockout (KO) mice is characterized by Ca2+ overloaded and swollen SR and by changes in the properties of SR Ca2+ release. We therefore investigated whether RyR1 gating behaviour is modified in the SR from Tric‐a KO mice by incorporating native RyR1 into planar phospholipid bilayers under voltage‐clamp conditions. We find that RyR1 channels from Tric‐a KO mice respond normally to cytosolic Ca2+, ATP, adenine, caffeine and to luminal Ca2+. However, the channels are more sensitive to the inactivating effects of divalent cations, thus, in the presence of Mg2+, ATP is inadequate as an activator. Additionally, channels are not characteristically activated by protein kinase A even though the phosphorylation levels of Ser2844 are similar to controls. The results of the present study suggest that TRIC‐A functions as an excitatory modulator of RyR1 channels within the SR terminal cisternae. Importantly, this regulatory action of TRIC‐A appears to be independent of (although additive to) any indirect consequences to RyR1 activity that arise as a result of K+ fluxes across the SR via TRIC‐A.
    May 23, 2017   doi: 10.1113/JP273550   open full text
  • Increased transient Na+ conductance and action potential output in layer 2/3 prefrontal cortex neurons of the fmr1−/y mouse.
    Brandy N. Routh, Rahul K. Rathour, Michael E. Baumgardner, Brian E. Kalmbach, Daniel Johnston, Darrin H. Brager.
    The Journal of Physiology. May 23, 2017
    Key points Layer 2/3 neurons of the prefrontal cortex display higher gain of somatic excitability, responding with a higher number of action potentials for a given stimulus, in fmr1−/y mice. In fmr1−/y L2/3 neurons, action potentials are taller, faster and narrower. Outside‐out patch clamp recordings revealed that the maximum Na+ conductance density is higher in fmr1−/y L2/3 neurons. Measurements of three biophysically distinct K+ currents revealed a depolarizing shift in the activation of a rapidly inactivating (A‐type) K+ conductance. Realistic neuronal simulations of the biophysical observations recapitulated the elevated action potential and repetitive firing phenotype. Abstract Fragile X syndrome is the most common form of inherited mental impairment and autism. The prefrontal cortex is responsible for higher order cognitive processing, and prefrontal dysfunction is believed to underlie many of the cognitive and behavioural phenotypes associated with fragile X syndrome. We recently demonstrated that somatic and dendritic excitability of layer (L) 5 pyramidal neurons in the prefrontal cortex of the fmr1−/y mouse is significantly altered due to changes in several voltage‐gated ion channels. In addition to L5 pyramidal neurons, L2/3 pyramidal neurons play an important role in prefrontal circuitry, integrating inputs from both lower brain regions and the contralateral cortex. Using whole‐cell current clamp recording, we found that L2/3 pyramidal neurons in prefrontal cortex of fmr1−/y mouse fired more action potentials for a given stimulus compared with wild‐type neurons. In addition, action potentials in fmr1−/y neurons were significantly larger, faster and narrower. Voltage clamp of outside‐out patches from L2/3 neurons revealed that the transient Na+ current was significantly larger in fmr1−/y neurons. Furthermore, the activation curve of somatic A‐type K+ current was depolarized. Realistic conductance‐based simulations revealed that these biophysical changes in Na+ and K+ channel function could reliably reproduce the observed increase in action potential firing and altered action potential waveform. These results, in conjunction with our prior findings on L5 neurons, suggest that principal neurons in the circuitry of the medial prefrontal cortex are altered in distinct ways in the fmr1−/y mouse and may contribute to dysfunctional prefrontal cortex processing in fragile X syndrome.
    May 23, 2017   doi: 10.1113/JP274258   open full text
  • Exercise training reverses age‐induced diastolic dysfunction and restores coronary microvascular function.
    Kazuki Hotta, Bei Chen, Bradley J. Behnke, Payal Ghosh, John N. Stabley, Jeremy A. Bramy, Jaime L. Sepulveda, Michael D. Delp, Judy M. Muller‐Delp.
    The Journal of Physiology. May 23, 2017
    Key points In a rat model of ageing that is free of atherosclerosis or hypertension, E/A, a diagnostic measure of diastolic filling, decreases, and isovolumic relaxation time increases, indicating that both active and passive ventricular relaxation are impaired with advancing age. Resting coronary blood flow and coronary functional hyperaemia are reduced with age, and endothelium‐dependent vasodilatation declines with age in coronary resistance arterioles. Exercise training reverses age‐induced declines in diastolic and coronary microvascular function. Thus, microvascular dysfunction and inadequate coronary perfusion are likely mechanisms of diastolic dysfunction in aged rats. Exercise training, initiated at an advanced age, reverses age‐related diastolic and microvascular dysfunction; these data suggest that late‐life exercise training can be implemented to improve coronary perfusion and diastolic function in the elderly. Abstract The risk for diastolic dysfunction increases with advancing age. Regular exercise training ameliorates age‐related diastolic dysfunction; however, the underlying mechanisms have not been identified. We investigated whether (1) microvascular dysfunction contributes to the development of age‐related diastolic dysfunction, and (2) initiation of late‐life exercise training reverses age‐related diastolic and microvascular dysfunction. Young and old rats underwent 10 weeks of exercise training or remained as sedentary, cage‐controls. Isovolumic relaxation time (IVRT), early diastolic filling (E/A), myocardial performance index (MPI) and aortic stiffness (pulse wave velocity; PWV) were evaluated before and after exercise training or cage confinement. Coronary blood flow and vasodilatory responses of coronary arterioles were evaluated in all groups at the end of training. In aged sedentary rats, compared to young sedentary rats, a 42% increase in IVRT, a 64% decrease in E/A, and increased aortic stiffness (PWV: 6.36 ± 0.47 vs.4.89 ± 0.41, OSED vs. YSED, P < 0.05) was accompanied by impaired coronary blood flow at rest and during exercise. Endothelium‐dependent vasodilatation was impaired in coronary arterioles from aged rats (maximal relaxation to bradykinin: 56.4 ± 5.1% vs. 75.3 ± 5.2%, OSED vs. YSED, P < 0.05). After exercise training, IVRT, a measure of active ventricular relaxation, did not differ between old and young rats. In old rats, exercise training reversed the reduction in E/A, reduced aortic stiffness, and eliminated impairment of coronary blood flow responses and endothelium‐dependent vasodilatation. Thus, age‐related diastolic and microvascular dysfunction are reversed by late‐life exercise training. The restorative effect of exercise training on coronary microvascular function may result from improved endothelial function.
    May 23, 2017   doi: 10.1113/JP274172   open full text
  • Beneficial effects of leptin treatment in a setting of cardiac dysfunction induced by transverse aortic constriction in mouse.
    Nieves Gómez‐Hurtado, Alejandro Domínguez‐Rodríguez, Philippe Mateo, María Fernández‐Velasco, Almudena Val‐Blasco, Rafael Aizpún, Jessica Sabourin, Ana María Gómez, Jean‐Pierre Benitah, Carmen Delgado.
    The Journal of Physiology. May 23, 2017
    Key points Leptin, is a 16 kDa pleiotropic peptide not only primarily secreted by adipocytes, but also produced by other tissues, including the heart. Controversy exists regarding the adverse and beneficial effects of leptin on the heart We analysed the effect of a non‐hypertensive dose of leptin on cardiac function, [Ca2+]i handling and cellular electrophysiology, which participate in the genesis of pump failure and related arrhythmias, both in control mice and in mice subjected to chronic pressure‐overload by transverse aorta constriction. We find that leptin activates mechanisms that contribute to cardiac dysfunction under physiological conditions. However, after the establishment of pressure overload, an increase in leptin levels has protective cardiac effects with respect to rescuing the cellular heart failure phenotype. These beneficial effects of leptin involve restoration of action potential duration via normalization of transient outward potassium current and sarcoplasmic reticulum Ca2+ content via rescue of control sarcoplasmic/endoplasmic reticulum Ca2+ ATPase levels and ryanodine receptor function modulation, leading to normalization of Ca2+ handling parameters. Abstract Leptin, is a 16 kDa pleiotropic peptide not only primary secreted by adipocytes, but also produced by other tissues, including the heart. Evidence indicates that leptin may have either adverse or beneficial effects on the heart. To obtain further insights, in the present study, we analysed the effect of leptin treatment on cardiac function, [Ca2+]i handling and cellular electrophysiology, which participate in the genesis of pump failure and related arrhythmias, both in control mice and in mice subjected to chronic pressure‐overload by transverse aorta constriction (TAC). Three weeks after surgery, animals received either leptin (0.36 mg kg–1 day–1) or vehicle via osmotic minipumps for 3 weeks. Echocardiographic measurements showed that, although leptin treatment was deleterious on cardiac function in sham, leptin had a cardioprotective effect following TAC. [Ca2+]i transient in cardiomyocytes followed similar pattern. Patch clamp experiments showed prolongation of action potential duration (APD) in TAC and leptin‐treated sham animals, whereas, following TAC, leptin reduced the APD towards control values. APD variations were associated with decreased transient outward potassium current and Kv4.2 and KChIP2 protein expression. TAC myocytes showed a higher incidence of triggered activities and spontaneous Ca2+ waves. These proarrhythmic manifestations, related to Ca2+/calmodulin‐dependent protein kinase II and ryanodine receptor phosphorylation, were reduced by leptin. The results of the present study demonstrate that, although leptin treatment was deleterious on cardiac function in control animals, leptin had a cardioprotective effect following TAC, normalizing cardiac function and reducing arrhythmogeneity at the cellular level.
    May 23, 2017   doi: 10.1113/JP274030   open full text
  • An increased extrasynaptic NMDA tone inhibits A‐type K+ current and increases excitability of hypothalamic neurosecretory neurons in hypertensive rats.
    Meng Zhang, Vinicia C. Biancardi, Javier E. Stern.
    The Journal of Physiology. May 23, 2017
    Key points A functional coupling between extrasynaptic NMDA receptors (eNMDARs) and the A‐type K+ current (IA) influences homeostatic firing responses of magnocellular neurosecretory cells (MNCs) to a physiological challenge. However, whether an altered eNMDAR–IA coupling also contributes to exacerbated MNC activity and neurohumoral activation during disease states is unknown. We show that activation of eNMDARs by exogenously applied NMDA inhibited IA in MNCs obtained from sham, but not in MNCs from renovascular hypertensive (RVH) rats. Neither the magnitude of the exogenously evoked NMDA current nor the expression of NMDAR subunits were altered in RVH rats. Conversely, we found that a larger endogenous glutamate tone, which was not due to blunted glutamate transport activity, led to the sustained activation of eNMDARs that tonically inhibited IA, contributing in turn to higher firing activity in RVH rats. Our studies show that exacerbated activation of eNMDARs by endogenous glutamate contributes to tonic inhibition of IA and enhanced MNC excitability in RVH rats. Abstract We recently showed that a functional coupling between extrasynaptic NMDA receptors (eNMDARs) and the A‐type K+ current (IA) influences the firing activity of hypothalamic magnocellular neurosecretory neurons (MNCs), as well as homeostatic adaptive responses to a physiological challenge. Here, we aimed to determine whether changes in the eNMDAR–IA coupling also contributed to exacerbated MNC activity during disease states. We used a combination of patch‐clamp electrophysiology and real‐time PCR in MNCs in sham and renovascular hypertensive (RVH) rats. Activation of eNMDARs by exogenously applied NMDA inhibited IA in sham rats, but this effect was largely blunted in RVH rats. The blunted response was not due to changes in eNMDAR expression and/or function, since neither NMDA current magnitude or reversal potential, nor the levels of NR1‐NR2A–D subunit expression were altered in RVH rats. Conversely, we found a larger endogenous glutamate tone, resulting in the sustained activation of eNMDARs that tonically inhibited IA and contributed also to higher ongoing firing activity in RVH rats. The enhanced endogenous glutamate tone in RVH rats was not due to blunted glutamate transporter activity. Rather, a higher transporter activity was observed, which possibly acted as a compensatory mechanism in the face of the elevated endogenous tone. In summary, our studies indicate that an elevated endogenous glutamate tone results in an exacerbated activation of eNMDARs, which in turn contributes to diminished IA magnitude and increased firing activity of MNCs from hypertensive rats.
    May 23, 2017   doi: 10.1113/JP274327   open full text
  • Hippocampal electrical stimulation disrupts associative learning when targeted at dentate spikes.
    Miriam S. Nokia, Irina Gureviciene, Tomi Waselius, Heikki Tanila, Markku Penttonen.
    The Journal of Physiology. May 23, 2017
    Key points Dentate spikes are fast fluctuations of hilar local‐field potentials that take place during rest and are thought to reflect input arriving from the entorhinal cortex to the hippocampus. During dentate spikes, neuronal firing in hippocampal input (dentate gyrus) and output (CA1/CA3) regions is uncoupled. To date, the behavioural significance of dentate spikes is unknown. Here, we provide evidence that disrupting the dentate spike‐related uncoupling of the dentate gyrus and the CA1/CA3 subregions for 1 h after training retards associative learning. We suggest dentate spikes play a significant role in memory consolidation. Abstract Hippocampal electrophysiological oscillations, namely theta and ripples, have been implicated in encoding and consolidation of new memories, respectively. According to existing literature, hippocampal dentate spikes are prominent, short‐duration (<30 ms), large‐amplitude (∼2–4 mV) fluctuations in hilar local‐field potentials that take place during awake immobility and sleep. Interestingly, previous studies indicate that during dentate spikes dentate gyrus granule cells increase their firing while firing of CA1 pyramidal cells are suppressed, thus resulting in momentary uncoupling of the two hippocampal subregions. To date, the behavioural significance of dentate spikes is unknown. Here, to study the possible role of dentate spikes in learning, we trained adult male Sprague–Dawley rats in trace eyeblink classical conditioning. For 1 h immediately following each conditioning session, one group of animals received hippocampal stimulation via the ventral hippocampal commissure (vHC) contingent on dentate spikes to disrupt the uncoupling between the dentate gyrus and the CA1 subregions. A yoked control group was stimulated during immobility, irrespective of brain state, and another control group was not stimulated at all. As a result, learning was impaired only in the group where vHC stimulation was administered contingent on dentate spikes. Our results suggest dentate spikes and/or the associated uncoupling of the dentate gyrus and the CA1 play a significant role in memory consolidation. Dentate spikes could possibly reflect reactivation and refinement of a memory trace within the dentate gyrus triggered by input from the entorhinal cortex.
    May 23, 2017   doi: 10.1113/JP274023   open full text
  • Human skeletal muscle fibroblasts stimulate in vitro myogenesis and in vivo muscle regeneration.
    Abigail L. Mackey, Mélanie Magnan, Bénédicte Chazaud, Michael Kjaer.
    The Journal of Physiology. May 23, 2017
    Key points Accumulation of skeletal muscle extracellular matrix is an unfavourable characteristic of many muscle diseases, muscle injury and sarcopenia. The extent of cross‐talk between fibroblasts, as the source of matrix protein, and satellite cells in humans is unknown. We studied this in human muscle biopsies and cell‐culture studies. We observed a strong stimulation of myogenesis by human fibroblasts in cell culture. In biopsies collected 30 days after a muscle injury protocol, fibroblast number increased to four times control levels, where fibroblasts were found to be preferentially located immediately surrounding regenerating muscle fibres. These novel findings indicate an important role for fibroblasts in supporting the regeneration of muscle fibres, potentially through direct stimulation of satellite cell differentiation and fusion, and contribute to understanding of cell–cell cross‐talk during physiological and pathological muscle remodelling. Abstract Accumulation of skeletal muscle extracellular matrix is an unfavourable characteristic of many muscle diseases, muscle injury and sarcopenia. In addition to the indispensable role satellite cells play in muscle regeneration, there is emerging evidence in rodents for a regulatory influence on fibroblast activity. However, the influence of fibroblasts on satellite cells and muscle regeneration in humans is unknown. The purpose of this study was to investigate this in vitro and during in vivo regeneration in humans. Following a muscle injury protocol in young healthy men (n = 7), the number of fibroblasts (TCF7L2+), satellite cells (Pax7+), differentiating myogenic cells (myogenin+) and regenerating fibres (neonatal/embryonic myosin+) was determined from biopsy cross‐sections. Fibroblasts and myogenic precursor cells (MPCs) were also isolated from human skeletal muscle (n = 4) and co‐cultured using different cell ratios, with the two cell populations either in direct contact with each other or separated by a permeable membrane. MPC proliferation, differentiation and fusion were assessed from cells stained for BrdU, desmin and myogenin. On biopsy cross‐sections, fibroblast number was seen to increase, along with myogenic cell number, by d7 and increase further by d30, where fibroblasts were observed to be preferentially located immediately surrounding regenerating muscle fibres. In vitro, the presence of fibroblasts in direct contact with MPCs was found to moderately stimulate MPC proliferation and strongly stimulate both MPC differentiation and MPC fusion. It thus appears, in humans, that fibroblasts exert a strong positive regulatory influence on MPC activity, in line with observations during in vivo skeletal muscle regeneration.
    May 23, 2017   doi: 10.1113/JP273997   open full text
  • Mechanical tuning and amplification within the apex of the guinea pig cochlea.
    Alberto Recio‐Spinoso, John S. Oghalai.
    The Journal of Physiology. May 21, 2017
    Key points A popular conception of mammalian cochlear physiology is that tuned mechanical vibration of the basilar membrane defines the frequency response of the innervating auditory nerve fibres However, the data supporting these concepts come from vibratory measurements at cochlear locations tuned to high frequencies (>7 kHz). Here, we measured the travelling wave in regions of the guinea pig cochlea that respond to low frequencies (<2 kHz) and found that mechanical tuning was broad and did not match auditory nerve tuning characteristics. Non‐linear amplification of the travelling wave functioned over a broad frequency range and did not substantially sharpen frequency tuning. Thus, the neural encoding of low‐frequency sounds, which includes most of the information conveyed by human speech, is not principally determined by basilar membrane mechanics. Abstract The popular notion of mammalian cochlear function is that auditory nerves are tuned to respond best to different sound frequencies because basilar membrane vibration is mechanically tuned to different frequencies along its length. However, this concept has only been demonstrated in regions of the cochlea tuned to frequencies >7 kHz, not in regions sensitive to lower frequencies where human speech is encoded. Here, we overcame historical technical limitations and non‐invasively measured sound‐induced vibrations at four locations distributed over the apical two turns of the guinea pig cochlea. In turn 3, the responses demonstrated low‐pass filter characteristics. In turn 2, the responses were low‐pass‐like, in that they occasionally did have a slight peak near the corner frequency. The corner frequencies of the responses were tonotopically tuned and ranged from 384 to 668 Hz. Non‐linear gain, or amplification of the vibrations in response to low‐intensity stimuli, was found both below and above the corner frequencies. Post mortem, cochlear gain disappeared. The non‐linear gain was typically 10–30 dB and was broad‐band rather than sharply tuned. However, the gain did reach nearly 50 dB in turn 2 for higher stimulus frequencies, nearly the amount of gain found in basal cochlear regions. Thus, our data prove that mechanical responses do not match neural responses and that cochlear amplification does not appreciably sharpen frequency tuning for cochlear regions that respond to frequencies <2 kHz. These data indicate that the non‐linear processing of sound performed by the guinea pig cochlea varies substantially between the cochlear apex and base.
    May 21, 2017   doi: 10.1113/JP273881   open full text
  • Compensatory and decompensatory alterations in cardiomyocyte Ca2+ dynamics in hearts with diastolic dysfunction following aortic banding.
    Sara Gattoni, Åsmund Treu Røe, Jan Magnus Aronsen, Ivar Sjaastad, William E. Louch, Nicolas P. Smith, Steven A. Niederer.
    The Journal of Physiology. May 21, 2017
    Key points At the cellular level cardiac hypertrophy causes remodelling, leading to changes in ionic channel, pump and exchanger densities and kinetics. Previous studies have focused on quantifying changes in channels, pumps and exchangers without quantitatively linking these changes with emergent cellular scale functionality. Two biophysical cardiac cell models were created, parameterized and validated and are able to simulate electrophysiology and calcium dynamics in myocytes from control sham operated rats and aortic‐banded rats exhibiting diastolic dysfunction. The contribution of each ionic pathway to the calcium kinetics was calculated, identifying the L‐type Ca2+ channel and sarco/endoplasmic reticulum Ca2+ATPase as the principal regulators of systolic and diastolic Ca2+, respectively. Results show that the ability to dynamically change systolic Ca2+, through changes in expression of key Ca2+ modelling protein densities, is drastically reduced following the aortic banding procedure; however the cells are able to compensate Ca2+ homeostasis in an efficient way to minimize systolic dysfunction. Abstract Elevated left ventricular afterload leads to myocardial hypertrophy, diastolic dysfunction, cellular remodelling and compromised calcium dynamics. At the cellular scale this remodelling of the ionic channels, pumps and exchangers gives rise to changes in the Ca2+ transient. However, the relative roles of the underlying subcellular processes and the positive or negative impact of each remodelling mechanism are not fully understood. Biophysical cardiac cell models were created to simulate electrophysiology and calcium dynamics in myocytes from control rats (SHAM) and aortic‐banded rats exhibiting diastolic dysfunction. The model parameters and framework were validated and the fitted parameters demonstrated to be unique for explaining our experimental data. The contribution of each ionic pathway to the calcium kinetics was calculated, identifying the L‐type Ca2+ channel (LCC) and the sarco/endoplasmic reticulum Ca2+‐ATPase (SERCA) as the principal regulators of systolic and diastolic Ca2+, respectively. In the aortic banding model, the sensitivity of systolic Ca2+ to LCC density and diastolic Ca2+ to SERCA density decreased by 16‐fold and increased by 23%, respectively, relative to the SHAM model. The energy cost of ionic homeostasis is maintained across the two models. The models predict that changes in ionic pathway densities in compensated aortic banding rats maintain Ca2+ function and efficiency. The ability to dynamically alter systolic function is significantly diminished, while the capacity to maintain diastolic Ca2+ is moderately increased.
    May 21, 2017   doi: 10.1113/JP273879   open full text
  • Maternal nutrient restriction during pregnancy and lactation leads to impaired right ventricular function in young adult baboons.
    Anderson H. Kuo, Cun Li, Hillary F. Huber, Matthias Schwab, Peter W. Nathanielsz, Geoffrey D. Clarke.
    The Journal of Physiology. May 18, 2017
    Key points Maternal nutrient restriction induces intrauterine growth restriction (IUGR) and leads to heightened cardiovascular risks later in life. We report right ventricular (RV) filling and ejection abnormalities in IUGR young adult baboons using cardiac magnetic resonance imaging. Both functional and morphological indicators of poor RV function were seen, many of which were similar to effects of ageing, but also with a few key differences. We observed more pronounced RV changes compared to our previous report of the left ventricle, suggesting there is likely to be a component of isolated RV abnormality in addition to expected haemodynamic sequelae from left ventricular dysfunction. In particular, our findings raise the suspicion of pulmonary hypertension after IUGR. This study establishes that IUGR also leads to impairment of the right ventricle in addition to the left ventricle classically studied. Abstract Maternal nutrient restriction induces intrauterine growth restriction (IUGR), increasing later life chronic disease including cardiovascular dysfunction. Our left ventricular (LV) CMRI studies in IUGR baboons (8 M, 8 F, 5.7 years – human equivalent approximately 25 years), control offspring (8 M, 8 F, 5.6 years), and normal elderly (OLD) baboons (6 M, 6 F, mean 15.9 years) revealed long‐term LV abnormalities in IUGR offspring. Although it is known that right ventricular (RV) function is dependent on LV health, the IUGR right ventricle remains poorly studied. We examined the right ventricle with cardiac magnetic resonance imaging in the same cohorts. We observed decreased ejection fraction (49 ± 2 vs. 33 ± 3%, P < 0.001), cardiac index (2.73 ± 0.27 vs. 1.89 ± 0.20 l min−1 m−2, P < 0.05), early filling rate/body surface area (BSA) (109.2 ± 7.8 vs. 44.6 ± 7.3 ml s−1 m−2, P < 0.001), wall thickening (61 ± 3 vs. 44 ± 5%, P < 0.05), and longitudinal shortening (26 ± 3 vs. 15 ± 2%, P < 0.01) in IUGR animals with increased chamber volumes. Many, but not all, of these changes share similarities to normal older animals. Our findings suggest IUGR‐induced pulmonary hypertension should be further investigated and that atrial volume, pulmonic outflow and interventricular septal motion may provide valuable insights into IUGR cardiovascular physiology. Overall, our findings reaffirm that gestational and neonatal challenges can result in long‐term programming of poor offspring cardiovascular health. To our knowledge, this is the first study reporting IUGR‐induced programmed adult RV dysfunction in an experimental primate model.
    May 18, 2017   doi: 10.1113/JP273928   open full text
  • Phosphatidylinositol 4,5‐bisphosphate (PIP2) modulates afterhyperpolarizations in oxytocin neurons of the supraoptic nucleus.
    Matthew K. Kirchner, Robert C. Foehring, Lie Wang, Giri Kumar Chandaka, Joseph C. Callaway, William E. Armstrong.
    The Journal of Physiology. May 15, 2017
    Key points Afterhyperpolarizations (AHPs) generated by repetitive action potentials in supraoptic magnocellular neurons regulate repetitive firing and spike frequency adaptation but relatively little is known about PIP2’s control of these AHPs. We examined how changes in PIP2 levels affected AHPs, somatic [Ca2+]i, and whole cell Ca2+ currents. Manipulations of PIP2 levels affected both medium and slow AHP currents in oxytocin (OT) neurons of the supraoptic nucleus. Manipulations of PIP2 levels did not modulate AHPs by influencing Ca2+ release from IP3‐triggered Ca2+ stores, suggesting more direct modulation of channels by PIP2. PIP2 depletion reduced spike‐evoked Ca2+ entry and voltage‐gated Ca2+ currents. PIP2 appears to influence AHPs in OT neurons by reducing Ca2+ influx during spiking. Abstract Oxytocin (OT)‐ and vasopressin (VP)‐secreting magnocellular neurons of the supraoptic nucleus (SON) display calcium‐dependent afterhyperpolarizations (AHPs) following a train of action potentials that are critical to shaping the firing patterns of these cells. Previous work demonstrated that the lipid phosphatidylinositol 4,5‐bisphosphate (PIP2) enabled the slow AHP component (sAHP) in cortical pyramidal neurons. We investigated whether this phenomenon occurred in OT and VP neurons of the SON. Using whole cell recordings in coronal hypothalamic slices from adult female rats, we demonstrated that inhibition of PIP2 synthesis with wortmannin robustly blocked both the medium and slow AHP currents (ImAHP and IsAHP) of OT, but not VP neurons with high affinity. We further tested this by introducing a water‐soluble PIP2 analogue (diC8‐PIP2) into neurons, which in OT neurons not only prevented wortmannin's inhibitory effect, but slowed rundown of the ImAHP and IsAHP. Inhibition of phospholipase C (PLC) with U73122 did not inhibit either ImAHP or IsAHP in OT neurons, consistent with wortmannin's effects not being due to reducing diacylglycerol (DAG) or IP3 availability, i.e. PIP2 modulation of AHPs is not likely to involve downstream Ca2+ release from inositol 1,4,5‐trisphosphate (IP3)‐triggered Ca2+‐store release, or channel modulation via DAG and protein kinase C (PKC). We found that wortmannin reduced [Ca2+]i increase induced by spike trains in OT neurons, but had no effect on AHPs evoked by uncaging intracellular Ca2+. Finally, wortmannin selectively reduced whole cell Ca2+ currents in OT neurons while leaving VP neurons unaffected. The results indicate that PIP2 modulates both the ImAHP and IsAHP in OT neurons, most likely by controlling Ca2+ entry through voltage‐gated Ca2+ channels opened during spike trains.
    May 15, 2017   doi: 10.1113/JP274219   open full text
  • Refuting the myth of non‐response to exercise training: ‘non‐responders’ do respond to higher dose of training.
    David Montero, Carsten Lundby.
    The Journal of Physiology. May 14, 2017
    Key points The prevalence of cardiorespiratory fitness (CRF) non‐response gradually declines in healthy individuals exercising 60, 120, 180, 240 or 300 min per week for 6 weeks. Following a successive identical 6‐week training period but comprising 120 min of additional exercise per week, CRF non‐response is universally abolished. The magnitude of CRF improvement is primarily attributed to changes in haemoglobin mass. The potential for CRF improvement may be present and unveiled with appropriate exercise training stimuli in healthy individuals without exception. Abstract One in five adults following physical activity guidelines are reported to not demonstrate any improvement in cardiorespiratory fitness (CRF). Herein, we sought to establish whether CRF non‐response to exercise training is dose‐dependent, using a between‐ and within‐subject study design. Seventy‐eight healthy adults were divided into five groups (1–5) respectively comprising one, two, three, four and five 60 min exercise sessions per week but otherwise following an identical 6‐week endurance training (ET) programme. Non‐response was defined as any change in CRF, determined by maximal incremental exercise power output (Wmax), within the typical error of measurement (±3.96%). Participants classified as non‐responders after the ET intervention completed a successive 6‐week ET period including two additional exercise sessions per week. Maximal oxygen consumption (V̇O2 max ), haematology and muscle biopsies were assessed prior to and after each ET period. After the first ET period, Wmax increased (P < 0.05) in groups 2, 3, 4 and 5, but not 1. In groups 1, 2, 3, 4 and 5, 69%, 40%, 29%, 0% and 0% of individuals, respectively, were non‐responders. After the second ET period, non‐response was eliminated in all individuals. The change in V̇O2 max with exercise training independently determined Wmax response (partial correlation coefficient, rpartial ≥ 0.74, P < 0.001). In turn, total haemoglobin mass was the strongest independent determinant of V̇O2 max (rpartial = 0.49, P < 0.001). In conclusion, individual CRF non‐response to exercise training is abolished by increasing the dose of exercise and primarily a function of haematological adaptations in oxygen‐carrying capacity.
    May 14, 2017   doi: 10.1113/JP273480   open full text
  • When size matters: transient receptor potential vanilloid 4 channel as a volume‐sensor rather than an osmo‐sensor.
    Trine L. Toft‐Bertelsen, David Križaj, Nanna MacAulay.
    The Journal of Physiology. May 14, 2017
    Key points Mammalian cells are frequently exposed to stressors causing volume changes. The transient receptor potential vanilloid 4 (TRPV4) channel translates osmotic stress into ion flux. The molecular mechanism coupling osmolarity to TRPV4 activation remains elusive. TRPV4 responds to isosmolar cell swelling and osmolarity translated via different aquaporins. TRPV4 functions as a volume‐sensing ion channel irrespective of the origin of the cell swelling. Abstract Transient receptor potential channel 4 of the vanilloid subfamily (TRPV4) is activated by a diverse range of molecular cues, such as heat, lipid metabolites and synthetic agonists, in addition to hyposmotic challenges. As a non‐selective cation channel permeable to Ca2+, it transduces physical stress in the form of osmotic cell swelling into intracellular Ca2+‐dependent signalling events. Its contribution to cell volume regulation might include interactions with aquaporin (AQP) water channel isoforms, although the proposed requirement for a TRPV4–AQP4 macromolecular complex remains to be resolved. To characterize the elusive mechanics of TRPV4 volume‐sensing, we expressed the channel in Xenopus laevis oocytes together with AQP4. Co‐expression with AQP4 facilitated the cell swelling induced by osmotic challenges and thereby activated TRPV4‐mediated transmembrane currents. Similar TRPV4 activation was induced by co‐expression of a cognate channel, AQP1. The level of osmotically‐induced TRPV4 activation, although proportional to the degree of cell swelling, was dependent on the rate of volume changes. Importantly, isosmotic cell swelling obtained by parallel activation of the co‐expressed water‐translocating Na+/K+/2Cl− cotransporter promoted TRPV4 activation despite the absence of the substantial osmotic gradients frequently employed for activation. Upon simultaneous application of an osmotic gradient and the selective TRPV4 agonist GSK1016790A, enhanced TRPV4 activation was observed only with subsaturating stimuli, indicating that the agonist promotes channel opening similar to that of volume‐dependent activation. We propose that, contrary to the established paradigm, TRPV4 is activated by increased cell volume irrespective of the molecular mechanism underlying cell swelling. Thus, the channel functions as a volume‐sensor, rather than as an osmo‐sensor.
    May 14, 2017   doi: 10.1113/JP274135   open full text
  • Sensitivity to ischaemia of single sympathetic nerve fibres innervating the dorsum of the human foot.
    W. J. Z'Graggen, R. Solà, N. E. Graf, J. Serra, H. Bostock.
    The Journal of Physiology. May 14, 2017
    Key points Changes in nerve conduction velocity following an impulse (i.e. velocity recovery cycles) reflect after‐potentials, and can provide an indication of altered nerve membrane properties. This study used microneurography to assess the effects of ischaemia on single human sympathetic fibres innervating the dorsum of the foot. It was found that velocity recovery cycles can distinguish whether a sympathetic nerve fibre is depolarized or not. The method may be used to detect membrane depolarization of sympathetic nerve fibres in human patients when autonomic neuropathy is suspected. Abstract The aim of this study was to determine whether velocity recovery cycles (VRCs) could detect the effects of ischaemia on sympathetic nerve fibres. VRCs of human sympathetic nerve fibres of the superficial peroneal nerve innervating the dorsum of the foot were recorded by microneurography in seven healthy volunteers. Sympathetic nerve fibres were identified by studying their response to manoeuvres increasing sympathetic outflow and by measuring activity‐dependent slowing at 2 Hz stimulation. VRCs were assessed at rest, during 30 min of induced limb ischaemia and during 20 min of recovery after ischaemia. From each VRC was measured the relative refractory period (RRP), the supernormality and the time to peak supernormality (SN@). During ischaemia, RRP increased from the baseline value of 37.4 ± 8.7 ms (mean ± SEM) to 67.1 ± 12.1 ms (P < 0.01) and SN@ increased from 68.6 ± 9.8 ms to 133.8 ± 11.0 ms (P < 0.005). The difference between SN@ and RRP separated ischaemic from non‐ischaemic sympathetic nerve fibres. It is concluded that these sympathetic nerve fibres are sensitive to ischaemia, and that VRCs provide a method to study changes of axonal membrane potential of human sympathetic nerve fibres in vivo.
    May 14, 2017   doi: 10.1113/JP274324   open full text
  • The impact of age and frailty on ventricular structure and function in C57BL/6J mice.
    H. A. Feridooni, A. E. Kane, O. Ayaz, A. Boroumandi, N. Polidovitch, R. G. Tsushima, R. A. Rose, S. E. Howlett.
    The Journal of Physiology. May 14, 2017
    Key points Heart size increases with age (called hypertrophy), and its ability to contract declines. However, these reflect average changes that may not be present, or present to the same extent, in all older individuals. That aging happens at different rates is well accepted clinically. People who are aging rapidly are frail and frailty is measured with a ‘frailty index’. We quantified frailty with a validated mouse frailty index tool and evaluated the impacts of age and frailty on cardiac hypertrophy and contractile dysfunction. Hypertrophy increased with age, while contractions, calcium currents and calcium transients declined; these changes were graded by frailty scores. Overall health status, quantified as frailty, may promote maladaptive changes associated with cardiac aging and facilitate the development of diseases such as heart failure. To understand age‐related changes in heart structure and function, it is essential to know both chronological age and the health status of the animal. Abstract On average, cardiac hypertrophy and contractile dysfunction increase with age. Still, individuals age at different rates and their health status varies from fit to frail. We investigated the influence of frailty on age‐dependent ventricular remodelling. Frailty was quantified as deficit accumulation in adult (≈7 months) and aged (≈27 months) C57BL/6J mice by adapting a validated frailty index (FI) tool. Hypertrophy and contractile function were evaluated in Langendorff‐perfused hearts; cellular correlates/mechanisms were investigated in ventricular myocytes. FI scores increased with age. Mean cardiac hypertrophy increased with age, but values in the adult and aged groups overlapped. When plotted as a function of frailty, hypertrophy was graded by FI score (r = 0.67–0.55, P < 0.0003). Myocyte area also correlated positively with FI (r = 0.34, P = 0.03). Left ventricular developed pressure (LVDP) plus rates of pressure development (+dP/dt) and decay (−dP/dt) declined with age and this was graded by frailty (r = −0.51, P = 0.0007; r = −0.48, P = 0.002; r = −0.56, P = 0.0002 for LVDP, +dP/dt and −dP/dt). Smaller, slower contractions graded by FI score were also seen in ventricular myocytes. Contractile dysfunction in cardiomyocytes isolated from frail mice was attributable to parallel changes in underlying Ca2+ transients. These changes were not due to reduced sarcoplasmic reticulum stores, but were graded by smaller Ca2+ currents (r = −0.40, P = 0.008), lower gain (r = −0.37, P = 0.02) and reduced expression of Cav1.2 protein (r = −0.68, P = 0.003). These results show that cardiac hypertrophy and contractile dysfunction in naturally aging mice are graded by overall health and suggest that frailty, in addition to chronological age, can help explain heterogeneity in cardiac aging.
    May 14, 2017   doi: 10.1113/JP274134   open full text
  • Endogenous nitric oxide formation in cardiac myocytes does not control respiration during β‐adrenergic stimulation.
    Michael Kohlhaas, Alexander G. Nickel, Stefanie Bergem, Barbara Casadei, Ulrich Laufs, Christoph Maack.
    The Journal of Physiology. May 14, 2017
    Key points In the heart, endothelial nitric oxide (NO) controls oxygen consumption in the working heart through paracrine mechanisms. While cardiac myocytes contain several isoforms of NO synthases, it is unclear whether these can control respiration in an intracrine fashion. A long‐standing controversy is whether a NOS exists within mitochondria. By combining fluorescence technologies with electrical field stimulation or the patch‐clamp technique in beating cardiac myocytes, we identified a neuronal NO synthase (nNOS) as the most relevant source of intracellular NO during β‐adrenergic stimulation, while no evidence for a mitochondria‐located NOS was obtained. The amounts of NO produced by non‐mitochondrial nNOS were insufficient to regulate respiration during β‐adrenergic stimulation, arguing against intracrine control of respiration by NO within cardiac myocytes. Abstract Endothelial nitric oxide (NO) controls cardiac oxygen (O2) consumption in a paracrine way by slowing respiration at the mitochondrial electron transport chain. While NO synthases (NOSs) are also expressed in cardiac myocytes, it is unclear whether they control respiration in an intracrine way. Furthermore, the existence of a mitochondrial NOS is controversial. Here, by combining fluorescence imaging with electrical field stimulation, the patch‐clamp method and knock‐out technology, we determined the sources and consequences of intracellular NO formation during workload transitions in isolated murine and guinea pig cardiac myocytes and mitochondria. Using 4‐amino‐5‐methylamino‐2′,7′‐difluorofluorescein diacetate (DAF) as a fluorescent NO‐sensor that locates to the cytosol and mitochondria, we observed that NO increased by ∼12% within 3 min of β‐adrenergic stimulation in beating cardiac myocytes. This NO stems from neuronal NOS (nNOS), but not endothelial (eNOS). After patch clamp‐mediated dialysis of cytosolic DAF, the remaining NO signals (mostly mitochondrial) were blocked by nNOS deletion, but not by inhibiting the mitochondrial Ca2+ uniporter with Ru360. While in isolated mitochondria exogenous NO inhibited respiration and reduced the NAD(P)H redox state, pyridine nucleotide redox states were unaffected by pharmacological or genetic disruption of endogenous nNOS or eNOS during workload transitions in cardiac myoctyes. We conclude that under physiological conditions, nNOS is the most relevant source for NO in cardiac myocytes, but this nNOS is not located in mitochondria and does not control respiration. Therefore, cardiac O2 consumption is controlled by endothelial NO in a paracrine, but not intracrine, fashion.
    May 14, 2017   doi: 10.1113/JP273750   open full text
  • Vasopressin casts light on the suprachiasmatic nucleus.
    Takahiro Tsuji, Andrew J. Allchorne, Meng Zhang, Chiharu Tsuji, Vicky A. Tobin, Rafael Pineda, Androniki Raftogianni, Javier E. Stern, Valery Grinevich, Gareth Leng, Mike Ludwig.
    The Journal of Physiology. May 14, 2017
    Key points A subpopulation of retinal ganglion cells expresses the neuropeptide vasopressin. These retinal ganglion cells project predominately to our biological clock, the suprachiasmatic nucleus (SCN). Light‐induced vasopressin release enhances the responses of SCN neurons to light. It also enhances expression of genes involved in photo‐entrainment of biological rhythms. Abstract In all animals, the transition between night and day engages a host of physiological and behavioural rhythms. These rhythms depend not on the rods and cones of the retina, but on retinal ganglion cells (RGCs) that detect the ambient light level in the environment. These project to the suprachiasmatic nucleus (SCN) of the hypothalamus to entrain circadian rhythms that are generated within the SCN. The neuropeptide vasopressin has an important role in this entrainment. Many SCN neurons express vasopressin, and it has been assumed that the role of vasopressin in the SCN reflects the activity of these cells. Here we show that vasopressin is also expressed in many retinal cells that project to the SCN. Light‐evoked vasopressin release contributes to the responses of SCN neurons to light, and enhances expression of the immediate early gene c‐fos in the SCN, which is involved in photic entrainment of circadian rhythms.
    May 14, 2017   doi: 10.1113/JP274025   open full text
  • Contribution of small conductance K+ channels to sinoatrial node pacemaker activity: insights from atrial‐specific Na+/Ca2+ exchange knockout mice.
    Angelo G. Torrente, Rui Zhang, Heidi Wang, Audrey Zaini, Brian Kim, Xin Yue, Kenneth D. Philipson, Joshua I. Goldhaber.
    The Journal of Physiology. May 13, 2017
    Key points Repolarizing currents through K+ channels are essential for proper sinoatrial node (SAN) pacemaking, but the influence of intracellular Ca2+ on repolarization in the SAN is uncertain. We identified all three isoforms of Ca2+‐activated small conductance K+ (SK) channels in the murine SAN. SK channel blockade slows repolarization and subsequent depolarization of SAN cells. In the atrial‐specific Na+/Ca2+ exchanger (NCX) knockout mouse, cellular Ca2+ accumulation during spontaneous SAN pacemaker activity produces intermittent hyperactivation of SK channels, leading to arrhythmic pauses alternating with bursts of pacing. These findings suggest that Ca2+‐sensitive SK channels can translate changes in cellular Ca2+ into a repolarizing current capable of modulating pacemaking. SK channels are a potential pharmacological target for modulating SAN rate or treating SAN dysfunction, particularly under conditions characterized by abnormal increases in diastolic Ca2+. Abstract Small conductance K+ (SK) channels have been implicated as modulators of spontaneous depolarization and electrical conduction that may be involved in cardiac arrhythmia. However, neither their presence nor their contribution to sinoatrial node (SAN) pacemaker activity has been investigated. Using quantitative PCR (q‐PCR), immunostaining and patch clamp recordings of membrane current and voltage, we identified all three SK isoforms (SK1, SK2 and SK3) in mouse SAN. Inhibition of SK channels with the specific blocker apamin prolonged action potentials (APs) in isolated SAN cells. Apamin also slowed diastolic depolarization and reduced pacemaker rate in isolated SAN cells and intact tissue. We investigated whether the Ca2+‐sensitive nature of SK channels could explain arrhythmic SAN pacemaker activity in the atrial‐specific Na+/Ca2+ exchange (NCX) knockout (KO) mouse, a model of cellular Ca2+ overload. SAN cells isolated from the NCX KO exhibited higher SK current than wildtype (WT) and apamin prolonged their APs. SK blockade partially suppressed the arrhythmic burst pacing pattern of intact NCX KO SAN tissue. We conclude that SK channels have demonstrable effects on SAN pacemaking in the mouse. Their Ca2+‐dependent activation translates changes in cellular Ca2+ into a repolarizing current capable of modulating regular pacemaking. This Ca2+ dependence also promotes abnormal automaticity when these channels are hyperactivated by elevated Ca2+. We propose SK channels as a potential target for modulating SAN rate, and for treating patients affected by SAN dysfunction, particularly in the setting of Ca2+ overload.
    May 13, 2017   doi: 10.1113/JP274249   open full text
  • Calcium‐activated BKCa channels govern dynamic membrane depolarizations of horizontal cells in rodent retina.
    Xiaoping Sun, Arlene A. Hirano, Nicholas C. Brecha, Steven Barnes.
    The Journal of Physiology. May 13, 2017
    Key points Large conductance, Ca2+‐activated K+ (BKCa) channels play important roles in mammalian retinal neurons, including photoreceptors, bipolar cells, amacrine cells and ganglion cells, but they have not been identified in horizontal cells. BKCa channel blockers paxilline and iberiotoxin, as well as Ca2+ free solutions and divalent cation Cav channel blockers, eliminate the outwardly rectifying current, while NS1619 enhances it. In symmetrical 150 mm K+, single channels had a conductance close to 250 pS, within the range of all known BKCa channels. In current clamped horizontal cells, BKCa channels subdue depolarizing membrane potential excursions, reduce the average resting potential and decrease oscillations. The results show that BKCa channel activation puts a ceiling on horizontal cell depolarization and regulates the temporal responsivity of the cells. Abstract Large conductance, calcium‐activated potassium (BKCa) channels have numerous roles in neurons including the regulation of membrane excitability, intracellular [Ca2+] regulation, and neurotransmitter release. In the retina, they have been identified in photoreceptors, bipolar cells, amacrine cells and ganglion cells, but have not been conclusively identified in mammalian horizontal cells. We found that outward current recorded between −30 and +60 mV is carried primarily in BKCa channels in isolated horizontal cells of rats and mice. Whole‐cell outward currents were maximal at +50 mV and declined at membrane potentials positive to this value. This current was eliminated by the selective BKCa channel blocker paxilline (100 nm), iberiotoxin (10 μm), Ca2+ free solutions and divalent cation Cav channel blockers. It was activated by the BKCa channel activator NS1619 (30 μm). Single channel recordings revealed the conductance of the channels to be 244 ± 11 pS (n = 17; symmetrical 150 mm K+) with open probability being both voltage‐ and Ca2+‐dependent. The channels showed fast activation kinetics in response to Ca2+ influx and inactivation gating that could be modified by intracellular protease treatment, which suggests β subunit involvement. Under current clamp, block of BKCa current increased depolarizing membrane potential excursions, raising the average resting potential and producing oscillations. BKCa current activation with NS1619 inhibited oscillations and hyperpolarized the resting potential. These effects underscore the functional role of BKCa current in limiting depolarization of the horizontal cell membrane potential and suggest actions of these channels in regulating the temporal responsivity of the cells.
    May 13, 2017   doi: 10.1113/JP274132   open full text
  • Non‐muscle (NM) myosin heavy chain phosphorylation regulates the formation of NM myosin filaments, adhesome assembly and smooth muscle contraction.
    Wenwu Zhang, Susan J. Gunst.
    The Journal of Physiology. May 08, 2017
    Key points Non‐muscle (NM) and smooth muscle (SM) myosin II are both expressed in smooth muscle tissues, however the role of NM myosin in SM contraction is unknown. Contractile stimulation of tracheal smooth muscle tissues stimulates phosphorylation of the NM myosin heavy chain on Ser1943 and causes NM myosin filament assembly at the SM cell cortex. Expression of a non‐phosphorylatable NM myosin mutant, NM myosin S1943A, in SM tissues inhibits ACh‐induced NM myosin filament assembly and SM contraction, and also inhibits the assembly of membrane adhesome complexes during contractile stimulation. NM myosin regulatory light chain (RLC) phosphorylation but not SM myosin RLC phosphorylation is regulated by RhoA GTPase during ACh stimulation, and NM RLC phosphorylation is required for NM myosin filament assembly and SM contraction. NM myosin II plays a critical role in airway SM contraction that is independent and distinct from the function of SM myosin. Abstract The molecular function of non‐muscle (NM) isoforms of myosin II in smooth muscle (SM) tissues and their possible role in contraction are largely unknown. We evaluated the function of NM myosin during contractile stimulation of canine tracheal SM tissues. Stimulation with ACh caused NM myosin filament assembly, as assessed by a Triton solubility assay and a proximity ligation assay aiming to measure interactions between NM myosin monomers. ACh stimulated the phosphorylation of NM myosin heavy chain on Ser1943 in tracheal SM tissues, which can regulate NM myosin IIA filament assembly in vitro. Expression of the non‐phosphorylatable mutant NM myosin S1943A in SM tissues inhibited ACh‐induced endogenous NM myosin Ser1943 phosphorylation, NM myosin filament formation, the assembly of membrane adhesome complexes and tension development. The NM myosin cross‐bridge cycling inhibitor blebbistatin suppressed adhesome complex assembly and SM contraction without inhibiting NM myosin Ser1943 phosphorylation or NM myosin filament assembly. RhoA inactivation selectively inhibited phosphorylation of the NM myosin regulatory light chain (RLC), NM myosin filament assembly and contraction, although it did not inhibit SM RLC phosphorylation. We conclude that the assembly and activation of NM myosin II is regulated during contractile stimulation of airway SM tissues by RhoA‐mediated NM myosin RLC phosphorylation and by NM myosin heavy chain Ser1943 phosphorylation. NM myosin II actomyosin cross‐bridge cycling regulates the assembly of membrane adhesome complexes that mediate the cytoskeletal processes required for tension generation. NM myosin II plays a critical role in airway SM contraction that is independent and distinct from the function of SM myosin.
    May 08, 2017   doi: 10.1113/JP273906   open full text
  • Phosphate increase during fatigue affects crossbridge kinetics in intact mouse muscle at physiological temperature.
    M. Nocella, G. Cecchi, B. Colombini.
    The Journal of Physiology. May 08, 2017
    Key points Actomyosin ATP hydrolysis occurring during muscle contraction releases inorganic phosphate [Pi] in the myoplasm. High [Pi] reduces force and affects force kinetics in skinned muscle fibres at low temperature. These effects decrease at high temperature, raising the question of their importance under physiological conditions. This study provides the first analysis of the effects of Pi on muscle performance in intact mammalian fibres at physiological temperature. Myoplasmic [Pi] was raised by fatiguing the fibres with a series of tetanic contractions. [Pi] increase reduces muscular force mainly by decreasing the force of the single molecular motor, the crossbridge, and alters the crossbridge response to fast length perturbation indicating faster kinetics. These results are in agreement with schemes of actomyosin ATPase and the crossbridge cycle including a low‐ or no‐force state and show that fibre length changes perturb the Pi‐sensitive force generation of the crossbridge cycle. Abstract Actomyosin ATP hydrolysis during muscle contraction releases inorganic phosphate, increasing [Pi] in the myoplasm. Experiments in skinned fibres at low temperature (10–12°C) have shown that [Pi] increase depresses isometric force and alters the kinetics of actomyosin interaction. However, the effects of Pi decrease with temperature and this raises the question of the role of Pi under physiological conditions. The present experiments were performed to investigate this point. Intact fibre bundles isolated from the flexor digitorum brevis of C57BL/6 mice were stimulated with a series of tetanic contractions at 1.5 s intervals at 33°C. As show previously the most significant change induced by a bout of contractile activity similar to the initial 10 tetani of the series was an increase of [Pi] without significant Ca2+ or pH changes. Measurements of force, stiffness and responses to fast stretches and releases were therefore made on the 10th tetanus of the series and compared with control. We found that (i) tetanic force at the 10th tetanus was ∼20% smaller than control without a significant decrease of crossbridge stiffness; and (ii) the force recovery following quick stretches and releases was faster than in control. These results indicate that at physiological temperature the increase of [Pi] occurring during early fatigue reduces tetanic force mainly by depressing the individual crossbridge force and accelerating crossbridge kinetics.
    May 08, 2017   doi: 10.1113/JP273672   open full text
  • Maternal chronic hypoxia increases expression of genes regulating lung liquid movement and surfactant maturation in male fetuses in late gestation.
    Erin V. McGillick, Sandra Orgeig, Beth J. Allison, Kirsty L. Brain, Youguo Niu, Nozomi Itani, Katie L. Skeffington, Andrew D. Kane, Emilio A. Herrera, Dino A. Giussani, Janna L. Morrison.
    The Journal of Physiology. May 07, 2017
    Key points Chronic fetal hypoxaemia is a common pregnancy complication associated with intrauterine growth restriction that may influence respiratory outcome at birth. We investigated the effect of maternal chronic hypoxia for a month in late gestation on signalling pathways regulating fetal lung maturation and the transition to air‐breathing at birth using isobaric hypoxic chambers without alterations to maternal food intake. Maternal chronic hypoxia in late gestation increases fetal lung expression of genes regulating hypoxia signalling, lung liquid reabsorption and surfactant maturation, which may be an adaptive response in preparation for the successful transition to air‐breathing at birth. In contrast to other models of chronic fetal hypoxaemia, late gestation onset fetal hypoxaemia promotes molecular regulation of fetal lung maturation. This suggests a differential effect of timing and duration of fetal chronic hypoxaemia on fetal lung maturation, which supports the heterogeneity observed in respiratory outcomes in newborns following exposure to chronic hypoxaemia in utero. Abstract Chronic fetal hypoxaemia is a common pregnancy complication that may arise from maternal, placental and/or fetal factors. Respiratory outcome of the infant at birth likely depends on the duration, timing and severity of the hypoxaemic insult. We have isolated the effect of maternal chronic hypoxia (MCH) for a month in late gestation on fetal lung development. Pregnant ewes were exposed to normoxia (21% O2) or hypoxia (10% O2) from 105 to 138 days of gestation (term ∼145 days). At 138 days, gene expression in fetal lung tissue was determined by quantitative RT‐PCR. Cortisol concentrations were determined in fetal plasma and lung tissue. Numerical density of surfactant protein positive cells was determined by immunohistochemistry. MCH reduced maternal PaO2 (106 ± 2.9 vs. 47 ± 2.8 mmHg) and fetal body weight (4.0 ± 0.4 vs. 3.2 ± 0.9 kg). MCH increased fetal lung expression of the anti‐oxidant marker CAT and decreased expression of the pro‐oxidant marker NOX‐4. MCH increased expression of genes regulating hypoxia signalling and feedback (HIF‐3α, KDM3A, SLC2A1, EGLN‐3). There was no effect of MCH on fetal plasma/lung tissue cortisol concentrations, nor genes regulating glucocorticoid signalling (HSD11B‐1, HSD11B‐2, NR3C1, NR3C2). MCH increased expression of genes regulating sodium (SCNN1‐B, ATP1‐A1, ATP1‐B1) and water (AQP‐4) movement in the fetal lung. MCH promoted surfactant maturation (SFTP‐B, SFTP‐D, ABCA3) at the molecular level, but did not alter the numerical density of surfactant positive cells in lung tissue. MCH in late gestation promotes molecular maturation of the fetal lung, which may be an adaptive response in preparation for the successful transition to air‐breathing at birth.
    May 07, 2017   doi: 10.1113/JP273842   open full text
  • Both standing and postural threat decrease Achilles’ tendon reflex inhibition from tendon electrical stimulation.
    Brian C. Horslen, J. Timothy Inglis, Jean‐Sébastien Blouin, Mark G. Carpenter.
    The Journal of Physiology. May 04, 2017
    Key points Golgi tendon organs (GTOs) and associated Ib reflexes contribute to standing balance, but the potential impacts of threats to standing balance on Ib reflexes are unknown. Tendon electrical stimulation to the Achilles’ tendon was used to probe changes in Ib inhibition in medial gastrocnemius with postural orientation (lying prone vs. upright standing; experiment 1) and height‐induced postural threat (standing at low and high surface heights; experiment 2). Ib inhibition was reduced while participants stood upright, compared to lying prone (42.2%); and further reduced when standing in the high, compared to low, threat condition (32.4%). These experiments will impact future research because they demonstrate that tendon electrical stimulation can be used to probe Ib reflexes in muscles engaged in standing balance. These results provide novel evidence that human short‐latency GTO‐Ib reflexes are dependent upon both task, as evidenced by changes with postural orientation, and context, such as height‐induced postural threat during standing. Abstract Golgi tendon organ Ib reflexes are thought to contribute to standing balance control, but it is unknown if they are modulated when people are exposed to a postural threat. We used a novel application of tendon electrical stimulation (TStim) to elicit Ib inhibitory reflexes in the medial gastrocnemius, while actively engaged in upright standing balance, to examine (a) how Ib reflexes to TStim are influenced by upright stance, and (b) the effects of height‐induced postural threat on Ib reflexes during standing. TStim evoked short‐latency (<47 ms) inhibition apparent in trigger‐averaged rectified EMG, which was quantified in terms of area, duration and mean amplitude of inhibition. In order to validate the use of TStim in a standing model, TStim‐Ib inhibition was compared from conditions where participants were lying prone vs. standing upright. TStim evoked Ib inhibition in both conditions; however, significant reductions in Ib inhibition area (42.2%) and duration (32.9%) were observed during stance. Postural threat, manipulated by having participants stand at LOW (0.8 m high, 0.6 m from edge) and HIGH (3.2 m, at edge) elevated surfaces, significantly reduced Ib inhibition area (32.4%), duration (16.4%) and amplitude (24.8%) in the HIGH, compared to LOW, threat condition. These results demonstrate TStim is a viable technique for investigating Ib reflexes in standing, and confirm Ib reflexes are modulated with postural orientation. The novel observation of reduced Ib inhibition with elevated postural threat reveals that human Ib reflexes are context dependent, and the human Ib reflex pathways are modulated by threat or emotional processing centres of the CNS.
    May 04, 2017   doi: 10.1113/JP273935   open full text
  • Mechanically sensitive Aδ nociceptors that innervate bone marrow respond to changes in intra‐osseous pressure.
    Sara Nencini, Jason Ivanusic.
    The Journal of Physiology. May 04, 2017
    Key points Sensory neurons that innervate the bone marrow provide the CNS with information about pain associated with bone disease and pathology, but little is known of their function. Here we use a novel in vivo bone–nerve electrophysiological preparation to study how they respond to noxious mechanical stimulation delivered by increasing intra‐osseous pressure. We provide evidence that sensory neurons that innervate the bone marrow respond to high threshold noxious mechanical stimulation, have response properties consistent with a role in nociception, provide information about different features of an intra‐osseous pressure stimulus and express the Piezo2 mechano‐transducer molecule. Our findings show how some bone marrow nociceptors signal pain in bony diseases and pathologies that involve a mechanical disturbance or increased intra‐osseous pressure, and that the Piezo2 mechano‐transducer may be involved. Abstract Whilst the sensory neurons and nerve terminals that innervate bone marrow have a morphology and molecular phenotype consistent with a role in nociception, little is known about their physiology or the mechanisms that generate and maintain bone pain. In the present study, we provide evidence that Aδ nociceptors that innervate the bone marrow respond to high threshold noxious mechanical stimulation, exhibit fatigue in response to prior stimulation and in some cases can be sensitized by capsaicin. They can be classified on the basis of their response properties as either phasic–tonic units that appear to code for different intensities of intra‐osseous pressure, or phasic units that code for the rate of change in intra‐osseous pressure. Three different subclasses of mechanically sensitive Aδ units were observed: phasic units that were sensitized by capsaicin, phasic units that were not sensitized by capsaicin and phasic–tonic units (that were not sensitized by capsaicin). These could also, in part, be distinguished by differences in their thresholds for activation, mean discharge frequency, latency to peak activation and peak‐to‐peak action potential amplitude. The majority of small‐diameter myelinated sensory neurons projecting to the bone marrow expressed Piezo2. Our findings indicate that Aδ mechano‐nociceptors are likely to play an important role in generating and maintaining pain in response to bony pathologies that involve a mechanical disturbance or increased intra‐osseous pressure, and imply that Piezo2 signalling may be involved in mechano‐transduction in these receptors.
    May 04, 2017   doi: 10.1113/JP273877   open full text
  • SIRT1 may play a crucial role in overload‐induced hypertrophy of skeletal muscle.
    Erika Koltai, Zoltán Bori, Clovis Chabert, Hervé Dubouchaud, Hisashi Naito, Shuichi Machida, Kelvin JA Davies, Zsolt Murlasits, Andrew C Fry, Istvan Boldogh, Zsolt Radak.
    The Journal of Physiology. April 28, 2017
    Key points Silent mating type information regulation 2 homologue 1 (SIRT1) activity and content increased significantly in overload‐induced hypertrophy. SIRT1‐mediated signalling through Akt, the endothelial nitric oxide synthase mediated pathway, regulates anabolic process in the hypertrophy of skeletal muscle. The regulation of catabolic signalling via forkhead box O 1 and protein ubiquitination is SIRT1 dependent. Overload‐induced changes in microRNA levels regulate SIRT1 and insulin‐like growth factor 1 signalling. Abstract Significant skeletal muscle mass guarantees functional wellbeing and is important for high level performance in many sports. Although the molecular mechanism for skeletal muscle hypertrophy has been well studied, it still is not completely understood. In the present study, we used a functional overload model to induce plantaris muscle hypertrophy by surgically removing the soleus and gastrocnemius muscles in rats. Two weeks of muscle ablation resulted in a 40% increase in muscle mass, which was associated with a significant increase in silent mating type information regulation 2 homologue 1 (SIRT1) content and activity (P < 0.001). SIRT1‐regulated Akt, endothelial nitric oxide synthase and GLUT4 levels were also induced in hypertrophied muscles, and SIRT1 levels correlated with muscle mass, paired box protein 7 (Pax7), proliferating cell nuclear antigen (PCNA) and nicotinamide phosphoribosyltransferase (Nampt) levels. Alternatively, decreased forkhead box O 1 (FOXO1) and increased K48 polyubiquitination also suggest that SIRT1 could be involved in the catabolic process of hypertrophy. Furthermore, increased levels of K63 and muscle RING finger 2 (MuRF2) protein could also be important enhancers of muscle mass. We report here that the levels of miR1 and miR133a decrease in hypertrophy and negatively correlate with muscle mass, SIRT1 and Nampt levels. Our results reveal a strong correlation between SIRT1 levels and activity, SIRT1‐regulated pathways and overload‐induced hypertrophy. These findings, along with the well‐known regulatory roles that SIRT1 plays in modulating both anabolic and catabolic pathways, allow us to propose the hypothesis that SIRT1 may actually play a crucial causal role in overload‐induced hypertrophy of skeletal muscle. This hypothesis will now require rigorous direct and functional testing.
    April 28, 2017   doi: 10.1113/JP273774   open full text
  • Impact of perinatal exposure to sucrose or high fructose corn syrup (HFCS‐55) on adiposity and hepatic lipid composition in rat offspring.
    Carla R. Toop, Beverly S. Muhlhausler, Kerin O'Dea, Sheridan Gentili.
    The Journal of Physiology. April 26, 2017
    Perinatal exposure to excess maternal intake of added sugars, including fructose and sucrose, is associated with an increased risk of obesity and type 2 diabetes in adult life. However, it is unknown to what extent the type of sugar and the timing of exposure affect these outcomes. The aim of this study was to determine the impact of exposure to maternal consumption of a 10% w/v beverage containing sucrose or high fructose corn syrup‐55 (HFCS‐55) during the prenatal and/or suckling periods on offspring at 3 and 12 weeks, utilising a cross‐fostering approach in a rodent model. Perinatal sucrose exposure decreased plasma glucose concentrations in offspring at 3 weeks, but did not alter glucose tolerance. Increased adiposity was observed in 3‐week‐old offspring exposed to sucrose or HFCS‐55 during suckling, with increased hepatic fat content in HFCS‐55‐exposed offspring. In terms of specific fatty acids, hepatic monounsaturated (omega‐7 and ‐9) fatty acid content was elevated at weaning, and was most pronounced in sucrose offspring exposed during both the prenatal and suckling periods, and HFCS‐55 offspring exposed during suckling only. By 12 weeks, the effects on adiposity and hepatic lipid composition were largely normalised. However, exposure to either sucrose or HFCS‐55 during the prenatal period only was associated with elevated plasma free fatty acids at weaning, and this effect persisted until 12 weeks. This study suggests that the type of sugar and the timing of exposure (prenatal or suckling periods) are both important for determining the impact on metabolic health outcomes in the offspring. This article is protected by copyright. All rights reserved
    April 26, 2017   doi: 10.1113/JP274066   open full text
  • Functional impact of an oculopharyngeal muscular dystrophy mutation in PABPN1.
    Maricela García‐Castañeda, Ana Victoria Vega, Rocío Rodríguez, Maria Guadalupe Montiel‐Jaen, Bulmaro Cisneros, Angel Zarain‐Herzberg, Guillermo Avila.
    The Journal of Physiology. April 25, 2017
    Key points Mutations in the gene encoding poly(A)‐binding protein nuclear 1 (PABPN1) result in oculopharyngeal muscular dystrophy (OPMD). This disease is of late‐onset, but the underlying mechanism is unclear. Ca2+ stimulates muscle growth and contraction and, because OPMD courses with muscle atrophy and weakness, we hypothesized that the homeostasis of Ca2+ is altered in this disorder. C2C12 myotubes were transfected with cDNAs encoding either PABPN1 or the PABPN1‐17A OPMD mutation. Subsequently, they were investigated concerning not only excitation–contraction coupling (ECC) and intracellular levels of Ca2+, but also differentiation stage and nuclear structure. PABPN1‐17A gave rise to: inhibition of Ca2+ release during ECC, depletion of sarcoplasmic reticulum Ca2+ content, reduced expression of ryanodine receptors, altered nuclear morphology and incapability to stimulate myoblast fusion. PABPN1‐17A failed to inhibit ECC in adult muscle fibres, suggesting that its effects are primarily related to muscle regeneration. Abstract Oculopharyngeal muscular dystrophy (OPMD) is linked to mutations in the gene encoding poly(A)‐binding protein nuclear 1 (PABPN1). OPMD mutations consist of an expansion of a tract that contains 10 alanines (to 12–17). This disease courses with muscle weakness that begins in adulthood, but the underlying mechanism is unclear. In the present study, we investigated the functional effects of PABPN1 and an OPMD mutation (PABPN1‐17A) using myotubes transfected with cDNAs encoding these proteins (GFP‐tagged). PABPN1 stimulated myoblast fusion (100%), whereas PABPN1‐17A failed to mimic this effect. Additionally, the OPMD mutation markedly altered nuclear morphology; specifically, it led to nuclei with a more convoluted and ovoid shape. Although PABPN1 and PABPN1‐17A modified the expression of sarcoplasmic/endoplasmic reticulum Ca2+‐ATPase and calsequestrin, the corresponding changes did not have a clear impact on [Ca2+]. Interestingly, neither L‐type Ca2+ channels, nor voltage‐gated sarcoplasmic reticulum (SR) Ca2+ release (VGCR) was altered by PABPN1. However, PABPN1‐17A produced a selective inhibition of VGCR (50%). This effect probably arises from both lower expression of RyR1 and depletion of SR Ca2+. The latter, however, was not related to inhibition of store‐operated Ca2+ entry. Both PABPN1 constructs promoted a moderated decrease in cytosolic [Ca2+], which apparently results from down‐regulation of excitation‐coupled Ca2+ entry. On the other hand, PABPN1‐17A did not alter ECC in muscle fibres, suggesting that adult muscle is less prone to developing deleterious effects. These results demonstrate that PABPN1 proteins regulate essential processes during myotube formation and support the notion that OPMD involves disruption of myogenesis, nuclear structure and homeostasis of Ca2+.
    April 25, 2017   doi: 10.1113/JP273948   open full text
  • In vitro characterization of cell‐level neurophysiological diversity in the rostral nucleus reuniens of adult mice.
    Darren A. Walsh, Jonathan T. Brown, Andrew D. Randall.
    The Journal of Physiology. April 25, 2017
    Key points The nucleus reuniens (Re), a nucleus of the midline thalamus, is part of a cognitive network including the hippocampus and the medial prefrontal cortex. To date, very few studies have examined the electrophysiological properties of Re neurons at a cellular level. The majority of Re neurons exhibit spontaneous action potential firing at rest. This is independent of classical amino‐acid mediated synaptic transmission. When driven by various forms of depolarizing current stimulus, Re neurons display considerable diversity in their firing patterns. As a result of the presence of a low threshold Ca2+ channel, spike output functions are strongly modulated by the prestimulus membrane potential. Finally, we describe a novel form of activity‐dependant intrinsic plasticity that eliminates the high‐frequency burst firing present in many Re neurons. These results provide a comprehensive summary of the intrinsic electrophysiological properties of Re neurons allowing us to better consider the role of the Re in cognitive processes. Abstract The nucleus reuniens (Re) is the largest of the midline thalamic nuclei. We have performed a detailed neurophysiological characterization of neurons in the rostral Re of brain slices prepared from adult male mice. At resting potential (−63.7 ± 0.6 mV), ∼90% of Re neurons fired action potentials, typically continuously at ∼8 Hz. Although Re neurons experience a significant spontaneous barrage of fast, amino‐acid‐mediate synaptic transmission, this was not predominantly responsible for spontaneous spiking because firing persisted in the presence of glutamate and GABA receptor antagonists. With resting potential preset to −80 mV, −20 pA current injections revealed a mean input resistance of 615 MΩ and a mean time constant of 38 ms. Following cessation of this stimulus, a significant rebound potential was seen that was sometimes sufficiently large to trigger a short burst of very high frequency (100–300 Hz) firing. In most cells, short (2 ms), strong (2 nA) current injections elicited a single spike followed by a large afterdepolarizing potential which, when suprathreshold, generated high‐frequency spiking. Similarly, in the majority of cells preset at −80 mV, 500 ms depolarizing current injections to cells led to a brief initial burst of very high‐frequency firing, although this was lost when cells were preset at −72 mV. Biophysical and pharmacological experiments indicate a prominent role for T‐type Ca2+ channels in the high‐frequency bursting of Re neurons. Finally, we describe a novel form of activity‐dependent intrinsic plasticity that persistently eliminates the burst firing potential of Re neurons.
    April 25, 2017   doi: 10.1113/JP273915   open full text
  • Evidence that 5‐HT stimulates intracellular Ca2+ signalling and activates pannexin‐1 currents in type II cells of the rat carotid body.
    Sindhubarathi Murali, Min Zhang, Colin A. Nurse.
    The Journal of Physiology. April 25, 2017
    Key points 5‐HT is a neuromodulator released from carotid body (CB) chemoreceptor (type I) cells and facilitates the sensory discharge following chronic intermittent hypoxia (CIH). In the present study, we show that, in addition to type I cells, adjacent glial‐like type II cells express functional, ketanserin‐sensitive 5‐HT2 receptors, and their stimulation increases cytoplasmic Ca2+ derived from intracellular stores. In type II cells, 5‐HT activated a ketanserin‐sensitive inward current (I5‐HT) that was similar to that (IUTP) activated by the P2Y2R agonist, UTP. As previously shown for IUTP, I5‐HT was inhibited by BAPTA‐AM and carbenoxolone (5 μm), a putative blocker of ATP‐permeable pannexin (Panx)‐1 channels; IUTP was reversibly inhibited by the specific Panx‐1 mimetic peptide channel blocker, 10Panx peptide. Paracrine stimulation of type II cells by 5‐HT, leading to ATP release via Panx‐1 channels, may contribute to CB excitability, especially in pathophysiological conditions associated with CIH (e.g. obstructive sleep apnoea). Abstract Carotid body (CB) chemoreceptor (type I) cells can synthesize and release 5‐HT and increased autocrine–paracrine 5‐HT2 receptor signalling contributes to sensory long‐term facilitation during chronic intermittent hypoxia (CIH). However, recent studies suggest that adjacent glial‐like type II cells can respond to CB paracrine signals by elevating intracellular calcium (Δ[Ca2+]i) and activating carbenoxolone‐sensitive, ATP‐permeable, pannexin (Panx)‐1‐like channels. In the present study, using dissociated rat CB cultures, we found that 5‐HT induced Δ[Ca2+]i responses in a subpopulation of type I cells, as well as in most (∼67%) type II cells identified by their sensitivity to the P2Y2 receptor agonist, UTP. The 5‐HT‐induced Ca2+ response in type II cells was dose‐dependent (EC50 ∼183 nm) and largely inhibited by the 5‐HT2A receptor blocker, ketanserin (1 μm), and also arose mainly from intracellular stores. 5‐HT also activated an inward current (I5‐HT) in type II cells (EC50 ∼200 nm) that was reversibly inhibited by ketanserin (1–10 nm), the Ca2+ chelator BAPTA‐AM (5 μm), and low concentrations of carbenoxolone (5 μm), a putative Panx‐1 channel blocker. I5‐HT reversed direction at approximately −11 mV and was indistinguishable from the UTP‐activated current (IUTP). Consistent with a role for Panx‐1 channels, IUTP was reversibly inhibited by the specific Panx‐1 mimetic peptide blocker 10Panx (100 μm), although not by its scrambled control peptide (scPanx). Because ATP is an excitatory CB neurotransmitter, it is possible that the contribution of enhanced 5‐HT signalling to the increased sensory discharge during CIH may occur, in part, by a boosting of ATP release from type II cells via Panx‐1 channels.
    April 25, 2017   doi: 10.1113/JP273473   open full text
  • Characterisation and functional mapping of surface potentials in the rat dorsal column nuclei.
    Alastair J. Loutit, Ted Maddess, Stephen J. Redmond, John W. Morley, Greg J. Stuart, Jason R. Potas.
    The Journal of Physiology. April 25, 2017
    Key points The brainstem dorsal column nuclei (DCN) process sensory information arising from the body before it reaches the brain and becomes conscious. Despite significant investigations into sensory coding in peripheral nerves and the somatosensory cortex, little is known about how sensory information arising from the periphery is represented in the DCN. Following stimulation of hind‐limb nerves, we mapped and characterised the evoked electrical signatures across the DCN surface. We show that evoked responses recorded from the DCN surface are highly reproducible and are unique to nerves carrying specific sensory information. Abstract The brainstem dorsal column nuclei (DCN) play a role in early processing of somatosensory information arising from a variety of functionally distinct peripheral structures, before being transmitted to the cortex via the thalamus. To improve our understanding of how sensory information is represented by the DCN, we characterised and mapped low‐ (<200 Hz) and high‐frequency (550–3300 Hz) components of nerve‐evoked DCN surface potentials. DCN surface potentials were evoked by electrical stimulation of the left and right nerves innervating cutaneous structures (sural nerve), or a mix of cutaneous and deep structures (peroneal nerve), in 8‐week‐old urethane‐anaesthetised male Wistar rats. Peroneal nerve‐evoked DCN responses demonstrated low‐frequency events with significantly longer durations, more high‐frequency events and larger magnitudes compared to responses evoked from sural nerve stimulation. Hotspots of low‐ and high‐frequency DCN activity were found ipsilateral to stimulated nerves but were not symmetrically organised. In conclusion, we find that sensory inputs from peripheral nerves evoke unique and characteristic DCN activity patterns that are highly reproducible both within and across animals.
    April 25, 2017   doi: 10.1113/JP273759   open full text
  • Early structural and functional signature of 3‐day human skeletal muscle disuse using the dry immersion model.
    Rémi Demangel, Loïc Treffel, Guillaume Py, Thomas Brioche, Allan F. Pagano, Marie‐Pierre Bareille, Arnaud Beck, Laurence Pessemesse, Robin Candau, Claude Gharib, Angèle Chopard, Catherine Millet.
    The Journal of Physiology. April 23, 2017
    Key points Our study contributes to the characterization of muscle loss and weakness processes induced by a sedentary life style, chronic hypoactivity, clinical bed rest, immobilization and microgravity. This study, by bringing together integrated and cellular evaluation of muscle structure and function, identifies the early functional markers and biomarkers of muscle deconditioning. Three days of muscle disuse in healthy adult subjects is sufficient to significantly decrease muscle mass, tone and force, and to induce changes in function relating to a weakness in aerobic metabolism and muscle fibre denervation. The outcomes of this study should be considered in the development of an early muscle loss prevention programme and/or the development of pre‐conditioning programmes required before clinical bed rest, immobilization and spaceflight travel. Abstract Microgravity and hypoactivity are associated with skeletal muscle deconditioning. The decrease of muscle mass follows an exponential decay, with major changes in the first days. The purpose of the study was to dissect out the effects of a short‐term 3‐day dry immersion (DI) on human quadriceps muscle function and structure. The DI model, by suppressing all support zones, accurately reproduces the effects of microgravity. Twelve healthy volunteers (32 ± 5 years) completed 3 days of DI. Muscle function was investigated through maximal voluntary contraction (MVC) tests and muscle viscoelasticity. Structural experiments were performed using MRI analysis and invasive experiments on muscle fibres. Our results indicated a significant 9.1% decrease of the normalized MVC constant (P = 0.048). Contraction and relaxation modelization kinetics reported modifications related to torque generation (kACT = −29%; P = 0.014) and to the relaxation phase (kREL = +34%; P = 0.040) after 3 days of DI. Muscle viscoelasticity was also altered. From day one, rectus femoris stiffness and tone decreased by, respectively, 7.3% (P = 0.002) and 10.2% (P = 0.002), and rectus femoris elasticity decreased by 31.5% (P = 0.004) after 3 days of DI. At the cellular level, 3 days of DI translated into a significant atrophy of type I muscle fibres (−10.6 ± 12.1%, P = 0.027) and an increased proportion of hybrid, type I/IIX fibre co‐expression. Finally, we report an increase (6‐fold; P = 0.002) in NCAM+ muscle fibres, showing an early denervation process. This study is the first to report experiments performed in Europe investigating human short‐term DI‐induced muscle adaptations, and contributes to deciphering the early changes and biomarkers of skeletal muscle deconditioning.
    April 23, 2017   doi: 10.1113/JP273895   open full text
  • Direct current stimulation boosts synaptic gain and cooperativity in vitro.
    Asif Rahman, Belen Lafon, Lucas C. Parra, Marom Bikson.
    The Journal of Physiology. April 23, 2017
    Key points Direct current stimulation (DCS) polarity specifically modulates synaptic efficacy during a continuous train of presynaptic inputs, despite synaptic depression. DCS polarizes afferent axons and postsynaptic neurons, boosting cooperativity between synaptic inputs. Polarization of afferent neurons in upstream brain regions may modulate activity in the target brain region during transcranial DCS (tDCS). A statistical theory of coincident activity predicts that the diffuse and weak profile of current flow can be advantageous in enhancing connectivity between co‐active brain regions. Abstract Transcranial direct current stimulation (tDCS) produces sustained and diffuse current flow in the brain with effects that are state dependent and outlast stimulation. A mechanistic explanation for tDCS should capture these spatiotemporal features. It remains unclear how sustained DCS affects ongoing synaptic dynamics and how modulation of afferent inputs by diffuse stimulation changes synaptic activity at the target brain region. We tested the effect of acute DCS (10–20 V m−1 for 3–5 s) on synaptic dynamics with constant rate (5–40 Hz) and Poisson‐distributed (4 Hz mean) trains of presynaptic inputs. Across tested frequencies, sustained synaptic activity was modulated by DCS with polarity‐specific effects. Synaptic depression attenuates the sensitivity to DCS from 1.1% per V m−1 to 0.55%. DCS applied during synaptic activity facilitates cumulative neuromodulation, potentially reversing endogenous synaptic depression. We establish these effects are mediated by both postsynaptic membrane polarization and afferent axon fibre polarization, which boosts cooperativity between synaptic inputs. This potentially extends the locus of neuromodulation from the nominal target to afferent brain regions. Based on these results we hypothesized the polarization of afferent neurons in upstream brain regions may modulate activity in the target brain region during tDCS. A multiscale model of transcranial electrical stimulation including a finite element model of brain current flow, numerical simulations of neuronal activity, and a statistical theory of coincident activity predicts that the diffuse and weak profile of current flow can be advantageous. Thus, we propose that specifically because tDCS is diffuse, weak and sustained it can boost connectivity between co‐active brain regions.
    April 23, 2017   doi: 10.1113/JP273005   open full text
  • Preservation of skeletal muscle mitochondrial content in older adults: relationship between mitochondria, fibre type and high‐intensity exercise training.
    Victoria L. Wyckelsma, Itamar Levinger, Michael J. McKenna, Luke E. Formosa, Michael T. Ryan, Aaron C. Petersen, Mitchell J. Anderson, Robyn M. Murphy.
    The Journal of Physiology. April 23, 2017
    Key points Ageing is associated with an upregulation of mitochondrial dynamics proteins mitofusin 2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49) in human skeletal muscle with the increased abundance of Mfn2 being exclusive to type II muscle fibres. These changes occur despite a similar content of mitochondria, as measured by COXIV, NDUFA9 and complexes in their native states (Blue Native PAGE). Following 12 weeks of high‐intensity training (HIT), older adults exhibit a robust increase in mitochondria content, while there is a decline in Mfn2 in type II fibres. We propose that the upregulation of Mfn2 and MiD49 with age may be a protective mechanism to protect against mitochondrial dysfunction, in particularly in type II skeletal muscle fibres, and that exercise may have a unique protective effect negating the need for an increased turnover of mitochondria. Abstract Mitochondrial dynamics proteins are critical for mitochondrial turnover and maintenance of mitochondrial health. High‐intensity interval training (HIT) is a potent training modality shown to upregulate mitochondrial content in young adults but little is known about the effects of HIT on mitochondrial dynamics proteins in older adults. This study investigated the abundance of protein markers for mitochondrial dynamics and mitochondrial content in older adults compared to young adults. It also investigated the adaptability of mitochondria to 12 weeks of HIT in older adults. Both older and younger adults showed a higher abundance of mitochondrial respiratory chain subunits COXIV and NDUFA9 in type I compared with type II fibres, with no difference between the older adults and young groups. In whole muscle homogenates, older adults had higher mitofusin‐2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49) contents compared to the young group. Also, older adults had higher levels of Mfn2 in type II fibres compared with young adults. Following HIT in older adults, MiD49 and Mfn2 levels were not different in whole muscle and Mfn2 content decreased in type II fibres. Increases in citrate synthase activity (55%) and mitochondrial respiratory chain subunits COXIV (37%) and NDUFA9 (48%) and mitochondrial respiratory chain complexes (∼70–100%) were observed in homogenates and/or single fibres. These findings reveal (i) a similar amount of mitochondria in muscle from young and healthy older adults and (ii) a robust increase of mitochondrial content following 12 weeks of HIT exercise in older adults.
    April 23, 2017   doi: 10.1113/JP273950   open full text
  • mGluR1 enhances efferent inhibition of inner hair cells in the developing rat cochlea.
    Zhanlei Ye, Juan D. Goutman, Sonja J. Pyott, Elisabeth Glowatzki.
    The Journal of Physiology. April 21, 2017
    Key points Spontaneous activity of the sensory inner hair cells shapes maturation of the developing ascending (afferent) auditory system before hearing begins. Just before the onset of hearing, descending (efferent) input from cholinergic neurons originating in the brainstem inhibit inner hair cell spontaneous activity and may further refine maturation. We show that agonist activation of the group I metabotropic glutamate receptor mGluR1 increases the strength of this efferent inhibition by enhancing the presynaptic release of acetylcholine. We further show that the endogenous release of glutamate from the inner hair cells may increase the strength of efferent inhibition via the activation of group I metabotropic glutamate receptors. Thus, before the onset of hearing, metabotropic glutamate signalling establishes a local negative feedback loop that is positioned to regulate inner hair cell excitability and refine maturation of the auditory system. Abstract Just before the onset of hearing, the inner hair cells (IHCs) receive inhibitory efferent input from cholinergic medial olivocochlear (MOC) neurons originating in the brainstem. This input may serve a role in the maturation of the ascending (afferent) auditory system by inhibiting spontaneous activity of the IHCs. To investigate the molecular mechanisms regulating these IHC efferent synapses, we combined electrical stimulation of the efferent fibres with patch clamp recordings from the IHCs to measure efferent synaptic strength. By examining evoked responses, we show that activation of metabotropic glutamate receptors (mGluRs) by general and group I‐specific mGluR agonists enhances IHC efferent inhibition. This enhancement is blocked by application of a group I mGluR1‐specific antagonist, indicating that enhancement of IHC efferent inhibition is mediated by group I mGluRs and specifically by mGluR1s. By comparing spontaneous and evoked responses, we show that group I mGluR agonists act presynaptically to increase neurotransmitter release without affecting postsynaptic responsiveness. Moreover, endogenous glutamate released from the IHCs also enhances IHC efferent inhibition via the activation of group I mGluRs. Finally, immunofluorescence analysis indicates that the efferent terminals are sufficiently close to IHC glutamate release sites to allow activation of mGluRs on the efferent terminals by glutamate spillover. Together, these results suggest that glutamate released from the IHCs activates group I mGluRs (mGluR1s), probably present on the efferent terminals, which, in turn, enhances release of acetylcholine and inhibition of the IHCs. Thus, mGluRs establish a local negative feedback loop positioned to regulate IHC activity and maturation of the ascending auditory system in the developing cochlea.
    April 21, 2017   doi: 10.1113/JP272604   open full text
  • Vasopressin V1a receptors mediate the hypertensive effects of [Pyr1]apelin‐13 in the rat rostral ventrolateral medulla.
    Philip R. Griffiths, Stephen J. Lolait, Louise E. Harris, Julian F. R. Paton, Anne‐Marie O'Carroll.
    The Journal of Physiology. April 21, 2017
    Key points Dysfunctions in CNS regulation of arterial blood pressure lead to an increase in sympathetic nerve activity that participates in the pathogenesis of hypertension. The apelin‐apelin receptor system affects arterial blood pressure homeostasis; however, the central mechanisms underlying apelin‐mediated changes in sympathetic nerve activity and blood pressure have not been clarified. We explored the mechanisms involved in the regulation of [Pyr1]apelin‐13‐mediated cardiovascular control within the rostral ventrolateral medulla (RVLM) using selective receptor antagonists. We show that [Pyr1]apelin‐13 acts as a modulating neurotransmitter in the normotensive RVLM to affect vascular tone through interaction with the vasopressin V1a receptor but that [Pyr1]apelin‐13‐induced sympathoexcitation is independent of angiotensin II receptor type 1, oxytocin, ionotropic glutamate and GABAA receptors. Our data confirm a role for the apelin peptide system in cardiovascular regulation at the level of the RVLM and highlight that this system is a possible potential therapeutic target for the treatment of hypertension. Abstract Apelin is a ubiquitous peptide that can elevate arterial blood pressure (ABP) yet understanding of the mechanisms involved remain incomplete. Bilateral microinjection of [Pyr1]apelin‐13 into the rostral ventrolateral medulla (RVLM), a major source of sympathoexcitatory neurones, increases ABP and sympathetic nerve activity. We aimed to investigate the potential involvement of neurotransmitter systems through which the apelin pressor response may occur within the RVLM. Adult male Wistar rats were anaesthetized and ABP was monitored via a femoral arterial catheter. Bilateral RVLM microinjection of [Pyr1]apelin‐13 significantly increased ABP (9 ± 1 mmHg) compared to saline (−1 ± 2mmHg; P < 0.001), which was blocked by pretreatment with the apelin receptor antagonist, F13A (0 ± 1 mmHg; P < 0.01). The rise in ABP was associated with an increase in the low frequency spectra of systolic BP (13.9 ± 4.3% total power; P < 0.001), indicative of sympathetic vasomotor activation. The [Pyr1]apelin‐13‐mediated pressor response and the increased low frequency spectra of systolic BP response were fully maintained despite RVLM pretreatment with the angiotensin II type 1 receptor antagonist losartan, the oxytocin receptor antagonist desGly‐NH2, d(CH2)5[D‐Tyr2,Thr4]OVT, the ionotropic glutamate receptor antagonist kynurenate or the GABAA antagonist bicuculline (P > 0.05). By contrast, the [Pyr1]apelin‐13 induced pressor and sympathoexcitatory effects were abolished by pretreatment of the RVLM with the vasopressin V1a receptor antagonist, SR 49059 (−1 ± 1 mmHg; 1.1 ± 1.1% total power, respectively; P < 0.001). These findings suggest that the pressor action of [Pyr1]apelin‐13 in the RVLM of normotensive rats is not mediated via angiotensin II type 1 receptor, oxytocin, ionotropic glutamate or GABAA receptors but instead involves a close relationship with the neuropeptide modulator vasopressin.
    April 21, 2017   doi: 10.1113/JP274178   open full text
  • Angiotensin II activates CaV1.2 Ca2+ channels through β‐arrestin2 and casein kinase 2 in mouse immature cardiomyocytes.
    Toshihide Kashihara, Tsutomu Nakada, Katsuhiko Kojima, Toshikazu Takeshita, Mitsuhiko Yamada.
    The Journal of Physiology. April 20, 2017
    Key points Angiotensin II (AngII) is crucial in cardiovascular regulation in perinatal mammalians. Here we show that AngII increases twitch Ca2+ transients of mouse immature but not mature cardiomyocytes by robustly activating CaV1.2 L‐type Ca2+ channels through a novel signalling pathway involving angiotensin type 1 (AT1) receptors, β‐arrestin2 and casein kinase 2. A β‐arrestin‐biased AT1 receptor agonist, TRV027, was as effective as AngII in activating L‐type Ca2+ channels. Our results help understand the molecular mechanism by which AngII regulates the perinatal circulation and also suggest that β‐arrestin‐biased AT1 receptor agonists may be valuable therapeutics for paediatric heart failure. Abstract Angiotensin II (AngII), the main effector peptide of the renin–angiotensin system, plays important roles in cardiovascular regulation in the perinatal period. Despite the well‐known stimulatory effect of AngII on vascular contraction, little is known about regulation of contraction of the immature heart by AngII. Here we found that AngII significantly increased the peak amplitude of twitch Ca2+ transients by robustly activating L‐type CaV1.2 Ca2+ (CaV1.2) channels in mouse immature but not mature cardiomyocytes. This response to AngII was mediated by AT1 receptors and β‐arrestin2. A β‐arrestin‐biased AT1 receptor agonist was as effective as AngII in activating CaV1.2 channels. Src‐family tyrosine kinases (SFKs) and casein kinase 2α’β (CK2α’β) were sequentially activated when AngII activated CaV1.2 channels. A cyclin‐dependent kinase inhibitor, p27Kip1 (p27), inhibited CK2α’β, and AngII removed this inhibitory effect through phosphorylating tyrosine 88 of p27 via SFKs in cardiomyocytes. In a human embryonic kidney cell line, tsA201 cells, overexpression of CK2α’β but not c‐Src directly activated recombinant CaV1.2 channels composed of C‐terminally truncated α1C, the distal C‐terminus of α1C, β2C and α2δ1 subunits, by phosphorylating threonine 1704 located at the interface between the proximal and the distal C‐terminus of CaV1.2α1C subunits. Co‐immunoprecipitation revealed that CaV1.2 channels, CK2α’β and p27 formed a macromolecular complex. Therefore, stimulation of AT1 receptors by AngII activates CaV1.2 channels through β‐arrestin2 and CK2α’β, thereby probably exerting a positive inotropic effect in the immature heart. Our results also indicated that β‐arrestin‐biased AT1 receptor agonists may be used as valuable therapeutics for paediatric heart failure in the future.
    April 20, 2017   doi: 10.1113/JP273883   open full text
  • Chronic alcohol feeding potentiates hormone‐induced calcium signalling in hepatocytes.
    Paula J. Bartlett, Anil Noronha Antony, Amit Agarwal, Mauricette Hilly, Victoria L. Prince, Laurent Combettes, Jan B. Hoek, Lawrence D. Gaspers.
    The Journal of Physiology. April 18, 2017
    Key points Chronic alcohol consumption causes a spectrum of liver diseases, but the pathogenic mechanisms driving the onset and progression of disease are not clearly defined. We show that chronic alcohol feeding sensitizes rat hepatocytes to Ca2+‐mobilizing hormones resulting in a leftward shift in the concentration–response relationship and the transition from oscillatory to more sustained and prolonged Ca2+ increases. Our data demonstrate that alcohol‐dependent adaptation in the Ca2+ signalling pathway occurs at the level of hormone‐induced inositol 1,4,5 trisphosphate (IP3) production and does not involve changes in the sensitivity of the IP3 receptor or size of internal Ca2+ stores. We suggest that prolonged and aberrant hormone‐evoked Ca2+ increases may stimulate the production of mitochondrial reactive oxygen species and contribute to alcohol‐induced hepatocyte injury. Abstract ‘Adaptive’ responses of the liver to chronic alcohol consumption may underlie the development of cell and tissue injury. Alcohol administration can perturb multiple signalling pathways including phosphoinositide‐dependent cytosolic calcium ([Ca2+]i) increases, which can adversely affect mitochondrial Ca2+ levels, reactive oxygen species production and energy metabolism. Our data indicate that chronic alcohol feeding induces a leftward shift in the dose–response for Ca2+‐mobilizing hormones resulting in more sustained and prolonged [Ca2+]i increases in both cultured hepatocytes and hepatocytes within the intact perfused liver. Ca2+ increases were initiated at lower hormone concentrations, and intercellular calcium wave propagation rates were faster in alcoholics compared to controls. Acute alcohol treatment (25 mm) completely inhibited hormone‐induced calcium increases in control livers, but not after chronic alcohol‐feeding, suggesting desensitization to the inhibitory actions of ethanol. Hormone‐induced inositol 1,4,5 trisphosphate (IP3) accumulation and phospholipase C (PLC) activity were significantly potentiated in hepatocytes from alcohol‐fed rats compared to controls. Removal of extracellular calcium, or chelation of intracellular calcium did not normalize the differences in hormone‐stimulated PLC activity, indicating calcium‐dependent PLCs are not upregulated by alcohol. We propose that the liver ‘adapts’ to chronic alcohol exposure by increasing hormone‐dependent IP3 formation, leading to aberrant calcium increases, which may contribute to hepatocyte injury.
    April 18, 2017   doi: 10.1113/JP273891   open full text
  • Geniculohypothalamic GABAergic projections gate suprachiasmatic nucleus responses to retinal input.
    Lydia Hanna, Lauren Walmsley, Abigail Pienaar, Michael Howarth, Timothy M. Brown.
    The Journal of Physiology. April 11, 2017
    Key points Visual input to the suprachiasmatic nucleus circadian clock is critical for animals to adapt their physiology and behaviour in line with the solar day. In addition to direct retinal projections, the clock receives input from the visual thalamus, although the role of this geniculohypothalamic pathway in circadian photoreception is poorly understood. In the present study, we develop a novel brain slice preparation that preserves the geniculohypothalamic pathway to show that GABAergic thalamic neurons inhibit retinally‐driven activity in the central clock in a circadian time‐dependent manner. We also show that in vivo manipulation of thalamic signalling adjusts specific features of the hypothalamic light response, indicating that the geniculohypothalamic pathway is primarily activated by crossed retinal inputs. Our data provide a mechanism by which geniculohypothalamic signals can adjust the magnitude of circadian and more acute hypothalamic light responses according to time‐of‐day and establish an important new model for future investigations of the circadian visual system. Abstract Sensory input to the master mammalian circadian clock, the suprachiasmatic nucleus (SCN), is vital in allowing animals to optimize physiology and behaviour alongside daily changes in the environment. Retinal inputs encoding changes in external illumination provide the principle source of such information. The SCN also receives input from other retinorecipient brain regions, primarily via the geniculohypothalamic tract (GHT), although the contribution of these indirect projections to circadian photoreception is currently poorly understood. To address this deficit, in the present study, we established an in vitro mouse brain slice preparation that retains connectivity across the extended circadian system. Using multi‐electrode recordings, we first confirm that this preparation retains intact optic projections to the SCN, thalamus and pretectum and a functional GHT. We next show that optogenetic activation of GHT neurons selectively suppresses SCN responses to retinal input, and also that this effect exhibits a pronounced day/night variation and involves a GABAergic mechanism. This inhibitory action was not associated with overt circadian rhythmicity in GHT output, indicating modulation at the SCN level. Finally, we use in vivo electrophysiological recordings alongside pharmacological inactivation or optogenetic excitation to show that GHT signalling actively modulates specific features of the SCN light response, indicating that GHT cells are primarily activated by crossed retinal projections. Taken together, our data establish a new model for studying network communication in the extended circadian system and provide novel insight into the roles of GHT‐signalling, revealing a mechanism by which thalamic activity can help gate retinal input to the SCN according to time of day.
    April 11, 2017   doi: 10.1113/JP273850   open full text
  • Baroreflex control of renal sympathetic nerve activity in early heart failure assessed by the sequence method.
    Renata Maria Lataro, Luiz Eduardo Virgilio Silva, Carlos Alberto Aguiar Silva, Helio Cesar Salgado, Rubens Fazan.
    The Journal of Physiology. April 07, 2017
    Key points The integrity of the baroreflex control of sympathetic activity in heart failure (HF) remains under debate. We proposed the use of the sequence method to assess the baroreflex control of renal sympathetic nerve activity (RSNA). The sequence method assesses the spontaneous arterial pressure (AP) fluctuations and their related changes in heart rate (or other efferent responses), providing the sensitivity and the effectiveness of the baroreflex. Effectiveness refers to the fraction of spontaneous AP changes that elicits baroreflex‐mediated variations in the efferent response. Using three different approaches, we showed that the baroreflex sensitivity between AP and RSNA is not altered in early HF rats. However, the sequence method provided evidence that the effectiveness of baroreflex in changing RSNA in response to AP changes is markedly decreased in HF. The results help us better understand the baroreflex control of the sympathetic nerve activity. Abstract In heart failure (HF), the reflex control of the heart rate is known to be markedly impaired; however, the baroreceptor control of the sympathetic drive remains under debate. Applying the sequence method to a series of arterial pressure (AP) and renal sympathetic nerve activity (RSNA), we demonstrated a clear dysfunction in the baroreflex control of sympathetic activity in rats with early HF. We analysed the baroreflex control of the sympathetic drive using three different approaches: AP vs. RSNA curve, cross‐spectral analysis and sequence method between AP and RSNA. The sequence method also provides the baroreflex effectiveness index (BEI), which represents the percentage of AP ramps that actually produce a reflex response. The methods were applied to control rats and rats with HF induced by myocardial infarction. None of the methods employed to assess the sympathetic baroreflex gain were able to detect any differences between the control and the HF group. However, rats with HF exhibited a lower BEI compared to the controls. Moreover, an optimum delay of 1 beat was observed, i.e. 1 beat is required for the RSNA to respond after AP changing, which corroborates with the findings related to the timing between these two variables. For delay 1, the BEI of the controls was 0.45 ± 0.03, whereas the BEI of rats with HF was 0.29 ± 0.09 (P < 0.05). These data demonstrate that while the gain of the baroreflex is not affected in early HF, its effectiveness is markedly decreased. The analysis of the spontaneous changes in AP and RSNA using the sequence method provides novel insights into arterial baroreceptor reflex function.
    April 07, 2017   doi: 10.1113/JP274065   open full text
  • A Western‐style obesogenic diet alters maternal metabolic physiology with consequences for fetal nutrient acquisition in mice.
    Barbara Musial, Owen R. Vaughan, Denise S. Fernandez‐Twinn, Peter Voshol, Susan E. Ozanne, Abigail L. Fowden, Amanda N. Sferruzzi‐Perri.
    The Journal of Physiology. April 05, 2017
    Key points In the Western world, obesogenic diets containing high fat and high sugar (HFHS) are commonly consumed during pregnancy, although their effects on the metabolism of the mother, in relation to feto‐placental glucose utilization and growth, are unknown. In the present study, the consumption of an obesogenic HFHS diet compromised maternal glucose tolerance and insulin sensitivity in late pregnancy in association with dysregulated lipid and glucose handling by the dam. These maternal metabolic changes induced by HFHS feeding were related to altered feto‐placental glucose metabolism and growth. A HFHS diet during pregnancy therefore causes maternal metabolic dysfunction with consequences for maternal nutrient allocation for fetal growth. These findings have implications for the health of women and their infants, who consume obesogenic diets during pregnancy. Abstract In the Western world, obesogenic diets containing high fat and high sugar (HFHS) are commonly consumed during pregnancy. However, the impacts of a HFHS diet during pregnancy on maternal insulin sensitivity and signalling in relation to feto‐placental growth and glucose utilization are unknown. The present study examined the effects of a HFHS diet during mouse pregnancy on maternal glucose tolerance and insulin resistance, as well as, on feto‐placental glucose metabolism. Female mice were fed a control or HFHS diet from day (D) 1 of pregnancy (term = D20.5). At D16 or D19, dams were assessed for body composition, metabolite and hormone concentrations, tissue abundance of growth and metabolic signalling pathways, glucose tolerance and utilization and insulin sensitivity. HFHS feeding perturbed maternal insulin sensitivity in late pregnancy; hepatic insulin sensitivity was higher, whereas sensitivity of the skeletal muscle and white adipose tissue was lower in HFHS than control dams. These changes were accompanied by increased adiposity and reduced glucose production and glucose tolerance of HFHS dams. The HFHS diet also disturbed the hormone and metabolite milieu and altered expression of growth and metabolic signalling pathways in maternal tissues. Furthermore, HFHS feeding was associated with impaired feto‐placental glucose metabolism and growth. A HFHS diet during pregnancy therefore causes maternal metabolic dysfunction with consequences for maternal nutrient allocation for fetal growth. These findings have implications for the health of women and their infants, who consume HFHS diets during pregnancy.
    April 05, 2017   doi: 10.1113/JP273684   open full text
  • mTOR folate sensing links folate availability to trophoblast cell function.
    Fredrick J. Rosario, Theresa L. Powell, Thomas Jansson.
    The Journal of Physiology. April 04, 2017
    Folate is a water‐soluble B vitamin that is essential for cellular methylation reactions and DNA synthesis and repair. Low maternal folate levels in pregnancy are associated with fetal growth restriction, however the underlying mechanisms are poorly understood. Mechanistic target of rapamycin (mTOR) links nutrient availability to cell growth and function by regulating gene expression and protein translation. Here we show that mTOR functions as a folate sensor in primary human trophoblast (PHT) cells. Folate deficiency in PHT cells caused inhibition of mTOR signalling and decreased the activity of key amino acid transporters. Folate sensing by mTOR in PHT cells involves both mTOR Complex 1 and 2 and requires the proton‐coupled folate transporter (PCFT, SLC46A1). The involvement of PCFT in mTOR folate sensing is not dependent on its function as a plasma membrane folate transporter. Increasing levels of homocysteine had no effect on PHT mTOR signalling, suggesting that mTOR senses low folate rather than high homocysteine. In addition, we demonstrate that maternal serum folate is positively correlated to placental mTORC1 and mTORC2 signalling activity in human pregnancy. We have identified a previously unknown molecular link between folate availability and cell function involving PCFT and mTOR signalling. We propose that mTOR folate sensing in trophoblast cells matches placental nutrient transport, and therefore fetal growth, to folate availability. These findings may have implications for our understanding of how altered folate availability causes human diseases such as fetal growth restriction, fetal malformations and cancer. This article is protected by copyright. All rights reserved
    April 04, 2017   doi: 10.1113/JP272424   open full text
  • Differential serotonergic modulation across the main and accessory olfactory bulbs.
    Zhenbo Huang, Nicolas Thiebaud, Debra Ann Fadool.
    The Journal of Physiology. March 31, 2017
    Key points There are serotonergic projections to both the main (MOB) and the accessory olfactory bulb (AOB). Current‐clamp experiments demonstrate that serotonergic afferents are largely excitatory for mitral cells (MCs) in the MOB where 5‐HT2A receptors mediate a direct excitatory action. Serotonergic afferents are predominately inhibitory for MCs in the AOB. There are two types of inhibition: indirect inhibition mediated through the 5‐HT2 receptors on GABAergic interneurons and direct inhibition via the 5‐HT1 receptors on MCs. Differential 5‐HT neuromodulation of MCs across the MOB and AOB could contribute to select behaviours such as olfactory learning or aggression. Abstract Mitral cells (MCs) contained in the main (MOB) and accessory (AOB) olfactory bulb have distinct intrinsic membrane properties but the extent of neuromodulation across the two systems has not been widely explored. Herein, we investigated a widely distributed CNS modulator, serotonin (5‐HT), for its ability to modulate the biophysical properties of MCs across the MOB and AOB, using an in vitro, brain slice approach in postnatal 15–30 day mice. In the MOB, 5‐HT elicited three types of responses in 93% of 180 cells tested. Cells were either directly excited (70%), inhibited (10%) or showed a mixed response (13%)– first inhibition followed by excitation. In the AOB, 82% of 148 cells were inhibited with 18% of cells showing no response. Albeit located in parallel partitions of the olfactory system, 5‐HT largely elicited MC excitation in the MOB while it evoked two different kinetic rates of MC inhibition in the AOB. Using a combination of pharmacological agents, we found that the MC excitatory responses in the MOB were mediated by 5‐HT2A receptors through a direct activation. In comparison, 5‐HT‐evoked inhibitory responses in the AOB arose due to a polysynaptic, slow‐onset inhibition attributed to 5‐HT2 receptor activation exciting GABAergic interneurons. The second type of inhibition had a rapid onset as a result of direct inhibition mediated by the 5‐HT1 class of receptors. The distinct serotonergic modulation of MCs between the MOB and AOB could provide a molecular basis for differential chemosensory behaviours driven by the brainstem raphe nuclei into these parallel systems.
    March 31, 2017   doi: 10.1113/JP273945   open full text
  • Computational analysis of the human sinus node action potential: model development and effects of mutations.
    Alan Fabbri, Matteo Fantini, Ronald Wilders, Stefano Severi.
    The Journal of Physiology. March 30, 2017
    Key points We constructed a comprehensive mathematical model of the spontaneous electrical activity of a human sinoatrial node (SAN) pacemaker cell, starting from the recent Severi–DiFrancesco model of rabbit SAN cells. Our model is based on electrophysiological data from isolated human SAN pacemaker cells and closely matches the action potentials and calcium transient that were recorded experimentally. Simulated ion channelopathies explain the clinically observed changes in heart rate in corresponding mutation carriers, providing an independent qualitative validation of the model. The model shows that the modulatory role of the ‘funny current’ (If) in the pacing rate of human SAN pacemaker cells is highly similar to that of rabbit SAN cells, despite its considerably lower amplitude. The model may prove useful in the design of experiments and the development of heart‐rate modulating drugs. Abstract The sinoatrial node (SAN) is the normal pacemaker of the mammalian heart.  Over several decades, a large amount of data on the ionic mechanisms underlying the spontaneous electrical activity of SAN pacemaker cells has been obtained, mostly in experiments on single cells isolated from rabbit SAN. This wealth of data has allowed the development of mathematical models of the electrical activity of rabbit SAN pacemaker cells. The present study aimed to construct a comprehensive model of the electrical activity of a human SAN pacemaker cell using recently obtained electrophysiological data from human SAN pacemaker cells.  We based our model on the recent Severi–DiFrancesco model of a rabbit SAN pacemaker cell. The action potential and calcium transient of the resulting model are close to the experimentally recorded values. The model has a much smaller ‘funny current’ (If) than do rabbit cells, although its modulatory role is highly similar. Changes in pacing rate upon the implementation of mutations associated with sinus node dysfunction agree with the clinical observations. This agreement holds for both loss‐of‐function and gain‐of‐function mutations in the HCN4, SCN5A and KCNQ1 genes, underlying ion channelopathies in If, fast sodium current and slow delayed rectifier potassium current, respectively. We conclude that our human SAN cell model can be a useful tool in the design of experiments and the development of drugs that aim to modulate heart rate.
    March 30, 2017   doi: 10.1113/JP273259   open full text
  • Uteroplacental insufficiency reduces rat plasma leptin concentrations and alters placental leptin transporters: ameliorated with enhanced milk intake and nutrition.
    Jessica F. Briffa, Rachael O'Dowd, Karen M. Moritz, Tania Romano, Lisa R. Jedwab, Andrew J. McAinch, Deanne H. Hryciw, Mary E. Wlodek.
    The Journal of Physiology. March 29, 2017
    Key points Uteroplacental insufficiency compromises maternal mammary development, milk production and pup organ development; this is ameliorated by cross‐fostering, which improves pup growth and organ development and prevents adult diseases in growth‐restricted (Restricted) offspring by enhancing postnatal nutrition. Leptin is transported to the fetus from the mother by the placenta; we report reduced plasma leptin concentrations in Restricted fetuses associated with sex‐specific alterations in placental leptin transporter expression. Pup plasma leptin concentrations were also reduced during suckling, which may suggest reduced milk leptin transport or leptin reabsorption. Mothers suckled by Restricted pups had impaired mammary development and changes in milk fatty acid composition with no alterations in milk leptin; cross‐fostering restored pup plasma leptin concentrations, which may be correlated to improved milk composition and intake. Increased plasma leptin and altered milk fatty acid composition in Restricted pups suckling mothers with normal lactation may improve postnatal growth and prevent adult diseases. Abstract Uteroplacental insufficiency reduces birth weight and adversely affects fetal organ development, increasing adult disease risk. Cross‐fostering improves postnatal nutrition and restores these deficits. Mothers with growth‐restricted pups have compromised milk production and composition; however, the impact cross‐fostering has on milk production and composition is unknown. Plasma leptin concentrations peak during the completion of organogenesis, which occurs postnatally in rats. Leptin is transferred to the fetus via the placenta and to the pup via the lactating mammary gland. This study investigated the effect of uteroplacental insufficiency on pup plasma leptin concentrations and placental leptin transporters. We additionally examined whether cross‐fostering improves mammary development, milk composition and pup plasma leptin concentrations. Fetal growth restriction was induced by bilateral uterine vessel ligation surgery on gestation day 18 in Wistar Kyoto rats (termed uteroplacental insufficiency surgery mothers). Growth‐restricted (Restricted) fetuses had reduced plasma leptin concentrations, persisting throughout lactation, and sex‐specific alterations in placental leptin transporters. Mothers suckled by Restricted pups had impaired mammary development, altered milk fatty acid composition and increased plasma leptin concentrations, despite no changes in milk leptin. Milk intake was reduced in Restricted pups suckling uteroplacental insufficiency surgery mothers compared to Restricted pups suckling sham‐operated mothers. Cross‐fostering Restricted pups onto a sham‐operated mother improved postnatal growth and restored plasma leptin concentrations compared to Restricted pups suckling uteroplacental insufficiency surgery mothers. Uteroplacental insufficiency alters leptin homeostasis. This is ameliorated with cross‐fostering and enhanced milk fatty acid composition and consumption, which may protect the pups from developing adverse health conditions in adulthood.
    March 29, 2017   doi: 10.1113/JP273825   open full text
  • A calcium‐dependent pathway underlies activity‐dependent plasticity of electrical synapses in the thalamic reticular nucleus.
    Jessica Sevetson, Sarah Fittro, Emily Heckman, Julie S. Haas.
    The Journal of Physiology. March 29, 2017
    Recent results have demonstrated modification of electrical synapse strength by varied forms of neuronal activity. However, the mechanisms underlying plasticity induction in central mammalian neurons are unclear. Here we show that the two established inductors of plasticity at electrical synapses in the thalamic reticular nucleus – paired burst spiking in coupled neurons, and mGluR‐dependent tetanization of synaptic input – are separate pathways that converge at a common downstream endpoint. Using occlusion experiments and pharmacology in patched pairs of coupled neurons in vitro, we show that burst‐induced depression depends on calcium entry via voltage‐gated channels, is blocked by BAPTA chelation, and recruits intracellular calcium release on its way to activation of phosphatase activity. In contrast, mGluR‐dependent plasticity is independent of calcium entry or calcium dynamics. Together, these results show that the spiking‐initiated mechanisms underlying electrical synapse plasticity are similar to those that induce plasticity at chemical synapses, and offer the possibility that calcium‐regulated mechanisms may also lead to alternate outcomes, such as potentiation. Because these mechanistic elements are widely found in mature neurons, we expect them to apply broadly to electrical synapses across the brain, acting as the crucial link between neuronal activity and electrical synapse strength. This article is protected by copyright. All rights reserved
    March 29, 2017   doi: 10.1113/JP274049   open full text
  • Leg vascular and skeletal muscle mitochondrial adaptations to aerobic high‐intensity exercise training are enhanced in the early postmenopausal phase.
    Michael Nyberg, Jon Egelund, Camilla M. Mandrup, Caroline B. Andersen, Karen M. B. E. Hansen, Ida‐Marie F. Hergel, Nicholai Valbak‐Andersen, Ruth Frikke‐Schmidt, Bente Stallknecht, Jens Bangsbo, Ylva Hellsten.
    The Journal of Physiology. March 29, 2017
    Key points Exercise training effectively improves vascular and skeletal muscle function; however, these effects of training may be blunted in postmenopausal women as a result of the loss of oestrogens. Accordingly, the capacity to deliver oxygen to the active muscles may also be impaired in postmenopausal women. In both premenopausal and recent postmenopausal women, exercise training was shown to improve leg vascular and skeletal muscle mitochondrial function. Interestingly, these effects were more pronounced in postmenopausal women. Skeletal muscle oxygen supply and utilization were similar in the two groups of women. These findings suggest that the early postmenopausal phase is associated with an enhanced capacity of the leg vasculature and skeletal muscle mitochondria to adapt to exercise training and that the ability to deliver oxygen to match the demand of the active muscles is preserved in the early phase following the menopausal transition. Abstract Exercise training leads to favourable adaptations within skeletal muscle; however, this effect of exercise training may be blunted in postmenopausal women as a result of the loss of oestrogens. Furthermore, postmenopausal women may have an impaired vascular response to acute exercise. We examined the haemodynamic response to acute exercise in matched pre‐ and postmenopausal women before and after 12 weeks of aerobic high intensity exercise training. Twenty premenopausal and 16 early postmenopausal (mean ± SEM: 3.1 ± 0.5 years after final menstrual period) women only separated by 4 years of age (mean ± SEM: 50 ± 0 years vs. 54 ± 1 years) were included. Before training, leg blood flow, O2 delivery, O2 uptake and lactate release during knee‐extensor exercise were similar in pre‐ and postmenopausal women. Exercise training reduced (P < 0.05) leg blood flow, O2 delivery, O2 uptake, lactate release, blood pressure and heart rate during the same absolute workloads in postmenopausal women. These effects were not detected in premenopausal women. Quadriceps muscle protein contents of mitochondrial complex II, III and IV; endothelial nitric oxide synthase (eNOS); cyclooxygenase (COX)‐1; COX‐2; and oestrogen‐related receptor α (ERRα) were increased (P < 0.05) with training in postmenopausal women, whereas only the levels of mitochondrial complex V, eNOS and COX‐2 were increased (P < 0.05) in premenopausal women. These findings demonstrate that vascular and skeletal muscle mitochondrial adaptations to aerobic high intensity exercise training are more pronounced in recent post‐ compared to premenopausal women, possibly as an effect of enhanced ERRα signalling. Also, the hyperaemic response to acute exercise appears to be preserved in the early postmenopausal phase.
    March 29, 2017   doi: 10.1113/JP273871   open full text
  • The role played by oxidative stress in evoking the exercise pressor reflex in health and simulated peripheral artery disease.
    Jonathan E. Harms, J. Matthew Kuczmarski, Joyce Kim, Gail D. Thomas, Marc P. Kaufman.
    The Journal of Physiology. March 28, 2017
    Contraction of muscle evokes the exercise pressor reflex (EPR), which is expressed partly by increases in heart rate and arterial pressure. Patients with peripheral artery disease (PAD) show an exaggerated EPR, sometimes report pain when walking and are at risk for cardiac arrthymias. Previous research suggested that reactive oxygen species (ROS) mediate the exaggerated EPR associated with PAD. To examine the effects of ROS on the EPR, we infused a superoxide scavenger, tiron, into the superficial epigastric artery of decerebrated rats. In some, we simulated PAD by ligating a femoral artery for 72 h before the experiment. The peak EPR in “ligated” rats during saline infusion averaged 31 ± 4 mmHg, whereas the peak EPR in these rats during tiron infusion averaged 13 ± 2 mmHg (n = 12; P < 0.001); the attenuating effect of tiron on the EPR was partly reversed when saline was reinfused into the superficial epigastric artery (21 ± 2; P < 0.01 vs tiron). The peak EPR in “ligated” rats was also attenuated (n = 7; P < 0.01) by infusion of gp91ds‐tat, a peptide which blocks the activity of NAD(P)H oxidase. Tiron infusion had no effect on the EPR in rats with patent femoral arteries (n = 9). Western blots showed that the triceps surae muscles of “ligated” rats expressed more Nox2 and p67phox, which are components of NADPH oxidase, than did triceps surae muscles of “freely perfused” rats. Tiron added to muscle homogenates reduced ROS production in vitro. Our results provide further evidence that ROS mediates the exaggeration of EPR in rats with simulated PAD. This article is protected by copyright. All rights reserved
    March 28, 2017   doi: 10.1113/JP273816   open full text
  • Quantitative analysis of the Ca2+‐dependent regulation of delayed rectifier K+ current IKs in rabbit ventricular myocytes.
    Daniel C. Bartos, Stefano Morotti, Kenneth S. Ginsburg, Eleonora Grandi, Donald M. Bers.
    The Journal of Physiology. March 28, 2017
    Key points [Ca2+]i enhanced rabbit ventricular slowly activating delayed rectifier K+ current (IKs) by negatively shifting the voltage dependence of activation and slowing deactivation, similar to perfusion of isoproterenol. Rabbit ventricular rapidly activating delayed rectifier K+ current (IKr) amplitude and voltage dependence were unaffected by high [Ca2+]i. When measuring or simulating IKs during an action potential, IKs was not different during a physiological Ca2+ transient or when [Ca2+]i was buffered to 500 nm. Abstract The slowly activating delayed rectifier K+ current (IKs) contributes to repolarization of the cardiac action potential (AP). Intracellular Ca2+ ([Ca2+]i) and β‐adrenergic receptor (β‐AR) stimulation modulate IKs amplitude and kinetics, but details of these important IKs regulators and their interaction are limited. We assessed the [Ca2+]i dependence of IKs in steady‐state conditions and with dynamically changing membrane potential and [Ca2+]i during an AP. IKs was recorded from freshly isolated rabbit ventricular myocytes using whole‐cell patch clamp. With intracellular pipette solutions that controlled free [Ca2+]i, we found that raising [Ca2+]i from 100 to 600 nm produced similar increases in IKs as did β‐AR activation, and the effects appeared additive. Both β‐AR activation and high [Ca2+]i increased maximally activated tail IKs, negatively shifted the voltage dependence of activation, and slowed deactivation kinetics. These data informed changes in our well‐established mathematical model of the rabbit myocyte. In both AP‐clamp experiments and simulations, IKs recorded during a normal physiological Ca2+ transient was similar to IKs measured with [Ca2+]i clamped at 500–600 nm. Thus, our study provides novel quantitative data as to how physiological [Ca2+]i regulates IKs amplitude and kinetics during the normal rabbit AP. Our results suggest that micromolar [Ca2+]i, in the submembrane or junctional cleft space, is not required to maximize [Ca2+]i‐dependent IKs activation during normal Ca2+ transients. [Ca2+]i enhanced rabbit ventricular slowly activating delayed rectifier K+ current (IKs) by negatively shifting the voltage dependence of activation and slowing deactivation, similar to perfusion of isoproterenol. Rabbit ventricular rapidly activating delayed rectifier K+ current (IKr) amplitude and voltage dependence were unaffected by high [Ca2+]i. When measuring or simulating IKs during an action potential, IKs was not different during a physiological Ca2+ transient or when [Ca2+]i was buffered to 500 nm. Membrane topology of a Kv7.1 α‐subunit and regulatory proteins.
    March 28, 2017   doi: 10.1113/JP273676   open full text
  • Nutritional status‐dependent endocannabinoid signalling regulates the integration of rat visceral information.
    Abdessattar Khlaifia, Isabelle Matias, Daniela Cota, Fabien Tell.
    The Journal of Physiology. March 27, 2017
    Key points Vagal sensory inputs transmit information from the viscera to brainstem neurones located in the nucleus tractus solitarii to set physiological parameters. These excitatory synapses exhibit a CB1 endocannabinoid‐induced long‐term depression (LTD) triggered by vagal fibre stimulation. We investigated the impact of nutritional status on long‐term changes in this long‐term synaptic plasticity. Food deprivation prevents LTD induction by disrupting CB1 receptor signalling. Short‐term refeeding restores the capacity of vagal synapses to express LTD. Ghrelin and cholecystokinin, respectively released during fasting and refeeding, play a key role in the control of LTD via the activation of energy sensing pathways such as AMPK and the mTOR and ERK pathways. Abstract Communication form the viscera to the brain is essential to set physiological homoeostatic parameters but also to drive more complex behaviours such as mood, memory and emotional states. Here we investigated the impact of the nutritional status on long‐term changes in excitatory synaptic transmission in the nucleus tractus solitarii, a neural hub integrating visceral signals. These excitatory synapses exhibit a CB1 endocannabinoid (eCB)‐induced long‐term depression (LTD) triggered by vagal fibre stimulation. Since eCB signalling is known to be an important component of homoeostatic regulation of the body and is regulated during various stressful conditions, we tested the hypothesis that food deprivation alters eCB signalling in central visceral afferent fibres. Food deprivation prevents eCB‐LTD induction due to the absence of eCB signalling. This loss was reversed by blockade of ghrelin receptors. Activation of the cellular fuel sensor AMP‐activated protein kinase or inhibition of the mechanistic target of rapamycin pathway abolished eCB‐LTD in free‐fed rats. Signals associated with energy surfeit, such as short‐term refeeding, restore eCB‐LTD induction, which in turn requires activation of cholecystokinin receptors and the extracellular signal‐regulated kinase pathway. These data suggest a tight link between eCB‐LTD in the NTS and nutritional status and shed light on the key role of eCB in the integration of visceral information.
    March 27, 2017   doi: 10.1113/JP273484   open full text
  • Article update.

    The Journal of Physiology. March 26, 2017
    There is no abstract available for this paper.
    March 26, 2017   doi: 10.1113/JP273893   open full text
  • Endothelin‐1 mediates natriuresis but not polyuria during vitamin D‐induced acute hypercalcaemia.
    Natsuko Tokonami, Lydie Cheval, Isabelle Monnay, Guillaume Meurice, Johannes Loffing, Eric Feraille, Pascal Houillier.
    The Journal of Physiology. March 23, 2017
    Key points Hypercalcaemia can occur under various pathological conditions, such as primary hyperparathyroidism, malignancy or granulomatosis, and it induces natriuresis and polyuria in various species via an unknown mechanism. A previous study demonstrated that hypercalcaemia induced by vitamin D in rats increased endothelin (ET)‐1 expression in the distal nephron, which suggests the involvement of the ET system in hypercalcaemia‐induced effects. In the present study, we demonstrate that, during vitamin D‐induced hypercalcaemia, the activation of ET system by increased ET‐1 is responsible for natriuresis but not for polyuria. Vitamin D‐treated hypercalcaemic mice showed a blunted response to amiloride, suggesting that epithelial sodium channel function is inhibited. We have identified an original pathway that specifically mediates the effects of vitamin D‐induced hypercalcaemia on sodium handling in the distal nephron without affecting water handling. Abstract Acute hypercalcaemia increases urinary sodium and water excretion; however, the underlying molecular mechanism remains unclear. Because vitamin D‐induced hypercalcaemia increases the renal expression of endothelin (ET)‐1, we hypothesized that ET‐1 mediates the effects of hypercalcaemia on renal sodium and water handling. Hypercalcaemia was induced in 8‐week‐old, parathyroid hormone‐supplemented, male mice by oral administration of dihydrotachysterol (DHT) for 3 days. DHT‐treated mice became hypercalcaemic and displayed increased urinary water and sodium excretion compared to controls. mRNA levels of ET‐1 and the transcription factors CCAAT‐enhancer binding protein β and δ were specifically increased in the distal convoluted tubule and downstream segments in DHT‐treated mice. To examine the role of the ET system in hypercalcaemia‐induced natriuresis and polyuria, mice were treated with the ET‐1 receptor antagonist macitentan, with or without DHT. Mice treated with both macitentan and DHT displayed hypercalcaemia and polyuria similar to that in mice treated with DHT alone; however, no increase in urinary sodium excretion was observed. To identify the affected sodium transport mechanism, we assessed the response to various diuretics in control and DHT‐treated hypercalcaemic mice. Amiloride, an inhibitor of the epithelial sodium channel (ENaC), increased sodium excretion to a lesser extent in DHT‐treated mice compared to control mice. Mice treated with either macitentan+DHT or macitentan alone had a similar response to amiloride. In summary, vitamin D‐induced hypercalcaemia increases the renal production of ET‐1 and decreases ENaC activity, which is probably responsible for the rise in urinary sodium excretion but not for polyuria.
    March 23, 2017   doi: 10.1113/JP273610   open full text
  • Prolactin regulation of oxytocin neurone activity in pregnancy and lactation.
    Rachael A. Augustine, Sharon R. Ladyman, Gregory T. Bouwer, Yousif Alyousif, Tony J. Sapsford, Victoria Scott, Ilona C. Kokay, David R. Grattan, Colin H. Brown.
    The Journal of Physiology. March 23, 2017
    Key points During lactation, prolactin promotes milk synthesis and oxytocin stimulates milk ejection. In virgin rats, prolactin inhibits the activity of oxytocin‐secreting neurones. We found that prolactin inhibition of oxytocin neurone activity is lost in lactation, and that some oxytocin neurones were excited by prolactin in lactating rats. The change in prolactin regulation of oxytocin neurone activity was not associated with a change in activation of intracellular signalling pathways known to couple to prolactin receptors. The change in prolactin regulation of oxytocin neurone activity in lactation might allow coordinated activation of both populations of neurones when required for successful lactation. Abstract Secretion of prolactin for milk synthesis and oxytocin for milk secretion is required for successful lactation. In virgin rats, prolactin inhibits oxytocin neurones but this effect would be counterproductive during lactation when secretion of both hormones is required for synthesis and delivery of milk to the newborn. Hence, we determined the effects of intracerebroventricular (i.c.v.) prolactin on oxytocin neurones in urethane‐anaesthetised virgin, pregnant and lactating rats. Prolactin (2 μg) consistently inhibited oxytocin neurones in virgin and pregnant rats (by 1.9 ± 0.4 and 1.8 ± 0.5 spikes s−1, respectively), but not in lactating rats; indeed, prolactin excited six of 27 oxytocin neurones by >1 spike s−1 in lactating rats but excited none in virgin or pregnant rats (χ22 = 7.2, P = 0.03). Vasopressin neurones were unaffected by prolactin (2 μg) in virgin rats but were inhibited by 1.1 ± 0.2 spikes s−1 in lactating rats. Immunohistochemistry showed that i.c.v. prolactin increased oxytocin expression in virgin and lactating rats and increased signal transducer and activator of transcription 5 phosphorylation to a similar extent in oxytocin neurones of virgin and lactating rats. Western blotting showed that i.c.v. prolactin did not affect phosphorylation of extracellular regulated kinase 1 or 2, or of Akt in the supraoptic or paraventricular nuclei of virgin or lactating rats. Hence, prolactin inhibition of oxytocin neurones is lost in lactation, which might allow concurrent elevation of prolactin secretion from the pituitary gland and activation of oxytocin neurones for synthesis and delivery of milk to the newborn.
    March 23, 2017   doi: 10.1113/JP273712   open full text
  • Frequency and function in the basal ganglia: the origins of beta and gamma band activity.
    Alexander Blenkinsop, Sean Anderson, Kevin Gurney.
    The Journal of Physiology. March 23, 2017
    Neural oscillations in the basal ganglia are well studied yet remain poorly understood. Behavioural correlates of spectral activity are well described, yet a quantitative hypothesis linking time domain dynamics and spectral properties to basal ganglia function has been lacking. We show, for the first time, that a unified description is possible by interpreting previously ignored structure in data describing GPi responses to cortical stimulation. These data were used to expose a pair of distinctive neuronal responses to the stimulation. This observation formed the basis for a new mathematical model of the BG, quantitatively fitted to the data, which describes the dynamics in the data, and is validated against other stimulus protocol experiments. A key new result is that when the model is run using inputs hypothesised to occur during the performance of a motor task, beta and gamma frequency oscillations emerge naturally during static‐force and movement respectively, consistent with experimental local field potentials. This new model predicts that the pallido‐striatum connection has a key role in the generation of beta band activity, and that the gamma band activity associated with motor task performance has its origins in the pallido‐subthalamic feedback loop. The network's functionality as a selection‐mechanism also occurs as an emergent property, and closer fits to the data gave better selection properties. The model provides a coherent framework for the study of spectral, temporal and functional analyses of the BG and therefore lays the foundation for an integrated approach to study BG pathologies such as Parkinson's disease in silico. This article is protected by copyright. All rights reserved
    March 23, 2017   doi: 10.1113/JP273760   open full text
  • Protein kinase A regulates C‐terminally truncated CaV1.2 in Xenopus oocytes: roles of N‐ and C‐termini of the α1C subunit.
    Shimrit Oz, Ines Pankonien, Anouar Belkacemi, Veit Flockerzi, Enno Klussmann, Hannelore Haase, Nathan Dascal.
    The Journal of Physiology. March 23, 2017
    Key points β‐Adrenergic stimulation enhances Ca2+ entry via L‐type CaV1.2 channels, causing stronger contraction of cardiac muscle cells. The signalling pathway involves activation of protein kinase A (PKA), but the molecular details of PKA regulation of CaV1.2 remain controversial despite extensive research. We show that PKA regulation of CaV1.2 can be reconstituted in Xenopus oocytes when the distal C‐terminus (dCT) of the main subunit, α1C, is truncated. The PKA upregulation of CaV1.2 does not require key factors previously implicated in this mechanism: the clipped dCT, the A kinase‐anchoring protein 15 (AKAP15), the phosphorylation sites S1700, T1704 and S1928, or the β subunit of CaV1.2. The gating element within the initial segment of the N‐terminus of the cardiac isoform of α1C is essential for the PKA effect. We propose that the regulation described here is one of two or several mechanisms that jointly mediate the PKA regulation of CaV1.2 in the heart. Abstract β‐Adrenergic stimulation enhances Ca2+ currents via L‐type, voltage‐gated CaV1.2 channels, strengthening cardiac contraction. The signalling via β‐adrenergic receptors (β‐ARs) involves elevation of cyclic AMP (cAMP) levels and activation of protein kinase A (PKA). However, how PKA affects the channel remains controversial. Recent studies in heterologous systems and genetically engineered mice stress the importance of the post‐translational proteolytic truncation of the distal C‐terminus (dCT) of the main (α1C) subunit. Here, we successfully reconstituted the cAMP/PKA regulation of the dCT‐truncated CaV1.2 in Xenopus oocytes, which previously failed with the non‐truncated α1C. cAMP and the purified catalytic subunit of PKA, PKA‐CS, injected into intact oocytes, enhanced CaV1.2 currents by ∼40% (rabbit α1C) to ∼130% (mouse α1C). PKA blockers were used to confirm specificity and the need for dissociation of the PKA holoenzyme. The regulation persisted in the absence of the clipped dCT (as a separate protein), the A kinase‐anchoring protein AKAP15, and the phosphorylation sites S1700 and T1704, previously proposed as essential for the PKA effect. The CaVβ2b subunit was not involved, as suggested by extensive mutagenesis. Using deletion/chimeric mutagenesis, we have identified the initial segment of the cardiac long‐N‐terminal isoform of α1C as a previously unrecognized essential element involved in PKA regulation. We propose that the observed regulation, that exclusively involves the α1C subunit, is one of several mechanisms underlying the overall PKA action on CaV1.2 in the heart. We hypothesize that PKA is acting on CaV1.2, in part, by affecting a structural ‘scaffold’ comprising the interacting cytosolic N‐ and C‐termini of α1C.
    March 23, 2017   doi: 10.1113/JP274015   open full text
  • T‐type calcium channels contribute to NMDA receptor independent synaptic plasticity in hippocampal regular‐spiking oriens‐alveus interneurons.
    Elizabeth Nicholson, Dimitri M. Kullmann.
    The Journal of Physiology. March 22, 2017
    Key points Regular‐spiking interneurons in the hippocampal stratum oriens exhibit a form of long‐term potentiation of excitatory transmission that is independent of NMDA receptors but requires co‐activation of Ca2+‐permeable AMPA receptors and group I metabotropic glutamate receptors. We show that T‐type Ca2+ channels are present in such interneurons. Blockade of T‐type currents prevents the induction of long‐term potentiation, and also interferes with long‐lasting potentiation induced either by postsynaptic trains of action potentials or by pairing postsynaptic hyperpolarization with activation of group I metabotropic receptors. Several Ca2+ sources thus converge on the induction of NMDA receptor independent synaptic plasticity. Abstract NMDA receptor independent long‐term potentiation (LTP) in hippocampal stratum oriens‐alveus (O/A) interneurons requires co‐activation of postsynaptic group I metabotropic glutamate receptors (mGluRs) and Ca2+‐permeable AMPA receptors. The rectification properties of such AMPA receptors contribute to the preferential induction of LTP at hyperpolarized potentials. A persistent increase in excitatory transmission can also be triggered by exogenous activation of group I mGluRs at the same time as the interneuron is hyperpolarized, or by postsynaptic trains of action potentials in the absence of presynaptic stimulation. In the present study, we identify low‐threshold transient (T‐type) channels as a further source of Ca2+ that contributes to synaptic plasticity. T‐type Ca2+ currents were detected in mouse regular‐spiking O/A interneurons. Blocking T‐type currents pharmacologically prevented LTP induced by high‐frequency stimulation of glutamatergic axons, or by application of the group I mGluR agonist dihydroxyphenylglycine, paired with postsynaptic hyperpolarization. T‐type current blockade also prevented synaptic potentiation induced by postsynaptic action potential trains. Several sources of Ca2+ thus converge on NMDA receptor independent LTP induction in O/A interneurons.
    March 22, 2017   doi: 10.1113/JP273695   open full text
  • Voltage‐sensitive conductances increase the sensitivity of rod photoresponses following pigment bleaching.
    Johan Pahlberg, Rikard Frederiksen, Gabriel E. Pollock, Kiyoharu J. Miyagishima, Alapakkam P. Sampath, M. Carter Cornwall.
    The Journal of Physiology. March 22, 2017
    Key points Following substantial bleaching of the visual pigment, the desensitization of the rod photovoltage is not as substantial as the desensitization of the rod outer segment photocurrent. The block of cation conductances during the internal dialysis of Cs+ further desensitizes the photovoltage thereby eliminating its difference in desensitization with the rod outer segment photocurrent. Bleached visual pigment produced an acceleration of the rod photovoltage with respect to the outer segment photocurrent, which is eliminated upon internal dialysis of Cs+. Abstract A majority of our visual experience occurs during the day when a substantial fraction of the visual pigment in our photoreceptor cells is bleached. Under these conditions it is widely believed that rods are saturated and do not contribute substantially to downstream signalling. However, behavioural experiments on subjects with only rod function reveals that these individuals unexpectedly retain substantial vision in daylight. We sought to understand this discrepancy by characterizing the sensitivity of rod photoresponses following exposure to bright bleaching light. Measurements of the rod outer segment photocurrent in transgenic mice, which have only rod function, revealed the well‐studied reduction in the sensitivity of rod photoresponses following pigment bleaching. However, membrane voltage measurements showed that the desensitization of the photovoltage was considerably less than that of the outer segment photocurrent following equivalent pigment bleaching. This discrepancy was largely eliminated during the blockade of cation channels due to the internal dialysis of Cs+, which increased the bleach‐induced desensitization of the photovoltage and slowed its temporal characteristics. Thus, sensitization of the photovoltage by rod inner segment conductances appears to extend the operating range of rod phototransduction following pigment bleaching.
    March 22, 2017   doi: 10.1113/JP273398   open full text
  • Diuretic‐sensitive electroneutral Na+ movement and temperature effects on central axons.
    Meneka Kanagaratnam, Christopher Pendleton, Danilo Almeida Souza, Joseph Pettit, James Howells, Mark D. Baker.
    The Journal of Physiology. March 22, 2017
    Key points Optic nerve axons get less excitable with warming. F‐fibre latency does not shorten at temperatures above 30°C. Action potential amplitude falls when the Na+‐pump is blocked, an effect speeded by warming. Diuretics reduce the rate of action potential fall in the presence of ouabain. Our data are consistent with electroneutral entry of Na+ occurring in axons and contributing to setting the resting potential. Abstract Raising the temperature of optic nerve from room temperature to near physiological has effects on the threshold, refractoriness and superexcitability of the shortest latency (fast, F) nerve fibres, consistent with hyperpolarization. The temperature dependence of peak impulse latency was weakened at temperatures above 30°C suggesting a temperature‐sensitive process that slows impulse propagation. The amplitude of the supramaximal compound action potential gets larger on warming, whereas in the presence of bumetanide and amiloride (blockers of electroneutral Na+ movement), the action potential amplitude consistently falls. This suggests a warming‐induced hyperpolarization that is reduced by blocking electroneutral Na+ movement. In the presence of ouabain, the action potential collapses. This collapse is speeded by warming, and exposure to bumetanide and amiloride slows the temperature‐dependent amplitude decline, consistent with a warming‐induced increase in electroneutral Na+ entry. Blocking electroneutral Na+ movement is predicted to be useful in the treatment of temperature‐dependent symptoms under conditions with reduced safety factor (Uhthoff's phenomenon) and provide a route to neuroprotection.
    March 22, 2017   doi: 10.1113/JP273963   open full text
  • Lysophosphatidic acid‐induced itch is mediated by signalling of LPA5 receptor, phospholipase D and TRPA1/TRPV1.
    Hiroki Kittaka, Kunitoshi Uchida, Naomi Fukuta, Makoto Tominaga.
    The Journal of Physiology. March 22, 2017
    Key points Lysophosphatidic acid (LPA) is an itch mediator, but not a pain mediator by a cheek injection model. Dorsal root ganglion neurons directly respond to LPA depending on transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1). LPA‐induced itch‐related behaviours are decreased in TRPA1‐knockout (KO), TRPV1KO or TRPA1TRPV1 double KO mice. TRPA1 and TRPV1 channels are activated by intracellular LPA, but not by extracellular LPA following LPA5 receptor activation with an activity of Ca2+‐independent phospholipase A2 and phospholipase D. Intracellular LPA interaction sites of TRPA1 are KK672–673 and KR977–978 (K: lysine, R: arginine). Abstract Intractable and continuous itch sensations often accompany diseases such as atopic dermatitis, neurogenic lesions, uremia and cholestasis. Lysophosphatidic acid (LPA) is an itch mediator found in cholestatic itch patients and it induces acute itch and pain in experimental rodent models. However, the molecular mechanism by which LPA activates peripheral sensory neurons remains unknown. In this study, we used a cheek injection method in mice to reveal that LPA induced itch‐related behaviours but not pain‐related behaviours. The LPA‐induced itch behaviour and cellular effects were dependent on transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1), which are important for itch signal transduction. We also found that, among the six LPA receptors, the LPA5 receptor had the greatest involvement in itching. Furthermore, we demonstrated that phospholipase D (PLD) plays a critical role downstream of LPA5 and that LPA directly and intracellularly activates TRPA1 and TRPV1. These results suggest a unique mechanism by which cytoplasmic LPA produced de novo could activate TRPA1 and TRPV1. We conclude that LPA‐induced itch is mediated by LPA5, PLD, TRPA1 and TRPV1 signalling, and thus targeting TRPA1, TRPV1 or PLD could be effective for cholestatic itch interventions.
    March 22, 2017   doi: 10.1113/JP273961   open full text
  • Requirement of extracellular Ca2+ binding to specific amino acids for heat‐evoked activation of TRPA1.
    Erkin Kurganov, Shigeru Saito, Claire Tanaka Saito, Makoto Tominaga.
    The Journal of Physiology. March 22, 2017
    Key points We found that extracellular Ca2+, but not other divalent cations (Mg2+ and Ba2+) or intracellular Ca2+, is involved in heat‐evoked activation of green anole (ga) TRPA1. Heat‐evoked activation of chicken (ch) and rat snake (rs) TRPA1 does not depend solely on extracellular Ca2+. Neutralization of acidic amino acids on the outer surface of TRPA1 by extracellular Ca2+ is important for heat‐evoked large activation of gaTRPA1, chTRPA1 and rsTRPA1. Abstract Transient receptor potential ankyrin 1 (TRPA1) is a homotetrameric non‐selective cation‐permeable channel that has six transmembrane domains and cytoplasmic N‐ and C‐termini. The N‐terminus is characterized by an unusually large number of ankyrin repeats. Although the 3‐dimensional structure of human TRPA1 has been determined, and TRPA1 channels from insects to birds are known to be activated by heat stimulus, the mechanism for temperature‐dependent TRPA1 activation is unclear. We previously reported that extracellular Ca2+, but not intracellular Ca2+, plays an important role in heat‐evoked TRPA1 activation in green anole lizards (gaTRPA1). Here we focus on extracellular Ca2+‐dependent heat sensitivity of gaTRPA1 by comparing gaTRPA1 with heat‐activated TRPA1 channels from rat snake (rsTRPA1) and chicken (chTRPA1). In the absence of extracellular Ca2+, rsTRPA1 and chTRPA1 are activated by heat and generate small inward currents. A comparison of extracellular amino acids in TRPA1 identified three negatively charged amino acid residues (glutamate and aspartate) near the outer pore vestibule that are involved in heat‐evoked TRPA1 activation in the presence of extracellular Ca2+. These results suggest that neutralization of acidic amino acids by extracellular Ca2+ is important for heat‐evoked activation of gaTRPA1, chTRPA1, and rsTRPA1, which could clarify mechanisms of heat‐evoked channel activation.
    March 22, 2017   doi: 10.1113/JP274083   open full text
  • Cardiac diastolic and autonomic dysfunction are aggravated by central chemoreflex activation in heart failure with preserved ejection fraction rats.
    Camilo Toledo, David C. Andrade, Claudia Lucero, Alexis Arce‐Alvarez, Hugo S. Díaz, Valentín Aliaga, Harold D. Schultz, Noah J. Marcus, Mónica Manríquez, Marcelo Faúndez, Rodrigo Del Rio.
    The Journal of Physiology. March 19, 2017
    Key points Heart failure with preserved ejection fraction (HFpEF) is associated with disordered breathing patterns, and sympatho‐vagal imbalance. Although it is well accepted that altered peripheral chemoreflex control plays a role in the progression of heart failure with reduced ejection fraction (HFrEF), the pathophysiological mechanisms underlying deterioration of cardiac function in HFpEF are poorly understood. We found that central chemoreflex is enhanced in HFpEF and neuronal activation is increased in pre‐sympathetic regions of the brainstem. Our data showed that activation of the central chemoreflex pathway in HFpEF exacerbates diastolic dysfunction, worsens sympatho‐vagal imbalance and markedly increases the incidence of cardiac arrhythmias in rats with HFpEF. Abstract Heart failure (HF) patients with preserved ejection fraction (HFpEF) display irregular breathing, sympatho‐vagal imbalance, arrhythmias and diastolic dysfunction. It has been shown that tonic activation of the central and peripheral chemoreflex pathway plays a pivotal role in the pathophysiology of HF with reduced ejection fraction. In contrast, no studies to date have addressed chemoreflex function or its effect on cardiac function in HFpEF. Therefore, we tested whether peripheral and central chemoreflexes are hyperactive in HFpEF and if chemoreflex activation exacerbates cardiac dysfunction and autonomic imbalance. Sprague‐Dawley rats (n = 32) were subjected to sham or volume overload to induce HFpEF. Resting breathing variability, chemoreflex gain, cardiac function and sympatho‐vagal balance, and arrhythmia incidence were studied. HFpEF rats displayed [mean ± SD; chronic heart failure (CHF) vs. Sham, respectively] a marked increase in the incidence of apnoeas/hypopnoeas (20.2 ± 4.0 vs. 9.7 ± 2.6 events h−1), autonomic imbalance [0.6 ± 0.2 vs. 0.2 ± 0.1 low/high frequency heart rate variability (LF/HFHRV)] and cardiac arrhythmias (196.0 ± 239.9 vs. 19.8 ± 21.7 events h−1). Furthermore, HFpEF rats showed increase central chemoreflex sensitivity but not peripheral chemosensitivity. Accordingly, hypercapnic stimulation in HFpEF rats exacerbated increases in sympathetic outflow to the heart (229.6 ± 43.2% vs. 296.0 ± 43.9% LF/HFHRV, normoxia vs. hypercapnia, respectively), incidence of cardiac arrhythmias (196.0 ± 239.9 vs. 576.7 ± 472.9 events h−1) and diastolic dysfunction (0.008 ± 0.004 vs. 0.027 ± 0.027 mmHg μl−1). Importantly, the cardiovascular consequences of central chemoreflex activation were related to sympathoexcitation since these effects were abolished by propranolol. The present results show that the central chemoreflex is enhanced in HFpEF and that acute activation of central chemoreceptors leads to increases of cardiac sympathetic outflow, cardiac arrhythmogenesis and impairment in cardiac function in rats with HFpEF.
    March 19, 2017   doi: 10.1113/JP273558   open full text
  • The influence of adrenergic stimulation on sex differences in left ventricular twist mechanics.
    Alexandra M. Williams, Rob E. Shave, William S. Cheyne, Neil D. Eves.
    The Journal of Physiology. March 19, 2017
    Key points Sex differences in left ventricular (LV) mechanics occur during acute physiological challenges; however, it is unknown whether sex differences in LV mechanics are fundamentally regulated by differences in adrenergic control. Using two‐dimensional echocardiography and speckle tracking analysis, this study compared LV mechanics in males and females matched for LV length during post‐exercise ischaemia (PEI) and β1‐adrenergic receptor blockade. Our data demonstrate that while basal rotation was increased in males, LV twist was not significantly different between the sexes during PEI. In contrast, during β1‐adrenergic receptor blockade, LV apical rotation, twist and untwisting velocity were reduced in males compared to females. Significant relationships were observed between LV twist and LV internal diameter and sphericity index in females, but not males. These findings suggest that LV twist mechanics may be more sensitive to alterations in adrenergic stimulation in males, but more highly influenced by ventricular structure and geometry in females. Abstract Sex differences in left ventricular (LV) mechanics exist at rest and during acute physiological stress. Differences in cardiac autonomic and adrenergic control may contribute to sex differences in LV mechanics and LV haemodynamics. Accordingly, this study aimed to investigate sex differences in LV mechanics with altered adrenergic stimulation achieved through post‐handgrip‐exercise ischaemia (PEI) and β1‐adrenergic receptor (AR) blockade. Twenty males (23 ± 5 years) and 20 females (22 ± 3 years) were specifically matched for LV length (males: 8.5 ± 0.5 cm, females: 8.2 ± 0.6 cm, P = 0.163), and two‐dimensional speckle‐tracking echocardiography was used to assess LV structure and function at baseline, during PEI and following administration of 5 mg bisoprolol (β1‐AR antagonist). During PEI, LV end‐diastolic volume and stroke volume were increased in both groups (P < 0.001), as was end‐systolic wall stress (P < 0.001). LV twist and apical rotation were not altered from baseline or different between the sexes; however, basal rotation increased in males (P = 0.035). During β1‐AR blockade, LV volumes were unchanged but blood pressure and heart rate were reduced in both groups (P < 0.001). LV apical rotation (P = 0.036) and twist (P = 0.029) were reduced in males with β1‐AR blockade but not females, resulting in lower apical rotation (males: 6.8 ± 2.1 deg, females: 8.8 ± 2.3 deg, P = 0.007) and twist (males: 8.6 ± 1.9 deg, females: 10.7 ± 2.8 deg, P = 0.008), and slower untwisting velocity (males: 68.2 ± 22.1 deg s−1, females: 82.0 ± 18.7 deg s−1, P = 0.046) compared to females. LV twist mechanics are reduced in males compared to females during reductions to adrenergic stimulation, providing preliminary evidence that LV twist mechanics may be more sensitive to adrenergic control in males than in females.
    March 19, 2017   doi: 10.1113/JP273368   open full text
  • Mechanisms of pruritogen‐induced activation of itch nerves in isolated mouse skin.
    F. Ru, H. Sun, D. Jurcakova, R. A. Herbstsomer, J. Meixong, X. Dong, B. J. Undem.
    The Journal of Physiology. March 19, 2017
    Key points Chloroquine (CQ) stimulates itch nerves and causes intense scratching in mice by activating the G‐protein coupled receptor (GPCR) MrgprA3; it is not known how stimulation of MrgprA3 (or other GPCRs) leads to activation of the itch nerve terminals in the skin, but previous studies have found that transient receptor potential A1 (TRPA1) gene deletion blocks CQ‐induced scratching. In the present study we used a novel dorsal skin–nerve preparation to evaluate mechanisms underlying CQ‐ and histamine‐induced action potential discharge in itch nerve terminals. We found that CQ activation of the nerves requires the beta3 isoform of phospholipase C, but TRPA1 or other TRP channel are not required. Evidence is provided for a role for calcium‐activated chloride channels such as TMEM16a in GPCR‐activation of itch nerve terminals. The mechanism by which TRP channels participate in pruritogen‐induced scratching may involve sites of action other than the primary afferent terminals. Abstract Chloroquine (CQ) and histamine are pruritogens commonly used to study itch in the mouse. A novel skin–nerve preparation was used to evaluate chloroquine (CQ)‐ and histamine‐induced activation of afferent nerves in the dorsal thoracic skin of the mouse. All CQ sensitive nerves were C‐fibres, and were also sensitive to histamine. The response to CQ, but not histamine, was largely absent in mrgpr‐cluster Δ−/− mice, supporting the hypothesis that CQ evokes itch largely via stimulation of MrgprA3 receptors. The CQ‐induced action potential discharge was largely absent in phospholipase Cβ3 knockout animals. The CQ and histamine responses were not influenced by removal of TRPA1, TRPV1, TRPC3 or TRPC6, nor by the TRP channel blocker Ruthenium Red. The bouts of scratching in response to CQ were not different between wild‐type and TRPA1‐deficient mice. A selective inhibitor of the calcium‐activated chloride channel TMEM16A, N‐((4‐methoxy)‐2‐naphthyl)‐5‐nitroanthranilic acid (MONNA), inhibited CQ‐induced action potential discharge at itch nerve terminals and bouts of scratching by about 50%. Although TRPA1 and TRPV1 channels may be involved in the scratching responses to intradermal pruritogens, this is unlikely to be due to an effect at the nerve terminals, where chloride channels may play a more important role.
    March 19, 2017   doi: 10.1113/JP273795   open full text
  • Loss of protohaem IX farnesyltransferase in mature dentate granule cells impairs short‐term facilitation at mossy fibre to CA3 pyramidal cell synapses.
    Sam A. Booker, Graham R. Campbell, Karolina S. Mysiak, Peter J. Brophy, Peter C. Kind, Don J. Mahad, David J. A. Wyllie.
    The Journal of Physiology. March 15, 2017
    Key points Neurodegenerative disorders can exhibit dysfunctional mitochondrial respiratory chain complex IV activity. Conditional deletion of cytochrome c oxidase, the terminal enzyme in the respiratory electron transport chain of mitochondria, from hippocampal dentate granule cells in mice does not affect low‐frequency dentate to CA3 glutamatergic synaptic transmission. High‐frequency dentate to CA3 glutamatergic synaptic transmission and feedforward inhibition are significantly attenuated in cytochrome c oxidase‐deficient mice. Intact presynaptic mitochondrial function is critical for the short‐term dynamics of mossy fibre to CA3 synaptic function. Abstract Neurodegenerative disorders are characterized by peripheral and central symptoms including cognitive impairments which have been associated with reduced mitochondrial function, in particular mitochondrial respiratory chain complex IV or cytochrome c oxidase activity. In the present study we conditionally removed a key component of complex IV, protohaem IX farnesyltransferase encoded by the COX10 gene, in granule cells of the adult dentate gyrus. Utilizing whole‐cell patch‐clamp recordings from morphologically identified CA3 pyramidal cells from control and complex IV‐deficient mice, we found that reduced mitochondrial function did not result in overt deficits in basal glutamatergic synaptic transmission at the mossy‐fibre synapse because the amplitude, input–output relationship and 50 ms paired‐pulse facilitation were unchanged following COX10 removal from dentate granule cells. However, trains of stimuli given at high frequency (> 20 Hz) resulted in dramatic reductions in short‐term facilitation and, at the highest frequencies (> 50 Hz), also reduced paired‐pulse facilitation, suggesting a requirement for adequate mitochondrial function to maintain glutamate release during physiologically relevant activity patterns. Interestingly, local inhibition was reduced, suggesting the effect observed was not restricted to synapses with CA3 pyramidal cells via large mossy‐fibre boutons, but rather to all synapses formed by dentate granule cells. Therefore, presynaptic mitochondrial function is critical for the short‐term dynamics of synapse function, which may contribute to the cognitive deficits observed in pathological mitochondrial dysfunction.
    March 15, 2017   doi: 10.1113/JP273581   open full text
  • The role of dentate nuclei in human oculomotor control: insights from cerebrotendinous xanthomatosis.
    Francesca Rosini, Elena Pretegiani, Andrea Mignarri, Lance M. Optican, Valeria Serchi, Nicola Stefano, Marco Battaglini, Lucia Monti, Maria T. Dotti, Antonio Federico, Alessandra Rufa.
    The Journal of Physiology. March 14, 2017
    Key points A cerebellar dentate nuclei (DN) contribution to volitional oculomotor control has recently been hypothesized but not fully understood. Cerebrotendinous xanthomatosis (CTX) is a rare neurometabolic disease typically characterized by DN damage. In this study, we compared the ocular movement characteristics of two sets of CTX patients, with and without brain MRI evidence of DN involvement, with a set of healthy subjects. Our results suggest that DN participate in voluntary behaviour, such as the execution of antisaccades, and moreover are involved in controlling the precision of the ocular movement. The saccadic abnormalities related to DN involvement were independent of global and regional brain atrophy. Our study confirms the relevant role of DN in voluntary aspects of oculomotion and delineates specific saccadic abnormalities that could be used to detect the involvement of DN in other cerebellar disorders. Abstract It is well known that the medial cerebellum controls saccadic speed and accuracy. In contrast, the role of the lateral cerebellum (cerebellar hemispheres and dentate nuclei, DN) is less well understood. Cerebrotendinous xanthomatosis (CTX) is a lipid storage disorder due to mutations in CYP27A1, typically characterized by DN damage. CTX thus provides a unique opportunity to study DN in human oculomotor control. We analysed horizontal and vertical visually guided saccades and horizontal antisaccades of 19 CTX patients. Results were related to the presence/absence of DN involvement and compared with those of healthy subjects. To evaluate the contribution of other areas, abnormal saccadic parameters were compared with global and regional brain volumes. CTX patients executed normally accurate saccades with normal main sequence relationships, indicating that the brainstem and medial cerebellar structures were functionally spared. Patients with CTX executed more frequent multistep saccades and directional errors during the antisaccade task than controls. CTX patients with DN damage showed less precise saccades with longer latencies, and more frequent directional errors, usually not followed by corrections, than either controls or patients without DN involvement. These saccadic abnormalities related to DN involvement but were independent of global and regional brain atrophy. We hypothesize that two different cerebellar networks contribute to the metrics of a movement: the medial cerebellar structures determine accuracy, whereas the lateral cerebellar structures control precision. The lateral cerebellum (hemispheres and DN) also participates in modulating goal directed gaze behaviour, by prioritizing volitional over reflexive movements.
    March 14, 2017   doi: 10.1113/JP273670   open full text
  • Calmodulin and ATP support activity of the Cav1.2 channel through dynamic interactions with the channel.
    Etsuko Minobe, Masayuki X. Mori, Masaki Kameyama.
    The Journal of Physiology. March 13, 2017
    Key points Cav1.2 channels maintain activity through interactions with calmodulin (CaM). In this study, activities of the Cav1.2 channel (α1C) and of mutant‐derivatives, C‐terminal deleted (α1CΔ) and α1CΔ linked with CaM (α1CΔCaM), were compared in the inside‐out mode. α1CΔ with CaM, but not without CaM, and α1CΔCaM were active, suggesting that CaM induced channel activity through a dynamic interaction with the channel, even without the distal C‐tail. ATP induced α1C activity with CaM and enhanced activity of the mutant channels. Okadaic acid mimicked the effect of ATP on the wildtype but not mutant channels. These results supported the hypothesis that CaM and ATP maintain activity of Cav1.2 channels through their dynamic interactions. ATP effects involve mechanisms both related and unrelated to channel phosphorylation. CaM‐linked channels are useful tools for investigating Cav1.2 channels in the inside‐out mode; the fast run‐down is prevented by only ATP and the slow run‐down is nearly absent. Abstract Calmodulin (CaM) plays a critical role in regulation of Cav1.2 Ca2+ channels. CaM binds to the channel directly, maintaining channel activity and regulating it in a Ca2+‐dependent manner. To explore the molecular mechanisms involved, we compared the activity of the wildtype channel (α1C) and mutant derivatives, C‐terminal deleted (α1C∆) and α1C∆ linked to CaM (α1C∆CaM). These were co‐expressed with β2a and α2δ subunits in HEK293 cells. In the inside‐out mode, α1C and α1C∆ showed minimal open‐probabilities in a basic internal solution (run‐down), whereas α1C∆ with CaM and α1C∆CaM maintained detectable channel activity, confirming that CaM was necessary, but not sufficient, for channel activity. Previously, we reported that ATP was required to maintain channel activity of α1C. Unlike α1C, the mutant channels did not require ATP for activation in the early phase (3–5 min). However, α1C∆ with CaM + ATP and α1C∆CaM with ATP maintained activity, even in the late phase (after 7–9 min). These results suggested that CaM and ATP interacted dynamically with the proximal C‐terminal tail of the channel and, thereby, produced channel activity. In addition, okadaic acid, a protein phosphatase inhibitor, could substitute for the effects of ATP on α1C but not on the mutant channels. These results supported the hypothesis that CaM and ATP maintain activity of Cav1.2 channels, further indicating that ATP has dual effects. One maintains phosphorylation of the channel and the other becomes apparent when the distal carboxyl‐terminal tail is removed.
    March 13, 2017   doi: 10.1113/JP273736   open full text
  • Hypothyroidism in utero stimulates pancreatic beta cell proliferation and hyperinsulinaemia in the ovine fetus during late gestation.
    Shelley E. Harris, Miles J. Blasio, Melissa A. Davis, Amy C. Kelly, Hailey M. Davenport, F. B. Peter Wooding, Dominique Blache, David Meredith, Miranda Anderson, Abigail L. Fowden, Sean W. Limesand, Alison J. Forhead.
    The Journal of Physiology. March 13, 2017
    Key points Thyroid hormones are important regulators of growth and maturation before birth, although the extent to which their actions are mediated by insulin and the development of pancreatic beta cell mass is unknown. Hypothyroidism in fetal sheep induced by removal of the thyroid gland caused asymmetric organ growth, increased pancreatic beta cell mass and proliferation, and was associated with increased circulating concentrations of insulin and leptin. In isolated fetal sheep islets studied in vitro, thyroid hormones inhibited beta cell proliferation in a dose‐dependent manner, while high concentrations of insulin and leptin stimulated proliferation. The developing pancreatic beta cell is therefore sensitive to thyroid hormone, insulin and leptin before birth, with possible consequences for pancreatic function in fetal and later life. The findings of this study highlight the importance of thyroid hormones during pregnancy for normal development of the fetal pancreas. Abstract Development of pancreatic beta cell mass before birth is essential for normal growth of the fetus and for long‐term control of carbohydrate metabolism in postnatal life. Thyroid hormones are also important regulators of fetal growth, and the present study tested the hypotheses that thyroid hormones promote beta cell proliferation in the fetal ovine pancreatic islets, and that growth retardation in hypothyroid fetal sheep is associated with reductions in pancreatic beta cell mass and circulating insulin concentration in utero. Organ growth and pancreatic islet cell proliferation and mass were examined in sheep fetuses following removal of the thyroid gland in utero. The effects of triiodothyronine (T3), insulin and leptin on beta cell proliferation rates were determined in isolated fetal ovine pancreatic islets in vitro. Hypothyroidism in the sheep fetus resulted in an asymmetric pattern of organ growth, pancreatic beta cell hyperplasia, and elevated plasma insulin and leptin concentrations. In pancreatic islets isolated from intact fetal sheep, beta cell proliferation in vitro was reduced by T3 in a dose‐dependent manner and increased by insulin at high concentrations only. Leptin induced a bimodal response whereby beta cell proliferation was suppressed at the lowest, and increased at the highest, concentrations. Therefore, proliferation of beta cells isolated from the ovine fetal pancreas is sensitive to physiological concentrations of T3, insulin and leptin. Alterations in these hormones may be responsible for the increased beta cell proliferation and mass observed in the hypothyroid sheep fetus and may have consequences for pancreatic function in later life.
    March 13, 2017   doi: 10.1113/JP273555   open full text
  • KATP channel inhibition blunts electromechanical decline during hypoxia in left ventricular working rabbit hearts.
    Kara Garrott, Sarah Kuzmiak‐Glancy, Anastasia Wengrowski, Hanyu Zhang, Jack Rogers, Matthew W. Kay.
    The Journal of Physiology. March 13, 2017
    Key points Heart function is critically dependent upon the balance of energy production and utilization. Sarcolemmal ATP‐sensitive potassium channels (KATP channels) in cardiac myocytes adjust contractile function to compensate for the level of available energy. Understanding the activation of KATP channels in working myocardium during high‐stress situations is crucial to the treatment of cardiovascular disease, especially ischaemic heart disease. Using a new optical mapping approach, we measured action potentials from the surface of excised contracting rabbit hearts to assess when sarcolemmal KATP channels were activated during physiologically relevant workloads and during gradual reductions in myocardial oxygenation. We demonstrate that left ventricular pressure is closely linked to KATP channel activation and that KATP channel inhibition with a low concentration of tolbutamide prevents electromechanical decline when oxygen availability is reduced. As a result, KATP channel inhibition probably exacerbates a mismatch between energy demand and energy production when myocardial oxygenation is low. Abstract Sarcolemmal ATP‐sensitive potassium channel (KATP channel) activation in isolated cells is generally understood, although the relationship between myocardial oxygenation and KATP activation in excised working rabbit hearts remains unknown. We optically mapped action potentials (APs) in excised rabbit hearts to test the hypothesis that hypoxic changes would be more severe in left ventricular (LV) working hearts (LWHs) than Langendorff (LANG) perfused hearts. We further hypothesized that KATP inhibition would prevent those changes. Optical APs were mapped when measuring LV developed pressure (LVDP), coronary flow rate and oxygen consumption in LANG and LWHs. Hearts were paced to increase workload and perfusate was deoxygenated to study the effects of myocardial hypoxia. A subset of hearts was perfused with 1 μm tolbutamide (TOLB) to identify the level of AP duration (APD) shortening attributed to KATP channel activation. During sinus rhythm, APD was shorter in LWHs compared to LANG hearts. APD in both LWHs and LANG hearts dropped steadily during deoxygenation. With TOLB, APDs in LWHs were longer at all workloads and APD reductions during deoxygenation were blunted in both LWHs and LANG hearts. At 50% perfusate oxygenation, APD and LVDP were significantly higher in LWHs perfused with TOLB (199 ± 16 ms; 92 ± 5.3 mmHg) than in LWHs without TOLB (109 ± 14 ms, P = 0.005; 65 ± 6.5 mmHg, P = 0.01). Our results indicate that KATP channels are activated to a greater extent in perfused hearts when the LV performs pressure–volume work. The results of the present study demonstrate the critical role of KATP channels in modulating myocardial function over a wide range of physiological conditions.
    March 13, 2017   doi: 10.1113/JP273873   open full text
  • Relationship between cortical state and spiking activity in the lateral geniculate nucleus of marmosets.
    Alexander N.J. Pietersen, Soon Keen Cheong, Brandon Munn, Pulin Gong, Paul R. Martin, Samuel G. Solomon.
    The Journal of Physiology. March 10, 2017
    Key points How parallel are the primate visual pathways? In the present study, we demonstrate that parallel visual pathways in the dorsal lateral geniculate nucleus (LGN) show distinct patterns of interaction with rhythmic activity in the primary visual cortex (V1). In the V1 of anaesthetized marmosets, the EEG frequency spectrum undergoes transient changes that are characterized by fluctuations in delta‐band EEG power. We show that, on multisecond timescales, spiking activity in an evolutionary primitive (koniocellular) LGN pathway is specifically linked to these slow EEG spectrum changes. By contrast, on subsecond (delta frequency) timescales, cortical oscillations can entrain spiking activity throughout the entire LGN. Our results are consistent with the hypothesis that, in waking animals, the koniocellular pathway selectively participates in brain circuits controlling vigilance and attention. Abstract The major afferent cortical pathway in the visual system passes through the dorsal lateral geniculate nucleus (LGN), where nerve signals originating in the eye can first interact with brain circuits regulating visual processing, vigilance and attention. In the present study, we investigated how ongoing and visually driven activity in magnocellular (M), parvocellular (P) and koniocellular (K) layers of the LGN are related to cortical state. We recorded extracellular spiking activity in the LGN simultaneously with local field potentials (LFP) in primary visual cortex, in sufentanil‐anaesthetized marmoset monkeys. We found that asynchronous cortical states (marked by low power in delta‐band LFPs) are linked to high spike rates in K cells (but not P cells or M cells), on multisecond timescales. Cortical asynchrony precedes the increases in K cell spike rates by 1–3 s, implying causality. At subsecond timescales, the spiking activity in many cells of all (M, P and K) classes is phase‐locked to delta waves in the cortical LFP, and more cells are phase‐locked during synchronous cortical states than during asynchronous cortical states. The switch from low‐to‐high spike rates in K cells does not degrade their visual signalling capacity. By contrast, during asynchronous cortical states, the fidelity of visual signals transmitted by K cells is improved, probably because K cell responses become less rectified. Overall, the data show that slow fluctuations in cortical state are selectively linked to K pathway spiking activity, whereas delta‐frequency cortical oscillations entrain spiking activity throughout the entire LGN, in anaesthetized marmosets.
    March 10, 2017   doi: 10.1113/JP273569   open full text
  • The TRPM7 channel kinase regulates store‐operated calcium entry.
    Malika Faouzi, Tatiana Kilch, F. David Horgen, Andrea Fleig, Reinhold Penner.
    The Journal of Physiology. March 10, 2017
    Key points Pharmacological and molecular inhibition of transient receptor potential melastatin 7 (TRPM7) reduces store‐operated calcium entry (SOCE). Overexpression of TRPM7 in TRPM7−/− cells restores SOCE. TRPM7 is not a store‐operated calcium channel. TRPM7 kinase rather than channel modulates SOCE. TRPM7 channel activity contributes to the maintenance of store Ca2+ levels at rest. Abstract The transient receptor potential melastatin 7 (TRPM7) is a protein that combines an ion channel with an intrinsic kinase domain, enabling it to modulate cellular functions either by conducting ions through the pore or by phosphorylating downstream proteins via its kinase domain. In the present study, we report store‐operated calcium entry (SOCE) as a novel target of TRPM7 kinase activity. TRPM7‐deficient chicken DT40 B lymphocytes exhibit a strongly impaired SOCE compared to wild‐type cells as a result of reduced calcium release activated calcium currents, and independently of potassium channel regulation, membrane potential changes or changes in cell‐cycle distribution. Pharmacological blockade of TRPM7 with NS8593 or waixenicin A in wild‐type B lymphocytes results in a significant decrease in SOCE, confirming that TRPM7 activity is acutely linked to SOCE, without TRPM7 representing a store‐operated channel itself. Using kinase‐deficient mutants, we find that TRPM7 regulates SOCE through its kinase domain. Furthermore, Ca2+ influx through TRPM7 is essential for the maintenance of endoplasmic reticulum Ca2+ concentration in resting cells, and for the refilling of Ca2+ stores after a Ca2+ signalling event. We conclude that the channel kinase TRPM7 and SOCE are synergistic mechanisms regulating intracellular Ca2+ homeostasis.
    March 10, 2017   doi: 10.1113/JP274006   open full text
  • Perinatal nicotine exposure impairs the maturation of glutamatergic inputs in the auditory brainstem.
    Veronika J. Baumann, Ursula Koch.
    The Journal of Physiology. March 10, 2017
    Key points Chronic perinatal nicotine exposure causes abnormal auditory brainstem responses and auditory processing deficits in children and animal models. The effect of perinatal nicotine exposure on synaptic maturation in the auditory brainstem was investigated in granule cells in the ventral nucleus of the lateral lemniscus, which receive a single calyx‐like input from the cochlear nucleus. Perinatal nicotine exposure caused a massive reduction in the amplitude of the excitatory input current. This caused a profound decrease in the number and temporal precision of spikes in these neurons. Perinatal nicotine exposure delayed the developmental downregulation of functional nicotinic acetylcholine receptors on these neurons. Abstract Maternal smoking causes chronic nicotine exposure during early development and results in auditory processing deficits including delayed speech development and learning difficulties. Using a mouse model of chronic, perinatal nicotine exposure we explored to what extent synaptic inputs to granule cells in the ventral nucleus of the lateral lemniscus are affected by developmental nicotine treatment. These neurons receive one large calyx‐like input from octopus cells in the cochlear nucleus and play a role in sound pattern analysis, including speech sounds. In addition, they exhibit high levels of α7 nicotinic acetylcholine receptors, especially during early development. Our whole‐cell patch‐clamp experiments show that perinatal nicotine exposure causes a profound reduction in synaptic input amplitude. In contrast, the number of inputs innervating each neuron and synaptic release properties of this calyx‐like synapse remained unaltered. Spike number and spiking precision in response to synaptic stimulation were greatly diminished, especially for later stimuli during a stimulus train. Moreover, chronic nicotine exposure delayed the developmental downregulation of functional nicotinic acetylcholine receptors on these neurons, indicating a direct action of nicotine in this brain area. This presumably direct effect of perinatal nicotine exposure on synaptic maturation in the auditory brainstem might be one of the underlying causes for auditory processing difficulties in children of heavy smoking mothers.
    March 10, 2017   doi: 10.1113/JP274059   open full text
  • Exploratory assessment of left ventricular strain–volume loops in severe aortic valve diseases.
    Hugo G. Hulshof, Arie P. Dijk, Keith P. George, Maria T. E. Hopman, Dick H. J. Thijssen, David L. Oxborough.
    The Journal of Physiology. March 09, 2017
    Key points Severe aortic valve diseases are common cardiac abnormalities that are associated with poor long‐term survival. Before any reduction in left ventricular (LV) function, the left ventricle undergoes structural remodelling under the influence of changing haemodynamic conditions. In this study, we combined temporal changes in LV structure (volume) with alterations in LV functional characteristics (strain, ԑ) into a ԑ–volume loop, in order to provide novel insight into the haemodynamic cardiac consequences of aortic valve diseases in those with preserved LV ejection fraction. We showed that our novel ԑ–volume loop and the specific loop characteristics provide additional insight into the functional and mechanical haemodynamic consequences of severe aortic valve diseases (with preserved LV ejection fraction). Finally, we showed that the ԑ–volume loop characteristics provide discriminative capacity compared with conventional measures of LV function. Abstract The purpose of this study was to examine left ventricular (LV) strain (ԑ)–volume loops to provide novel insight into the haemodynamic cardiac consequences of aortic valve stenosis (AS) and aortic valve regurgitation (AR). Twenty‐seven participants were retrospectively recruited: AR (n = 7), AS (n = 10) and control subjects (n = 10). Standard transthoracic echocardiography was used to obtain apical four‐chamber images to construct ԑ–volume relationships, which were assessed using the following parameters: early systolic ԑ (ԑ_ES); slope of ԑ–volume relationship during systole (Sslope); end‐systolic peak ԑ (peak ԑ); and diastolic uncoupling (systolic ԑ–diastolic ԑ at same volume) during early diastole (UNCOUP_ED) and late diastole (UNCOUP_LD). Receiver operating characteristic curves were used to determine the ability to detect impaired LV function. Although LV ejection fraction was comparable between groups, longitudinal peak ԑ was reduced compared with control subjects. In contrast, ԑ_ES and Sslope were lower in both pathologies compared with control subejcts (P < 0.01), but also different between AS and AR (P < 0.05). UNCOUP_ED and UNCOUP_LD were significantly higher in both patient groups compared with control subjects (P < 0.05). Receiver operating characteristic curves revealed that loop characteristics (AUC = 0.99, 1.00 and 1.00; all P < 0.01) were better able then peak ԑ (AUC = 0.75, 0.89 and 0.76; P = 0.06, <0.01 and 0.08, respectively) and LV ejection fraction (AUC = 0.56, 0.69 and 0.69; all P > 0.05) to distinguish AS vs control, AR vs control and AS vs AR groups, respectively. Temporal changes in ԑ–volume characteristics provide novel insight into the haemodynamic cardiac impact of AS and AR. Contrary to traditional measures (i.e. ejection fraction, peak ԑ), these novel measures successfully distinguish between the haemodynamic cardiac impact of AS and AR.
    March 09, 2017   doi: 10.1113/JP273526   open full text
  • Low pHo boosts burst firing and catecholamine release by blocking TASK‐1 and BK channels while preserving Cav1 channels in mouse chromaffin cells.
    Laura Guarina, David H. F. Vandael, Valentina Carabelli, Emilio Carbone.
    The Journal of Physiology. March 02, 2017
    Key points Mouse chromaffin cells (MCCs) generate spontaneous burst‐firing that causes large increases of Ca2+‐dependent catecholamine release, and is thus a key mechanism for regulating the functions of MCCs. With the aim to uncover a physiological role for burst‐firing we investigated the effects of acidosis on MCC activity. Lowering the extracellular pH (pHo) from 7.4 to 6.6 induces cell depolarizations of 10–15 mV that generate bursts of ∼330 ms at 1–2 Hz and a 7.4‐fold increase of cumulative catecholamine‐release. Burst‐firing originates from the inhibition of the pH‐sensitive TASK‐1‐channels and a 60% reduction of BK‐channel conductance at pHo 6.6. Blockers of the two channels (A1899 and paxilline) mimic the effects of pHo 6.6, and this is reverted by the Cav1 channel blocker nifedipine. MCCs act as pH‐sensors. At low pHo, they depolarize, undergo burst‐firing and increase catecholamine‐secretion, generating an effective physiological response that may compensate for the acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue. Abstract Mouse chromaffin cells (MCCs) generate action potential (AP) firing that regulates the Ca2+‐dependent release of catecholamines (CAs). Recent findings indicate that MCCs possess a variety of spontaneous firing modes that span from the common ‘tonic‐irregular’ to the less frequent ‘burst’ firing. This latter is evident in a small fraction of MCCs but occurs regularly when Nav1.3/1.7 channels are made less available or when the Slo1β2‐subunit responsible for BK channel inactivation is deleted. Burst firing causes large increases of Ca2+‐entry and potentiates CA release by ∼3.5‐fold and thus may be a key mechanism for regulating MCC function. With the aim to uncover a physiological role for burst‐firing we investigated the effects of acidosis on MCC activity. Lowering the extracellular pH (pHo) from 7.4 to 7.0 and 6.6 induces cell depolarizations of 10–15 mV that generate repeated bursts. Bursts at pHo 6.6 lasted ∼330 ms, occurred at 1–2 Hz and caused an ∼7‐fold increase of CA cumulative release. Burst firing originates from the inhibition of the pH‐sensitive TASK‐1/TASK‐3 channels and from a 40% BK channel conductance reduction at pHo 7.0. The same pHo had little or no effect on Nav, Cav, Kv and SK channels that support AP firing in MCCs. Burst firing of pHo 6.6 could be mimicked by mixtures of the TASK‐1 blocker A1899 (300 nm) and BK blocker paxilline (300 nm) and could be prevented by blocking L‐type channels by adding 3 μm nifedipine. Mixtures of the two blockers raised cumulative CA‐secretion even more than low pHo (∼12‐fold), showing that the action of protons on vesicle release is mainly a result of the ionic conductance changes that increase Ca2+‐entry during bursts. Our data provide direct evidence suggesting that MCCs respond to low pHo with sustained depolarization, burst firing and enhanced CA‐secretion, thus mimicking the physiological response of CCs to acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue.
    March 02, 2017   doi: 10.1113/JP273735   open full text
  • Calcium‐calmodulin‐dependent protein kinase mediates the intracellular signalling pathways of cardiac apoptosis in mice with impaired glucose tolerance.
    Marilen Federico, Enrique L. Portiansky, Leandro Sommese, Francisco J. Alvarado, Paula G. Blanco, Carolina N. Zanuzzi, John Dedman, Marcia Kaetzel, Xander H. T. Wehrens, Alicia Mattiazzi, Julieta Palomeque.
    The Journal of Physiology. March 02, 2017
    Key points Spontaneous sarcoplasmic reticulum (SR) Ca2+ release events increased in fructose‐rich diet mouse (FRD) myocytes vs. control diet (CD) mice, in the absence of significant changes in SR Ca2+ load. In HEK293 cells, hyperglycaemia significantly enhanced [3H]ryanodine binding and Ca2+/calmodulin‐dependent protein kinase II (CaMKII) phosphorylation of RyR2‐S2814 residue vs. normoglycaemia. These increases were prevented by CaMKII inhibition. FRD significantly augmented cardiac apoptosis in WT vs. CD‐WT mice, which was prevented by co‐treatment with the reactive oxygen species scavenger Tempol. Oxidative stress was also increased in FRD‐SR‐autocamide inhibitory peptide (AIP) mice, expressing the SR‐targeted CaMKII inhibitor AIP, without any significant enhancement of apoptosis vs. CD‐SR‐AIP mice. FRD produced mitochondrial swelling and membrane depolarization in FRD‐WT mice but not in FRD‐S2814A mice, in which the CaMKII site on ryanodine receptor 2 was ablated. FRD decreased mitochondrial area, mean Feret diameter and the mean distance between SR and the outer mitochondrial membrane vs. CD hearts. This remodelling was prevented in AC3I mice, with cardiac‐targeted CaMKII inhibition. Abstract The impact of cardiac apoptosis in pre‐diabetic stages of diabetic cardiomyopathy is unknown. We show that myocytes from fructose‐rich diet (FRD) animals exhibit arrhythmias produced by exacerbated Ca2+/calmodulin‐protein kinase (CaMKII) activity, ryanodine receptor 2 (RyR2) phosphorylation and sarcoplasmic reticulum (SR) Ca2+ leak. We tested the hypothesis that this mechanism also underlies cardiac apoptosis in pre‐diabetes. We generated a pre‐diabetic model in FRD mice. FRD mice showed an increase in oxidative stress, hypertrophy and systolic dysfunction. FRD myocytes exhibited enhanced SR Ca2+ spontaneous events in the absence of SR Ca2+ load alterations vs. control‐diet (CD) myocytes. In HEK293 cells, hyperglycaemia significantly enhanced [3H]ryanodine binding and CaMKII phosphorylation of RyR2‐S2814 residue vs. normoglycaemia. CaMKII inhibition prevented hyperglycaemia‐induced alterations. FRD also evoked cardiac apoptosis in WT mice vs. CD‐WT mice. Co‐treatment with the reactive oxygen species scavenger Tempol prevented FRD‐induced apoptosis in WT mice. In contrast, FRD enhanced oxidative stress but not apoptosis in FRD‐SR‐AIP mice, in which a CaMKII inhibitor is targeted to the SR. FRD produced mitochondrial membrane depolarization in WT mice but not in S2814A mice, in which the CaMKII phosphorylation site on RyR2 was ablated. Furthermore, FRD decreased mitochondrial area, mean Feret diameter and mean SR–mitochondrial distance vs. CD‐WT hearts. This remodelling was prevented in AC3I mice, with cardiac‐targeted CaMKII inhibition. CaMKII phosphorylation of RyR2, SR Ca2+ leak and mitochondrial membrane depolarization are critically involved in the apoptotic pathway of the pre‐diabetic heart. The FRD‐induced decrease in SR–mitochondrial distance is likely to additionally favour Ca2+ transit between the two organelles.
    March 02, 2017   doi: 10.1113/JP273714   open full text
  • Visceral and somatic pain modalities reveal NaV1.7‐independent visceral nociceptive pathways.
    James R. F. Hockley, Rafael González‐Cano, Sheridan McMurray, Miguel A. Tejada‐Giraldez, Cian McGuire, Antonio Torres, Anna L. Wilbrey, Vincent Cibert‐Goton, Francisco R. Nieto, Thomas Pitcher, Charles H. Knowles, José Manuel Baeyens, John N. Wood, Wendy J. Winchester, David C. Bulmer, Cruz Miguel Cendán, Gordon McMurray.
    The Journal of Physiology. March 01, 2017
    Key points Voltage‐gated sodium channels play a fundamental role in determining neuronal excitability. Specifically, voltage‐gated sodium channel subtype NaV1.7 is required for sensing acute and inflammatory somatic pain in mice and humans but its significance in pain originating from the viscera is unknown. Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for NaV1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling. These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality‐specific manner and help to direct drug discovery efforts towards novel visceral analgesics. Abstract Voltage‐gated sodium channel NaV1.7 is required for acute and inflammatory pain in mice and humans but its significance for visceral pain is unknown. Here we examine the role of NaV1.7 in visceral pain processing and the development of referred hyperalgesia using a conditional nociceptor‐specific NaV1.7 knockout mouse (NaV1.7Nav1.8) and selective small‐molecule NaV1.7 antagonist PF‐5198007. NaV1.7Nav1.8 mice showed normal nociceptive behaviours in response to intracolonic application of either capsaicin or mustard oil, stimuli known to evoke sustained nociceptor activity and sensitization following tissue damage, respectively. Normal responses following induction of cystitis by cyclophosphamide were also observed in both NaV1.7Nav1.8 and littermate controls. Loss, or blockade, of NaV1.7 did not affect afferent responses to noxious mechanical and chemical stimuli in nerve–gut preparations in mouse, or following antagonism of NaV1.7 in resected human appendix stimulated by noxious distending pressures. However, expression analysis of voltage‐gated sodium channel α subunits revealed NaV1.7 mRNA transcripts in nearly all retrogradely labelled colonic neurons, suggesting redundancy in function. By contrast, using comparative somatic behavioural models we identify that genetic deletion of NaV1.7 (in NaV1.8‐expressing neurons) regulates noxious heat pain threshold and that this can be recapitulated by the selective NaV1.7 antagonist PF‐5198007. Our data demonstrate that NaV1.7 (in NaV1.8‐expressing neurons) contributes to defined pain pathways in a modality‐dependent manner, modulating somatic noxious heat pain, but is not required for visceral pain processing, and advocate that pharmacological block of NaV1.7 alone in the viscera may be insufficient in targeting chronic visceral pain.
    March 01, 2017   doi: 10.1113/JP272837   open full text
  • Pre‐ischaemic mitochondrial substrate constraint by inhibition of malate‐aspartate shuttle preserves mitochondrial function after ischaemia–reperfusion.
    Nichlas Riise Jespersen, Takashi Yokota, Nicolaj Brejnholt Støttrup, Andreas Bergdahl, Kim Bolther Pælestik, Jonas Agerlund Povlsen, Flemming Dela, Hans Erik Bøtker.
    The Journal of Physiology. February 27, 2017
    Key points Pre‐ischaemic administration of aminooxiacetate (AOA), an inhibitor of the malate‐aspartate shuttle (MAS), provides cardioprotection against ischaemia–reperfusion injury. The underlying mechanism remains unknown. We examined whether transient inhibition of the MAS during ischaemia and early reperfusion by AOA treatment could prevent mitochondrial damage at later reperfusion. The AOA treatment preserved mitochondrial respiratory capacity with reduced mitochondrial oxidative stress during late reperfusion to the same extent as ischaemic preconditioning (IPC). However, AOA treatment, but not IPC, reduced the myocardial interstitial concentration of tricarboxylic acid cycle intermediates at the onset of reperfusion. The results obtained in the present study demonstrate that metabolic regulation by inhibition of the MAS at the onset of reperfusion may be beneficial for the preservation of mitochondrial function during late reperfusion in an IR‐injured heart. Abstract Mitochondrial dysfunction plays a central role in ischaemia–reperfusion (IR) injury. Pre‐ischaemic administration of aminooxyacetate (AOA), an inhibitor of the malate‐aspartate shuttle (MAS), provides cardioprotection against IR injury, although the underlying mechanism remains unknown. We hypothesized that a transient inhibition of the MAS during ischaemia and early reperfusion could preserve mitochondrial function at later phase of reperfusion in the IR‐injured heart to the same extent as ischaemic preconditioning (IPC), which is a well‐validated cardioprotective strategy against IR injury. In the present study, we show that pre‐ischaemic administration of AOA preserved mitochondrial complex I‐linked state 3 respiration and fatty acid oxidation during late reperfusion in IR‐injured isolated rat hearts. AOA treatment also attenuated the excessive emission of mitochondrial reactive oxygen species during state 3 with complex I‐linked substrates during late reperfusion, which was consistent with reduced oxidative damage in the IR‐injured heart. As a result, AOA treatment reduced infarct size after reperfusion. These protective effects of MAS inhibition on the mitochondria were similar to those of IPC. Intriguingly, the protection of mitochondrial function by AOA treatment appears to be different from that of IPC because AOA treatment, but not IPC, downregulated myocardial tricarboxilic acid (TCA)‐cycle intermediates at the onset of reperfusion. MAS inhibition thus preserved mitochondrial respiratory capacity and decreased mitochondrial oxidative stress during late reperfusion in the IR‐injured heart, at least in part, via metabolic regulation of TCA cycle intermediates in the mitochondria at the onset of reperfusion.
    February 27, 2017   doi: 10.1113/JP273408   open full text
  • Cardiac sympathetic afferent reflex control of cardiac function in normal and chronic heart failure states.
    Han‐Jun Wang, George J. Rozanski, Irving H. Zucker.
    The Journal of Physiology. February 27, 2017
    Key points Cardiac sympathetic afferents are considered to be essential pathways for transmission of cardiac nociception to the central nervous system during myocardial ischaemia. However, a potential contribution of the CSAR control of cardiac dysfunction in both normal and chronic heart failure (CHF) states remains unknown. We found that activation of the CSAR evokes little increase in cardiac contractility with an exaggerated peripheral vasoconstriction in the CHF state. CSAR inhibition by epicardial lidocaine decreased cardiac contractility to a greater extent in CHF rats than sham rats. Furthermore, we also found that epicardial lidocaine paradoxically decreased left ventricular end‐diastolic pressure (LVEDP) and left ventricular end‐diastolic volume (preload) in CHF rats, which was not observed in sham rats. Chronic ablation of the CSAR by epicardial application of the afferent neurotoxin, RTX, selectively lowered diastolic blood pressure CHF rats. The observation suggests that CSAR has a differential effect on cardiac function in normal and CHF states. CSAR activation in normal state causes significant increase in cardiac contractility and cardiac output. Abstract The enhanced ‘cardiac sympathetic afferent reflex’ (CSAR) critically contributes to the exaggerated global sympathetic tone in chronic heart failure (CHF). However, a potential contribution of the cardio‐cardiac reflex control of cardiac function in both normal and CHF states remains unknown. In this study, we evaluated the effects of direct activation or inhibition of the CSAR on cardiac function by pressure–volume (P–V) loop analysis in ∼12‐week sham‐operated and myocardial infarcted (MI) rats. In sham rats, acute CSAR activation by epicardial application of bradykinin (BK) increased heart rate (HR), left ventricular systolic pressure (LVSP), the maximum first derivative of left ventricular pressure (dp/dtmax), and the slope of the end‐systolic P–V relationship (ESPVR), suggesting that acute CSAR activation in the normal state enhances myocardial contractility. CSAR activation also decreased left ventricular (LV) systolic and diastolic volumes with little effect on LV end‐diastolic pressure (LVEDP) or the end‐diastolic P–V relationship (EDPVR) in sham rats. Compared to sham, CHF rats exhibit a reduced increase in the slope of the ESPVR and dp/dtmax in response to BK, indicating a poor contractile response to CSAR activation. Interestingly, BK application in CHF rats increased cardiac systolic and diastolic volumes and further increased the elevated LVEDP, neither of which was seen in sham rats. Following CSAR inhibition by epicardial lidocaine, blood pressure, HR, LVSP, dp/dt, LVEDP and ESPVR decreased in CHF rats whereas lidocaine had little effect in sham rats, indicating that the CSAR is tonically active in CHF and contributes to cardiac dysfunction. Furthermore, we found that epicardial lidocaine paradoxically decreased LV end‐diastolic volume (preload) in CHF rats, which was not observed in sham rats. The decreased preload by lidocaine in CHF rats may be due to a reduction in peripheral vascular resistance since epicardial lidocaine significantly lowered peripheral (renal) sympathetic nerve activity in CHF rats but not in sham rats. Furthermore, chronic ablation of CSAR by epicardial application of a selective afferent neurotoxin, resiniferatoxin, selectively lowered diastolic blood pressure both at daytime and night‐time with less effect on systolic blood pressure in CHF rats. Our data suggest that there is an imbalance between cardiac and peripheral responses to CSAR in CHF animals compared to sham‐operated controls.
    February 27, 2017   doi: 10.1113/JP273764   open full text
  • Inhibition of oxytocin and vasopressin neuron activity in rat hypothalamic paraventricular nucleus by relaxin‐3–RXFP3 signalling.
    Alan Kania, Anna Gugula, Agnieszka Grabowiecka, Camila Ávila, Tomasz Blasiak, Zenon Rajfur, Marian H. Lewandowski, Grzegorz Hess, Elena Timofeeva, Andrew L. Gundlach, Anna Blasiak.
    The Journal of Physiology. February 27, 2017
    Key points Relaxin‐3 is a stress‐responsive neuropeptide that acts at its cognate receptor, RXFP3, to alter behaviours including feeding. In this study, we have demonstrated a direct, RXFP3‐dependent, inhibitory action of relaxin‐3 on oxytocin and vasopressin paraventricular nucleus (PVN) neuron electrical activity, a putative cellular mechanism of orexigenic actions of relaxin‐3. We observed a Gαi/o‐protein‐dependent inhibitory influence of selective RXFP3 activation on PVN neuronal activity in vitro and demonstrated a direct action of RXFP3 activation on oxytocin and vasopressin PVN neurons, confirmed by their abundant expression of RXFP3 mRNA. Moreover, we demonstrated that RXFP3 activation induces a cadmium‐sensitive outward current, which indicates the involvement of a characteristic magnocellular neuron outward potassium current. Furthermore, we identified an abundance of relaxin‐3‐immunoreactive axons/fibres originating from the nucleus incertus in close proximity to the PVN, but associated with sparse relaxin‐3‐containing fibres/terminals within the PVN. Abstract The paraventricular nucleus of the hypothalamus (PVN) plays an essential role in the control of food intake and energy expenditure by integrating multiple neural and humoral inputs. Recent studies have demonstrated that intracerebroventricular and intra‐PVN injections of the neuropeptide relaxin‐3 or selective relaxin‐3 receptor (RXFP3) agonists produce robust feeding in satiated rats, but the cellular and molecular mechanisms of action associated with these orexigenic effects have not been identified. In the present studies, using rat brain slices, we demonstrated that relaxin‐3, acting through its cognate G‐protein‐coupled receptor, RXFP3, hyperpolarized a majority of putative magnocellular PVN neurons (88%, 22/25), including cells producing the anorexigenic neuropeptides, oxytocin and vasopressin. Importantly, the action of relaxin‐3 persisted in the presence of tetrodotoxin and glutamate/GABA receptor antagonists, indicating its direct action on PVN neurons. Similar inhibitory effects on PVN oxytocin and vasopressin neurons were produced by the RXFP3 agonist, RXFP3‐A2 (82%, 80/98 cells). In situ hybridization histochemistry revealed a strong colocalization of RXFP3 mRNA with oxytocin and vasopressin immunoreactivity in rat PVN neurons. A smaller percentage of putative parvocellular PVN neurons was sensitive to RXFP3‐A2 (40%, 16/40 cells). These data, along with a demonstration of abundant peri‐PVN and sparse intra‐PVN relaxin‐3‐immunoreactive nerve fibres, originating from the nucleus incertus, the major source of relaxin‐3 neurons, identify a strong inhibitory influence of relaxin‐3–RXFP3 signalling on the electrical activity of PVN oxytocin and vasopressin neurons, consistent with the orexigenic effect of RXFP3 activation observed in vivo.
    February 27, 2017   doi: 10.1113/JP273787   open full text
  • Cortical and reticular contributions to human precision and power grip.
    Toshiki Tazoe, Monica A. Perez.
    The Journal of Physiology. February 27, 2017
    Key points The corticospinal tract contributes to the control of finger muscles during precision and power grip. We explored the neural mechanisms contributing to changes in corticospinal excitability during these gripping configurations. Motor evoked potentials (MEPs) elicited by cortical, but not by subcortical, stimulation were more suppressed during power grip compared with precision grip and index finger abduction. Intracortical inhibition was more reduced during power grip compared with the other tasks. An acoustic startle cue, a stimulus that engages the reticular system, suppressed MEP size during power grip to a lesser extent than during the other tasks at a cortical level and this positively correlated with changes in intracortical inhibition. Our findings suggest that changes in corticospinal excitability during gross more than fine finger manipulations are largely cortical in origin and that the reticular system contributed, at least in part, to these effects. Abstract It is well accepted that the corticospinal tract contributes to the control of finger muscles during precision and power grip in humans but the neural mechanisms involved remain poorly understood. Here, we examined motor evoked potentials elicited by cortical and subcortical stimulation of corticospinal axons (MEPs and CMEPs, respectively) and the activity in intracortical circuits (suppression of voluntary electromyography) and spinal motoneurons (F‐waves) in an intrinsic hand muscle during index finger abduction, precision grip and power grip. We found that the size of MEPs, but not CMEPs, was more suppressed during power grip compared with precision grip and index finger abduction, suggesting a cortical origin for these effects. Notably, intracortical inhibition was more reduced during power grip compared with the other tasks. To further examine the origin of changes in intracortical inhibition we assessed the contribution of the reticular system, which projects to cortical neurons, and projects to spinal motoneurons controlling hand muscles. An acoustic startle cue, which engages the reticular system, suppressed MEP size during power grip to a lesser extent than during the other tasks and this positively correlated with changes in intracortical inhibition. A startle cue decreased intracortical inhibition, but not CMEPs, during power grip. F‐waves remained unchanged across conditions. Our novel findings show that changes in corticospinal excitability present during power grip compared with fine finger manipulations are largely cortical in origin and suggest that the reticular system contributed, at least in part, to these effects.
    February 27, 2017   doi: 10.1113/JP273679   open full text
  • Enteric glial activity regulates secretomotor function in the mouse colon but does not acutely affect gut permeability.
    Vladimir Grubišić, Brian D. Gulbransen.
    The Journal of Physiology. February 22, 2017
    Key points The role of enteric glial cell activity in the acute regulation of epithelial barrier and secretomotor functions of the intestines under physiological conditions is not clear. We used transgenic mice to modify glial activity and found that enteric glia significantly contribute to the neurogenic ion transport while glial activity does not appear to play a major role in the acute regulation of barrier function. The selective activation of glial activity evoked electrogenic ion transport primarily through neural pathways and was sufficient to drive electrogenic ion transport to an extent equal to the direct activation of neurogenic ion transport. These findings provide novel insight into the cellular mechanisms that control fluid transport homeostasis in the intestine and might provide novel therapeutic avenues for functional diarrheal diseases. Abstract Enteric glial cells are often implicated in the regulation of epithelial barrier and secretomotor functions of the intestines. But whether glial cell activity regulates these functions acutely under physiological conditions is not clear. We addressed this issue by using transgenic animal models to modify the activity of enteric glia, either reducing glial expression of connexin 43 in Sox10::CreERT2+/−/Cx43f/f mice or activating glial calcium responses in GFAP::hM3Dq mice, and tested the effects on colonic barrier function and electrogenic ion transport in Ussing chambers. We assessed neuronal‐dependent and ‐independent contributions by activating or inhibiting neurogenic activity with veratridine and tetrodotoxin, respectively. Our results show that the reduction of glial Cx43 expression in Sox10::CreERT2+/−/Cx43f/f mice significantly reduced neurogenic ion transport. The selective glial activation in tissues from GFAP::hM3Dq mice evoked electrogenic ion transport to an extent equal to the direct activation of neurogenic ion transport with veratridine and glial driven responses consisted of both tetrodotoxin‐sensitive and ‐insensitive components. The selective glial stimulation did not affect transmural ion conductance or cell‐impermeant dye flux but the baseline ion conductance was more variable in Sox10::CreERT2+/−/Cx43f/f tissues. Together, our findings show that glial activity contributes to the regulation of electrogenic ion transport in the intestine through effects on neurons and possibly direct effects on epithelial cells. However, glial activity does not appear to play a major role in the acute regulation of barrier function. These findings provide novel insight into the cellular mechanisms that control fluid transport homeostasis in the intestine.
    February 22, 2017   doi: 10.1113/JP273492   open full text
  • Presynaptic inhibition of transient receptor potential vanilloid type 1 (TRPV1) receptors by noradrenaline in nociceptive neurons.
    Saikat Chakraborty, Vincent Elvezio, Martin Kaczocha, Mario Rebecchi, Michelino Puopolo.
    The Journal of Physiology. February 22, 2017
    Key points The transient receptor potential vanilloid type 1 (TRPV1) receptor is a polymodal molecular integrator in the pain pathway expressed in Aδ‐ and C‐fibre nociceptors and is responsible for the thermal hyperalgesia associated with inflammatory pain. Noradrenaline strongly inhibited the activity of TRPV1 channels in dorsal root ganglia neurons. The effect of noradrenaline was reproduced by clonidine and antagonized by yohimbine, consistent with contribution of α2 adrenergic receptors. The inhibitory effect of noradrenaline on TRPV1 channels was dependent on calcium influx and linked to calcium/calmodulin‐dependent protein kinase II. In spinal cord slices, clonidine reduced the frequency of capsaicin‐induced miniature EPSCs in the presence of tetrodotoxin and ω‐conotoxin‐MVIIC, consistent with inhibition of presynaptic TRPV1 channels by α2 adrenergic receptors. We suggest that modulation of presynaptic TRPV1 channels in nociceptive neurons by descending noradrenergic inputs may constitute a mechanism for noradrenaline to modulate incoming noxious stimuli in the dorsal horn of the spinal cord. Abstract The transient receptor potential vanilloid type 1 (TRPV1) receptor is a well‐known contributor to nociceptor excitability. To address whether noradrenaline can down‐regulate TRPV1 channel activity in nociceptors and reduce their synaptic transmission, the effects of noradrenaline and clonidine were tested on the capsaicin‐activated current recorded from acutely dissociated small diameter (<27 μm) dorsal root ganglia (DRG) neurons and on miniature (m)EPSCs recorded from large lamina I neurons in horizontal spinal cord slices. Noradrenaline or clonidine inhibited the capsaicin‐activated current by ∼60%, and the effect was reversed by yohimbine, confirming that it was mediated by activation of α2 adrenergic receptors. Similarly, clonidine reduced the frequency of capsaicin‐induced mEPSCs by ∼60%. Inhibition of capsaicin‐activated current by noradrenaline was mediated by GTP binding proteins, and was highly dependent on calcium influx. The inhibitory effect of noradrenaline on the capsaicin‐activated current was not affected either by blocking the activity of protein kinase A with H89, or by blocking the activity of protein kinase C with bisindolylmaleimide II. In contrast, when the calcium/calmodulin‐dependent protein kinase II (CaMKII) was blocked with KN‐93, the inhibitory effect of noradrenaline on the capsaicin‐activated current was greatly reduced, suggesting that activation of adrenergic receptors in DRG neurons is preferentially linked to CaMKII activity. We suggest that modulation of TRPV1 channels by noradrenaline in nociceptive neurons is a mechanism whereby noradrenaline may suppress incoming noxious stimuli at the primary synaptic afferents in the dorsal horn of the spinal cord.
    February 22, 2017   doi: 10.1113/JP273455   open full text
  • Rapid limb‐specific modulation of vestibular contributions to ankle muscle activity during locomotion.
    Patrick A. Forbes, Mark Vlutters, Christopher J. Dakin, Herman der Kooij, Jean‐Sébastien Blouin, Alfred C. Schouten.
    The Journal of Physiology. February 22, 2017
    Key points The vestibular influence on human walking is phase‐dependent and modulated across both limbs with changes in locomotor velocity and cadence. Using a split‐belt treadmill, we show that vestibular influence on locomotor activity is modulated independently in each limb. The independent vestibular modulation of muscle activity from each limb occurs rapidly at the onset of split‐belt walking, over a shorter time course relative to the characteristic split‐belt error‐correction mechanisms (i.e. muscle activity and kinematics) associated with locomotor adaptation. Together, the present results indicate that the nervous system rapidly modulates the vestibular influence of each limb separately through processes involving ongoing sensory feedback loops. These findings help us understand how vestibular information is used to accommodate the variable and commonplace demands of locomotion, such as turning or navigating irregular terrain. Abstract During walking, the vestibular influence on locomotor activity is phase‐dependent and modulated in both limbs with changes in velocity. It is unclear, however, whether this bilateral modulation is due to a coordinated mechanism between both limbs or instead through limb‐specific processes that remain masked by the symmetric nature of locomotion. Here, human subjects walked on a split‐belt treadmill with one belt moving at 0.4 m s−1 and the other moving at 0.8 m s−1 while exposed to an electrical vestibular stimulus. Muscle activity was recorded bilaterally around the ankles of each limb and used to compare vestibulo‐muscular coupling between velocity‐matched and unmatched tied‐belt walking. In general, response magnitudes decreased by ∼20–50% and occurred ∼13–20% earlier in the stride cycle at the higher belt velocity. This velocity‐dependent modulation of vestibular‐evoked muscle activity was retained during split‐belt walking and was similar, within each limb, to velocity‐matched tied‐belt walking. These results demonstrate that the vestibular influence on ankle muscles during locomotion can be adapted independently to each limb. Furthermore, modulation of vestibular‐evoked muscle responses occurred rapidly (∼13–34 strides) after onset of split‐belt walking. This rapid adaptation contrasted with the prolonged adaptation in step length symmetry (∼128 strides) as well as EMG magnitude and timing (∼40–100 and ∼20–70 strides, respectively). These results suggest that vestibular influence on ankle muscle control is adjusted rapidly in sensorimotor control loops as opposed to longer‐term error correction mechanisms commonly associated with split‐belt adaptation. Rapid limb‐specific sensorimotor feedback adaptation may be advantageous for asymmetric overground locomotion, such as navigating irregular terrain or turning.
    February 22, 2017   doi: 10.1113/JP272614   open full text
  • The statistics of the vestibular input experienced during natural self‐motion differ between rodents and primates.
    Jérome Carriot, Mohsen Jamali, Maurice J. Chacron, Kathleen E. Cullen.
    The Journal of Physiology. February 22, 2017
    Key points In order to understand how the brain's coding strategies are adapted to the statistics of the sensory stimuli experienced during everyday life, the use of animal models is essential. Mice and non‐human primates have become common models for furthering our knowledge of the neuronal coding of natural stimuli, but differences in their natural environments and behavioural repertoire may impact optimal coding strategies. Here we investigated the structure and statistics of the vestibular input experienced by mice versus non‐human primates during natural behaviours, and found important differences. Our data establish that the structure and statistics of natural signals in non‐human primates more closely resemble those observed previously in humans, suggesting similar coding strategies for incoming vestibular input. These results help us understand how the effects of active sensing and biomechanics will differentially shape the statistics of vestibular stimuli across species, and have important implications for sensory coding in other systems. Abstract It is widely believed that sensory systems are adapted to the statistical structure of natural stimuli, thereby optimizing coding. Recent evidence suggests that this is also the case for the vestibular system, which senses self‐motion and in turn contributes to essential brain functions ranging from the most automatic reflexes to spatial perception and motor coordination. However, little is known about the statistics of self‐motion stimuli actually experienced by freely moving animals in their natural environments. Accordingly, here we examined the natural self‐motion signals experienced by mice and monkeys: two species commonly used to study vestibular neural coding. First, we found that probability distributions for all six dimensions of motion (three rotations, three translations) in both species deviated from normality due to long tails. Interestingly, the power spectra of natural rotational stimuli displayed similar structure for both species and were not well fitted by power laws. This result contrasts with reports that the natural spectra of other sensory modalities (i.e. vision, auditory and tactile) instead show a power‐law relationship with frequency, which indicates scale invariance. Analysis of natural translational stimuli revealed important species differences as power spectra deviated from scale invariance for monkeys but not for mice. By comparing our results to previously published data for humans, we found the statistical structure of natural self‐motion stimuli in monkeys and humans more closely resemble one another. Our results thus predict that, overall, neural coding strategies used by vestibular pathways to encode natural self‐motion stimuli are fundamentally different in rodents and primates.
    February 22, 2017   doi: 10.1113/JP273734   open full text
  • Rapid frequency‐dependent changes in free mitochondrial calcium concentration in rat cardiac myocytes.
    Rob C. I. Wüst, Michiel Helmes, Jody L. Martin, Thomas J. T. der Wardt, René J. P. Musters, Jolanda der Velden, Ger J. M. Stienen.
    The Journal of Physiology. February 22, 2017
    Key points Calcium ions regulate mitochondrial ATP production and contractile activity and thus play a pivotal role in matching energy supply and demand in cardiac muscle. The magnitude and kinetics of the changes in free mitochondrial calcium concentration in cardiac myocytes are largely unknown. Rapid stimulation frequency‐dependent increases but relatively slow decreases in free mitochondrial calcium concentration were observed in rat cardiac myocytes. This asymmetry caused a rise in the mitochondrial calcium concentration with stimulation frequency. These results provide insight into the mechanisms of mitochondrial calcium uptake and release that are important in healthy and diseased myocardium. Abstract Calcium ions regulate mitochondrial ATP production and contractile activity and thus play a pivotal role in matching energy supply and demand in cardiac muscle. Little is known about the magnitude and kinetics of the changes in free mitochondrial calcium concentration in cardiomyocytes. Using adenoviral infection, a ratiometric mitochondrially targeted Förster resonance energy transfer (FRET)‐based calcium indicator (4mtD3cpv, MitoCam) was expressed in cultured adult rat cardiomyocytes and the free mitochondrial calcium concentration ([Ca2+]m) was measured at different stimulation frequencies (0.1–4 Hz) and external calcium concentrations (1.8–3.6 mm) at 37°C. Cytosolic calcium concentrations were assessed under the same experimental conditions in separate experiments using Fura‐4AM. The increases in [Ca2+]m during electrical stimulation at 0.1 Hz were rapid (rise time = 49 ± 2 ms), while the decreases in [Ca2+]m occurred more slowly (decay half time = 1.17 ± 0.07 s). Model calculations confirmed that this asymmetry caused the rise in [Ca2+]m during diastole observed at elevated stimulation frequencies. Inhibition of the mitochondrial sodium–calcium exchanger (mNCE) resulted in a rise in [Ca2+]m at baseline and, paradoxically, in an acceleration of Ca2+ release. In conclusion: rapid increases in [Ca2+]m allow for fast adjustment of mitochondrial ATP production to increases in myocardial demand on a beat‐to‐beat basis and mitochondrial calcium release depends on mNCE activity and mitochondrial calcium buffering.
    February 22, 2017   doi: 10.1113/JP273589   open full text
  • EAG channels expressed in microvillar photoreceptors are unsuited to diurnal vision.
    Esa‐Ville Immonen, Andrew S. French, Päivi H. Torkkeli, Hongxia Liu, Mikko Vähäsöyrinki, Roman V. Frolov.
    The Journal of Physiology. February 22, 2017
    Key points The principles underlying the evolutionary selection of ion channels for expression in sensory neurons are unclear. Photoreceptor depolarization in the diurnal Drosophila melanogaster is predominantly provided by light‐activated transient receptor potential (TRP) channels, whereas repolarization is mediated by sustained voltage‐gated K+ channels of the Shab family. In the present study, we show that phototransduction in the nocturnal cockroach Periplaneta americana is predominantly mediated by TRP‐like channels, whereas membrane repolarization is based on EAG channels. Although bright light stimulates Shab channels in Drosophila, further restricting depolarization and improving membrane bandwidth, it strongly suppresses EAG conductance in Periplaneta. This light‐dependent inhibition (LDI) is caused by calcium and is abolished by chelating intracellular calcium or suppressing eag gene expression. LDI increases membrane resistance, augments gain and reduces the signalling bandwidth. This makes EAG unsuitable for light response conditioning during the day and might have resulted in the evolutionary replacement of EAG by other delayed rectifiers in diurnal insects. Abstract The principles underlying evolutionary selection of ion channels for expression in sensory neurons are unclear. Among species possessing microvillar photoreceptors, the major ionic conductances have only been identified in Drosophila melanogaster. In Drosophila, depolarization is provided by light‐activated transient receptor potential (TRP) channels with a minor contribution from TRP‐like (TRPL) channels, whereas repolarization is mediated by sustained voltage‐gated K+ (Kv) channels of the Shab family. Bright light stimulates Shab channels, further restricting depolarization and improving membrane bandwidth. In the present study, data obtained using a combination of electrophysiological, pharmacological and molecular knockdown techniques strongly suggest that in photoreceptors of the nocturnal cockroach Periplaneta americana the major excitatory channel is TRPL, whereas the predominant delayed rectifier is EAG, a ubiquitous but enigmatic Kv channel. By contrast to the diurnal Drosophila, bright light strongly suppresses EAG conductance in Periplaneta. This light‐dependent inhibition (LDI) is caused by calcium entering the cytosol and is amplified following inhibition of calcium extrusion, and it can also be abolished by chelating intracellular calcium or suppressing eag gene expression by RNA interference. LDI increases membrane resistance, augments gain and reduces the signalling bandwidth, impairing information transfer. LDI is also observed in the nocturnal cricket Gryllus integer, whereas, in the diurnal water strider Gerris lacustris, the delayed rectifier is up‐regulated by light. Although LDI is not expected to reduce delayed rectifier current in the normal illumination environment of nocturnal cockroaches and crickets, it makes EAG unsuitable for light response conditioning during the day, and might have resulted in the evolutionary replacement of EAG by other delayed rectifiers in diurnal insects.
    February 22, 2017   doi: 10.1113/JP273612   open full text
  • Late gestational intermittent hypoxia induces metabolic and epigenetic changes in male adult offspring mice.
    Abdelnaby Khalyfa, Rene Cortese, Zhuanhong Qiao, Honggang Ye, Riyue Bao, Jorge Andrade, David Gozal.
    The Journal of Physiology. February 22, 2017
    Key points Late gestation during pregnancy has been associated with a relatively high prevalence of obstructive sleep apnoea (OSA). Intermittent hypoxia, a hallmark of OSA, could impose significant long‐term effects on somatic growth, energy homeostasis and metabolic function in offspring. Here we show that late gestation intermittent hypoxia induces metabolic dysfunction as reflected by increased body weight and adiposity index in adult male offspring that is paralleled by epigenomic alterations and inflammation in visceral white adipose tissue. Fetal perturbations by OSA during pregnancy impose long‐term detrimental effects manifesting as metabolic dysfunction in adult male offspring. Abstract Pregnancy, particularly late gestation (LG), has been associated with a relatively high prevalence of obstructive sleep apnoea (OSA). Intermittent hypoxia (IH), a hallmark of OSA, could impose significant long‐term effects on somatic growth, energy homeostasis, and metabolic function in offspring. We hypothesized that IH during late pregnancy (LG‐IH) may increase the propensity for metabolic dysregulation and obesity in adult offspring via epigenetic modifications. Time‐pregnant female C57BL/6 mice were exposed to LG‐IH or room air (LG‐RA) during days 13–18 of gestation. At 24 weeks, blood samples were collected from offspring mice for lipid profiles and insulin resistance, indirect calorimetry was performed and visceral white adipose tissues (VWAT) were assessed for inflammatory cells as well as for differentially methylated gene regions (DMRs) using a methylated DNA immunoprecipitation on chip (MeDIP‐chip). Body weight, food intake, adiposity index, fasting insulin, triglycerides and cholesterol levels were all significantly higher in LG‐IH male but not female offspring. LG‐IH also altered metabolic expenditure and locomotor activities in male offspring, and increased number of pro‐inflammatory macrophages emerged in VWAT along with 1520 DMRs (P < 0.0001), associated with 693 genes. Pathway analyses showed that genes affected by LG‐IH were mainly associated with molecular processes related to metabolic regulation and inflammation. LG‐IH induces metabolic dysfunction as reflected by increased body weight and adiposity index in adult male offspring that is paralleled by epigenomic alterations and inflammation in VWAT. Thus, perturbations to fetal environment by OSA during pregnancy can have long‐term detrimental effects on the fetus, and lead to persistent metabolic dysfunction in adulthood.
    February 22, 2017   doi: 10.1113/JP273570   open full text
  • Residual force enhancement is regulated by titin in skeletal and cardiac myofibrils.
    Nabil Shalabi, Anabelle Cornachione, Felipe de Souza Leite, Srikar Vengallatore, Dilson E. Rassier.
    The Journal of Physiology. February 19, 2017
    Key points When a skeletal muscle is stretched while it contracts, the muscle produces a relatively higher force than the force from an isometric contraction at the same length: a phenomenon referred to as residual force enhancement. Residual force enhancement is puzzling because it cannot be directly explained by the classical force–length relationship and the sliding filament theory of contraction, the main paradigms in the muscle field. We used custom‐built instruments to measure residual force enhancement in skeletal myofibrils, and, for the first time, in cardiac myofibrils. Our data report that residual force enhancement is present in skeletal muscles, but not cardiac muscles, and is regulated by the different isoforms of the titin protein filaments. Abstract When a skeletal muscle contracts isometrically, the muscle produces a force that is relative to the final isometric sarcomere length (SL). However, when the same final SL is reached by stretching the muscle while it contracts, the muscle produces a relatively higher force: a phenomenon commonly referred to as residual force enhancement. In this study, we investigated residual force enhancement in rabbit skeletal psoas myofibrils and, for the first time, cardiac papillary myofibrils. A custom‐built atomic force microscope was used in experiments that stretched myofibrils before and after inhibiting myosin and actin interactions to determine whether the different cardiac and skeletal titin isoforms regulate residual force enhancement. At SLs ranging from 2.24 to 3.13 μm, the skeletal myofibrils enhanced the force by an average of 9.0%, and by 29.5% after hindering myosin and actin interactions. At SLs ranging from 1.80 to 2.29 μm, the cardiac myofibrils did not enhance the force before or after hindering myosin and actin interactions. We conclude that residual force enhancement is present only in skeletal muscles and is dependent on the titin isoforms.
    February 19, 2017   doi: 10.1113/JP272983   open full text
  • A critical period of corticomuscular and EMG–EMG coherence detection in healthy infants aged 9–25 weeks.
    Anina Ritterband‐Rosenbaum, Anna Herskind, Xi Li, Maria Willerslev‐Olsen, Mikkel Damgaard Olsen, Simon Francis Farmer, Jens Bo Nielsen.
    The Journal of Physiology. February 15, 2017
    Key points The early postnatal development of functional corticospinal connections in human infants is not fully clarified. Corticospinal drive to upper and lower limb muscle shows developmental changes with an increased functional coupling in infants between 9 and 25 weeks in the beta frequency band. The changes in functional coupling coincide with the developmental period where fidgety movements are present in healthy infants. Data support a possible sensitive period where functional connections between corticospinal tract fibres and spinal motoneurones undergo activity‐dependent reorganization. Abstract The early postnatal development of functional corticospinal connections in human infants is not fully clarified. We used EEG and EMG to investigate the development of corticomuscular and intramuscular coherence as indicators of functional corticospinal connectivity in healthy infants aged 1–66 weeks. EEG was recorded over leg and hand area of motor cortex. EMG recordings were made from right ankle dorsiflexor and right wrist extensor muscles. Quantification of the amount of corticomuscular coherence in the 20–40 Hz frequency band showed a significantly larger coherence for infants aged 9–25 weeks compared to younger and older infants. Coherence between paired EMG recordings from tibialis anterior muscle in the 20–40 Hz frequency band was also significantly larger for the 9–25 week age group. A low‐amplitude, broad‐duration (40–50 ms) central peak of EMG–EMG synchronization was observed for infants younger than 9 weeks, whereas a short‐lasting (10–20 ms) central peak was observed for EMG–EMG synchronization in older infants. This peak was largest for infants aged 9–25 weeks. These data suggest that the corticospinal drive to lower and upper limb muscles shows significant developmental changes with an increase in functional coupling in infants aged 9–25 weeks, a period which coincides partly with the developmental period of normal fidgety movements. We propose that these neurophysiological findings may reflect the existence of a sensitive period where the functional connections between corticospinal tract fibres and spinal motoneurones undergo activity‐dependent reorganization. This may be relevant for the timing of early therapy interventions in infants with pre‐ and perinatal brain injury.
    February 15, 2017   doi: 10.1113/JP273090   open full text
  • Bicarbonate sensing in mouse cortical astrocytes during extracellular acid/base disturbances.
    Shefeeq M. Theparambil, Zinnia Naoshin, Sabrina Defren, Jana Schmaelzle, Tobias Weber, Hans‐Peter Schneider, Joachim W. Deitmer.
    The Journal of Physiology. February 15, 2017
    Key points The present study suggests that the electrogenic sodium–bicarbonate cotransporter, NBCe1, supported by carbonic anhydrase II, CAII, provides an efficient mechanism of bicarbonate sensing in cortical astrocytes. This mechanism is proposed to play a major role in setting the pHi responses to extracellular acid/base challenges in astrocytes. A decrease in extracellular [HCO3−] during isocapnic acidosis and isohydric hypocapnia, or an increase in intracellular [HCO3−] during hypercapnic acidosis, was effectively sensed by NBCe1, which carried bicarbonate out of the cells under these conditions, and caused an acidification and sodium fall in WT astrocytes, but not in NBCe1‐knockout astrocytes. Isocapnic acidosis, hypercapnic acidosis and isohydric hypocapnia evoked inward currents in NBCe1‐ and CAII‐expressing Xenopus laevis oocytes, but not in native oocytes, suggesting that NBCe1 operates in the outwardly directed mode under these conditions consistent with our findings in astrocytes. We propose that bicarbonate sensing of astrocytes may have functional significance during extracellular acid/base disturbances in the brain, as it not only alters intracellular pH/[HCO3−]‐dependent functions of astrocytes, but also modulates the extracellular pH/[HCO3−] in brain tissue. Abstract Extracellular acid/base status of the mammalian brain undergoes dynamic changes during many physiological and pathological events. Although intracellular pH (pHi) of astrocytes responds to extracellular acid/base changes, the mechanisms mediating these changes have remained unresolved. We have previously shown that the electrogenic sodium–bicarbonate cotransporter, NBCe1, is a high‐affinity bicarbonate carrier in cortical astrocytes. In the present study, we investigated whether NBCe1 plays a role in bicarbonate sensing in astrocytes, and in determining the pHi responses to extracellular acid/base challenges. We measured changes in intracellular H+ and Na+ in astrocytes from wild‐type (WT) and from NBCe1‐knockout (KO) mice, using ion‐selective dyes, during isocapnic acidosis, hypercapnic acidosis and hypocapnia. We also analysed NBCe1‐mediated membrane currents in Xenopus laevis oocytes under similar conditions. Comparing WT and NBCe1‐KO astrocytes, we could dissect the contribution of NBCe1, of diffusion of CO2 across the cell membrane and, after blocking carbonic anhydrase (CA) activity with ethoxyzolamide, of the role of CA, for the amplitude and rate of acid/base fluxes. Our results suggest that NBCe1 transport activity in astrocytes, supported by CA activity, renders astrocytes bicarbonate sensors in the mouse cortex. NBCe1 carried bicarbonate into and out of the cell by sensing the variations of transmembrane [HCO3−], irrespective of the changes in intra‐ and extracellular pH, and played a major role in setting pHi responses to the extracellular acid/base challenges. We propose that bicarbonate sensing of astrocytes may have potential functional significance during extracellular acid/base alterations in the brain.
    February 15, 2017   doi: 10.1113/JP273394   open full text
  • Effect of gravity and microgravity on intracranial pressure.
    Justin S. Lawley, Lonnie G. Petersen, Erin J. Howden, Satyam Sarma, William K. Cornwell, Rong Zhang, Louis A. Whitworth, Michael A. Williams, Benjamin D. Levine.
    The Journal of Physiology. February 14, 2017
    Key Points Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure on Earth. Gravity has a profound effect on fluid distribution and pressure within the human circulation. In contrast to prevailing theory, we observed that microgravity reduces central venous and intracranial pressure. This being said, intracranial pressure is not reduced to the levels observed in the 90 deg seated upright posture on Earth. Thus, over 24 h in zero gravity, pressure in the brain is slightly above that observed on Earth, which may explain remodelling of the eye in astronauts. Abstract Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure (ICP). This syndrome is considered the most mission‐critical medical problem identified in the past decade of manned spaceflight. We recruited five men and three women who had an Ommaya reservoir inserted for the delivery of prophylactic CNS chemotherapy, but were free of their malignant disease for at least 1 year. ICP was assessed by placing a fluid‐filled 25 gauge butterfly needle into the Ommaya reservoir. Subjects were studied in the upright and supine position, during acute zero gravity (parabolic flight) and prolonged simulated microgravity (6 deg head‐down tilt bedrest). ICP was lower when seated in the 90 deg upright posture compared to lying supine (seated, 4 ± 1 vs. supine, 15 ± 2 mmHg). Whilst lying in the supine posture, central venous pressure (supine, 7 ± 3 vs. microgravity, 4 ± 2 mmHg) and ICP (supine, 17 ± 2 vs. microgravity, 13 ± 2 mmHg) were reduced in acute zero gravity, although not to the levels observed in the 90 deg seated upright posture on Earth. Prolonged periods of simulated microgravity did not cause progressive elevations in ICP (supine, 15 ± 2 vs. 24 h head‐down tilt, 15 ± 4 mmHg). Complete removal of gravity does not pathologically elevate ICP but does prevent the normal lowering of ICP when upright. These findings suggest the human brain is protected by the daily circadian cycles in regional ICPs, without which pathology may occur.
    February 14, 2017   doi: 10.1113/JP273557   open full text
  • Adenosine receptors regulate gap junction coupling of the human cerebral microvascular endothelial cells hCMEC/D3 by Ca2+ influx through cyclic nucleotide‐gated channels.
    Almke Bader, Willem Bintig, Daniela Begandt, Anne Klett, Ina G. Siller, Carola Gregor, Frank Schaarschmidt, Babette Weksler, Ignacio Romero, Pierre‐Olivier Couraud, Stefan W. Hell, Anaclet Ngezahayo.
    The Journal of Physiology. February 14, 2017
    Key points Gap junction channels are essential for the formation and regulation of physiological units in tissues by allowing the lateral cell‐to‐cell diffusion of ions, metabolites and second messengers. Stimulation of the adenosine receptor subtype A2B increases the gap junction coupling in the human blood–brain barrier endothelial cell line hCMEC/D3. Although the increased gap junction coupling is cAMP‐dependent, neither the protein kinase A nor the exchange protein directly activated by cAMP were involved in this increase. We found that cAMP activates cyclic nucleotide‐gated (CNG) channels and thereby induces a Ca2+ influx, which leads to the increase in gap junction coupling. The report identifies CNG channels as a possible physiological link between adenosine receptors and the regulation of gap junction channels in endothelial cells of the blood–brain barrier. Abstract The human cerebral microvascular endothelial cell line hCMEC/D3 was used to characterize the physiological link between adenosine receptors and the gap junction coupling in endothelial cells of the blood–brain barrier. Expressed adenosine receptor subtypes and connexin (Cx) isoforms were identified by RT‐PCR. Scrape loading/dye transfer was used to evaluate the impact of the A2A and A2B adenosine receptor subtype agonist 2‐phenylaminoadenosine (2‐PAA) on the gap junction coupling. We found that 2‐PAA stimulated cAMP synthesis and enhanced gap junction coupling in a concentration‐dependent manner. This enhancement was accompanied by an increase in gap junction plaques formed by Cx43. Inhibition of protein kinase A did not affect the 2‐PAA‐related enhancement of gap junction coupling. In contrast, the cyclic nucleotide‐gated (CNG) channel inhibitor l‐cis‐diltiazem, as well as the chelation of intracellular Ca2+ with BAPTA, or the absence of external Ca2+, suppressed the 2‐PAA‐related enhancement of gap junction coupling. Moreover, we observed a 2‐PAA‐dependent activation of CNG channels by a combination of electrophysiology and pharmacology. In conclusion, the stimulation of adenosine receptors in hCMEC/D3 cells induces a Ca2+ influx by opening CNG channels in a cAMP‐dependent manner. Ca2+ in turn induces the formation of new gap junction plaques and a consecutive sustained enhancement of gap junction coupling. The report identifies CNG channels as a physiological link that integrates gap junction coupling into the adenosine receptor‐dependent signalling of endothelial cells of the blood–brain barrier.
    February 14, 2017   doi: 10.1113/JP273150   open full text
  • ANO1 in intramuscular interstitial cells of Cajal plays a key role in the generation of slow waves and tone in the internal anal sphincter.
    C. A. Cobine, E. E. Hannah, M. H. Zhu, H. E. Lyle, J. R. Rock, K. M. Sanders, S. M. Ward, K. D. Keef.
    The Journal of Physiology. February 14, 2017
    Key points The internal anal sphincter develops tone important for maintaining high anal pressure and continence. Controversy exists regarding the mechanisms underlying tone development. We examined the hypothesis that tone depends upon electrical slow waves (SWs) initiated in intramuscular interstitial cells of Cajal (ICC‐IM) by activation of Ca2+‐activated Cl− channels (ANO1, encoded by Ano1) and voltage‐dependent L‐type Ca2+ channels (CavL, encoded by Cacna1c). Measurement of membrane potential and contraction indicated that ANO1 and CavL have a central role in SW generation, phasic contractions and tone, independent of stretch. ANO1 expression was examined in wildtype and Ano1/+egfp mice with immunohistochemical techniques. Ano1 and Cacna1c expression levels were examined by quantitative PCR in fluorescence‐activated cell sorting. ICC‐IM were the predominant cell type expressing ANO1 and the most likely candidate for SW generation. SWs in ICC‐IM are proposed to conduct to smooth muscle where Ca2+ entry via CavL results in phasic activity that sums to produce tone. Abstract The mechanism underlying tone generation in the internal anal sphincter (IAS) is controversial. We examined the hypothesis that tone depends upon generation of electrical slow waves (SWs) initiated in intramuscular interstitial cells of Cajal (ICC‐IM) by activation of Ca2+‐activated Cl− channels (encoded by Ano1) and voltage‐dependent L‐type Ca2+ channels (encoded by Cacna1c). Phasic contractions and tone in the IAS were nearly abolished by ANO1 and CavL antagonists. ANO1 antagonists also abolished SWs as well as transient depolarizations that persisted after addition of CavL antagonists. Tone development in the IAS did not require stretch of muscles, and the sensitivity of contraction to ANO1 antagonists was the same in stretched versus un‐stretched muscles. ANO1 expression was examined in wildtype and Ano1/+egfp mice with immunohistochemical techniques. Dual labelling revealed that ANO1 expression could be resolved in ICC but not smooth muscle cells (SMCs) in the IAS and rectum. Ano1, Cacna1c and Kit gene expression were the same in extracts of IAS and rectum muscles. In IAS cells isolated with fluorescence‐activated cell sorting, Ano1 expression was 26.5‐fold greater in ICC than in SMCs while Cacna1c expression was only 2‐fold greater in SMCs than in ICC. These data support a central role for ANO1 and CavL in the generation of SWs and tone in the IAS. ICC‐IM are the probable cellular candidate for ANO1 currents and SW generation. We propose that ANO1 and CavL collaborate to generate SWs in ICC‐IM followed by conduction to adjacent SMCs where phasic calcium entry through CavL sums to produce tone.
    February 14, 2017   doi: 10.1113/JP273618   open full text
  • Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers.
    Louise M. Burke, Megan L. Ross, Laura A. Garvican‐Lewis, Marijke Welvaert, Ida A. Heikura, Sara G. Forbes, Joanne G. Mirtschin, Louise E. Cato, Nicki Strobel, Avish P. Sharma, John A. Hawley.
    The Journal of Physiology. February 14, 2017
    Key points Three weeks of intensified training and mild energy deficit in elite race walkers increases peak aerobic capacity independent of dietary support. Adaptation to a ketogenic low carbohydrate, high fat (LCHF) diet markedly increases rates of whole‐body fat oxidation during exercise in race walkers over a range of exercise intensities. The increased rates of fat oxidation result in reduced economy (increased oxygen demand for a given speed) at velocities that translate to real‐life race performance in elite race walkers. In contrast to training with diets providing chronic or periodised high carbohydrate availability, adaptation to an LCHF diet impairs performance in elite endurance athletes despite a significant improvement in peak aerobic capacity. Abstract We investigated the effects of adaptation to a ketogenic low carbohydrate (CHO), high fat diet (LCHF) during 3 weeks of intensified training on metabolism and performance of world‐class endurance athletes. We controlled three isoenergetic diets in elite race walkers: high CHO availability (g kg−1 day−1: 8.6 CHO, 2.1 protein, 1.2 fat) consumed before, during and after training (HCHO, n = 9); identical macronutrient intake, periodised within or between days to alternate between low and high CHO availability (PCHO, n = 10); LCHF (< 50 g day−1 CHO; 78% energy as fat; 2.1 g kg−1 day−1 protein; LCHF, n = 10). Post‐intervention, V̇O2 peak during race walking increased in all groups (P < 0.001, 90% CI: 2.55, 5.20%). LCHF was associated with markedly increased rates of whole‐body fat oxidation, attaining peak rates of 1.57 ± 0.32 g min−1 during 2 h of walking at ∼80% V̇O2 peak . However, LCHF also increased the oxygen (O2) cost of race walking at velocities relevant to real‐life race performance: O2 uptake (expressed as a percentage of new V̇O2 peak ) at a speed approximating 20 km race pace was reduced in HCHO and PCHO (90% CI: −7.047, −2.55 and −5.18, −0.86, respectively), but was maintained at pre‐intervention levels in LCHF. HCHO and PCHO groups improved times for 10 km race walk: 6.6% (90% CI: 4.1, 9.1%) and 5.3% (3.4, 7.2%), with no improvement (−1.6% (−8.5, 5.3%)) for the LCHF group. In contrast to training with diets providing chronic or periodised high‐CHO availability, and despite a significant improvement in V̇O2 peak , adaptation to the topical LCHF diet negated performance benefits in elite endurance athletes, in part due to reduced exercise economy.
    February 14, 2017   doi: 10.1113/JP273230   open full text
  • Vagal denervation inhibits the increase in pulmonary blood flow during partial lung aeration at birth.
    Justin A. R. Lang, James T. Pearson, Corinna Binder‐Heschl, Megan J. Wallace, Melissa L. Siew, Marcus J. Kitchen, Arjan B. te Pas, Robert A. Lewis, Graeme R. Polglase, Mikiyasu Shirai, Stuart B. Hooper.
    The Journal of Physiology. February 14, 2017
    Key points Lung aeration at birth significantly increases pulmonary blood flow, which is unrelated to increased oxygenation or other spatial relationships that match ventilation to perfusion. Using simultaneous X‐ray imaging and angiography in near‐term rabbits, we investigated the relative contributions of the vagus nerve and oxygenation to the increase in pulmonary blood flow at birth. Vagal denervation inhibited the global increase in pulmonary blood flow induced by partial lung aeration, although high inspired oxygen concentrations can partially mitigate this effect. The results of the present study indicate that a vagal reflex may mediate a rapid global increase in pulmonary blood flow in response to partial lung aeration. Abstract Air entry into the lungs at birth triggers major cardiovascular changes, including a large increase in pulmonary blood flow (PBF) that is not spatially related to regional lung aeration. To investigate the possible underlying role of a vagally‐mediated stimulus, we used simultaneous phase‐contrast X‐ray imaging and angiography in near‐term (30 days of gestation) vagotomized (n = 15) or sham‐operated (n = 15) rabbit kittens. Rabbits were imaged before ventilation, when one lung was ventilated (unilateral) with 100% nitrogen (N2), air or 100% oxygen (O2), and after all kittens were switched to unilateral ventilation in air and then ventilation of both lungs using air. Compared to control kittens, vagotomized kittens had little or no increase in PBF in both lungs following unilateral ventilation when ventilation occurred with 100% N2 or with air. However, relative PBF did increase in vagotomized animals ventilated with 100% O2, indicating the independent stimulatory effects of local oxygen concentration and autonomic innervation on the changes in PBF at birth. These findings demonstrate that vagal denervation inhibits the previously observed increase in PBF with partial lung aeration, although high inspired oxygen concentrations can partially mitigate this effect.
    February 14, 2017   doi: 10.1113/JP273682   open full text
  • Bicarbonate‐rich fluid secretion predicted by a computational model of guinea‐pig pancreatic duct epithelium.
    Makoto Yamaguchi, Martin C. Steward, Kieran Smallbone, Yoshiro Sohma, Akiko Yamamoto, Shigeru B. H. Ko, Takaharu Kondo, Hiroshi Ishiguro.
    The Journal of Physiology. February 08, 2017
    Key points The ductal system of the pancreas secretes large volumes of alkaline fluid containing HCO3− concentrations as high as 140 mm during hormonal stimulation. A computational model has been constructed to explore the underlying ion transport mechanisms. Parameters were estimated by fitting the model to experimental data from guinea‐pig pancreatic ducts. The model was readily able to secrete 140 mm HCO3−. Its capacity to do so was not dependent upon special properties of the cystic fibrosis transmembrane conductance regulator (CFTR) anion channels and solute carrier family 26 member A6 (SLC26A6) anion exchangers. We conclude that the main requirement for secreting high HCO3− concentrations is to minimize the secretion of Cl− ions. These findings help to clarify the mechanism responsible for pancreatic HCO3− secretion, a vital process that prevents the formation of protein plugs and viscous mucus in the ducts, which could otherwise lead to pancreatic disease. Abstract A computational model of guinea‐pig pancreatic duct epithelium was developed to determine the transport mechanism by which HCO3− ions are secreted at concentrations in excess of 140 mm. Parameters defining the contributions of the individual ion channels and transporters were estimated by least‐squares fitting of the model predictions to experimental data obtained from isolated ducts and intact pancreas under a range of experimental conditions. The effects of cAMP‐stimulated secretion were well replicated by increasing the activities of the basolateral Na+‐HCO3− cotransporter (NBC1) and apical Cl−/HCO3− exchanger (solute carrier family 26 member A6; SLC26A6), increasing the basolateral K+ permeability and apical Cl− and HCO3− permeabilities (CFTR), and reducing the activity of the basolateral Cl−/HCO3− exchanger (anion exchanger 2; AE2). Under these conditions, the model secreted ∼140 mm HCO3− at a rate of ∼3 nl min−1 mm−2, which is consistent with experimental observations. Alternative 1:2 and 1:1 stoichiometries for Cl−/HCO3− exchange via SLC26A6 at the apical membrane were able to support a HCO3−‐rich secretion. Raising the HCO3−/Cl− permeability ratio of CFTR from 0.4 to 1.0 had little impact upon either the secreted HCO3− concentration or the volume flow. However, modelling showed that a reduction in basolateral AE2 activity by ∼80% was essential in minimizing the intracellular Cl− concentration following cAMP stimulation and thereby maximizing the secreted HCO3− concentration. The addition of a basolateral Na+‐K+‐2Cl− cotransporter (NKCC1), assumed to be present in rat and mouse ducts, raised intracellular Cl− and resulted in a lower secreted HCO3− concentration, as is characteristic of those species. We conclude therefore that minimizing the driving force for Cl− secretion is the main requirement for secreting 140 mm HCO3−.
    February 08, 2017   doi: 10.1113/JP273306   open full text
  • Western diet induces colonic nitrergic myenteric neuropathy and dysmotility in mice via saturated fatty acid‐ and lipopolysaccharide‐induced TLR4 signalling.
    François Reichardt, Benoit Chassaing, Behtash Ghazi Nezami, Ge Li, Sahar Tabatabavakili, Simon Mwangi, Karan Uppal, Bill Liang, Matam Vijay‐Kumar, Dean Jones, Andrew T. Gewirtz, Shanthi Srinivasan.
    The Journal of Physiology. February 08, 2017
    Key points A high‐fat diet (60% kcal from fat) is associated with motility disorders inducing constipation and loss of nitrergic myenteric neurons in the proximal colon. Gut microbiota dysbiosis, which occurs in response to HFD, contributes to endotoxaemia. High levels of lipopolysaccharide lead to apoptosis in cultured myenteric neurons that express Toll‐like receptor 4 (TLR4). Consumption of a Western diet (WD) (35% kcal from fat) for 6 weeks leads to gut microbiota dysbiosis associated with altered bacterial metabolites and increased levels of plasma free fatty acids. These disorders precede the nitrergic myenteric cell loss observed in the proximal colon. Mice lacking TLR4 did not exhibit WD‐induced myenteric cell loss and dysmotility. Lipopolysaccharide‐induced in vitro enteric neurodegeneration requires the presence of palmitate and may be a result of enhanced NO production. The present study highlights the critical role of plasma saturated free fatty acids that are abundant in the WD with respect to driving enteric neuropathy and colonic dysmotility. Abstract The consumption of a high‐fat diet (HFD) is associated with myenteric neurodegeneration, which in turn is associated with delayed colonic transit and constipation. We examined the hypothesis that an inherent increase in plasma free fatty acids (FFA) in the HFD together with an HFD‐induced alteration in gut microbiota contributes to the pathophysiology of these disorders. C57BL/6 mice were fed a Western diet (WD) (35% kcal from fat enriched in palmitate) or a purified regular diet (16.9% kcal from fat) for 3, 6, 9 and 12 weeks. Gut microbiota dysbiosis was investigated by fecal lipopolysaccharide (LPS) measurement and metabolomics (linear trap quadrupole‐Fourier transform mass spectrometer) analysis. Plasma FFA and LPS levels were assessed, in addition to colonic and ileal nitrergic myenteric neuron quantifications and motility. Compared to regular diet‐fed control mice, WD‐fed mice gained significantly more weight without blood glucose alteration. Dysbiosis was exhibited after 6 weeks of feeding, as reflected by increased fecal LPS and bacterial metabolites and concomitant higher plasma FFA. The numbers of nitrergic myenteric neurons were reduced in the proximal colon after 9 and 12 weeks of WD and this was also associated with delayed colonic transit. WD‐fed Toll‐like receptor 4 (TLR4)−/− mice did not exhibit myenteric cell loss or dysmotility. Finally, LPS (0.5–2 ng·ml–1) and palmitate (20 and 30 μm) acted synergistically to induce neuronal cell death in vitro, which was prevented by the nitric oxide synthase inhibitor NG‐nitro‐l‐arginine methyl ester. In conclusion, WD‐feeding results in increased levels of FFA and microbiota that, even in absence of hyperglycaemia or overt endotoxaemia, synergistically induce TLR4‐mediated neurodegeneration and dysmotility.
    February 08, 2017   doi: 10.1113/JP273269   open full text
  • Effect of reproductive ageing on pregnant mouse uterus and cervix.
    Rima Patel, James D. Moffatt, Evangelia Mourmoura, Luc Demaison, Paul T. Seed, Lucilla Poston, Rachel M. Tribe.
    The Journal of Physiology. February 08, 2017
    Key points Older pregnant women have a greater risk of operative delivery, still birth and post‐term induction. This suggests that maternal age can influence the timing of birth and processes of parturition. We have found that increasing maternal age in C57BL/6J mice is associated with prolongation of gestation and length of labour. Older pregnant mice also had delayed progesterone withdrawal and impaired myometrial function. Uterine ageing and labour dysfunction should be investigated further in older primigravid women. Abstract Advanced maternal age (≥35 years) is associated with increased rates of operative delivery, stillbirth and post‐term labour induction. The physiological causes remain uncertain, although impaired myometrial function has been implicated. To investigate the hypothesis that maternal age directly influences successful parturition, we assessed the timing of birth and fetal outcome in pregnant C57BL/6J mice at 3 months (young) and 5 months (intermediate) vs. 8 months (older) of age using infrared video recording. Serum progesterone profiles, myometrium and cervix function, and mitochondrial electron transport chain complex enzymatic activities were also examined. Older pregnant mice had a longer mean gestation and labour duration (P < 0.001), as well as reduced litter size (P < 0.01) vs. 3‐month‐old mice. Older mice did not exhibit the same decline in serum progesterone concentrations as younger mice. Cervical tissues from older mice were more distensible than younger mice (P < 0.05). Oxytocin receptor and connexin‐43 mRNA expression were reduced in the myometrium from 8‐month‐old vs. 3‐month‐old mice (P < 0.05 and P < 0.01 respectively) in tandem with more frequent but shorter duration spontaneous myometrial contractions (P < 0.05) and an attenuated contractile response to oxytocin. Myometrial mitochondrial copy number was reduced in older mice, although there were no age‐induced changes to the enzymatic activities of the mitochondrial electron transport chain complexes. In conclusion, 8‐month‐old mice provide a useful model of reproductive ageing. The present study has identified potential causes of labour dysfunction amenable to investigation in older primigravid women.
    February 08, 2017   doi: 10.1113/JP273350   open full text
  • N‐Cadherin, a novel and rapidly remodelling site involved in vasoregulation of small cerebral arteries.
    Zhe Sun, Min Li, Zhaohui Li, Michael A. Hill, Gerald A. Meininger.
    The Journal of Physiology. February 07, 2017
    Key points N‐cadherin formed punctate adherens junctions (AJ) along the borders between vascular smooth muscle cells (VSMCs) in the pressurized rat superior cerebellar artery. The formation of N‐cadherin AJs in the vessel wall depends on the intraluminal pressure and was responsive to treatment with phenylephrine (PE) (10−5 m) and ACh (10−5 m). N‐cadherin‐coated beads were able to induce clustering of N‐cadherin‐enhanced green fluorescent protein (EGFP) on the plasma membrane of isolated VSMCs, whereas treatment with PE (10−5 m) or sodium nitroprusside (10−5 m) induced a significant increase or decrease in the N‐cadherin‐EGFP clustering, respectively. Application of pulling force (∼1 nN) to the N‐cadherin‐coated beads via an atomic force microscope induced a localized mechanical response from the VSMCs that opposed the pulling. Abstract N‐cadherin is the major cell–cell adhesion molecule in vascular smooth muscle cells (VSMCs). We tested the hypothesis that N‐cadherin is part of a novel mechanosensory mechanism in VSMCs and plays an active role in both the arteriolar myogenic response and during changes in vascular tone induced by vasomotor agonists. Intact and pressurized rat superior cerebellar arteries were labelled for confocal immunofluorescence imaging. N‐cadherin formed punctate adherens junctions (AJ) along the borders between VSMCs. When the lumen pressure was raised from 50 to 90 mmHg, both the density and the average size of N‐cadherin AJs increased significantly. Similarly, arteriolar constriction with phenylephrine (PE) (10–5 m) induced a significant increase of N‐cadherin AJ density at 50 mmHg, whereas vasodilatation induced by ACh (10–5 m) was accompanied by a significant decrease in density and size of N‐cadherin AJs. An atomic force microscope (AFM) was employed to further examine the mechano‐responsive properties of N‐cadherin adhesion sites in isolated VSMCs. AFM probes with an attached N‐cadherin‐coated microbead (5 μm) induced a progressive clustering of N‐cadherin‐enhanced green fluorescent protein (EGFP) on the VSMC surface. Application of pulling force (∼1 nN) to the N‐cadherin‐coated‐beads with the AFM induced a localized mechanical response from the VSMCs that opposed the pulling. Treatment with PE (10–5 m) or sodium nitroprusside (10–5 m) induced a significant increase or decrease of the N‐cadherin‐EGFP clustering, respectively. These observations provide compelling evidence that N‐cadherin AJs are sensitive to pressure and vasomotor agonists in VSMCs and support a functional role of N‐cadherin AJs in vasomotor regulation.
    February 07, 2017   doi: 10.1113/JP272995   open full text
  • The quantal catecholamine release from mouse chromaffin cells challenged with repeated ACh pulses is regulated by the mitochondrial Na+/Ca2+ exchanger.
    Angela López‐Gil, Carmen Nanclares, Iago Méndez‐López, Carmen Martínez‐Ramírez, Cristóbal los Rios, J. Fernando Padín‐Nogueira, Mayte Montero, Luis Gandía, Antonio G. García.
    The Journal of Physiology. February 07, 2017
    Key points Upon repeated application of short ACh pulses to C57BL6J mouse chromaffin cells, the amperometrically monitored secretory responses promptly decayed to a steady‐state level of around 25% of the initial response. A subsequent K+ pulse, however, overcame such decay. These data suggest that mouse chromaffin cells have a ready release‐vesicle pool that is selectively recruited by the physiological neurotransmitter ACh. The ACh‐sensitive vesicle pool is refilled and maintained by the rate of Ca2+ delivery from mitochondria to the cytosol, through the mitochondrial Na+/Ca2+ exchanger (mNCX). ITH12662, a novel blocker of the mNCX, prevented the decay of secretion elicited by ACh pulses and delayed the rate of [Ca2+]c clearance. This regulatory pathway may be physiologically relevant in situations of prolonged stressful conflicts where a sustained catecholamine release is regulated by mitochondrial Ca2+ circulation through the mNCX, which couples respiration and ATP synthesis to long‐term stimulation of chromaffin cells by endogenously released ACh. Abstract Using caged‐Ca2+ photorelease or paired depolarising pulses in voltage‐clamped chromaffin cells (CCs), various pools of secretory vesicles with different readiness to undergo exocytosis have been identified. Whether these pools are present in unclamped CCs challenged with ACh, the physiological neurotransmitter at the splanchnic nerve‐CC synapse, is unknown. We have explored here whether an ACh‐sensitive ready‐release vesicle pool (ASP) is present in C57BL6J mouse chromaffin cells (MCCs). Single cells were fast perfused with a Tyrode solution at 37°C, and challenged with 12 sequential ACh pulses (100 μm, 2 s, every 30 s) plus a K+ pulse given at the end (75 mm K+). After the first 2–3 ACh pulses the amperometrically monitored secretory responses promptly decayed to a steady‐state level of around 25% of the initial response. The last K+ pulse, however, overcame such decay. Repeated ACh pulses to voltage‐clamped cells elicited non‐desensitising nicotinic currents. Also, the [Ca2+]c transients elicited by repeated ACh pulses that were superimposed on a stable baseline elevation did not undergo decay. The novel blocker of the mitochondrial Na+/Ca2+ exchanger (mNCX) ITH12662 prevented the decay of secretion elicited by ACh pulses and delayed the rate of [Ca2+]c clearance. The experiments are compatible with the idea that C57BL6J MCCs have an ASP vesicle pool that is selectively recruited by the physiological neurotransmitter ACh and is regulated by the rate of Ca2+ delivery from mitochondria to the cytosol, through the mNCX.
    February 07, 2017   doi: 10.1113/JP273339   open full text
  • Diltiazem prevents stress‐induced contractile deficits in cardiomyocytes, but does not reverse the cardiomyopathy phenotype in Mybpc3‐knock‐in mice.
    Frederik Flenner, Birgit Geertz, Silke Reischmann‐Düsener, Florian Weinberger, Thomas Eschenhagen, Lucie Carrier, Felix W. Friedrich.
    The Journal of Physiology. February 07, 2017
    Key points Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac illness and can lead to diastolic dysfunction, sudden cardiac death and heart failure. Treatment of HCM patients is empirical and current pharmacological treatments are unable to stop disease progression or reverse hypertrophy. In this study, we tested if the non‐dihydropyridine Ca2+ channel blocker diltiazem, which previously showed potential to stop disease progression, can improve the phenotype of a HCM mouse model (Mybpc3‐targeted knock‐in), which is based on a mutation commonly found in patients. Diltiazem improved contractile function of isolated ventricular cardiomyocytes acutely, but chronic application did not improve the phenotype of adult mice with a fully developed HCM. Our study shows that diltiazem has beneficial effects in HCM, but long‐term treatment success is likely to depend on characteristics and cause of HCM and onset of treatment. Abstract Left ventricular hypertrophy, diastolic dysfunction and fibrosis are the main features of hypertrophic cardiomyopathy (HCM). Guidelines recommend β‐adrenoceptor or Ca2+ channel antagonists as pharmacological treatment. The Ca2+ channel blocker diltiazem recently showed promising beneficial effects in pre‐clinical HCM, particularly in patients carrying MYBPC3 mutations. In the present study we evaluated whether diltiazem could ameliorate or reverse the disease phenotype in cells and in vivo in an Mybpc3‐targeted knock‐in (KI) mouse model of HCM. Sarcomere shortening and Ca2+ transients were measured in KI and wild‐type (WT) cardiomyocytes in basal conditions (1‐Hz pacing) and under stress conditions (30 nm isoprenaline, 5‐Hz pacing) with or without pre‐treatment with 1 μm diltiazem. KI cardiomyocytes exhibited lower diastolic sarcomere length (dSL) at baseline, a tendency to a stronger positive inotropic response to isoprenaline than WT, a marked reduction of dSL and a tendency towards arrhythmias under stress conditions. Pre‐treatment of cardiomyocytes with 1 μm diltiazem reduced the drop in dSL and arrhythmia frequency in KI, and attenuated the positive inotropic effect of isoprenaline. Furthermore, diltiazem reduced the contraction amplitude at 5 Hz but did not affect diastolic Ca2+ load and Ca2+ transient amplitude. Six months of diltiazem treatment of KI mice did not reverse cardiac hypertrophy and dysfunction, activation of the fetal gene program or fibrosis. In conclusion, diltiazem blunted the response to isoprenaline in WT and KI cardiomyocytes and improved diastolic relaxation under stress conditions in KI cardiomyocytes. This beneficial effect of diltiazem in cells did not translate in therapeutic efficacy when applied chronically in KI mice.
    February 07, 2017   doi: 10.1113/JP273769   open full text
  • Angiotensin II reduces the surface abundance of KV1.5 channels in arterial myocytes to stimulate vasoconstriction.
    Michael W. Kidd, Simon Bulley, Jonathan H. Jaggar.
    The Journal of Physiology. February 05, 2017
    Key points Several different voltage‐dependent K+ (KV) channel isoforms are expressed in arterial smooth muscle cells (myocytes). Vasoconstrictors inhibit KV currents, but the isoform selectivity and mechanisms involved are unclear. We show that angiotensin II (Ang II), a vasoconstrictor, stimulates degradation of KV1.5, but not KV2.1, channels through a protein kinase C‐ and lysosome‐dependent mechanism, reducing abundance at the surface of mesenteric artery myocytes. The Ang II‐induced decrease in cell surface KV1.5 channels reduces whole‐cell KV1.5 currents and attenuates KV1.5 function in pressurized arteries. We describe a mechanism by which Ang II stimulates protein kinase C‐dependent KV1.5 channel degradation, reducing the abundance of functional channels at the myocyte surface. Abstract Smooth muscle cells (myocytes) of resistance‐size arteries express several different voltage‐dependent K+ (KV) channels, including KV1.5 and KV2.1, which regulate contractility. Myocyte KV currents are inhibited by vasoconstrictors, including angiotensin II (Ang II), but the mechanisms involved are unclear. Here, we tested the hypothesis that Ang II inhibits KV currents by reducing the plasma membrane abundance of KV channels in myocytes. Angiotensin II (applied for 2 h) reduced surface and total KV1.5 protein in rat mesenteric arteries. In contrast, Ang II did not alter total or surface KV2.1, or KV1.5 or KV2.1 cellular distribution, measured as the percentage of total protein at the surface. Bisindolylmaleimide (BIM; a protein kinase C blocker), a protein kinase C inhibitory peptide or bafilomycin A (a lysosomal degradation inhibitor) each blocked the Ang II‐induced decrease in total and surface KV1.5. Immunofluorescence also suggested that Ang II reduced surface KV1.5 protein in isolated myocytes; an effect inhibited by BIM. Arteries were exposed to Ang II or Ang II plus BIM (for 2 h), after which these agents were removed and contractility measurements performed or myocytes isolated for patch‐clamp electrophysiology. Angiotensin II reduced both whole‐cell KV currents and currents inhibited by Psora‐4, a KV1.5 channel blocker. Angiotensin II also reduced vasoconstriction stimulated by Psora‐4 or 4‐aminopyridine, another KV channel inhibitor. These data indicate that Ang II activates protein kinase C, which stimulates KV1.5 channel degradation, leading to a decrease in surface KV1.5, a reduction in whole‐cell KV1.5 currents and a loss of functional KV1.5 channels in myocytes of pressurized arteries.
    February 05, 2017   doi: 10.1113/JP272893   open full text
  • Four‐week cold acclimation in adult humans shifts uncoupling thermogenesis from skeletal muscles to brown adipose tissue.
    Denis P. Blondin, Amani Daoud, Taryn Taylor, Hans C. Tingelstad, Véronic Bézaire, Denis Richard, André C. Carpentier, Albert W. Taylor, Mary‐Ellen Harper, Céline Aguer, François Haman.
    The Journal of Physiology. February 05, 2017
    Key points Muscle‐derived thermogenesis during acute cold exposure in humans consists of a combination of cold‐induced increases in skeletal muscle proton leak and shivering. Daily cold exposure results in an increase in brown adipose tissue oxidative capacity coupled with a decrease in the cold‐induced skeletal muscle proton leak and shivering intensity. Improved coupling between electromyography‐determined muscle activity and whole‐body heat production following cold acclimation suggests a maintenance of ATPase‐dependent thermogenesis and decrease in skeletal muscle ATPase independent thermogenesis. Although daily cold exposure did not change the fibre composition of the vastus lateralis, the fibre composition was a strong predictor of the shivering pattern evoked during acute cold exposure. Abstract We previously showed that 4 weeks of daily cold exposure in humans can increase brown adipose tissue (BAT) volume by 45% and oxidative metabolism by 182%. Surprisingly, we did not find a reciprocal reduction in shivering intensity when exposed to a mild cold (18°C). The present study aimed to determine whether changes in skeletal muscle oxidative metabolism or shivering activity could account for these unexpected findings. Nine men participated in a 4 week cold acclimation intervention (10°C water circulating in liquid‐conditioned suit, 2 h day–1, 5 days week–1). Shivering intensity and pattern were measured continuously during controlled cold exposure (150 min at 4 °C) before and after the acclimation. Muscle biopsies from the m. vastus lateralis were obtained to measure oxygen consumption rate and proton leak of permeabilized muscle fibres. Cold acclimation elicited a modest 21% (P < 0.05) decrease in whole‐body and m. vastus lateralis shivering intensity. Furthermore, cold acclimation abolished the acute cold‐induced increase in proton leak. Although daily cold exposure did not change the fibre composition of the m. vastus lateralis, fibre composition was a strong predictor of the shivering pattern evoked during acute cold. We conclude that muscle‐derived thermogenesis during acute cold exposure in humans is not only limited to shivering, but also includes cold‐induced increases in proton leak. The efficiency of muscle oxidative phosphorylation improves with cold acclimation, suggesting that reduced muscle thermogenesis occurs through decreased proton leak, in addition to decreased shivering intensity as BAT capacity and activity increase. These changes occur with no net difference in whole‐body thermogenesis.
    February 05, 2017   doi: 10.1113/JP273395   open full text
  • Differential regulation of blood flow‐induced neovascularization and mural cell recruitment by vascular endothelial growth factor and angiopoietin signalling.
    Oliver A. Stone, James G. Carter, P. Charles Lin, Ewa Paleolog, Maria J. C. Machado, David O. Bates.
    The Journal of Physiology. February 02, 2017
    Key points Combining nitric oxide (NO)‐mediated increased blood flow with angiopoietin‐1–Tie2 receptor signalling induces arteriolargenesis – the formation of arterioles from capillaries – in a model of physiological angiogenesis. This NO–Tie‐mediated arteriolargenesis requires endogenous vascular endothelial growth factor (VEGF) signalling. Inhibition of VEGF signalling increases pericyte coverage in microvessels. Together these findings indicate that generation of functional neovasculature requires close titration of NO–Tie2 signalling and localized VEGF induction, suggesting that the use of exogenous VEGF expression as a therapeutic for neovascularization may not be successful. Abstract Signalling through vascular endothelial growth factor (VEGF) receptors and the tyrosine kinase with IgG and EGF domains‐2 (Tie2) receptor by angiopoietins is required in combination with blood flow for the formation of a functional vascular network. We tested the hypothesis that VEGF and angiopoietin‐1 (Ang1) contribute differentially to neovascularization induced by nitric oxide (NO)‐mediated vasodilatation, by comparing the phenotype of new microvessels in the mesentery during induction of vascular remodelling by over‐expression of endothelial nitric oxide synthase in the fat pad of the adult rat mesentery during inhibition of angiopoietin signalling with soluble Tie2 (sTie2) and VEGF signalling with soluble Fms‐like tyrosine kinase receptor‐1 (sFlt1). We found that NO‐mediated angiogenesis was blocked by inhibition of VEGF with sFlt1 (from 881 ± 98% increase in functional vessel area to 279 ± 72%) and by inhibition of angiopoietin with sTie2 (to 337 ± 67%). Exogenous angiopoietin‐1 was required to induce arteriolargenesis (8.6 ± 1.3% of vessels with recruitment of vascular smooth muscle cells; VSMCs) in the presence of enhanced flow. sTie2 and sFlt1 both inhibited VSMC recruitment (both 0%), and VEGF inhibition increased pericyte recruitment to newly formed vessels (from 27 ± 2 to 54 ± 3% pericyte ensheathment). We demonstrate that a fine balance of VEGF and angiopoietin signalling is required for the formation of a functional vascular network. Endogenous VEGF signalling prevents excess neovessel pericyte coverage, and is required for VSMC recruitment during increased nitric oxide‐mediated vasodilatation and angiopoietin signalling (NO–Tie‐mediated arteriogenesis). Therapeutic vascular remodelling paradigms may therefore require treatments that modulate blood flow to utilize endogenous VEGF, in combination with exogenous Ang1, for effective neovascularization.
    February 02, 2017   doi: 10.1113/JP273430   open full text
  • Non‐uniform phase sensitivity in spatial frequency maps of the human visual cortex.
    Reza Farivar, Simon Clavagnier, Bruce C. Hansen, Ben Thompson, Robert F. Hess.
    The Journal of Physiology. February 02, 2017
    Key points Just as a portrait painting can come from a collection of coarse and fine details, natural vision can be decomposed into coarse and fine components. Previous studies have shown that the early visual areas in the brain represent these components in a map‐like fashion. Other studies have shown that these same visual areas can be sensitive to how coarse and fine features line up in space. We found that the brain actually jointly represents both the scale of the feature (fine, medium, or coarse) and the alignment of these features in space. The results suggest that the visual cortex has an optimized representation particularly for the alignment of fine details, which are crucial in understanding the visual scene. Abstract Complex natural scenes can be decomposed into their oriented spatial frequency (SF) and phase relationships, both of which are represented locally at the earliest stages of cortical visual processing. The SF preference map in the human cortex, obtained using synthetic stimuli, is orderly and correlates strongly with eccentricity. In addition, early visual areas show sensitivity to the phase information that describes the relationship between SFs and thereby dictates the structure of the image. Taken together, two possibilities arise for the joint representation of SF and phase: either the entirety of the cortical SF map is uniformly sensitive to phase, or a particular set of SFs is selectively phase sensitive – for example, greater phase sensitivity for higher SFs that define fine‐scale edges in a complex scene. To test between these two possibilities, we constructed a novel continuous natural scene video whereby phase information was maintained in one SF band but scrambled elsewhere. By shifting the central frequency of the phase‐aligned band in time, we mapped the phase‐sensitive SF preference of the visual cortex. Using functional magnetic resonance imaging, we found that phase sensitivity in early visual areas is biased toward higher SFs. Compared to a SF map of the same scene obtained using linear‐filtered stimuli, a much larger patch of areas V1 and V2 is sensitive to the phase alignment of higher SFs. The results of early areas cannot be explained by attention. Our results suggest non‐uniform sensitivity to phase alignment in population‐level SF representations, with phase alignment being particularly important for fine‐scale edge representations of natural scenes.
    February 02, 2017   doi: 10.1113/JP273206   open full text
  • High‐fat diet induces protein kinase A and G‐protein receptor kinase phosphorylation of β2‐adrenergic receptor and impairs cardiac adrenergic reserve in animal hearts.
    Qin Fu, Yuting Hu, Qingtong Wang, Yongming Liu, Ning Li, Bing Xu, Sungjin Kim, Nipavan Chiamvimonvat, Yang K. Xiang.
    The Journal of Physiology. February 02, 2017
    Key points Patients with diabetes show a blunted cardiac inotropic response to β‐adrenergic stimulation despite normal cardiac contractile reserve. Acute insulin stimulation impairs β‐adrenergically induced contractile function in isolated cardiomyocytes and Langendorff‐perfused hearts. In this study, we aimed to examine the potential effects of hyperinsulinaemia associated with high‐fat diet (HFD) feeding on the cardiac β2‐adrenergic receptor signalling and the impacts on cardiac contractile function. We showed that 8 weeks of HFD feeding leads to reductions in cardiac functional reserve in response to β‐adrenergic stimulation without significant alteration of cardiac structure and function, which is associated with significant changes in β2‐adrenergic receptor phosphorylation at protein kinase A and G‐protein receptor kinase sites in the myocardium. The results suggest that clinical intervention might be applied to subjects in early diabetes without cardiac symptoms to prevent further cardiac complications. Abstract Patients with diabetes display reduced exercise capability and impaired cardiac contractile reserve in response to adrenergic stimulation. We have recently uncovered an insulin receptor and adrenergic receptor signal network in the heart. The aim of this study was to understand the impacts of high‐fat diet (HFD) on the insulin–adrenergic receptor signal network in hearts. After 8 weeks of HFD feeding, mice exhibited diabetes, with elevated insulin and glucose concentrations associated with body weight gain. Mice fed an HFD had normal cardiac structure and function. However, the HFD‐fed mice displayed a significant elevation of phosphorylation of the β2‐adrenergic receptor (β2AR) at both the protein kinase A site serine 261/262 and the G‐protein‐coupled receptor kinase site serine 355/356 and impaired adrenergic reserve when compared with mice fed on normal chow. Isolated myocytes from HFD‐fed mice also displayed a reduced contractile response to adrenergic stimulation when compared with those of control mice fed normal chow. Genetic deletion of the β2AR led to a normalized adrenergic response and preserved cardiac contractile reserve in HFD‐fed mice. Together, these data indicate that HFD promotes phosphorylation of the β2AR, contributing to impairment of cardiac contractile reserve before cardiac structural and functional remodelling, suggesting that early intervention in the insulin–adrenergic signalling network might be effective in prevention of cardiac complications in diabetes.
    February 02, 2017   doi: 10.1113/JP273314   open full text
  • The internal representation of head orientation differs for conscious perception and balance control.
    Brian H. Dalton, Brandon G. Rasman, J. Timothy Inglis, Jean‐Sébastien Blouin.
    The Journal of Physiology. February 01, 2017
    Key points We tested perceived head‐on‐feet orientation and the direction of vestibular‐evoked balance responses in passively and actively held head‐turned postures. The direction of vestibular‐evoked balance responses was not aligned with perceived head‐on‐feet orientation while maintaining prolonged passively held head‐turned postures. Furthermore, static visual cues of head‐on‐feet orientation did not update the estimate of head posture for the balance controller. A prolonged actively held head‐turned posture did not elicit a rotation in the direction of the vestibular‐evoked balance response despite a significant rotation in perceived angular head posture. It is proposed that conscious perception of head posture and the transformation of vestibular signals for standing balance relying on this head posture are not dependent on the same internal representation. Rather, the balance system may operate under its own sensorimotor principles, which are partly independent from perception. Abstract Vestibular signals used for balance control must be integrated with other sensorimotor cues to allow transformation of descending signals according to an internal representation of body configuration. We explored two alternative models of sensorimotor integration that propose (1) a single internal representation of head‐on‐feet orientation is responsible for perceived postural orientation and standing balance or (2) conscious perception and balance control are driven by separate internal representations. During three experiments, participants stood quietly while passively or actively maintaining a prolonged head‐turned posture (>10 min). Throughout the trials, participants intermittently reported their perceived head angular position, and subsequently electrical vestibular stimuli were delivered to elicit whole‐body balance responses. Visual recalibration of head‐on‐feet posture was used to determine whether static visual cues are used to update the internal representation of body configuration for perceived orientation and standing balance. All three experiments involved situations in which the vestibular‐evoked balance response was not orthogonal to perceived head‐on‐feet orientation, regardless of the visual information provided. For prolonged head‐turned postures, balance responses consistent with actual head‐on‐feet posture occurred only during the active condition. Our results indicate that conscious perception of head‐on‐feet posture and vestibular control of balance do not rely on the same internal representation, but instead treat sensorimotor cues in parallel and may arrive at different conclusions regarding head‐on‐feet posture. The balance system appears to bypass static visual cues of postural orientation and mainly use other sensorimotor signals of head‐on‐feet position to transform vestibular signals of head motion, a mechanism appropriate for most daily activities.
    February 01, 2017   doi: 10.1113/JP272998   open full text
  • Dynamics of volume‐averaged intracellular Ca2+ in a rat CNS nerve terminal during single and repetitive voltage‐clamp depolarizations.
    Kun‐Han Lin, Holger Taschenberger, Erwin Neher.
    The Journal of Physiology. February 01, 2017
    Key points The intracellular concentration of free calcium ions ([Ca2+]i) in a nerve terminal controls both transmitter release and synaptic plasticity. The rapid triggering of transmitter release depends on the local micro‐ or nanodomain of highly elevated [Ca2+]i in the vicinity of open voltage‐gated Ca2+ channels, whereas short‐term synaptic plasticity is often controlled by global changes in residual [Ca2+]i, averaged over the whole nerve terminal volume. Here we describe dynamic changes of such global [Ca2+]i in the calyx of Held – a giant mammalian glutamatergic nerve terminal, which is particularly suited for biophysical studies. We provide quantitative data on Ca2+ inflow, Ca2+ buffering and Ca2+ clearance. These data allow us to predict changes in [Ca2+]i in the nerve terminal in response to a wide range of stimulus protocols at high temporal resolution and provide a basis for the modelling of short‐term plasticity of glutamatergic synapses. Abstract Many aspects of short‐term synaptic plasticity (STP) are controlled by relatively slow changes in the presynaptic intracellular concentration of free calcium ions ([Ca2+]i) that occur in the time range of a few milliseconds to several seconds. In nerve terminals, [Ca2+]i equilibrates diffusionally during such slow changes, such that the globally measured, residual [Ca2+]i that persists after the collapse of local domains is often the appropriate parameter governing STP. Here, we study activity‐dependent dynamic changes in global [Ca2+]i at the rat calyx of Held nerve terminal in acute brainstem slices using patch‐clamp and microfluorimetry. We use low concentrations of a low‐affinity Ca2+ indicator dye (100 μm Fura‐6F) in order not to overwhelm endogenous Ca2+ buffers. We first study voltage‐clamped terminals, dialysed with pipette solutions containing minimal amounts of Ca2+ buffers, to determine Ca2+ binding properties of endogenous fixed buffers as well as the mechanisms of Ca2+ clearance. Subsequently, we use pipette solutions including 500 μm EGTA to determine the Ca2+ binding kinetics of this chelator. We provide a formalism and parameters that allow us to predict [Ca2+]i changes in calyx nerve terminals in response to a wide range of stimulus protocols. Unexpectedly, the Ca2+ affinity of EGTA under the conditions of our measurements was substantially lower (KD = 543 ± 51 nm) than measured in vitro, mainly as a consequence of a higher than previously assumed dissociation rate constant (2.38 ± 0.20 s−1), which we need to postulate in order to model the measured presynaptic [Ca2+]i transients.
    February 01, 2017   doi: 10.1113/JP272773   open full text
  • Degeneracy in the regulation of short‐term plasticity and synaptic filtering by presynaptic mechanisms.
    Chinmayee L. Mukunda, Rishikesh Narayanan.
    The Journal of Physiology. February 01, 2017
    Key points We develop a new biophysically rooted, physiologically constrained conductance‐based synaptic model to mechanistically account for short‐term facilitation and depression, respectively through residual calcium and transmitter depletion kinetics. We address the specific question of how presynaptic components (including voltage‐gated ion channels, pumps, buffers and release‐handling mechanisms) and interactions among them define synaptic filtering and short‐term plasticity profiles. Employing global sensitivity analyses (GSAs), we show that near‐identical synaptic filters and short‐term plasticity profiles could emerge from disparate presynaptic parametric combinations with weak pairwise correlations. Using virtual knockout models, a technique to address the question of channel‐specific contributions within the GSA framework, we unveil the differential and variable impact of each ion channel on synaptic physiology. Our conclusions strengthen the argument that parametric and interactional complexity in biological systems should not be viewed from the limited curse‐of‐dimensionality standpoint, but from the evolutionarily advantageous perspective of providing functional robustness through degeneracy. Abstract Information processing in neurons is known to emerge as a gestalt of pre‐ and post‐synaptic filtering. However, the impact of presynaptic mechanisms on synaptic filters has not been quantitatively assessed. Here, we developed a biophysically rooted, conductance‐based model synapse that was endowed with six different voltage‐gated ion channels, calcium pumps, calcium buffer and neurotransmitter‐replenishment mechanisms in the presynaptic terminal. We tuned our model to match the short‐term plasticity profile and band‐pass structure of Schaffer collateral synapses, and performed sensitivity analyses to demonstrate that presynaptic voltage‐gated ion channels regulated synaptic filters through changes in excitability and associated calcium influx. These sensitivity analyses also revealed that calcium‐ and release‐control mechanisms were effective regulators of synaptic filters, but accomplished this without changes in terminal excitability or calcium influx. Next, to perform global sensitivity analysis, we generated 7000 randomized models spanning 15 presynaptic parameters, and computed eight different physiological measurements in each of these models. We validated these models by applying experimentally obtained bounds on their measurements, and found 104 (∼1.5%) models to match the validation criteria for all eight measurements. Analysing these valid models, we demonstrate that analogous synaptic filters emerge from disparate combinations of presynaptic parameters exhibiting weak pairwise correlations. Finally, using virtual knockout models, we establish the variable and differential impact of different presynaptic channels on synaptic filters, underlining the critical importance of interactions among different presynaptic components in defining synaptic physiology. Our results have significant implications for protein‐localization strategies required for physiological robustness and for degeneracy in long‐term synaptic plasticity profiles.
    February 01, 2017   doi: 10.1113/JP273482   open full text
  • Non‐chemosensitive parafacial neurons simultaneously regulate active expiration and airway patency under hypercapnia in rats.
    Alan A. Britto, Davi J. A. Moraes.
    The Journal of Physiology. February 01, 2017
    Key points Hypercapnia or parafacial respiratory group (pFRG) disinhibition at normocapnia evokes active expiration in rats by recruitment of pFRG late‐expiratory (late‐E) neurons. We show that hypercapnia simultaneously evoked active expiration and exaggerated glottal dilatation by late‐E synaptic excitation of abdominal, hypoglossal and laryngeal motoneurons. Simultaneous rhythmic expiratory activity in previously silent pFRG late‐E neurons, which did not express the marker of ventral medullary CO2‐sensitive neurons (transcription factor Phox2b), was also evoked by hypercapnia. Hypercapnia‐evoked active expiration, neural and neuronal late‐E activities were eliminated by pFRG inhibition, but not after blockade of synaptic excitation. Hypercapnia produces disinhibition of non‐chemosensitive pFRG late‐E neurons to evoke active expiration and concomitant cranial motor respiratory responses controlling the oropharyngeal and upper airway patency. Abstract Hypercapnia produces active expiration in rats and the recruitment of late‐expiratory (late‐E) neurons located in the parafacial respiratory group (pFRG) of the ventral medullary brainstem. We tested the hypothesis that hypercapnia produces active expiration and concomitant cranial respiratory motor responses controlling the oropharyngeal and upper airway patency by disinhibition of pFRG late‐E neurons, but not via synaptic excitation. Phrenic nerve, abdominal nerve (AbN), cranial respiratory motor nerves, subglottal pressure, and medullary and spinal neurons/motoneurons were recorded in in situ preparations of juvenile rats. Hypercapnia evoked AbN active expiration, exaggerated late‐E discharges in cranial respiratory motor outflows, and glottal dilatation via late‐E synaptic excitation of abdominal, hypoglossal and laryngeal motoneurons. Simultaneous rhythmic late‐E activity in previously silent pFRG neurons, which did not express the marker of ventral medullary CO2‐sensitive neurons (transcription factor Phox2b), was also evoked by hypercapnia. In addition, hypercapnia‐evoked AbN active expiration, neural and neuronal late‐E activities were eliminated by pFRG inhibition, but not after blockade of synaptic excitation. On the other hand, pFRG inhibition did not affect either hypercapnia‐induced inspiratory increases in respiratory motor outflows or CO2 sensitivity of the more medial Phox2b‐positive neurons in the retrotrapezoid nucleus (RTN). Our data suggest that neither RTN Phox2b‐positive nor other CO2‐sensitive brainstem neurons activate Phox2b‐negative pFRG late‐E neurons under hypercapnia to produce AbN active expiration and concomitant cranial motor respiratory responses controlling the oropharyngeal and upper airway patency. Hypercapnia produces disinhibition of non‐chemosensitive pFRG late‐E neurons in in situ preparations of juvenile rats to activate abdominal, hypoglossal and laryngeal motoneurons.
    February 01, 2017   doi: 10.1113/JP273335   open full text
  • A novel mechanism of tandem activation of ryanodine receptors by cytosolic and SR luminal Ca2+ during excitation–contraction coupling in atrial myocytes.
    Joshua T. Maxwell, Lothar A. Blatter.
    The Journal of Physiology. February 01, 2017
    Key points In atrial myocytes excitation–contraction coupling is strikingly different from ventricle because atrial myocytes lack a transverse tubule membrane system: Ca2+ release starts in the cell periphery and propagates towards the cell centre by Ca2+‐induced Ca2+ release from the sarcoplasmic reticulum (SR) Ca2+ store. The cytosolic Ca2+ sensitivity of the ryanodine receptor (RyRs) Ca2+ release channel is low and it is unclear how Ca2+ release can be activated in the interior of atrial cells. Simultaneous confocal imaging of cytosolic and intra‐SR calcium revealed a transient elevation of store Ca2+ that we termed ‘Ca2+ sensitization signal’. We propose a novel paradigm of atrial ECC that is based on tandem activation of the RyRs by cytosolic and luminal Ca2+ through a ‘fire–diffuse–uptake–fire’ (or FDUF) mechanism: Ca2+ uptake by SR Ca2+ pumps at the propagation front elevates Ca2+ inside the SR locally, leading to luminal RyR sensitization and lowering of the cytosolic Ca2+ activation threshold. Abstract In atrial myocytes Ca2+ release during excitation–contraction coupling (ECC) is strikingly different from ventricular myocytes. In many species atrial myocytes lack a transverse tubule system, dividing the sarcoplasmic reticulum (SR) Ca2+ store into the peripheral subsarcolemmnal junctional (j‐SR) and the much more abundant central non‐junctional (nj‐SR) SR. Action potential (AP)‐induced Ca2+ entry activates Ca2+‐induced Ca2+ release (CICR) from j‐SR ryanodine receptor (RyR) Ca2+ release channels. Peripheral elevation of [Ca2+]i initiates CICR from nj‐SR and sustains propagation of CICR to the cell centre. Simultaneous confocal measurements of cytosolic ([Ca2+]i; with the fluorescent Ca2+ indicator rhod‐2) and intra‐SR ([Ca2+]SR; fluo‐5N) Ca2+ in rabbit atrial myocytes revealed that Ca2+ release from j‐SR resulted in a cytosolic Ca2+ transient of higher amplitude compared to release from nj‐SR; however, the degree of depletion of j‐SR [Ca2+]SR was smaller than nj‐SR [Ca2+]SR. Similarly, Ca2+ signals from individual release sites of the j‐SR showed a larger cytosolic amplitude (Ca2+ sparks) but smaller depletion (Ca2+ blinks) than release from nj‐SR. During AP‐induced Ca2+ release the rise of [Ca2+]i detected at individual release sites of the nj‐SR preceded the depletion of [Ca2+]SR, and during this latency period a transient elevation of [Ca2+]SR occurred. We propose that Ca2+ release from nj‐SR is activated by cytosolic and luminal Ca2+ (tandem RyR activation) via a novel ‘fire—diffuse–uptake–fire’ (FDUF) mechanism. This novel paradigm of atrial ECC predicts that Ca2+ uptake by sarco‐endoplasmic reticulum Ca2+‐ATPase (SERCA) at the propagation front elevates local [Ca2+]SR, leading to luminal RyR sensitization and lowering of the activation threshold for cytosolic CICR.
    February 01, 2017   doi: 10.1113/JP273611   open full text
  • Compensatory axon sprouting for very slow axonal die‐back in a transgenic model of spinal muscular atrophy type III.
    Esther Udina, Charles T. Putman, Luke R. Harris, Neil Tyreman, Victoria E. Cook, Tessa Gordon.
    The Journal of Physiology. January 25, 2017
    Key points Smn+/− transgenic mouse is a model of the mildest form of spinal muscular atrophy. Although there is a loss of spinal motoneurons in 11‐month‐old animals, muscular force is maintained. This maintained muscular force is mediated by reinnervation of the denervated fibres by surviving motoneurons. The spinal motoneurons in these animals do not show an increased susceptibility to death after nerve injury and they retain their regenerative capacity. We conclude that the hypothesized immaturity of the neuromuscular system in this model cannot explain the loss of motoneurons by systematic die‐back. Abstract Spinal muscular atrophy (SMA) is a common autosomal recessive disorder in humans and is the leading genetic cause of infantile death. Patients lack the SMN1 gene with the severity of the disease depending on the number of copies of the highly homologous SMN2 gene. Although motoneuron death in the Smn+/− transgenic mouse model of the mildest form of SMA, SMA type III, has been reported, we have used retrograde tracing of sciatic and femoral motoneurons in the hindlimb with recording of muscle and motor unit isometric forces to count the number of motoneurons with intact neuromuscular connections. Thereby, we investigated whether incomplete maturation of the neuromuscular system induced by survival motoneuron protein (SMN) defects is responsible for die‐back of axons relative to survival of motoneurons. First, a reduction of ∼30% of backlabelled motoneurons began relatively late, at 11 months of age, with a significant loss of 19% at 7 months. Motor axon die‐back was affirmed by motor unit number estimation. Loss of functional motor units was fully compensated by axonal sprouting to retain normal contractile force in four hindlimb muscles (three fast‐twitch and one slow‐twitch) innervated by branches of the sciatic nerve. Second, our evaluation of whether axotomy of motoneurons in the adult Smn+/− transgenic mouse increases their susceptibility to cell death demonstrated that all the motoneurons survived and they sustained their capacity to regenerate their nerve fibres. It is concluded the systematic die‐back of motoneurons that innervate both fast‐ and slow‐twitch muscle fibres is not related to immaturity of the neuromuscular system in SMA.
    January 25, 2017   doi: 10.1113/JP273404   open full text
  • Loop G in the GABAA receptor α1 subunit influences gating efficacy.
    Daniel T. Baptista‐Hon, Simona Gulbinaite, Tim G. Hales.
    The Journal of Physiology. January 25, 2017
    Key points The functional importance of residues in loop G of the GABAA receptor has not been investigated. D43 and T47 in the α1 subunit are of particular significance as their structural modification inhibits activation by GABA. While the T47C substitution had no significant effect, non‐conservative substitution of either residue (D43C or T47R) reduced the apparent potency of GABA. Propofol potentiated maximal GABA‐evoked currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors. Non‐stationary variance analysis revealed a reduction in maximal GABA‐evoked Popen, suggesting impaired agonist efficacy. Further analysis of α1(T47R)β2γ2 receptors revealed that the efficacy of the partial agonist THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridine‐3‐ol) relative to GABA was impaired. GABA‐, THIP‐ and propofol‐evoked currents mediated by α1(T47R)β2γ2 receptors deactivated faster than those mediated by α1β2γ2 receptors, indicating that the mutation impairs agonist‐evoked gating. Spontaneous gating caused by the β2(L285R) mutation was also reduced in α1(T47R)β2(L285R)γ2 compared to α1β2(L285R)γ2 receptors, confirming that α1(T47R) impairs gating independently of agonist activation. Abstract The modification of cysteine residues (substituted for D43 and T47) by 2‐aminoethyl methanethiosulfonate in the GABAA α1 subunit loop G is known to impair activation of α1β2γ2 receptors by GABA and propofol. While the T47C substitution had no significant effect, non‐conservative substitution of either residue (D43C or T47R) reduced the apparent potency of GABA. Propofol (1 μm), which potentiates sub‐maximal but not maximal GABA‐evoked currents mediated by α1β2γ2 receptors, also potentiated maximal currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors. Furthermore, the peak open probabilities of α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors were reduced. The kinetics of macroscopic currents mediated by α1(D43C)β2γ2 and α1(T47R)β2γ2 receptors were characterised by slower desensitisation and faster deactivation. Similar changes in macroscopic current kinetics, together with a slower activation rate, were observed with the loop D α1(F64C) substitution, known to impair both efficacy and agonist binding, and when the partial agonist THIP (4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridine‐3‐ol) was used to activate WT or α1(T47R)β2γ2 receptors. Propofol‐evoked currents mediated by α1(T47R)β2γ2 and α1(F64C)β2γ2 receptors also exhibited faster deactivation than their WT counterparts, revealing that these substitutions impair gating through a mechanism independent of orthosteric binding. Spontaneous gating caused by the introduction of the β2(L285R) mutation was also reduced in α1(T47R)β2(L285R)γ2 compared to α1β2(L285R)γ2 receptors, confirming that α1(T47R) impairs gating independently of activation by any agonist. These findings implicate movement of the GABAA receptor α1 subunit's β1 strand during agonist‐dependent and spontaneous gating. Immobilisation of the β1 strand may provide a mechanism for the inhibition of gating by inverse agonists such as bicuculline.
    January 25, 2017   doi: 10.1113/JP273752   open full text
  • The age‐dependent association between aortic pulse wave velocity and telomere length.
    Barry J. McDonnell, Yasmin, Lee Butcher, John R. Cockcroft, Ian B. Wilkinson, Jorge D. Erusalimsky, Carmel M. McEniery.
    The Journal of Physiology. January 24, 2017
    Key points Age significantly modifies the relationship between aortic pulse wave velocity and telomere length. The differential relationships observed between aortic pulse wave velocity and telomere length in younger and older individuals suggest that the links between cellular and vascular ageing reflect a complex interaction between genetic and environmental factors acting over the life‐course. Abstract Ageing is associated with marked large artery stiffening. Telomere shortening, a marker of cellular ageing, is linked with arterial stiffening. However, the results of existing studies are inconsistent, possibly because of the confounding influence of variable exposure to cardiovascular risk factors. Therefore, we investigated the relationship between telomere length (TL) and aortic stiffness in well‐characterized, younger and older healthy adults, who were pre‐selected on the basis of having either low or high aortic pulse wave velocity (aPWV), a robust measure of aortic stiffness. Demographic, haemodynamic and biochemical data were drawn from participants in the Anglo‐Cardiff Collaborative Trial. Two age groups with an equal sex ratio were examined: those aged <30 years (younger) or >50 years (older). Separately for each age group and sex, DNA samples representing the highest (n = 125) and lowest (n = 125) extremes of aPWV (adjusted for blood pressure) were selected for analysis of leukocyte TL. Ultimately, this yielded complete phenotypic data on 904 individuals. In younger subjects, TL was significantly shorter in those with high aPWV vs. those with low aPWV (P = 0.017). By contrast, in older subjects, TL was significantly longer in those with high aPWV (P = 0.001). Age significantly modified the relationship between aPWV and TL (P < 0.001). Differential relationships are observed between aPWV and TL, with an inverse association in younger individuals and a positive association in older individuals. The links between cellular and vascular ageing reflect a complex interaction between genetic and environmental factors acting over the life‐course.
    January 24, 2017   doi: 10.1113/JP273689   open full text
  • Parasympathetic withdrawal increases heart rate after 2 weeks at 3454 m altitude.
    Christoph Siebenmann, Peter Rasmussen, Mike Hug, Stefanie Keiser, Daniela Flück, James P. Fisher, Matthias P. Hilty, Marco Maggiorini, Carsten Lundby.
    The Journal of Physiology. January 24, 2017
    Key points Heart rate is increased in chronic hypoxia and we tested whether this is the result of increased sympathetic nervous activity, reduced parasympathetic nervous activity, or a non‐autonomic mechanism. In seven lowlanders, heart rate was measured at sea level and after 2 weeks at high altitude after individual and combined pharmacological inhibition of sympathetic and/or parasympathetic control of the heart. Inhibition of parasympathetic control of the heart alone or in combination with inhibition of sympathetic control abolished the high altitude‐induced increase in heart rate. Inhibition of sympathetic control of the heart alone did not prevent the high altitude‐induced increase in heart rate. These results indicate that a reduced parasympathetic nervous activity is the main mechanism underlying the elevated heart rate in chronic hypoxia. Abstract Chronic hypoxia increases resting heart rate (HR), but the underlying mechanism remains incompletely understood. We investigated the relative contributions of the sympathetic and parasympathetic nervous systems, along with potential non‐autonomic mechanisms, by individual and combined pharmacological inhibition of muscarinic and/or β‐adrenergic receptors. In seven healthy lowlanders, resting HR was determined at sea level (SL) and after 15–18 days of exposure to 3454 m high altitude (HA) without drug intervention (control, CONT) as well as after intravenous administration of either propranolol (PROP), or glycopyrrolate (GLYC), or PROP and GLYC in combination (PROP+GLYC). Circulating noradrenaline concentration increased from 0.9 ± 0.4 nmol l−1 at SL to 2.7 ± 1.5 nmol l−1 at HA (P = 0.03). The effect of HA on HR depended on the type of autonomic inhibition (P = 0.006). Specifically, HR was increased at HA from 64 ± 10 to 74 ± 12 beats min−1 during the CONT treatment (P = 0.007) and from 52 ± 4 to 59 ± 5 beats min−1 during the PROP treatment (P < 0.001). In contrast, HR was similar between SL and HA during the GLYC treatment (110 ± 7 and 112 ± 5 beats min−1, P = 0.28) and PROP+GLYC treatment (83 ± 5 and 85 ± 5 beats min−1, P = 0.25). Our results identify a reduction in cardiac parasympathetic activity as the primary mechanism underlying the elevated HR associated with 2 weeks of exposure to hypoxia. Unexpectedly, the sympathoactivation at HA that was evidenced by increased circulating noradrenaline concentration had little effect on HR, potentially reflecting down‐regulation of cardiac β‐adrenergic receptor function in chronic hypoxia. These effects of chronic hypoxia on autonomic control of the heart may concern not only HA dwellers, but also patients with disorders that are associated with hypoxaemia.
    January 24, 2017   doi: 10.1113/JP273726   open full text
  • The Ca2+ sensitizer CK‐2066260 increases myofibrillar Ca2+ sensitivity and submaximal force selectively in fast skeletal muscle.
    Darren T. Hwee, Arthur J. Cheng, James J. Hartman, Aaron C. Hinken, Ken Lee, Nickie Durham, Alan J. Russell, Fady I. Malik, Håkan Westerblad, Jeffrey R. Jasper.
    The Journal of Physiology. January 24, 2017
    Key points We report that the small molecule CK‐2066260 selectively slows the off‐rate of Ca2+ from fast skeletal muscle troponin, leading to increased myofibrillar Ca2+ sensitivity in fast skeletal muscle. Rodents dosed with CK‐2066260 show increased hindlimb muscle force and power in response to submaximal rates of nerve stimulation in situ. CK‐2066260 has no effect on free cytosolic [Ca2+] during contractions of isolated muscle fibres. We conclude that fast skeletal muscle troponin sensitizers constitute a potential therapy to address an unmet need of improving muscle function in conditions of weakness and premature muscle fatigue. Abstract Skeletal muscle dysfunction occurs in many diseases and can lead to muscle weakness and premature muscle fatigue. Here we show that the fast skeletal troponin activator, CK‐2066260, counteracts muscle weakness by increasing troponin Ca2+ affinity, thereby increasing myofibrillar Ca2+ sensitivity. Exposure to CK‐2066260 resulted in a concentration‐dependent increase in the Ca2+ sensitivity of ATPase activity in isolated myofibrils and reconstituted hybrid sarcomeres containing fast skeletal muscle troponin C. Stopped‐flow experiments revealed a ∼2.7‐fold decrease in the Ca2+ off‐rate of isolated troponin complexes in the presence of CK‐2066260 (6 vs. 17 s−1 under control conditions). Isolated mouse flexor digitorum brevis fibres showed a rapidly developing, reversible and concentration‐dependent force increase at submaximal stimulation frequencies. This force increase was not accompanied by any changes in the free cytosolic [Ca2+] or its kinetics. CK‐2066260 induced a slowing of relaxation, which was markedly larger at 26°C than at 31°C and could be linked to the decreased Ca2+ off‐rate of troponin C. Rats dosed with CK‐2066260 showed increased hindlimb isometric and isokinetic force in response to submaximal rates of nerve stimulation in situ producing significantly higher absolute forces at low isokinetic velocities, whereas there was no difference in force at the highest velocities. Overall muscle power was increased and the findings are consistent with a lack of effect on crossbridge kinetics. In conclusion, CK‐2066260 acts as a fast skeletal troponin activator that may be used to increase muscle force and power in conditions of muscle weakness.
    January 24, 2017   doi: 10.1113/JP273248   open full text
  • Resveratrol supplementation of high‐fat diet‐fed pregnant mice promotes brown and beige adipocyte development and prevents obesity in male offspring.
    Tiande Zou, Daiwen Chen, Qiyuan Yang, Bo Wang, Mei‐Jun Zhu, Peter W. Nathanielsz, Min Du.
    The Journal of Physiology. January 24, 2017
    Key points Maternal high‐fat diet impairs brown adipocyte function and correlates with obesity in offspring. Maternal resveratrol administration recovers metabolic activity of offspring brown adipose tissue. Maternal resveratrol promotes beige adipocyte development in offspring white adipose tissue. Maternal resveratrol intervention protects offspring against high‐fat diet‐induced obesity. Abstract Promoting beige/brite adipogenesis and thermogenic activity is considered as a promising therapeutic approach to reduce obesity and metabolic syndrome. Maternal obesity impairs offspring brown adipocyte function and correlates with obesity in offspring. We previously found that dietary resveratrol (RES) induces beige adipocyte formation in adult mice. Here, we evaluated further the effect of resveratrol supplementation of pregnant mice on offspring thermogenesis and energy expenditure. Female C57BL/6 J mice were fed a control diet (CON) or a high‐fat diet (HFD) with or without 0.2% (w/w) RES during pregnancy and lactation. Male offspring were weaned onto a HFD and maintained on this diet for 11 weeks. The offspring thermogenesis and related regulatory factors in adipose tissue were evaluated. At weaning, HFD offspring had lower thermogenesis in brown and white adipose tissues compared with CON offspring, which was recovered by maternal RES supplementation, along with the appearance of multilocular brown/beige adipocytes and elevated thermogenic gene expression. Adult offspring of RES‐treated mothers showed increased energy expenditure and insulin sensitivity when on an obesogenic diet compared with HFD offspring. The elevated metabolic activity was correlated with enhanced brown adipose function and white adipose tissue browning in HFD+RES compared with HFD offspring. In conclusion, RES supplementation of HFD‐fed dams during pregnancy and lactation promoted white adipose browning and thermogenesis in offspring at weaning accompanied by persistent beneficial effects in protecting against HFD‐induced obesity and metabolic disorders.
    January 24, 2017   doi: 10.1113/JP273478   open full text
  • Dynamical effects of calcium‐sensitive potassium currents on voltage and calcium alternans.
    Matthew Kennedy, Donald M. Bers, Nipavan Chiamvimonvat, Daisuke Sato.
    The Journal of Physiology. January 24, 2017
    Key points A mathematical model of a small conductance Ca2+‐activated potassium (SK) channel was developed and incorporated into a physiologically detailed ventricular myocyte model. Ca2+‐sensitive K+ currents promote negative intracellular Ca2+ to membrane voltage (CAi2+→ Vm) coupling. Increase of Ca2+‐sensitive K+ currents can be responsible for electromechanically discordant alternans and quasiperiodic oscillations at the cellular level. At the tissue level, Turing‐type instability can occur when Ca2+‐sensitive K+ currents are increased. Abstract Cardiac alternans is a precursor to life‐threatening arrhythmias. Alternans can be caused by instability of the membrane voltage (Vm), instability of the intracellular Ca2+ ( Ca i2+) cycling, or both. Vm dynamics and Ca i2+ dynamics are coupled via Ca2+‐sensitive currents. In cardiac myocytes, there are several Ca2+‐sensitive potassium (K+) currents such as the slowly activating delayed rectifier current (IKs) and the small conductance Ca2+‐activated potassium (SK) current (ISK). However, the role of these currents in the development of arrhythmias is not well understood. In this study, we investigated how these currents affect voltage and Ca2+ alternans using a physiologically detailed computational model of the ventricular myocyte and mathematical analysis. We define the coupling between Vm and Ca i2+ cycling dynamics ( Ca i2+→Vm coupling) as positive (negative) when a larger Ca2+ transient at a given beat prolongs (shortens) the action potential duration (APD) of that beat. While positive coupling predominates at baseline, increasing IKs and ISK promote negative Ca i2+→Vm coupling at the cellular level. Specifically, when alternans is Ca2+‐driven, electromechanically (APD–Ca2+) concordant alternans becomes electromechanically discordant alternans as IKs or ISK increase. These cellular level dynamics lead to different types of spatially discordant alternans in tissue. These findings help to shed light on the underlying mechanisms of cardiac alternans especially when the relative strength of these currents becomes larger under pathological conditions or drug administrations.
    January 24, 2017   doi: 10.1113/JP273626   open full text
  • Evidence that an internal schema adapts swallowing to upper airway requirements.
    Seng Mun Wong, Rickie J. Domangue, Sidney Fels, Christy L. Ludlow.
    The Journal of Physiology. January 18, 2017
    Key points To swallow food and liquid safely, airway protection is essential. Upward and forward movements of the hyoid and larynx in the neck during swallowing vary in magnitude between individuals. In healthy human adults, hyoid and laryngeal movements during swallowing were scaled by differences in initial upper airway area before swallowing. Individuals increased laryngeal elevation during swallowing in response to increased airway opening before swallowing. We show that when upper airway protection requirements change, individuals use an internal sensorimotor scaling system to adapt movements to maintain swallow safety. Abstract Hyoid and laryngeal movements contribute to laryngeal vestibule closure and upper oesophageal sphincter opening during swallowing. Evidence of an internal sensorimotor scaling system allowing individuals to achieve these functional goals is lacking. In speech, speakers adjust their articulatory movement magnitude according to the movement distance required to reach an articulatory target for intelligible speech. We investigated if swallowing is similar in that movement amplitude may be scaled by the functional goal for airway protection during swallowing, rather than by head and neck size. We hypothesized that healthy individuals adapt to their own anatomy by adjusting hyo‐laryngeal movements to achieve closure of the upper airway. We also investigated if individuals would automatically compensate for changes in their initial hyo‐laryngeal positions and area when head position was changed prior to swallowing. Videofluoroscopy was performed in 31 healthy adults. Using frame‐by‐frame motion analysis, anterior and superior hyoid and laryngeal displacement, and hyo‐laryngeal area were measured prior to and during swallowing. Kinematic measurements during swallowing were examined for relationships with pharyngeal neck length, and initial hyo‐laryngeal positions, length and area before swallowing. During swallowing, individuals altered laryngeal elevation magnitude to exceed hyoid elevation based on hyo‐laryngeal length before swallowing. Anterior laryngeal displacement was related to initial larynx distance from the spine, while hyoid elevation was predicted by pharyngeal neck length and initial hyoid distance from the mandible prior to the swallow. In conclusion, individuals automatically adapt hyo‐laryngeal movement during swallowing based on targets required for closing the hyo‐laryngeal area for safe swallowing.
    January 18, 2017   doi: 10.1113/JP272368   open full text
  • Membrane‐associated guanylate kinase dynamics reveal regional and developmental specificity of synapse stability.
    Jonathan M. Levy, Roger A. Nicoll.
    The Journal of Physiology. January 18, 2017
    Key points The membrane‐associated guanylate kinase (MAGUK) family of synaptic scaffolding proteins anchor glutamate receptors at CNS synapses. MAGUK removal via RNAi‐mediated knockdown in the CA1 hippocampal region in immature animals causes rapid and lasting reductions in glutamatergic transmission. In mature animals, the same manipulation has little acute effect. The hippocampal dentate gyrus, a region with ongoing adult neurogenesis, is sensitive to MAGUK loss in mature animals, behaving like an immature CA1. Over long time courses, removal of MAGUKs in CA1 causes reductions in glutamatergic transmission, indicating that synapses in mature animals require MAGUKs for anchoring glutamate receptors, but are much more stable. These results demonstrate regional and developmental control of synapse stability and suggest the existence of a sensitive period of heightened hippocampal plasticity in CA1 of pre‐adolescent rodents, and in dentate gyrus throughout maturity. Abstract Fast excitatory transmission in the brain requires localization of glutamate receptors to synapses. The membrane‐associated guanylate kinase (MAGUK) family of synaptic scaffolding proteins is critical for localization of glutamate receptors to synapses. Although the MAGUKs are well‐studied in reduced preparations and young animals, few data exist on their role in adult animals. Here, we present a detailed developmental study of the role of the MAGUKs during rat development. We first confirmed by knockdown experiments that MAGUKs are essential for glutamatergic transmission in young animals and cultured slices, and an increase in postsynaptic density protein 95 (PSD‐95) by overexpression caused correlated increases in glutamatergic transmission. We found that CA1 synapses in adults, in contrast, were largely unaffected by overexpression of MAGUKs, and although adult CA1 synapses required MAGUKs to the same degree as synapses in young animals, this was only apparent over long time scales of knockdown. We additionally showed that overexpression of MAGUKs is likely to function to accelerate the developmental strengthening of excitatory transmission. Finally, we showed that adult dentate gyrus appears similar to immature CA1, demonstrating regional and developmental control of MAGUK dynamics. Together, these results demonstrate a period of juvenile instability at CA1 synapses, followed by a period of adult stability in which synapses are acutely unaffected by changes in MAGUK abundance.
    January 18, 2017   doi: 10.1113/JP273147   open full text
  • Does trans‐spinal and local DC polarization affect presynaptic inhibition and post‐activation depression?
    D. Kaczmarek, J. Ristikankare, E. Jankowska.
    The Journal of Physiology. January 17, 2017
    Key points Trans‐spinal polarization was recently introduced as a means to improve deficient spinal functions. However, only a few attempts have been made to examine the mechanisms underlying DC actions. We have now examined the effects of DC on two spinal modulatory systems, presynaptic inhibition and post‐activation depression, considering whether they might weaken exaggerated spinal reflexes and enhance excessively weakened ones. Direct current effects were evoked by using local intraspinal DC application (0.3–0.4 μA) in deeply anaesthetized rats and were compared with the effects of trans‐spinal polarization (0.8–1.0 mA). Effects of local intraspinal DC were found to be polarity dependent, as locally applied cathodal polarization enhanced presynaptic inhibition and post‐activation depression, whereas anodal polarization weakened them. In contrast, both cathodal and anodal trans‐spinal polarization facilitated them. The results suggest some common DC‐sensitive mechanisms of presynaptic inhibition and post‐activation depression, because both were facilitated or depressed by DC in parallel. Abstract Direct current (DC) polarization has been demonstrated to alleviate the effects of various deficits in the operation of the central nervous system. However, the effects of trans‐spinal DC stimulation (tsDCS) have been investigated less extensively than the effects of transcranial DC stimulation, and their cellular mechanisms have not been elucidated. The main objectives of this study were, therefore, to extend our previous analysis of DC effects on the excitability of primary afferents and synaptic transmission by examining the effects of DC on two spinal modulatory feedback systems, presynaptic inhibition and post‐activation depression, in an anaesthetized rat preparation. Other objectives were to compare the effects of locally and trans‐spinally applied DC (locDC and tsDCS). Local polarization at the sites of terminal branching of afferent fibres was found to induce polarity‐dependent actions on presynaptic inhibition and post‐activation depression, as cathodal locDC enhanced them and anodal locDC depressed them. In contrast, tsDCS modulated presynaptic inhibition and post‐activation depression in a polarity‐independent fashion because both cathodal and anodal tsDCS facilitated them. The results show that the local presynaptic actions of DC might counteract both excessively strong and excessively weak monosynaptic actions of group Ia and cutaneous afferents. However, they indicate that trans‐spinally applied DC might counteract the exaggerated spinal reflexes but have an adverse effect on pathologically weakened spinal activity by additional presynaptic weakening. The results are also relevant for the analysis of the basic properties of presynaptic inhibition and post‐activation depression because they indicate that some common DC‐sensitive mechanisms contribute to them.
    January 17, 2017   doi: 10.1113/JP272902   open full text
  • Gaze‐evoked nystagmus induced by alcohol intoxication.
    Fausto Romano, Alexander A. Tarnutzer, Dominik Straumann, Stefano Ramat, Giovanni Bertolini.
    The Journal of Physiology. January 17, 2017
    Key points The cerebellum is the core structure controlling gaze stability. Chronic cerebellar diseases and acute alcohol intoxication affect cerebellar function, inducing, among others, gaze instability as gaze‐evoked nystagmus. Gaze‐evoked nystagmus is characterized by increased centripetal eye‐drift. It is used as an important diagnostic sign for patients with cerebellar degeneration and to assess the ‘driving while intoxicated’ condition. We quantified the effect of alcohol on gaze‐holding using an approach allowing, for the first time, the comparison of deficits induced by alcohol intoxication and cerebellar degeneration. Our results showed that alcohol intoxication induces a two‐fold increase of centripetal eye‐drift. We establish analysis techniques for using controlled alcohol intake as a model to support the study of cerebellar deficits. The observed similarity between the effect of alcohol and the clinical signs observed in cerebellar patients suggests a possible pathomechanism for gaze‐holding deficits. Abstract Gaze‐evoked nystagmus (GEN) is an ocular‐motor finding commonly observed in cerebellar disease, characterized by increased centripetal eye‐drift with centrifugal correcting saccades at eccentric gaze. With cerebellar degeneration being a rare and clinically heterogeneous disease, data from patients are limited. We hypothesized that a transient inhibition of cerebellar function by defined amounts of alcohol may provide a suitable model to study gaze‐holding deficits in cerebellar disease. We recorded gaze‐holding at varying horizontal eye positions in 15 healthy participants before and 30 min after alcohol intake required to reach 0.6‰ blood alcohol content (BAC). Changes in ocular‐motor behaviour were quantified measuring eye‐drift velocity as a continuous function of gaze eccentricity over a large range (±40 deg) of horizontal gaze angles and characterized using a two‐parameter tangent model. The effect of alcohol on gaze stability was assessed analysing: (1) overall effects on the gaze‐holding system, (2) specific effects on each eye and (3) differences between gaze angles in the temporal and nasal hemifields. For all subjects, alcohol consumption induced gaze instability, causing a two‐fold increase [2.21 (0.55), median (median absolute deviation); P = 0.002] of eye‐drift velocity at all eccentricities. Results were confirmed analysing each eye and hemifield independently. The alcohol‐induced transient global deficit in gaze‐holding matched the pattern previously described in patients with late‐onset cerebellar degeneration. Controlled intake of alcohol seems a suitable disease model to study cerebellar GEN. With alcohol resulting in global cerebellar hypofunction, we hypothesize that patients matching the gaze‐holding behaviour observed here suffered from diffuse deficits in the gaze‐holding system as well.
    January 17, 2017   doi: 10.1113/JP273204   open full text
  • Neurovascular mechanisms underlying augmented cold‐induced reflex cutaneous vasoconstriction in human hypertension.
    Jody L. Greaney, W. Larry Kenney, Lacy M. Alexander.
    The Journal of Physiology. January 16, 2017
    Key points In hypertensive adults (HTN), cardiovascular risk increases disproportionately during environmental cold exposure. Despite ample evidence of dysregulated sympathetic control of the peripheral vasculature in hypertension, no studies have examined integrated neurovascular function during cold stress in HTN. The findings of the present study show that whole‐body cold stress elicits greater increases in sympathetic outflow directed to the cutaneous vasculature and, correspondingly, greater reductions in skin blood flow in HTN. We further demonstrate an important role for non‐adrenergic sympathetic co‐transmitters in mediating the vasoconstrictor response to cold stress in hypertension. In the context of thermoregulation and the maintenance of core temperature, sympathetically‐mediated control of the cutaneous vasculature is not only preserved, but also exaggerated in hypertension. Given the increasing prevalence of hypertension, clarifying the mechanistic underpinnings of hypertension‐induced alterations in neurovascular function during cold exposure is clinically relevant. Abstract Despite ample evidence of dysregulated sympathetic control of the peripheral vasculature in hypertension, no studies have examined integrated neurovascular function during cold stress in hypertensive adults (HTN). We hypothesized that (i) whole‐body cooling would elicit greater cutaneous vasoconstriction and greater increases in skin sympathetic nervous system activity (SSNA) in HTN (n = 14; 56 ± 2 years) compared to age‐matched normotensive adults (NTN; n = 14; 55 ± 2 years) and (ii) augmented reflex vasoconstriction in HTN would be mediated by an increase in cutaneous vascular adrenergic sensitivity and a greater contribution of non‐adrenergic sympathetic co‐transmitters. SSNA (peroneal microneurography) and red cell flux (laser Doppler flowmetry; dorsum of foot) were measured during whole‐body cooling (water‐perfused suit). Sympathetic adrenergic‐ and non‐adrenergic‐dependent contributions to reflex cutaneous vasoconstriction and vascular adrenergic sensitivity were assessed pharmacologically using intradermal microdialysis. Cooling elicited greater increases in SSNA (NTN: +64 ± 13%baseline vs. HTN: +194 ± 26%baseline; P < 0.01) and greater reductions in skin blood flow (NTN: −16 ± 2%baseline vs. HTN: −28 ± 3%baseline; P < 0.01) in HTN compared to NTN, reflecting an increased response range for sympathetic reflex control of cutaneous vasoconstriction in HTN. Norepinephrine dose–response curves showed no HTN‐related difference in cutaneous adrenergic sensitivity (logEC50; NTN: −7.4 ± 0.3 log M vs. HTN: −7.5 ± 0.3 log M; P = 0.84); however, non‐adrenergic sympathetic co‐transmitters mediated a significant portion of the vasoconstrictor response to cold stress in HTN. Collectively, these findings indicate that hypertension increases the peripheral cutaneous vasoconstrictor response to cold via greater increases in skin sympathetic outflow coupled with an increased reliance on non‐adrenergic neurotransmitters.
    January 16, 2017   doi: 10.1113/JP273487   open full text
  • Post‐translational cleavage of Hv1 in human sperm tunes pH‐ and voltage‐dependent gating.
    Thomas K. Berger, David M. Fußhöller, Normann Goodwin, Wolfgang Bönigk, Astrid Müller, Nasim Dokani Khesroshahi, Christoph Brenker, Dagmar Wachten, Eberhard Krause, U. Benjamin Kaupp, Timo Strünker.
    The Journal of Physiology. January 15, 2017
    Key points In human sperm, proton flux across the membrane is controlled by the voltage‐gated proton channel Hv1. We show that sperm harbour both Hv1 and an N‐terminally cleaved isoform termed Hv1Sper. The pH‐control of Hv1Sper and Hv1 is distinctively different. Hv1Sper and Hv1 can form heterodimers that combine features of both constituents. Cleavage and heterodimerization of Hv1 might represent an adaptation to the specific requirements of pH control in sperm. Abstract In human sperm, the voltage‐gated proton channel Hv1 controls the flux of protons across the flagellar membrane. Here, we show that sperm harbour Hv1 and a shorter isoform, termed Hv1Sper. Hv1Sper is generated from Hv1 by removal of 68 amino acids from the N‐terminus by post‐translational proteolytic cleavage. The pH‐dependent gating of the channel isoforms is distinctly different. In both Hv1 and Hv1Sper, the conductance–voltage relationship is determined by the pH difference across the membrane (∆pH). However, simultaneous changes in intracellular and extracellular pH that leave ΔpH constant strongly shift the activation curve of Hv1Sper but not that of Hv1, demonstrating that cleavage of the N‐terminus tunes pH sensing in Hv1. Moreover, we show that Hv1 and Hv1Sper assemble as heterodimers that combine features of both constituents. We suggest that cleavage and heterodimerization of Hv1 represents an adaptation to the specific requirements of pH control in sperm.
    January 15, 2017   doi: 10.1113/JP273189   open full text
  • Causal relationships between neurons of the nucleus incertus and the hippocampal theta activity in the rat.
    Sergio Martínez‐Bellver, Ana Cervera‐Ferri, Aina Luque‐García, Joana Martínez‐Ricós, Alfonso Valverde‐Navarro, Manuel Bataller, Juan Guerrero, Vicent Teruel‐Marti.
    The Journal of Physiology. January 10, 2017
    Key points The nucleus incertus is a key node of the brainstem circuitry involved in hippocampal theta rhythmicity. Synchronisation exists between the nucleus incertus and hippocampal activities during theta periods. By the Granger causality analysis, we demonstrated a directional information flow between theta rhythmical neurons in the nucleus incertus and the hippocampus in theta‐on states. The electrical stimulation of the nucleus incertus is also able to evoke a phase reset of the hippocampal theta wave. Our data suggest that the nucleus incertus is a key node of theta generation and the modulation network. Abstract In recent years, a body of evidence has shown that the nucleus incertus (NI), in the dorsal tegmental pons, is a key node of the brainstem circuitry involved in hippocampal theta rhythmicity. Ascending reticular brainstem system activation evokes hippocampal theta rhythm with coupled neuronal activity in the NI. In a recent paper, we showed three populations of neurons in the NI with differential firing during hippocampal theta activation. The objective of this work was to better evaluate the causal relationship between the activity of NI neurons and the hippocampus during theta activation in order to further understand the role of the NI in the theta network. A Granger causality analysis was run to determine whether hippocampal theta activity with sensory‐evoked theta depends on the neuronal activity of the NI, or vice versa. The analysis showed causal interdependence between the NI and the hippocampus during theta activity, whose directional flow depended on the different neuronal assemblies of the NI. Whereas type I and II NI neurons mainly acted as receptors of hippocampal information, type III neuronal activity was the predominant source of flow between the NI and the hippocampus in theta states. We further determined that the electrical activation of the NI was able to reset hippocampal waves with enhanced theta‐band power, depending on the septal area. Collectively, these data suggest that hippocampal theta oscillations after sensory activation show dependence on NI neuron activity, which could play a key role in establishing optimal conditions for memory encoding.
    January 10, 2017   doi: 10.1113/JP272841   open full text
  • Potassium currents in the heart: functional roles in repolarization, arrhythmia and therapeutics.
    Nipavan Chiamvimonvat, Ye Chen‐Izu, Colleen E. Clancy, Isabelle Deschenes, Dobromir Dobrev, Jordi Heijman, Leighton Izu, Zhilin Qu, Crystal M. Ripplinger, Jamie I. Vandenberg, James N. Weiss, Gideon Koren, Tamas Banyasz, Eleonora Grandi, Michael C. Sanguinetti, Donald M. Bers, Jeanne M. Nerbonne.
    The Journal of Physiology. January 05, 2017
    Abstract This is the second of the two White Papers from the fourth UC Davis Cardiovascular Symposium Systems Approach to Understanding Cardiac Excitation–Contraction Coupling and Arrhythmias (3–4 March 2016), a biennial event that brings together leading experts in different fields of cardiovascular research. The theme of the 2016 symposium was ‘K+ channels and regulation’, and the objectives of the conference were severalfold: (1) to identify current knowledge gaps; (2) to understand what may go wrong in the diseased heart and why; (3) to identify possible novel therapeutic targets; and (4) to further the development of systems biology approaches to decipher the molecular mechanisms and treatment of cardiac arrhythmias. The sessions of the Symposium focusing on the functional roles of the cardiac K+ channel in health and disease, as well as K+ channels as therapeutic targets, were contributed by Ye Chen‐Izu, Gideon Koren, James Weiss, David Paterson, David Christini, Dobromir Dobrev, Jordi Heijman, Thomas O'Hara, Crystal Ripplinger, Zhilin Qu, Jamie Vandenberg, Colleen Clancy, Isabelle Deschenes, Leighton Izu, Tamas Banyasz, Andras Varro, Heike Wulff, Eleonora Grandi, Michael Sanguinetti, Donald Bers, Jeanne Nerbonne and Nipavan Chiamvimonvat as speakers and panel discussants. This article summarizes state‐of‐the‐art knowledge and controversies on the functional roles of cardiac K+ channels in normal and diseased heart. We endeavour to integrate current knowledge at multiple scales, from the single cell to the whole organ levels, and from both experimental and computational studies.
    January 05, 2017   doi: 10.1113/JP272883   open full text
  • Influence of menstrual phase and arid vs. humid heat stress on autonomic and behavioural thermoregulation during exercise in trained but unacclimated women.
    Tze‐Huan Lei, Stephen R. Stannard, Blake G. Perry, Zachary J. Schlader, James D. Cotter, Toby Mündel.
    The Journal of Physiology. January 04, 2017
    Key points Despite an attenuated fluctuation in ovarian hormone concentrations in well‐trained women, one in two of such women believe their menstrual cycle negatively impacts training and performance. Forthcoming large international events will expose female athletes to hot environments, and studies evaluating aerobic exercise performance in such environments across the menstrual cycle are sparse, with mixed findings. We have identified that autonomic heat loss responses at rest and during fixed‐intensity exercise in well‐trained women are not affected by menstrual cycle phase, but differ between dry and humid heat. Furthermore, exercise performance is not different across the menstrual cycle, yet is lower in humid heat, in conjunction with reduced evaporative cooling. Menstrual cycle phase does not appear to affect exercise performance in the heat in well‐trained women, but humidity impairs performance, probably due to reduced evaporative power. Abstract We studied thermoregulatory responses of ten well‐trained [V̇O2 max , 57 (7) ml min−1 kg−1] eumenorrheic women exercising in dry and humid heat, across their menstrual cycle. They completed four trials, each of resting and cycling at fixed intensities (125 and 150 W), to assess autonomic regulation, then self‐paced intensity (30 min work trial), to assess behavioural regulation. Trials were in early‐follicular (EF) and mid‐luteal (ML) phases in dry (DRY) and humid (HUM) heat matched for wet bulb globe temperature (WBGT, 27°C). During rest and fixed‐intensity exercise, rectal temperature was ∼0.2°C higher in ML than EF (P < 0.01) independent of environment (P = 0.66). Mean skin temperature did not differ between menstrual phases (P ≥ 0.13) but was higher in DRY than HUM (P < 0.01). Local sweat rate and/or forearm blood flow differed as a function of menstrual phase and environment (interaction: P ≤ 0.01). Exercise performance did not differ between phases [EF: 257 (37), ML: 255 (43) kJ, P = 0.62], but was 7 (9)% higher in DRY than HUM [263 (39), 248 (40) kJ; P < 0.01] in conjunction with equivalent autonomic regulation and thermal strain but higher evaporative cooling [16 (6) W m2; P < 0.01]. In well‐trained women exercising in the heat: (1) menstrual phase did not affect performance, (2) humidity impaired performance due to reduced evaporative cooling despite matched WBGT and (3) behavioural responses nullified thermodynamic and autonomic differences associated with menstrual phase and dry vs. humid heat.
    January 04, 2017   doi: 10.1113/JP273176   open full text
  • Extracellular protons enable activation of the calcium‐dependent chloride channel TMEM16A.
    Silvia Cruz‐Rangel, José J. Jesús‐Pérez, Iván A. Aréchiga‐Figueroa, Aldo A. Rodríguez‐Menchaca, Patricia Pérez‐Cornejo, H. Criss Hartzell, Jorge Arreola.
    The Journal of Physiology. January 03, 2017
    Key points The calcium‐activated chloride channel TMEM16A provides a pathway for chloride ion movements that are key in preventing polyspermy, allowing fluid secretion, controlling blood pressure, and enabling gastrointestinal activity. TMEM16A is opened by voltage‐dependent calcium binding and regulated by permeant anions and intracellular protons. Here we show that a low proton concentration reduces TMEM16A activity while maximum activation is obtained when the external proton concentration is high. In addition, protonation conditions determine the open probability of TMEM16A without changing its calcium sensitivity. External glutamic acid 623 (E623) is key for TMEM16A's ability to respond to external protons. At physiological pH, E623 is un‐protonated and TMEM16A is activated when intracellular calcium increases; however, under acidic conditions E623 is partially protonated and works synergistically with intracellular calcium to activate the channel. These findings are critical for understanding physiological and pathological processes that involve changes in pH and chloride flux via TMEM16A. Abstract Transmembrane protein 16A (TMEM16A), also known as ANO1, the pore‐forming subunit of a Ca2+‐dependent Cl− channel (CaCC), is activated by direct, voltage‐dependent, binding of intracellular Ca2+. Endogenous CaCCs are regulated by extracellular protons; however, the molecular basis of such regulation remains unidentified. Here, we evaluated the effects of different extracellular proton concentrations ([H+]o) on mouse TMEM16A expressed in HEK‐293 cells using whole‐cell and inside‐out patch‐clamp recordings. We found that increasing the [H+]o from 10−10 to 10−5.5 m caused a progressive increase in the chloride current (ICl) that is described by titration of a protonatable site with pK = 7.3. Protons regulate TMEM16A in a voltage‐independent manner, regardless of channel state (open or closed), and without altering its apparent Ca2+ sensitivity. Noise analysis showed that protons regulate TMEM16A by tuning its open probability without modifying the single channel current. We found a robust reduction of the proton effect at high [Ca2+]i. To identify protonation targets we mutated all extracellular glutamate and histidine residues and 4 of 11 aspartates. Most mutants were sensitive to protons. However, mutation that substituted glutamic acid (E) for glutamine (Q) at amino acid position 623 (E623Q) displayed a titration curve shifted to the left relative to wild type channels and the ICl was nearly insensitive to proton concentrations between 10−5.5 and 10−9.0 m. Additionally, ICl of the mutant containing an aspartic acid (D) to asparagine (N) substitution at position 405 (D405N) mutant was partially inhibited by a proton concentration of 10−5.5 m, but 10−9.0 m produced the same effect as in wild type. Based on our findings we propose that external protons titrate glutamic acid 623, which enables voltage activation of TMEM16A at non‐saturating [Ca2+]i.
    January 03, 2017   doi: 10.1113/JP273111   open full text
  • Altering intracellular pH reveals the kinetic basis of intraburst gating in the CFTR Cl− channel.
    Jeng‐Haur Chen, Weiyi Xu, David N. Sheppard.
    The Journal of Physiology. January 03, 2017
    Key points The cystic fibrosis transmembrane conductance regulator (CFTR), which is defective in the genetic disease cystic fibrosis (CF), forms a gated pathway for chloride movement regulated by intracellular ATP. To understand better CFTR function, we investigated the regulation of channel openings by intracellular pH. We found that short‐lived channel closures during channel openings represent subtle changes in the structure of CFTR that are regulated by intracellular pH, in part, at ATP‐binding site 1 formed by the nucleotide‐binding domains. Our results provide a framework for future studies to understand better the regulation of channel openings, the dysfunction of CFTR in CF and the action of drugs that repair CFTR gating defects. Abstract Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP‐gated Cl− channel defective in the genetic disease cystic fibrosis (CF). The gating behaviour of CFTR is characterized by bursts of channel openings interrupted by brief, flickery closures, separated by long closures between bursts. Entry to and exit from an open burst is controlled by the interaction of ATP with two ATP‐binding sites, sites 1 and 2, in CFTR. To understand better the kinetic basis of CFTR intraburst gating, we investigated the single‐channel activity of human CFTR at different intracellular pH (pHi) values. When compared with the control (pHi 7.3), acidifying pHi to 6.3 or alkalinizing pHi to 8.3 and 8.8 caused small reductions in the open‐time constant (τo) of wild‐type CFTR. By contrast, the fast closed‐time constant (τcf), which describes the short‐lived closures that interrupt open bursts, was greatly increased at pHi 5.8 and 6.3. To analyse intraburst kinetics, we used linear three‐state gating schemes. All data were satisfactorily modelled by the C1 ↔ O ↔ C2 kinetic scheme. Changing the intracellular ATP concentration was without effect on τo, τcf and their responses to pHi changes. However, mutations that disrupt the interaction of ATP with ATP‐binding site 1, including K464A, D572N and the CF‐associated mutation G1349D all abolished the prolongation of τcf at pHi 6.3. Taken together, our data suggest that the regulation of CFTR intraburst gating is distinct from the ATP‐dependent mechanism that controls channel opening and closing. However, our data also suggest that ATP‐binding site 1 modulates intraburst gating.
    January 03, 2017   doi: 10.1113/JP273205   open full text
  • The effect of α1‐adrenergic blockade on post‐exercise brachial artery flow‐mediated dilatation at sea level and high altitude.
    Michael M. Tymko, Joshua C. Tremblay, Alex B. Hansen, Connor A. Howe, Chris K. Willie, Mike Stembridge, Daniel J. Green, Ryan L. Hoiland, Prajan Subedi, James D. Anholm, Philip N. Ainslie.
    The Journal of Physiology. December 29, 2016
    Key points Our objective was to quantify endothelial function (via brachial artery flow‐mediated dilatation) at sea level (344 m) and high altitude (3800 m) at rest and following both maximal exercise and 30 min of moderate‐intensity cycling exercise with and without administration of an α1‐adrenergic blockade. Brachial endothelial function did not differ between sea level and high altitude at rest, nor following maximal exercise. At sea level, endothelial function decreased following 30 min of moderate‐intensity exercise, and this decrease was abolished with α1‐adrenergic blockade. At high altitude, endothelial function did not decrease immediately after 30 min of moderate‐intensity exercise, and administration of α1‐adrenergic blockade resulted in an increase in flow‐mediated dilatation. Our data indicate that post‐exercise endothelial function is modified at high altitude (i.e. prolonged hypoxaemia). The current study helps to elucidate the physiological mechanisms associated with high‐altitude acclimatization, and provides insight into the relationship between sympathetic nervous activity and vascular endothelial function. Abstract We examined the hypotheses that (1) at rest, endothelial function would be impaired at high altitude compared to sea level, (2) endothelial function would be reduced to a greater extent at sea level compared to high altitude after maximal exercise, and (3) reductions in endothelial function following moderate‐intensity exercise at both sea level and high altitude are mediated via an α1‐adrenergic pathway. In a double‐blinded, counterbalanced, randomized and placebo‐controlled design, nine healthy participants performed a maximal‐exercise test, and two 30 min sessions of semi‐recumbent cycling exercise at 50% peak output following either placebo or α1‐adrenergic blockade (prazosin; 0.05 mg kg −1). These experiments were completed at both sea‐level (344 m) and high altitude (3800 m). Blood pressure (finger photoplethysmography), heart rate (electrocardiogram), oxygen saturation (pulse oximetry), and brachial artery blood flow and shear rate (ultrasound) were recorded before, during and following exercise. Endothelial function assessed by brachial artery flow‐mediated dilatation (FMD) was measured before, immediately following and 60 min after exercise. Our findings were: (1) at rest, FMD remained unchanged between sea level and high altitude (placebo P = 0.287; prazosin: P = 0.110); (2) FMD remained unchanged after maximal exercise at sea level and high altitude (P = 0.244); and (3) the 2.9 ± 0.8% (P = 0.043) reduction in FMD immediately after moderate‐intensity exercise at sea level was abolished via α1‐adrenergic blockade. Conversely, at high altitude, FMD was unaltered following moderate‐intensity exercise, and administration of α1‐adrenergic blockade elevated FMD (P = 0.032). Our results suggest endothelial function is differentially affected by exercise when exposed to hypobaric hypoxia. These findings have implications for understanding the chronic impacts of hypoxaemia on exercise, and the interactions between the α1‐adrenergic pathway and endothelial function.
    December 29, 2016   doi: 10.1113/JP273183   open full text
  • Differences in TRPC3 and TRPC6 channels assembly in mesenteric vascular smooth muscle cells in essential hypertension.
    Inés Álvarez‐Miguel, Pilar Cidad, M. Teresa Pérez‐García, José Ramón López‐López.
    The Journal of Physiology. December 29, 2016
    Key points Canonical transient receptor potential (TRPC)3 and TRPC6 channels of vascular smooth muscle cells (VSMCs) mediate stretch‐ or agonist‐induced cationic fluxes, contributing to membrane potential and vascular tone. Native TRPC3/C6 channels can form homo‐ or heterotetrameric complexes, which can hinder individual TRPC channel properties. The possibility that the differences in their association pattern may change their contribution to vascular tone in hypertension is unexplored. Functional characterization of heterologously expressed channels showed that TRPC6‐containing complexes exhibited Pyr3/Pyr10‐sensitive currents, whereas TRPC3‐mediated currents were blocked by anti‐TRPC3 antibodies. VSMCs from hypertensive (blood pressure high; BPH) mice have larger cationic basal currents insensitive to Pyr10 and sensitive to anti‐TRPC3 antibodies. Consistently, myography studies showed a larger Pyr3/10‐induced vasodilatation in BPN (blood pressure normal) mesenteric arteries. We conclude that the increased TRPC3 channel expression in BPH VSMCs leads to changes in TRPC3/C6 heteromultimeric assembly, with a higher TRPC3 channel contribution favouring depolarization of hypertensive VSMCs. Abstract Increased vascular tone in essential hypertension involves a sustained rise in total peripheral resistance. A model has been proposed in which the combination of membrane depolarization and higher L‐type Ca2+ channel activity generates augmented Ca2+ influx into vascular smooth muscle cells (VSMCs), contraction and vasoconstriction. The search for culprit ion channels responsible for membrane depolarization has provided several candidates, including members of the canonical transient receptor potential (TRPC) family. TRPC3 and TRPC6 are diacylglycerol‐activated, non‐selective cationic channels contributing to stretch‐ or agonist‐induced depolarization. Conflicting information exists regarding changes in TRPC3/TRPC6 functional expression in hypertension. However, although TRPC3‐TRPC6 channels can heteromultimerize, the possibility that differences in their association pattern may change their functional contribution to vascular tone is largely unexplored. We probe this hypothesis using a model of essential hypertension (BPH mice; blood pressure high) and its normotensive control (BPN mice; blood pressure normal). First, non‐selective cationic currents through homo‐ and heterotetramers recorded from transfected Chinese hamster ovary cells indicated that TRPC currents were sensitive to the selective antagonist Pyr10 only when TRPC6 was present, whereas intracellular anti‐TRPC3 antibody selectively blocked TRPC3‐mediated currents. In mesenteric VSMCs, basal and agonist‐induced currents were more sensitive to Pyr3 and Pyr10 in BPN cells. Consistently, myography studies showed a larger Pyr3/10‐induced vasodilatation in BPN mesenteric arteries. mRNA and protein expression data supported changes in TRPC3 and TRPC6 proportions and assembly, with a higher TRPC3 channel contribution in BPH VSMCs that could favour cell depolarization. These differences in functional and pharmacological properties of TRPC3 and TRPC6 channels, depending on their assembly, could represent novel therapeutical opportunities.
    December 29, 2016   doi: 10.1113/JP273327   open full text
  • Differential roles of two delayed rectifier potassium currents in regulation of ventricular action potential duration and arrhythmia susceptibility.
    Ryan A. Devenyi, Francis A. Ortega, Willemijn Groenendaal, Trine Krogh‐Madsen, David J. Christini, Eric A. Sobie.
    The Journal of Physiology. December 28, 2016
    Key points Arrhythmias result from disruptions to cardiac electrical activity, although the factors that control cellular action potentials are incompletely understood. We combined mathematical modelling with experiments in heart cells from guinea pigs to determine how cellular electrical activity is regulated. A mismatch between modelling predictions and the experimental results allowed us to construct an improved, more predictive mathematical model. The balance between two particular potassium currents dictates how heart cells respond to perturbations and their susceptibility to arrhythmias. Abstract Imbalances of ionic currents can destabilize the cardiac action potential and potentially trigger lethal cardiac arrhythmias. In the present study, we combined mathematical modelling with information‐rich dynamic clamp experiments to determine the regulation of action potential morphology in guinea pig ventricular myocytes. Parameter sensitivity analysis was used to predict how changes in ionic currents alter action potential duration, and these were tested experimentally using dynamic clamp, a technique that allows for multiple perturbations to be tested in each cell. Surprisingly, we found that a leading mathematical model, developed with traditional approaches, systematically underestimated experimental responses to dynamic clamp perturbations. We then re‐parameterized the model using a genetic algorithm, which allowed us to estimate ionic current levels in each of the cells studied. This unbiased model adjustment consistently predicted an increase in the rapid delayed rectifier K+ current and a drastic decrease in the slow delayed rectifier K+ current, and this prediction was validated experimentally. Subsequent simulations with the adjusted model generated the clinically relevant prediction that the slow delayed rectifier is better able to stabilize the action potential and suppress pro‐arrhythmic events than the rapid delayed rectifier. In summary, iterative coupling of simulations and experiments enabled novel insight into how the balance between cardiac K+ currents influences ventricular arrhythmia susceptibility.
    December 28, 2016   doi: 10.1113/JP273191   open full text
  • Tonotopic action potential tuning of maturing auditory neurons through endogenous ATP.
    Saša Jovanovic, Tamara Radulovic, Claudio Coddou, Beatrice Dietz, Jana Nerlich, Stanko S. Stojilkovic, Rudolf Rübsamen, Ivan Milenkovic.
    The Journal of Physiology. December 28, 2016
    Key points Following the genetically controlled formation of neuronal circuits, early firing activity guides the development of sensory maps in the auditory, visual and somatosensory system. However, it is not clear whether the activity of central auditory neurons is specifically regulated depending on the position within the sensory map. In the ventral cochlear nucleus, the first central station along the auditory pathway, we describe a mechanism through which paracrine ATP signalling enhances firing in a cell‐specific and tonotopically‐determined manner. Developmental down‐regulation of P2X2/3R currents along the tonotopic axis occurs simultaneously with an increase in AMPA receptor currents, suggesting a high‐to‐low frequency maturation pattern. Facilitated action potential (AP) generation, measured as higher firing rate, shorter EPSP‐AP delay in vivo and shorter AP latency in slice experiments, is consistent with increased synaptic efficacy caused by ATP. The long lasting change in intrinsic neuronal excitability is mediated by the heteromeric P2X2/3 receptors. Abstract Synaptic refinement and strengthening are activity‐dependent processes that establish orderly arranged cochleotopic maps throughout the central auditory system. The maturation of auditory brainstem circuits is guided by action potentials (APs) arising from the inner hair cells in the developing cochlea. The AP firing of developing central auditory neurons can be modulated by paracrine ATP signalling, as shown for the cochlear nucleus bushy cells and principal neurons in the medial nucleus of the trapezoid body. However, it is not clear whether neuronal activity may be specifically regulated with respect to the nuclear tonotopic position (i.e. sound frequency selectivity). Using slice recordings before hearing onset and in vivo recordings with iontophoretic drug applications after hearing onset, we show that cell‐specific purinergic modulation follows a precise tonotopic pattern in the ventral cochlear nucleus of developing gerbils. In high‐frequency regions, ATP responsiveness diminished before hearing onset. In low‐to‐mid frequency regions, ATP modulation persisted after hearing onset in a subset of low‐frequency bushy cells (characteristic frequency< 10 kHz). Down‐regulation of P2X2/3R currents along the tonotopic axis occurs simultaneously with an increase in AMPA receptor currents, thus suggesting a high‐to‐low frequency maturation pattern. Facilitated AP generation, measured as higher firing frequency, shorter EPSP‐AP delay in vivo, and shorter AP latency in slice experiments, is consistent with increased synaptic efficacy caused by ATP. Finally, by combining recordings and pharmacology in vivo, in slices, and in human embryonic kidney 293 cells, it was shown that the long lasting change in intrinsic neuronal excitability is mediated by the P2X2/3R.
    December 28, 2016   doi: 10.1113/JP273272   open full text
  • Inwardly rectifying K+ channels are major contributors to flow‐induced vasodilatation in resistance arteries.
    Sang Joon Ahn, Ibra S. Fancher, Jing‐Tan Bian, Chong Xu Zhang, Sarah Schwab, Robert Gaffin, Shane A. Phillips, Irena Levitan.
    The Journal of Physiology. December 26, 2016
    Key points Endothelial inwardly rectifying K+ (Kir2.1) channels regulate flow‐induced vasodilatation via nitric oxide (NO) in mouse mesenteric resistance arteries. Deficiency of Kir2.1 channels results in elevated blood pressure and increased vascular resistance. Flow‐induced vasodilatation in human resistance arteries is also regulated by inwardly rectifying K+ channels. This study presents the first direct evidence that Kir channels play a critical role in physiological endothelial responses to flow. Abstract Inwardly rectifying K+ (Kir) channels are known to be sensitive to flow, but their role in flow‐induced endothelial responses is not known. The goal of this study is to establish the role of Kir channels in flow‐induced vasodilatation and to provide first insights into the mechanisms responsible for Kir signalling in this process. First, we establish that primary endothelial cells isolated from murine mesenteric arteries express functional Kir2.1 channels sensitive to shear stress. Then, using the Kir2.1+/− heterozygous mouse model, we establish that downregulation of Kir2.1 results in significant decrease in shear‐activated Kir currents and inhibition of endothelium‐dependent flow‐induced vasodilatation (FIV) assayed in pressurized mesenteric arteries pre‐constricted with endothelin‐1. Deficiency in Kir2.1 also results in the loss of flow‐induced phosphorylation of eNOS and Akt, as well as inhibition of NO generation. All the effects are fully rescued by endothelial cell (EC)‐specific overexpression of Kir2.1. A component of FIV that is Kir independent is abrogated by blocking Ca2+‐sensitive K+ channels. Kir2.1 has no effect on endothelium‐independent and K+‐induced vasodilatation in denuded arteries. Kir2.1+/− mice also show increased mean blood pressure measured by carotid artery cannulation and increased microvascular resistance measured using a tail‐cuff. Importantly, blocking Kir channels also inhibits flow‐induced vasodilatation in human subcutaneous adipose microvessels. Endothelial Kir channels contribute to FIV of mouse mesenteric arteries via an NO‐dependent mechanism, whereas Ca2+‐sensitive K+ channels mediate FIV via an NO‐independent pathway. Kir2 channels also regulate vascular resistance and blood pressure. Finally, Kir channels also contribute to FIV in human subcutaneous microvessels.
    December 26, 2016   doi: 10.1113/JP273255   open full text
  • Evidence of viscerally‐mediated cold‐defence thermoeffector responses in man.
    Nathan B. Morris, Davide Filingeri, Mark Halaki, Ollie Jay.
    The Journal of Physiology. December 26, 2016
    Key points Visceral thermoreceptors that modify thermoregulatory responses are widely accepted in animal but not human thermoregulation models. Recently, we have provided evidence of viscerally‐mediated sweating alterations in humans during exercise brought about by warm and cool fluid ingestion. In the present study, we characterize the modification of shivering and whole‐body thermal sensation during cold stress following the administration of a graded thermal stimuli delivered to the stomach via fluid ingestion at 52, 37, 22 and 7°C. Despite no differences in core and skin temperature, fluid ingestion at 52°C rapidly decreased shivering and sensations of cold compared to 37°C, whereas fluid ingestion at 22 and 7°C led to equivalent increases in these responses. Warm and cold fluid ingestion independently modifies cold defence thermoeffector responses, supporting the presence of visceral thermoreceptors in humans. However, the cold‐defence thermoeffector response patterns differed from previously identified hot‐defence thermoeffectors. Abstract Sudomotor activity is modified by both warm and cold fluid ingestion during heat stress, independently of differences in core and skin temperatures, suggesting independent viscerally‐mediated modification of thermoeffectors. The present study aimed to determine whether visceral thermoreceptors modify shivering responses to cold stress. Ten males (mean ± SD: age 27 ± 5 years; height 1.73 ± 0.06 m, weight 78.4 ± 10.7 kg) underwent whole‐body cooling via a water perfusion suit at 5°C, on four occasions, to induce a steady‐state shivering response, at which point two aliquots of 1.5 ml kg–1 (SML) and 3.0 ml kg–1 (LRG), separated by 20 min, of water at 7, 22, 37 or 52°C were ingested. Rectal, mean skin and mean body temperature (Tb), electromyographic activity (EMG), metabolic rate (M) and whole‐body thermal sensation on a visual analogue scale (WBTS) ranging from 0 mm (very cold) to 200 mm (very hot) were all measured throughout. Tb was not different between all fluid temperatures following SML fluid ingestion (7°C: 35.7 ± 0.5°C; 22°C: 35.6 ± 0.5°C; 37°C: 35.5 ± 0.4°C; 52°C: 35.5 ± 0.4°C; P = 0.27) or LRG fluid ingestion (7°C: 35.3 ± 0.6°C; 22°C: 35.3 ± 0.5°C; 37°C: 35.2 ± 0.5°C; 52°C: 35.3 ± 0.5°C; P = 0.99). With SML fluid ingestion, greater metabolic rates and cooler thermal sensations were observed with ingestion at 7°C (M: 179 ± 55 W, WBTS: 29 ± 21 mm) compared to 52°C (M: 164 ± 34 W, WBTS: 51 ± 28 mm; all P < 0.05). With LRG ingestion, compared to shivering and thermal sensations with ingestion at 37°C (M: 215 ± 47 W, EMG: 3.9 ± 2.5% MVC, WBTS: 33 ± 2 mm), values were different (all P < 0.05) following ingestion at 7°C (M: 269 ± 77 W, EMG: 5.5 ± 0.9% MVC, WBTS: 14 ± 12 mm), 22°C (M: 270 ± 86 W, EMG: 5.6 ± 1.0% MVC, WBTS: 18 ± 19 mm) and 52°C (M: 179 ± 34 W, EMG: 3.3 ± 2.1% MVC, WBTS: 53 ± 28 mm). In conclusion, fluid ingestion at 52°C decreased shivering and the sensation of coolness, whereas fluid ingestion at 22 and 7°C increased shivering and sensations of coolness to similar levels, independently of core and skin temperature.
    December 26, 2016   doi: 10.1113/JP273052   open full text
  • Cardiac remodelling in a baboon model of intrauterine growth restriction mimics accelerated ageing.
    Anderson H. Kuo, Cun Li, Jinqi Li, Hillary F. Huber, Peter W. Nathanielsz, Geoffrey D. Clarke.
    The Journal of Physiology. December 17, 2016
    Key points Rodent models of intrauterine growth restriction (IUGR) successfully identify mechanisms that can lead to short‐term and long‐term detrimental cardiomyopathies but differences between rodent and human cardiac physiology and placental‐fetal development indicate a need for models in precocial species for translation to human development. We developed a baboon model for IUGR studies using a moderate 30% global calorie restriction of pregnant mothers and used cardiac magnetic resonance imaging to evaluate offspring heart function in early adulthood. Impaired diastolic and systolic cardiac function was observed in IUGR offspring with differences between male and female subjects, compared to their respective controls. Aspects of cardiac impairment found in the IUGR offspring were similar to those found in normal controls in a geriatric cohort. Understanding early cardiac biomarkers of IUGR using non‐invasive imaging in this susceptible population, especially taking into account sexual dimorphisms, will aid recognition of the clinical presentation, development of biomarkers suitable for use in humans and management of treatment strategies. Abstract Extensive rodent studies have shown that reduced perinatal nutrition programmes chronic cardiovascular disease. To enable translation to humans, we developed baboon offspring cohorts from mothers fed ad libitum (control) or 70% of the control ad libitum diet in pregnancy and lactation, which were growth restricted at birth. We hypothesized that intrauterine growth restriction (IUGR) offspring hearts would show impaired function and a premature ageing phenotype. We studied IUGR baboons (8 male, 8 female, 5.7 years), control offspring (8 male, 8 female, 5.6 years – human equivalent approximately 25 years), and normal elderly (OLD) baboons (6 male, 6 female, mean 15.9 years). Left ventricular (LV) morphology and systolic and diastolic function were evaluated with cardiac MRI and normalized to body surface area. Two‐way ANOVA by group and sex (with P < 0.05) indicated ejection fraction, 3D sphericity indices, cardiac index, normalized systolic volume, normalized LV wall thickness, and average filling rate differed by group. Group and sex differences were found for normalized LV wall thickening and normalized myocardial mass, without interactions. Normalized peak LV filling rate and diastolic sphericity index were not correlated in control but strongly correlated in OLD and IUGR baboons. IUGR programming in baboons produces myocardial remodelling, reduces systolic and diastolic function, and results in the emergence of a premature ageing phenotype in the heart. To our knowledge, this is the first demonstration of the specific characteristics of cardiac programming and early life functional decline with ageing in an IUGR non‐human primate model. Further studies across the life span will determine progression of cardiac dysfunction.
    December 17, 2016   doi: 10.1113/JP272908   open full text
  • Human motoneurone excitability is depressed by activation of serotonin 1A receptors with buspirone.
    Jessica M. D'Amico, Annie A. Butler, Martin E. Héroux, Florence Cotel, Jean‐François M. Perrier, Jane E. Butler, Simon C. Gandevia, Janet L. Taylor.
    The Journal of Physiology. December 17, 2016
    Key points In the adult turtle spinal cord, action potential generation in motoneurones is inhibited by spillover of serotonin to extrasynaptic serotonin 1A (5‐HT1A) receptors at the axon initial segment. We explored whether ingestion of the 5‐HT1A receptor partial agonist, buspirone, decreases motoneurone excitability in humans. Following ingestion of buspirone, two tests of motoneurone excitability showed decreases. F‐wave areas and persistence in an intrinsic muscle of the hand were reduced, as was the area of cervicomedullary motor evoked potentials in biceps brachii. Our findings suggest that activation of 5‐HT1A receptors depresses human motoneurone excitability. Such a depression could contribute to decreased motoneurone output during fatiguing exercise if there is high serotonergic drive to the motoneurones. Abstract Intense serotonergic drive in the turtle spinal cord results in serotonin spillover to the axon initial segment of the motoneurones where it activates serotonin 1A (5‐HT1A) receptors and inhibits generation of action potentials. We examined whether activation of 5‐HT1A receptors decreases motoneurone excitability in humans by determining the effects of a 5‐HT1A receptor partial agonist, buspirone, on F waves and cervicomedullary motor evoked potentials (CMEPs). In a placebo‐controlled double‐blind study, 10 participants were tested on two occasions where either placebo or 20 mg of buspirone was administered orally. The ulnar nerve was stimulated supramaximally to evoke F waves in abductor digiti minimi (ADM). CMEPs and the maximal M wave were elicited in biceps brachii by cervicomedullary stimulation and brachial plexus stimulation, respectively. Following buspirone intake, F‐wave area and persistence, as well as CMEP area, were significantly decreased. The mean post‐pill difference in normalized F‐wave areas and persistence between buspirone and placebo days was –27% (–42, –12; 95% confidence interval) and –9% (–16, –2), respectively. The mean post‐pill difference in normalized CMEP area between buspirone and placebo days showed greater variation and was –31% (–60, –2). In conclusion, buspirone reduces motoneurone excitability in humans probably via activation of 5‐HT1A receptors at the axon initial segment. This has implications for motor output during high drive to the motoneurones when serotonin may spill over to these inhibitory receptors and consequently inhibit motoneurone output. Such a mechanism could potentially contribute to fatigue with exercise.
    December 17, 2016   doi: 10.1113/JP273200   open full text
  • A unifying hypothesis for M1 muscarinic receptor signalling in pyramidal neurons.
    Sameera Dasari, Corey Hill, Allan T. Gulledge.
    The Journal of Physiology. December 17, 2016
    Key points Phasic release of acetylcholine (ACh) in the neocortex facilitates attentional processes. Acting at a single metabotropic receptor subtype, ACh exerts two opposing actions in cortical pyramidal neurons: transient inhibition and longer‐lasting excitation. Cholinergic inhibitory responses depend on calcium release from intracellular calcium stores, and run down rapidly at resting membrane potentials when calcium stores become depleted. We demonstrate that cholinergic excitation promotes calcium entry at subthreshold membrane potentials to rapidly refill calcium stores, thereby maintaining the fidelity of inhibitory cholinergic signalling. We propose a ‘unifying hypothesis’ for M1 receptor signalling whereby inhibitory and excitatory responses to ACh in pyramidal neurons represent complementary mechanisms governing rapid calcium cycling between the endoplasmic reticulum, the cytosol and the extracellular space. Abstract Gq‐coupled M1‐type muscarinic acetylcholine (ACh) receptors (mAChRs) mediate two distinct electrophysiological responses in cortical pyramidal neurons: transient inhibition driven by calcium‐dependent small conductance potassium (‘SK’) channels, and longer‐lasting and voltage‐dependent excitation involving non‐specific cation channels. Here we examine the interaction of these two cholinergic responses with respect to their contributions to intracellular calcium dynamics, testing the ‘unifying hypothesis’ that rundown of inhibitory SK responses at resting membrane potentials (RMPs) reflects depletion of intracellular calcium stores, while mAChR‐driven excitation acts to refill those stores by promoting voltage‐dependent entry of extracellular calcium. We report that fidelity of cholinergic SK responses requires the continued presence of extracellular calcium. Inhibitory responses that diminished after repetitive ACh application at RMPs were immediately rescued by pairing mAChR stimulation with subthreshold depolarization (∼10 mV from RMPs) initiated with variable delay (up to 500 ms) after ACh application, but not by subthreshold depolarization preceding mAChR stimulation. Further, rescued SK responses were time‐locked to ACh application, rather than to the timing of subsequent depolarizing steps, suggesting that cholinergic signal transduction itself is not voltage‐sensitive, but that depolarization facilitates rapid cycling of extracellular calcium through the endoplasmic reticulum to activate SK channels. Consistent with this prediction, rescue of SK responses by subthreshold depolarization required the presence of extracellular calcium. Our results demonstrate that, in addition to gating calcium release from intracellular stores, mAChR activation facilitates voltage‐dependent refilling of calcium stores, thereby maintaining the ongoing fidelity of SK‐mediated inhibition in response to phasic release of ACh.
    December 17, 2016   doi: 10.1113/JP273627   open full text
  • Consequences of maternal omega‐3 polyunsaturated fatty acid supplementation on respiratory function in rat pups.
    Luana Tenorio‐Lopes, Cécile Baldy, Alexandra Jochmans‐Lemoine, Océane Mercier, Olivier Pothier‐Piccinin, Tommy Seaborn, Vincent Joseph, Isabelle Marc, Richard Kinkead.
    The Journal of Physiology. December 16, 2016
    Key points Incomplete development of the neural circuits that control breathing contributes to respiratory disorders in pre‐term infants. Manifestations include respiratory instability, prolonged apnoeas and poor ventilatory responses to stimuli. Based on evidence suggesting that omega‐3 polyunsaturated fatty acids (n‐3 PUFA) improves brain development, we determined whether n‐3 PUFA supplementation (via the maternal diet) improves respiratory function in 10–11‐day‐old rat pups. n‐3 PUFA treatment prolonged apnoea duration but augmented the relative pulmonary surface area and the ventilatory response to hypoxia. During hypoxia, the drop in body temperature measured in treated pups was 1 °C less than in controls. n‐3 PUFA treatment also reduced microglia cell density in the brainstem. Although heterogeneous, the results obtained in rat pups constitute a proof of concept that n‐3 PUFA supplementation can have positive effects on neonatal respiration. This includes a more sustained hypoxic ventilatory response and a decreased respiratory inhibition during laryngeal chemoreflex. Abstract Most pre‐term infants present respiratory instabilities and apnoeas as a result of incomplete development of the neural circuits that control breathing. Because omega‐3 polyunsaturated fatty acids (n‐3 PUFA) benefit brain development, we hypothesized that n‐3 PUFA supplementation (via the maternal diet) improves respiratory function in rat pups. Pups received n‐3 PUFA supplementation from an enriched diet (13 g kg−1 of n‐3 PUFA) administered to the mother from birth until the experiments were performed (postnatal days 10–11). Controls received a standard diet (0.3 g kg−1 of n‐3 PUFA). Breathing was measured in intact pups at rest and during hypoxia (FiO2 = 0.12; 20 min) using whole body plethysmography. The duration of apnoeas induced by stimulating the laryngeal chemoreflex (LCR) was measured under anaesthesia. Lung morphology was compared between groups. Maternal n‐3 PUFA supplementation effectively raised n‐3 PUFA levels above control levels both in the blood and brainstem of pups. In intact, resting pups, n‐3 PUFA increased the frequency and duration of apnoeas, especially in females. During hypoxia, n‐3 PUFA supplemented pups hyperventilated 23% more than controls; their anapyrexic response was 1 °C less than controls. In anaesthetized pups, n‐3 PUFA shortened the duration of LCR‐induced apnoeas by 32%. The relative pulmonary surface area of n‐3 PUFA supplemented pups was 12% higher than controls. Although n‐3 PUFA supplementation augments apnoeas, there is no clear evidence of deleterious consequences on these pups. Based on the improved lung architecture and responses to respiratory challenges, this neonatal treatment appears to be beneficial to the offspring. However, further experiments are necessary to establish its overall safety.
    December 16, 2016   doi: 10.1113/JP273471   open full text
  • High resolution three‐dimensional reconstruction of fibrotic skeletal muscle extracellular matrix.
    Allison R. Gillies, Mark A. Chapman, Eric A. Bushong, Thomas J. Deerinck, Mark H. Ellisman, Richard L. Lieber.
    The Journal of Physiology. December 14, 2016
    Key points Fibrosis occurs secondary to many skeletal muscle diseases and injuries, and can alter muscle function. It is unknown how collagen, the most abundant extracellular structural protein, alters its organization during fibrosis. Quantitative and qualitative high‐magnification electron microscopy shows that collagen is organized into perimysial cables which increase in number in a model of fibrosis, and cables have unique interactions with collagen‐producing cells. Fibrotic muscles are stiffer and have a higher concentration of collagen‐producing cells. These results improve our understanding of the organization of fibrotic skeletal muscle extracellular matrix and identify novel structures that might be targeted by antifibrotic therapy. Abstract Skeletal muscle extracellular matrix (ECM) structure and organization are not well understood, yet the ECM plays an important role in normal tissue homeostasis and disease processes. Fibrosis is common to many muscle diseases and is typically quantified based on an increase in ECM collagen. Through the use of multiple imaging modalities and quantitative stereology, we describe the structure and composition of wild‐type and fibrotic ECM, we show that collagen in the ECM is organized into large bundles of fibrils, or collagen cables, and the number of these cables (but not their size) increases in desmin knockout muscle (a fibrosis model). The increase in cable number is accompanied by increased muscle stiffness and an increase in the number of collagen producing cells. Unique interactions between ECM cells and collagen cables were also observed and reconstructed by serial block face scanning electron microscopy. These results demonstrate that the muscle ECM is more highly organized than previously reported. Therapeutic strategies for skeletal muscle fibrosis should consider the organization of the ECM to target the structures and cells contributing to fibrotic muscle function.
    December 14, 2016   doi: 10.1113/JP273376   open full text
  • Brainstem sources of cardiac vagal tone and respiratory sinus arrhythmia.
    David G.S. Farmer, Mathias Dutschmann, Julian F.R. Paton, Anthony E. Pickering, Robin M. McAllen.
    The Journal of Physiology. December 14, 2016
    Key points Cardiac vagal tone is a strong predictor of health, although its central origins are unknown. Respiratory‐linked fluctuations in cardiac vagal tone give rise to respiratory sinus arryhthmia (RSA), with maximum tone in the post‐inspiratory phase of respiration. In the present study, we investigated whether respiratory modulation of cardiac vagal tone is intrinsically linked to post‐inspiratory respiratory control using the unanaesthetized working heart‐brainstem preparation of the rat. Abolition of post‐inspiration, achieved by inhibition of the pontine Kolliker‐Fuse nucleus, removed post‐inspiratory peaks in efferent cardiac vagal activity and suppressed RSA, whereas substantial cardiac vagal tone persisted. After transection of the caudal pons, part of the remaining tone was removed by inhibition of nucleus of the solitary tract. We conclude that cardiac vagal tone depends upon at least 3 sites of the pontomedullary brainstem and that a significant proportion arises independently of RSA. Abstract Cardiac vagal tone is a strong predictor of health, although its central origins are unknown. The rat working heart‐brainstem preparation shows strong cardiac vagal tone and pronounced respiratory sinus arrhythmia. In this preparation, recordings from the cut left cardiac vagal branch showed efferent activity that peaked in post‐inspiration, ∼0.5 s before the cyclic minimum in heart rate (HR). We hypothesized that respiratory modulation of cardiac vagal tone and HR is intrinsically linked to the generation of post‐inspiration. Neurons in the pontine Kölliker‐Fuse nucleus (KF) were inhibited with bilateral microinjections of isoguvacine (50–70 nl, 10 mm) to remove the post‐inspiratory phase of respiration. This also abolished the post‐inspiratory peak of cardiac vagal discharge (and cyclical HR modulation), although a substantial level of activity remained. In separate preparations with intact cardiac vagal branches but sympathetically denervated by thoracic spinal pithing, cardiac chronotropic vagal tone was quantified by HR compared to its final level after systemic atropine (0.5 μm). Bilateral KF inhibition removed 88% of the cyclical fluctuation in HR but, on average, only 52% of the chronotropic vagal tone. Substantial chronotropic vagal tone also remained after transection of the brainstem through the caudal pons. Subsequent bilateral isoguvacine injections into the nucleus of the solitary tract further reduced vagal tone: remaining sources were untraced. We conclude that cardiac vagal tone depends on neurons in at least three sites of the pontomedullary brainstem, and much of it arises independently of respiratory sinus arrhythmia.
    December 14, 2016   doi: 10.1113/JP273164   open full text
  • Acetylcholine released by endothelial cells facilitates flow‐mediated dilatation.
    Calum Wilson, Matthew D. Lee, John G. McCarron.
    The Journal of Physiology. December 14, 2016
    Key points The endothelium plays a pivotal role in the vascular response to chemical and mechanical stimuli. The endothelium is exquisitely sensitive to ACh, although the physiological significance of ACh‐induced activation of the endothelium is unknown. In the present study, we investigated the mechanisms of flow‐mediated endothelial calcium signalling. Our data establish that flow‐mediated endothelial calcium responses arise from the autocrine action of non‐neuronal ACh released by the endothelium. Abstract Circulating blood generates frictional forces (shear stress) on the walls of blood vessels. These frictional forces critically regulate vascular function. The endothelium senses these frictional forces and, in response, releases various vasodilators that relax smooth muscle cells in a process termed flow‐mediated dilatation. Although some elements of the signalling mechanisms have been identified, precisely how flow is sensed and transduced to cause the release of relaxing factors is poorly understood. By imaging signalling in large areas of the endothelium of intact arteries, we show that the endothelium responds to flow by releasing ACh. Once liberated, ACh acts to trigger calcium release from the internal store in endothelial cells, nitric oxide production and artery relaxation. Flow‐activated release of ACh from the endothelium is non‐vesicular and occurs via organic cation transporters. ACh is generated following mitochondrial production of acetylCoA. Thus, we show ACh is an autocrine signalling molecule released from endothelial cells, and identify a new role for the classical neurotransmitter in endothelial mechanotransduction.
    December 14, 2016   doi: 10.1113/JP272927   open full text
  • Nitric oxide synthase and cyclooxygenase modulate β‐adrenergic cutaneous vasodilatation and sweating in young men.
    Naoto Fujii, Brendan D. McNeely, Glen P. Kenny.
    The Journal of Physiology. December 12, 2016
    Key points β‐Adrenergic receptor agonists such as isoproterenol induce cutaneous vasodilatation and sweating in humans, but the mechanisms underpinning this response remain unresolved. Using intradermal microdialysis, we evaluated the roles of nitric oxide synthase (NOS) and cyclooxygenase (COX) in β‐adrenergic cutaneous vasodilatation and sweating elicited by administration of isoproterenol. We show that while NOS contributes to β‐adrenergic cutaneous vasodilatation, COX restricts cutaneous vasodilatation. We also show that combined inhibition of NOS and COX augments β‐adrenergic sweating These new findings advance our basic knowledge regarding the physiological control of cutaneous blood flow and sweating, and provide important and new information to better understand the physiological significance of β‐adrenergic receptors in the skin. Abstract β‐Adrenergic receptor agonists such as isoproterenol can induce cutaneous vasodilatation and sweating in humans, but the mechanisms underpinning this response remain unresolved. We evaluated the hypotheses that (1) nitric oxide synthase (NOS) contributes to β‐adrenergic cutaneous vasodilatation, whereas cyclooxygenase (COX) limits the vasodilatation, and (2) COX contributes to β‐adrenergic sweating. In 10 young males (25 ± 5 years), cutaneous vascular conductance (CVC) and sweat rate were evaluated at four intradermal forearm skin sites infused with (1) lactated Ringer solution (control), (2) 10 mm Nω‐nitro‐l‐arginine (l‐NNA), a non‐specific NOS inhibitor, (3) 10 mm ketorolac, a non‐specific COX inhibitor, or (4) a combination of l‐NNA and ketorolac. All sites were co‐administered with a high dose of isoproterenol (100 μm) for 3 min to maximally induce β‐adrenergic sweating (β‐adrenergic sweating is significantly blunted by subsequent activations). Approximately 60 min after the washout period, three incremental doses of isoproterenol were co‐administered (1, 10 and 100 μm each for 25 min). Increases in CVC induced by the first and second 100 μm isoproterenol were attenuated by l‐NNA alone, and those in response to all doses of isoproterenol were reduced by l‐NNA with co‐infusion of ketorolac (all P ≤ 0.05). Ketorolac alone augmented increases in CVC induced by 10 μm and by the second 100 μm isoproterenol (both P ≤ 0.05). While isoproterenol‐induced sweating was not affected by the separate administration of l‐NNA or ketorolac (all P > 0.05), their combined administration augmented sweating elicited by the first 3 min of 100 μm isoproterenol (P = 0.05). We show that while NOS contributes to β‐adrenergic cutaneous vasodilatation, COX restrains the vasodilatation. Finally, combined inhibition of NOS and COX augments β‐adrenergic sweating.
    December 12, 2016   doi: 10.1113/JP273502   open full text
  • FoxO‐dependent atrogenes vary among catabolic conditions and play a key role in muscle atrophy induced by hindlimb suspension.
    Lorenza Brocca, Luana Toniolo, Carlo Reggiani, Roberto Bottinelli, Marco Sandri, Maria Antonietta Pellegrino.
    The Journal of Physiology. December 12, 2016
    Key points Muscle atrophy is a debilitating condition that affects a high percentage of the population with a negative impact on quality of life. Dissecting the molecular level of the atrophy process, and the similarities/dissimilarities among different catabolic conditions, is a necessary step for designing specific countermeasures to attenuate/prevent muscle loss. The FoxO family transcription factors represent one of the most important regulators of atrophy programme stimulating the expression of many atrophy‐related genes. The findings of the present study clearly indicate that the signalling network controlling the atrophy programme is specific for each catabolic condition. Abstract Muscle atrophy is a complex process that is in common with many different catabolic diseases including disuse/inactivity and ageing. The signalling pathways that control the atrophy programme in the different disuse/inactivity conditions have not yet been completely dissected. The inhibition of FoxO is considered to only partially spare muscle mass after denervation. The present study aimed: (i) to determine the involvement of FoxOs in hindlimb suspension disuse model; (ii) to define whether the molecular events of protein breakdown are shared among different unloaded muscles; and finally (iii) to compare the data obtained in this model with another model of inactivity such as denervation. Both wild‐type and muscle‐specific FoxO1,3,4 knockout (FoxO1,3,4−/−) mice were unloaded for 3 and 14 days and muscles were characterized by functional, morphological, biochemical and molecular assays. The data obtained show that FoxOs are required for muscle loss and force drop during unloading. Moreover, we found that FoxO‐dependent atrogenes vary in different unloaded muscles and that they diverge from denervation. The findings of the present study clearly indicate that the signalling network that controls the atrophy programme is specific for each catabolic condition.
    December 12, 2016   doi: 10.1113/JP273097   open full text
  • Interplay among distinct Ca2+ conductances drives Ca2+ sparks/spontaneous transient outward currents in rat cerebral arteries.
    Ahmed M. Hashad, Neil Mazumdar, Monica Romero, Anders Nygren, Kamran Bigdely‐Shamloo, Osama F. Harraz, Jose L. Puglisi, Edward J. Vigmond, Sean M. Wilson, Donald G. Welsh.
    The Journal of Physiology. December 12, 2016
    Key points Distinct Ca2+ channels work in a coordinated manner to grade Ca2+ spark/spontaneous transient outward currents (STOCs) in rat cerebral arteries. The relative contribution of each Ca2+ channel to Ca2+ spark/STOC production depends upon their biophysical properties and the resting membrane potential of smooth muscle. Na+/Ca2+ exchanger, but not TRP channels, can also facilitate STOC production. Abstract Ca2+ sparks are generated in a voltage‐dependent manner to initiate spontaneous transient outward currents (STOCs), events that moderate arterial constriction. In this study, we defined the mechanisms by which membrane depolarization increases Ca2+ sparks and subsequent STOC production. Using perforated patch clamp electrophysiology and rat cerebral arterial myocytes, we monitored STOCs in the presence and absence of agents that modulate Ca2+ entry. Beginning with CaV3.2 channel inhibition, Ni2+ was shown to decrease STOC frequency in cells held at hyperpolarized (−40 mV) but not depolarized (−20 mV) voltages. In contrast, nifedipine, a CaV1.2 inhibitor, markedly suppressed STOC frequency at −20 mV but not −40 mV. These findings aligned with the voltage‐dependent profiles of L‐ and T‐type Ca2+ channels. Furthermore, computational and experimental observations illustrated that Ca2+ spark production is intimately tied to the activity of both conductances. Intriguingly, this study observed residual STOC production at depolarized voltages that was independent of CaV1.2 and CaV3.2. This residual component was insensitive to TRPV4 channel modulation and was abolished by Na+/Ca2+ exchanger blockade. In summary, our work highlights that the voltage‐dependent triggering of Ca2+ sparks/STOCs is not tied to a single conductance but rather reflects an interplay among multiple Ca2+ permeable pores with distinct electrophysiological properties. This integrated orchestration enables smooth muscle to grade Ca2+ spark/STOC production and thus precisely tune negative electrical feedback.
    December 12, 2016   doi: 10.1113/JP273329   open full text
  • Minimum number of myosin motors accounting for shortening velocity under zero load in skeletal muscle.
    Luca Fusi, Valentina Percario, Elisabetta Brunello, Marco Caremani, Pasquale Bianco, Joseph D. Powers, Massimo Reconditi, Vincenzo Lombardi, Gabriella Piazzesi.
    The Journal of Physiology. December 12, 2016
    Key points Myosin filament mechanosensing determines the efficiency of the contraction by adapting the number of switched ON motors to the load. Accordingly, the unloaded shortening velocity (V0) is already set at the end of latency relaxation (LR), ∼10 ms after the start of stimulation, when the myosin filament is still in the OFF state. Here the number of actin‐attached motors per half‐myosin filament (n) during V0 shortening imposed either at the end of LR or at the plateau of the isometric contraction is estimated from the relation between half‐sarcomere compliance and force during the force redevelopment after shortening. The value of n decreases progressively with shortening and, during V0 shortening starting at the end of LR, is 1–4. Reduction of n is accounted for by a constant duty ratio of 0.05 and a parallel switching OFF of motors, explaining the very low rate of ATP utilization found during unloaded shortening. Abstract The maximum velocity at which a skeletal muscle can shorten (i.e. the velocity of sliding between the myosin filament and the actin filament under zero load, V0) is already set at the end of the latency relaxation (LR) preceding isometric force generation, ∼10 ms after the start of electrical stimulation in frog muscle fibres at 4°C. At this time, Ca2+‐induced activation of the actin filament is maximal, while the myosin filament is in the OFF state characterized by most of the myosin motors lying on helical tracks on the filament surface, making them unavailable for actin binding and ATP hydrolysis. Here, the number of actin‐attached motors per half‐thick filament during V0 shortening (n) is estimated by imposing, on tetanized single fibres from Rana esculenta (at 4°C and sarcomere length 2.15 μm), small 4 kHz oscillations and determining the relation between half‐sarcomere (hs) compliance and force during the force development following V0 shortening. When V0 shortening is superimposed on the maximum isometric force T0, n decreases progressively with the increase of shortening (range 30–80 nm per hs) and, when V0 shortening is imposed at the end of LR, n can be as low as 1–4. Reduction of n is accounted for by a constant duty ratio of the myosin motor of ∼0.05 and a parallel switching OFF of the thick filament, providing an explanation for the very low rate of ATP utilization during extended V0 shortening.
    December 12, 2016   doi: 10.1113/JP273299   open full text
  • Molecular recognition at cholinergic synapses: acetylcholine versus choline.
    Iva Bruhova, Anthony Auerbach.
    The Journal of Physiology. December 12, 2016
    Key points Neuromuscular acetylcholine (ACh) receptors have a high affinity for the neurotransmitter ACh and a low affinity for its metabolic product choline. At each transmitter binding site three aromatic groups determine affinity, and together provide ∼50% more binding energy for ACh than for choline. Deprotonation of αY190 by a nearby lysine strengthens the interaction between this aromatic ring and both ACh and choline. H‐bonds position ACh and choline differently in the aromatic cage to generate the different affinities. Abstract Acetylcholine (ACh) released at the vertebrate nerve‐muscle synapse is hydrolysed rapidly to choline (Cho), so endplate receptors (AChRs) are exposed to high concentrations of both of these structurally related ligands. To understand how these receptors distinguish ACh and Cho, we used single‐channel electrophysiology to measure resting affinities (binding free energies) of these and other agonists in adult‐type mouse AChRs having a mutation(s) at the transmitter‐binding sites. The aromatic rings of αY190, αW149 and αY198 each provide ∼50% less binding energy for Cho compared to ACh. At αY198 a phenylalanine substitution had no effect, but at αY190 this substitution caused a large, agonist‐independent loss in binding energy that depended on the presence of αK145. The results suggest that (1) αY190 is deprotonated by αK145 to strengthen the interaction between this benzene ring and the agonist's quaternary ammonium (QA) and (2) AChRs respond strongly to ACh because an H‐bond positions the QA to interact optimally with the rings, and weakly to Cho because a different H‐bond tethers the ligand to misalign the QA and form weaker interactions with the aromatic groups. The results suggest that the difference in ACh versus Cho binding energies is determined by different ligand positions within a fixed protein structure.
    December 12, 2016   doi: 10.1113/JP273291   open full text
  • Oscillatory dynamics and functional connectivity during gating of primary somatosensory responses.
    Alex I. Wiesman, Elizabeth Heinrichs‐Graham, Nathan M. Coolidge, James E. Gehringer, Max J. Kurz, Tony W. Wilson.
    The Journal of Physiology. December 12, 2016
    Key Points Sensory gating is important for preventing excessive environmental stimulation from overloading neural resources. Gating in the human somatosensory cortices is a critically understudied topic, particularly in the lower extremities. We utilize the unique capabilities of magnetoencephalographic neuroimaging to quantify the normative neural population responses and dynamic functional connectivity of somatosensory gating in the lower extremities of healthy human participants. We show that somatosensory processing is subserved by a robust gating effect in the oscillatory domain, as well as a dynamic effect on interhemispheric functional connectivity between primary sensory cortices. These results provide novel insight into the dynamic neural mechanisms that underlie the processing of somatosensory information in the human brain, and will be vital in better understanding the neural responses that are aberrant in gait‐related neurological disorders (e.g. cerebral palsy). Abstract Sensory gating (SG) is a phenomenon in which neuronal responses to subsequent similar stimuli are weaker, and is considered to be an important mechanism for preventing excessive environmental stimulation from overloading shared neural resources. Although gating has been demonstrated in multiple sensory systems, the neural dynamics and developmental trajectory underlying SG remain poorly understood. In the present study, we adopt a data‐driven approach to map the spectrotemporal amplitude and functional connectivity (FC) dynamics that support gating in the somatosensory system (somato‐SG) in healthy children and adolescents using magnetoencephalography (MEG). These data underwent time‐frequency decomposition and the significant signal changes were imaged using a beamformer. Voxel time series were then extracted from the peak voxels and these signals were examined in the time and time‐frequency domains, and then subjected to dynamic FC analysis. The results obtained indicate a significant decrease in the amplitude of the neural response following the second stimulation relative to the first in the primary somatosensory cortex (SI). A significant decrease in response latency was also found between stimulations, and each stimulation induced a sharp decrease in FC between somatosensory cortical areas. Furthermore, there were no significant correlations between somato‐SG metrics and age. We conclude that somato‐SG can be observed in SI in both the time and oscillatory domains, with rich dynamics and alterations in inter‐hemispheric FC, and that this phenomenon has already matured by early childhood. A better understanding of these dynamics may provide insight to the numerous psychiatric and neurologic conditions that have been associated with aberrant SG across multiple modalities.
    December 12, 2016   doi: 10.1113/JP273192   open full text
  • Cholinergic modulation of the parafacial respiratory group.
    Rozlyn C. T. Boutin, Zaki Alsahafi, Silvia Pagliardini.
    The Journal of Physiology. December 11, 2016
    Key points This study investigates the effects of cholinergic transmission on the expiratory oscillator, the parafacial respiratory group (pFRG) in urethane anaesthetized adult rats. Local inhibition of the acetyl cholinesterase enzyme induced activation of expiratory abdominal muscles and active expiration. Local application of the cholinomimetic carbachol elicited recruitment of late expiratory neurons, expiratory abdominal muscle activity and active expiration. This effect was antagonized by local application of the muscarinic antagonists scopolamine, J104129 and 4DAMP. We observed distinct physiological responses between the more medial chemosensitive region of the retrotrapezoid nucleus and the more lateral region of pFRG. These results support the hypothesis that pFRG is under cholinergic neuromodulation and the region surrounding the facial nucleus contains a group of neurons with distinct physiological roles. Abstract Active inspiration and expiration are opposing respiratory phases generated by two separate oscillators in the brainstem: inspiration driven by a neuronal network located in the preBötzinger complex (preBötC) and expiration driven by a neuronal network located in the parafacial respiratory group (pFRG). While continuous activity of the preBötC is necessary for maintaining ventilation, the pFRG behaves as a conditional expiratory oscillator, being silent in resting conditions and becoming rhythmically active in the presence of increased respiratory drive (e.g. hypoxia, hypercapnia, exercise and through release of inhibition). Recent evidence from our laboratory suggests that expiratory activity in the principal expiratory pump muscles, the abdominals, is modulated in a state‐dependent fashion, frequently occurring during periods of REM sleep. We hypothesized that acetylcholine, a neurotransmitter released in wakefulness and REM sleep by mesopontine structures, contributes to the activation of pFRG neurons and thus acts to promote the recruitment of expiratory abdominal muscle activity. We investigated the stimulatory effect of cholinergic neurotransmission on pFRG activity and recruitment of active expiration in vivo under anaesthesia. We demonstrate that local application of the acetylcholinesterase inhibitor physostigmine into the pFRG potentiated expiratory activity. Furthermore, local application of the cholinomimetic carbachol into the pFRG activated late expiratory neurons and induced long lasting rhythmic active expiration. This effect was completely abolished by pre‐application of the muscarinic antagonist scopolamine, and more selective M3 antagonists 4DAMP and J104129. We conclude that cholinergic muscarinic transmission contributes to excitation of pFRG neurons and promotes both active recruitment of abdominal muscles and active expiratory flow.
    December 11, 2016   doi: 10.1113/JP273012   open full text
  • Sildenafil therapy for fetal cardiovascular dysfunction during hypoxic development: studies in the chick embryo.
    Nozomi Itani, Katie L. Skeffington, Christian Beck, Dino A. Giussani.
    The Journal of Physiology. December 11, 2016
    Key points Common complications of pregnancy, such as chronic fetal hypoxia, trigger a fetal origin of cardiovascular dysfunction and programme cardiovascular disease in later life. Sildenafil treatment protects placental perfusion and fetal growth, but whether the effects of sildenafil transcend the placenta to affect the fetus is unknown. Using the chick embryo model, here we show that sildenafil treatment directly protects the fetal cardiovascular system in hypoxic development, and that the mechanisms of sildenafil protection include reduced oxidative stress and increased nitric oxide bioavailability; Sildenafil does not protect against fetal growth restriction in the chick embryo, supporting the idea that the protective effect of sildenafil on fetal growth reported in mammalian studies, including humans, is secondary to improved placental perfusion. Therefore, sildenafil may be a good candidate for human translational antioxidant therapy to protect the chronically hypoxic fetus in adverse pregnancy. Abstract There is a need for developing clinically translatable therapy for preventing fetal origins of cardiovascular disease in pregnancy complicated by chronic fetal hypoxia. Evidence shows that sildenafil protects placental perfusion and fetal growth. However, whether beneficial effects of sildenafil transcend onto the fetal heart and circulation in complicated development is unknown. We isolated the direct effects of sildenafil on the fetus using the chick embryo and hypothesised that sildenafil also protects fetal cardiovascular function in hypoxic development. Chick embryos (n = 11 per group) were incubated in normoxia or hypoxia (14% O2) from day 1 and treated with sildenafil (4 mg kg−1 day−1) from day 13 of the 21‐day incubation. Hypoxic incubation increased oxidative stress (4‐hydroxynonenal, 141.1 ± 17.6% of normoxic control), reduced superoxide dismutase (60.7 ± 6.3%), increased phosphodiesterase type 5 expression (167 ± 13.7%) and decreased nitric oxide bioavailability (54.7 ± 6.1%) in the fetal heart, and promoted peripheral endothelial dysfunction (70.9 ± 5.6% AUC of normoxic control; all P < 0.05). Sildenafil treatment after onset of chronic hypoxia prevented the increase in phosphodiesterase expression (72.5 ± 22.4%), protected against oxidative stress (94.7 ± 6.2%) and normalised nitric oxide bioavailability (115.6 ± 22.3%) in the fetal heart, and restored endothelial function in the peripheral circulation (89.8 ± 2.9%). Sildenafil protects the fetal heart and circulation directly in hypoxic development via mechanisms including decreased oxidative stress and enhanced nitric oxide bioavailability. Sildenafil may be a good translational candidate for human antioxidant therapy to prevent fetal origins of cardiovascular dysfunction in adverse pregnancy.
    December 11, 2016   doi: 10.1113/JP273393   open full text
  • Cerebral haemodynamic response to somatosensory stimulation in near‐term fetal sheep.
    S. Nakamura, D. W. Walker, F. Y. Wong.
    The Journal of Physiology. December 11, 2016
    Key points Cerebral haemodynamic response to neural stimulation has been extensively investigated in animal and clinical studies, in both adult and paediatric populations, but little is known about cerebral haemodynamic functional response in the fetal brain. The present study describes the cerebral haemodynamic response measured by near‐infrared spectroscopy to somatosensory stimulation in fetal sheep. The cerebral haemodynamic response in the fetal sheep brain changes from a positive (increase in oxyhaemoglobin (oxyHb)) response pattern to a negative or biphasic response pattern when the duration of somatosensory stimulation is increased, probably due to cerebral vasoconstriction with prolonged stimulations. In contrast to adult studies, we have found that changes in fetal cerebral blood flow and oxyHb are positively increased in response to somatosensory stimulation during hypercapnia. We propose this is related to reduced vascular resistance and recruitment of cerebral vasculature in the fetal brain during hypercapnia. Abstract Functional hyperaemia induced by a localised increase in neuronal activity has been suggested to occur in the fetal brain owing to a positive blood oxygen level‐dependent (BOLD) signal recorded by functional magnetic resonance imaging following acoustic stimulation. To study the effect of somatosensory input on local cerebral perfusion we used near‐infrared spectroscopy (NIRS) in anaesthetised, partially exteriorised fetal sheep where the median nerve was stimulated with trains of pulses (2 ms, 3.3 Hz) for durations of 1.8, 4.8 and 7.8 s. Signal averaging of cerebral NIRS responses to 20 stimulus trains repeated every 60 s revealed that a short duration of stimulation (1.8 s) increased oxyhaemoglobin in the contralateral cortex consistent with a positive functional response, whereas longer durations of stimulation (4.8, 7.8 s) produced more variable oxyhaemoglobin responses including positive, negative and biphasic patterns of change. Mean arterial blood pressure and cerebral perfusion as monitored by laser Doppler flowmetry always showed small, but coincident increases following median nerve stimulation regardless of the type of response detected by the NIRS in the contralateral cortex. Hypercapnia significantly increased the baseline total haemoglobin and deoxyhaemoglobin, and in 7 of 8 fetal sheep positively increased the changes in contralateral total haemoglobin and oxyhaemoglobin in response to the 7.8 s stimulus train, compared to the response recorded during normocapnia. These results show that activity‐driven changes in cerebral perfusion and oxygen delivery are present in the fetal brain, and persist even during periods of hypercapnia‐induced cerebral vasodilatation.
    December 11, 2016   doi: 10.1113/JP273163   open full text
  • Effect of maternal position on fetal behavioural state and heart rate variability in healthy late gestation pregnancy.
    Peter R. Stone, Wendy Burgess, Jordan P. R. McIntyre, Alistair J. Gunn, Christopher A. Lear, Laura Bennet, Edwin A. Mitchell, John M. D. Thompson,.
    The Journal of Physiology. December 11, 2016
    Key points Fetal behavioural state in healthy late gestation pregnancy is affected by maternal position. Fetal state 1F is more likely to occur in maternal supine or right lateral positions. Fetal state 4F is less likely to occur when the woman lies supine or semi‐recumbent. Fetal state change is more likely when the woman is supine or semi‐recumbent. Fetal heart rate variability is affected by maternal position with variability reduced in supine and semi‐recumbent positions. Abstract Fetal behavioural states (FBS) are measures of fetal wellbeing. In acute hypoxaemia, the human fetus adapts to a lower oxygen consuming state with changes in the cardiotocograph and reduced fetal activity. Recent studies of late gestation stillbirth described the importance of sleep position in the risk of intrauterine death. We designed this study to assess the effects of different maternal positions on FBS in healthy late gestation pregnancies under controlled conditions. Twenty‐nine healthy women had continuous fetal ECG recordings under standardized conditions in four randomly allocated positions, left lateral, right lateral, supine and semi‐recumbent. Two blinded observers, assigned fetal states in 5 min blocks. Measures of fetal heart rate variability were calculated from ECG beat to beat data. Compared to state 2F, state 4F was less likely to occur when women were semi‐recumbent [odds ratio (OR) = 0.11, 95% confidence interval (95% CI) 0.02, 0.55], and supine (OR = 0.27, 95% CI 0.07, 1.10). State 1F was more likely on the right (OR = 2.36, 95% CI 1.11, 5.04) or supine (OR = 4.99, 95% CI 2.41, 10.43) compared to the left. State change was more likely when the mother was semi‐recumbent (OR = 2.17, 95% CI 1.19, 3.95) or supine (OR = 2.67, 95% CI 1.46, 4.85). There was a significant association of maternal position to mean fetal heart rate. The measures of heart rate variability (SDNN and RMSSD) were reduced in both semi‐recumbent and supine positions. In healthy late gestation pregnancy, maternal position affects FBS and heart rate variability. These effects are likely fetal adaptations to positions which may produce a mild hypoxic stress.
    December 11, 2016   doi: 10.1113/JP273201   open full text
  • Acute and chronic effects of noradrenergic enhancement on transcranial direct current stimulation‐induced neuroplasticity in humans.
    Hsiao‐I. Kuo, Walter Paulus, Giorgi Batsikadze, Asif Jamil, Min‐Fang Kuo, Michael A. Nitsche.
    The Journal of Physiology. December 07, 2016
    Key points Chronic administration of the selective noradrenaline reuptake inhibitor (NRI) reboxetine (RBX) increased and prolonged the long‐term potentiation‐like plasticity induced by anodal transcranial direct current stimulation (tDCS) for over 24 h. Chronic administration of RBX converted cathodal tDCS‐induced long‐term depression‐like plasticity into facilitation for 120 min. Chronic noradrenergic activity enhancement on plasticity of the human brain might partially explain the delayed therapeutic impact of selective NRIs in depression and other neuropsychiatric diseases. Abstract Noradrenaline affects cognition and motor learning processes via its impact on long‐term potentiation (LTP) and depression (LTD). We aimed to explore the impact of single dose and chronic administration of the selective noradrenaline reuptake inhibitor (NRI) reboxetine (RBX) on plasticity induced by transcranial direct current stimulation (tDCS) in healthy humans via a double‐blinded, placebo‐controlled, randomized crossover study. Sixteen healthy volunteers received placebo or single dose RBX (8 mg) before anodal or cathodal tDCS of the primary motor cortex. Afterwards, the same subjects took RBX (8 mg day−1) consecutively for 21 days. During this period, two additional interventions were performed (RBX with anodal or cathodal tDCS), to explore the impact of chronic RBX treatment on plasticity. Plasticity was monitored by motor‐evoked potential amplitudes elicited by transcranial magnetic stimulation. Chronic administration of RBX increased and prolonged the LTP‐like plasticity induced by anodal tDCS for over 24 h. Chronic RBX significantly converted cathodal tDCS‐induced LTD‐like plasticity into facilitation, as compared to the single dose condition, for 120 min after stimulation. The results show a prominent impact of chronic noradrenergic enhancement on plasticity of the human brain that might partially explain the delayed therapeutic impact of selective NRIs in depression and other neuropsychiatric diseases.
    December 07, 2016   doi: 10.1113/JP273137   open full text
  • Distinct subcellular mechanisms for the enhancement of the surface membrane expression of SK2 channel by its interacting proteins, α‐actinin2 and filamin A.
    Zheng Zhang, Hannah A. Ledford, Seojin Park, Wenying Wang, Sassan Rafizadeh, Hyo Jeong Kim, Wilson Xu, Ling Lu, Victor C. Lau, Anne A. Knowlton, Xiao‐Dong Zhang, Ebenezer N. Yamoah, Nipavan Chiamvimonvat.
    The Journal of Physiology. December 07, 2016
    Key points Ion channels are transmembrane proteins that are synthesized within the cells but need to be trafficked to the cell membrane for the channels to function. Small‐conductance, Ca2+‐activated K+ channels (SK, KCa2) are unique subclasses of K+ channels that are regulated by Ca2+ inside the cells; they are expressed in human atrial myocytes and responsible for shaping atrial action potentials. We have previously shown that interacting proteins of SK2 channels are important for channel trafficking to the membrane. Using total internal reflection fluorescence (TIRF) and confocal microscopy, we studied the mechanisms by which the surface membrane localization of SK2 (KCa2.2) channels is regulated by their interacting proteins. Understanding the mechanisms of SK channel trafficking may provide new insights into the regulation controlling the repolarization of atrial myocytes. Abstract The normal function of ion channels depends critically on the precise subcellular localization and the number of channel proteins on the cell surface membrane. Small‐conductance, Ca2+‐activated K+ channels (SK, KCa2) are expressed in human atrial myocytes and are responsible for shaping atrial action potentials. Understanding the mechanisms of SK channel trafficking may provide new insights into the regulation controlling the repolarization of atrial myocytes. We have previously demonstrated that the C‐ and N‐termini of SK2 channels interact with the actin‐binding proteins α‐actinin2 and filamin A, respectively. However, the roles of the interacting proteins on SK2 channel trafficking remain incompletely understood. Using total internal reflection fluorescence (TIRF) microscopy, we studied the mechanisms of surface membrane localization of SK2 (KCa2.2) channels. When SK2 channels were co‐expressed with filamin A or α‐actinin2, the membrane fluorescence intensity of SK2 channels increased significantly. We next tested the effects of primaquine and dynasore on SK2 channels expression. Treatment with primaquine significantly reduced the membrane expression of SK2 channels. In contrast, treatment with dynasore failed to alter the surface membrane expression of SK2 channels. Further investigations using constitutively active or dominant‐negative forms of Rab GTPases provided additional insights into the distinct roles of the two cytoskeletal proteins on the recycling processes of SK2 channels from endosomes. α‐Actinin2 facilitated recycling of SK2 channels from both early and recycling endosomes while filamin A probably aids the recycling of SK2 channels from recycling endosomes.
    December 07, 2016   doi: 10.1113/JP272942   open full text
  • Effects of postural change from supine to head‐up tilt on the skin sympathetic nerve activity component synchronised with the cardiac cycle in warmed men.
    Yu Ogawa, Yoshi‐ichiro Kamijo, Shigeki Ikegawa, Shizue Masuki, Hiroshi Nose.
    The Journal of Physiology. December 07, 2016
    Key points Humans are unique in controlling body temperature in a hot environment by a large amount of skin blood flow; however, the decrease in total peripheral resistance due to systemic cutaneous vasodilatation and the reduction of venous return to the heart due to blood pooling in the cutaneous vein threatens blood pressure maintenance in the upright position, and occasionally causes heat syncope. Against this condition, cutaneous vasodilatation is reportedly suppressed to maintain arterial pressure; however, the nerve activity responsible for this phenomenon has not been identified. In the present study, we found that the skin sympathetic nerve activity component that was synchronised with the cardiac cycle increased in hyperthermia, but the increase was suppressed when the posture was changed from supine to head‐up tilt. The profile of the component agreed with that of cutaneous vasodilatation. Thus, the component might contribute to the prevention of heat syncope in humans. Abstract In humans, the cutaneous vasodilatation response to hyperthermia has been suggested to be suppressed by baroreflexes to maintain arterial pressure when the posture is changed from supine to upright, and if the reflexes do not function sufficiently, it can cause heat syncope. However, the efferent signals of the reflexes have not been identified. To identify the signals, we continuously measured skin sympathetic nerve activity (SSNA; microneurography), right atrial volume (RAV; echocardiography, the baroreceptors for the reflexes are reportedly located in the right atrium), cutaneous vascular conductance on the chest (CVCchest; laser Doppler flowmetry), and oesophageal temperature (Toes; thermocouple) in young men before and after passive warming with a perfusion suit, during which periods the posture was changed from supine to 30 deg head‐up tilt positions. During these periods, we also simultaneously measured muscle sympathetic nerve activity (MSNA) to distinguish the SSNA from MSNA. We found that an increase in Toes by ∼0.7°C (P < 0.0001) increased the total SSNA (P < 0.005); however, the head‐up tilt in hyperthermia did not change the total SSNA (P > 0.26) although an increase in CVCchest (P < 0.019) was suppressed and RAV was reduced (P < 0.008). In contrast, the SSNA component synchronised with the cardiac cycle increased in hyperthermia (P < 0.015), but decreased with the postural change (P < 0.017). The SSNA component during the postural change before and after warming was highly correlated with the CVCchest (r = 0.817, P < 0.0001), but the MSNA component was not (r = 0.359, P = 0.085). Thus, the SSNA component synchronised with the cardiac cycle appeared to be involved in suppressing cutaneous vasodilatation during postural changes.
    December 07, 2016   doi: 10.1113/JP273281   open full text
  • Store‐operated interactions between plasmalemmal STIM1 and TRPC1 proteins stimulate PLCβ1 to induce TRPC1 channel activation in vascular smooth muscle cells.
    Jian Shi, Francesc Miralles, Lutz Birnbaumer, William A. Large, Anthony P. Albert.
    The Journal of Physiology. December 07, 2016
    Key points Depletion of Ca2+ stores activates store‐operated channels (SOCs), which mediate Ca2+ entry pathways that regulate cellular processes such as contraction, proliferation and gene expression. In vascular smooth muscle cells (VSMCs), stimulation of SOCs composed of canonical transient receptor potential channel 1 (TRPC1) proteins requires G protein α q subunit (Gαq)/phospholipase C (PLC)β1/protein kinase C (PKC) activity. We studied the role of stromal interaction molecule 1 (STIM1) in coupling store depletion to this activation pathway using patch clamp recording, GFP‐PLCδ1‐PH imaging and co‐localization techniques. Store‐operated TRPC1 channel and PLCβ1 activities were inhibited by STIM1 short hairpin RNA (shRNA) and absent in TRPC1−/− cells, and store‐operated PKC phosphorylation of TRPC1 was inhibited by STIM1 shRNA. Store depletion induced interactions between STIM1 and TRPC1, Gαq and PLCβ1, which required STIM1 and TRPC1. Similar effects were produced with noradrenaline. These findings identify a new activation mechanism of TRPC1‐based SOCs in VSMCs, and a novel role for STIM1, where store‐operated STIM1‐TRPC1 interactions stimulate Gαq/PLCβ1/PKC activity to induce channel gating. Abstract In vascular smooth muscle cells (VSMCs), stimulation of canonical transient receptor potential channel 1 (TRPC1) protein‐based store‐operated channels (SOCs) mediates Ca2+ entry pathways that regulate contractility, proliferation and migration. It is therefore important to understand how these channels are activated. Studies have shown that stimulation of TRPC1‐based SOCs requires G protein α q subunit (Gαq)/phospholipase C (PLC)β1 activities and protein kinase C (PKC) phosphorylation, although it is unclear how store depletion stimulates this gating pathway. The present study examines this issue by focusing on the role of stromal interaction molecule 1 (STIM1), an endo/sarcoplasmic reticulum Ca2+ sensor. Store‐operated TRPC1 channel activity was inhibited by TRPC1 and STIM1 antibodies and STIM1 short hairpin RNA (shRNA) in wild‐type VSMCs, and was absent in TRPC1−/− VSMCs. Store‐operated PKC phosphorylation of TRPC1 was reduced by knockdown of STIM1. Moreover, store‐operated PLCβ1 activity measured with the fluorescent phosphatidylinositol 4,5‐bisphosphate/inositol 1,4,5‐trisphosphate biosensor GFP‐PLCδ1‐PH was reduced by STIM1 shRNA and absent in TRPC1−/− cells. Immunocytochemistry, co‐immunoprecipitation and proximity ligation assays revealed that store depletion activated STIM1 translocation from within the cell to the plasma membrane (PM) where it formed STIM1‐TRPC1 complexes, which then associated with Gαq and PLCβ1. Noradrenaline also evoked TRPC1 channel activity and associations between TRPC1, STIM1, Gαq and PLCβ1, which were inhibited by STIM1 knockdown. Effects of N‐terminal and C‐terminal STIM1 antibodies on TRPC1‐based SOCs and STIM1 staining suggest that channel activation may involve insertion of STIM1 into the PM. The findings of the present study identify a new activation mechanism of TRPC1‐based SOCs in VSMCs, and a novel role for STIM1, in which store‐operated STIM1‐TRPC1 interactions stimulate PLCβ1 activity to induce PKC phosphorylation of TRPC1 and channel gating.
    December 07, 2016   doi: 10.1113/JP273302   open full text
  • N‐Acetylcysteine, a glutathione precursor, reverts vascular dysfunction and endothelial epigenetic programming in intrauterine growth restricted guinea pigs.
    Emilio A. Herrera, Francisca Cifuentes‐Zúñiga, Esteban Figueroa, Cristian Villanueva, Cherie Hernández, René Alegría, Viviana Arroyo‐Jousse, Estefania Peñaloza, Marcelo Farías, Ricardo Uauy, Paola Casanello, Bernardo J. Krause.
    The Journal of Physiology. December 04, 2016
    Key points Intrauterine growth restriction (IUGR) is associated with vascular dysfunction, oxidative stress and signs of endothelial epigenetic programming of the umbilical vessels. There is no evidence that this epigenetic programming is occurring on systemic fetal arteries. In IUGR guinea pigs we studied the functional and epigenetic programming of endothelial nitric oxide synthase (eNOS) (Nos3 gene) in umbilical and systemic fetal arteries, addressing the role of oxidative stress in this process by maternal treatment with N‐acetylcysteine (NAC) during the second half of gestation. The present study suggests that IUGR endothelial cells have common molecular markers of programming in umbilical and systemic arteries. Notably, maternal treatment with NAC restores fetal growth by increasing placental efficiency and reverting the functional and epigenetic programming of eNOS in arterial endothelium in IUGR guinea pigs. Abstract In humans, intrauterine growth restriction (IUGR) is associated with vascular dysfunction, oxidative stress and signs of endothelial programming in umbilical vessels. We aimed to determine the effects of maternal antioxidant treatment with N‐acetylcysteine (NAC) on fetal endothelial function and endothelial nitric oxide synthase (eNOS) programming in IUGR guinea pigs. IUGR was induced by implanting ameroid constrictors on uterine arteries of pregnant guinea pigs at mid gestation, half of the sows receiving NAC in the drinking water (from day 34 until term). Fetal biometry and placental vascular resistance were followed by ultrasound throughout gestation. At term, umbilical arteries and fetal aortae were isolated to assess endothelial function by wire‐myography. Primary cultures of endothelial cells (ECs) from fetal aorta, femoral and umbilical arteries were used to determine eNOS mRNA levels by quantitative PCR and analyse DNA methylation in the Nos3 promoter by pyrosequencing. Doppler ultrasound measurements showed that NAC reduced placental vascular resistance in IUGR (P < 0.05) and recovered fetal weight (P < 0.05), increasing fetal‐to‐placental ratio at term (∼40%) (P < 0.001). In IUGR, NAC treatment restored eNOS‐dependent relaxation in aorta and umbilical arteries (P < 0.05), normalizing eNOS mRNA levels in EC fetal and umbilical arteries (P < 0.05). IUGR‐derived ECs had a decreased DNA methylation (∼30%) at CpG −170 (from the transcription start site) and this epigenetic signature was absent in NAC‐treated fetuses (P < 0.001). These data show that IUGR‐ECs have common molecular markers of eNOS programming in umbilical and systemic arteries and this effect is prevented by maternal treatment with antioxidants.
    December 04, 2016   doi: 10.1113/JP273396   open full text
  • Molecular mechanisms of Slo2 K+ channel closure.
    M. Hunter Giese, Alison Gardner, Angela Hansen, Michael C. Sanguinetti.
    The Journal of Physiology. December 02, 2016
    Key points Intracellular Na+‐activated Slo2 potassium channels are in a closed state under normal physiological conditions, although their mechanisms of ion permeation gating are not well understood. A cryo‐electron microscopy structure of Slo2.2 suggests that the ion permeation pathway of these channels is closed by a single constriction of the inner pore formed by the criss‐crossing of the cytoplasmic ends of the S6 segments (the S6 bundle crossing) at a conserved Met residue. Functional characterization of mutant Slo2 channels suggests that hydrophobic interactions between Leu residues in the upper region of the S6 segments contribute to stabilizing the inner pore in a non‐conducting state. Mutation of the conserved Met residues in the S6 segments to the negatively‐charged Glu did not induce constitutive opening of Slo2.1 or Slo2.2, suggesting that ion permeation of Slo2 channels is not predominantly gated by the S6 bundle crossing. Abstract Large conductance K+‐selective Slo2 channels are in a closed state unless activated by elevated [Na+]i. Our previous studies suggested that the pore helix/selectivity filter serves as the activation gate in Slo2 channels. In the present study, we evaluated two other potential mechanisms for stabilization of Slo2 channels in a closed state: (1) dewetting and collapse of the inner pore (hydrophobic gating) and (2) constriction of the inner pore by tight criss‐crossing of the cytoplasmic ends of the S6 α‐helical segments. Slo2 channels contain two conserved Leu residues in each of the four S6 segments that line the inner pore region nearest the bottom of the selectivity filter. To evaluate the potential role of these residues in hydrophobic gating, Leu267 and Leu270 in human Slo2.1 were each replaced by 15 different residues. The relative conductance of mutant channels was highly dependent on hydrophilicity and volume of the amino acid substituted for Leu267 and was maximal with L267H. Consistent with their combined role in hydrophobic gating, replacement of both Leu residues with the isosteric but polar residue Asn (L267N/L270N) stabilized channels in a fully open state. In a recent cryo‐electron microscopy structure of chicken Slo2.2, the ion permeation pathway of the channel is closed by a constriction of the inner pore formed by criss‐crossing of the S6 segments at a conserved Met. Inconsistent with the S6 segment crossing forming the activation gate, replacement of the homologous Met residues in human Slo2.1 or Slo2.2 with the negatively‐charged Glu did not induce constitutive channel opening.
    December 02, 2016   doi: 10.1113/JP273225   open full text
  • Synaptic vesicle pool‐specific modification of neurotransmitter release by intravesicular free radical generation.
    Olusoji A. T. Afuwape, Catherine R. Wasser, Thomas Schikorski, Ege T. Kavalali.
    The Journal of Physiology. December 02, 2016
    Key points Synaptic transmission is mediated by the release of neurotransmitters from synaptic vesicles in response to stimulation or through the spontaneous fusion of a synaptic vesicle with the presynaptic plasma membrane. There is growing evidence that synaptic vesicles undergoing spontaneous fusion versus those fusing in response to stimuli are functionally distinct. In this study, we acutely probe the effects of intravesicular free radical generation on synaptic vesicles that fuse spontaneously or in response to stimuli. By targeting vesicles that preferentially release spontaneously, we can dissociate the effects of intravesicular free radical generation on spontaneous neurotransmission from evoked neurotransmission and vice versa. Taken together, these results further advance our knowledge of the synapse and the nature of the different synaptic vesicle pools mediating neurotransmission. Abstract Earlier studies suggest that spontaneous and evoked neurotransmitter release processes are maintained by synaptic vesicles which are segregated into functionally distinct pools. However, direct interrogation of the link between this putative synaptic vesicle pool heterogeneity and neurotransmission has been difficult. To examine this link, we tagged vesicles with horseradish peroxidase (HRP) – a haem‐containing plant enzyme – or antibodies against synaptotagmin‐1 (syt1). Filling recycling vesicles in hippocampal neurons with HRP and subsequent treatment with hydrogen peroxide (H2O2) modified the properties of neurotransmitter release depending on the route of HRP uptake. While strong depolarization‐induced uptake of HRP suppressed evoked release and augmented spontaneous release, HRP uptake during mild activity selectively impaired evoked release, whereas HRP uptake at rest solely potentiated spontaneous release. Expression of a luminal HRP‐tagged syt1 construct and subsequent H2O2 application resulted in a similar increase in spontaneous release and suppression as well as desynchronization of evoked release, recapitulating the canonical syt1 loss‐of‐function phenotype. An antibody targeting the luminal domain of syt1, on the other hand, showed that augmentation of spontaneous release and suppression of evoked release phenotypes are dissociable depending on whether the antibody uptake occurred at rest or during depolarization. Taken together, these findings indicate that vesicles that maintain spontaneous and evoked neurotransmitter release preserve their identity during recycling and syt1 function in suppression of spontaneous neurotransmission can be acutely dissociated from syt1 function to synchronize synaptic vesicle exocytosis upon stimulation.
    December 02, 2016   doi: 10.1113/JP273115   open full text
  • Synaptic reliability and temporal precision are achieved via high quantal content and effective replenishment: auditory brainstem versus hippocampus.
    Elisa G Krächan, Alexander U Fischer, Jürgen Franke, Eckhard Friauf.
    The Journal of Physiology. December 02, 2016
    Key points Auditory brainstem neurons involved in sound source localization are equipped with several morphological and molecular features that enable them to compute interaural level and time differences. As sound source localization works continually, synaptic transmission between these neurons should be reliable and temporally precise, even during sustained periods of high‐frequency activity. Using patch‐clamp recordings in acute brain slices, we compared synaptic reliability and temporal precision in the seconds–minute range between auditory and two types of hippocampal synapses; the latter are less confronted with temporally precise high‐frequency transmission than the auditory ones. We found striking differences in synaptic properties (e.g. continually high quantal content) that allow auditory synapses to reliably release vesicles at much higher rate than their hippocampal counterparts. Thus, they are indefatigable and also in a position to transfer information with exquisite temporal precision and their performance appears to be supported by very efficient replenishment mechanisms. Abstract At early stations of the auditory pathway, information is encoded by precise signal timing and rate. Auditory synapses must maintain the relative timing of events with submillisecond precision even during sustained and high‐frequency stimulation. In non‐auditory brain regions, e.g. telencephalic ones, synapses are activated at considerably lower frequencies. Central to understanding the heterogeneity of synaptic systems is the elucidation of the physical, chemical and biological factors that determine synapse performance. In this study, we used slice recordings from three synapse types in the mouse auditory brainstem and hippocampus. Whereas the auditory brainstem nuclei experience high‐frequency activity in vivo, the hippocampal circuits are activated at much lower frequencies. We challenged the synapses with sustained high‐frequency stimulation (up to 200 Hz for 60 s) and found significant performance differences. Our results show that auditory brainstem synapses differ considerably from their hippocampal counterparts in several aspects, namely resistance to synaptic fatigue, low failure rate and exquisite temporal precision. Their high‐fidelity performance supports the functional demands and appears to be due to the large size of the readily releasable pool and a high release probability, which together result in a high quantal content. In conjunction with very efficient vesicle replenishment mechanisms, these properties provide extremely rapid and temporally precise signalling required for neuronal communication at early stations of the auditory system, even during sustained activation in the minute range.
    December 02, 2016   doi: 10.1113/JP272799   open full text
  • TRPV4 participates in pressure‐induced inhibition of renin secretion by juxtaglomerular cells.
    François Seghers, Xavier Yerna, Nadège Zanou, Olivier Devuyst, Rudi Vennekens, Bernd Nilius, Philippe Gailly.
    The Journal of Physiology. December 02, 2016
    Key points Increase in blood pressure in the renal afferent arteriole is known to induce an increase in cytosolic calcium concentration ([Ca2+]i) of juxtaglomerular (JG) cells and to result in a decreased secretion of renin. Mechanical stimulation of As4.1 JG cells induces an increase in [Ca2+]i that is inhibited by HC067047 and RN1734, two inhibitors of TRPV4, or by siRNA‐mediated repression of TRPV4. Inhibition of TRPV4 impairs pressure‐induced decrease in renin secretion. Compared to wild‐type mice, Trpv4−/− mice present increased resting plasma levels of renin and aldosterone and present a significantly altered pressure–renin relationship. We suggest that TRPV4 channel participates in mechanosensation at the juxtaglomerular apparatus. Abstract The renin–angiotensin system is a crucial blood pressure regulation system. It consists of a hormonal cascade where the rate‐limiting enzyme is renin, which is secreted into the blood flow by renal juxtaglomerular (JG) cells in response to low pressure in the renal afferent arteriole. In contrast, an increase in blood pressure results in a decreased renin secretion. This is accompanied by a transitory increase in [Ca2+]i of JG cells. The inverse relationship between [Ca2+]i and renin secretion has been called the ‘calcium paradox’ of renin release. How increased pressure induces a [Ca2+]i transient in JG cells, is however, unknown. We observed that [Ca2+]i transients induced by mechanical stimuli in JG As4.1 cells were completely abolished by HC067047 and RN1734, two inhibitors of TRPV4. They were also reduced by half by siRNA‐mediated repression of TRPV4 but not after repression or inhibition of TRPV2 or Piezo1 ion channels. Interestingly, the stimulation of renin secretion by the adenylate cyclase activator forskolin was totally inhibited by cyclic stretching of the cells. This effect was mimicked by stimulation with GSK1016790A and 4αPDD, two activators of TRPV4 and inhibited in the presence of HC067047. Moreover, in isolated perfused kidneys from Trpv4−/− mice, the pressure–renin relationship was significantly altered. In vivo, Trpv4−/− mice presented increased plasma levels of renin and aldosterone compared to wild‐type mice. Altogether, our results suggest that TRPV4 is involved in the pressure‐induced entry of Ca2+ in JG cells, which inhibits renin release and allows the negative feedback regulation on blood pressure.
    December 02, 2016   doi: 10.1113/JP273595   open full text
  • Effects of continuous positive airway pressure and isocapnic‐hypoxia on cerebral autoregulation in patients with obstructive sleep apnoea.
    Xavier Waltz, Andrew E. Beaudin, Patrick J. Hanly, Georgios D. Mitsis, Marc J. Poulin.
    The Journal of Physiology. December 01, 2016
    Key points Altered cerebral autoregulation (CA) in obstructive sleep apnoea (OSA) patients may contribute to increased stroke risk in this population; the gold standard treatment for OSA is continuous positive airway pressure, which improves cerebrovascular regulation and may decrease the risk of stroke. Isocapnic‐hypoxia impairs CA in healthy subjects, but it remains unknown in OSA whether impaired CA is further exacerbated by isocapnic‐hypoxia and whether it is improved by treatment with continuous positive airway pressure. During normoxia, CA was altered in the more severe but not in the less severe OSA patients, while, in contrast, during isocapnic‐hypoxia, CA was similar between groups and tended to improve in patients with more severe OSA compared to normoxia. From a clinical perspective, one month of continuous positive airway pressure treatment does not improve CA. From a physiological perspective, this study suggests that sympathetic overactivity may be responsible for altered CA in the more severe OSA patients. Abstract Cerebral autoregulation (CA) impairment may contribute to the increased risk of stroke associated with obstructive sleep apnoea (OSA). It is unknown if impaired CA is further exacerbated by isocapnic‐hypoxia and whether it is improved by treatment of OSA with continuous positive airway pressure (CPAP). CA was assessed during wakefulness in 53 OSA patients (50.3 ± 9.3 years) and 21 controls (49.8 ± 8.6 years) at baseline and following a minimum of 1 month of effective CPAP therapy (OSA patients, n = 40). Control participants (n = 21) performed a follow‐up visit to control for time effects within OSA patients between baseline and the post‐CPAP visit. Beat‐by‐beat middle cerebral artery blood flow velocity and mean arterial blood pressure (MBP), and breath‐by‐breath end‐tidal partial pressure of CO2 (P ET ,CO2) were monitored. CA was determined during normoxia and isocapnic‐hypoxia using transfer function (phase and gain) and coherence analysis (including multiple and partial coherence (using MBP and P ET ,CO2 as inputs)) in the very low frequency range (0.03–0.07 Hz). OSA patients were divided into two subgroups (less severe and more severe) based upon the median respiratory disturbance index (RDI). During normoxia, the more severe OSA patients (RDI 45.9 ± 10.3) exhibited altered CA compared to controls and the less severe OSA patients (RDI 24.5 ± 5.9). In contrast, during isocapnic‐hypoxia, CA was similar between groups. CPAP had no effect on CA. In conclusion, CA is altered in the more severe OSA patients during normoxia but not during isocapnic‐hypoxia and CPAP treatment does not impact CA.
    December 01, 2016   doi: 10.1113/JP272967   open full text
  • Magnetic resonance imaging biomarkers of exercise‐induced improvement of oxidative stress and inflammation in the brain of old high‐fat‐fed ApoE−/− mice.
    Erica N. Chirico, Vanessa Di Cataldo, Fabien Chauveau, Alain Geloën, David Patsouris, Benoît Thézé, Cyril Martin, Hubert Vidal, Jennifer Rieusset, Vincent Pialoux, Emmanuelle Canet‐Soulas.
    The Journal of Physiology. December 01, 2016
    Key points Vascular brain lesions and atherosclerosis are two similar conditions that are characterized by increased inflammation and oxidative stress. Non‐invasive imaging in a murine model of atherosclerosis showed vascular brain damage and peripheral inflammation. In this study, exercise training reduced magnetic resonance imaging‐detected abnormalities, insulin resistance and markers of oxidative stress and inflammation in old ApoE−/− mice. Our results demonstrate the protective effect of exercise on neurovascular damage in the ageing brain of ApoE−/− mice. Abstract Vascular brain lesions, present in advanced atherosclerosis, share pathological hallmarks with peripheral vascular lesions, such as increased inflammation and oxidative stress. Physical activity reduces these peripheral risk factors, but its cerebrovascular effect is less documented, especially by non‐invasive imaging. Through a combination of in vivo and post‐mortem techniques, we aimed to characterize vascular brain damage in old ApoE−/− mice fed a high‐cholesterol (HC) diet with dietary controlled intake. We then sought to determine the beneficial effects of exercise training on oxidative stress and inflammation in the brain as a treatment option in an ageing atherosclerosis mouse model. Using in vivo magnetic resonance imaging (MRI) and biological markers of oxidative stress and inflammation, we evaluated the occurrence of vascular abnormalities in the brain of HC‐diet fed ApoE−/− mice >70 weeks old, its association with local and systemic oxidative stress and inflammation, and whether both can be modulated by exercise. Exercise training significantly reduced both MRI‐detected abnormalities (present in 71% of untrained vs. 14% of trained mice) and oxidative stress (lipid peroxidation, 9.1 ± 1.4 vs. 5.2 ± 0.9 μmol mg−1; P < 0.01) and inflammation (interleukin‐1β, 226.8 ± 27.1 vs. 182.5 ± 21.5 pg mg−1; P < 0.05) in the brain, and the mortality rate. Exercise also decreased peripheral insulin resistance, oxidative stress and inflammation, but significant associations were seen only within brain markers. Highly localized vascular brain damage is a frequent finding in this ageing atherosclerosis model, and exercise is able to reduce this outcome and improve lifespan. In vivo MRI evaluated both the neurovascular damage and the protective effect of exercise.
    December 01, 2016   doi: 10.1113/JP271903   open full text
  • Peroxisome proliferator‐activated receptor‐γ coactivator 1 α1 induces a cardiac excitation–contraction coupling phenotype without metabolic remodelling.
    Maija Mutikainen, Tomi Tuomainen, Nikolay Naumenko, Jenni Huusko, Boris Smirin, Svetlana Laidinen, Krista Kokki, Heidi Hynynen, Seppo Ylä‐Herttuala, Merja Heinäniemi, Jorge L. Ruas, Pasi Tavi.
    The Journal of Physiology. December 01, 2016
    Key points Transcriptional co‐activator PGC‐1α1 has been shown to regulate energy metabolism and to mediate metabolic adaptations in pathological and physiological cardiac hypertrophy but other functional implications of PGC‐1α1 expression are not known. Transgenic PGC‐1α1 overexpression within the physiological range in mouse heart induces purposive changes in contractile properties, electrophysiology and calcium signalling but does not induce substantial metabolic remodelling. The phenotype of the PGC‐1α1 transgenic mouse heart recapitulates most of the functional modifications usually associated with the exercise‐induced heart phenotype, but does not protect the heart against load‐induced pathological hypertrophy. Transcriptional effects of PGC‐1α1 show clear dose‐dependence with diverse changes in genes in circadian clock, heat shock, excitability, calcium signalling and contraction pathways at low overexpression levels, while metabolic genes are recruited at much higher PGC‐1α1 expression levels. These results imply that the physiological role of PGC‐1α1 is to promote a beneficial excitation–contraction coupling phenotype in the heart. Abstract The transcriptional coactivator PGC‐1α1 has been identified as a central factor mediating metabolic adaptations of the heart. However, to what extent physiological changes in PGC‐1α1 expression levels actually contribute to the functional adaptation of the heart is still mostly unresolved. The aim of this study was to characterize the transcriptional and functional effects of physiologically relevant, moderate PGC‐1α1 expression in the heart. In vivo and ex vivo physiological analysis shows that expression of PGC‐1α1 within a physiological range in mouse heart does not induce the expected metabolic alterations, but instead induces a unique excitation–contraction (EC) coupling phenotype recapitulating features typically seen in physiological hypertrophy. Transcriptional screening of PGC‐1α1 overexpressing mouse heart and myocyte cultures with higher, acute adenovirus‐induced PGC‐1α1 expression, highlights PGC‐1α1 as a transcriptional coactivator with a number of binding partners in various pathways (such as heat shock factors and the circadian clock) through which it acts as a pleiotropic transcriptional regulator in the heart, to both augment and repress the expression of its target genes in a dose‐dependent fashion. At low levels of overexpression PGC‐1α1 elicits a diverse transcriptional response altering the expression state of circadian clock, heat shock, excitability, calcium signalling and contraction pathways, while metabolic targets of PGC‐1α1 are recruited at higher PGC‐1α1 expression levels. Together these findings demonstrate that PGC‐1α1 elicits a dual effect on cardiac transcription and phenotype. Further, our results imply that the physiological role of PGC‐1α1 is to promote a beneficial EC coupling phenotype in the heart.
    December 01, 2016   doi: 10.1113/JP272847   open full text
  • Active integration of glutamatergic input to the inferior olive generates bidirectional postsynaptic potentials.
    Derek L. F. Garden, Arianna Rinaldi, Matthew F. Nolan.
    The Journal of Physiology. November 29, 2016
    Key points We establish experimental preparations for optogenetic investigation of glutamatergic input to the inferior olive. Neurones in the principal olivary nucleus receive monosynaptic extra‐somatic glutamatergic input from the neocortex. Glutamatergic inputs to neurones in the inferior olive generate bidirectional postsynaptic potentials (PSPs), with a fast excitatory component followed by a slower inhibitory component. Small conductance calcium‐activated potassium (SK) channels are required for the slow inhibitory component of glutamatergic PSPs and oppose temporal summation of inputs at intervals ≤ 20 ms. Active integration of synaptic input within the inferior olive may play a central role in control of olivo‐cerebellar climbing fibre signals. Abstract The inferior olive plays a critical role in motor coordination and learning by integrating diverse afferent signals to generate climbing fibre inputs to the cerebellar cortex. While it is well established that climbing fibre signals are important for motor coordination, the mechanisms by which neurones in the inferior olive integrate synaptic inputs and the roles of particular ion channels are unclear. Here, we test the hypothesis that neurones in the inferior olive actively integrate glutamatergic synaptic inputs. We demonstrate that optogenetically activated long‐range synaptic inputs to the inferior olive, including projections from the motor cortex, generate rapid excitatory potentials followed by slower inhibitory potentials. Synaptic projections from the motor cortex preferentially target the principal olivary nucleus. We show that inhibitory and excitatory components of the bidirectional synaptic potentials are dependent upon AMPA (GluA) receptors, are GABAA independent, and originate from the same presynaptic axons. Consistent with models that predict active integration of synaptic inputs by inferior olive neurones, we find that the inhibitory component is reduced by blocking large conductance calcium‐activated potassium channels with iberiotoxin, and is abolished by blocking small conductance calcium‐activated potassium channels with apamin. Summation of excitatory components of synaptic responses to inputs at intervals ≤ 20 ms is increased by apamin, suggesting a role for the inhibitory component of glutamatergic responses in temporal integration. Our results indicate that neurones in the inferior olive implement novel rules for synaptic integration and suggest new principles for the contribution of inferior olive neurones to coordinated motor behaviours.
    November 29, 2016   doi: 10.1113/JP273424   open full text
  • Rate of rise in diastolic blood pressure influences vascular sympathetic response to mental stress.
    Khadigeh El Sayed, Vaughan G. Macefield, Sarah L. Hissen, Michael J. Joyner, Chloe E. Taylor.
    The Journal of Physiology. November 29, 2016
    Key points Research indicates that individuals may experience a rise (positive responders) or fall (negative responders) in muscle sympathetic nerve activity (MSNA) during mental stress. In this study, we examined the early blood pressure responses (including the peak, time of peak and rate of rise in blood pressure) to mental stress in positive and negative responders. Negative MSNA responders to mental stress exhibit a more rapid rise in diastolic pressure at the onset of the stressor, suggesting a baroreflex‐mediated suppression of MSNA. In positive responders there is a more sluggish rise in blood pressure during mental stress, which appears to be MSNA‐driven. This study suggests that whether MSNA has a role in the pressor response is dependent upon the reactivity of blood pressure early in the task. Abstract Research indicates that individuals may experience a rise (positive responders) or fall (negative responders) in muscle sympathetic nerve activity (MSNA) during mental stress. The aim was to examine the early blood pressure response to stress in positive and negative responders and thus its influence on the direction of change in MSNA. Blood pressure and MSNA were recorded continuously in 21 healthy young males during 2 min mental stressors (mental arithmetic, Stroop test) and physical stressors (cold pressor, handgrip exercise, post‐exercise ischaemia). Participants were classified as negative or positive responders according to the direction of the mean change in MSNA during the stressor tasks. The peak changes, time of peak and rate of changes in blood pressure were compared between groups. During mental arithmetic negative responders experienced a significantly greater rate of rise in diastolic blood pressure in the first minute of the task (1.3 ± 0.5 mmHg s−1) compared with positive responders (0.4 ± 0.1 mmHg s−1; P = 0.03). Similar results were found for the Stroop test. Physical tasks elicited robust parallel increases in blood pressure and MSNA across participants. It is concluded that negative MSNA responders to mental stress exhibit a more rapid rise in diastolic pressure at the onset of the stressor, suggesting a baroreflex‐mediated suppression of MSNA. In positive responders there is a more sluggish rise in blood pressure during mental stress, which appears to be MSNA‐driven. This study suggests that whether MSNA has a role in the pressor response is dependent upon the reactivity of blood pressure early in the task.
    November 29, 2016   doi: 10.1113/JP272963   open full text
  • Role of dynamin‐related protein 1‐mediated mitochondrial fission in resistance of mouse C2C12 myoblasts to heat injury.
    Tianzheng Yu, Patricia Deuster, Yifan Chen.
    The Journal of Physiology. November 29, 2016
    Key points Understanding how skeletal muscles respond to high temperatures may help develop strategies for improving exercise tolerance and preventing heat injury. Mitochondria regulate cell survival by constantly changing their morphology through fusion and fission in response to environmental stimuli. Little is known about the involvement of mitochondrial dynamics in tolerance of skeletal muscle against heat stress. Mild heat acclimation and moderate heat shock appear to have different effects on the mitochondrial morphology and fission protein Drp1 in skeletal muscle cells. Mitochondrial integrity plays a key role in cell survival under heat stress. Abstract The regulation of mitochondrial morphology is closely coupled to cell survival during stress. We examined changes in the mitochondrial morphology of mouse C2C12 skeletal muscle cells in response to heat acclimation and heat shock exposure. Acclimated cells showed a greater survival rate during heat shock exposure than non‐acclimated cells, and were characterized by long interconnected mitochondria and reduced expression of dynamin‐related protein 1 (Drp1) for their mitochondrial fractions. Exposure of C2C12 muscle cells to heat shock led to apoptotic death featuring activation of caspase 3/7, release of cytochrome c and loss of cell membrane integrity. Heat shock also caused excessive mitochondrial fragmentation, loss of mitochondrial membrane potential and production of reactive oxygen species in C2C12 cells. Western blot and immunofluorescence image analysis revealed translocation of Drp1 to mitochondria from the cytosol in C2C12 cells exposed to heat shock. Mitochondrial division inhibitor 1 or Drp1 gene silencer reduced mitochondrial fragmentation and increased cell viability during exposure to heat shock. These results suggest that Drp1‐dependent mitochondrial fission may regulate susceptibility to heat‐induced apoptosis in muscle cells and that Drp1 may serve as a target for the prevention of heat‐related injury.
    November 29, 2016   doi: 10.1113/JP272885   open full text
  • Impact of vesicular glutamate leakage on synaptic transmission at the calyx of Held.
    Chihiro Takami, Kohgaku Eguchi, Tetsuya Hori, Tomoyuki Takahashi.
    The Journal of Physiology. November 29, 2016
    Key points It is controversial whether glutamate can leak out of vesicles in the nerve terminal. To address this issue, we abolished vesicular glutamate uptake by washing out presynaptic cytosolic glutamate or by blocking vacuolar ATPase activity using bafilomycin A1. In the absence of vesicular glutamate uptake, both spontaneous and nerve‐evoked EPSCs underwent a rundown, suggesting that vesicular glutamate can leak out of vesicles. However, the rundown of evoked EPSCs was caused mainly by accumulation of unfilled vesicles after exocytic release of glutamate, suggesting a minor influence of glutamate leakage on synaptic transmission. Abstract Glutamate leaks out of synaptic vesicles when the transvesicular proton gradient is dissipated in isolated vesicle preparations. In the nerve terminal, however, it is controversial whether glutamate can leak out of vesicles. To address this issue, we abolished vesicular glutamate uptake by washing out presynaptic cytosolic glutamate in whole‐cell dialysis or by blocking vacuolar ATPase using bafilomycin A1 (Baf) at the calyx of Held in mouse brainstem slices. Presynaptic glutamate washout or Baf application reduced the mean amplitude and frequency of spontaneous miniature (m)EPSCs and the mean amplitude of EPSCs evoked every 10 min. The percentage reduction of mEPSC amplitude was much less than that of EPSC amplitude or mEPSC frequency, and tended to reach a plateau. The mean amplitude of mEPSCs after glutamate washout or Baf application remained high above the detection limit, deduced from the reduction of mEPSC amplitude by the AMPA receptor blocker 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione. Membrane capacitance measurements from presynaptic terminals indicated no effect of glutamate washout on exocytosis or endocytosis of synaptic vesicles. We conclude that glutamate can leak out of vesicles unless it is continuously taken up from presynaptic cytosol. However, the magnitude of glutamate leakage was small and had only a minor effect on synaptic responses. In contrast, prominent rundowns of EPSC amplitude and mEPSC frequency observed after glutamate washout or Baf application are likely to be caused by accumulation of unfilled vesicles in presynaptic terminals retrieved after spontaneous and evoked glutamate release.
    November 29, 2016   doi: 10.1113/JP273467   open full text
  • Physiological roles of Kv2 channels in entorhinal cortex layer II stellate cells revealed by Guangxitoxin‐1E.
    Christoph Hönigsperger, Maximiliano J. Nigro, Johan F. Storm.
    The Journal of Physiology. November 13, 2016
    Key points Kv2 channels underlie delayed‐rectifier potassium currents in various neurons, although their physiological roles often remain elusive. Almost nothing is known about Kv2 channel functions in medial entorhinal cortex (mEC) neurons, which are involved in representing space, memory formation, epilepsy and dementia. Stellate cells in layer II of the