--- - |2+ Key Points Summary Concurrent 5‐HT2A (Q pathway) and 5‐HT7 (S pathway) serotonin receptor activation cancels phrenic motor facilitation due to mutual cross‐talk inhibition Spinal PKCδ or PKA inhibition restores phrenic motor facilitation with concurrent Q and S pathway activation, demonstrating a key role for these kinases in cross‐talk inhibition Spinal PKCδ inhibition enhances adenosine‐dependent severe AIH‐induced pLTF (S pathway), consistent with relief of cross‐talk inhibition. Abstract Intermittent spinal serotonin receptor activation elicits long‐lasting phrenic motor facilitation (pMF), a form of respiratory motor plasticity. When activated alone, spinal Gq protein‐coupled serotonin 2A receptors (5‐HT2A) initiate pMF by a mechanism that requires ERK‐MAP kinase signaling and new BDNF protein synthesis (Q pathway). Spinal Gs protein‐coupled serotonin 7 (5‐HT7) and adenosine 2A (A2A) receptor activation also elicits pMF, but via distinct mechanisms (S pathway) that require Akt signaling and new TrkB protein synthesis. Although studies have shown inhibitory cross‐talk interactions between these competing pathways, underlying cellular mechanisms are unknown. Hypothesis: a) concurrent 5‐HT2A and 5‐HT7 activation undermines pMF; b) protein kinase A (PKA); and c) NADPH oxidase mediate inhibitory interactions between Q (5‐HT2A) and S (5‐HT7) pathways. Selective 5‐HT2A (DOI‐hydrochloride) and 5HT7 (AS‐19) agonists were administered intrathecally at C4 (3 injections, 5‐min intervals) in anesthetized, vagotomized and ventilated male rats. With either spinal 5‐HT2A or 5‐HT7 activation alone, phrenic amplitude progressively increased (pMF). In contrast, concurrent 5‐HT2A and 5‐HT7 activation failed to elicit pMF. 5‐HT2A‐induced Q pathway was restored by inhibiting PKA activity (Rp‐8‐Br‐cAMPS). NADPH oxidase inhibition did not prevent cross‐talk inhibition. Therefore, we investigated alternative mechanisms to explain Q to S pathway inhibition. Spinal PKC inhibition with Gö6983 or PKCδ peptide inhibitor restored 5‐HT7‐induced S pathway to pMF, revealing PKCδ as the relevant isoform. Spinal PKCδ inhibition enhanced S pathway‐dependent form of pMF elicited by severe AIH. We suggest that powerful constraints between 5‐HT2A and 5‐HT7 or A2A receptor‐induced pMF are mediated by PKCδ and PKA, respectively. This article is protected by copyright. All rights reserved - 'The Journal of Physiology, Volume 0, Issue ja, -Not available-. '