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The degree of acute descending control of spinal nociception in an area of primary hyperalgesia is dependent on the peripheral domain of afferent input

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The Journal of Physiology

Published online on

Abstract

Descending controls of spinal nociceptive processing play a critical role in the development of inflammatory hyperalgesia. Acute peripheral nociceptor sensitisation drives spinal sensitisation, and activates spino‐supraspinal‐spinal loops leading to descending inhibitory and facilitatory controls of spinal neuronal activity, that further modify the extent and degree of the pain state. The afferent inputs from hairy and glabrous skin are distinct with respect to both the profile of primary afferents classes and the degree of their peripheral sensitisation. It is not known whether these differences in afferent input differentially engage descending control systems to different extents or in different ways. Injection of Complete Freund's adjuvant resulted in inflammation and swelling of hairy hindpaw skin in rats, a transient thermal hyperalgesia lasting <2 hours, and long‐lasting primary mechanical hyperalgesia (at least 7d). Much longer lasting thermal hyperalgesia was apparent in glabrous skin (1h up to >72h). In hairy skin, transient hyperalgesia was associated with sensitisation of withdrawal reflexes to thermal activation of either A‐ and C‐nociceptors. The transience of the hyperalgesia was attributable to a rapidly engaged descending inhibitory noradrenergic mechanism, which affected withdrawal responses to both A‐ and C‐nociceptor activation and this could be reversed by intrathecal administration of yohimbine (alpha‐2‐adrenoceptor antagonist). In glabrous skin, yohimbine had no effect on an equivalent thermal inflammatory hyperalgesia. We conclude that acute inflammation and peripheral nociceptor sensitisation in hind paw hairy skin, but not glabrous skin, rapidly activates a descending inhibitory noradrenergic system. This may result from differences in the engagement of descending controls systems following sensitisation of different primary afferent classes that innervate glabrous and hairy skin. This article is protected by copyright. All rights reserved