Sympathetic Modulation of Activity in Aδ- and C-Primary Nociceptive Afferents After Intradermal Injection of Capsaicin in Rats

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Abstract
Neuropathic and inflammatory pain can be modulated by the sympathetic nervous system. In some pain models, sympathetic postganglionic efferents are involved in the modulation of nociceptive transmission in the periphery. The purpose of this study is to examine the sensitization of Aδ- and C-primary afferent nociceptors induced by intradermal injection of capsaicin (CAP) to see whether the presence of sympathetic efferents is essential for the sensitization. Single primary afferent discharges were recorded from the tibial nerve after the fiber types were identified by conduction velocity in anesthetized rats. An enhanced response of some Aδ- and most C-primary afferent fibers to mechanical stimuli was seen in sham-sympathectomized rats after CAP (1%, 15 μl) injection, but the enhanced responses of both Aδ- and C-fibers were reduced after sympathetic postganglionic efferents were removed. Peripheral pretreatment with norepinephrine by intraarterial injection could restore and prolong the CAP-induced enhancement of responses under sympathectomized conditions. In sympathetically intact rats, pretreatment with an α1-adrenergic receptor antagonist (terazosin) blocked completely the enhanced responses of C-fibers after CAP injection in sympathetically intact rats without significantly affecting the enhanced responses of Aδ-fibers. In contrast, a blockade of α2-adrenergic receptors by yohimbine only slightly reduced the CAP-evoked enhancement of responses. We conclude that the presence of sympathetic efferents is essential for the CAP-induced sensitization of Aδ- and C-primary afferent fibers to mechanical stimuli and that α1-adrenergic receptors play a major role in the sympathetic modulation of C-nociceptor sensitivity in the periphery.