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Paracrine-like excitation of low-threshold mechanoceptive C-fibers innervating rat hairy skin is mediated by substance P via NK-1 receptors

Faculty of Pharmacology, School of Medicine, Zhejiang University, Hangzhou, Zhejiang 310058, PR China
Brain Research Bulletin (Impact Factor: 2.97). 02/2008; DOI: 10.1016/j.brainresbull.2007.08.003

ABSTRACT We reported previously that C-fibers innervating rat skin can be excited by short trains of electrical shocks (‘tetanus’) applied to neighboring nerves. Since these nerves were disconnected from the CNS, the cross-talk is located peripherally. Here we tested if low-threshold mechanoceptive (LTM) C-fibers can be excited by this cross-talk and if this process is mediated by substance P (SP) via neurokinin-1 (NK-1) receptors. In urethane anesthetized rats we found that 80% (56/71) of LTM C-fibers, recorded in the lateral cutaneous branch of the dorsal ramus (CBDR) of T10 spinal nerve, were excited by a 10 s, 20 Hz tetanus of the T9 CBDR. Compared to the spontaneous pre-tetanic firing frequency of 1.62 ± 0.40 impulses/30 s, the frequency significantly increased to 3.74 ± 0.99, 3.17 ± 0.69 and 2.92 ± 0.63 impulses/30 s, at 30, 60 and 90 s after the tetanus, respectively, and declined to the baseline frequency thereafter. When injected into their receptive fields, SP mimicked the tetanically induced increase of firing rate, whereas the NK-1 receptor antagonist WIN 51708 blocked the excitation in most fibers. The excitation was significantly diminished in adult rats that were neonatally treated with capsaicin, a treatment that destroys most SP-expressing afferent fibers. Thus, we conclude that peptidergic primary afferents are functionally linked with adjacent LTM C-fibers in a non-synaptic, paracrine-like signaling pathway via SP and NK-1 receptors, and perhaps also other agents as well. We propose that this cross-talk has evolved as a mechanism regulating the mechanoceptive characteristics of LTM C-fibers, presumably contributing to pain sensation elicited by tactile stimuli (‘allodynia’).

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