Aδ and C Primary Afferents Convey Dorsal Root Reflexes After Intradermal Injection of Capsaicin in Rats

Abstract

Antidromic activity was recorded in anesthetized rats from single afferent fibers in the proximal ends of cut dorsal root filaments at the L4–6 level and tested for responses to acute cutaneous inflammation produced by intradermal injection of capsaicin. This antidromic activity included low-frequency spontaneous firing and dorsal root reflex (DRR) discharges evoked by applying von Frey hairs to the skin of the foot. DRRs could be recorded from both small myelinated (Aδ) and unmyelinated (C) afferent fibers, as well as from large myelinated (Aβ) fibers. After capsaicin was injected intradermally into the plantar skin of the foot, a significant enhancement of DRR activity was seen in Aδ and C fibers but not in Aβ fibers, and this increase lasted for ∼1 h. This study supports the hypothesis that centrally mediated antidromic activity in Aδ and C primary afferent fibers contributes to the development of neurogenic inflammation, presumably by release of inflammatory substances in the periphery.