A delta and C primary afferents convey dorsal root reflexes after intradermal injection of capsaicin in rats

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 intrad ermal injection of capsaicin. This antidromic activity included low-frequen cy spontaneous firing and dorsal root reflex (DRR) discharges evoked by app lying von Frey hairs to the skin of the foot. DRRs could be recorded from b oth small myelinated (A delta) and unmyelinated (C) afferent fibers, as wel l as from large myelinated (A beta) fibers. After capsaicin was injected in tradermally into the plantar skin of the foot, a significant enhancement of DRR activity was seen in A delta and C fibers but not in A beta fibers, an d this increase lasted for similar to1 h. This study supports the hypothesi s that centrally mediated antidromic activity in A delta and C primary affe rent fibers contributes to the development of neurogenic inflammation, pres umably by release of inflammatory substances in the periphery.