Tetrodotoxin-resistant conductivity and spinal effects of cutaneous C-fibre afferents in the rat

The effect of the sodium channel blocking agent tetrodotoxin (TTX) on signa l processing in afferent fibres of dorsal roots was tested in Sprague-Dawle y rats. TTX applied to the dorsal roots L4-L6 blocked the fast afferent vol leys from the sural nerve, which was stimulated electrically with supramaxi mal strength for A-fibres. Afferent C-fibre compound action potentials (CAP s) elicited by electrical stimulation of the dorsal root L5 peripherally fr om the TTX block or by electrical stimulation of the sural nerve likewise d isappeared from the recording. Cord dorsum potentials (CDPs) recorded at th e dorsal root entry zone of L4 were blocked completely if elicited by A-fib re volleys. In contrast, CDPs elicited by C-fibre stimulation persisted wit h longer latency and reduced amplitude in the first part of the CDP. During TTX block, C-fibre potentials could also be recorded from dorsal root fila ments after stimulation of the sural nerve or the dorsal root L5 peripheral ly of the TTX-block. The results suggest that in the axonal membrane of cut aneous C-afferents, both TTX sensitive and TTX-resistant voltage gated sodi um channels exist, the latter being responsible for the propagation of sign als in a portion of C-fibres after TTX application. The TTX-resistant porti on of the afferent potential does not seem to contribute much to the affere nt C-fibre CAP before TTX application, but its central effects seem to be o verproportionally strong. (C) 2001 Elsevier Science Ireland Ltd and the Jap an Neuroscience Society. All rights reserved.