The effect of capsaicin application to a peripheral nerve on impulse conduction in functionally identified afferent nerve fibres.

Capsaicin applied locally to a coccygeal or saphenous nerve of rats was shown to block impulse conduction in unmyelinated afferent nerve fibres. Shortly after application of capsaicin (1% dissolved in 10% Tween 80 in paraffin oil) conduction of C-fibre--but not A-fibre--compound action potentials across the application site was markedly diminished. No recovery occurred during the period of observation, i.e. up to 2 h afterwards. C-Fibre compound action potentials stimulated and recorded proximal to the application site seemed to be unaffected. Solvent application had no effect. Recording from single units revealed that nociceptive C-fibres responding to strong mechanical and heat stimulation (MH units, polymodal nociceptors) were blocked. In contrast, unmyelinated cold fibres were not affected. The selective block of MH units indicates different membrane properties of unmyelinated MH and cold units not only at the receptive nerve endings, but also at the axons, to which capsaicin was applied.