Altered excitability of afferent C-fibres of the rat distal to a nerve site exposed to capsaicin.

Excitability of afferent C-fibres in saphenous and coccygeal nerves of rats was studied distal to a nerve site treated with 1% capsaicin for half an hour one day before. This treatment induced the following changes. (a) Fewer C-fibres than in control nerves were excited by natural skin stimulation among those driven by electrical nerve stimulation. Comparison of compound action potentials derived from capsaicin-treated and from untreated contralateral coccygeal nerves indicated that the total number of excitable C-fibres was also diminished. (b) Whereas in a normal rat nerve virtually all unmyelinated nociceptors responding to heat stimulation are also driven by strong mechanical stimuli (MH-nociceptors, polymodal nociceptors), most of them were found to respond only to heat stimulation after local capsaicon treatment of the nerve.