Long-Term Reduction in the Number of C-Fibre Nociceptors Following Capsaicin Treatment of a Cutaneous Nerve in Adult Rats

Abstract

The structure and function of C- and A-fibres have been studied 3 - 12 months following a single 30 min exposure of the saphenous nerve of adult rats to 1% capsaicin. Examination of nerve cross-sections showed that the number of C-fibres was reduced by 36%, but A-fibres were unaffected. The remaining C-fibres included many that were unusually small in size. The changes were not restricted to the treatment site but were found over at least a further 10 mm proximal and distal to it. In treated nerves, the C-fibre component of the compound action potential was reduced in size relative to the Aalphabeta component. Single unit studies revealed that an unusually high proportion of C-fibres had no cutaneous receptive field (54%, compared with 28% in controls). There was no such change for A-fibres. The conduction velocity range for C-fibre units in treated nerves was almost normal, but this sample contained an unusually low proportion of polymodal nociceptors. Allowing for the fall in total numbers and the reduction in units with cutaneous receptive fields, it was calculated that overall the numbers of C-polymodal nociceptor units had fallen by 74%. No significant changes in number had occurred in the other classes of C-afferents. The reduction in the numbers of nociceptive C-fibres is likely to be the direct cause of the reduction in pain responses and in neurogenic inflammation that several groups have reported following treatment of adult rats with capsaicin. Reduction in the numbers of C-fibre is also likely to be the reason for reduced neuropeptide levels seen in tissues innervated by treated nerves.