Effect of neonatal treatment with capsaicin on the numbers and properties of cutaneous afferent units from the hairy skin of the rat.

Neonatal treatment with capsaicin caused a 40% reduction in the number of C-fibres in the rat saphenous nerve. Within the reduced total the proportion of nociceptive C fibres was similar in both capsaicin-treated and untreated rats. No change was found in the conduction velocities or firing thresholds (to pressure and to heat) in A-fibre and C-fibre nociceptors. The perimeters of axons were also unchanged by capsaicin. From a comparison of the morphometric and conduction velocity (v) data it is suggested that in the rat saphenous nerve v (m/s) = 1.9 X axon perimeter (micron) for A fibres and v = 0.44 X square root of the axon perimeter for C fibres.