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Excitability of afferent C-fibres in a skin nerve was studied in 28 rats treated with capsaicin on day two of their lives (80 mg/kg, s.c.). At the age of 4-5 months they were subjected to experiments in which recordings were made from single C-fibre units of saphenous nerve. The effectivity of neonatal capsaicin treatment in blocking reactions to the irritating action of the toxin was evaluated with the wiping test. Five untreated rats served as controls. In capsaicin treated animals afferent cutaneous C-fibres were diminished by more than 50%. No significant change was found, however, in the spectrum of kinds of cutaneous receptors. In particular, the ratio between nociceptors responding to heat and mechanical stimuli (polymodal nociceptors, MH-units) and other C-fibre receptors, such as sensitive cold units was not changed significantly. Furthermore, in the animals neonatally treated with capsaicin conduction velocities of afferent C-fibres and thresholds to mechanical and thermal stimuli were not significantly altered. In both capsaicin treated and control samples, the spike conduction in most of the mechano- and heat sensitive C-nociceptors was blocked by acute topical application of this toxin to the nerve stem. It is concluded that part of the afferent C-fibres survived neonatal capsaicin treatment, even in those rats that showed complete intensitivity to the toxin in behavioural tests.

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