Regionally selective elimination of cutaneous thermal nociception in rats by neonatal capsaicin.

We have studied thermal and mechanical nociception in anesthetized adult rats that had received subcutaneous injections of capsaicin as neonates. This treatment appeared not to have impaired mechanonociception in any of the body regions that were examined. In contrast, thermal nociception was present in some locations and absent in others. Its total disappearance from back skin correlated with the loss of C fiber responsiveness in the dorsal cutaneous nerves (DCNs); electrical excitation of these nerves now failed to evoke the delayed component of the cutaneus trunci muscle reflex, although the early reflex response, elicited by excitation of the mechanosensitive A delta axons, was unaffected. Heat nociception was similarly eliminated from the nonpad glabrous skin of the paws. When the stimulus was applied selectively to the pads, however, a normal foot-withdrawal response was evoked. The probable basis of this differential heat responsiveness in the paws was revealed by the use of the Evans Blue technique in untreated animals; antidromic activation of the paw nerves to excite their C fiber population led to dye extravasation in the glabrous skin around the pads but to almost none in the pads themselves, indicating a relative absence in the pads of a nociceptive C fiber innervation. In capsaicin-treated animals, the tail-flick test revealed a significantly reduced responsiveness to noxious heat in the tail. These results may explain some of the anomalies among the reports of the effects of neonatal capsaicin. It appears that in strategically exposed locations exemplified by the paw pads and tail, capsaicin-resistant axons that could be the relatively fast-conducting A delta ones are involved in the signalling of noxious heat.(ABSTRACT TRUNCATED AT 250 WORDS)