Differences in the mechanisms of the thermoregulatory impairment induced by capsaicin in newborn and adult rats.

The function of heat defence was compared in rats pretreated as adults (A-rats) and neonates (N-rats) with capsaicin. The thermoregulatory impairment as tested by whole body heating was similar in A-rats and N-rats. The thermosensitivity and chemosensitivity of the preoptic region (RPO) seemed to be normal in N-rats. A deficiency of RPO mechanisms could be demonstrated in A-rats. It is suggested that the preoptic effects of capsaicin have but a minor significance for the thermoregulatory impairment. It seems that the main cause of decreased heat tolerance is a reduction of peripheral warm sensation due to degeneration of unmyelinated C-fibre primary neurons in both A-rats and N-rats. The results do not support the primary importance of preoptic warm sensation in physiological thermoregulation.