Neurophysiological and thermoregulatory effects of capsaicin.

The effects, in rats, of peripherally administered capsaicin were evaluated on body temperature and on brain neurophysiological activity. Capsaicin (5 mg/kg, SC) produced a rapid, long-lasting fall in body temperature. A second dose given 4.5 hr after the first showed a moderate decrement in hypothermic effect. The same dose of capsaicin caused discernible changes in deep EEG activity in the anterior hypothalamus (AH), medial habenula (HB), substantia nigra (SN), and dorsal raphe (DR). The EEG changes were more pronounced at doses of 10 and 20 mg/kg. Capsaicin in all doses modified sensory evoked responses recorded from all four brain areas. Response amplitudes after acoustic stimulation were increased at all doses of capsaicin in a dose related pattern. Photic evoked responses, like body temperature effects, showed diminished effects with second and third doses. Peripherally administered capsaicin, therefore, can affect body temperature and brain electrical activity.