CAPSAICIN ACTIVATES HEAT-LOSS AND HEAT-PRODUCTION SIMULTANEOUSLY AND INDEPENDENTLY IN RATS

Subcutaneous administration of capsaicin (5 mg/kg) immediately increas ed the temperature of the tail skin (T-sk) for 2 h in urethan-anesthet ized rats, suggesting an increase in heat loss. O-2 consumption, an in dex of heat production, also immediately increased after the capsaicin injection, and this increase lasted for >10 h. Colonic temperature (T -co) decreased within 1 h after the injection, and this decrease was f ollowed by a long-lasting hyperthermic period. Adrenal demedullation l argely attenuated the capsaicin-induced increase in O-2 consumption, a nd sympathetic denervation of the interscapular brown adipose tissue p artly attenuated the increase in O-2 consumption. However, capsaicin-i nduced heat loss was normal in these rats. In rats with cutaneous vaso dilation maximized by warming and administration of hexamethonium, cap saicin did not further increase T-sk but normally induced heat product ion, and T-co gradually rose without a hypothermic period. Thus capsai cin simultaneously increased heat loss and heat production, and inhibi tion of one response did not affect the other. These findings suggest that capsaicin simultaneously activates independent networks for heat loss and heat production.