Lack of integrative control of heat production and heat loss after capsaicin administration.

Body temperature is usually regulated by opposing controls of heat production and heat loss. However, systemic administration of capsaicin activates heat loss and heat production simultaneously. Because capsaicin receptors are located mainly on primary sensory neurons and body temperature is regulated by the central nervous system, we investigated the brain mechanisms involved in these capsaicin-induced thermal responses. For this purpose, we examined the effects of spinalization and decerebration on these responses in artificially ventilated, urethane-anesthetized rats. Cervical spinal transection largely attenuated both responses, showing the critical involvement of the brain. Colonic temperature (Tc) did not change after the capsaicin administration to the spinalized rats. Decerebration between the hypothalamus and midbrain prevented the capsaicin-induced heat loss and enhanced the capsaicin-induced heat production. Consequently, Tc increased without a hypothermic period. The results show that capsaicin activates brainstem-controlled heat production and forebrain-controlled heat loss separately.