Temperature- and capsaicin-sensitive nerve fibers in brown adipose tissue attenuate thermogenesis in the rat

We examined the function of putative sensory fibers that are contained in i ntercostal nerves and innervate interscapular brown adipose tissue (IBAT) i n urethane-anesthetized rats. Warming the IBAT to 40-44 degrees C with two small heaters placed bilaterally on the skin above it attenuated the subseq uent noradrenaline-induced thermogenesis (NIT) of the IBAT. In this range o f warming, higher fBAT temperatures resulted in more attenuation. Denervati on of IBAT blocked the effect of thermal stimulation on the NIT. Thus, acti vation of nerve fibers in IBAT that are sensitive to warmth or to the nocic eptive effects of heat probably attenuated the NIT. Since the putative sens ory fibers in the IBAT contain calcitonin gene-related peptide (CGRP) and s ubstance P, which are thought to act in peripheral tissues, we tested the e ffects of injection of these neuropeptides into the IBAT. Administration of 5.2 nmol. CGRP but not substance P or vehicle saline mimicked the effect o f thermal stimulation of IBAT. As the neuropeptide-containing primary senso ry neurons are characterized by their sensitivity to capsaicin, we also tes ted its effects (1 mg/kg, s.c.) and found that it also attenuated the NIT. Denervation of the IBAT or pretreatment with capsazepine, a capsaicin recep tor antagonist, blocked the effect of capsaicin. We propose that temperatur e- and capsaicin-sensitive nerve fibers release CGRP to attenuate the NIT o f brown adipocytes.