Skin cooling‐evoked responses of rostral raphe pallidus (rRPa) neurons with slowly conducting spinal axons – putative sympathetic premotor neurons for brown adipose tissue (BAT)

Activation of neurons in the rRPa or the parapyramidal area increases sympathetic outflow (SNA) to BAT and BAT thermogenesis and inhibition of neuronal activity in these areas reverses increases in BAT SNA evoked by skin cooling, by application of prostaglandin to the preoptic area or by leptin. In combination with pseudorabies virus infection of rRPa neurons after inoculation of BAT, these data suggest that the rRPa contains sympathetic premotor neurons controlling BAT SNA. To identify putative BAT sympathetic premotor neurons, extracellular recordings were made in the rRPa of urethane/chloralose anesthetized, artificially ventilated, paralyzed rats during cooling of the trunk skin by circulating cold water through a copper coil. To determine the conduction velocity of their spinal axons, antidromic stimulation was applied to the IML of the third thoracic segment. Based on the difference in peak latencies of potentials evoked in BAT SNA from the rRPa and the T3 IML (separation: 40 mm), the expected conduction velocity of BAT sympathetic premotor neurons in rRPa would be about 0.6 m/s. Putative sympathetic premotor neurons were identified in rRPa which had spinal conduction velocities between 0.4 to 0.8 m/s and which were excited during episodes of skin cooling that increased BAT SNA and inhibited during episodes of skin rewarming that reversed the BAT sympathoexcitation. These results provide further support for the localization of BAT sympathetic premotor neurons in rRPa. Supported by NIH grants NS40987 and DK57838.