Differential regulation of brown adipose and splanchnic sympathetic outflows in rat: Roles of raphe and rostral ventrolateral medulla neurons

1. The medullary premotor neurons determining the sympathetic outflow regul ating cardiac function and vasoconstriction are located in the rostral vent rolateral medulla (RVLM). The present study sought evidence for an alternat ive location for the sympathetic premotor neurons determining the sympathet ic nerve activity (SNA) controlling brown adipose tissue (BAT) metabolism a nd thermogenesis. 2. The tonic discharge on sympathetic nerves is determined by the inputs to functionally specific sympathetic preganglionic neurons from supraspinal p opulations of premotor neurons. Under normothermic conditions, BAT SNA was nearly silent, while splanchnic (SPL) SNA, controlling mesenteric vasoconst riction, exhibited sustained large-amplitude hursts, 3, The rostral raphe pallidus (RPa) contains potential sympathetic premotor neurons that project to the region of sympathetic preganglionic neurons in the thoracic spinal cord. Disinhibition of neurons in RPa elicited a drama tic increase in BAT SNA, with only a small rise in SPL SNA, 4. Splanchnic SNA was strongly influenced by the baroreceptor reflex, as in dicated by a high coherence with the arterial pressure wave, a significant amplitude modulation over the time-course of the cardiac cycle and a marked inhibition of SPL SNA during a sustained increase in arterial pressure. Wh en activated, the bursts in BAT SNA exhibited no correlation with arterial pressure and were not affected by increases in arterial pressure. 5, Because these characteristics and reflex responses in sympathetic outflo w have been shown to arise from the on-going or altered discharge of sympat hetic premotor neurons, the marked differences between SPL and BAT SNA prov ide strong evidence supporting the hypothesis that vasoconstriction and the rmogenesis (metabolism) are controlled by distinct populations of sympathet ic premotor neurons, the former in the RVLM and the latter, potentially, in the RPa.