Medullary pathways mediating specific sympathetic responses to activation of dorsomedial hypothalamus

We sought to determine which medullary sympathetic premotor neurons mediate the cardiovascular and thermogenic effects resulting from activation of neurons in the dorsomedial hypothalamus (DMH) in urethane/chloralose-anesthetized, artificially ventilated rats. Unilateral disinhibition of neurons in the DMH with microinjection of bicuculline (2 mM, 30 nl) caused significant increases in brown adipose tissue sympathetic nerve activity (BAT SNA, +828±169% of control, n=16), cardiac SNA (+516±82% of control, n=16), renal SNA (RSNA, +203±25% of control, n=28) and, accompanied by increases in BAT temperature (+1.6±0.3°C, n=11), end-tidal CO₂ (+0.7±0.1%, n=15), heart rate (+113±7 beats/min, n=32), arterial pressure (+19±2 mm Hg, n=32) and plasma epinephrine and norepinephrine concentrations. Inhibition of neurons in the rostral raphe pallidus (RPa) with microinjection of muscimol (6 mM, 60 nl) abolished the increases in BAT SNA and BAT temperature and reduced the tachycardia induced by disinhibition of DMH neurons. Inhibition of neurons in the RVLM with microinjection of muscimol (6 mM, 60 nl) markedly reduced the increase in RSNA, but did not affect the evoked tachycardia or the increase in arterial pressure. Combined glutamic acid decarboxylase (GAD-67) immunocytochemistry and pseudorabies viral retrograde tracing from BAT indicated close appositions between GABAergic terminals and DMH neurons in sympathetic pathways to BAT. In conclusion, these results demonstrate the existence of a tonically active, GABAergic inhibitory input to neurons in the DMH and that blockade of this inhibition increases sympathetic outflow to thermogenic and cardiovascular targets by activating functionally specific populations of sympathetic premotor neurons: the excitation of BAT SNA and BAT thermogenesis is mediated through putative sympathetic premotor neurons in the RPa, while the activation in RSNA is dependent on those in RVLM. These data increase our understanding of the central pathways mediating changes in sympathetically mediated thermogenesis that is activated in thermoregulation, stress responses and energy balance.