Preoptic-raphé connections for thermoregulatory vasomotor control

Abstract

Blood flow to glabrous skin such as the rat's tail determines heat dissipation from the body and is regulated by sympathetic vasoconstrictor nerves. Tail vasoconstrictor activity is tonically inhibited by neurons in two distinct preoptic regions, rostromedial (RMPO) and caudolateral (CLPO) regions, whose actions may be via direct projections to medullary raphé premotor neurons. In urethaneanesthe tized rats, we sought single preoptic neurons that were antidromically activated from the medullary raphé and could subserve this function. Nine of 45 raphé-projecting preoptic neurons, predominantly in the CLPO, showed spontaneous activity under warm conditions and were inhibited by cooling the trunk skin (warm-responsive). Unexpectedly, 14 raphé-projecting preoptic neurons (mostly in the RMPO) were activated by skin cooling (cold-responsive), suggesting that an excitatory pathway from this region could contribute to tail vasoconstriction. Supporting this, neuronal disinhibition in theRMPOby microinjecting the GABAA receptor antagonist bicuculline (0.5 mM, 15 nl) caused a rapid increase in tail sympathetic nerve activity (SNA). Similar injections into the CLPO were without effect. Electrical stimulation of theRMPOalso activated tail SNA, with a latency_25mslonger than to stimulation of the medullary raphé. Injection of the glutamate receptor antagonist kynurenate (50 mM, 120 nl) into the medullary raphé suppressed tail SNA responses to both RMPO bicuculline and skin cooling. These findings suggest that both inhibitory and excitatory descending drives regulate tail vasoconstriction in the cold and that warm- and cold-responsive raphé-projecting preoptic neurons may mediate these actions. © 2011 the authors.