Heat loss responses and blockade of prostaglandin E2-induced thermogenesis elicited by α1-adrenergic activation in the rostromedial preoptic area

The unilateral microinjection of noradrenaline (NA), but not vehicle solution, into the rostromedial preoptic area (POA) elicited simultaneous increases in cutaneous temperatures of the tail and sole of the foot and decreases in the whole-body O2 consumption rate, heart rate, and colonic temperature in urethane–chloralose-anesthetized rats, suggesting a coordinate increase in heat loss and decrease in heat production. The magnitude of these responses increased dose-dependently over the range of 1–100 pmol, except for the metabolic and bradycardic responses. Similar hypothermic responses were elicited by the microinjection of 40 pmol methoxamine (an α1-adrenergic agonist), but not by that of clonidine (an α2-agonist) or isoproterenol (a β-agonist). Sites at which microinjection of NA elicited hypothermic responses were in the vicinity of the organum vasculosum of the lamina terminalis including the median preoptic nucleus, whereas no thermal or metabolic response was elicited when NA was microinjected into the lateral POA or caudal part of the medial POA. The microinjection of 130 fmol prostaglandin (PG) E2 into the NA-sensitive site always elicited thermogenic, tachycardic, and hyperthermic responses. Furthermore, the PGE2-induced febrile responses were greatly attenuated by prior administration of NA at the same site. These results demonstrate that NA in the rostromedial POA exerts α1-adrenoceptor-mediated hypothermic effects and opposes PGE2-induced fever.