Posterior hypothalamic influences on cardiovascular effects of aortic nerve stimulation

The purpose of this study was to examine the influence of stimulation of the posterior hypothalamus on the baroreflex responses produced by stimulation of the aortic depressor nerve. Animals were initially anesthetized and implanted with a bipolar electrode in the posterior hypothalamus. Three to 5 days later, animals were anesthetized with urethan, and the left aortic depressor nerve was dissected and placed on a bipolar platinum-iridium electrode. The effects of electrical stimulation of the posterior hypothalamus (0, 160, and 280 microA) were examined in baroreflex-intact and acutely sinoaortic baroreceptor-denervated animals, and the responses to aortic nerve stimulation (2, 8, 16, and 32 Hz) were examined during each level of hypothalamic stimulation. The first set of experiments was performed in baroreceptor-intact animals; e.g., in animals with arterial baroreceptor inputs intact from both carotid sinus regions in addition to intact right aortic baroreceptor afferent pathways. In that group, stimulation of the posterior hypothalamus attenuated the bradycardia and depressor effects of aortic nerve stimulation. When influences from other baroreceptor inputs were removed with acute sinoaortic baroreceptor denervation, posterior hypothalamic stimulation interrupted the reflex bradycardia due to aortic nerve stimulation; however, the depressor response to aortic nerve stimulation was not attenuated. Similar to the arterial pressure response, hypothalamic stimulation did not attenuate the decreases in mesenteric and iliac vascular resistance produced by aortic nerve stimulation in the baroreflex-denervated group. We conclude that posterior hypothalamic stimulation attenuates baroreflex-mediated bradycardia but does not alter baroreflex control of arterial pressure and peripheral vascular resistance.