Electrophysiological study of cardiovascular neurons in the rostral ventrolateral medulla in rats.

In urethane-anesthetized rats, electrophysiological recordings of spontaneously active neurons in the vasopressor area of the rostral ventrolateral medulla were analyzed for participation in cardiovascular regulation. A total of 138 units were found which were inhibited by transient increases in mean arterial pressure elicited by intravenous injection of norepinephrine, aortic occlusion, or electrical stimulation of the hypothalamus, with 100% inhibition occurring at 148 mm Hg. Histograms of postsystolic activity showed that these units had pulse-synchronous rhythms which grew more prominent as arterial pressure increased. Furthermore, electrical stimulation of the superior laryngeal nerve, which elicited a vasodepressor response, strongly inhibited these units. Thus, these neurons were termed negatively correlated cardiovascular units. At least half of these cells project to or through the thoracic spinal cord. In addition, arterial pressure sensitivity is conveyed through carotid sinus and aortic arch afferents. Approximately half of the cardiovascular units are also excited by hypothalamic stimulation. Finally, analysis of neighboring cells showed that it is possible to distinguish between cardiovascular and respiratory units. These data are consistent with the concept of a medullary center which supports tonic sympathetic vasomotor tone and which mediates baroreceptor reflexes, as well as vascular responses of the defense reaction.