Involvement of the area postrema in the regulation of sympathetic outflow to the cardiovascular system.

The circumventricular organs of the brain have been implicated in the central regulation of the cardiovascular system. The area postrema, which is the only circumventricular organ in the hindbrain, has received less attention than the others, but recent studies suggest that it may play an important role in the regulation of the cardiovascular system. Studies in rats and rabbits indicate that angiotensin II (Ang II)-dependent hypertension is abolished by lesioning of the area postrema. Additional studies indicate that the hypertension associated with Ang II involves a resetting of the arterial baroreflex to a higher pressure. This upward resetting requires an interaction of neurons in the area postrema with barosensitive neurons terminating in the medial nucleus tractus solitarius (mNTS). Another peptide, arginine vasopressin (AVP), has been shown to enhance the sympathoinhibitory influence of the arterial baroreflex via an action at the area postrema. Studies in rabbits suggest that the sympathoinhibitory response is due to resetting of the baroreflex to a lower pressure. Electrophysiological studies, using an in vitro brain slice preparation, have shown that activation of area postrema neurons projecting to the mNTS alters the responsiveness of mNTS neurons to afferent inputs. It is postulated that alpha-adrenergic mechanisms are involved in these interactions.