Pressor responses and adrenomedullary catecholamine release during brain stimulation in the rat

Electrical stimulation of the diencephalon and rostral mesencephalon produced a biphasic pressor response consisting of an initial sharp rise followed by a more prolonged elevation in blood pressure. Active sites from which comparable responses could be evoked were found in the ventral medial and lateral hypothalamus, in the region of the parafascicularis nucleus, and in the central gray matter and the tegmentum of the mesencephalon. Together with the biphasic pressor response brain stimulation also increased plasma catecholamine (CA) concentrations 10-fold. Either bilateral adrenalectomy or demedullation selectively abolished the secondary phase of the pressor response and greatly reduced the plasma CA response to stimulation. After chemical destruction of the terminals of the sympathetic neurones by intravenous administration of 6-hydroxydopamine (6-OHDA), the primary pressor phase was eliminated whereas the secondary phase as well as plasma CA concentration reached higher levels than in normal rats. 6-OHDA treatment combined with adrenalectomy completely abolished both pressor and CA responses. The results demonstrate that adrenomedullary mechanisms capable of increasing blood pressure significantly can be activated by electrical stimulation of different sites in the diencephalon and mesencephalon.