Inhibition of tonic cardiac sympathetic activity by stimulation of brain septal region

Abstract

Low-intensity electrical stimulation in the region of the medial or lateral septal nuclei of the brain significantly decreased right ventricular contractile force, heart rate, and systemic blood pressure in each of 46 anesthetized cats. These effects developed within 6 sec after onset of stimulation and persisted for about 5 min after cessation of the 10- to 40-sec periods of stimulation. The depressor responses could be elicited repeatedly and were unaffected by bilateral vagotomy, by large doses of cholinergic blocking drugs, or by the antihistamine, diphenhydramine. The decreases in contractile force and heart rate induced by septal stimulation were abolished by extirpation of the stellate ganglia, by tetraethylammonium (TEA) and by the beta adrenergic blocking drug, dichloroisoproterenol (DCI). The characteristic fall in blood pressure induced by septal stimulation was unaffected by stellate ganglionectomy or by DCI. Both TEA and the alpha adrenergic blocking drug, phenoxybenzamine, decreased blood pressure to levels lower than those produced by septal stimulation, and stimulation of the septal nuclei after TEA or phenoxybenzamine did not reduce pressure further. Unlike TEA, phenoxybenzamine had no influence on the decreases in force and rate induced by septal stimulation. The reductions in force and rate were not secondary to hypotension for they were still obtained when aortic pressure was kept constant. These findings suggest that the reductions in contractile force, heart rate, and blood pressure induced by septal stimulation are the result of a decrease in sympathetic tone to the heart and blood vessels.