Studies on the mechanism of the cardiovascular action of central injections of histamine.

The role of the sympathetic nervous system in the mediation of cardiovascular function, following administration of histamine centrally was investigated. Intracerebroventricular (i.c.v.) injections of histamine into the lateral ventricle of conscious rats caused a dose-dependent increase in blood pressure as well as a dose-dependent fall in heart rate. The pressure and heart rate responses were both attenuated by pretreatment with chlorpheniramine. Peripheral blockade of autonomic ganglia with hexamethonium failed to block the centrally-mediated pressor response to histamine. Peripheral chemical sympathectomy with 6-hydroxydopamine also did not inhibit the pressor response to histamine. In this latter group of animals the pressor response to histamine was potentiated. It is concluded that in the conscious rat, increased in blood pressure resulting from central stimulation with histamine are not mediated solely through the activation of the sympathetic nervous system and are, at least in part, due to the release of a vasopressor substance.