The cardiovascular effects of intraventricularly administered histamine in the anaesthetised rat

In urethane-anaestetised rats intraventricular (i.c.v.) injections of histamine (0.1-10.0mug) elicited dose-related rises in both the resting blood pressure and heart rate. These cardiovascular effects of histamine were antagonised in a dose-dependent manner by i.c.v. pretreatments with the histamine H1-receptor antagonists mepyramine (10, 50 and 100 mug) and diphenylpyraline (100 and 200mug). Pretreatment with the histamine H2-receptor antagonist metiamide (100 and 200 mug i.c.v.) failed to modify either of the responses. A dose-related antagonism of the hypertensive response to histamine i.c.v. was elicited by phentolamine (100 and 200 mug i.c.v.) but the positive chronotropic effect was not modified by this pretreatment. The cardiovascular responses to histamine i.c.v. were abolished by mecamylamine (5.0 mg/kg i.v.) and greatly reduced by 6-hydroxydopamine (3 X 250 mug i.c.v.), but only the tachycardia was significantly modified by atropine (100 mug i.c.v.) and propranolol (1 mg/kg i.v.). Propranolol (100 mug i.c.v.), bilateral vagotomy, or acute bilateral adrenal demedullation failed to modify the cardiovascular responses to histamine i.c.v. The results suggest that histamine is able to modify the resting blood pressure and heart rate by independent central modes of action, which involve central adrenergic and cholinergic mechanisms.