Central mechanisms of the isoprenaline-induced hypotension in anesthetized and conscious rats

Isoprenaline was microinjected into the cerebroventricular spaces of anesthetized or unanesthetized rats and the blood pressor effects recorded prior to or after intraventricular administration of pharmacologic antagonists. Injection of 1-20 micrograms of isoprenaline produced only depressor responses in both groups of animals. Pretreatment with 400-200 micrograms of propranolol partially antagonized the depressor effect of isoprenaline, whereas pretreatment with 40 micrograms of phenoxybenzamine or 100 micrograms phentolamine potentiated the depressor response to isoprenaline. The involvement of central beta-adrenoceptors and a propranolol-insensitive mechanism in the depressor response to intracerebroventricular isoprenaline is proposed. Indirect evidence of a central alpha-adrenoceptor mediation of pressor mechanisms counteracting the depressor responses elicited by beta-adrenoceptor stimulation is presented.