Role of central alpha 1- and alpha 2-adrenoceptors on the dipsogenic and cardiovascular effect of angiotensin II.

We investigated the effects of previous central treatment with prazosin (an alpha 1-adrenoceptor antagonist) or clonidine (an alpha 2-adrenoceptor agonist) on the dipsogenic, pressor and tachycardic responses produced by intracerebroventricular (ICV) injection of angiotensin II (AII) in conscious rats. Holtzman rats with a chronic cannula implanted in the lateral ventricle were tested for dipsogenic and cardiovascular (arterial pressure and heart rate) responses in separate experiments. Previous ICV treatment with clonidine (20, 40, 80 and 120 nmol) abolished the pressor, tachycardic and dipsogenic effects of ICV AII. After all doses of prazosin (40, 80 and 120 nmol), AII induced bradycardic responses, but only the 80 and 120 nmol doses of prazosin reduced the pressor responses to AII. Prazosin produced no alteration in the dipsogenic effect of AII. The results show that the periventricular alpha 1-adrenoceptors are involved only in the cardiovascular responses produced by central AII, whereas clonidine acting through alpha 2-adrenergic and/or imidazole receptors can modulate all actions of AII.