Differential cardiovascular effects of central clonidine and B-HT 920 in conscious rats

Abstract

The effect of intracerebroventricular (i.c.v.) injection of the α2-adrenoceptor agonists clonidine and B-HT 920 on mean arterial pressure (MAP), heart rate (HR), and plasma concentrations of noradrenaline and adrenaline was examined in conscious unrestrained rats. The injection of 1.0 μg clonidine significantly decreased MAP and slightly decreased HR. Plasma noradrenaline and adrenaline levels were slightly but not significantly decreased after the injection of 1 μg clonidine. In contrast, the injection of 0.1–10.0 μg B-HT 920 increased MAP and decreased HR. Plasma noradrenaline and adrenaline levels were slightly increased after the injection of the 1- and 10-μg doses. The i.c. v. injection of the α2-antagonist rauwolscine slightly but not significantly increased MAP and plasma noradrenaline and adrenaline levels. The responses to i.c. v. injection of clonidine and B-HT 920 were not changed by prior administration of rauwolscine. Neither the pressor response to B-HT 920 nor the depressor response to clonidine was abolished by rauwolscine, suggesting that neither response was mediated by α2-adrenoceptors.