Adrenoceptor Involvement in the Cardiovascular Responses to B-HT 920 in Sinoaortic Denervated Rats

Abstract

1. A study was made relating the involvement of α-adrenoceptors in the cardiovascular responses to intracerebroventricular (ICV) injection of B-HT 920, a clonidine-type drug, in conscious sham-operated and sinoaortic-denervated rats. 2. Wistar rats were used, 7 days after the sham operation or sinoaortic denervation. For ICV injection of drugs, a guide cannula had been previously implanted in the left lateral ventricle. 3. In sham-operated rats, cardiovascular responses to B-HT 920 (10–60 μg) were increased blood pressure and bradycardia; but, in sinoaortic-denervated rats, after the pressor response, a decrease in blood pressure also was seen. The responses to this agent were greater in sinoaortic-denervated rats than in sham-operated animals. Treatment with the α 2-adrenoceptor antagonist yohimbine (30 μg), the imidazoline receptor antagonist idazoxan (15 μg) and the α 1A-adrenoceptor antagonist 5-methylurapidil (15 μg) blocked the responses to B-HT 920 (30 μg). The α 1-adrenoceptor antagonist prazosin (15 μg) and the α 1B-adrenoceptor antagonist chloroethylclonidine (100 μg) did not modify the responses to agonist. 4. Sinoaortic denervation enhances the cardiovascular responses to B-HT 920. Moreover, the effects of ICV administration of B-HT 920 could be mediated by several types of brain receptors: imidazoline receptors and α 1A- and α 2-adrenoceptors.