Adrenoceptor involvement in the cardiovascular responses to B-HT 920 in sinoaortic denervated rats

1. A study was made relating the involvement of alpha-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. Wister 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 mu g) were increased blood pressure and bradycardia; but, in sinoaortic-denervated rats, after the presser 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 alpha(2)-adrenoceptor antagonist yohimbine (30 mu g), the imidazoline receptor antagonist idazoxan (15 mu g) and the alpha(1A)-adrenoceptor antagonist 5-methylurapidil (15 pg) blocked the responses to B-HT 920 (30 mu g). The alpha(1)-adrenoceptor antagonist prazosin (15 mu g) and the alpha(1B)-adrenoceptor antagonist chloroethylclonidine (100 mu g) 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 alpha(1A)- and alpha(2)-adrenoceptors. (C) 1998 Elsevier Science Inc.