1. A study was made relating the involvement of alpha-
adrenoceptors in the cardiovascular responses to intracerebroventricular (i.c.v.) 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 i.c.v. 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 microg) 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 alpha2-adrenoceptor antagonist
yohimbine (30 microg), the
imidazoline receptor antagonist
idazoxan (15 microg) and the alpha1A-adrenoceptor antagonist
5-methylurapidil (15 microg) blocked the responses to
B-HT 920 (30 microg). The alpha1-adrenoceptor antagonist
prazosin (15 microg) and the alpha1B-adrenoceptor antagonist
chloroethylclonidine (100 microg) did not modify the responses to agonist. 4. Sinoaortic
denervation enhances the cardiovascular responses to
B-HT 920. Moreover, the effects of i.c.v. administration of
B-HT 920 could be mediated by several types of brain
receptors: imidazoline receptors and alpha1A- and alpha2-adrenoceptors.