CENTRAL ALPHA(1) AND ALPHA(2)-ADRENOCEPTORS AND BRAIN CHOLINERGIC STIMULATION IN SINOAORTIC DENERVATED RATS

The central alpha-adrenoceptor role in cardiovascular responses to int racerebroventricular (i.c.v.) injection of neostigmine, a tertiary ant icholinesterase, was studied in conscious sham-operated and sinoaortic -denervated rats. Neostigmine (0.1-1 mu g i.c.v.) showed dose-dependen t presser and bradycardiac effects in vehicle-pretreated sham-operated rats but only an increased presser effect in sinoaortic-denervated an imals. The pretreatment with the catecholaminergic neurotoxin, 6-hydro xydopamine (250 mu g i.c.v.), given 72 h previous to the corresponding operation, blunted the cardiovascular effects of neostigmine in both groups of rats. Prazosin (10 and 30 mu g i.c.v.), an alpha(1)-adrenoce ptor antagonist, prevented the presser response to neostigmine (0.3 mu g i.c.v.) in sham-operated and sinoaortic-denervated rats. Yohimbine, an alpha(2)-adrenoceptor antagonist (10 and 30 mu g i.c.v.), only pre vented the bradycardia induced by neostigmine (0.3 mu g i.c.v.) in the sham-operated rats. 6-Hydroxydopamine pretreatment lowered the norepi nephrine content in hypothalamus, midbrain, medulla oblongata and spin al cord, but did not modify it in the pens, in sham-operated rats and sinoaortic-denervated animals. The present results suggested that brai n alpha(1)-adrenoceptors would mediate the presser response to neostig mine (i.c.v.) in sham-operated and sinoaortic-denervated rats and cent ral alpha(2)-adrenoceptors mediate the bradycardia in sham-operated ra ts. This work lends support to the view that cardiovascular responses to brain cholinergic stimulation in sham-operated and sinoaortic-dener vated rats could be mediated by a central catecholaminergic activation .