Characterization of α-adrenoceptors in the nucleus reticularis gigantocellularis involved in the cardiovascular depressant effects of guanabenz in the rat

The participation of α-adrenoceptors in the nucleus reticularis gigantocellularis in the hypotensive, negative inotropic and chronotropic effects induced by guanabenz, was examined in rats anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Pretreatment with α-adrenoceptor antagonists yohimbine (10 μg), phentolamine (2.5 μg) and phenoxybenzamine (20 μg), which were injected bilaterally into the nucleus reticularis gigantocellularis, significantly antagonized the cardiovascular suppressant effects normally produced by systemic administration of guanabenz (10 μg/kg, i.v.). Pretreatment with prazosin (0.25 μg) did not affect the vasodepressive, but significantly attenuated the bradycardic actions of guanabenz. The general trend of "antagonization potency" shown by the α-adrenergic blockers, against the cardiovascular effects of guanabenz, was in the order: yohimbine > phentolamine > phenoxybenzamine > prazosin. It is concluded that while the α₂-adrenoceptors in the nucleus reticularis gigantocellularis are more critically involved in the antihypertensive actions of guanabenz, the possibility exists that α₁-adrenoceptors may also participate, in part.