Evidence for involvement of ? 2Adreonceptors in the nucleus ambiguus in baroreflex-mediated bradycardia

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Abstract
Microinjection of noradrenaline and clonidine into the nucleus ambiguus elicited dose-dependent bradycardia with insignificant alteration of blood pressure. Phenylephrine failed to elicit any cardiovascular effect. The bradycardic effects of noradrenaline and clonidine were antagonized by piperoxan but not by phenoxybenzamine. Adrenergic neurone blockade with local guanethidine pretreatment also abolished the response to clonidine. No significant cardiovascular effect of clonidine microinjection into the nucleus ambiguus was observed in bilaterally vagotomized animals. The baroreflex bradycardia induced by volume loading was abolished by yohimbine and piperoxan by volume loading was abolished by yohimbine and piperoxan but not by phenoxybenzamine, microinjected bilaterally into the nucleus ambiguus. These results demonstrate the presence of cardioinhihibitory, presynaptic α2-adrenoceptors in the nucleus ambiguus and their involvement in baroreflex bradycardia.