Involvement of alpha2-adrenoceptors of nucleus tractus solitarius in baroreflex mediated bradycardia.

Microinjections of noradrenaline and clonidine into nucleus tractus solitarius produced dose dependent bradycardia without significant decrease in blood pressure in chloralose anaesthetized cats. Phenylephrine failed to produce any significant alteration of heart rate or blood pressure. Piperoxan microinjection into nucleus tractus solitarius elicited a mild but significant tachycardia and could also block the noradrenaline and clonidine responses. Phenoxybenzamine however neither affected resting heart rate and blood pressure nor antagonized the responses of noradrenaline and clonidine. Guanethidine pretreatment of nucleus tractus solitarius also abolished the clonidine response. Baroreceptor reflex activation induced bradycardia was inhibited by yohimbine or piperoxane injected into the cisterna magna or microinjected bilaterally into the nucleus tractus solitarius. Pretreatment of nucleus tractus solitarius with phenoxybenzamine by either route, did not affect the reflex bradycardia. It is concluded that the alpha-adrenoceptors of nucleus tractus solitarius involved in the decrease in heart rate during baroreceptor activation are alpha2 in nature.