An analysis of the alpha-adrenoceptor modulation of vasomotor tone at the level of lateral medullary pressor area (LMPA).

Microinjection of noradrenaline and clonidine into lateral medullary pressor area (LMPA) of chloralose anaesthetized cats produced dose dependent decrease in blood pressure without affecting heart rate, while phenylephrine did not elicit any cardiovascular response. Selective alpha 2-adrenoceptor antagonists idazoxan and piperoxan, microinjected locally, blocked the effects of the agonists but prazosin and phenoxybenzamine, which are relatively selective for alpha 1-adrenoceptors, failed to do so. Clonidine did not elicit any response in guanethidine pretreated cats but noradrenaline microinjected into LMPA of these animals induced a pressor response which was blocked by prazosin pretreatment. It is concluded that catecholaminergic fibres impinging upon this area inhibit the activity of the inhibitory second order baroreceptor neurone by activating alpha 1-adrenoceptors while alpha 2-adrenoceptors situated presynaptically on these inhibitory catecholaminergic nerve terminals are responsible for the manifestation of the hypotensive effect of clonidine and exogenously administered noradrenaline.