Evidence for involvement of central noradrenergic neurons in the cardiovascular depression induced by morphine in the rat.

Morphine caused in the anaesthetized rat reduction in brain noradrenaline (NA) turnover, hypotension and bradycardia, similarly to the antihypertensive, alpha-adrenergic agonist, clonidine. All effects of morphine were antagonized by naloxone, as well as the alpha-receptor antagonist, yohimbine. In contrast, naloxone did not affect the circulatory depression and reduction in brain NA utilization by clonidine which both previously have been found to be antagonized by yohimbine. In contrast to clonidine, morphine even in high doses did not facilitate the flexor reflex activity of acutely spinalized rats. Pretreatment with protriptylin largely attenuated the circulatory depressive effects of morphine, as it has previously been found to block the corresponding effects of clonidine. Thus, the morphine-induced cardiovascular depressive effects are primarily elicited by activation of opiate receptors. However, the inhibition of brain NA neurotransmission by morphine appears critically involved in the mediation of the circulatory depression.