Changes in catecholaminergic pathways innervating the rat heart ventricle during morphine dependence. Involvement of alpha(1)- and alpha(2)-adrenoceptors

In the present study, we examined the effects of alpha(1)- and the alpha(2) -adrenoceptors blockade on the changes in the ventricular content of catech olamines in rats withdrawn from morphine. Rats were given morphine by s.c. implantation of morphine pellets for 5 days. On the seventh day, morphine w ithdrawal was induced by s.c. administration of naloxone (1 mg/kg), and rat s were killed 30 min later. Pretreatment with yohimbine (alpha(2)-adrenocep tor) or prazosin (alpha(1)-adrenoceptor) 15 min prior to naloxone administr ation attenuated some of the behavioural signs of morphine withdrawal. In a ddition, biochemical analysis indicated that yohimbine completely abolished the withdrawal-induced increase in noradrenaline acid dopamine turnover in the right ventricle. By contrast, prazosin did not block the hyperactivity of catecholaminergic neurons in the heart during withdrawal. These data su ggest that the hyperactivity of catecholaminergic neurons in the heart duri ng morphine withdrawal is dependent upon alpha(2)-adrenoceptor activation. In addition, the present results rule out the involvement of alpha(1)-adren oceptors. (C) 2000 Elsevier Science B.V. All rights reserved.