Effects of morphine withdrawal on catecholaminergic neurons on heart rightventricle; implication of dopamine receptors

The purpose of our study was to examine the effects of D-1-and D-2-dopamine receptors blockade on the changes in the ventricular content of catecholam ines in rats withdrawn from morphine. Rats were given morphine by subcutane ous (sc) implantation of morphine pellets for 5 days. On the eighth day, mo rphine withdrawal was induced by sc administration of naloxone (1 mg/kg), a nd rats were killed 30 min later. Pretreatment with SCH 23390 (dopamine D-1 , D-5 receptor antagonist) 15 min prior to naloxone administration suppress ed some the behavioural signs of morphine withdrawal, whereas eticlopride ( dopamine D-2, D-3, D-4 receptor antagonist) did not. In addition, biochemic al analysis indicate that SCH 23390 completely abolished the withdrawal-ind uced increase in noradrenaline and dopamine turnover in the right ventricle . By contrast, eticlopride did not block the hyperactivity of catecholamine rgic neurons in the heart during morphine withdrawal. These data suggest th at the hyperactivity of catecholaminergic neurons in the heart during morph ine withdrawal is dependent upon D-1 dopamine receptor activation. In addit ion, our results exclude the involvement of D-2 dopamine receptors.