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RESEARCH PRODUCT

Lack of Specific Effects of Selective D1 and D2 Dopamine Antagonists vs. Risperidone on Morphine-Induced Hyperactivity

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Abstract RODRIGUEZ-ARIAS, M., I. BROSETA, M. A. AGUILAR AND J. MINARRO. Lack of specific effects of selective D 1 and D 2 dopamine antagonists on morphine-induced hyperactivity. PHARMACOL BIOCHEM BEHAV 66 (1) 189–197, 2000.—In the present study, three different dopamine antagonists were challenged in order to counteract hyperactivity induced by 50 mg/kg of morphine. A wide range of doses of morphine (50, 25, 12.5, 6.25, or 3.12 mg/kg) were evaluated on spontaneous locomotor activity. A significant increase was observed only with the two higher doses tested (25 and 50 mg/kg). No decrease was found with any of the doses used at any period of time. After analyzing doses of SCH 23390 (0.5, 0.1, and 0.05 mg/kg), raclopride (0.5, 0.25, and 0.125 mg/kg) and risperidone (0.1, 0.05, and 0.025 mg/kg) administered alone, only the 0.5 mg/kg dose of SCH 23390 decreased locomotor activity. The three compounds counteracted morphine-induced hyperactivity, but with SCH 23390 it was only achieved with the dose of 0.5 mg/kg, which also decreased spontaneous locomotor activity and induced catalepsy. On the other hand, raclopride and risperidone neutralized morphine-induced hyperactivity at doses that did not affect locomotor activity, although the former induced catalepsy when administered with morphine. It is concluded that although the blockade of D 1 and D 2 DA receptors decreases morphine-induced hyperactivity, this action is not specific, contrary to the action of risperidone, which counteracts this hyperactivity without any other motor effects.