Involvement of D1-and D2-Like Dopamine Receptors Within the Rat Nucleus Accumbens in the Maintenance of Morphine Rewarding Properties in the Rats

Previous studies on drug abuse have shown that response to drug-associated cues exist during prolonged abstinence. In succession to previous investigations in our laboratory on morphine dependence and our research on acquisition and expression phases of morphine-conditioned place preference (CPP), in this study we attempt to determine the effects of intraaccumbal administration of SCH-23390, as a D1-like receptor antagonist, and sulpiride, as a D2-like receptor antagonist, in the maintenance of morphine-induced CPP in rats. Seventy-nine adult male Wistar rats weighing 200-280 g were bilaterally implanted with cannulas into the nucleus accumbens. During the 3-day conditioning phase, the animals received daily subcutaneous administration of morphine (5 mg/kg). CPP score and locomotor activity of animals were recorded by Ethovision software. Different doses (0.25, 1, 4 mu g per 0.5 mu L vehicle) of D1- and D2-like antagonists were bilateral injected daily after the expression phase and during the extinction phase. Our findings revealed that intraaccumbal administration of D1-like and D2-like antagonists after the CPP test shortened the extinction phase in the rats. The results suggested that the existence of the dopamine receptors in the nucleus accumbens was important for the maintenance of morphine-rewarding properties during the extinction phase. Therefore, dopamine receptors may be considered as a promising therapeutic agent in preventing the maintenance of morphine-rewarding effects in dependent individuals.