Opiate agonists microinjected into the nucleus accumbens enhance sucrose drinking in rats

Previous studies have indicated that central opioid peptides and opiate receptors play an important role in the modulation of ingestive behaviors. The nucleus accumbens (Acb), a forebrain region involved in reinforcement, contains high levels of opiate receptors. The present investigation was undertaken to determine the relative involvement of opiate receptor subtypes in sucrose drinking. Morphine (0, 0.5, 5 micrograms/0.5 microliter), the mu agonist D-Ala2, NMe-Phe4, Glyo 15-enkephalin (DAMGO; 0, 0.025, 0.25 and 2.5 micrograms/0.5 microliter), the delta agonist D-Pen2,5-enkephalin (DPEN; 0, 0.031, 0.31, 3.1 micrograms/0.5 microliter), and the kappa agonists U50488H (0, 0.0186, 0.186, 1.86 micrograms/0.5 microliter), and dynorphin (0, 0.05, 0.5, 5 micrograms/0.5 microliter) were microinfused into Acb. Intake of 5% sucrose, drinking duration, locomotion, rearing, and grooming were measured in a 30-min session in rats previously adapted to sucrose. After microinjection into Acb, morphine induced a robust increase in both sucrose intake and drinking duration at the low dose. DAMGO enhanced sucrose drinking at lower doses, and suppressed drinking at the highest dose. DPEN also increased sucrose intake in a dose-dependent manner. U50488H and dynorphin had no effect on sucrose drinking. In addition, it was demonstrated that intra-Acb administration of DAMGO specifically enhanced palatable sweet solution drinking, leaving water intake unchanged. Although mu and delta agonists tended to increase spontaneous motor activity, the pattern of effects indicated that increases in ingestion could not be simply attributed to general arousal. These findings demonstrate that both mu and delta receptors within the accumbens may have an important modulatory role in ingestion of palatable substances.