Pharmacology of sucrose-reinforced place-preference conditioning: Effects of naltrexone

Two experiments investigated the role of the opioid system in sucrose-reinf orced conditioned place preferences (CPPs) in rats. Experiment 1 examined t he effects of a general opioid antagonist, naltrexone, on the expression of a CPP acquired in the absence of the drug. Subjects were trained to associ ate one compartment of a two-compartment chamber with sucrose and the other compartment with water. Rats displayed a preference for the sucrose-associ ated compartment in a choice test without sugar or water available followin g vehicle saline treatment. Naltrexone doses of 2.5 and 5.0 mg/kg reduced t his preference for the sucrose-associated compartment. Experiment 2 examine d the effects of naltrexone on the acquisition as well as the expression of CPPS. Different groups of rats received daily injections of either saline, 0.1. 1.0, or 5.0 mg/kg of naltrexone prior to each training session, and t hen these groups were given a choice test for the CPP after saline or naltr exone injections. Although naltrexone treatment attenuated the expression o f CPPs in each group relative to saline treatment, there were no group diff erences during these tests in the magnitude of the preferences. Moreover, a ll groups displayed equal acquisition of CPPs despite the fact that naltrex one dose dependently decreased sucrose intake during the training phase. To gether, the results indicate that the opioid system modulates the expressio n but not the acquisition of sucrose-reinforced CPPs. (C) 2000 Elsevier Sci ence Inc.