DISCRIMINATIVE STIMULUS PROPERTIES OF COCAINE, ALONE AND IN COMBINATION WITH BUPRENORPHINE, MORPHINE AND NALTREXONE

Rats were trained to discriminate a dose of 10.0 mg/kg cocaine from saline. During substitution tests, both cocaine (5.6-10.0 mg/kg) and d-amphetamine (1.0-3.0 mg/kg) produced greater than 80% responding on the cocaine-appropriate lever. In contrast, buprenorphine (0.03-0.56 mg/kg), morphine (0.3-10.0 mg/kg) and naltrexone (1.0-10.0 mg/kg) failed to substitute for the cocaine stimulus, up to doses that substantially decreased rate of responding. When the cocaine dose-effect curve was redetermined in the presence of selected doses of buprenorphine, the amount of cocaine-appropriate responding following a low dose of cocaine (1.0 mg/kg) was increased slightly whereas cocaine-appropriate responding following higher doses of cocaine (3.0 and 5.6 mg/kg) was reduced slightly. Responding following the training dose of cocaine (10.0 mg/kg) was not changed. These results indicate that buprenorphine produced only small alterations in cocaine's discriminative stimulus effects and that the nature of these alterations differed depending on the dose of cocaine examined.