Discriminative Stimulus Effects of Naltrexone in Rats with Limited Access to Sucrose

Abstract

In most operant paradigms, large doses of naloxone are typically required to establish naloxone as a discriminative stimulus in opioid‐naïve subjects. Chronic, but limited access to sucrose increases endorphin and dopamine function in rats. We wondered if rats given limited access to sucrose could be trained to recognize doses of naltrexone (NTX) that are typically not discriminable in operant paradigms. Prior to discrimination training, rats were trained to lever press and were then given access to sucrose (25% or 32%) for 12 hours a day for two weeks. Rats were then given daily injections of either saline or NTX (3.2 mg/kg, 15 min PT) 1 hour after sucrose access. After the training session subjects were returned to their home cage for the remainder of the 12 hour sucrose‐access period. Subjects learned to discriminate NTX from saline in a mean of 72 sessions (Md = 63, range 27‐135 sessions). No significant differences in acquisition of the NTX‐saline discrimination or initial NTX dose‐effect functions were noted between sucrose concentrations. These results provide further support for the notion that chronic sucrose consumption increases endogenous endorphin function.