ORALLY DELIVERED METHADONE AS A REINFORCER FOR RHESUS-MONKEYS - THE RELATIONSHIP BETWEEN DRUG CONCENTRATION AND CHOICE

The relative reinforcing effects of orally delivered methadone were st udied in five male rhesus monkeys. Drug deliveries were available unde r either a fixed-ratio (FR) or a fixed-interval (FI) schedule. Three c oncentrations of methadone, low (0.05 mg/ml), intermediate (0.2 mg/ml) , and high (0.8 mg/ml) were delivered in 0.65 mi volumes. In the first experiment, monkeys were presented with a choice paradigm. Under inde pendent FR schedules responding led to a delivery of either a methadon e solution or the water vehicle. For each concentration, deliveries of a methadone solution maintained higher response rates than did delive ries of water. In the second experiment, methadone concentrations were tested in pairs in the following sequence: high vs. low, high vs. int ermediate, intermediate vs. low, high vs. intermediate (retest), and h igh vs. low (retest). The retest of the last two pairs was designed to counterbalance the test sequence, so that order effects, if they exis ted, could be detected. Regardless of the schedule, the higher concent ration of the methadone pair maintained a greater response rate than d id the lower concentration. The present results are consistent with th e generalization based on other studies that over a broad range of con centrations and across pharmacological classes, reinforcement schedule s, and routes of administration, reinforcing effects increase with inc reases in drug concentration.