Evidence for an underlying opponent process during morphine elicited hyperactivity in the hamster

Two experiments investigated the effects of naloxone on morphine elicited hyperactivity in the hamster. In Experiment 1, naloxone (0.4 mg/kg) administered two hours after morphine (15 mg/kg) produced sedation in animals running at high rates under the influence of morphine. Saline control animals running at comparable rates were unaffected by naloxone. In Experiment 2, naloxone administered two hours after morphine converted morphine elicited hyperactivity into sedation. These results are discussed in terms of a modified dual-action hypothesis which holds that morphine elicited hyperactivity masks an underlying opponent process.