DISCRIMINATION AMONG MORPHINE, SALINE AND NALTREXONE IN RHESUS-MONKEYS RECEIVING MORPHINE SUBCHRONICALLY

Discriminative control was established among morphine, saline and nalt rexone in rhesus monkeys receiving morphine every other day. Three hou rs prior to sessions subjects received saline or 3.2 mg/kg morphine; i mmediately prior to sessions they received saline or 0.01 mg/kg of nal trexone. There were dose-related generalizations to each training cond ition: morphine generalized to the morphine plus saline lever; small d oses of naltrexone reversed effects of morphine and larger doses occas ioned responding on the morphine plus naltrexone lever; in one monkey still larger doses occasioned responding on the saline plus saline lev er. When saline was administered 3h earlier, naltrexone had no effect in one subject and occasioned responding on the morphine plus naltrexo ne lever in a second subject. Nalbuphine substituted for morphine plus saline in one monkey and for morphine plus naltrexone in a second mon key; ketamine did not substitute for either training drug. That stimul us control was established between no drug and a combination of morphi ne and naltrexone suggests the latter condition did not represent the absence of morphine. In addition to demonstrating stimulus control for three conditions in rhesus monkeys, the current study suggests opioid antagonists might have novel discriminative stimulus effects at opioi d receptors even under conditions where signs of withdrawal are not ev ident.