ACUTE MORPHINE-DEPENDENCE IN THE HAMSTER

Two experiments investigated naloxone-precipitated withdrawal following a brief course of morphine administration in hamsters. In Experiment 1, observable withdrawal symptoms (e.g., wet-dog shakes) were elicited by two doses of naloxone (0.4 and 1.0 mg/kg) following four and eight daily injections of morphine (15 mg/kg), a regimen that replicated previous studies in our laboratory using a locomotor activity paradigm. At the lower dose of naloxone, the frequency of withdrawal signs was greater after eight than after four morphine injections. In Experiment 2, observable withdrawal symptoms were elicited by the same two doses of naloxone, 70 min after a single morphine injection. These results suggest that acute dependence in the hamster, as in other species, begins to develop with the first morphine exposure.