Hypothalamic self-stimulation under the chronic morphine treatment in the rat.

The influence of morphine on self-stimulation (SS) under chronic morphine administration (30 injections for 15 days) in increasing doses (from 20 to 20 mg/kg/injection) has been studied. Initial doses had a depressive effect varying in degree in different animals. Subsequent injections activated "reward" behavior. This activation is suggested as one of the neurophysiological mechanisms of morphine abuse and addiction.