Lack of sensitization or tolerance to the facilitating effect of ventral tegmental area morphine on lateral hypothalamic brain stimulation reward

Unilateral microinjections of morphine (2.5 μg/0.5 μl) into the ventral tegmental area potentiated the rewarding effects of lateral hypothalamic brain stimulation, causing parallel leftward shifts of the functions relating response rate to stimulation frequency. Testing was repeated eight times with no evidence of reliable day-to-day changes either in the direction of sensitization or in the direction of tolerance to the reward-facilitating effects of the drug. The injection dose was sufficient to cause contraversive circling that continued long after the period of reward testing; as with the reward-facilitation, there was neither tolerance nor sensitization to this locomotor effect of the drug. The same dose given to control animals that were not tested for self-stimulation caused progressively stronger circling with repeated testing. Lack of sensitization of the reward-facilitating effects of ventral tegmental morphine raises the possibility of some degree of independence of the brain mechanisms of drug-induced locomotion and of reward-facilitation.