Reinforcing effects of morphine microinjection into the ventral tegmental area

A neural substrate for the reinforcing property of an opiate drug was identified in the ventral tegmental area (VTA) by establishing conditioned reinforcement to salient environmental stimuli paired with intracerebral microinjections of morphine. Bilateral microinjections of morphine into the VTA in doses of 0.2 microgram and 1.0 microgram produced a subsequent change in place preference to a distinctive compartment previously associated with the stimulant effects of morphine. Microinjection of 1.0 microgram morphine at sites 2.5 mm dorsal to the VTA had no effect. Pretreatment with naloxone (2 mg/kg) antagonized the reinforcing effects of 1.0 microgram morphine as this group showed no significant change in place preference. Nor did control groups receiving microinjections of sterile physiological saline. Taken together, these data suggest that opiate receptors, located in the ventral tegmental area, play an important role in mediating the reinforcing effects of morphine. The possible involvement of dopaminergic neurons in these effects is discussed.