Dopaminergic mediation of reward produced by direct injection of enkephalin into the ventral tegmental area of the rat.

A conditioned place preference paradigm was employed to demonstrate the rewarding property of unilateral injections of 200 ng (D-ala2)-met5-enkephalinamide into the ventral tegmental area of the rat brain. This effect was attenuated in a dose-related manner by systemic injections of the dopamine receptor blocker haloperidol. In addition, selective lesions of the ascending dopamine (DA) pathways ipsilateral to the injection site blocked the rewarding effect when DA levels were reduced by more than 90%. Similar lesions in the contralateral hemisphere had no influence on this behavior. These data suggest that forebrain DA pathways can mediate some of the rewarding properties of opioid drugs.