Self-administration of methionine enkephalin into the nucleus accumbens.

Microinfusions of the endogenous opiate neurohumor, methionine enkephalin, into the nucleus accumbens initiated a reinforcing stimulus in a dose-related manner. The reinforcing nature of this intracranial self-administration was evaluated with intermittent schedules of reinforcement and a two-lever discrimination procedure. Opiate receptors are likely responsible for the initiation of this reinforcing stimulus since naloxone effectively blocked self-administration. These data suggest the mediation of opiate reinforcement through interactions with opiate receptors in brain regions outside the ventral tegmental area, questioning the current dopamine hypothesis for the initiation of these reinforcement processes.