Opioid-neuroleptic interaction in brainstem self-stimulation.

Injection of morphine into the ventral tegmental area (but not dorsal to it) induced a dose-dependent decrease in the frequency threshold for midline metencephalic brain stimulation reward. Facilitating doses of ventral tegmental morphine also reversed, in 4 of 6 animals, the threshold-increasing effects of pimozide (0.35 mg/kg, i.p.). This reversal was itself reversed by naloxone (2 mg/kg, i.p.), suggesting a direct action of morphine at ventral tegmental opiate receptors. These data fit with electrophysiological evidence that ventral tegmental morphine stimulates or disinhibits dopamine impulse flow, which would result in increased synaptic dopamine concentrations and decreased synaptic pimozide effectiveness. In the remaining two animals, the combined neuroleptic-opiate treatment resulted in a complete cessation of responding that was not reversed by a 5-fold increase in stimulation frequency. This finding suggested a complete inactivation of the reward mechanism, which might be expected from the interaction of high doses of two drugs that are each known to be capable of producing depolarization inactivation of dopaminergic neurons. These data confirm that brainstem self-stimulation, like medial forebrain bundle self-stimulation, depends critically on the function of the mesocorticolimbic dopamine system.