Low-dose apomorphine attenuates morphine-induced enhancement of brain stimulation reward.

Thresholds for brain stimulation reward (BSR) delivered to the medial forebrain bundle-lateral hypothalamus were determined by means of a rate free psychophysical method. Lower doses of apomorphine (0.5 to 0.2 mg/kg) produced modest elevations in BSR thresholds. A 0.4 mg/kg dose of apomorphine resulted in emergence of stereotypic behaviors and the loss of stimulus control. Morphine's BSR threshold lowering effects were significantly blocked by the concurrent administration of a 0.1 mg/kg dose of apomorphine. These results support the hypothesis that presynaptic dopamine neurons are involved in the mediation of morphine's reinforcing effects and that dopamine autoreceptor agonists may be of some use in the pharmacotherapy of opiate abuse.