Alcohol drinking in rats during and following morphine injections.

Adult male Sprague-Dawley rats given daily SC injections of 2.5 mg/kg or 10 mg/kg morphine sulfate (for eight days) decreased preference for a 5% (v/v) ethanol solution, and then increased ethanol preference during the postinjection period, relative to a saline control group. These bidirectional effects were directly related to morphine dosage. The results were interpreted as supporting the theory that alcohol drinking is reinforced through its interaction with the endogenous opioid system and can compensate for deficiencies in opioid receptor activity.