Morphine and diprenorphine together potentiate intake of alcoholic beverages.

Water-deprived rats were given a daily opportunity to take water or an ethanol solution. Prior to some opportunities to drink, some were injected with morphine (across procedures either 2.0, 7.5, or 20.0 mg/kg), diprenorphine (from 0.001 to 10.0 mg/kg), or a combination of diprenorphine and morphine. The small dose of morphine increased intake of alcoholic beverage and the large dose decreased intake, confirming previous observations. Diprenorphine, across a wide range of doses, increased intake of ethanol solution. Morphine and diprenorphine together produced more intake than either given alone. Diprenorphine reversed the depressing effects of large doses of morphine on intake of ethanol solution. Since diprenorphine is an antagonist with respect to opioid analgesia and behavioral depression and an agonist with respect to intake of alcoholic beverages, and since it potentiates the small dose morphine effect, it is concluded that only some effects of morphine are related to opioid-potentiation of intake of alcoholic beverages.