Persistence and specificity of small doses of morphine on intake of alcoholic beverages.

Subsequent to water deprivation, male rats were given daily, 1.5-hr opportunities to take either water or a sweetened ethanol solution (ES). Each day, 15 min before the session, rats received a subcutaneous injection of either morphine (1.0 mg/kg) or saline. Across daily sessions, rats given saline gradually increased their intake of ES, until they were eventually taking about 2.0 to 3.0 g of ethanol/kg a session. Rats receiving morphine took greater amounts of ES from nearly the first opportunities. Additional tests assessed the effects of small doses of morphine on intakes of some sucrose solutions, and sweetened solutions containing methanol or propanol. The data support the conclusion that small doses of morphine persistently increase intake of ES across many days (up to 100) of testing, but that the effect is not unique to ES. Even though morphine's effects are not specific to ethanol, the fact that morphine persistently increases intake of ES is of interest with respect to theories of alcoholism.