Consumption of ethanol solution is potentiated by morphine and attenuated by naloxone persistently across repeated daily administrations.

For 29 days, rats were given a daily opportunity to take a sweetened ethanol solution (ES) or water when deprived of water. Under this regimen, and throughout all procedures, rats gained weight normally. They eventually took considerable amounts of ethanol (E). Across the next days of the regimen, one group was given an injection of morphine (MOR), 2.5 mg/kg, 30 min before each of 6 consecutive daily opportunities to drink. MOR enhanced intake of ES on every day of injections. Naloxone (NX) (2.5 mg/kg, 10 min before opportunity to drink) was given to rats having had many daily opportunities to take ES. NX reduced intakes day after day. Rats given NX from the first opportunity to take ES did not develop an avidity for ES as other rats given placebo-injections. MOR increased intakes among rats not deprived of water, among rats housed in groups as well as individually, among rats taking unsweetened ES, and among rats presented with various flavors of ES as the only solution during an hour-long session. The data confirm and extend the conclusion that small doses of MOR enhance E-intake, indicate that the effects of opioids on E-intake do not tolerate with repeated administrations, and generally support the idea that an endogenous opioid system is involved in the reinforcement derived from E.