Eighty-three male rats were maintained on a daily regimen involving 22 h of deprivation of fluids followed by 2 h of access to water and a sweetened alcoholic beverage (12% ethanol, 5% sucrose). After about 3 weeks of such a regimen, rats eventually take considerable amounts of ethanol daily. In the present study, a series of injections of opioids was given subsequent to establishing stable daily intakes of ethanol. Specifically, before a day's opportunity to take fluids, some rats were given a small dose of morphine (2.0 mg/kg), while others were given a dose of naloxone (4.0 mg/kg). When morphine was given 0.5 h before the opportunity to drink, intake of ethanol was increased. However, when morphine was given 4.0 h before the opportunity, intake of ethanol was decreased. Nearly opposite effects were observed when naloxone was given. Other experiments tested the effects of giving morphine 4.0 h before the opportunity to drink plus the effects of a small dose of naloxone or plus the effects of a small dose of morphine. Morphine given 4.0 h before potentiated the effects of a small dose of naloxone and attenuated the effects of another dose of morphine. The effects of morphine were also shown to be similar among rats taking a solution of ethanol and water rather than a sweetened solution. These data provide support for the idea that surfeits, not deficits, in opioidergic activity increase propensity to take alcoholic beverages.
