Chronically administered morphine, circadian cyclicity, and intake of an alcoholic beverage

Subsequent to 22 hr of deprivation of water, 40 male rats were given daily access to water and a sweetened ethanol solution for 2 hr each day for 24 days. Twenty of the rats had this opportunity to take fluids during the light-phase of the circadian cycle while the other twenty were given it during the dark. Prior to the 24 days, some rats (half of each group) were implanted subcutaneously with osmotic pumps giving them constant infusions of morphine (0.6 mg/kg/hr). Across daily sessions, all rats gradually increased their intakes of ethanol with the greatest increases among those having morphine and taking fluids during the light-phase. The rats which drank during the dark gained more weight than the others and the most marked gains in weight were by those receiving no morphine. The data support the conclusion that some general disruption resulting from the schedule of presentation of fluids during the day qualifies as a setting condition for heightened intake of ethanol.