Effect of menstrual cycle on ethanol drinking in rhesus monkeys.

backgroundSex differences in the abuse-related effects of alcohol have been demonstrated in the clinic and in preclinical animal models. Less is known about the influence of menstrual cycle phase on drinking.

methodsIn this study, we examined the relationship between menstrual cycle phase and intake of ethanol (EtOH) in five adult female rhesus monkeys. Subjects consumed a 4% EtOH solution in their home cage 6 h per day, 5 days per week and pressed a lever to receive food pellets during the drinking session. Menstrual cycle was determined with vaginal swabs 5-7 days per week. To facilitate comparison with previous studies, the cycle was divided three different ways for analysis.

resultsFirst, no significant difference was observed when EtOH intake was compared between phases defined as "follicular" (days 5-10) and "luteal" (19-24). Second, when the cycle was further divided into four phases [early follicular (days 1-7), late follicular (8-14), early luteal (15-21) and late luteal (22-next cycle)], significant differences were detected, with intake highest in phases that bracket menses and lowest in the late follicular phase. Finally, EtOH intake during "mid-cycle" (days 12-16) was significantly lower than during "menses" (days 1-5) and "late luteal" (last 5 days). Effect sizes were small to moderate, although absolute differences in EtOH intake (g/kg) were <15%. Food-maintained responding was not different across phases.

conclusionsMenstrual cycle has modest but statistically significant and selective effects on EtOH drinking, with higher EtOH intake observed in the peri-menstrual period compared to the middle of the cycle.