Effect of menstrual cycle on ethanol drinking in rhesus monkeys.

Background: Sex 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.

Methods: In 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.

Results: First, 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.

Conclusions: Menstrual 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.