Effect of menstrual cycle on ethanol drinking in rhesus monkeys

Highlights

• Daily ethanol intake ∼3.0 g/kg did not alter menstrual cycle regularity or duration.

• More ethanol was drunk during peri-menstrual vs. peri-ovulatory phases of the cycle.

• Food-maintained responding was similar across the cycle.

Abstract

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.

Introduction

Excessive alcohol drinking and alcohol use disorder (AUD) persist as major public health problems, resulting in ∼80,000 deaths and costing >$223 billion per year in the US alone (Kanny et al., 2013). The development of effective medications requires an understanding of the how characteristics of individual patients affects their response to medication. For example, sex differences in the effects of alcohol are well-documented (see Becker and Koob, 2016); understanding how both alcohol and putative medications differentially affect men and women will permit development of sex-specific pharmacotherapies. Less is known about the interaction of menstrual cycle phase with the abuse-related effects of alcohol. Considering that interactions between drugs and gonadal hormones likely underlie male-female differences, it is of further interest to determine whether hormonal fluctuations across the menstrual cycle alter the effects of alcohol.

The question of whether drinking differs across the menstrual cycle has long been of interest. Early clinical studies, primarily consisting of retrospective reports and reviews of medical charts, supported the hypothesis that women increase alcohol intake just prior to or during menses to relieve associated dysphoria (e.g. Belfer et al., 1971; Podolsky, 1963). These studies had several limitations (reviewed in Tate and Charett, 1991), and early prospective reports found an opposite or no association between menstrual cycle phase and alcohol drinking (e.g., Griffin et al., 1987; Harvey and Beckman, 1985; Jones and Jones, 1984; Mello et al., 1986; Sutker et al., 1983). Other studies found no correlation between symptom severity and alcohol intake (e.g., Tate and Charette, 1991; Tate and Charette, 1991). Several reviews have identified methodological differences that likely underlie the discrepant results observed in human studies over the past six decades (e.g., Carroll et al., 2015; Gill, 1997; Lammers et al., 1995; Tate and Charette, 1991).

Studies in animal models lack some of the confounds inherent in human studies and allow for greater experimental and environmental control. Nonhuman primates have particular advantages that make them ideal research subjects in models of AUD (Grant and Bennett, 2003; Phillips et al., 2014; Weerts et al., 2007). Although important knowledge can be gained about the neuropharmacology of ethanol (EtOH) in rodents, the ability to generate clinically relevant phenotypes related to long-term drinking is limited in rodent species. Regarding studies of the interaction between menstrual cycle and alcohol drinking, nonhuman primate models are particularly valuable. Unlike rodents, which have a 4-day estrous cycle, the menstrual cycle of macaques (which include cynomolgus and rhesus monkeys) is similar to humans, with a duration of approximately 28 days and well-characterized fluctuations in estradiol and progesterone (Appt, 2004; Goodman et al., 1977; Jewitt and Dukelow, 1972). Only one study in nonhuman primates has directly assessed whether alcohol self-administration differs across the menstrual cycle (Mello et al., 1986). Female rhesus monkeys were trained to press a lever to receive intravenous (i.v.) injections of EtOH (0.12 g/kg per injection). Although monkeys who self-administered less than 1.5 g/kg per day tended to consume less in the middle of the cycle compared to the peri-menstrual period, different relationships between phase and intake were observed in the majority of monkeys who consumed more than 1.5 g/kg per day. Other relevant data include studies that used a drug discrimination procedure, which models the subjective effects of drugs. Grant and colleagues trained cynomolgus monkeys to discriminate orally administered EtOH (1 or 2 g/kg) from water (Grant et al., 1997; Green et al., 1999). Monkeys were more sensitive to the discriminative stimulus effects of EtOH during the luteal phase (around day 17 of cycle), but that effect diminished when the higher training dose was used.

In the present report, we examined data from five adult female rhesus monkeys to determine whether EtOH intake differed across the menstrual cycle. One noted confound in previous studies was variation in or absence of definition of specific menstrual cycle phases and inconsistencies as to which portions of the cycle were compared (e.g., Carroll et al., 2015). Thus, we analyzed data in three ways. First, we assessed whether drinking differed between “follicular” and “luteal” phases, defined as days 5–10 and 19–24 of the cycle, respectively. Next, we further divided the cycle into four divisions: early follicular (days 1–7), late follicular (days 8–14), early luteal (days 15–21) and late luteal (days 22-onset of next cycle) to better match the changing milieu of ovarian hormones. Finally, we compared drinking at three points of the cycle, similar to Mello et al. (1986). We compared the first 5 days of the cycle (“menstruation”), 5 days in the middle of the cycle (“mid-cycle,” days 12–16) and the last 5 days of the cycle (“late luteal” phase).