Estrogen Receptor α Regulates Ethanol Excitation of Ventral Tegmental Area Neurons and Binge Drinking in Female Mice.

Elevations in estrogen (17ß-estradiol, E2) are associated with increased alcohol drinking by women and experimentally in rodents. E2 alters the activity of the dopamine system, including the VTA and its projection targets, which plays an important role in binge drinking. A previous study demonstrated that, during high E2 states, VTA neurons in female mice are more sensitive to ethanol excitation. However, the mechanisms responsible for the ability of E2 to enhance ethanol sensitivity of VTA neurons have not been investigated. In this study, we used selective agonists and antagonists to examine the role of ER subtypes (ERα and ERß) in regulating the ethanol sensitivity of VTA neurons in female mice and found that ERα promotes the enhanced ethanol response of VTA neurons. We also demonstrated that enhancement of ethanol excitation requires the activity of the metabotropic glutamate receptor, mGluR1, which is known to couple with ERα at the plasma membrane. To investigate the behavioral relevance of these findings, we administered lentivirus-expressing short hairpin RNAs targeting either ERα or ERß into the VTA and found that knockdown of each receptor in the VTA reduced binge-like ethanol drinking in female, but not male, mice. Reducing ERα in the VTA had a more dramatic effect on binge-like drinking than reducing ERß, consistent with the ability of ERα to alter ethanol sensitivity of VTA neurons. These results provide important insight into sex-specific mechanisms that drive excessive alcohol drinking.SIGNIFICANCE STATEMENT Estrogen has potent effects on the dopamine system and increases the vulnerability of females to develop addiction to substances, such as alcohol. We investigated the mechanisms by which estrogen increases the response of neurons in the VTA to ethanol. We found that activation of the ERα increased the ethanol-induced excitation of VTA neurons. 17ß-Estradiol-mediated enhancement of ethanol-induced excitation required the metabotropic glutamate receptor mGluR1. We also demonstrated that ERs in the VTA regulate binge-like alcohol drinking by female, but not male, mice. The influence of ERs on binge drinking in female mice suggests that treatments for alcohol use disorder in women may need to account for this sex difference.

Estradiol enhances ethanol reward in female mice through activation of ERα and ERß.

Alcohol use disorder (AUD) manifests differently in men and women, but little is known about sex differences in the brain's response to ethanol. It is known that the steroid hormone 17ß-estradiol (E2) regulates voluntary ethanol consumption in female rodents. However, the role of E2 as a regulator of ethanol reward has not been investigated. In this study, we tested for the effects of E2 and agonists selective for the classical estrogen receptors, ERα and ERß, on ethanol reward in ovariectomized (OVX) mice using the conditioned place preference (CPP) test. E2 enhanced ethanol CPP and, while specific activation of either receptor alone had no effect, co-activation of ERα and ERß also enhanced ethanol CPP, suggesting that E2 enhances ethanol reward in female mice through actions at both ERα and ERß. These results have implications for sex differences in the development of AUD, suggesting that women may find ethanol more rewarding than men because of higher circulating E2 levels.

Ovarian Hormones Contribute to High Levels of Binge-Like Drinking by Female Mice.

BACKGROUND: Recently, the incidence of binge drinking by women has increased. Binge drinking is detrimental to women's health, yet the biological mechanisms that promote excessive drinking by women are not well understood. One method of assessing binge-like ethanol (EtOH) consumption in mice is the drinking in the dark (DID) test, in which mice drink sufficient EtOH to achieve intoxication. In this study, we directly compared male, female, and ovariectomized (OVX) mice for DID and tested whether 17ß-estradiol (E2) contributes to DID. We also measured whether DID varies throughout the estrous cycle and whether repeated intermittent DID impacts the estrous cycle. METHODS: Male, female, and OVX C57BL/6J mice were tested for DID for 2 hours per day on days 1 to 3 and for 4 hours on day 4 using a single bottle containing 20% EtOH. To measure the effects of E2 on DID, OVX mice were treated with estradiol benzoate (EB) or vehicle daily starting 2 weeks prior to the drinking test and throughout the DID procedure. In a separate group of experiments, EtOH consumption and estrous cycle phase were measured in freely cycling mice that were drinking EtOH or water 5 days per week for 2 or 6 weeks. RESULTS: Female mice consumed more EtOH than male and OVX mice. Treatment with EB increased EtOH consumption by OVX mice compared with vehicle-treated controls. However, EtOH intake did not vary across the estrous cycle, nor did long-term DID alter the estrous cycle. CONCLUSIONS: These results demonstrate that ovarian hormones, specifically E2, contribute to increased EtOH consumption by female mice in the DID test. Although ovarian hormones contribute to this behavior, EtOH consumption is not affected by estrous cycle phase in freely cycling mice. This study provides a framework for understanding the factors that contribute to binge drinking in females.

Studying Sex Differences in Animal Models of Addiction: An Emphasis on Alcohol-Related Behaviors.

Animal models are essential for understanding the biological factors that contribute to drug and alcohol addiction and discovering new pharmacotherapies to treat these disorders. Alcohol (ethanol) is the most commonly abused drug in the world, and as the prevalence of alcohol use disorder (AUD) increases, so does the need for effective pharmacotherapies. In particular, treatments with high efficacy in the growing number of female AUD sufferers are needed. Female animals remain underrepresented in biomedical research and sex differences in the brain's response to alcohol are poorly understood. To help bridge the gender gap in addiction research, this Review discusses strategies that researchers can use to examine sex differences in the context of several common animal models of AUD. Self-administration, two-bottle choice, drinking in the dark, and conditioned place preference are discussed, with a focus on the role of estrogen as a mediator of sex differences in alcohol-related behaviors.

Sex differences in cocaine conditioned place preference in C57BL/6J mice.

Men and women respond differently to the subjective effects of cocaine and cocaine-associated cues, which has implications for the development and maintenance of cocaine addiction. Preclinical studies performed in rats, modeling various aspects of cocaine addiction, have largely validated these results, indicating that female rats may be more sensitive to the rewarding properties of cocaine. The molecular mechanisms leading to sex differences in cocaine reward have largely not been determined, although sex hormones are thought to play a role. The mouse is commonly used as a model organism to study the molecular and genetic factors that influence a variety of psychiatric disorders. In particular, the inbred C57BL/6 mouse strain is often used for behavioral studies related to substance abuse. To begin to understand the hormonal, molecular and genetic mechanisms that might affect cocaine reward, we directly compared male and female C57BL/6J mice in cocaine conditioned place preference (CPP), a test that examines the rewarding and cue-associated properties of drugs of abuse. We conditioned mice at three doses of cocaine and examined preference and extinction of preference. We found that the acquisition of cocaine CPP did not differ between male and female mice. However, extinction of cocaine CPP was delayed in male mice compared with females at the lowest dose of cocaine. We conclude that sex differences in cocaine CPP can be observed in C57BL/6J mice at very low doses of cocaine.