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

Neuronal biomarkers as potential therapeutic targets for drug addiction related to sex differences in the brain: Opportunities for personalized treatment approaches

Biological sex disparities manifest at various stages of drug addiction, including craving, substance abuse, abstinence, and relapse. These discrepancies are underpinned by notable distinctions in neurobiological substrates, encompassing brain structures, functions, and neurotransmitter systems implicated in drug addiction. Neuronal biomarkers, such as neurotransmitters, signaling proteins, and genes may be associated with the diagnosis, prognosis, and treatment outcomes in both biological sexes afflicted by drug abuse. Sex differences in the neural reward system, mainly through dopaminergic transmission during drug abuse, can be attributed to modifications in neurotransmitter systems and signaling pathways. This results in distinct patterns of neural activation and responsiveness to addictive substances in males and females. Sex hormones, the estrus/menstrual cycle, and cerebral neurochemistry contribute to the progression of psychological and physiological dependence in both male and female individuals grappling with addiction. Moreover, the alteration of sex hormone balance and neurotransmitter release plays a pivotal role in substance use disorders, subsequently modulating cognitive functions pertinent to reward, including memory formation, decision-making, and locomotor activity. Comparative investigations reveal distinctions in brain region volume, gene expression, neuronal firing, and circuitry in substance use disorders affecting individuals of both biological sexes. This review examines prevalent substance use disorders to elucidate the impact of sex hormones as therapeutic biomarkers on the mesocorticolimbic neurotransmitter systems via diverse mechanisms within the addicted brain. We underscore the imperative necessity of considering these variations to gain a deeper comprehension of addiction mechanisms and potentially discern sex-specific neuronal biomarkers for tailored therapeutic interventions.

Identification of Phosphodiesterase-7A (PDE7A) as a Novel Target for Reducing Ethanol Consumption in Mice

BACKGROUND

Ethanol elicits a rapid stimulatory effect and a subsequent, prolonged sedative response, which are potential predictors of EtOH consumption by decreasing adenosine signaling; this phenomenon also reflects the obvious sex difference. cAMP (cyclic Adenosine Monophosphate)-PKA (Protein Kinase A) signaling pathway modulation can influence the stimulatory and sedative effects induced by EtOH in mice. This study's objective is to clarify the role of phosphodiesterase (PDE) in mediating the observed sex differences in EtOH responsiveness between male and female animals.

METHODS

EtOH was administered i.p. for 7 days to identify the changes in PDE isoforms in response to EtOH treatment. Additionally, EtOH consumption and preference of male and female C57BL/6J mice were assessed using the drinking-in-the-dark and 2-bottle choice tests. Further, pharmacological inhibition of PDE7A heterozygote knockout mice was performed to investigate its effects on EtOH-induced stimulation and sedation in both male and female mice. Finally, Western blotting analysis was performed to evaluate the alterations in cAMP-PKA/Epac2 pathways.

RESULTS

EtOH administration resulted in an immediate upregulation in PDE7A expression in female mice, indicating a strong association between PDE7A and EtOH stimulation. Through the pharmacological inhibition of PDE7A KD mice, we have demonstrated for the first time, to our knowledge, that PDE7A selectively attenuates EtOH responsiveness and consumption exclusively in female mice, whichmay be associated with the cAMP-PKA/Epac2 pathway and downstream phosphorylation of CREB and ERK1/2.

CONCLUSIONS

Inhibition or knockdown of PDE7A attenuates EtOH responsivenessand consumption exclusively in female mice, which is associated with alterations in the cAMP-PKA/Epac2 signaling pathways, thereby highlighting its potential as a novel therapeutic target for alcohol use disorder.

Sex Differences in Alcohol Use: Is It All About Hormones?

Risky alcohol use and alcohol use disorders (AUD) are a rising problem in women, yet a major disparity in our understanding of what drives alcohol consumption in women remains. Historically biomedical research has focused on male subjects; however, recent increases in reporting of females, have highlighted major differences between the sexes. Here we review the current literature of the effect of gonadal steroid hormones (estrogens, androgens, and progestins), neurosteriods, and neurobiological factors on alcohol use in clinical and preclinical studies of both sexes. Further, we briefly discuss how fundamental sex differences in genetics, metabolism, neuroimmune, and stress responses may influence sex differences in alcohol intake. Comparing the sexes could aid in the discovery of novel therapeutics to treat AUD, and implementation of current treatment options in women.

Conditioned preferences: Gated by experience, context, and endocrine systems

Central to the navigation of an ever-changing environment is the ability to form positive associations with places and conspecifics. The functions of location and social conditioned preferences are often studied independently, limiting our understanding of their interplay. Furthermore, a de-emphasis on natural functions of conditioned preferences has led to neurobiological interpretations separated from ecological context. By adopting a naturalistic and ethological perspective, we uncover complexities underlying the expression of conditioned preferences. Development of conditioned preferences is a combination of motivation, reward, associative learning, and context, including for social and spatial environments. Both social- and location-dependent reward-responsive behaviors and their conditioning rely on internal state-gating mechanisms that include neuroendocrine and hormone systems such as opioids, dopamine, testosterone, estradiol, and oxytocin. Such reinforced behavior emerges from mechanisms integrating past experience and current social and environmental conditions. Moreover, social context, environmental stimuli, and internal state gate and modulate motivation and learning via associative reward, shaping the conditioning process. We highlight research incorporating these concepts, focusing on the integration of social neuroendocrine mechanisms and behavioral conditioning. We explore three paradigms: 1) conditioned place preference, 2) conditioned social preference, and 3) social conditioned place preference. We highlight nonclassical species to emphasize the naturalistic applications of these conditioned preferences. To fully appreciate the complex integration of spatial and social information, future research must identify neural networks where endocrine systems exert influence on such behaviors. Such research promises to provide valuable insights into conditioned preferences within a broader naturalistic context.

Role of estrogen in sex differences in memory, emotion and neuropsychiatric disorders

Estrogen regulates a wide range of neuronal functions in the brain, such as dendritic spine formation, remodeling of synaptic plasticity, cognition, neurotransmission, and neurodevelopment. Estrogen interacts with intracellular estrogen receptors (ERs) and membrane-bound ERs to produce its effect via genomic and non-genomic pathways. Any alterations in these pathways affect the number, size, and shape of dendritic spines in neurons associated with psychiatric diseases. Increasing evidence suggests that estrogen fluctuation causes changes in dendritic spine density, morphology, and synapse numbers of excitatory and inhibitory neurons differently in males and females. In this review, we discuss the role of estrogen hormone in rodents and humans based on sex differences. First, we explain estrogen role in learning and memory and show that a high estrogen level alleviates the deficits in learning and memory. Secondly, we point out that estrogen produces a striking difference in emotional memories in men and women, which leads them to display sex-specific differences in underlying neuronal signaling. Lastly, we discuss that fluctuations in estrogen levels in men and women are related to neuropsychiatric disorders, including schizophrenia, autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), bipolar disorder (BPD), major depressive disorder (MDD), substance use disorder (SUD), and anxiety disorders.

Influence of Inherited Seizure Susceptibility on Intermittent Voluntary Alcohol Consumption and Alcohol Withdrawal Seizures in Genetically Epilepsy-Prone Rats (GEPR-3s)

BACKGROUND

The link between epilepsy and alcohol consumption is complex, with conflicting reports. To enhance our understanding of this link, we conducted a study to determine how inherited seizure susceptibility affects voluntary alcohol consumption and influences alcohol withdrawal seizures in male and female genetically epilepsy-prone rats (GEPR-3s) compared to Sprague Dawley (SD) rats.

METHODS

In the first experiment, animals were given access to two bottles simultaneously, one containing water and the other 7.5%, 15%, or 30% (v/v) alcohol three times a week for each dose after acclimation to drinking water. In a second experiment, animals were tested for acoustically evoked alcohol seizures 24 h after the last session of voluntary alcohol consumption.

RESULTS

Analysis revealed that GEPR-3s (males and females) had lower alcohol intake and preference than SD rats, particularly at lower alcohol concentrations. However, female GEPR-3s consumed more alcohol and had a higher alcohol preference than males. Furthermore, withdrawal from voluntary alcohol consumption facilitated the onset and duration of seizures in GEPR-3s.

CONCLUSIONS

Our study suggests that genetic seizure susceptibility in GEPR-3s is negatively associated with alcohol consumption. However, withdrawal from low to moderate amounts of alcohol intake can promote epileptogenesis in the epileptic GEPR-3s.

Behavioral characterization of co-exposure to cannabinoids and hormonal contraceptives in female rats

Hormonal contraceptives are among the most widely used drugs by young healthy women to block ovulation and avoid pregnancy. They reduce the ovarian secretion of estradiol and progesterone, hormones that also modulate neuronal plasticity, cognitive functions, emotions and mood. Cannabis is the most commonly used illicit drug worldwide and its use is increasing among young women, many of which regularly take the "pill". Despite evidence of a bidirectional interaction between the endocannabinoid system and gonadal hormones, only very few studies have examined the consequences of cannabis consumption in young females under hormonal contraceptives treatment. To fill this gap, this study evaluated the behavioral effects of co-exposure to chronic 1) hormonal contraceptives, i.e., ethinyl estradiol (EE) plus levonorgestrel (LNG), one of the synthetic estrogen-progestin combinations of hormonal contraceptives, and 2) cannabinoid receptor agonist, i.e., WIN 55,212-2 (WIN), on motor activity, emotional state and cognitive functions in young adult female rats (8-11/experimental group). Hormonal and cannabinoid treatment started at post-natal day (PND) 52 and 56, respectively, while behavioral testing occurred between PND 84-95. The results show that chronic EE-LNG treatment, at doses (0.020 and 0.060 mg/rat, respectively) known to drastically reduce plasma progesterone levels, and the contextual exposure to WIN, at a dose (12.5 μg/kg/infusion) known to be rewarding in the rat, alters the hormonal milieu but does not cause further changes in locomotor activity compared to EE-LNG or WIN alone, and does not modify anxiety-like state (as measured by the elevated plus maze and the marble burying tests) and cognitive abilities (as measured by the novel object recognition and the prepulse inhibition tests) in young adult female rats. Although exposure to EE-LNG and WIN tends to increase the duration of immobility and to reduce the time spent swimming in the forced swimming test, there was not a significant additive effect suggestive of a depressive-like state. These findings allow deepening the current knowledge on the interaction between cannabinoid agonists and hormonal contraceptives and suggest that low, rewarding doses of cannabinoids do not significantly alter the motor and cognitive skills and do not induce anxiety or depressive-like states in females that use hormonal contraceptives.

Removal of the ovaries suppresses ethanol drinking and promotes aversion-resistance in C57BL/6J female mice

RATIONALE

Female rodents consume more ethanol (EtOH) than males and exhibit greater aversion-resistant drinking in some paradigms. Ovarian hormones promote EtOH drinking but the contribution of ovarian hormones to aversion-resistant drinking has not been assessed.

OBJECTIVES

We aimed to investigate the role of ovarian hormones to aversion-resistant drinking in female mice in a drinking in the dark (DID) task.

METHODS

Female C57BL/6 J mice first underwent an ovariectomy (OVX, n = 16) or sham (SHAM, n = 16) surgery. Four weeks following surgery, mice underwent a DID paradigm where they were given access to water and 15% EtOH 3 h into the dark cycle for up to 4 h across 15 drinking sessions. To assess frontloading behavior, bottles were weighed at 30 min, 2 h, and 4 h. Aversion-resistance was tested by adding escalating concentrations of quinine (0, 100, 250, and 500 µM) to the 15% EtOH bottle on sessions 16 - 19.

RESULTS

Removal of the ovaries reduced EtOH consumption in OVX subjects. When assessing aversion-resistant EtOH drinking, mice with ovarian hormones (SHAM) reduced consumption of 250 and 500 µM quinine in EtOH, while OVX subjects exhibited aversion-resistance at all quinine concentrations. OVX mice had greater frontloading for quinine + EtOH at higher concentrations of quinine.

CONCLUSIONS

These results indicate that circulating ovarian hormones may be protective against the development of aversion-resistant EtOH drinking and call for further investigation of the role of ovarian hormones in models of addictive behavior.

Stress and gonadal steroid influences on alcohol drinking and withdrawal, with focus on animal models in females

Sexually dimorphic effects of alcohol, following binge drinking, chronic intoxication, and withdrawal, are documented at the level of the transcriptome and in behavioral and physiological responses. The purpose of the current review is to update and to expand upon contributions of the endocrine system to alcohol drinking and withdrawal in females, with a focus on animal models. Steroids important in the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes, the reciprocal interactions between these axes, the effects of chronic alcohol use on steroid levels, and the genomic and rapid membrane-associated effects of steroids and neurosteroids in models of alcohol drinking and withdrawal are described. Importantly, comparison between males and females highlight some divergent effects of sex- and stress-steroids on alcohol drinking- and withdrawal-related behaviors, and the distinct differences in response emphasize the importance of considering sex in the development of novel pharmacotherapies for the treatment of alcohol use disorder.

Sex/Gender Differences in the Time-Course for the Development of Substance Use Disorder: A Focus on the Telescoping Effect

Sex/gender effects have been demonstrated for multiple aspects of addiction, with one of the most commonly cited examples being the "telescoping effect" where women meet criteria and/or seek treatment of substance use disorder (SUD) after fewer years of drug use as compared with men. This phenomenon has been reported for multiple drug classes including opioids, psychostimulants, alcohol, and cannabis, as well as nonpharmacological addictions, such as gambling. However, there are some inconsistent reports that show either no difference between men and women or opposite effects and a faster course to addiction in men than women. Thus, the goals of this review are to evaluate evidence for and against the telescoping effect in women and to determine the conditions/populations for which the telescoping effect is most relevant. We also discuss evidence from preclinical studies, which strongly support the validity of the telescoping effect and show that female animals develop addiction-like features (e.g., compulsive drug use, an enhanced motivation for the drug, and enhanced drug-craving/vulnerability to relapse) more readily than male animals. We also discuss biologic factors that may contribute to the telescoping effect, such as ovarian hormones, and its neurobiological basis focusing on the mesolimbic dopamine reward pathway and the corticomesolimbic glutamatergic pathway considering the critical roles these pathways play in the rewarding/reinforcing effects of addictive drugs and SUD. We conclude with future research directions, including intervention strategies to prevent the development of SUD in women. SIGNIFICANCE STATEMENT: One of the most widely cited gender/sex differences in substance use disorder (SUD) is the "telescoping effect," which reflects an accelerated course in women versus men for the development and/or seeking treatment for SUD. This review evaluates evidence for and against a telescoping effect drawing upon data from both clinical and preclinical studies. We also discuss the contribution of biological factors and underlying neurobiological mechanisms and highlight potential targets to prevent the development of SUD in women.

Alcohol use and motives for drinking across the menstrual cycle in a psychiatric outpatient sample

BACKGROUND

Females who misuse alcohol experience high rates of negative physical and mental health consequences. Existing findings are inconsistent but suggest a relationship between ovarian hormones and alcohol use. We aim to clarify how alcohol use and drinking motives vary across the menstrual cycle in female psychiatric outpatients using the luteinizing hormone (LH)-confirmed cycle phase.

METHODS

Daily self-reports (n = 3721) were collected from 94 naturally cycling females, recruited for past-month suicidal ideation, during the baseline phase of three parent clinical trials between February 2017 and May 2022. Multilevel logistic and linear models estimated the relationship between the cycle phase (with LH-surge confirmed ovulation) and daily alcohol use or drinking motives, moderated by the weekend. Models were adjusted for age, legal drinking status, substance use disorder, and the COVID-19 pandemic, and included random effects.

RESULTS

Participants were generally more likely to drink in the midluteal (vs. perimenstrual) phase, but more likely to drink heavily on weekends in periovulatory and perimenstrual (vs. midluteal) phases. Social motives for drinking were significantly higher on weekends in the periovulatory, mid-follicular, and midluteal phases (vs. weekdays), but this finding was non-significant in the perimenstrual phase. Participants rated drinking to cope higher in the perimenstrual phase (vs. midluteal phase), regardless of the weekend.

CONCLUSION

In a psychiatric sample with LH-surge-confirmed ovulation, we find an increased likelihood to drink heavily in periovulatory and perimenstrual phases on weekends. We also find that the perimenstrual phase is associated with increased drinking to cope, and relatively lower weekend social drinking. Finally, random effects across models suggest individual differences in the extent to which the cycle influences drinking. Our findings stress (1) predictable phases of increased high-risk alcohol use across the menstrual cycle, and (2) the importance of individual assessment of cyclical changes in alcohol use to predict and prevent ovulation- and menses-related surges in heavy drinking.

Gonadal hormones and sex chromosome complement differentially contribute to ethanol intake, preference, and relapse-like behaviour in four core genotypes mice

Alcohol use and high-risk alcohol drinking behaviours among women are rapidly rising. In rodent models, females typically consume more ethanol (EtOH) than males. Here, we used the four core genotypes (FCG) mouse model to investigate the influence of gonadal hormones and sex chromosome complement on EtOH drinking behaviours. FCG mice were given access to escalating concentrations of EtOH in a two-bottle, 24-h continuous access drinking paradigm to assess consumption and preference. Relapse-like behaviour was measured by assessing escalated intake following repeated cycles of deprivation and re-exposure. Twenty-four-hour EtOH consumption was greater in mice with ovaries (Sry-), relative to those with testes, and in mice with the XX chromosome complement, relative to those with XY sex chromosomes. EtOH preference was higher in XX versus XY mice. For both consumption and preference, the influences of the Sry gene and sex chromosomes were concentration dependent. Escalated intake following repeated cycles of deprivation and re-exposure emerged only in XX mice (vs. XY). Mice with ovaries (Sry- FCG mice and C57BL/6J females) were also found to consume more water than mice with testes. These results demonstrate that aspects of EtOH drinking behaviour may be independently regulated by sex hormones and chromosomes and inform our understanding of the neurobiological mechanisms which contribute to EtOH dependence in male and female mice. Future investigation of the contribution of sex chromosomes to EtOH drinking behaviours is warranted. We used the FCG mouse model to investigate the influence of gonadal hormones and sex chromosome complement on EtOH drinking behaviours, including the alcohol deprivation effect. Escalated intake following repeated cycles of deprivation and re-exposure emerged only in XX mice (vs. XY). These results demonstrate that aspects of EtOH drinking behaviour may be independently regulated by sex hormones and chromosomes.

Sex differences in stress-induced alcohol intake: a review of preclinical studies focused on amygdala and inflammatory pathways

Clinical studies suggest that women are more likely than men to relapse to alcohol drinking in response to stress; however, the mechanisms underlying this sex difference are not well understood. A number of preclinical behavioral models have been used to study stress-induced alcohol intake. Here, we review paradigms used to study effects of stress on alcohol intake in rodents, focusing on findings relevant to sex differences. To date, studies of sex differences in stress-induced alcohol drinking have been somewhat limited; however, there is evidence that amygdala-centered circuits contribute to effects of stress on alcohol seeking. In addition, we present an overview of inflammatory pathways leading to microglial activation that may contribute to alcohol-dependent behaviors. We propose that sex differences in neuronal function and inflammatory signaling in circuits centered on the amygdala are involved in sex-dependent effects on stress-induced alcohol seeking and suggest that this is an important area for future studies.

Effect of a brain-penetrant selective estrogen receptor degrader (SERD) on binge drinking in female mice

Background

Greater circulating levels of the steroid hormone 17β‐estradiol (E2) are associated with higher levels of binge drinking in women. In female mice, estrogen receptors in the ventral tegmental area, a dopaminergic region of the brain involved in the motivation to consume ethanol, regulate binge‐like ethanol intake. We recently developed a brain‐penetrant selective estrogen receptor degrader (SERD), YL3‐122, that could be used to test the behavioral role of brain estrogen receptors. We hypothesized that treating female mice with this compound would reduce binge‐like ethanol drinking.

Methods

Female C57BL/6J mice were treated systemically with YL3‐122 and a related SERD with low brain penetrance, XR5‐27, and tested for binge‐like ethanol consumption in the drinking in the dark (DID) test. Mice were also tested for sucrose and water consumption and blood ethanol clearance after treatment with the SERDs. Finally, the effect of ethanol exposure on Esr1 gene expression was measured in the ventral tegmental area (VTA), prefrontal cortex (PFC), and ventral hippocampus (vHPC) of male and female mice by quantitative real‐time PCR after 4 DID sessions.

Results

YL3‐122 reduced ethanol consumption when mice were in diestrus but not estrus. YL3‐122 also decreased sucrose consumption but did not alter water intake or blood ethanol clearance. XR5‐27 did not affect any of these measures. Binge‐like ethanol drinking resulted in increased Esr1 transcript in the VTA of both sexes, male vHPC, and female PFC.

Conclusions

These results indicate that SERD treatment can decrease binge‐like ethanol drinking in female mice. Thus, it could be a novel strategy to reduce binge drinking in women, with the caveat that effectiveness may depend on menstrual cycle phase. In addition, Esr1 transcript is increased by binge ethanol exposure in both sexes but in a brain region‐specific manner.

Protective effect of estradiol copreservation against kidney ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) is the major cause of delayed graft function (DGF) during the posttransplantation period. Estradiol (E2) prevents IRI-induced kidney dysfunction and tissue injury. However, many side effects limit E2's in vivo application. Recent evidence uncovers E2's expanded use in the field of transplantation. We aimed to study if and how E2 exerts protective activity during the period of kidney organ preservation. The autologous kidney transplant model in rats was first established. Rats were divided into 5 groups: normal group (N), sham group (sham), static cold storage (SCS) 4 hours group (control), SCS 4 hours + ethanol (1 µL/mL) group (solvent), and SCS 4 hours + ethanol (1 µL/mL) + E2 (1000 ng/mL) group (E2). ERα expression under hypothermia was measured by western blotting. Moreover, biochemical analyses of plasma levels of creatinine, BUN, estradiol, and testosterone were examined. Among all groups, kidney tissues were collected and processed for further western blot analysis about ERα, eNOS, Bcl-2, and Bax expression, histological analyses such as H&E staining to evaluate pathological severity. In addition, a TUNEL assay is performed to evaluate apoptosis. E2 copreservation upregulated ERα expression under hypothermia. Moreover, E2 copreservation reduced levels of creatinine and BUN in plasma but without affecting estradiol and testosterone. Further, E2 copreservation increased expression of eNOS and antiapoptotic Bcl-2 and decreases expression of proapoptotic Bax. E2 copreservation significantly inhibited IRI-induced apoptosis and evidently improved pathological severity in the kidney of rats. E2 copreservation exerts protective activity against IRI-induced pro-inflammatory and proapoptotic effects in kidneys during organ preservation time and improves transplanted kidney function.

Structural, functional, and behavioral significance of sex and gonadal hormones in the basolateral amygdala: A review of preclinical literature

The basolateral amygdala (BLA) is intimately involved in the development of neuropsychiatric disorders such as anxiety and alcohol use disorder (AUD). These disorders have clear sex biases, with women more likely to develop an anxiety disorder and men more likely to develop AUD. Preclinical models have largely confirmed these sex-specific vulnerabilities and emphasize the effects of sex hormones on behaviors influenced by the BLA. This review will discuss sex differences in BLA-related behaviors and highlight potential mechanisms mediated by altered BLA structure and function, including the composition of GABAergic interneuron subpopulations, glutamatergic pyramidal neuron morphology, glutamate/GABA neurotransmission, and neuromodulators. Further, sex hormones differentially organize dimorphic circuits during sensitive developmental periods (organizational effects) and initiate more transient effects throughout adulthood (activational effects). Current literature indicates that estradiol and allopregnanolone, a neuroactive progestogen, generally reduce BLA-related behaviors through a variety of mechanisms, including activation of estrogen receptors or facilitation of GABA_A-mediated inhibition, respectively. This enhanced GABAergic inhibition may protect BLA pyramidal neurons from the excitability associated with anxiety and alcohol withdrawal. Understanding sex differences and the effects of sex hormones on BLA structure and function may help explain sex-specific vulnerabilities in BLA-related behaviors and ultimately improve treatments for anxiety and AUD.

Studying Sex Differences in Rodent Models of Addictive Behavior

Animal models of addictive behaviors are useful for uncovering neural mechanisms involved in the development of dependence and for identifying risk factors for drug abuse. One such risk factor is biological sex, which strongly moderates drug self-administration behavior in rodents. Female rodents are more likely to acquire drug self-administration behaviors, consume higher amounts of drug, and reinstate drug-seeking behavior more readily. Despite this female vulnerability, preclinical addiction research has largely been done in male animals. The study of sex differences in rodent models of addictive behavior is increasing, however, as more investigators are choosing to include both male and female animals in experiments. This commentary is meant to serve as an introductory guide for preclinical investigators new to the study of sex differences in addiction. We provide an overview of self-administration models, a broad view of female versus male self-administration behaviors, and suggestions for study design and implementation. Inclusion of female subjects in preclinical addiction research is timely, as problem drug and alcohol use in women is increasing. With proper attention, design, and analysis, the study of sex differences in addiction has the potential to uncover novel neural mechanisms and lead to greater translational success for addiction research. © 2021 Wiley Periodicals LLC.

Sex Differences in the Neurobiology of Alcohol Use Disorder

Sex differences may play a critical role in modulating how chronic or heavy alcohol use impacts the brain to cause the development of alcohol use disorder (AUD). AUD is a multifaceted and complex disorder driven by changes in key neurobiological structures that regulate executive function, memory, and stress. A three-stage framework of addiction (binge/intoxication; withdrawal/negative affect; preoccupation/anticipation) has been useful for conceptualizing the complexities of AUD and other addictions. Initially, alcohol drinking causes short-term effects that involve signaling mediated by several neurotransmitter systems such as dopamine, corticotropin releasing factor, and glutamate. With continued intoxication, alcohol leads to dysfunctional behaviors that are thought to be due in part to alterations of these and other neurotransmitter systems, along with alterations in neural pathways connecting prefrontal and limbic structures. Using the three-stage framework, this review highlights examples of research examining sex differences in drinking and differential modulation of neural systems contributing to the development of AUD. New insights addressing the role of sex differences in AUD are advancing the field forward by uncovering the complex interactions that mediate vulnerability.

The Endocrine System and Alcohol Drinking in Females

Sexually dimorphic effects of alcohol exposure throughout life have been documented in clinical and preclinical studies. In the past, rates of alcohol use disorder (AUD) were higher in men than in women, but over the past 10 years, the difference between sexes in prevalence of AUD and binge drinking has narrowed. Recent evidence adds to historical data regarding the influence of sex steroids on alcohol drinking and the interaction with stress-related steroids. This review considers the contribution of the endocrine system to alcohol drinking in females, with a focus on the hypothalamic pituitary gonadal axis and the hypothalamic pituitary adrenal axis and their reciprocal interactions. Emphasis is given to preclinical studies that examined genomic and rapid membrane effects of estrogen, progesterone, glucocorticoids, and GABAergic neurosteroids for their effects on alcohol drinking and models of relapse. Pertinent comparisons to data in males highlight divergent effects of sex and stress steroids on alcohol drinking and emphasize the importance of considering sex in the development of novel pharmacotherapeutic targets for the treatment of AUD. For instance, pharmacological strategies targeting the corticotropin releasing factor and glucocorticoid receptor systems may be differentially effective in males and females, whereas strategies to enhance GABAergic neurosteroids may represent a biomarker of treatment efficacy in both sexes.

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

Elevations in estrogen (17β-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 ERβ) 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 ERβ 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 ERβ, 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. 17β-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.

Sex differences in the effect of the FKBP5 inhibitor SAFit2 on anxiety and stress-induced reinstatement following cocaine self-administration

Cocaine use and withdrawal prompt stress system responses. Stress and the negative affective state produced by cocaine withdrawal are major triggers for relapse. FKBP5 is a co-chaperone of the glucocorticoid receptor and regulates HPA axis negative feedback. The role of FKBP5 in cocaine-related behaviors has not been studied. The FKBP5 inhibitor SAFit2 was used to examine the role of FKBP5 in anxiety-like behavior during early cocaine withdrawal and in stress-induced reinstatement following cocaine self-administration in male and female rats. Withdrawal from cocaine self-administration resulted in heightened anxiety-like behavior in female rats, which was significantly attenuated by SAFit2 administration. SAFit2 pretreatment prior to stress-induced reinstatement to cocaine seeking significantly reduced active lever presses of males. In female rats, SAFit2 administration prevented stress-induced reinstatement for rats in metestrus or diestrus, but not proestrus or estrus phases at the time of reinstatement. These data suggest an important role for FKBP5 in stress-related behaviors following cocaine self-administration, particularly in females.

Cue-alcohol associative learning in female rats

The ability of environmental cues to trigger alcohol-seeking behaviors is believed to facilitate problematic alcohol use. We previously showed that the development of this cue-evoked alcohol approach reflects cue-alcohol learning and memory in the adult male rat; however, we do not know whether the same is true for similarly aged female rats. Consequently, adult Long-Evans female rats were allowed to drink unsweetened alcohol in the home cage (Monday, Wednesday, Friday; 24-h two-bottle choice; 5 weeks) and were subsequently split into two experimental groups: Paired and Unpaired. Groups were matched for ingested doses and alcohol bottle preference across the pre-conditioning home cage period. Both groups were trained in conditioning chambers using a Pavlovian procedure. For the Paired group, the chamber houselight was illuminated to signal access to an alcohol sipper. Houselight onset was yoked for the Unpaired group, but access to the alcohol sipper was scheduled to occur only during the intervening periods (in the absence of light). We found that in the Paired, but not Unpaired group, an alcohol approach reaction was conditioned to houselight illumination, and the level of cue-conditioned reactivity predicted drinking behavior within trials. Groups experienced equivalently low but non-negligible blood alcohol concentrations over the course of conditioning sessions. We conclude that cue-triggered alcohol-seeking behavior in adult female rats reflects associative learning about the relationship between alcohol availability and houselight illumination.

Sex Differences in Ethanol Reward Seeking Under Conflict in Mice

BACKGROUND

Alcohol use disorders are characterized by inflexible alcohol seeking that occurs despite adverse consequences. Males and females are differentially sensitive to ethanol (EtOH) reward, but it is unclear whether sex differences in EtOH seeking under reward-aversion conflict are present.

METHODS

To investigate sex differences in EtOH seeking under conflict, adult male and female C57BL/6J mice underwent chronic intermittent EtOH (CIE) exposure by vapor inhalation or served as air-exposed controls. After CIE, mice were trained in a modified EtOH conditioned place preference paradigm. During 3 conditioning sessions, 2 g/kg EtOH was administered prior to confinement in the "EtOH-paired" chamber. On alternating days, saline was injected prior to confinement in the "saline-paired" chamber. After conditioning, mice experienced a footshock in the EtOH-paired chamber. EtOH-seeking behavior was assessed before and after footshock.

RESULTS

Control and CIE-exposed males reduced the time spent in and increased latency to enter the reward-paired chamber following footshock. Control females did not alter EtOH-seeking behavior following footshock. CIE-exposed females spent more time in the EtOH-paired chamber at baseline. However, following a footshock, CIE-exposed females significantly reduced the time spent in and increased latency to enter the EtOH-paired chamber.

CONCLUSIONS

Nondependent female mice exhibited aversion-resistant alcohol seeking to a greater degree than males. Chronic EtOH exposure did not impact EtOH seeking in males. In females, CIE enhanced EtOH seeking in the absence of conflict, but reduced EtOH seeking after an aversive experience. While these sex-specific effects of CIE are not present when reward seeking is assessed in the absence of an aversive experience, multiple factors may underlie the differences in reward seeking despite adverse consequences, including reward- and aversion-related learning and decision making under conflict. These data highlight the importance of considering sex as a variable influencing EtOH seeking and provide a greater understanding of how sex interacts with EtOH exposure to alter behavior.

Estradiol-Induced Potentiation of Dopamine Release in Dorsal Striatum Following Amphetamine Administration Requires Estradiol Receptors and mGlu5

Estradiol potentiates behavioral sensitization to cocaine as well as self-administration of cocaine and other drugs of abuse in female rodents. Furthermore, stimulated dopamine (DA) in the dorsolateral striatum (DLS) is rapidly enhanced by estradiol, and it is hypothesized that this enhanced DA release mediates the more rapid escalation of drug taking seen in females, compared with males. The mechanisms mediating the effect of estradiol to enhance stimulated DA release were investigated in this study. Using in vivo microdialysis and high performance liquid chromatography coupled with electrochemical detection, we first examined the effect of estradiol on amphetamine-induced DA increase in the DLS of ovariectomized rats. We then tested whether the potentiation of this DA increase could be blocked by the estradiol receptor antagonist, ICI 182,780 (ICI), or an antagonist to the metabotropic glutamate receptor subtype 5 (mGlu5), 2-methyl-6-(phenylethynyl)pyridine (MPEP). There is evidence that estradiol receptors collaborate with mGlu5 within caveoli in DLS and mGlu5 is hypothesized to mediate many of the effects of estradiol in the addiction processes in females. Our data show that estradiol enhances the DA response to amphetamine. Either ICI or MPEP prevented the effect of estradiol to enhance DA release. Importantly, our results also showed that neither ICI or MPEP alone is able to influence the DA response to amphetamine when estradiol is not administrated, suggesting that ICI and MPEP act via estradiol receptors. Together, our findings demonstrate that estradiol potentiates amphetamine-stimulated DA release in the DLS and this effect requires both estradiol receptors and mGlu5.

Sexual dimorphism in the neural impact of stress and alcohol

Alcohol use disorder is a widespread mental illness characterized by periods of abstinence followed by recidivism, and stress is the primary trigger of relapse. Despite the higher prevalence of alcohol use disorder in males, the relationship between stress and behavioral features of relapse, such as craving, is stronger in females. Given the greater susceptibility of females to stress-related psychiatric disorders, understanding sexual dimorphism in the relationship between stress and alcohol use is essential to identifying better treatments for both male and female alcoholics. This review addresses sex differences in the impact of stressors on alcohol drinking and seeking in rodents and humans. As these behavioral differences in alcohol use and relapse originate from sexual dimorphism in neuronal function, the impact of stressors and alcohol, and their interaction, on molecular adaptations and neural activity in males and females will also be discussed. Together, the data reviewed herein, arising from a symposium titled "Sex matters in stress-alcohol interactions" presented at the Fourth Volterra Conference on Stress and Alcohol, will highlight the importance of identifying sex differences to improve treatments for comorbid stress and alcohol use disorder in both sexes.