Estrogen receptor alpha is required in GABAergic, but not glutamatergic, neurons to masculinize behavior

Reproductive function and behaviors: an update on the role of neural estrogen receptors alpha and beta

Infertility is becoming a major public health problem, with increasing frequency due to medical, environmental and societal causes. The increasingly late age of childbearing, growing exposure to endocrine disruptors and other reprotoxic products, and increasing number of medical reproductive dysfunctions (endometriosis, polycystic ovary syndrome, etc.) are among the most common causes. Fertility relies on fine-tuned control of both neuroendocrine function and reproductive behaviors, those are critically regulated by sex steroid hormones. Testosterone and estradiol exert organizational and activational effects throughout life to establish and activate the neural circuits underlying reproductive function. This regulation is mediated through estrogen receptors (ERs) and androgen receptor (AR). Estradiol acts mainly via nuclear estrogen receptors ERα and ERβ. The aim of this review is to summarize the genetic studies that have been undertaken to comprehend the specific contribution of ERα and ERβ in the neural circuits underlying the regulation of the hypothalamic-pituitary-gonadal axis and the expression of reproductive behaviors, including sexual and parental behavior. Particular emphasis will be placed on the neural role of these receptors and the underlying sex differences.

The decision of male medaka to mate or fight depends on two complementary androgen signaling pathways

Adult male animals typically court and attempt to mate with females, while attacking other males. Emerging evidence from mice indicates that neurons expressing the estrogen receptor ESR1 in behaviorally relevant brain regions play a central role in mediating these mutually exclusive behavioral responses to conspecifics. However, the findings in mice are unlikely to apply to vertebrates in general because, in many species other than rodents and some birds, androgens-rather than estrogens-have been implicated in male behaviors. Here, we report that male medaka (Oryzias latipes) lacking one of the two androgen receptor subtypes (Ara) are less aggressive toward other males and instead actively court them, while those lacking the other subtype (Arb) are less motivated to mate with females and conversely attack them. These findings indicate that, in male medaka, the Ara- and Arb-mediated androgen signaling pathways facilitate appropriate behavioral responses, while simultaneously suppressing inappropriate responses, to males and females, respectively. Notably, males lacking either receptor retain the ability to discriminate the sex of conspecifics, suggesting a defect in the subsequent decision-making process to mate or fight. We further show that Ara and Arb are expressed in intermingled but largely distinct populations of neurons, and stimulate the expression of different behaviorally relevant genes including galanin and vasotocin, respectively. Collectively, our results demonstrate that male teleosts make adaptive decisions to mate or fight as a result of the activation of one of two complementary androgen signaling pathways, depending on the sex of the conspecific that they encounter.

Effects of estrogen on spatial navigation and memory

RATIONALE

Animal studies suggest that the so-called "female" hormone estrogen enhances spatial navigation and memory. This contradicts the observation that males generally out-perform females in spatial navigation and tasks involving spatial memory. A closer look at the vast number of studies actually reveals that performance differences are not so clear.

OBJECTIVES

To help clarify the unclear performance differences between men and women and the role of estrogen, we attempted to isolate organizational from activational effects of estrogen on spatial navigation and memory.

METHODS

In a double-blind, placebo-controlled study, we tested the effects of orally administered estradiol valerate (E2V) in healthy, young women in their low-hormone menstrual cycle phase, compared to healthy, young men. Participants performed several first-person, environmentally rich, 3-D computer games inspired by spatial navigation and memory paradigms in animal research.

RESULTS

We found navigation behavior suggesting that sex effects dominated any E2 effects with men performing better with allocentric strategies and women with egocentric strategies. Increased E2 levels did not lead to general improvements in spatial ability in either sex but to behavioral changes reflecting navigation flexibility.

CONCLUSION

Estrogen-driven differences in spatial cognition might be better characterized on a spectrum of navigation flexibility rather than by categorical performance measures or skills.

Let's talk about sex: Mechanisms of neural sexual differentiation in Bilateria

In multicellular organisms, sexed gonads have evolved that facilitate release of sperm versus eggs, and bilaterian animals purposefully combine their gametes via mating behaviors. Distinct neural circuits have evolved that control these physically different mating events for animals producing eggs from ovaries versus sperm from testis. In this review, we will describe the developmental mechanisms that sexually differentiate neural circuits across three major clades of bilaterian animals-Ecdysozoa, Deuterosomia, and Lophotrochozoa. While many of the mechanisms inducing somatic and neuronal sex differentiation across these diverse organisms are clade-specific rather than evolutionarily conserved, we develop a common framework for considering the developmental logic of these events and the types of neuronal differences that produce sex-differentiated behaviors. This article is categorized under: Congenital Diseases > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

Comparative analysis of gonadal hormone receptor expression in the postnatal house mouse, meadow vole, and prairie vole brain

The socially monogamous prairie vole (Microtus ochrogaster) and promiscuous meadow vole (Microtus pennsylvanicus) are closely related, but only prairie voles display long-lasting pair bonds, biparental care, and selective aggression towards unfamiliar individuals after pair bonding. These social behaviors in mammals are largely mediated by steroid hormone signaling in the social behavior network (SBN) of the brain. Hormone receptors are reproducible markers of sex differences that can provide more information than anatomy alone and can even be at odds with anatomical dimorphisms. We reasoned that behaviors associated with social monogamy in prairie voles may emerge in part from unique expression patterns of steroid hormone receptors in this species, and that these expression patterns would be more similar across males and females in prairie than in meadow voles or the laboratory mouse. To obtain insight into steroid hormone signaling in the developing prairie vole brain, we assessed expression of estrogen receptor alpha (Esr1), estrogen receptor beta (Esr2), and androgen receptor (Ar) within the SBN, using in situ hybridization at postnatal day 14 in mice, meadow, and prairie voles. We found species-specific patterns of hormone receptor expression in the hippocampus and ventromedial hypothalamus, as well as species differences in the sex bias of these markers in the principal nucleus of the bed nucleus of the stria terminalis. These findings suggest the observed differences in gonadal hormone receptor expression may underlie species differences in the display of social behaviors.

Practical solutions for including sex as a biological variable (SABV) in preclinical neuropsychopharmacological research

Recently, many funding agencies have released guidelines on the importance of considering sex as a biological variable (SABV) as an experimental factor, aiming to address sex differences and avoid possible sex biases to enhance the reproducibility and translational relevance of preclinical research. In neuroscience and pharmacology, the female sex is often omitted from experimental designs, with researchers generalizing male-driven outcomes to both sexes, risking a biased or limited understanding of disease mechanisms and thus potentially ineffective therapeutics. Herein, we describe key methodological aspects that should be considered when sex is factored into in vitro and in vivo experiments and provide practical knowledge for researchers to incorporate SABV into preclinical research. Both age and sex significantly influence biological and behavioral processes due to critical changes at different timepoints of development for males and females and due to hormonal fluctuations across the rodent lifespan. We show that including both sexes does not require larger sample sizes, and even if sex is included as an independent variable in the study design, a moderate increase in sample size is sufficient. Moreover, the importance of tracking hormone levels in both sexes and the differentiation between sex differences and sex-related strategy in behaviors are explained. Finally, the lack of robust data on how biological sex influences the pharmacokinetic (PK), pharmacodynamic (PD), or toxicological effects of various preclinically administered drugs to animals due to the exclusion of female animals is discussed, and methodological strategies to enhance the rigor and translational relevance of preclinical research are proposed.

Steroid hormone-mediated regulation of sexual and aggressive behaviour by non-genomic signalling

Sex and aggression are well studied examples of social behaviours that are common to most animals and are mediated by an evolutionary conserved group of interconnected nuclei in the brain called the social behaviour network. Though glucocorticoids and in particular estrogen regulate these social behaviours, their effects in the brain are generally thought to be mediated by genomic signalling, a slow transcriptional regulation mediated by nuclear hormone receptors. In the last decade or so, there has been renewed interest in understanding the physiological significance of rapid, non-genomic signalling mediated by steroids. Though the identity of the membrane hormone receptors that mediate this signalling is not clearly understood and appears to be different in different cell types, such signalling contributes to physiologically relevant behaviours such as sex and aggression. In this short review, we summarise the evidence for this phenomenon in the rodent, by focusing on estrogen and to some extent, glucocorticoid signalling. The use of these signals, in relation to genomic signalling is manifold and ranges from potentiation of transcription to the possible transduction of environmental signals.

Pregnancy programs the brain for mothering

Progesterone and estrogen exert separate effects on the brains of pregnant mice.

Neural Control of Sexually Dimorphic Social Behavior: Connecting Development to Adulthood

Rapid advances in the neural control of social behavior highlight the role of interconnected nodes engaged in differential information processing to generate behavior. Many innate social behaviors are essential to reproductive fitness and therefore fundamentally different in males and females. Programming these differences occurs early in development in mammals, following gonadal differentiation and copious androgen production by the fetal testis during a critical period. Early-life programming of social behavior and its adult manifestation are separate but yoked processes, yet how they are linked is unknown. This review seeks to highlight that gap by identifying four core mechanisms (epigenetics, cell death, circuit formation, and adult hormonal modulation) that could connect developmental changes to the adult behaviors of mating and aggression. We further propose that a unique social behavior, adolescent play, bridges the preweaning to the postpubertal brain by engaging the same neural networks underpinning adult reproductive and aggressive behaviors.

Microglia phagocytosis mediates the volume and function of the rat sexually dimorphic nucleus of the preoptic area

The sexually dimorphic nucleus of the preoptic area (SDN-POA) is the oldest and most robust sex difference reported in mammalian brain and is singular for its presence across a wide range of species from rodents to ungulates to man. This small collection of Nissl-dense neurons is reliably larger in volume in males. Despite its notoriety and intense interrogation, both the mechanism establishing the sex difference and the functional role of the SDN have remained elusive. Convergent evidence from rodent studies led to the conclusion that testicular androgens aromatized to estrogens are neuroprotective in males and that higher apoptosis (naturally occurring cell death) in females determines their smaller SDN. In several species, including humans, a smaller SDN correlates with a preference for mating with males. We report here that this volume difference is dependent upon a participatory role of phagocytic microglia which engulf more neurons in the female SDN and assure their destruction. Selectively blocking microglia phagocytosis temporarily spared neurons from apoptotic death and increased SDN volume in females without hormone treatment. Increasing the number of neurons in the SDN in neonatal females resulted in loss of preference for male odors in adulthood, an effect paralleled by dampened excitation of SDN neurons as evidenced by reduced immediate early gene (IEG) expression when exposed to male urine. Thus, the mechanism establishing a sex difference in SDN volume includes an essential role for microglia, and SDN function as a regulator of sexual partner preference is confirmed.

Genetic dissection of steroid-hormone modulated social behavior: Novel paralogous genes are a boon for discovery

Research across species has led to important discoveries on the functions of steroid hormones in the regulation of behavior. However, like in many fields, advancements in transgenic and mutagenic technology allowed mice to become the premier genetic model for conducting many experiments to understand how steroids control social behavior. Since there has been a general lack of parallel methodological developments in other species, many of the findings cannot be generalized. This is especially the case for teleost fish, in which a whole-genome duplication produced novel paralogs for key steroid hormone signaling genes. In this review, we summarize technical advancements over the history of the field of neuroendocrinology that have led to important insights in our understanding of the control of social behavior by steroids. We demonstrate that early mouse genetic models to understand these mechanisms suffered from several issues that were remedied by more precise transgenic technological advancements. We then highlight the importance of CRISPR/Cas9 gene editing tools that will in time bridge the gap between mice and non-traditional model species for understanding principles of steroid hormone action in the modulation of social behavior. We specifically highlight the role of teleost fish in bridging this gap because they are 1) highly genetically tractable and 2) provide a novel advantage in achieving precise genetic control. The field of neuroendocrinology is entering a new "gene editing revolution" that will lead to novel discoveries about the roles of steroid hormones in the regulation and evolutionary trajectories of social behavior.

Comparative Transcriptional Analyses in the Nucleus Accumbens Identifies RGS2 as a Key Mediator of Depression-Related Behavior

BACKGROUND

Major depressive disorder is one of the most commonly diagnosed mental illnesses worldwide, with a higher prevalence in women than in men. Although currently available pharmacological therapeutics help many individuals, they are not effective for most. Animal models have been important for the discovery of molecular alterations in stress and depression, but difficulties in adapting animal models of depression for females has impeded progress in developing novel therapeutic treatments that may be more efficacious for women.

METHODS

Using the California mouse social defeat model, we took a multidisciplinary approach to identify stress-sensitive molecular targets that have translational relevance for women. We determined the impact of stress on transcriptional profiles in male and female California mouse nucleus accumbens (NAc) and compared these results with data from postmortem samples of the NAc from men and women diagnosed with major depressive disorder.

RESULTS

Our cross-species computational analyses identified Rgs2 (regulator of G protein signaling 2) as a transcript downregulated by social defeat stress in female California mice and in women with major depressive disorder. RGS2 plays a key role in signal regulation of neuropeptide and neurotransmitter receptors. Viral vector-mediated overexpression of Rgs2 in the NAc restored social approach and sucrose preference in stressed female California mice.

CONCLUSIONS

These studies show that Rgs2 acting in the NAc has functional properties that translate to changes in anxiety- and depression-related behavior. Future studies should investigate whether targeting Rgs2 represents a novel target for treatment-resistant depression in women.

The Role of Transposable Elements in Sexual Development

Up to 50% of most mammalian genomes are made up of transposable elements (TEs) that have the potential to mobilize around the genome. Despite this prevalence, research on TEs is only beginning to gain traction within the field of neuroscience. While TEs have long been regarded as "junk" or parasitic DNA, it has become evident that they are adaptive DNA and RNA regulatory elements. In addition to their vital role in normal development, TEs can also interact with steroid receptors, which are key elements to sexual development. In this review, we provide an overview of the involvement of TEs in processes related to sexual development- from TE activity in the germline to TE accumulation in sex chromosomes. Moreover, we highlight sex differences in TE activity and their regulation of genes related to sexual development. Finally, we speculate on the epigenetic mechanisms that may govern TEs' role in sexual development. In this context, we emphasize the need to further the understanding of sexual development through the lens of TEs including in a variety of organs at different developmental stages, their molecular networks, and evolution.

Gene regulation by gonadal hormone receptors underlies brain sex differences

Oestradiol establishes neural sex differences in many vertebrates1-3 and modulates mood, behaviour and energy balance in adulthood4-8. In the canonical pathway, oestradiol exerts its effects through the transcription factor oestrogen receptor-α (ERα)9. Although ERα has been extensively characterized in breast cancer, the neuronal targets of ERα, and their involvement in brain sex differences, remain largely unknown. Here we generate a comprehensive map of genomic ERα-binding sites in a sexually dimorphic neural circuit that mediates social behaviours. We conclude that ERα orchestrates sexual differentiation of the mouse brain through two mechanisms: establishing two male-biased neuron types and activating a sustained male-biased gene expression program. Collectively, our findings reveal that sex differences in gene expression are defined by hormonal activation of neuronal steroid receptors. The molecular targets we identify may underlie the effects of oestradiol on brain development, behaviour and disease.

Deletion of neural estrogen receptor alpha induces sex differential effects on reproductive behavior in mice

Estrogen receptor (ER) α is involved in several estrogen-modulated neural and peripheral functions. To determine its role in the expression of female and male reproductive behavior, a mouse line lacking the ERα in the nervous system was generated. Mutant females did not exhibit sexual behavior despite normal olfactory preference, and had a reduced number of progesterone receptor-immunoreactive neurons in the ventromedial hypothalamus. Mutant males displayed a moderately impaired sexual behavior and unaffected fertility, despite evidences of altered organization of sexually dimorphic populations in the preoptic area. In comparison, males deleted for both neural ERα and androgen receptor (AR) displayed greater sexual deficiencies. Thus, these data highlight a predominant role for neural ERα in females and a complementary role with the AR in males in the regulation of sexual behavior, and provide a solid background for future analyses of neuronal versus glial implication of these signaling pathways in both sexes.

Neural deletion of the estrogen receptor, ERα, inhibits sexual behavior in female mice, but only has moderately effect in male mice. These results contrast with previous studies using global ERα knockouts, which found that ERα is mandatory for reproductive behavior in both sexes.

A functional cellular framework for sex and estrous cycle-dependent gene expression and behavior

Sex hormones exert a profound influence on gendered behaviors. How individual sex hormone-responsive neuronal populations regulate diverse sex-typical behaviors is unclear. We performed orthogonal, genetically targeted sequencing of four estrogen receptor 1-expressing (Esr1+) populations and identified 1,415 genes expressed differentially between sexes or estrous states. Unique subsets of these genes were distributed across all 137 transcriptomically defined Esr1+ cell types, including estrous stage-specific ones, that comprise the four populations. We used differentially expressed genes labeling single Esr1+ cell types as entry points to functionally characterize two such cell types, BNSTprTac1/Esr1 and VMHvlCckar/Esr1. We observed that these two cell types, but not the other Esr1+ cell types in these populations, are essential for sex recognition in males and mating in females, respectively. Furthermore, VMHvlCckar/Esr1 cell type projections are distinct from those of other VMHvlEsr1 cell types. Together, projection and functional specialization of dimorphic cell types enables sex hormone-responsive populations to regulate diverse social behaviors.

Running the Female Power Grid Across Lifespan Through Brain Estrogen Signaling

The role of central estrogen in cognitive, metabolic, and reproductive health has long fascinated the lay public and scientists alike. In the last two decades, insight into estrogen signaling in the brain and its impact on female physiology is beginning to catch up with the vast information already established for its actions on peripheral tissues. Using newer methods to manipulate estrogen signaling in hormone-sensitive brain regions, neuroscientists are now identifying the molecular pathways and neuronal subtypes required for controlling sex-dependent energy allocation. However, the immense cellular complexity of these hormone-sensitive brain regions makes it clear that more research is needed to fully appreciate how estrogen modulates neural circuits to regulate physiological and behavioral end points. Such insight is essential for understanding how natural or drug-induced hormone fluctuations across lifespan affect women's health.

The potential role of stress and sex steroids in heritable effects of sevoflurane

Most surgical procedures require general anesthesia, which is a reversible deep sedation state lacking all perception. The induction of this state is possible because of complex molecular and neuronal network actions of general anesthetics (GAs) and other pharmacological agents. Laboratory and clinical studies indicate that the effects of GAs may not be completely reversible upon anesthesia withdrawal. The long-term neurocognitive effects of GAs, especially when administered at the extremes of ages, are an increasingly recognized health concern and the subject of extensive laboratory and clinical research. Initial studies in rodents suggest that the adverse effects of GAs, whose actions involve enhancement of GABA type A receptor activity (GABAergic GAs), can also extend to future unexposed offspring. Importantly, experimental findings show that GABAergic GAs may induce heritable effects when administered from the early postnatal period to at least young adulthood, covering nearly all age groups that may have children after exposure to anesthesia. More studies are needed to understand when and how the clinical use of GAs in a large and growing population of patients can result in lower resilience to diseases in the even larger population of their unexposed offspring. This minireview is focused on the authors' published results and data in the literature supporting the notion that GABAergic GAs, in particular sevoflurane, may upregulate systemic levels of stress and sex steroids and alter expressions of genes that are essential for the functioning of these steroid systems. The authors hypothesize that stress and sex steroids are involved in the mediation of sex-specific heritable effects of sevoflurane.

ESR1 gene polymorphism (rs827421) as a potential genetic marker for constitutional delay of growth and puberty in Egyptian adolescents

Constitutional delay of growth and puberty (CDGP) is a variant of normal pubertal timing and progress. It is the most common form of delayed puberty in both genders. The genetic director of CDGP is ill-understood despite the positive family history result noted in those patients. The current study aimed at assessing the role of estrogen receptor 1 (ESR1) gene variant (rs827421) in Egyptian adolescents with CDGP. A cross-sectional study with follow-up part was carried out on 6760 children aged 4 to15 years. The study focused generally on children aged 13-15 years in order to evaluate the prevalence of delayed puberty in relation to all ages in general and to their peers in specific. Assessment of serum TSH, FSH, and LH was conducted on all participants, along with the measurement of serum-free testosterone for males and estradiol for females. Genotyping of ESR1 (rs827421) was done to all subjects through the use of TaqMan discrimination assay by real-time PCR. ESR1 (rs827421) GG genotype and G allele were significantly dominant among CDGP adolescents in comparison with controls (OR = 25.67 and 6.90). As regards follow-up of testicular size, AA genotype was significantly associated with increased size in the right and left testis compared to other genotypes (P = 0.021 and 0.006, respectively). Moreover, AA genotype showed significantly higher Tanner stage in both males and females in comparison with other genotypes. Serum estradiol level was significantly higher in AA genotype group than other genotypes groups. ESR1 gene polymorphism can be considered a potential genetic marker for CDGP in both sexes in a sample of Egyptian adolescents.

17β-estradiol promotes acute refeeding in hungry mice via membrane-initiated ERα signaling

Objective

Estrogen protects animals from obesity through estrogen receptor α (ERα), partially by inhibiting overeating in animals fed ad libitum. However, the effects of estrogen on feeding behavior in hungry animals remain unclear. In this study, we examined the roles of 17β-estradiol (E2) and ERα in the regulation of feeding in hungry female animals and explored the underlying mechanisms.

Methods

Wild-type female mice with surgical depletion of endogenous estrogens were used to examine the effects of E2 supplementation on acute refeeding behavior after starvation. ERα-C451A mutant mice deficient in membrane-bound ERα activity and ERα-AF2⁰ mutant mice lacking ERα transcriptional activity were used to further examine mechanisms underlying acute feeding triggered by either fasting or central glucopenia (induced by intracerebroventricular injections of 2-deoxy-D-glucose). We also used electrophysiology to explore the impact of these ERα mutations on the neural activities of ERα neurons in the hypothalamus.

Results

In the wild-type female mice, ovariectomy reduced fasting-induced refeeding, which was restored by E2 supplementation. The ERα-C451A mutation, but not the ERα-AF2⁰ mutation, attenuated acute feeding induced by either fasting or central glucopenia. The ERα-C451A mutation consistently impaired the neural responses of hypothalamic ERα neurons to hypoglycemia.

Conclusion

In addition to previous evidence that estrogen reduces deviations in energy balance by inhibiting eating at a satiated state, our findings demonstrate the unexpected role of E2 that promotes eating in hungry mice, also contributing to the stability of energy homeostasis. This latter effect specifically requires membrane-bound ERα activity.

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Endogenous E2 is required to maintain acute refeeding in hungry female mice after starvation.

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Membrane-bound ERα activity in female mice is required for efficient refeeding after starvation.

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Membrane-bound ERα activity is required for hypothalamic ERα neurons to respond to hypoglycemia.

Neonatal exposure to chlordecone alters female social behaviors and central estrogen alpha receptor expression in socially monogamous mandarin voles

Chlordecone (CD) is one of the common persistent organic pollutants in nature and has a profound impact on the environment and on public health. Accumulating evidence has demonstrated that neonatal exposure of CD influences adult physiology and behavior due to its estrogenic properties. Using socially monogamous mandarin voles as an experimental animal model, the present study aimed to evaluate the impact of neonatal exposure to CD on female social behaviors and central estrogen receptor alpha (ERα) expression in adulthood. After receiving a single subcutaneous injection with sesame seed oil (female control group), 17 beta-estradiol (E₂ group), or CD group on postnatal Day 1, the social behaviors of adult animals and ERα expression in specific brain regions were assessed. The data indicated that CD or E₂-treated female animals displayed increased affiliative behaviors and decreased aggressive behaviors with regard to the unfamiliar females in the social interaction test. In addition, CD or E₂-treated female voles exhibited significant preferences to females over males in the sexual preference test. Moreover, CD-treated female animals exhibited higher levels of ERα expression in the bed nucleus of the stria terminalis, the central amygdala, the medial amygdala and the medial preoptic area compared with those of the control voles. The results suggested that neonatal exposure to CD may masculinize female social behaviors, possibly via CD-induced changes in the ERα expression of relevant brain regions.

Dietary phytoestrogens modulate aggression and activity in social behavior circuits of male mice

Phytoestrogens comprise biologically active constituents of human and animal diet that can impact on systemic and local estrogen functions in the brain. Here we report on the importance of dietary phytoestrogens for maintaining activity in a brain circuit controlling aggressive and social behavior of male mice. After six weeks of low-phytoestrogen chronic diet (diadzein plus genistein <20 μg/g) a reduction of intermale aggression and altered territorial marking behavior could be observed, compared to littermates on a standard soy-bean based diet (300 μg/g). Further, mice on low-phyto diet displayed a decrease in sociability and a reduced preference for social odors, indicating a general disturbance of social behavior. Underlying circuits were investigated by analysing the induction of the activity marker c-Fos upon social encounter. Low-phyto diet led to a markedly reduced c-Fos induction in the medial as well as the cortical amygdala, the lateral septum, medial preoptic area and bed nucleus of the stria terminalis. No difference between groups was observed in the olfactory bulb. Together our data suggest that dietary phytoestrogens critically modulate social behavior circuits in the male mouse brain.

Signatures of sex: Sex differences in gene expression in the vertebrate brain

Women and men differ in disease prevalence, symptoms, and progression rates for many psychiatric and neurological disorders. As more preclinical studies include both sexes in experimental design, an increasing number of sex differences in physiology and behavior have been reported. In the brain, sex-typical behaviors are thought to result from sex-specific patterns of neural activity in response to the same sensory stimulus or context. These differential firing patterns likely arise as a consequence of underlying anatomic or molecular sex differences. Accordingly, gene expression in the brains of females and males has been extensively investigated, with the goal of identifying biological pathways that specify or modulate sex differences in brain function. However, there is surprisingly little consensus on sex-biased genes across studies and only a handful of robust candidates have been pursued in the follow-up experiments. Furthermore, it is not known how or when sex-biased gene expression originates, as few studies have been performed in the developing brain. Here we integrate molecular genetic and neural circuit perspectives to provide a conceptual framework of how sex differences in gene expression can arise in the brain. We detail mechanisms of gene regulation by steroid hormones, highlight landmark studies in rodents and humans, identify emerging themes, and offer recommendations for future research. This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Sex Determination.

Deficiency of the palmitoyl acyltransferase ZDHHC7 impacts brain and behavior of mice in a sex-specific manner

The palmitoyl acyltransferase ZDHHC7 belongs to the DHHC family responsible for the covalent attachment of palmitic acid (palmitoylation) to target proteins. Among synaptic proteins, its main targets are sex steroid receptors such as the estrogen receptors. When palmitoylated, these couple to membrane microdomains and elicit non-genomic rapid responses. Such coupling is found particularly in cortico-limbic brain areas which impact structure, function, and behavioral outcomes. Thus far, the functional role of ZDHHC7 has not been investigated in this context. To directly analyze an impact of ZDHHC7 on brain anatomy, microstructure, connectivity, function, and behavior, we generated a mutant mouse in which the Zdhhc7 gene is constitutively inactivated. Male and female Zdhhc7^-/- mice were phenotypically compared with wild-type mice using behavioral tests, electrophysiology, protein analyses, and neuroimaging with diffusion tensor-based fiber tractography. Zdhhc7-deficiency impaired excitatory transmission, synaptic plasticity at hippocampal Schaffer collateral CA1 synapses, and hippocampal structural connectivity in both sexes in similar manners. Effects on both sexes but in different manners appeared in medial prefrontal cortical synaptic transmission and in hippocampal microstructures. Finally, Zdhhc7-deficiency affected anxiety-related behaviors exclusively in females. Our data demonstrated the importance of Zdhhc7 for assembling proper brain structure, function, and behavior on a system level in mice in a sex-related manner. Given the prominent role of sex-specificity also in humans and associated mental disorders, Zdhhc7^-/- mice might provide a promising model for in-depth investigation of potentially underlying sex-specifically altered mechanisms.

Sex Differences in the Epigenome: A Cause or Consequence of Sexual Differentiation of the Brain?

Females and males display differences in neural activity patterns, behavioral responses, and incidence of psychiatric and neurological diseases. Sex differences in the brain appear throughout the animal kingdom and are largely a consequence of the physiological requirements necessary for the distinct roles of the two sexes in reproduction. As with the rest of the body, gonadal steroid hormones act to specify and regulate many of these differences. It is thought that transient hormonal signaling during brain development gives rise to persistent sex differences in gene expression via an epigenetic mechanism, leading to divergent neurodevelopmental trajectories that may underlie sex differences in disease susceptibility. However, few genes with a persistent sex difference in expression have been identified, and only a handful of studies have employed genome-wide approaches to assess sex differences in epigenomic modifications. To date, there are no confirmed examples of gene regulatory elements that direct sex differences in gene expression in the brain. Here, we review foundational studies in this field, describe transcriptional mechanisms that could act downstream of hormone receptors in the brain, and suggest future approaches for identification and validation of sex-typical gene programs. We propose that sexual differentiation of the brain involves self-perpetuating transcriptional states that canalize sex-specific development.

Neural control of sexually dimorphic social behaviors

Sexually reproducing animals display sex differences in behavior. Although many of these sex differences in behavior are acquired with experience, sexually dimorphic behaviors such as mating and aggression are innate in the sense that they can be displayed without prior training or experience. In this review, we present recent advances in our understanding of the neural control of such innate sexually dimorphic social behaviors, with a focus on sexual behavior and aggression in flies and mice. We provide a brief overview of fundamental processes that regulate sexual differentiation in these animals to provide a framework within which more recent advances can be understood. We discuss advances in sensory, neuromodulatory, neural circuit, and experiential regulation of sexually dimorphic social behaviors.

Fast, furious and enduring: Sensitive versus critical periods in sexual differentiation of the brain

Understanding critical periods in brain development and how they impact adult functioning is a primary goal of neuroscience. The sexual differentiation of the brain is a unique critical period in that it is initiated by endogenous production of a critical signaling molecule in only one sex, testosterone in fetal males. Females, by contrast, do not produce testosterone but are highly responsive to it and remain sensitive to its masculinizing effects well past the close of the critical period in males. Compared to other well characterized critical periods, such as those for the visual system or barrel cortex, the masculinization of the brain is telescoped into a few short days and initiated prenatally. The slightly longer and postnatal sensitive period in females provides a valuable tool for understanding this challenging but fundamental developmental process.

A GABAergic cell type in the lateral habenula links hypothalamic homeostatic and midbrain motivation circuits with sex steroid signaling

The lateral habenula (LHb) has a key role in integrating a variety of neural circuits associated with reward and aversive behaviors. There is limited information about how the different cell types and neuronal circuits within the LHb coordinate physiological and motivational states. Here, we report a cell type in the medial division of the LHb (LHbM) in male rats that is distinguished by: (1) a molecular signature for GABAergic neurotransmission (Slc32a1/VGAT) and estrogen receptor (Esr1/ERα) expression, at both mRNA and protein levels, as well as the mRNA for vesicular glutamate transporter Slc17a6/VGLUT2, which we term the GABAergic estrogen-receptive neuron (GERN); (2) its axonal projection patterns, identified by in vivo juxtacellular labeling, to both local LHb and to midbrain modulatory systems; and (3) its somatic expression of receptors for vasopressin, serotonin and dopamine, and mRNA for orexin receptor 2. This cell type is anatomically located to receive afferents from midbrain reward (dopamine and serotonin) and hypothalamic water and energy homeostasis (vasopressin and orexin) circuits. These afferents shared the expression of estrogen synthase (aromatase) and VGLUT2, both in their somata and axon terminals. We demonstrate dynamic changes in LHbM VGAT+ cell density, dependent upon gonadal functional status, that closely correlate with motivational behavior in response to predator and forced swim stressors. The findings suggest that the homeostasis and reward-related glutamatergic convergent projecting pathways to LHbMC employ a localized neurosteroid signaling mechanism via axonal expression of aromatase, to act as a switch for GERN excitation/inhibition output prevalence, influencing depressive or motivated behavior.