A brain sexual dimorphism controlled by adult circulating androgens

Sex and mental health are related to subcortical brain microstructure

Some mental health problems such as depression and anxiety are more common in females, while others such as autism and attention deficit/hyperactivity (AD/H) are more common in males. However, the neurobiological origins of these sex differences are poorly understood. Animal studies have shown substantial sex differences in neuronal and glial cell structure, while human brain imaging studies have shown only small differences, which largely reflect overall body and brain size. Advanced diffusion MRI techniques can be used to examine intracellular, extracellular, and free water signal contributions and provide unique insights into microscopic cellular structure. However, the extent to which sex differences exist in these metrics of subcortical gray matter structures implicated in psychiatric disorders is not known. Here, we show large sex-related differences in microstructure in subcortical regions, including the hippocampus, thalamus, and nucleus accumbens in a large sample of young adults. Unlike conventional T1-weighted structural imaging, large sex differences remained after adjustment for age and brain volume. Further, diffusion metrics in the thalamus and amygdala were associated with depression, anxiety, AD/H, and antisocial personality problems. Diffusion MRI may provide mechanistic insights into the origin of sex differences in behavior and mental health over the life course and help to bridge the gap between findings from experimental, epidemiological, and clinical mental health research.

Cohabitation with receptive females under D2-type agonism in adulthood restores partner preference and brain dimorphism in the SDN-POA following neonatal gonadectomy in male rats

Perinatal testosterone, or its metabolite estradiol, organize the brain toward a male phenotype. Male rodents with insufficient testosterone during this period fail to display sexual behavior and partner preference for receptive females in adulthood. However, cohabitation with non-reproductive conspecifics under the influence of a D2 agonist facilitates the expression of conditioned partner preference via Pavlovian learning in gonadally intact male rats. In the present experiment, three groups of neonatal PD1 males (N = 12/group) were either gonadectomized (GDX), sham-GDX, or left intact and evaluated for social preferences and sexual behaviors as adults. We then examined whether the effects of GDX could be reversed by conditioning the males via cohabitation with receptive females under the effects of the D2 agonist quinpirole (QNP) or saline, along with the size of some brain regions, such as the sexually dimorphic nucleus of the preoptic area (SDN-POA), suprachiasmatic nucleus (SCN), posterior dorsal medial amygdala (MeApd) and ventromedial hypothalamus (VMH). Results indicated that neonatal GDX resulted in the elimination of male-typical sexual behavior, an increase in same-sex social preference, and a reduction of the area of the SDN-POA. However, GDX-QNP males that underwent exposure to receptive females in adulthood increased their social preference for females and recovered the size in the SDN-POA. Although neonatal GDX impairs sexual behavior and disrupts partner preference and brain dimorphism in adult male rats, Pavlovian conditioning under enhanced D2 agonism ameliorates the effects on social preference and restores brain dimorphism in the SDN-POA without testosterone.

Sex and interspecies differences in ESR2-expressing cell distributions in mouse and rat brains

BACKGROUND

ESR2, a nuclear estrogen receptor also known as estrogen receptor β, is expressed in the brain and contributes to the actions of estrogen in various physiological phenomena. However, its expression profiles in the brain have long been debated because of difficulties in detecting ESR2-expressing cells. In the present study, we aimed to determine the distribution of ESR2 in rodent brains, as well as its sex and interspecies differences, using immunohistochemical detection with a well-validated anti-ESR2 antibody (PPZ0506).

METHODS

To determine the expression profiles of ESR2 protein in rodent brains, whole brain sections from mice and rats of both sexes were subjected to immunostaining for ESR2. In addition, to evaluate the effects of circulating estrogen on ESR2 expression profiles, ovariectomized female mice and rats were treated with low or high doses of estrogen, and the resulting numbers of ESR2-immunopositive cells were analyzed. Welch's t-test was used for comparisons between two groups for sex differences, and one-way analysis of variance followed by the Tukey-Kramer test were used for comparisons among multiple groups with different estrogen treatments.

RESULTS

ESR2-immunopositive cells were observed in several subregions of mouse and rat brains, including the preoptic area, extended amygdala, hypothalamus, mesencephalon, and cerebral cortex. Their distribution profiles exhibited sex and interspecies differences. In addition, low-dose estrogen treatment in ovariectomized female mice and rats tended to increase the numbers of ESR2-immunopositive cells, whereas high-dose estrogen treatment tended to decrease these numbers.

CONCLUSIONS

Immunohistochemistry using the well-validated PPZ0506 antibody revealed a more localized expression of ESR2 protein in rodent brains than has previously been reported. Furthermore, there were marked sex and interspecies differences in its distribution. Our histological analyses also revealed estrogen-dependent changes in ESR2 expression levels in female brains. These findings will be helpful for understanding the ESR2-mediated actions of estrogen in the brain.

Sex differences in androgen receptor, estrogen receptor alpha, and c-Fos co-expression with corticotropin releasing factor expressing neurons in restrained adult mice

Gonadal hormone actions through androgen receptor (AR) and estrogen receptor alpha (ERα) regulate sex differences in hypothalamic-pituitary-adrenal (HPA) axis responsivity and stress-related behaviors. Here we tested whether corticotropin releasing factor (CRF) expressing neurons, which are widely known to regulate neuroendocrine and behavioral stress responses, co-express AR and ERα as a potential mechanism for gonadal hormone regulation of these responses. Using Crh-IRES-Cre::Ai9 reporter mice we report high co-localization of AR in CRF neurons within the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BST), medial amygdala (MeA), and ventromedial hypothalamus (VMH), moderate levels within the central amygdala (CeA) and low levels in the paraventricular hypothalamus (PVN). Sex differences in CRF/AR co-expression were found in the principal nucleus of the BST (BSTmpl), CeA, MeA, and VMH (males>females). CRF co-localization with ERα was generally lower relative to AR co-localization. However, high co-expression was found within the MPOA, AVPV, and VMH, with moderate co-expression in the arcuate nucleus (ARC), BST, and MeA and low levels in the PVN and CeA. Sex differences in CRF/ERα co-localization were found in the BSTmpl and PVN (males>females). Finally, we assessed neural activation of CRF neurons in restraint-stressed mice and found greater CRF/c-Fos co-expression in females in the BSTmpl and periaqueductal gray, while co-expression was higher in males within the ARC and dorsal CA1. Given the known role of CRF in regulating behavioral stress responses and the HPA axis, AR/ERα co-expression and sex-specific activation of CRF cell groups indicate potential mechanisms for modulating sex differences in these functions.

Effects of postnatal exposure to cadmium on male sexual incentive motivation and copulatory behavior: Estrogen and androgen receptors expression in adult brain rat

There are numerous evidence showing that cadmium (Cd) is an endocrine disruptor that exerts multiple toxic effects at different reproductive levels, including male sexual behavior (MSB). The effect of early exposure to Cd on sexual incentive motivation (SIM) and MSB in adult stage, and the immunoreactivity of receptors for hormones such as estrogens and androgens in brain regions that are relevant for the SIM and MSB display, have not been studied until now. The present study evaluated the effects of 0.5 and 1 mg/kg CdCl2 from day 1-56 of postnatal life on SIM and MSB in adults rats, as well as serum testosterone concentrations, Cd concentration in blood, testis, and brain areas, and the immunoreactivity in estrogen receptors (ER-α and -β), and androgen receptor (AR) in the olfactory bulbs (OB), medial preoptic area (mPOA), and medial amygdala (MeA). Our results showed that both doses of Cd decreased SIM and MSB, accompanied by low serum concentrations of testosterone. Also, there was a significant reduction in immunoreactivity of ER-α and AR in mPOA, and a significant reduction in AR in MeA on male rats treated with Cd 1 mg/kg. These results show that exposure to high doses of Cd in early postnatal life could alter the correct integration of hormonal signals in the brain areas that regulate and display SIM and MSB in adult male rats.

Intergenerational Perioperative Neurocognitive Disorder

Accelerated neurocognitive decline after general anesthesia/surgery, also known as perioperative neurocognitive disorder (PND), is a widely recognized public health problem that may affect millions of patients each year. Advanced age, with its increasing prevalence of heightened stress, inflammation, and neurodegenerative alterations, is a consistent contributing factor to the development of PND. Although a strong homeostatic reserve in young adults makes them more resilient to PND, animal data suggest that young adults with pathophysiological conditions characterized by excessive stress and inflammation may be vulnerable to PND, and this altered phenotype may be passed to future offspring (intergenerational PND). The purpose of this narrative review of data in the literature and the authors' own experimental findings in rodents is to draw attention to the possibility of intergenerational PND, a new phenomenon which, if confirmed in humans, may unravel a big new population that may be affected by parental PND. In particular, we discuss the roles of stress, inflammation, and epigenetic alterations in the development of PND. We also discuss experimental findings that demonstrate the effects of surgery, traumatic brain injury, and the general anesthetic sevoflurane that interact to induce persistent dysregulation of the stress response system, inflammation markers, and behavior in young adult male rats and in their future offspring who have neither trauma nor anesthetic exposure (i.e., an animal model of intergenerational PND).

The Curious Case of the Naked Mole-Rat: How Extreme Social and Reproductive Adaptations Might Influence Sex Differences in the Brain

Research in the neurobiology of sex differences is inherently influenced by the study species that are used. Some traditional animal research models, such as rats and mice, show certain sex differences in the brain that have been foundational to neurobiological research. However, subsequent work has demonstrated that these differences are not always generalizable, especially to species with different social structures and sex-associated roles or behaviors. One such example is the naked mole-rat (Heterocephalus glaber), which has an unusual social structure among mammals. Naked mole-rats live in large groups where reproduction is restricted to a dominant female, called the "queen," and often only one breeding male. All other animals in the group, the "subordinates," are socially suppressed from reproduction and remain in a prepubescent state as adults, unless they are removed from the presence of the queen. These subordinates show little to no sex differences in external morphology, neural morphology, or behavior. However, there are a suite of neurobiological differences between subordinate and breeding naked mole-rats. After naked mole-rats attain breeding status, many of the classically sexually differentiated brain regions increase in volume (paraventricular nucleus, medial amygdala, bed nucleus of the stria terminalis). There are additionally social status differences in sex hormone receptor expression in the brain, as well as other changes in gene expression, some of which also show sex differences - though not always in the predicted direction based on other rodent studies. Data from naked mole-rats show that it is critical to consider the evolved social structure of a species when studying sex differences in the brain.

Social Network Plasticity of Mice Parental Behavior

Neural plasticity occurs during developmental stages and is essential for sexual differentiation of the brain and the ensuing sex-dependent behavioral changes in adults. Maternal behavior is primarily affected by sex-related differences in the brain; however, chronic social isolation even in mature male mice can induce maternal retrieving and crouching behavior when they are first exposed to pups. Social milieus influence the inherent behavior of adults and alter the molecular architecture in the brain, thereby allowing higher levels of associated gene expression and molecular activity. This review explores the possibility that although the development of neural circuits is closely associated with maternal behavior, the brain can still retain its neuroplasticity in adults from a neuromolecular perspective. In addition, neuronal machinery such as neurotransmitters and neuropeptides might influence sociobehavioral changes. This review also discusses that the neural circuits regulating behaviors such as parenting and infanticide (including neglect behavior), might be controlled by neural relay on melanin concentrating hormone (MCH)–oxytocin in the hypothalamus during the positive and negative mode of action in maternal behavior. Furthermore, MCH–oxytocin neural relay might contribute to the anxiolytic effect on maternal behavior, which is involved with reward circuits.

Higher Rate of Male Sexual Displays Correlates with Larger Ventral Posterior Amygdala Volume and Neuron Soma Volume in Wild-Caught Common Side-Blotched Lizards, Uta stansburiana

Several areas of the vertebrate brain are involved in facilitating and inhibiting the production of sexual behaviors and displays. In the laboratory, a higher rate of sexual displays is correlated with a larger ventral posterior amygdala (VPA), an area of the brain involved in the expression of sexual display behaviors, as well as larger VPA neuronal somas. However, it remains unclear if individuals in the field reflect similar patterns, as there are likely many more selective pressures in the field that may also modulate the VPA architecture. In this study, we examined variation in VPA volume and neuron soma volume in wild-caught common side-blotched lizards (Uta stansburiana) from two different populations. In a population from Nevada, males experience high predation pressure and have decreased sexual display rates during the breeding season, whereas a population in Oregon has lower levels of predation and higher rates of male sexual displays. We found that wild-caught males from the population with lower display rates also exhibited decreased VPA volume and VPA neuron cell soma volume, which may suggest that decreased display rate, possibly due to increased predation rate, covaries with VPA attributes.

Brain-Wide Synaptic Inputs to Aromatase-Expressing Neurons in the Medial Amygdala Suggest Complex Circuitry for Modulating Social Behavior

Here, we reveal an unbiased view of the brain regions that provide specific inputs to aromatase-expressing cells in the medial amygdala, neurons that play an outsized role in the production of sex-specific social behaviors, using rabies tracing and light sheet microscopy. While the downstream projections from these cells are known, the specific inputs to the aromatase-expressing cells in the medial amygdala remained unknown. We observed established connections to the medial amygdala (e.g., bed nucleus of the stria terminalis and accessory olfactory bulb) indicating that aromatase neurons are a major target cell type for efferent input including from regions associated with parenting and aggression. We also identified novel and unexpected inputs from areas involved in metabolism, fear and anxiety, and memory and cognition. These results confirm the central role of the medial amygdala in sex-specific social recognition and social behavior, and point to an expanded role for its aromatase-expressing neurons in the integration of multiple sensory and homeostatic factors, which are likely used to modulate many other social behaviors.

Amygdala and subregion volumes are associated with photoperiod and seasonal depressive symptoms: A cross-sectional study in the UK Biobank cohort

Although seasonal changes in amygdala volume have been demonstrated in animals, seasonal differences in human amygdala subregion volumes have yet to be investigated. Amygdala volume has also been linked to depressed mood. Therefore, we hypothesised that differences in photoperiod would predict differences in amygdala or subregion volumes and that this association would be linked to depressed mood. 10,033 participants ranging in age from 45 to 79 years were scanned by MRI in a single location. Amygdala subregion volumes were obtained using automated processing and segmentation algorithms. A mediation analysis tested whether amygdala volume mediated the relationship between photoperiod and mood. Photoperiod was positively associated with total amygdala volume (p < .001). Multivariate (GLM) analyses revealed significant effects of photoperiod across all amygdala subregion volumes for both hemispheres (p < .001). Post hoc univariate regression analyses revealed significant associations of photoperiod with each amygdala subregion volume (p < .001). PLS showed the highest loadings of amygdala subregions in lateral nucleus, ABN, basal nucleus, CAT, PLN, AAA, central nucleus, cortical nucleus and medial nucleus for left hemisphere and ABN, lateral nucleus, CAT, PLN, cortical nucleus, AAA, central nucleus and medial nucleus for right hemisphere. There were no significant associations between photoperiod and mood nor between mood scores and amygdala volumes, and due to the lack of these associations, the mediation hypothesis was not supported. This study is the first to demonstrate an association between photoperiod and amygdala volume. These findings add to the evidence supporting the role of photoperiod on brain structural plasticity.

Posterodorsal Medial Amygdala Regulation of Female Social Behavior: GABA versus Glutamate Projections

Social behaviors, including reproductive behaviors, often display sexual dimorphism. Lordosis, the measure of female sexual receptivity, is one of the most apparent sexually dimorphic reproductive behaviors. Lordosis is regulated by estrogen and progesterone (P4) acting within a hypothalamic-limbic circuit, consisting of the arcuate, medial preoptic, and ventromedial nuclei of the hypothalamus. Social cues are integrated into the circuit through the amygdala. The posterodorsal part of the medial amygdala (MeApd) is involved in sexually dimorphic social and reproductive behaviors, and sends projections to hypothalamic neuroendocrine regions. GABA from the MeApd appears to facilitate social behaviors, while glutamate may play the opposite role. To test these hypotheses, adult female vesicular GABA transporter (VGAT)-Cre and vesicular glutamate transporter 2 (VGluT2)-Cre mice were transfected with halorhodopsin (eNpHR)-expressing or channelrhodopsin-expressing adeno-associated viruses (AAVs), respectively, in the MeApd. The lordosis quotient (LQ) was measured following either photoinhibition of VGAT or photoexcitation of VGluT2 neurons, and brains were assessed for c-Fos immunohistochemistry (IHC). Photoinhibition of VGAT neurons in the MeApd decreased LQ, and decreased c-Fos expression within VGAT neurons, within the MeApd as a whole, and within the ventrolateral part of the ventromedial nucleus (VMHvl). Photoexcitation of VGluT2 neurons did not affect LQ, but did increase time spent self-grooming, and increased c-Fos expression within VGluT2 neurons in the MeApd. Neither condition altered c-Fos expression in the medial preoptic nucleus (MPN) or the arcuate nucleus (ARH). These data support a role for MeApd GABA in the facilitation of lordosis. Glutamate from the MeApd does not appear to be directly involved in the lordosis circuit, but appears to direct behavior away from social interactions.SIGNIFICANCE STATEMENT Lordosis, the measure of female sexual receptivity, is a sexually dimorphic behavior regulated within a hypothalamic-limbic circuit. Social cues are integrated through the amygdala, and the posterodorsal part of the medial amygdala (MeApd) is involved in sexually dimorphic social and reproductive behaviors. Photoinhibition of GABAergic neurons in the MeApd inhibited lordosis, while photoactivation of glutamate neurons had no effect on lordosis, but increased self-grooming. These data support a role for MeApd GABA in the facilitation of social behaviors and MeApd glutamate projections in anti-social interactions.

Brain structural and neuroendocrine basis of sex differences in epilepsy

This chapter reviews the current information about sex differences in epilepsy and potential mechanisms underlying sex differences in seizure susceptibility and epilepsy. The susceptibility to and occurrence of seizures are generally higher in men than women. There is gender-specific epilepsies such as catamenial epilepsy, a neuroendocrine condition in which seizures are most often clustered around the perimenstrual or periovulatory period in adult women. Structural differences in cerebral morphology, the structural and functional circuits may render men and women differentially vulnerable to seizure disorders and epileptogenic processes. Changes in seizure sensitivity are evident at puberty, pregnancy, and menopause, often attributed to circulating steroid hormones and neurosteroids as well as neuroplasticity in receptor systems. An improved understanding of the sexual dimorphism in neural circuits and the neuroendocrine basis of sex differences or resistance to protective drugs is essential to develop sex-specific therapies for seizure conditions.

The social network: Neural control of sex differences in reproductive behaviors, motivation, and response to social isolation

Social animal species present a vast repertoire of social interactions when encountering conspecifics. Reproduction-related behaviors, such as mating, parental care, and aggression, are some of the most rewarding types of social interactions and are also the most sexually dimorphic ones. This review focuses on rodent species and summarizes recent advances in neuroscience research that link sexually dimorphic reproductive behaviors to sexual dimorphism in their underlying neuronal circuits. Specifically, we present a few possible mechanisms governing sexually-dimorphic behaviors, by hypothalamic and reward-related brain regions. Sex differences in the neural response to social isolation in adulthood are also discussed, as well as future directions for comparative studies with naturally solitary species.

Pheromone effects on the human hypothalamus in relation to sexual orientation and gender

Pheromones are chemicals that serve communicational purposes within a species. In most terrestrial mammals, pheromones are detected by either the olfactory epithelium or the vomeronasal organ and processed by various downstream structures including the medial amygdala and the hypothalamus to regulate motivated behaviors and endocrine responses. The search for human pheromones began in the 1970s. Whereas bioactive ligands are yet to be identified, there has been accumulating evidence that human body odors exert a range of pheromone-like effects on the recipients, including triggering innate behavioral responses, modulating endocrine levels, signaling social information, and affecting mood and cognition. In parallel, results from recent brain imaging studies suggest that body odors evoke distinct neural responses from those observed with common nonsocial odors. Two endogenous steroids androsta-4,16,- dien-3-one and estra-1,3,5(10),16-tetraen-3-ol are considered by some as candidates for human sex pheromones. The two substances produce sexually dimorphic effects on human perception, mood, and physiological arousal. Moreover, they reportedly elicit different hypothalamic response patterns in manners contingent on the recipients' sex and sexual orientation. Neuroendocrine mechanisms underlying the effects of human chemosignals are not yet clear and await future detailed analyses.

Sex Differences in the Development of the Rodent Corticolimbic System

In recent years, a growing body of research has shown sex differences in the prevalence and symptomatology of psychopathologies, such as depression, anxiety, and fear-related disorders, all of which show high incidence rates in early life. This has highlighted the importance of including female subjects in animal studies, as well as delineating sex differences in neural processing across development. Of particular interest is the corticolimbic system, comprising the hippocampus, amygdala, and medial prefrontal cortex. In rodents, these corticolimbic regions undergo dynamic changes in early life, and disruption to their normative development is believed to underlie the age and sex-dependent effects of stress on affective processing. In this review, we consolidate research on sex differences in the hippocampus, amygdala, and medial prefrontal cortex across early development. First, we briefly introduce current principles on sexual differentiation of the rodent brain. We then showcase corticolimbic regional sex differences in volume, morphology, synaptic organization, cell proliferation, microglia, and GABAergic signaling, and explain how these differences are influenced by perinatal and pubertal gonadal hormones. In compiling this research, we outline evidence of what and when sex differences emerge in the developing corticolimbic system, and illustrate how temporal dynamics of its maturational trajectory may differ in male and female rodents. This will help provide insight into potential neural mechanisms underlying sex-specific critical windows for stress susceptibility and behavioral emergence.

Sex Differences in Biophysical Signatures across Molecularly Defined Medial Amygdala Neuronal Subpopulations

The medial amygdala (MeA) is essential for processing innate social and non-social behaviors, such as territorial aggression and mating, which display in a sex-specific manner. While sex differences in cell numbers and neuronal morphology in the MeA are well established, if and how these differences extend to the biophysical level remain unknown. Our previous studies revealed that expression of the transcription factors, Dbx1 and Foxp2, during embryogenesis defines separate progenitor pools destined to generate different subclasses of MEA inhibitory output neurons. We have also previously shown that Dbx1-lineage and Foxp2-lineage neurons display different responses to innate olfactory cues and in a sex-specific manner. To examine whether these neurons also possess sex-specific biophysical signatures, we conducted a multidimensional analysis of the intrinsic electrophysiological profiles of these transcription factor defined neurons in the male and female MeA. We observed striking differences in the action potential (AP) spiking patterns across lineages, and across sex within each lineage, properties known to be modified by different voltage-gated ion channels. To identify the potential mechanism underlying the observed lineage-specific and sex-specific differences in spiking adaptation, we conducted a phase plot analysis to narrow down putative ion channel candidates. Of these candidates, we found a subset expressed in a lineage-biased and/or sex-biased manner. Thus, our results uncover neuronal subpopulation and sex differences in the biophysical signatures of developmentally defined MeA output neurons, providing a potential physiological substrate for how the male and female MeA may process social and non-social cues that trigger innate behavioral responses.

Synaptic Connections of Aromatase Circuits in the Medial Amygdala Are Sex Specific

The brains of male and female mice are shaped by genetics and hormones during development. The enzyme aromatase helps establish sex differences in social behaviors and in the neural circuits that produce these behaviors. The medial amygdala of mice contains a large population of aromatase neurons and is a critical hub in the social behavior network. Moreover, the neural representation of social stimuli in the medial amygdala displays clear sex differences that track developmental changes in social behaviors. Here, we identify a potential anatomic basis for those sex differences. We found that sensory input from the accessory olfactory bulb (AOB) to aromatase neurons is derived nearly exclusively from the anterior AOB, which selectively responds to chemosensory cues from conspecific animals. Through the coordinated use of mouse transgenics and viral-based circuit-tracing strategies, we demonstrate a clear sex difference in the volume of synapses connecting the accessory olfactory bulb to aromatase-expressing neurons in the medial amygdala of male versus female mice. This difference in anatomy likely mediates, at least in part, sex differences in medial amygdala-mediated social behaviors.

Effects of Prenatal Exposure to a Low-Dose of Bisphenol A on Sex Differences in Emotional Behavior and Central Alpha2-Adrenergic Receptor Binding

Prenatal exposure to bisphenol A (BPA) influences the development of sex differences neurologically and behaviorally across many species of vertebrates. These effects are a consequence of BPA’s estrogenic activity and its ability to act as an endocrine disrupter even, at very low doses. When exposure to BPA occurs during critical periods of development, it can interfere with the normal activity of sex steroids, impacting the fate of neurons, neural connectivity and the development of brain regions sensitive to steroid activity. Among the most sensitive behavioral targets of BPA action are behaviors that are characterized by a sexual dimorphism, especially emotion and anxiety related behaviors, such as the amount of time spent investigating a novel environment, locomotive activity and arousal. Moreover, in some species of rodents, BPA exposure affected males’ sexual behaviors. Interestingly, these behaviors are at least in part modulated by the catecholaminergic system, which has been reported to be a target of BPA action. In the present study we investigated the influence of prenatal exposure of mice to a very low single dose of BPA on emotional and sexual behaviors and on the density and binding characteristics of alpha₂ adrenergic receptors. Alpha₂ adrenergic receptors are widespread in the central nervous system and they can act as autoreceptors, inhibiting the release of noradrenaline and other neurotransmitters from presynaptic terminals. BPA exposure disrupted sex differences in behavioral responses to a novel environment, but did not affect male mice sexual behavior. Importantly, BPA exposure caused a change in the binding affinity of alpha₂ adrenergic receptors in the locus coeruleus and medial preoptic area (mPOA) and it eliminated the sexual dimorphism in the density of the receptors in the mPOA.

The neurogenetics of sexually dimorphic behaviors from a postdevelopmental perspective

Most sexually reproducing animal species are characterized by two morphologically and behaviorally distinct sexes. The genetic, molecular and cellular processes that produce sexual dimorphisms are phylogenetically diverse, though in most cases they are thought to occur early in development. In some species, however, sexual dimorphisms are manifested after development is complete, suggesting the intriguing hypothesis that sex, more generally, might be considered a continuous trait that is influenced by both developmental and postdevelopmental processes. Here, we explore how biological sex is defined at the genetic, neuronal and behavioral levels, its effects on neuronal development and function, and how it might lead to sexually dimorphic behavioral traits in health and disease. We also propose a unifying framework for understanding neuronal and behavioral sexual dimorphisms in the context of both developmental and postdevelopmental, physiological timescales. Together, these two temporally separate processes might drive sex-specific neuronal functions in sexually mature adults, particularly as it pertains to behavior in health and disease.

In vivo magnetic resonance imaging reveals the effect of gonadal hormones on morphological and functional brain sexual dimorphisms in adult sheep

Sex differences in the brain and behavior are produced by the perinatal action of testosterone, which is converted into estradiol by the enzyme aromatase in the brain. Although magnetic resonance imaging (MRI) has been widely used in humans to study these differences, the use of animal models, where hormonal status can be properly manipulated, is necessary to explore the mechanisms involved. We used sheep, a recognized model in the field of neuroendocrinology, to assess brain morphological and functional sex differences and their regulation by adult gonadal hormones. To this end, we performed voxel-based morphometry and a resting-state functional MRI approach to assess sex differences in gonadally intact animals. We demonstrated significant sex differences in gray matter concentration (GMC) at the level of the gonadotropic axis, i.e., not only within the hypothalamus and pituitary but also within the hippocampus and the amygdala of intact animals. We then performed the same analysis one month after gonadectomy and found that some of these differences were reduced, especially in the hypothalamus and amygdala. By contrast, we found few differences in the organization of the functional connectome between males and females either before or after gonadectomy. As a whole, our study identifies brain regions that are sexually dimorphic in the sheep brain at the resolution of the MRI and highlights the role of gonadal hormones in the maintenance of these differences.

Supraphysiologic-dose anabolic-androgenic steroid use: A risk factor for dementia?

Supraphysiologic-dose anabolic-androgenic steroid (AAS) use is associated with physiologic, cognitive, and brain abnormalities similar to those found in people at risk for developing Alzheimer's Disease and its related dementias (AD/ADRD), which are associated with high brain β-amyloid (Aβ) and hyperphosphorylated tau (tau-P) protein levels. Supraphysiologic-dose AAS induces androgen abnormalities and excess oxidative stress, which have been linked to increased and decreased expression or activity of proteins that synthesize and eliminate, respectively, Aβ and tau-P. Aβ and tau-P accumulation may begin soon after initiating supraphysiologic-dose AAS use, which typically occurs in the early 20s, and their accumulation may be accelerated by other psychoactive substance use, which is common among non-medical AAS users. Accordingly, the widespread use of supraphysiologic-dose AAS may increase the numbers of people who develop dementia. Early diagnosis and correction of sex-steroid level abnormalities and excess oxidative stress could attenuate risk for developing AD/ADRD in supraphysiologic-dose AAS users, in people with other substance use disorders, and in people with low sex-steroid levels or excess oxidative stress associated with aging.

Kisspeptin and the control of emotions, mood and reproductive behaviour

Reproduction is fundamental for the survival of all species and requires meticulous synchronisation of a diverse complement of neural, endocrine and related behaviours. The reproductive hormone kisspeptin (encoded by the KISS1/Kiss1 gene) is now a well-established orchestrator of reproductive hormones, acting upstream of gonadotrophin-releasing hormone (GnRH) at the apex of the hypothalamic–pituitary–gonadal (HPG) reproductive axis. Beyond the hypothalamus, kisspeptin is also expressed in limbic and paralimbic brain regions, which are areas of the neurobiological network implicated in sexual and emotional behaviours. We are now forming a more comprehensive appreciation of extra-hypothalamic kisspeptin signalling and the complex role of kisspeptin as an upstream mediator of reproductive behaviours, including olfactory-driven partner preference, copulatory behaviour, audition, mood and emotion. An increasing body of research from zebrafish to humans has implicated kisspeptin in the integration of reproductive hormones with an overall positive influence on these reproductive behaviours. In this review, we critically appraise the current literature regarding kisspeptin and its control of reproductive behaviour. Collectively, these data significantly enhance our understanding of the integration of reproductive hormones and behaviour and provide the foundation for kisspeptin-based therapies to treat related disorders of body and mind.

Sex-specific changes in postnatal GH and PRL secretion in somatotrope LEPR-null mice

The developing pituitary is a rapidly changing environment that is constantly meeting the physiological demands of the growing organism. During early postnatal development, the anterior pituitary is refining patterns of anterior hormone secretion in response to numerous genetic factors. Our laboratory previously developed a somatotrope leptin receptor (LEPR) deletion mouse model that had decreased lean body mass, disrupted metabolism, decreased GH stores and was GH deficient as an adult. To understand how deletion of LEPR in somatotropes altered GH, we turned our attention to postnatal development. The current study examines GH, PRL, TSH, ACTH, LH and FSH secretion during postnatal days 4, 5, 8, 10 and 15 and compares age and sex differences. The LEPR mutants have dysregulation of GH (P < 0.03) and a reduced developmental prolactin peak in males (P < 0.04) and females (P < 0.002). There were no differences in weight between groups, and the postnatal leptin surge appeared to be normal. Percentages of immunolabeled GH cells were reduced in mutants compared with controls in all age groups by 35-61% in males and 41-44% in females. In addition, we measured pituitary expression of pituitary transcription factors, POU1F1 and PROP1. POU1F1 was reduced in mutant females at PND 10 (P < 0.009) and PND 15 (P < 0.02) but increased in males at PND 10 (P < 0.01). PROP1 was unchanged in female mutants but showed developmental increases at PND 5 (P < 0.02) and PND 15 (P < 0.01). These studies show that the dysfunction caused by LEPR deletion in somatotropes begins as early as neonatal development and involves developing GH and prolactin cells (somatolactotropes).

Effects of Estrogens on Central Nervous System Neurotransmission: Implications for Sex Differences in Mental Disorders

Nearly one of every five US individuals aged 12 years old or older lives with certain types of mental disorders. Men are more likely to use various types of substances, while women tend to be more susceptible to mood disorders, addiction, and eating disorders, all of which are risks associated with suicidal attempts. Fundamental sex differences exist in multiple aspects of the functions and activities of neurotransmitter-mediated neural circuits in the central nervous system (CNS). Dysregulation of these neural circuits leads to various types of mental disorders. The potential mechanisms of sex differences in the CNS neural circuitry regulating mood, reward, and motivation are only beginning to be understood, although they have been largely attributed to the effects of sex hormones on CNS neurotransmission pathways. Understanding this topic is important for developing prevention and treatment of mental disorders that should be tailored differently for men and women. Studies using animal models have provided important insights into pathogenesis, mechanisms, and new therapeutic approaches of human diseases, but some concerns remain to be addressed. The purpose of this chapter is to integrate human and animal studies involving the effects of the sex hormones, estrogens, on CNS neurotransmission, reward processing, and associated mental disorders. We provide an overview of existing evidence for the physiological, behavioral, cellular, and molecular actions of estrogens in the context of controlling neurotransmission in the CNS circuits regulating mood, reward, and motivation and discuss related pathology that leads to mental disorders.

Effects of perinatal bisphenol A exposure on the volume of sexually-dimorphic nuclei of juvenile rats: A CLARITY-BPA consortium study

Bisphenol A (BPA) is a high volume endocrine disrupting chemical found in a wide variety of products including plastics and epoxy resins. Human exposure is nearly ubiquitous, and higher in children than adults. Because BPA has been reported to interfere with sex steroid hormone signaling, there is concern that developmental exposure, even at levels below the current FDA No Observed Adverse Effect Level (NOAEL) of 5mg/kg body weight (bw)/day, can disrupt brain sexual differentiation. The current studies were conducted as part of the CLARITY-BPA (Consortium Linking Academic and Regulatory Insights on BPA Toxicity) program and tested the hypothesis that perinatal BPA exposure would induce morphological changes in hormone sensitive, sexually dimorphic brain regions. Sprague-Dawley rats were randomly assigned to 5 groups: BPA (2.5, 25, or 2500μg/kgbw/day), a reference estrogen (0.5μg ethinylestradiol (EE₂)/kgbw/day), or vehicle. Exposure occurred by gavage to the dam from gestational day 6 until parturition, and then to the offspring from birth through weaning. Unbiased stereology was used to quantify the volume of the sexually dimorphic nucleus (SDN), the anteroventral periventricular nucleus (AVPV), the posterodorsal portion of the medial amygdala (MePD), and the locus coeruleus (LC) at postnatal day 28. No appreciable effects of BPA were observed on the volume of the SDN or LC. However, AVPV volume was enlarged in both sexes, even at levels below the FDA NOAEL. Collectively, these data suggest the developing brain is vulnerable to endocrine disruption by BPA at exposure levels below previous estimates by regulatory agencies.

Prenatal Influences on Human Sexual Orientation: Expectations versus Data

In non-human vertebrate species, sexual differentiation of the brain is primarily driven by androgens such as testosterone organizing the brains of males in a masculine fashion early in life, while the lower levels of androgen in developing females organize their brains in a feminine fashion. These principles may be relevant to the development of sexual orientation in humans, because retrospective markers of prenatal androgen exposure, namely digit ratios and otoacoustic emissions, indicate that lesbians, on average, were exposed to greater prenatal androgen than were straight women. Thus, the even greater levels of prenatal androgen exposure experienced by fetal males may explain why the vast majority of them grow up to be attracted to women. However, the same markers indicate no significant differences between gay and straight men in terms of average prenatal androgen exposure, so the variance in orientation in men cannot be accounted for by variance in prenatal androgen exposure, but may be due to variance in response to prenatal androgens. These data contradict several popular notions about human sexual orientation. Sexual orientation in women is said to be fluid, sometimes implying that only social influences in adulthood are at work, yet the data indicate prenatal influences matter as well. Gay men are widely perceived as under-masculinized, yet the data indicate they are exposed to as much prenatal androgen as straight men. There is growing sentiment to reject "binary" conceptions of human sexual orientations, to emphasize instead a spectrum of orientations. Yet the data indicate that human sexual orientation is sufficiently polarized that groups of lesbians, on average, show evidence of greater prenatal androgen exposure than groups of straight women, while groups of gay men have, on average, a greater proportion of brothers among their older siblings than do straight men.

Neuroimmunology and neuroepigenetics in the establishment of sex differences in the brain

The study of sex differences in the brain is a topic of neuroscientific study that has broad reaching implications for culture, society and biomedical science. Recent research in rodent models has led to dramatic shifts in our views of the mechanisms underlying the sexual differentiation of the brain. These include the surprising discoveries of a role for immune cells and inflammatory mediators in brain masculinization and a role for epigenetic suppression in brain feminization. How and to what degree these findings will translate to human brain development will be questions of central importance in future research in this field.

The neuroendocrine basis of sex differences in epilepsy

Epilepsy affects people of all ages and both genders. Sex differences are well known in epilepsy. Seizure susceptibility and the incidence of epilepsy are generally higher in men than women. In addition, there are gender-specific epilepsies such as catamenial epilepsy, a neuroendocrine condition in which seizures are most often clustered around the perimenstrual or periovulatory period in adult women with epilepsy. Changes in seizure sensitivity are also evident at puberty, pregnancy, and menopause. Sex differences in seizure susceptibility and resistance to antiseizure drugs can be studied in experimental models. An improved understanding of the neuroendocrine basis of sex differences or resistance to protective drugs is essential to develop targeted therapies for sex-specific seizure conditions. This article provides a brief overview of the current status of sex differences in seizure susceptibility and the potential mechanisms underlying the gender differences in seizure sensitivity.

Kisspeptin in the medial amygdala and sexual behavior in male rats

The medial amygdala (MeA) is crucial for sexual behavior; kisspeptin (Kiss1) also plays a role in sexual function. Kisspeptin receptor (Kiss1r) knockout mice display no sexual behavior. Recently Kiss1 and Kiss1r have been discovered in the posterodorsal subnucleus of the medial amygdala (MePD). We hypothesised that Kiss1 in the MePD may have an influence on male sexual behavior. To test this we bilaterally cannulated the MePD and infused kisspeptin-10 in male rats. This caused the rats to have multiple erections, an effect specific to Kiss1 receptor activation, because Kiss1r antagonism blocked the erectile response. When Kiss1 was infused into the lateral cerebroventricle, there were no observed erections. We also measured the plasma levels of LH when Kiss1 is infused into the MePD or lateral cerebroventricle; Kiss1 increased plasma LH to comparable levels when infused into both sites. We conclude that Kiss1 has a role in male sexual behavior, which is specific to the MePD.

Estrogen- and progesterone-mediated structural neuroplasticity in women: evidence from neuroimaging

There is substantial evidence that the ovarian sex hormones, estrogen and progesterone, which vary considerably over the course of the human female lifetime, contribute to changes in brain structure and function. This structured, quantitative literature reviews aims to summarize neuroimaging literature addressing physiological variation in brain macro- and microstructure across an array of hormonal transitions including the menstrual cycle, use of hormonal contraceptives, pregnancy, and menopause. Twenty-five studies reporting structural neuroimaging of women, addressing variation across hormonal states, were identified from a structured search of PUBMED and were systematically reviewed. Although the studies are heterogenous with regard to methodology, overall the results point to overlapping areas of hormone related effects on brain structure particularly affecting the structures of the limbic system. These findings are in keeping with functional data that point to a role for estrogen and progesterone in mediating emotional processing.

Sex and laterality differences in medial amygdala neurons and astrocytes of adult mice

The posterodorsal aspect of the medial amygdala (MePD) in rats is sexually dimorphic, being larger and containing more and larger neurons in males than in females. It is also highly lateralized, with the right MePD larger than the left in both sexes, but with the smaller left MePD actually containing more and larger neurons than the larger right. Astrocytes are also strikingly sexually differentiated, with male-biased numbers and lateralized favoring the right in the rat MePD. However, comparable information is scant for mice where genetic tools offer greater experimental power. Hence, we examined the MePD from adult male and female C57Bl/6(J) mice. We now report that the MePD is larger in males than in females, with the MePD in males containing more astrocytes and neurons than in females. However, we did not find sex differences in astrocyte complexity or overall glial number nor effects of laterality in either measure. While the mouse MePD is generally less lateralized than in rats, we did find that the sex difference in astrocyte number is only on the right because of a significant lateralization in females, with significantly fewer astrocytes on the right than the left but only in females. A sex difference in neuronal soma size favoring males was also evident, but only on the left. Sex differences in the number of neurons and astrocytes common to both rodent species may represent core morphological features that critically underlie the expression of sex-specific behaviors that depend on the MePD. J. Comp. Neurol. 524:2492-2502, 2016. © 2016 Wiley Periodicals, Inc.

Brain and cognition abnormalities in long-term anabolic-androgenic steroid users

BACKGROUND

Anabolic-androgenic steroid (AAS) use is associated with psychiatric symptoms including increased aggression as well as with cognitive dysfunction. The brain effects of long-term AAS use have not been assessed in humans.

METHODS

This multimodal magnetic resonance imaging study of the brain compared 10 male weightlifters reporting long-term AAS use with 10 age-matched weightlifters reporting no AAS exposure. Participants were administered visuospatial memory tests and underwent neuroimaging. Brain volumetric analyses were performed; resting-state fMRI functional connectivity (rsFC) was evaluated using a region-of-interest analysis focused on the amygdala; and dorsal anterior cingulate cortex (dACC) metabolites were quantified by proton magnetic resonance spectroscopy (MRS).

RESULTS

AAS users had larger right amygdala volumes than nonusers (P=0.002) and reduced rsFC between right amygdala and frontal, striatal, limbic, hippocampal, and visual cortical areas. Left amygdala volumes were slightly larger in AAS users (P=0.061) but few group differences were detected in left amygdala rsFC. AAS users also had lower dACC scyllo-inositol levels (P=0.004) and higher glutamine/glutamate ratios (P=0.028), possibly reflecting increased glutamate turnover. On a visuospatial cognitive task, AAS users performed more poorly than nonusers, with the difference approaching significance (P=0.053).

CONCLUSIONS

Long-term AAS use is associated with right amygdala enlargement and reduced right amygdala rsFC with brain areas involved in cognitive control and spatial memory, which could contribute to the psychiatric effects and cognitive dysfunction associated with AAS use. The MRS abnormalities we detected could reflect enhanced glutamate turnover and increased vulnerability to neurotoxic or neurodegenerative processes, which could contribute to AAS-associated cognitive dysfunction.

Inhibitory and multisynaptic spines, and hemispherical synaptic specialization in the posterodorsal medial amygdala of male and female rats

The density of dendritic spines is sexually dimorphic and variable throughout the female estrous cycle in the rat posterodorsal medial amygdala (MePD), a relevant area for the modulation of reproductive behavior in rats. The local synaptic activity differs between hemispheres in prepubertal animals. Here we used serial section transmission electron microscopy to produce 3D reconstructions of dendritic shafts and spines to characterize synaptic contacts on MePD neurons of both hemispheres in adult males and in females along the estrous cycle. Pleomorphic spines and nonsynaptic filopodia occur in the MePD. On average, 8.6% of dendritic spines received inputs from symmetric gamma-aminobutyric acid (GABA)-immunoreactive terminals, whereas 3.6% received two synaptic contacts on the spine head, neck, or base. Presynaptic terminals in female right MePD had a higher density of synaptic vesicles and docked vesicles than the left MePD, suggesting a higher rate of synaptic vesicle release in the right MePD of female rats. In contrast, males did not show laterality in any of those parameters. The proportion of putative inhibitory synapses on dendritic shafts in the right MePD of females in proestrus was higher than in the left MePD, and higher than in the right MePD in males, or in females in diestrus or estrus. This work shows synaptic laterality depending on sex and estrous cycle phase in mature MePD neurons. Most likely, sexual hormone effects are lateralized in this brain region, leading to higher synaptic activity in the right than in the left hemisphere of females, mediating timely neuroendocrine and social/reproductive behavior.

Sex-specific impairment and recovery of spatial learning following the end of chronic unpredictable restraint stress: potential relevance of limbic GAD

Chronic restraint stress alters hippocampal-dependent spatial learning and memory in a sex-dependent manner, impairing spatial performance in male rats and leaving intact or facilitating performance in female rats. Moreover, these stress-induced spatial memory deficits improve following post-stress recovery in males. The current study examined whether restraint administered in an unpredictable manner would eliminate these sex differences and impact a post-stress period on spatial ability and limbic glutamic acid decarboxylase (GAD65) expression. Male (n=30) and female (n=30) adult Sprague-Dawley rats were assigned to non-stressed control (Con), chronic stress (Str-Imm), or chronic stress given a post-stress recovery period (Str-Rec). Stressed rats were unpredictably restrained for 21 days using daily non-repeated combinations of physical context, duration, and time of day. Then, all rats were tested on the radial arm water maze (RAWM) for 2 days and given one retention trial on the third day, with brains removed 30min later to assess GAD65 mRNA. In Str-Imm males, deficits occurred on day 1 of RAWM acquisition, an impairment that was not evident in the Str-Rec group. In contrast, females did not show significant outcomes following chronic stress or post-stress recovery. In males, amygdalar GAD65 expression negatively correlated with RAWM performance on day 1. In females, hippocampal CA1 GAD65 positively correlated with RAWM performance on day 1. These results demonstrate that GABAergic function may contribute to the sex differences observed following chronic stress. Furthermore, unpredictable restraint and a recovery period failed to eliminate the sex differences on spatial learning and memory.

Relationships among sex, season and testosterone in the expression of androgen receptor mRNA and protein in the green anole forebrain

Sexual behavior in male green anole lizards is regulated by a seasonal increase in testosterone (T). However, T is much more effective at activating behavioral, morphological and biochemical changes related to reproduction in the breeding season (BS; spring) compared to nonbreeding season (NBS; fall). An increase in androgen receptor (AR) during the BS is one potential mechanism for this differential responsiveness. AR expression has not been investigated in specific brain regions across seasons in anoles. The present studies were designed to determine relative AR expression in areas important for male (preoptic area, ventromedial amygdala) and female (ventromedial hypothalamus) sexual behavior, as well as whether T upregulates AR in the anole brain. In situ hybridization and Western blot analyses were performed in unmanipulated animals across sex and season, as well as in gonadectomized animals with and without T treatment. Among hormone-manipulated animals, more cells expressing AR mRNA were detected in females than males in the amygdala. T treatment increased the volume of the ventromedial hypothalamus of gonadectomized animals in the BS, but not the NBS. AR protein in dissections of the hypothalamus and preoptic area was increased in males compared to females specifically in the BS. Additionally, among females, it was increased in the NBS compared to the BS. Collectively, these results indicate that differences in central AR expression probably do not facilitate a seasonal responsiveness to T. However, they are consistent with a role for AR in regulating some differences between sexes in the display of reproductive behaviors.

Sex-specific processing of social cues in the medial amygdala

Animal–animal recognition within, and across species, is essential for predator avoidance and social interactions. Despite its essential role in orchestrating responses to animal cues, basic principles of information processing by the vomeronasal system are still unknown. The medial amygdala (MeA) occupies a central position in the vomeronasal pathway, upstream of hypothalamic centers dedicated to defensive and social responses. We have characterized sensory responses in the mouse MeA and uncovered emergent properties that shed new light onto the transformation of vomeronasal information into sex- and species-specific responses. In particular, we show that the MeA displays a degree of stimulus selectivity and a striking sexually dimorphic sensory representation that are not observed in the upstream relay of the accessory olfactory bulb (AOB). Furthermore, our results demonstrate that the development of sexually dimorphic circuits in the MeA requires steroid signaling near the time of puberty to organize the functional representation of sensory stimuli.

DOI:http://dx.doi.org/10.7554/eLife.02743.001

Many animals emit and detect chemicals known as pheromones to communicate with other members of their own species. Animals also rely on chemical signals from other species to warn them, for example, that a predator is nearby. Many of these chemical signals—which are present in sweat, tears, urine, and saliva—are detected by a structure called the vomeronasal organ, which is located at the base of the nasal cavity.

When this organ detects a particular chemical signal, it broadcasts this information to a network of brain regions that generates an appropriate behavioral response. Two structures within this network, the accessory olfactory bulb and the medial amygdala, play an important role in modifying this signal before it reaches its final destination—a region of the brain called the hypothalamus. Activation of the hypothalamus by the signal triggers changes in the animal's behavior. Although the anatomical details of this pathway have been widely studied, it is not clear how information is actually transmitted along it.

Now, Bergan et al. have provided insights into this process by recording signals in the brains of anesthetized mice exposed to specific stimuli. Whereas neurons in the accessory olfactory bulb responded similarly in male and female mice, those in the medial amygdala showed a preference for female urine in male mice, and a preference for male urine in the case of females. This is the first direct demonstration of differences in sensory processing in the brains of male and female mammals.

These differences are thought to result from the actions of sex hormones, particularly estrogen, on brain circuits during development. Consistent with this, neurons in the medial amygdala of male mice with reduced levels of estrogen showed a reduced preference for female urine compared to control males. Similarly, female mice that had been previously exposed to high levels of estrogen as pups showed a reduced preference for male urine compared to controls.

In addition to increasing understanding of how chemical signals—including pheromones—influence the responses of rodents to other animals, the work of Bergan et al. has provided clues to the neural mechanisms that underlie sex-specific differences in behaviors.

DOI:http://dx.doi.org/10.7554/eLife.02743.002

Sex-specific processing of social cues in the medial amygdala

Animal-animal recognition within, and across species, is essential for predator avoidance and social interactions. Despite its essential role in orchestrating responses to animal cues, basic principles of information processing by the vomeronasal system are still unknown. The medial amygdala (MeA) occupies a central position in the vomeronasal pathway, upstream of hypothalamic centers dedicated to defensive and social responses. We have characterized sensory responses in the mouse MeA and uncovered emergent properties that shed new light onto the transformation of vomeronasal information into sex- and species-specific responses. In particular, we show that the MeA displays a degree of stimulus selectivity and a striking sexually dimorphic sensory representation that are not observed in the upstream relay of the accessory olfactory bulb (AOB). Furthermore, our results demonstrate that the development of sexually dimorphic circuits in the MeA requires steroid signaling near the time of puberty to organize the functional representation of sensory stimuli.DOI: http://dx.doi.org/10.7554/eLife.02743.001.

Copulation or sensory cues from the female augment Fos expression in arginine vasopressin neurons of the posterodorsal medial amygdala of male rats

Background

The posterodorsal part of the medial amygdala is essential for processing reproductively salient sensory information in rodents. This is the initial brain structure where information from olfactory system and male hormones intersect. The neurochemical identity of the neurons participating in the sensory processing in medial amygdala remains presently undetermined. Many neurons in this brain structure express arginine vasopressin in a testosterone-dependent manner, suggesting that this neuropeptide is maintained by the androgenic milieu.

Method

Here we use Fos, a protein expressed by recently active neurons, to quantify activation of arginine vasopressin neurons after exposure to odor from physically inaccessible female. We compare it to mating with accessible female and to reproductively innocuous odor.

Results

We show that inaccessible female activate arginine vasopressin neurons in the male posterodorsal medial amygdala. The magnitude of activation is not further enhanced when physical access with resultant mating is granted, even though it remains undetermined if same population of AVP neurons is activated by both inaccessible female and copulation. We also show that arginine vasopressin activation cannot be fully accounted for by mere increase in the number of Fos and AVP neurons.

Conclusion

These observations posit a role for the medial amygdala arginine vasopressin in reproductive behaviors, suggesting that these neurons serve as integrative node between the hormonal status of the animal and the availability of reproductive opportunities.

Organization of multisynaptic circuits within and between the medial and the central extended amygdala

The central and medial extended amygdala comprises the central (CEA) and medial nuclei of the amygdala (MEA), respectively, together with anatomically connected regions of the bed nucleus of the stria terminalis (BST). To reveal direct and multisynaptic connections within the central and medial extended amygdala, monosynaptic and transneuronal viral tracing experiments were performed in adult male rats. In the first set of experiments, a cocktail of anterograde and retrograde tracers was iontophoretically delivered into the medial CEA (CEAm), anterodorsal MEA (MEAad), or posterodorsal MEA (MEApd), revealing direct, topographically organized projections between distinct amygdalar and BST subnuclei. In the second set of experiments, the retrograde transneuronal tracer pseudorabies virus (PRV) was microinjected into the CEAm or MEAad. After 48 hours of survival, there were no significant differences between monosynaptic and PRV cases in the subnuclear distribution or proportions of retrogradely labeled BST neurons. However, after 60 hours of survival, CEAm-injected cases displayed an increased proportion of labeled neurons within the anteromedial group of BST subnuclei (amgBST) and within the posterior BST, which do not directly innervate the CEA. MEApd-injected 60-hour cases displayed a significantly increased proportion of retrograde labeling in the amgBST compared with monosynaptic and 48-hour cases, whereas MEAad-injected cases displayed no proportional changes over time. Thus, multisynaptic circuits within the medial extended amygdala overlap the direct connections making up this anatomical unit, whereas the multisynaptic boundaries of the central extended amygdala extend into BST subnuclei previously identified as part of the medial extended amygdala.

Sex-biased methylome and transcriptome in human prefrontal cortex

Brain function and cognitive performance differ between men and women in some measures. The phenotypic variation may be partially due to sex differences in epigenomes and transcriptomes in specific brain regions [e.g. the prefrontal cortex (PFC)]. Genome-wide DNA methylation and gene expression were examined in postmortem PFC of 32 males and 14 females (all were Caucasians) using Illumina's 450K Methylation and HT-12 v4 Gene Expression BeadChips, respectively. Multiple linear regression, Pearson correlation and DAVID functional annotation analyses were applied to investigate sex-biased DNA methylation and gene expression, DNA methylation-gene expression correlation and gene ontology (GO) annotations overrepresented by differentially methylated and expressed genes. A total of 22 124 CpGs showed differential methylation between males and females (2.6 × 10(-38) ≤ Pnominal ≤ 0.05), and the P-values of 8357 CpGs withstood multiple-testing correction (q < 0.05). A total of 1489 genes showed differential expression between males and females (4.1 × 10(-36) ≤ Pnominal ≤ 0.05), and the P-values of 35 genes survived multiple-testing correction (q < 0.05). A significant correlation (Pcorrelation < 0.05) was observed between methylation levels of 585 differentially methylated CpGs (Pnominal ≤ 0.05) and expression levels of 188 differentially expressed host genes (Pnominal < 0.05). The GO terms enriched by these 188 genes (134 on autosomes and 54 on sex chromosomes) were assigned to 24 clusters, and 33 genes involved in the top cluster (enrichment score: 4.7) mainly participate in ribosome structure and function, RNA binding and protein translation. This study demonstrated sex-specific methylomic and transcriptomic profiles in the human PFC. Our findings suggest that sex-biased DNA methylation and gene expression could be either the cause or consequence of differential brain development between males and females.

Modulation of testosterone-dependent male sexual behavior and the associated neuroplasticity

Steroids modulate the transcription of a multitude of genes and ultimately influence numerous aspects of reproductive behaviors. Our research investigates how one single steroid, testosterone, is able to trigger this vast number of physiological and behavioral responses. Testosterone potency can be changed locally via aromatization into 17β-estradiol which then activates estrogen receptors of the alpha and beta sub-types. We demonstrated that the independent activation of either receptor activates different aspects of male sexual behavior in Japanese quail. In addition, several studies suggest that the specificity of testosterone action on target genes transcription is related to the recruitment of specific steroid receptor coactivators. We demonstrated that the specific down-regulation of the coactivators SRC-1 or SRC-2 in the medial preoptic nucleus by antisense techniques significantly inhibits steroid-dependent male-typical copulatory behavior and the underlying neuroplasticity. In conclusion, our results demonstrate that the interaction between several steroid metabolizing enzymes, steroid receptors and their coactivators plays a key role in the control of steroid-dependent male sexual behavior and the associated neuroplasticity in quail.

Sexual Dimorphism in the Brain of the Monogamous California Mouse (Peromyscus californicus)

Sex differences in behavior and morphology are usually assumed to be stronger in polygynous species compared to monogamous species. A few brain structures have been identified as sexually dimorphic in polygynous rodent species, but it is less clear whether these differences persist in monogamous species. California mice are among the 5% or less of mammals that are considered to be monogamous and as such provide an ideal model to examine sexual dimorphism in neuroanatomy. In the present study we compared the volume of hypothalamic- and limbic-associated regions in female and male California mice for sexual dimorphism. We also used tyrosine hydroxylase (TH) immunohistochemistry to compare the number of dopamine neurons in the ventral tegmental area (VTA) in female and male California mice. Additionally, tract tracing was used to accurately delineate the boundaries of the VTA. The total volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA), the principal nucleus of the bed nucleus of the stria terminalis (BNST), and the posterodorsal medial amygdala (MEA) was larger in males compared to females. In the SDN-POA we found that the magnitude of sex differences in the California mouse were intermediate between the large differences observed in promiscuous meadow voles and rats and the absence of significant differences in monogamous prairie voles. However, the magnitude of sex differences in MEA and the BNST were comparable to polygynous species. No sex differences were observed in the volume of the whole brain, the VTA, the nucleus accumbens or the number of TH-ir neurons in the VTA. These data show that despite a monogamous social organization, sexual dimorphisms that have been reported in polygynous rodents extend to California mice. Our data suggest that sex differences in brain structures such as the SDN-POA persist across species with different social organizations and may be an evolutionarily conserved characteristic of mammalian brains.

Testosterone administration in women increases amygdala responses to fearful and happy faces

Data from both rodents and humans show that testosterone reduces fear. This effect is hypothesized to result from testosterone's down regulating effects on the amygdala, a key region in the detection of threat and instigator of fight-or-flight behavior. However, neuroimaging studies employing testosterone administration in humans have consistently shown increased amygdala responsivity. Yet, no study to date has investigated specifically how testosterone affects the amygdala response to fearful emotional expressions. Such stimuli signal the presence of environmental threat and elicit robust amygdala responses that have consistently been associated with anxious traits. In the present study, we therefore used functional magnetic resonance imaging combined with a single administration of 0.5mg testosterone in 12 healthy women to assess testosterone's effects on amygdala responses to dynamic fearful (and happy control) faces. Our results show that both stimuli activate the amygdala. Notably, testosterone increased the amygdala response to both stimuli, and to an equal degree. Thus, testosterone appears not to reduce fear by attenuating the amygdala response toward signals of threat. Data further show that testosterone selectively increases activation of the superficial amygdala (SFA) and, to a lesser extent, the basolateral amygdala (BLA). No effect was found in the central nucleus, which is involved in the generation of autonomic fear responses. Both the SFA and BLA are considered input regions, and enhanced activation by testosterone might reflect the role of this hormone in adaptive responding to socially relevant stimuli. Furthermore, literature on the distinct roles of the SFA and BLA in fear processing show that increased activation of these subregions of the amygdala is consistent with a fear reducing effect of testosterone.

Visualisation and characterisation of oestrogen receptor α-positive neurons expressing green fluorescent protein under the control of the oestrogen receptor α promoter

Oestrogen receptor (ER)α plays important roles in the development and function of various neuronal systems through activation by its ligands, oestrogens. To visualise ERα-positive neurons, we generated transgenic (tg) mice expressing green fluorescent protein (GFP) under the control of the ERα promoter. In three independent tg lines, GFP-positive neurons were observed in areas previously reported to express ERα mRNA, including the lateral septum, bed nucleus of the stria terminalis, medial preoptic nucleus (MPO), hypothalamus, and amygdala. In these areas, GFP signals mostly overlapped with ERα immunoreactivity. GFP fluorescence was seen in neurites and cell bodies of neurons. In addition, the network and detailed structure of neurites were visible in dissociated and slice cultures of hypothalamic neurons. We examined the effect of oestrogen deprivation by ovariectomy on the structure of the GFP-positive neurons. The area of ERα-positive cell bodies in the bed nucleus of the stria terminalis and MPO was measured by capturing the GFP signal and was found to be significantly smaller in ovariectomy mice than in control mice. When neurons in the MPO were infected with an adeno-associated virus that expressed small hairpin RNA targeting the ERα gene, an apparent induction of GFP was observed in this area, suggesting a negative feedback mechanism in which ERα controls expression of the ERα gene itself. Thus, the ERα promoter-GFP tg mice will be useful to analyse the development and plastic changes of the structure of ERα-expressing neurons and oestrogen and its receptor-mediated neuronal responses.