Gonadal steroids promote glial differentiation and alter neuronal morphology in the developing hypothalamus in a regionally specific manner

Using Organoids to Model Sex Differences in the Human Brain

Sex differences are widespread during neurodevelopment and play a role in neuropsychiatric conditions such as autism, which is more prevalent in males than females. In humans, males have been shown to have larger brain volumes than females with development of the hippocampus and amygdala showing prominent sex differences. Mechanistically, sex steroids and sex chromosomes drive these differences in brain development, which seem to peak during prenatal and pubertal stages. Animal models have played a crucial role in understanding sex differences, but the study of human sex differences requires an experimental model that can recapitulate complex genetic traits. To fill this gap, human induced pluripotent stem cell-derived brain organoids are now being used to study how complex genetic traits influence prenatal brain development. For example, brain organoids from individuals with autism and individuals with X chromosome-linked Rett syndrome and fragile X syndrome have revealed prenatal differences in cell proliferation, a measure of brain volume differences, and excitatory-inhibitory imbalances. Brain organoids have also revealed increased neurogenesis of excitatory neurons due to androgens. However, despite growing interest in using brain organoids, several key challenges remain that affect its validity as a model system. In this review, we discuss how sex steroids and the sex chromosomes each contribute to sex differences in brain development. Then, we examine the role of X chromosome inactivation as a factor that drives sex differences. Finally, we discuss the combined challenges of modeling X chromosome inactivation and limitations of brain organoids that need to be taken into consideration when studying sex differences.

Astrogenesis in the hypothalamus: A life-long process contributing to the development and plasticity of neuroendocrine networks

Astrocytes are now recognized as integral components of neural circuits, regulating their maturation, activity and plasticity. Neuroendocrinology has provided fertile ground for revealing the diverse strategies used by astrocytes to regulate the physiological and behavioural outcomes of neural circuit activity in response to internal and environmental inputs. However, the development of astrocytes in the hypothalamus has received much less attention than in other brain regions such as the cerebral cortex and spinal cord. In this review, we synthesize our current knowledge of astrogenesis in the hypothalamus across various life stages. A distinctive feature of hypothalamic astrogenesis is that it persists all throughout lifespan, and involves multiple cellular sources corresponding to radial glial cells during early development, followed by tanycytes, parenchymal progenitors and locally dividing astrocytes. Astrogenesis in the hypothalamus is closely coordinated with the maturation of hypothalamic neurons. This coordination is exemplified by recent findings in neurons producing gonadotropin-releasing hormone, which actively shape their astroglial environment during infancy to integrate functionally into their neural network and facilitate sexual maturation, a process vulnerable to endocrine disruption. While hypothalamic astrogenesis shares common principles with other brain regions, it also exhibits specific features in its dynamics and regulation, both at the inter- and intra-regional levels. These unique properties emphasize the importance of further exploration. Additionally, we discuss the experimental strategies used to assess astrogenesis in the hypothalamus and their potential bias and limitations. Understanding the mechanisms of hypothalamic astrogenesis throughout life will be crucial for comprehending the development and function of the hypothalamus under both physiological and pathological conditions.

Sex-dependent effect of sublethal copper concentrations on de novo cholesterol synthesis in astrocytes and their possible links to variations in cholesterol and amyloid precursor protein levels in neuronal membranes

BACKGROUND

Cholesterol (Cho) is an essential lipophilic molecule in cells; however, both its decrease and its increase may favor the development of neurological diseases such as Alzheimer's disease (AD). Although copper (Cu) is an essential trace metal for cells, the increased plasma concentration of its free form has been linked with AD development and severity. AD affects aged people, but its prevalence and severity are higher in women than in men. We have previously shown that Cu promotes Cho de novo synthesis in immature neurons as well as increased Cho in membrane rafts and Aβ levels in culture medium, but there are no results yet regarding sex differences in the effects of sublethal Cu exposure on Cho de novo synthesis.

METHODS

We examined the potential sex-specific impact of sublethal Cu concentrations on de novo Cho synthesis in primary cultures of male and female astrocytes. We also explored whether this had any correlation with variations in Cho and APP levels within neuronal membrane rafts.

RESULTS

Flow cytometry analysis demonstrated that Cu treatment leads to a greater increase in ROS levels in female astrocytes than in males. Furthermore, through RT-PCR analysis, we observed an upregulation of SREBP-2 and HMGCR. Consistently, we observed an increase in de novo Cho synthesis. Finally, western blot analysis indicated that the levels of ABCA1 increase after Cu treatment, accompanied by a higher release of radiolabeled Cho and an elevation in Cho and APP levels in neuronal membrane rafts. Importantly, all these results were significantly more pronounced in female astrocytes than in males.

CONCLUSIONS

Our findings confirm that Cu stimulates Cho synthesis in astrocytes, both in a ROS-dependent and -independent manner. Moreover, female astrocytes displayed elevated levels of HMGCR, and de novo Cho synthesis compared to males following TBH and Cu treatments. This corresponds with higher levels of Cho released into the culture medium and a more significant Cho and APP rise within neuronal rafts. We consider that the increased risk of AD in females partly arises from sex-specific responses to metals and/or exogenous substances, impacting key enzyme regulation in various biochemical pathways, including HMGCR.

Neuroestradiol and neuronal development: Not an exclusive male tale anymore

The brain synthesizes a variety of neurosteroids, including neuroestradiol. Inhibition of neuroestradiol synthesis results in alterations in basic neurodevelopmental processes, such as neurogenesis, neuroblast migration, neuritogenesis and synaptogenesis. Although the neurodevelopmental actions of neuroestradiol are exerted in both sexes, some of them are sex-specific, such as the well characterized effects of neuroestradiol derived from the metabolism of testicular testosterone during critical periods of male brain development. In addition, recent findings have shown sex-specific actions of neuroestradiol on neuroblast migration, neuritic growth and synaptogenesis in females. Among other factors, the epigenetic regulation exerted by X linked genes, such as Kdm6a/Utx, may determine sex-specific actions of neuroestradiol in the female brain. This review evidences the impact of neuroestradiol on brain formation in both sexes and highlights the interaction of neural steriodogenesis, hormones and sex chromosomes in sex-specific brain development.

Contribution of birds to the study of sexual differentiation of brain and behavior

Behavioral neuroendocrinology has largely relied on mammalian models to understand the relationship between hormones and behavior, even if this discipline has historically used a larger diversity of species than other fields. Recent advances revealed the potential of avian models in elucidating the neuroendocrine bases of behavior. This paper provides a review focused mainly on the contributions of our laboratory to the study of sexual differentiation in Japanese quail and songbirds. Quail studies have firmly established the role of embryonic estrogens in the sexual differentiation of male copulatory behavior. While most sexually differentiated features identified in brain structure and physiology result from the different endocrine milieu of adults, a few characteristics are organized by embryonic estrogens. Among them, a sex difference was identified in the number and morphology of microglia which is not associated with sex differences in the concentration/expression of neuroinflammatory molecules. The behavioral role of microglia and neuroinflammatory processes requires further investigations. Sexual differentiation of singing in zebra finches is not mediated by the same endocrine mechanisms as male copulatory behavior and "direct" genetic effect, i.e., not mediated by gonadal steroids have been identified. Epigenetic contributions have also been considered. Finally sex differences in specific aspects of singing behavior have been identified in canaries after treatment of adults with exogenous testosterone suggesting that these aspects of song are differentiated during ontogeny. Integration of quail and songbirds as alternative models has thus expanded understanding of the interplay between hormones and behavior in the control of sexual differentiation.

Testosterone is Sufficient to Impart Susceptibility to Isoflurane Neurotoxicity in Female Neonatal Rats

BACKGROUND

Volatile anesthetic exposure during development leads to long-term cognitive deficits in rats which are dependent on age and sex. Female rats are protected relative to male rats for the same exposure on postnatal day 7. Here we test our hypothesis that androgens can modulate chloride cotransporter expression to alter the susceptibility to neurotoxicity from GABAergic drugs using female rats with exogenous testosterone exposure.

METHODS

Female rats were injected with testosterone (100 μg/animal) or vehicle on postnatal days 1 to 6. On postnatal day 7, the animals were randomized to either isoflurane exposure or sham. Spatial memory was assessed with the Barnes maze starting on postnatal day 41. Western blots were run from testosterone treated postnatal day 7 animals to measure levels of chloride cotransporters sodium-potassium-chloride symporter (NKCC1) and chloride-potassium symporter 5 (KCC2).

RESULTS

Exogenous testosterone modulated isoflurane anesthetic neurotoxicity in female rats based on poor performance in the probe trial of the Barnes Maze. By contrast, females with vehicle and isoflurane exposure were able to differentiate the goal position. These behavioral differences corresponded to differences in the protein levels of NKCC1 and KCC2 after exogenous testosterone exposure, with NKCC1 increasing ( P <0.001) and KCC2 decreasing ( P =0.003) relative to female controls.

CONCLUSIONS

The expression of chloride cotransporters, NKCC1 and KCC2, is altered by testosterone in female rats and corresponds to a cognitive deficit after isoflurane exposure. This confirms the role of androgens in perinatal anesthetic neurotoxicity and supports our hypothesis that the developing GABAergic system plays a critical role in the underlying mechanism.

Distribution and Morphological Characteristics of Oligodendrocytes in Selected Areas of the Brain of Male and Female Red Kangaroos (Macropus rufus)

This study was carried out on six adult red kangaroos of both sexes. To determine the location of the oligodendrocytes (OLGs) of the hippocampus (Hip) and corpus callosum (CC), the method of impregnation of the neuroglia with silver salts was applied. The iron distribution in the OLGs was determined by the histochemical method. The Nissl method was used to determine the location of the brain structure and to analyze the number of OLGs. In the Hip, these cells are located one beside another, mainly in blood vessels and neurons; in the neocortex (NC), they are located in layers I–VI; and in the CC, they are arranged in characteristic rows and accompany both nerve fibers and blood vessels. The analysis of the results obtained by the chosen methods in the Hip, NC, and CC in males and females did not show statistically significant differences in the distribution and location of the red kangaroo OLGs. The involvement of these cells is a physiological process that proceeds in a similar manner throughout the life of individuals and actively influences the metabolism of neurons and myelin.

Prenatal Low-Protein and Low-Calorie Diets Differentially Alter Arcuate Nucleus Morphology in Newborn Male Rats

Background

Malnutrition during the early stages of development produces alterations that can compromise the functioning of the hypothalamic circuits that regulate food intake. The purpose of this study is to analyze the effects that a low-protein and low-calorie diet has on the morphology of the arcuate nucleus (ARC) of the hypothalamus in newborn male and female rats.

Methods

On gestational day 6 (G6), six pregnant rats were divided into two groups. One group was made up of three pregnant rats, which were fed ad libitum with a control diet (20% casein), and the other one was made up of three pregnant rats, which were fed ad libitum with a low-protein diet (8% casein) and 30% of a calorie-restricted diet. On the day of birth, pups were sacrificed, resulting in four experimental groups: control male, control female, low-protein and low-calorie diet male, and low-protein and low-calorie diet female (n = 5 in each group). The volume and number of neurons, together with the neuronal density and number of apoptotic cells, were measured.

Results

Males on a low-protein and low-calorie diet showed a significant increase in the number of neurons and in the neuronal density of the ARC with regard to the rest of the groups studied. These increases were also reflected in the posterior part of the nucleus. Although the existence of sexual dimorphism was not detected in any of the parameters studied in the control groups, the number of neurons and neuronal density showed differences between males and females fed with a low-protein and low-calorie diets due to the increase in the number of neurons shown by the male. No significant differences were found in the number of apoptotic cells.

Conclusion

Our results show that a low-protein and low-calorie diet during the prenatal stage produces alterations in the ARC of the hypothalamus in newborn animals and, more importantly, that the effects of malnutrition are evident in males but not in females. Therefore, it is essential to follow a balanced diet during the early stages of life to ensure optimal development of the neural circuits that regulate eating.

Early Life Events and Maturation of the Dentate Gyrus: Implications for Neurons and Glial Cells

The dentate gyrus (DG), an important part of the hippocampus, plays a significant role in learning, memory, and emotional behavior. Factors potentially influencing normal development of neurons and glial cells in the DG during its maturation can exert long-lasting effects on brain functions. Early life stress may modify maturation of the DG and induce lifelong alterations in its structure and functioning, underlying brain pathologies in adults. In this paper, maturation of neurons and glial cells (microglia and astrocytes) and the effects of early life events on maturation processes in the DG have been comprehensively reviewed. Early postnatal interventions affecting the DG eventually result in an altered number of granule neurons in the DG, ectopic location of neurons and changes in adult neurogenesis. Adverse events in early life provoke proinflammatory changes in hippocampal glia at cellular and molecular levels immediately after stress exposure. Later, the cellular changes may disappear, though alterations in gene expression pattern persist. Additional stressful events later in life contribute to manifestation of glial changes and behavioral deficits. Alterations in the maturation of neuronal and glial cells induced by early life stress are interdependent and influence the development of neural nets, thus predisposing the brain to the development of cognitive and psychiatric disorders.

Sexual Dimorphism in Kisspeptin Signaling

Kisspeptin (KP) and kisspeptin receptor (KPR) are essential for the onset of puberty, development of gonads, and maintenance of gonadal function in both males and females. Hypothalamic KPs and KPR display a high degree of sexual dimorphism in expression and function. KPs act on KPR in gonadotropin releasing hormone (GnRH) neurons and induce distinct patterns of GnRH secretion in males and females. GnRH acts on the anterior pituitary to secrete gonadotropins, which are required for steroidogenesis and gametogenesis in testes and ovaries. Gonadal steroid hormones in turn regulate the KP neurons. Gonadal hormones inhibit the KP neurons within the arcuate nucleus and generate pulsatile GnRH mediated gonadotropin (GPN) secretion in both sexes. However, the numbers of KP neurons in the anteroventral periventricular nucleus and preoptic area are greater in females, which release a large amount of KPs in response to a high estrogen level and induce the preovulatory GPN surge. In addition to the hypothalamus, KPs and KPR are also expressed in various extrahypothalamic tissues including the liver, pancreas, fat, and gonads. There is a remarkable difference in circulating KP levels between males and females. An increased level of KPs in females can be linked to increased numbers of KP neurons in female hypothalamus and more KP production in the ovaries and adipose tissues. Although the sexually dimorphic features are well characterized for hypothalamic KPs, very little is known about the extrahypothalamic KPs. This review article summarizes current knowledge regarding the sexual dimorphism in hypothalamic as well as extrahypothalamic KP and KPR system in primates and rodents.

Sex-specific differences in KCC2 localisation and inhibitory synaptic transmission in the rat hippocampus

Sexual differentiation of the brain is influenced by testosterone and its metabolites during the perinatal period, when many aspects of brain development, including the maturation of GABAergic transmission, occur. Whether and how testosterone signaling during the perinatal period affects GABAergic transmission is unclear. Here, we analyzed GABAergic circuit functional markers in male, female, testosterone-treated female, and testosterone-insensitive male rats after the first postnatal week and in young adults. In the hippocampus, mRNA levels of proteins associated with GABA signaling were not significantly affected at postnatal day (P) 7 or P40. Conversely, membrane protein levels of KCC2, which are critical for determining inhibition strength, were significantly higher in females compared to males and testosterone-treated females at P7. Further, female and testosterone-insensitive male rats at P7 showed higher levels of the neurotrophin BDNF, which is a powerful regulator of neuronal function, including GABAergic transmission. Finally, spontaneous GABAergic currents in hippocampal CA1 pyramidal cells were more frequent in females and testosterone-insensitive males at P40. Overall, these results show that perinatal testosterone levels modulate GABAergic circuit function, suggesting a critical role of perinatal sex hormones in regulating network excitability in the adult hippocampus.

Precocious Puberty in Boys: A Study Based on Five Years of Data from a Single Center in Northern China

OBJECTIVE

To evaluate the clinical features and etiology of precocious puberty (PP) in Chinese boys.

METHODS

In this study, data from boys who were referred for evaluation of PP from 2015 to 2020 at a tertiary hospital in Northern China were retrospectively analyzed.

RESULTS

Eighty-two boys were diagnosed with PP from 2015 to 2020. Sixty-two patients (75.6%) were diagnosed with central PP (CPP), and twenty patients (24.4%) were diagnosed with peripheral PP (PPP). In the CPP group, forty-nine cases were classified as idiopathic CPP, and thirteen patients had pathogenic CPP. The top three causes of PPP were congenital adrenal hyperplasia, germ cell tumors and familial male-limited PP.

CONCLUSION

The etiology of PP in males is diverse. The majority of CPP cases in Chinese boys are idiopathic rather than organic.

Mitochondrial bioenergetics of astrocytes in Fragile X Syndrome: new perspectives from culture conditions and sex effects

Fragile X syndrome is a genetic disorder that is characterized by a range of cognitive and behavioural deficits, including mild-moderate intellectual disability. The disease is characterized by an X-linked mutation of the Fmr1 gene, which causes silencing of the gene coding for FMRP, a translational regulator integral for neurodevelopment. Mitochondrial dysfunction has been recently associated with FXS, with reports of increases in oxidative stress markers, reactive oxygen species, and lipid peroxidation being present in brain tissue. Astrocytes, a prominent glial cell within the CNS, plays a large role in regulating oxidative homeostasis within the developing brain and dysregulation of astrocyte redox balance in FXS may contribute to oxidative stress. Astrocyte function and mitochondrial bioenergetics is significantly influenced by oxygen availability as well as circulating sex hormones; yet these parameters are rarely considered during in vitro experimentation. Given that the brain normally develops in a range of hypoxic conditions and FXS is a sex-linked genetic disorder, we investigated how different oxygen levels (normoxic versus hypoxic) and biological sex affected mitochondrial bioenergetics of astrocytes in FXS. Our results show demonstrate that both mitochondrial respiration capacity and reactive oxygen species emission are altered with Fmr1 deletion in astrocytes and these changes were dependent upon both sexual dimorphism and oxygen availability.

The development and physiological and pathophysiological functions of resident macrophages and glial cells

In the past, brain function and the onset and progression of neurological diseases have been studied in a neuron-centric manner. However, in recent years the focus of many neuroscientists has shifted to other cell types that promote neurodevelopment and contribute to the functionality of neuronal networks in health and disease. Particularly microglia and astrocytes have been implicated in actively contributing to and controlling neuronal development, neuroinflammation, and neurodegeneration. Here, we summarize the development of brain-resident macrophages and astrocytes and their core functions in the developing brain. We discuss their contribution and intercellular crosstalk during tissue homeostasis and pathophysiology. We argue that in-depth knowledge of non-neuronal cells in the brain could provide novel therapeutic targets to reverse or contain neurological diseases.

Microglial and Astrocytic Function in Physiological and Pathological Conditions: Estrogenic Modulation

There are sexual differences in the onset, prevalence, and outcome of numerous neurological diseases. Thus, in Alzheimer’s disease, multiple sclerosis, and major depression disorder, the incidence in women is higher than in men. In contrast, men are more likely to present other pathologies, such as amyotrophic lateral sclerosis, Parkinson’s disease, and autism spectrum. Although the neurological contribution to these diseases has classically always been studied, the truth is that neurons are not the only cells to be affected, and there are other cells, such as glial cells, that are also involved and could be key to understanding the development of these pathologies. Sexual differences exist not only in pathology but also in physiological processes, which shows how cells are differentially regulated in males and females. One of the reasons these sexual differences may occur could be due to the different action of sex hormones. Many studies have shown an increase in aromatase levels in the brain, which could indicate the main role of estrogens in modulating proinflammatory processes. This review will highlight data about sex differences in glial physiology and how estrogenic compounds, such as estradiol and tibolone, could be used as treatment in neurological diseases due to their anti-inflammatory effects and the ability to modulate glial cell functions.

Microglia and sexual differentiation of the developing brain: A focus on extrinsic factors

Microglia, the innate immune cells of the brain, have recently been removed from the position of mere sentinels and promoted to the role of active sculptors of developing circuits and cells. Alongside their functions in normal brain development, microglia coordinate sexual differentiation of the brain, a set of processes which vary by region and endpoint like that of microglia function itself. In this review, we highlight the ways microglia are both targets and drivers of brain sexual differentiation. We examine the factors that may drive sex differences in microglia, with a special focus on how changing microenvironments in the developing brain dictate microglia phenotypes and discuss how their diverse functions sculpt lasting sex-specific changes in the brain. Finally, we consider how sex-specific early life environments contribute to epigenetic programming and lasting sex differences in microglia identity.

Crmp4-KO Mice as an Animal Model for Investigating Certain Phenotypes of Autism Spectrum Disorders

Previous research has demonstrated that the collapsin response mediator protein (CRMP) family is involved in the formation of neural networks. A recent whole-exome sequencing study identified a de novo variant (S541Y) of collapsin response mediator protein 4 (CRMP4) in a male patient with autism spectrum disorder (ASD). In addition, Crmp4-knockout (KO) mice show some phenotypes similar to those observed in human patients with ASD. For example, compared with wild-type mice, Crmp4-KO mice exhibit impaired social interaction, abnormal sensory sensitivities, broader distribution of activated (c-Fos expressing) neurons, altered dendritic formation, and aberrant patterns of neural gene expressions, most of which have sex differences. This review summarizes current knowledge regarding the role of CRMP4 during brain development and discusses the possible contribution of CRMP4 deficiencies or abnormalities to the pathogenesis of ASD. Crmp4-KO mice represent an appropriate animal model for investigating the mechanisms underlying some ASD phenotypes, such as impaired social behavior, abnormal sensory sensitivities, and sex-based differences, and other neurodevelopmental disorders associated with sensory processing disorders.

Microglial Phagocytosis of Newborn Cells Is Induced by Endocannabinoids and Sculpts Sex Differences in Juvenile Rat Social Play

Brain sex differences are established developmentally and generate enduring changes in circuitry and behavior. Steroid-mediated masculinization of the rat amygdala during perinatal development produces higher levels of juvenile rough-and-tumble play by males. This sex difference in social play is highly conserved across mammals, yet the mechanisms by which it is established are unknown. Here, we report that androgen-induced increases in endocannabinoid tone promote microglia phagocytosis during a critical period of amygdala development. Phagocytic microglia engulf more viable newborn cells in males; in females, less phagocytosis allows more astrocytes to survive to the juvenile age. Blocking complement-dependent phagocytosis in males increases astrocyte survival and prevents masculinization of play. Moreover, increased astrocyte density in the juvenile amygdala reduces neuronal excitation during play. These findings highlight novel mechanisms of brain development whereby endocannabinoids induce microglia phagocytosis to regulate newborn astrocyte number and shape the sexual differentiation of social circuitry and behavior. VIDEO ABSTRACT.

Neurobiological characteristics underlying metabolic differences between males and females

The hypothalamus is the main integrating center for metabolic control. Our understanding of how hypothalamic circuits function to control appetite and energy expenditure has increased dramatically in recent years, due to the rapid rise in the incidence of obesity and the search for effective treatments. Increasing evidence indicates that these treatments will most likely differ between males and females. Indeed, sex differences in metabolism have been demonstrated at various levels, including in two of the most studied neuronal populations involved in metabolic control: the anorexigenic proopiomelanocortin neurons and the orexigenic neuropeptide Y/Agouti-related protein neurons. Here we review what is known to date regarding the sex differences in these two neuronal populations, as well as other neuronal populations involved in metabolic control and glial cells.

Screening of quorum sensing peptides for biological effects in neuronal cells

Quorum sensing peptides (QSP) are an important class of bacterial peptides which can have an effect on human host cells. These peptides are used by bacteria to communicate with each other. Some QSP are able to cross the blood-brain barrier and reach the brain parenchyma. However, nothing is known about the effects of these peptides in the brain. Therefore, 85 quorum sensing peptides were screened on six different neuronal cell lines using MTT toxicity, neurite differentiation, cytokine production and morphology as biological outcomes. This primary screening resulted in 22 peptides with effects observed on neuronal cell lines, indicating a possible role in the gut-brain axis. Four peptides (Q138, Q143, Q180 and Q212) showed induction of neurite outgrowth while two peptides (Q162 and Q208) inhibited NGF-induced neurite outgrowth in PC12 cells. Eight peptides (Q25, Q135, Q137, Q146, Q151, Q165, Q208 and Q298) induced neurite outgrowth in human SH-SY5Y neuroblastoma cells. Two peptides (Q13 and Q52) were toxic for SH-SY5Y cells and one (Q123) for BV-2 microglia cells based on the MTT assay. Six peptides had an effect on BV-2 microglia, Q180, Q184 and Q191 were able to induce IL-6 expression and Q164, Q192 and Q208 induced NO production. Finally, Q75 and Q147 treated C8D1A astrocytes demonstrated a higher fraction of round cells. Overall, these in vitro screening study results indicate for the first time possible effects of QSP on neuronal cells.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Sex differences in the neuroendocrine control of metabolism and the implication of astrocytes

Males and females have distinct propensities to develop obesity and its related comorbidities, partially due to gonadal steroids. There are sex differences in hypothalamic neuronal circuits, as well as in astrocytes, that participate in metabolic control and the development of obesity-associated complications. Astrocytes are involved in nutrient transport and metabolism, glucose sensing, synaptic remodeling and modulation of neuronal signaling. They express receptors for metabolic hormones and mediate effects of these metabolic signals on neurons, with astrogliosis occurring in response to high fat diet and excess weight gain. However, most studies of obesity have focused on males. Recent reports indicate that male and female astrocytes respond differently to metabolic signals and this could be involved in the differential response to high fat diet and the onset of obesity-associated pathologies. Here we focus on the sex differences in response to obesogenic paradigms and the possible role of hypothalamic astrocytes in this phenomenon.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Sex differences and estrogen regulation of BDNF gene expression, but not propeptide content, in the developing hippocampus

Sex differences in adult brain function are frequently determined developmentally through the actions of steroid hormones during sensitive periods of prenatal and early postnatal life. In rodents, various cellular end points of the developing brain are affected by estradiol that is derived from the aromatization of circulating testosterone and/or synthesized within the brain. We have previously described a sex difference in neurogenesis in the hippocampus of neonatal rats that is modulated by estradiol. In this report, we examined a potential downstream regulator of the effects of estradiol on hippocampal cell proliferation by measuring gene expression of brain-derived neurotrophin (BDNF) in male and female neonatal rats in response to estradiol. Males had higher baseline BDNF gene expression in dentate gyrus and CA1 regions of the hippocampus compared with females. Neonatal administration of exogenous estradiol resulted in opposite effects on BDNF expression in these areas of the neonatal hippocampus, such that BDNF transcripts increased in CA1 but decreased in dentate. Blocking endogenous estradiol signaling by antagonizing estrogen receptors decreased BDNF expression in the dentate of males, but not females, and had no effect in CA1. Interestingly, this sex difference and response to estradiol was not mirrored by translational output, as no differences in BDNF precursor peptide were observed. The sex- and region-specific effects of estradiol on BDNF expression in the neonatal hippocampus suggest a complex functional relationship between these pleiotropic factors in regulating developmental neurogenesis. © 2016 Wiley Periodicals, Inc.

Polycystic ovary syndrome: Understanding the role of the brain

Polycystic ovary syndrome (PCOS) is a prevalent endocrine disorder and the leading cause of anovulatory infertility. Characterised by hyperandrogenism, menstrual dysfunction and polycystic ovaries, PCOS is a broad-spectrum disorder unlikely to stem from a single common origin. Although commonly considered an ovarian disease, the brain is now a prime suspect in both the ontogeny and pathology of PCOS. We discuss here the neuroendocrine impairments present in PCOS that implicate involvement of the brain and review evidence gained from pre-clinical models of the syndrome about the specific brain circuitry involved. In particular, we focus on the impact that developmental androgen excess and adult hyperandrogenemia have in programming and regulating brain circuits important in the central regulation of fertility. The studies discussed here provide compelling support for the importance of the brain in PCOS ontogeny and pathophysiology and highlight the need for a better understanding of the underlying mechanisms involved.

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.

Proteomic analyses of limbic regions in neonatal male, female and androgen receptor knockout mice

Background

It is well-established that organizational effects of sex steroids during early development are fundamental for sex-typical displays of, for example, mating and aggressive behaviors in rodents and other species. Male and female brains are known to differ with respect to neuronal morphology in particular regions of the brain, including the number and size of neurons, and the density and length of dendrites in nuclei of hypothalamus and amygdala. The aim of the present study was to use global proteomics to identify proteins differentially expressed in hypothalamus/amygdala during early development (postnatal day 8) of male, female and conditional androgen receptor knockout (AR^NesDel) male mice, lacking androgen receptors specifically in the brain. Furthermore, verification of selected sexually dimorphic proteins was performed using targeted proteomics.

Results

Our proteomic approach, iTRAQ, allowed us to investigate expression differences in the 2998 most abundantly expressed proteins in our dissected tissues. Approximately 170 proteins differed between the sexes, and 38 proteins between AR^NesDel and control males (p < 0.05). In line with previous explorative studies of sexually dimorphic gene expression we mainly detected subtle protein expression differences (fold changes <1.3). The protein MARCKS (myristoylated alanine rich C kinase substrate), having the largest fold change of the proteins selected from the iTRAQ analyses and of known importance for synaptic transmission and dendritic branching, was confirmed by targeted proteomics as differentially expressed between the sexes.

Conclusions

Overall, our results provide solid evidence that a large number of proteins show sex differences in their brain expression and could potentially be involved in brain sexual differentiation. Furthermore, our finding of a sexually dimorphic expression of MARCKS in the brain during development warrants further investigation on the involvement in sexual differentiation of this protein.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-016-0332-1) contains supplementary material, which is available to authorized users.

The immune system as a novel regulator of sex differences in brain and behavioral development

Sexual differentiation of the brain occurs early in life as a result of sex-typical hormone action and sex chromosome effects. Immunocompetent cells are being recognized as underappreciated regulators of sex differences in brain and behavioral development, including microglia, astrocytes, and possibly other less well studied cell types, including T cells and mast cells. Immunocompetent cells in the brain are responsive to steroid hormones, but their role in sex-specific brain development is an emerging field of interest. This Review presents a summary of what is currently known about sex differences in the number, morphology, and signaling profile of immune cells in the developing brain and their role in the early-life programming of sex differences in brain and behavior. We review what is currently known about sex differences in the response to early-life perturbations, including stress, inflammation, diet, and environmental pollutants. We also discuss how and why understanding sex differences in the developing neuroimmune environment may provide insight into understanding the etiology of several neurodevelopmental disorders. This Review also highlights what remains to be discovered in this emerging field of developmental neuroimmunology and underscores the importance of filling in these knowledge gaps. © 2016 Wiley Periodicals, Inc.

Genetic programs of the developing tuberal hypothalamus and potential mechanisms of their disruption by environmental factors

The hypothalamus is a critical regulator of body homeostasis, influencing the autonomic nervous system and releasing trophic hormones to modulate the endocrine system. The developmental mechanisms that govern formation of the mature hypothalamus are becoming increasingly understood as research in this area grows, leading us to gain appreciation for how these developmental programs are susceptible to disruption by maternal exposure to endocrine disrupting chemicals or other environmental factors in utero. These vulnerabilities, combined with the prominent roles of the various hypothalamic nuclei in regulating appetite, reproductive behaviour, mood, and other physiologies, create a window whereby early developmental disruption can have potent long-term effects. Here we broadly outline our current understanding of hypothalamic development, with a particular focus on the tuberal hypothalamus, including what is know about nuclear coalescing and maturation. We finish by discussing how exposure to environmental or maternally-derived factors can perhaps disrupt these hypothalamic developmental programs, and potentially lead to neuroendocrine disease states.

Role of non-neuronal cells in body weight and appetite control

The brain is composed of neurons and non-neuronal cells, with the latter encompassing glial, ependymal and endothelial cells, as well as pericytes and progenitor cells. Studies aimed at understanding how the brain operates have traditionally focused on neurons, but the importance of non-neuronal cells has become increasingly evident. Once relegated to supporting roles, it is now indubitable that these diverse cell types are fundamental for brain development and function, including that of metabolic circuits, and they may play a significant role in obesity onset and complications. They participate in processes of neurogenesis, synaptogenesis, and synaptic plasticity of metabolic circuits both during development and in adulthood. Some glial cells, such as tanycytes and astrocytes, transport circulating nutrients and metabolic factors that are fundamental for neuronal viability and activity into and within the hypothalamus. All of these cell types express receptors for a variety of metabolic factors and hormones, suggesting that they participate in metabolic function. They are the first line of defense against any assault to neurons. Indeed, microglia and astrocytes participate in the hypothalamic inflammatory response to high fat diet (HFD)-induced obesity, with this process contributing to inflammatory-related insulin and leptin resistance. Moreover, HFD-induced obesity and hyperleptinemia modify hypothalamic astroglial morphology, which is associated with changes in the synaptic inputs to neuronal metabolic circuits. Astrocytic contact with the microvasculature is increased by HFD intake and this could modify nutrient/hormonal uptake into the brain. In addition, progenitor cells in the hypothalamus are now known to have the capacity to renew metabolic circuits, and this can be affected by HFD intake and obesity. Here, we discuss our current understanding of how non-neuronal cells participate in physiological and physiopathological metabolic control.

Using sex differences in the developing brain to identify nodes of influence for seizure susceptibility and epileptogenesis

Sexual differentiation of the developing brain organizes the neural architecture differently between males and females, and the main influence on this process is exposure to gonadal steroids during sensitive periods of prenatal and early postnatal development. Many molecular and cellular processes are influenced by steroid hormones in the developing brain, including gene expression, cell birth and death, neurite outgrowth and synaptogenesis, and synaptic activity. Perturbations in these processes can alter neuronal excitability and circuit activity, leading to increased seizure susceptibility and the promotion of pathological processes that constitute epileptogenesis. In this review, we will provide a general overview of sex differences in the early developing brain that may be relevant for altered seizure susceptibility in early life, focusing on limbic areas of the brain. Sex differences that have the potential to alter the progress of epileptogenesis are evident at molecular and cellular levels in the developing brain, and include differences in neuronal excitability, response to environmental insult, and epigenetic control of gene expression. Knowing how these processes differ between the sexes can help us understand fundamental mechanisms underlying gender differences in seizure susceptibility and epileptogenesis.

Sex differences in anxiety and depression: role of testosterone

Compelling evidence exists for pervasive sex differences in pathological conditions, including anxiety and depressive disorders, with females more than twice as likely to be afflicted. Gonadal hormones may be a major factor in this disparity, given that women are more likely to experience mood disturbances during times of hormonal flux, and testosterone may have protective benefits against anxiety and depression. In this review we focus on the effects of testosterone in males and females, revealed in both human and animal studies. We also present possible neurobiological mechanisms underlying testosterone's mostly protective benefits, including the brain regions, neural circuits, and cellular and molecular pathways involved. While the precise underlying mechanisms remain unclear, both activational and organizational effects of testosterone appear to contribute to these effects. Future clinical studies are necessary in order to better understand when and how testosterone therapy may be effective in both sexes.

Gonadal steroid modulation of sleep and wakefulness in male and female rats is sexually differentiated and neonatally organized by steroid exposure

The paucity of clinical and preclinical studies investigating sex differences in sleep has resulted in mixed findings as to the exact nature of these differences. Although gonadal steroids are known to modulate sleep in females, less is known about males. Moreover, little evidence exists concerning the origin of these sex differences in sleep behavior. Thus, the goal of this study was to directly compare the sensitivity of sleep behavior in male and female Sprague Dawley rats to changes in the gonadal steroid milieu and to test whether the sex differences in sleep are the result of brain sexual differentiation or differences in circulating gonadal steroids. Here we report the magnitude of change in sleep behavior induced by either estradiol (E2) or testosterone (T) was greater in females compared with males, suggesting that sleep behavior in females is more sensitive to the suppressive effects of gonadal steroids. Furthermore, we demonstrated that the organizational effects of early gonadal steroid exposure result in male-like responsivity to gonadal steroids and directly alter the activity of the ventrolateral preoptic area (VLPO), an established sleep-promoting nucleus, in adult masculinized females. Moreover, the nonaromatizable androgen dihydrotestosterone did not suppress sleep in either males or females, suggesting that the T-mediated effect in females was due to the aromatization of T into E2. Together our data suggest that, like sex behavior, sex differences in sleep follow the classical organizational/activational effects of gonadal steroids.

MeCP2 regulates GFAP expression within the developing brain

Mutations in MECP2 cause Rett syndrome (RTT), an X-linked neurodevelopmental disorder that primarily affects females. Individuals with RTT have increased glial fibrillary acidic protein (GFAP) expression in the brain. GFAP is an intermediate filament protein that is expressed predominately within astrocytes in the CNS. MeCP2 binds to methylated regions of the GFAP promoter region and suppresses GFAP expression in vitro. Therefore, we wanted to determine if transiently reducing MeCP2 expression would increase GFAP expression in the developing rat brain. Male and female rats received infusions of either MeCP2 or control siRNA targeting the amygdala during the first 3 days of postnatal life. Brains were collected after 6h or 2 weeks following the last infusion. MeCP2 siRNA increased GFAP mRNA and protein within the female, but not the male, amygdala on postnatal day (PN) 2. Two weeks following the infusion, levels returned to normal. MeCP2 siRNA targeting the hypothalamus also increases GFAP mRNA within the female hypothalamus on PN2, suggesting that the regulation is not brain region-specific. It appears that MeCP2 does not regulate all astrocyte markers in the developing female brain, but specifically regulates GFAP expression, as levels of S100β and vimentin were not altered in the female amygdala at either time point. These data contribute to the idea that the role of MeCP2 differs in the developing male versus female brain. Further elucidating the regulation and function of GFAP can contribute to our understanding of MeCP2 function and perhaps RTT etiology.

Androgen receptors mediate masculinization of astrocytes in the rat posterodorsal medial amygdala during puberty

Astrocytes in the posterodorsal portion of the medial amygdala (MePD) are sexually dimorphic in adult rats: males have more astrocytes in the right MePD and more elaborate processes in the left MePD than do females. Functional androgen receptors (ARs) are required for masculinization of MePD astrocytes, as these measures are demasculinized in adult males carrying the testicular feminization mutation (Tfm) of the AR gene, which renders AR dysfunctional. We now report that the number of astrocytes is already sexually dimorphic in the right MePD of juvenile 25-day-old (P25) rats. Because Tfm males have as many astrocytes as wild-type males at this age, this prepubertal sexual dimorphism is independent of ARs. After P25, astrocyte number increases in the MePD of all groups, but activation of ARs augments this increase in the right MePD, where more astrocytes are added in males than in Tfm males. Consequently, by adulthood, females and Tfm males have equivalent numbers of astrocytes in the right MePD. Sexual dimorphism in astrocyte arbor complexity in the left MePD arises after P25, and is entirely AR-dependent. Thus, masculinization of MePD astrocytes is a result of both AR-independent processes before the juvenile period and AR-dependent processes afterward.

Losses and gains: chronic pain and altered brain morphology

As in many fields of neuroscience, alterations in brain morphology, and specifically gray matter volume and cortical thickness, have been repeatedly linked to chronic pain disorders. Numerous studies have shown changes in cortical and subcortical brain regions suggesting a dynamic process that may be a result of chronic pain or contributing to a more generalized phenomenon in chronic pain including comorbid anxiety and depression. In this review, we provide a perspective of pain as an innate state of pain based on alterations in structure and by inference, brain function. A better neurobiological understanding of gray matter changes will contribute to our understanding of how structural changes contribute to chronic pain (disease driver) and how these changes may be reversed (disease modification or treatment).

Post-proliferative immature radial glial cells female-specifically express aromatase in the medaka optic tectum

Aromatase, the key enzyme responsible for estrogen biosynthesis, is present in the brain of all vertebrates. Much evidence has accumulated that aromatase is highly and exclusively expressed in proliferating mature radial glial cells in the brain of teleost fish even in adulthood, unlike in other vertebrates. However, the physiological significance of this expression remains unknown. We recently found that aromatase is female-specifically expressed in the optic tectum of adult medaka fish. In the present study, we demonstrated that, contrary to the accepted view of the teleost brain, female-specific aromatase-expressing cells in the medaka optic tectum represent a transient subset of post-proliferative immature radial glial cells in the neural stem cell lineage. This finding led us to hypothesize that female-specific aromatase expression and consequent estrogen production causes some sex difference in the life cycle of tectal cells. As expected, the female tectum exhibited higher expression of genes indicative of cell proliferation and radial glial maturation and lower expression of an anti-apoptotic gene than did the male tectum, suggesting a female-biased acceleration of the cell life cycle. Complicating the interpretation of this result, however, is the additional observation that estrogen administration masculinized the expression of these genes in the optic tectum, while simultaneously stimulating aromatase expression. Taken together, these results provide evidence that a unique subpopulation of neural stem cells female-specifically express aromatase in the optic tectum and suggest that this aromatase expression and resultant estrogen synthesis have an impact on the life cycle of tectal cells, whether stimulatory or inhibitory.

17β-Estradiol alters the response of subfornical organ neurons that project to supraoptic nucleus to plasma angiotensin II and hypernatremia

This study was done in urethane anesthetized, ovariectomized (OVX) female rats that were either implanted or not implanted with silastic capsules containing17β-estradiol (E2) to investigate the effect of systemic changes in E2 on the discharge rate of subfornical organ (SFO) neurons that projected to supraoptic nucleus (SON) and responded to changes in plasma levels of angiotensin II (ANG II) or hypernatremia. Extracellular single unit recordings were made from 146 histologically verified single units in SFO. Intra-carotid infusions of ANG II excited ~57% of these neurons, whereas ~23% were excited by hypertonic NaCl. Basal discharge rate of neurons excited by ANG II or hypertonic NaCl was significantly lower in OVX+E2 rats compared to OVX only animals. The response of SFO neurons antidromically activated by SON stimulation to intra-carotid injections of ANG II or hypertonic NaCl was greater in the OVX only compared to the OVX+E2 rats. Intra-carotid injections of E2 in either group attenuated not only the basal discharge of these neurons, but also their response to ANG II or hypertonic NaCl. In all cases this inhibitory effect of E2 was blocked by an intra-carotid injection of the E2 receptor antagonist ICI-182780, although ICI-182780 did not alter the neuron's response to ANG II or hypertonic NaCl. Additionally, ICI-182780 in the OVX+E2 animals significantly raised the basal discharge of SFO neurons and their response to ANG II or hypertonic NaCl. These data indicate that E2 alters the response of SFO neurons to ANG II or NaCl that project to SON, and suggest that E2 functions in the female to regulate neurohypophyseal function in response to circulating ANG II and plasma hypernatremia.

Perinatal testosterone exposure and maternal care effects on the female rat's development and sexual behaviour

Natural variations in maternal care have profound influences on offspring behaviour, brain activity and hormone release. Measuring the amount of time that a rat dam spends licking/grooming (LG) her pups during their first week of life allows for characterisation of distinctive Low, Mid and High LG phenotypes. We have previously found that female offspring of High LG mothers are less sexually receptive, less motivated to mate and show a later onset of puberty relative to Low LG offspring. Given that High LG females are exposed to greater levels of testosterone in utero, we hypothesise that differences in sexual behaviour between High and Low LG female offspring are driven in part by differences in prenatal hormone exposure. To test this hypothesis, pregnant dams pre-characterised as Low, Mid, or High LG mothers were implanted with testosterone or placebo on gestational day (GD) 16. Offspring body weight and anogenital index were assessed at GD 21 and in adulthood. Age of vaginal opening and oestrous cyclicity were assessed to determine the timing of pubertal onset. Testosterone exposure removed the difference between LG phenotypes in pubertal onset by delaying vaginal opening and the appearance of first pro-oestrus. In adulthood, sexual behaviour in a paced mating chamber after sham surgery or ovariectomy with steroid-replacement was examined. Our findings show that Low, Mid and High LG female offspring are differentially affected by perinatal testosterone exposure, and that this exposure removes the precocial pubertal onset of Low LG offspring and increases the sexual proceptivity and receptivity of High LG offspring. These results suggest that maternal programming of the female reproductive system may be mediated, in part, through differences in perinatal testosterone exposure, instead of solely through maternal behaviour.

Enhanced striatal β1-adrenergic receptor expression following hormone loss in adulthood is programmed by both early sexual differentiation and puberty: a study of humans and rats

After reproductive senescence or gonadectomy, changes occur in neural gene expression, ultimately altering brain function. The endocrine mechanisms underlying these changes in gene expression beyond immediate hormone loss are poorly understood. To investigate this, we measured changes in gene expression the dorsal striatum, where 17β-estradiol modulates catecholamine signaling. In human caudate, quantitative PCR determined a significant elevation in β1-adrenergic receptor (β1AR) expression in menopausal females when compared with similarly aged males. No differences were detected in β2-adrenergic and D1- and D2-dopamine receptor expression. Consistent with humans, adult ovariectomized female rats exhibited a similar increase in β1AR expression when compared with gonadectomized males. No sex difference in β1AR expression was detected between intact adults, prepubertal juveniles, or adults gonadectomized before puberty, indicating the necessity of pubertal development and adult ovariectomy. Additionally, increased β1AR expression in adult ovariectomized females was not observed if animals were masculinized/defeminized with testosterone injections as neonates. To generate a model system for assessing functional impact, increased β1AR expression was induced in female-derived cultured striatal neurons via exposure to and then removal of hormone-containing serum. Increased β1AR action on cAMP formation, cAMP response element-binding protein phosphorylation and gene expression was observed. This up-regulation of β1AR action was eliminated with 17β-estradiol addition to the media, directly implicating this hormone as a regulator of β1AR expression. Beyond having implications for the known sex differences in striatal function and pathologies, these data collectively demonstrate that critical periods early in life and at puberty program adult gene responsiveness to hormone loss after gonadectomy and potentially reproductive senescence.

The development of kisspeptin circuits in the Mammalian brain

The neuropeptide kisspeptin, encoded by the Kiss1 gene, is required for mammalian puberty and fertility. Examining the development of the kisspeptin system contributes to our understanding of pubertal progression and adult reproduction and sheds light on possible mechanisms underlying the development of reproductive disorders, such as precocious puberty or hypogonadotropic hypogonadism. Recent work, primarily in rodent models, has begun to study the development of kisspeptin neurons and their regulation by sex steroids and other factors at early life stages. In the brain, kisspeptin is predominantly expressed in two areas of the hypothalamus, the anteroventral periventricular nucleus and neighboring periventricular nucleus (pre-optic area in some species) and the arcuate nucleus. Kisspeptin neurons in these two hypothalamic regions are differentially regulated by testosterone and estradiol, both in development and in adulthood, and also display differences in their degree of sexual dimorphism. In this chapter, we discuss what is currently known and not known about the ontogeny, maturation, and sexual differentiation of kisspeptin neurons, as well as their regulation by sex steroids and other factors during development.

The organizational and aromatization hypotheses apply to rapid, nonclassical hormone action: neonatal masculinization eliminates rapid estradiol action in female hippocampal neurons

Early exposure to the steroid sex hormone testosterone and its estrogen metabolite estradiol masculinize neural tissue during a developmental critical period. Many aspects of neuron anatomy and physiology are permanently altered, including later sensitivity to estradiol. Although it is well established that early hormone exposure alters neuronal responsiveness regarding classical estradiol actions (i.e. acting via nuclear estrogen receptors), it has not yet been determined whether it also alters neuronal processing of nonclassical estrogen receptor signaling, including the actions of membrane-associated estrogen receptors. Hence, we tested whether membrane estrogen receptor regulation of cAMP response element binding protein (CREB) phosphorylation observed in female (but not male) hippocampal pyramidal neurons is due to the lack of androgen and/or estrogen exposure in females during this critical period. Female rat neonates on postnatal d 0 and 1 were systemically injected with one of four compounds: vehicle, testosterone, the nonaromatizable androgen dihydrotestosterone, or estradiol. On postnatal d 2, primary hippocampal neuron cultures were generated from these animals. After 8-9 d in culture, we assessed whether estradiol affected CREB phosphorylation. Neurons from female neonates exposed to testosterone lacked estradiol signaling to CREB. In contrast, dihydrotestosterone injections of female neonates did not disrupt estradiol regulation of CREB. Estradiol injections of female neonates, however, eliminated estradiol signaling to CREB. These findings indicate that testosterone aromatization to estradiol leads to a masculinization/defeminization process whereby hippocampal neurons fail to exhibit rapid estradiol signaling to CREB. Broadly, these findings extend the organizational and aromatization hypotheses to rapid, nonclassical hormone action.

Sex and steroid hormones in early brain injury

Brain injury during development can have severe, long-term consequences. Using an array of animal models, we have an understanding of the etiology of perinatal brain injury. However, we have only recently begun to address the consequences of endogenous factors such as genetic sex and developmental steroid hormone milieu. Our limited understanding has sometimes led researchers to make over-generalizing and potentially dangerous statements regarding treatment for brain injury. Therefore this review acts as a cautionary tale, speaking to our need to understand the effects of sex and steroid hormone environment on the response to brain trauma in the neonate.

Prostaglandin E2 is an endogenous modulator of cerebellar development and complex behavior during a sensitive postnatal period

Prostaglandins are lipid-derived molecules that mediate the generation of fever in the central nervous system. In addition to their proinflammatory role, prostaglandins also impact neuronal development and synaptic plasticity, sometimes in a sex-specific manner. The cerebellum has a high expression of prostaglandin receptors during development, but the role that these molecules play during normal cerebellar maturation is unknown. We demonstrate here that disrupting prostaglandin synthesis with cyclo-oxygenase inhibitors during a time-sensitive window in early postnatal life alters cerebellar Purkinje cell development in rats, resulting in initially increased dendritic growth in both sexes. We show that this results in later cerebellar atrophy in males only, resulting in a sex-specific loss of cerebellar volume. Further, although performance in motor tasks is spared, social interaction and the sensory threshold are altered in males developmentally exposed to cyclo-oxygenase inhibitors. This work demonstrates a previously unknown role for prostaglandins in cerebellar development and emphasizes the role that the cerebellum plays outside motor tasks, in cognitive and sensory domains that may help to explain its connection to complex neurodevelopmental disorders such as autism.

Astrocytes in the rat medial amygdala are responsive to adult androgens

The posterodorsal medial amygdala (MePD) exhibits numerous sex differences including differences in volume and in the number and morphology of neurons and astroctyes. In adulthood, gonadal hormones, including both androgens and estrogens, have been shown to play a role in maintaining the masculine character of many of these sex differences, but whether adult gonadal hormones maintain the increased number and complexity of astrocytes in the male MePD was unknown. To answer this question we examined astrocytes in the MePD of male and female Long Evans rats that were gonadectomized as adults and treated for 30 days with either testosterone or a control treatment. At the end of treatment brains were collected and immunostained for glial fibrillary acidic protein. Stereological analysis revealed that adult androgen levels influenced the number and complexity of astrocytes in the MePD of both sexes, but the specific effects of androgens were different in males and females. However, sex differences in the number and complexity of adult astrocytes persisted even in the absence of gonadal hormones in adulthood, suggesting that androgens also act earlier in life to determine these adult sex differences. Using immunofluorescence and confocal microscopy, we found robust androgen receptor immunostaining in a subpopulation of MePD astrocytes, suggesting that testosterone may act directly on MePD astrocytes to influence their structure and function.

Sex, glia, and development: interactions in health and disease

Microglia and astrocytes are the primary immune cells within the central nervous system. Microglia influence processes including neural development, synaptic plasticity and cognition; while their activation and production of immune molecules can induce stereotyped sickness behaviors or pathologies including cognitive dysfunction. Given their role in health and disease, we propose that glia may also be a critical link in understanding the etiology of many neuropsychiatric disorders that present with a strong sex-bias in their symptoms or prevalence. Specifically, males are more likely to be diagnosed with disorders that have distinct developmental origins such as autism or schizophrenia. In contrast, females are more likely to be diagnosed with disorders that present later in life, after the onset of adolescence, such as depression and anxiety disorders. In this review we will summarize the evidence suggesting that sex differences in the colonization and function of glia within the normal developing brain may contribute to distinct windows of vulnerability between males and females. We will also highlight the current gaps in our knowledge as well as the future directions and considerations of research aimed at understanding the link between neuroimmune function and sex differences in mental health disorders.

Loss of the androgen receptor cofactor p44/WDR77 induces astrogliosis

Astrogliosis is induced by neuronal damage and is also a pathological feature of the major aging-related neurodegenerative disorders. The mechanisms that control the cascade of astrogliosis have not been well established. In a previous study, we identified a novel androgen receptor (AR)-interacting protein, p44/WDR77, that plays a critical role in the proliferation and differentiation of prostate epithelial cells. In the present study, we found that deletion of the p44/WDR77 gene caused premature death with dramatic astrogliosis in mouse brain. We further found that p44/WDR77 is expressed in astrocytes and that loss of p44/WDR77 expression in astrocytes leads to growth arrest and astrogliosis. The astrocyte activation induced by deletion of the p44/WDR77 gene was associated with upregulation of p21(Cip1) expression and NF-κB activation. Silencing p21(Cip1) or NF-κB p65 expression with short hairpin RNA (shRNA) abolished astrocyte activation and rescued the astrocyte growth inhibition induced by deletion of the p44/WDR77 gene. Our results reveal a novel role for p44/WDR77 in the control of astrocyte activation through p21(Cip1) and NF-κB signaling.

Sex differences in the neurobiology of fear conditioning and extinction: a preliminary fMRI study of shared sex differences with stress-arousal circuitry

BACKGROUND

The amygdala, hippocampus, medial prefrontal cortex (mPFC) and brain-stem subregions are implicated in fear conditioning and extinction, and are brain regions known to be sexually dimorphic. We used functional magnetic resonance imaging (fMRI) to investigate sex differences in brain activity in these regions during fear conditioning and extinction.

METHODS

Subjects were 12 healthy men comparable to 12 healthy women who underwent a 2-day experiment in a 3 T MR scanner. Fear conditioning and extinction learning occurred on day 1 and extinction recall occurred on day 2. The conditioned stimuli were visual cues and the unconditioned stimulus was a mild electric shock. Skin conductance responses (SCR) were recorded throughout the experiment as an index of the conditioned response. fMRI data (blood-oxygen-level-dependent [BOLD] signal changes) were analyzed using SPM8.

RESULTS

Findings showed no significant sex differences in SCR during any experimental phases. However, during fear conditioning, there were significantly greater BOLD-signal changes in the right amygdala, right rostral anterior cingulate (rACC) and dorsal anterior cingulate cortex (dACC) in women compared with men. In contrast, men showed significantly greater signal changes in bilateral rACC during extinction recall.

CONCLUSIONS

These results indicate sex differences in brain activation within the fear circuitry of healthy subjects despite similar peripheral autonomic responses. Furthermore, we found that regions where sex differences were previously reported in response to stress, also exhibited sex differences during fear conditioning and extinction.