Sex differences in the anteroventral periventricular nucleus of the preoptic area and in the related effects of androgen in prenatal rats

Sex differences in proliferation and attrition of pubertally born cells in the rat posterior dorsal medial amygdala

The rodent posterodorsal medial amygdala (MePD) evaluates and assigns valence to social sensory stimuli. The perception of social stimuli evolves during puberty, when the focus of social interactions shifts from kin to peers. Using the cell birthdate marker bromo-deoxyuridine (BrdU), we previously discovered that more pubertally born cells are added to the rat MePD in males than females. Here we addressed several questions that remained unanswered by our previous work. First, to determine whether there are sex differences in cell proliferation within the MePD, we examined BrdU-immunoreactive (-ir) cells at 2 and 4 h following BrdU administration on postnatal day 30 (P30). The density of BrdU-ir cells was greater in males than in females, indicating greater proliferation in males. Proliferation was substantiated by double-label immunohistochemistry showing that MePD BrdU-ir cells colocalize proliferating cell nuclear antigen, but not the cell death marker Caspase3. We next studied longer time points (2-21 days) following BrdU administration on P30 and found that the rate of cell attrition is higher in males. Finally, triple-label immunohistochemistry of P30-born MePD cells revealed that some of these cells differentiate into neurons or astrocytes within three weeks of cell birth, with no discernable sex differences. The demonstration of pubertal neuro- and glio-genesis in the MePD of male and female rats adds a new dimension to developmental plasticity of the MePD that may contribute to pubertal changes in the perception of social stimuli in both sexes.

Sexually Dimorphic Formation of the Preoptic Area and the Bed Nucleus of the Stria Terminalis by Neuroestrogens

Testicular androgens during the perinatal period play an important role in the sexual differentiation of the brain of rodents. Testicular androgens transported into the brain act via androgen receptors or are the substrate of aromatase, which synthesizes neuroestrogens that act via estrogen receptors. The latter that occurs in the perinatal period significantly contributes to the sexual differentiation of the brain. The preoptic area (POA) and the bed nucleus of the stria terminalis (BNST) are sexually dimorphic brain regions that are involved in the regulation of sex-specific social behaviors and the reproductive neuroendocrine system. Here, we discuss how neuroestrogens of testicular origin act in the perinatal period to organize the sexually dimorphic structures of the POA and BNST. Accumulating data from rodent studies suggest that neuroestrogens induce the sex differences in glial and immune cells, which play an important role in the sexually dimorphic formation of the dendritic synapse patterning in the POA, and induce the sex differences in the cell number of specific neuronal cell groups in the POA and BNST, which may be established by controlling the number of cells dying by apoptosis or the phenotypic organization of living cells. Testicular androgens in the peripubertal period also contribute to the sexual differentiation of the POA and BNST, and thus their aromatization to estrogens may be unnecessary. Additionally, we discuss the notion that testicular androgens that do not aromatize to estrogens can also induce significant effects on the sexually dimorphic formation of the POA and BNST.

Does GPER1 Play a Role in Sexual Dimorphism?

Estrogens are critical in driving sex-typical social behaviours that are ethologically relevant in mammals. This is due to both production of local estrogens and signaling by these ligands, particularly in an interconnected set of nuclei called the social behavioural network (SBN). The SBN is a sexually dimorphic network studied predominantly in rodents that is thought to underlie the display of social behaviour in mammals. Signalling by the predominant endogenous estrogen, 17β-estradiol, can be either via the classical genomic or non-classical rapid pathway. In the classical genomic pathway, 17β-estradiol binds the intracellular estrogen receptors (ER) α and β which act as ligand-dependent transcription factors to regulate transcription. In the non-genomic pathway, 17β-estradiol binds a putative plasma membrane ER (mER) such as GPR30/GPER1 to rapidly signal via kinases or calcium flux. Though GPER1's role in sexual dimorphism has been explored to a greater extent in cardiovascular physiology, less is known about its role in the brain. In the last decade, activation of GPER1 has been shown to be important for lordosis and social cognition in females. In this review we will focus on several mechanisms that may contribute to sexually dimorphic behaviors including the colocalization of these estrogen receptors in the SBN, interplay between the signaling pathways activated by these different estrogen receptors, and the role of these receptors in development and the maintenance of the SBN, all of which remain underexplored.

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.

Neuroimmunology of the female brain across the lifespan: Plasticity to psychopathology

The female brain is highly dynamic and can fundamentally remodel throughout the normal ovarian cycle as well as in critical life stages including perinatal development, pregnancy and old-age. As such, females are particularly vulnerable to infections, psychological disorders, certain cancers, and cognitive impairments. We will present the latest evidence on the female brain; how it develops through the neonatal period; how it changes through the ovarian cycle in normal individuals; how it adapts to pregnancy and postpartum; how it responds to illness and disease, particularly cancer; and, finally, how it is shaped by old age. Throughout, we will highlight female vulnerability to and resilience against disease and dysfunction in the face of environmental challenges.

Both neural and global androgen receptor overexpression affect sexual dimorphism in the mouse brain

Testosterone is the main endocrine mechanism mediating sexual differentiation of the mammalian brain, although testosterone signalling is complex and important mechanistic questions remain. Notably, the extent to which testosterone acts via androgen receptors (AR) in this process remains unknown and it is also not clear where testosterone acts in the body to produce sexual dimorphisms in neuroanatomy. To address these questions, we used a transgenic mouse model of Cre/loxP-driven AR overexpression in which AR was induced selectively in neural tissue (Nestin-cre) or in all tissues (CMV-cre). We then studied sexually dimorphic features of several well-characterised sexual dimorphisms: calbindin-immunoreactive neurones in the medial preoptic area (CALB-SDN), tyrosine hydroxylase neurones in the anteroventral periventricular nucleus, and vasopressin-immunoreactive neurones originating in the bed nucleus of the stria terminalis and their projections in the lateral septum. We additionally evaluated oestrogen receptor α immunoreactivity in these nuclei. Briefly, we found that global but not neural overexpression of AR resulted in masculinisation of CALB-SDN nucleus volume, cell number and cell size in transgenic females. Furthermore, neural AR overexpression resulted in increased oestrogen receptor α staining in females compared to males in the medial preoptic area. AR overexpression did not affect other measures. Overall, the results of the present study provide support for the hypothesis that androgenic mechanisms external to the nervous system can affect sexual differentiation of the brain.

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.

Small cells with big implications: Microglia and sex differences in brain development, plasticity and behavioral health

Brain sex differences are programmed largely by sex hormone secretions and direct sex chromosome effects in early life, and are subsequently modulated by early life experiences. The brain's resident immune cells, called microglia, actively contribute to brain development. Recent research has shown that microglia are sexually dimorphic, especially during early life, and may participate in sex-specific organization of the brain and behavior. Likewise, sex differences in immune cells and their signaling in the adult brain have been found, although in most cases their function remains unclear. Additionally, immune cells and their signaling have been implicated in many disorders in which brain development or plasticity is altered, including autism, schizophrenia, pain disorders, major depression, and postpartum depression. This review summarizes what is currently known about sex differences in neuroimmune function in development and during other major phases of brain plasticity, as well as the current state of knowledge regarding sex-specific neuroimmune function in psychiatric disorders.

Estrogenic regulation of social behavior and sexually dimorphic brain formation

It has long been known that the estrogen, 17β-estradiol (17β-E), plays a central role for female reproductive physiology and behavior. Numerous studies have established the neurochemical and molecular basis of estrogenic induction of female sexual behavior, i.e., lordosis, in animal models. In addition, 17β-E also regulates male-type sexual and aggressive behavior. In males, testosterone secreted from the testes is irreversibly aromatized to 17β-E in the brain. We discuss the contribution of two nuclear receptor isoforms, estrogen receptor (ER)α and ERβ to the estrogenic regulation of sexually dimorphic brain formation and sex-typical expression of these social behaviors. Furthermore, 17β-E is a key player for social behaviors such as social investigation, preference, recognition and memory as well as anxiety-related behaviors in social contexts. Recent studies also demonstrated that not only nuclear receptor-mediated genomic signaling but also membrane receptor-mediated non-genomic actions of 17β-E may underlie the regulation of these behaviors. Finally, we will discuss how rapidly developing research tools and ideas allow us to investigate estrogenic action by emphasizing behavioral neural networks.

Microglia and Beyond: Innate Immune Cells As Regulators of Brain Development and Behavioral Function

Innate immune cells play a well-documented role in the etiology and disease course of many brain-based conditions, including multiple sclerosis, Alzheimer's disease, traumatic brain and spinal cord injury, and brain cancers. In contrast, it is only recently becoming clear that innate immune cells, primarily brain resident macrophages called microglia, are also key regulators of brain development. This review summarizes the current state of knowledge regarding microglia in brain development, with particular emphasis on how microglia during development are distinct from microglia later in life. We also summarize the effects of early life perturbations on microglia function in the developing brain, the role that biological sex plays in microglia function, and the potential role that microglia may play in developmental brain disorders. Finally, given how new the field of developmental neuroimmunology is, we highlight what has yet to be learned about how innate immune cells shape the development of brain and behavior.

Microglia: Driving critical periods and sexual differentiation of the brain

The proverbial role of microglia during brain development is shifting from passive members of the brain's immune system to active participants that are able to dictate enduring outcomes. Despite these advances, little attention has been paid to one of the most critical components of early brain development-sexual differentiation. Mounting evidence suggests that the normal developmental functions microglia perform-cell number regulation and synaptic connectivity-may be involved in the sex-specific patterning of the brain during these early sensitive periods, and may have lasting sex-dependent and sex-independent effects on behavior. In this review, we outline the known functions of microglia during developmental sensitive periods, and highlight the role they play in the establishment of sex differences in brain and behavior. We also propose a framework for how researchers can incorporate microglia in their study of sex differences and vice versa. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 580-592, 2018.

Sexual Differentiation and Sex Differences in Neural Development

Sex determination occurs at the moment of conception, as a result of XX or XY chromosome pairing. From that point, the body undergoes the process of sexual differentiation, inducing the development of physical characteristics that are easily distinguishable between the sexes and are often reflected in one's physical appearance and gender identity. Although less apparent, the brain also undergoes sexual differentiation. Sex differences in the brain are organized during a critical period of neural development and have an instrumental role in determining the physiology and behavior of an individual throughout the lifespan. Understanding the extent of sex differences in neurodevelopment also influences our understanding of the potential risk for a number of neurodevelopmental, neurological, and mental health disorders that exhibit strong sex biases. Advances made in our understanding of sexually dimorphic brain nuclei, sex differences in neural cell communication, and sex differences in the communication between the brain and peripheral organs are all research fields that have provided valuable information related to the physiological and behavioral outcomes of sex differences in brain development. More recently, investigations into the impact of epigenetic mechanisms on sexual differentiation of the brain have indicated that changes in gene expression, via epigenetic modifications, also contribute to sexual differentiation of the developing brain. Still, there are a number of important questions and ideas that have arisen from our current understanding of sex differences in neurodevelopmental processes that necessitate more time and attention in this field.

Temporal Expression Patterns of Genes Related to Sex Steroid Action in Sexually Dimorphic Nuclei During Puberty

Sex steroids play a major role in sexually dimorphic brain development during not only the perinatal period but also the pubertal period. We previously showed that, in male mice, the estrogen receptor-α (Esr1) and aromatase (Cyp19a1) genes are essential to the sexually dimorphic formation of the anteroventral periventricular nucleus (AVPV) and the principal nucleus of the bed nucleus of the stria terminalis (BNSTp), but the estrogen receptor-β (Esr2) gene is not necessary. We also showed that the androgen receptor (Ar) gene is essential to the sexually dimorphic formation of the BNSTp. These genes are expressed in the AVPV and BNSTp of perinatal mice. However, it remains unknown whether these genes are expressed in the AVPV and BNSTp during puberty, and whether the expression, if any, differs by sex, age, and brain region. Here, we dissected the AVPV and BNSTp from Nissl-stained brain sections of male and female mice on postnatal day (PD) 20 (prepuberty), PD30 (puberty onset in females), PD40 (puberty onset in males), and PD60 (young adult) using a laser microdissection system. We then examined the mRNA levels of Esr1, Esr2, Cyp19a1, and Ar in these brain regions. In the AVPV, Esr1 mRNA levels were greater in females than males during PD20-60. Esr2 and Ar mRNA expressions did not differ between sexes. Ar mRNA levels were higher at PD30 than PD20. Cyp19a1 mRNA was not detected in the AVPV at PD20-60. In the BNSTp, Esr1 and Esr2 mRNA levels were higher in females than in males during PD20-60, although the mRNA levels of Cyp19a1 and Ar did not differ between sexes. Additionally, we revealed that orchiectomy at PD20 induced a failure of normal formation of the male BNSTp and testosterone replacement in the prepubertal period rescued the effect of orchiectomy at PD20. Taken together, it is suggested that pubertal testosterone transported to the AVPV is not converted to estradiol there and does not act via ESR1 and ESR2. By contrast, the formation of the male BNSTp may be affected by testicular testosterone during puberty via AR and/or via ESR1 after conversion to estradiol by CYP19A1.

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.

Gonadal hormone modulation of intracellular calcium as a mechanism of neuroprotection

Hormones have wide-ranging effects throughout the nervous system, including the ability interact with and modulate many aspects of intracellular calcium regulation and calcium signaling. Indeed, these interactions specifically may help to explain the often opposing or paradoxical effects of hormones, such as their ability to both promote and prevent neuronal cell death during development, as well as reduce or exacerbate damage following an insult or injury in adulthood. Here, we review the basic mechanisms underlying intracellular calcium regulation-perhaps the most dynamic and flexible of all signaling molecules-and discuss how gonadal hormones might manipulate these mechanisms to coordinate diverse cellular responses and achieve disparate outcomes. Additional future research that specifically addresses questions of sex and hormone effects on calcium signaling at different ages will be critical to understanding hormone-mediated neuroprotection.

Neurons and Glial Cells Are Added to the Female Rat Anteroventral Periventricular Nucleus During Puberty

The anteroventral periventricular nucleus (AVPV) orchestrates the neuroendocrine-positive feedback response that triggers ovulation in female rodents. The AVPV is larger and more cell-dense in females than in males, and during puberty, only females develop the capacity to show a positive feedback response. We previously reported a potential new mechanism to explain this female-specific gain of function during puberty, namely a female-biased sex difference in the pubertal addition of new cells to the rat AVPV. Here we first asked whether this sex difference is due to greater cell proliferation and/or survival in females. Female and male rats received the cell birthdate marker 5-bromo-2'-deoxyuridine (BrdU; 200 mg/kg, ip) on postnatal day (P) 30; brains were collected at short and long intervals after BrdU administration to assess cell proliferation and survival, respectively. Overall, females had more BrdU-immunoreactive cells in the AVPV than did males, with no sex differences in the rate of cell attrition over time. Thus, the sex difference in pubertal addition of AVPV cells appears to be due to greater cell proliferation in females. Next, to determine the phenotype of pubertally born AVPV cells, daily BrdU injections were given to female rats on P28-56, and tissue was collected on P77 to assess colocalization of BrdU and markers for mature neurons or glia. Of the pubertally born AVPV cells, approximately 15% differentiated into neurons, approximately 19% into astrocytes, and approximately 23% into microglia. Thus, both neuro- and gliogenesis occur in the pubertal female rat AVPV and potentially contribute to maturation of female reproductive function.

Pre- and postnatal bisphenol A treatment does not alter the number of tyrosine hydroxylase-positive cells in the anteroventral periventricular nucleus (AVPV) of weanling male and female rats

Exposure to Bisphenol A (BPA) may interfere with brain sexual differentiation. Altered numbers of tyrosine hydroxylase (TH) cells in the rodent anteroventral periventricular nucleus (AVPV) after developmental BPA treatment have been reported; however, definitive conclusions are lacking. The current study incorporated many of the guidelines suggested for endocrine disrupter research. Specifically, ethinyl estradiol (EE2) served as a reference estrogen, exogenous environmental estrogen exposure was controlled, BPA was administered orally, and subjects consumed a low phytoestrogen diet. Here, on gestational days 6-21, Sprague-Dawley rats (10-15/group) were gavaged with 2.5 or 25.0 µg BPA/kg/day or 5.0 or 10.0 µg EE2/kg/day or the vehicle (5 ml of 0.3% aqueous carboxymethylcellulose/kg/day). A naïve control group was weighed and restrained, but not gavaged. Beginning on postnatal day (PND) 1 and continuing until PND 21, the 4 pups/sex/litter were orally treated with the same dose their dam had received. On PND 21, 1/sex/litter was perfused and the brain removed. TH immunoreactivity (TH-ir) was counted in 8 images/pup by a technician blind to treatment status. ANOVA results indicated significantly higher TH-ir cells/mm(2) in females (main effect of sex: p<0.01); however, there was no significant effect of treatment or a significant interaction of treatment with sex. In a separate untreated group of PND 21 Sprague-Dawley pups, AVPV volume was quantified and no significant sexual dimorphism was apparent. Similar to our reported results of behavioral assessments, the BPA treatment paradigm used here (2.5 or 25.0 µg BPA/kg/day administered orally) does not appear to cause significant alterations in AVPV TH-ir.

Regional difference in sex steroid action on formation of morphological sex differences in the anteroventral periventricular nucleus and principal nucleus of the bed nucleus of the stria terminalis

Sex steroid action is critical to form sexually dimorphic nuclei, although it is not fully understood. We previously reported that masculinization of the principal nucleus of the bed nucleus of the stria terminalis (BNSTp), which is larger and has more neurons in males than in females, involves aromatized testosterone that acts via estrogen receptor-α (ERα), but not estrogen receptor-β (ERβ). Here, we examined sex steroid action on the formation of the anteroventral periventricular nucleus (AVPV) that is larger and has more neurons in females. Morphometrical analysis of transgenic mice lacking aromatase, ERα, or ERβ genes revealed that the volume and neuron number of the male AVPV were significantly increased by deletion of aromatase and ERα genes, but not the ERβ gene. We further examined the AVPV and BNSTp of androgen receptor knockout (ARKO) mice. The volume and neuron number of the male BNSTp were smaller in ARKO mice than those in wild-type mice, while no significant effect of ARKO was found on the AVPV and female BNSTp. We also examined aromatase, ERα, and AR mRNA levels in the AVPV and BNSTp of wild-type and ARKO mice on embryonic day (ED) 18 and postnatal day (PD) 4. AR mRNA in the BNSTp and AVPV of wild-type mice was not expressed on ED18 and emerged on PD4. In the AVPV, the aromatase mRNA level was higher on ED18, although the ERα mRNA level was higher on PD4 without any effect of AR gene deletion. Aromatase and ERα mRNA levels in the male BNSTp were significantly increased on PD4 by AR gene deletion. These results suggest that estradiol signaling via ERα during the perinatal period and testosterone signaling via AR during the postnatal period are required for masculinization of the BNSTp, whereas the former is sufficient to defeminize the AVPV.

Species differences in androgen receptor expression in the medial preoptic and anterior hypothalamic areas of adult male and female rodents

The medial preoptic and anterior hypothalamic areas (MPO/AH) are important androgen targets regulating homeostasis, neuroendocrinology and circadian rhythm as well as instinctive and sociosexual behaviors. Although species differences between rats and mice have been pointed out in terms of morphology and physiology, detailed distributions of androgen receptor (AR) have never been compared between the two rodents. In the present study, AR distribution was examined immunohistochemically in serial sections of the MPO/AH and compared for adult rats and mice. Western blotting and immunohistochemistry clearly demonstrated that AR expression in the brain was stronger in mice than in rats and was stronger in males than in females. In addition, we found (1) an "obliquely elongated calbindin-ir cell island" in mice medial preoptic nucleus (MPN) expressed AR intensely, as well as the sexually dimorphic nucleus in the MPN (SDN-MPN) in rats, strongly supporting a "putative SDN-MPN" previously proposed in mice; (2) AR expression in the suprachiasmatic nucleus (SCN) was much more prominent in mice than in rats and differed in localization between the two species; (3) a mouse-specific AR-ir cell cluster was newly identified as the "tear drop nucleus (TDN)", with male-dominant sexual dimorphism; and (4) two rat-specific AR-ir cell clusters were also newly identified as the "rostral and caudal nebular islands", with male-dominant sexual dimorphism. The present results may provide basic morphological evidence underlying species differences in androgen-modified psychological, physiological and endocrinergic responses. Above all, the findings of the mouse-specific TDN and differing AR expression in the SCN might explain not only species difference in gonadal modification of circadian rhythm, but also distinct structural bases in the context of transduction of SCN oscillation. The current study could also serve as a caution that data on androgen-sensitive functions obtained from one species should not always be directly applied to others among rodents.

Estrogen receptor β expression in the mouse forebrain: age and sex differences

Estrogen receptors regulate multiple brain functions, including stress, sexual, and memory-associated behaviors as well as controlling neuroendocrine and autonomic function. During development, estrogen signaling is involved in programming adult sex differences in physiology and behavior. Expression of estrogen receptor α changes across development in a region-specific fashion. By contrast, estrogen receptor β (ERβ) is expressed in many brain regions, yet few studies have explored sex and developmental differences in its expression, largely because of the absence of selective reagents for anatomical localization of the protein. This study utilized bacterial artificial chromosome transgenic mice expressing ERβ identified by enhanced green fluorescent protein (EGFP) to compare expression levels and distribution of ERβ in the male and female mouse forebrain on the day of birth (P0), on postnatal day 4 (P4), and on P21. By using qualitative analysis, we mapped the distribution of ERβ-EGFP and found developmental alterations in ERβ expression within the cortex, hippocampus, and hypothalamic regions including the arcuate, ventromedial, and paraventricular nuclei. We also report a sex difference in ERβ in the bed nucleus of the stria terminalis, with males showing greater expression at P4 and P21. Another sex difference was found in the anteroventral periventricular nucleus of P21, but not P0 or P4, mice, in which ERβ-EGFP-immunoreactive cells were densely clustered near the third ventricle in females but not males. These developmental changes and sex differences in ERβ indicate a mechanism through which estrogens might differentially affect brain functions or program adult physiology at select times during development.

Sex differences in circadian timing systems: implications for disease

Virtually every eukaryotic cell has an endogenous circadian clock and a biological sex. These cell-based clocks have been conceptualized as oscillators whose phase can be reset by internal signals such as hormones, and external cues such as light. The present review highlights the inter-relationship between circadian clocks and sex differences. In mammals, the suprachiasmatic nucleus (SCN) serves as a master clock synchronizing the phase of clocks throughout the body. Gonadal steroid receptors are expressed in almost every site that receives direct SCN input. Here we review sex differences in the circadian timing system in the hypothalamic-pituitary-gonadal axis (HPG), the hypothalamic-adrenal-pituitary (HPA) axis, and sleep-arousal systems. We also point to ways in which disruption of circadian rhythms within these systems differs in the sexes and is associated with dysfunction and disease. Understanding sex differentiated circadian timing systems can lead to improved treatment strategies for these conditions.

Differential control of sex differences in estrogen receptor α in the bed nucleus of the stria terminalis and anteroventral periventricular nucleus

The principal nucleus of the bed nucleus of the stria terminalis (BNSTp) and anteroventral periventricular nucleus of the hypothalamus (AVPV) are sexually dimorphic, hormone-sensitive forebrain regions. Here we report a profound sex difference in estrogen receptor-α (ERα) immunoreactivity (IR) in the BNSTp, with robust ERα IR in females and the near absence of labeling in males. This sex difference is due to the suppression of ERα IR by testicular hormones in adulthood: it was not present at birth and was not altered by neonatal treatment of females with estradiol; gonadectomy of adult males increased ERα IR to that of females, whereas gonadectomy of adult females had no effect. Treating gonadally intact males with an aromatase inhibitor partially feminized ERα IR in the BNSTp, suggesting that testicular suppression required aromatization. By contrast, in AVPV we found a modest sex difference in ERα IR that was relatively insensitive to steroid manipulations in adulthood. ERα IR in AVPV was, however, masculinized in females treated with estradiol at birth, suggesting that the sex difference is due to organizational effects of estrogens. The difference in ERα IR in the BNSTp of males and females appears to be at least in part due to greater expression of mRNA of the ERα gene (Esr1) in females. The sex difference in message is smaller than the difference in immunoreactivity, however, suggesting that posttranscriptional mechanisms also contribute to the pronounced suppression of ERα IR and presumably to functions mediated by ERα in the male BNSTp.

Cell death atlas of the postnatal mouse ventral forebrain and hypothalamus: effects of age and sex

Naturally occurring cell death is essential to the development of the mammalian nervous system. Although the importance of developmental cell death has been appreciated for decades, there is no comprehensive account of cell death across brain areas in the mouse. Moreover, several regional sex differences in cell death have been described for the ventral forebrain and hypothalamus, but it is not known how widespread the phenomenon is. We used immunohistochemical detection of activated caspase-3 to identify dying cells in the brains of male and female mice from postnatal day (P) 1 to P11. Cell death density, total number of dying cells, and regional volume were determined in 16 regions of the hypothalamus and ventral forebrain (the anterior hypothalamus, arcuate nucleus, anteroventral periventricular nucleus, medial preoptic nucleus, paraventricular nucleus, suprachiasmatic nucleus, and ventromedial nucleus of the hypothalamus; the basolateral, central, and medial amygdala; the lateral and principal nuclei of the bed nuclei of the stria terminalis; the caudate-putamen; the globus pallidus; the lateral septum; and the islands of Calleja). All regions showed a significant effect of age on cell death. The timing of peak cell death varied between P1 to P7, and the average rate of cell death varied tenfold among regions. Several significant sex differences in cell death and/or regional volume were detected. These data address large gaps in the developmental literature and suggest interesting region-specific differences in the prevalence and timing of cell death in the hypothalamus and ventral forebrain.

Collapsin response mediator protein 4 affects the number of tyrosine hydroxylase-immunoreactive neurons in the sexually dimorphic nucleus in female mice

In the sexually dimorphic anteroventral periventricular nucleus (AVPV) of the hypothalamus, females have a greater number of tyrosine hydroxylase-immunoreactive (TH-ir) and kisspeptin-immunoreactive (kisspeptin-ir) neurons than males. In this study, we used proteomics analysis and gene-deficient mice to identify proteins that regulate the number of TH-ir and kisspeptin-ir neurons in the AVPV. Analysis of protein expressions in the rat AVPV on postnatal day 1 (PD1; the early phase of sex differentiation) using two-dimensional fluorescence difference gel electrophoresis followed by MALDI-TOF-MS identified collapsin response mediator protein 4 (CRMP4) as a protein exhibiting sexually dimorphic expression. Interestingly, this sexually differential expressions of CRMP4 protein and mRNA in the AVPV was not detected on PD6. Prenatal testosterone exposure canceled the sexual difference in the expression of Crmp4 mRNA in the rat AVPV. Next, we used CRMP4-knockout (CRMP4-KO) mice to determine the in vivo function of CRMP4 in the AVPV. Crmp4 knockout did not change the number of kisspeptin-ir neurons in the adult AVPV in either sex. However, the number of TH-ir neurons was increased in the AVPV of adult female CRMP4-KO mice as compared with the adult female wild-type mice. During development, no significant difference in the number of TH-ir neurons was detected between sexes or genotypes on embryonic day 15, but a female-specific increase in TH-ir neurons was observed in CRMP4-KO mice on PD1, when the sex difference was not yet apparent in wild-type mice. These results indicate that CRMP4 regulates the number of TH-ir cell number in the female AVPV.

Organizational and activational effects of sex steroids on kisspeptin neuron development

Kisspeptin, encoded by the Kiss1 gene, is a neuropeptide required for puberty and adult reproductive function. Understanding the regulation and development of the kisspeptin system provides valuable knowledge about the physiology of puberty and adult fertility, and may provide insights into human pubertal or reproductive disorders. Recent studies, particularly in rodent models, have assessed how kisspeptin neurons develop and how hormonal and non-hormonal factors regulate this developmental process. Exposure to sex steroids (testosterone and estradiol) during critical periods of development can induce organizational (permanent) effects on kisspeptin neuron development, with respect to both sexually dimorphic and non-sexually dimorphic aspects of kisspeptin biology. In addition, sex steroids can also impart activational (temporary) effects on kisspeptin neurons and Kiss1 gene expression at various times during neonatal and peripubertal development, as they do in adulthood. Here, we discuss the current knowledge--and in some cases, lack thereof--of the influence of hormones and other factors on kisspeptin neuronal development.

Location, location, location: genetic regulation of neural sex differences

Sex differences in many behaviors such as cognition, mood, and motor skills are well-documented in animals and humans and are regulated by many neural circuits. Sexual dimorphisms within cell populations in these circuits play critical roles in the production of these behavioral dichotomies. Here we focus on three proteins that have well described sexual dimorphisms; calbindin-D28k, a calcium binding protein, tyrosine hydroxylase, the rate limiting enzyme involved in dopamine synthesis and vasopressin, a neuropeptide with central and peripheral sites of action. We describe the sex differences in subpopulations of these proteins, with particular emphasis on laboratory mice. Our thrust is to examine genetic bases of sex differences and how the use of genetically modified models has advanced our understanding of this topic. Regional sex differences in the expression of these three proteins are driven by sex chromosome complement, steroid receptors or in some instances both. While studies of sex differences attributable to sex chromosome genes are still few in number it is exciting to note that this variable factors into expression differences for all three of these proteins. Different genetic mechanisms, which elaborate sex differences, may be employed stochastically in different cell populations. Alternately, general patterns involving the timing of differentiation of the sex differences, relative to the "critical period" in hormonal differences between males and female neonates may emerge. In conclusion, future directions in this area should include examination of the importance of location, timing, steroidal receptor/sex chromosome gene synergy and epigenetics in molding neural sex differences.

Gender development and the human brain

Convincing evidence indicates that prenatal exposure to the gonadal hormone, testosterone, influences the development of children's sex-typical toy and activity interests. In addition, growing evidence shows that testosterone exposure contributes similarly to the development of other human behaviors that show sex differences, including sexual orientation, core gender identity, and some, though not all, sex-related cognitive and personality characteristics. In addition to these prenatal hormonal influences, early infancy and puberty may provide additional critical periods when hormones influence human neurobehavioral organization. Sex-linked genes could also contribute to human gender development, and most sex-related characteristics are influenced by socialization and other aspects of postnatal experience, as well. Neural mechanisms underlying the influences of gonadal hormones on human behavior are beginning to be identified. Although the neural mechanisms underlying experiential influences remain largely uninvestigated, they could involve the same neural circuitry as that affected by hormones.

The genetics of sex differences in brain and behavior

Biological differences between men and women contribute to many sex-specific illnesses and disorders. Historically, it was argued that such differences were largely, if not exclusively, due to gonadal hormone secretions. However, emerging research has shown that some differences are mediated by mechanisms other than the action of these hormone secretions and in particular by products of genes located on the X and Y chromosomes, which we refer to as direct genetic effects. This paper reviews the evidence for direct genetic effects in behavioral and brain sex differences. We highlight the 'four core genotypes' model and sex differences in the midbrain dopaminergic system, specifically focusing on the role of Sry. We also discuss novel research being done on unique populations including people attracted to the same sex and people with a cross-gender identity. As science continues to advance our understanding of biological sex differences, a new field is emerging that is aimed at better addressing the needs of both sexes: gender-based biology and medicine. Ultimately, the study of the biological basis for sex differences will improve healthcare for both men and women.

Prenatal PCBs disrupt early neuroendocrine development of the rat hypothalamus

Neonatal exposure to endocrine disrupting chemicals (EDCs) such as polychlorinated biphenyls (PCBs) can interfere with hormone-sensitive developmental processes, including brain sexual differentiation. We hypothesized that disruption of these processes by gestational PCB exposure would be detectable as early as the day after birth (postnatal day (P) 1) through alterations in hypothalamic gene and protein expression. Pregnant Sprague-Dawley rats were injected twice, once each on gestational days 16 and 18, with one of the following: DMSO vehicle; the industrial PCB mixture Aroclor 1221 (A1221); a reconstituted mixture of the three most prevalent congeners found in humans, PCB138, PCB153, and PCB180; or estradiol benzoate (EB). On P1, litter composition, anogenital distance (AGD), and body weight were assessed. Pups were euthanized for immunohistochemistry of estrogen receptor α (ERα) or TUNEL labeling of apoptotic cells or quantitative PCR of 48 selected genes in the preoptic area (POA). We found that treatment with EB or A1221 had a sex-specific effect on developmental apoptosis in the neonatal anteroventral periventricular nucleus (AVPV), a sexually dimorphic hypothalamic region involved in the regulation of reproductive neuroendocrine function. In this region, exposed females had increased numbers of apoptotic nuclei, whereas there was no effect of treatment in males. For ERα, EB treatment increased immunoreactive cell numbers and density in the medial preoptic nucleus (MPN) of both males and females, while A1221 and the PCB mixture had no effect. PCR analysis of gene expression in the POA identified nine genes that were significantly altered by prenatal EDC exposure, in a manner that varied by sex and treatment. These genes included brain-derived neurotrophic factor, GABA(B) receptors-1 and -2, IGF-1, kisspeptin receptor, NMDA receptor subunits NR2b and NR2c, prodynorphin, and TGFα. Collectively, these results suggest that the disrupted sexual differentiation of the POA by prenatal EDC exposures is already evident as early as the day after birth, effects that may change the trajectory of postnatal development and compromise adult reproductive function.

Similar numbers of neurons are generated in the male and female rat preoptic area in utero

The birth date of neurons comprising the sexually dimorphic nucleus of the rat preoptic area (SDN-POA) was determined by bromodeoxyuridine (BrdU) injections at a prescribed time during the embryonic period. Calbindin immunostaining was used as a marker to identity the SDN-POA. The animals were bred from dams injected with BrdU on days 14, 16 or 18 of pregnancy (fertilization defined as day 1). On day 15 after birth (PD), all offspring were euthanized and brain sections were prepared for histology. Neurogenesis in the SDN-POA began around embryonic day (ED) 14 and culminated on ED 18, whereas the preoptic neurons surrounding the SDN-POA generated earlier than did those of the SDN-POA. Although the SDN-POA was significantly larger in males than in females at PD15, the total numbers of neurons comprising the SDN-POA were not significantly different between sexes. Similar aggregates of somatostatin mRNA-positive cells in the central portion of the SDN-POA were observed in both sexes at PD8. On PD15, the aggregates became scattered in males, whereas the aggregates in females remained congested. These data suggest that sexual dimorphism in the SDN-POA results from male-specific postnatal radial spreading of cells rather than cell proliferation during embryonic neurogenesis.

Central role of TRAF-interacting protein in a new model of brain sexual differentiation

Sexually dimorphic brain nuclei underlie gender-specific neural functions and susceptibility to disease, but the developmental basis of dimorphisms is poorly understood. In these studies, we focused on the anteroventral periventricular nucleus (AVPV), a nucleus that is larger in females and critical for the female-typical cyclic surge pattern of luteinizing hormone (LH) release. Sex differences in the size and function of the AVPV result from apoptosis that occurs preferentially in the developing male. To identify upstream pathways responsible for sexual differentiation of the AVPV, we used targeted apoptosis microarrays and in vivo and in vitro follow-up studies. We found that the tumor necrosis factor alpha (TNFalpha)-TNF receptor 2 (TNFR2)-NFkappaB cell survival pathway is active in postnatal day 2 (PND2) female AVPV and repressed in male counterparts. Genes encoding key members of this pathway were expressed exclusively in GABAergic neurons. One gene in particular, TNF receptor-associated factor 2 (TRAF2)-inhibiting protein (trip), was higher in males and it inhibited both TNFalpha-dependent NFkappaB activation and bcl-2 gene expression. The male AVPV also had higher levels of bax and bad mRNA, but neither of these genes was regulated by either TNFalpha or TRIP. Finally, the trip gene was not expressed in the sexually dimorphic nucleus of the preoptic area (SDN-POA), a nucleus in which apoptosis is higher in females than males. These findings form the basis of a new model of sexual differentiation of the AVPV that may also apply to the development of other sexually dimorphic nuclei.

Estrogen induces caspase-dependent cell death during hypothalamic development

The sexually dimorphic population of dopamine neurons in the anteroventral periventricular nucleus of the preoptic region of the hypothalamus (AVPV) develops postnatally under the influence of testosterone, which is aromatized to estrogen. There are fewer dopaminergic neurons labeled with tyrosine hydroxylase (TH) in the male AVPV than the female, and sex steroids determine this sex difference, yet the role of cell death in specifying numbers of dopaminergic neurons in the AVPV is unknown. Estradiol treatment of the AVPV, in vivo and in vitro, was used to manipulate TH-ir cell number. In vitro, concurrent treatment with the estrogen receptor antagonist ICI 182,780 rescued TH-ir cells. Cyclosporin A, an inhibitor of cell death dependent on the opening of a mitochondrial permeability transition pore also blocked TH-ir cell loss. In vivo, estradiol increased the number of apoptotic profiles, both TUNEL and Hoechst labeled nuclei, in the AVPV. This increased apoptosis was also dependent on the presence of the alpha form of the estrogen receptor. To test for caspase dependent TH-ir cell loss, the pancaspase inhibitor ZVAD (N-benzyloxycabonyl-Val-Ala-Asp-fluoromethylketone) was used to rescue TH-ir cells from estradiol-mediated reduction in number. Together, these data suggest that an intrinsic cell death pathway is activated by estrogen to regulate TH-ir cell number. Thus, in contrast to the more widespread neuroprotective actions of sex steroids in the mammalian nervous system, in the AVPV estrogen regulates dopaminergic neuron number through a caspase-dependent mechanism of apoptotic cell death.

Sex differences and the roles of sex steroids in apoptosis of sexually dimorphic nuclei of the preoptic area in postnatal rats

The brain contains several sexually dimorphic nuclei that exhibit sex differences with respect to cell number. It is likely that the control of cell number by apoptotic cell death in the developing brain contributes to creating sex differences in cell number in sexually dimorphic nuclei, although the mechanisms responsible for this have not been determined completely. The milieu of sex steroids in the developing brain affects sexual differentiation in the brain. The preoptic region of rats has two sexually dimorphic nuclei. The sexually dimorphic nucleus of the preoptic area (SDN-POA) has more neurones in males, whereas the anteroventral periventricular nucleus (AVPV) has a higher cell density in females. Sex differences in apoptotic cell number arise in the SDN-POA and AVPV of rats in the early postnatal period, and an inverse correlation exists between sex differences in apoptotic cell number and the number of living cells in the mature period. The SDN-POA of postnatal male rats exhibits a higher expression of anti-apoptotic Bcl-2 and lower expression of pro-apoptotic Bax compared to that in females and, as a potential result, apoptotic cell death via caspase-3 activation more frequently occurs in the SDN-POA of females. The patterns of expression of Bcl-2 and Bax in the SDN-POA of postnatal female rats are changed to male-typical ones by treatment with oestrogen, which is normally synthesised from testicular androgen and affects the developing brain in males. In the AVPV of postnatal rats, apoptotic regulation also differs between the sexes, although Bcl-2 expression is increased and Bax expression and caspase-3 activity are decreased in females. The mechanisms of apoptosis possibly contributing to the creation of sex differences in cell number and the roles of sex steroids in apoptosis are discussed.

Control of cell number in the sexually dimorphic brain and spinal cord

The hormonal control of cell death is currently the best-established mechanism for creating sex differences in cell number in the brain and spinal cord. For example, males have more cells than do females in the principal nucleus of the bed nucleus of the stria terminalis (BNSTp) and spinal nucleus of the bulbocavernosus (SNB), whereas females have a cell number advantage in the anteroventral periventricular nucleus (AVPV). In each case, the difference in cell number in adulthood correlates with a sex difference in the number of dying cells at some point in development. Mice with over- or under-expression of cell death genes have been used to test more directly the contribution of cell death to neural sex differences, to identify molecular mechanisms involved, and to determine the behavioural consequences of suppressing developmental cell death. Bax is a pro-death gene of the Bcl-2 family that is singularly important for apoptosis in neural development. In mice lacking bax, the number of cells in the BNSTp, SNB and AVPV are significantly increased, and sex differences in total cell number in each of these regions are eliminated. Cells rescued by bax gene deletion in the BNSTp express markers of differentiated neurones and the androgen receptor. On the other hand, sex differences in other phenotypically identified populations, such as vasopressin-expressing neurones in the BNSTp or dopaminergic neurones in AVPV, are not affected by either bax deletion or bcl-2 over-expression. Possible mechanisms by which testosterone may regulate cell death in the nervous system are discussed, as are the behavioural effects of eliminating sex differences in neuronal cell number.

Kisspeptin/metastin: a key molecule controlling two modes of gonadotrophin-releasing hormone/luteinising hormone release in female rats

Kisspeptin (also known as metastin), a hypothalamic peptide, has attracted attention as a key molecule in the release of gonadotrophin-releasing hormone (GnRH) in various mammalian species, such as rodents, sheep and primates. Two populations of kisspeptin neurones in the brain may control two modes of GnRH release to time the onset of puberty and regulate oestrous cyclicity in rats and mice. One population of kisspeptin neurones, located in the anteroventral periventricular nucleus, appears to be responsible for the induction of the GnRH surge that leads to the luteinising hormone surge and ovulation. The other, located in the hypothalamic arcuate nucleus, appears to be involved in generating GnRH pulses, resulting in luteinising hormone pulses followed by follicular development and steroidogenesis in the ovary. The present review focuses on the physiological role of the two populations of kisspeptin neurones in controlling gonadal functions by generating the two modes of GnRH release in a female rat model.

Steroid-induced sexual differentiation of the developing brain: multiple pathways, one goal

Hormone exposure, including testosterone and its metabolite estradiol, induces a myriad of effects during a critical period of brain development that are necessary for brain sexual differentiation. Nuclear volume, neuronal morphology, and astrocyte complexity are examples of the wide range of effects by which testosterone and estradiol can induce permanent changes in the function of neurons for the purpose of reproduction in adulthood. This review will examine the multitude of mechanisms by which steroid hormones induce these permanent changes in brain structure and function. Elucidating how steroids alter brain development sheds light on how individual variation in neuronal phenotype is established during a critical period.

Developmental programming and endocrine disruptor effects on reproductive neuroendocrine systems

The ability of a species to reproduce successfully requires the careful orchestration of developmental processes during critical time points, particularly the late embryonic and early postnatal periods. This article begins with a brief presentation of the evidence for how gonadal steroid hormones exert these imprinting effects upon the morphology of sexually differentiated hypothalamic brain regions, the mechanisms underlying these effects, and their implications in adulthood. Then, I review the evidence that aberrant exposure to hormonally-active substances such as exogenous endocrine-disrupting chemicals (EDCs), may result in improper hypothalamic programming, thereby decreasing reproductive success in adulthood. The field of endocrine disruption has shed new light on the discipline of basic reproductive neuroendocrinology through studies on how early life exposures to EDCs may alter gene expression via non-genomic, epigenetic mechanisms, including DNA methylation and histone acetylation. Importantly, these effects may be transmitted to future generations if the germline is affected via transgenerational, epigenetic actions. By understanding the mechanisms by which natural hormones and xenobiotics affect reproductive neuroendocrine systems, we will gain a better understanding of normal developmental processes, as well as develop the potential ability to intervene when development is disrupted.

Neuronal nitric oxide synthase and calbindin delineate sex differences in the developing hypothalamus and preoptic area

Throughout the hypothalamus there are several regions known to contain sex differences in specific cellular, neurochemical, or cell grouping characteristics. The current study examined the potential origin of sex differences in calbindin expression in the preoptic area and hypothalamus as related to sources of nitric oxide. Specific cell populations were defined by immunoreactive (ir) calbindin and neuronal nitric oxide synthase (nNOS) in the preoptic area/anterior hypothalamus (POA/AH), anteroventral periventricular nucleus (AVPv), and ventromedial nucleus of the hypothalamus (VMN). The POA/AH of adult mice was characterized by a striking sex difference in the distribution of cells with ir-calbindin. Examination of the POA/AH of androgen receptor deficient Tfm mice suggests that this pattern was in part androgen receptor dependent, since Tfm males had reduced ir-calbindin compared with wild-type males and more similar to wild-type females. At P0 ir-calbindin was more prevalent than in adulthood, with males having significantly more ir-calbindin and nNOS than have females. Cells that contained either ir-calbindin or ir-nNOS in the POA/AH were in adjacent cell groups, suggesting that NO derived from the enzymatic activity of nNOS may influence the development of ir-calbindin cells. In the region of AVPv, at P0, there was a sex difference with males having more ir-nNOS fibers than have females while ir-calbindin was not detected. In the VMN, at P0, ir-nNOS was greater in females than in males, with no significant difference in ir-calbindin. We suggest that NO as an effector molecule and calbindin as a molecular biomarker illuminate key aspects of sexual differentiation in the developing mouse brain.

Development of sex differences in the principal nucleus of the bed nucleus of the stria terminalis of mice: role of Bax-dependent cell death

Neuron number in the principal nucleus of the bed nucleus of the stria terminalis (BNSTp) is greater in adult male mice than in females. Deletion of the proapoptotic gene, Bax, increases the number of BNSTp cells in adulthood and eliminates the sex difference in cell number. Here, we map the ontogeny of sex differences in nuclear volume and cell number in the BNSTp of neonatal mice, and evaluate the role of cell death in the development of these differences. We find that BNSTp volume and cell number do not differ between male and female wild-type mice on postnatal days P3, P5, or P7. Sex differences emerge after the first postnatal week and both measures are significantly greater in males than in females on P9 and P11. Cell death, assessed by TUNEL staining, was observed in the BNSTp of both sexes from P1-P8. Females had more TUNEL-positive cells than males from approximately P3-P6, with the maximum number of dying cells observed on P5/P6. To test whether the Bax gene is required for sexually dimorphic cell death in the BNSTp, TUNEL cells were counted on P6 in Bax -/- mice and their Bax +/+ siblings. Bax gene deletion nearly abolished TUNEL-positive cells in the BNSTp of both sexes. Together, these findings support the interpretation that the sex difference in BNSTp cell number seen in adulthood is due to Bax-dependent, sexually dimorphic cell death during the first week of life.

AVPV neurons containing estrogen receptor-beta in adult male rats are influenced by soy isoflavones

BACKGROUND

Isoflavones, the most abundant phytoestrogens in soy foods, are structurally similar to 17beta-estradiol. It is known that 17beta-estradiol induces apoptosis in anteroventral periventricular nucleus (AVPV) in rat brain. Also, there is evidence that consumption of soy isoflavones reduces the volume of AVPV in male rats. Therefore, in this study, we examined the influence of dietary soy isoflavones on apoptosis in AVPV of 150 day-old male rats fed either a soy isoflavone-free diet (Phyto-free) or a soy isoflavone-rich diet (Phyto-600).

RESULTS

The occurrence of apoptosis in AVPV was examined by TUNEL staining. The incidence of apoptosis was about 10 times higher in the Phyto-600 group (33.1 +/- 1.7%) than in the Phyto-free group (3.6 +/- 1.0%). Furthermore, these apoptotic cells were identified as neurons by dual immunofluorescent staining of GFAP and NeuN as markers of astrocytes and neurons, respectively. Then the dopaminergic neurons in AVPV were detected by immunohistochemistry staining of tyrosine hydroxylase (TH). No significant difference in the number of TH neurons was observed between the diet treatment groups. When estrogen receptor (ER) alpha and beta were examined by immunohistochemistry, we observed a 22% reduction of ERbeta-positive cell numbers in AVPV with consumption of soy isoflavones, whereas no significant change in ERalpha-positive cell numbers was detected. Furthermore, almost all the apoptotic cells were ERbeta-immunoreactive (ir), but not ERalpha-ir. Last, subcutaneous injections of equol (a major isoflavone metabolite) that accounts for approximately 70-90% of the total circulating plasma isoflavone levels did not alter the volume of AVPV in adult male rats.

CONCLUSION

In summary, these findings provide direct evidence that consumption of soy isoflavones, but not the exposure to equol, influences the loss of ERbeta-containing neurons in male AVPV.

Sex differences in the level of Bcl-2 family proteins and caspase-3 activation in the sexually dimorphic nuclei of the preoptic area in postnatal rats

In developing rats, sex differences in the number of apoptotic cells are found in the central division of the medial preoptic nucleus (MPNc), which is a significant component of the sexually dimorphic nucleus of the preoptic area, and in the anteroventral periventricular nucleus (AVPV). Specifically, male rats have more apoptotic cells in the developing AVPV, whereas females have more apoptotic cells in the developing MPNc. To determine the mechanisms for the sex differences in apoptosis in these nuclei, we compared the expression of the Bcl-2 family members and active caspase-3 in postnatal female and male rats. Western blot analyses for the Bcl-2 family proteins were performed using preoptic tissues isolated from the brain on postnatal day (PD) 1 (day of birth) or on PD8. In the AVPV-containing tissues of PD1 rats, there were significant sex differences in the level of Bcl-2 (female > male) and Bax (female < male) proteins, but not of Bcl-xL or Bad proteins. In the MPNc-containing tissues of PD8 rats, there were significant sex differences in the protein levels for Bcl-2 (female < male), Bax (female > male), and Bad (female < male), but not for Bcl-xL. Immunohistochemical analyses showed significant sex differences in the number of active caspase-3-immunoreactive cells in the AVPV on PD1 (female < male) and in the MPNc on PD8 (female > male). We further found that active caspase-3-immunoreactive cells of the AVPV and MPNc were immunoreactive for NeuN, a neuronal marker. These results suggest that there are sex differences in the induction of apoptosis via the mitochondrial pathway during development of the AVPV and MPNc.

The aryl hydrocarbon receptor pathway and sexual differentiation of neuroendocrine functions

Historically, much of the research on health effects of environmental pollutants focused on ascertaining whether compounds were carcinogenic. More recent findings show that environmental contaminants also exert insidious effects by disrupting hormone action. Of particular concern are findings that developmental exposure to dioxins, chemicals that act through the aryl hydrocarbon receptor pathway, permanently alters sexually differentiated neural functions in animal models. In this review, we focus on mechanisms through which dioxins disrupt neuroendocrine development as exemplified by effects on a brain region critical for ovulation in rodents. We also provide evidence that dysregulation of GABAergic neural development may be a general mechanism underlying a broad spectrum of effects seen after perinatal dioxin exposure.

Early post-hatching sex differences in cell proliferation and survival in the quail telencephalic ventricular zone and intermediate medial mesopallium

Previous studies indicated that avian telencephalic areas related to learned behavior, such as song perception and production, are sexually dimorphic. Our study focused on the eventual occurrence of dimorphism in the intermediate medial mesopallium, an area associated with learning in non-singing birds. During early post-hatching life (days 1 and 5) cell proliferation and survival of newborn cells were studied by means of 5-bromo-2-deoxy-uridine immunocytochemistry. Programmed cell death (apoptosis) was investigated at post-hatching day 10. The ventricular zone, intermediate medial part of mesopallium and lateral septal area was analyzed using stereological methods for cell counts. Short-term experiments revealed significantly higher numbers of newborn cells in male ventricular zone of mesopallium compared to female at post-hatching day 1. Long-term survival until post-hatching day 20 showed significantly higher numbers of labeled cells in the male compared to female intermediate medial part of mesopallium, which is the final destination of migrating cells born in the overlying ventricular zone. The vast majority of these early post-hatching newborn cells residing in the intermediate medial part of mesopallium expressed a neuronal phenotype. In addition to neurogenesis, higher numbers of apoptotic figures were found in the male intermediate medial part of mesopallium at post-hatching day 10, suggesting that cell death plays a role in the control of telencephalic regional cell density in males. Our findings indicate that sex-specific mechanisms possibly stimulate increased cell genesis and survival, as well as the counteracting event of increased apoptotic cell death that characterized the male intermediate medial part of mesopallium.

Estrogen and progesterone do not activate Fos in AVPV or LHRH neurons in male rats

In rodents, females but not males, in response to escalating levels of estrogen, express a luteinizing hormone (LH) surge that is prompted by a surge in luteinizing hormone-releasing hormone (LHRH). It cannot take place if estrogen-sensitive afferents located in the anteroventral periventricular nucleus (AVPV) are either absent or disabled. Males appear to lack the ability to exhibit an LH surge, but it is unclear what level of the CNS contributes to this dimorphic response. This study was conducted to determine whether estrogen followed by progesterone treatment (E + P) of gonadectomized males evokes Fos activation in LHRH and AVPV neurons as it does in females. The results indicated that, consistent with the males' inability to express an LH surge in response to E + P treatment, LHRH and AVPV neurons in males failed to show increased Fos activation. Examination of neuron nuclear antigen (NeuN, a neuron-specific marker), estrogen receptor (ERalpha) and progesterone receptor (PR) neurons in AVPV neurons indicated that, while essentially all the neurons of the caudal AVPV in males and females are steroid responsive, the male possessed half the number of steroid responsive neurons within the caudal AVPV (where activation of Fos is maximal in females) compared to the female. Together, these data indicate that the male lacks a substantial population of steroid receptive AVPV neurons and is unable to respond to the presence of E and P and activate either AVPV or LHRH neurons.

Cell death and sexual differentiation of the nervous system

Sex differences in nuclear volume or neuron number often are attributed to the hormonal control of cell death. In the spinal nucleus of the bulbocavernosus, the central portion of the medial preoptic nucleus, and the principal nucleus of the bed nucleus of the stria terminalis testicular hormones decrease cell death during perinatal life, resulting in a male advantage in neuron number in adulthood. Conversely, males have more dying cells during development and fewer neurons in adulthood than do females in the anteroventral periventricular nucleus of the hypothalamus. This review discusses several limitations and unresolved issues in the literature on sexually dimorphic cell death, and identifies molecular mechanisms by which gonadal steroids may control cell survival. In particular, evidence is presented for the hormonal regulation of neurotrophic factors and involvement of Bcl-2 family proteins in the determination of sex differences in neuron number.

Comparative study of the sources of neuronal projections to the site of gonadotrophin-releasing hormone perikarya and to the anteroventral periventricular nucleus in female rats

The rat ovulatory cycle is dependent on the preoptic region encompassing the gonadotrophin-releasing hormone (GnRH) perikarya and the anteroventral periventricular nucleus (AVPV). Retrograde tract tracing was used to identify and compare the sources of inputs to these sites in female rats. Within the telencephalon and diencephalon, the incidence of retrograde labelling from both sites was moderate to abundant in the ventral lateral septum, posteromedial bed nucleus of the stria terminalis, amygdalohippocampal area and the periventricular, medial preoptic, anterodorsal preoptic, dorsomedial suprachiasmatic, arcuate, and posterior ventrolateral ventromedial hypothalamic nuclei. In these regions, the incidence of retrograde labelling was either greater from the AVPV than from the GnRH perikarya site or similar from both sites. In the medial amygdaloid, parastrial, striohypothalamic, and ventral premammillary nuclei, the retrograde labelling from the AVPV greatly exceeded the sparse incidence from the GnRH perikarya site. In contrast, retrograde labelling from the GnRH perikarya site predominated in the median preoptic, lateroanterior and dorsomedial hypothalamic nuclei, subparaventricular zone, and retrochiasmatic area; it was abundant in the AVPV. Caudal to the diencephalon, retrograde labelling from either site was sparse, except in the lateral parabrachial nucleus, which displayed a particularly high incidence from the GnRH perikarya site. Other mesencephalic regions labelled from either site included the periaqueductal gray and dorsal and median raphe nuclei. The most caudal labelling was found in the ventrolateral medulla and region of the solitary tract nucleus; this was almost exclusively from the GnRH perikarya site. These findings further elucidate the neuroanatomical connections underlying the control of the ovulatory cycle.

Deletion of Bax eliminates sex differences in the mouse forebrain

Several of the best-studied sex differences in the mammalian brain are ascribed to the hormonal control of cell death. This conclusion is based primarily on correlations between pyknotic cell counts in development and counts of mature neurons in adulthood; the molecular mechanisms of hormone-regulated, sexually dimorphic cell death are unknown. We asked whether Bax, a member of the Bcl-2 family of proteins that is required for cell death in many developing neurons, might be essential for sex differences in neuron number. We compared Bax knockout mice and their WT siblings, focusing on two regions of the mouse forebrain that show opposite patterns of sexual differentiation: the principal nucleus of the bed nucleus of the stria terminalis, in which males have more neurons than do females, and the anteroventral periventricular nucleus (AVPV), where females have more neurons overall and many more dopaminergic neurons than do males. Testosterone, or its metabolites, is responsible for the sex differences in both nuclei. A null mutation of the Bax gene completely eliminated sex differences in overall cell number in both the principal nucleus of the bed nucleus of the stria terminalis and AVPV. Thus, Bax-dependent cell death is required for sexual differentiation of cell number, regardless of whether testosterone decreases or increases cell death. In contrast, the sex difference in AVPV dopaminergic cell number, as measured by tyrosine hydroxylase immunohistochemistry, was not affected by Bax gene deletion, demonstrating heterogeneity of mechanisms controlling cell number within a single nucleus.