Sex differences in progesterone receptor expression: a potential mechanism for estradiol-mediated sexual differentiation

Relating sex-bias in human cortical and hippocampal microstructure to sex hormones

Determining sex-bias in brain structure is of great societal interest to improve diagnostics and treatment of brain-related disorders. So far, studies on sex-bias in brain structure predominantly focus on macro-scale measures, and often ignore factors determining this bias. Here we study sex-bias in cortical and hippocampal microstructure in relation to sex hormones. Investigating quantitative intracortical profiling in-vivo using the T1w/T2w ratio in 1093 healthy females and males of the cross-sectional Human Connectome Project young adult sample, we find that regional cortical and hippocampal microstructure differs between males and females and that the effect size of this sex-bias varies depending on self-reported hormonal status in females. Microstructural sex-bias and expression of sex hormone genes, based on an independent post-mortem sample, are spatially coupled. Lastly, sex-bias is most pronounced in paralimbic areas, with low laminar complexity, which are predicted to be most plastic based on their cytoarchitectural properties. Albeit correlative, our study underscores the importance of incorporating sex hormone variables into the investigation of brain structure and plasticity.

Validation of a new Custom Polyclonal Progesterone Receptor Antibody for Immunohistochemistry in the Female Mouse Brain

Immunohistochemical visualization of progesterone receptor (PR)-expressing cells in the brain is a powerful technique to investigate the role of progesterone in the neuroendocrine regulation of fertility. A major obstacle to the immunohistochemical visualization of progesterone-sensitive cells in the rodent brain has been the discontinuation of the commercially produced A0098 rabbit polyclonal PR antibody by DAKO. To address the unavailability of this widely used PR antibody, we optimized and evaluated 4 alternative commercial PR antibodies and found that each lacked the specificity and/or sensitivity to immunohistochemically label PR-expressing cells in paraformaldehyde-fixed female mouse brain sections. As a result, we developed and validated a new custom RC269 PR antibody, directed against the same 533-547 amino acid sequence of the human PR as the discontinued A0098 DAKO PR antibody. Immunohistochemical application of the RC269 PR antibody on paraformaldehyde-fixed mouse brain sections resulted in nuclear PR labeling that was highly distinguishable from background, specific to its antigen, highly regulated by estradiol, matched the known distribution of PR protein expression in the female mouse hypothalamus, and nearly identical to that of the discontinued A0098 DAKO PR antibody. In summary, the RC269 PR antibody is a specific and sensitive antibody to immunohistochemically visualize PR-expressing cells in the mouse brain.

Role of Hormones During Gestation and Early Development: Pathways Involved in Developmental Programming

Accumulating evidence suggests that an altered maternal milieu and environmental insults during the intrauterine and perinatal periods of life affect the developing organism, leading to detrimental long-term outcomes and often to adult pathologies through programming effects. Hormones, together with growth factors, play critical roles in the regulation of maternal-fetal and maternal-neonate interfaces, and alterations in any of them may lead to programming effects on the developing organism. In this chapter, we will review the role of sex steroids, thyroid hormones, and insulin-like growth factors, as crucial factors involved in physiological processes during pregnancy and lactation, and their role in developmental programming effects during fetal and early neonatal life. Also, we will consider epidemiological evidence and data from animal models of altered maternal hormonal environments and focus on the role of different tissues in the establishment of maternal and fetus/infant interaction. Finally, we will identify unresolved questions and discuss potential future research directions.

Kisspeptin neurons as a key player bridging the endocrine system and sexual behavior in mammals

Reproductive behaviors are sexually differentiated: for example, male rodents show mounting behavior, while females in estrus show lordosis behavior as sex-specific sexual behaviors. Kisspeptin neurons govern reproductive function via direct stimulation of gonadotropin-releasing hormone (GnRH) and subsequent gonadotropin release for gonadal steroidogenesis in mammals. First, we discuss the role of hypothalamic kisspeptin neurons as an indispensable regulator of sexual behavior by stimulating the synthesis of gonadal steroids, which exert "activational effects" on the behavior in adulthood. Second, we discuss the central role of kisspeptin neurons that are directly involved in neural circuits controlling sexual behavior in adulthood. We then focused on the role of perinatal hypothalamic kisspeptin neurons in the induction of perinatal testosterone secretion for its "organizational effects" on masculinization/defeminization of the male brain in rodents during a critical period. We subsequently concluded that kisspeptin neurons are key players in bridging the endocrine system and sexual behavior in mammals.

Sex-specific effects of neonatal progestin receptor antagonism on juvenile social play behavior in rats

Developing mammals are exposed to progesterone through several sources; however, the role of progesterone in early development is not well understood. Males express more progestin receptors (PRs) than females within several brain regions during early postnatal life, suggesting that PRs may be important for the organization of the sex differences in the brain and behavior. Indeed, previous studies showed cognitive impairments in male rats treated neonatally with a PR antagonist. In the present study, we examined the role of PRs in organizing juvenile behaviors. Social play behavior and social discrimination were examined in juvenile male and female rats that had been treated with CDB, a PR antagonist, during the first week of postnatal life. Interestingly, neonatal PR antagonism altered different juvenile behaviors in males and females. A transient disruption in PR signaling during development had no effect on social discrimination but increased play initiation and pins in females. These data suggest that PRs play an important role in the organization of sex differences in some social behaviors.

Maternal Exposure to Dibutyl Phthalate (DBP) or Diethylstilbestrol (DES) Leads to Long-Term Changes in Hypothalamic Gene Expression and Sexual Behavior

Xenobiotic exposure during pregnancy and lactation has been linked to perinatal changes in male reproductive outcomes and other endocrine parameters. This pilot study wished to assess whether brief maternal exposure of rats to xenobiotics dibutyl phthalate (DBP) or diethylstilbestrol (DES) might also cause long-term changes in hypothalamic gene expression or in reproductive behavior of the resulting offspring. Time-mated female Sprague Dawley rats were given either DBP (500 mg/kg body weight, every second day from GD14.5 to PND6), DES (125 µg/kg body weight at GD14.5 and GD16.5 only), or vehicle (n = 8–12 per group) and mild endocrine disruption was confirmed by monitoring postnatal anogenital distance. Hypothalamic RNA from male and female offspring at PND10, PND24 and PND90 was analyzed by qRT-PCR for expression of aromatase, oxytocin, vasopressin, ER-alpha, ER-beta, kisspeptin, and GnRH genes. Reproductive behavior was monitored in male and female offspring from PND60 to PND90. Particularly, DES treatment led to significant changes in hypothalamic gene expression, which for the oxytocin gene was still evident at PND90, as well as in sexual behavior. In conclusion, maternal xenobiotic exposure may not only alter endocrine systems in offspring but, by impacting on brain development at a critical time, can have long-term effects on male or female sexual behavior.

Transgenerational effects of polychlorinated biphenyls: 2. Hypothalamic gene expression in rats†

Polychlorinated biphenyls (PCBs) are endocrine-disrupting chemicals (EDCs) with well-established effects on reproduction and behavior in developmentally-exposed (F1) individuals. Because of evidence for transgenerational effects of EDCs on the neuroendocrine control of reproductive physiology, we tested the hypothesis that prenatal PCB exposure leads to unique hypothalamic gene-expression profiles in three generations. Pregnant Sprague-Dawley rats were treated on gestational days 16 and 18 with the PCB mixture Aroclor 1221 (A1221), vehicle (3% DMSO in sesame oil), or estradiol benzoate (EB, 50 μg/kg), the latter a positive control for estrogenic effects of A1221. Maternal- and paternal-lineage F2 and F3 generations were bred using untreated partners. The anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC), involved in the hypothalamic control of reproduction, were dissected from F1 to F3 females and males, RNA extracted, and gene expression measured in a qPCR array. We detected unique gene-expression profiles in each generation, which were sex- and lineage-specific. In the AVPV, treatment significantly changed 10, 25, and 11 transcripts in F1, F2, and F3 generations, whereas 10, 1, and 12 transcripts were changed in these generations in the ARC. In the F1 AVPV and ARC, most affected transcripts were decreased by A1221. In the F2 AVPV, most effects of A1221 were observed in females of the maternal lineage, whereas only Pomc expression changed in the F2 ARC (by EB). The F3 AVPV and ARC were mainly affected by EB. It is notable that results in one generation do not predict results in another, and that lineage was a major determinant in results. Thus, transient prenatal exposure of F1 rats to A1221 or EB can alter hypothalamic gene expression across three generations in a sex- and lineage-dependent manner, leading to the conclusion that the legacy of PCBs continues for generations.

The synthetic steroid tibolone exerts sex-specific regulation of astrocyte phagocytosis under basal conditions and after an inflammatory challenge

BACKGROUND

Tibolone is a synthetic steroid used in clinical practice for the treatment of climacteric symptoms and osteoporosis. Active metabolites of tibolone, generated in target tissues, have an affinity for estrogen and androgen receptors. Astrocytes are direct targets for estrogenic compounds and previous studies have shown that tibolone protects brain cortical neurons in association with a reduction in reactive astrogliosis in a mouse model of traumatic brain injury. Since phagocytosis is a crucial component of the neuroprotective function exerted by astrocytes, in the present study, we have assessed whether tibolone regulates phagocytosis in primary astrocytes incubated with brain-derived cellular debris.

METHODS

Male and female astrocyte cell cultures were obtained from newborn (P0-P2) female and male Wistar rats. Astrocytic phagocytosis was first characterized using carboxylate beads, Escherichia coli particles, or brain-derived cellular debris. Then, the effect of tibolone on the phagocytosis of Cy3-conjugated cellular debris was quantified by measuring the intensity of Cy3 dye-emitted fluorescence in a given GFAP immunoreactive area. Before the phagocytosis assays, astrocytes were incubated with tibolone in the presence or absence of estrogen or androgen receptor antagonists or an inhibitor of the enzyme that synthesizes estradiol. The effect of tibolone on phagocytosis was analyzed under basal conditions and after inflammatory stimulation with lipopolysaccharide.

RESULTS

Tibolone stimulated phagocytosis of brain-derived cellular debris by male and female astrocytes, with the effect being more pronounced in females. The effect of tibolone in female astrocytes was blocked by a selective estrogen receptor β antagonist and by an androgen receptor antagonist. None of these antagonists affected tibolone-induced phagocytosis in male astrocytes. In addition, the inhibition of estradiol synthesis in the cultures enhanced the stimulatory effect of tibolone on phagocytosis in male astrocytes but blocked the effect of the steroid in female cells under basal conditions. However, after inflammatory stimulation, the inhibition of estradiol synthesis highly potentiated the stimulation of phagocytosis by tibolone, particularly in female astrocytes.

CONCLUSIONS

Tibolone exerts sex-specific regulation of phagocytosis in astrocytes of both sexes, both under basal conditions and after inflammatory stimulation.

Bisphenol A (BPA) induces progesterone receptor expression in an estrogen receptor α-dependent manner in perinatal brain

Bisphenol A (BPA) is a xenoestrogen that is prevalent in the environment of industrialized nations due its use in the production of many plastic household items. Virtually all adults in the U.S. have detectable levels of BPA in urine and it can be measured in fetal serum and in breastmilk, making developmental exposure a particular concern. The present study utilizes a progesterone receptor (PR) expression bioassay to assess the estrogen receptor α (ERα)-dependent effects of BPA in fetal rodent brain following maternal exposure. Maternal ingestion of 10 μg/kg/day, but not 50 μg/kg/day, BPA from gestational day 14-22 significantly increased levels of PR immunoreactivity (PRir) in the medial preoptic nucleus (MPN) of female offspring. PR expression in the perinatal MPN is highly dependent on the activation of ERα, but not ERβ, by estrogens. Indeed, injections of BPA (5 μg/kg) to neonates from postnatal day 2-4 (P2-4) significantly increased PR expression in the MPN of postnatal day 5 females compared to the MPN of females administered the oil vehicle. However, pretreatment with the ER antagonist, ICI 182,780 from P1-4 significantly attenuated the effects of BPA on PR expression, indicating an ERα-dependent mechanism. The present results also demonstrate a non-monotonic effect of BPA on the direct expression of a transcription factor in developing brain.

Genetics of sex differences in neuroanatomy and function

Sex differences are observed at many distinct biologic levels, such as in the anatomy and functioning of the brain, behavior, and susceptibility to neuropsychiatric disorders. Previously, these differences were believed to entirely result from the secretion of gonadal hormones; however, recent research has demonstrated that differences are also the consequence of direct or nonhormonal effects of genes located on the sex chromosomes. This chapter reviews the four core genotype model that separates the effects of hormones and sex chromosomes and highlights a few genes that are believed to be partly responsible for sex dimorphism of the brain, in particular, the Sry gene. Genetics of the brain's neurochemistry is discussed and the susceptibility to certain neurologic and psychiatric disorders is reviewed. Lastly, we discuss the sex-specific genetic contribution in disorders of sexual development. The precise molecular mechanisms underlying these differences are currently not entirely known. An increased knowledge and understanding of the role of candidate genes will undeniably be of great aid in elucidating the molecular basis of sex-biased disorders and potentially allow for more sex-specific therapies.

Progesterone Actions During Central Nervous System Development

Although progesterone is a steroid hormone mainly associated with female reproductive functions, such as uterine receptivity and maintenance of pregnancy, accumulating data have shown its physiological actions to extend to several non-reproductive functions in the central nervous system (CNS) both in males and females. In fact, progesterone is de novo synthesized in specific brain regions by neurons and glial cells and is involved in the regulation of various molecular and cellular processes underlying myelination, neuroprotection, neuromodulation, learning and memory, and mood. Furthermore, progesterone has been reported to be implicated in critical developmental events, such as cell differentiation and neural circuits formation. This view is supported by the increase in progesterone synthesis observed during pregnancy in both the placenta and the fetal brain. In the present review, we will focus on progesterone actions during CNS development.

Expression of progesterone receptor, estrogen receptors α and β, and kisspeptin in the hypothalamus during perinatal development of gonad-lacking steroidogenic factor-1 knockout mice

Gonadal hormones contribute to brain sexual differentiation. We analyzed expression of progesterone receptor (PR), estrogen receptor-α (ERα), ERβ, and kisspeptin, in the preoptic area (POA) and/or the arcuate nucleus (ARC), in gonad-lacking steroidogenic factor-1 knockout (KO) mice during perinatal development. At postnatal-day (P) 0-P7, POA PR levels were higher in wild-type (WT) males compared with WT females, while those in KO males were lower than in WT males and similar to those in WT and KO females. At P14-P21, PR levels in all groups increased similarly. POA ERα levels were similar in all groups at embryonic-day (E) 15.5-P14. Those in WT but not KO males reduced during postnatal development to be significantly lower compared with females at P21. POA ERβ levels were higher in WT males than in WT females, while those in KO males were lower than in WT males and similar to those in WT and KO females at P0-P21. POA kisspeptin expression was female-biased in WT mice, while levels in KO females were lower compared with WT females and similar to those in WT and KO males. ARC kisspeptin levels were equivalent among groups at E15.5-P0. At P7-P21, ARC levels in WT but not KO males became lower compared with WT females. Diethylstilbestrol exposure during P0-P6 and P7-P13 increased POA PR and ERβ, and decreased POA ERα and ARC kisspeptin levels at P7 and/or P14 in both sexes of KO mice. These data further understanding of gonadal hormone action on neuronal marker expression during brain sexual development.

Progesterone receptor expression in cajal-retzius cells of the developing rat dentate gyrus: Potential role in hippocampus-dependent memory

The development of medial temporal lobe circuits is critical for subsequent learning and memory functions later in life. The present study reports the expression of progesterone receptor (PR), a powerful transcription factor of the nuclear steroid receptor superfamily, in Cajal-Retzius cells of the molecular layer of the dentate gyrus of rats. PR was transiently expressed from the day of birth through postnatal day 21, but was absent thereafter. Although PR immunoreactive (PR-ir) cells did not clearly express typical markers of mature neurons, they possessed an ultrastructural morphology consistent with neurons. PRir cells did not express markers for GABAergic neurons, neuronal precursor cells, nor radial glia. However, virtually all PR cells co-expressed the calcium binding protein, calretinin, and the glycoprotein, reelin, both reliable markers for Cajal-Retzius neurons, a transient population of developmentally critical pioneer neurons that guide synaptogenesis of perforant path afferents and histogenesis of the dentate gyrus. Indeed, inhibition of PR activity during the first two weeks of life impaired adult performance on both the novel object recognition and object placement memory tasks, two behavioral tasks hypothesized to describe facets of episodic-like memory in rodents. These findings suggest that PR plays an unexplored and important role in the development of hippocampal circuitry and adult memory function.

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.

Androgenic mechanisms of sexual differentiation of the nervous system and behavior

Testicular androgens are the major endocrine factor promoting masculine phenotypes in vertebrates, but androgen signaling is complex and operates via multiple signaling pathways and sites of action. Recently, selective androgen receptor mutants have been engineered to study androgenic mechanisms of sexual differentiation of the nervous system and behavior. The focus of these studies has been to evaluate androgenic mechanisms within the nervous system by manipulating androgen receptor conditionally in neural tissues. Here we review both the effects of neural loss of AR function as well as the effects of neural overexpression of AR in relation to global AR mutants. Although some studies have conformed to our expectations, others have proved challenging to assumptions underlying the dominant hypotheses. Notably, these studies have called into question both the primacy of direct, neural mechanisms and also the linearity of the relationship between androgenic dose and sexual differentiation of brain and behavior.

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.

Epigenetic impacts of endocrine disruptors in the brain

The acquisition of reproductive competence is organized and activated by steroid hormones acting upon the hypothalamus during critical windows of development. This review describes the potential role of epigenetic processes, particularly DNA methylation, in the regulation of sexual differentiation of the hypothalamus by hormones. We examine disruption of these processes by endocrine-disrupting chemicals (EDCs) in an age-, sex-, and region-specific manner, focusing on how perinatal EDCs act through epigenetic mechanisms to reprogram DNA methylation and sex steroid hormone receptor expression throughout life. These receptors are necessary for brain sexual differentiation and their altered expression may underlie disrupted reproductive physiology and behavior. Finally, we review the literature on histone modifications and non-coding RNA involvement in brain sexual differentiation and their perturbation by EDCs. By putting these data into a sex and developmental context we conclude that perinatal EDC exposure alters the developmental trajectory of reproductive neuroendocrine systems in a sex-specific manner.

Progesterone from maternal circulation binds to progestin receptors in fetal brain

Steroid hormones activate nuclear receptors which, as transcription factors, can regulate critical aspects of neural development. Many regions of the rat forebrain, midbrain and hindbrain express progestin receptors (PR) during perinatal life, suggesting that progesterone may play an important role in the development of the brain. An immunohistochemical approach using two antibodies with differential recognition of ligand-bound PR was used to examine whether fetuses are exposed to maternal progesterone during pregnancy and whether progesterone from maternal circulation can bind to PR within the fetal brain. Findings demonstrate that maternal and fetal serum progesterone levels are positively correlated at the end of gestation, suggesting a common source of progesterone in mothers and fetuses (e.g., the maternal ovary). Additional findings suggest that administration of exogenous progesterone to mothers not only increases fetal serum progesterone levels within 2 h, but appears to increase ligand-bound PR in fetal brain. These findings suggest that progesterone of maternal origin may play a previously overlooked role in neural development. In addition, there are implications for the ongoing prophylactic use of synthetic progestins in pregnant women for the prevention of premature birth. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 767-774, 2017.

Estradiol differentially induces progesterone receptor isoforms expression through alternative promoter regulation in a mouse embryonic hypothalamic cell line

Progesterone receptor (PR) presents two main isoforms (PR-A and PR-B) that are regulated by two specific promoters and transcribed from alternative transcriptional start sites. The molecular regulation of PR isoforms expression in embryonic hypothalamus is poorly understood. The aim of the present study was to assess estradiol regulation of PR isoforms in a mouse embryonic hypothalamic cell line (mHypoE-N42), as well as the transcriptional status of their promoters. MHypoE-N42 cells were treated with estradiol for 6 and 12 h. Then, Western blot, real-time quantitative reverse transcription polymerase chain reaction, and chromatin and DNA immunoprecipitation experiments were performed. PR-B expression was transiently induced by estradiol after 6 h of treatment in an estrogen receptor alpha (ERα)-dependent manner. This induction was associated with an increase in ERα phosphorylation (serine 118) and its recruitment to PR-B promoter. After 12 h of estradiol exposure, a downregulation of this PR isoform was associated with a decrease of specific protein 1, histone 3 lysine 4 trimethylation, and RNA polymerase II occupancy on PR-B promoter, without changes in DNA methylation and hydroxymethylation. In contrast, there were no estradiol-dependent changes in PR-A expression that could be related with the epigenetic marks or the transcription factors evaluated. We demonstrate that PR isoforms are differentially regulated by estradiol and that the induction of PR-B expression is associated to specific transcription factors interactions and epigenetic changes in its promoter in embryonic hypothalamic cells.

Hypothalamic Effects of Tamoxifen on Oestrogen Regulation of Luteinising Hormone and Prolactin Secretion in Female Rats

Oestradiol (E2) acts in the hypothalamus to regulate luteinising hormone (LH) and prolactin (PRL) secretion. Tamoxifen (TX) has been extensively used as a selective oestrogen receptor modulator, although its neuroendocrine effects remain poorly understood. In the present study, we investigated the hypothalamic effects of TX in rats under low or high circulating E2 levels. Ovariectomised (OVX) rats treated with oil, E2 or TX, or E2 plus TX, were evaluated for hormonal secretion and immunohistochemical analyses in hypothalamic areas. Both E2 and TX reduced LH levels, whereas TX blocked the E2 -induced surges of LH and PRL. TX prevented the E2 -induced expression of progesterone receptor (PR) in the anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC), although it did not alter PR expression in OVX rats. TX blocked the E2 induction of c-Fos in AVPV neurones, consistent with the suppression of LH surge. However, TX failed to prevent E2 inhibition of kisspeptin expression in the ARC. In association with the blockade of PRL surge, TX increased the phosphorylation of tyrosine hydroxylase (TH) in the median eminence of OVX, E2 -treated rats. TX also precluded the E2 -induced increase in TH expression in the ARC. In all immunohistochemical analyses, TX treatment in OVX rats caused no measurable effect on the hypothalamus. Thus, TX is able to prevent the positive- but not negative-feedback effect of E2 on the hypothalamus. TX also blocks the effects of E2 on tuberoinfundibular dopaminergic neurones and PRL secretion. These findings further characterise the anti-oestrogenic actions of TX in the hypothalamus and provide new information on the oestrogenic regulation of LH and PRL.

Neural Computation and Neuromodulation Underlying Social Behavior

Social behaviors are as diverse as the animals that employ them, with some behaviors, like affiliation and aggression, expressed in nearly all social species. Whether discussing a "family" of beavers or a "murder" of crows, the elaborate language we use to describe social animals immediately hints at patterns of behavior typical of each species. Neuroscience has now revealed a core network of regions of the brain that are essential for the production of social behavior. Like the behaviors themselves, neuromodulation and hormonal changes regulate the underlying neural circuits on timescales ranging from momentary events to an animal's lifetime. Dynamic and heavily interconnected social circuits provide a distinct challenge for developing a mechanistic understanding of social behavior. However, advances in neuroscience continue to generate an explanation of social behavior based on the electrical activity and synaptic connections of neurons embedded in defined neural circuits.

Sexually dimorphic neurons in the ventromedial hypothalamus govern mating in both sexes and aggression in males

Sexual dimorphisms in the brain underlie behavioral sex differences, but the function of individual sexually dimorphic neuronal populations is poorly understood. Neuronal sexual dimorphisms typically represent quantitative differences in cell number, gene expression, or other features, and it is unknown whether these dimorphisms control sex-typical behavior exclusively in one sex or in both sexes. The progesterone receptor (PR) controls female sexual behavior, and we find many sex differences in number, distribution, or projections of PR-expressing neurons in the adult mouse brain. Using a genetic strategy we developed, we have ablated one such dimorphic PR-expressing neuronal population located in the ventromedial hypothalamus (VMH). Ablation of these neurons in females greatly diminishes sexual receptivity. Strikingly, the corresponding ablation in males reduces mating and aggression. Our findings reveal the functions of a molecularly defined, sexually dimorphic neuronal population in the brain. Moreover, we show that sexually dimorphic neurons can control distinct sex-typical behaviors in both sexes.

Progesterone receptor expression in the brain of the socially monogamous and paternal male prairie vole

Differences in the social organization and behavior of male mammals are attributable to species differences in neurochemistry, including differential expression of steroid hormone receptors. However, the distribution of progestin receptors (PR) in a socially monogamous and spontaneously parental male rodent has never been examined. Here we determined if PR exists and is regulated by testicular hormones in forebrain sites traditionally influencing socioreproductive behaviors in male prairie voles (Microtus ochrogaster). We hypothesized that PR expression in male prairie voles would differ from that described in other male rodents because PR activity inhibits parental behaviors and social memory in laboratory mice and rats. Adult male prairie voles received a sham surgery, were gonadectomized, or were gonadectomized and implanted with a testosterone-filled capsule. PR immunoreactivity (PRir) was measured four weeks later in areas of the hypothalamus and extended amygdala. A group of gonadally intact female prairie voles was included to reveal possible sex differences. We found considerable PRir in all sites examined. Castration reduced PRir in males' medial preoptic nucleus, anteroventral periventricular nucleus, ventromedial hypothalamus, and posterodorsal medial amygdala, and it was maintained in these sites by testosterone. This is the first study to examine PR expression in brain sites involved in socioreproductive behaviors in a socially monogamous and spontaneously paternal male rodent. Our results mostly reveal cross-species conservation in the distribution and hormone sensitivity of PR expression. Because PR interferes with aspects of sociality in other male rodents, PR may eventually be found to have different neurobiological actions in male prairie voles.

Gender-specific differences in the central nervous system's response to anesthesia

Males and females are physiologically distinct in their responses to various anesthetic agents. The brain and central nervous system (CNS), the main target of anesthesia, are sexually dimorphic from birth and continue to differentiate throughout life. Accordingly, gender has a substantial impact on the influence of various anesthetic agents in the brain and CNS. Given the vast differences in the male and female CNS, it is surprising to find that females are often excluded from basic and clinical research studies of anesthesia. In animal research, males are typically studied to avoid the complication of breeding, pregnancy, and hormonal changes in females. In clinical studies, females are also excluded for the variations that occur in the reproductive cycle. Being that approximately half of the surgical population is female, the exclusion of females in anesthesia-related research studies leaves a huge knowledge gap in the literature. In this review, we examine the reported sex-specific differences in the central nervous system's response to anesthesia. Furthermore, we suggest that anesthesia researchers perform experiments on both sexes to further evaluate such differences. We believe a key goal of research studying the interaction of the brain and anesthesia should include the search for knowledge of sex-specific mechanisms that will improve anesthetic care and management in both sexes.

Maternal hypothyroidism decreases progesterone receptor expression in the cortical subplate of foetal rat brain

Steroid hormones exert profound effects on the development of brain areas controlling complex cognitive function in adulthood. One class, progestins, may contribute by acting on the progestin receptor (PR), which is transiently expressed in a critical layer of developing cortex: the subplate. PR expression in the subplate coincides with the establishment of ongoing cortical connectivity and may play an important organisational role. Identification of the factor(s) that regulate the precise timing of PR expression within subplate may help elucidate the function of PR. Thyroid hormone may interact with hormone response elements within the PR gene. The present study examined the effects of maternal hypothyroidism on levels of PR immunoreactivity (PR-IR) within the foetal subplate. Pregnant rats were made hypothyroid by the administration of methimazole and potassium perchlorate in drinking water. Maternal hypothyroidism significantly decreased PR-IR within the foetal subplate. Using the incorporation of 5-bromo-2'-deoxyuridine (BrDU) during subplate cell neurogenesis (embryonic day 13.5) to determine subplate cell survival in hypothyroid animals, we found that decreases in PR-IR cannot be attributed to significant subplate cell loss but are more likely the result of altered PR expression. Gestational thyroxine replacement to hypothyroid dams prevented the decrease in PR-IR within the subplate. These results identify thyroid hormone as a potential factor in the regulation of PR expression in the developing brain. These results are consistent with the idea that endocrine cross-talk between progesterone and thyroid hormone may be one mechanism by which maternal hypothyroidism alters normal cortical development.

Sex differences in brain epigenetics

Sexual differentiation of the brain takes place during a perinatal-sensitive time window as a result of gonadal hormone-induced activational and organizational effects on neuronal substrates. Increasing evidence suggests that epigenetic mechanisms can contribute to the establishment and maintenance of some aspects of these processes, and that these epigenetic mechanisms may themselves be under the control of sex hormones. Epigenetic programming of neuroendocrine and behavioral phenotypes frequently occurs sex specifically, pointing to sex differences in brain epigenetics as a possible determinant. Understanding how sex-specific epigenomes and sex-biased responses to environmental cues contribute to the development of brain diseases might provide new insights for epigenetic therapy.

Sexual differentiation of the rodent brain: dogma and beyond

Steroid hormones of gonadal origin act on the neonatal brain to produce sex differences that underlie adult reproductive physiology and behavior. Neuronal sex differences occur on a variety of levels, including differences in regional volume and/or cell number, morphology, physiology, molecular signaling, and gene expression. In the rodent, many of these sex differences are determined by steroid hormones, particularly estradiol, and are established by diverse downstream effects. One brain region that is potently organized by estradiol is the preoptic area (POA), a region critically involved in many behaviors that show sex differences, including copulatory and maternal behaviors. This review focuses on the POA as a case study exemplifying the depth and breadth of our knowledge as well as the gaps in understanding the mechanisms through which gonadal hormones produce lasting neural and behavioral sex differences. In the POA, multiple cell types, including neurons, astrocytes, and microglia are masculinized by estradiol. Multiple downstream molecular mediators are involved, including prostaglandins, various glutamate receptors, protein kinase A, and several immune signaling molecules. Moreover, emerging evidence indicates epigenetic mechanisms maintain sex differences in the POA that are organized perinatally and thereby produce permanent behavioral changes. We also review emerging strategies to better elucidate the mechanisms through which genetics and epigenetics contribute to brain and behavioral sex differences.

Sexually dimorphic testosterone secretion in prenatal and neonatal mice is independent of kisspeptin-Kiss1r and GnRH signaling

Kisspeptin, encoded by the Kiss1 gene, stimulates GnRH secretion and is therefore critical for sex steroid secretion at puberty and in adulthood. However, kisspeptin's role in regulating sex steroid secretion earlier in development is unexplored. In rodents, testosterone (T) levels are higher in prenatal and newborn males than females. We determined whether kisspeptin-Kiss1r and GnRH signaling plays a role in sexually dimorphic perinatal T secretion in mice. Our results demonstrate that 1) T levels in newborn males are elevated at 4 h but not 20 h after birth, but hypothalamic Kiss1 and neurokinin B (NKB) levels in males are not different between these time points (and both are lower than in females); 2) serum T levels in newborn Kiss1r knockout (KO) males are higher than in newborn females and similar to wild-type (WT) males; 3) perinatal hypothalamic progesterone receptor (Pgr) expression, which is dependent on circulating levels of gonadally produced T, is significantly higher in prenatal and newborn Kiss1r KO and WT males than similarly aged females; 4) multiple measures of testicular growth and function are not different between developing Kiss1r KO and WT mice until after postnatal d 5; and 5) GnRH neurons of newborn males do not exhibit high c-fos coexpression, and newborn hypogonadal (hpg) male mice (lacking GnRH) secrete elevated T, similar to newborn WT males. We conclude that, unlike in puberty and adulthood, elevated T secretion in prenatal and neonatal mice is independent of both kisspeptin and GnRH signaling, and the necessity of kisspeptin-Kiss1r signaling for testicular function is first apparent after d 5.

Anatomically-specific actions of oestrogen receptor in the developing female rat brain: effects of oestradiol and selective oestrogen receptor modulators on progestin receptor expression

Steroid hormones largely exert their actions by activating nuclear receptors, which, as transcription factors, powerfully influence fundamental processes of neural development. Often, steroid receptor action demonstrates remarkable specificity under different developmental, anatomical or hormonal conditions. Yet, the mechanisms underlying such specificity are poorly understood. The present study examined the anatomically-specific regulation of progestin receptor (PR) expression by oestrogen receptor (ER) activation in the ventromedial nucleus (VMN) of the hypothalamus and the medial preoptic nucleus (MPN) of the neonatal female rat brain, using the selective oestrogen receptor modulators (SERMs), tamoxifen and ICI 182780 (ICI), in the presence or absence of oestradiol benzoate (EB) treatment. The results demonstrate that PR immunoreactivity (PR-ir) in the neonatal female MPN was significantly increased by EB and this increase was abolished by either tamoxifen or ICI treatment. In contrast, within the VMN of the same animals, EB had no effect on PR-ir and the SERMs only modestly decreased PR-ir. Interestingly, ICI acted as a true antagonist regardless of EB treatment, whereas tamoxifen acted as an ER agonist in the absence of EB in the MPN, but not the VMN, representing one of the first in vivo demonstrations of tissue-specific and oestradiol-independent effects of tamoxifen on ER activation. The present results indicate that PR expression is highly dependent on oestradiol and its receptor in the MPN, although it is independent of both oestradiol and ER activation within the neonatal VMN. These findings demonstrate the anatomically-specific actions of oestradiol and its receptor to induce PR in two brain regions controlling different aspects of female reproductive behaviours in adulthood.

Sexual differentiation and the neuroendocrine hypothesis of autism

The phenotypic expression of autism spectrum disorders varies widely in severity and characteristics and it is, therefore, likely that a number of etiological factors are involved. However, one finding which has been found consistently is that there is a greater incidence of autism in boys than girls. Recently, attention has been given to the extreme male hypothesis-that is that autism behaviors are an extreme form of typical male behaviors, including lack of empathy and language deficits but an increase in so-called systemizing behaviors, such as attention to detail and collecting. This points to the possibility that an alteration during sexual differentiation of the brain may occur in autism. During sexual differentiation of the brain, two brain regions are highly sexually dimorphic-the amygdala and the hypothalamus. Both of these regions are also implicated in the neuroendocrine hypothesis of autism, wherein a balance between oxytocin and cortisol may contribute to the disorder. We are thus proposing that the extreme male hypothesis and the neuroendocrine hypothesis are in fact compatible in that sexual differentiation of the brain towards an extreme male phenotype would result in the neuroendocrine changes proposed in autism. We have preliminary data, treating developing rat pups with the differentiating hormone 17-β estradiol during a critical time and showing changes in social behaviors and oxytocin, to support this hypothesis. Further studies should be undertaken to confirm the role of extremes of normal sexual differentiation in producing the neuroendocrine changes associated with autism.

Organized for sex - steroid hormones and the developing hypothalamus

Steroid hormones of gonadal origin act on the neonatal brain, particularly the hypothalamus, to produce sex differences that underlie copulatory behavior. Neuroanatomical sex differences include regional volume, cell number, connectivity, morphology, physiology, neurotransmitter phenotype and molecular signaling, all of which are determined by the action of steroid hormones, particularly by estradiol in males, and are established by diverse downstream effects. Sex differences in distinct hypothalamic regions can be organized by the same steroid hormone, but the direction of a sex difference is often specific to one region or cell type, illustrating the wide range of effects that steroid hormones have on the developing brain. Substantial progress has been made in elucidating the downstream mechanisms through which gonadal hormones sexually differentiate the brain, but gaps remain in establishing the precise relationship between changes in neuronal morphology and behavior. A complete understanding of sexual differentiation will require integrating the diverse mechanisms across multiple brain regions into a functional network that regulates behavioral output.

Epigenetic underpinnings of developmental sex differences in the brain

Sexual differentiation of the brain is a crucial developmental process that enables the lifelong expression of sexually dimorphic behaviors, including those necessary for successful reproduction. During a perinatal sensitive period, gonadal hormones defeminize and masculinize the male brain, and a lack of gonadal steroids allows for feminization in the female. This hormonally-induced differentiation permanently alters neural structures, creating highly dimorphic brain regions; however, the mechanism by which hormones exert their long-lasting effects are still largely unknown. Epigenetic processes such as DNA methylation and histone modifications serve as an interface for environmental stimuli to exert control over the genome. These modifications have the capacity to activate or repress gene expression, thereby shaping the developmental outcomes of cells, circuits, and structures in the brain. Sex differences in methylation, methyl-binding proteins, and chromatin modifications indicate that epigenetic mechanism may be important for sexual differentiation of the brain. The data outlined in this review provide evidence that gonadal hormones in the neonatal brain influence epigenetic processes such as DNA methylation and histone acetylation, but also emphasize the primitive status of our current understanding of epigenetics and sexual differentiation and the brain.

Postnatal and adult exposure to estradiol differentially influences adult neurogenesis in the main and accessory olfactory bulb of female mice

Neurons incorporated into the adult main olfactory bulb (MOB) and accessory olfactory bulb (AOB) derive from the subventricular zone (SVZ). Despite some recent studies on the role of olfactory neurogenesis in sociosexual behaviors mediated by hormones, data on the implication of estrogens are still lacking. Taking advantage of female aromatase-knockout (ArKO) mice, which are unable to produce estradiol across their life span, we investigated the role of estradiol exposure during early postnatal and adult periods on adult neurogenesis in the MOB and AOB. We found that proliferation of progenitor cells in the adult female SVZ was not influenced by estradiol. However, whereas adult exposure to estradiol influences the turnover of MOB newborn neurons, the survival of those in the AOB depends on exposure to estradiol during the early postnatal period. Finally, based on their expression of Zif268, we showed that newborn neurons in the MOB responded to sociosexual odors, albeit to a lesser extent in ArKO females, suggesting a contribution of estradiol during the early postnatal period to this response. Together, these results suggest that the survival and functional integration of newborn neurons in the adult female MOB and AOB are differentially influenced by estrogens from the early postnatal period to adulthood.

Developmental and hormone-induced epigenetic changes to estrogen and progesterone receptor genes in brain are dynamic across the life span

Sexual differentiation of the rodent brain occurs during a perinatal critical period when androgen production from the male testis is locally converted to estradiol in neurons, resulting in masculinization of adult sexual behavior. Adult brain responses to hormones are programmed developmentally by estradiol exposure, but the mechanism(s) by which these changes are permanently organized remains poorly understood. Activation of steroid receptors plays a major role in organization of the brain, and we hypothesized that estradiol-induced alteration of steroid-receptor gene methylation is a critical component to this process. Estrogen receptor (ER)-α and ER-β and progesterone receptor are expressed at high levels within the preoptic area (POA) and the mediobasal hypothalamus, two brain regions critical for the expression of male and female sexual behavior. The percent methylation on the ER-α promoter increased markedly across development. During the critical period of sexual differentiation, females had significantly increased methylation than males or females masculinized with estradiol at two CpG sites. By adulthood, the neonatal sex difference and hormonal modulation of methylation were replaced with a new pattern at a different CpG site on the ER-α promoter. In contrast, the percent methylation on the progesterone receptor and ER-β promoter did not change developmentally but was modulated by hormones and exhibited only late emerging transient sex differences. These data indicate that sex differences in the methylation pattern of genes important for sexual behavior are epigenetically modified during development, but the specific changes observed do not endure and are not necessarily temporally associated with neonatal hormone exposure.

Cellular and subcellular localization of estrogen and progestin receptor immunoreactivities in the mouse hippocampus

Estrogen receptor-alpha (ERalpha), estrogen receptor-beta (ERbeta), and progestin receptor (PR) immunoreactivities are localized to extranuclear sites in the rat hippocampal formation. Because rats and mice respond differently to estradiol treatment at a cellular level, the present study examined the distribution of ovarian hormone receptors in the dorsal hippocampal formation of mice. For this, antibodies to ERalpha, ERbeta, and PR were localized by light and electron immunomicroscopy in male and female mice across the estrous cycle. Light microscopic examination of the mouse hippocampal formation showed sparse nuclear ERalpha and PR immunoreactivity (-ir) most prominently in the CA1 region and diffuse ERbeta-ir primarily in the CA1 pyramidal cell layer as well as in a few interneurons. Ultrastructural analysis additionally revealed discrete extranuclear ERalpha-, ERbeta-, and PR-ir in neuronal and glial profiles throughout the hippocampal formation. Although extranuclear profiles were detected in all animal groups examined, the amount and types of profiles varied with sex and estrous cycle phase. ERalpha-ir was highest in diestrus females, particularly in dendritic spines, axons, and glia. Similarly, ERbeta-ir was highest in estrus and diestrus females, mainly in dendritic spines and glia. Conversely, PR-ir was highest during proestrus, mostly in axons. Except for very low levels of extranuclear ERbeta-ir in mossy fiber terminals in mice, the labeling patterns in the mice for all three antibodies were similar to the ultrastructural labeling found previously in rats, suggesting that regulation of these receptors is well conserved across the two species.

Of mice and rats: key species variations in the sexual differentiation of brain and behavior

Mice and rats are important mammalian models in biomedical research. In contrast to other biomedical fields, work on sexual differentiation of brain and behavior has traditionally utilized comparative animal models. As mice are gaining in popularity, it is essential to acknowledge the differences between these two rodents. Here we review neural and behavioral sexual dimorphisms in rats and mice, which highlight species differences and experimental gaps in the literature, that are needed for direct species comparisons. Moving forward, investigators must answer fundamental questions about their chosen organism, and attend to both species and strain differences as they select the optimal animal models for their research questions.

Reduced prepubertal expression of progesterone receptor in the hypothalamus of female aromatase knockout mice

Previous research using alpha-fetoprotein knockout and aromatase knockout (ArKO) female mice suggested that the developing hypothalamic mechanisms that later control feminine sexual behavior are protected prenatally from estradiol, whereas shortly after birth, they may be stimulated by this same sex hormone. In the present study, we found that the amount of progesterone receptor immunoreactivity (PR-ir) in the anteroventral periventricular nucleus and medial part of the medial preoptic nucleus was significantly lower in ArKO female mice than in wild-type (WT) females at several prepubertal ages including postnatal d 15 (P15), P15, P20, and P25 but not neonatally at P0, P5, or P10. Likewise, PR-ir in the lateral subdivision of the ventromedial hypothalamic nucleus was significantly lower at P25 in ArKO vs. WT female mice but not at earlier postnatal ages. PR-ir was consistently higher in male than in female WT mice in the anteroventral periventricular nucleus and medial preoptic nucleus over P0-P10 and in the ventromedial hypothalamic nucleus over P0-P20. In these brain regions across these latter ages, PR-ir in male ArKO mice was significantly lower than in WT males and resembled the values seen in WT females, confirming previous reports that estradiol formed in the developing male hypothalamus from testicular testosterone is responsible for male-typical levels of neural PR expression. Thus, estradiol induces both female- and male-typical expression of PR postnatally in the mouse hypothalamus. Future experiments will determine whether this estradiol-induced PR expression contributes to either female- or male-typical brain and behavioral differentiation.

Binge-pattern alcohol exposure during puberty induces sexually dimorphic changes in genes regulating the HPA axis

Maternal alcohol consumption during critical periods of fetal brain development leads to devastating long-term consequences on adult reproductive physiology, cognitive function, and social behaviors. However, very little is known about the long-term consequences of alcohol consumption during puberty, which is perhaps an equally dynamic and critical period of brain development. Alcohol abuse during adulthood has been linked with an increase in clinically diagnosed anxiety disorders, yet the etiology and neurochemical mechanisms of alcohol-induced anxiety behavior is unknown. In this study, we determined the effects of binge ethanol exposure during puberty on two critical central regulators of stress and anxiety behavior: corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP). Our results showed that ethanol increased plasma corticosterone (CORT) levels in both sexes, yet binge-treated animals had significantly lower CORT levels than animals exposed to a single dose, suggesting that the hypothalamo-pituitary-adrenal (HPA) axis habituated to the repeated stressful stimuli of ethanol. Binge ethanol exposure also significantly increased CRH and AVP gene expression in the paraventricular nucleus of males, but not females. Overall, our results demonstrate that binge ethanol exposure during puberty changes the central expression of stress-related genes in a sex-specific manner, potentially leading to permanent dysregulation of the HPA axis and long-term behavioral consequences.

The kisspeptin/neurokinin B/dynorphin (KNDy) cell population of the arcuate nucleus: sex differences and effects of prenatal testosterone in sheep

Recent work in sheep has identified a neuronal subpopulation in the arcuate nucleus that coexpresses kisspeptin, neurokinin B, and dynorphin (referred to here as KNDy cells) and that mediate the negative feedback influence of progesterone on GnRH secretion. We hypothesized that sex differences in progesterone negative feedback are due to sexual dimorphism of KNDy cells and compared neuropeptide and progesterone receptor immunoreactivity in this subpopulation between male and female sheep. In addition, because sex differences in progesterone negative feedback and neurokinin B are due to the influence of testosterone (T) during fetal life, we determined whether prenatal T exposure would mimic sex differences in KNDy cells. Adult rams had nearly half the number of kisspeptin, neurokinin B, dynorphin, and progesterone receptor-positive cells in the arcuate nucleus as did females, but the percentage of KNDy cells colocalizing progesterone receptors remained high in both sexes. Prenatal T treatment also reduced the number of dynorphin, neurokinin B, and progesterone receptor-positive cells in the female arcuate nucleus; however, the number of kisspeptin cells remained high and at levels comparable to control females. Thus, sex differences in kisspeptin in the arcuate nucleus, unlike that of dynorphin and neurokinin B, are not due solely to exposure to prenatal T, suggesting the existence of different critical periods for multiple peptides coexpressed within the same neuron. In addition, the imbalance between inhibitory (dynorphin) and stimulatory (kisspeptin) neuropeptides in this subpopulation provides a potential explanation for the decreased ability of progesterone to inhibit GnRH neurons in prenatal T-treated ewes.

Females follow a more "compact" early human brain development model than males. A case-control study of preterm neonates

The pattern of sexual differentiation of the human brain is not well understood, particularly at the early stages of development when intense growth and multiple maturational phenomena overlap and interrelate. A case-control study of 20 preterm males and females matched for age was conducted. Three-dimensional images were acquired with 3 T MRI. The cerebral volume and the cortical folding area (FA), defined as the surface area of the interface between cortical gray and white matter, were compared between males and females. Females had smaller cerebra than males even after removing the influence of overall size differences between the subjects. The cortical FA increased in relation to volume by a power of 4/3 in both groups. Females had larger cortical FA compared with males with similar cerebral volumes. The study provides in vivo evidence of sexually dimorphic early human brain development. The relatively more "compact" female model may well relate to sex differences in neural circuitry and cognitive domains.

Endocrine-disrupting chemicals: an Endocrine Society scientific statement

There is growing interest in the possible health threat posed by endocrine-disrupting chemicals (EDCs), which are substances in our environment, food, and consumer products that interfere with hormone biosynthesis, metabolism, or action resulting in a deviation from normal homeostatic control or reproduction. In this first Scientific Statement of The Endocrine Society, we present the evidence that endocrine disruptors have effects on male and female reproduction, breast development and cancer, prostate cancer, neuroendocrinology, thyroid, metabolism and obesity, and cardiovascular endocrinology. Results from animal models, human clinical observations, and epidemiological studies converge to implicate EDCs as a significant concern to public health. The mechanisms of EDCs involve divergent pathways including (but not limited to) estrogenic, antiandrogenic, thyroid, peroxisome proliferator-activated receptor gamma, retinoid, and actions through other nuclear receptors; steroidogenic enzymes; neurotransmitter receptors and systems; and many other pathways that are highly conserved in wildlife and humans, and which can be modeled in laboratory in vitro and in vivo models. Furthermore, EDCs represent a broad class of molecules such as organochlorinated pesticides and industrial chemicals, plastics and plasticizers, fuels, and many other chemicals that are present in the environment or are in widespread use. We make a number of recommendations to increase understanding of effects of EDCs, including enhancing increased basic and clinical research, invoking the precautionary principle, and advocating involvement of individual and scientific society stakeholders in communicating and implementing changes in public policy and awareness.

Brain development, environment and sex: what can we learn from studying graviperception, gravitransduction and the gravireaction of the developing CNS to altered gravity?

As man embarks on space exploration and contemplates space habitation, there is a critical need for basic understanding of the impact of the environmental factors of space, and in particular gravity, on human survival, health, reproduction and development. This review summarizes our present knowledge on the effect of altered gravity on the developing CNS with respect to the response of the developing CNS to altered gravity (gravireaction), the physiological changes associated with altered gravity that could contribute to this effect (gravitransduction), and the possible mechanisms involved in the detection of altered gravity (graviperception). Some of these findings transcend gravitational research and are relevant to our understanding of the impact of environmental factors on CNS development on Earth.

New tricks by an old dogma: mechanisms of the Organizational/Activational Hypothesis of steroid-mediated sexual differentiation of brain and behavior

The hormonal regulation of sexual behavior has been the topic of study for over 50 years and yet controversies persist regarding the importance of early versus late events and the identity of the critical neural and cellular substrates. We have taken a mechanistic approach toward the masculinizing actions of the gonadal steroid estradiol, as a means to understand how organization of the neuroarchitechture during a perinatal sensitive period exerts enduring influences on adult behavior. We have identified important roles for prostaglandins, FAK and paxillin, PI3 kinase and glutamate, and determined that cell-to-cell signaling is a critical component of the early organizational events. We have further determined that the mechanisms mediating different components of sexual behavior are distinct and regionally specific. The multitude of mechanisms by which the steroid estradiol, exerts divergent effects on the developing nervous system provides for a multitude of phenotypes which can vary significantly both within and between the sexes.

Regulation of progestin receptors in medial amygdala: estradiol, phytoestrogens and sex

Exposure to estrogens during critical developmental periods and in adulthood affects sex differences in the brain. We examined the roles of estradiol (E2) and phytoestrogens, and their interactions, on potential sex differences in brain. We used aromatase knockout (ArKO) mice, which cannot produce endogenous estrogens, along with wild type (WT) littermates. Mice were gestated, raised and maintained on a diet either rich in phytoestrogens or a diet virtually void of soy-derived phytoestrogens. Adult males and females were gonadectomized and received implants filled with 17-beta-estradiol to induce progestin receptors (PR), while controls received empty implants. Mice were sacrificed five days later and brain sections containing the posterodorsal medial amygdala (MePD) were processed for PR immunoreactivity. Activation of sex differences in PR required adult E2 treatment. A diet high in phytoestrogens was required for expression of sex differences in PR after E2 treatment. Our data underscore the important contribution of dietary phytoestrogens for the development of sex differences in PR-ir in the adult mouse medial amygdala. We hypothesize that both aromatization of androgens to estrogens and dietary sources of additional estrogens are part of the normal requirement for sex differences in the rodent brain.

Progestin receptor is transiently expressed perinatally in neurons of the rat isocortex

Steroid hormones influence the development of numerous brain regions, including some that are not classically considered steroid-sensitive. For example, nuclear receptors for both androgen and estrogen have been detected in neonatal cortical cells. High levels of progestin binding and progestin receptor (PR) mRNA have also been reported in early perinatal isocortex. PR expression coincides with high levels of de novo progesterone produced within the cortex, suggesting that PR and its ligand influence the important developmental cortical processes occurring shortly after birth. In order to better understand the role PR plays in cortical development, we used the cellular-level resolution of immunohistochemistry and in situ hybridization (ISH) to characterize changes in perinatal PR expression within specific cortical lamina. PR immunoreactivity (PR-ir) was examined at embryonic days (E) 18, 20, 21, 22, and postnatal days (P) 1, 3, 6, 9, 13, and 27. We find that PR-ir is transiently expressed in specific lamina of frontal, parietal, temporal, and occipital cortex. PR-ir was observed in subplate cells on E18, in increasingly superficial lamina (primarily lamina V, then II/III) during early postnatal development, and was absent by P27. Double-labeling immunohistochemistry indicated that PR-ir colocalizes with the neuronal marker, microtubule associated protein-2, but not with the glial marker, nestin, nor with gamma-aminobutyric acid. These results suggest that specific subpopulations of cortical neurons may be transiently sensitive to progesterone, and that progesterone and its receptor may play a critical role in the fundamental mechanisms underlying normal cortical development.