Sex differences of hypothalamic prolactin cells develop independently of the presence of sex steroids

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

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

Sex-limited chromosomes and non-reproductive traits

Sex chromosomes are typically viewed as having originated from a pair of autosomes, and differentiated as the sex-limited chromosome (e.g. Y) has degenerated by losing most genes through cessation of recombination. While often thought that degenerated sex-limited chromosomes primarily affect traits involved in sex determination and sex cell production, accumulating evidence suggests they also influence traits not sex-limited or directly involved in reproduction. Here, we provide an overview of the effects of sex-limited chromosomes on non-reproductive traits in XY, ZW or UV sex determination systems, and discuss evolutionary processes maintaining variation at sex-limited chromosomes and molecular mechanisms affecting non-reproductive traits.

Article Commentary: Sex- and Gender-Based Medicine: The Need for Precise Terminology

As our knowledge of sex- and gender-based medicine (SGBM) continues to grow, attention to precision in the use of related terminology is critical. Unfortunately, the terms sex and gender are often used interchangeably and incorrectly, both within and outside of the typical binary construct. On behalf of the Sex and Gender Women's Health Collaborative (SGWHC), a national organization whose mission is the integration of SGBM into research, health professions education, and clinical practice, our objective was to develop recommendations for the accurate use of SGBM terminology in research and clinical practice across medical specialties and across health professions. In addition, we reviewed the origins and evolution of SGBM terminology and described terms used when referring to individuals outside the typical binary categorization of sex and gender. Standardization and precision in the use of sex and gender terminology will lead to a greater understanding and appropriate translation of sex and gender evidence to patient care along with an accurate assessment of the impact sex and gender have on patient outcomes. In addition, it is critical to acknowledge that SGBM terminology will continue to evolve and become more precise as our knowledge of sex and gender differences in health and disease progresses.

Oestradiol synthesized by female neurons generates sex differences in neuritogenesis

Testosterone produced by the foetal testis is converted by male neurons to oestradiol, which masculinizes neuronal morphology. Female neurons are known to synthesize oestradiol in absence of exogenous testosterone. However, the role of neuronal oestradiol on the differentiation of foetal female neurons is unknown. Here we show that, due to endogenous neuronal oestradiol synthesis, female hippocampal neurons have higher expression of the neuritogenic protein Neurogenin 3 and enhanced neuritogenesis than males. Exogenous application of testosterone or its metabolite dihydrotestosterone increases Neurogenin 3 expression and promotes neuritogenesis in males, but reduces these parameters in females. Together our data indicate that gonadal-independent oestradiol synthesis by female neurons participates in the generation of sex differences in hippocampal neuronal development.

Neurogenin 3 mediates sex chromosome effects on the generation of sex differences in hypothalamic neuronal development

The organizational action of testosterone during critical periods of development is the cause of numerous sex differences in the brain. However, sex differences in neuritogenesis have been detected in primary neuronal hypothalamic cultures prepared before the peak of testosterone production by fetal testis. In the present study we assessed the hypothesis of that cell-autonomous action of sex chromosomes can differentially regulate the expression of the neuritogenic gene neurogenin 3 (Ngn3) in male and female hypothalamic neurons, generating sex differences in neuronal development. Neuronal cultures were prepared from male and female E14 mouse hypothalami, before the fetal peak of testosterone. Female neurons showed enhanced neuritogenesis and higher expression of Ngn3 than male neurons. The silencing of Ngn3 abolished sex differences in neuritogenesis, decreasing the differentiation of female neurons. The sex difference in Ngn3 expression was determined by sex chromosomes, as demonstrated using the four core genotypes mouse model, in which a spontaneous deletion of the testis-determining gene Sry from the Y chromosome was combined with the insertion of the Sry gene onto an autosome. In addition, the expression of Ngn3, which is also known to mediate the neuritogenic actions of estradiol, was increased in the cultures treated with the hormone, but only in those from male embryos. Furthermore, the hormone reversed the sex differences in neuritogenesis promoting the differentiation of male neurons. These findings indicate that Ngn3 mediates both cell-autonomous actions of sex chromosomes and hormonal effects on neuritogenesis.

Growth hormone and prolactin regulate human neural stem cell regenerative activity

We have previously shown that the growth hormone (GH)/prolactin (PRL) axis has a significant role in regulating neuroprotective and/or neurorestorative mechanisms in the brain and that these effects are mediated, at least partly, via actions on neural stem cells (NSCs). Here, using NSCs with properties of neurogenic radial glia derived from fetal human forebrains, we show that exogenously applied GH and PRL promote the proliferation of NSCs in the absence of epidermal growth factor or basic fibroblast growth factor. When applied to differentiating NSCs, they both induce neuronal progenitor proliferation, but only PRL has proliferative effects on glial progenitors. Both GH and PRL also promote NSC migration, particularly at higher concentrations. Since human GH activates both GH and PRL receptors, we hypothesized that at least some of these effects may be mediated via the latter. Migration studies using receptor-specific antagonists confirmed that GH signals via the PRL receptor promote migration. Mechanisms of receptor signaling in NSC proliferation, however, remain to be elucidated. In summary, GH and PRL have complex stimulatory and modulatory effects on NSC activity and as such may have a role in injury-related recovery processes in the brain.

Pituitary growth hormone network responses are sexually dimorphic and regulated by gonadal steroids in adulthood

There are well-recognized sex differences in many pituitary endocrine axes, usually thought to be generated by gonadal steroid imprinting of the neuroendocrine hypothalamus. However, the recognition that growth hormone (GH) cells are arranged in functionally organized networks raises the possibility that the responses of the network are different in males and females. We studied this by directly monitoring the calcium responses to an identical GH-releasing hormone (GHRH) stimulus in populations of individual GH cells in slices taken from male and female murine GH-eGFP pituitary glands. We found that the GH cell network responses are sexually dimorphic, with a higher proportion of responding cells in males than in females, correlated with greater GH release from male slices. Repetitive waves of calcium spiking activity were triggered by GHRH in some males, but were never observed in females. This was not due to a permanent difference in the network architecture between male and female mice; rather, the sex difference in the proportions of GH cells responding to GHRH were switched by postpubertal gonadectomy and reversed with hormone replacements, suggesting that the network responses are dynamically regulated in adulthood by gonadal steroids. Thus, the pituitary gland contributes to the sexually dimorphic patterns of GH secretion that play an important role in differences in growth and metabolism between the sexes.

Neural mechanisms underlying sex-specific behaviors in vertebrates

From invertebrates to humans, males and females of a given species display identifiable differences in behaviors, mostly but not exclusively pertaining to sexual and social behaviors. Within a species, individuals preferentially exhibit the set of behaviors that is typical of their sex. These behaviors include a wide range of coordinated and genetically pre-programmed social and sexual displays that ensure successful reproductive strategies and the survival of the species. What are the mechanisms underlying sex-specific brain function? Although sexually dimorphic behaviors represent the most extreme examples of behavioral variability within a species, the basic principles underlying the sex specificity of brain activity are largely unknown. Moreover, with few exceptions, the quest for fundamental differences in male and female brain structures and circuits that would parallel that of sexual behaviors and peripheral organs has so far uncovered modest quantitative rather than the expected clear qualitative differences. As will be detailed in this review, recent advances have directly challenged the established notion of the unique role of steroid hormones in organizing and activating male- and female-specific brain circuits and have uncovered new mechanisms underlying the neural control of sex-specific behaviors.

Prolactin expression in the sheep brain

Accumulating evidence in rodents suggests that a prolactin locally synthesized and released within the brain can act together with that taken up from the circulation to modulate neuroendocrine responses. The present study was designed to identify the regional patterns of prolactin expression in the adult and developing sheep brain. Specifically, we tested the hypothesis that prolactin is expressed in regions of the adult and fetal sheep brain that are critical in the development of neuroendocrine homeostatic and behavioral functions. The expression of prolactin protein in sheep brain was demonstrated by Western blot analysis and brain prolactin mRNA was detected and sequenced using RT-PCR. In situ hybridization histochemistry revealed that prolactin mRNA was expressed in the medial preoptic area, periventricular preoptic nucleus, bed nucleus of the stria terminalis, and in the paraventricular nucleus of the hypothalamus, particularly the ventral region. The neuroanatomical distribution of prolactin mRNA was best visualized in the fetus and prolactin-immunoreactive neurons could also be identified in late gestation fetuses. Brain prolactin mRNA was expressed as early as day 60 of gestation and increased as the fetus aged and peaked at day 135 (term = 147 days). Prolactin mRNA expression did not exhibit a sex difference in the preoptic area, but in the amygdala prolactin mRNA was significantly higher in females than in males at day 100 of gestation. We conclude that prolactin expressed in adult and fetal sheep brain could be involved in neurodevelopment and/or modulation of the neuroendocrine stress axis, although it is too early to rule out other possibilities given the diverse actions that have been attributed to prolactin.

Estrogen receptors: how do they signal and what are their targets

During the past decade there has been a substantial advance in our understanding of estrogen signaling both from a clinical as well as a preclinical perspective. Estrogen signaling is a balance between two opposing forces in the form of two distinct receptors (ER alpha and ER beta) and their splice variants. The prospect that these two pathways can be selectively stimulated or inhibited with subtype-selective drugs constitutes new and promising therapeutic opportunities in clinical areas as diverse as hormone replacement, autoimmune diseases, prostate and breast cancer, and depression. Molecular biological, biochemical, and structural studies have generated information which is invaluable for the development of more selective and effective ER ligands. We have also become aware that ERs do not function by themselves but require a number of coregulatory proteins whose cell-specific expression explains some of the distinct cellular actions of estrogen. Estrogen is an important morphogen, and many of its proliferative effects on the epithelial compartment of glands are mediated by growth factors secreted from the stromal compartment. Thus understanding the cross-talk between growth factor and estrogen signaling is essential for understanding both normal and malignant growth. In this review we focus on several of the interesting recent discoveries concerning estrogen receptors, on estrogen as a morphogen, and on the molecular mechanisms of anti-estrogen signaling.

Tissue-specific expression and regulation of sexually dimorphic genes in mice

We report a comprehensive analysis of gene expression differences between sexes in multiple somatic tissues of 334 mice derived from an intercross between inbred mouse strains C57BL/6J and C3H/HeJ. The analysis of a large number of individuals provided the power to detect relatively small differences in expression between sexes, and the use of an intercross allowed analysis of the genetic control of sexually dimorphic gene expression. Microarray analysis of 23,574 transcripts revealed that the extent of sexual dimorphism in gene expression was much greater than previously recognized. Thus, thousands of genes showed sexual dimorphism in liver, adipose, and muscle, and hundreds of genes were sexually dimorphic in brain. These genes exhibited highly tissue-specific patterns of expression and were enriched for distinct pathways represented in the Gene Ontology database. They also showed evidence of chromosomal enrichment, not only on the sex chromosomes, but also on several autosomes. Genetic analyses provided evidence of the global regulation of subsets of the sexually dimorphic genes, as the transcript levels of a large number of these genes were controlled by several expression quantitative trait loci (eQTL) hotspots that exhibited tissue-specific control. Moreover, many tissue-specific transcription factor binding sites were found to be enriched in the sexually dimorphic genes.

The genetic basis for sex differences in human behaviour: role of the sex chromosomes

The nature of the mechanisms underlying observed sex differences in human behaviour continues to be debated. This review concentrates on the thesis that genes on the sex chromosomes other than those directly controlling sex determination, and whose functions are, at least in part, independent from hormonal influences, play a significant role in determining gender differences in behaviour. To provide an adequate basis for examining this issue, the current understanding of the nature of sex determination, differences in behaviour and the influences of sex hormones are evaluated. The possible contribution to behavioural differences of those X-linked genes which escape inactivation, or which may be subjected to imprinting, is discussed. The review concludes with a summary of the genetic basis for two sexually disparate types of behaviour.

Are XX and XY brain cells intrinsically different?

In mammals and birds, the sex of the gonads is determined by genes on the sex chromosomes. For example, the mammalian Y-linked gene Sry causes testis differentiation. The testes then secrete testosterone, which acts on the brain (often after conversion to estradiol) to cause masculine patterns of development. If this were the only reason for sex differences in neural development, then XX and XY brain cells would have to be deemed otherwise equivalent. This equivalence is doubtful because of recent experimental results demonstrating that some XX and XY tissues, including the brain, are sexually dimorphic even when they develop in a similar endocrine environment. Although X and Y genes probably influence brain phenotype in a sex-specific manner, much more information is needed to identify the magnitude and character of these effects.

Sex differences in mouse cortical thickness are independent of the complement of sex chromosomes

Although the morphology of the cerebral cortex is known to be sexually dimorphic in several species, to date this difference has not been investigated in mice. The present study is the first to report that the mouse cerebral cortex is thicker in males than in females. We further asked if this sex difference is the result of gonadal hormones, or alternatively is induced by a direct effect of genes encoded on the sex chromosomes. The traditional view of mammalian neural sexual differentiation is that androgens or their metabolites act during early development to masculinize the brain, whereas a feminine brain develops in the relative absence of sex steroids. We used mice in which the testis determination gene Sry was inherited independently from the rest of the Y chromosome to produce XX animals that possessed either ovaries or testes, and XY animals that possessed either testes or ovaries. Thus, the design allowed assessment of the role of sex chromosome genes, independent of gonadal hormones, in the ontogeny of sex differences in the mouse cerebral cortex. When a sex difference was present, mice possessing testes were invariably masculine in the morphology of the cerebral cortex, independent of the complement of their sex chromosomes (XX vs. XY), and mice with ovaries always displayed the feminine phenotype. These data suggest that sex differences in cortical thickness are under the control of gonadal steroids and not sex chromosomal complement. However, it is unclear whether it is the presence of testicular secretions or the absence of ovarian hormones that is responsible for the thicker male cerebral cortex.

A model system for study of sex chromosome effects on sexually dimorphic neural and behavioral traits

We tested the hypothesis that genes encoded on the sex chromosomes play a direct role in sexual differentiation of brain and behavior. We used mice in which the testis-determining gene (Sry) was moved from the Y chromosome to an autosome (by deletion of Sry from the Y and subsequent insertion of an Sry transgene onto an autosome), so that the determination of testis development occurred independently of the complement of X or Y chromosomes. We compared XX and XY mice with ovaries (females) and XX and XY mice with testes (males). These comparisons allowed us to assess the effect of sex chromosome complement (XX vs XY) independent of gonadal status (testes vs ovaries) on sexually dimorphic neural and behavioral phenotypes. The phenotypes included measures of male copulatory behavior, social exploration behavior, and sexually dimorphic neuroanatomical structures in the septum, hypothalamus, and lumbar spinal cord. Most of the sexually dimorphic phenotypes correlated with the presence of ovaries or testes and therefore reflect the hormonal output of the gonads. We found, however, that both male and female mice with XY sex chromosomes were more masculine than XX mice in the density of vasopressin-immunoreactive fibers in the lateral septum. Moreover, two male groups differing only in the form of their Sry gene showed differences in behavior. The results show that sex chromosome genes contribute directly to the development of a sex difference in the brain.

Differential effect of oestradiol and astroglia-conditioned media on the growth of hypothalamic neurons from male and female rat brains

To determine whether soluble products from different CNS regions differ in their ability to support oestrogen-stimulated neurite growth, hypothalamic neurons from sexually segregated embryos were cultured with astroglia-conditioned medium (CM) derived from cortex, striatum and mesencephalon, with or without 17-beta-oestradiol 100 nM added to the medium. After 48 h in vitro, neurite outgrowth was quantified by morphometric analysis. Astroglia-CM from mesencephalon (a target for the axons of hypothalamic neurons) induced the greatest axogenic response in males and in this case only a neuritogenic effect could be demonstrated for oestradiol. On the other hand, astroglia-CM from regions that do not receive projections from ventromedial hypothalamus inhibited axon growth. A sexual difference in the response of hypothalamic neurons to astroglia-CM and oestradiol was found; growth of neurons from female foetuses was increased by astroglia-CM from mesencephalon, but no neuritogenic effect could be demonstrated for oestradiol in these cultures. Blot immunobinding demonstrated the presence of receptors for neurotrophic factors in cultures of hypothalamic neurons; Western blot analysis of these cultures demonstrated that oestradiol increased the concentration of trkB and IGF-I Rbeta, whereas trkA was not detected and the concentration of trkC was not modified. These results support the hypothesis that target regions produce some factor(s) that stimulate the growth of axons from projecting neurons and further indicate that in the case of males this effect is modulated by oestradiol, perhaps mediated through the upregulation of trkB and IGF-I receptors.

Isolation of sex-specific cDNAs from fetal mouse brain using mRNA differential display and representational difference analysis

Comparing female and male brain structures reveals a variety of sex differences in many vertebrates. These differences are manifested throughout the brain, in regions such as the hypothalamus, the preoptic area and the amygdala. Some are thought to be induced during the fetal period by the effect of steroid hormones produced in the gonads. It is well-established that fetal androgens, probably through the conversion to estrogen by the enzyme aromatase, masculinize the nervous system and set adult mounting behavior in rodents. However, less is known about molecular mechanisms involved in gender-specific development of the brain. We have taken a broad approach to isolate sex-specific genes from fetal brain. mRNAs from 18.5 days post-coitum (dpc) female and male mouse brain were screened with the classical and the recently developed signal peptide differential display (SPDD) and with representational difference analysis of cDNA (cDNA-RDA). Two sex-specific cDNAs were isolated, F29 and M17, corresponding to the female-specific Xist gene and the male-specific Smcy gene, respectively.

Gender differences in brain and behavior: hormonal and neural bases

This article briefly discusses the difficulties in determining the brain-behavior relationship and reviews the literature on some potential mechanisms underlying gender differences in behavioral responses. Mechanisms that are discussed include genetic effects, organizational effects of gonadal hormones, genomic actions of steroids, nongenomic effects of steroids, and environmental influences. The review is an introduction to the articles presented in this special volume on gender differences in brain and behavior.

Gender-specific steroid metabolism in neural differentiation

1. Both the neuroendocrine system and the brain mechanisms underlying gender-specific behavior are known to be organized by steroid sex hormones, androgen and estrogen, during specific sensitive phases of early fetal and perinatal development. The factors that control these phasic effects of the hormones on brain development are still not understood. Processes of masculinization and defeminization are thought to be involved in the sex differentiation of mammalian reproductive behavior. 2. The P450 aromatase, converting androgen to estrogen, is a key enzyme in the development of neural systems, and the activity of this enzyme is likely to be one of the factors determining brain sex differentiation. 3. We have examined the localization and regulation of brain aromatase using the mouse as a model. Measurement of testosterone conversion to estradiol-17 beta, using a sensitive radiometric 3H2O assay, indicates that estrogens are formed more actively in the male mouse brain than in the female during both the prenatal and the neonatal periods. In primary cell cultures of embryonic mouse hypothalamus there are sex differences in aromatase activity during early and late embryogenesis, with a higher capacity for estrogen formation in the male than the female. These sex differences are regionally specific in the brain, since on gender differences in aromatase activity are detectable in cortical cells. 4. Aromatase activity in the mouse brain is neuronal rather than glial. Using a specific antibody to the mouse aromatase, immunoreactivity is restricted to neuronal soma and neurites in hypothalamic cultures. There are more neurons containing expressed aromatase in the male hypothalamus than in the female. Therefore, gender-specific differences in embryonic aromatase activity are neuronal. 5. Testosterone increases aromatase activity specifically in hypothalamic neurons, but has no effect on cortical cells. The neuronal aromatase activity appears to be sensitive to the inductive effects of androgen only in the later stages of embryonic development. Androgen also increases the numbers of aromatase-immunoreactive neurons in the hypothalamus. 6. This work suggests that the embryonic male hypothalamus and other androgen target areas contain a network of neurons which has the capacity to provide estrogen for the sexual differentiation of brain mechanisms of behavior. The phasic activity of the key enzyme, aromatase, during development is influenced by androgen. What determines the developmental action of androgen and the other factors involved in the regulation and expression of this neuronal enzyme still have to be established.

Experimental analysis of sexual differentiation of the zebra finch brain

Classical theories of sexual differentiation of brain and behavior hold that sex differences in the brain arise because of the action of gonadal steroid hormones. In mammals, testosterone secretion by the testes stimulates a masculine pattern of neural differentiation, whereas feminine patterns of development occur in the absence of testicular secretions. In some bird species, estrogen secreted by the ovary is thought to trigger feminine patterns of neural development, whereas masculine development occurs in the absence of ovaries. Sexual differentiation of the neural circuit for song in zebra finches is not easily explained by these theories. Although female zebra finches can be masculinized by treatments with estrogen, it has proven difficult to prevent masculine neural development in genetic males by treating them with inhibitors of estrogen synthesis. Moreover, when genetic female embryos are treated with inhibitors of estrogen synthesis, they develop significant amounts of testicular tissue that causes little or no masculinization of the song system. Thus, testicular secretions alone appear to be insufficient to cause masculine neural differentiation, and other factors need to be invoked. These factors may include ovarian secretions that inhibit masculine development, or direct genetic (nonhormonal) effects on neural differentiation.

Human dermal fibroblast cells express prolactin in vitro

This study demonstrates the synthesis and release of prolactin (PRL) from dermal fibroblasts (>98%) in vitro, suggesting a potential local source of PRL in skin. PRL release was first detected in confluent cultures (0.25 x 10(6) plated cells) on or before day 18 and increased to a maximal level of 2 ng/72 h by day 30. Medroxyprogesterone acetate and estradiol (E2) had no effect on PRL release, but prostaglandin E2 (PGE2) reduced the time required for PRL induction to 6-9 days. The steroids and PGE2 together were synergistic, reaching maximal values of approximately 10 ng/72 h after 2 or more weeks of treatment. Dibutyryl-cyclic AMP, a second messenger in prostaglandin signal transduction, was also synergistic with medroxyprogesterone acetate and E2, but induced significant PRL expression in the absence of the steroids (28 and 12 ng/72 h, respectively). The increase in PRL release was not a result of increased cell proliferation, because the PRL-secreting cultures had 32.2 +/- 8.8% less DNA (N = 3 individuals, 93% confidence limit) than control cultures after 3 weeks of treatment with dibutyryl-cyclic AMP, medroxyprogesterone acetate, and E2. Dermal fibroblast PRL was immunologically and electrophoretically identical to decidual and pituitary PR-Ls, and Northern blot analysis demonstrated a PRL mRNA size of 1.15 kb. Maximal PRL release from fibroblast cells was 32.0 +/- 6.1 ng/72 h (mean +/- SD at 95% confidence limit) for a donor population representing both males (n = 15) and females (n = 7) between the ages of 20-week gestation to 52 years. In contrast to term decidual fibroblast cells that also express PRL, dermal fibroblasts did not co-express insulin-like growth factor-binding protein-1.

Roles of steroid hormones and their receptors in structural organization in the nervous system

Due to their chemical properties, steroid hormones cross the blood-brain barrier where they have profound effects on neuronal development and reorganization both in invertebrates and vertebrates, including humans mediated through their receptors. Steroids play a crucial role in the organizational actions of cellular differentiation representing sexual dimorphism and apoptosis, and in the activational effects of phenotypic changes in association with structural plasticity. Their sites of action are primarily the genes themselves but some are coupled with membrane-bound receptor/ion channels. The effects of steroid hormones on gene transcription are not direct, and other cellular components interfere with their receptors through cross-talk and convergence of the signaling pathways in neurons. These genomic and non-genomic actions account for the divergent effects of steroid hormones on brain function as well as on their structure. This review looks again at and updates the tremendous advances made in recent decades on the study of the role of steroid (gonadal and adrenal) hormones and their receptors on developmental processes and plastic changes in the nervous system.

Neuritogenic effect of estradiol on rat ventromedial hypothalamic neurons co-cultured with homotopic or heterotopic glia

In sexually segregated cultures of dissociated neurons taken from ventromedial hypothalamus of rat fetuses at embryonic day 16 (E16), it is demonstrated that only neurons from males respond with increased axonal growth to the addition of 17-beta-estradiol 100 nM (E2) to the culture medium. Moreover, this response is contingent upon co-culture with heterotopic glia from a target region (amygdala), whereas in the presence of homotopic glia or in cultures without glia, E2 has no effect. It is concluded that before neurons are exposed to gonadal steroids in utero there is a sexual difference in the response to E2, probably explained by earlier maturation of neurons from males as compared to females. The possibility that the observed axogenic effect may be the consequence of an interaction among E2, cells equipped with specific receptors, and glia-producing trophic factors is discussed.

Sexual differentiation of brain and behavior in quail and zebra finches: studies with a new aromatase inhibitor, R76713

In many species of vertebrates, major sex differences affect reproductive behavior and endocrinology. Most of these differences do not result from a direct genomic action but develop following early exposure to a sexually differentiated endocrine milieu. In rodents, the female reproductive phenotype mostly develops in the absence of early steroid influence and male differentiation is imposed by the early action of testosterone, acting at least in part through its central conversion into estrogens or aromatization. This pattern of differentiation does not seem to be applicable to avian species. In Japanese quail (Coturnix japonica), injection of estrogens into male embryos causes a permanent loss of the capacity to display male-type copulatory behavior when exposed to testosterone in adulthood. Based on this experimental result, it was proposed that the male reproductive phenotype is "neutral" in birds (i.e. develops in the absence of endocrine influence) and that endogenous estradiol secreted by the ovary of the female embryo is responsible for the physiological demasculinization of females. This model could be recently confirmed. Females indeed display a higher level of circulating estrogens that males during the second part of their embyronic life. In addition, treatment of female embryos with the potent aromatase inhibitor, R76713 or racemic vorozole which suppresses the endogenous secretion of estrogens maintains in females the capacity to display the full range of male copulatory behaviors. The brain mechanisms that control this sexually differentiated behavior have not been identified so far but recent data suggest that they should primarily concern a sub-population of aromatase-immunoreactive neurons located in the lateral parts of the sexually dimorphic preoptic nucleus. The zebra finch (Taeniopygia guttata) exhibits a more complex, still partly unexplained, differentiation pattern. In this species, early treatment with exogenous estrogens produces a masculinization of singing behavior in females and a demasculinization of copulatory behavior in males.(ABSTRACT TRUNCATED AT 400 WORDS)

Aromatase-immunoreactivity is localised specifically in neurones in the developing mouse hypothalamus and cortex

Local formation of oestrogens from androgens by aromatase cytochrome P-450 within brain cells is crucial for the sexual differentiation of the mammalian CNS. Aromatase activity has been detected in several brain regions of the developing rodent brain. In the present study, we used a mouse-specific, peptide-generated, polyclonal aromatase antibody to determine whether neurones and/or glial cells in the developing brain are involved in androgen aromatization and if aromatase-immunoreactive (Arom-IR) cells exhibit a sex-specific distribution and regional-specific morphological characteristics. For these experiments, gender-specific cell cultures were prepared from embryonic day 15 mouse hypothalamus and cortex. Specificity of the immunoreaction was confirmed by Western-blot analysis and by inhibition of aromatase activity using tissue homogenates from mouse ovaries and male newborn hypothalamus and from male hypothalamic cultures with known aromatase activity, respectively. Arom-IR cells were found in both hypothalamic and cortical cultures. Double-labeling experiments revealed that Arom-IR cells co-stained only for the neuronal marker MAP II, but never for glial markers. Therefore aromatase immunoreactivity is specifically neuronal. Regional differences in the morphology of Arom-IR neurones were observed between both brain regions. In hypothalamic cultures, IR-neurones represented a heterologous population of phenotypes (magnocellular, small bipolar and multipolar neurones with long processes showing varicose-like structures or without processes). Cortical Arom-IR neurones were always oval in shape with short or no IR-processes. Sexual dimorphisms in numbers of Arom-IR neurones were found in the hypothalamus with significantly higher cell numbers in male cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Gender-specific brain formation of oestrogen in behavioural development

Steroid sex hormones have an organisational role in the development of brain mechanisms underlying gender-specific behaviour. Although peaks in gonadal androgen occur at developmental stages that coincide with sensitive periods for the differentiation of both structural sex differences in the brain and sexual behaviour, the factors that control the phasic effects of steroids are still not understood. Aromatase, converting androgen to oestrogen, is a key enzyme in development, and regulation of the activity of this enzyme is likely to be one of the factors determining availability of oestrogen effective for brain differentiation. Measurement of testosterone metabolism in vitro shows that in the mouse oestrogens are formed actively in the neonatal brain during male development. In cultured cells of the embryonic mouse hypothalamus there are sex differences in hypothalamic aromatase activity both during early embryonic and later perinatal development, with a higher capacity for oestrogen formation in the male than in the female. The sex differences are regionally specific, since no differences in aromatase activity are detectable in cultured cortical cells between male and female. Aromatase activity is neuronal rather than astroglial. Using a specific antibody to the mouse aromatase, immunoreactivity is also restricted to neuronal soma and neurites in hypothalamic cultures. Therefore, gender-specific differences in aromatase regulation are probably restricted to neurons. Testosterone increases oestrogen formation specifically in cultured hypothalamic neurones, but has no effect on cortical cells. Although there is a sex difference in early embryonic neuronal aromatase, aromatase activity appears to be sensitive to androgen only in later embryonic development. What determines the phasic sensitivity of the developing brain aromatase system to androgen has still to be determined.

Aromatase-immunoreactive neurons in the adult female chicken brain detected using a specific antibody

Estrogen formation in the brain catalysed by the cytochrome P450arom is required for the control of estrogen-dependent neural mechanisms regulating reproductive behaviour. A polyclonal antibody was raised against a 15-amino acid fragment of the chicken ovarian P450arom protein, to localise aromatase-immunoreactive (AR-IR) cells in the adult female chicken brain. Specificity of antibody reaction was established by Western blot and by inhibition of aromatase activity in homogenates of chicken ovarian follicles determined by a radiometric assay. The AR-IR material in the brain was localised in the perikarya and some of their adjacent cytoplasmatic processes. Intense immunoreactivity was observed in the preoptic region as well as in other hypothalamic nuclei. AR-IR cells were also found in extrahypothalamic areas; in particular, in the area entorhinalis and hippocampus. These results confirm histologically that aromatization of testosterone in the adult female chicken brain occurs in preoptic nuclei closely associated with the regulation of reproductive behaviour. The mapping of AR-IR cells in the female chicken brain now allows study of its regulation under different physiological and environmental conditions, and its relation to classic target areas expressing estrogen receptors.