Sex differences in the brain: the relation between structure and function

Sexually dimorphic effects of a prenatal immune challenge on social play and vasopressin expression in juvenile rats

Background

Infectious diseases and inflammation during pregnancy increase the offspring’s risk for behavioral disorders. However, how immune stress affects neural circuitry during development is not well known. We tested whether a prenatal immune challenge interferes with the development of social play and with neural circuits implicated in social behavior.

Methods

Pregnant rats were given intraperitoneal injections of the bacterial endotoxin lipopolysaccharide (LPS – 100 μg /kg) or saline on the 15th day of pregnancy. Offspring were tested for social play behaviors between postnatal days 26–40. Brains were harvested on postnatal day 45 and processed for arginine vasopressin (AVP) mRNA in situ hybridization.

Results

In males, LPS treatment reduced the frequency of juvenile play behavior and reduced AVP mRNA expression in the medial amygdala and bed nucleus of the stria terminalis. These effects were not found in females. LPS treatment did not change AVP mRNA expression in the suprachiasmatic nucleus, paraventricular nucleus, or supraoptic nucleus of either sex, nor did it affect the sex difference in the size of the sexually dimorphic nucleus of the preoptic area.

Conclusions

Given AVP’s central role in regulating social behavior, the sexually dimorphic effects of prenatal LPS treatment on male AVP mRNA expression may contribute to the sexually dimorphic effect of LPS on male social play and may, therefore, increase understanding of factors that contribute to sex differences in social psychopathology.

Sex differences in behavioral decision-making and the modulation of shared neural circuits

Animals prioritize behaviors according to their physiological needs and reproductive goals, selecting a single behavioral strategy from a repertoire of possible responses to any given stimulus. Biological sex influences this decision-making process in significant ways, differentiating the responses animals choose when faced with stimuli ranging from food to conspecifics. We review here recent work in invertebrate models, including C. elegans, Drosophila, and a variety of insects, mollusks and crustaceans, that has begun to offer intriguing insights into the neural mechanisms underlying the sexual modulation of behavioral decision-making. These findings show that an animal's sex can modulate neural function in surprisingly diverse ways, much like internal physiological variables such as hunger or thirst. In the context of homeostatic behaviors such as feeding, an animal's sex and nutritional status may converge on a common physiological mechanism, the functional modulation of shared sensory circuitry, to influence decision-making. Similarly, considerable evidence suggests that decisions on whether to mate or fight with conspecifics are also mediated through sex-specific neuromodulatory control of nominally shared neural circuits. This work offers a new perspective on how sex differences in behavior emerge, in which the regulated function of shared neural circuitry plays a crucial role. Emerging evidence from vertebrates indicates that this paradigm is likely to extend to more complex nervous systems as well. As men and women differ in their susceptibility to a variety of neuropsychiatric disorders affecting shared behaviors, these findings may ultimately have important implications for human health.

Gender development and the human brain

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

Male or Female? Brains are Intersex

The underlying assumption in popular and scientific publications on sex differences in the brain is that human brains can take one of two forms “male” or “female,” and that the differences between these two forms underlie differences between men and women in personality, cognition, emotion, and behavior. Documented sex differences in brain structure are typically taken to support this dimorphic view of the brain. However, neuroanatomical data reveal that sex interacts with other factors in utero and throughout life to determine the structure of the brain, and that because these interactions are complex, the result is a multi-morphic, rather than a dimorphic, brain. More specifically, here I argue that human brains are composed of an ever-changing heterogeneous mosaic of “male” and “female” brain characteristics (rather than being all “male” or all “female”) that cannot be aligned on a continuum between a “male brain” and a “female brain.” I further suggest that sex differences in the direction of change in the brain mosaic following specific environmental events lead to sex differences in neuropsychiatric disorders.

Testostérone et contrôle central de l’érection

La testostérone orchestre l’organisation périnatale et l’activation adulte des structures nerveuses cérébrales et spinales impliquées dans l’expression du comportement sexuel mâle. Cette revue décrit brièvement les différents effets de la testostérone dans la régulation de la motivation sexuelle et de l’érection, et les modèles génétiques générés, jusqu’à présent, dans le but d’élucider ses mécanismes d’action centraux.

Social status affects the degree of sex difference in the songbird brain

It is thought that neural sex differences are functionally related to sex differences in the behaviour of vertebrates. A prominent example is the song control system of songbirds. Inter-specific comparisons have led to the hypothesis that sex differences in song nuclei size correlate with sex differences in song behaviour. However, only few species with similar song behaviour in both sexes have been investigated and not all data fit the hypothesis. We investigated the proposed structure – function relationship in a cooperatively breeding and duetting songbird, the white-browed sparrow weaver (Plocepasser mahali). This species lives in groups of 2–10 individuals, with a dominant breeding pair and male and female subordinates. While all male and female group members sing duet and chorus song, a male, once it has reached the dominant position in the group, sings an additional type of song that comprises a distinct and large syllable repertoire. Here we show for both types of male – female comparisons a male-biased sex difference in neuroanatomy of areas of the song production pathway (HVC and RA) that does not correlate with the observed polymorphism in song behaviour. In contrast, in situ hybridisation of mRNA of selected genes expressed in the song nucleus HVC reveals a gene expression pattern that is either similar between sexes in female – subordinate male comparisons or female-biased in female – dominant male comparisons. Thus, the polymorphic gene expression pattern would fit the sex- and status-related song behaviour. However, this implies that once a male has become dominant it produces the duetting song with a different neural phenotype than subordinate males.

Sex differences in aromatase gene expression in the medaka brain

The brain of teleost fish exhibits a significant degree of sexual plasticity, even in adulthood. This unique feature is almost certainly attributable to a teleost-specific sexual differentiation process of the brain, which remains largely unknown. To dissect the molecular basis of sexual differentiation of the teleost brain, we searched for genes differentially expressed between both sexes in the medaka brain. One gene identified in the screen, cyp19a1b, which encodes the steroidogenic enzyme aromatase, was selected for further analysis. As opposed to the situation in most vertebrates, medaka cyp19a1b is expressed at higher levels in the adult female brain than the male brain. The female-biased expression in the brain is consistent regardless of reproductive or diurnal cycle. Medaka cyp19a1b is expressed throughout the ventricular zones in wide areas of the brain, where, in most regions, females have a greater degree of expression compared to males, with the optic tectum exhibiting the most conspicuous predominance in females. Contrary to what is known in mammals, cyp19a1b expression exhibits neither a transient elevation nor a sex difference in medaka embryos. It is not until just before the onset of puberty that cyp19a1b expression in the medaka brain is sexually differentiated. Finally, cyp19a1b expression in the medaka brain is not under the direct control of sex chromosome genes but relies mostly, if not solely, on oestrogen derived from the gonad. These unique properties of aromatase expression in the brain probably contribute substantially to the less rigid sexual differentiation process, thus ensuring remarkable sexual plasticity in the teleost brain.

Sexual differentiation of human behavior: effects of prenatal and pubertal organizational hormones

A key question concerns the extent to which sexual differentiation of human behavior is influenced by sex hormones present during sensitive periods of development (organizational effects), as occurs in other mammalian species. The most important sensitive period has been considered to be prenatal, but there is increasing attention to puberty as another organizational period, with the possibility of decreasing sensitivity to sex hormones across the pubertal transition. In this paper, we review evidence that sex hormones present during the prenatal and pubertal periods produce permanent changes to behavior. There is good evidence that exposure to high levels of androgens during prenatal development results in masculinization of activity and occupational interests, sexual orientation, and some spatial abilities; prenatal androgens have a smaller effect on gender identity, and there is insufficient information about androgen effects on sex-linked behavior problems. There is little good evidence regarding long-lasting behavioral effects of pubertal hormones, but there is some suggestion that they influence gender identity and perhaps some sex-linked forms of psychopathology, and there are many opportunities to study this issue.

The genetics of sex differences in brain and behavior

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

Cooperation of sex chromosomal genes and endocrine influences for hypothalamic sexual differentiation

There is little debate that mammalian sexual differentiation starts from the perspective of two primary sexes that correspond to differential sex chromosomes (X versus Y) that lead to individuals with sex typical characteristics. Sex steroid hormones account for most aspects of brain sexual differentiation, however, a growing literature has raised important questions about the role of sex chromosomal genes separate from sex steroid actions. Several important model animals are being used to address these issues and, in particular, they are taking advantage of molecular genetic approaches using different mouse strains. The current review examines the cooperation of genetic and endocrine influences from the perspective of behavioral and morphological hypothalamic sexual differentiation, first in adults and then in development. In the final analysis, there is an ongoing need to account for the influence of hormones in the context of underlying genetic circumstances and null hormone conditions.

A lumpers versus splitters approach to sexual differentiation of the brain

Over 50 years of rigorous empirical attention to the study of sexual differentiation of the brain has produced sufficient data to reveal fundamental guiding principles, but has also required the generation of new hypotheses to explain non-conforming observations. An early emphasis on the powerful impact and essential role of gonadal steroids is now complemented by an appreciation for genetic contributions to sex differences in the brain. The organizing effects of early steroid hormones on reproductively relevant brain regions and endpoints are largely dependent upon neuronal aromatization of androgens to estrogens. The effect of estradiol is mediated via estrogen receptors (ER). The presence or absence of ER can restrict hormone action to select cells and either prevent or invoke cell death. Alternatively, ER activation can initiate signaling cascades that induce cell-to-cell communication and thereby transduce organizational steroid effects to large numbers of cells. However, the specific details by which cell death and cell-to-cell communication are achieved appear to be locally, even cellularly, unique and specific to that particular subpopulation. As the field moves forward the increasingly specific and detailed elucidation of mechanism challenges us to generate new guiding principles in order to gain a holistic understanding of how the brain develops in males and females.

Prepubertal gonadotropin-releasing hormone analog leads to exaggerated behavioral and emotional sex differences in sheep

In mammals, sex specialization is reflected by differences in brain anatomy and function. Measurable differences are documented in reproductive behavior, cognition, and emotion. We hypothesized that gonadotropin-releasing hormone (GnRH) plays a crucial role in controlling the extent of the brain's sex specificity and that changes in GnRH action during critical periods of brain development, such as puberty, will result in altered sex-specific behavioral and physiological patterns. We blocked puberty in half of the 48 same-sex Scottish mule Texel cross sheep twins with GnRH analog (GnRHa) goserelin acetate every 3 weeks, beginning just before puberty. To determine the effects of GnRHa treatment on sex-specific behavior and emotion regulation in different social contexts, we employed the food acquisition task (FAT) and measurement of heart rate variability (HRV). ANOVA revealed significant sex and sex×treatment interaction effects, suggesting that treated males were more likely to leave their companions to acquire food than untreated, while the opposite effect was observed in females. Concordant results were seen in HRV; treated males displayed higher HRV than untreated, while the reverse pattern was found in females, as shown by significant sex and sex×treatment interaction effects. We conclude that long-term prepubertal GnRHa treatment significantly affected sex-specific brain development, which impacted emotion and behavior regulation in sheep. These results suggest that GnRH is a modulator of cognitive function in the developing brain and that the sexes are differentially affected by GnRH modulation.

Cell death and sexual differentiation of behavior: worms, flies, and mammals

Sex differences in the nervous system are found throughout the animal kingdom. Here, we discuss three prominent genetic models: nematodes, fruit flies, and mice. In all three, differential cell death is central to sexual differentiation and shared molecular mechanisms have been identified. Our knowledge of the precise function of neural sex differences lags behind. One fruitful approach to the 'function' question is to contrast sexual differentiation in standard laboratory animals with differentiation in species exhibiting unique social and reproductive organizations. Advanced genetic strategies are also addressing this question in worms and flies, and may soon be applicable to vertebrates.

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.

Coming of age in the kisspeptin era: sex differences, development, and puberty

The status of the neuroendocrine reproductive axis differs dramatically during various stages of development, and also differs in several critical ways between the sexes, including its earlier pubertal activation in females than males and the presence of neural circuitry that generates preovulatory hormone surges in females but not males. The reproductive axis is controlled by various hormonal and neural pathways that converge upon forebrain gonadotropin-releasing hormone (GnRH) neurons, and many of the critical age and sex differences in the reproductive axis likely reflect differences in the "upstream" circuits and factors that regulate the GnRH system. Recently, the neural kisspeptin system has been implicated as an important regulator of GnRH neurons. Here I discuss the evidence supporting a critical role of kisspeptin signaling at different stages of life, including early postnatal and pubertal development, as well as in adulthood, focusing primarily on information gleaned from mammalian studies. I also evaluate key aspects of sexual differentiation and development of the brain as it relates to the Kiss1 system, with special emphasis on rodents. In addition to discussing recent advances in the field of kisspeptin biology, this paper will highlight a number of unanswered questions and future challenges for kisspeptin investigators, and will stress the importance of studying the kisspeptin system in both males and females, as well as in multiple species.

Vasopressin cell groups exhibit strongly divergent responses to copulation and male-male interactions in mice

Arginine vasopressin (AVP) and its nonmammalian homolog arginine vasotocin influence social behaviors ranging from affiliation to resident-intruder aggression. Although numerous sites of action have been established for these behavioral effects, the involvement of specific AVP cell groups in the brain is poorly understood, and socially elicited Fos responses have not been quantified for many of the AVP cell groups found in rodents. Surprisingly, this includes the AVP population in the posterior part of the medial bed nucleus of the stria terminalis (BSTMP), which has been extensively implicated, albeit indirectly, in various aspects of affiliation and other social behaviors. We examined the Fos responses of eight hypothalamic and three extra-hypothalamic AVP-immunoreactive (-ir) cell groups to copulation, nonaggressive male-male interaction, and aggressive male-male interaction in both dominant and subordinate C57BL/6J mice. The BSTMP cells exhibited a response profile that was unlike all other cell groups: from a control baseline of approximately 5% of AVP-ir neurons colocalizing with Fos, colocalization increased significantly to approximately 12% following nonaggressive male-male interaction, and to approximately 70% following copulation. Aggressive interactions did not increase colocalization beyond the level observed in nonaggressive male mice. These results suggest that BSTMP neurons in mice may increase AVP-Fos colocalization selectively in response to affiliation-related stimuli, similar to findings in finches. In contrast, virtually all other cell groups were responsive to negative aspects of interaction, either through elevated AVP-Fos colocalization in subordinate animals, positive correlations of AVP-Fos colocalization with bites received, and/or negative correlations of AVP-Fos colocalization with dominance. These findings greatly expand what is known of the contributions of specific brain AVP cell groups to social behavior.

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.

Sexual differentiation and development of forebrain reproductive circuits

Males and females exhibit numerous anatomical and physiological differences in the brain that often underlie important sex differences in physiology or behavior, including aspects relating to reproduction. Neural sex differences are both region-specific and trait-specific and may consist of divergences in synapse morphology, neuron size and number, and specific gene expression levels. In most cases, sex differences are induced by the sex steroid hormonal milieu during early perinatal development. In rodents, the hypothalamic anteroventral periventricular nucleus (AVPV) is sexually differentiated as a result of postnatal sex steroids, and also specific neuronal populations in this nucleus are sexually dimorphic, with females possessing more kisspeptin, dopaminergic, and GABA/glutamate neurons than males. The ability of female rodents, but not males, to display an estrogen-induced luteinizing hormone (LH) surge is consistent with the higher levels of these neuropeptides in the AVPV of females. Of these AVPV populations, the recently identified kisspeptin system has been most strongly implicated as a crucial component of the sexually dimorphic LH surge mechanism, though GABA and glutamate have also received some attention. New findings have suggested that the sexual differentiation and development of kisspeptin neurons in the AVPV is mediated by developmental estradiol signaling. Although apoptosis is the most common process implicated in neuronal sexual differentiation, it is currently unknown how developmental estradiol acts to differentiate specific neuronal populations in the AVPV, such as kisspeptin or dopaminergic neurons.

From mental disorder to iatrogenic hypogonadism: dilemmas in conceptualizing gender identity variants as psychiatric conditions

The categorization of gender identity variants (GIVs) as "mental disorders" in the Diagnostic and Statistical Manual of Mental Disorders (DSM) of the American Psychiatric Association is highly controversial among professionals as well as among persons with GIV. After providing a brief history of GIV categorizations in the DSM, this paper presents some of the major issues of the ongoing debate: GIV as psychopathology versus natural variation; definition of "impairment" and "distress" for GID; associated psychopathology and its relation to stigma; the stigma impact of the mental-disorder label itself; the unusual character of "sex reassignment surgery" as a psychiatric treatment; and the consequences for health and mental-health services if the disorder label is removed. Finally, several categorization options are examined: Retaining the GID category, but possibly modifying its grouping with other syndromes; narrowing the definition to dysphoria and taking "disorder" out of the label; categorizing GID as a neurological or medical rather than a psychiatric disorder; removing GID from both the DSM and the International Classification of Diseases (ICD); and creating a special category for GIV in the DSM. I conclude that-as also evident in other DSM categories-the decision on the categorization of GIVs cannot be achieved on a purely scientific basis, and that a consensus for a pragmatic compromise needs to be arrived at that accommodates both scientific considerations and the service needs of persons with GIVs.

6-hydroxydopamine lesions enhance progesterone-facilitated lordosis of rats and hamsters, independent of effects on motor behavior

The Ventral Tegmental Area (VTA) is an important brain area for progesterone (P(4))'s effects to facilitate female sexual behavior of rodents. We investigated the importance of dopaminergic neurons in the VTA, and two dopaminergic projection sites, the Nucleus Accumbens (NAc), and Caudate Nucleus of the Striatum (CN), in modulating P(4)-facilitated sex and motor behavior. Ovariectomized (ovx) rats and hamsters, administered estradiol benzoate (10 microg) and P(4) (0, 50, 100, 200, or 500 microg), were tested for motor behavior in a chamber that automatically records horizontal beam breaks, and for sexual behavior in response to a sexually-experienced male. Animals were tested once a week until each P(4) dosage was received; animals then had bilateral 6-hydroxydopamine (6-OHDA) or sham lesions to the VTA, NAc, or CN and were re-tested at each P(4) dosage on subsequent weeks. Fixed brains were stained with cresyl violet and processed for dopamine transporter (DAT) immunoreactivity. The number of cresyl violet stained cells was significantly lower in all 6-OHDA infusion sites compared to non-6-OHDA infusion sites of rats and hamsters. Also, in rats, the number of DAT-immunoreactive neurons was lower in all 6-OHDA infusion sites compared to non-6-OHDA infusion sites. In rats, 6-OHDA but not sham, lesions to the VTA, NAc, or CN produced P(4)-dependent increases in lordosis quotients and resulted in modest increases in motor behavior. In hamsters, 6-OHDA, but not sham, lesions to the VTA, NAc, or CN produced P(4)-dependent increases in total lordosis durations and produced modest decreases in motor behavior. This suggests that the dopaminergic output neurons of midbrain VTA may play an important role in modulation of P(4)-facilitated sexual lordosis among rodents.