The genetics of sex differences in brain and behavior

The biological basis of human sexual orientation: is there a role for epigenetics?

Sexual orientation is one of the largest sex differences in humans. The vast majority of the population is heterosexual, that is, they are attracted to members of the opposite sex. However, a small but significant proportion of people are bisexual or homosexual and experience attraction to members of the same sex. The origins of the phenomenon have long been the subject of scientific study. In this chapter, we will review the evidence that sexual orientation has biological underpinnings and consider the involvement of epigenetic mechanisms. We will first discuss studies that show that sexual orientation has a genetic component. These studies show that sexual orientation is more concordant in monozygotic twins than in dizygotic ones and that male sexual orientation is linked to several regions of the genome. We will then highlight findings that suggest a link between sexual orientation and epigenetic mechanisms. In particular, we will consider the case of women with congenital adrenal hyperplasia (CAH). These women were exposed to high levels of testosterone in utero and have much higher rates of nonheterosexual orientation compared to non-CAH women. Studies in animal models strongly suggest that the long-term effects of hormonal exposure (such as those experienced by CAH women) are mediated by epigenetic mechanisms. We conclude by describing a hypothetical framework that unifies genetic and epigenetic explanations of sexual orientation and the continued challenges facing sexual orientation research.

Moderating variables of music training-induced neuroplasticity: a review and discussion

A large body of literature now exists to substantiate the long-held idea that musicians' brains differ structurally and functionally from non-musicians' brains. These differences include changes in volume, morphology, density, connectivity, and function across many regions of the brain. In addition to the extensive literature that investigates these differences cross-sectionally by comparing musicians and non-musicians, longitudinal studies have demonstrated the causal influence of music training on the brain across the lifespan. However, there is a large degree of inconsistency in the findings, with discordance between studies, laboratories, and techniques. A review of this literature highlights a number of variables that appear to moderate the relationship between music training and brain structure and function. These include age at commencement of training, sex, absolute pitch (AP), type of training, and instrument of training. These moderating variables may account for previously unexplained discrepancies in the existing literature, and we propose that future studies carefully consider research designs and methodologies that control for these variables.

Homosexuality via canalized sexual development: a testing protocol for a new epigenetic model

We recently synthesized and reinterpreted published studies to advance an epigenetic model for the development of homosexuality (HS). The model is based on epigenetic marks laid down in response to the XX vs. XY karyotype in embryonic stem cells. These marks boost sensitivity to testosterone in XY fetuses and lower it in XX fetuses, thereby canalizing sexual development. Our model predicts that a subset of these canalizing epigenetic marks stochastically carry over across generations and lead to mosaicism for sexual development in opposite-sex offspring--the homosexual phenotype being one such outcome. Here, we begin by outlining why HS has been under-appreciated as a commonplace phenomenon in nature, and how this trend is currently being reversed in the field of neurobiology. We next briefly describe our epigenetic model of HS, develop a set of predictions, and describe how epigenetic profiles of human stem cells can provide for a strong test of the model.

Gender identity development in adolescence

This article is part of a Special Issue "Puberty and Adolescence".This article aims to provide an outline of what is currently known on trajectories, and contributing factors to gender identity development in adolescence. We give a historical overview of the concept of gender identity, and describe general identity development in adolescence, gender identity development in the general population and in gender variant youth. Possible psychosocial (such as child and parental characteristics) and biological factors (such as the effects of prenatal exposure to gonadal hormones and the role of genetics) contributing to a gender variant identity are discussed. Studies focusing on a number of psychosocial and biological factors separately, indicate that each of these factors influence gender identity formation, but little is known about the complex interplay between the factors, nor about the way individuals themselves contribute to the process. Research into normative and gender variant identity development of adolescents is clearly lagging behind. However, studies on persons with gender dysphoria and disorders of sex development, show that the period of adolescence, with its changing social environment and the onset of physical puberty, seems to be crucial for the development of a non-normative gender identity.

The differential role of androgens in early human sex development

Sexual development in humans is only partly understood at the molecular level. It is dependent on genetic control primarily induced by the sex chromosomal differences between males and females. This leads to the development of the gonads, whereby afterwards the differentiation of the apparent phenotype is controlled by hormone action. Sex steroids may exert permanent and temporary effects. Their organizational features of inducing permanent changes in phenotype occur through genetic control of downstream genes. In this, androgens are the key elements for the differentiation of male internal and external genitalia as well as other sexual organs and general body composition, acting through a single androgen receptor. The androgen receptor is a nuclear transcription factor modulating DNA transcription of respective target genes and thereby driving development and growth in a stringent manner. The specificity of androgen action seems to be a strictly time-controlled process with the androgen receptor acting in concert with different metabolites and an array of cofactors modulating the cellular response and thereby permanently altering the phenotype of any given individual. For every cell programmed by androgens, a specific 'androgen response index' must be proposed.

Sex differences in emotion-related cognitive processes in irritable bowel syndrome and healthy control subjects

Greater responsiveness of emotional arousal circuits in relation to delivered visceral pain has been implicated as underlying central pain amplification in irritable bowel syndrome (IBS), with female subjects showing greater responses than male subjects. Functional magnetic resonance imaging was used to measure neural responses to an emotion recognition paradigm, using faces expressing negative emotions (fear and anger). Sex and disease differences in the connectivity of affective and modulatory cortical circuits were studied in 47 IBS (27 premenopausal female subjects) and 67 healthy control subjects (HCs; 38 premenopausal female subjects). Male subjects (IBS+HC) showed greater overall brain responses to stimuli than female subjects in prefrontal cortex, insula, and amygdala. Effective connectivity analyses identified major sex- and disease-related differences in the functioning of brain networks related to prefrontal regions, cingulate, insula, and amygdala. Male subjects had stronger connectivity between anterior cingulate subregions, amygdala, and insula, whereas female subjects had stronger connectivity to and from the prefrontal modulatory regions (medial/dorsolateral cortex). Male IBS subjects demonstrate greater engagement of cortical and affect-related brain circuitry compared to male control subjects and female subjects, when viewing faces depicting emotions previously shown to elicit greater behavioral and brain responses in male subjects.

Impacts of early intervention with fluoxetine following early neonatal immune activation on depression-like behaviors and body weight in mice

Several reports have suggested that early neonatal immune activation adversely influences the hypothalamic-pituitary-adrenal (HPA) axis development in humans and animal models. In addition, there have been several studies indicating that early intervention with fluoxetine (FLX) can alter HPA axis development and function, and prevent occurrence of behavioral abnormalities induced by common early-life insults. The present study aims to investigate the effects of early intervention with FLX following early neonatal immune activation on depression-like behaviors and body weight in mice. Neonatal mice in their postnatal days (PNDs) 3 and 5 received either lipopolysaccharide (LPS; 50 μg/kg, s.c.) or saline treatment, then male and female mice of both neonatal intervention groups received oral administration of FLX (5 and 10 mg/kg/day) or water via regular drinking bottles during the periadolescent period (PNDs 35-65). The results showed that neonatal LPS exposure elevated depression-like behaviors accompanied by increasing corticosterone levels in adulthood and decreasing body weight during neonatal and adolescent periods. Furthermore, the periadolescent FLX treatment inhibited the depression-like behaviors induced by neonatal infection in both sexes. This study obtained some experimental evidence indicating the potential adverse impacts of the FLX on normal behavioral development in male control animals. In conclusion, our findings suggest that an early pharmacological intervention with FLX may prevent emergence of depression-like behaviors induced by neonatal immune challenge without any detrimental effect on health in a sex- and dose-dependent manner in mice.

Neonatal NMDA receptor blockade alters anxiety- and depression-related behaviors in a sex-dependent manner in mice

There is increasing evidence that N-methyl-D-aspartate (NMDA) receptor blockade in the neonatal period has a long-lasting influence on brain and behavior development and has been linked to an increased risk for neuropsychiatric disorders in later life. We sought to determine whether postnatal NMDA receptor blockade can affect normal development of body weight, corticosterone levels, anxiety- and depression-related behaviors in male and female mice in adulthood. For this purpose, male and female NMRI mice were treated with either saline or phencyclidine (PCP; 5 and 10 mg/kg, s.c.) on postnatal days (PND) 7, 9, and 11, and then subjected to different behavioral tests, including open field, elevated plus-maze, elevated zero-maze, light-dark box, tail suspension test and forced swimming test in adulthood. The results indicated that neonatal PCP treatment reduced body weight during neonatal and adulthood periods, and did not alter baseline corticosterone levels in both male and female mice. Moreover, this study obtained some experimental evidence showing the PCP at dose of 10 mg/kg increases stress-induced corticosterone levels, anxiety- and depression-related behaviors in males, while decreasing levels of anxiety without any significant effect on depression in female mice in adulthood. These data support the argument that neonatal NMDA receptor blockade can lead to behavioral abnormalities and psychiatric diseases in adulthood. Collectively, our findings suggest that neonatal exposure to PCP may have profound effects on the development of anxiety- and depression-related behaviors in a sex- and dose-dependent manner in mice.

Distinct neuroendocrine mechanisms control neural activity underlying sex differences in sexual motivation and performance

Sexual behavior can be usefully parsed into an appetitive and a consummatory component. Both appetitive and consummatory male-typical sexual behaviors (respectively, ASB and CSB) are activated in male Japanese quail by testosterone (T) acting in the medial preoptic nucleus (POM), but never observed in females. This sex difference is based on a demasculinization (= organizational effect) by estradiol during embryonic life for CSB, but a differential activation by T in adulthood for ASB. Males expressing rhythmic cloacal sphincter movements (RCSMs; a form of ASB) or allowed to copulate display increased Fos expression in POM. We investigated Fos brain responses in females exposed to behavioral tests after various endocrine treatments. T-treated females displayed RCSM, but never copulated when exposed to another female. Accordingly they showed an increased Fos expression in POM after ASB but not CSB tests. Females treated with the aromatase inhibitor Vorozole in ovo and T in adulthood displayed both male-typical ASB and CSB, and Fos expression in POM was increased after both types of tests. Thus, the neural circuit mediating ASB is present or can develop in both sexes, but is inactive in females unless they are exposed to exogenous T. In contrast, the neural mechanism mediating CSB is not normally present in females, but can be preserved by blocking the embryonic production of estrogens. Overall these data confirm the difference in endocrine controls and probably neural mechanisms supporting ASB and CSB in quail, and highlight the complexity of mechanisms underlying sexual differentiation of behavior.

Instability in X chromosome inactivation patterns in AMD: a new risk factor?

Years ago, it was thought that a genetic component was the fundamental cause of a number retinopathy diseases including age related macular degeneration (AMD). Since then, information has emerged about novel genes that contribute to various forms of AMD and other retinopathies that have been eluding researchers for years. In the genetic sense, only the APOE 2 and 4 genes have been found to be a risk factor for sporadic AMD. But, a recent Genome wide association study (GWAS) revealed that an alteration of five SNIPs on the X chromosome in a gene named DIAPH2 may be a susceptibility gene for AMD. Furthermore, the gene DIAPH2 showed to have a polygenic pleiotropy for premature ovarian failure (POF) and AMD in a cohort of women. POF is highly associated with X chromosome skewing, an epigenetic alteration of the inactivation process of the X chromosome. These findings suggest a hypothesis that an epigenetic alteration on the inactivation centres of the X chromosome (or skewing) relates not only to aging, but might be a novel property that affects women with AMD more often than men.

Sex determines which section of the SLC6A4 gene is linked to obsessive-compulsive symptoms in normal Chinese college students

Previous case-control and family-based association studies have implicated the SLC6A4 gene in obsessive-compulsive disorder (OCD). Little research, however, has examined this gene's role in obsessive-compulsive symptoms (OCS) in community samples. The present study genotyped seven tag SNPs and two common functional tandem repeat polymorphisms (5-HTTLPR and STin2), which together cover the whole SLC6A4 gene, and investigated their associations with OCS in normal Chinese college students (N = 572). The results revealed a significant gender main effect and gender-specific genetic effects of the SLC6A4 gene on OCS. Males scored significantly higher on total OCS and its three dimensions than did females (ps < .01). The 5-HTTLPR in the promoter region showed a female-specific genetic effect, with the l/l and l/s genotypes linked to higher OCS scores than the s/s genotype (ps < .05). In contrast, a conserved haplotype polymorphism (rs1042173| rs4325622| rs3794808| rs140701| rs4583306| rs2020942) covering from intron 3 to the 3' UTR of the SLC6A4 gene showed male-specific genetic effects, with the CGAAGG/CGAAGG genotype associated with lower OCS scores than the other genotypes (ps < .05). These effects remained significant after controlling for OCS-related factors including participants' depressive and anxiety symptoms as well as stressful life events, and correction for multiple tests. These results are discussed in terms of their implications for our understanding of the sex-specific role of the different sections of the SLC6A4 gene in OCD.

Rodents rely on Merkel cells for texture discrimination tasks

The cutaneous somatosensory system contains multiple types of mechanoreceptors that detect different mechanical stimuli (Johnson, 2001). These stimuli, either alone or in combination, are ultimately interpreted by the brain as different aspects of the sense of touch. Psychophysical and electrophysiological experiments in humans and other mammals implicate one of these mechanoreceptors, the Merkel cell/neurite complex, in two-point discrimination and the detection of curvature, shape, and texture (Johnson and Lamb, 1981; Johnson et al., 2000; Johnson, 2001). However, whether Merkel cell/neurite complex function is required for the detection of these stimuli is unknown. We genetically engineered mice that lack Merkel cells (Maricich et al., 2009; Morrison et al., 2009) to directly test the hypothesis that Merkel cell/neurite complexes are necessary to perform these types of sensory discrimination tasks. We found that mice devoid of Merkel cells could not detect textured surfaces with their feet while other measures of motor and sensory function were unaffected. Interestingly, these mice retained the ability to discriminate both texture and shape using their whiskers, suggesting that other somatosensory afferents can functionally substitute for Merkel cell/neurite complexes in this sensory organ. These findings suggest that Merkel cell/neurite complexes are essential for texture discrimination tasks involving glabrous skin but not whiskers.

"Straight-acting gays": the relationship between masculine consciousness, anti-effeminacy, and negative gay identity

Some gay men are preoccupied with traditional notions of masculinity and express negative feelings towards effeminate behavior in gay men. Various scholars have speculated that such attitudes by gay men reflect internalized negative feelings about being gay. Thus, we sought to assess the importance of masculinity among gay men, to compare their ideal versus perceived masculinity-femininity, to ask how gay men assess masculinity, and to test whether masculine consciousness and anti-effeminacy could predict negative feelings about being gay. Results from an online survey of 751 gay men in the United States (MAge=32.64 years, SD=11.94) showed that the majority rated masculinity for themselves and in a same-sex partner as important, and they ideally wished that their behavior was more masculine (Cohen's d=.42) and less feminine (d=.42) than they perceived it to be. Furthermore, one's behavior was more important than how one looks when assessing masculinity. A multiple regression analysis showed that the degree to which they were preoccupied with masculinity and expressed anti-effeminacy accounted for 30% of the variance in negative feelings about being gay. These finding further support the idea that masculinity is an important construct for gay men and that masculine consciousness and anti-effeminacy are related to negative feelings about being gay.

A review on sex differences in processing emotional signals

Interest in sex-related differences in psychological functioning has again come to the foreground with new findings about their possible functional basis in the brain. Sex differences may be one way how evolution has capitalized on the capacity of homologous brain regions to process social information between men and women differently. This paper focuses specifically on the effects of emotional valence, sex of the observed and sex of the observer on regional brain activations. We also discuss the effects of and interactions between environment, hormones, genes and structural differences of the brain in the context of differential brain activity patterns between men and women following exposure to seen expressions of emotion and in this context we outline a number of methodological considerations for future research. Importantly, results show that although women are better at recognizing emotions and express themselves more easily, men show greater responses to threatening cues (dominant, violent or aggressive) and this may reflect different behavioral response tendencies between men and women as well as evolutionary effects. We conclude that sex differences must not be ignored in affective research and more specifically in affective neuroscience.

Neuroprotective effects of estrogens and androgens in CNS inflammation and neurodegeneration

Multiple sclerosis (MS) is a disease characterized by inflammation and demyelination. Currently, the cause of MS is unknown. Experimental autoimmune encephalomyelitis (EAE) is the most common mouse model of MS. Treatments with the sex hormones, estrogens and androgens, are capable of offering disease protection during EAE and are currently being used in clinical trials of MS. Beyond endogenous estrogens and androgens, treatments with selective estrogen receptor modulators (SERMs) for estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) are also capable of providing disease protection. This protection includes, but is not limited to, prevention of clinical disease, reduction of CNS inflammation, protection against demyelination, and protection against axonal loss. In EAE, current efforts are focused on using conditional cell specific knockouts of sex hormone receptors to identify the in vivo targets of these estrogens and androgens as well as downstream molecules responsible for disease protection.

Sexual dimorphism in ischemic stroke: lessons from the laboratory

Ischemic stroke is emerging as a major health problem for elderly women. Women have lower stroke incidence than men until an advanced age, when the epidemiology of ischemic stroke shifts and incidence rises dramatically in women. Experimental models of rodent stroke have replicated this clinical epidemiology, with exacerbated injury in older compared with young female rodents. Many of the detrimental effects of aging on ischemic stroke outcome in females can be replicated by ovariectomy, suggesting that hormones such as estrogen play a neuroprotective role. However, emerging data suggest that the molecular mechanisms leading to ischemic cell death differ in the two sexes, and these effects may be independent of circulating hormone levels. This article highlights recent clinical and experimental literature on sex differences in stroke outcomes and mechanisms.

Sex-based differences in gene expression in hippocampus following postnatal lead exposure

The influence of sex as an effect modifier of childhood lead poisoning has received little systematic attention. Considering the paucity of information available concerning the interactive effects of lead and sex on the brain, the current study examined the interactive effects of lead and sex on gene expression patterns in the hippocampus, a structure involved in learning and memory. Male or female rats were fed either 1500 ppm lead-containing chow or control chow for 30 days beginning at weaning.Blood lead levels were 26.7±2.1 μg/dl and 27.1±1.7 μg/dl for females and males, respectively. The expression of 175 unique genes was differentially regulated between control male and female rats. A total of 167 unique genes were differentially expressed in response to lead in either males or females. Lead exposure had a significant effect without a significant difference between male and female responses in 77 of these genes. In another set of 71 genes, there were significant differences in male vs. female response. A third set of 30 genes was differentially expressed in opposite directions in males vs. females, with the majority of genes expressed at a lower level in females than in males. Highly differentially expressed genes in males and females following lead exposure were associated with diverse biological pathways and functions. These results show that a brief exposure to lead produced significant changes in expression of a variety of genes in the hippocampus and that the response of the brain to a given lead exposure may vary depending on sex.

Minireview: Hormones and human sexual orientation

Many people believe that sexual orientation (homosexuality vs. heterosexuality) is determined by education and social constraints. There are, however, a large number of studies indicating that prenatal factors have an important influence on this critical feature of human sexuality. Sexual orientation is a sexually differentiated trait (over 90% of men are attracted to women and vice versa). In animals and men, many sexually differentiated characteristics are organized during early life by sex steroids, and one can wonder whether the same mechanism also affects human sexual orientation. Two types of evidence support this notion. First, multiple sexually differentiated behavioral, physiological, or even morphological traits are significantly different in homosexual and heterosexual populations. Because some of these traits are known to be organized by prenatal steroids, including testosterone, these differences suggest that homosexual subjects were, on average, exposed to atypical endocrine conditions during development. Second, clinical conditions associated with significant endocrine changes during embryonic life often result in an increased incidence of homosexuality. It seems therefore that the prenatal endocrine environment has a significant influence on human sexual orientation but a large fraction of the variance in this behavioral characteristic remains unexplained to date. Genetic differences affecting behavior either in a direct manner or by changing embryonic hormone secretion or action may also be involved. How these biological prenatal factors interact with postnatal social factors to determine life-long sexual orientation remains to be determined.

Sexual orientation, fraternal birth order, and the maternal immune hypothesis: a review

In 1996, psychologists Ray Blanchard and Anthony Bogaert found evidence that gay men have a greater number of older brothers than do heterosexual men. This "fraternal birth order" (FBO) effect has been replicated numerous times, including in non-Western samples. More recently, strong evidence has been found that the FBO effect is of prenatal origin. Although there is no direct support for the exact prenatal mechanism, the most plausible explanation may be immunological in origin, i.e., a mother develops an immune reaction against a substance important in male fetal development during pregnancy, and that this immune effect becomes increasingly likely with each male gestation. This immune effect is hypothesized to cause an alteration in (some) later born males' prenatal brain development. The target of the immune response may be molecules (i.e., Y-linked proteins) on the surface of male fetal brain cells, including in sites of the anterior hypothalamus, which has been linked to sexual orientation in other research. Antibodies might bind to these molecules and thus alter their role in typical sexual differentiation, leading some later born males to be attracted to men as opposed to women. Here we review evidence in favor of this hypothesis, including recent research showing that mothers of boys develop an immune response to one Y-linked protein (i.e., H-Y antigen; SMCY) important in male fetal development, and that this immune effect becomes increasingly likely with each additional boy to which a mother gives birth. We also discuss other Y-linked proteins that may be relevant if this hypothesis is correct. Finally, we discuss issues in testing the maternal immune hypothesis of FBO.

Sexual differentiation of the human brain: relation to gender identity, sexual orientation and neuropsychiatric disorders

During the intrauterine period a testosterone surge masculinizes the fetal brain, whereas the absence of such a surge results in a feminine brain. As sexual differentiation of the brain takes place at a much later stage in development than sexual differentiation of the genitals, these two processes can be influenced independently of each other. Sex differences in cognition, gender identity (an individual's perception of their own sexual identity), sexual orientation (heterosexuality, homosexuality or bisexuality), and the risks of developing neuropsychiatric disorders are programmed into our brain during early development. There is no evidence that one's postnatal social environment plays a crucial role in gender identity or sexual orientation. We discuss the relationships between structural and functional sex differences of various brain areas and the way they change along with any changes in the supply of sex hormones on the one hand and sex differences in behavior in health and disease on the other.