Effect of Fetal Pituitary-Testes Suppression on Brain Sexual Differentiation and Reproductive Function in Male Sheep

We previously demonstrated that treating fetal lambs on gestational day 62 with the long-acting gonadotrophin-releasing hormone (GnRH) antagonist degarelix (DG) suppresses pituitary-testicular function during midgestation. The objective of this study was to investigate whether impaired gonadotrophic drive during this fetal period has enduring effects on sexual differentiation and reproductive function in adult male sheep. We assessed the effects of prenatal administration of DG, with or without testosterone (T) replacement, on various sexually dimorphic behavioral traits in adult rams, including sexual partner preferences, as well as neuroendocrine responsiveness and testicular function. Our findings revealed that DG treatment had no effect on genital differentiation or somatic growth. There were some indications that DG treatment suppressed juvenile play behavior and adult sexual motivation; however, male-typical sexual differentiation of reproductive behavior, sexual partner preference, and gonadotropin feedback remained unaffected and appeared to be fully masculinized and defeminized. DG-treated rams showed an increased LH response to GnRH stimulation and a decreased T response to human chorionic gonadotropin stimulation, suggesting impaired Leydig cell function and reduced T feedback. Both effects were reversed by cotreatment with T propionate. DG treatment also suppressed the expression of CYP17 messenger RNA, a key enzyme for T biosynthesis. Despite the mild hypogonadism induced by DG treatment, ejaculate volume, sperm motility, and sperm morphology were not affected. In summary, these results suggest that blocking GnRH during midgestation does not have enduring effects on brain sexual differentiation but does negatively affect the testes' capacity to synthesize T.

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

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

Actions of Peripubertal Gonadal Steroids in the Formation of Sexually Dimorphic Brain Regions in Mice

The calbindin-sexually dimorphic nucleus (CALB-SDN) and calbindin-principal nucleus of the bed nucleus of the stria terminalis (CALB-BNSTp) show male-biased sex differences in calbindin neuron number. The ventral part of the BNSTp (BNSTpv) exhibits female-biased sex differences in noncalbindin neuron number. We previously reported that prepubertal gonadectomy disrupts the masculinization of the CALB-SDN and CALB-BNSTp and the feminization of the BNSTpv. This study aimed to determine the action mechanisms of testicular androgens on the masculinization of the CALB-SDN and CALB-BNSTp and whether ovarian estrogens are the hormones that have significant actions in the feminization of the BNSTpv. We performed immunohistochemical analyses of calbindin and NeuN, a neuron marker, in male mice orchidectomized on postnatal day 20 (PD20) and treated with cholesterol, testosterone, estradiol, or dihydrotestosterone during PD20-70, female mice ovariectomized on PD20 and treated with cholesterol or estradiol during PD20-70, and PD70 mice gonadectomized on PD56. Calbindin neurons number in the CALB-SDN and CALB-BNSTp in males treated with testosterone or dihydrotestosterone, but not estradiol, was significantly larger than that in cholesterol-treated males. Noncalbindin neuron number in the BNSTpv in estradiol-treated females was significantly larger than that in cholesterol-treated females. Gonadectomy on PD56 had no significant effect on neuron numbers. Additionally, an immunohistochemical analysis revealed the expression of androgen receptors in the CALB-SDN and CALB-BNSTp of PD30 males and estrogen receptors-α in the BNSTpv of PD30 females. These results suggest that peripubertal testicular androgens act to masculinize the CALB-SDN and CALB-BNSTp without aromatization, and peripubertal ovarian estrogens act to feminize the BNSTpv.

The Complex Relationships between Sex and the Brain

In the past decennia, our understanding of the sexual differentiation of the mammalian brain has dramatically changed. The simple model according to which testosterone masculinizes the brain of males away from a default female form, was replaced with a complex scenario, according to which sex effects on the brain of both females and males are exerted by genetic, hormonal, and environmental factors. These factors act via multiple partly independent mechanisms that may vary according to internal and external factors. These observations led to the "mosaic" hypothesis-the expectation of high variability in the degree of "maleness"/"femaleness" of different features within a single brain. Here, we briefly review animal data that form the basis of current understanding of sexual differentiation; present, in this context, the results of co-analyses of human brain measures obtained by magnetic resonance imaging or postmortem; discuss criticisms and controversies of the mosaic hypothesis and implications for research; and conclude that co-analysis of several (preferably, many) features and going back from the group level to that of the individual would advance our understanding of the relations between sex and the brain in health and disease.

Neurobiology of gender identity and sexual orientation

Sexual identity and sexual orientation are independent components of a person's sexual identity. These dimensions are most often in harmony with each other and with an individual's genital sex, although not always. The present review discusses the relationship of sexual identity and sexual orientation to prenatal factors that act to shape the development of the brain and the expression of sexual behaviours in animals and humans. One major influence discussed relates to organisational effects that the early hormone environment exerts on both gender identity and sexual orientation. Evidence that gender identity and sexual orientation are masculinised by prenatal exposure to testosterone and feminised in it absence is drawn from basic research in animals, correlations of biometric indices of androgen exposure and studies of clinical conditions associated with disorders in sexual development. There are, however, important exceptions to this theory that have yet to be resolved. Family and twin studies indicate that genes play a role, although no specific candidate genes have been identified. Evidence that relates to the number of older brothers implicates maternal immune responses as a contributing factor for male sexual orientation. It remains speculative how these influences might relate to each other and interact with postnatal socialisation. Nonetheless, despite the many challenges to research in this area, existing empirical evidence makes it clear that there is a significant biological contribution to the development of an individual's sexual identity and sexual orientation.

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

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

Effects of Long-Term Flutamide Treatment During Development on Sexual Behaviour and Hormone Responsiveness in Rams

Testosterone exposure during midgestation differentiates neural circuits controlling sex-specific behaviours and patterns of gonadotrophin secretion in male sheep. Testosterone acts through androgen receptors (AR) and/or after aromatisation to oestradiol and binding to oestrogen receptors. The present study assessed the role of AR activation in male sexual differentiation. We compared rams that were exposed to the AR antagonist flutamide (Flu) throughout the critical period (i.e. days 30-90 of gestation) to control rams and ewes that received no prenatal treatments. The external genitalia of all Flu rams were phenotypically female. Testes were positioned s.c. in the inguinal region of the abdomen, exhibited seasonally impaired androgen secretion and were azospermic. Flu rams displayed male-typical precopulatory and mounting behaviours but could not intromit or ejaculate because they lacked a penis. Flu rams exhibited greater mounting behaviour than control rams and, similar to controls, showed sexual partner preferences for oestrous ewes. Neither control, nor Flu rams responded to oestradiol treatments with displays of female-typical receptive behaviour or LH surge responses, whereas all control ewes responded as expected. The ovine sexually dimorphic nucleus in Flu rams was intermediate in volume between control rams and ewes and significantly different from both. These results indicate that prenatal anti-androgen exposure is not able to block male sexual differentiation in sheep and suggest that compensatory mechanisms intervene to maintain sufficient androgen stimulation during development.

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

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

Wired on steroids: sexual differentiation of the brain and its role in the expression of sexual partner preferences

The preference to seek out a sexual partner of the opposite sex is robust and ensures reproduction and survival of the species. Development of female-directed partner preference in the male is dependent on exposure of the developing brain to gonadal steroids synthesized during critical periods of sexual differentiation of the central nervous system. In the absence of androgen exposure, a male-directed partner preference develops. The development and expression of sexual partner preference has been extensively studied in rat, ferret, and sheep model systems. From these models it is clear that gonadal testosterone, often through estrogenic metabolites, cause both masculinization and defeminization of behavior during critical periods of brain development. Changes in the steroid environment during these critical periods result in atypical sexual partner preference. In this manuscript, we review the major findings which support the hypothesis that the organizational actions of sex steroids are responsible for sexual differentiation of sexual partner preferences in select non-human species. We also explore how this information has helped to frame our understanding of the biological influences on human sexual orientation and gender identity.

Developing a project-oriented introduction to neuroscience lab at hope college

The Introduction to Neuroscience course at Hope College includes a three-hour laboratory period each week. Seven of the fifteen weeks of the lab are used for a lab project that is focused on understanding the effects of gonadal hormones on brain and behavior. Students perform ovariectomies and implant sham, estradiol, or testosterone capsules in rats and then carry out five experiments: 1) Sexual Behavior, 2) Spatial Learning using the Morris Water Maze, 3) The Size of the Sexually Dimorphic Nucleus, 4) Phosphorylation of NMDA Receptors, and 5) Long Term Potentiation in Hippocampal Slices. The experiments are designed to provide the students with experiences at different levels of neuroscience, while improving their skills in statistics, using the primary literature, and scientific writing. The students generate interesting and statistically significant data which they summarize in a journal style lab reports. Using a Self Assessment of Learning Gains tool, we learned that students perceive the lab project improves their ability to A) pose questions from more than one disciplinary perspective that can be addressed by collecting and evaluating scientific evidence, B) learn about complex science problems that require insight from more than one discipline, C) extract main points from a scientific article and develop a coherent summary, and D) write reports using scientific data as evidence. Based on our results, we believe an extended lab project in an introductory neuroscience course can be used to engage students in neuroscience topics and help them develop the skills and habits of neuroscientists.

The neurobiology of sexual partner preferences in rams

The question of what causes a male animal to seek out and choose a female as opposed to another male mating partner is unresolved and remains an issue of considerable debate. The most developed biologic theory is the perinatal organizational hypothesis, which states that perinatal hormone exposure mediates sexual differentiation of the brain. Numerous animal experiments have assessed the contribution of perinatal testosterone and/or estradiol exposure to the development of a male-typical mate preference, but almost all have used hormonally manipulated animals. In contrast, variations in sexual partner preferences occur spontaneously in domestic rams, with as many as 8% of the population exhibiting a preference for same-sex mating partners (male-oriented rams). Thus, the domestic ram is an excellent experimental model to study possible links between fetal neuroendocrine programming of neural mechanisms and adult sexual partner preferences. In this review, we present an overview of sexual differentiation in relation to sexual partner preferences. We then summarize results that test the relevance of the organizational hypothesis to expression of same-sex sexual partner preferences in rams. Finally, we demonstrate that the sexual differentiation of brain and behavior in sheep does not depend critically on aromatization of testosterone to estradiol.

The ram as a model for behavioral neuroendocrinology

The sheep offers a unique model to study male sexual behavior and sexual partner preference. Rams are seasonal breeders and show the greatest libido during short days coincident with the resumption of ovarian cyclicity in the ewe. Threshold concentrations of testosterone are required for the acquisition and display of adult sexual behavior. In addition, estrogens produced from circulating testosterone by cytochrome P450 aromatase in the preoptic area are critical for the maintenance of sexual behaviors in rams. Sex differences in adult reproductive behaviors and hormone responsiveness are the result of permanent organizational effects exerted by testosterone and its metabolites on brain development. Early exposure to ewes enhances ram sexual performance, but cannot prevent some rams from exhibiting male-oriented sexual partner preferences. Neurochemical and neuroanatomical studies suggest that male-oriented ram behavior may be a consequence of individual variations in brain sexual differentiation.