Sex on the brain

An anthropometric study of sexual orientation and gender identity in Thailand

The biodevelopment of psychological sex differentiation is putatively reflected in several anthropometrics. We examined eight anthropometrics in 1404 Thai participants varying in sex, sexual orientation, and gender identity/expression: heterosexual men and women, gay men, lesbian women, bisexual women, sao praphet song (transgender birth-assigned males), toms (transgender birth-assigned females), and dees (birth-assigned females attracted to toms). Exploratory factor analyses indicated the biomarkers should be analyzed independently. Using regressions, in birth-assigned males, less male-typical second-to-fourth digit ratios in the left hand were associated with sexual orientation towards men regardless of gender identity/expression, whereas shorter height and long-bone growth in the arms and legs were more evident among sao praphet song-who are both sexually oriented towards men and markedly feminine. In birth-assigned females, there were no clear sexual orientation effects, but there were possible gender-related effects. Groups of individuals who tend to be more masculine (i.e., toms, lesbians) showed more male-typical patterns on weight and leg length than some groups of individuals who tend to be less masculine (i.e., heterosexual women, dees). Thus, it appears the various anthropometrics inform separate biodevelopmental processes that differentially relate to sexual orientation and gender identity/expression depending on the measure in question as well as birth-assigned sex.

Both neural and global androgen receptor overexpression affect sexual dimorphism in the mouse brain

Testosterone is the main endocrine mechanism mediating sexual differentiation of the mammalian brain, although testosterone signalling is complex and important mechanistic questions remain. Notably, the extent to which testosterone acts via androgen receptors (AR) in this process remains unknown and it is also not clear where testosterone acts in the body to produce sexual dimorphisms in neuroanatomy. To address these questions, we used a transgenic mouse model of Cre/loxP-driven AR overexpression in which AR was induced selectively in neural tissue (Nestin-cre) or in all tissues (CMV-cre). We then studied sexually dimorphic features of several well-characterised sexual dimorphisms: calbindin-immunoreactive neurones in the medial preoptic area (CALB-SDN), tyrosine hydroxylase neurones in the anteroventral periventricular nucleus, and vasopressin-immunoreactive neurones originating in the bed nucleus of the stria terminalis and their projections in the lateral septum. We additionally evaluated oestrogen receptor α immunoreactivity in these nuclei. Briefly, we found that global but not neural overexpression of AR resulted in masculinisation of CALB-SDN nucleus volume, cell number and cell size in transgenic females. Furthermore, neural AR overexpression resulted in increased oestrogen receptor α staining in females compared to males in the medial preoptic area. AR overexpression did not affect other measures. Overall, the results of the present study provide support for the hypothesis that androgenic mechanisms external to the nervous system can affect sexual differentiation of the brain.

Male-typical courtship, spawning behavior, and olfactory sensitivity are induced to different extents by androgens in the goldfish suggesting they are controlled by different neuroendocrine mechanisms

Male-typical reproductive behaviors vary greatly between different species of fishes with androgens playing a variety of roles that appear especially important in the gonochorist cypriniform fishes. The goldfish is an important model for the cypriniformes and while it is clear that male goldfish are fully feminized by prostaglandin F2α(PGF2α), it is not clear whether females will exhibit normal levels of male-typical reproductive behaviors as well as olfactory function when treated with androgens. To answer this question, we exposed sexually-regressed adult female goldfish to several types of androgen and monitored their tendencies to court (inspect females) and mate (spawn, or attempt to release gametes) while monitoring their olfactory sensitivity until changes in these attributes were maximized. Untreated adult males (intact) were included to determine the extent of masculinization. Treatments included the natural androgens, 11-ketotestosterone and testosterone (KT and T), administered via capsules (KT+T-implanted fish); the artificial androgen, methyltestosterone (MT), administered via capsules (MT-C); and MT administered in the fishes' water (MT-B). Male-typical olfactory sensitivity to a pheromone (15keto-PGF2α) increased in all androgen-treated groups and by week 6 was fully equivalent to that of males. Male-typical courtship behavior increased in all androgen-treated groups although slowly, and only MT-B females came to exhibit levels equivalent to those of males after 18weeks. In contrast, male-typical mating activity increased only slightly, with MT-B females reaching levels one-third that of males after 30weeks. We conclude that while androgens fully masculinize olfactory sensitivity and courtship behavior in goldfish, mating behavior is controlled by a different neuroendocrine mechanism(s) that has yet to be fully elucidated.

Turning sex inside-out: Peripheral contributions to sexual differentiation of the central nervous system

Sexual differentiation of the nervous system occurs via the interplay of genetics, endocrinology and social experience through development. Much of the research into mechanisms of sexual differentiation has been driven by an implicit theoretical framework in which these causal factors act primarily and directly on sexually dimorphic neural populations within the central nervous system. This review will examine an alternative explanation by describing what is known about the role of peripheral structures and mechanisms (both neural and non-neural) in producing sex differences in the central nervous system. The focus of the review will be on experimental evidence obtained from studies of androgenic masculinization of the spinal nucleus of the bulbocavernosus, but other systems will also be considered.

Androgen receptor expression and morphology of forebrain and neuromuscular systems in male green anoles displaying individual differences in sexual behavior

Investigating individual differences in sexual performance in unmanipulated males is important for understanding natural relationships between behavior and morphology, and the mechanisms regulating them. Among male green anole lizards, some court and copulate frequently (studs) and others do not (duds). To evaluate potential factors underlying differences in the level of these behaviors, morphology and androgen receptor expression in neuromuscular courtship and copulatory structures, as well as in the preoptic area and amygdala, were compared in males displaying varying degrees of sexual function. This study revealed that individual differences in behavior among unmanipulated males, in particular the extension of a throat fan (dewlap) used during courtship, were positively correlated with the size of fibers in the associated muscle and with soma size in the amygdala. The physiological response to testosterone, as indicated by the height of cells in an androgen-sensitive portion of the kidney, was also correlated with male sexual behavior, and predicted it better than plasma androgen levels. Androgen receptor expression was not related to the display of courtship or copulation in any of the tissues examined. The present data indicate that higher levels of male courtship behavior result in (or are the result of) enhanced courtship muscle and amygdala morphology, and that androgen-sensitive tissue in studs may be more responsive to testosterone than duds. However, some mechanism(s) other than androgen receptor expression likely confer this difference in responsiveness.

Courtship and copulation in the adult male green anole: effects of season, hormone and female contact on reproductive behavior and morphology

Interactions among reproductive season, testosterone (T) and female presence were investigated on the structure and function of forebrain and neuromuscular systems controlling courtship and copulation in the green anole lizard. Under breeding (BS) or non-breeding (NBS) environmental conditions, male green anoles were implanted with either T or blank capsules and exposed to one of three female stimulus conditions: physical, visual or no female contact. T and at least visual exposure to females increased courtship displays (extension of a throat fan, or dewlap), and these effects were greater during the BS than NBS. T also facilitated copulation, and did so to a greater extent in the BS. The hormone increased soma size in the preoptic area (POA) and amygdala (AMY), and in the AMY the effects were greater in the BS than NBS. Cross-sectional areas of copulatory organs and associated muscle fibers were enhanced by T, and more so in the BS than NBS. However, no effects on morphology of dewlap motoneurons or muscles or copulatory motoneurons were detected. Thus, (1) changes in behavior and neural and/or muscular morphology are not always parallel and (2) differences in responsiveness to T exist across seasons and among tissues.

Steroid-dependent plasticity in the medial amygdala

Behavioral sex differences have traditionally been thought to arise from gonadal steroids during a neonatal sensitive period. However, it is possible to sex-reverse certain behaviors by reversing the levels of circulating androgen in adult males and females. These results suggest that the sexually dimorphic substrates of sex behavior are subject to a high degree of plasticity, even in adulthood. I have found that circulating androgen exerts a trophic effect on the Nissl-stained morphology of an important nucleus in the control of sex behavior, namely, the posterodorsal subnucleus of the medial amygdala. First, sex-reversing the level of circulating androgen reversed the sex difference in soma size and regional volume of the posterodorsal subnucleus of the medial amygdala in adult rats. Interestingly, activation of both androgen and estrogen receptors was necessary for the post-castration maintenance of a masculine phenotype in terms of posterodorsal subnucleus of the medial amygdala cell size, whereas only estrogen receptor activity was necessary to maintain a masculine posterodorsal subnucleus of the medial amygdala volume. Then, we showed that seasonal variation in androgen was correlated with morphologic plasticity in the posterodorsal subnucleus of the medial amygdala of the Siberian hamster. However, if the experimental males were housed with females, their posterodorsal subnucleus of the medial amygdalas failed to regress in response to winter-like short daylengths. Furthermore, when male hamsters were castrated and treated with testosterone, the posterodorsal subnucleus of the medial amygdala responded to the hormone only if the animals were in summer-like photoperiods. Overall, these findings indicate that circulating androgens are critical for the maintenance of greater posterodorsal subnucleus of the medial amygdala regional volumes and soma sizes, and that environmental variables can regulate testosterone secretion and responsiveness.

Cross-sex hormone treatment does not change sex-sensitive cognitive performance in gender identity disorder patients

Cognitive performance in untreated early onset gender identity disorder (GID) patients might correspond to their born sex and not to their perceived gender. As a current mode of intervention, cross-sex hormone treatment causes considerable physical changes in GID patients. We asked, as has been suggested, whether this treatment skews cognitive performance towards that of the acquired sex. Somatically healthy male and female early onset GID patients were neuropsychologically tested before, 3 and 12 months after initiating cross-sex hormone treatment, whereas untreated healthy subjects without GID served as controls (C). Performance was assessed by testing six cognitive abilities (perception, arithmetic, rotation, visualization, logic, and verbalization), and controlled for age, education, born sex, endocrine differences and treatment by means of repeated measures analysis of variance. GID patients and controls showed an identical time-dependent improvement in cognitive performance. The slopes were essentially parallel for males and females. There was no significant three-way interaction of born sex by group by time for the six investigated cognitive abilities. Only education and age significantly influenced this improvement. Despite the substantial somatic cross-sex changes in GID patients, no differential effect on cognition over time was found between C and GID participants. The cognitive performance of cross-sex hormone-treated GID patients was virtually identical to that of the control group. The documented test-retest effect should be taken into consideration when evaluating treatment effects generally in psychiatry.

Seasonal plasticity in the copulatory neuromuscular system of green anole lizards: a role for testosterone in muscle but not motoneuron morphology

The copulatory system of green anoles is highly sexually dimorphic. Males possess bilateral copulatory organs called hemipenes, each independently controlled by two muscles: the transversus penis (TPN) and retractor penis magnus (RPM). The TPN everts the hemipene through the cloaca and the RPM retracts it. Adult females do not possess hemipenes or either of these two muscles. The spinal nucleus projecting to the TPN and RPM contains more and larger motoneurons in males than females. Because anoles breed seasonally, two experiments were designed to test whether adult copulatory morphology varies with environmental condition, and if so, whether the effect is mediated by testicular androgens. Three groups of adult males were used in each experiment: males from breeding environmental conditions with reproductive testes (BS); males in breeding conditions with regressed testes (BS-X); and males in nonbreeding conditions with regressed testes (NBS). Experiment 1 compared gonadally intact males and Experiment 2 compared castrated males treated with either testosterone (T) or an empty implant. In both experiments, copulatory and control motoneurons appeared smaller in NBS males, but T did not affect their size. In contrast, while hemipene and RPM muscle fiber size were not plastic across season in gonadally intact males, T in castrated males significantly increased both measures under BS and BS-X, but not NBS, conditions. These results demonstrate that neuron soma size might change on a general level and environmental cues can mediate T-induced changes in peripheral structures, suggesting that plasticity across copulatory system components is regulated by different mechanisms.

Effects of testosterone on the development of neuromuscular systems and their target tissues involved in courtship and copulation in green anoles (Anolis carolinensis)

Male green anole lizards court females using a red throat fan (dewlap) and copulate by intromitting one of two penises (hemipenes). These structures begin sexually monomorphic, but by adulthood males have larger dewlaps, only males have hemipenes, and many of the neuromuscular components of both systems show male-biased dimorphisms. We hypothesized that testosterone (T), which increases in juvenile males but not females about a month after hatching, facilitates masculinization. To test this idea, on post-hatching day 30, gonadally intact females received either a blank or T implant, and males were either castrated or sham-castrated. At day 90, juveniles were euthanized and the length of the cartilage and cross-sectional areas of the muscle fibers and motoneurons required for dewlap extension were examined. We also measured the cross-sectional areas of the hemipenes and associated muscle fibers and motoneurons, and counted the motoneurons. T-treated females had longer cartilages and larger dewlap muscle fibers compared to those with blank implants. No effects on motoneurons were detected, and no females possessed hemipenes or associated musculature. In males, castration produced shorter dewlap cartilages and smaller hemipenes; other measures were not affected by treatment. These data indicate that components of the dewlap system differentiate relatively late in development, that T likely mediates the process, and that although components of the copulatory system are plastic in juvenile males, sexual differentiation of peripheral features is complete before day 30. The data also suggest that target structures (dewlap cartilage and hemipenes), compared to their neuromuscular effectors, are particularly sensitive to developmental T exposure.

Sex-sensitive cognitive performance in untreated patients with early onset gender identity disorder

BACKGROUND

We explored whether the cognitive performance of gender identity disorder patients (GID) was comparable to that of their biological sex or skewed towards that of their gender identity.

METHOD

We tested four potentially sex-sensitive cognitive factors (rotation, visualization, perception, and verbalization) as well as two neutral factors (logic and arithmetic) in GID patients from Norway (GID-N, n = 33) or the USA (GID-US, n = 19) and in a control group (C, n = 29). The testing was undertaken prior to cross sex hormone treatment. Four-way ANOVA was applied in the final analysis of the cognitive performance and its dependency on different predictors (age, biological sex, education, group).

RESULTS

In both GID groups as well as in the control group (C) males excelled in visualization and rotation, also when controlling for potential confounders (biological sex, group, age and education). No female advantage was detected. Furthermore, no interaction between biological sex and group assignment was revealed in the samples.

CONCLUSION

In this study the cognitive pattern of GID patients is consistent with that of their biological sex and not that of their gender identity.

Variation in aromatase activity in the medial preoptic area and plasma progesterone is associated with the onset of paternal behavior

The effects of aromatase within the brain on sexual behavior have been studied in a wide variety of species. Relatively few non-mating behaviors have been considered, despite evidence that estrogen affects many social behaviors. Testosterone promotes paternal behavior in California mouse (Peromyscus californicus) fathers, acting primarily via aromatization to estradiol. Virgin male California mice rarely exhibit paternal behavior, so we investigated whether aromatase in the brain changed with the onset of paternal behavior in California mouse fathers. In the medial preoptic area (MPOA), a brain area known to regulate parental behavior in rodents, we found that fathers had significantly more aromatase activity than mated males without pups, suggesting that an increase in estrogen production in this brain area contributes to the onset of paternal behavior. We also found that progesterone (P(4)) levels were lower in fathers compared to sexually inexperienced males and that P(4) was negatively correlated with aromatase activity in the MPOA. These P(4) findings agree with a recent study that found an inhibitory effect of P(4) on paternal behavior. Overall, we found that aromatase activity and P(4) levels change in association with an important life history transition, and may provide a mechanistic basis for plasticity in paternal behavior.

Functional genomics of neural and behavioral plasticity

How does the environment, particularly the social environment, influence brain and behavior and what are the underlying physiologic, molecular, and genetic mechanisms? Adaptations of brain and behavior to changes in the social or physical environment are common in the animal world, either as short-term (i.e., modulatory) or as long-term modifications (e.g., via gene expression changes) in behavioral or physiologic properties. The study of the mechanisms and constraints underlying these dynamic changes requires model systems that offer plastic phenotypes as well as a sufficient level of quantifiable behavioral complexity while being accessible at the physiological and molecular level. In this article, I explore how the new field of functional genomics can contribute to an understanding of the complex relationship between genome and environment that results in highly plastic phenotypes. This approach will lead to the discovery of genes under environmental control and provide the basis for the study of the interrelationship between an individual's gene expression profile and its social phenotype in a given environmental context.

The ratio of second- and fourth-digit lengths and congenital adrenal hyperplasia due to 21-hydroxylase deficiency

Congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency results in excessive androgen exposure in the gestational period and various degrees of masculinization of the external genitalia in female foetuses. Intrauterine gonadal steroids are not only essential for the development of the genital organs but also affect some other extragenital organ development. The second to fourth digit (2D/4D) ratio shows a sexually dimorphic pattern with longer fourth digit from second digit in men compared to women. A low 2D/4D ratio is associated with high sperm count, testosterone levels and reproductive success in men. A high 2D/4D ratio is associated with high oestrogen levels in women. Second and fourth digit ratio has also found to be correlated with sexual orientation, left hand preference autism and some adult onset diseases such as breast cancer and myocardial infarction. We found lower 2D/4D ratio in female patients with 21-hydroxylase deficiency compared to healthy girls (p=0.000) and equal 2D/4D ratio for female patients when compared to male controls. Male patients with 21-hydroxylase deficiency had significantly lower 2D/4D ratio than female and male controls in the right hand. Healthy boys had lower 2D/4D ratio than healthy girls. It is concluded that 2D/4D ratio established by intrauterine androgen levels influences the sexually dimorphic digit pattern.

Evidence that androgen acts through NMDA receptors to affect motoneurons in the rat spinal nucleus of the bulbocavernosus

In adult male rats, spinal nucleus of the bulbocavernosus (SNB) motoneurons shrink after castration and are restored in size after androgen treatment. Sixty-day-old Sprague Dawley males were castrated and implanted with SILASTIC capsules containing testosterone (T) or nothing, and osmotic minipumps continuously infusing MK-801, a noncompetitive NMDA receptor antagonist, or saline. Twenty-five days later, bulbocavernosus muscles were injected with the retrograde tracer cholera toxin-horseradish peroxidase conjugate (CT-HRP) to label SNB cells. As seen previously, among saline-treated rats, SNB somata of T-treated castrates were significantly larger than those of castrates receiving blank capsules (p < 0.0001). MK-801 treatment blocked this effect of T on the SNB. MK-801 had no effect on non-androgen-responsive spinal motoneurons in the neighboring retrodorsolateral nucleus (RDLN), nor did the drug affect SNB soma size in the absence of androgen treatment. Motoneuronal soma size in Nissl stain revealed the same pattern of results seen with CT-HRP fills. In situ hybridization indicated that SNB motoneurons express mRNA for the NMDA receptor subunits R1, R2a, and R2b. Castration reduced the expression of R1 mRNA in SNB motoneurons, an effect that was blocked by androgen replacement in castrates. R2A and R2B mRNA expression in SNB cells was not affected by androgen manipulations. Likewise, androgen manipulations had no effect on the expression of any NMDA receptor subtypes in RDLN motoneurons. These results suggest that androgen affects the size of SNB motoneurons by influencing their expression of the NMDA receptor, and therefore the response of the motoneurons to endogenous glutamate.

Effects of perinatal picrotoxin and sexual experience on heterosexual and homosexual behavior in male rats

The effects of perinatal picrotoxin (0.75 mg/kg) on heterosexual and homosexual behavior of male rats, sexually experienced or not, were studied. The following data were obtained: (1) at birth, body weight and anogenital distance were not modified by the treatment; (2) during lactation, both treatment and sex interfered with body weight as well as in adult age; (3) as experimental animals were trained, the heterosexual behavior was improved; (4) picrotoxin treatment reduced lordotic response of homosexual behavior in inexperienced male rats and (5) the heterosexual experience with female rats inhibited homosexual behavior of both experimental and control animals. These results suggest that perinatal maternal picrotoxin exposure improved heterosexual behavior in male rats and the sexual experience reveals this effect. In addition, picrotoxin did not induce feminization in experimental inexperienced rats. Finally, the sexual experience per se promotes changes in brain regions related to male behavioral and sexual aspects.

Post-weaning social isolation of male rats reduces the volume of the medial amygdala and leads to deficits in adult sexual behavior

At 21 days of age, gonadally intact male Long Evans rats were weaned and placed into standard laboratory conditions (three per cage) or housed singly. They were tested for noncontact erections and sexual performance at 90 and 220 days of age. Rats raised in isolation displayed significantly fewer noncontact erections in response to sensory cues from an estrous female and fewer intromissions when allowed to mate with a female than did males raised in groups. The volume of the posterodorsal component of the medial amygdala (MePD) and the size of neurons within the MePD were significantly smaller in the isolated males than in socially housed males. Similarly, neurons in the sexually dimorphic nucleus of the preoptic area (SDN-POA) were smaller in isolate animals than in controls. As both MePD volume and SDN-POA soma size are responsive to sex steroids, these differences could result if the isolates experienced lower testosterone levels. Finally, the volume of the overall medial amygdala (MeA) correlated significantly with the number of noncontact erections, a relationship that was not explained by housing condition. These findings highlight the role of social experience as a factor in the sexual differentiation of the brain and suggest a positive relationship between the volume of a brain structure and the display of sexual behaviors.

Adult plasticity in hormone-sensitive motoneuron morphology: methodological/behavioral confounds

Changes in androgen levels can alter the structure of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), a motor nucleus that innervates perineal muscles involved in copulatory behavior. While sexual activity can alter androgen levels in normal males, it has no effect on SNB motoneuron soma size or dendritic morphology (Beversdorf, Kurz, and Sengelaub, 1990). However, Breedlove (1997) reported reductions in the size of SNB somata, nuclei, and target muscles of copulating versus noncopulating castrated rats maintained on subphysiological testosterone. To reconcile the results obtained using intact versus implant paradigms, we tested the hypothesis that the implant/behavior paradigm could produce differences in hormone levels, potentially confounding sexual behavior effects on the morphology of this androgen-sensitive neuromuscular system. Young adult male rats were castrated and immediately given 5-mm Silastic implants containing crystalline testosterone. One week later, blood samples were drawn and the males were housed with receptive females (copulators) or nonreceptive females (noncopulators) or housed alone (singles). After 27 days, blood samples were drawn again, and SNB target muscles and spinal cords removed. No differences in target muscle weight or SNB somata and nuclei size were observed between copulators, noncopulators, or singles; as expected, all measures were significantly reduced relative to intact males. Radioimmunoassay showed that testosterone declined differentially over the course of the behavioral manipulation across groups, being greatest in copulators and least pronounced in single males. These data indicate that differences in sexual or housing experience can alter testosterone titers under these implant conditions, potentially confounding hormone-sensitive measures of morphology.

The orthodox view of brain sexual differentiation

The standard view of sexual differentiation of the brain, derived primarily from work with mammals, is that the same steroidal signal which permanently masculinizes the body early in life, androgen, also permanently masculinizes the nervous system. This oversimplified view overlooks the rich diversity of mechanisms produced by natural selection. We review the mechanisms underlying sexual differentiation of what may be the simplest mammalian model, the spinal nucleus of the bulbocavernosus (SNB), a system that is intimately associated with sexual differentiation of the periphery. Indeed, in many instances, early androgen can permanently masculinize the SNB system but, surprisingly, these early influences may depend to some extent on social mediating factors. Furthermore, in adulthood, androgen continues to affect the SNB system in diverse ways, acting on several different loci, indicating a life-long plasticity in even this simple system. Finally, there is evidence that adult androgens interact with social experience in order to affect the SNB system. Thus the SNB system displays a far richer array of interactions than the standard view of sexual differentiation would predict. Examination of other systems and other species, as depicted in the following reports, reveals a far more complicated, and far more interesting perspective on how the brains and behaviors of males and females diverge.

Sexual orientation and handedness in men and women: a meta-analysis

Recent findings suggest that sexual orientation has an early neurodevelopmental basis. Handedness, a behavioral marker of early neurodevelopment, has been associated with sexual orientation in some studies but not in others. The authors conducted a meta-analysis of 20 studies that compared the rates of non-right-handedness in 6,987 homosexual (6,182 men and 805 women) and 16,423 heterosexual (14,808 men and 1,615 women) participants. Homosexual participants had 39% greater odds of being non-right-handed. The corresponding values for homosexual men (20 contrasts) and women (9 contrasts) were 34% and 91%, respectively. The results support the notion that sexual orientation in some men and women has an early neurodevelopmental basis, but the factors responsible for the handedness-sexual orientation association require elucidation. The authors discuss 3 possibilities: cerebral laterality and prenatal exposure to sex hormones, maternal immunological reactions to the fetus, and developmental instability.

Male-to-female transsexuals have female neuron numbers in a limbic nucleus

Transsexuals experience themselves as being of the opposite sex, despite having the biological characteristics of one sex. A crucial question resulting from a previous brain study in male-to-female transsexuals was whether the reported difference according to gender identity in the central part of the bed nucleus of the stria terminalis (BSTc) was based on a neuronal difference in the BSTc itself or just a reflection of a difference in vasoactive intestinal polypeptide innervation from the amygdala, which was used as a marker. Therefore, we determined in 42 subjects the number of somatostatin-expressing neurons in the BSTc in relation to sex, sexual orientation, gender identity, and past or present hormonal status. Regardless of sexual orientation, men had almost twice as many somatostatin neurons as women (P < 0.006). The number of neurons in the BSTc of male-to-female transsexuals was similar to that of the females (P = 0.83). In contrast, the neuron number of a female-to-male transsexual was found to be in the male range. Hormone treatment or sex hormone level variations in adulthood did not seem to have influenced BSTc neuron numbers. The present findings of somatostatin neuronal sex differences in the BSTc and its sex reversal in the transsexual brain clearly support the paradigm that in transsexuals sexual differentiation of the brain and genitals may go into opposite directions and point to a neurobiological basis of gender identity disorder.

Testosterone and estrogen affect neuronal differentiation but not proliferation in early embryonic cortex of the rat: the possible roles of androgen and estrogen receptors

We examined the effect of testosterone (T) and 17 beta-estradiol (E) on differentiation and proliferation of cultured neurons from the cortex of 14-day-rat embryos (E14) using immunocytochemistry. We found that the cultures receiving E had significantly more neurons with longer neurites than the control cultures, while both fewer and less differentiated neurons were seen after 24 h of incubation with T. However, neither T nor E changed the number of cells positive for BrdU, a proliferation marker. We also found that the androgen receptor (AR) was markedly expressed in the neurons, whereas the expression of estrogen (ER(alpha)) receptor was barely detectable. These results suggest that E and T differ in effect on differentiation, while neither affect proliferation in early developmental cortex. Furthermore, since the AR is expressed in the cortical neurons by E14, the inhibitory effect of T on differentiation may be receptor-mediated, while the stimulatory effects of estrogen in the cortex do not appear to involve nuclear ER(alpha) at this developmental stage.

Sexual dimorphisms in a neuromuscular system regulating courtship in the green anole lizard: effects of season and androgen treatment

During the breeding season, male green anoles (Anolis carolinensis) court females by extending a red throat fan called a dewlap. Motoneurons controlling this sexually dimorphic behavior are located in two portions of the brain stem: (a) the vagal portion of nucleus ambiguus (AmbX), and (b) the region containing the glossopharyngeal portion of nucleus ambiguus and the ventral motor nucleus of the facial nerve (AmbIX/VIImv). These motoneurons project to the ceratohyoideus muscle via the ramus pharyngo-laryngeus IX+X. To investigate the effects of season on and androgen regulation of neural and peripheral structures controlling dewlap extension, two experiments were conducted: (a) During the breeding and nonbreeding seasons, motoneuron number, soma size, and nucleus size were investigated in intact males and females and in castrated males treated with a testosterone propionate (TP) or a blank Silastic capsule. (b) Cross-sectional area of the nerve and muscle fiber size, number, and density were investigated in the four treatment groups during the breeding season only. No significant differences were found in motoneuron number. In the breeding season, subtle male-biased sex differences existed in both AmbX and AmbIX/VIImv soma size. Nerve cross-sectional area and muscle fiber size and number were substantially larger in males than females. Muscle fiber density was higher in females. No consistent effects due to season or androgen treatment were detected, although characteristics of motoneurons were in some cases slightly larger in the nonbreeding season. These results suggest that, while parallels to behavior exist between the sexes, morphological changes in adulthood in the dewlap motoneurons and muscle do not normally regulate courtship behavior in the male green anole.

Contributions of social cues and photoperiod to seasonal plasticity in the adult avian song control system

In seasonally breeding birds, the vernal growth of the song system is thought to result primarily from increased daylength and the associated increase in circulating testosterone. Other environmental factors such as social cues between mates influence the timing of reproduction, but less is known about how social cues might affect the song system and song behavior. We used white-crowned sparrows (Zonotrichia leucophrys gambelii) to test the hypothesis that the presence of a female in breeding condition influences song nuclei and song behavior of adult males. There were four treatment groups: (1) eight males housed individually in the same room on long days and paired with estradiol-implanted females; (2) eight males housed similarly on long days but without females; (3) four males isolated on long days; and (4) four males isolated on short days. The volumes of two song nuclei, HVc and RA, were significantly larger in males housed with females than in any other treatment group. Males isolated on short days had smaller HVc, RA, and area X volumes than all other groups. The volumes of Rt (a thalamic nucleus not involved in song) and the telencephalon did not differ among groups. Plasma androgen levels did not differ among the three long-day, social treatment groups at the times sampled, but were lower in the short-day isolates. Males paired with females sang at a higher maximum rate than males housed together, who sang at a higher rate than long-day isolates. These results suggest that seasonal plasticity in the adult song system is influenced by social cues.

Effects of sexual behavioral manipulation on brain plasticity in adult rats

The purpose of the present study was to determine the effects of sexual behaviorial manipulation on brain plasticity in adult male rats. Adult male Sprague-Dawley rats that copulated during male sexual behavior testing were divided into four groups: control male; gonadectomized (Gdx) male; sexually active male; and sexually nonactive male. Female animals were used as an additional control group. At the end of a 12-week experimental period, the animals were again tested for male sexual behavior and tested for sexual motivation. Sexual behavior manipulations over the 12-week period resulted in significant differences in mount latency, mount frequency, intromission latency, intromission frequency, ejaculation latency, and the postejaculation interval. In the motivation test, significant differences in the number of approaches, contacts, and crossings of an electrified grid separating the test animal from a receptive female were also observed. Sexually dimorphic nucleus of the preoptic area (SDN-POA) volumes in sexually nonactive males were significantly smaller than in control males or sexually active males. Anteroventral periventricular nucleus (AVPV) volumes in the male groups were not significantly altered by sexual behavioral manipulations, however, the nonactive AVPV vol. was the only vol. not significantly different from the control female vol. These data demonstrate that in the adult rat, sexual behavioral manipulations resulted in significant alterations in behavior and in the vol. of the SDN-POA and that the effect of sexual behavior on the AVPV needs to be further investigated.

Sexual differentiation of the vertebrate brain: principles and mechanisms

A wide variety of sexual dimorphisms, structural differences between the sexes, have been described in the brains of many vertebrate species, including humans. In animal models of neural sexual dimorphism, gonadal steroid hormones, specifically androgens, play a crucial role in engendering these differences by masculinizing the nervous system of males. Usually, the androgen must act early in life, often during the fetal period to masculinize the nervous system and behavior. However, there are a few examples of androgen, in adulthood, masculinizing both the structure of the nervous system and behavior. In the modal pattern, androgens are required both during development and adulthood to fully masculinize brain structure and behavior. In rodent models of neural sexual dimorphism, it is often the aromatized metabolites of androgen, i.e., estrogens, which interact with estrogen receptors to masculinize the brain, but there is little evidence that aromatized metabolites of androgen play this role in primates, including humans. There are other animal models where androgens themselves masculinize the nervous system through interaction with androgen receptors. In the course of masculinizing the nervous system, steroids can affect a wide variety of cellular mechanisms, including neurogenesis, cell death, cell migration, synapse formation, synapse elimination, and cell differentiation. In animal models, there are no known examples where only a single neural center displays sexual dimorphism. Rather, each case of sexual dimorphism seems to be part of a distributed network of sexually dimorphic neuronal populations which normally interact with each other. Finally, there is ample evidence of sexual dimorphism in the human brain, as sex differences in behavior would require, but there has not yet been any definitive proof that steroids acting early in development directly masculinize the human brain.

Sex steroids and libido

The effects of steroidal hormones on sexual desire and motivation are a question still under debate. This paper reviews up-to-date knowledge regarding physiological imprinting and activation by endogenous hormones of central nervous system areas involved in libido during intrauterine life and puberty. The endocrine environment probably continues to play a role during fertile life and the postmenopausal period, but this effect is often overridden by psychological and social factors. The impairment of sexual interest during estrogen-progestin treatment is an infrequent but relevant side-effect whose possible underlying mechanisms are discussed. Both endocrine and psychorelational elements may interact. From the biological point of view, androgen and oxytocin level modification and loss of estrogen fluctuations have been considered, but also the history of hormone-related mood changes could be a risk factor. On the psychological side, both the profound repercussions of the contraceptive choice and consequent responsibility, as well as the high value attributed to sexual experience are probably facilitating elements in the loss of libido under treatment.