Effects of the testicular feminization mutation (tfm) of the androgen receptor gene on BSTMPM volume and morphology in rats

Clomiphene citrate treatment during perinatal development alters adult partner preference, mating behaviour and androgen receptor and vasopressin in the male mandarin vole Microtus mandarinus

During development, many aspects of behaviour, including partner preferences and sexual behaviour, are "organized" by neural aromatization of androgen and oestrogen. This study aimed to analyse these processes in the mandarin vole (Microtus mandarinus); this is a novel mammalian model exhibiting strong monogamous pair bonds. Male pups were treated daily with a sesame oil only (MC) or the oestrogen receptor blocker-clomiphene citrate sesame oil mixture (MT) from prenatal day 14 to postnatal day 10. Female pups were treated daily with sesame oil only (FC). Partner preferences, sexual behaviour, and the expression of androgen receptor (AR) and arginine vasopressin (AVP) were examined when animals were 3 months old. The MT and FC groups exhibited male-directed partner preferences and feminized behaviour. AR-immunoreactive neurons (AR-IRs) in the medial preoptic area (mPOA), bed nucleus of stria terminalis (BNST), and medial amygdaloid nucleus (MeA) were reduced in MT males compared to MC males, and there was no significant difference in the number of AR-IRs between MT males and FC females. AVP-immunoreactive neurons (AVP-IRs) in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) were reduced in MT males compared to MC males, and there were no significant differences in the number of AVP-IRs between MT males and FC females. The results indicate a significant role of hormone signalling in the development of male mate preference in the novel monogamous mammal model.

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.

Oestrogen and androgen receptor activation contribute to the masculinisation of oxytocin receptors in the bed nucleus of the stria terminalis of rats

Oxytocin (OT) often regulates social behaviours in sex-specific ways, and this may be a result of sex differences in the brain OT system. Adult male rats show higher OT receptor (OTR) binding in the posterior bed nucleus of the stria terminalis (pBNST) than adult female rats. In the present study, we investigated the mechanisms that lead to this sex difference. First, we found that male rats have higher OTR mRNA expression in the pBNST than females at postnatal day (P) 35 and P60, which demonstrates the presence of the sex difference in OTR binding density at message level. Second, the sex difference in OTR binding density in the pBNST was absent at P0 and P3, but was present by P5. Third, systemic administration of the oestrogen receptor (ER) antagonist fulvestrant at P0 and P1 dose-dependently reduced OTR binding density in the pBNST of 5-week-old male rats, but did not eliminate the sex difference in OTR binding density. Fourth, pBNST-OTR binding density was lower in androgen receptor (AR) deficient genetic male rats compared to wild-type males, but higher compared to wild-type females. Finally, systemic administration of the histone deacetylase inhibitor valproic acid at P0 and P1 did not alter pBNST-OTR binding density in 5-week-old male and female rats. Interestingly, neonatal ER antagonism, AR deficiency, and neonatal valproic acid treatment each eliminated the sex difference in pBNST size. Overall, we demonstrate a role for neonatal ER and AR activation in setting up the sex difference in OTR binding density in the pBNST, which may underlie sexual differentiation of the pBNST and social behaviour.

Estrogenic regulation of social behavior and sexually dimorphic brain formation

It has long been known that the estrogen, 17β-estradiol (17β-E), plays a central role for female reproductive physiology and behavior. Numerous studies have established the neurochemical and molecular basis of estrogenic induction of female sexual behavior, i.e., lordosis, in animal models. In addition, 17β-E also regulates male-type sexual and aggressive behavior. In males, testosterone secreted from the testes is irreversibly aromatized to 17β-E in the brain. We discuss the contribution of two nuclear receptor isoforms, estrogen receptor (ER)α and ERβ to the estrogenic regulation of sexually dimorphic brain formation and sex-typical expression of these social behaviors. Furthermore, 17β-E is a key player for social behaviors such as social investigation, preference, recognition and memory as well as anxiety-related behaviors in social contexts. Recent studies also demonstrated that not only nuclear receptor-mediated genomic signaling but also membrane receptor-mediated non-genomic actions of 17β-E may underlie the regulation of these behaviors. Finally, we will discuss how rapidly developing research tools and ideas allow us to investigate estrogenic action by emphasizing behavioral neural networks.

Non-neural androgen receptors affect sexual differentiation of brain and behaviour

Although gonadal testosterone is the principal endocrine factor that promotes masculine traits in mammals, the development of a male phenotype requires local production of both androgenic and oestrogenic signals within target tissues. Much of our knowledge concerning androgenic components of testosterone signalling in sexual differentiation comes from studies of androgen receptor (Ar) loss of function mutants. Here, we review these studies of loss of Ar function and of AR overexpression either globally or selectively in the nervous system of mice. Global and neural mutations affect socio-sexual behaviour and the neuroanatomy of these mice in a sexually differentiated manner. Some masculine traits are affected by both global and neural mutation, indicative of neural mediation, whereas other masculine traits are affected only by global mutation, indicative of an obligatory non-neural androgen target. These results support a model in which multiple sites of androgen action coordinate to produce masculine phenotypes. Furthermore, AR overexpression does not always have a phenotype opposite to that of loss of Ar function mutants, indicative of a nonlinear relationship between androgen dose and masculine phenotype in some cases. Potential mechanisms of Ar gene function in non-neural targets in producing masculine phenotypes are discussed.

Gonadal Hormone-Dependent Sexual Differentiation of a Female-Biased Sexually Dimorphic Cell Group in the Principal Nucleus of the Bed Nucleus of the Stria Terminalis in Mice

We recently reported a female-biased sexually dimorphic area in the mouse brain in the boundary region between the preoptic area and the bed nucleus of the stria terminalis (BNST). We reexamined this area and found that it is a ventral part of the principal nucleus of the BNST (BNSTp). The BNSTp is a male-biased sexually dimorphic nucleus, but the ventral part of the BNSTp (BNSTpv) exhibits female-biased sex differences in volume and neuron number. The volume and neuron number of the BNSTpv were increased in males by neonatal orchiectomy and decreased in females by treatment with testosterone, dihydrotestosterone, or estradiol within 5 days after birth. Sex differences in the volume and neuron number of the BNSTpv emerged before puberty. These sex differences became prominent in adulthood with increasing volume in females and loss of neurons in males during the pubertal/adolescent period. Prepubertal orchiectomy did not affect the BNSTpv, although prepubertal ovariectomy reduced the volume increase and induced loss of neurons in the female BNSTpv. In contrast, the volume and neuron number of male-biased sexually dimorphic nuclei that are composed of mainly calbindin neurons and are located in the preoptic area and BNST were decreased by prepubertal orchiectomy but not affected by prepubertal ovariectomy. Testicular testosterone during the postnatal period may defeminize the BNSTpv via binding directly to the androgen receptor and indirectly to the estrogen receptor after aromatization, although defeminization may proceed independently of testicular hormones in the pubertal/adolescent period. Ovarian hormones may act to feminize the BNSTpv during the pubertal/adolescent period.

Androgenic mechanisms of sexual differentiation of the nervous system and behavior

Testicular androgens are the major endocrine factor promoting masculine phenotypes in vertebrates, but androgen signaling is complex and operates via multiple signaling pathways and sites of action. Recently, selective androgen receptor mutants have been engineered to study androgenic mechanisms of sexual differentiation of the nervous system and behavior. The focus of these studies has been to evaluate androgenic mechanisms within the nervous system by manipulating androgen receptor conditionally in neural tissues. Here we review both the effects of neural loss of AR function as well as the effects of neural overexpression of AR in relation to global AR mutants. Although some studies have conformed to our expectations, others have proved challenging to assumptions underlying the dominant hypotheses. Notably, these studies have called into question both the primacy of direct, neural mechanisms and also the linearity of the relationship between androgenic dose and sexual differentiation of brain and behavior.

A Sexually Dimorphic Area of the Dorsal Hypothalamus in Mice and Common Marmosets

We found a novel sexually dimorphic area (SDA) in the dorsal hypothalamus (DH) of mice. The SDA-DH was sandwiched between 2 known male-biased sexually dimorphic nuclei, the principal nucleus of the bed nucleus of the stria terminalis and the calbindin-sexually dimorphic nucleus, and exhibited a female-biased sex difference in neuronal cell density. The density of neurons in the SDA-DH was increased in male mice by orchidectomy on the day of birth and decreased in female mice by treatment with testosterone, dihydrotestosterone, or estradiol within 5 days after birth. These findings indicate that the SDA-DH is defeminized under the influence of testicular testosterone, which acts via both directly by binding to the androgen receptor, and indirectly by binding to the estrogen receptor after aromatization. We measured the activity of SDA-DH neurons with c-Fos, a neuronal activity marker, in female mice during maternal and sexual behaviors. The number of c-Fos-expressing neurons in the SDA-DH of female mice was negatively correlated with maternal behavior performance. However, the number of c-Fos-expressing neurons did not change during female sexual behavior. These findings suggest that the SDA-DH contains a neuronal cell population, the activity of which decreases in females exhibiting higher performance of maternal behavior, but it may contribute less to female sexual behavior. Additionally, we examined the brain of common marmosets and found an area that appears to be homologous with the mouse SDA-DH. The sexually dimorphic structure identified in this study is not specific to mice and may be found in other species.

Regional difference in sex steroid action on formation of morphological sex differences in the anteroventral periventricular nucleus and principal nucleus of the bed nucleus of the stria terminalis

Sex steroid action is critical to form sexually dimorphic nuclei, although it is not fully understood. We previously reported that masculinization of the principal nucleus of the bed nucleus of the stria terminalis (BNSTp), which is larger and has more neurons in males than in females, involves aromatized testosterone that acts via estrogen receptor-α (ERα), but not estrogen receptor-β (ERβ). Here, we examined sex steroid action on the formation of the anteroventral periventricular nucleus (AVPV) that is larger and has more neurons in females. Morphometrical analysis of transgenic mice lacking aromatase, ERα, or ERβ genes revealed that the volume and neuron number of the male AVPV were significantly increased by deletion of aromatase and ERα genes, but not the ERβ gene. We further examined the AVPV and BNSTp of androgen receptor knockout (ARKO) mice. The volume and neuron number of the male BNSTp were smaller in ARKO mice than those in wild-type mice, while no significant effect of ARKO was found on the AVPV and female BNSTp. We also examined aromatase, ERα, and AR mRNA levels in the AVPV and BNSTp of wild-type and ARKO mice on embryonic day (ED) 18 and postnatal day (PD) 4. AR mRNA in the BNSTp and AVPV of wild-type mice was not expressed on ED18 and emerged on PD4. In the AVPV, the aromatase mRNA level was higher on ED18, although the ERα mRNA level was higher on PD4 without any effect of AR gene deletion. Aromatase and ERα mRNA levels in the male BNSTp were significantly increased on PD4 by AR gene deletion. These results suggest that estradiol signaling via ERα during the perinatal period and testosterone signaling via AR during the postnatal period are required for masculinization of the BNSTp, whereas the former is sufficient to defeminize the AVPV.

The mechanisms underlying sexual differentiation of behavior and physiology in mammals and birds: relative contributions of sex steroids and sex chromosomes

From a classical viewpoint, sex-specific behavior and physiological functions as well as the brain structures of mammals such as rats and mice, have been thought to be influenced by perinatal sex steroids secreted by the gonads. Sex steroids have also been thought to affect the differentiation of the sex-typical behavior of a few members of the avian order Galliformes, including the Japanese quail and chickens, during their development in ovo. However, recent mammalian studies that focused on the artificial shuffling or knockout of the sex-determining gene, Sry, have revealed that sex chromosomal effects may be associated with particular types of sex-linked differences such as aggression levels, social interaction, and autoimmune diseases, independently of sex steroid-mediated effects. In addition, studies on naturally occurring, rare phenomena such as gynandromorphic birds and experimentally constructed chimeras in which the composition of sex chromosomes in the brain differs from that in the other parts of the body, indicated that sex chromosomes play certain direct roles in the sex-specific differentiation of the gonads and the brain. In this article, we review the relative contributions of sex steroids and sex chromosomes in the determination of brain functions related to sexual behavior and reproductive physiology in mammals and birds.

New knockout model confirms a role for androgen receptors in regulating anxiety-like behaviors and HPA response in mice

Men are less likely than women to suffer from anxiety disorders. Because gonadal hormones play a crucial role in many behavioral sex differences, they may underlie sex differences in human anxiety. In rodents, testosterone (T) exerts anxiolytic effects via the androgen receptor (AR): we found that male mice with a naturally-occurring mutation rendering the AR dysfunctional, referred to as spontaneous testicular feminization mutation (sTfm), showed more anxiety-like behaviors than wildtype (WT) males. Here, we used Cre-lox recombination technology to create another dysfunctional allele for AR. These induced Tfm (iTfm) animals also displayed more anxiety-like behaviors than WTs. We further found that AR-modulation of these behaviors interacts with circadian phase. When tested in the resting phase, iTfms appeared more anxious than WTs in the open field, novel object and elevated plus maze tests, but not the light/dark box. However, when tested during the active phase (lights off), iTfms showed more anxiety-related behavior than WTs in all four tests. Finally, we confirmed a role of T acting via AR in regulating HPA axis activity, as WT males with T showed a lower baseline and overall corticosterone response, and a faster return to baseline following mild stress than did WT males without T or iTfms. These findings demonstrate that this recombined AR allele is a valuable model for studying androgenic modulation of anxiety, that the anxiolytic effects of AR in mice are more prominent in the active phase, and that HPA axis modulation by T is AR dependent.

The organizational role of testicular hormones and the androgen receptor in anxiety-related behaviors and sensorimotor gating in rats

Perinatal exposure to testosterone (T), which can act upon both the androgen receptor (AR) and, via aromatization of T into estrogens, upon estrogen receptors, organizes many adult behaviors in rodents. We compared behaviors in wild-type (WT) male rats and AR-deficient rats with the testicular feminization mutation (Tfm), which on the day of birth were either gonadectomized (Neo-Gdx) or sham operated. In adulthood, all rats were either gonadectomized or sham operated and implanted with T capsules to equilibrate circulating androgens. In each of four tests of behavior related to anxiety (open field, novel object exposure, light/dark box, and elevated plus maze), Neo-Gdx rats showed decreased indices of anxiety and increased activity compared with rats sham operated on the day of birth, with no differences between WT or Tfm males within treatment groups. These results indicate that testicular hormones act in development to increase adult indices of anxiety and decrease activity in males and that functional ARs are not required for this effect. Acoustic startle response was also reduced by Neo-Gdx, suggesting that postnatal testicular secretions potentiate this behavior as well. Adult corticosterone levels and sensorimotor gating, as measured by prepulse inhibition of the acoustic startle response, were increased by neonatal castration in both WT and Tfm rats. These findings indicate a role of T before adulthood in the organization of anxiety-related behaviors, activity, the hypothalamic-pituitary-adrenal axis, and sensorimotor gating in rats, all of which appears to be AR independent.

Organized for sex - steroid hormones and the developing hypothalamus

Steroid hormones of gonadal origin act on the neonatal brain, particularly the hypothalamus, to produce sex differences that underlie copulatory behavior. Neuroanatomical sex differences include regional volume, cell number, connectivity, morphology, physiology, neurotransmitter phenotype and molecular signaling, all of which are determined by the action of steroid hormones, particularly by estradiol in males, and are established by diverse downstream effects. Sex differences in distinct hypothalamic regions can be organized by the same steroid hormone, but the direction of a sex difference is often specific to one region or cell type, illustrating the wide range of effects that steroid hormones have on the developing brain. Substantial progress has been made in elucidating the downstream mechanisms through which gonadal hormones sexually differentiate the brain, but gaps remain in establishing the precise relationship between changes in neuronal morphology and behavior. A complete understanding of sexual differentiation will require integrating the diverse mechanisms across multiple brain regions into a functional network that regulates behavioral output.

Effects of aromatase or estrogen receptor gene deletion on masculinization of the principal nucleus of the bed nucleus of the stria terminalis of mice

The principal nucleus of the bed nucleus of the stria terminalis (BNSTp) is a sexually dimorphic nucleus, and the male BNSTp is larger and has more neurons than the female BNSTp. To assess the roles of neuroestrogen synthesized from testicular androgen by brain aromatase in masculinization of the BNSTp, we performed morphometrical analyses of the adult BNSTp in aromatase knockout (ArKO), estrogen receptor-α knockout (αERKO), and estrogen receptor-β knockout (βERKO) mice and their respective wild-type littermates. In wild-type littermates, the BNSTp of males had a larger volume and greater numbers of neuronal and glial cells than did that of females. The volume and neuron number of the BNSTp in ArKO and αERKO males and glial cell number of the BNSTp in αERKO males were significantly smaller than those of wild-type male littermates, and they were not significantly different from those in female mice with either gene knockout. In contrast, there was no significant morphological difference in the BNSTp between βERKO and wild-type mice. Next, we examined the BNSTp of ArKO males subcutaneously injected with estradiol benzoate (EB) on postnatal days 1, 2, and 3 (1.5 μg/day). EB-treated ArKO males had a significantly greater number of BNSTp neurons than did oil-treated ArKO males. The number of BNSTp neurons in EB-treated ArKO males was comparable to that in wild-type males. These findings suggested that masculinization of the BNSTp in mice involves the actions of neuroestrogen that was synthesized by aromatase and that this estrogen mostly binds to ERα during the postnatal period.

Control of cell number in the bed nucleus of the stria terminalis of mice: role of testosterone metabolites and estrogen receptor subtypes

INTRODUCTION

The bed nucleus of the stria terminalis (BNST) exhibits several sex differences that may be related to male sexual behavior and gender identity. In mice and rats, sex differences in the principal nucleus of the BNST (BNSTp) are due to sexually dimorphic cell death during perinatal life. Although testosterone treatment of newborn female rats increases BNSTp cell number, the relevant hormone metabolite(s) are not known, and the effect of testosterone on the development of BNSTp cell number in mice has not been examined.

AIM

To identify the sex hormone metabolites and receptors controlling cell number, volume, and cell size in the BNSTp of mice.

METHODS

In the first experiment, C57BL/6J male mice were injected on the day of birth with peanut oil; females were injected with testosterone propionate (TP), estradiol benzoate (EB), dihydrotestosterone propionate (DHTP), or oil alone, and the BNSTp of all animals was examined in adulthood. In the second experiment, to compare effects of EB to the effects of estrogen receptor subtype specific agonists, newborn female mice were injected with EB, propyl-pyrazole-triol (PPT, a selective estrogen receptor alpha [ERalpha] agonist), or diarylpropionitrile (DPN, a selective estrogen receptor beta [ERbeta] agonist).

MAIN OUTCOME MEASURES

Nuclear volume measurements and stereological cell counts in the BNSTp in adulthood.

RESULTS

TP treatment of newborn females completely masculinized both BNSTp volume and cell number. EB masculinized neuron number, whereas DHTP had no effect on volume or cell number. In the second experiment, EB again fully masculinized neuron number in the BNSTp and in this study also masculinized BNSTp volume. PPT and DPN each significantly increased cell number, but neither completely mimicked the effects of EB.

CONCLUSIONS

We conclude that estrogenic metabolites of testosterone control sexually dimorphic cell survival in the BNSTp and that activation of both ERalpha and ERbeta may be required for complete masculinization of this brain region.

Sex differences and the roles of sex steroids in apoptosis of sexually dimorphic nuclei of the preoptic area in postnatal rats

The brain contains several sexually dimorphic nuclei that exhibit sex differences with respect to cell number. It is likely that the control of cell number by apoptotic cell death in the developing brain contributes to creating sex differences in cell number in sexually dimorphic nuclei, although the mechanisms responsible for this have not been determined completely. The milieu of sex steroids in the developing brain affects sexual differentiation in the brain. The preoptic region of rats has two sexually dimorphic nuclei. The sexually dimorphic nucleus of the preoptic area (SDN-POA) has more neurones in males, whereas the anteroventral periventricular nucleus (AVPV) has a higher cell density in females. Sex differences in apoptotic cell number arise in the SDN-POA and AVPV of rats in the early postnatal period, and an inverse correlation exists between sex differences in apoptotic cell number and the number of living cells in the mature period. The SDN-POA of postnatal male rats exhibits a higher expression of anti-apoptotic Bcl-2 and lower expression of pro-apoptotic Bax compared to that in females and, as a potential result, apoptotic cell death via caspase-3 activation more frequently occurs in the SDN-POA of females. The patterns of expression of Bcl-2 and Bax in the SDN-POA of postnatal female rats are changed to male-typical ones by treatment with oestrogen, which is normally synthesised from testicular androgen and affects the developing brain in males. In the AVPV of postnatal rats, apoptotic regulation also differs between the sexes, although Bcl-2 expression is increased and Bax expression and caspase-3 activity are decreased in females. The mechanisms of apoptosis possibly contributing to the creation of sex differences in cell number and the roles of sex steroids in apoptosis are discussed.

The androgen receptor is selectively involved in organization of sexually dimorphic social behaviors in mice

It is well established that sexually dimorphic neural regions are organized by steroid hormones during development. In many species, neonatal males are exposed to more testosterone than their female littermates, and ultimately it is the estradiol, produced by aromatization of testosterone, that affects sexual differentiation. However, the androgen receptor also plays an important role in the masculinization of brain and behavior. Here we tested the hypothesis that sexually dimorphic social and odor preference behaviors can be differentiated by a nonaromatizable androgen during development by treating female mice on the day of birth (PN0) with dihydrotestosterone (DHT). Control mice received a single vehicle injection on PN0. Adults were gonadectomized, treated with estradiol, and tested for social behaviors. In contrast with control females, females treated on PN0 with DHT, like male controls, exhibited a preference for female-soiled vs. male-soiled bedding, a preference to investigate a female vs. a male and reduced c-Fos-immunoreactivity (ir) in several neural areas after exposure to male-soiled bedding. However, females treated with DHT on PN0 had normal female-typical sexual behavior. The number of calbindin-ir cells in the preoptic area is sexually dimorphic (males more than females), but females given DHT on PN0 had intermediate numbers of calbindin-ir neurons, not significantly different from control males or females. Our data demonstrate that organization of social and olfactory preferences in mice can be affected by perinatal DHT and lends support to the role of androgen receptor in organization of sexual differentiation of brain and behaviors.

A role for the androgen receptor in the sexual differentiation of the olfactory system in mice

Olfactory signals play a central role in the identification of a mating partner in rodents, and the behavioral response to these cues varies markedly between the sexes. As several other sexually dimorphic traits, this response is thought to differentiate as a result of exposure of the developing individual to gonadal steroids, but both the identity of the specific steroid signal and the neural structures targeted for differentiation on this particular case are largely unknown. The present review summarizes results obtained in our lab using genetic males affected by the testicular feminization syndrome (Tfm) as experimental model, and that led to the identification of a role for non-aromatized gonadal steroids acting through the androgen receptor (AR) in the differentiation of olfactory cues processing in mice. The existing literature about AR-mediated sexual differentiation of the CNS in animal models is discussed, along with potential targets for the action of non-aromatized gonadal steroids in either one of the subsystems that detect and process olfactory information in rodents.

The role of androgen receptors in the masculinization of brain and behavior: what we've learned from the testicular feminization mutation

Many studies demonstrate that exposure to testicular steroids such as testosterone early in life masculinizes the developing brain, leading to permanent changes in behavior. Traditionally, masculinization of the rodent brain is believed to depend on estrogen receptors (ERs) and not androgen receptors (ARs). According to the aromatization hypothesis, circulating testosterone from the testes is converted locally in the brain by aromatase to estrogens, which then activate ERs to masculinize the brain. However, an emerging body of evidence indicates that the aromatization hypothesis cannot fully account for sex differences in brain morphology and behavior, and that androgens acting on ARs also play a role. The testicular feminization mutation (Tfm) in rodents, which produces a nonfunctional AR protein, provides an excellent model to probe the role of ARs in the development of brain and behavior. Tfm rodent models indicate that ARs are normally involved in the masculinization of many sexually dimorphic brain regions and a variety of behaviors, including sexual behaviors, stress response and cognitive processing. We review the role of ARs in the development of the brain and behavior, with an emphasis on what has been learned from Tfm rodents as well as from related mutations in humans causing complete androgen insensitivity.