Sex Differences in Neuronal Morphology Influenced Hormonally throughout Life

Does the syrinx, a peripheral structure, constrain effects of sex steroids on behavioral sex reversal in adult canaries?

We previously confirmed that effects of testosterone (T) on singing activity and on the volume of brain song control nuclei are sexually differentiated in adult canaries: females are limited in their ability to respond to T as males do. Here we expand on these results by focusing on sex differences in the production and performance of trills, i.e., rapid repetitions of song elements. We analyzed >42,000 trills recorded over a period of 6 weeks from 3 groups of castrated males and 3 groups of photoregressed females that received Silastic™ implants filled with T, T plus estradiol or left empty as control. Effects of T on the number of trills, trill duration and percent of time spent trilling were all stronger in males than females. Irrespective of endocrine treatment, trill performance assessed by vocal deviations from the trill rate versus trill bandwidth trade-off was also higher in males than in females. Finally, inter-individual differences in syrinx mass were positively correlated with specific features of trills in males but not in females. Given that T increases syrinx mass and syrinx fiber diameter in males but not in females, these data indicate that sex differences in trilling behavior are related to sex differences in syrinx mass and syrinx muscle fiber diameter that cannot be fully suppressed by sex steroids in adulthood. Sexual differentiation of behavior thus reflects organization not only of the brain but also of peripheral structures.

Does the syrinx, a peripheral structure, constrain effects of sex steroids on behavioral sex reversal in adult canaries?

We previously confirmed that effects of testosterone (T) on singing activity and on the volume of brain song control nuclei are sexually differentiated in adult canaries: females are limited in their ability to respond to T as males do. Here we expand on these results by focusing on sex differences in the production and performance of trills, i.e., rapid repetitions of song elements. We analyzed more than 42,000 trills recorded over a period of 6 weeks from 3 groups of castrated males and 3 groups of photoregressed females that received Silasticâ"¢ implants filled with T, T plus estradiol or left empty as control. Effects of T on the number of trills, trill duration and percent of time spent trilling were all stronger in males than females. Irrespective of endocrine treatment, trill performance assessed by vocal deviations from the trill rate versus trill bandwidth trade-off was also higher in males than in females. Finally, inter-individual differences in syrinx mass were positively correlated with trill production in males but not in females. Given that T increases syrinx mass and syrinx fiber diameter in males but not in females, these data indicate that sex differences in trilling behavior are related to sex differences in syrinx mass and syrinx muscle fiber diameter that cannot be fully reversed by sex steroids in adulthood. Sexual differentiation of behavior thus reflects organization not only of the brain but also of peripheral structures.

Molecular correlates of hypothalamic development in songbird ontogeny in comparison with the telencephalon

Development of the songbird brain provides an excellent experimental model for understanding the regulation of sex differences in ontogeny. Considering the regulatory role of the hypothalamus in endocrine, in particular reproductive, physiology, we measured the structural (volume) and molecular correlates of hypothalamic development during ontogeny of male and female zebra finches. We quantified by relative quantitative polymerase chain reaction (rqPCR) the expression of 14 genes related to thyroid and steroid hormones actions as well as 12 genes related to brain plasticity at four specific time points during ontogeny and compared these expression patterns with the expression of the same genes as detected by transcriptomics in the telencephalon. These two different methodological approaches detected specific changes with age and demonstrated that in a substantial number of cases changes observed in both brain regions are nearly identical. Other genes however had a tissue-specific developmental pattern. Sex differences or interactions of sex by age were detected in the expression of a subset of genes, more in hypothalamus than telencephalon. These results correlate with multiple known aspects of the developmental and reproductive physiology but also raise a number of new functional questions.

Steroidogenic factor 1 and the central nervous system

Steroidogenic factor 1 (SF-1; officially designated NR5a1) is a member of a nuclear receptor superfamily with important roles in the development of endocrine systems. Studies with global and tissue-specific (i.e. central nervous system) knockout mice have revealed several roles of SF-1 in brain. These include morphological effects on the development of the ventromedial nucleus of the hypothalamus and functional effects on body weight regulation through modulation of physical activity, anxiety-like behaviours and female sexual behaviours. Although such defects are almost certainly a result of the absence of SF-1 acting as a transcription factor in the hypothalamus, global SF-1 knockout mice also represent a model for studying the sex differences in the brain that develop in the absence of exposure to foetal sex steroid hormones as a result of the absence of gonads. In the present review, current knowledge of the roles of SF-1 protein in the central nervous system is discussed.

Sex differences in brain developing in the presence or absence of gonads

Brain sexual differentiation results from the interaction of genetic and hormonal influences. This study used a unique agonadal mouse model to determine relative contributions of genetic and gonadal hormone influences in the differentiation of selected brain regions. SF-1 knockout (SF-1 KO) mice are born without gonads and adrenal glands and are not exposed to endogenous sex steroids during fetal/neonatal development. Consequently, male and female SF-1 KO mice are born with female external genitalia and if left on their own, die shortly after birth due to adrenal insufficiency. In this study, SF-1 KO mice were rescued by neonatal adrenal transplantation to examine their brain morphology in adult life. To determine potential brain loci that might mediate functional sex differences, we examined the area and distribution of immunoreactive calbindin and neuronal nitric oxide synthase in the preoptic area (POA) and ventromedial nucleus of the hypothalamus, two areas previously reported to be sexually dimorphic in the mammalian brain. A sex difference in the positioning of cells containing immunoreactive calbindin in a group within the POA was clearly gonad dependent based on the elimination of the sex difference in SF-1 KO mice. Several other differences in the area of ventromedial hypothalamus and in POA were maintained in male and female SF-1 KO mice, suggesting gonad-independent genetic influences on sexually dimorphic brain development.

Neuronal nitric oxide synthase and calbindin delineate sex differences in the developing hypothalamus and preoptic area

Throughout the hypothalamus there are several regions known to contain sex differences in specific cellular, neurochemical, or cell grouping characteristics. The current study examined the potential origin of sex differences in calbindin expression in the preoptic area and hypothalamus as related to sources of nitric oxide. Specific cell populations were defined by immunoreactive (ir) calbindin and neuronal nitric oxide synthase (nNOS) in the preoptic area/anterior hypothalamus (POA/AH), anteroventral periventricular nucleus (AVPv), and ventromedial nucleus of the hypothalamus (VMN). The POA/AH of adult mice was characterized by a striking sex difference in the distribution of cells with ir-calbindin. Examination of the POA/AH of androgen receptor deficient Tfm mice suggests that this pattern was in part androgen receptor dependent, since Tfm males had reduced ir-calbindin compared with wild-type males and more similar to wild-type females. At P0 ir-calbindin was more prevalent than in adulthood, with males having significantly more ir-calbindin and nNOS than have females. Cells that contained either ir-calbindin or ir-nNOS in the POA/AH were in adjacent cell groups, suggesting that NO derived from the enzymatic activity of nNOS may influence the development of ir-calbindin cells. In the region of AVPv, at P0, there was a sex difference with males having more ir-nNOS fibers than have females while ir-calbindin was not detected. In the VMN, at P0, ir-nNOS was greater in females than in males, with no significant difference in ir-calbindin. We suggest that NO as an effector molecule and calbindin as a molecular biomarker illuminate key aspects of sexual differentiation in the developing mouse brain.

Genes controlling hypothalamic development and sexual differentiation

Steroid hormones dramatically influence the development of numerous sites in the nervous system. Basic mechanisms in neural development provide foci for understanding how factors related to sex can alter the ontogeny of these regions. Sex differences in neurogenesis, cell migration, cell differentiation, cell death, and synaptogenesis are being addressed. Any and all of these events serve as likely targets for genetic or gonadal steroid-dependent mechanisms throughout development. Although the majority of sexually dimorphic characteristics in brain have been described in older animals, many hormonal mechanisms that determine sexually differentiated brain characteristics occur during critical perinatal periods. Genes suggested to contribute to the development of specific hypothalamic nuclear groups have rarely been examined in the context of sex. The identification of sex differences in the expression of some of these genes may suggest early and likely transient molecular events that set the stage for later amplification by hormone actions. Sex differences in the positioning of cells in the developing hypothalamus further suggest that cell migration may be one key target for early gene actions that impact long-term susceptibility to brain sexual differentiation.

Oestrogen receptor alpha is essential for female-directed chemo-investigatory behaviour but is not required for the pheromone-induced luteinizing hormone surge in male mice

The expression of normal masculine sexual behaviour requires testosterone. Testosterone can bind to androgen receptors, either in its native form, or after reduction to other androgen metabolites. In addition, testosterone can be aromatized to oestrogen, and bind to oestrogen receptor alpha and/or beta. Male copulatory behaviour is deficient in mice lacking functional oestrogen receptor alpha gene (ERalphaKO mice). We sought to determine which aspect(s) of masculine sexual behaviour is compromised in the ERalphaKOs. Specifically, we asked whether ERalphaKO males have reduced motivation and/or an inability to recognize oestrous females. We found significant differences between mice of different genotypes in the amount of chemo-investigatory behaviour displayed and in the target of their investigation. Wild-type males spent more time investigating ovariectomized, oestradiol-treated females, than either males, or ovariectomized females that had not received hormone priming. ERalphaKO males spent little time investigating any of the stimulus mice and showed no preferences. To test the hypothesis that this lack of chemo-investigatory behaviour is due to the inability of ERalphaKO males to detect and respond to female pheromones, we exposed males to chemosensory cues (soiled bedding) from females. Males resided in clean, or female-soiled, cage bedding for 60 min. Next, blood was collected and plasma luteinizing hormone (LH) assayed. We also assessed Fos-like immunoreactivity (Fos-ir) in several neural regions involved in processing chemosensory cues. Despite the fact that male ERalphaKOs spend little time engaged in chemo-investigation of females, their neuroendocrine responses to female-soiled bedding were similar to those seen in wild-type males. Our data suggest that the normal coupling between the neuroendocrine response to females and the generation of sexual behaviour is disrupted in ERalphaKO mice. Responses to female pheromones do not require ERalpha. However, normal male sexual performance requires the ERalpha gene.

Sex differences in cell migration in the preoptic area/anterior hypothalamus of mice

The preoptic area/anterior hypothalamus (POA/AH) sits as a boundary region rostral to the classical diencephalic hypothalamus and ventral to the telencephalic septal region. Numerous studies have pointed to the region's importance for sex-dependent functions. Previous studies suggested that migratory guidance cues within this region might be particularly unique in their diversity. To better understand the early development and differentiation of the POA/AH, cytoarchitectural, birthdate, immunocytochemical, and cell migration studies were conducted in vivo and in vitro using embryonic C57BL/6J mice. A medial preoptic nucleus became discernible using Nissl stain in males and females between embryonic days (E) E15 and E17. Cells containing immunoreactive estrogen receptor-alpha were detected in the POA/AH by E13, and increased in number with age in both sexes. From E15 to E17, examination of the radial glial fiber pattern by immunocytochemistry confirmed the presence of dual orientations for migratory guidance ventral to the anterior commissure (medial-lateral and dorsal-ventral) and uniform orientation more caudally (medial-lateral). Video microscopy studies followed the migration of DiI-labeled cells in coronal 250-microm brain slices from E15 mice maintained in serum-free media for 1-3 days. Analyses showed significant migration along a dorsal-ventral orientation in addition to medial-lateral. The video analyses showed significantly more medial-lateral migration in males than females in the caudal POA/AH. In vivo, changes in the distribution of cells labeled by the mitotic indicator bromodeoxyuridine (BrdU) suggested their progressive migration through the POA/AH. BrdU analyses also indicated significant movement from dorsal to ventral regions ventral to the anterior commissure. The significant dorsal-ventral migration of cells in the POA/AH provides additional support for the notion that the region integrates developmental information from both telencephalic and diencephalic compartments. The sex difference in the orientation of migration of cells in the caudal POA/AH suggests one locus for the influence of gonadal steroids in the embryonic mouse forebrain.

Sex with knockout models: behavioral studies of estrogen receptor alpha

Estrogens are an important class of steroid hormones, having multiple targets, in the body and brain, and exerting ubiquitous effects on behavior. At present, two estrogen receptors (ERalpha and beta) have been cloned and sequenced in mammals. In the brain these receptors are regionally specific, but both have widespread distributions, which are largely non-overlapping. Given the newly emerging complexities of estrogen's mechanisms of action it is important to distinguish which pathways are involved in modifying which behaviors. We use a knockout mouse, lacking functional copies of the estrogen receptor alpha (ERalpha) gene, to study the mechanisms by which estrogens mediate behaviors. There are pronounced ramifications of ERalpha gene disruption on behavior. First, female ERalpha knockout (ERalphaKO) mice do not display normal feminine sexual behavior. Second, treatment of adult mice with androgens promotes masculine sexual behavior in both sexes. However, male-typical sexual behavior is severely compromised in male and female ERalphaKOs. Third, male ERalphaKOs do not exhibit the same social preferences for female mice as do wildtype (WT) littermates. Thus, the ERalpha is essential for normal expression of sexual behaviors. In addition, gonadectomized ERalphaKO and WT mice rapidly learn to escape from the Morris water maze. Exogenous estrogen treatment prevents WT females from learning this task, yet, has no effect in ERalphaKO mice, suggesting that estrogens effects on learning in adult females involves the ERalpha. Based on these data we hypothesize that ERalpha mediates many of the effects of estrogen on sexual behavior, learning, and memory.

The preoptic area/anterior hypothalamus of different strains of mice: sex differences and development

While sex differences in neural morphology in the preoptic area/anterior hypothalamus (POA/AH) have been demonstrated in many species, their existence in mice have been controversial. Given the increased use of transgenic and gene-disrupted mice, we characterized sex differences using Nissl stains, and the immunocytochemical location of estrogen receptor-alpha (ER-alpha) and galanin in the POA/AH of two widely used strains, C57BL/6 and 129SvEv, and a mixed strain (C57BL/6x129Sv); the wild-type littermates of steroidogenic factor-1 (SF-1) gene-disrupted mice. Cell grouping was not a reliable marker of sex. In adults, cells located beneath the anterior commissure (AC) were reliably larger in females than males in 129SvEv, but not in the other strains. Caudally, cells in a group medial to the medial extension of the bed nucleus of the stria terminalis (BST) were significantly larger in males than females in C57BL/6J and SF-1 gene-disrupted wild-types. Cell groups discernible by embryonic day (E) 18 were not sexually dimorphic for cell size in C57BL/6J mice at E18 or postnatal day (P) 4. The pattern of distribution of cells containing ER-alpha was similar among the strains, reduced in the group medial to the BST; a pattern established by P0. Galanin-containing cells and fibers were seen from E15 to adulthood ventral to the AC. Caudally, a smaller group ventromedial to the BST was found only in 129SvEv adults. Sex differences in neural morphology which develop within the POA/AH depend upon multiple factors, particularly including genetic background.

Special relationship of gamma-aminobutyric acid to the ventromedial nucleus of the hypothalamus during embryonic development

The ventromedial nucleus of the hypothalamus (VMH) is a key nucleus for regulating homeostatic, neuroendocrine, and behavioral functions. We conducted immunocytochemical analyses by using antisera directed against gamma-aminobutyric acid (GABA), its synthetic enzyme glutamic acid decarboxylase (GAD67), GABA-A receptor subunits (alpha2, beta3, epsilon), estrogen receptor-alpha, and Neuropeptide Y (NPY) in the region of the VMH in embryonic mice to identify potential patterning elements for VMH formation. Cells and fibers containing GABA and GAD67 encircled the primordial VMH as early as embryonic day 13 (E13) when the cytoarchitecture of the VMH was not recognizable by Nissl stain. At E16-17 the cytoarchitecture of the VMH became recognizable by Nissl stain as GABAergic fibers invaded the nucleus, continued postnatally, and by adulthood the density of GABAergic fibers was greater inside than outside the VMH. GABA-A receptor subunit expression (beta3 by E13 and alpha2 by E15) within the primordial VMH suggested potential sensitivity to the surrounding GABA signal. Brain slices were used to test whether fibers from distal or proximal sites influenced VMH development. Coronal Vibratome slices were prepared and maintained in vitro for 0-3 days. Nissl stain analyses showed a uniform distribution of cells in the region of the VMH on the day of plating (E15). After 3 days in vitro, cellular aggregation suggesting VMH formation was seen. Nuclear formation in vitro suggests that key factors resided locally within the coronal plane of the slices. It is suggested that either GABA intrinsic to the region nearby the VMH directly influences the development and organization of the VMH, or along with other markers provides an early indicator of pattern determination that precedes the cellular organization of the VMH.

Sex differences in the vomeronasal system

In the early eighties we found sex differences in the vomeronasal organ (VNO) and hypothesized that the vomeronasal system (VNS), a complex neural network involved in the control of reproductive behavior, might be sexually dimorphic. At that time sex differences had already been described for some structures that receive VNO input, such as the medial amygdala, the medial preoptic area, the ventromedial hypothalamic nucleus, and the ventral region of the premammillary nucleus. Since then, we have shown sex differences in the accessory olfactory bulb (AOB), the bed nucleus of the accessory olfactory tract (BAOT), and the bed nucleus of the stria terminalis (BST). When new VNS connections were found, all of them ended in nuclei that present sex differences. In general, sex differences in the olfactory system show two morphological patterns: one in which males present greater morphological measures than females, and just the opposite. To explain the morphometric measures of males in the latter, it has been hypothesized that androgens serve as inhibitors. Our work on the involvement of the GABA(A) receptor in the development of AOB and maternal behavior sex differences also suggests that neonatal changes in neuronal membrane permeability to the ion Cl- differences. This might be the first animal model to help us to understand the situation in which human genetic and gonadal sex do not agree with brain and behavioral sex. Finally, we stress that sex differences in the VNS constitute a neurofunctional model for understanding sex differences in reproductive behaviors.

Estrogen receptor function as revealed by knockout studies: neuroendocrine and behavioral aspects

Estrogens are an important class of steroid hormones, involved in the development of brain, skeletal, and soft tissues. These hormones influence adult behaviors, endocrine state, and a host of other physiological functions. Given the recent cloning of a second estrogen receptor (ER) cDNA (the ER beta), work on alternate spliced forms of ER alpha, and the potential for membrane estrogen receptors, an animal with a null background for ER alpha function is invaluable for distinguishing biological responses of estrogens working via the ER alpha protein and those working via another ER protein. Data generated to date, and reviewed here, indicate that there are profound ramifications of the ER alpha disruption on behavior and neuroendocrine function. First, data on plasma levels of estradiol (E2), testosterone (T), and luteinizing hormone (LH) in wild-type (WT) versus ER alpha- mice confirm that ER alpha is essential in females for normal regulation of the hypothalamic-pituitary gonadal axis. Second, ovariectomized female ER alpha- mice do not display sexual receptivity when treated with a hormonal regime of estrogen and progesterone that induces receptivity in WT littermates. Finally, male sexual behaviors are disrupted in ER alpha- animals. Given decades of data on these topics our findings may seem self-evident. However, these data represent the most direct test currently possible of the specific role of the ER alpha protein on behavior and neuroendocrinology. The ER alpha- mouse can be used to ascertain the specific functions of ER alpha, to suggest functions for the other estrogen receptors, and to study indirect effects of ER alpha on behavior via actions on other receptors, neurotransmitters, and neuropeptides.

In vitro models for the effects of sex hormones on neurons

The results of these two in vitro models share some striking similarities. In both, estrogen was able to induce or promote the formation of either dendrites themselves in hippocampal neurons or dendritic specializations in PC12 neurites, and these specializations were then able to induce interneural interactions. In both models, androgen was able to promote the development of axons that branched frequently, while not directly fostering interneuronal contact. These findings recapitulate in part some of the effects of estrogen and androgen on neurons in vivo and suggest the inherent ability of cells of neural crest origin to respond to these hormones with specific neural morphogenetic programs designed to alter interneuronal communication. In these ways, it seems likely that both sex hormones are acting as neural growth factors in cells that express the appropriate receptor, leading to stereotyped changes in neural growth and pattern formation. Through the examination of such subcellular mechanisms, we hope to further understand the effects of sex hormones on brain development and the ontogeny of behavioral, cognitive, and reproductive differences between the sexes.

Telencephalic and diencephalic origin of radial glial processes in the developing preoptic area/anterior hypothalamus

Neuronal birth-dating studies using [3H] thymidine have indicated that neurons in the preoptic area/anterior hypothalamus (POA/AH) are derived primarily from progenitors in proliferative zones surrounding the third ventricle. Radial glial processes are potential guides for neuronal migration, and their presence and orientation during development may provide further information about the origin of cells in the POA/AH. In addition to determining the orientation of radial glial fibers, we examined the relationship of neurons with identified birth dates to radial glial processes in the developing POA/AH of ferrets. Neuronal birth dates were determined by injecting ferret fetuses with bromodeoxyuridine (BrdU) at several different gestational ages; brains were taken from ferret kits at subsequent prenatal ages. Sections were processed for immunocytochemistry to reveal vimentin or glial fibrillary acidic protein in radial glial, or BrdU-labeled cell nuclei. Numerous radial glial processes extended from the lateral ventricles through ventral portions of the septal region to the pial surface of the POA/AH. These fibers both encapsulated and coursed ventrally through and around the anterior commissure of ferret, rat, and mouse fetuses. These ventrally directed fibers were less evident at older ages. In double-labeled sections from ferrets, BrdU-labeled cells in the dorsal POA/AH were often aligned in the same dorsal-ventral orientation as adjacent radial glial fibers. We suggest that a subset of neurons, originating in telencephalic proliferative zones, migrates ventrally along radial glial guides into the dorsal POA/AH.

Failure to find pregnancy effects on cortical thickness

There are reports by Diamond and her colleagues, based on measurements made shortly after parturition, that pregnancy increases the size of the cerebral cortex in rats. As a prelude to examining the cellular basis for this phenomenon, we measured the size of the cortex in three coronal planes in hooded rats on the last day of their pregnancy and in control, nonpregnant female littermates. No differences were found between the pregnant and control rats. We conclude that either the events on the day of parturition alter the size of the cortex or the effects of pregnancy on the cortex are not reliable.