The interaction of genotype and exogenous neonatal androgen and estrogen: sex behavior in female mice

Gene expression profile of the neonatal female mouse brain after administration of testosterone propionate

INTRODUCTION

Clinical care decisions for peripubertal adolescents with gender dysphoria (GD) should be made carefully. Furthermore, the identification of biomarkers is very important for rapid and accurate diagnosis of GD in young people.

AIM

The aim of this study was to investigate gene expression profiles during masculinization of the neonatal female mouse brain by testosterone and to identify biomarkers related to GD.

METHODS

Microarray analysis was performed using RNAs extracted from the brains of neonatal mice treated by intraperitoneal injection of testosterone propionate during the sexual determination period. Sequence motif enrichment analysis for sex hormone receptor responsive elements was performed for the flanking regions of genes that showed significant expression changes following administration of testosterone propionate.

MAIN OUTCOME MEASURES

We revealed a gene set with marked changes in expression during brain masculinization of neonatal female mice following administration of testosterone propionate.

RESULTS

We identified 334 genes that showed differential expression in the masculinized neonatal female brain after testosterone propionate treatment. Interestingly, most of these genes are not reported to be expressed in a sexually dimorphic manner. Moreover, sequence motif enrichment analysis suggested that masculinization of the neonatal female brain by testosterone was controlled more by estrogen receptors than androgen receptors.

CONCLUSIONS

Differences in genes that are expressed differentially following administration of testosterone injection from known sexually dimorphic genes suggest that many GD-related genes are upregulated during female brain masculinization. The gene set identified in this study provides a basis to better understand the mechanisms of GD and delineate its associated biomarkers.

Differential efferent projections of the anterior, posteroventral, and posterodorsal subdivisions of the medial amygdala in mice

The medial amygdaloid nucleus (Me) is a key structure in the control of sociosexual behavior in mice. It receives direct projections from the main and accessory olfactory bulbs (AOB), as well as an important hormonal input. To better understand its behavioral role, in this work we investigate the structures receiving information from the Me, by analysing the efferent projections from its anterior (MeA), posterodorsal (MePD) and posteroventral (MePV) subdivisions, using anterograde neuronal tracing with biotinylated and tetrametylrhodamine-conjugated dextranamines. The Me is strongly interconnected with the rest of the chemosensory amygdala, but shows only moderate projections to the central nucleus and light projections to the associative nuclei of the basolateral amygdaloid complex. In addition, the MeA originates a strong feedback projection to the deep mitral cell layer of the AOB, whereas the MePV projects to its granule cell layer. The Me (especially the MeA) has also moderate projections to different olfactory structures, including the piriform cortex (Pir). The densest outputs of the Me target the bed nucleus of the stria terminalis (BST) and the hypothalamus. The MeA and MePV project to key structures of the circuit involved in the defensive response against predators (medial posterointermediate BST, anterior hypothalamic area, dorsomedial aspect of the ventromedial hypothalamic nucleus), although less dense projections also innervate reproductive-related nuclei. In contrast, the MePD projects mainly to structures that control reproductive behaviors [medial posteromedial BST, medial preoptic nucleus, and ventrolateral aspect of the ventromedial hypothalamic nucleus], although less dense projections to defensive-related nuclei also exist. These results confirm and extend previous results in other rodents and suggest that the medial amygdala is anatomically and functionally compartmentalized.

Of mice and rats: key species variations in the sexual differentiation of brain and behavior

Mice and rats are important mammalian models in biomedical research. In contrast to other biomedical fields, work on sexual differentiation of brain and behavior has traditionally utilized comparative animal models. As mice are gaining in popularity, it is essential to acknowledge the differences between these two rodents. Here we review neural and behavioral sexual dimorphisms in rats and mice, which highlight species differences and experimental gaps in the literature, that are needed for direct species comparisons. Moving forward, investigators must answer fundamental questions about their chosen organism, and attend to both species and strain differences as they select the optimal animal models for their research questions.

Estrogen masculinizes neural pathways and sex-specific behaviors

Sex hormones are essential for neural circuit development and sex-specific behaviors. Male behaviors require both testosterone and estrogen, but it is unclear how the two hormonal pathways intersect. Circulating testosterone activates the androgen receptor (AR) and is also converted into estrogen in the brain via aromatase. We demonstrate extensive sexual dimorphism in the number and projections of aromatase-expressing neurons. The masculinization of these cells is independent of AR but can be induced in females by either testosterone or estrogen, indicating a role for aromatase in sexual differentiation of these neurons. We provide evidence suggesting that aromatase is also important in activating male-specific aggression and urine marking because these behaviors can be elicited by testosterone in males mutant for AR and in females subjected to neonatal estrogen exposure. Our results suggest that aromatization of testosterone into estrogen is important for the development and activation of neural circuits that control male territorial behaviors.

Sex differences in the brain: the relation between structure and function

In the fifty years since the organizational hypothesis was proposed, many sex differences have been found in behavior as well as structure of the brain that depend on the organizational effects of gonadal hormones early in development. Remarkably, in most cases we do not understand how the two are related. This paper makes the case that overstating the magnitude or constancy of sex differences in behavior and too narrowly interpreting the functional consequences of structural differences are significant roadblocks in resolving this issue.

Deletion of the Bax gene disrupts sexual behavior and modestly impairs motor function in mice

Cell death is a nearly ubiquitous feature of the developing nervous system, and differential death in males and females contributes to several well studied sex differences in neuron number. Nonetheless, the functional importance of neuronal cell death has been subjected to few direct tests. Bax, a pro-apoptotic protein, is required for cell death in many neural regions. Deletion of the Bax gene in mice increases neuron number in several areas and eliminates sex differences in cell number in the brain and spinal cord. Here, sexual and motor behaviors were examined in Bax-/- mice and their wild-type siblings to test the functional consequences of preventing Bax-dependent cell death. Animals were gonadectomized in adulthood and provided with ovarian hormones or with testosterone for tests of feminine and masculine sexual behaviors, respectively. Wild-type mice exhibited a sex difference in feminine sexual behavior, with high lordosis scores in females and low scores in males. This sex difference was eliminated by Bax deletion, with very low receptivity exhibited by both male and female Bax-/- mice. Masculine sexual behavior was not sexually dimorphic among wild-type mice, but mounts and pelvic thrusts were nearly eliminated in Bax-/- mice of both sexes. Motor strength and performance at low speeds on a RotaRod apparatus did not differ by sex or Bax gene status. However, Bax-/- animals exhibited impairments on the RotaRod at higher speeds. Thus, developmental cell death may be required for masculine and feminine sexual behaviors and the fine tuning of motor coordination.

Of mice and missing data: what we know (and need to learn) about male sexual behavior

With recent advances in molecular genetics, the popularity of mice as subjects for behavioral neuroscience is increasing at an exponential rate. Unfortunately, the existing body of knowledge on sexual behavior in male mice is not large and many basic gaps exist. The assumption that what is true of rats is also true of mice is a dangerous one that can misdirect and, in the worst case, impede progress. We summarize the current knowledge about the sexual behavior of male mice, with an emphasis on hormonal bases of these behaviors. Behavioral differences between strains, developmental actions of steroids, activational actions of steroids given peripherally and in the brain, and data generated in various receptor knockout and related mice are discussed. In addition, suggestions are made for the standardization of experimental protocols used in investigations of the sexual physiology and behavior of male mice in order to facilitate between-experiment and between-laboratory comparisons and to expedite the growth of knowledge in this area.

Infanticide: accounting for genetic variation in mice

Infanticide, the killing of young, is one of a number of sexually-dimorphic traits in mice that is dependent upon androgen stimulation during perinatal life and during adulthood. Genotype also influences infanticide in that males of some strains of mice (C57BL/6J) exhibit high levels of this behavior while males of other strains (DBA/2J) seldom kill young. The experiments conducted here show that strain differences in pup killing behavior exhibited by males are not related to postweaning social factors nor are they due to differences in perinatal, pubertal, or adult levels of circulating hormones. These results, in combination with those previously reported, suggest that strain differences in the tendency of mice to kill young may instead depend upon the interaction of genotypic features such as prenatal hormone titers and/or sensitivity to these hormones, as well as on extra organismic factors such as intrauterine position. A model for understanding the manner in which genes and hormones may interact to influence infanticide and other hormone dependent sexually-dimorphic behaviors in mice is presented.

The genetics of hormonal influences on male sexual behavior of mice and rats

This review focuses on the intersection of genes and hormones as they relate to the development of male sexual behavior. Three major hypotheses are discussed: (1) Some differences in adult male sexual behavior are due to gene differences that influence brain differentiation. Genes that influence brain differentiation may do so by affecting the elaboration of testosterone (i.e., H-Y antigen) or the sensitivity to testosterone (i.e., Tfm mutation and autosomal variations) during neonatal and/or prenatal life. (2) Some differences in male sexual behavior are due to gene differences that influence adult levels of testosterone or sensitivity to testosterone and its metabolites. (3) There is a gene(s) on the Y chromosome that influences the development of sexual behavior that is associated with the arousal mechanism. A possible hormonal mechanism of this Y chromosomal gene(s) is discussed.

Differentiation of coital behavior in mammals: a comparative analysis

The evidence reviewed suggests that in all mammalian species the adult male's ability to display masculine coital behavior depends in part on exposure of the developing brain to testicular testosterone or its metabolites. In many mammals, particularly rodents, ruminants, and some carnivores, perinatal exposure to androgen also causes behavioral defeminization, i.e., reduced capacity to display typically feminine coital behavior in response to gonadal hormones in adulthood. The data reviewed suggest that no such process occurs in certain other mammalian species, including ferret, rhesus monkey, marmoset, and man. Testicular androgen may cause behavioral defeminization only in those species in which expression of feminine sexual behavior normally depends on the neural action of progesterone, acting synergistically with estradiol; new data support this claim in the ferret. The possible contribution of estrogenic and 5 alpha-reduced androgenic metabolites of testosterone to the occurrence of behavioral masculinization and defeminization is considered in those mammalian species for which data are available.

Neonatal androgen and mounting behaviour in female house mice

Strains of intact, mature female mice were found to differ appreciably in their levels of performance of masculine copulatory patterns when presented with a stimulus oestrous female. Balb/c and DBA/2 strains, which showed few responses to stimulus females, were still unresponsive when given exogenous androgen, whereas the C57BL/6Fa strain and its R1 hybrids with the DBA/2 strain, showed more pre-injection masculine responses which increased to consistent and very high levels following androgen treatment. In a second study, injection of Balb/c females with testosterone propionate on the day of birth did not increase the number of masculine responses displayed, even after androgen treatment at maturity.

Estrogen target sites in the brain of the chick embryo

Autoradiograms prepared from brains of chick embryos after injection of [3H]estradiol demonstrate the existence of target cells for estrogen in the medial preoptic and ventral hypothalamic regions as early as day 10 of incubation. Target cells also appear in telencephalic locations during later stages of embryonic development. These hormone-concentrating cells probably are the anatomical substrate for the formative action of sex steroids during embryonic life on certain brain functions such as control of sexual and aggressive behaviour and gonadotropin secretion.