Abstract
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.
Introduction
Sex differences in the structures of the brain are considered to underlie sex-specific functions of the brain and brain functions that differ between sexes or genders. The mechanisms by which the brain is sexually differentiated have not yet been completely elucidated; however, they have long been studied using animal models, especially rodents. Based on accumulated data, androgens secreted from the testes during the perinatal period are converted to estrogens in the brain, wherein the neuroestrogens masculinize and defeminize the brain. Neuroestrogens are essential but not sole factors in the sexual differentiation of the brain. There are other factors that significantly contribute to the brain sexual differentiation. The processes of brain sexual differentiation require sex chromosome genes’ expression in the brain (; ) and gonadal steroids secreted during the peripubertal period (; ). However, there is no doubt that neuroestrogens of testicular origin play an important role in the sexual differentiation of the brain. In this mini review, we focused on two sexually dimorphic brain regions: the preoptic area (POA) and the bed nucleus of the stria terminalis (BNST), which are involved in the regulation of sexually dimorphic social behaviors and reproductive neuroendocrine functions. First, we give an overview of the sex differences in the POA and BNST of the rodent brain. Second, we discuss how neuroestrogens masculinize or defeminize the POA and BNST. Third, we further discuss the notion that testicular androgens that do not aromatize into estrogens can also induce the sexually dimorphic formation of the POA and BNST.
Sex Differences in the POA and BNST
The POA and BNST show morphological sex differences that are related to sex-specific brain functions (Figure 1). The number of dendritic spine synapses in the POA is twofold greater in male rats than in females; the POA masculinized by neuroestrogens, resulting in a greater number of dendritic spine synapses, plays an important role in the control of male sexual behavior (, ; ; ). The male-biased sex difference in dendritic spine synapses in the POA is established by the crosstalk between neuroendocrine and immune systems where microglia and mast cells have significant roles [see reviews (; ) and the next section].
FIGURE 1
In the POA of rats and mice, there are two sexually dimorphic nuclei that have been identified to date. The sexually dimorphic nucleus of the POA (SDN-POA) exhibits male-biased sex differences in volume and the number of neurons (, ). The SDN-POA of male rats has been suggested to be related to partner preference (; ) and sexual arousal (; ; ); however, the physiological functions of the SDN-POA require further investigation. Approximately half of the SDN-POA neurons express calbindin-D28K (Calb) (), a calcium-binding protein that functions as a buffer, sensor, and transporter of calcium (). A cluster of Calb neurons in the SDN-POA is termed the calbindin-sexually dimorphic nucleus (CALB-SDN), which has more Calb neurons in males than in females (; ; ). Although the physiological roles of Calb neurons remain unclear, Calb neurons in male rats are activated during sexual behavior ().
Another sexually dimorphic nucleus in the POA is the anteroventral periventricular nucleus (AVPV), which is larger and contains more neurons in females than in males (). The AVPV of rats contains neurons expressing kisspeptin, neurons producing dopamine, and neurons producing both GABA and glutamate, and exhibits a female-biased sex difference in the number of these neurons (; ; ). In the AVPV of mice, approximately half of kisspeptin neurons produce dopamine and vice versa (). In the female AVPV, kisspeptin neurons expressing estrogen receptor α (ERα) are a target of the positive feedback actions of ovarian estrogens to induce ovulation (; ). Furthermore, kisspeptin neurons in the AVPV of female mice are key players in orchestrating successful reproduction by synchronizing copulation with ovulation (). Dual-phenotype GABA/glutamate neurons in the AVPV of rats and mice interact with gonadotropin-releasing hormone neurons to excite or inhibit their activity (; ). Dopamine neurons in the AVPV of female mice enhance maternal behavior, whereas dopamine neurons in the AVPV of male mice do not affect parental behavior, but suppress intermale aggression ().
The principal nucleus of the BNST (BNSTp) is a subnucleus of the BNST showing male-biased sex differences in size and neuron number (, ). Like the SDN-POA, the BNSTp contains more Calb neurons in male mice than in female mice (). The subregion of the BNSTp that contains many Calb neurons and exhibits the male-biased sex difference in the number of Calb neurons is hereinafter referred to as CALB-BNSTp. BNSTp neurons expressing aromatase in male mice are necessary to distinguish the conspecific sexes and ensure social interactions (). However, the physiological functions of Calb neurons in the BNSTp remain unclear. Unlike the CALB-BNSTp, the ventral part of the BNSTp (BNSTpv) contains few Calb neurons without sex differences, but the BNSTpv is larger and has more non-Calb neurons in female mice than in males (; ). Thus, the BNSTp is composed of a region exhibiting male-biased sex differences in Calb neurons and a region exhibiting female-biased sex differences in non-Calb neurons.
Neuroestrogens of Testicular Origin are Significant Factors for Sexually Dimorphic Formation of the POA and BNST
Neuroestrogens originating from testicular androgens affect the POA and BNST in the perinatal period to organize sexually dimorphic structures in a variety of modes of action (Figure 2). As mentioned before, the POA of rats has more dendritic spines in males than in females. The increased number of dendritic spines in the male POA is induced by estrogens in the perinatal period (, ). The mechanisms responsible for the masculinization of dendritic spine patterning by estrogens are considered to be as follows. First, neuroestrogens originating from testicular androgens during the perinatal period affect mast cells in the POA via ER to stimulate histamine release, which then stimulates microglia in the POA to release prostaglandin E2, which triggers POA neurons to increase dendritic spine synapses via induction of glutamate receptor signaling (; ; , , ). Thus, microglia and mast cells have critical roles in the masculinization of dendritic spine patterning. The POA of postnatal males has twice as many ameboid microglia, a class of microglia with a more activated morphological profile, compared with postnatal females (). The male-biased sex difference in ameboid microglia is regulated by neuroestrogens of testicular origin, because treatment with estradiol increased ameboid microglia in the POA of postnatal females (). The male POA has more activated mast cells than the female POA in the perinatal period, and approximately half of the mast cells in both sexes express ERα (). Additionally, astrocytes in the POA of postnatal rats exhibit a sex difference in morphology: astrocytes in males have longer and more primary processes, and the astrocyte morphology is masculinized by neuroestrogens in the postnatal period (). Astrocytes release chemical transmitters, including glutamate, and are involved in synapse formation (). Taken together, the sexually dimorphic synapse formation may follow the sexual differentiation of these non-neuronal cells.
FIGURE 2
The masculinization of the SDN-POA/CALB-SDN requires the actions of estrogens that are synthesized in the brain from testicular androgens and act via ERα during the postnatal period (
Neuroestrogens originating from testicular androgens in the perinatal period reduce the total number of neurons to defeminize the AVPV in rats and mice (
Estrogens that are synthesized in the brain from testicular androgens and act via ERα during the postnatal period masculinize the BNSTp by increasing the volume and neuron number in rats and mice (
In rodents, the critical time window in which neuroestrogens effectively induce brain sexual differentiation is limited to the perinatal period. Nevertheless, the effects of neuroestrogens persist until adulthood. The long-lasting effects are considered to be due to epigenetic changes in gene expression [see reviews (
Sexually Dimorphic Formation of the POA and BNST Requires Neuroestrogens of Testicular Origin and Testicular Androgens
Masculinization of the BNSTp is disrupted in rats with reduced functional androgen receptors (ARs) (
Conclusion
Perinatal testicular androgens induce masculinizing and defeminizing effects on the POA and BNST of rodents through binding to ER after conversion to estrogens in the brain rather than by binding to AR directly, resulting in sex differences in glial and immune cells, dendritic synapse patterning, and specific neuronal cell groups. Interactions among immune cells, glial cells, and neuronal cells under the influence of neuroestrogens is a prerequisite for producing the sex difference in dendritic synapse patterning in the POA. Sex differences in specific neuronal cell groups in the SDN-POA, AVPV, and BNSTp may be established by controlling the number of dying cells by apoptosis or phenotypic organization of living cells that are influenced by neuroestrogens. Neuroestrogens binding to ER modulate the expression of the target genes at the transcriptional level, but also modulate gene expression by epigenetic regulation, which ensures the long-lasting effects of neuroestrogens beyond the perinatal period. Testicular androgens in the peripubertal period also contribute to the sexual differentiation of the POA and BNST, but aromatizing them to estrogens may not be necessary. Thus, peripubertal testicular androgens can act via AR directly to masculinize the sexually dimorphic nuclei.
Statements
Author contributions
MM and ST prepared the manuscript. Both authors contributed to the article and approved the submitted version.
Funding
This work was supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 20K07257) to ST and Grant for Basic Science Research Projects from the Sumitomo Foundation (No. 180084) to ST.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
sexual differentiation of the brain, sexually dimorphic nucleus, sex difference, androgens, estrogens, preoptic area, bed nucleus of the stria terminalis
Citation
Tsukahara S and Morishita M (2020) Sexually Dimorphic Formation of the Preoptic Area and the Bed Nucleus of the Stria Terminalis by Neuroestrogens. Front. Neurosci. 14:797. doi: 10.3389/fnins.2020.00797
Received
24 March 2020
Accepted
07 July 2020
Published
29 July 2020
Volume
14 - 2020
Edited by
Takayoshi Ubuka, Waseda University, Japan
Reviewed by
Mitsuhiro Kawata, Kyoto Prefectural University of Medicine, Japan; Carol F. Elias, University of Michigan, United States
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© 2020 Tsukahara and Morishita.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Shinji Tsukahara, stsuka@mail.saitama-u.ac.jp
This article was submitted to Neuroendocrine Science, a section of the journal Frontiers in Neuroscience
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