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

Litter Environment Affects Behavior and Brain Metabolic Activity of Adult Knockout Mice

In mammals, the formative environment for social and anxiety-related behaviors is the family unit; in the case of rodents, this is the litter and the mother-young bond. A deciding factor in this environment is the sex ratio of the litter and, in the case of mice lacking functional copies of gene(s), the ratio of the various genotypes in the litter. Both Sex and Genotype ratios of the litter affect the nature and quality of the individual's behavior later in adulthood, as well as metabolic activity in brain nuclei that underlie these behaviors. Mice were raised in litters reconstituted shortly after to birth to control for sex ratio and genotype ratio (wild type pups versus pups lacking a functional estrogen receptor α). In both males and females, the Sex and Genotype of siblings in the litter affected aggressive behaviors as well as patterns of metabolic activity in limbic nuclei in the social behavior network later in adulthood. Further, this pattern in males varied depending upon the Genotype of their brothers and sisters. Principal Components Analysis revealed two components comprised of several amygdalar and hypothalamic nuclei; the VMH showed strong correlations in both clusters, suggesting its pivotal nature in the organization of two neural networks

Estrogen Receptors in the Medial Amygdala Inhibit the Expression of Male Prosocial Behavior

Studies using estrogen receptor alpha(ER) knock-out mice indicate that ER alpha masculinizes male behavior. Recent studies of ER alpha and male prosocial behavior have shown an inverse relationship between ER alpha expression in regions of the brain that regulate social behavior, including the medial amygdala (MeA), and the expression of male prosocial behavior. These studies have lead to the hypothesis that low levels of ER alpha are necessary to permit the expression of high levels of male prosocial behavior. To test this, viral vectors were used to enhance ER alpha in male prairie voles (Microtus ochrogaster), which display high levels of prosocial behavior and low levels of MeA ER alpha. Adult male prairie voles were transfected with ER alpha in the MeA (MeA-ER alpha) or the caudate-putamen (ER alpha control) or luciferase (MeAsitespecific control), and 3 weeks later tested for spontaneous alloparental behavior and partner preference. Enhancing ER alpha in the MeA altered/reduced male prosocial behavior. Only one-third of MeA-ER alpha males, compared with all control males, were alloparental. Me-A-ER alpha males also displayed a significant preference for a novel female. This is a critical finding because the manipulations of neuropeptides, oxytocin and vasopressin, can inhibit the formation of a partner preference, but do not lead to the formation of a preference for a novel female. The results support the hypothesis that low levels of ER alpha are necessary for high levels of male prosocial behavior, and provide the first direct evidence that site-specific ER alpha expression plays a critical role in the expression of male prosocial behavior

The Role of Estrogen Receptor β (ERβ) in the Establishment of Hierarchical Social Relationships in Male Mice

Acquisition of social dominance is important for social species including mice, for preferential access to foods and mates. Male mice establish social rank through agonistic behaviors, which are regulated by gonadal steroid hormone, testosterone, as its original form and aromatized form. It is well known that estrogen receptors (ERs), particularly ER α (ERα), mediate effects of aromatized testosterone, i.e., 17β-estradiol, but precise role played by ER β (ERβ) is still unclear. In the present study, we investigated effects of ERβ gene disruption on social rank establishment in male mice. Adult male ERβ knockout (βERKO) mice and their wild type (WT) littermates were paired based on genotype- and weight-matched manner and tested against each other repeatedly during 7 days experimental period. They underwent 4 trials of social interaction test in neutral cage (homogeneous set test) every other day. Along repeated trials, WT but not βERKO pairs showed a gradual increase of agonistic behaviors including aggression and tail rattling, and a gradual decrease of latency to social rank determination in tube test conducted after each trial of the social interaction test. Analysis of behavioral transition further suggested that WT winners in the tube test showed one-sided aggression during social interaction test suggesting WT pairs went through a process of social rank establishment. On the other hand, a dominant-subordinate relationship in βERKO pairs was not as apparent as that in WT pairs. Moreover, βERKO mice showed lower levels of aggressive behavior than WT mice in social interaction tests. These findings collectively suggest that ERβ may play a significant role in the establishment and maintenance of hierarchical social relationships among male mice

Estrogen Receptor-Alpha in the Bed Nucleus of the Stria Terminalis Regulates Social Affiliation in Male Prairie Voles (Microtus Ochrogaster)

Estrogen receptor alpha (ER alpha) typically masculinizes male behavior, while low levels of ER alpha in the medial amygdala (MeA) and the bed nucleus of the stria terminalis (BST) are associated with high levels of male prosocial behavior. In the males of the highly social prairie vole (Microtus ochrogaster), increasing ER alpha in the MeA inhibited the expression of spontaneous alloparental behavior and produced a preference for novel females. To test for the effects of increased ER alpha in the BST, a viral vector was used to enhance ER alpha expression in the BST of adult male prairie voles. Following treatment, adult males were tested for alloparental behavior with 1-3-day- old pups, and for heterosexual social preference and affiliation. Treatment did not affect alloparental behavior as 73% of ER alpha-BST males and 62.5% of control males were alloparental. Increasing ER alpha in the BST affected heterosexual affiliation, with ER alpha-BST males spending significantly less total time in side-by-side contact with females relative to time spent with control males. ER alpha-BST males did not show a preference for either the familiar or novel female. These findings differed significantly from those reported in ER alpha-MeA enhanced males, where ER alpha inhibited alloparental behavior and produced a preference for a novel female. The findings from this study suggest two things: first, that increased ER alpha in the BST decreases social affiliation and second, that altering ER alpha in different regions of the social neural circuit differentially impacts the expression of social behavior

Estrogen Receptor-α in the Bed Nucleus of the Stria Terminalis Regulates Social Affiliation in Male Prairie Voles (Microtus ochrogaster)

Estrogen receptor alpha (ERα) typically masculinizes male behavior, while low levels of ERα in the medial amygdala (MeA) and the bed nucleus of the stria terminalis (BST) are associated with high levels of male prosocial behavior. In the males of the highly social prairie vole (Microtus ochrogaster), increasing ERα in the MeA inhibited the expression of spontaneous alloparental behavior and produced a preference for novel females. To test for the effects of increased ERα in the BST, a viral vector was used to enhance ERα expression in the BST of adult male prairie voles. Following treatment, adult males were tested for alloparental behavior with 1–3-day-old pups, and for heterosexual social preference and affiliation. Treatment did not affect alloparental behavior as 73% of ERα-BST males and 62.5% of control males were alloparental. Increasing ERα in the BST affected heterosexual affiliation, with ERα-BST males spending significantly less total time in side-by-side contact with females relative to time spent with control males. ERα-BST males did not show a preference for either the familiar or novel female. These findings differed significantly from those reported in ERα-MeA enhanced males, where ERα inhibited alloparental behavior and produced a preference for a novel female. The findings from this study suggest two things: first, that increased ERα in the BST decreases social affiliation and second, that altering ERα in different regions of the social neural circuit differentially impacts the expression of social behavior

The Role of Estrogen Receptor β in the Dorsal Raphe Nucleus on the Expression of Female Sexual Behavior in C57BL/6J Mice

17β-Estradiol (E2) regulates the expression of female sexual behavior by acting through estrogen receptor (ER) α and β. Previously, we have shown that ERβ knockout female mice maintain high level of lordosis expression on the day after behavioral estrus when wild-type mice show a clear decline of the behavior, suggesting ERβ may be involved in inhibitory regulation of lordosis. However, it is not identified yet in which brain region(s) ERβ may mediate an inhibitory action of E2. In this study, we have focused on the dorsal raphe nucleus (DRN) that expresses ERβ in higher density than ERα. We site specifically knocked down ERβ in the DRN in ovariectomized mice with virally mediated RNA interference method. All mice were tested weekly for a total of 3 weeks for their lordosis expression against a stud male in two consecutive days: day 1 with the hormonal condition mimicking the day of behavioral estrus, and day 2 under the hormonal condition mimicking the day after behavioral estrus. We found that the level of lordosis expression in ERβ knockdown (βERKD) mice was not different from that of control mice on day 1. However, βERKD mice continuously showed elevated levels of lordosis behavior on day 2 tests, whereas control mice showed a clear decline of the behavior on day 2. These results suggest that the expression of ERβ in the DRN may be involved in the inhibitory regulation of sexual behavior on the day after behavioral estrus in cycling female mice

内側視索前野を中心とした養育行動の神経内分泌学的基盤に関わる研究の動向

Maternal behaviors are defined as adult behavior directed to offspring to provide necessary conditions, such as nutrition, thermoregulation, and cleanness, for their survival. Since postpartum females in mice and rats show higher frequency and duration of maternal behaviors compared to virgin females, it is hypothesized that changes in neuronal circuitry induced by the fluctuation of gonadal steroid hormones during gestation and parturition may be involved in the induction of the behavior. The medial preoptic area (MPOA) of the hypothalamus plays a central role in the regulation of maternal behavior. Gonadal steroid hormones facilitate the expression of maternal behavior by binding to their receptors expressed in the MPOA. In this review, we will first overview recent findings, with the use of recently developed techniques, on properties of the MPOA neurons activated during maternal behavior. We then discuss possible interaction of the MPOA with other brain systems, such as dopaminergic and oxytocinergic, in the regulation of maternal behavior

Abolition of male sexual behaviors in mice lacking estrogen receptors alpha and beta (alpha beta ERKO)

Male mice with a knockout of the estrogen receptor (ER)-α gene, a ligand-activated transcription factor, showed reduced levels of intromissions and no ejaculations whereas simple mounting behavior was not affected. In contrast, all components of sexual behaviors were intact in male mice lacking the novel ER-β gene. Here we measure the extent of phenotype in mice that lack both ER-α and ER-β genes (αβERKO). αβERKO male mice did not show any components of sexual behaviors, including simple mounting behavior. Nor did they show ultrasonic vocalizations during behavioral tests with receptive female mice. On the other hand, reduced aggressive behaviors of αβERKO mice mimicked those of single knockout mice of ER-α gene (αERKO). They showed reduced levels of lunge and bite aggression, but rarely showed offensive attacks. Thus, either one of the ERs is sufficient for the expression of simple mounting in male mice, indicating a redundancy in function. Offensive attacks, on the other hand, depend specifically on the ER-α gene. Different patterns of natural behaviors require different patterns of functions by ER genes

Long-Lasting Consequences of Neonatal Maternal Separation on Social Behaviors in Ovariectomized Female Mice

Maternal separation (MS) stress is known to induce long-lasting alterations in emotional and anxiety-related behaviors, but effects on social behaviors are not well defined. The present study examined MS effects on female social behaviors in the social investigation (SIT) and social preference (SPT) tests, in addition to non-social behaviors in the open-field (OFT) and light-dark transition (LDT) tests in C57BL/6J mice. All females were tested as ovariectomized to eliminate confounding effects of endogenous estrogen during behavioral testing. Daily MS (3 hr) from postnatal day 1 to 14 did not affect anxiety levels in LDT, but were elevated in OFT with modified behavioral responses to the novel environment. Furthermore, MS altered social investigative behaviors and preference patterns toward unfamiliar stimulus mice in SIT and short- and long-term SPT paradigms. In SIT, MS reduced social investigation duration and increased number of stretched approaches towards both female and male unfamiliar stimulus mice, suggesting increased social anxiety levels in MS females. Similarly, MS heightened levels of social anxiety during short-term SPT but no MS effect on social preference was found. On the other hand, MS females displayed a distinctive preference for female stimuli, unlike control females, when tested for long-term SPT over a prolonged period of 5 days. Evaluation of FosB expression in the paraventricular nucleus, medial and central amygdala following stimulus exposure demonstrated greater number of FosB immunopositive cells in all three brain regions in MS females compared to control females. These results suggest that MS females might differ in neuroendocrine responses toward unfamiliar female and male opponents, which may be associated with modifications in social behaviors found in the present study. Taken together, this study provides new evidence that early life stress modifies female social behaviors by highlighting alterations in behavioral responses to situations involving social as well as non-social novelty