Estrogen-induced memory enhancements are blocked by acute bisphenol A in adult female rats: role of dendritic spines

Neurodevelopment effects of early life bisphenol-A exposure on visual memory: Insights into recovery dynamics

Bisphenol A (BPA), a ubiquitous endocrine disruptor, is implicated in the cognitive deficits observed in both children and animals. Especially, BPA-induced spatial memory deterioration during the whole development phase of rodents has been well delineated. However, whether BPA exposure on the different development phases exerts similar effects on the prefrontal cortex (PFC) dependent visual memory is still elusive. Here, we chose two exposure windows, the whole gestation and lactation phases (E0∼P21) and the whole juvenile and adolescent phases (P22∼P60), for exposing rats to BPA. The visual memory of those rats was accessed by object recognition testing in the open field after BPA exposure and a constant recovery interval. The results revealed a substantial decline of visual memory under both exposure conditions, accompanied by an increase in anxiety-like behavior in BPA-exposed rats. Notably, after a 20-day recovery period, those behavioral changes induced by BPA exposure during P22∼60, not E0∼P21, were reversed compared to the control rats. According to morphological analysis of those rats after recovery, we found that the spine density of pyramidal neurons in the PFC were significant decreased in rats with BPA exposure during E0∼P21 and there was no difference between rats with or without BPA exposure during P22∼P60. Additionally, a similar change trend in excitatory receptors expression was observed under both exposure conditions. After an additional 20 days of recovery, the behavioral changes in rats with perinatal BPA exposure reverted to the normal status. Our present findings illuminate the dynamic effects of BPA on PFC-dependent functions across two crucial early developmental stages of life.

Evidence that ovarian hormones, but not diet and exercise, contribute to the sex disparity in post-traumatic stress disorder

Females are twice as likely as males to receive a diagnosis of post-traumatic stress disorder (PTSD). One hypothesis for this sex disparity is that ovarian hormones, including estrogen and progesterone, contribute to PTSD risk. Alternatively, sex differences in lifestyle factors, such as diet and exercise, may play a role in PTSD risk. Using data from the Atlantic Partnership for Tomorrow's Health (PATH) cohort (n = 16,899), the relationship between endogenous hormone fluctuations (e.g., menarche, pregnancy, and menopause), exogenous hormone use (e.g., hormonal contraception and hormone replacement therapy (HRT)) and lifestyle variables (diet and exercise habits, as measured by the Mediterranean Diet Adherence Screener, Healthy Eating Index, and International Physical Activity Questionnaire) with PTSD diagnosis and treatment were analyzed. While several hormonal variables, including contraceptive use, higher total number of pregnancies, younger menarche age, and having undergone menopause increased the risk of PTSD, no lifestyle variables contributed to an increased risk of PTSD diagnosis. These findings support the theory that ovarian hormones contribute to the sex-linked disparity in PTSD diagnosis.

Estradiol Receptors Inhibit Long-Term Potentiation in the Dorsomedial Striatum

Estradiol, a female sex hormone and the predominant form of estrogen, has diverse effects throughout the brain including in learning and memory. Estradiol modulates several types of learning that depend on the dorsomedial striatum (DMS), a subregion of the basal ganglia involved in goal-directed learning, cued action-selection, and motor skills. A cellular basis of learning is synaptic plasticity, and the presence of extranuclear estradiol receptors ERα, ERβ, and G-protein-coupled estrogen receptor (GPER) throughout the DMS suggests that estradiol may influence rapid cellular actions including those involved in plasticity. To test whether estradiol affects synaptic plasticity in the DMS, corticostriatal long-term potentiation (LTP) was induced using theta-burst stimulation (TBS) in ex vivo brain slices from intact male and female C57BL/6 mice. Extracellular field recordings showed that female mice in the diestrous stage of the estrous cycle exhibited LTP similar to male mice, while female mice in estrus did not exhibit LTP. Furthermore, antagonists of ERα or GPER rescued LTP in estrous females and agonists of ERα or GPER reduced LTP in diestrous females. In males, activating ERα but not GPER reduced LTP. These results uncover an inhibitory action of estradiol receptors on cellular learning in the DMS and suggest a cellular mechanism underlying the impairment in certain types of DMS-based learning observed in the presence of high estradiol. Because of the dorsal striatum's role in substance use disorders, these findings may provide a mechanism underlying an estradiol-mediated progression from goal-directed to habitual drug use.

Steroid Hormone Interaction with Dendritic Spines: Implications for Neuropsychiatric Disease

Dendritic spines, key sites for neural plasticity, are influenced by gonadal steroids. In this chapter, we review the effects of gonadal steroids on dendritic spine density in areas important to cognitive function, the hippocampus, and prefrontal cortex. Most of these animal model studies investigated the effects of estrogen in females, but we also include more recent data on androgen effects in both males and females. The underlying genomic and non-genomic mechanisms related to gonadal steroid-induced spinogenesis are also reviewed. Subsequently, we discuss possible reasons for the observed sex differences in many neuropsychiatric diseases, which appear to be caused, in part, by aberrant synaptic connections that may involve dendritic spine pathology. Overall, knowledge concerning the regulation of dendritic spines by gonadal hormones has grown since the initial discoveries in the 1990s, and current research points to a potential role for aberrant spine functioning in many neuropsychiatric disorders.

APOE4 homozygote females are resistant to the beneficial effects of 17β-estradiol on memory and CA1 dendritic spine density in the EFAD mouse model of Alzheimer's disease

Female APOE4 carriers are at greatest risk of Alzheimer's disease (AD). The potent estrogen 17β-estradiol (E2) may mediate AD risk, as the onset of memory decline coincides with the menopausal transition. Whether APOE genotype mediates E2's effects on memory and neuronal morphology is poorly understood. We used the APOE+/+/5xFAD+/- (EFAD) mouse model to examine how APOE3 homozygote (E3FAD), APOE3/4 heterozygote (E3/4FAD), and APOE4 homozygote (E4FAD) genotypes modulate effects of E2 on object and spatial memory consolidation, dendritic spine density, and dorsal hippocampal estrogen receptor expression in 6-month-old ovariectomized EFAD mice. Dorsal hippocampal E2 infusion enhanced memory consolidation and increased CA1 apical spine density in E3FAD and E3/4FAD, but not E4FAD, mice. CA1 basal mushroom spines were also increased by E2 in E3FADs. E4FAD mice exhibited reduced CA1 and mPFC basal spine density, and increased dorsal hippocampal ERα protein, independent of E2. Overall, E2 benefitted hippocampal memory and structural plasticity in females bearing one or no APOE4 allele, whereas two APOE4 alleles impeded the memory-enhancing and spinogenic effects of E2.

Sex differences in cognition following variations in endocrine status

Spatial memory, mediated primarily by the hippocampus, is responsible for orientation in space and retrieval of information regarding location of objects and places in an animal's environment. Since the hippocampus is dense with steroid hormone receptors and is capable of robust neuroplasticity, it is not surprising that changes in spatial memory performance occur following a variety of endocrine alterations. Here, we review cognitive changes in both spatial and nonspatial memory tasks following manipulations of the hypothalamic-pituitary-adrenal and gonadal axes and after exposure to endocrine disruptors in rodents. Chronic stress impairs male performance on numerous behavioral cognitive tasks and enhances or does not impact female cognitive function. Sex-dependent changes in cognition following stress are influenced by both organizational and activational effects of estrogen and vary depending on the developmental age of the stress exposure, but responses to gonadal hormones in adulthood are more similar than different in the sexes. Also discussed are possible underlying neural mechanisms for these steroid hormone-dependent, cognitive effects. Bisphenol A (BPA), an endocrine disruptor, given at low levels during adolescent development, impairs spatial memory in adolescent male and female rats and object recognition memory in adulthood. BPA's negative effects on memory may be mediated through alterations in dendritic spine density in areas that mediate these cognitive tasks. In summary, this review discusses the evidence that endocrine status of an animal (presence or absence of stress hormones, gonadal hormones, or endocrine disruptors) impacts cognitive function and, at times, in a sex-specific manner.

Androgens Enhance Recognition Memory and Dendritic Spine Density in the Hippocampus and Prefrontal Cortex of Ovariectomized Female Rats

Estrogen replacement has been repeatedly shown to enhance memory and increase dendritic spine density in the hippocampus and prefrontal cortex of ovariectomized (OVX) female rats. Given the potential deleterious effects of chronic estrogen administration, the present study assessed cognitive function using recognition memory tasks and measured dendritic spine density in the CA1 region of the hippocampus and medial prefrontal cortex after subchronic androgen replacement to adult OVX female rats. All androgens enhanced recognition memory in OVX rats, but object placement (OP) and object recognition (OR) results differed. Only testosterone enhanced OR. Testosterone had no effect on OP while dehydroepiandrosterone (DHEA), dihydrotestosterone (DHT) and androstenedione (AD) enhanced OP. Dendritic spine density was increased by both TP and DHEA in both brain areas (DHT and AD were not tested). Lastly, we used the aromatase inhibitor, letrozole, to discriminate between potential androgenic and estrogenic effects of androgens on behavior. Letrozole alone did not alter recognition memory in OVX rats and did not block the effects of either TP or DHEA on recognition memory suggesting that effects were mediated via androgenic mechanisms. The present results expand previous information on gonadal hormone actions and show that, in addition to estrogens, androgens also improve memory and increase spine density in brains of OVX female rats. While requiring further investigation, these observations provide a basis for therapeutic interventions in the treatment of menopausal, age or disease related memory loss.

Sex-specific lipid dysregulation in the Abca7 knockout mouse brain

Alzheimer’s disease is a devastating neurodegenerative disease that affects more women than men. The pathomechanism underlying the sex disparity, especially in the brain, is unclear. ABCA7 is one of the strongest susceptibility genes for Alzheimer’s disease. It mediates the transport of lipids across membranes and is associated with pathways related to amyloid-β neuropathology. However, the role of ABCA7 in the regulation of brain lipids is largely unknown. Sex-specific differences in the pathological link between brain lipid dysregulation and amyloid-β are also unknown. Here, we undertook quantitative discovery lipidomics of male and female Abca7 knockout (n = 52) and wild type (n = 35) mouse brain using sophisticated liquid chromatography/mass spectrometry. We identified 61 lipid subclasses in the mouse brain and found sex-specific differences in lipids that were altered with Abca7 deletion. The altered lipids belong to cellular pathways that control cell signalling, sterol metabolism, mitochondrial function and neuroprotection. We also investigated the relationship between lipids and amyloid-β levels in the Abca7 knockout mice and found elevated free cholesterol only in female mice that was significantly correlated with amyloid-β42 levels. In male Abca7 knockout mice, the neuroprotective ganglioside GD1a levels were elevated and inversely correlated with amyloid-β42 levels. Collectively, these results demonstrate that Abca7 deletion leads to sex-specific lipid dysregulation in the brain, providing insight into the underlying sex disparity in the aetiology of Alzheimer’s disease.

Does Bisphenol A Confer Risk of Neurodevelopmental Disorders? What We Have Learned from Developmental Neurotoxicity Studies in Animal Models

Substantial evidence indicates that bisphenol A (BPA), a ubiquitous environmental chemical used in the synthesis of polycarbonate plastics and epoxy resins, can impair brain development. Clinical and epidemiological studies exploring potential connections between BPA and neurodevelopmental disorders in humans have repeatedly identified correlations between early BPA exposure and developmental disorders, such as attention deficit/hyperactivity disorder and autism spectrum disorder. Investigations using invertebrate and vertebrate animal models have revealed that developmental exposure to BPA can impair multiple aspects of neuronal development, including neural stem cell proliferation and differentiation, synapse formation, and synaptic plasticity-neuronal phenotypes that are thought to underpin the fundamental changes in behavior-associated neurodevelopmental disorders. Consistent with neuronal phenotypes caused by BPA, behavioral analyses of BPA-treated animals have shown significant impacts on behavioral endophenotypes related to neurodevelopmental disorders, including altered locomotor activity, learning and memory deficits, and anxiety-like behavior. To contextualize the correlations between BPA and neurodevelopmental disorders in humans, this review summarizes the current literature on the developmental neurotoxicity of BPA in laboratory animals with an emphasis on neuronal phenotypes, molecular mechanisms, and behavioral outcomes. The collective works described here predominantly support the notion that gestational exposure to BPA should be regarded as a risk factor for neurodevelopmental disorders.

A Putative Role for Ubiquitin-Proteasome Signaling in Estrogenic Memory Regulation

Sex steroid hormones such as 17β-estradiol (E₂) are critical neuromodulators of hippocampal synaptic plasticity and hippocampus-dependent memory in both males and females. However, the mechanisms through which E₂ regulates memory formation in both sexes remain unclear. Research to date suggests that E₂ regulates hippocampus-dependent memory by activating numerous cell-signaling cascades to promote the synthesis of proteins that support structural changes at hippocampal synapses. However, this work has largely overlooked the equally important contributions of protein degradation mediated by the ubiquitin proteasome system (UPS) in remodeling the synapse. Despite being critically implicated in synaptic plasticity and successful formation of long-term memories, it remains unclear whether protein degradation mediated by the UPS is necessary for E₂ to exert its beneficial effects on hippocampal plasticity and memory formation. The present article provides an overview of the receptor and signaling mechanisms so far identified as critical for regulating hippocampal E₂ and UPS function in males and females, with a particular emphasis on the ways in which these mechanisms overlap to support structural integrity and protein composition of hippocampal synapses. We argue that the high degree of correspondence between E₂ and UPS activity warrants additional study to examine the contributions of ubiquitin-mediated protein degradation in regulating the effects of sex steroid hormones on cognition.

Estradiol Regulates Polysialylated Form of the Neural Cell Adhesion Molecule Expression and Connectivity of O-LM Interneurons in the Hippocampus of Adult Female Mice

The estrous cycle is caused by the changing concentration of ovarian hormones, particularly 17β-estradiol, a hormone whose effect on excitatory circuits has been extensively reported. However, fewer studies have tried to elucidate how this cycle, or this hormone, affects the plasticity of inhibitory networks and the structure of interneurons. Among these cells, somatostatin-expressing O-LM neurons of the hippocampus are especially interesting. They have a role in the modulation of theta oscillations, and they receive direct input from the entorhinal cortex, which place them in the center of hippocampal function. In this study, we report that the expression of polysialylated form of the neural cell adhesion molecule (PSA-NCAM) in the hippocampus, a molecule involved in the plasticity of somatostatin-expressing interneurons in the adult brain, fluctuated through the different stages of the estrous cycle. Likewise, these stages and the expression of PSA-NCAM affected the density of dendritic spines of O-LM cells. We also describe that 17β-estradiol replacement of adult ovariectomized female mice caused an increase in the perisomatic inhibitory puncta in O-LM interneurons as well as an increase in their axonal bouton density. Interestingly, this treatment also induced a decrease in their dendritic spine density, specifically in O-LM interneurons lacking PSA-NCAM expression. Finally, using an ex vivo real-time assay with entorhinal-hippocampal organotypic cultures, we show that this hormone decreased the dynamics in spinogenesis, altogether highlighting the modulatory effect that 17β-estradiol has on inhibitory circuits.

Perinatal Exposure of Bisphenol A Differently Affects Dendritic Spines of Male and Female Grown-Up Adult Hippocampal Neurons

Perinatal exposure to Bisphenol A (BPA) at a very low dose may modulate the development of synapses of the hippocampus during growth to adulthood. Here, we demonstrate that perinatal exposure to 30 μg BPA/kg per mother’s body weight/day significantly altered the dendritic spines of the grownup rat hippocampus. The density of the spine was analyzed by imaging of Lucifer Yellow-injected CA1 glutamatergic neurons in adult hippocampal slices. In offspring 3-month male hippocampus, the total spine density was significantly decreased by BPA exposure from 2.26 spines/μm (control, no BPA exposure) to 1.96 spines/μm (BPA exposure). BPA exposure considerably changed the normal 4-day estrous cycle of offspring 3-month females, resulting in a 4∼5 day estrous cycle with 2-day estrus stages in most of the subjects. In the offspring 3-month female hippocampus, the total spine density was significantly increased by BPA exposure at estrus stage from 2.04 spines/μm (control) to 2.25 spines/μm (BPA exposure). On the other hand, the total spine density at the proestrus stage was moderately decreased from 2.33 spines/μm (control) to 2.19 spines/μm (BPA exposure). Thus, after the perinatal exposure to BPA, the total spine density in males became lower than that in females. Concerning the BPA effect on the morphology of spines, the large-head spine was significantly changed with its significant decrease in males and moderate change in females.

Neural basis for estrous cycle-dependent control of female behaviors

Females display changes in distinct behaviors along the estrous cycle. Levels of circulating ovarian sex steroid hormones peak around ovulation, which occur around estrus phase of the cycle. This increase of sex hormones is thought to be important for changes in behaviors, however, neural circuit mechanisms of periodic behavioral changes in females are not understood well. Different lines of research indicate sex hormonal effects on several forms of neuronal plasticity. This review provides an overview of behavioral and plastic changes that occur in an estrous cycle-dependent manner and explores the current research linking these changes to understand neural circuit mechanisms that control female behaviors.

Protective effect of alpha-lipoic acid on bisphenol A-induced learning and memory impairment in developing mice: nNOS and keap1/Nrf2 pathway

The adverse effects of bisphenol A (BPA) on learning and memory may be related with oxidative stress, but the mechanisms are unclear. This study aimed to investigate the mechanism of damaged learning and memory caused by BPA through inducing oxidative stress, as well as to explore whether alpha-lipoic acid (ALA) show a protective action. Female mice were exposed to 0.1 μg/mL BPA, 0.2 μg/mL BPA, 0.6 mg/mL ALA, and 0.2 BPA + ALA through drinking water for 8 weeks. The results showed that ALA protected against the impairment of spatial, recognition, and avoidance memory caused by BPA. ALA replenished the reduce of hippocampus coefficient, serum estradiol (E2) level, and hippocampal neurotransmitters levels induced by BPA. ALA alleviated BPA-induced oxidative stress and hippocampal histological changes. BPA exposure reduced the levels of synaptic structural proteins and PKC/ERK/CREB pathway proteins, and ALA improved these reductions. ALA altered the protein levels of nNOS and keap1/Nrf2 pathway affected by BPA. Our results suggested that impairments of learning and memory caused by BPA was related to the damage of hippocampal synapses mediated by oxidative stress, and ALA protected learning and memory by reducing the oxidative stress induced by BPA through regulating the nNOS and keap1/Nrf2 pathway.

Long-term oral administration of a novel estrogen receptor beta agonist enhances memory and alleviates drug-induced vasodilation in young ovariectomized mice

Development of estrogen therapies targeting the β (ERβ) but not α (ERα) estrogen receptor is critically needed for the treatment of negative menopausal symptoms, as ERα activation increases health risks like cancer. Here, we determined the effects of long-term oral treatment with EGX358, a novel highly selective ERβ agonist, on memory, vasodilation, and affect in young ovariectomized mice. Mice were orally gavaged daily for 9 weeks with vehicle, 17β-estradiol (E₂), the ERβ agonist diarylpropionitrile (DPN), or EGX358 at doses that enhance memory when delivered acutely. Tail skin temperature was recorded as a proxy for vasodilation following injection of vehicle or senktide, a tachykinin receptor 3 agonist used to model hot flashes. Anxiety-like behavior was assessed in the open field (OF) and elevated plus maze (EPM), and depression-like behavior was measured in the tail suspension (TST) and forced swim tests (FST). Finally, memory was assessed in object recognition (OR) and object placement (OP) tasks. E₂, DPN, and EGX358 reduced senktide-mediated increases in tail skin temperature compared to vehicle. All three treatments also enhanced memory in the OR and OP tasks, whereas vehicle did not. Although E₂ increased time spent in the center of the OF, no other treatment effects were observed in the OF, EPM, TST, or FST. These data suggest that long-term ERβ activation can reduce hot flash-like symptoms and enhance spatial and object recognition memories in ovariectomized mice. Thus, the highly selective ERβ agonist EGX358 may be a promising avenue for reducing menopause-related hot flashes and memory dysfunction.

HOXA5 Expression Is Elevated in Breast Cancer and Is Transcriptionally Regulated by Estradiol

HOXA5 is a homeobox-containing gene associated with the development of the lung, gastrointestinal tract, and vertebrae. Here, we investigate potential roles and the gene regulatory mechanism in HOXA5 in breast cancer cells. Our studies demonstrate that HOXA5 expression is elevated in breast cancer tissues and in estrogen receptor (ER)-positive breast cancer cells. HOXA5 expression is critical for breast cancer cell viability. Biochemical studies show that estradiol (E2) regulates HOXA5 gene expression in cultured breast cancer cells in vitro. HOXA5 expression is also upregulated in vivo in the mammary tissues of ovariectomized female rats. E2-induced HOXA5 expression is coordinated by ERs. Knockdown of either ERα or ERβ downregulated E2-induced HOXA5 expression. Additionally, ER co-regulators, including CBP/p300 (histone acetylases) and MLL-histone methylases (MLL2, MLL3), histone acetylation-, and H3K4 trimethylation levels are enriched at the HOXA5 promoter in present E2. In summary, our studies demonstrate that HOXA5 is overexpressed in breast cancer and is transcriptionally regulated via estradiol in breast cancer cells.

Bazedoxifene - a promising brain active SERM that crosses the blood brain barrier and enhances spatial memory

Over 20 years of accumulated evidence has shown that the major female sex hormone 17β-estradiol can enhance cognitive functioning. However, the utility of estradiol as a therapeutic cognitive enhancer is hindered by its unwanted peripheral effects (carcinogenic). Selective estrogen receptor modulators (SERMs) avoid the unwanted effects of estradiol by acting as estrogen receptor antagonists in some tissues such as breast and uterus, but as agonists in others such as bone, and are currently used for the treatment of osteoporosis. However, understanding of their actions in the brain are limited. The third generation SERM bazedoxifene has recently been FDA approved for clinical use with an improved biosafety profile. However, whether bazedoxifene can enter the brain and enhance cognition is unknown. Using mice, the current study aimed to explore if bazedoxifene can 1) cross the blood-brain barrier, 2) rescue ovariectomy-induced hippocampal-dependent spatial memory deficit, and 3) activate neural estrogen response element (ERE)-dependent gene transcription. Using liquid chromatography-mass spectrometry (LC-MS), we firstly demonstrate that a peripheral injection of bazedoxifene can enter the brain. Secondly, we show that an acute intraperitoneal injection of bazedoxifene can rescue ovariectomy-induced spatial memory deficits. And finally, using the ERE-luciferase reporter mouse, we show in vivo that bazedoxifene can activate the ERE in the brain. The evidence shown here suggest bazedoxifene could be a viable cognitive enhancer with promising clinical applicability.

Rapid Estrogenic and Androgenic Neurosteroids Effects in the Induction of Long-Term Synaptic Changes: Implication for Early Memory Formation

Mounting experimental evidence demonstrate that sex neuroactive steroids (neurosteroids) are essential for memory formation. Neurosteroids have a profound impact on the function and structure of neural circuits and their local synthesis is necessary for the induction of both long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission and for neural spine formation in different areas of the central nervous system (CNS). Several studies demonstrated that in the hippocampus, 17β-estradiol (E2) is necessary for inducing LTP, while 5α-dihydrotestosterone (DHT) is necessary for inducing LTD. This contribution has been proven by administering sex neurosteroids in rodent models and by using blocking agents of their synthesis or of their specific receptors. The general opposite role of sex neurosteroids in synaptic plasticity appears to be dependent on their different local availability in response to low or high frequency of synaptic stimulation, allowing the induction of bidirectional synaptic plasticity. The relevant contribution of these neurosteroids to synaptic plasticity has also been described in other brain regions involved in memory processes such as motor learning, as in the case of the vestibular nuclei, the cerebellum, and the basal ganglia, or as the emotional circuit of the amygdala. The rapid effects of sex neurosteroids on neural synaptic plasticity need the maintenance of a tonic or phasic local steroid synthesis determined by neural activity but might also be influenced by circulating hormones, age, and gender. To disclose the exact mechanisms how sex neurosteroids participate in finely tuning long-term synaptic changes and spine remodeling, further investigation is required.

It takes a neural village: Circuit-based approaches for estrogenic regulation of episodic memory

Cognitive behaviors, such as episodic memory formation, are complex processes involving coordinated activity in multiple brain regions. However, much of the research on hormonal regulation of cognition focuses on manipulation of one region at a time or provides a single snapshot of how a systemic treatment affects multiple brain regions without investigating how these regions might interact to mediate hormone effects. Here, we use estrogenic regulation of episodic memory as an example of how circuit-based approaches may be incorporated into future studies of hormones and cognition. We first review basic episodic memory circuitry, rapid mechanisms by which 17β-estradiol can alter circuit activity, and current knowledge about 17β-estradiol's effects on episodic memory. Next, we outline approaches that researchers can employ to consider circuit effects in their estrogen research and provide examples of how these methods have been used to examine hormonal regulation of memory and other behaviors.

Sex Differences in the Development of the Rodent Corticolimbic System

In recent years, a growing body of research has shown sex differences in the prevalence and symptomatology of psychopathologies, such as depression, anxiety, and fear-related disorders, all of which show high incidence rates in early life. This has highlighted the importance of including female subjects in animal studies, as well as delineating sex differences in neural processing across development. Of particular interest is the corticolimbic system, comprising the hippocampus, amygdala, and medial prefrontal cortex. In rodents, these corticolimbic regions undergo dynamic changes in early life, and disruption to their normative development is believed to underlie the age and sex-dependent effects of stress on affective processing. In this review, we consolidate research on sex differences in the hippocampus, amygdala, and medial prefrontal cortex across early development. First, we briefly introduce current principles on sexual differentiation of the rodent brain. We then showcase corticolimbic regional sex differences in volume, morphology, synaptic organization, cell proliferation, microglia, and GABAergic signaling, and explain how these differences are influenced by perinatal and pubertal gonadal hormones. In compiling this research, we outline evidence of what and when sex differences emerge in the developing corticolimbic system, and illustrate how temporal dynamics of its maturational trajectory may differ in male and female rodents. This will help provide insight into potential neural mechanisms underlying sex-specific critical windows for stress susceptibility and behavioral emergence.

Impairment of learning and memory induced by perinatal exposure to BPA is associated with ERα-mediated alterations of synaptic plasticity and PKC/ERK/CREB signaling pathway in offspring rats

The effect of bisphenol A (BPA) on learning and memory has attracted much attention recently, but its underlying mechanism remains unclear. We aimed to investigate whether the impairment of learning and memory induced by perinatal exposure to BPA was associated with the hippocampal estrogen receptor α (ERα)-mediated synaptic plasticity and PKC/ERK/CREB signaling pathway in different sex offspring rats. Pregnant Sprague-Dawley rats were treated with BPA (1 and 10 μg/mL) through drinking water from gestational day (GD) 6 to postnatal day (PND) 21. After weaning, offspring drank BPA-free water until PND 56. Morris water maze, placement and object recognition, and step-down passive avoidance task were performed. The serum estradiol (E2) levels, histopathology of hippocampus, and the expression of learning and memory related proteins were measured. The results showed that spatial and recognition memory were impaired in BPA-exposed female and male offspring, but the impaired passive avoidance memory presented only in males, not in females. The serum E2 levels were increased in BPA-exposed females and males. BPA altered the morphology and quantity of hippocampal neurons. The levels of ERα, NMDA receptor subunit 2B (NR2B), p-NR2B, AMPA receptor 1 (GluA1), p-GluA1, PSD-95, synapsin I, PKC, p-ERK and p-CREB protein expression were decreased in BPA exposed females and males, and there were interactions of sex × BPA exposure in ERα, p-NR2B and p-ERK levels. These findings suggested that perinatal exposure to BPA has sex-specific effects on learning and memory, which is associated with ERα-mediated impairment of synaptic plasticity and down-regulation of PKC/ERK/CREB signaling pathway.

A potential role for dendritic spines in bisphenol-A induced memory impairments during adolescence and adulthood

Developmental exposure to Bisphenol A (BPA), an endocrine disrupting chemical, alters many behaviors and neural parameters in rodents and non-human-primates. The effects of BPA are mediated via gonadal hormone, primarily, estrogen receptors, and are not limited to the perinatal period since recent studies show impairments further into development. The studies described in this chapter address the effects of BPA administration during early adolescence on memory and dendritic spine density in intact male and female rats as well as ovariectomized (OVX) rats in late adolescence and show that some of these adolescent induced changes endure into adulthood. In general, BPA impairs spatial memory and induces decreases in dendritic spine density in the hippocampus and the medial prefrontal cortex, two areas important for memory. The effects of adolescent BPA in intact females are compared to OVX females in an attempt to address the importance of estrogens in the mechanism(s) underlying the profound neuronal alterations occurring during adolescent development. In addition, potential mechanisms by which acute and chronic BPA induce structural alterations are discussed. These studies suggest a complex interaction between low doses of BPA, gonadal state and neural development.

Bisphenol-A Exposure during Gestation and Lactation Causes Visual Perception Deficits in Rat Pups Following a Decrease in Interleukin 1β Expression in the Primary Visual Cortex

Bisphenol-A (BPA) exposure can affect cognitive functions of rodents and humans. However, whether information inputs for these functions in the brain are perturbed by BPA remains unclear. Here, visual perception in rats was assessed by testing their ability to discriminate between vertical and horizontal grating. We found that BPA exposure (1 mg/kg/day) during gestation and lactation markedly decreased the grating discrimination ability in rat pups (postnatal 21 days). The results of neuronal functions in the primary visual cortex (V1) showed that the orientation selectivity and signal extraction ability and fidelity were notably decreased after BPA exposure. These effects were accompanied by a reduction in synaptic plasticity (i.e., spine density and maturity) in the V1. According to inflammatory factor expression and glial cell morphology, no increase in inflammatory activation was observed after BPA exposure. However, BPA-exposed rat pups exhibited a significant decrease in IL-1β expression in the V1, accompanied by a decline in P38 phosphorylation. After local injection of IL-1β (10 ng/ml) in the V1, these two visual properties recovered to normal levels. Thus, our findings imply that physiological dysfunction of IL-1β may contribute to orientation perception deficits in BPA-exposed rats.

Estrogenic regulation of memory: The first 50 years

This review highlights fifty years of progress in research on estradiol's role in regulating behavior(s). It was initially thought that estradiol was only involved in regulating estrus/menstrual cycles and concomitant sexual behavior, but it is now clear that estradiol also influences the higher order neural function of cognition. We provide a brief overview of estradiol's regulation of memory and some mechanisms which underlie its effects. Given systemically or directly into the hippocampus, to ovariectomized female rodents, estradiol or specific agonists, enhance learning and/or memory in a variety of rodent cognitive tasks. Acute (within minutes) or chronic (days) treatments enhance cognitive functions. Under the same treatment conditions, dendritic spine density on pyramidal neurons in the CA1 area of the hippocampus and medial prefrontal cortex increase which suggests that these changes are an important component of estrogen's ability to impact memory processes. Noradrenergic, dopaminergic and serotoninergic activity are also altered in these areas following estrogen treatments. Memory enhancements and increased spine density by estrogens are not limited to females but are also present in castrate males. In the next fifty years, neuroscientists need to determine how currently described neural changes mediate improved memory, how interactions among areas important for memory promote memory and the potential significance of neurally derived estrogens in normal cognitive processing. Answering these questions may provide significant advances for treatment of dementias as well as age and neuro-degenerative disease related memory loss.

The Role of Hormonal and Reproductive Status in the Treatment of Anxiety Disorders in Women

Exposure therapy, a key treatment for anxiety disorders, can be modelled in the laboratory using Pavlovian fear extinction. Understanding the hormonal and neurobiological mechanisms underlying fear extinction in females, who are twice more likely than males to present with anxiety disorders, may aid in optimising exposure therapy outcomes in this population. This chapter will begin by discussing the role of the sex hormones, estradiol and progesterone, in fear extinction in females. We will also propose potential mechanisms by which these hormones may modulate fear extinction. The second half of this chapter will discuss the long-term hormonal, neurological and behavioural changes that arise from pregnancy and motherhood and how these changes may alter the features of fear extinction in females. Finally, we will discuss implications of this research for the treatment of anxiety disorders in women with and without prior reproductive experience.

G Protein-Coupled Estrogen Receptor: Rapid Effects on Hippocampal-Dependent Spatial Memory and Synaptic Plasticity

In the hippocampus, estrogen regulates gene transcription linked to neuronal growth, neuroprotection, and the maintenance of memory function (1-3). The mechanism is likely to involve genomic regulation through classic estrogen receptor (ER) signaling cascades that influence transcription. Estrogens binding to classic nuclear ERs, alpha (ERα) and beta (ERβ), and have pleotropic effects on development, behavior, and neurophysiological functions, including synaptic plasticity and memory consolidation (4-7). In addition to ERα and ERβ, estrogen can also initiate activation of classical second messenger signaling cascades to influence the activity of G-proteins and a host of kinases, resulting in rapid changes in physiology (8-14). These rapid effects of estrogen are commonly mediated by membrane receptors. In the late 90s, multiple laboratories cloned cDNA/gene for an orphan G-protein-coupled receptor with very low homology with other G-protein-coupled receptors and named it G-protein-coupled receptor 30 (GPR30) (15-20). Later in 2007, the International Union of Basic and Clinical Pharmacology designated GPR30 as G protein-coupled estrogen receptor (GPER) (21); GPER is a seven-transmembrane G-protein-coupled receptor, predominantly expressed on the cell membrane (22). Interestingly, GPER is reported to mediate many of the rapid responses of estradiol in the adult brain, and is widely distributed in the mammalian brain including the plasma membrane of hippocampal neurons (23-31). GPER modulates second messenger signaling cascades involving Gα_s- and Gα_i/o-associated increase in cyclic adenosine monophosphate and phosphoinositide 3-kinase or Src protein kinase respectively (32, 33). Activation of GPER is also associated with phospholipase C, and the inositol receptor and ryanodine receptor-mediated increase in intracellular calcium (24, 34). This commentary is concentrated specifically on the possible rapid effects of GPER in hippocampal-dependent spatial memory function and synaptic plasticity.

Oestradiol, threat conditioning and extinction, post-traumatic stress disorder, and prolonged exposure therapy: A common link

The accumulating evidence regarding the impact of estradiol on learning and memory synergized studies to examine its influence on enhancing animal's ability to quell fear and anxiety. In this review, we first provide a foundational platform regarding the impact of oestradiol on cellular mechanisms of learning and memory and we review recent advances from rodent and human data showing that oestrogen enhances extinction learning across species. We then propose clinical application to these data. We discuss the potential role of oestradiol variance on the aetiology, maintenance and treatment for post-traumatic stress disorder. Specifically, we argue that the use of oestradiol as an adjunct to prolonged exposure (PE) therapy for PTSD may provide a new treatment approach for enhancing the efficacy of PE in women with PTSD. This could advance our understanding of the mechanisms of PTSD and help tailor sex-specific treatments for this disorder.

Estrogen activates Alzheimer's disease genes

Introduction

Women are at increased risk for Alzheimer's disease (AD), but the reason why remains unknown. One hypothesis is that low estrogen levels at menopause increases vulnerability to AD, but this remains unproven.

Methods

We compared neuronal genes upregulated by estrogen in ovariectomized female rhesus macaques with a database of >17,000 diverse gene sets and applied a rare variant burden test to exome sequencing data from 1208 female AD patients with the age of onset < 75 years and 2162 female AD controls.

Results

We found a striking overlap between genes upregulated by estrogen in macaques and genes downregulated in the human postmortem AD brain, and we found that estrogen upregulates the APOE gene and that progesterone acts antagonistically to estrogen genome-wide. We also found that female patients with AD have excess rare mutations in the early menopause gene MCM8.

Discussion

We show with genomic data that the menopausal loss of estrogen could underlie the increased risk for AD in women.

An alternative theory for hormone effects on sex differences in PTSD: The role of heightened sex hormones during trauma

Women are at least twice as susceptible to developing post-traumatic stress disorder (PTSD) compared to men. Although most research seeking to explain this discrepancy has focussed on the role of oestradiol during fear extinction learning, the role of progesterone has been overlooked, despite relatively consistent findings being reported concerning the role of progesterone during consolidation of emotional and intrusive memories. In this review article, we outline literature supporting the role of progesterone on memory formation, with particular emphasis on potential memory-enhancing properties of progesterone when subjects are placed under stress. It is possible that progesterone directly and indirectly exerts memory-enhancing effects at the time of trauma, which is an effect that may not be necessarily captured during non-stressful paradigms. We propose a model whereby progesterone's steroidogenic relationship to cortisol and brain-derived neurotrophic factor in combination with elevated oestradiol may enhance emotional memory consolidation during trauma and therefore present a specific vulnerability to PTSD formation in women, particularly during the mid-luteal phase of the menstrual cycle.

Dorsal Hippocampal Actin Polymerization Is Necessary for Activation of G-Protein-Coupled Estrogen Receptor (GPER) to Increase CA1 Dendritic Spine Density and Enhance Memory Consolidation

Activation of the membrane estrogen receptor G-protein-coupled estrogen receptor (GPER) in ovariectomized mice via the GPER agonist G-1 mimics the beneficial effects of 17β-estradiol (E₂) on hippocampal CA1 spine density and memory consolidation, yet the cell-signaling mechanisms mediating these effects remain unclear. The present study examined the role of actin polymerization and c-Jun N-terminal kinase (JNK) phosphorylation in mediating effects of dorsal hippocampally infused G-1 on CA1 dendritic spine density and consolidation of object recognition and spatial memories in ovariectomized mice. We first showed that object learning increased apical CA1 spine density in the dorsal hippocampus (DH) within 40 min. We then found that DH infusion of G-1 increased both CA1 spine density and phosphorylation of the actin polymerization regulator cofilin, suggesting that activation of GPER may increase spine morphogenesis through actin polymerization. As with memory consolidation in our previous work (Kim et al., 2016), effects of G-1 on CA1 spine density and cofilin phosphorylation depended on JNK phosphorylation in the DH. Also consistent with our previous findings, E₂-induced cofilin phosphorylation was not dependent on GPER activation. Finally, we found that infusion of the actin polymerization inhibitor, latrunculin A, into the DH prevented G-1 from increasing apical CA1 spine density and enhancing both object recognition and spatial memory consolidation. Collectively, these data demonstrate that GPER-mediated hippocampal spinogenesis and memory consolidation depend on JNK and cofilin signaling, supporting a critical role for actin polymerization in the GPER-induced regulation of hippocampal function in female mice.SIGNIFICANCE STATEMENT Emerging evidence suggests that G-protein-coupled estrogen receptor (GPER) activation mimics effects of 17β-estradiol on hippocampal memory consolidation. Unlike canonical estrogen receptors, GPER activation is associated with reduced cancer cell proliferation; thus, understanding the molecular mechanisms through which GPER regulates hippocampal function may provide new avenues for the development of drugs that provide the cognitive benefits of estrogens without harmful side effects. Here, we demonstrate that GPER increases CA1 dendritic spine density and hippocampal memory consolidation in a manner dependent on actin polymerization and c-Jun N-terminal kinase phosphorylation. These findings provide novel insights into the role of GPER in mediating hippocampal morphology and memory consolidation, and may suggest first steps toward new therapeutics that more safely and effectively reduce memory decline in menopausal women.

Memory Function, Neurological, and Immunological Biomarkers in Allergic Asthmatic Mice Intratracheally Exposed to Bisphenol A

Bisphenol A (BPA) is a major constituent of plastic products, including epoxy resin containers, mobile phones, dental sealants, as well as electronic and medical equipment. BPA is recognized as an endocrine system-disrupting chemical which has toxic effects on the brain and reproductive system. However, little is known about the effects of co-exposure of BPA with allergens on the memory function and neurological as well as immunological biomarker levels. In this study, we examined the effects of intratracheal instillation of BPA on the memory function and neuroimmune biomarker levels using a mouse model of allergic asthma. Male C3H/HeJ Jcl mice were given three doses of BPA (0.0625 pmol, 1.25 pmol, and 25 pmol BPA/animal) intratracheally once a week, and ovalbumin (OVA) intratracheally every other week from 5 to 11 weeks old. At 11 weeks of age, a novel object recognition test was conducted after the final administration of OVA, and the hippocampi and hypothalami of the animals were collected after 24 h. The expression levels of the memory function-related genes N-methyl-D-aspartate (NMDA) receptor subunits, inflammatory cytokines, microglia markers, estrogen receptor-alpha, and oxytocin receptor were examined by real-time RT-PCR (real-time reverse transcription polymerase chain reaction) and immunohistochemical methods. Impairment of the novel object recognition ability was observed in the high-dose BPA-exposed mice with allergic asthma. In addition, the allergic asthmatic mice also showed downregulation of neurological biomarkers, such as NMDA receptor subunit NR2B in the hippocampus but no significant effect on immunological biomarkers in the hypothalamus. These findings suggest that exposure to high-dose BPA triggered impairment of memory function in the allergic asthmatic mice. This is the first study to show that, in the presence of allergens, exposure to high-dose BPA may affect memory by modulating the memory function-related genes in the hippocampus.

The Impact of Estradiol on Neurogenesis and Cognitive Functions in Alzheimer's Disease

Alzheimer's disease (AD) is described as cognitive and memory impairments with a sex-related epidemiological profile, affecting two times more women than men. There is emerging evidence that alternations in the hippocampal neurogenesis occur at the early stage of AD. Therapies that may effectively slow, stop, or regenerate the dying neurons in AD are being extensively investigated in the last few decades, but none has yet been found to be effective. The regulation of endogenous neurogenesis is one of the main therapeutic targets for AD. Mounting evidence indicates that the neurosteroid estradiol (17β-estradiol) plays a supporting role in neurogenesis, neuronal activity, and synaptic plasticity of AD. This effect may provide preventive and/or therapeutic approaches for AD. In this article, we discuss the molecular mechanism of potential estradiol modulatory action on endogenous neurogenesis, synaptic plasticity, and cognitive function in AD.

Estrogen-Dependent Functional Spine Dynamics in Neocortical Pyramidal Neurons of the Mouse

Surgical ovariectomy has been shown to reduce spine density in hippocampal CA1 pyramidal cells of rodents, and this reduction is reversed by 17β-estradiol (E2) treatment in a model of human estrogen replacement therapy. Here, we report reduction of spine density in apical dendrites of layer 5 pyramidal neurons of several neocortical regions that is reversed by subsequent E2 treatment in ovariectomized (OVX) female Thy1M-EGFP mice. We also found that OVX-associated reduction of spine density in somatosensory cortex was accompanied by a reduction in miniature EPSC (mEPSC) frequency (but not mIPSC frequency), indicating a change in functional synapses. OVX-associated spine loss in somatosensory cortex was also rescued by an agonist of the G-protein-linked estrogen receptor (GPER) but not by agonists of the classic estrogen receptors ERα/ERβ, whereas the opposite selectivity was found in area CA1. Acute treatment of neocortical slices with E2 also rescued the OVX-associated reduction in mEPSC frequency, which could be mimicked by a GPER agonist and abolished by a GPER antagonist. Time-lapse in vivo two-photon imaging showed that OVX-associated reduction in spine density is achieved by both an increase in spine loss rate and a decrease in spine gain rate and that subsequent rescue by E2 reversed both of these processes. Crucially, the spines added after E2 rescue were no more likely to reappear at or nearby the sites of pre-OVX spines than those in control mice treated with vehicle. Thus, a model of estrogen replacement therapy, although restoring spine density and dynamics, does not entirely restore functional connectivity.SIGNIFICANCE STATEMENT Estrogen replacement therapy following menopause or surgical removal of the ovaries is a widespread medical practice, yet little is known about the consequences of such treatment for cells in the brain. Here, we show that estrogen replacement reverses some of the effects of surgical removal of the ovaries on the structure and function of brain cells in the mouse. Yet, importantly, the fine wiring of the brain is not returned to the presurgery state by estrogen treatment, suggesting lasting functional consequences.

Chemogenetic Suppression of Medial Prefrontal-Dorsal Hippocampal Interactions Prevents Estrogenic Enhancement of Memory Consolidation in Female Mice

The importance of the dorsal hippocampus (DH) in mediating the memory-enhancing effects of the sex-steroid hormone 17β-estradiol (E₂) is well established. However, estrogen receptors (ERs) are highly expressed in other brain regions that support memory formation, including the medial prefrontal cortex (mPFC). The mPFC and DH interact to mediate the formation of several types of memory, and behavioral tasks that recruit the mPFC are enhanced by systemic E₂ administration, making this region a prime candidate for investigating circuit-level questions regarding the estrogenic regulation of memory. Further, infusion of E₂ directly into the DH increases dendritic spine density in both the DH and mPFC, and this effect depends upon rapid activation of cell-signaling pathways in the DH, demonstrating a previously unexplored interaction between the DH and mPFC that led us to question the role of the mPFC in object memory consolidation and the necessity of DH-mPFC interactions in the memory-enhancing effects of E₂. Here, we found that infusion of E₂ directly into the mPFC of ovariectomized mice increased mPFC apical spine density and facilitated object recognition and spatial memory consolidation, demonstrating that E₂ in the mPFC increases spinogenesis and enhances on memory consolidation. Next, chemogenetic suppression of the mPFC blocked the beneficial effects of DH-infused E₂ on memory consolidation, indicating that systems-level DH-mPFC interactions are necessary for the memory-enhancing effects of E₂. Together, these studies provide evidence that E₂ in the mPFC mediates memory formation, and reveal that the DH and mPFC act in concert to support the memory-enhancing effects of E₂ in female mice.

Structural plasticity of the hippocampus in response to estrogens in female rodents

It is well established that estrogens affect neuroplasticity in a number of brain regions. In particular, estrogens modulate and mediate spine and synapse formation as well as neurogenesis in the hippocampal formation. In this review, we discuss current research exploring the effects of estrogens on dendritic spine plasticity and neurogenesis with a focus on the modulating factors of sex, age, and pregnancy. Hormone levels, including those of estrogens, fluctuate widely across the lifespan from early life to puberty, through adulthood and into old age, as well as with pregnancy and parturition. Dendritic spine formation and modulation are altered both by rapid (likely non-genomic) and classical (genomic) actions of estrogens and have been suggested to play a role in the effects of estrogens on learning and memory. Neurogenesis in the hippocampus is influenced by age, the estrous cycle, pregnancy, and parity in female rodents. Furthermore, sex differences exist in hippocampal cellular and molecular responses to estrogens and are briefly discussed throughout. Understanding how structural plasticity in the hippocampus is affected by estrogens and how these effects can influence function and be influenced by other factors, such as experience and sex, is critical and can inform future treatments in conditions involving the hippocampus.

Effects of adolescent Bisphenol-A exposure on memory and spine density in ovariectomized female rats: Adolescence vs adulthood

The endocrine disruptor, Bisphenol-A (BPA), alters many behavioral and neural parameters in rodents. BPA administration to gonadally intact adolescent rats increases anxiety, impairs spatial memory, and decreases dendritic spine density when measured in adulthood. Since BPA's action seems to be mediated through gonadal steroid receptors, the current experiments were done in ovariectomized (OVX) female rats to examine the effects on behavior and spine density of adolescent BPA exposure under controlled hormone conditions. OVX (postnatal day, PND, 21) female Sprague-Dawley rats (n = 66) received subcutaneous injections of BPA (40 μg/kg/bodyweight), 17β-Estradiol (E2, 50 μg/kg/bodyweight), or saline during adolescence (PND 38-49). Following the last injection brains were processed for Golgi impregnation (Exp1), behavioral and spine density in adolescence (Exp2), or in adulthood (Exp3). In Exp1, E2 increased spine density in CA1 pyramidal cells and BPA decreased spine density in granule cells of the dentate gyrus (DG). In Exp2, BPA impaired spatial memory on the object placement (OP) task, E2 increased spine density in CA1, BPA decreased spine density in the DG and the medial prefrontal cortex (mPFC). When measured in adulthood (Exp3), BPA impaired OP and object recognition (OR) performance, E2 increased spine density in CA1, and BPA decreased spine density in CA1, the mPFC and the DG. Results provide novel data on the effects of adolescent BPA in an OVX model and are compared to data in intact animals and within the context of understanding the importance of the profound neuronal alterations occurring during adolescent development.

Molecular mechanism of endocrine-disruptive effects induced by Bisphenol A: The role of transmembrane G-protein estrogen receptor 1 and integrin αvβ3

Bisphenol A (BPA) is one of the highest volume industrial products worldwide and has been widely used to make various products as the intermediates of polycarbonate plastics and epoxy resins. Inevitably, general population has been widely exposed to BPA due to extensive use of BPA-containing products. BPA has similar chemical structure with the natural estrogen and has been shown to induce a variety of estrogen-like endocrine effects on organism in vivo or in vitro. High doses of BPA tend to act as antagonist of estrogen receptors (ERs) by directly regulating the genomic transcription. However, BPA at environmentally relevant low-dose always disrupt the biological function via a non-genomic manner mediated by membrane receptors, rather than ERs. Although some studies had investigated the non-genomic effects of low-dose BPA, the exact molecular mechanism still remains unclear. Recently, we found that membrane G protein-coupled estrogen receptor 1 and integrin αvβ3 and its relative signal pathways participate in the induction of male germ cell proliferation and thyroid transcription disruption by the low-dose BPA. A profound understanding for the mechanism of action of the environmentally relevant BPA exposure not only contributes to objectively evaluate and predict the potential influence to human health, but also provides theoretical basis and methodological support for assessing health effects trigged by other estrogen-like environmental endocrine disruptors. Based mainly on our recent findings, this review outlines the research progress of molecular mechanism on endocrine disrupting effects of environmental low-dose BPA, existing problems and some consideration for future studies.

Concerns related to ED-mediated effects of Bisphenol A and their regulatory consideration

The extensive database on BPA provides strong evidence of its adverse effects on reproductive, neurobehavioural, metabolic functions and mammary gland. Disruption of estrogenic pathway is central in the mediation of these effects although other modes of action may be involved. BPA has a weak affinity for ERα/β but interaction with extranuclearly located pathways activated by estrogens such as ERRγ and GPER reveals how BPA can act at low doses. The effects are observed later in life after developmental exposure and are associated with pathologies of major societal concern in terms of severity, incidence, impact on quality of life, burden on public health system. The complexity of the dose response raise uncertainties on the possibility to establish safe levels and the scope of ED-mediated effects of BPA may be wider. These concerns fulfill the requirements for ED identification under REACH regulation.

Impairment of learning and memory performances induced by BPA: Evidences from the literature of a MoA mediated through an ED

Many rodent studies and a few non-human primate data report impairments of spatial and non-spatial memory induced by exposure to bisphenol A (BPA), which are associated with neural modifications, particularly in processes involved in synaptic plasticity. BPA-induced alterations involve disruption of the estrogenic pathway as established by reversal of BPA-induced effects with estrogenic receptor antagonist or by interference of BPA with administered estradiol in ovariectomized animals. Sex differences in hormonal impregnation during critical periods of development and their influence on maturation of learning and memory processes may explain the sexual dimorphism observed in BPA-induced effects in some studies. Altogether, these data highly support the plausibility that alteration of learning and memory and synaptic plasticity by BPA is essentially mediated by disturbance of the estrogenic pathways. As memory function in humans involves similar signaling pathways, this mode of action of BPA has the potential to alter human cognitive abilities.

Estradiol-induced enhancement of fear extinction in female rats: The role of NMDA receptor activation

Converging cross-species evidence indicates that fear extinction (the laboratory basis of exposure therapy for anxiety disorders) in females is modulated by endogenous and exogenous estradiol. The mechanisms underlying estradiol's influences on fear extinction are largely undefined. However, one likely candidate is the NMDA-receptor (NMDAr), activation of which is necessary for estradiol-mediated enhancements in structural and functional neural plasticity, as well as extinction consolidation in males. Here, we demonstrate that systemic co-administration of the non-competitive NMDAr antagonist, MK801, blocked the enhancement of fear extinction by systemic estradiol in ovariectomized rats. In intact rats, MK801 during diestrus (rising estradiol) prevented the enhancement in extinction recall in rats that received extinction training during proestrus (peak estradiol). Systemic administration of the partial NMDAr agonist D-cycloserine (DCS) prior to extinction training facilitated extinction in ovariectomized rats, mimicking the effects of estradiol. In intact rats, DCS administered on the afternoon of proestrus and the morning of estrus (declining estradiol) facilitated extinction in rats that received extinction training during metestrus (low estradiol). Finally, DCS also facilitated extinction in ovariectomized rats when administered immediately after extinction training. Combined, these findings suggest that endogenous and exogenous estradiol enhance fear extinction via NMDAr-dependent mechanisms. Moreover, these findings raise the possibility that fear extinction deficits during periods of low endogenous estradiol levels can be reversed by increasing NMDAr activation via DCS administration, either well prior to, or immediately after, extinction training.

Mechanisms underlying the rapid effects of estradiol and progesterone on hippocampal memory consolidation in female rodents

Contribution to Special Issue on Fast effects of steroids. Although rapid effects of 17β‑estradiol (E₂) and progesterone on cellular functions have been observed for several decades, a proliferation of data in recent years has demonstrated the importance of these actions to cognition. In particular, an emerging literature has demonstrated that these hormones promote the consolidation of spatial and object recognition memories in rodents via rapid activation of numerous cellular events including cell signaling, histone modifications, and local protein translation in the hippocampus. This article provides an overview of the evidence demonstrating that E₂ and progesterone enhance hippocampal memory consolidation in female rodents, and then discusses numerous molecular mechanisms thus far shown to mediate the beneficial effects of these hormones on memory formation.

Rapid effects on memory consolidation and spine morphology by estradiol in female and male rodents

Contribution to Special Issue on Fast effects of steroids. Rapid, neurosteroid-like effects of estrogens on memory consolidation during recognition memory tasks in both male and female rodents are described. We discuss how these mnemonic changes are related to rapid estrogenic effects on dendritic spine density, the distribution of spine types and the expression of PSD95 and GluA2 within spines in the hippocampus and medial prefrontal cortex, two areas critical for learning and memory. Overall, these data lead to the conclusion that estrogens are capable of exerting rapid and potent influences on memory and spine morphology in both sexes. The demonstration of estrogenic effects in males, which are used in the majority of memory studies, may provide a model for better understanding how hormone dependent changes in signaling pathways mediating memory and spinogenesis are coordinated to promote memory consolidation.

Estradiol and hippocampal memory in female and male rodents

Estrogens influence nearly every aspect of hippocampal function, including memory formation. Although this research has traditionally focused on ovariectomized females, more recent work is providing insights into the ways in which estrogens regulate hippocampal function in both sexes. This review provides an overview of estrogenic regulation of hippocampal function in female and male rodents, with a particular emphasis on memory formation. Where applicable, we discuss the involvement of specific estrogen receptors and molecular mechanisms that mediate these effects. The review concludes by suggesting gaps in the literature that need to be filled to provide greater insights into potential sex differences in the effects of estrogens on hippocampal function.

Estrogenic regulation of memory consolidation: A look beyond the hippocampus, ovaries, and females

The potent estrogen 17β-estradiol (E2) has long been known to regulate the hippocampus and hippocampal-dependent memories in females, and research from the past decade has begun to shed light on the molecular mechanisms through which E2 mediates memory formation in females. Although E2 can also regulate hippocampal function in males, relatively little is known about how E2 influences memory formation in males, or whether sex differences in underlying mechanisms exist. This review, based on a talk given in April 2017 at the American University symposium entitled, "Sex Differences: From Neuroscience to the Clinic and Beyond", first provides an overview of the molecular mechanisms in the dorsal hippocampus through which E2 enhances memory consolidation in ovariectomized female mice. Next, newer research is described demonstrating key roles for the prefrontal cortex and de novo hippocampal E2 synthesis to the memory-enhancing effects of E2 in females. The review then discusses the effects of de novo and exogenous E2 on hippocampal memory consolidation in both sexes, and putative sex differences in the underlying molecular mechanisms through which E2 enhances memory formation. The review concludes by discussing the importance and implications of sex differences in the molecular mechanisms underlying E2-induced memory consolidation for human health.

Rapid nongenomic modulation by neurosteroids of dendritic spines in the hippocampus: Androgen, oestrogen and corticosteroid

Memories are stored in synapses that consist of axon terminals and dendritic spines. Dendritic spines are postsynaptic structures of synapses and are essential for synaptic plasticity and cognition. Therefore, extensive investigations concerning the functions and structures of spines have been performed. Sex steroids and stress steroids have been shown to modulate hippocampal synapses. Although the rapid modulatory action of sex steroids on synapses has been studied in hippocampal neurones over several decades, the essential molecular mechanisms have not been fully understood. Here, a description of kinase-dependent signalling mechanisms is provided that can explain the rapid nongenomic modulation of dendritic spinogenesis in rat and mouse hippocampal slices by the application of sex steroids, including dihydrotestosterone, testosterone, oestradiol and progesterone. We also indicate the role of synaptic (classic) sex steroid receptors that trigger these rapid synaptic modulations. Moreover, we describe rapid nongenomic spine modulation by applying corticosterone, which is an acute stress model of the hippocampus. The explanations for the results obtained are mainly based on the optical imaging of dendritic spines. Comparisons are also performed with results obtained from other types of imaging, including electron microscopic imaging. Relationships between spine modulation and modulation of cognition are discussed. We recognise that most of rapid effects of exogenously applied oestrogen and androgen were observed in steroid-depleted conditions, including acute slices of the hippocampus, castrated male animals and ovariectomised female animals. Therefore, the previously observed effects can be considered as a type of recovery event, which may be essentially similar to hormone replacement therapy under hormone-decreased conditions. On the other hand, in gonadally intact young animals with high levels of endogenous sex hormones, further supplementation of sex hormones might not be effective, whereas the infusion of blockers for steroid receptors or kinases may be effective, with respect to suppressing sex hormone functions, thus providing useful information regarding molecular mechanisms.

17β-estradiol (E2) promotes growth and stability of new dendritic spines via estrogen receptor β pathway in intact mouse cortex

The steroid hormone 17β-estradiol (E2) remodels neural circuits at the synaptic level in the mammalian hippocampus and cortex. However, the underlying mechanism of synapse dynamics remains unclear. To elucidate the mechanism, we traced individual dendritic spines on layer V pyramidal neurons of the primary sensory cortex in adult female mice under E2 intervention using two-photon in vivo imaging microscopy. We confirmed the increase of the spine density upon E2 treatment in the intact mouse cortex. Furthermore, we found that this increase is due to the promotion of spine formation and the stability of newly formed spines. E2 treatment doesn't alter the elimination rate of pre-existing spines. Our results also indicate that the activation of the estrogen receptor β (ERβ) mimics the effects of E2 administration on spine dynamics. Taken together, our findings suggest that estrogen promotes growth and stability of new dendritic spines via the ERβ pathway in the intact cortex of female mice.

Estradiol rapidly increases GluA2-mushroom spines and decreases GluA2-filopodia spines in hippocampus CA1

Hippocampal dendritic spine density rapidly increases following estradiol (E2 ) treatment, but the types of spines and trafficking of synaptic markers have received little investigation. We assessed rapid effects of E2 over time on the density of four spine types (stubby, filopodial, long thin, and mushroom) and trafficking of AMPA receptor subunit GluA2 and PSD95 on tertiary, apical dendrites in CA1. Castrated male rats received 20 μg kg-1 of E2 or vehicle and were sacrificed 30 or 120 min later. Images of Golgi-Cox impregnated and PSD95/GluA2 stained dendrites were captured under the confocal microscope and quantified with IMARIS-XT. Stubby and filopodial spine densities did not change following treatment. Long-thin spines significantly decreased at 30 min while mushroom spines significantly increased at 120 min. GluA2, PSD95, and GluA2/PSD95 colocalization levels in stubby or long thin spines did not change, but filopodial spines had significantly reduced GluA2 levels at 30 min. Mushroom spines showed significantly increased levels for GluA2, PSD95 and GluA2/PSD95 colocalization at 120 min. Because GluA2 is important for memory consolidation, current results present novel data suggesting that trafficking of GluA2 to mushroom spines provides one mechanism contributing to estradiol's ability to enhance learning and memory by the PI3 signaling pathway.

The importance of studying sex differences in disease: The example of multiple sclerosis

To date, scientific research has often focused on one sex, with assumptions that study of the other sex would yield similar results. However, many diseases affect males and females differently. The sex of a patient can affect the risk for both disease susceptibility and progression. Such differences can be brought to the laboratory bench to be investigated, potentially bringing new treatments back to the clinic. This method of research, known as a "bedside to bench to bedside" approach, has been applied to studying sex differences in multiple sclerosis (MS). Females have greater susceptibly to MS, while males have worse disease progression. These two characteristics of the disease are influenced by the immune system and the nervous system, respectively. Thus, sex differences in each system must be studied. Personalized medicine has been at the forefront of research recently, and studying sex differences in disease fits with this initiative. This review will discuss the known sex differences in MS and highlight how investigating them can lead to new insights and potential treatments for both men and women. © 2016 Wiley Periodicals, Inc.