Estrogen and the developing mammalian brain

Estrogen Signaling Influences Nephron Segmentation of the Zebrafish Embryonic Kidney

Despite significant advances in understanding nephron segment patterning, many questions remain about the underlying genes and signaling pathways that orchestrate renal progenitor cell fate choices and regulate differentiation. In an effort to identify elusive regulators of nephron segmentation, our lab conducted a high-throughput drug screen using a bioactive chemical library and developing zebrafish, which are a conserved vertebrate model and particularly conducive to large-scale screening approaches. 17β-estradiol (E2), which is the dominant form of estrogen in vertebrates, was a particularly interesting hit from this screen. E2 has been extensively studied in the context of gonad development, but roles for E2 in nephron development were unknown. Here, we report that exogenous estrogen treatments affect distal tubule composition, namely, causing an increase in the distal early segment and a decrease in the neighboring distal late. These changes were noted early in development but were not due to changes in cell dynamics. Interestingly, exposure to the xenoestrogens ethinylestradiol and genistein yielded the same changes in distal segments. Further, upon treatment with an estrogen receptor 2 (Esr2) antagonist, PHTPP, we observed the opposite phenotypes. Similarly, genetic deficiency of the Esr2 analog, esr2b, revealed phenotypes consistent with that of PHTPP treatment. Inhibition of E2 signaling also resulted in decreased expression of essential distal transcription factors, irx3b and its target irx1a. These data suggest that estrogenic compounds are essential for distal segment fate during nephrogenesis in the zebrafish pronephros and expand our fundamental understanding of hormone function during kidney organogenesis.

Sex-specific Effects of Endocrine-disrupting Chemicals on Brain Monoamines and Cognitive Behavior

The period of brain sexual differentiation is characterized by the development of hormone-sensitive neural circuits that govern the subsequent presentation of sexually dimorphic behavior in adulthood. Perturbations of hormones by endocrine-disrupting chemicals (EDCs) during this developmental period interfere with an organism's endocrine function and can disrupt the normative organization of male- or female-typical neural circuitry. This is well characterized for reproductive and social behaviors and their underlying circuitry in the hypothalamus and other limbic regions of the brain; however, cognitive behaviors are also sexually dimorphic, with their underlying neural circuitry potentially vulnerable to EDC exposure during critical periods of brain development. This review provides recent evidence for sex-specific changes to the brain's monoaminergic systems (dopamine, serotonin, norepinephrine) after developmental EDC exposure and relates these outcomes to sex differences in cognition such as affective, attentional, and learning/memory behaviors.

Embryonic exposure to butylparaben and propylparaben induced developmental toxicity and triggered anxiety-like neurobehavioral response associated with oxidative stress and apoptosis in the head of zebrafish larvae

Parabens are synthetic antimicrobial compounds used as a preservative for extending the shelf life of food, pharmaceutical and cosmetic products. The alkyl chain length of the paraben esters positively correlates with their antimicrobial property. Hence, long-chain paraben esters, namely butylparaben and propylparaben, are used in combination as they have better solubility and antimicrobial efficacy. Extensive use of parabens has now resulted in the ubiquitous presence of these compounds in various human and environmental matrices. During early life, exposure to environmental contaminants is known to cause oxidative-stress mediated apoptosis in developing organs. The brain being one of the high oxygen-consuming, metabolically active and lipid-rich organ, it is primarily susceptible to reactive oxygen species (ROS) and lipid peroxidation (LP) induced neuronal cell death. The primary cause for the impairment in cognitive and emotional neurobehvioural outcomes in neurodegenerative disease was found to be associated with neuronal apoptosis. The present study aimed to study butylparaben and propylparaben's effect on zebrafish during early embryonic stages. Besides this, the association between alteration in anxiety-like neurobehavioral response with oxidative stress and antioxidant status in head region was also studied. The study results showed variation in the toxic signature left by butylparaben and propylparaben on developmental parameters such as hatching rate, survival and non-lethal malformations in a time-dependent manner. Data from the light-dark preference test showed embryonic exposure to butylparaben and propylparaben to trigger anxiety-like behavior in zebrafish larvae. In addition, a significant increase in intracellular ROS and LP levels correlated with suppressed antioxidant enzymes: superoxide dismutases (SOD), catalases (CAT), Glutathione peroxidase (GPx), glutathione S-transferase (GST), and Glutathione (GSH) activity in the head region of the zebrafish larvae. Acetylcholinesterase (AChE) activity was also suppressed in the exposed groups, along with increased nitric oxide production. The overall observations show increased oxidative stress indices correlating with upregulated expression of apoptotic cells in a dose-dependent manner. Collectively, our findings reveal butylparaben and propylparaben as an anxiogenic neuroactive compound capable of inducing anxiety-like behavior through a mechanism involving oxidative-stress-induced apoptosis in the head of zebrafish larvae, which suggests a potential hazard to the early life of zebrafish and this can be extrapolated to human health as well.

Estrogen, Cognitive Performance, and Functional Imaging Studies: What Are We Missing About Neuroprotection?

Menopause transition can be interpreted as a vulnerable state characterized by estrogen deficiency with detrimental systemic effects as the low-grade chronic inflammation that appears with aging and partly explains age-related disorders as cancer, diabetes mellitus and increased risk of cognitive impairment. Over the course of a lifetime, estrogen produces several beneficial effects in healthy neurological tissues as well as cardioprotective effects, and anti-inflammatory effects. However, clinical evidence on the efficacy of hormone treatment in menopausal women has failed to confirm the benefit reported in observational studies. Unambiguously, enhanced verbal memory is the most robust finding from longitudinal and cross-sectional studies, what merits consideration for future studies aiming to determine estrogen neuroprotective efficacy. Estrogen related brain activity and functional connectivity remain, however, unexplored. In this context, the resting state paradigm may provide valuable information about reproductive aging and hormonal treatment effects, and their relationship with brain imaging of functional connectivity may be key to understand and anticipate estrogen cognitive protective effects. To go in-depth into the molecular and cellular mechanisms underlying rapid-to-long lasting protective effects of estrogen, we will provide a comprehensive review of cognitive tasks used in animal studies to evaluate the effect of hormone treatment on cognitive performance and discuss about the tasks best suited to the demonstration of clinically significant differences in cognitive performance to be applied in human studies. Eventually, we will focus on studies evaluating the DMN activity and responsiveness to pharmacological stimulation in humans.

In utero exposure to endocrine disruptors and developmental neurotoxicity: Implications for behavioural and neurological disorders in adult life

Endocrine disrupting chemicals (EDCs) are a class of environmental toxicants that interfere with the endocrine system, resulting in developmental malformations, reproductive disorders, and alterations to immune and nervous system function. The emergence of screening studies identifying these chemicals in fetal developmental matrices such as maternal blood, placenta and amniotic fluid has steered research focus towards elucidation of in utero effects of exposure to these chemicals, as their capacity to cross the placenta and reach the fetus was established. The presence of EDCs, a majority of which are estrogen mimics, in the fetal environment during early development could potentially affect neurodevelopment, with implications for behavioural and neurological disorders in adult life. This review summarizes studies in animal models and human cohorts that aim to elucidate mechanisms of action of EDCs in the context of neurodevelopment and disease risk in adult life. This is a significant area of study as early brain development is heavily mediated by estrogen and could be particularly sensitive to EDC exposure. A network analysis presented using genes summarized in this review, further show a significant association with disorders such as major depressive disorder, alcoholic disorder, psychotic disorders and autism spectrum disorder. Functional outcomes such as alterations in memory, behaviour, cognition, learning memory, feeding behaviour and regulation of ion transport are also highlighted. Interactions between genes, receptors and signaling pathways like NMDA glutamate receptor activity, 5-hydroxytryptamine receptor activity, Ras-activated Ca₂⁺ influx and Grin2A interactions, provide further potential mechanisms of action of EDCs in mediating brain function. Taken together with the growing pool of human and animal studies, this review summarizes current status of EDC neurotoxicity research, limitations and future directions of study for researchers.

GnRH protective effects against amyloid β-induced cognitive decline: A potential role of the 17β-estradiol

INTRODUCTION

The 17β-estradiol (E2) enhances hippocampal dendritic spine synapses, facilitates learning processes, and exerts neuroprotection. Brain estrogen decline has been reported in Alzheimer's disease. The role of GnRH in modulating steroid biosynthesis convinced us to examine whether hippocampal GnRH administration could enhance the local E2 levels and overcome the development of cognition decline in amyloid β (Aβ) neurotoxicity. To explore if GnRH acts through regulating E2 synthesis, letrozole, an aromatase inhibitor, has been applied in combination with GnRH.

METHODS

Female rats received an intracerebroventricular injection of Aβ. The GnRH and, or letrozole were injected into the CA1 for 14 consecutive days. Working memory, novel object recognition memory, and anxiety-like behavior were evaluated. Serum and hippocampal E2 levels were measured. Hippocampal mRNA expression of GnRH (GnRH-R) and E2 (ERα and ERβ) receptors was assessed. GnRH effect on the excitability of pyramidal cells was studied by in vivo single-unit recording.

RESULTS

GnRH increased hippocampal E2 levels, evoked an increase in the spontaneous firing of pyramidal neurons, and caused mRNA overexpression of hippocampal GnRH receptors. GnRH prevented the adverse effects of Aβ on working memory, NOR index, and anxiogenic behavior. Letrozole did not reverse GnRH modulatory effects on hippocampal E2 levels and neuroprotection.

CONCLUSION

GnRH prevented the Aβ-induced memory deficit, which may be mediated through hippocampal E2 levels enhancement. The electrophysiological analysis revealed the enhanced neuronal excitability in the CA1 region. All these data suggest that GnRH might be a promising candidate that reduces anxiety and improves memory indices in the context of Aβ neurotoxicity.

A comprehensive review of the neurobehavioral effects of bisphenol S and the mechanisms of action: New insights from in vitro and in vivo models

The normal brain development and function are delicately driven by an ever-changing milieu of steroid hormones arising from fetal, placental, and maternal origins. This reliance on the neuroendocrine system sets the stage for the exquisite sensitivity of the central nervous system to the adverse effects of endocrine-disrupting chemicals (EDCs). Bisphenol A (BPA) is one of the most common EDCs which has been a particular focus of environmental concern for decades due to its widespread nature and formidable threat to human and animal health. The heightened regulatory actions and the scientific and public concern over the adverse health effects of BPA have led to its replacement with a suite of structurally similar but less known alternative chemicals. Bisphenol S (BPS) is the main substitute for BPA that is increasingly being used in a wide array of consumer and industrial products. Although it was considered to be a safe BPA alternative, mounting evidence points to the deleterious effects of BPS on a wide range of neuroendocrine functions in animals. In addition to its reproductive toxicity, recent experimental efforts indicate that BPS has a considerable potential to induce neurotoxicity and behavioral dysfunction. This review analyzes the current state of knowledge regarding the neurobehavioral effects of BPS and discusses its potential mode of actions on several aspects of the neuroendocrine system. We summarize the role of certain hormones and their signaling pathways in the regulation of brain and behavior and discuss how BPS induces neurotoxicity through interactions with these pathways. Finally, we review potential links between BPS exposure and aberrant neurobehavioral functions in animals and identify key knowledge gaps and hypotheses for future research.

4-tert-Octylphenol Exposure Disrupts Brain Development and Subsequent Motor, Cognition, Social, and Behavioral Functions

The endocrine-disrupting chemical 4-tert-octylphenol (OP) is a widespread estrogenic chemical used in consumer products such as epoxy resins and polycarbonate plastic. However, the effects of OP on brain development are unknown. The present study examined the effects of OP on neuron and neurobehavioral development in mice. By using primary cortical neuron cultures, we found that OP-treated showed a decreased length of axons and dendrites and an increased number of primary and secondary dendrites. OP reduced bromodeoxyuridine (BrdU), mitotic marker Ki67, and phospho-histone H3 (p-Histone-H3), resulting in a reduction of neuronal progenitor proliferation in offspring mouse brain. Moreover, OP induced apoptosis in neuronal progenitor cells in offspring mouse brain. Furthermore, offspring mice from OP-treated dams showed abnormal cognitive, social, and anxiety-like behaviors. Taken together, these results suggest that perinatal exposure to OP disrupts brain development and behavior in mice.

Sexual Dimorphism in Titi Monkeys' Digit (2D:4D) Ratio is Associated with Maternal Urinary Sex Hormones During Pregnancy

The second-to-fourth digit (2D:4D) ratio is a sexually-dimorphic biomarker for prenatal sex hormone exposure. We investigated whether titi monkeys (Plecturocebus cupreus) exhibit sexually-dimorphic 2D:4D ratio, and whether variation in 2D:4D ratio correlates with maternal testosterone and estrogen levels during early pregnancy. Subjects were 61 adult titi monkeys (32 males, 29 females). For 26 subjects, maternal urine samples were collected approximately 15-20 weeks before birth and assayed for testosterone and estrone conjugate (E1 C). Titi monkeys exhibited a human-like pattern of sexual dimorphism in right-hand 2D:4D ratio, with females exhibiting higher 2D:4D ratio than males (β = -0.29, p = 0.023). For left-hand 2D:4D ratio, high levels of maternal E1 C predicted low offspring 2D:4D ratio (β = -0.48, p = 0.009). For right-hand 2D:4D ratio, high levels of testosterone (β = -0.53, p = 0.005) and testosterone-to-E1 C ratio (β = -0.41, p = 0.028) predicted low offspring 2D:4D ratio. For 2D:4D ratio asymmetry (right-hand - left-hand), high levels of testosterone (β = -0.43, p = 0.03) and testosterone-to-E1 C ratio (β = -0.53, p = 0.003) predicted low (right-biased) asymmetry. This is the first report of sexually-dimorphic 2D:4D ratio in New World monkeys, and the results support a growing literature suggesting prenatal sex hormones may modulate offspring 2D:4D ratio.

Modulatory effect of 17β-estradiol on myeloid cell infiltration into the male rat brain after ischemic stroke

Ischemic stroke is the leading cause of human disability and mortality in the world. Neuroinflammation is the main pathological event following ischemia which contributes to secondary brain tissue damage and is driven by infiltration of circulating immune cells such as macrophages. Because of neuroprotective properties against ischemic brain damage, estrogens have the potential to become of therapeutic interest. However, the exact mechanisms of neuroprotection and signaling pathways is not completely understood. In the current study, 12-week-old male Wistar rats underwent an experimental ischemia by occluding the middle cerebral artery transiently (tMCAO) for 1 h. Male rats subjected to tMCAO were randomly assigned to receive 17β-estradiol or vehicle treatment. The animals were sacrificed 72 h post tMCAO, transcardially perfused and the brains were proceeded either for TTC staining and gene analysis or for flow cytometry (CD45, CD11b, CD11c, CD40). We found that 17β-estradiol substitution significantly reduced the cortical infarct which was paralleled by an improved Garcia test scoring. Flow cytometry revealed that CD45+ cells as well as CD45+CD11b+CD11c+ cells were massively increased in tMCAO animals and numbers were nearly restored to sham levels after 17β-estradiol treatment. Gene expression analysis showed a reperfusion time-dependent upregulation of the markers CD45, CD11b and the activation marker CD40. The reduction in gene expression after 72 h of reperfusion and simultaneous 17β-estradiol substitution did not reach statistical significance. These data indicate that 17β-estradiol alleviated the cerebral ischemia-reperfusion injury and selectively suppressed the activation of the neuroinflammatory cascade via reduction of the number of activated microglia or infiltrated monocyte-derived macrophages in brain.

Single-cell RNA-seq analysis revealed long-lasting adverse effects of tamoxifen on neurogenesis in prenatal and adult brains

The CreER/LoxP system is widely accepted to track neural lineages and study gene functions upon tamoxifen (TAM) administration. We have observed that prenatal TAM treatment caused high rates of delayed delivery and fetal mortality. This substance could produce undesired results, leading to data misinterpretation. Here, we report that administration of TAM during early stages of cortical neurogenesis promoted precocious neural differentiation, while it inhibited neural progenitor cell (NPC) proliferation. The TAM-induced inhibition of NPC proliferation led to deficits in cortical neurogenesis, dendritic morphogenesis, synaptic formation, and cortical patterning in neonatal and postnatal offspring. Mechanistically, by employing single-cell RNA-sequencing (scRNA-seq) analysis combined with in vivo and in vitro assays, we show TAM could exert these drastic effects mainly through dysregulating the Wnt-Dmrta2 signaling pathway. In adult mice, administration of TAM significantly attenuated NPC proliferation in both the subventricular zone and the dentate gyrus. This study revealed the cellular and molecular mechanisms for the adverse effects of TAM on corticogenesis, suggesting that care must be taken when using the TAM-induced CreER/LoxP system for neural lineage tracing and genetic manipulation studies in both embryonic and adult brains.

Neural sex steroids and hippocampal synaptic plasticity

It was a widely held belief that sex steroids, namely testosterone and 17β-estradiol (E2) of gonadal origin, control synaptic plasticity in the hippocampus. A new paradigm emerged when it was shown that these sex steroids are synthesized in the hippocampus. The inhibition of sex steroids in the hippocampus impairs synaptic plasticity sex-dependently in this region of the brain. In gonadectomized animals and in hippocampal cultures, inhibition of estradiol synthesis in female animals and in cultures from female animals, and inhibition of dihydrotestosterone synthesis in male animals and in cultures of male animals, cause synapse loss and impair LTP in the hippocampus, but not vice versa. Since the hippocampal cultures originated from perinatal animals, and due to the similarity of in vivo and in vitro findings, it appears that hippocampal neurons are differentiated in a sex-specific manner during the perinatal period when sexual imprinting takes place.

Perinatal Exposure to Triclosan Results in Abnormal Brain Development and Behavior in Mice

Triclosan (TCS) is one of the most common endocrine-disrupting chemicals (EDCs) present in household and personal wash products. Recently, concerns have been raised about the association between abnormal behavior in children and exposure to EDC during gestation. We hypothesized that exposure to TCS during gestation could affect brain development. Cortical neurons of mice were exposed in vitro to TCS. In addition, we examined in vivo whether maternal TCS administration can affect neurobehavioral development in the offspring generation. We determined that TCS can impair dendrite and axon growth by reducing average length and numbers of axons and dendrites. Additionally, TCS inhibited the proliferation of and promoted apoptosis in neuronal progenitor cells. Detailed behavioral analyses showed impaired acquisition of spatial learning and reference memory in offspring derived from dams exposed to TCS. The TCS-treated groups also showed cognition dysfunction and impairments in sociability and social novelty preference. Furthermore, TCS-treated groups exhibited increased anxiety-like behavior, but there was no significant change in depression-like behaviors. In addition, TCS-treated groups exhibited deficits in nesting behavior. Taken together, our results indicate that perinatal exposure to TCS induces neurodevelopment disorder, resulting in abnormal social behaviors, cognitive impairment, and deficits in spatial learning and memory in offspring.

Developmental exposure to low levels of ethinylestradiol affects social play in juvenile male rats

Juvenile social play contributes to the development of adult social and emotional skills in humans and non-human animals and is therefore a useful endpoint to study the effects of endocrine disrupters on behavior in animal models. Ethinylestradiol (EE₂), a widely produced, powerful synthetic estrogen is widespread in the environment mainly because it is a component of the contraceptive pill. To understand whether clinical or environmental exposure to EE₂ during critical perinatal periods can affect male social play, we exposed 72 male Sprague-Dawley rats to EE₂ or vehicle either during gestation (from gestation day (GD) 5 through 20) or during lactation (from postnatal day (PND) 1 through 21). Two doses of EE₂ were used to treat the dams: a lower dose in the range of possible environmental exposure (4 ng/kg/day) and a higher dose similar to that received during contraceptive treatment (400 ng/kg/day). Social play was observed between PND 40 and 45. A principal component analysis (PCA) of frequencies of behavioral items observed during play sessions allowed to allocate behaviors to the two main components that we named aggressive-like play and defensive-like play. Aggressive-like play was increased by gestational and decreased by lactational exposure. Defensive-like play was decreased by treatment. For both types of play the lower dose (4 ng/kg/day) was as effective as the higher one. Total social activity was increased by gestational and decreased by lactational exposure. These findings provide further evidence that exposure to low and to very low doses of EE₂ during critical periods of development can affect essential aspects of social behavior, and that the timing of exposure is critical to understand its developmental action.

Estrogen Formation and Inactivation Following TBI: What we Know and Where we Could go

Traumatic brain injury (TBI) is responsible for various neuronal and cognitive deficits as well as psychosocial dysfunction. Characterized by damage inducing neuroinflammation, this response can cause an acute secondary injury that leads to widespread neurodegeneration and loss of neurological function. Estrogens decrease injury induced neuroinflammation and increase cell survival and neuroprotection and thus are a potential target for use following TBI. While much is known about the role of estrogens as a neuroprotective agent following TBI, less is known regarding their formation and inactivation following damage to the brain. Specifically, very little is known surrounding the majority of enzymes responsible for the production of estrogens. These estrogen metabolizing enzymes (EME) include aromatase, steroid sulfatase (STS), estrogen sulfotransferase (EST/SULT1E1), and some forms of 17β-hydroxysteroid dehydrogenase (HSD17B) and are involved in both the initial conversion and interconversion of estrogens from precursors. This article will review and offer new prospective and ideas on the expression of EMEs following TBI.

Non-reproductive Functions of Aromatase in the Central Nervous System Under Physiological and Pathological Conditions

The modulation of brain function and behavior by steroid hormones was classically associated with their secretion by peripheral endocrine glands. The discovery that the brain expresses the enzyme aromatase, which produces estradiol from testosterone, expanded this traditional concept. One of the best-studied roles of brain estradiol synthesis is the control of reproductive behavior. In addition, there is increasing evidence that estradiol from neural origin is also involved in a variety of non-reproductive functions. These include the regulation of neurogenesis, neuronal development, synaptic transmission, and plasticity in brain regions not directly related with the control of reproduction. Central aromatase is also involved in the modulation of cognition, mood, and non-reproductive behaviors. Furthermore, under pathological conditions aromatase is upregulated in the central nervous system. This upregulation represents a neuroprotective and likely also a reparative response by increasing local estradiol levels in order to maintain the homeostasis of the neural tissue. In this paper, we review the non-reproductive functions of neural aromatase and neural-derived estradiol under physiological and pathological conditions. We also consider the existence of sex differences in the role of the enzyme in both contexts.

Involvement of Estrogen and Its Receptors in Morphological Changes in the Eyes of the Japanese Eel, Anguilla japonica, in the Process of Artificially-Induced Maturation

During the long migration from river habitats to the spawning ground, the Japanese eel undergoes sexual maturation. This spawning migration occurs concurrently with morphological changes, such as increases in eye size; however, the mechanisms by which sex steroids and their receptors influence these changes in peripheral tissues remain unclear. The aim of this study was to investigate changes in the eyes of female Japanese eels during sexual maturation, and our research focused on estrogen receptor (ER)α and ERβ transcripts. During ovarian development, the gonadosomatic index increased and yolk-laden oocytes developed rapidly. These changes occurred in conjunction with a steady increase in plasma levels of estradiol-17β (E₂). Concomitant increases in transcript levels of ERα and ERβ in eye, brain, pituitary, and ovary were also observed. Fluorescence in-situ hybridization analyses revealed that ERα and ERβ transcripts were present in the choriocapillary layer and photoreceptor layer of the eyes, and the analysis also revealed that their signals in these layers became stronger in mature females compared to those observed in immature females, suggesting that under the influence of gonadotropins, morphological changes in the eyes are regulated by E₂ through the activation of its receptors. In conclusion, E₂ plays a crucial role in physiological adaptations that occur in peripheral tissues during the spawning migration.

Sex Hormones Regulate SHANK Expression

Autism spectrum disorders (ASD) have a higher prevalence in male individuals compared to females, with a ratio of affected boys compared to girls of 4:1 for ASD and 11:1 for Asperger syndrome. Mutations in the SHANK genes (comprising SHANK1, SHANK2 and SHANK3) coding for postsynaptic scaffolding proteins have been tightly associated with ASD. As early brain development is strongly influenced by sex hormones, we investigated the effect of dihydrotestosterone (DHT) and 17β-estradiol on SHANK expression in a human neuroblastoma cell model. Both sex hormones had a significant impact on the expression of all three SHANK genes, which could be effectively blocked by androgen and estrogen receptor antagonists. In neuron-specific androgen receptor knock-out mice (Ar^NesCre), we found a nominal significant reduction of all Shank genes at postnatal day 7.5 in the cortex. In the developing cortex of wild-type (WT) CD1 mice, a sex-differential protein expression was identified for all Shanks at embryonic day 17.5 and postnatal day 7.5 with significantly higher protein levels in male compared to female mice. Together, we could show that SHANK expression is influenced by sex hormones leading to a sex-differential expression, thus providing novel insights into the sex bias in ASD.

Neonatal exposure to oestradiol increases dopaminergic transmission in nucleus accumbens and morphine-induced conditioned place preference in adult female rats

Steroid sex hormones produce physiological effects in reproductive tissues and also in nonreproductive tissues, such as the brain, particularly in cortical, limbic and midbrain areas. Dopamine (DA) neurones involved in processes such as prolactin secretion (tuberoinfundibular system), motor circuit regulation (nigrostriatal system) and driving of motivated behaviour (mesocorticolimbic system) are specially regulated by sex hormones. Indeed, sex hormones promote neurochemical and behavioural effects induced by drugs of abuse by tuning midbrain DA neurones in adult animals. However, the long-term effects induced by neonatal exposure to sex hormones on dopaminergic neurotransmission have not been fully studied. The present study aimed to determine whether a single neonatal exposure with oestradiol valerate (EV) results in a programming of dopaminergic neurotransmission in the nucleus accumbens (NAcc) of adult female rats. To answer this question, electrophysiological, neurochemical, cellular, molecular and behavioural techniques were used. The data show that frequency but not amplitude of the spontaneous excitatory postsynaptic current is significantly increased in NAcc medium spiny neurones of EV-treated rats. In addition, DA content and release are both increased in the NAcc of EV-treated rats, caused by an increased synthesis of this neurotransmitter. These results are functionally associated with a higher percentage of EV-treated rats conditioned to morphine, a drug of abuse, compared to controls. In conclusion, neonatal programming with oestradiol increases NAcc dopaminergic neurotransmission in adulthood, which may be associated with increased reinforcing effects of drugs of abuse.

Hypothalamic gene expression changes in F1 California mice (Peromyscus californicus) parents developmentally exposed to bisphenol A or ethinyl estradiol

Bisphenol A (BPA) is a pervasive industrial chemical used in many common household items. To examine how early exposure to BPA and ethinyl estradiol (EE, estrogen in birth control pill) might affect biparental care, effects of these chemicals in male and female California mice (Peromyscus californicus), who are monogamous and biparental, were examined. California mice exposed during pre- and peri-natal life to BPA at an environmentally relevant concentration or EE show later disrupted biparental behaviors. The hypothalamus is an important brain region for regulating parental behaviors. Thus, it was hypothesized compromised biparental care might be partially due to hypothalamic gene alterations. To address this question, brains from F1 parenting female and male California mice from controls, BPA- and EE-exposed groups were collected at postnatal day (PND) 2, and RNA was isolated from hypothalamic micropunches. Gene expression was examined in this brain region for genes affected by BPA exposure and attributed to governing parental care in rodents and humans. BPA-exposed California mice showed increased hypothalamic expression of Kiss1, Esr1 and Esr2 relative to AIN control and EE-exposed parents in the case of Esr2. Notably, current studies represent the first report to show that early exposure to BPA can induce longstanding effects on hypothalamic gene expression in parenting male and female rodents. Absence of such hypothalamic gene expression changes in EE-exposed parents indicates early BPA exposure may induce later transcriptomic effects through estrogen receptor-independent pathways. BPA-driven changes in hypothalamic function of California mice might contribute to decreased biparental investment, which could result in F2 multigenerational effects.

Estradiol and the Development of the Cerebral Cortex: An Unexpected Role?

The cerebral cortex undergoes rapid folding in an "inside-outside" manner during embryonic development resulting in the establishment of six discrete cortical layers. This unique cytoarchitecture occurs via the coordinated processes of neurogenesis and cell migration. In addition, these processes are fine-tuned by a number of extracellular cues, which exert their effects by regulating intracellular signaling pathways. Interestingly, multiple brain regions have been shown to develop in a sexually dimorphic manner. In many cases, estrogens have been demonstrated to play an integral role in mediating these sexual dimorphisms in both males and females. Indeed, 17β-estradiol, the main biologically active estrogen, plays a critical organizational role during early brain development and has been shown to be pivotal in the sexually dimorphic development and regulation of the neural circuitry underlying sex-typical and socio-aggressive behaviors in males and females. However, whether and how estrogens, and 17β-estradiol in particular, regulate the development of the cerebral cortex is less well understood. In this review, we outline the evidence that estrogens are not only present but are engaged and regulate molecular machinery required for the fine-tuning of processes central to the cortex. We discuss how estrogens are thought to regulate the function of key molecular players and signaling pathways involved in corticogenesis, and where possible, highlight if these processes are sexually dimorphic. Collectively, we hope this review highlights the need to consider how estrogens may influence the development of brain regions directly involved in the sex-typical and socio-aggressive behaviors as well as development of sexually dimorphic regions such as the cerebral cortex.

Neuroanatomical and molecular correlates of cognitive and behavioural outcomes in hypogonadal males

Robust epidemiological, clinical and laboratory evidence supports emerging roles for the sex steroids in such domains as neurodevelopment, behaviour, learning and cognition. Regions of the mammalian brain that are involved in cognitive development and memory do not only express the classical nuclear androgen receptor, but also the non-genomic membrane receptor, which is a G protein-coupled receptor that mediates some rapid effects of the androgens on neurogenesis and synaptic plasticity. Under physiological conditions, hippocampal neurons do express the enzyme aromatase, and therefore actively aromatize testosterone to oestradiol. Although glial expression of the aromatase enzyme is minimal, increased expression following injury suggests a role for sex steroids in neuroprotection. It is therefore plausible to deduce that low levels of circulating androgens in males would perturb neuronal functions in relation to cognition and memory, as well as neural repair following injury. The present review is an overview of some roles of the sex steroids on cognitive function in males, and the neuroanatomical and molecular underpinnings of some behavioural and cognitive deficits characteristic of such genetic disorders noted for low androgen levels, including Klinefelter syndrome, Bardet-Biedl syndrome, Kallman syndrome and Prader-Willi syndrome. Recent literature in relation to some behavioural and cognitive changes secondary to surgical and pharmacological castration are also appraised.

Estrogen serum concentration affects blood immune cell composition and polarization in human females under controlled ovarian stimulation

Estrogens modulate the immune system and possess anti-inflammatory properties. In line, immune cells express a variety of estrogen receptors (ER) including ER-alpha and -beta. In the present study, we examined the influence of 17beta-estradiol (E2) serum concentrations on blood leukocyte composition and their ex vivo polarization/activation status by FACS analysis in sub-fertile human females under controlled ovarian stimulation (COS). Using a set of cell-type and polarization-specific markers, we demonstrate that increased 17ß-estradiol (E2) serum concentrations yield an overall increase in leukocytes, neutrophils and monocytes but decreased lymphocytes. There was a clear ratio shift towards an increase in M2 monocytes with a protective quality and an increase in T-helper cells compared to a decrease in cytotoxic T-cells. These data support experimental findings and clinical trials, i.e. related to multiple sclerosis and other autoimmune-related diseases, that have shown a down-regulation of CD8(+) T cells and up-regulation of T-regulatory cells. Further studies have to pinpoint to which extent the immune system/-responsiveness of otherwise healthy female patients is affected by medium-term systemic E2 variations.

Late pregnancy is vulnerable period for exposure to BPA

Bisphenol A (BPA) is among the better-known endocrine disruptors. BPA is used in various food-contacting materials and is easily eluted into food; as a result, we are exposed to BPA on a daily basis. In adults, BPA is metabolized and eliminated rapidly from the body. However, numerous reports suggest that fetuses and young children are susceptible to BPA. One of the concerning adverse effects of BPA is disruption of behavior, especially anxiety-like behavior. In order to study the mechanism of influences on offspring, it is important to clarify the most vulnerable gestation period. We hypothesized that offspring in late pregnancy would be more susceptible to BPA, because late pregnancy is a critical time for functional brain development. In this study, C57BL/6 mouse fetuses were exposed prenatally by oral dosing of pregnant dams, once daily from gestational day 5.5 to 12.5 (early pregnancy) or 11.5 to 18.5 (late pregnancy), with BPA (0 or 10 mg/kg body weight). Following birth and weaning, the resulting pups were tested using an elevated plus maze at postnatal week 10. The behavior of the offspring was altered by prenatal BPA exposure during late pregnancy but not during early pregnancy. These results indicated that offspring are more vulnerable to exposure to BPA in late pregnancy.

Neuroprotection by Estrogen and Progesterone in Traumatic Brain Injury and Spinal Cord Injury

In recent years there has been a growing body of clinical and laboratory evidence demonstrating the neuroprotective effects of estrogen and progesterone after traumatic brain injury (TBI) and spinal cord injury (SCI). In humans, women have been shown to have a lower incidence of morbidity and mortality after TBI compared with age-matched men. Similarly, numerous laboratory studies have demonstrated that estrogen and progesterone administration is associated with a mortality reduction, improvement in neurological outcomes, and a reduction in neuronal apoptosis after TBI and SCI. Here, we review the evidence that supports hormone-related neuroprotection and discuss possible underlying mechanisms. Estrogen and progesterone-mediated neuroprotection are thought to be related to their effects on hormone receptors, signaling systems, direct antioxidant effects, effects on astrocytes and microglia, modulation of the inflammatory response, effects on cerebral blood flow and metabolism, and effects on mediating glutamate excitotoxicity. Future laboratory research is needed to better determine the mechanisms underlying the hormones' neuroprotective effects, which will allow for more clinical studies. Furthermore, large randomized clinical control trials are needed to better assess their role in human neurodegenerative conditions.

Control of aromatase in hippocampal neurons

Our knowledge on estradiol-induced modulation of synaptic function in the hippocampus is widely based on results following the application of the steroid hormone to either cell cultures, or after the treatment of gonadectomized animals, thus ignoring local neuronal estrogen synthesis. We and others, however, have shown that hippocampus-derived estradiol also controls synaptic plasticity in the hippocampus. Estradiol synthesis in the hippocampus is regulated by several mechanisms, which are reviewed in this report. The regulation of the activity of aromatase, the final enzyme of estrogen biosynthesis, by Ca(2+) transients, is of particular interest. Aromatase becomes inactivated as soon as it is phosphorylated by Ca(2+)-dependent kinases upon calcium release from internal stores. Accordingly, thapsigargin dephosphorylates aromatase and stimulates estradiol synthesis by depletion of internal Ca(2+) stores. Vice versa, letrozole, an aromatase inhibitor, phosphorylates aromatase and reduces estradiol synthesis. Treatment of the cultures with 17β-estradiol results in phosphorylation of the enzyme and increased aromatase protein expression, which suggests that estradiol synthesis in hippocampal neurons is regulated in an autocrine manner.

Memory impairment is associated with the loss of regular oestrous cycle and plasma oestradiol levels in an activity-based anorexia animal model

OBJECTIVES

Patients with anorexia nervosa (AN) suffer from neuropsychological deficits including memory impairments. Memory partially depends on 17β-oestradiol (E2), which is reduced in patients with AN. We assessed whether memory functions correlate with E2 plasma levels in the activity-based anorexia (ABA) rat model.

METHODS

Nine 4-week-old female Wistar rats were sacrificed directly after weight loss of 20-25% (acute starvation), whereas 17 animals had additional 2-week weight-holding (chronic starvation). E2 serum levels and novel object recognition tasks were tested before and after starvation and compared with 21 normally fed controls.

RESULTS

Starvation disrupted menstrual cycle and impaired memory function, which became statistically significant in the chronic state (oestrous cycle (P < 0.001), E2 levels (P = 0.011) and object recognition memory (P = 0.042) compared to controls). E2 reduction also correlated with the loss of memory in the chronic condition (r = 0.633, P = 0.020).

CONCLUSIONS

Our results demonstrate that starvation reduces the E2 levels which are associated with memory deficits in ABA rats. These effects might explain reduced memory capacity in patients with AN as a consequence of E2 deficiency and the potentially limited effectiveness of psychotherapeutic interventions in the starved state. Future studies should examine whether E2 substitution could prevent cognitive deficits and aid in earlier readiness for therapy.

17α-Ethinylestradiol (EE2) treatment of wild roach (Rutilus rutilus) during early life development disrupts expression of genes directly involved in the feedback cycle of estrogen

Fish are more sensitive to introduced disturbances from synthetic endocrine disrupting compounds during early life phases compared with mature stages. 17α-Ethinylestradiol (EE2), which is the active compound in human oral contraceptives and hormone replacement therapies, is today ever present in the effluents from sewage treatment plants. EE2 targets and interacts with the endogenous biological systems of exposed vertebrates resulting in to large extents unknown short- and long-term effects. We investigated how EE2 exposure affects expression profiles of a large number of target genes during early life of roach (Rutilus rutilus). We exposed fertilized roach eggs collected from a lake in Southern Sweden to EE2 for 12weeks together with 1+-year-old roach in aquaria. We measured the gene expression of the estrogen receptor (esr)1/2a/2b, androgen receptor (ar), vitellogenin, cytochrome P450 (cyp)19a1a/1b in fertilized eggs; newly hatched larvae; 12-week-old fry; and juvenile wild roach (1+-year-old). Results shows that an EE2 concentration as low as 0.5ng/L significantly affects gene expression during early development. Gene expression responses vary both among life stages and molecular receptors. We also show that the gene profile of the estrogen feedback cycle to a large extent depends on the relationship between the three esr genes and the two cyp19a1 genes, which are all up-regulated with age. Results indicate that a disruption of the natural activity of the dominant esr gene could lead to detrimental biological effects if EE2 exposure occurs during development, even if this exposure occurred for only a short period.

Autism Spectrum Disorder Risk in Relation to Maternal Mid-Pregnancy Serum Hormone and Protein Markers from Prenatal Screening in California

We examined prenatal screening markers and offspring autism spectrum disorder (ASD) using California statewide data on singleton births in 1996 and 2002. Second trimester levels of unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and maternal serum alpha-fetoprotein (MSAFP) were compared between mothers of children with ASD (n = 2586) and of non-cases (n = 600,103). Adjusted odds ratios (AOR) were calculated by logistic regression. Lower uE3 (AOR for < 10th percentile vs. 25th-74th percentiles = 1.21, 95 % CI 1.06-1.37), and higher MSAFP (AOR = 1.21, 95 % CI 1.07-1.37 for > 90th percentile) were significantly associated with ASD. A U-shaped relationship was seen for hCG (AOR = 1.16, 95 % CI 1.02-1.32 for < 10th percentile; AOR = 1.19, 95 % CI 1.05-1.36 for > 90th percentile). Our results further support prenatal hormone involvement in ASD risk.

Programming of Dopaminergic Neurons by Neonatal Sex Hormone Exposure: Effects on Dopamine Content and Tyrosine Hydroxylase Expression in Adult Male Rats

We sought to determine the long-term changes produced by neonatal sex hormone administration on the functioning of midbrain dopaminergic neurons in adult male rats. Sprague-Dawley rats were injected subcutaneously at postnatal day 1 and were assigned to the following experimental groups: TP (testosterone propionate of 1.0 mg/50 μL); DHT (dihydrotestosterone of 1.0 mg/50 μL); EV (estradiol valerate of 0.1 mg/50 μL); and control (sesame oil of 50 μL). At postnatal day 60, neurochemical studies were performed to determine dopamine content in substantia nigra-ventral tegmental area and dopamine release in nucleus accumbens. Molecular (mRNA expression of tyrosine hydroxylase) and cellular (tyrosine hydroxylase immunoreactivity) studies were also performed. We found increased dopamine content in substantia nigra-ventral tegmental area of TP and EV rats, in addition to increased dopamine release in nucleus accumbens. However, neonatal exposure to DHT, a nonaromatizable androgen, did not affect midbrain dopaminergic neurons. Correspondingly, compared to control rats, levels of tyrosine hydroxylase mRNA and protein were significantly increased in TP and EV rats but not in DHT rats, as determined by qPCR and immunohistochemistry, respectively. Our results suggest an estrogenic mechanism involving increased tyrosine hydroxylase expression, either by direct estrogenic action or by aromatization of testosterone to estradiol in substantia nigra-ventral tegmental area.

Salutary Effects of Estrogen Sulfate for Traumatic Brain Injury

Estrogen plays an important role as a neuroprotector in the central nervous system (CNS), directly interacting with neurons and regulating physiological properties of non-neuronal cells. Here we evaluated estrogen sulfate (E2-SO4) for traumatic brain injury (TBI) using a Sprague-Dawley rat model. TBI was induced via lateral fluid percussion (LFP) at 24 h after craniectomy. E2-SO4 (1 mg/kg BW in 1 mL/kg BW) or saline (served as control) was intravenously administered at 1 h after TBI (n=5/group). Intracranial pressure (ICP), cerebral perfusion pressure (CPP), and partial brain oxygen pressure (pbtO2) were measured for 2 h (from 23 to 25 h after E2-SO4 injection). Brain edema and diffuse axonal injury (DAI) were assessed by diffusion tensor imaging (DTI), and cerebral glycolysis was measured by (18)F-labeled fluorodeoxyglucose (FDG) positron emission tomography (PET) imaging, at 1 and 7 days after E2-SO4 injection. E2-SO4 significantly decreased ICP, while increasing CPP and pbtO2 (p<0.05) as compared with vehicle-treated TBI rats. The edema size in the brains of the E2-SO4 treated group was also significantly smaller than that of vehicle-treated group at 1 day after E2-SO4 injection (p=0.04), and cerebral glycolysis of injured region was also increased significantly during the same time period (p=0.04). However, E2-SO4 treatment did not affect DAI (p>0.05). These findings demonstrated the potential benefits of E2-SO4 in TBI.

Regulation of brain microglia by female gonadal steroids

Microglial cells are the primary mediators of the CNS immune defense system and crucial for shaping inflammatory responses. They represent a highly dynamic cell population which is constantly moving and surveying their environment. Acute brain damage causes a local attraction and activation of this immune cell type which involves neuron-to-glia and glia-to-glia interactions. The prevailing view attributes microglia a "negative" role such as defense and debris elimination. More topical studies also suggest a protective and "positive" regulatory function. Estrogens and progestins exert anti-inflammatory and neuroprotective effects in the CNS in acute and chronic brain diseases. Recent work revealed that microglial cells express subsets of classical and non-classical estrogen and progesterone receptors in a highly dynamic way. In this review article, we would like to stress the importance of microglia for the spreading of neural damage during hypoxia, their susceptibility to functional modulation by sex steroids, the potency of sex hormones to switch microglia from a pro-inflammatory M1 to neuroprotective M2 phenotype, and the regulation of pro- and anti-inflammatory properties including the inflammasome. We will further discuss the possibility that the neuroprotective action of sex steroids in the brain involves an early and direct modulation of local microglia cell function. This article is part of a Special Issue entitled 'Sex steroids and brain disorders'.

The effects of steroid hormone exposure on direct gene regulation

Steroid hormones have been widely overlooked as controllers of gene expression. Through the mechanisms of gene expression (DNA methylation, histone methylation, and RNAi), we discuss the impact of normal reproductive templates on the pulsatility and amplitude of potential gene-regulating treatment protocols. By examining the interactions of estradiol (E2) and progesterone (P4) in women, we propose that changes in physiologic reproductive hormone templates of exposure and timing can affect fertility and even cancer through the silencing or amplification of gene products; such as P53 and Bcl-2 in women. We suggest that uncontrolled hormone levels, due to aging and/or the environment, may be restored to a normal youthful template of gene expression through the fluctuating exogenous application of E2 and P4 that mimic the normal hormonal milieu of reproductive health. Furthermore, we hypothesize that restoration of normal hormone levels may lead to a lower risk of the chronic illnesses of aging and a better quality of life in patients suffering those conditions.

PELP1: a review of PELP1 interactions, signaling, and biology

Proline, glutamic acid, and leucine rich protein 1 (PELP1) is a large multi-domain protein that has been shown to modulate an increasing number of pathways and biological processes. The first reports describing the cloning and characterization of PELP1 showed that it was an estrogen receptor coactivator. PELP1 has now been shown to be a coregulator for a growing number of transcription factors. Furthermore, recent reports have shown that PELP1 is a member of chromatin remodeling complexes. In addition to PELP1 nuclear functions, it has been shown to have cytoplasmic signaling functions as well. In the cytoplasm PELP1 acts as a scaffold molecule and mediates rapid signaling from growth factor and hormone receptors. PELP1 signaling ultimately plays a role in cancer biology by increasing proliferation and metastasis, among other cellular processes. Here we will review (1) the cloning and characterization of PELP1 expression, (2) interacting proteins, (3) PELP1 signaling, and (4) PELP1-mediated biology.

Tualang honey supplement improves memory performance and hippocampal morphology in stressed ovariectomized rats

Recently, our research team has reported that Tualang honey was able to improve immediate memory in postmenopausal women comparable with that of estrogen progestin therapy. Therefore the aim of the present study was to examine the effects of Tualang honey supplement on hippocampal morphology and memory performance in ovariectomized (OVX) rats exposed to social instability stress. Female Sprague-Dawley rats were divided into six groups: (i) sham-operated controls, (ii) stressed sham-operated controls, (iii) OVX rats, (iv) stressed OVX rats, (v) stressed OVX rats treated with 17β-estradiol (E2), and (vi) stressed OVX rats treated with Tualang honey. These rats were subjected to social instability stress procedure followed by novel object recognition (NOR) test. Right brain hemispheres were subjected to Nissl staining. The number and arrangement of pyramidal neurons in regions of CA1, CA2, CA3 and the dentate gyrus (DG) were recorded. Two-way ANOVA analyses showed significant interactions between stress and OVX in both STM and LTM test as well as number of Nissl-positive cells in all hippocampal regions. Both E2 and Tualang honey treatments improved both short-term and long-term memory and enhanced the neuronal proliferation of hippocampal CA2, CA3 and DG regions compared to that of untreated stressed OVX rats.

Neuroprotection by gonadal steroid hormones in acute brain damage requires cooperation with astroglia and microglia

The neuroactive steroids 17β-estradiol and progesterone control a broad spectrum of neural functions. Besides their roles in the regulation of classical neuroendocrine loops, they strongly influence motor and cognitive systems, behavior, and modulate brain performance at almost every level. Such a statement is underpinned by the widespread and lifelong expression pattern of all types of classical and non-classical estrogen and progesterone receptors in the CNS. The life-sustaining power of neurosteroids for tattered or seriously damaged neurons aroused interest in the scientific community in the past years to study their ability for therapeutic use under neuropathological challenges. Documented by excellent studies either performed in vitro or in adequate animal models mimicking acute toxic or chronic neurodegenerative brain disorders, both hormones revealed a high potency to protect neurons from damage and saved neural systems from collapse. Unfortunately, neurons, astroglia, microglia, and oligodendrocytes are comparably target cells for both steroid hormones. This hampers the precise assignment and understanding of neuroprotective cellular mechanisms activated by both steroids. In this article, we strive for a better comprehension of the mutual reaction between these steroid hormones and the two major glial cell types involved in the maintenance of brain homeostasis, astroglia and microglia, during acute traumatic brain injuries such as stroke and hypoxia. In particular, we attempt to summarize steroid-activated cellular signaling pathways and molecular responses in these cells and their contribution to dampening neuroinflammation and neural destruction. This article is part of a Special Issue entitled 'CSR 2013'.

Neurosteroidogenic enzymes and their regulation in the early brain of the protogynous grouper Epinephelus coioides during gonadal sex differentiation

The regulatory role of neurosteroids in the early brain during gonadal sex differentiation is unclear. The aim of this study was to investigate the expression and cellular localization of key steroidogenic enzymes in the early brain of the protogynous orange-spotted grouper Epinephelus coioides and the temporal expressions has been correlated with gonadal sex differentiation. In this study, we showed that peak neurosteroidogenesis occurs in the early brain during gonadal sex differentiation. The temporal expressions of key enzymes, cyp11a1 (cytochrome P450 side chain cleavage), hsd3b1 (3β-hydroxysteroid dehydrogenase) and cyp17a1 (cytochrome P450c17) were studied at different developmental ages (from 90- to 150-dah: days after hatching) using quantitative real-time PCR (q-PCR). q-PCR analysis indicated that the transcript expressions of cyp11a1, hsd3b1 and cyp17a1 were increased in the brain around the period of gonadal sex differentiation. Further, in situ hybridization (ISH) analysis showed that cyp11a1, hsd3b1 and cyp17a1 transcripts were widely expressed in several discrete brain regions, especially the intense expression in the forebrain, with an overall similar expression pattern. High density in the cyp19a1b/Cyp19a1b expression was detected in radial glial cells. Thus, the expression of grouper cyp19a1b/Cyp19a1b is restricted to radial glial cells, suggesting estrogens can modulate their activity. Next, by combining Cyp19a1b immunohistochemistry (IHC) with florescence ISH (FISH) of cyp11a1, hsd3b1 and cyp17a1, we showed that sub-cellular localization of cyp11a1, hsd3b1 and cyp17a1 transcripts, in partial, appeared to be in Cyp19a1b radial glial cell soma. Further, exogenous estradiol (E(2)) increased the expression of cyp17a1 and cyp19a1b/Cyp19a1b in the brain of grouper. Consequently, our results illustrated that the locally synthesized E(2) upregulated neurosteroidogenic enzymes in the early brain and suggest a role for these enzymes in the neurogenic process during gonadal sex differentiation.

Brain-derived neurotrophic factor-estrogen interactions in the hippocampal mossy fiber pathway: implications for normal brain function and disease

The neurotrophin brain-derived neurotrophic factor (BDNF) and the steroid hormone estrogen exhibit potent effects on hippocampal neurons during development and in adulthood. BDNF and estrogen have also been implicated in the etiology of diverse types of neurological disorders or psychiatric illnesses, or have been discussed as potentially important in treatment. Although both are typically studied independently, it has been suggested that BDNF mediates several of the effects of estrogen in the hippocampus, and that these interactions play a role in the normal brain as well as disease. Here we focus on the mossy fiber (MF) pathway of the hippocampus, a critical pathway in normal hippocampal function, and a prime example of a location where numerous studies support an interaction between BDNF and estrogen in the rodent brain. We first review the temporal and spatially regulated expression of BDNF and estrogen in the MFs, as well as their receptors. Then we consider the results of studies that suggest that 17β-estradiol alters hippocampal function by its influence on BDNF expression in the MF pathway. We also address the hypothesis that estrogen influences the hippocampus by mechanisms related not only to the mature form of BDNF, acting at trkB receptors, but also by regulating the precursor, proBDNF, acting at p75NTR. We suggest that the interactions between BDNF and 17β-estradiol in the MFs are potentially important in the normal function of the hippocampus, and have implications for sex differences in functions that depend on the MFs and in diseases where MF plasticity has been suggested to play an important role, Alzheimer's disease, epilepsy and addiction.

Estrogen-regulated synaptogenesis in the hippocampus: sexual dimorphism in vivo but not in vitro

Hippocampal neurons are capable of synthesizing estradiol de novo. Estradiol synthesis can be suppressed by aromatase inhibitors and by knock-down of steroid acute regulatory protein (StAR), whereas elevated levels of substrates of steroidogenesis enhance estradiol synthesis. In rat hippocampal cultures, the expression of estrogen receptors (ERs) and synaptic proteins, as well as synapse density, correlated positively with aromatase activity, regardless of whether the cultures originated from males or females. All effects induced by the inhibition of aromatase activity were rescued by application of estradiol to the cultures. In vivo, however, systemic application of letrozole, an aromatase inhibitor, induced synapse loss in female rats, but not in males. Furthermore, in the female hippocampus, density of spines and spine synapses varied with the estrus cycle. In addressing this in vivo-in vitro discrepancy, we found that gonadotropin-releasing hormone (GnRH) regulated estradiol synthesis via an aromatase-mediated mechanism and consistently regulated spine synapse density and the expression of synaptic proteins. Along these lines, GnRH receptor density was higher in the hippocampus than in the cortex and hypothalamus, and estrus cyclicity of spinogenesis was found in the hippocampus, but not in the cortex. Since GnRH receptor expression also varies with the estrus cycle, the sexual dimorphism in estrogen-regulated spine synapse density in the hippocampus very likely results from differences in the GnRH responsiveness of the male and the female hippocampus. This article is part of a Special Issue entitled 'Neurosteroids'.

Estrogen and the regulation of mitochondrial structure and function in the brain

The mitochondrion is the unquestionable cellular compartment that actively preserves most of the cell functions, such as lipid metabolism, ion homeostasis, energy and ROS production, steroid biosynthesis, and control of apoptotic signaling. Thus, this cell organelle depicts a major drop-in centre for regulatory processes within a cell irrespective of the organ or tissue. However, brain tissue is unique in spite of everything due to its extremely high energy demand and sensitivity to oxidative stress. This makes brain cells, in particular neurons, considerably vulnerable against toxins and challenges that attack the mitochondrial structural organization and energetic performance. Estrogens are known to regulate a multitude of cellular functions in neural cells under physiological conditions but also play a protective role under neuropathological circumstances. In recent years, it became evident that estrogens affect distinct cellular processes by interfering with the bioenergetic mitochondrial compartment. According to the general view, estrogens indirectly regulate the mitochondrion through the control of genomic transcription of mitochondrial-located proteins and modulation of cytoplasmic signaling cascades that act upon mitochondrial physiology. More recent but still arguable data suggest that estrogens might directly signal to the mitochondrion either through classical steroid receptors or novel types of receptors/proteins associated with the mitochondrial compartment. This would allow estrogens to more rapidly modulate the function of a mitochondrion than hitherto discussed. Assuming that this novel perception of steroid action is correct, estrogen might influence the energetic control centre through long-lasting nuclear-associated processes and rapid mitochondria-intrinsic temporary mechanisms. In this article, we would like to particularly accentuate the novel conceptual approach of this duality comprising that estrogens govern the mitochondrial structural integrity and functional capacity by different cellular signaling routes. This article is part of a Special Issue entitled 'Neurosteroids'.