Neuroprotection by estradiol: A role of aromatase against spine synapse loss after blockade of GABAA receptors

Novel studies on Isolation, purification and characterization of dibenzonitro compound from Glycosmis pentaphylla (Retz.) DC. and effect in downregulating neuronal cancers

To isolate Letrozole from Glycosmis pentaphylla (Retz.) DC. and to determine its effect on regulating the proliferation, cell cycle distribution, apoptosis and key mechanisms in human neuroblastoma cell lines. Letrozole was isolated through column chromatographic technique and its effect was checked on human neuroblastoma cell lines, IMR 32. The effects of Letrozole on cell viability were measured by MTT assay, and the cell cycle distribution was determined by flow cytometry. The expression changes in mRNA of proliferating cell nuclear antigen (PCNA), cyclin D1 and Bcl-xL were taken from real-time PCR analysis and the protein levels were detected by Western blotting. The results of the present study showed that Letrozole, isolated from leaves of G. pentaphylla could cause significant inhibitory effect on proliferation of IMR 32 cells in a dose dependent manner. Cell arrest was obtained at S phase with the treatment of Letrozole. Apart from this, the expression of PCNA, cyclin D1 and Bcl-xL were decreased both at mRNA and protein levels for the same treatment. Letrozole can inhibit proliferation, induce cell arrest and cause apoptosis in IMR 32 cell lines. The decreased expression of PCNA, cyclin D1 and Bcl-xL induced by Letrozole contributes to the above effects in vitro. This is the first report on the isolation of Letrozole from G. pentaphylla.

Sex hormones and risk of epilepsy: A bidirectional Mendelian randomization study

Background

Multiple evidence has suggested complex interaction between sex hormones and epilepsy. However, whether there exists a causal association and the effect direction remains controversial. Here we aimed to examine the causative role of hormones in the risk of epilepsy and vice versa.

Methods

We conducted a bidirectional two-sample Mendelian randomization analysis using summary statistics from genome-wide association studies of major sex hormones including testosterone (N = 425,097), estradiol (N = 311,675) and progesterone (N = 2,619), together with epilepsy (N = 44,889). We further performed sex-stratified analysis, and verified the significant results using summary statistics from another study on estradiol in males (N = 206,927).

Results

Genetically determined higher estradiol was associated with a reduced risk of epilepsy (OR: 0.90, 95% CI: 0.83-0.98, P = 9.51E-03). In the sex-stratified analysis, the protective effect was detected in males (OR: 0.92, 95% CI: 0.88-0.97, P = 9.18E-04), but not in females. Such association was further verified in the replication stage (OR: 0.44, 95% CI: 0.23-0.87, P = 0.017). In contrast, no association was identified between testosterone, progesterone and the risk of epilepsy. In the opposite direction, epilepsy was not causally associated with sex hormones.

Conclusion

These results demonstrated higher estradiol could reduce the risk of epilepsy, especially in males. Future development of preventive or therapeutic interventions in clinical trials could attach importance to this.

Sex-dependent responsiveness of hippocampal neurons to sex neurosteroids: A role of Arc/Arg3.1

Sex steroids, such as estradiol (E₂ ) and dihydrotestosterone (DHT), regulate hippocampal plasticity and memory in a sex-dependent manner. Because the activity-regulated cytoskeleton protein Arc/Arg3.1 is essential for long-term memory formation and synaptic plasticity, we investigated the expression of Arc/Arg3.1 with respect to its responsiveness to E₂ and DHT in male and female hippocampal neurons. For the first time, we show that, in hippocampal neurons, Arc/Arg3.1 expression is sex-dependently regulated by sex steroids. No difference in the expression between sexes was observed under control conditions. Using a quantitative real-time polymerase chain reaction, western blot analysis and quantitative immunoreactivity, upregulation of Arc/Arg3.1 protein expression was observed in specifically female hippocampal neurons after application of E₂ to the cultures. Conversely, upregulation of Arc/Arg3.1 was seen in specifically male neurons after application of DHT. A quantitative real-time PCR revealed that the sex-dependency was most pronounced on the mRNA level. Most importantly, the effects of E₂ in cultures of female animals were abolished when neuron-derived E₂ synthesis was inhibited. Our results point to a potentially important role of Arc/Arg3.1 regarding sex-dependency in sex steroid-induced synaptic plasticity in the hippocampus.

Aromatase in the Human Brain

The aromatase cytochrome P450 (P450arom) enzyme, or estrogen synthase, which is coded by the CYP19A1 gene, is widely expressed in a subpopulation of excitatory and inhibitory neurons, astrocytes, and other cell types in the human brain. Experimental studies in laboratory animals indicate a prominent role of brain aromatization of androgens to estrogens in regulating different brain functions. However, the consequences of aromatase expression in the human brain remain poorly understood. Here, we summarize the current knowledge about aromatase expression in the human brain, abundant in the thalamus, amygdala, hypothalamus, cortex, and hippocampus and discuss its role in the regulation of sensory integration, body homeostasis, social behavior, cognition, language, and integrative functions. Since brain aromatase is affected by neurodegenerative conditions and may participate in sex-specific manifestations of autism spectrum disorders, major depressive disorder, multiple sclerosis, stroke, and Alzheimer's disease, we discuss future avenues for research and potential clinical and therapeutic implications of the expression of aromatase in the human brain.

Sex neurosteroids: Hormones made by the brain for the brain

In general, hippocampal neurons are capable of synthesizing sex steroids de novo from cholesterol, since the brain is equipped with all the enzymes required for the synthesis of estradiol and testosterone, the end products of sex steroidogenesis. Regarding estradiol, its synthesis in hippocampal neurons is homeostatically controlled by Ca²⁺ transients and is regulated by GnRH. Locally synthesized estradiol and testosterone maintain synaptic transmission and synaptic connectivity. Remarkably, the neurosteroid estradiol is effective in females, but not in males, and vice versa dihydrotestosterone (DHT) is effective in males, but not in females. Experimentally induced inhibition of estradiol synthesis in females and DHT synthesis in males resp. results in synapse loss, impaired LTP, and downregulation of synaptic proteins. GnRH-induced increase in estradiol synthesis appears to provide a link between the hypothalamus and the hippocampus, which may underlie estrous cyclicity of spine density in the female hippocampus. Hippocampal neurons are sex-dependently differentiated with respect to the responsiveness of hippocampal neurons to sex neurosteroids.

Anabolic steroids and their effects of on neuronal density in cortical areas and hippocampus of mice

Anabolic substances have been increasingly used by bodybuilders and athletes with the goal of improving performance and aesthetics. However, this practice has caused some concern to physicians and researchers because of unknowledge of consequences that the indiscriminate and illicit use of these substances can cause. Thus, this study analyzed the effects of two commercially available anabolic steroids (AS), Winstrol Depot® (Stanozolol) and Deposteron® (Testosterone Cypionate), in the neuronal density of limbic, motor and sensory regions on the cerebral cortex and in CA1, CA2, CA3 regions of the hippocampus. A total of 60 Swiss mice were used (30 males and 30 females), separated into three groups: control and two experimental groups, which received the AAS. From each brain, homotypic and semi-serial samples were taken in frontal sections from areas established for the study. The results showed that females treated with testosterone cypionate presented a reduction in all regions tested and the ones treated with Stanozolol showed a decrease in some hippocampal areas. Regarding male animals, stanozolol led to a decrease in neuron number in one hippocampal region. These data allow us to conclude that supra-physiological doses of steroids used in this study, can cause considerable damage to nervous tissue with ultrastructural and consequently behavioral impairment. These changes could interfere with the loss of physical yield and performance of athletes and non-athletes and may cause irreparable damage to individuals making irresponsible use of anabolic steroids.

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.

Molecular mechanisms and cellular events involved in the neuroprotective actions of estradiol. Analysis of sex differences

Estradiol, either from peripheral or central origin, activates multiple molecular neuroprotective and neuroreparative responses that, being mediated by estrogen receptors or by estrogen receptor independent mechanisms, are initiated at the membrane, the cytoplasm or the cell nucleus of neural cells. Estrogen-dependent signaling regulates a variety of cellular events, such as intracellular Ca²⁺ levels, mitochondrial respiratory capacity, ATP production, mitochondrial membrane potential, autophagy and apoptosis. In turn, these molecular and cellular actions of estradiol are integrated by neurons and non-neuronal cells to generate different tissue protective responses, decreasing blood-brain barrier permeability, oxidative stress, neuroinflammation and excitotoxicity and promoting synaptic plasticity, axonal growth, neurogenesis, remyelination and neuroregeneration. Recent findings indicate that the neuroprotective and neuroreparative actions of estradiol are different in males and females and further research is necessary to fully elucidate the causes for this sex difference.

Letrozole induces worse hippocampal synaptic and dendritic changes and spatial memory impairment than ovariectomy in adult female mice

Estrogens (E2) derived from ovaries and/or local de novo synthesis in the hippocampus profoundly regulate hippocampal structure and function, but the importance of local E2 versus ovarian E2 on hippocampal synaptic plasticity and spatial memory has not been well elucidated. The present study used ovariectomy (OVX) and intraperitoneal injection of an E2 synthase inhibitor, letrozole (LET), in adult female mice to investigate changes in hippocampal steroid receptor coactivator-1 (SRC-1), postsynaptic proteins, and actin polymerization dynamics with these treatments. Changes in the CA1 spine density, synapse density and spatial learning and memory after OVX and LET were also investigated. As a result, OVX and LET showed similar regulation of the expression of GluR1, spinophilin and p-Cofilin, but LET tended to induce more significant changes in SRC-1, PSD95, Rictor, Cofilin and actin depolymerization. More significant decreases in F-actin/G-actin, CA1 spine density and synapse density were also observed after LET than after OVX. Notably, LET-treated mice showed worse learning and memory impairment than OVX mice. Taken together, these results demonstrated that circulating E2 played a limited role and that hippocampus-derived E2 played a more important role in the regulation of hippocampal synaptic plasticity and hippocampus-based spatial learning and memory.

Toward the Language Oscillogenome

Language has been argued to arise, both ontogenetically and phylogenetically, from specific patterns of brain wiring. We argue that it can further be shown that core features of language processing emerge from particular phasal and cross-frequency coupling properties of neural oscillations; what has been referred to as the language ‘oscillome.’ It is expected that basic aspects of the language oscillome result from genetic guidance, what we will here call the language ‘oscillogenome,’ for which we will put forward a list of candidate genes. We have considered genes for altered brain rhythmicity in conditions involving language deficits: autism spectrum disorders, schizophrenia, specific language impairment and dyslexia. These selected genes map on to aspects of brain function, particularly on to neurotransmitter function. We stress that caution should be adopted in the construction of any oscillogenome, given the range of potential roles particular localized frequency bands have in cognition. Our aim is to propose a set of genome-to-language linking hypotheses that, given testing, would grant explanatory power to brain rhythms with respect to language processing and evolution.

Estrogen Signaling as a Therapeutic Target in Neurodevelopmental Disorders

Estrogens, the primary female sex hormones, were originally characterized through their important role in sexual maturation and reproduction. However, recent studies have shown that estrogens play critical roles in a number of brain functions, including cognition, learning and memory, neurodevelopment, and adult neuroplasticity. A number of studies from both clinical as well as preclinical research suggest a protective role of estrogen in neurodevelopmental disorders including autism spectrum disorder (ASD) and schizophrenia. Alterations in the levels of estrogen receptors have been found in subjects with ASD or schizophrenia, and adjunctive estrogen therapy has been shown to be effective in enhancing the treatment of schizophrenia. This review summarizes the findings on the role of estrogen in the pathophysiology of neurodevelopmental disorders with a focus on ASD and schizophrenia. We also discuss the potential of estrogen as a therapeutic target in the above disorders.

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.

Nanoparticle-Based and Bioengineered Probes and Sensors to Detect Physiological and Pathological Biomarkers in Neural Cells

Nanotechnology, a rapidly evolving field, provides simple and practical tools to investigate the nervous system in health and disease. Among these tools are nanoparticle-based probes and sensors that detect biochemical and physiological properties of neurons and glia, and generate signals proportionate to physical, chemical, and/or electrical changes in these cells. In this context, quantum dots (QDs), carbon-based structures (C-dots, grapheme, and nanodiamonds) and gold nanoparticles are the most commonly used nanostructures. They can detect and measure enzymatic activities of proteases (metalloproteinases, caspases), ions, metabolites, and other biomolecules under physiological or pathological conditions in neural cells. Here, we provide some examples of nanoparticle-based and genetically engineered probes and sensors that are used to reveal changes in protease activities and calcium ion concentrations. Although significant progress in developing these tools has been made for probing neural cells, several challenges remain. We review many common hurdles in sensor development, while highlighting certain advances. In the end, we propose some future directions and ideas for developing practical tools for neural cell investigations, based on the maxim "Measure what is measurable, and make measurable what is not so" (Galileo Galilei).

Sexual neurosteroids and synaptic plasticity in the hippocampus

Sexual neurosteroids (SN), namely 17β-estradiol (E2) and 5α-dehydrotestosterone (DHT), are synthesized in the hippocampus, where they induce circuit modifications by changing the number of excitatory spine synapses in a paracrine and sex-specific manner. The mechanisms of this sex-specific synapse turnover, which are likely to affect cognitive functions, are poorly understood. We found that hippocampal neurons synthesize estradiol, which maintains LTP and synapses in females but not in males. In females, inhibition of estradiol synthesis results in impairment of LTP and synapse loss. These effects were not seen in males. The essential role of local estrogen on the stability and maintenance of connectivity in the hippocampus is consistent with age-related cognitive decline in women after menopause. In male animals the regulation of synaptic stability and plasticity by locally synthesized sexual steroids remains to be clarified. This article is part of a Special Issue entitled SI: Brain and Memory.

Oestradiol-induced synapse formation in the female hippocampus: roles of oestrogen receptor subtypes

During the oestrus cycle, varying spine synapse density correlates positively with varying local synthesis of oestradiol in the hippocampus. In this context, the roles of the oestrogen receptor (ER) subtypes ERα and β are not fully understood. In the present study, we used neonatal hippocampal slice cultures from female rats because these cultures synthesise oestradiol and express both receptor subtypes, and inhibition of oestradiol synthesis in these cultures results in spine synapse loss. Using electron microscopy, we tested the effects on spine synapse density in response to agonists of both ERα and ERβ. Application of agonists to the cultures had no effect. After inhibition of oestradiol synthesis, however, agonists of ERα induced spine synapse formation, whereas ERβ agonists led to a reduction in spine synapse density in the CA1 region of these cultures. Consistently, up-regulation of ERβ in the hippocampus of adult female aromatase-deficient mice is paralleled by hippocampus-specific spine synapse loss in this mutant. Finally, we found an increase in spine synapses in the adult female ERβ knockout mouse, but no effect in the adult female ERα knockout mouse. Our data suggest antagonistic roles of ERβ and ERα in spine synapse formation in the female hippocampus, which may contribute to oestrus cyclicity of spine synapse density in the hippocampus.

Dendritic spines: the locus of structural and functional plasticity

The introduction of high-resolution time lapse imaging and molecular biological tools has changed dramatically the rate of progress towards the understanding of the complex structure-function relations in synapses of central spiny neurons. Standing issues, including the sequence of molecular and structural processes leading to formation, morphological change, and longevity of dendritic spines, as well as the functions of dendritic spines in neurological/psychiatric diseases are being addressed in a growing number of recent studies. There are still unsettled issues with respect to spine formation and plasticity: Are spines formed first, followed by synapse formation, or are synapses formed first, followed by emergence of a spine? What are the immediate and long-lasting changes in spine properties following exposure to plasticity-producing stimulation? Is spine volume/shape indicative of its function? These and other issues are addressed in this review, which highlights the complexity of molecular pathways involved in regulation of spine structure and function, and which contributes to the understanding of central synaptic interactions in health and disease.

Insights into rapid modulation of neuroplasticity by brain estrogens

Converging evidence from cellular, electrophysiological, anatomic, and behavioral studies suggests that the remodeling of synapse structure and function is a critical component of cognition. This modulation of neuroplasticity can be achieved through the actions of numerous extracellular signals. Moreover, it is thought that it is the integration of different extracellular signals regulation of neuroplasticity that greatly influences cognitive function. One group of signals that exerts powerful effects on multiple neurologic processes is estrogens. Classically, estrogens have been described to exert their effects over a period of hours to days. However, there is now increasing evidence that estrogens can rapidly influence multiple behaviors, including those that require forebrain neural circuitry. Moreover, these effects are found in both sexes. Critically, it is now emerging that the modulation of cognition by rapid estrogenic signaling is achieved by activation of specific signaling cascades and regulation of synapse structure and function, cumulating in the rewiring of neural circuits. The importance of understanding the rapid effects of estrogens on forebrain function and circuitry is further emphasized as investigations continue to consider the potential of estrogenic-based therapies for neuropathologies. This review focuses on how estrogens can rapidly influence cognition and the emerging mechanisms that underlie these effects. We discuss the potential sources and the biosynthesis of estrogens within the brain and the consequences of rapid estrogenic-signaling on the remodeling of neural circuits. Furthermore, we argue that estrogens act via distinct signaling pathways to modulate synapse structure and function in a manner that may vary with cell type, developmental stage, and sex. Finally, we present a model in which the coordination of rapid estrogenic-signaling and activity-dependent stimuli can result in long-lasting changes in neural circuits, contributing to cognition, with potential relevance for the development of novel estrogenic-based therapies for neurodevelopmental or neurodegenerative disorders.

Endocrine regulation of estrogen synthesis in the hippocampus?

Estradiol synthesis in the ovaries is regulated via feedback mechanisms mediated by gonadotrophin-releasing hormone (GnRH) and gonadotrophins, secreted by the hypothalamus and the pituitary, respectively. Estradiol synthesis also takes place in the hippocampus. In hippocampal slice cultures of female animals, GnRH regulates estradiol synthesis dose-dependently. Hence, both hippocampal and ovarian estradiol synthesis are synchronized by GnRH. Hippocampus-derived estradiol is essential to synapse stability and maintenance because it stabilizes the spine cytoskeleton of hippocampal neurons. Inhibition of hippocampal estradiol synthesis in mice, however, results in loss of spines and spine synapses in females, but not in males. Stereotaxic application of GnRH to the hippocampus of female rats confirms the regulatory role of GnRH on estradiol synthesis and synapse density in the female hippocampus in vivo. This regulatory role of GnRH necessarily results in estrus cyclicity of spine density in the hippocampus of females.

Flutamide Enhances Neuroprotective Effects of Testosterone during Experimental Cerebral Ischemia in Male Rats

Testosterone has been shown to worsen histological and neurological impairment during cerebral ischemia in animal models. Cell culture studies revealed that testosterone is implicated in protecting neural and glial cells against insults, and they started to elucidate testosterone pathways that underlie these protective effects. These studies support the hypothesis that testosterone can be neuroprotective throughout an episode of cerebral ischemia. Therefore, we evaluated the mechanisms underlying the shift between testosterone protective and deleterious effects via block testosterone aromatization and androgen receptors in rats subjected to 60-minute middle cerebral artery occlusion. Fifty rats were divided into five equal groups: gonadally intact male; castrated male; intact male + flutamide; intact male + letrozole; intact male + combination flutamide and letrozole. Our results indicated that castration has the ability to reduce histological damage and to improve neurological score 24 hours after middle cerebral artery occlusion. Moreover, flutamide improved histologic and neurological impairment better than castration. Letrozole induced increases in striatal infarct volume and seizures in gonadally intact rats. Combination of flutamide and letrozole showed that letrozole can reverse beneficial effects of flutamide. In conclusion, it seems that the beneficial effects of flutamide are the prevention of the deleterious effects and enhancement of neuroprotective effects of testosterone during cerebral ischemia.

Increasing hippocampal estrogen receptor alpha levels via viral vectors increases MAP kinase activation and enhances memory in aging rats in the absence of ovarian estrogens

We previously demonstrated that aged ovariectomized rats that had received prior estradiol treatment in middle-age exhibited increased levels of estrogen receptor alpha (ERα) in the hippocampus as well as enhanced hippocampal dependent memory as compared to aged rats that had not received mid-life estradiol treatment. These effects persisted long after the estradiol treatment had been terminated. The goal of the current experiment was to determine if increased expression of ERα in the hippocampus, in the absence of exogenously administered estrogens, can impact the hippocampus and cognitive function in aging ovariectomized rats. Middle-aged rats were trained for 24 days on an eight-arm radial maze spatial memory task. All rats were then ovariectomized. Forty days later, rats received either lentiviral delivery to the hippocampus of the gene encoding ERα (lenti-ERα) or a control virus. Rats were tested on delay trials in the radial-maze in which delays of varying lengths were imposed between the fourth and fifth arm choices. Following behavior testing, hippocampi were immunostained using western blotting for ERα, the ERα-regulated protein choline acetyltransferase, and phosphorylation of the ERα-regulated kinases, ERK/MAPK and Akt. Results revealed that aging ovariectomized rats that received delivery of lenti-ERα to the hippocampus exhibited enhanced spatial memory as indicated by increased arm-choice accuracy across delays as compared to ovariectomized rats that received control virus. Western blot data revealed that lenti-ERα delivery significantly increased levels of ERα and phosphorylated ERK/MAPK and had no impact on levels of ChAT or phosphorylation of Akt. Results indicate that increasing hippocampal levels of ERα in aging females in the absence of ovarian or exogenously administered estrogens leads to increases in phosphorylation of ERK/MAPK as well as in enhanced memory.

Sexually dimorphic transcriptomic responses in the teleostean hypothalamus: a case study with the organochlorine pesticide dieldrin

Organochlorine pesticides (OCPs) such as dieldrin are a persistent class of aquatic pollutants that cause adverse neurological and reproductive effects in vertebrates. In this study, female and male largemouth bass (Micropterus salmoides) (LMB) were exposed to 3mg dieldrin/kg feed in a 2 month feeding exposure (August-October) to (1) determine if the hypothalamic transcript responses to dieldrin were conserved between the sexes; (2) characterize cell signaling cascades underlying dieldrin neurotoxicity; and (3) determine whether or not co-feeding with 17β-estradiol (E(2)), a hormone with neuroprotective roles, mitigates responses in males to dieldrin. Despite also being a weak estrogen, dieldrin treatments did not elicit changes in reproductive endpoints (e.g. gonadosomatic index, vitellogenin, or plasma E(2)). Sub-network (SNEA) and gene set enrichment analysis (GSEA) revealed that neuro-hormone networks, neurotransmitter and nuclear receptor signaling, and the activin signaling network were altered by dieldrin exposure. Most striking was that the majority of cell pathways identified by the gene set enrichment were significantly increased in females while the majority of cell pathways were significantly decreased in males fed dieldrin. These data suggest that (1) there are sexually dimorphic responses in the teleost hypothalamus; (2) neurotransmitter systems are a target of dieldrin at the transcriptomics level; and (3) males co-fed dieldrin and E(2) had the fewest numbers of genes and cell pathways altered in the hypothalamus, suggesting that E(2) may mitigate the effects of dieldrin in the central nervous system.

Aromatase inhibition abolishes LTP generation in female but not in male mice

Inhibitors of aromatase, the final enzyme of estradiol synthesis, are suspected of inducing memory deficits in women. In previous experiments, we found hippocampal spine synapse loss in female mice that had been treated with letrozole, a potent aromatase inhibitor. In this study, we therefore focused on the effects of letrozole on long-term potentiation (LTP), which is an electrophysiological parameter of memory and is known to induce spines, and on phosphorylation of cofilin, which stabilizes the spine cytoskeleton and is required for LTP in mice. In acute slices of letrozole-treated female mice with reduced estradiol serum concentrations, impairment of LTP started as early as after 6 h of treatment and progressed further, together with dephosphorylation of cofilin in the same slices. Theta-burst stimulation failed to induce LTP after 1 week of treatment. Impairment of LTP was followed by spine and spine synapse loss. The effects were confirmed in vitro by using hippocampal slice cultures of female mice. The sequence of effects in response to letrozole were similar in ovariectomized female and male mice, with, however, differences as to the degree of downregulation. Our data strongly suggest that impairment of LTP, followed by loss of mushroom spines and spine synapses in females, may have implications for memory deficits in women treated with letrozole.

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'.

Membrane estrogen receptors activate the metabotropic glutamate receptors mGluR5 and mGluR3 to bidirectionally regulate CREB phosphorylation in female rat striatal neurons

Along with its ability to directly regulate gene expression, estradiol influences cell signaling and brain functions via rapid, membrane-initiated events. In the female rat striatum, estradiol activates membrane-localized estrogen receptors to influence synaptic neurotransmission, calcium channel activity, and behaviors related to motor control. Yet, the mechanism by which estradiol acts to rapidly affect striatal physiology has remained elusive. Here we find that membrane estrogen receptors (ERs) couple to the metabotropic glutamate receptors mGluR5 and mGluR3, providing the framework to understand how membrane estrogen receptors affect striatal function. Using CREB phosphorylation as a downstream measure of ER/mGluR activation, membrane-localized estrogen receptor α (ERα) activates mGluR5 signaling to mediate mitogen-activated protein kinase (MAPK)-dependent CREB phosphorylation. Further, ERα and estrogen receptor β (ERβ) activate mGluR3 to attenuate L-type calcium channel-dependent CREB signaling. Interestingly, while this fundamental mechanism of ER/mGluR signaling was initially characterized in hippocampal neurons, estrogen receptors in striatal neurons are paired with a different set of mGluRs, resulting in the potential to functionally isolate membrane-initiated estrogen signaling across brain regions via use of specific mGluR modulators. These results provide both a mechanism for the rapid actions of estrogens within the female striatum, as well as demonstrate that estrogen receptors can interact with a more diverse set of surface membrane receptors than previously recognized.

Estrogen regulates cytoskeletal flexibility, cellular metabolism and synaptic proteins: A proteomic study

Estrogen (E2) influences brain function to induce gender differences in neuronal processes. In contrast to its well-described effects on signaling systems and gene transcription factors, our knowledge of E2-regulated protein networks is rather limited. Thus, we examined changes in protein expression patterns in the whole brains of ovariectomized mice after 24h estrogen exposure using two-dimensional differential gel electrophoresis. Interpretation of our network-based hypothesis suggested that E2 regulates synaptic proteins and processes, increases cytoskeletal flexibility and alters glucose consumption in the brain. We verified the predicted reduced basal synaptic activity using in vivo microdialysis in conscious mice, showing that E2 decreases the extracellular concentrations of certain amino acids in two different brain areas (in the striatum and in the hypothalamus) and that this is independent from the E2 receptor densities. Our data reveal that E2 induces minor, but substantial changes to functionally different protein networks at the whole brain level, and as a cumulative effect, it adjusts the brain steady-state condition to a more flexible state.

Aromatase in the brain of teleost fish: expression, regulation and putative functions

Unlike that of mammals, the brain of teleost fish exhibits an intense aromatase activity due to the strong expression of one of two aromatase genes (aromatase A or cyp19a1a and aromatase B or cyp19a1b) that arose from a gene duplication event. In situ hybridization, immunohistochemistry and expression of GFP (green fluorescent protein) in transgenic tg(cyp19a1b-GFP) fish demonstrate that aromatase B is only expressed in radial glial cells (RGC) of adult fish. These cells persist throughout life and act as progenitors in the brain of both developing and adult fish. Although aromatase B-positive radial glial cells are most abundant in the preoptic area and the hypothalamus, they are observed throughout the entire central nervous system and spinal cord. In agreement with the fact that brain aromatase activity is correlated to sex steroid levels, the high expression of cyp19a1b is due to an auto-regulatory loop through which estrogens and aromatizable androgens up-regulate aromatase expression. This mechanism involves estrogen receptor binding on an estrogen response element located on the cyp19a1b promoter. Cell specificity is achieved by a mandatory cooperation between estrogen receptors and unidentified glial factors. Given the emerging roles of estrogens in neurogenesis, the unique feature of the adult fish brain suggests that, in addition to classical functions on brain sexual differentiation and sexual behaviour, aromatase expression in radial glial cells could be part of the mechanisms authorizing the maintenance of a high proliferative activity in the brain of fish.

Aromatase inhibitors induce spine synapse loss in the hippocampus of ovariectomized mice

Recently, inhibition of estrogen synthesis by aromatase inhibitors has become a favored therapy for breast cancer in postmenopausal women. Estrogen is, however, important for synapse formation in the hippocampus. Inhibition of aromatase induces spine synapse loss in organotypic hippocampal slice cultures. We therefore studied the effect of systemic treatment with the potent aromatase inhibitor letrozole on spine synapse formation and synaptic proteins in the hippocampi of female mice for periods of 7 d and 4 wk. In cyclic, letrozole-treated females and in ovariectomized, letrozole-treated females, the number of spine synapses was significantly reduced in the hippocampus but not in the prefrontal or cerebellar cortex. Consequently, the expression of the N-methyl-D-aspartate receptor NR1 was significantly down-regulated after treatment with letrozole. In cyclic animals the expression of the synaptic proteins synaptophysin and spinophilin was down-regulated in response to letrozole. In ovariectomized animals, however, protein expression was down-regulated after 7 d of treatment, whereas the expression was up-regulated after 4 wk of treatment. Our results indicate that systemic inhibition of aromatase in mice affects structural synaptic plasticity in the hippocampus. This may contribute to cognitive deficits in postmenopausal women treated with aromatase inhibitors.

Estrus cyclicity of spinogenesis: underlying mechanisms

Hippocampal spine density varies with the estrus cycle. The cyclic change in estradiol levels in serum was hypothesized to underlie this phenomenon, since treatment of ovariectomized animals with estradiol induced an increase in spine density in hippocampal dendrites of rats, as compared to ovariectomized controls. In contrast, application of estradiol to hippocampal slice cultures did not promote spinogenesis. In addressing this discrepancy, we found that hippocampal neurons themselves 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) and enhanced by substrates of steroidogenesis. Expression of estrogen receptors (ERs) and synaptic proteins, synaptogenesis, and long-term potentiation (LTP) correlated positively with aromatase activity in hippocampal cultures without any difference between genders. All effects due to inhibition of aromatase activity were rescued by application of estradiol to the cultures. Most importantly, gonadotropin-releasing hormone (GnRH) increased estradiol synthesis dose-dependently via an aromatase-mediated mechanism and consistently increased spine synapse density and spinophilin expression. As a consequence, our data suggest that cyclic fluctuations in spine synapse density result from pulsative release of GnRH from the hypothalamus and its effect on hippocampal estradiol synthesis, rather than from varying levels of serum estradiol. This hypothesis is further supported by higher GnRH receptor (GnRH-R) density in the hippocampus than in the cortex and hypothalamus and the specificity of estrus cyclicity of spinogenesis in the hippocampus, as compared to the cortex.

The opposing roles of estradiol on synaptic protein expression in hippocampal cultures

Estrogen-induced synaptic plasticity was frequently shown by an increase of spines at apical dendrites of CA1 pyramidal neurons after systemic application of estradiol to ovariectomized rats. Surprisingly, exogenous application of estradiol to hippocampal cultures had no effect on spines and on spine synapses, although quantitative immunohistochemistry revealed an upregulation of spinophilin and of synaptophysin, in these cultures. The role of synaptophysin as a presynaptic marker and of spinophilin as a postsynaptic marker, appears questionable from these discrepancies. In contrast, synaptopodin, a marker protein of "mature" mushroom-shaped spines, was downregulated after treatment of hippocampal cultures with estradiol. Synaptopodin is strongly associated to the spine apparatus, a spine-specific cell organelle, which is present in 80% of all mushroom-shaped spines. Consistently, we found a reduction in the number of spines, containing a spine apparatus in response to estradiol, suggesting that the presence of a spine apparatus in many but not all spines is very likely a result of their dynamic character. In summary, synaptic proteins appear to be regulated by estradiol, independent of its function on spine and spine synapse formation.

Cholesterol-promoted synaptogenesis requires the conversion of cholesterol to estradiol in the hippocampus

Cholesterol of glial origin promotes synaptogenesis (Mauch et al., (2001) Science 294:1354-1357). Because in the hippocampus local estradiol synthesis is essential for synaptogenesis, we addressed the question of whether cholesterol-promoted synapse formation results from the function of cholesterol as a precursor of estradiol synthesis in this brain area. To this end, we treated hippocampal cultures with cholesterol, estradiol, or with letrozole, a potent aromatase inhibitor. Cholesterol increased neuronal estradiol release into the medium, the number of spine synapses in hippocampal slice cultures, and immunoreactivity of synaptic proteins in dispersed cultures. Simultaneous application of cholesterol and letrozole or blockade of estrogen receptors by ICI 182 780 abolished cholesterol-induced synapse formation. As a further approach, we inhibited the access of cholesterol to the first enzyme of steroidogenesis by knock-down of steroidogenic acute regulatory protein, the rate-limiting step in steroidogenesis. A rescue of reduced synaptic protein expression in transfected cells was achieved by estradiol but not by cholesterol. Our data indicate that in the hippocampus cholesterol-promoted synapse formation requires the conversion of cholesterol to estradiol.

Aromatase expression in the normal and epileptic human hippocampus

Aromatase is a key enzyme in estrogen biosynthesis that is involved in neuronal plasticity in the rodent hippocampus. Although aromatase mRNA expression has been detected in the human hippocampus, its cellular distribution has yet to be determined. Here, we have examined the immunohistochemical distribution of aromatase in the normal and the epileptic and sclerotic human hippocampus. In both the normal and epileptic hippocampus, aromatase was detected in numerous CA1-CA3 pyramidal neurons, in granule cells of the dentate gyrus and in interneurons that co-expressed the calcium-binding proteins calbindin, calretinin or parvalbumin. However, only a small subpopulation of astrocytes was immunoreactive for aromatase in either the normal and epileptic hippocampus. The widespread expression of aromatase in a large population of neurons in the normal and damaged hippocampus suggests that local estrogen formation may play an important role in human hippocampal function.

The role of neurosteroids in the pathophysiology and treatment of catamenial epilepsy

Catamenial epilepsy is a multifaceted neuroendocrine condition in which seizures are clustered around specific points in the menstrual cycle, most often around perimenstrual or periovulatory period. Generally, a twofold or greater increase in seizure frequency during a particular phase of the menstrual cycle could be considered as catamenial epilepsy. Based on this criteria, recent clinical studies indicate that catamenial epilepsy affects 31-60% of the women with epilepsy. Three types of catamenial seizures (perimenstrual, periovulatory and inadequate luteal) have been identified. However, there is no specific drug available today for catamenial epilepsy, which has not been successfully treated with conventional antiepileptic drugs. Elucidation of the pathophysiology of catamenial epilepsy is a prerequisite to develop specific targeted approaches for treatment or prevention of the disorder. Cyclical changes in the circulating levels of estrogens and progesterone play a central role in the development of catamenial epilepsy. There is emerging evidence that endogenous neurosteroids with anticonvulsant or proconvulsant effects could play a critical role in catamenial epilepsy. It is thought that perimenstrual catamenial epilepsy is associated with the withdrawal of anticonvulsant neurosteroids. Progesterone and other hormonal agents have been shown in limited trials to be moderately effective in catamenial epilepsy, but may cause endocrine side effects. Synthetic neurosteroids, which enhance the tonic GABA-A receptor function, might provide an effective approach for the catamenial epilepsy therapy without producing hormonal side effects.

Selective transcriptional regulation of aromatase gene by vitamin D, dexamethasone, and mifepristone in human glioma cells

The human aromatase gene (CYP19A1) is controlled by multiple promoters that give rise to different aromatase transcripts. Its regulation has been studied in cells from multiple origins, including placenta, bone, adipose tissue, and breast cancer. However, little is known about its regulation in cells from neural origin. We assessed whether vitamin D, dexamethasone, and the glucocorticoid receptor antagonist mifepristone regulate the aromatase gene in human glioma, neuroblastoma, and breast cancer cells. The results show that these compounds enhance the activity of different aromatase promoters in glioma cells, but not in neuroblastoma and breast cancer cells. Vitamin D increased the expression of I.3, I.7, and I.4 aromatase transcripts and induced de novo expression of the I.6 transcript; dexamethasone increased the expression of I.4, PII, and I.3 transcripts and mifepristone increased the expression of PII and I.3 aromatase transcripts. The cell specific regulation of CYP19A1 by vitamin D, dexamethasone, and mifepristone opens the possibility for cellular selective modulation of estrogen biosynthesis within the brain.

Caveolin proteins and estrogen signaling in the brain

Best described outside the nervous system, caveolins are structural proteins that form caveolae, functional microdomains at the plasma membrane that cluster related signaling molecules. Caveolin-associated proteins include G protein-coupled receptors and G proteins, receptor tyrosine kinases, as well as protein kinases, ion channels and various other signaling enzymes. Not surprisingly, a wide array of biological disorders are thought to be rooted in caveolin dysfunction. In addition, caveolins traffic and cluster estrogen receptors to caveolae. Interactions between the estrogen receptors ERalpha and ERbeta with caveolins appear critical in many non-neuronal cell types, e.g., disruption of normal function may underlie many forms of breast cancer. Recent findings suggest caveolins may also play an essential role in membrane estrogen receptor function in the nervous system. Not only are they expressed in neurons and glia, but different caveolin isoforms also appear necessary to generate distinct functional signaling complexes. With membrane estrogen receptors responsible for the efficient activation of a multitude of intracellular signaling pathways, which in turn influence a wide variety of nervous system functions, caveolin proteins are poised to act as the central coordinators of these processes.

Aromatase in the brain: not just for reproduction anymore

Aromatase, the enzyme that synthesises oestrogens from androgen precursors, is expressed in the brain, where it has been classically associated with the regulation of neuroendocrine events and behaviours linked with reproduction. Recent findings, however, have revealed new unexpected roles for brain aromatase, indicating that the enzyme regulates synaptic activity, synaptic plasticity, neurogenesis and the response of neural tissue to injury, and may contribute to control nonreproductive behaviours, mood and cognition. Therefore, the function of brain aromatase is not restricted to the regulation of reproduction as previously thought.

Aromatase distribution in the monkey temporal neocortex and hippocampus

Numerous studies have shown that neuronal plasticity in the hippocampus and neocortex is regulated by estrogen and that aromatase, the key enzyme for estrogen biosynthesis, is present in cerebral cortex. Although the expression pattern of aromatase mRNA has been described in the monkey brain, its precise cellular distribution has not been determined. In addition, the degree to which neuronal aromatase is affected by gonadal estrogen has not been investigated. In this study, we examined the immunohistochemical distribution of aromatase in young ovariectomized female rhesus monkeys with or without long-term cyclic estradiol treatment. Both experimental groups showed that aromatase is localized in a large population of CA1-3 pyramidal cells, in granule cells of the dentate gyrus and in some interneurons in which it was co-expressed with the calcium-binding proteins calbindin, calretinin, and parvalbumin. Moreover, numerous pyramidal cells were immunoreactive for aromatase in the neocortex, whereas only small subpopulations of neocortical interneurons were immunoreactive for aromatase. The widespread expression of the protein in a large neuronal population suggests that local intraneuroral estrogen synthesis may contribute to estrogen-induced synaptic plasticity in monkey hippocampus and neocortex of female rhesus monkeys. In addition, the apparent absence of obvious differences in aromatase distribution between the two experimental groups suggests that these localization patterns are not dependent on plasma estradiol levels.

Gonadotropin-releasing hormone regulates spine density via its regulatory role in hippocampal estrogen synthesis

Spine density in the hippocampus changes during the estrus cycle and is dependent on the activity of local aromatase, the final enzyme in estrogen synthesis. In view of the abundant gonadotropin-releasing hormone receptor (GnRH-R) messenger RNA expression in the hippocampus and the direct effect of GnRH on estradiol (E2) synthesis in gonadal cells, we asked whether GnRH serves as a regulator of hippocampal E2 synthesis. In hippocampal cultures, E2 synthesis, spine synapse density, and immunoreactivity of spinophilin, a reliable spine marker, are consistently up-regulated in a dose-dependent manner at low doses of GnRH but decrease at higher doses. GnRH is ineffective in the presence of GnRH antagonists or aromatase inhibitors. Conversely, GnRH-R expression increases after inhibition of hippocampal aromatase. As we found estrus cyclicity of spine density in the hippocampus but not in the neocortex and GnRH-R expression to be fivefold higher in the hippocampus compared with the neocortex, our data strongly suggest that estrus cycle–dependent synaptogenesis in the female hippocampus results from cyclic release of GnRH.

Nanomolar concentrations of anabolic-androgenic steroids amplify excitotoxic neuronal death in mixed mouse cortical cultures

The use of anabolic-androgenic steroids (AASs) in the world of sport has raised a major concern for the serious, sometimes life-threatening, side effects associated with these drugs. Most of the CNS effects are of psychiatric origin, and whether or not AASs are toxic to neurons is yet unknown. We compared the effect of testosterone with that of the AASs, 19-nortestosterone (nandrolone), stanozolol, and gestrinone, on excitotoxic neuronal death induced by N-methyl-d-aspartate (NMDA) in primary cultures of mouse cortical cells. In the most relevant experiments, steroids were applied to the cultures once daily during the 4 days preceding the NMDA pulse. Under these conditions, testosterone amplified excitotoxic neuronal death only at very high concentrations (10 muM), whereas it was protective at concentrations of 10 nM and inactive at intermediate concentrations. Low concentrations of testosterone became neurotoxic in the presence of the aromatase inhibitors, i.e. anastrozole and aminoglutethimide, suggesting that the intrinsic toxicity of testosterone was counterbalanced by its aromatization into 17beta-estradiol. As opposed to testosterone, nortestosterone, stanozolol and gestrinone amplified NMDA toxicity at nanomolar concentrations; their action was insensitive to aromatase inhibitors, but was abrogated by the androgen receptor antagonist, flutamide. None of the AASs were toxic in the absence of NMDA. These data suggest that AASs increase neuronal vulnerability to an excitotoxic insult and may therefore facilitate neuronal death associated with acute or chronic CNS disorders.

Mass spectrometric assay and physiological-pharmacological activity of androgenic neurosteroids

Steroid hormones play a key role in the pathophysiology of several brain disorders. Testosterone modulates neuronal excitability, but the underlying mechanisms are obscure. There is emerging evidence that testosterone-derived "androgenic neurosteroids", 3alpha-androstanediol and 17beta-estradiol, mediate the testosterone effects on neural excitability and seizure susceptibility. Testosterone undergoes metabolism to neurosteroids via two distinct pathways. Aromatization of the A-ring converts testosterone into 17beta-estradiol. Reduction of testosterone by 5alpha-reductase generates 5alpha-dihydrotestosterone, which is then converted to 3alpha-androstanediol, a powerful GABA(A) receptor-modulating neurosteroid with anticonvulsant properties. Although the 3alpha-androstanediol is an emerging neurosteroid in the brain, there is no specific and sensitive assay for determination of 3alpha-androstanediol in biological samples. This article describes the development and validation of mass spectrometric assay of 3alpha-androstanediol, and the molecular mechanisms underlying the testosterone modulation of seizure susceptibility. A liquid chromatography-tandem mass spectrometry assay to measure 3alpha-androstanediol is validated with excellent linearity, specificity, sensitivity, and reproducibility. Testosterone modulation of seizure susceptibility is demonstrated to occur through its conversion to neurosteroids with "anticonvulsant" and "proconvulsant" actions and hence the net effect of testosterone on neural excitability and seizure activity depends on the levels of distinct testosterone metabolites. The proconvulsant effect of testosterone is associated with increases in plasma 17beta-estradiol concentrations. The 5alpha-reduced metabolites of testosterone, 5alpha-dihydrotestosterone and 3alpha-androstanediol, had powerful anticonvulsant activity. Overall, the testosterone-derived neurosteroids 3alpha-androstanediol and 17beta-estradiol could contribute to the net cellular actions of testosterone in the brain. Because 3alpha-androstanediol is a potent positive allosteric modulator of GABA(A) receptors, it could serve as an endogenous neuromodulator of neuronal excitability in men. The 3alpha-androstanediol assay is an important tool in this area because of the growing interest in the potential to use adjuvant aromatase inhibitor therapy to improve treatment of epilepsy.

Isolation and culture of adult neurons and neurospheres

Here we present a protocol for extraction and culture of neurons from adult rat or mouse CNS. The method proscribes an optimized protease digestion of slices, control of osmolarity and pH outside the incubator with Hibernate and density gradient separation of neurons from debris. This protocol produces yields of millions of cortical, hippocampal neurons or neurosphere progenitors from each brain. The entire process of neuron isolation and culture takes less than 4 h. With suitable growth factors, adult neuron regeneration of axons and dendrites in culture proceeds over 1-3 weeks to allow controlled studies in pharmacology, electrophysiology, development, regeneration and neurotoxicology. Adult neurospheres can be collected in 1 week as a source of neuroprogenitors ethically preferred over embryonic or fetal sources. This protocol emphasizes two differences between neuron differentiation and neurosphere proliferation: adhesion dependence and the differentiating power of retinyl acetate.

In search of neuroprotective therapies based on the mechanisms of estrogens

Although estradiol is a neuroprotective factor, estrogen therapy in older women increases the risk of adverse cognitive outcomes and poses additional peripheral risks, requiring careful use of estrogenic compounds as treatments for neurodegenerative conditions or neural injury. Potential alternatives to estrogen therapy to promote neuroprotection might include treatment with molecules that are able to interact with estrogen receptors, with alternative mechanisms of action, or with molecules that induce local estradiol synthesis in the brain, or a combination of all. However, before considering the broad clinical applications, more basic research is required to clarify the mechanisms of action and potential risks of some of these estrogen-based treatments.