Increased estrogen receptor beta expression correlates with decreased spine formation in the rat hippocampus

Exogenous estradiol and oxytocin modulate sex differences in hippocampal reactivity during the encoding of episodic memories

Considerable evidence supports sex differences in episodic memory. The hormones estradiol and oxytocin both affect episodic memory and may contribute to these sex differences, but possible underlying hormonal interactions have not been tested in a sample involving both sexes. To this end, we conducted a randomized, placebo-controlled, parallel-group functional magnetic resonance imaging (fMRI) study including healthy free-cycling women (n = 111) and men (n = 115). The fMRI session was conducted under four experimental conditions: 1. transdermal estradiol (2 mg) and intranasal oxytocin (24 IU), 2. transdermal placebo and intranasal oxytocin, 3. transdermal estradiol and intranasal placebo, 4. transdermal placebo and intranasal placebo. Participants were scanned during the encoding of positive, neutral, and negative scenes. Recognition memory was tested three days following the scanning sessions without additional treatments. Under placebo, women showed a significantly better recognition memory and increased hippocampal responses to subsequently remembered items independent of the emotional valence compared to men. The separate treatments with either hormone significantly diminished this mnemonic sex difference and reversed the hippocampal activation pattern. However, the combined treatments produced no significant effect. Collectively, the results suggest that both hormones play a crucial role in modulating sex differences in episodic memory. Furthermore, possible antagonistic interactions between estradiol and oxytocin could explain previously observed opposing hormonal effects in women and men.

Restoring Age-Related Cognitive Decline through Environmental Enrichment: A Transcriptomic Approach

Cognitive decline is one of the greatest health threats of old age and the maintenance of optimal brain function across a lifespan remains a big challenge. The hippocampus is considered particularly vulnerable but there is cross-species consensus that its functional integrity benefits from the early and continuous exercise of demanding physical, social and mental activities, also referred to as environmental enrichment (EE). Here, we investigated the extent to which late-onset EE can improve the already-impaired cognitive abilities of lifelong deprived C57BL/6 mice and how it affects gene expression in the hippocampus. To this end, 5- and 24-month-old mice housed in standard cages (5mSC and 24mSC) and 24-month-old mice exposed to EE in the last 2 months of their life (24mEE) were subjected to a Barnes maze task followed by next-generation RNA sequencing of the hippocampal tissue. Our analyses showed that late-onset EE was able to restore deficits in spatial learning and short-term memory in 24-month-old mice. These positive cognitive effects were reflected by specific changes in the hippocampal transcriptome, where late-onset EE affected transcription much more than age (24mSC vs. 24mEE: 1311 DEGs, 24mSC vs. 5mSC: 860 DEGs). Remarkably, a small intersection of 72 age-related DEGs was counter-regulated by late-onset EE. Of these, Bcl3, Cttnbp2, Diexf, Esr2, Grb10, Il4ra, Inhba, Rras2, Rps6ka1 and Socs3 appear to be particularly relevant as key regulators involved in dendritic spine plasticity and in age-relevant molecular signaling cascades mediating senescence, insulin resistance, apoptosis and tissue regeneration. In summary, our observations suggest that the brains of aged mice in standard cage housing preserve a considerable degree of plasticity. Switching them to EE proved to be a promising and non-pharmacological intervention against cognitive decline.

Estradiol administration modulates neural emotion regulation

Variations of sex hormones during the menstrual cycle can lead to changes in emotion processing. The ability to successfully regulate one's emotions is associated with better social abilities and mental health. While women show better performance in fear extinction learning under high estradiol (E2) compared to women under low E2 levels, little is known about the effect of E2 on emotion regulation. We explored whether E2 modulates emotion regulation in a functional magnetic resonance imaging paradigm and administered E2 valerate to 32 young naturally cycling women during their early follicular phase in a double-blind, placebo-controlled within-subject design. This standardized experimental control allowed us to explore the specific effect of E2 on emotion regulation while controlling for other hormones varying throughout the menstrual cycle. Behaviorally, no difference between conditions appeared. However, on the neural level, E2 administration was associated with lower activation in the right lingual- and left calcarine gyrus, right orbitofrontal cortex and left hippocampus relative to placebo. With respect to the main effect of down-regulation higher activation of the right superior frontal gyrus and left dorsomedial prefrontal cortex was seen; which is in accordance to previous literature. An interaction between drug condition and emotion regulation appeared for the left inferior frontal gyrus extending into the middle frontal gyrus indicating lower activation during down-regulation in the E2 condition than the placebo condition. On the behavioral level, women reported less negative affect in the E2 condition. The results fit well to a previously described psychoneuroendocrinological model in which E2 plays an important modulatory role on emotional processes and risk factors of mental health in women.

Linear and inverted U-shaped dose-response functions describe estrogen effects on hippocampal activity in young women

In animals, 17-beta-estradiol (E2) enhances hippocampal plasticity in a dose-dependent, monotonically increasing manner, but this relationship can also exhibit an inverted U-shaped function. To investigate E2’s dose-response function in the human hippocampus, we pharmacologically increased E2 levels in 125 naturally cycling women (who were in their low-hormone menstruation phase) to physiological (equivalent to menstrual cycle peak) and supraphysiological (equivalent to levels during early pregnancy) concentrations in a placebo-controlled design. Twenty-four hours after first E2 intake, we measured brain activity during encoding of neutral and negative pictures and then tested recognition memory 24 h after encoding. Here we report that E2 exhibits both a monotonically increasing relationship with hippocampal activity as well as an inverted U-shaped relationship, depending on the hippocampal region. Hippocampal activity exhibiting a U-shaped relationship inflects at supraphysiological E2 levels, suggesting that while E2 within physiological ranges stimulates hippocampal activity, supraphysiological ranges show opposite effects.

While estrogen is known to change hippocampal activity in animals, it is not known if this effect extends to humans. Here, authors vary the doses of estrogen in young women and show that the effects on hippocampal activity can be described by linear and inverted-U shaped dose-response functions.

A High-Cholesterol Diet Increases 27-Hydroxycholesterol and Modifies Estrogen Receptor Expression and Neurodegeneration in Rabbit Hippocampus

Hypercholesterolemia has been implicated in numerous health problems from cardiovascular disease to neurodegeneration. High serum cholesterol levels in midlife have been associated with an increased risk of developing Alzheimer's disease (AD) later in life which suggests that the pathways leading to AD pathology might be activated decades before the symptoms of the disease are detected. Cholesterol-fed animals, particularly cholesterol-fed rabbits, exhibit brain pathology similar to the changes found in brains of AD patients. Dietary cholesterol, which cannot pass the blood-brain barrier, is thought to influence central nervous system homeostasis by increased transport of its circulatory breakdown product, 27-hydroxycholesterol (27-OHC), into the brain. 27-OHC is an endogenous selective estrogen receptor modulator. Estrogen-mediated non-reproductive functions require estrogen receptors (ERs) and include modulation of mitochondrial function and structure, as well as regulation of synaptogenesis in the brain. ERs are located in brain areas affected early in AD pathogenesis, including the hippocampus. Here we report that increase in serum cholesterol, induced by feeding rabbits a high-cholesterol diet, is associated with higher levels of 27-OHC in the brain as well as increased levels of neurodegeneration in the hippocampus. Furthermore, these results are accompanied by changes in expression of ERs in the hippocampus as well as a decrease in hippocampal mitochondria. These findings provide an important insight into one of the possible mechanisms involved in the development of AD, and shed light on the processes that may antedate amyloid-β and tau phosphorylation changes currently hypothesized to cause AD symptomology and pathology.

Role of Estrogen and Other Sex Hormones in Brain Aging. Neuroprotection and DNA Repair

Aging is an inevitable biological process characterized by a progressive decline in physiological function and increased susceptibility to disease. The detrimental effects of aging are observed in all tissues, the brain being the most important one due to its main role in the homeostasis of the organism. As our knowledge about the underlying mechanisms of brain aging increases, potential approaches to preserve brain function rise significantly. Accumulating evidence suggests that loss of genomic maintenance may contribute to aging, especially in the central nervous system (CNS) owing to its low DNA repair capacity. Sex hormones, particularly estrogens, possess potent antioxidant properties and play important roles in maintaining normal reproductive and non-reproductive functions. They exert neuroprotective actions and their loss during aging and natural or surgical menopause is associated with mitochondrial dysfunction, neuroinflammation, synaptic decline, cognitive impairment and increased risk of age-related disorders. Moreover, loss of sex hormones has been suggested to promote an accelerated aging phenotype eventually leading to the development of brain hypometabolism, a feature often observed in menopausal women and prodromal Alzheimer's disease (AD). Although data on the relation between sex hormones and DNA repair mechanisms in the brain is still limited, various investigations have linked sex hormone levels with different DNA repair enzymes. Here, we review estrogen anti-aging and neuroprotective mechanisms, which are currently an area of intense study, together with the effect they may have on the DNA repair capacity in the brain.

Role of Estrogen and Other Sex Hormones in Brain Aging. Neuroprotection and DNA Repair

Aging is an inevitable biological process characterized by a progressive decline in physiological function and increased susceptibility to disease. The detrimental effects of aging are observed in all tissues, the brain being the most important one due to its main role in the homeostasis of the organism. As our knowledge about the underlying mechanisms of brain aging increases, potential approaches to preserve brain function rise significantly. Accumulating evidence suggests that loss of genomic maintenance may contribute to aging, especially in the central nervous system (CNS) owing to its low DNA repair capacity. Sex hormones, particularly estrogens, possess potent antioxidant properties and play important roles in maintaining normal reproductive and non-reproductive functions. They exert neuroprotective actions and their loss during aging and natural or surgical menopause is associated with mitochondrial dysfunction, neuroinflammation, synaptic decline, cognitive impairment and increased risk of age-related disorders. Moreover, loss of sex hormones has been suggested to promote an accelerated aging phenotype eventually leading to the development of brain hypometabolism, a feature often observed in menopausal women and prodromal Alzheimer's disease (AD). Although data on the relation between sex hormones and DNA repair mechanisms in the brain is still limited, various investigations have linked sex hormone levels with different DNA repair enzymes. Here, we review estrogen anti-aging and neuroprotective mechanisms, which are currently an area of intense study, together with the effect they may have on the DNA repair capacity in the brain.

Low dietary soy isoflavonoids increase hippocampal spine synapse density in ovariectomized rats

High dietary intake of plant estrogens (phytoestrogens) can affect brain structure and function. The effects of phytoestrogen intake within the range of normal animal and human dietary consumption, however, remain uncertain. The aim of the present study was to determine the effects of the isoflavonoids present in a standard low phytoestrogen laboratory rat chow on spine synapse density in the stratum radiatum of area CA1 of the hippocampus. Weanling rats (22days old) were fed either standard chow (Teklad 2018), a nutritionally comparable diet without soy (Teklad 2016) or a custom diet containing Teklad 2016 supplemented with the principal soy isoflavonoids, daidzein and genistein, for 40days. Rats were ovariectomized at 54days of age. Eight days later, spine synapse density on the apical dendrites of hippocampal pyramidal neurons in the stratum radiatum of area CA1 was measured by electron microscopic stereological analysis. Animals maintained on Teklad 2016 exhibited an approximately 60% lower CA1 spine synapse density than animals consuming Teklad 2018. Replacing genistein and daidzein in Teklad 2016 returned synapse density to levels indistinguishable from those in animals on Teklad 2018. These results indicate that the isoflavonoids in a standard laboratory rat diet exert significant effects on spine synapse density in the CA1 region of the hippocampus. Since changes in spine synapse density in this region of the hippocampus have been linked to cognitive performance and mood state, these data suggest that even relatively low daily consumption of soy phytoestrogens may be sufficient to influence hippocampal function.

Slow-pressor angiotensin II hypertension and concomitant dendritic NMDA receptor trafficking in estrogen receptor β-containing neurons of the mouse hypothalamic paraventricular nucleus are sex and age dependent

The incidence of hypertension increases after menopause. Similar to humans, "slow-pressor" doses of angiotensin II (AngII) increase blood pressure in young males, but not in young female mice. However, AngII increases blood pressure in aged female mice, paralleling reproductive hormonal changes. These changes could influence receptor trafficking in central cardiovascular circuits and contribute to hypertension. Increased postsynaptic N-methyl-D-aspartate (NMDA) receptor activity in the hypothalamic paraventricular nucleus (PVN) is crucial for the sympathoexcitation driving AngII hypertension. Estrogen receptors β (ERβs) are present in PVN neurons. We tested the hypothesis that changes in ovarian hormones with age promote susceptibility to AngII hypertension, and influence NMDA receptor NR1 subunit trafficking in ERβ-containing PVN neurons. Transgenic mice expressing enhanced green fluorescent protein (EGFP) in ERβ-containing cells were implanted with osmotic minipumps delivering AngII (600 ng/kg/min) or saline for 2 weeks. AngII increased blood pressure in 2-month-old males and 18-month-old females, but not in 2-month-old females. By electron microscopy, NR1-silver-intensified immunogold (SIG) was mainly in ERβ-EGFP dendrites. At baseline, NR1-SIG density was greater in 2-month-old females than in 2-month-old males or 18-month-old females. After AngII infusion, NR1-SIG density was decreased in 2-month-old females, but increased in 2-month-old males and 18-month-old females. These findings suggest that, in young female mice, NR1 density is decreased in ERβ-PVN dendrites thus reducing NMDA receptor activity and preventing hypertension. Conversely, in young males and aged females, NR1 density is upregulated in ERβ-PVN dendrites and ultimately leads to the neurohumoral dysfunction driving hypertension.

Object recognition memory deficit and depressive-like behavior caused by chronic ovariectomy can be transitorialy recovered by the acute activation of hippocampal estrogen receptors

It is well known that estradiol (E2) replacement therapy is effective on restoring memory deficits and mood disorders that may occur during natural menopause or after surgical ovarian removal (ovariectomy, OVX). However, it is still unknown the effectiveness of acute and localized E2 administration on the effects of chronic OVX. Here we tested the hypothesis that the intra-hippocampal E2 infusion, as well as specific agonists of estrogen receptors (ERs) alpha (ERα) and beta (ERβ), are able to mend novel object recognition (NOR) memory deficit and depressive-like behavior caused by 12 weeks of OVX. We found that both ERα and ERβ activation, at earlier stages of consolidation, recovered the NOR memory deficit caused by 12 w of OVX. Conversely, only the ERβ activation was effective in decreasing the depressive-like behavior caused by 12 w of OVX. Furthermore, we investigated the effect of OVX on hippocampal volume and ERs expression. The structural MRI showed no alteration in the hippocampus volume of 12 w OVX animals. Interestingly, ERα expression in the hippocampus decreased after one week of OVX, but increased in 12 w OVX animals. Overall, we may conclude that the chronic estrogen deprivation, induced by 12 weeks of OVX, modulates the hippocampal ERα expression and induces NOR memory deficit and depressive-like behaviors. Nonetheless, it is noteworthy that the acute effects of E2 on NOR memory and depressive-like behavior are still apparent even after 12 weeks of OVX.

Learning and memory: Steroids and epigenetics

Memory formation and utilization is a complex process involving several brain structures in conjunction as the hippocampus, the amygdala and the adjacent cortical areas, usually defined as medial temporal lobe structures (MTL). The memory processes depend on the formation and modulation of synaptic connectivity affecting synaptic strength, synaptic plasticity and synaptic consolidation. The basic neurocognitive mechanisms of learning and memory are shortly recalled in the initial section of this paper. The effect of sex hormones (estrogens, androgens and progesterone) and of adrenocortical steroids on several aspects of memory processes are then analyzed on the basis of animal and human studies. A specific attention has been devoted to the different types of steroid receptors (membrane or nuclear) involved and on local metabolic transformations when required. The review is concluded by a short excursus on the steroid activated epigenetic mechanisms involved in memory formation.

Estrogen receptors, the hippocampus, and memory

Estradiol effects on memory depend on hormone levels and the interaction of different estrogen receptors within neural circuits. Estradiol induces gene transcription and rapid membrane signaling mediated by estrogen receptor-alpha (ERα), estrogen receptor-beta (ERβ), and a recently characterized G-protein coupled estrogen receptor, each with distinct distributions and ability to influence estradiol-dependent signaling. Investigations using receptor specific agonists suggest that all three receptors rapidly activate kinase-signaling and have complex dose-dependent influences on memory. Research employing receptor knockout mice demonstrate that ERα maintains transcription and memory as estradiol levels decline. This work indicates a regulatory role of ERβ in transcription and cognition, which depends on estradiol levels and the function of ERα. The regulatory role of ERβ is due in part to ERβ acting as a negative regulator of ERα-mediated transcription. Vector-mediated expression of estrogen receptors in the hippocampus provides an innovative research approach and suggests that memory depends on the relative expression of ERα and ERβ interacting with estradiol levels. Notably, the ability of estradiol to improve cognition declines with advanced age along with decreased expression of estrogen receptors. Thus, it will be important for future research to determine the mechanisms that regulate estrogen receptor expression during aging.

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.

Carboxypeptidase E protects hippocampal neurons during stress in male mice by up-regulating prosurvival BCL2 protein expression

Prolonged chronic stress causing elevated plasma glucocorticoids leads to neurodegeneration. Adaptation to stress (allostasis) through neuroprotective mechanisms can delay this process. Studies on hippocampal neurons have identified carboxypeptidase E (CPE) as a novel neuroprotective protein that acts extracellularly, independent of its enzymatic activity, although the mechanism of action is unclear. Here, we aim to determine if CPE plays a neuroprotective role in allostasis in mouse hippocampus during chronic restraint stress (CRS), and the molecular mechanisms involved. Quantitative RT-PCR/in situ hybridization and Western blots were used to assay for mRNA and protein. After mild CRS (1 h/d for 7 d), CPE protein and mRNA were significantly elevated in the hippocampal CA3 region, compared to naïve littermates. In addition, luciferase reporter assays identified a functional glucocorticoid regulatory element within the cpe promoter that mediated the up-regulation of CPE expression in primary hippocampal neurons following dexamethasone treatment, suggesting that circulating plasma glucocorticoids could evoke a similar effect on CPE in the hippocampus in vivo. Overexpression of CPE in hippocampal neurons, or CRS in mice, resulted in elevated prosurvival BCL2 protein/mRNA and p-AKT levels in the hippocampus; however, CPE(-/-) mice showed a decrease. Thus, during mild CRS, CPE expression is up-regulated, possibly contributed by glucocorticoids, to mediate neuroprotection of the hippocampus by enhancing BCL2 expression through AKT signaling, and thereby maintaining allostasis.

Bidirectional modulatory effect of 17β-estradiol on NMDA receptors via ERα and ERβ in the dentate gyrus of juvenile male rats

The neurosteroid 17β-estradiol (E2) is synthesized by aromatase in both male and female hippocampi and is known to modulate hippocampal synaptic functions. However, as some contradictory findings regarding the modulatory effects of E2 have been reported in the literature, its physiological role and mechanism of action in the hippocampus remain controversial. Our recent study showed that a low E2 dose (1 nM) increased the amplitude of NMDA receptor-mediated EPSCs (NMDAR-EPSCs) and lowered the threshold for the induction of NMDA receptor-dependent long-term potentiation (NMDAR-LTP), while a high E2 dose (7 nM) exerted opposite effects in the dentate gyrus of juvenile male rat hippocampal slices. The present study is a follow-up that explores the underlying mechanism of this bidirectional effect of E2. We found that the ERα agonist PPT reproduced the actions of the low E2 dose on NMDAR-EPSCs and NMDAR-LTP, while the ERβ agonist DPN reproduced the actions of the high E2 dose. Moreover, PPT, but not DPN, restored the decrease in NMDAR-EPSCs induced by the aromatase inhibitor letrozole, suggesting that E2 synthesized constitutively in the hippocampus enhances NMDA receptor function via ERα. The PPT-induced enhancement in NMDAR-EPSCs was mediated by Src family kinase, but was not caused by NR2B modulation. These findings demonstrate that E2 exerts condition-dependent bidirectional effects on NMDA receptor-mediated transmission and, thus, synaptic plasticity via ERα and ERβ in the dentate gyrus of juvenile male rats.

Contribution of estrogen receptors alpha and beta in the brain response to traumatic brain injury

OBJECT

Although there is evidence that estradiol has neuroprotective effects after traumatic brain injury (TBI) in female rats, it is unclear which estrogen receptor (ER) subtype, ERα or ERβ, mediates this effect. The authors therefore examined the roles of the different ERs in this effect. Here the authors used the ERα selective agonist propyl pyrazole triol (PPT) and the ERβ selective agonist diarylpropionitrile (DPN) alone and in combination in female rats to investigate this question.

METHODS

Before the ovariectomized animals were injured using the Marmarou TBI technique, they were randomly divided into the following 9 groups: control, sham, TBI, vehicle, E1 (physiological dose of 17-β estradiol), E2 (pharmacological dose of 17-β estradiol), PPT, DPN, and PPT+DPN. Levels of blood-brain barrier (BBB) disruption (5 hours) and water content (24 hours) were evaluated after TBI. In groups receiving drugs or vehicle, treatment was administered as a single dose intraperitoneally 30 minutes after induction of TBI.

RESULTS

Results showed that brain edema or brain water content after TBI was lower (p < 0.001) in the E2, PPT, DPN, and PPT+DPN groups than it was in the vehicle group. After trauma, the Evans blue dye content or BBB permeability was significantly higher in the TBI and vehicle groups (p < 0.001) than in the E2, PPT, DPN, and PPT+DPN groups. The inhibitory effects of PPT+DPN on brain water content, neurological scores, and Evans blue dye content were the highest for all groups. Although both PPT and DPN increased neurological scores after TBI, PPT appears to be more effective in increasing neurological scores.

CONCLUSIONS

Neuroprotective effects of estradiol on brain edema, BBB permeability, and neurological scores are mediated through both ERα and ERβ. This may suggest a therapeutic potential in the brain trauma for ER-specific agonists.

Is basic research providing answers if adjuvant anti-estrogen treatment of breast cancer can induce cognitive impairment?

Adjuvant treatment of cancer by chemotherapy is associated with cognitive impairment in some cancer survivors. Breast cancer patients are frequently also receiving endocrine therapy with selective estrogen receptor modulators (SERMs) and/or aromatase inhibitors (AIs) to suppress the growth of estradiol sensitive breast tumors. Estrogens are well-known, however, to target brain areas involved in the regulation of cognitive behavior. In this review clinical and basic preclinical research is reviewed on the actions of estradiol, SERMs and AIs on brain and cognitive functioning to see if endocrine therapy potentially induces cognitive impairment and in that respect may contribute to the detrimental effects of chemotherapy on cognitive performance in breast cancer patients. Although many clinical studies may be underpowered to detect changes in cognitive function, current basic and clinical reports suggest that there is little evidence that AIs may have a lasting detrimental effect on cognitive performance in breast cancer patients. The clinical data on SERMs are not conclusive, but some studies do suggest that tamoxifen administration may form a risk for cognitive functioning particularly in older women. An explanation may come from basic preclinical research which indicates that tamoxifen often acts agonistic in the absence of estradiol but antagonistic in the presence of endogenous estradiol. It could be hypothesized that the negative effects of tamoxifen in older women is related to the so-called window of opportunity for estrogen. Administration of SERMs beyond this so-called window of opportunity may not be effective or might even have detrimental effects similar to estradiol.

Estrogen effects on the brain: actions beyond the hypothalamus via novel mechanisms

From its origins in how the brain controls the endocrine system via the hypothalamus and pituitary gland, neuroendocrinology has evolved into a science that now includes hormone action on many aspects of brain function. These actions involve the whole central nervous system and not just the hypothalamus. Advances in our understanding of cellular and molecular actions of steroid hormones have gone beyond the important cell nuclear actions of steroid hormone receptors to include signaling pathways that intersect with other mediators such as neurotransmitters and neuromodulators. This has, in turn, broadened the search for and identification of steroid receptors to include nonnuclear sites in synapses, dendrites, mitochondria, and glial cells, as well as cell nuclei. The study of estrogen receptors and estrogen actions on processes related to cognition, mood, autonomic regulation, pain, and neuroprotection, among other functions, has led the way in this new view of hormone actions on the brain. In this review, we summarize past and current work in our laboratory on this topic. This exciting and growing field involving many laboratories continues to reshape our ideas and approaches to neuroendocrinology both at the bench and the bedside.

Role of estrogen receptor alpha and beta expression and signaling on cognitive function during aging

This review presents evidence for the idea that the expression of estrogen receptor alpha and beta (ERα and ERβ) interacts with the level of estradiol (E2) to influence the etiology of age-related cognitive decline and responsiveness to E2 treatments. There is a nonmonotonic dose response curve for E2 influences on behavior and transcription. Evidence is mounting to indicate that the dose response curve is shifted according to the relative expression of ERα and ERβ. Recent work characterizing age-related changes in the expression of ERα and ERβ in the hippocampus, as well as studies using mutant mice, and viral mediated delivery of estrogen receptors indicate that an age-related shift in ERα/ERβ expression, combined with declining gonadal E2 can impact transcription, cell signaling, neuroprotection, and neuronal growth. Finally, the role of ERα/ERβ on rapid E2 signaling and synaptogenesis as it relates to hippocampal aging is discussed.

Reduction of dendritic spines and elevation of GABAergic signaling in the brains of mice treated with an estrogen receptor β ligand

An estrogen receptor (ER) β ligand (LY3201) with a preference for ERβ over ERα was administered in s.c. pellets releasing 0.04 mg/d. The brains of these mice were examined 3 d after treatment had begun. Although estradiol-17β is known to increase spine density and glutaminergic signaling, as measured by Golgi staining, a clear reduction in spines was evident on the dendritic branches in LY3201-treated mice but no morphological alteration and no difference in the number of dendritic spines on dendritic stems were observed. In the LY3201-treatment group, there was higher expression of glutamic acid decarboxylase (GAD) in layer V of cortex and in the CA1 of hippocampus, more GAD(+) terminals surrounding the pyramidal neurons and less glutamate receptor (NMDAR) on the neurons in layer V. There were no alterations in expression of Iba1 or in Olig2 or CNPase. However, GFAP(+) astrocytes were increased in the LY3201-treatment group. There were also more projections characteristic of activated astrocytes and increased expression of glutamine synthetase (GS). No expression of ERβ was detectable in the nuclei of astrocytes. Clearly, LY3201 caused a shift in the balance between excitatory and inhibitory neurotransmission in favor of inhibition. This shift was due in part to increased synthesis of GABA and increased removal of glutamate from the synaptic cleft by astrocytes. The data reveal that treatment with a selective ERβ agonist results in changes opposite to those reported in estradiol-17β-treated mice and suggests that ERα and ERβ play opposing roles in the brain.

Estradiol acts via estrogen receptors alpha and beta on pathways important for synaptic plasticity in the mouse hippocampal formation

Estradiol affects hippocampal-dependent spatial memory and underlying structural and electrical synaptic plasticity in female mice and rats. Using estrogen receptor (ER) alpha and beta knockout mice and wild-type littermates, we investigated the role of ERs in estradiol effects on multiple pathways important for hippocampal plasticity and learning. Six hours of estradiol administration increased immunoreactivity for phosphorylated Akt throughout the hippocampal formation, whereas 48 h of estradiol increased immunoreactivity for phosphorylated TrkB receptor. Estradiol effects on phosphorylated Akt and TrkB immunoreactivities were abolished in ER alpha and ER beta knockout mice. Estradiol also had distinct effects on immunoreactivity for post-synaptic density 95 (PSD-95) and brain derived-neurotrophic factor (BDNF) mRNA in ER alpha and beta knockout mice. Thus, estradiol acts through both ERs alpha and beta in several subregions of the hippocampal formation. The different effects of estradiol at 6 and 48 h indicate that several mechanisms of estrogen receptor signaling contribute to this female hormone's influence on hippocampal synaptic plasticity. By further delineating these mechanisms, we will better understand and predict the effects of endogenous and exogenous ovarian steroids on mood, cognition, and other hippocampal-dependent behaviors.

Differential expression of estrogen receptors in two hippocampal regions during the estrous cycle of the rat

In the hippocampus, estrogens increase dendritic arborization, long-term potentiation, neuroprotection, and participate in many functions related with learning, memory, and affective behaviors. The presence of both estrogen receptors alpha (ERα) and beta (ERβ) isoforms has been described in the hippocampus where they play different physiological roles. The aim of this study was to investigate, by using both techniques immunohistochemistry and Western Blot, the expression pattern of ERα and ERβ in the hippocampus of the rat along the estrous cycle. Western blot analysis was used to confirm the specificity of the antibodies used against ERα and ERβ and its relative content in the hippocampus. Results from immunohistochemical studies indicate that ERβ expression increased more than the ERα in CA1 and CA3 regions during all phases of the estrous cycle. ERβ immunoreactivity was mainly located in the nucleus and predominantly expressed in CA1 during estrous and metestrus, and in CA3 during diestrus. ERα was more abundant during estrous in comparison to other phases of the cycle in CA1 region, while it was more abundant during metestrus in CA3. Interestingly, the immunolocalization of ERα subtype was both cytoplasmic and nuclear. The overall results indicate that there is a differential expression, cellular localization, and distribution of both ER subtypes in CA1 and CA3 regions, suggesting different roles for these two receptors in the hippocampus along the estrous cycle.

Estrogen and aging affect the synaptic distribution of estrogen receptor β-immunoreactivity in the CA1 region of female rat hippocampus

Estradiol (E) mediates increased synaptogenesis in the hippocampal CA1 stratum radiatum (sr) and enhances memory in young and some aged female rats, depending on dose and age. Young female rats express more estrogen receptor α (ERα) immunolabeling in CA1sr spine synapse complexes than aged rats and ERα regulation is E sensitive in young but not aged rats. The current study examined whether estrogen receptor β (ERβ) expression in spine synapse complexes may be altered by age or E treatment. Young (3-4 months) and aged (22-23 months) female rats were ovariectomized 7 days prior to implantation of silastic capsules containing either vehicle (cholesterol) or E (10% in cholesterol) for 2 days. ERβ immunoreactivity (ir) in CA1sr was quantitatively analyzed using post-embedding electron microscopy. ERβ-ir was more prominent post-synaptically than pre-synaptically and both age and E treatment affected its synaptic distribution. While age decreased the spine synaptic complex localization of ERβ-ir (i.e., within 60 nm of the pre- and post-synaptic membranes), E treatment increased synaptic ERβ in both young and aged rats. In addition, the E treatment, but not age, increased dendritic shaft labeling. This data demonstrates that like ERα the levels of ERβ-ir decrease in CA1 axospinous synapses with age, however, unlike ERα the levels of ERβ-ir increase in these synapses in both young and aged rats in response to E. This suggests that synaptic ERβ may be a more responsive target to E, particularly in aged females.

Estrogen-mediated effects on cognition and synaptic plasticity: what do estrogen receptor knockout models tell us?

A plethora of evidence supports a beneficial role of estrogen in the brain. However, while these effects are hypothesized to be driven via the two main forms of estrogen receptors (ERα and ERß), the mechanism through which these receptors mediate estrogen's effects on cognition and plasticity remain unclear. Estrogen receptors are heterogeneously expressed in many cognition sensitive regions of the brain, have the ability to dimerize and heterodimerize, and are localized to both neurons and glia. In addition, while many of the known actions of estrogen through their receptor are mediated via the classical genomic regulatory mechanism of gene transcription, rapid non-genomic action of estrogens are also gaining relevance. These complex events make the mechanistic understanding of estrogen effects challenging. The development of transgenic estrogen receptor knockout mouse models has provided some much needed insight on the role of these receptors in mediating estrogen effects on cognition and synaptic plasticity. This review provides an overview of estrogen receptors in the brain and an update of knowledge gained from transgenic knockout models on cognition and synaptic plasticity.

Kalirin-7, an important component of excitatory synapses, is regulated by estradiol in hippocampal neurons

Estradiol enhances the formation of dendritic spines and excitatory synapses in hippocampal neurons in vitro and in vivo, but the underlying mechanisms are not fully understood. Kalirin-7 (Kal7), the major isoform of Kalirin in the adult hippocampus, is a Rho GDP/GTP exchange factor localized to postsynaptic densities. In the hippocampus, both Kal7 and estrogen receptor α (ERα) are highly expressed in a subset of interneurons. Over-expression of Kal7 caused an increase in spine density and size in hippocampal neurons. To determine whether Kalirin might play a role in the effects of estradiol on spine formation, Kal7 expression was examined in the hippocampus of ovariectomized rats. Estradiol replacement increased Kal7 staining in both CA1 pyramidal neurons and interneurons in ovariectomized rats. Estradiol treatment of cultured hippocampal neurons increased Kal7 levels at the postsynaptic side of excitatory synapses and increased the number of excitatory synapses along the dendrites of pyramidal neurons. These increases were mediated via ERα because a selective ERα agonist, but not a selective ERβ agonist, caused a similar increase in both Kal7 levels and excitatory synapse number in cultured hippocampal neurons. When Kal7 expression was reduced using a Kal7-specific shRNA, the density of excitatory synapses was reduced and estradiol was no longer able to increase synapse formation. Expression of exogenous Kal7 in hippocampal interneurons resulted in decreased levels of GAD65 staining. Inhibition of GABAergic transmission with bicuculline produced a robust increase in Kal7 expression. These studies suggest Kal7 plays a key role in the mechanisms of estradiol-mediated synaptic plasticity.

Estradiol and the relationship between dendritic spines, NR2B containing NMDA receptors, and the magnitude of long-term potentiation at hippocampal CA3-CA1 synapses

When circulating estrogen levels decline as a natural consequence of menopause and aging in women, there is an increased incidence of deficits in working memory. In many cases, these deficits are rescued by estrogen replacement therapy. These clinical data therefore highlight the importance of defining the biological pathways linking estrogen to the cellular substrates of learning and memory. It has been known for nearly two decades that estrogen enhances dendritic spine density on apical dendrites of CA1 pyramidal cells in hippocampus, a brain region required for learning. Interestingly, at synapses between CA3-CA1 pyramidal cells, estrogen has also been shown to enhance synaptic NMDA receptor current and the magnitude of long-term potentiation, a cellular correlate of learning and memory. Given that synapse density, NMDAR function, and long-term potentiation at CA3-CA1 synapses in hippocampus are associated with normal learning, it is likely that modulation of these parameters by estrogen facilitates the improvement in learning observed in rats, primates and humans following estrogen replacement. To facilitate the design of clinical strategies to potentially prevent or reverse the age-related decline in learning and memory during menopause, the relationship between the estrogen-induced morphological and functional changes in hippocampus must be defined and the role these changes play in facilitating learning must be elucidated. The aim of this report is to provide a summary of the proposed mechanisms by which this hormone increases synaptic function and in doing so, it briefly addresses potential mechanisms contributing to the estrogen-induced increase in synaptic morphology and plasticity, as well as important future directions.

Estrogen receptor alpha and beta specific agonists regulate expression of synaptic proteins in rat hippocampus

Changes in hippocampal CA1 dendritic spine density and synaptic number across the estrous cycle in female rats correlate with increased hippocampal-dependent cognitive performance in a manner that is dependent on estrogen receptors (ERs). Two isoforms of the estrogen receptor, alpha and beta are present in the rat hippocampus and distinct effects on cognitive behavior have been described for each receptor. The present study generated a profile of synaptic proteins altered by administration of estradiol benzoate, the ERalpha selective agonist PPT (1,3,5-tris (4-hydroxyphenyl)-4-propyl-1H-pyrazole) and the ERbeta selective agonist DPN (2,3-bis (4-hydroxyphenyl) propionitrile) alone and in combination in comparison to vehicle in the CA1 region of the dorsal hippocampus. In the stratum radiatum, estradiol, DPN, and PPT increased PSD-95 and AMPA-type glutamate receptor subunit GluR1. Only DPN administration regulated expression of AMPA receptor subunits GluR2 and GluR3, increasing and decreasing levels respectively. DPN also increased GluR2 expression in the other lamina of the CA1. These results support previous reports that estradiol and isoform specific agonists differentially activate ERalpha and ERbeta to regulate protein expression. The distinct effects of DPN and PPT administration on synaptic proteins suggest that the desired therapeutic outcome of estrogen may be accomplished by using specific estrogen receptor agonists. Moreover, the effects of estradiol treatment on PSD-95 expression are consistent with a growing body of evidence that this postsynaptic protein is a key marker of estrogen action related to spine synapse formation.

Red wine interferes with oestrogen signalling in rat hippocampus

Oestrogens have neuroprotective properties, resulting in memory and learning preservation. Red wine (RW) has been linked to neuroprotection, but mechanisms are largely unknown. The aim of this work was to test the effect of RW or 13% ethanol solution consumption on the expression of aromatase and estrogen receptors (ER) in the rat hippocampus. Beverages were supplied to male Wistar rats and after 8 weeks of treatment animals were euthanised, hippocampus was removed, aromatase expression assessed by western blotting and aromatase and ER transcription determined by RT-PCR. The effects of treatments on hippocampal aromatase activity were also determined, as well as the effect of several red wine polyphenols in hippocampal homogenates from untreated animals. Aromatase transcription was increased by ethanol (to 158+/-7%) but only significantly by RW (to 180+/-9%). No difference was found in ERalpha expression among groups, whereas RW significantly decreased ERbeta expression (to 63+/-10%). Resveratrol, quercetin, myricetin and kaempferol had no effect on aromatase activity and catechin (300 microM), epicatechin (200 microM), procyanidin extract (200 mg/L) and fractioned procyanidins (FI and FII; 200 mg/L) significantly decreased aromatase activity. The contribution of procyanidins in wine to the effect observed in aromatase was investigated in animals treated for the same period with these compounds (200 mg/L), although no effect was seen in aromatase activity, mRNA or protein levels, meaning that this group of compounds had little contribution, if any, to the effects observed. Nevertheless, the increase in aromatase expression induced by RW may corroborate the neuroprotective ability attributed to this beverage. Alterations in the relative abundance of ER expression may also play an important role in the protection.

Expression of estrogen receptor alpha and beta in reactive astrocytes at the male rat hippocampus after status epilepticus

Estrogen is neuroprotective against status epilepticus (SE)-induced hippocampal damage in female animals. In male animals, estrogen is converted from testosterone via aromatization the activity of which is upregulated by brain damage. However, it is controversial whether estrogen is neuroprotective or neuroinvasive against male hippocampal damage after SE. In order to understand the role of estrogen, it is important to elucidate the distribution manner of estrogen receptor (ER)alpha and beta as the targets of estrogen. In this study, we examined the time course changes of ERs in adult male rat hippocampus after SE using anti-ERalpha antibodies (MC-20 and PA1-309) and anti-ERbeta antibodies (PA1-310B and PA1-311). In control rats, both ERalpha and beta were expressed in the pyramidal cells predominantly at CA1 and CA3. ERalpha was expressed in the cytoplasm and the nucleus, whereas ERbeta was expressed in the cytoplasm of the pyramidal cells. After SE, according to the pyramidal cell loss at CA1, the number of ERalpha- and beta-immunoreactive pyramidal cells decreased up to day 21. On the other hand, reactive astrocytes, which newly appeared after SE and formed gliosis at CA1, were confirmed to express both ERs in the nucleus, cytoplasm, and process. There were no differences in immunoreactivity between antibodies. Our results indicate that endogenous estrogen affects the pyramidal cells through ERalpha and beta under normal circumstances in adult male rats, whereas the targets of estrogen shift to the reactive astrocytes through ERalpha and beta after SE.

Uncovering the mechanisms of estrogen effects on hippocampal function

Estrogens have direct effects on the brain areas controlling cognition. One of the most studied of these regions is the dorsal hippocampal formation, which governs the formation of spatial and episodic memories. In laboratory animals, most investigators report that estrogen enhances synaptic plasticity and improves performance on hippocampal-dependent cognitive behaviors. This review summarizes work conducted in our laboratory and others toward identifying estrogen's actions in the hippocampal formation, and the mechanisms for these actions. Physiologic and pharmacologic estrogen affects cognitive behavior in mammals, which may be applicable to human health and disease. The effects of estrogen in the hippocampal formation that lead to modulation of hippocampal function include effects on cell morphology, synapse formation, signaling, and excitability that have been studied in laboratory mice, rats, and primates. Finally, estrogen may signal through both nuclear and extranuclear hippocampal estrogen receptors to achieve its downstream effects.

Estrogen and aging affect synaptic distribution of phosphorylated LIM kinase (pLIMK) in CA1 region of female rat hippocampus

17beta-Estradiol (E) increases axospinous synapse density in the hippocampal CA1 region of young female rats, but not in aged rats. This may be linked to age-related alterations in signaling pathways activated by synaptic estrogen receptor alpha (ER-alpha) that potentially regulate spine formation, such as LIM-kinase (LIMK), an actin depolymerizing factor/cofilin kinase. We hypothesized that, as with ER-alpha, phospho-LIM-kinase (pLIMK) may be less abundant or responsive to E in CA1 synapses of aged female rats. To address this, cellular and subcellular distribution of pLIMK-immunoreactivity (IR) in CA1 was analyzed by light and electron microscopy in young and aged female rats that were ovariectomized and treated with either vehicle or E. pLIMK-IR was found primarily in perikarya within the pyramidal cell layer and dendritic shafts and spines in stratum radiatum (SR). While pLIMK-IR was occasionally present in terminals, post-embedding quantitative analysis of SR showed that pLIMK had a predominant post-synaptic localization and was preferentially localized within the postsynaptic density (PSD). The percentage of pLIMK-labeled synapses increased (30%) with E treatment (P<0.02) in young animals, and decreased (43%) with age (P<0.002) regardless of treatment. The pattern of distribution of pLIMK-IR within dendritic spines and synapses was unaffected by age or E treatment, with the exception of an E-induced increase in the non-synaptic core of spines in young females. These data suggest that age-related synaptic alterations similar to those seen with ER-alpha occur with signaling molecules such as pLIMK, and support the hypothesis that age-related failure of E treatment to increase synapse number in CA1 may be due to changes in the molecular profile of axospinous synapses with respect to signaling pathways linked to formation of additional spines and synapses in response to E.

Estrogen receptor beta is essential for sprouting of nociceptive primary afferents and for morphogenesis and maintenance of the dorsal horn interneurons

Estrogen is known to influence pain, but the specific roles of the two estrogen receptors (ERs) in the spinal cord are unknown. In the present study, we have examined the expression of ERalpha and ERbeta in the spinal cord and have looked for defects in pain pathways in ERbeta knockout (ERbeta(-/-)) mice. In the spinal cords of 10-month-old WT mice, ERbeta-positive cells were localized in lamina II, whereas ERalpha-positive cells were mainly localized in lamina I. In ERbeta(-/-) mice, there were higher levels of calcitonin gene-regulated peptide and substance P in spinal cord dorsal horn and isolectin B4 in the dorsal root ganglion. In the superficial layers of the spinal cord, there was a decrease in the number of calretinin (CR)-positive neurons, and in the outer layer II, there was a loss of calbindin-positive interneurons. During embryogenesis, ERbeta was first detectable in the spinal cord at embryonic day 13.5 (E13.5), and ERalpha was first detectable at E15.5. During middle and later embryonic stages, ERbeta was abundantly expressed in the superficial layers of the dorsal horn. ERalpha was also expressed in the dorsal horn but was limited to fewer neurons. Double staining for ERbeta and CR showed that, in the superficial dorsal horn of WT neonates [postnatal day 0 (P0)], most CR neurons also expressed ERbeta. At this stage, few CR-positive cells were detected in the dorsal horn of ERbeta(-/-) mice. Taken together, these findings suggest that, early in embryogenesis, ERbeta is involved in dorsal horn morphogenesis and in sensory afferent fiber projections to the dorsal horn and that ERbeta is essential for survival of dorsal horn interneurons throughout life.

Estradiol targets synaptic proteins to induce glutamatergic synapse formation in cultured hippocampal neurons: critical role of estrogen receptor-alpha

Estradiol mediates structural changes at synapses of the hippocampus, an area in the brain important for learning and memory. This study was designed to test the hypothesis that estradiol mediates subcellular changes of synaptic proteins to induce new synapses via an estrogen receptor (ER)-mediated process. To elucidate the mechanisms involved in glutamatergic synapse formation, we investigated effects of estradiol on synaptic proteins in cultured hippocampal neurons using immunocytochemistry and confocal microscopy. Synaptic protein distribution and size were identified with antibodies to the presynaptic vesicular glutamate transporter protein (vGlut1) and postsynaptic NMDA receptor (NR1 subunit). We observed an increase in synapse density, as detected by NR1 and vGlut1 colocalization, along dendrites of neurons cultured in steroid-stripped media and exposed to estradiol (10 nM) for 48 h. Additionally, the NR1 subunit was enriched at synaptic clusters. Immunocytochemistry and confocal imaging revealed punctate staining of extranuclear ERs along dendrites of hippocampal neurons expressing NR1. Estradiol increased the density of both ER-alpha and ER-beta protein clusters along dendrites. To test whether ERs play an important functional role in the estradiol-induced synaptogenesis, we used the ER antagonist [7alpha,17beta-[9[(4,4,5,5,5-pentafluoropentyl)sulfinyl]nonyl]estra-1,3,5(10)-triene-3,17-diol (ICI 182,780)] and the ER-alpha- and ER-beta-specific agonists [1,3,5-tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT) and 2,3-bis(4-hydroxyphenyl) propionitrile (DPN), respectively]. ICI 182,780 blocked the increase in synapse density. Treatment with PPT, but not DPN, induced significant increases in synapse density that mimicked treatment with estradiol. Together, our results demonstrate that estradiol stimulates glutamatergic synapse formation in the developing hippocampus through an ER-alpha-dependent mechanism. These findings carry profound implications regarding the potential of estrogen to influence learning, memory, and possibly hormone-modulated neurodegeneration.

Rapid modulation of long-term depression and spinogenesis via synaptic estrogen receptors in hippocampal principal neurons

Rapid modulation of hippocampal synaptic plasticity by estrogen has long been a hot topic, but analysis of molecular mechanisms via synaptic estrogen receptors has been seriously difficult. Here, two types of independent synaptic plasticity, long-term depression (LTD) and spinogenesis, were investigated, in response to 17beta-estradiol and agonists of estrogen receptors using hippocampal slices from adult male rats. Multi-electrode investigations demonstrated that estradiol rapidly enhanced LTD not only in CA1 but also in CA3 and dentate gyrus. Dendritic spine morphology analysis demonstrated that the density of thin type spines was selectively increased in CA1 pyramidal neurons within 2 h after application of 1 nm estradiol. This enhancement of spinogenesis was completely suppressed by mitogen-activated protein (MAP) kinase inhibitor. Only the estrogen receptor (ER) alpha agonist, (propyl-pyrazole-trinyl)tris-phenol (PPT), induced the same enhancing effect as estradiol on both LTD and spinogenesis in the CA1. The ERbeta agonist, (4-hydroxyphenyl)-propionitrile (DPN), suppressed LTD and did not affect spinogenesis. Because the mode of synaptic modulations by estradiol was mostly the same as that by the ERalpha agonist, a search was made for synaptic ERalpha using purified RC-19 antibody qualified using ERalpha knockout (KO) mice. Localization of ERalpha in spines of principal glutamatergic neurons was demonstrated using immunogold electron microscopy and immunohistochemistry. ERalpha was also located in nuclei, cytoplasm and presynapses.

Sex steroids and the dentate gyrus

In the late 1980s, the finding that the dentate gyrus contains more granule cells in the male than in the female of certain mouse strains provided the first indication that the dentate gyrus is a significant target for the effects of sex steroids during development. Gonadal hormones also play a crucial role in shaping the function and morphology of the adult brain. Besides reproduction-related processes, sex steroids participate in higher brain operations such as cognition and mood, in which the hippocampus is a critical mediator. Being part of the hippocampal formation, the dentate gyrus is naturally involved in these mechanisms and as such, this structure is also a critical target for the activational effects of sex steroids. These activational effects are the results of three major types of steroid-mediated actions. Sex steroids modulate the function of dentate neurons under normal conditions. In addition, recent research suggests that hormone-induced cellular plasticity may play a larger role than previously thought, particularly in the dentate gyrus. Specifically, the regulation of dentate gyrus neurogenesis and synaptic remodeling by sex steroids received increasing attention lately. Finally, the dentate gyrus is influenced by gonadal hormones in the context of cellular injury, and the work in this area demonstrates that gonadal hormones have neuroprotective potential. The expression of estrogen, progestin, and androgen receptors in the dentate gyrus suggests that sex steroids, which could be of gonadal origin and/or synthesized locally in the dentate gyrus, may act directly on dentate cells. In addition, gonadal hormones could also influence the dentate gyrus indirectly, by subcortical hormone-sensitive structures such as the cholinergic septohippocampal system. Importantly, these three sex steroid-related themes, functional effects in the normal dentate gyrus, mechanisms involving neurogenesis and synaptic remodeling, as well as neuroprotection, have substantial implications for understanding normal cognitive function, with clinical importance for epilepsy, Alzheimer's disease and mental disorders.