Duration of estrogen deprivation, not chronological age, prevents estrogen's ability to enhance hippocampal synaptic physiology

Hypersensitivity of the hippocampal CA3 region to stress-induced neurodegeneration and amyloidogenesis in a rat model of surgical menopause

Females who enter menopause prematurely via bilateral ovariectomy (surgical menopause) have a significantly increased risk for cognitive decline and dementia. To help elucidate the mechanisms underlying this phenomenon, we used an animal model of surgical menopause, long-term (10-week) bilateral ovariectomy in female rats. Herein, we demonstrate that long-term oestrogen deprivation dramatically increases sensitivity of the normally resistant hippocampal CA3 region to ischaemic stress, an effect that was gender-specific, as it was not observed in long-term orchiectomized males. Furthermore, the enhanced damage to the CA3 region correlated with a worse cognitive outcome after ischaemic stress. Long-term ovariectomized rats also displayed a robust hyperinduction of Alzheimer's disease-related proteins in the CA3 region and a switch in amyloid precursor protein processing from non-amyloidogenic to amyloidogenic following ischaemic stress CA3 hypersensitivity also extended to an Alzheimer's disease-relevant insult, as the CA3 region of long-term ovariectomized rats was profoundly hypersensitive to the neurotoxic effects of amyloid-β1-42, the most amyloidogenic form of the amyloid-β peptide. Additional studies revealed that CA3 region hypersensitivity, Alzheimer's disease-related protein induction, and amyloidogenesis are mediated by a NADPH oxidase/superoxide/c-Jun N-terminal kinase/c-Jun signalling pathway, involving both transcriptional and post-translational mechanisms. In addition, while 17β-oestradiol replacement at the end of the long-term oestrogen deprivation period could not prevent CA3 hypersensitivity and amyloidogenesis, if 17β-oestradiol was initiated at the time of ovariectomy and maintained throughout the 10-week oestrogen deprivation period, it completely prevented these events, providing support for the 'critical window' hypothesis for oestrogen replacement therapy benefit. Collectively, these findings may help explain the increased risk of cognitive decline and dementia observed in women following surgical menopause, and they provide increased support that early 17β-oestradiol replacement is critical in preventing the negative neural effects associated with bilateral ovariectomy.

Factors influencing the cognitive and neural effects of hormone treatment during aging in a rodent model

Whether hormone treatment alters brain structure or has beneficial effects on cognition during aging has recently become a topic of debate. Although previous research has indicated that hormone treatment benefits memory in menopausal women, several newer studies have shown no effect or detrimental effects. These inconsistencies emphasize the need to evaluate the role of hormones in protecting against age-related cognitive decline in an animal model. Importantly, many studies investigating the effects of estrogen and progesterone on cognition and related brain regions have used young adult animals, which respond differently than aged animals. However, when only the studies that have examined the effects of hormone treatment in an aging model are reviewed, there are still varied behavioral and neural outcomes. This article reviews some of the important factors that can influence the behavioral and neural outcomes of hormone treatment including the type of estrogen administered, whether or not estrogen is combined with progesterone and if so, the type of progesterone used, as well as the route, mode, and length of treatment. How these factors influence cognitive outcomes highlights the importance of study design and avoiding generalizations from a small number of studies. This article is part of a Special Issue entitled Hormone Therapy.

Regulation of STAT3 by miR-106a is linked to cognitive impairment in ovariectomized mice

MicroRNAs are abundantly expressed in the brain and play an important role in disorders of the brain, including cognitive impairment and Alzheimer's disease (AD). A growing body of evidence suggests that the janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway plays a key role in the pathogenesis of AD. However, it is unclear whether miRNAs are involved in this process. Therefore, we characterized the expression and role of miR-106a and JAK/STAT signaling in an ovariectomized (OVX) mouse model of cognitive impairment. Cognitive impairment, as indicated by escape latency and time spent in the platform quadrant in the Morris water maze test, was significantly reduced at 12 weeks post-OVX, compared to age-matched controls. Quantitative real-time PCR and Western blotting demonstrated that miR-106a was upregulated, and STAT3 and phospho-STAT3 were downregulated in the hippocampus at 12 weeks post-OVX, compared with age matched controls and the 6 and 8 weeks post-OVX groups. Transfection of human neuroblastoma SH-SY5Y cells with a miR-106a mimic reduced the expression of STAT3 mRNA, compared to control cells transfected with a scrambled mimic. STAT3 and phospho-STAT3 protein expression was upregulated or downregulated by a miR-106a inhibitor or miR-106a mimic, respectively, indicating that miR-106a negatively regulates STAT3. Luciferase reporter gene assays confirmed that miR-106a directly targets the 3' untranslated region (UTR) of STAT3. This study suggests that miR-106a negatively regulates STAT3 activation, and also that miR-106a may provide a marker of onset or potential therapeutic target for cognitive disturbances.

Estrogens, estrogen receptors, and female cognitive aging: the impact of timing

Estrogens have been shown to be protective agents against neurodegeneration and associated cognitive decline in aging females. However, clinical data have been equivocal as to the benefits to the brain and cognition of estrogen therapy in postmenopausal women. One factor that is proposed to be critical in determining the efficacy of hormone therapy is the timing of its initiation. The critical period or window of opportunity hypothesis proposes that following long-term ovarian hormone deprivation, the brain and cognition become insensitive to exogenously administered estrogens. In contrast, if estrogens are administered during a critical period near the time of cessation of ovarian function, they will exert beneficial effects. The focus of the current review is the examination of evidence from rodent models investigating the critical period hypothesis. A growing body of experimental data indicates that beneficial effects of 17β-estradiol (estradiol) on cognition and on cholinergic function and hippocampal plasticity, both of which have been linked to the ability of estradiol to exert beneficial effects on cognition, are attenuated if estradiol is administered following a period of long-term ovarian hormone deprivation. Further, emerging data implicate loss of estrogen receptor alpha (ERα) in the brain resulting from long-term hormone deprivation as a basis for the existence of the critical period. A unifying model is proposed by which the presence or absence of estrogens during a critical period following the cessation of ovarian function permanently alters the system resulting in decreased or increased risk, respectively, of neurodegeneration and cognitive decline.

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.

Progesterone-induced neuroprotection: factors that may predict therapeutic efficacy

Both progesterone and estradiol have well-described neuroprotective effects against numerous insults in a variety of cell culture models, animal models and in humans. However, the efficacy of these hormones may depend on a variety of factors, including the type of hormone used (ex. progesterone versus medroxyprogesterone acetate), the duration of the postmenopausal period prior to initiating the hormone intervention, and potentially, the age of the subject. The latter two factors relate to the proposed existence of a "window of therapeutic opportunity" for steroid hormones in the brain. While such a window of opportunity has been described for estrogen, there is a paucity of information to address whether such a window of opportunity exists for progesterone and its related progestins. Here, we review known cellular mechanisms likely to underlie the protective effects of progesterone and furthermore, describe key differences in the neurobiology of progesterone and the synthetic progestin, medroxyprogesterone acetate (MPA). Based on the latter, we offer a model that defines some of the key cellular and molecular players that predict the neuroprotective efficacy of progesterone. Accordingly, we suggest how changes in the expression or function of these cellular and molecular targets of progesterone with age or prolonged duration of hormone withdrawal (such as following surgical or natural menopause) may impact the efficacy of progesterone. This article is part of a Special Issue entitled Hormone Therapy.

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.

Estrogen promotes learning-related plasticity by modifying the synaptic cytoskeleton

Estrogen's acute, facilitatory effects on glutamatergic transmission and long-term potentiation (LTP) provide a potential explanation for the steroid's considerable influence on behavior. Recent work has identified mechanisms underlying these synaptic actions. Brief infusion of 17ß-estradiol (E2) into adult male rat hippocampal slices triggers actin polymerization within dendritic spines via a signaling cascade beginning with the GTPase RhoA and ending with inactivation of the filament-severing protein cofilin. Blocking this sequence, or actin polymerization itself, eliminates E2's effects on synaptic physiology. Notably, the theta burst stimulation used to induce LTP activates the same signaling pathway as E2 plus events that stabilize the reorganization of the sub-synaptic cytoskeleton. These observations suggest that E2 elicits a partial form of LTP, resulting in an increase of fast excitatory postsynaptic potentials (EPSPs) and a reduction in the threshold for lasting synaptic changes. While E2's effects on the cytoskeleton could be direct, results described here indicate that the hormone activates synaptic tropomyosin-related kinase B (TrkB) receptors for brain-derived neurotrophic factor (BDNF), a releasable neurotrophin that stimulates the RhoA to cofilin pathway. It is therefore possible that E2 acts via transactivation of neighboring receptors to modify the composition and structure of excitatory contacts. Finally, there is the question of whether a loss of acute synaptic actions contributes to the memory problems associated with estrogen depletion. Initial tests found that ovariectomy in middle-aged rats disrupts RhoA signaling, actin polymerization, and LTP consolidation. Acute applications of E2 reversed these defects, a result consistent with the idea that disturbances to actin management are one cause of behavioral effects that emerge with reductions in steroid levels.

Interactions between estradiol, BDNF and dendritic spines in promoting memory

Several lines of evidence have converged to indicate that memory formation involves plasticity of dendritic spines in the medial prefrontal cortex (PFC) and the hippocampus. Memory varies with estrogen levels throughout the lifespan of the female. Generally, increased levels of estrogen are related to greater dendritic spine density on pyramidal cells in the PFC and the hippocampus and to improved memory function. Brain-derived neurotrophic factor (BDNF) is a growth factor which increases dendritic spines and enhances memory function. Estrogens increase BDNF levels in the PFC and the hippocampus. In the present review we provide evidence that estradiol and BDNF may work in concert to enhance cognition. In adult females, fluctuations in recognition memory following ovariectomy and estradiol replacement, during the estrous cycle, in pregnancy and with aging are accompanied by similar changes in circulating estradiol, BDNF levels and spine density alterations in the PFC and the hippocampus. In addition, both estradiol and BDNF induce spine plasticity via rapid membrane effects and slower transcriptional regulation via the CREB pathway. Moreover, estradiol increases BDNF levels through action on nuclear receptors. While the exact mechanism(s) for the influence of estrogens and BDNF on memory remain unclear, this combination may provide the basis for new and more effective strategies for treating age-related and neurodegenerative memory loss.

Estradiol-induced increase in novel object recognition requires hippocampal NR2B-containing NMDA receptors

17β-estradiol (E2), at high circulating levels, enhances learning and memory in many women, making it a clinical treatment for hormone-related cognitive decline in aging. However, the mechanisms stimulated by E2, which are responsible for its cognitive enhancing effects, remain incompletely defined. Using an ovariectomized rat model, we previously reported that increasing plasma E2 enhances the magnitude of long-term potentiation (LTP) at hippocampal CA3-CA1 synapses, which is caused by a selective increase in current mediated by NR2B-containing NMDARs, leading to an increase in the NMDAR/AMPAR ratio. Whether the increase in NR2B current is causally related to the ability of E2 to enhance hippocampal dependent learning and memory has yet to be tested. Here, we find that E2 enhances performance in the novel object recognition (NOR) task with the same time course we previously showed E2 enhances the LTP magnitude, temporally linking the increase in LTP to enhanced learning and memory. Furthermore, using the selective NR2B subunit antagonist Ro25-6981, we find that the E2-enhanced NOR, like the enhanced LTP, requires hippocampal NR2B-containing NMDARs, specifically in area CA1. Finally, using whole-cell recordings and the phosphatase inhibitor orthovanadate, we investigated whether the E2-induced increase in NMDAR current is caused by an increase in the density of synaptic NMDARs and/or an increase in NMDAR subunit phosphorylation. We find that both mechanisms are responsible for the enhanced NMDAR current in E2-treated rats. Our results show that the E2-enhanced NOR requires a functional increase in NR2B-containing NMDARs, a requirement shared with the E2-enhanced LTP magnitude at CA3-CA1 synapses, supporting the hypothesis that the increase in LTP likely contributes to the enhanced learning and memory following an increase in plasma E2 levels.

Low doses of 17β-estradiol rapidly improve learning and increase hippocampal dendritic spines

While a great deal of research has been performed on the long-term genomic actions of estrogens, their rapid effects and implications for learning and memory are less well characterized. The often conflicting results of estrogenic effects on learning and memory may be due to complex and little understood interactions between genomic and rapid effects. Here, we investigated the effects of low, physiologically relevant, doses of 17β-estradiol on three different learning paradigms that assess social and non-social aspects of recognition memory and spatial memory, during a transcription independent period of memory maintenance. Ovariectomized female CD1 mice were subcutaneously administered vehicle, 1.5 μg/kg, 2 μg/kg, or 3 μg/kg of 17β-estradiol 15 minutes before social recognition, object recognition, or object placement learning. These paradigms were designed to allow the testing of learning effects within 40 min of hormone administration. In addition, using a different set of ovariectomized mice, we examined the rapid effects of 1.5 μg/kg, 2 μg/kg, or 3 μg/kg of 17β-estradiol on CA1 hippocampal dendritic spines. All 17β-estradiol doses tested impacted learning, memory, and CA1 hippocampal spines. 17β-Estradiol improved both social and object recognition, and may facilitate object placement learning and memory. In addition, 17β-estradiol increased dendritic spine density in the stratum radiatum subregion of the CA1 hippocampus, but did not affect dendritic spines in the lacunosum-moleculare, within 40 min of administration. These results demonstrate that the rapid actions of 17β-estradiol have important implications for general learning and memory processes that are not specific for a particular type of learning paradigm. These effects may be mediated by the rapid formation of new dendritic spines in the hippocampus.

Gonadectomy differentially regulates steroid receptor coactivator-1 and synaptic proteins in the hippocampus of adult female and male C57BL/6 mice

Hippocampus is one of the most important structures that mediates learning and memory, cognition, and mental behaviors and profoundly regulated by sex hormones in a sex-specific manner, but the mechanism of underlying sex differences regulation is still unclear. We have previously reported that in the male and female mice, steroid receptor coactivator-1 (SRC-1) and some key synaptic proteins share similar developmental profile in the hippocampus, but how circulating sex hormones affect hippocampal SRC-1 as well as these synaptic proteins remain unclear. In this study, we examined how gonad sex hormones regulate hippocampal SRC-1, synaptophysin, PSD-95, and AMPA receptor subtype GluR1 by using immunohistochemistry and Western blot. The results showed that in the female mice, ovariectomy affected hippocampal SRC-1 and GluR1 were only detected at 2 weeks post operation, then it recovered to sham level; synaptophysin was unaffected at any timepoint examined; significant decrease of PSD-95 was only detected at 4 weeks post operation. However, in the male hippocampus, SRC-1 and PSD-95 were decreased from one week and lasted to 4 weeks after orchidectomy, GluR1 decreased from 2 weeks after orchidectomy, but synaptophysin remained unchanged as in the females. Correlation analysis showed the profiles of SRC-1 were positively correlated with GluR1 of the females, PSD-95 and GluR1 of the males, respectively. The above results suggested a distinct regulatory mode between female and male gonad hormones in the regulation of hippocampal SRC-1 and synaptic proteins, which may be one of the mechanisms contributing to the dimorphism of hippocampus during development and ageing.

Estradiol attenuates ischemia-induced death of hippocampal neurons and enhances synaptic transmission in aged, long-term hormone-deprived female rats

Background

Transient global forebrain ischemia causes selective, delayed death of hippocampal CA1 pyramidal neurons, and the ovarian hormone 17β-estradiol (E2) reduces neuronal loss in young and middle-aged females. The neuroprotective efficacy of E2 after a prolonged period of hormone deprivation is controversial, and few studies examine this issue in aged animals given E2 treatment after induction of ischemia.

Methodology/Principal Findings

The present study investigated the neuroprotective effects of E2 administered immediately after global ischemia in aged female rats (15–18 months) after 6 months of hormone deprivation. We also used electrophysiological methods to assess whether CA1 synapses in the aging hippocampus remain responsive to E2 after prolonged hormone withdrawal. Animals were ovariohysterectomized and underwent 10 min global ischemia 6 months later. A single dose of E2 (2.25 µg) infused intraventricularly after reperfusion significantly increased cell survival, with 45% of CA1 neurons surviving vs 15% in controls. Ischemia also induced moderate loss of CA3/CA4 pyramidal cells. Bath application of 1 nM E2 onto brain slices derived from non-ischemic aged females after 6 months of hormone withdrawal significantly enhanced excitatory transmission at CA1 synapses evoked by Schaffer collateral stimulation, and normal long-term potentiation (LTP) was induced. The magnitude of LTP and of E2 enhancement of field excitatory postsynaptic potentials was indistinguishable from that recorded in slices from young rats.

Conclusions/Significance

The data demonstrate that 1) acute post-ischemic infusion of E2 into the brain ventricles is neuroprotective in aged rats after 6 months of hormone deprivation; and 2) E2 enhances synaptic transmission in CA1 pyramidal neurons of aged long-term hormone deprived females. These findings provide evidence that the aging hippocampus remains responsive to E2 administered either in vivo or in vitro even after prolonged periods of hormone withdrawal.

New Neurons in Aging Brains: Molecular Control by Small Non-Coding RNAs

Adult neurogenesis generates functional neurons from neural stem cells present in specific brain regions. It is largely confined to two main regions: the subventricular zone of the lateral ventricle, and the subgranular zone of the dentate gyrus (DG), in the hippocampus. With age, the function of the hippocampus and particularly the DG is impaired. For instance, adult neurogenesis is decreased with aging, in both proliferating and differentiation of newborn cells, while in parallel an age-associated decline in cognitive performance is often seen. Surprisingly, the synaptogenic potential of adult-born neurons is only marginally influenced by aging. Therefore, although proliferation, differentiation, and synaptogenesis of adult-born new neurons in the DG are closely related to each other, they are differentially affected by aging. In this review we discuss the crucial roles of a novel class of recently discovered modulators of gene expression, the small non-coding RNAs, in the regulation of adult neurogenesis. Multiple small non-coding RNAs are differentially expressed in the hippocampus. In particular a subgroup of the small non-coding RNAs, the microRNAs, fine-tune the progression of adult neurogenesis. This makes small non-coding RNAs appealing candidates to orchestrate the functional alterations in adult neurogenesis and cognition associated with aging. Finally, we summarize observations that link changes in circulating levels of steroid hormones with alterations in adult neurogenesis, cognitive decline, and vulnerability to psychopathology in advanced age, and discuss a potential interplay between steroid hormone receptors and microRNAs in cognitive decline in aging individuals.

Oestrogen signalling and neuroprotection in cerebral ischaemia

17β-Oestradiol (E(2)) is an important hormone signal that regulates multiple tissues and functions in the body. This review focuses on the neuroprotective actions of E(2) in the brain against cerebral ischaemia and the potential underlying mechanisms. A particular focus of the review will be on the role of E(2) to attenuate NADPH oxidase activation, superoxide and reactive oxygen species generation and reduce oxidative stress in the ischaemic brain as a potentially key neuroprotective mechanism. Evidence of a potential novel role of extranuclear oestrogen receptors in mediating E(2) signalling and neuroprotective actions is also discussed. An additional subject is the growing evidence indicating that periods of long-term oestrogen deprivation, such as those occurring after menopause or surgical menopause, may lead to loss or attenuation of E(2) signalling and neuroprotective actions in the brain, as well as enhanced sensitivity of the hippocampus to ischaemic stress damage. These findings have important implications with respect to the 'critical period hypothesis', which proposes that oestrogen replacement must be initiated at peri-menopause in humans to exert its beneficial cardiovascular and neural effects. The insights gained from these various studies will prove valuable for guiding future directions in the field.

Estrogen neuroprotection and the critical period hypothesis

17β-Estradiol (estradiol or E2) is implicated as a neuroprotective factor in a variety of neurodegenerative disorders. This review focuses on the mechanisms underlying E2 neuroprotection in cerebral ischemia, as well as emerging evidence from basic science and clinical studies, which suggests that there is a "critical period" for estradiol's beneficial effect in the brain. Potential mechanisms underlying the critical period are discussed, as are the neurological consequences of long-term E2 deprivation (LTED) in animals and in humans after natural menopause or surgical menopause. We also summarize the major clinical trials concerning postmenopausal hormone therapy (HT), comparing their outcomes with respect to cardiovascular and neurological disease and discussing their relevance to the critical period hypothesis. Finally, potential caveats, controversies and future directions for the field are highlighted and discussed throughout the review.

Chronic stress and a cyclic regimen of estradiol administration separately facilitate spatial memory: relationship with hippocampal CA1 spine density and dendritic complexity

This study investigated the effects of chronic restraint stress and repeated cyclic estradiol pulses on hippocampal CA3 and CA1 dendritic and/or spine morphology and spatial memory in female rats. Sprague-Dawley adult female rats were ovariectomized and then injected over 2 days with 17β-estradiol (10 μg, s.c.), which was repeated every 4-5 days. While all rats received similar estradiol injection histories, half of the rats were chronically restrained and/or given a final cyclic pulse of estradiol prior to testing on a hippocampal-dependent object placement (OP) task to assess spatial memory. OP testing was performed 2 days after the last restraint session, as well as when the last 2 estradiol pulses best captured the maximal effect on hippocampal CA1 spine density. The data revealed several novel findings: (a) chronic stress or estradiol separately facilitated spatial memory, but did not have the same effects when coadministered, (b) CA1 spine densities negatively correlated with spatial memory, and (c) repeated estradiol pulses failed to prevent stress-induced CA3 dendritic retraction. We also corroborated previous studies showing increased CA1 spine density following estradiol, chronic stress, and behavioral manipulations. The present study uniquely combined chronic stress, repeated estradiol pulses, hippocampal morphology, and behavior within the same animals, allowing for correlational analyses to be performed between CA1 spine morphology and spatial memory. We demonstrate novel findings that chronic stress or estradiol pulses independently facilitate spatial memory, but not when coadministered, and that these effects may involve a balance of CA1 apical spine expression that is independent of CA3 dendritic complexity.

Proteomic analysis of zebrafish brain tissue following exposure to the pesticide prochloraz

The hypothalamus-pituitary-gonadal (HPG) axis plays a central role in the maintenance of homeostasis and disruptions in its function can have important implications for reproduction and other critical biological processes. A number of compounds found in aquatic environments are known to affect the HPG axis. In the present study, we used two-dimensional electrophoresis to investigate the proteome of female and male zebrafish brain after 96 h exposure to the fungicide prochloraz. Prochloraz has known effects on a number of key HPG molecules, including antagonism of Cyp17 and Cyp19 (aromatase). Twenty-eight proteins were shown to be differentially expressed in the brains of females and 22 in males. Proteins were identified using LC-MS/MS and identities were examined relative to brain function in the context of changing steroid hormone levels. There was little overlap between sexes in proteins exhibiting differential expression. Proteins with known roles in metabolism, learning, neuroprotection, and calcium regulation were determined to be differentially regulated. Relationships between identified proteins were also examined using Ingenuity Pathway Analysis, and females were shown to exhibit enrichment of several metabolic pathways. We used differentially expressed proteins to establish a putative classifier consisting of three proteins that was able to discriminate prochloraz-exposed from control females. Putatively impacted brain functions and specific protein changes that were observed have the potential to be generalized to other that similarly impact steroid hormone levels.

C terminus of Hsc70-interacting protein (CHIP)-mediated degradation of hippocampal estrogen receptor-alpha and the critical period hypothesis of estrogen neuroprotection

Recent work suggests that timing of 17β-estradiol (E2) therapy may be critical for observing a beneficial neural effect. Along these lines, E2 neuroprotection, but not its uterotropic effect, was shown to be lost following long-term E2 deprivation (LTED), and this effect was associated with a significant decrease of estrogen receptor-α (ERα) in the hippocampus but not the uterus. The purpose of the current study was to determine the mechanism underlying the ERα decrease and to determine whether aging leads to a similar loss of hippocampal ERα and E2 sensitivity. The results of the study show that ERα in the rat hippocampal CA1 region but not the uterus undergoes enhanced interaction with the E3 ubiquitin ligase C terminus of heat shock cognate protein 70 (Hsc70)-interacting protein (CHIP) that leads to its ubiquitination/proteasomal degradation following LTED (10-wk ovariectomy). E2 treatment initiated before but not after LTED prevented the enhanced ERα-CHIP interaction and ERα ubiquitination/degradation and was fully neuroprotective against global cerebral ischemia. Administration of a proteasomal inhibitor or CHIP antisense oligonucleotides to knock down CHIP reversed the LTED-induced down-regulation of ERα. Further work showed that these observations extended to natural aging, because aged rats showed enhanced CHIP interaction; ubiquitination and degradation of both hippocampal ERα and ERβ; and, importantly, a correlated loss of E2 neuroprotection against global cerebral ischemia. In contrast, E2 administration to middle-aged rats was still capable of exerting neuroprotection. As a whole, the study provides support for a "critical period" for E2 neuroprotection of the hippocampus and provides important insight into the mechanism underlying the critical period.

Donepezil plus estradiol treatment enhances learning and delay-dependent memory performance by young ovariectomized rats with partial loss of septal cholinergic neurons

Effects of estrogen therapy on cognitive performance appear to diminish with age and time following the loss of ovarian function. We hypothesize that this is due to a reduction in basal forebrain cholinergic function and that treatment with a cholinergic enhancer can reverse the effect. This study tested whether combining the cholinesterase inhibitor donepezil with estradiol treatment can enhance/restore estradiol effects on cognitive performance in young ovariectomized rats with selective lesions of septal cholinergic neurons. 192IgG-saporin was injected directly into the medial septum to produce selective cholinergic lesions. Rats were then treated with donepezil (Don, daily injections of 3mg/kg/day, i.p.) or vehicle, and then with 17β-estradiol (E2, administered by silastic capsule implanted s.c.) or an empty capsule. Rats were trained on a delayed matching-to-position (DMP) T-maze task which previous studies have shown is sensitive to ovariectomy and estrogen replacement. Results show that neither estradiol nor donepezil alone significantly enhanced acquisition of the DMP task in rats with cholinergic lesions. Combination therapy was effective, however, depending on the severity of the lesion. Don+E2 significantly enhanced acquisition of the task in rats with partial lesions (<50% loss of cholinergic neurons), but not in rats with severe lesions. This effect was due largely to a reduction in perseverative behavior. Don+E2 also improved working memory in rats with partial lesions, as evidenced by significantly better performance than controls during increased intertrial delays. These findings suggest that even partial loss of septal cholinergic neurons can reduce effects of estrogen therapy on cognitive performance, and demonstrate that combining a cholinesterase inhibitor with estrogen therapy can help to restore beneficial effects on performance. We propose that combination therapy may have similar beneficial effects in women, particularly in older women who have not used estrogen therapy for many years and are beginning to show signs of cognitive impairment or early Alzheimer's disease.

Accelerated ovarian failure: a novel, chemically induced animal model of menopause

Current rodent models of menopause fail to adequately recapitulate the menopause transition. The intact aging model fails to achieve very low estrogen levels, and the ovariectomy model lacks a perimenopause phase. A new rodent model of accelerated ovarian failure (AOF) successfully replicates human perimenopause and postmenopause, including estrous acyclicity and fluctuating, followed by undetectable, estrogen levels, and allows for the dissociation of the effects of hormone levels from the effects of aging. In this model, an ovotoxic chemical, 4-vinylcyclohexene diepoxide (VCD), selective for primary and primordial follicles, is injected intraperitonelly in animals for 15 days. As the mature follicle population is depleted through natural cycling, ovarian failure follows increasing periods of acyclity. Administered at low doses, VCD specifically causes apoptotic cell death of primordial follicles but does not affect other peripheral tissues, including the liver and spleen, nor does it affect brain inflammation markers. In addition to reducing confounds associated with genetic and surgical manipulations, the AOF model maintains the presence of ovarian tissue which importantly parallels to the menopause transition in humans. The VCD injection procedure can be applied to studies using transgenic or knockout mice strains, or in other disease-state models (e.g., ischemia, atherosclerosis, or diabetes). This AOF model of menopause will generate new insights into women's health particularly in determining the critical periods (i.e., a window of opportunity) during perimenopause for restoring ovarian hormones for the most efficacious effect on memory and mood disorders as well as other menopausal symptoms.

Chronic treatment with the nitric oxide synthase inhibitor, L-NAME, attenuates estradiol-mediated improvement of learning and memory in ovariectomized rats

INTRODUCTION

The role of ovarian hormones and nitric oxide in learning and memory has been widely investigated.

OBJECTIVE

The present study was carried out to evaluate the effect of the nitric oxide synthase (NOS) inhibitor, N (G)-nitro-L-arginine methyl ester (L-NAME), on the ability of estradiol to improve learning in OVX rats using the Morris water maze.

METHODS

Forty rats were divided into five groups: (1) ovariectomized (OVX), (2) ovariectomized-estradiol (OVX-Est), (3) ovariectomized-L-NAME 10 (OVX-LN 10), (4) ovariectomized-L-NAME 50 (OVX-LN 50) and (5) ovariectomized-estradiol-L-NAME 50 (OVX-Est-LN 50). The animals in the OVX-Est group were treated with a weekly injection of estradiol valerate (2 mg/kg; i.m.). The OVX-LN 10 and OVX-LN 50 groups were treated with daily injections of 10 and 50 mg/kg L-NAME (i.p.), respectively. The animals in the OVX-Est-LN 50 group received a weekly injection of estradiol valerate and a daily injection of 50 mg/kg L-NAME. After 8 weeks, all animals were tested in the Morris water maze.

RESULTS

The animals in the OVX-Est group had a significantly lower latency in the maze than the OVX group (p<0.001). There was no significant difference in latency between the OVX-LN 10 and OVX-LN 50 groups in comparison with the OVX group. The latency in the OVX-Est-LN 50 group was significantly higher than that in the OVX-Est group (p<0.001).

CONCLUSION

These results show that L-NAME treatment attenuated estradiol-mediated enhancement of spatial learning and memory in OVX rats, but it had no significant effect in OVX rats without estrogen, suggesting an interaction of nitric oxide and estradiol in these specific brain functions.