Length of postovariectomy interval and age, but not estrogen replacement, regulate N-methyl-D-aspartate receptor mRNA levels in the hippocampus of female rats

Effects of estrogen and aging on synaptic morphology and distribution of phosphorylated Tyr1472 NR2B in the female rat hippocampus

Age and estrogens may impact the mobility of N-methyl-D-aspartate receptors (NMDARs) in hippocampal synapses. Here, we used serial section immunogold electron microscopy to examine whether phosphorylated tyrosine 1472 NR2B (pY1472), which is involved in the surface expression of NMDARs, is altered in the dorsal hippocampus of young (3-4 months old) and aged (∼24 months old) ovariectomized rats treated with 17β-estradiol or vehicle for 2 days. The number of gold particles labeling pY1472 was higher in presynaptic and postsynaptic compartments of aged rats with low estradiol (vehicle-treated) compared to other groups. In terminals, pY1472 levels were elevated in aged rats but reduced by estradiol treatment to levels seen in young rats. Conversely, the mitochondria number was lower in aged females but was restored to young levels by estradiol. In the postsynaptic density and dendritic spines, estradiol reduced pY1472 in young and aged rats. As phosphorylation at Y1472 blocks NR2B endocytosis, reduction of pY1472 by estradiol suggests another mechanism through which estrogen enhances synaptic plasticity by altering localization of NMDAR subunits within synapses.

Re-Opening the Critical Window for Estrogen Therapy

A decline in estradiol (E2)-mediated cognitive benefits denotes a critical window for the therapeutic effects of E2, but the mechanism for closing of the critical window is unknown. We hypothesized that upregulating the expression of estrogen receptor α (ERα) or estrogen receptor β (ERβ) in the hippocampus of aged animals would restore the therapeutic potential of E2 treatments and rejuvenate E2-induced hippocampal plasticity. Female rats (15 months) were ovariectomized, and, 14 weeks later, adeno-associated viral vectors were used to express ERα, ERβ, or green fluorescent protein (GFP) in the CA1 region of the dorsal hippocampus. Animals were subsequently treated for 5 weeks with cyclic injections of 17β-estradiol-3-benzoate (EB, 10 μg) or oil vehicle. Spatial memory was examined 48 h after EB/oil treatment. EB treatment in the GFP (GFP + EB) and ERβ (ERβ + EB) groups failed to improve episodic spatial memory relative to oil-treated animals, indicating closing of the critical window. Expression of ERβ failed to improve cognition and was associated with a modest learning impairment. Cognitive benefits were specific to animals expressing ERα that received EB treatment (ERα + EB), such that memory was improved relative to ERα + oil and GFP + EB. Similarly, ERα + EB animals exhibited enhanced NMDAR-mediated synaptic transmission compared with the ERα + oil and GFP + EB groups. This is the first demonstration that the window for E2-mediated benefits on cognition and hippocampal E2 responsiveness can be reinstated by increased expression of ERα.

SIGNIFICANCE STATEMENT

Estradiol is neuroprotective, promotes synaptic plasticity in the hippocampus, and protects against cognitive decline associated with aging and neurodegenerative diseases. However, animal models and clinical studies indicate a critical window for the therapeutic treatment such that the beneficial effects are lost with advanced age and/or with extended hormone deprivation. We used gene therapy to upregulate expression of the estrogen receptors ERα and ERβ and demonstrate that the window for estradiol's beneficial effects on memory and hippocampal synaptic function can be reinstated by enhancing the expression of ERα. Our findings suggest that the activity of ERα controls the therapeutic window by regulating synaptic plasticity mechanisms involved in memory.

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.

Long-term ovariectomy increases BDNF gene methylation status in mouse hippocampus

Estradiol (E) has been suggested to have a neuroprotective effect in young animals but has neutral or harmful effects when it is administered to aged animals. In the present study, we determined whether the post-ovariectomy (post-OVX) timeframe elapsed before the initiation of chronic E treatment is critical for the estrogenic induction of neurotrophins (brain-derived neurotrophic factor, BDNF, and synaptophysin, SYN) in the rodent hippocampus. Adult mice were OVX and, a short period (short-term E (STE) animals) or a long period (long-term E (LTE) animals) after the OVX, were daily treated with E. Control animals were treated with sesame oil (short-term control (STC) and long-term control (LTC) animals). Protein expression was determined using an immunohistochemical approach. Transcriptional activity in the hippocampus of individual BDNF promoters was assessed by real-time quantitative RT-PCR, and the methylation levels of regulatory regions were analyzed by methylation-specific PCR and combined bisulfite restriction analysis. STE animals showed increased BDNF and SYN protein expression and a higher activity of BDNF II, IV, and V promoters. In contrast, LTE animals did not show E induction of neurotrophins. In these animals, the methylation levels of regulatory sequences of the BDNF were higher than in the STE animals in a CpG island of promoter V and in the CRE regulatory site located in promoter IV. With this experiment, we determined that a prolonged period of hypoestrogenicity disrupts the E-induction of neurotrophins, and we postulated that DNA methylation is one of the epigenetic mechanisms that could explain the E-insensitivity of the BDNF after a long period post-OVX.

The influences of reproductive status and acute stress on the levels of phosphorylated delta opioid receptor immunoreactivity in rat hippocampus

In the hippocampus, ovarian hormones and sex can alter the trafficking of delta opioid receptors (DORs) and the proportion of DORs that colocalize with the stress hormone, corticotropin releasing factor. Here, we assessed the effects of acute immobilization stress (AIS) and sex on the phosphorylation of DORs in the rat hippocampus. We first localized an antibody to phosphorylated DOR (pDOR) at the SER363 carboxy-terminal residue, and demonstrated its response to an opioid agonist. By light microscopy, pDOR-immunoreactivity (ir) was located predominantly in CA2/CA3a pyramidal cell apical dendrites and in interneurons in CA1-3 stratum oriens and the dentate hilus. By electron microscopy, pDOR-ir primarily was located in somata and dendrites, associated with endomembranes, or in dendritic spines. pDOR-ir was less frequently found in mossy fibers terminals. Quantitative light microscopy revealed a significant increase in pDOR-ir in the CA2/CA3a region of male rats 1h following an injection of the opioid agonist morphine (20mg/kg, I.P). To look at the effects of stress on pDOR, we compared pDOR-ir in males and cycling females after AIS. The level of pDOR-ir in stratum radiatum of CA2/CA3a was increased in control estrus (elevated estrogen and progesterone) females compared to proestrus and diestrus females and males. However, immediately following 30min of AIS, no significant differences in pDOR levels were seen across estrous cycle phase or sex. These findings suggest that hippocampal levels of phosphorylated DORs vary with estrous cycle phase and that acute stress may dampen the differential effects of hormones on DOR activation in females.

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.

Aging alters the expression of genes for neuroprotection and synaptic function following acute estradiol treatment

This study used microarray analysis to examine age-related changes in gene expression 6 and 12 h following a single estradiol injection in ovariectomized mice. Estradiol-responsive gene expression at the 6 h time point was reduced in aged (18 months) animals compared with young (4 months) and middle-aged (MA, 12 months) mice. Examination of gene clustering within biological and functional pathways indicated that young and MA mice exhibited increased expression of genes for cellular components of the synapse and decreased expression of genes related to oxidative phosphorylation and mitochondrial dysfunction. At the 12 h time point, estradiol-responsive gene expression increased in aged animals and decreased in young and MA mice compared with the 6 h time point. Gene clustering analysis indicated that aged mice exhibited increased expression of genes for signaling pathways that are rapidly influenced by estradiol. The age differences in gene expression for rapid signaling pathways may relate to disparity in basal pathway activity and estradiol mediated activation of rapid signaling cascades.

Delta opioid receptors colocalize with corticotropin releasing factor in hippocampal interneurons

The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect, likely playing a critical role in the interaction between stress and drug addiction. Prior study findings suggest that the stress-related neuropeptide corticotropin releasing factor (CRF) and the delta opioid receptor (DOR) may localize to similar neuronal populations within HF lamina. Here, hippocampal sections of male and cycling female adult Sprague-Dawley rats were processed for immunolabeling using antisera directed against the DOR and CRF peptide, as well as interneuron subtype markers somatostatin or parvalbumin, and analyzed by fluorescence and electron microscopy. Both DOR- and CRF-labeling was observed in interneurons in the CA1, CA3, and dentate hilus. Males and normal cycling females displayed a similar number of CRF immunoreactive neurons co-labeled with DOR and a similar average number of CRF-labeled neurons in the dentate hilus and stratum oriens of CA1 and CA3. In addition, 70% of DOR/CRF dual-labeled neurons in the hilar region co-labeled with somatostatin, suggesting a role for these interneurons in regulating perforant path input to dentate granule cells. Ultrastructural analysis of CRF-labeled axon terminals within the hilar region revealed that proestrus females have a similar number of CRF-labeled axon terminals that contain DORs compared to males but an increased number of CRF-labeled axon terminals without DORs. Taken together, these findings suggest that while DORs are anatomically positioned to modulate CRF immunoreactive interneuron activity and CRF peptide release, their ability to exert such regulatory activity may be compromised in females when estrogen levels are high.

Ovarian hormone deficiency reduces intrinsic excitability and abolishes acute estrogen sensitivity in hippocampal CA1 pyramidal neurons

Premature and uncompensated loss of ovarian hormones following ovariectomy (OVX) elevates the risks of cognitive impairment and dementia. These risks are prevented with estrogen (E(2))-containing hormone replacement therapy initiated shortly following OVX but not after substantial delay. Currently, the cellular bases underlying these clinical findings are unknown. At the cellular level, intrinsic membrane properties regulate the efficiency of synaptic inputs to initiate output action potentials (APs), thereby affecting neuronal communication, hence cognitive processing. This study tested the hypothesis that in CA1 pyramidal neurons, intrinsic membrane properties and their acute regulation by E(2) require ovarian hormones for maintenance. Whole-cell current-clamp recordings were performed on neurons from ∼ 7-month-old OVX rats that experienced either short-term (10 d, control OVX) or long-term (5 months, OVX(LT)) ovarian hormone deficiency. The results reveal that long-term hormone deficiency reduced intrinsic membrane excitability (IE) as measured by the number of evoked APs and firing duration for a given current injection. This was accompanied by AP broadening, an increased slow afterhyperpolarization (sAHP), and faster accumulation of Na(V) channel inactivation during repetitive firing. In the control OVX neurons, E(2) acutely increased IE and reduced the sAHP. In contrast, acute regulation of IE by E(2) was absent in the OVX(LT) neurons. Since the degree of IE of hippocampal pyramidal neurons is positively related with hippocampus-dependent learning ability, and modulation of IE is observed following successful learning, these findings provide a framework for understanding hormone deficiency-related cognitive impairment and the critical window for therapy initiation.

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.

Hormonal regulation of delta opioid receptor immunoreactivity in interneurons and pyramidal cells in the rat hippocampus

Clinical and preclinical studies indicate that women and men differ in relapse vulnerability to drug-seeking behavior during abstinence periods. As relapse is frequently triggered by exposure of the recovered addict to objects previously associated with drug use and the formation of these associations requires memory systems engaged by the hippocampal formation (HF), studies exploring ovarian hormone modulation of hippocampal function are warranted. Previous studies revealed that ovarian steroids alter endogenous opioid peptide levels and trafficking of mu opioid receptors in the HF, suggesting cooperative interaction between opioids and estrogens in modulating hippocampal excitability. However, whether ovarian steroids affect the levels or trafficking of delta opioid receptors (DORs) in the HF is unknown. Here, hippocampal sections of adult male and normal cycling female Sprague-Dawley rats were processed for quantitative immunoperoxidase light microscopy and dual label fluorescence or immunoelectron microscopy using antisera directed against the DOR and neuropeptide Y (NPY). Consistent with previous studies in males, DOR-immunoreactivity (-ir) localized to select interneurons and principal cells in the female HF. In comparison to males, females, regardless of estrous cycle phase, show reduced DOR-ir in the granule cell layer of the dentate gyrus and proestrus (high estrogen) females, in particular, display reduced DOR-ir in the CA1 pyramidal cell layer. Ultrastructural analysis of DOR-labeled profiles in CA1 revealed that while females generally show fewer DORs in the distal apical dendrites of pyramidal cells, proestrus females, in particular, exhibit DOR internalization and trafficking towards the soma. Dual label studies revealed that DORs are found in NPY-labeled interneurons in the hilus, CA3, and CA1. While DOR colocalization frequency in NPY-labeled neuron somata was similar between animals in the hilus, proestrus females had fewer NPY-labeled neurons that co-labeled with DOR in stratum oriens of CA1 and CA3 when compared to males. Ultrastructural analysis of NPY-labeled axon terminals within stratum radiatum of CA1 revealed that NPY-labeled axon terminals contain DORs that are frequently found at or near the plasma membrane. As no differences were noted by sex or estrous cycle phase, DOR activation on NPY-labeled axon terminals would inhibit GABA release probability equally in males and females. Taken together, these findings suggest that ovarian steroids can impact hippocampal function through direct effects on DOR levels and trafficking in principal cells and broad indirect effects through reductions in DOR-ir in NPY-labeled interneurons, particularly in CA1.

Age- and hormone-regulation of opioid peptides and synaptic proteins in the rat dorsal hippocampal formation

Circulating estrogen levels and hippocampal-dependent cognitive functions decline with aging. Moreover, the responses of hippocampal synaptic structure to estrogens differ between aged and young rats. We recently reported that estrogens increase levels of post-synaptic proteins, including PSD-95, and opioid peptides leu-enkephalin and dynorphin in the hippocampus of young animals. However, the influence of ovarian hormones on synaptic protein and opioid peptide levels in the aging hippocampus is understudied. Here, young (3- to 5-month-old), middle-aged (9- to 12-month-old), and aged (about 22-month-old) female rats were ovariectomized and then, 4 weeks later, subcutaneously implanted with a silastic capsule containing vehicle or 17β-estradiol. After 48 h, rats were subcutaneously injected with progesterone or vehicle and sacrificed 1 day later. Coronal sections through the dorsal hippocampus were processed for quantitative peroxidase immunohistochemistry of leu-enkephalin, dynorphin, synaptophysin, and PSD-95. With age, females showed opposing changes in leu-enkephalin and dynorphin levels in the mossy fiber pathway, particularly within the hilus, and regionally specific changes in synaptic protein levels. 17β-estradiol, with or without progesterone, altered leu-enkephalin levels in the dentate gyrus and synaptophysin levels in the CA1 of young but not middle-aged or aged females. Additionally, 17β-estradiol decreased synaptophysin levels in the CA3 of middle-aged females. Our results support and extend previous findings indicating 17β-estradiol modulation of hippocampal opioid peptides and synaptic proteins while demonstrating regional and age-specific effects. Moreover, they lend credence to the "window of opportunity" hypothesis during which hormone replacement can modulate hippocampal structure and circuitry to improve cognitive outcomes.

Effects of estrogen and aging on the synaptic distribution of phosphorylated Akt-immunoreactivity in the CA1 region of the female rat hippocampus

The estrogen 17β-estradiol (E) increases the axospinous synaptic density and plasticity in the hippocampal CA1 region of young female rats but fails to do so in aged female rats. This E stimulus on synaptic plasticity is associated with the phosphorylation-dependent activation of Akt kinase. Our previous findings demonstrated that increased estrogen levels subsequently increase phosphorylated Akt (pAkt)-immunoreactivity (-IR) within the dendritic shafts and spines of pyramidal neurons in young female rats. Therefore, because Akt can promote cell survival and growth, we tested the hypothesis that the less plastic synapses of aged female rats would contain less E-stimulated pAkt-IR. Here, young (3-4 months) and aged (22-23 months) female rats were ovariectomized 7 days prior to a 48-h administration of either vehicle or E. The pAkt-IR synaptic distribution was then analyzed using post-embedding electron microscopy. In both young and aged rats, pAkt-IR was found in dendritic spines and terminals, and pAkt-IR was particularly abundant at the post-synaptic density. Quantitative analyses revealed that the percentage of pAkt-labeled synapses was significantly greater in young rats compared to aged rats. Nonetheless, E treatment significantly increased pAkt-IR in pre- and post-synaptic profiles of both young and aged rats, although the stimulus in young rats was notably more widespread. These data support the evidence that hormone-activated signaling associated with cell growth and survival is diminished in the aged brain. However, the observation that E can still increase pAkt-IR in aged synapses presents this signaling component as a candidate target for hormone replacement therapies.

BDNF upregulation rescues synaptic plasticity in middle-aged ovariectomized rats

Brain-derived neurotrophic factor (BDNF) has emerged as a possible broad-spectrum treatment for the plasticity losses found in rodent models of human conditions associated with memory and cognitive deficits. We have tested this strategy in the particular case of ovariectomy. The actin polymerization in spines normally found after patterned afferent stimulation was greatly reduced, along with the stabilization of long-term potentiation, in hippocampal slices prepared from middle-aged ovariectomized rats. Both effects were fully restored by a 60-minute infusion of 2 nM BDNF. Comparable rescue results were obtained after elevating endogenous BDNF protein levels in hippocampus with 4 daily injections of a short half-life ampakine (positive modulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate [AMPA]-type glutamate receptors). These results provide the first evidence that minimally invasive, mechanism-based drug treatments can ameliorate defects in spine plasticity caused by depressed estrogen levels.

Selective Vulnerabilities of N-methyl-D-aspartate (NMDA) Receptors During Brain Aging

N-methyl-D-aspartate (NMDA) receptors are present in high density within the cerebral cortex and hippocampus and play an important role in learning and memory. NMDA receptors are negatively affected by aging, but these effects are not uniform in many different ways. This review discusses the selective age-related vulnerabilities of different binding sites of the NMDA receptor complex, different subunits that comprise the complex, and the expression and functions of the receptor within different brain regions. Spatial reference, passive avoidance, and working memory, as well as place field stability and expansion all involve NMDA receptors. Aged animals show deficiencies in these functions, as compared to young, and some studies have identified an association between age-associated changes in the expression of NMDA receptors and poor memory performance. A number of diet and drug interventions have shown potential for reversing or slowing the effects of aging on the NMDA receptor. On the other hand, there is mounting evidence that the NMDA receptors that remain within aged individuals are not always associated with good cognitive functioning. This may be due to a compensatory response of neurons to the decline in NMDA receptor expression or a change in the subunit composition of the remaining receptors. These studies suggest that developing treatments that are aimed at preventing or reversing the effects of aging on the NMDA receptor may aid in ameliorating the memory declines that are associated with aging. However, we need to be mindful of the possibility that there may also be negative consequences in aged individuals.

The multiple roles of estrogens and the enzyme aromatase

Aromatase is the enzyme that catalyzes the last step of estrogen biosynthesis. It is expressed in many tissues such as the gonads, brain and adipose tissue. The regulation of the level and activity of aromatase determines the levels of estrogens that have endocrine, paracrine and autocrine effects on tissues. Estrogens play many roles in the body, regulating reproduction, metabolism and behavior. In the brain, cell survival and the activity of neurons are affected by estrogens and hence aromatase.

Age- and hormone-regulation of N-methyl-D-aspartate receptor subunit NR2b in the anteroventral periventricular nucleus of the female rat: implications for reproductive senescence

Glutamate, acting through its N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the hypothalamus, regulates reproductive neuroendocrine functions via direct and indirect actions upon gonadotrophin-releasing hormone (GnRH) neurones. Previous studies indicate that the NMDA receptor subunit NR2b undergoes changes in protein and gene expression in the hypothalamus in general, and on GnRH neurones in particular, during reproductive ageing. In the present study, we examined whether the NR2b-expressing cell population, both alone and in association with the NR1 subunit (i.e. the latter subunit is necessary for a functional NMDA receptor), is altered as a function of age and ⁄ or steroid hormone treatment. Studies focused on the anteroventral periventricular (AVPV) nucleus of the hypothalamus, a region critically involved in the control of reproduction. Young (3-5 months), middle-aged (9-12 months), and aged (approximately 22 months) female rats were ovariectomised and, 1 month later, they were treated sequentially with oestradiol plus progesterone, oestradiol plus vehicle, or vehicle plus vehicle, then perfused. Quantitative stereologic analysis of NR2b-immunoreactive cell numbers in the AVPV showed an age-associated decrease in the density of NR2b-immunoreactive cells, but no effect of hormone treatment. In a second study, immunofluorescent double labelling of NR2b and NR1 was analysed by confocal microscopy of fraction volume, a semi-quantitative measure of fluorescence intensity. No effect of ageing was detected for immunofluorescent NR1 or NR2b alone, whereas the NR2b fraction volume increased in the oestradiol plus vehicle group. With ageing, the fraction volume of the NR2b/NR1-colocalised subunits increased. Together with the stereology results, this suggests that, although fewer cells express the NR2b subunit in the ageing AVPV, a greater percentage of these subunits are co-expressed with NR1. Our results suggest that the subunit composition of NMDA receptors in the AVPV undergo both age- and hormonal-regulation, which may be related to previous observations of changes in functional responses of reproductive neuroendocrine systems to NMDA receptor modulators with ageing.

Estradiol and lithium chloride specifically alter NMDA receptor subunit NR1 mRNA and excitotoxicity in primary cultures

Glutamate facilitates calcium influx via NMDAR, and excess calcium influx increases excitotoxicity--a pathological characteristic of neurological diseases. Both 17beta-estradiol (E2) and lithium influence NMDAR expression/signaling and excitotoxicity. This led us to hypothesize that combined E2 and lithium will alter NMDAR expression and excitotoxicity. We tested this hypothesis using primary cell cultures from the cortex and hippocampus of C57BL/6J fetal mice pretreated with E2, lithium chloride (LiCl) and combined E2/LiCl for 12, 24 or 48 h. We examined cultures for brain cell type and changes in cell type caused by experimental procedures using glia and neuron gene specific primers. These cultures expressed increased glial fibrillary acidic protein (GFAP) mRNA with low neurofilament-heavy chain (NF-H) mRNA expression. Subsequent analysis of cortical cell cultures indicated that combined E2/LiCl decreased NR1 mRNA expression after a 12 and 48 h treatment period. Combined E2/LiCl also reduced NR1 mRNA expression in hippocampal cultures but only after a 48 h treatment period. LiCl-treated hippocampal cultures also reduced NR1 mRNA expression after a 24 and 48 h treatment. We next examined the response of 48 h pretreated cultures to a toxic level of glutamate. Excitotoxicity was measured using fluorescein diacetate/propidium iodide (FDA/PI) cell viability assay. Results from FDA/PI assay revealed that LiCl pretreatment increased viability for cortical cultures while E2 and combined E2/LiCl reduced viability. All pretreatments for hippocampal cultures failed to increase viability. Our results showed combined E2/LiCl reduced NR1 mRNA and prevented protection against glutamate excitotoxicity in glial primary cultures.

Sex steroids and cognitive function

Gonadal hormones, most notably oestradiol, enhance some aspects of cognitive function in animal and human models. However, the demonstrated effects are often not large and inconsistent across studies. Nonetheless, because increased numbers of women are living longer in a state of oestrogen deprivation, research on this topic continues to be important. This review traces major developments concerning hormonal influences on cognition and provides some insights from recent studies that may be fruitful for future research.

Estrogen alters spinal NMDA receptor activity via a PKA signaling pathway in a visceral pain model in the rat

Pain symptoms in several chronic pain disorders in women, including irritable bowel syndrome, fluctuate with the menstrual cycle suggesting a gonadal hormone component. In female rats, estrogens modulate visceral sensitivity although the underlying mechanism(s) are unknown. In the present study the effects of 17-beta estradiol on N-methyl-D-aspartate (NMDA) receptor signaling of colorectal nociceptive processing in the spinal cord were examined. Estrogen receptor alpha and the NR1 subunit of the NMDA receptor are co-expressed in dorsal horn neurons, supporting a direct action of estradiol on NMDA receptors. Intrathecal administration of the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (APV) dose-dependently attenuated the visceromotor response with greater potency in ovariectomized (OVx) rats compared to OVx with estradiol replacement (E2) rats. Estradiol significantly increased protein expression of NR1 in the lumbosacral spinal cord compared to OVx rats. Colorectal distention significantly increased phosphorylation of NR1ser-897, a PKA phosphorylation site on the NR1 subunit in E2, but not OVx rats. Intrathecal administration of a PKA inhibitor significantly attenuated the visceromotor response, decreased NR1 phosphorylation and increased the potency of APV to attenuate the visceromotor response compared to vehicle-treated E2 rats. These data suggest that estradiol increases spinal processing of visceral nociception by increasing NMDA receptor NR1 subunit expression and increasing site-specific receptor phosphorylation on the NR1 subunit contributing to an increase in NMDA receptor activity.

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.

NMDA receptor subunit NR2b: effects on LH release and GnRH gene expression in young and middle-aged female rats, with modulation by estradiol

BACKGROUND/AIMS

The loss of reproductive capacity during aging involves changes in the neural regulation of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons controlling reproduction. This neuronal circuitry includes glutamate receptors on GnRH neurons. Previously, we reported an increase in the expression of the NR2b subunit protein of the NMDA receptor on GnRH neurons in middle-aged compared to young female rats. Here, we examined the functional implications of the NR2b subunit on the onset of reproductive aging, using an NR2b-specific antagonist ifenprodil.

METHODS

Young (3-5 months) and middle-aged (10-13 months) female rats were ovariectomized (OVX), 17beta-estradiol (E2) or vehicle (cholesterol) treated, and implanted with a jugular catheter. Serial blood sampling was undertaken every 10 min for 4 h, with ifenprodil (10 mg/kg) or vehicle injected (i.p.) after 1 h of baseline sampling. The pulsatile release of pituitary LH and levels of GnRH mRNA in hypothalamus were quantified as indices of the reproductive axis.

RESULTS

Our results showed effects of ifenprodil on both endpoints. In OVX rats given cholesterol, neither age nor ifenprodil had any effects on LH release. In E2-treated rats, aging was associated with significant decreases in pulsatile LH release. Additionally, ifenprodil stimulated parameters of pulsatile LH release in both young and middle-aged animals. Ifenprodil had few effects on GnRH mRNA; the only significant effect of ifenprodil was found in the middle-aged, cholesterol group.

CONCLUSION

Together, these findings support a role for the NR2b subunit of the NMDAR in GnRH/LH regulation. Because most of these effects were exhibited on pituitary LH release in the absence of a concomitant change in GnRH gene expression, it is likely that NMDA receptors containing the NR2b subunit play a role in GnRH-induced LH release, independent of de novo GnRH gene expression.

Interactive effects of age and estrogen on cognition and pyramidal neurons in monkey prefrontal cortex

We previously reported that long-term cyclic estrogen (E) treatment reverses age-related impairment of cognitive function mediated by the dorsolateral prefrontal cortex (dlPFC) in ovariectomized (OVX) female rhesus monkeys, and that E induces a corresponding increase in spine density in layer III dlPFC pyramidal neurons. We have now investigated the effects of the same E treatment in young adult females. In contrast to the results for aged monkeys, E treatment failed to enhance dlPFC-dependent task performance relative to vehicle control values (group young OVX+Veh) but nonetheless led to a robust increase in spine density. This response was accompanied by a decline in dendritic length, however, such that the total number of spines per neuron was equivalent in young OVX+Veh and OVX+E groups. Robust effects of chronological age, independent of ovarian hormone status, were also observed, comprising significant age-related declines in dendritic length and spine density, with a preferential decrease in small spines in the aged groups. Notably, the spine effects were partially reversed by cyclic E administration, although young OVX+Veh monkeys still had a higher complement of small spines than did aged E treated monkeys. In summary, layer III pyramidal neurons in the dlPFC are sensitive to ovarian hormone status in both young and aged monkeys, but these effects are not entirely equivalent across age groups. The results also suggest that the cognitive benefit of E treatment in aged monkeys is mediated by enabling synaptic plasticity through a cyclical increase in small, highly plastic dendritic spines in the primate dlPFC.

Estradiol replacement increases the low-density lipoprotein receptor related protein (LRP) in the mouse brain

Numerous epidemiology studies have shown protective effects of hormone therapy (HT) on chronic neurological diseases. We have proposed that some of the neuroprotective effects of estrogen are mediated by apolipoprotein E (apoE). Polymorphisms of receptors for apoE modify the risk for dementia. To our knowledge, no reports exist showing CNS effects of estrogen replacement on members of the low-density lipoprotein receptor family. The current study focused on the effect of estradiol-17beta (E2) replacement on protein expression of two members of the receptor family, the low-density lipoprotein receptor (LDL-r) and low-density lipoprotein receptor related protein (LRP) in ovariectomized mice. Five days of E2 replacement significantly increased LRP expression in the hippocampus, olfactory bulb and neocortex but not in cerebellum. In contrast, E2 treatment decreased LDL-r protein expression in olfactory bulb. HT modification of both apoE and LRP could have wide-spread effects on cellular function given LRP's manifold signaling functions.

Estrogen alters spine number and morphology in prefrontal cortex of aged female rhesus monkeys

Long-term cyclic treatment with 17beta-estradiol reverses age-related impairment in ovariectomized rhesus monkeys on a test of cognitive function mediated by the prefrontal cortex (PFC). Here, we examined potential neurobiological substrates of this effect using intracellular loading and morphometric analyses to test the possibility that the cognitive benefits of hormone treatment are associated with structural plasticity in layer III pyramidal cells in PFC area 46. 17beta-Estradiol did not affect several parameters such as total dendritic length and branching. In contrast, 17beta-estradiol administration increased apical and basal dendritic spine density, and induced a shift toward smaller spines, a response linked to increased spine motility, NMDA receptor-mediated activity, and learning. These results document that, although the aged primate PFC is vulnerable in the absence of factors such as circulating estrogens, it remains responsive to long-term cyclic 17beta-estradiol treatment, and that increased dendritic spine density and altered spine morphology may contribute to the cognitive benefits of such treatment.

Estradiol replacement enhances working memory in middle-aged rats when initiated immediately after ovariectomy but not after a long-term period of ovarian hormone deprivation

The goal of the present study was to explore the effects of long-term hormone deprivation on the ability of subsequent estrogen replacement to affect cognition. Female rats, 12 months of age, underwent ovariectomies (n = 30) or sham surgeries (n = 10). Intact rats and 20 ovariectomized rats received cholesterol implants. Ten ovariectomized rats received implants containing 25% estradiol. Five months later, implants were replaced. Half of the ovariectomized rats with cholesterol implants received estradiol implants and half received new cholesterol implants. Rats with estradiol implants received new estradiol implants. Intact rats were ovariectomized and given estradiol implants. Beginning 1 wk later, working memory performance was assessed in an eight-arm radial maze across 24 d of acquisition and during eight additional trials in which a 2.5-h delay was imposed between the fourth and fifth arm choices. Estradiol replacement initiated immediately after ovariectomy at either 12 or 17 months of age significantly improved performance during acquisition and delay trials, compared with control treatment. When estradiol replacement was initiated at 17 months of age, 5 months after ovariectomy, no enhancements were evident. Uteri of rats that experienced delayed estradiol replacement weighed significantly more than uteri of ovariectomized controls but significantly less than uteri of rats that received immediate estradiol replacement. Uterine weight negatively correlated with mean errors during acquisition. These results indicate that whereas chronic estradiol replacement regimens positively affect working memory in middle-aged animals when initiated immediately after ovariectomy, estradiol replacement is not effective when initiated after long-term hormone deprivation.

Time course of response to estradiol replacement in ovariectomized mice: brain apolipoprotein E and synaptophysin transiently increase and glial fibrillary acidic protein is suppressed

The current study examined the effect of long-term estradiol replacement in ovariectomized mice. Estradiol-17beta (E2) pellets or vehicle pellets were implanted at the time of ovariectomy (OVX) in young adult female mice. Five mice from each group were sacrificed at 5, 14, 28 and 49 days after OVX and pellet replacement. Western blotting of homogenates from somatosensory cortex, hippocampus, olfactory bulb and cerebellum was performed to obtain concentrations of glial fibrillary acidic protein (GFAP), apolipoprotein E (apoE) and synaptophysin (SYN). At 5 days after OVX, GFAP levels were not affected by E2 replacement. In contrast to GFAP, synaptophysin and apoE concentrations were significantly elevated by 15% and 25%, respectively, in the E2-replaced group compared to the vehicle-replaced group at 5 days but by 14 days concentrations were equivalent. Late in the time course of this study, at 49 days, GFAP concentrations were higher in the E2-deprived mice but did not increase in the E2-replaced group. Immunocytochemistry for GFAP confirmed this observation. Of note was that these effects occurred in all four brain regions measured. These observations suggest that estradiol is able to suppress reactive gliosis. In addition, E2 replacement in OVX mice is associated with transiently higher levels of apoE and synaptophysin.

Estrogen and ovariectomy regulate mRNA and protein of glutamic acid decarboxylases and cation-chloride cotransporters in the adult rat hippocampus

17beta-Estradiol spatiotemporally regulates the gamma-aminobutyric acid (GABAergic) tone in the adult hippocampus. However, the complex estrogenic effect on the GABAergic system is still unclear. In adult central nervous system (CNS) neurons, GABA can induce both inhibitory and excitatory actions, which are predominantly controlled by the cation-chloride cotransporters NKCC1 and KCC2. We therefore studied the estrogenic regulation of two glutamate decarboxylase (GAD) isoforms, GAD65 and GAD67, as well as NKCC1 and KCC2 in the adult female rat hippocampus by immunohistochemistry and in situ hybridization. First, we focused on the duration after ovariectomy (OVX) and its effects on GAD65 protein levels. The basal number of GAD65-immunoreactive cells decreased after long-term (10 days) OVX compared to short-term (3 days) OVX. We found that, only after long-term OVX but not after short-term OVX, estradiol increased the number of GAD65-immunoreactive cells in the CA1 pyramidal cell layer. Furthermore, estradiol did not alter the GAD65-immunoreactive cell population in any other CA1 subregion. Second, we therefore focused on long-term OVX and the estrogenic regulation of GAD and cation-chloride cotransporter mRNA levels. In the pyramidal cell layer, estradiol affected GAD65, GAD67 and NKCC1 mRNA levels, but not KCC2 mRNA levels. Both GAD65 and NKCC1 mRNA levels increased within 24 h after estradiol treatment, followed by a subsequent increase in GAD67 mRNA levels. These findings suggest that basal levels of estrogen might contribute to a balance between the excitatory and inhibitory synaptic transmission onto CA1 pyramidal cells by regulating perisomatic GAD and NKCC1 expression in the adult hippocampus.

Changes in interneuronal phenotypes regulated by estradiol in the adult rat hippocampus: A potential role for neuropeptide Y

Ovarian hormones regulate pyramidal cell synapse formation and excitability and interneuronal GABAergic tone in the CA1 region of the adult female rat hippocampus. The role of 17beta-estradiol in these effects is complex and appears to involve a subset of hippocampal interneurons, which express different calcium-binding protein and neuropeptide phenotypes and nuclear estrogen receptor alpha. We found that, in the hippocampus, nuclear estrogen receptor alpha-immunoreactive interneurons co-express neuropeptide Y, calbindin-D28k and calretinin but do not parvalbumin or cholecystokinin. Moreover, a proportion of neuropeptide Y-immunoreactive interneurons co-expresses calbindin-D28k and calretinin. This pattern is similar in the presence or absence of 17beta-estradiol treatment in ovariectomized rats. We then used immunohistochemistry and in situ hybridization to determine whether 17beta-estradiol treatment regulates expression of CA1 interneuronal phenotypic markers via nuclear estrogen receptor alpha activation. We found that 17beta-estradiol treatment of ovariectomized rats increased neuropeptide Y mRNA levels (25%) and the neuropeptide Y mRNA-associated grain density per cell (11%), as well as the number of neuropeptide Y-immunoreactive cells (11%), predominantly in the pyramidal cell layer (stratum pyramidale). Treatment with CI628, a selective estrogen response modulator that acts as an antagonist for nuclear estrogen receptor, blocked 17beta-estradiol-induced increase of neuropeptide Y mRNA levels. 17beta-Estradiol treatment did not alter the number of parvalbumin, calretinin, and cholecystokinin immunoreactive cells, nor mRNA levels for parvalbumin and cholecystokinin. Therefore, the present study has identified neuropeptide Y expression as the main interneuronal phenotype that co-expresses nuclear estrogen receptor alpha and shown that neuropeptide Y is responsive to 17beta-estradiol in CA1 pyramidal cell layer. We suggest that 17beta-estradiol may regulate neuropeptide Y expression mediated by nuclear estrogen receptor alpha-dependent activation in a subset of hippocampal interneurons, and we speculate that subsequent neuropeptide Y release may indirectly contribute to regulate glutamate-dependent neuronal activity in the adult rat hippocampus.

Hormone therapy and risk for dementia: where do we go from here?

Prospective observational studies suggest that hormone therapy (HT) might confer protection against the development of Alzheimer's disease. In contrast, recent findings from the Women's Health Initiative Memory Study (WHIMS) indicated a doubling of the risk of all-cause dementia in women randomized to receive HT after age 64. The discrepancy between findings from observational studies and the WHIMS is commonly attributed to the lack of treatment bias in the randomized trial. However, there are other potentially important dfferences between the WHIMS and the observational studies. These include timing of initiation of HT and type of HT regimen used. The present review focuses on the clinical and basic science studies bearing on these clinically important issues. Additional clinical studies are needed to understand the external generalizability of the WHIMS results to populations of women for whom HT remains an indication.

Development and aging of N-methyl-D-aspartate receptor expression in the prefrontal/frontal cortex of mice

The present study was designed to determine whether the changes that occur during aging in the expression of the N-methyl-D-aspartate (NMDA) receptor and two NMDA receptor subunits, zeta1 and epsilon2, are a continuation of developmental processes and whether protein and mRNA expression patterns of the subunits are similar across the lifespan. The prefrontal/frontal cortex of C57BL/6 mice of eight different ages (7-8, 13-15, 30-32, 49-53, and 70-72 days and 4.5, 11, and 25 months of age) were used to examine NMDA-displaceable [(3)H]glutamate binding and mRNA in tissue sections and mRNA and protein from homogenates. The lateral prefrontal/frontal cortex of C57BL/6 mice showed more significant declines in density of agonist binding to NMDA receptors during both development and aging than the medial cortex. Changes in mRNA expression for the epsilon2 subunit across the lifespan appeared to be related to the changes in NMDA receptor binding in the lateral cortex, even though the protein expression of the epsilon2 subunit did not show the same pattern of expression as the mRNA during development. The changes in epsilon2 subunit mRNA expression during adult aging may be a continuation of developmental processes, but there was also evidence that expression levels plateaued during early adulthood. The developmental expression of the zeta1subunit in the prefrontal/frontal cortex was influenced by gender and there was no significant effect of adult aging on either the protein or mRNA expression of this subunit. Determining how the expression of the NMDA receptor and its subunits change throughout the lifespan can help us to better understand the processes affecting the receptor during aging. These results should be useful for designing interventions into the aging process to repair or prevent changes in the NMDA receptor and its associated functions, such as learning and memory.

Changes in the expression of the NR2B subunit during aging in macaque monkeys

Humans, non-human primates and rodents show declines in spatial memory abilities with increased age. Some of these declines in mice are related to changes in the expression of the epsilon2 (epsilon2) (NR2B) subunit of the N-methyl-D-aspartate receptor. The purpose of this study was to determine whether primates show changes during aging in the mRNA expression of the NR2B subunit. In situ hybridization was performed on tissue sections from three different ages of Rhesus monkeys (Macaca mulatta; 6-8, 10-12, and 24-26 years). There was a significant decrease in the mRNA expression of the NR2B subunit overall in the prefrontal cortex and in the caudate nucleus between young and old monkeys. There were no significant changes in NR2B mRNA expression in the hippocampus or the parahippocampal gyrus. The results in the prefrontal cortex, caudate and hippocampus were similar to those seen previously in C57BL/6 mice during aging, which suggests that mice may be useful as a model for primates to further examine the age-related changes in the expression of the NR2B subunit of the NMDA receptor in several important regions of the brain.

Estrogen modulates synapticN-methyl-D-aspartate receptor subunit distribution in the aged hippocampus

Estrogen interacts with N-methyl-D-aspartate (NMDA) receptors to regulate multiple aspects of morphological and functional plasticity. In hippocampus, estrogen increases both dendritic spine density and synapse number, and NMDA antagonists block these effects. Thus, estrogen-mediated hippocampal plasticity may be of particular importance in the context of age-related changes in endocrine status and cognitive performance. NR1 levels per synapse are increased in CA1 by estrogen in aged rats but not young rats, although no information is available on estrogen-induced synaptic alterations in other NMDA receptor subunits that might impact function. Therefore, the present study was designed to investigate the effect of estrogen on the synaptic and subsynaptic distributions of the NMDA receptor subunits, NR2A and NR2B in CA1 pyramidal cells, within the context of aging. Our results demonstrated that the overall synaptic levels of NR2A and NR2B are similar in young and aged female rats, regardless of estrogen treatment. However, in the aged CA1, estrogen restores NR2B levels back to young levels in the lateral portions of the active synaptic zone. Thus, estrogen may impact the mobility of NMDA receptors across the synapse and, in the process, restore a more youthful synaptic profile. These findings have important implications for the mechanism of estrogen-induced alterations in NMDA receptor-mediated processes, particularly in the context of aging.

Stereologic analysis of estrogen receptor alpha (ER alpha) expression in rat hypothalamus and its regulation by aging and estrogen

The estrogen receptor alpha (ERalpha) in the hypothalamus plays important roles in the regulation of reproductive development, physiology, and behavior. However, the expression of the ERalpha may change during aging or in response to varying estrogen levels. The present study measured changes in the numbers of ERalpha-expressing cells in specific hypothalamic and preoptic nuclei of ovariectomized female Sprague-Dawley rats at three ages (young [3-4 months], middle-aged [10-12 months], or old [24-26 months]) and with or without estrogen replacement. Numbers of ERalpha-immunoreactive neurons were quantified in four regions relevant to reproductive function: the anteroventral periventricular nucleus (AVPV), medial preoptic nucleus (MPN), arcuate nucleus (ARH), and ventromedial nucleus (VMN), using an unbiased stereologic approach. In the AVPV and VMN, significant age-related increases in the numbers of ERalpha-expressing cells from the middle-aged to the old group were detected, and no differences were observed in the MPN and ARH, indicating that ERalpha neuron number is maintained or even elevated during aging. No significant effects of estrogen on ERalpha cell number were detected in any of the four regions studied. Therefore, ERalpha cell number in the rat hypothalamus and preoptic area changes with aging in a region-specific manner.

Estradiol selectively affects processing of conspecifics' faces in female rhesus monkeys

Estrogen deficiency following ovariectomy or menopause increases the risk of developing diseases such as osteoporosis and may also lead to memory impairment. Although estrogen replacement therapy (ERT) alleviates many symptoms associated with estrogen loss, it is not clear whether it also benefits cognitive function. The effect of estrogens upon cognition can best be studied in an animal model of human menopause, in which estrogen levels can be experimentally manipulated. Six young ovariectomized female rhesus monkeys (6-9 years old) were tested on a battery of touchscreen-based cognitive tasks, including the Matching-to-Sample (MTS) task with mixed delays and the spatial, object, and face conditions of the Delayed Recognition Span Test (DRST). Monkeys were tested 5 days a week, one task per week, for a total of 8 months, while undergoing treatments with placebo and ethinyl estradiol (EE2) in alternating 28-days blocks. Blood samples were collected to verify EE2 levels. We also observed the monkeys by video monitor during test sessions and recorded locomotor activity and response topology. Performance on the face-DRST, a task that involved selecting the new face in an increasing array of rhesus monkey faces, was impaired by EE2 treatment, as compared to placebo. Other tasks were unaffected by EE2. There was no clear evidence of EE2 effects upon motor activity or anxiety. In order to test the reliability of our findings, we conducted an additional experiment in which the monkeys were again given the face-DRST with different categories of face stimuli for 4 months, while receiving placebo and EE2 in alternating 7-days blocks. They performed each task 4-5 days/week for 4 weeks with (1) the same rhesus monkey faces as in the first experiment, (2) human faces, (3) chimpanzee faces, and (4) novel rhesus monkey faces. Face-DRST performance did not vary as a function of treatment when human or chimpanzee faces were used as stimuli. In contrast, periods of EE2 treatment were associated with a lower performance for both sets of rhesus monkey faces. These findings suggest that EE2 treatment has a detrimental effect on processing faces of conspecifics by female rhesus monkeys. We speculate that estrogens may produce this effect by enhancing emotional reactivity to socially relevant stimuli.

Onset of estrogen replacement has a critical effect on synaptic density of CA1 hippocampus in ovariectomized adult rats

OBJECTIVES

The aim of this study was to evaluate differences between estrogen replacement therapy initiated either 4 or 12 days after ovariectomy on the synaptic density of the hippocampal CA1 field in rats.

DESIGN

Female, adult, Wistar rats were ovariectomized bilaterally under ether anesthesia and divided among the following groups: 1) estrogen (conjugated equine estrogen 50 microg in 0.5 mL of propylene glycol, daily, p.o. gavage, for 60 days), starting 4 days after ovariectomy (n = 5); 2) propylene glycol (0.5 mL daily, p.o. gavage, for 60 days), starting 4 days after ovariectomy (n = 4); 3) estrogen (conjugated equine estrogen 50 microg in 0.5 mL of propylene glycol, daily, p.o. gavage, for 45 days), starting 12 days after ovariectomy (n = 3); 4) propylene glycol (0.5 mL daily, p.o. gavage, for 45 days), starting 12 days after ovariectomy (n = 3). At the end of the treatment, the rats were processed for electron microscopy and light analysis.

RESULTS

Synaptic density in all of the CA1 strata subjected to evaluation was significantly higher in animals in which estrogen replacement was initiated 4 days after ovariectomy as compared with controls. In contrast, initiation of treatment after a 12-day interval did not result in recovery of synaptic density in any of the CA1 strata and was significantly lower than that of the animals subjected to hormone replacement after a 4-day delay (P < 0.01).

CONCLUSION

The delay for hormone replacement therapy might have critical implications for modulating synaptic density.

Age-related changes in estrogen receptor beta in rat hypothalamus: a quantitative analysis

Although the estrogen receptor beta (ER beta) is a major target for actions of estrogen on the brain, little is known about its neural expression during aging, when levels and the mode of estrogen release undergo substantial changes. Therefore, in the present study we examined effects of aging and estrogen treatment on the number of cells expressing the ER beta in female rats. Two regions relevant to reproductive function were analyzed: the anteroventral periventricular nucleus (AVPV) and the principal nucleus of the bed nucleus of the stria terminalis (pBST). The numbers of ER beta-expressing cells were quantified using an unbiased stereological approach. Female rats were used at three ages [young (3-4 months), middle-aged (10-12 months), and old (24-26 months)], with or without estrogen replacement. Because the estrogen milieu impacts the function of neurotransmitter receptors such as the N-methyl-D-aspartate receptor in the brain, we also investigated the colocalization of ER beta and the obligatory N-methyl-D-aspartate receptor subunit, NR1. We observed a significant age-related decrease in ER beta cell number in the AVPV, but not the pBST. No significant effect of estrogen on ER beta cell number was detected in either brain region at any age. Approximately 10% and 3% of cells expressing ER beta also coexpressed NR1 in AVPV and pBST, respectively, and this did not differ with age or treatment. Taken together, our results demonstrate 1) there are age-related changes in ER beta cell number that are region specific; 2) this expression is not altered by estrogen replacement; and 3) a subset of ER beta-positive cells coexpresses NR1.

Influence of estradiol, stress, and 5-HT2A agonist treatment on brain-derived neurotrophic factor expression in female rats

BACKGROUND

Estradiol affects neuronal plasticity, mood, and cognition. We examined the effects of the estrous cycle, acute and chronic estradiol treatments on BDNF mRNA expression in the hippocampus and cortex of female rats. The roles of 5-HT2A receptors and of stress on the BDNF mRNA regulation were also explored.

METHODS

BDNF mRNA levels were measured using in situ hybridization at proestrus and estrus, and following acute and chronic estradiol treatment of acutely and chronically ovariectomized (OVX) female rats. Some rats were pretreated with 5-HT2A agonist and antagonist, and another group was subjected to two-hour immobilization stress.

RESULTS

BDNF mRNA levels in the dentate gyrus and the medial prefrontal cortex were decreased during estrus, when estradiol levels are highest. Acute estradiol treatment decreased hippocampal BDNF mRNA in acutely OVX rats, but neither acute nor chronic estradiol had effect in chronically OVX rats. Estradiol pretreatment reduced the 5-HT2A receptor-mediated cortical upregulation in BDNF mRNA and did not effect the stress-induced down-regulation of BDNF mRNA in the dentate gyrus.

CONCLUSIONS

The duration of the estradiol treatment and the duration of the ovarian hormone deprivation are important factors in the regulation of BDNF synthesis and possibly in the functional outcome of estrogen treatment.

Colocalization and hormone regulation of estrogen receptor alpha and N-methyl-D-aspartate receptor in the hypothalamus of female rats

Effects of N-methyl-D-aspartate (NMDA) receptor (NMDAR) activation on neuroendocrine function can be modulated by the steroid hormone milieu. For example, the hypothalamic GnRH neurons, the primary cells regulating reproductive function, are stimulated by NMDAR agonists, and this is greatly potentiated by estrogen. We hypothesized that the actions of glutamate and estrogen may converge at target cells in the brain in which the NMDA and estrogen receptors (ERs) are coexpressed. To this end, we used quantitative stereological techniques to determine the colocalization of the obligatory NMDAR subunit, NR1, and the ERalpha, in the anteroventral periventricular nucleus and the medial preoptic nucleus, two critical regions for reproductive physiology and behavior. We observed extensive colocalization of ERalpha and NR1 in these brain regions (approximately 80%). In the anteroventral periventricular nucleus, treatment of ovariectomized rats with estrogen up-regulated the coexpression, whereas in the medial preoptic nucleus, estrogen had no effect, demonstrating a regional specificity to the estrogen sensitivity. The number of ERalpha cells that did not express NR1 was not altered by estrogen treatment in either brain region. Thus, we speculate that the extensive colocalization of ERalpha and the NMDAR provides an anatomical level at which estrogen and glutamate can act at target cells, and potentially synergize, to influence neuroendocrine and autonomic functions.

17beta-estradiol benzoate decreases the AHP amplitude in CA1 pyramidal neurons

Disruption of Ca(2+) homeostasis is hypothesized to mediate several electrophysiological markers of brain aging. Recent evidence indicates that estradiol can rapidly alter Ca(2+)-dependent processes in neurons through nongenomic mechanisms. In the current study, electrophysiological effects of 17beta-estradiol benzoate (EB) on the Ca(2+)-activated afterhyperpolarization (AHP) were investigated using intracellular sharp electrode recording in hippocampal slices from ovariectomized Fischer 344 female rats. The AHP amplitude was enhanced in aged (22-24 mo) compared with young (5-8 mo) rats and direct application of EB (100 pM) reduced the AHP in aged rats. The age-related difference was due, in part, to the increased AHP amplitude of aged animals, since an EB-mediated decrease in the AHP could be observed in young rats when the extracellular Ca(2+) was elevated to increase the AHP amplitude. In aged rats, bath application of EB occluded the ability of the L-channel blocker, nifedipine (10 microM), to attenuate the AHP. The results support a role for EB in modifying hippocampal Ca(2+)-dependent processes in a manner diametrically opposite that observed during aging, possibly through L-channel inhibition.

Estrogen and aging affect the subcellular distribution of estrogen receptor-alpha in the hippocampus of female rats

Estrogen replacement increases both the number of dendritic spines and the density of axospinous synapses in the hippocampal CA1 region in young rats, yet this is attenuated in aged rats. The estrogen receptor-alpha (ER-alpha) is localized within select spines of CA1 pyramidal cells in young animals and thus may be involved locally in this process. The present study investigated the effects of estrogen on the ultrastructural distribution of ER-alpha in the CA1 of young (3-4 months) and aged (22-23 months) Sprague Dawley rats using postembedding immunogold electron microscopy. Within dendritic spines, most ER-alpha immunoreactivity (IR) was seen in plasmalemmal and cytoplasmic regions of spine heads, with a smaller proportion within 60 nm of the postsynaptic density. In presynaptic terminals, ER-alpha-IR was clustered and often associated with synaptic vesicles. Significant effects of both aging and estrogen were observed. Quantitative analysis revealed that nonsynaptic pools of ER-alpha-IR within the presynaptic and postsynaptic compartments were decreased (35 and 27%, respectively) in the young estrogen-replaced animals compared with those that received vehicle. Such localized regulation of ER-alpha in response to circulating estrogen levels might directly affect synaptic signaling in CA1 pyramidal cells. No estrogen treatment-related differences were observed in the aged animals. However, 50% fewer spines contained ER-alpha in the aged compared with young hippocampus. These data suggest that the decreased responsiveness of hippocampal synapses to estrogen in aged animals may result from age-related decrements in ER-alpha levels and its subcellular localization vis-à-vis the synapse. Such a role for spinous ER-alpha has important implications for age-related attenuation of estrogen-induced hippocampal plasticity.

Age-related changes in hypothalamic gonadotropin-releasing hormone and N-methyl-D-aspartate receptor gene expression, and their regulation by oestrogen, in the female rat

During reproductive ageing, the oestrous cycles of female rats become irregular and eventually cease. The mechanisms for reproductive senescence in rodents are believed to involve changes in hypothalamic neurones, including gonadotropin-releasing hormone (GnRH) cells and their afferent inputs. In addition, effects of oestrogen on hypothalamic function may vary in animals of different ages. These issues were addressed using young (aged 4-5 months), middle-aged (12-14 months) and old (24-26 months) female Sprague-Dawley rats. Animals were ovariectomized and given oestrogen or vehicle replacement. They were killed and the preoptic area-anterior hypothalamus (POA-AH) and the medial basal hypothalamus-median eminence (MBH-ME) were dissected out, RNA extracted, and RNase protection assay used to quantify gene expression of several hypothalamic molecules. In the first experiment, GnRH RNA levels were measured in the POA-AH. No effects of ageing or oestrogen were observed on GnRH gene expression. This finding suggests that ageing and oestrogen may affect GnRH release from neuroterminals independently of de novo biosynthesis, and that this may involve other neurones that affect GnRH neurosecretory function. In the second experiment, we investigated changes in N-methyl-D-aspartate (NMDA) receptor subunit mRNA levels. These receptors play an important regulatory role in mediating effects of glutamate on GnRH function, and are themselves regulated by oestrogen and ageing. NMDA receptor subunit (NR) 1, 2a and 2b mRNA levels were quantified in the POA-AH and MBH-ME, the sites of GnRH perikarya and neuroterminals, respectively. In general, oestrogen had inhibitory effects on NR1 and NR2a, and differential effects on NR2b subunit mRNA levels. NMDA receptor subunit mRNA levels also changed during ageing: age-related decreases in NR1 mRNA occurred in the MBH-ME, and an age-related increase in NR2b mRNA occurred in the POA-AH. Taken together, these results demonstrate subunit- and region-specific changes in hypothalamic NMDA receptor subunit gene expression with oestrogen and ageing. These alterations could have implications for the physiological effects of glutamate on its NMDA receptor, and impact the regulation of reproductive and other neuroendocrine and autonomic functions by hypothalamic glutamatergic inputs.

Estrogen and exercise interact to regulate brain-derived neurotrophic factor mRNA and protein expression in the hippocampus

We investigated the possibility that estrogen and exercise interact in the hippocampus and regulate brain-derived neurotrophic factor (BDNF), a molecule increasingly recognized for its role in plasticity and neuron function. An important aspect of this study is to examine the effect of different time intervals between estrogen loss and estrogen replacement intervention. We demonstrate that in the intact female rat, physical activity increases hippocampal BDNF mRNA and protein levels. However, the exercise effect on BDNF up-regulation is reduced in the absence of estrogen, in a time-dependent manner. In addition, voluntary activity itself is stimulated by the presence of estrogen. In exercising animals, estrogen deprivation reduced voluntary activity levels, while estrogen replacement restored activity to normal levels. In sedentary animals, estrogen deprivation (ovariectomy) decreased baseline BDNF mRNA and protein, which were restored by estrogen replacement. Despite reduced activity levels in the ovariectomized condition, exercise increased BDNF mRNA levels in the hippocampus after short-term (3 weeks) estrogen deprivation. However, long-term estrogen-deprivation blunted the exercise effect. After 7 weeks of estrogen deprivation, exercise alone no longer affected either BDNF mRNA or protein levels. However, exercise in combination with long-term estrogen replacement increased BDNF protein above the effects of estrogen replacement alone. Interestingly, protein levels across all conditions correlated most closely with mRNA levels in the dentate gyrus, suggesting that expression of mRNA in this hippocampal region may be the major contributor to the hippocampal BDNF protein pool. The interaction of estrogen, physical activity and hippocampal BDNF is likely to be an important issue for maintenance of brain health, plasticity and general well-being, particularly in women.

Gonadotropin-releasing hormone neurons, NMDA receptors, and their regulation by steroid hormones across the reproductive life cycle

The effects of ovarian steroid hormones on gonadotropin-releasing hormone (GnRH) neurons have been studied for many years. In addition to their regulation by sex steroids, GnRH neurons are affected by inputs from neurotransmitters such as glutamate, acting via the NMDA receptor (NMDAR). Moreover, the NMDAR itself is subject to estrogen regulation. Thus, effects of ovarian steroids on GnRH neurons and the NMDAR, and their interactions, are under intense investigation. Messenger RNA and protein levels of GnRH and NMDAR subunits were measured in neuroendocrine brain regions in response to estrogen treatment, or across the reproductive cycle. Stimulatory effects of ovarian steroids on GnRH gene expression occur during the preovulatory LH surge in young adult rats, and this is abolished in middle-aged rats that have an attenuated LH surge. Effects of estrogen on GnRH neurons have also been studied in the ovariectomized, estrogen-primed rat, and while results vary between laboratories, there appear to be age-related changes in the sensitivity of GnRH neurons to estrogen. Estrogen also has effects on NMDAR mRNA levels. In intact rats, mRNA levels of NMDAR decrease during reproductive aging in the preoptic area, the site of GnRH perikarya, while in the medial basal hypothalamus-median eminence, the site of GnRH neuroterminals, levels of NMDAR subunit mRNAs increase with aging. Thus, glutamatergic inputs to GnRH perikarya and neuroterminals and other neuroendocrine cells may change during reproductive aging in intact rats. In ovariectomized rats, NMDAR subunit mRNA levels also undergo age-related changes, and respond to estrogen replacement in a subunit- and age-specific manner. Notably, there are major differences in NMDAR gene expression during aging between intact and ovariectomized rats, suggesting that ovarian factors other than estrogen play a role in the regulation of this receptor.