Estradiol protects against ischemic injury

Role of estrogen in central nuclei mediating stroke-induced changes in autonomic tone

The current investigation examined the role of estrogen in central autonomic regulatory nuclei on the autonomic dysfunction resulting from middle cerebral artery occlusion (MCAO). Experiments were done in anaesthetized male Sprague-Dawley rats. The effect of MCAO on autonomic tone was assessed by monitoring vagal and renal efferent nerve activities before and following systemic administration of either estrogen or saline and the bilateral microinjection of the estrogen receptor antagonist, ICI 182, 780, into several autonomic nuclei (the intrathecal space of the spinal cord, nucleus tractus solitarius, nucleus ambiguus, rostral ventrolateral medulla, parabrachial nucleus, central nucleus of the amygdala or ventral posteromedial thalamus). Autonomic reflex function was evoked using intravenous injection of increasing doses of phenylephrine (0.025-0.1 mg/kg) and the peak changes in heart rate and blood pressure were plotted to obtain the baroreflex sensitivity. The presence of ICI 182, 780 in the intrathecal space of the spinal cord, nucleus ambiguous, nucleus tractus solitarius, rostral ventrolateral medulla, parabrachial nucleus, or central nucleus of the amygdala prior to the administration of estrogen resulted in a significant attenuation (ranging from 79% to 94 %) in the estrogen-induced recovery of autonomic function following MCAO. Blocking estrogen receptors in the ventral posteromedial thalamus had no effect on the ability of estrogen to prevent the MCAO-induced changes in autonomic function. These results suggest that the estrogen-mediated recovery of autonomic function following MCAO is dependent on the availability of estrogen receptors in several forebrain and brainstem autonomic nuclei.

Stroke-induced changes in estrogen release and neuronal activity in the parabrachial nucleus of the male rat

Recent investigations have provided evidence to suggest exogenous estrogen administration into autonomic nuclei prevents or reverses the autonomic dysfunction observed after middle cerebral artery occlusion (MCAO) in male rats. Because estrogen seems to be a potent neuroprotectant against autonomic dysfunction, it is our hypothesis that endogenous estrogen levels within autonomic nuclei will increase in response to stroke. Therefore, in this investigation, in vivo microdialysis was used to simultaneously measure the concentration of estrogen in the plasma and in the parabrachial nucleus (PBN) of male Sprague-Dawley rats after MCAO. Analysis of dialysate samples before MCAO and in sham-operated controls revealed a baseline concentration of estrogen in the PBN (38 +/- 3 pg/mL; n = 36), which was significantly greater than that found in plasma (22 +/- 6 pg/mL; n = 6; P < .05). The concentration of estrogen in the PBN was significantly increased immediately after MCAO (85 +/- 4 pg/mL; n = 7; P < .05) but then decreased to below pre-MCAO values (12 +/- 2 pg/mL; n = 7; P < .05) by 90 minutes after MCAO and remained below baseline levels until the end of the experiment (240 minutes post-MCAO). No changes in plasma estrogen levels were detected at any time point after MCAO. In addition, extracellular electrophysiological recordings from PBN neurons revealed that MCAO resulted in an immediate decrease in the activity of PBN neurons, which was completely blocked after systemic estrogen injection. These results suggest that estrogen is released into the PBN in response to MCAO and that the source of estrogen seems to be primarily caused by terminal release as opposed to increased local synthesis.

Gender-related differences in apoptotic pathways after neonatal cerebral ischemia

Many central nervous system (CNS) diseases display sexual dimorphism, specifically a predilection for one gender or a gender-dependent response to treatment. Exposure to circulating sex steroids is felt to be a chief contributor to this phenomenon. However, CNS diseases of childhood and of the elderly also demonstrate gender predominance and/or sexual dimorphism response to therapies. In this short update, we provide information concerning one of the most interesting new emerging concepts related to the influence of the sex in the pathogenesis of developmental brain injuries leading to different levels of neuroprotection between genders after cerebral hypoxia-ischemia or ischemia.

Progesterone increases brain-derived neuroptrophic factor expression and protects against glutamate toxicity in a mitogen-activated protein kinase- and phosphoinositide-3 kinase-dependent manner in cerebral cortical explants

The higher prevalence and risk for Alzheimer's disease in women relative to men has been partially attributed to the precipitous decline in gonadal hormone levels that occurs in women following the menopause. Although considerable attention has been focused on the consequence of estrogen loss, and thus estrogen's neuroprotective potential, it is important to recognize that the menopause results in a precipitous decline in progesterone levels as well. In fact, progesterone is neuroprotective, although the precise mechanisms involved remain unclear. Based on our previous observation that progesterone elicits the phosphorylation of ERK and Akt, key effectors of the neuroprotective mitogen-activated protein kinase (MAPK) and phosphoinositide-3 kinase (PI3-K) pathways, respectively, we determined whether activation of either of these pathways was necessary for progesterone-induced protection. With organotypic explants (slice culture) of the cerebral cortex, we found that progesterone protected against glutamate-induced toxicity. Furthermore, these protective effects were inhibited by either the MEK1/2 inhibitor UO126 or the PI3-K inhibitor LY294002, supporting the requirement for both the MAPK and PI3-K pathways in progesterone-induced protection. In addition, at a concentration and duration of treatment consistent with our neuroprotection data, progesterone also increased the expression of brain-derived neurotrophic factor (BDNF), at the level of both protein and mRNA. This induction of BDNF may be relevant to the protective effects of progesterone, in that inhibition of Trk signaling, with K252a, inhibited the protective effects of progesterone. Collectively, these data suggest that progesterone is protective via multiple and potentially related mechanisms. (c) 2007 Wiley-Liss, Inc.

Modulation of the endocannabinoid system by focal brain ischemia in the rat is involved in neuroprotection afforded by 17beta-estradiol

Endogenous levels of the endocannabinoid anandamide, and the activities of the synthesizing and hydrolyzing enzymes, i.e. N-acylphosphatidylethanolamine-hydrolyzing phospholipase D and fatty acid amide hydrolase, respectively, were determined in the cortex and the striatum of rats subjected to transient middle cerebral artery occlusion. Anandamide content was markedly increased ( approximately 3-fold over controls; P < 0.01) in the ischemic striatum after 2 h of middle cerebral artery occlusion, but not in the cortex, and this elevation was paralleled by increased activity of N-acylphosphatidylethanolamine-hydrolyzing phospholipase D ( approximately 1.7-fold; P < 0.01), and reduced activity ( approximately 0.6-fold; P < 0.01) and expression ( approximately 0.7-fold; P < 0.05) of fatty acid amide hydrolase. These effects of middle cerebral artery occlusion were further potentiated by 1 h of reperfusion, whereas anandamide binding to type 1 cannabinoid and type 1 vanilloid receptors was not affected significantly by the ischemic insult. Additionally, the cannabinoid type 1 receptor antagonist SR141716, but not the receptor agonist R-(+)-WIN55,212-2, significantly reduced (33%; P < 0.05) cerebral infarct volume detected 22 h after the beginning of reperfusion. A neuroprotective intraperitoneal dose of 17beta-estradiol (0.20 mg x kg(-1)) that reduced infarct size by 43% also minimized the effect of brain ischemia on the endocannabinoid system, in an estrogen receptor-dependent manner. In conclusion, we show that the endocannabinoid system is implicated in the pathophysiology of transient middle cerebral artery occlusion-induced brain damage, and that neuroprotection afforded by estrogen is coincident with a re-establishment of anandamide levels in the ischemic striatum through a mechanism that needs to be investigated further.

Distribution and localization patterns of estrogen receptor-beta and insulin-like growth factor-1 receptors in neurons and glial cells of the female rat substantia nigra: localization of ERbeta and IGF-1R in substantia nigra

Although several studies have focused on the neuroprotective effects of estrogen (E2) on stroke, there have been tantalizing reports on the potential neuroprotective role of E2 in degenerative neuronal diseases such as Alzheimer's and Parkinson's (PD). In animal models of PD, E2 protects the nigrostriatal dopaminergic (DA) system against neurotoxins. However, little is known about the cellular and molecular mechanism(s) involved by which E2 elicits its neuroprotective effects on the nigrostriatal DA system. A preferred mechanism for neuroprotection is the interaction of E2 with specific neuroprotective growth factors and receptors. One such neuroprotective factor/receptor system is insulin-like growth factor-1 (IGF-1). E2 neuroprotective effects in the substantia nigra (SN) DA system have been shown to be dependent on IGF-1. To determine whether E2 also interacts with the IGF-1 receptor (IGF-1R) and to determine the cellular localization of estrogen receptor (ER) and IGF-1R, we compared the distribution of ER and IGF-1R in the SN. Stereological measurements revealed that 40% of the subpopulation of tyrosine hydroxylase-immunoreactive (TH-ir) SN pars compacta (SNpc) DA neurons are immunoreactive for estrogen receptor-beta (ERbeta). No immunolabeling for ERalpha was observed. In situ hybridization and immunocytochemistry studies confirmed the expression of IGF-1R mRNA and revealed that almost all TH-ir SNpc DA neurons were immunoreactive for IGF-1R, respectively. Moreover, one-third of glial fibrillary acidic protein (GFAP-ir) cells in the SN were ERbeta-ir, and 67% of GFAP-ir cells expressed IGF-1R-ir. Therefore, the localization of ERbeta and IGF-1R on SNpc DA neurons and astrocytes suggests a modulatory role of E2 on IGF-1R, and this modulation may affect neuroprotection.

Neurocognitive outcomes are not improved by 17beta-estradiol in postmenopausal women undergoing cardiac surgery

BACKGROUND AND PURPOSE

Neurocognitive dysfunction is an important source of patient morbidity and mortality after cardiac surgery that may disproportionately affect postmenopausal women. 17beta-Estradiol limits the extent of ischemic neuronal injury in a variety of experimental models. The purpose of this study was to evaluate whether perioperative administration of 17beta-estradiol to postmenopausal women reduces the frequency of neurocognitive dysfunction after cardiac surgery.

METHODS

One hundred seventy-four postmenopausal women not on estrogen replacement therapy who were undergoing primary coronary artery bypass graft surgery and/or valve surgery with cardiopulmonary bypass were prospectively randomized to receive in a double-blinded manner either 17beta-estradiol or placebo beginning the day before surgery and continuing for 5 days postoperatively. The patients were evaluated before and after surgery with the National Institutes of Health Stroke Scale and a psychometric test battery.

RESULTS

There were no differences in the frequency of neurocognitive dysfunction (primary outcome) between patients randomized to perioperative 17beta-estradiol (n=86) and those randomized to placebo (n=88) 4 to 6 weeks after surgery (17beta-estradiol, 22.4% versus placebo, 21.4%, P=0.45). The mean scores on tests of psychomotor speed were worse in women in the 17beta-estradiol group than in the placebo group at the 4- to 6-week (P=0.005) postoperative testing sessions.

CONCLUSIONS

Perioperative treatment with 17beta-estradiol did not result in improved neurocognitive outcomes in postmenopausal women undergoing cardiac surgery.

Human umbilical cord blood cells or estrogen may be beneficial in treating heatstroke

This current review summarized animal models of heatstroke experimentation that promote our current knowledge of therapeutic effects on cerebrovascular dysfunction, coagulopathy, and/or systemic inflammation with human umbilical cord blood cells (HUCBCs) or estrogen in the setting of heatstroke. Accumulating evidences have demonstrated that HUCBCs provide a promising new therapeutic method against neurodegenerative diseases, such as stroke, traumatic brain injury, and spinal cord injury as well as blood disease. More recently, we have also demonstrated that post- or pretreatment by HUCBCs may resuscitate heatstroke rats with by reducing circulatory shock, and cerebral nitric oxide overload and ischemic injury. Moreover, CD34+ cells sorted from HUCBCs may improve survival by attenuating inflammatory, coagulopathy, and multiorgan dysfunction during experimental heatstroke. Many researchers indicated pro- (e.g. tumor necrosis factor-alpha [TNF-alpha]) and anti-inflammatory (e.g. interleukin-10 [IL-10]) cytokines in the peripheral blood stream correlate with severity of circulatory shock, cerebral ischemia and hypoxia, and neuronal damage occurring in heatstroke. It has been shown that intravenous administration of CD34+ cells can secrete therapeutic molecules, such as neurotrophic factors, and attenuate systemic inflammatory reactions by decreasing serum TNF-alpha but increasing IL-10 during heatstroke. Another line of evidence has suggested that estrogen influences the severity of injury associated with cerebrovascular shock. Recently, we also successfully demonstrated estrogen resuscitated heatstroke rats by ameliorating systemic inflammation. Conclusively, HUCBCs or estrogen may be employed as a beneficial therapeutic strategy in prevention and repair of cerebrovascular dysfunction, coagulopathy, and/or systemic inflammation during heatstroke.

Retinal ganglion cell protection by 17-beta-estradiol in a mouse model of inherited glaucoma

Glaucoma is the second leading cause of blindness in the world. The ultimate cause of vision loss due to glaucoma is thought to be retinal ganglion cell (RGC) apoptosis. Neuroprotection of RGC is becoming an important approach of glaucoma therapy. Several lines of evidence suggest that estrogen has neurotrophic and neuroprotective properties. In this study, we examine the role of estrogen in preventing RGC loss in DBA/2J mouse, an in vivo model of an inherited (pigmentary) glaucoma. Two-month-old female DBA/2J mice were anesthetized and ovariectomized with or without subcutaneous 17beta-estradiol (betaE2) pellet implantation. RGC survival was evaluated from flat-mounted whole retinas by counting retrograde-labeled cells. The loss of nerve fibers and RGC were also evaluated in paraffin-fixed retinal cross sections. Biochemical alterations in the retinas of DBA/2J mice in response to systemic injection of betaE2 were also examined. We have made several important observations showing that: (1) betaE2 treatment reduced the loss of RGC and neurofibers through inhibition of ganglion cell apoptosis, (2) betaE2 activated Akt and cAMP-responsive-element-binding-protein (CREB), (3) betaE2 up-regulated thioredoxin-1 (Trx-1) expression, (4) betaE2 reduced the increased activations of mitogen-activated protein kinases (MAPK) and NF-kappaB, (5) betaE2 inhibited the increased interleukin-18 (IL-18) expression, and (6) treatment with tamoxifen, an estrogen receptor antagonist, blocked betaE2-mediated activation of Akt and inhibition of MAPK phosphorylation in the retinas of DBA/2J mice. These findings suggest the possible involvement of multiple biochemical events, including estrogen receptor/Akt/CREB/thioredoxin-1, and estrogen receptor/MAPK/NF-kappaB, in estrogen-mediated retinal ganglion cell protection.

Neurotrophic and neuroprotective actions of estrogen: basic mechanisms and clinical implications

Estrogen is an important hormone signal that regulates multiple tissues and functions in the body. This review focuses on the neurotrophic and neuroprotective actions of estrogen in the brain, with particular emphasis on estrogen actions in the hippocampus, cerebral cortex and striatum. Sex differences in the risk, onset and severity of neurodegenerative disease such as Alzheimer's disease, Parkinson's disease and stroke are well known, and the potential role of estrogen as a neuroprotective factor is discussed in this context. The review assimilates a complex literature that spans research in humans, non-human primates and rodent animal models and attempts to contrast and compare the findings across species where possible. Current controversies regarding the Women's Health Initiative (WHI) study, its ramifications, concerns and the new studies needed to address these concerns are also addressed. Signaling mechanisms underlying estrogen-induced neuroprotection and synaptic plasticity are reviewed, including the important concepts of genomic versus nongenomic mechanisms, types of estrogen receptor involved and their subcellular targeting, and implicated downstream signaling pathways and mediators. Finally, a multicellular mode of estrogen action in the regulation of neuronal survival and neurotrophism is discussed, as are potential future directions for the field.

Mitochondrial mechanisms of estrogen neuroprotection

Oxidative stress, bioenergetic failure and mitochondrial dysfunction are all implicated in the etiology of neurodegenerative diseases such as Alzheimer's disease (AD). The mitochondrial involvement in neurodegenerative diseases reflects the regulatory role mitochondrial failure plays in both necrotic cell death and apoptosis. The potent feminizing hormone, 17 beta-estradiol (E2), is neuroprotective in a host of cell and animal models of stroke and neurodegenerative diseases. The discovery that 17alpha-estradiol, an isomer of E2, is equally as neuroprotective as E2 yet is >200-fold less active as a hormone, has permitted development of novel, more potent analogs where neuroprotection is independent of hormonal potency. Studies of structure-activity relationships and mitochondrial function have led to a mechanistic model in which these steroidal phenols intercalate into cell membranes where they block lipid peroxidation reactions, and are in turn recycled. Indeed, the parental estrogens and novel analogs stabilize mitochondria under Ca(2+) loading otherwise sufficient to collapse membrane potential. The neuroprotective and mitoprotective potencies for a series of estrogen analogs are significantly correlated, suggesting that these compounds prevent cell death in large measure by maintaining functionally intact mitochondria. This therapeutic strategy is germane not only to sudden mitochondrial failure in acute circumstances, such as during a stroke or myocardial infarction, but also to gradual mitochondrial dysfunction associated with chronic degenerative disorders such as AD.

Dose dependence and therapeutic window for the neuroprotective effects of 17beta-estradiol when administered after cerebral ischemia

The present study was undertaken to determine if the neuroprotective effect of 17beta-estradiol (E(2)) when administrated after ischemia is dose-dependent and if the therapeutic window for estrogen can be prolonged. Ischemic injury was induced by permanent middle cerebral artery occlusion (p-MCAO). Administration of E(2) at 30 min after ischemia resulted in a reduction in lesion volume. A higher dose of E(2) extended the therapeutic window to 6h after cerebral ischemia in 33% of the rats. These findings suggest that postischemic treatment with estrogen affords protection against ischemic damage and that it acts within a clinically useful therapeutic window.

Timing of estrogen therapy after ovariectomy dictates the efficacy of its neuroprotective and antiinflammatory actions

Recent studies describing the seemingly contradictory actions of estrogens in ischemic stroke injury have led us to reevaluate the circumstances under which estrogen therapy (ET) provides benefits against cerebral stroke and decipher its mechanisms of action. One prominent feature that follows stroke injury is massive central and peripheral inflammatory responses. Evidence now suggests that postischemic inflammatory responses strongly contribute to the extent of brain injury, and 17beta-estradiol (E(2)) may protect the ischemic brain by exerting antiinflammatory actions. In an attempt to explain recently reported dichotomous effects of E(2) in stroke injury, we tested the hypothesis that an extended period of hypoestrogenicity both prevents E(2) from protecting the brain against ischemia and simultaneously suppresses its antiinflammatory actions. We report that E(2) exerts profound neuroprotective action when administered immediately upon ovariectomy, but not when administered after 10 weeks of hypoestrogenicity. Consistently, E(2) treatment given immediately at the time of ovariectomy attenuated central and peripheral production of proinflammatory cytokines after ischemic stroke. In contrast, E(2) did not suppress production of proinflammatory molecules when it was administered after 10 weeks postovariectomy. These results demonstrate that a prolonged period of hypoestrogenicity disrupts both neuroprotective and antiinflammatory actions of E(2). Our findings may help to explain the results of the Women's Health initiative that reported no beneficial effect of ET against stroke because the majority of the subjects initiated ET after an extended period of hypoestrogenicity.

Implications for estrogens in Parkinson's disease: an epidemiological approach

Evidence from experimental and epidemiological studies suggests a role of sex hormones in the pathogenic process leading to neurodegenerative diseases, (i.e., Alzheimer's and Parkinson's disease). The effects of sexual steroid hormones are complex and vary with the events of women's fertile life. Estrogens are supposed to influence dopamine synthesis, metabolism, and transport; however, there is no consensus regarding the direction, locus, and mechanism of the effect of estrogens on the dopaminergic system. A neuroprotective effect of estrogens has been demonstrated in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-animal models of Parkinson's disease (PD). Epidemiological studies indicate gender differences regarding the onset and the prognosis of PD. Most of the analytical studies explored the relationship between PD and exogenous estrogens. Only three studies investigated the role of endogenous estrogens in the risk of developing PD. These studies reported an increased risk of PD in conditions causing an early reduction in endogenous estrogens (early menopause, reduced fertile life length). Longer cumulative length of pregnancies has also been associated with an increased PD risk. A lack of consensus still exists on the effect of the type of menopause (surgical vs. natural) on PD risk. Finally, the effect of postmenopausal estrogen replacement therapy is still debated. Inconsistencies across studies are in part explained by the complexity of the mechanisms of action of sexual hormones and by the paucity of analytical studies.

Acute pretreatment with estradiol protects against CA1 cell loss and spatial learning impairments resulting from transient global ischemia

Estradiol can act to protect against hippocampal damage resulting from transient global ischemia, but little is known about the functional consequences of such neuroprotection. The present study examines whether acute estradiol administered prior to the induction of transient global ischemia protects against hippocampal cell death and deficits in performance on a spatial learning task. Ovariectomized female rats were primed with estradiol benzoate or oil vehicle 48 and 24 h prior to experiencing one of three durations of 4-vessel occlusion (0, 5, or 10 min). Performance on the cued and hidden platform versions of the Morris water maze was assessed 1 week following ischemia. On the cued platform task, neither hormone treatment nor ischemia significantly influenced acquisition. When tested on the hidden platform task, however, oil-treated rats exhibited impairments in spatial learning after either 5 or 10 min of ischemia while estradiol-treated rats showed no impairments after 5 min of ischemia and only mild impairments after 10 min of ischemia. Immediately following behavioral testing, rats were perfused and survival of CA1 pyramidal cells was assessed. Ischemia was associated with the loss of CA1 pyramidal cells but rats that received estradiol prior to ischemia showed less severe damage. Furthermore, the extent of cell loss was correlated with degree of spatial bias expressed on a probe trial following hidden platform training. These findings indicate that acute exposure to estradiol prior to ischemia is both neuroprotective and functionally protective.

Changes in estrogen receptor-alpha mRNA in the mouse cortex during development

Estrogen plays a critical role in brain development and is responsible for generating sex differences in cognition and emotion. Studies in rodent models have shown high levels of estrogen binding in non-reproductive areas of the brain during development, including the cortex and hippocampus, yet binding is diminished in the same areas of the adult brain. These binding studies demonstrated that estrogen receptors decline in the cortex during development but did not identify which of the two estrogen receptors was present. In the current study, we examined the expression of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) in the mouse cortex during the first month of life. Messenger RNA was isolated from cortical tissue taken from C57BL/6 mice on postnatal day (PND) 1, 4, 10, 18 and 25 and expression levels were determined by real-time PCR. ERalpha mRNA expression in the mouse cortex at PND 25 was significantly reduced as compared to PND 1 (p<0.01). ERbeta mRNA expression at PND 25 was significantly increased as compared to PND 1 (p<0.05). Although the increase in ERbeta mRNA was statistically significant, the ERbeta levels were extremely low in the isocortex compared to ERalpha mRNA levels, suggesting that ERalpha may play a more critical role in the developmental decrease of estradiol binding than ERbeta. Additionally, we measured ERalpha mRNA expression in organotypic explant cultures of cortex taken from PND 3 mice. Explants were maintained in vitro for 3 weeks. mRNA was isolated at several time points and ERalpha and ERbeta mRNA was measured by real-time RT-PCR. ERalpha and ERbeta mRNA levels reflected a similar pattern in vitro and in vivo, suggesting that signals outside the cortex are not needed for this developmental change. This study lays the groundwork for an understanding of the mechanisms of the developmental regulation of ERalpha mRNA.

Oestrogen and stroke: the potential for harm as well as benefit

Epidemiological studies point to a beneficial influence of the female reproductive hormones on stroke risk in that women have a lower incidence of stroke prior to the menopause compared with men, but this difference weakens with age and stroke risk in women rises after the menopause. However, recent Women's Health Initiative trials in post-menopausal women report an increased stroke risk on hormone replacement therapy. An influence of gender is also apparent on stroke outcome in animal models: female rats exposed to transient MCA (middle cerebral artery) occlusion sustain less brain damage than age-matched males, with loss of protection following ovariectomy. The major hormone thought to be responsible for beneficial influences on stroke incidence and outcome is oestrogen, and a large preclinical literature now exists where exogenously administered oestrogen has been studied in male and ovariectomized female rats using a range of stroke models and outcome measures. Most of these studies administer oestrogen prior to the stroke, use a model of transient ischaemia followed by reperfusion and report a significant oestrogen-induced neuroprotection. However, in some studies where the MCA is permanently occluded, oestrogen pre-treatment in ovariectomized female rats has been shown to significantly exacerbate ischaemic damage. Therefore preclinical results demonstrate harmful as well as beneficial influences of oestrogen on the ischaemic brain, highlighting the need for further study to elucidate the mechanisms responsible for both detrimental and beneficial influences. Ultimately, this could lead to the development of new classes of oestrogenic compounds with improved risk/benefit profiles, designed to selectively activate pathways inducing only the beneficial effects of oestrogen in vivo.

Membrane-initiated actions of estrogens in neuroendocrinology: emerging principles

Hormonal ligands for the nuclear receptor superfamily have at least two interacting mechanisms of action: 1) classical transcriptional regulation of target genes (genomic mechanisms); and 2) nongenomic actions that are initiated at the cell membrane, which could impact transcription. Although transcriptional mechanisms are increasingly well understood, membrane-initiated actions of these ligands are incompletely understood. Historically, this has led to a considerable divergence of thought in the molecular endocrine field. We have attempted to uncover principles of hormone action that are relevant to membrane-initiated actions of estrogens. There is evidence that the membrane-limited actions of hormones, particularly estrogens, involve the rapid activation of kinases and the release of calcium. Membrane actions of estrogens, which activate these rapid signaling cascades, can also potentiate nuclear transcription. These signaling cascades may occur in parallel or in series but subsequently converge at the level of modification of transcriptionally relevant molecules such as nuclear receptors and/or coactivators. In addition, other hormones or neurotransmitters may also activate cascades to crosstalk with estrogen receptor-mediated transcription. The idea of synergistic coupling between membrane-initiated and genomic actions of hormones fundamentally revises the paradigms of cell signaling in neuroendocrinology.

Estradiol prevents the injury-induced decrease of 90 ribosomal S6 kinase (p90RSK) and Bad phosphorylation

Estradiol prevents neuronal cell death through the activation of cell survival signals and the inhibition of apoptotic signals. This study investigated whether estradiol modulates the anti-apoptotic signal through the activation of Raf-MEK-ERK and its downstream targets, including 90 ribosomal S6 kinase (p90RSK) and Bad. Adult female rats were ovariectomied and treated with estradiol prior to middle cerebral artery occlusion (MCAO). Brains were collected 24h after MCAO and infarct volumes were analyzed. We confirmed that estradiol significantly reduces infarct volume and decreases the positive cells of TUNEL staining in the cerebral cortex. Estradiol prevents the injury-induced decrease of Raf-1, MEK1/2, and ERK1/2 phosphorylation. Also, it inhibits the injury-induced decrease of p90RSK and Bad phosphorylation. Further, in the presence of estradiol, the interaction of phospho-Bad and 14-3-3 increased, compared with that of oil-treated animals. Our findings suggest that estradiol prevents cell death due to brain injury and that Raf-MEK-ERK cascade activation and its downstream targets, p90RSK, Bad phosphorylation by estradiol mediated these protective effects.

Guarding the blood-brain barrier: a role for estrogen in the etiology of neurodegenerative disease

Although the effect of estrogen replacement therapy on the incidence of the neurodegenerative disease such as Alzheimer's disease is controversial, experimental studies indicate that estrogen replacement to young adult animals is neuroprotective and that perimenopausal estrogen replacement is associated with a decreased incidence of Alzheimer's disease. Estrogen affects a wide variety of cellular processes that can protect neuronal health. This article considers the disruption of the blood-brain barrier in Alzheimer's disease and forwards the hypothesis that estrogen may preserve neural health by maintaining the integrity of the blood-brain barrier.

Gender and Estrogen Manipulation Do Not Affect Traumatic Brain Injury in Mice

As epidemiological data have suggested that female patients may have improved clinical prognoses following traumatic brain injury (TBI) compared to males, we designed experiments to determine the role of gender and estrogen in TBI-induced brain injury and inflammation in rodents. To this end, male and female C57Bl/6 mice were separated into the following four groups: intact males, intact females with vehicle supplementation, ovariectomized females with vehicle supplementation, and ovariectomized females with estrogen supplementation. All mice were subjected to a controlled cortical impact model of TBI, and cortical injury, hippocampal degeneration, microglial activation, and brain cytokine expression were analyzed after injury. Additionally, the spleens were harvested and cytokine release from cultured splenic cells was measured in response to specific stimuli. Data indicate that TBI-induced cortical and hippocampal injury, as well as injury-related microglial activation were not significantly affected by gender or estrogen manipulation. Conversely, brain levels of MCP-1 and IL-6 were significantly increased in males and intact females following TBI, but not in female mice that had been ovariectomized and supplemented with either estrogen or vehicle. Evaluation of splenic responses showed that the spleen was only moderately affected by TBI, and furthermore that spleens isolated from mice that had been given estrogen supplementation showed significantly higher release of the anti-inflammatory cytokine IL-4, regardless of the presence of absence of TBI. Overall, these data indicate that while estrogen can modulate immune responses, and indeed can predispose splenic responses towards and anti-inflammatory phenotype, these effects do not translate to decreased brain injury or inflammation following TBI in mice.

Evidence to Implicate Early Modulation of Interleukin‐1β Expression in the Neuroprotection Afforded by 17β‐Estradiol in Male Rats Undergone Transient Middle Cerebral Artery Occlusion

Neuroprotection exerted by 17beta-estradiol (17beta-E(2)) has been widely investigated in animal models of acute cerebral ischemia. Estrogens interact with intracellular receptors (ERalpha and ERbeta) to modulate the transcription of target genes, including those implicated in neuronal survival. Neuroprotection may also occur via interaction with ER-like membrane receptors mediating rapid, non-genomic, actions or via receptor-independent mechanisms. There is also evidence that blockade of inflammatory factors may represent an important mechanism involved in estrogenic neuroprotection. Here we investigate whether reduced brain damage by acute pharmacological treatment with 17beta-E(2) in male rats subjected to transient (2h) middle cerebral artery occlusion (tMCAo) involves modulation of interleukin-1beta (IL-1beta), a proinflammatory cytokine strongly implicated in the pathophysiology of ischemic stroke. Administration of 17beta-E(2) (0.2mg/kg, i.p., 1h before tMCAo) results in significant reduction of brain infarct volume, and this is reverted by the ER antagonist ICI 182,780 (0.25mg/kg, i.p.) administered 1h before 17beta-E(2). Two hours MCAo followed by 2-h reperfusion results in a significant, threefold increase of IL-1beta levels in the cortical tissue ipsilateral to the ischemic damage. Interestingly, a pretreatment with a neuroprotective dose of 17beta-E(2) attenuates the cytokine elevation and this appears to occur through ER activation. In addition, neuroprotection by 17beta-E(2) is accompanied by reduced cytochrome c translocation both in the striatum and in the cortex as revealed by Western blotting 3h after reperfusion. In conclusion, we report the original observation that neuroprotection exerted by 17beta-E(2) in a rat model of transient focal brain ischemia is accompanied by reduced cytochrome c translocation to the cytosol and involves early modulation of IL-1beta production.

Role of astrocytes in estrogen-mediated neuroprotection

Recent work has suggested that the ovarian steroid hormone, 17beta-estradiol (E2), at physiological concentrations, may exert protective effects in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and acute ischemic stroke. While physiological concentrations of E2 have consistently been shown to be protective in vivo, direct protection of neurons remains controversial, suggesting that while direct protection of neurons may occur in some instances, an alternative or parallel pathway for protection may exist which could involve another cell type in the brain. In the present review, we summarize the data in support of a possible role for astrocytes in the mediation of neuroprotection by E2. We also summarize the data suggesting a non-classical estrogen receptor may underlie some of the protective effects of E2 by activating cellular signaling pathways, such as extracellular-regulated kinase (ERK) and phosphatidylinositol 3-kinase/Akt. A possible indirect pathway involving astrocytes may act in concert with the proposed direct pathway to achieve a widespread, global protection of both ER positive and negative neurons.

17beta-estradiol pretreatment prevents the global ischemic injury-induced decrease of Akt activation and bad phosphorylation in gerbils

Estradiol acts as a neuroprotective factor against ischemic brain injury. This study investigated whether estradiol modulates neuroprotective mechanism through the activation of Akt and its downstream target such as Bad in global ischemic injury. Adult female gerbils were ovariectomized and treated with estradiol prior to ischemic injury. Transient cerebral ischemia was accomplished by bilateral clipping of the common carotid artery for 5 min. Brains were collected on 1, 3, 5 day after injury. In hippocampal CA1 region of non-treated gerbils, most of neuronal cells exhibited pyknotic nuclei and showed the positive reaction of TUNEL staining on 5 day after injury. However, estradiol significantly reduced the neuronal cell death. Potential activation was measured by phosphorylation of Akt at Ser473 and Bad at Ser136 using western blot analysis. The levels of pAkt and pBad were significantly decreased in non-treated gerbils on 1-5 day after injury. However, estradiol prevents the global ischemic injury-induced decrease of pAkt and pBad. Our findings suggest that estradiol prevents cell death due to global ischemic injury and that Akt activation and Bad phosphorylation by estradiol mediated these protective effects.

Treatment with phytoestrogen alpha-zearalanol might protect neurons of hippocampus in ovariectomized rats

Although neuroprotective effects of estrogen on postmenopausal women have been recognized, an associated increased incidence of uterine and breast tumors has jeopardized the clinical use of estrogen. This study was designed to evaluate the neuroprotective effects of a novel phytoestrogen alpha-zearalanol (alpha-ZAL), on ovariectomized (OVX) rats. Adult Wistar rats were ovariectomized or sham-operated and treatment with equivalent doses of 17beta-estradiol or alpha-ZAL for 5 wk. Uteruses have been weighted and stained by hematoxylin and eosin for morphology analysis. The expression of synaptophysin and parvalbumin in hippocampus were evaluated by immunohistochemistry assays. Our experiments indicated that the synaptophysin and parvalbumin-positive areas were significantly decreased in the OVX group compared to the sham group, alpha-ZAL or 17beta-estradiol administration can reverse the effects. Although alpha-ZAL and 17beta-estradiol treatments reconciled uterus weight loss which was induced by ovariectomy, the effect of alpha-ZAL was less than 17beta-estradiol. This result suggests that alpha-ZAL may effectively abate neurons loss in the hippocampus while slightly promoting weight gain of the uterus.

Estrogen, cognition and female ageing

Starting from fetal life, estrogens are crucial in determining central gender dimorphism, and an estrogen-induced synaptic plasticity is well evident during puberty and seasonal changes as well as during the ovarian cycle. Estrogens act on the central nervous system (CNS) both through genomic mechanisms, modulating synthesis, release and metabolism of neurotransmitters, neuropeptides and neurosteroids, and through non-genomic mechanisms, influencing electrical excitability, synaptic function and morphological features. Therefore, estrogen's neuroactive effects are multifaceted and encompass a system that ranges from the chemical to the biochemical to the genomic mechanisms, protecting against a wide range of neurotoxic insults. Clinical evidences show that, during the climacteric period, estrogen withdrawal in the limbic system gives rise to modifications in mood, behaviour and cognition and that estrogen administration is able to improve mood and cognitive efficiency in post-menopause. Many biological mechanisms support the hypothesis that estrogens might protect against Alzheimer's disease (AD) by influencing neurotransmission, increasing cerebral blood flow, modulating growth proteins associated with axonal elongation and blunting the neurotoxic effects of beta-amyloid. On the contrary, clinical studies of estrogen replacement therapy (ERT) and cognitive function have reported controversial results, indicating a lack of efficacy of estrogens on cognition in post-menopausal women aged >or=65 years. These findings suggest the presence of a critical period for HRT-related neuroprotection and underlie the potential importance of early initiation of therapy for cognitive benefit. In this review, we shall first describe the multiple effects of steroids in the nervous system, which may be significant in the ageing process. A critical update of HRT use in women and a discussion of possible prospectives for steroid use are subsequently proposed.

Down-regulation of glutaredoxin by estrogen receptor antagonist renders female mice susceptible to excitatory amino acid mediated complex I inhibition in CNS

beta-N-oxalyl-amino-L-alanine, (L-BOAA), an excitatory amino acid, acts as an agonist of the AMPA subtype of glutamate receptors. It inhibits mitochondrial complex I in motor cortex and lumbosacral cord of male mice through oxidation of critical thiol groups, and glutaredoxin, a thiol disulfide oxido-reductase, helps maintain integrity of complex I. Since incidence of neurolathyrism is less common in women, we examined the mechanisms underlying the gender-related effects. Inhibition of complex I activity by L-BOAA was seen in male but not female mice. Pretreatment of female mice with estrogen receptor antagonist ICI 182,780 or tamoxifen sensitizes them to L-BOAA toxicity, indicating that the neuroprotection is mediated by estrogen receptors. L-BOAA triggers glutathione (GSH) loss in male mice but not in female mice, and only a small but significant increase in oxidized glutathione (GSSG) was seen in females. As a consequence, up-regulation of gamma-glutamyl cysteinyl synthase (the rate-limiting enzyme in glutathione synthesis) was seen only in male mouse CNS but not in females. Both glutathione reductase and glutaredoxin that reduce oxidized glutathione and protein glutathione mixed disulfides, respectively, were constitutively expressed at higher levels in females. Furthermore, glutaredoxin activity in female mice was down-regulated by estrogen antagonist indicating its regulation by estrogen receptor. The higher constitutive expression of glutathione reductase and glutaredoxin could potentially confer neuroprotection to female mice.

Estradiol reduces activity of the blood-brain barrier Na-K-Cl cotransporter and decreases edema formation in permanent middle cerebral artery occlusion

Estrogen has been shown to protect against stroke-induced brain damage, yet the mechanism is unknown. During the early hours of stroke, cerebral edema forms as increased transport of Na and Cl from blood into brain occurs across an intact blood-brain barrier (BBB). We showed previously that a luminal BBB Na-K-Cl cotransporter is stimulated by hypoxia and arginine vasopressin (AVP), factors present during cerebral ischemia, and that inhibition of the cotransporter by intravenous bumetanide greatly reduces edema in rats subjected to permanent middle cerebral artery occlusion (MCAO). The present study was conducted to determine whether estrogen protects in stroke at least in part by reducing activity of the BBB cotransporter, thereby decreasing edema formation. Ovariectomized rats were subjected to 210 mins of permanent MCAO after 7-day or 30-min pretreatment with 17beta-estradiol and then brain swelling and 2,3,5-triphenyltetrazolium chloride staining were assessed as measures of brain edema and lesion volume, respectively. Diffusion-weighed imaging was used to monitor permanent MCAO-induced decreases in apparent diffusion coefficient (ADC) values, an index of changes in brain water distribution and mobility. Na-K-Cl cotransporter activity of cerebral microvascular endothelial cells (CMECs) was assessed as bumetanide-sensitive K influx and cotransporter abundance by Western blot analysis after estradiol treatment. Estradiol significantly decreased brain swelling and lesion volume and attenuated the decrease in ADC values during permanent MCAO. Estradiol also abolished CMEC cotransporter stimulation by chemical hypoxia or AVP and decreased cotransporter abundance. These findings support the hypothesis that estrogen attenuates stimulation of BBB Na-K-Cl cotransporter activity, reducing edema formation during stroke.

Complex actions of sex steroids in adipose tissue, the cardiovascular system, and brain: Insights from basic science and clinical studies

Recent publications describing the results of the Women's Health Initiative (WHI) and other studies reporting the impact of hormone therapy on aging women have spurred reexamination of the broad use of estrogens and progestins during the postmenopausal years. Here, we review the complex pharmacology of these hormones, the diverse and sometimes opposite effects that result from the use of different estrogenic and progestinic compounds, given via different delivery routes in different concentrations and treatment sequence, and to women of different ages and health status. We examine our new and growing appreciation of the role of estrogens in the immune system and the inflammatory response, and we pose the concept that estrogen's interface with this system may be at the core of some of the effects on multiple physiological systems, such as the adipose/metabolic system, the cardiovascular system, and the central nervous system. We compare and contrast clinical and basic science studies as we focus on the actions of estrogens in these systems because the untoward effects of hormone therapy reported in the WHI were not expected. The broad interpretation and publicity of the results of the WHI have resulted in a general condemnation of all hormone replacement in postmenopausal women. In fact, careful review of the extensive literature suggests that data resulting from the WHI and other recent studies should be interpreted within the narrow context of the study design. We argue that these results should encourage us to perform new studies that take advantage of a dialogue between basic scientists and clinician scientists to ensure appropriate design, incorporation of current knowledge, and proper interpretation of results. Only then will we have a better understanding of what hormonal compounds should be used in which populations of women and at what stages of menopausal/postmenopausal life.

Estradiol prevents the injury-induced decrease of Akt/glycogen synthase kinase 3β phosphorylation

Estradiol prevents neuronal cell death through the activation of cell survival signals and the inhibition of apoptotic signals. This study investigated whether estradiol modulates the anti-apoptotic signal through the phosphorylation of Akt and its downstream target, glycogen synthase kinase 3beta (GSK3beta). Adult female rats were ovariectomized and treated with estradiol prior to middle cerebral artery occlusion (MCAO). Brains were collected 24 h after MCAO and infarct volumes were analyzed. Estradiol administration significantly reduced infarct volume and decreased the positive cells of TUNEL staining in the cerebral cortex. Potential activation was measured by phosphorylation of Akt at Ser(473) and GSK3beta at Ser(9) using Western blot analysis and immunohistochemistry. Estradiol prevented the injury-induced decrease of pAkt and pGSK3beta. Furthermore, pretreatment with estradiol decreased glutamate toxicity-induced cell death in a hippocampal cell line (HT22). Also, estradiol prevented the glutamate toxicity-induced decrease of pAkt and pGSK3beta in HT22 cells. Our findings suggest that estradiol plays a potent protective role against brain injury and that phosphorylation of Akt and GSK3beta by estradiol mediated these protective effects.

Estrogens and experimental ischemic stroke: a systematic review

Estrogens are believed to provide females with endogenous protection against cerebrovascular events although clinical trials studying long-term hormone replacement have yielded disappointing results. In contrast, estrogens might be neuroprotective after experimental ischemia. We performed a systematic review of controlled experimental studies that administered estrogens before, or after, cerebral ischemia and measured lesion volume. Relevant studies were found from searching PubMed, Embase and Web of Science. From 161 identified publications, 27 studies using 1,304 experimental subjects were analyzed using the Cochrane Review Manager software. Estrogens reduced lesion volume in a dose-dependent manner, after either transient (P<0.001) or permanent (P<0.001) ischemia and whether administered before or up to 4 h after ischemia onset; no studies assessed efficacy for later time periods. The effect size for estrogens decreased with increasing quality scores for studies of transient ischemia. Estrogens reduced lesion volume when administered to ovariectomized females and young adult males, but had no effect in intact females. Limited data were present for aged animals and the full dose-response relationship was not available in all experimental groups. On the basis of these data, estrogens are a candidate treatment for ischemic stroke, although further preclinical studies are also warranted.

Chronic 17beta-estradiol pretreatment and ischemia-induced hippocampal degeneration and memory impairments: a 6-month survival study

Exogenous administration of estrogen has been shown to significantly reduce ischemia-induced neuronal degeneration. However, the long-term impact of such treatment on neuronal protection and functional recovery remain largely unknown. The present study assessed the effects of a 15-day pretreatment with 17beta-estradiol on memory deficits and neuronal damage up to 6 months following a 10-min global ischemia in rats. Four groups of ovariectomized female rats [sham-operated and ischemic rats receiving a 15-day pretreatment of either the vehicle or 17beta-estradiol (100 microg/kg)] were tested. The 8-arm radial maze and object recognition tests served to evaluate the impact of 17beta-estradiol treatment on ischemia-induced spatial and recognition memory impairments, respectively. Testing in the radial maze was initiated at two distinct time intervals following reperfusion (7 and 120 days) to evaluate changes in memory functions over time. Our findings revealed long-lasting neuroprotective effects of 17beta-estradiol treatment on hippocampal CA1 pyramidal cells in ovariectomized ischemic rats (43.5% greater neuronal survival than observed in vehicle-treated ischemic animals). Importantly, this neuronal protection translated into significant improvements of recognition and spatial memory functions in estradiol-treated ischemic rats.

17β-Estradiol treatment following permanent focal ischemia does not influence recovery of sensorimotor function

The development of therapy to aid poststroke recovery is essential. The female hormone 17beta-estradiol has been shown to promote synaptogenesis; the purpose of this study was to attempt to harness these mechanisms to promote repair and recovery in the peri-infarct zone. Rats were ovariectomized, tested for sensorimotor function, and the middle cerebral artery permanently occluded (MCAO). Infarct volumes were calculated using MRI, and damage was equivalent in all animals prior to implantation of either 17beta-estradiol or placebo pellets. Animals were tested for functional recovery for 28 days and tissue processed for synaptic marker syntaxin immunohistochemistry. The stroke induced a significant behavioral deficit, which persisted out to 28 days, and was not significantly different between 17beta-estradiol and placebo treatment groups. There was no difference in syntaxin immunostaining between groups in either the peri-infarct cortex or in the dendritic CA1 reference region. In conclusion, 17beta-estradiol treatment, delivered poststroke, did not influence recovery of function or synaptogenesis.

17beta-estradiol is protective in spinal cord injury in post- and pre-menopausal rats

The neuroprotective effects of 17 beta -estradiol have been shown in models of central nervous system injury, including ischemia, brain injury, and more recently, spinal cord injury (SCI). Recent epidemiological trends suggest that SCIs in elderly women are increasing; however, the effects of menopause on estrogen-mediated neuroprotection are poorly understood. The objective of this study was to evaluate the effects of 17beta-estradiol and reproductive aging on motor function, neuronal death, and white matter sparing after SCI of post- and pre-menopausal rats. Two-month-old or 1- year-old female rats were ovariectomized and implanted with a silastic capsule containing 180 microg/mL of 17beta-estradiol or vehicle. Complete crush SCI at T8-9 was performed 1 week later. Additional animals of each age group were left ovary-intact but were spinal cord injured. The Basso, Beattie, Bresnahan (BBB) locomotor test was performed. Spinal cords were collected on post-SCI days 1, 7, and 21, and processed for histological markers. Administration of 17beta-estradiol to ovariectomized rats improved recovery of hind-limb locomotion, increased white matter sparing, and decreased apoptosis in both the post- and pre-menopausal rats. Also, ovary-intact 1-year-old rats did worse than ovary-intact 2-month-old rats, suggesting that endogenous estrogen confers neuroprotection in young rats, which is lost in older animals. Taken together, these data suggest that estrogen is neuroprotective in SCI and that the loss of endogenous estrogen-mediated neuroprotective seen in older rats can be attenuated with exogenous administration of 17beta-estradiol.

Protein expression changes in the nucleus accumbens and amygdala of inbred alcohol-preferring rats given either continuous or scheduled access to ethanol

Chronic ethanol (EtOH) drinking produces neuronal alterations within the limbic system. To investigate changes in protein expression levels associated with EtOH drinking, inbred alcohol-preferring (iP) rats were given one of three EtOH access conditions in their home-cages: continuous ethanol (CE: 24h/day, 7days/week access to EtOH), multiple scheduled access (MSA: four 1-h sessions during the dark cycle/day, 5 days/week) to EtOH, or remained EtOH-naïve. Both MSA and CE groups consumed between 6 and 6.5g of EtOH/kg/day after the 3rd week of access. On the first day of EtOH access for the seventh week, access was terminated at the end of the fourth MSA session for MSA rats and the corresponding time point (2300h) for CE rats. Ten h later, the rats were decapitated, brains extracted, the nucleus accumbens (NAcc) and amygdala (AMYG) microdissected, and protein isolated for 2-dimensional gel electrophoretic analyses. In the NAcc, MSA altered expression levels for 12 of the 14 identified proteins, compared with controls, with six of these proteins altered by CE access, as well. In the AMYG, CE access changed expression levels for 22 of the 27 identified proteins, compared with controls, with 8 of these proteins altered by MSA, as well. The proteins could be grouped into functional categories of chaperones, cytoskeleton, intracellular communication, membrane transport, metabolism, energy production, or neurotransmission. Overall, it appears that EtOH drinking and the conditions under which EtOH is consumed, differentially affect protein expression levels between the NAcc and AMYG. This may reflect differences in neuroanatomical and/or functional characteristics associated with EtOH self-administration and possibly withdrawal, between these two brain structures.

Neuroprotective effects of 17beta-estradiol and nonfeminizing estrogens against H2O2 toxicity in human neuroblastoma SK-N-SH cells

Neuroprotective effects of estrogens have been shown in various in vitro and in vivo models, but the mechanisms underlying protection by estrogen are not clear. Mounting evidence suggests antioxidant effects contribute to the neuroprotective effects of estrogens. In the present study, we assessed the protective effects of estrogens against H2O2-induced toxicity in human neuroblastoma cells and the potential mechanisms involved in this protection. We demonstrate that 17beta-estradiol (17beta-E2) increases cell survival against H2O2 toxicity in human neuroblastoma cells. 17beta-E2 effectively reduced lipid peroxidation induced by 5-min H2O2 exposure. Furthermore, 17beta-E2 exerts the protective effects by maintaining intracellular Ca2+ homeostasis, attenuating ATP depletion, ablating mitochondrial calcium overloading, and preserving mitochondrial membrane potential. Two nonfeminizing estrogens, 17alpha- and ent-estradiol, were as effective as 17beta-E2 in increasing cell survival, alleviating lipid peroxidation, preserving mitochondrial function, and maintaining intracellular glutathione levels and Ca2+ homeostasis against H2O2 insult. Moreover, the estrogen receptor antagonist fulvestrant (ICI 182,780) did not block effects of 17beta-E2, but increased cell survival and blunted intracellular Ca2+ increases. However, these estrogens failed to reduce cytosolic reactive oxygen species, even at concentrations as high as 10 microM. In conclusion, estrogens exert protective effects against oxidative stress by inhibiting lipid peroxidation and subsequently preserving Ca2+ homeostasis, mitochondrial membrane potential, and ATP levels.

Estrogen: a neuroprotective or proinflammatory hormone? Emerging evidence from reproductive aging models

Estrogen or hormone (estrogen + progestin) replacement is typically prescribed to women for relief from vasomotor symptoms at menopause. Observational studies have shown that such replacement also decreases the risk for Alzheimer's disease. Experimental data from a variety of animal models also suggest that estrogen replacement given to ovariectomized animals is largely neuroprotective. However, the recent intervention trial (Women's Health Initiative Memory Study; WHIMS) concluded that estrogen replacement and hormone replacement prescribed to postmenopausal women increased the risk for global cognitive impairment and dementia, respectively. This paper will examine evidence that the disparity in the human and animal data can be reconciled by consideration of the "reproductive" age of the individual receiving estrogen or hormone replacement. Our recent studies comparing the effects of estrogen replacement on young adult animals with those of estrogen replacement to reproductive senescent animals suggest that the estrogen replacement is beneficial when given to "surgically menopausal" (ovariectomized) animals. However, estrogen replacement appears to be deleterious to acyclic reproductive senescent animals, where target organs such as the brain have been in a prolonged estrogen-deficient state. The paper will also review aging and reproductive age-related changes in the estrogen receptor (ER) systems, specifically ER-alpha, as a potential mechanism for estrogen's deleterious effects in the reproductive senescent animal.

Investigative models for determining hormone therapy-induced outcomes in brain: evidence in support of a healthy cell bias of estrogen action

The profound disparities between the largely positive basic science findings of gonadal steroid action in brain and the adverse outcomes of recent hormone therapy clinical trials in women who are either aged postmenopausal or postmenopausal with Alzheimer's disease have led to an intense reassessment of gonadal hormone action and the model systems used in basic and clinical science. The power of model systems is their predictive validity for a target population--in this case, menopausal women considering the health benefits and risks of hormone therapy. Analysis of the model systems used across the basic to clinical research continuum separate into two broad classes: those that use prevention interventions in healthy organisms and those that use hormone interventions in organisms with compromised neurological function. Basic science analyses that led to elucidation of the neurotrophic and neuroprotective effects of estrogen and the underlying mechanisms of action typically used a prevention-based experimental paradigm. This paradigm relies on healthy neurons/brains/animals/humans as the starting foundation followed by exposure to estrogen/hormone followed by exposure to neurodegenerative insult. The prevention paradigm in basic science analyses parallels the analyses of Sherwin and colleagues (Psychoneuroendocrinology 13: 345-357, 1988), who investigated the cognitive impact of estrogen therapy in women with surgical- or pharmacological-induced menopause. Observational retrospective and prospective studies are also consistent with the healthy cell bias of estrogen action and a prevention paradigm of estrogen or hormone therapy intervention. For the most part, the epidemiological observational data indicate reduction in the risk of Alzheimer's disease in women who began estrogen or hormone therapy at the time of the menopause. In contrast, studies that fall within the second class, hormone intervention in organisms with compromised neurological function--that is, a treatment paradigm--exhibit a mixed profile. In a randomized double-blind clinical trial of estrogen therapy in a cohort of women aged 72 or more years and diagnosed with Alzheimer's disease, estrogen therapy resulted in a modest benefit in the short term (2 months) and adverse progression of disease in the long term (12 months). In the Women's Health Initiative Memory Study (WHIMS) cohort of women 65 or more years of age, with no indicators of neurological disease but with variable health status, estrogen and hormone therapy for 5 years increased the risk of developing Alzheimer's disease. These data would suggest that as the continuum of neurological health progresses from healthy to unhealthy, so too do the benefits of estrogen or hormone therapy. If neurons are healthy at the time of estrogen exposure, their response to estrogen is beneficial for both neurological function and survival. In contrast, if neurological health is compromised, estrogen exposure over time exacerbates neurological demise. Based on these and other data, a hypothesis of a healthy cell bias of gonadal hormone action is put forth. The healthy cell bias of estrogen action hypothesis provides a lens through which to assess the disparities in outcomes across the domains of scientific inquiry and to access future applications of estrogen and hormone therapeutic interventions.

Chronic treatment with low doses of estradiol affects pituitary and thyroid function in young and middle-aged ovariectomized rats

Menopause marks the end of the reproductive life span of women and is characterized by a dramatic drop of circulating estrogen. Hormone replacement therapy (HRT) has been successfully used to treat the symptoms of menopause because estrogen has strong suppressive effects on climacteric complaints. As the hypothalamo-pituitary-thyroid axis is a target for estrogen action we assessed the effect of ovariectomy (OVX) and long-term (3 months) estradiol (E2) treatment by means of subcutaneously (s.c.) implanted silastic capsules on pituitary and thyroid function in middle-aged female rats. Our results demonstrate that s.c. administration of E2 resulted in physiological serum concentrations of E2. Upon OVX, ERbeta mRNA in the pituitary was induced and amounts of transcripts decreased after E2 administration. Implanted E2 capsules led to a reduction of ERalpha mRNA in the pituitary. E2 treatment attenuated the OVX-induced increases in TSHbeta and TSHalpha mRNA. Chronic E2 treatment reduced total T4 levels in OVX animals. TSH and total T3 serum levels were not altered upon E2 treatment. Taken together, our results clearly demonstrate that both, the pituitary and the thyroid of middle-aged rats remain susceptible to the influence of OVX and E2 treatment.

Novel mechanisms for estrogen-induced neuroprotection

Estrogens are gonadal steroid hormones that are present in the circulation of both males and females and that can no longer be considered within the strict confines of reproductive function. In fact, the bone, the cardiovascular system, and extrahypothalamic regions of the brain are now well-established targets of estrogens. Among the numerous aspects of brain function regulated by estrogens are their effects on mood, cognitive function, and neuronal viability. Here, we review the supporting evidence for estrogens as neuroprotective agents and summarize the various mechanisms that may be involved in this effect, focusing particularly on the mitochondria as an important target. On the basis of this evidence, we discuss the clinical applicability of estrogens in treating various age-related disorders, including Alzheimer disease and stroke, and identify the caveats that must be considered.

Differential modulation of estrogen receptors (ERs) in ischemic brain injury: a role for ERalpha in estradiol-mediated protection against delayed cell death

Estradiol enhances plasticity and survival of the injured brain. Our previous work demonstrates that physiological levels of estradiol protect against cerebral ischemia in the young and aging brain through actions involving estrogen receptors (ERs) and alterations in gene expression. The major goal of this study was to establish mechanisms of neuroprotective actions induced by low levels of estradiol. We first examined effects of estradiol on the time-dependent evolution of ischemic brain injury. Because estradiol is known to influence apoptosis, we hypothesized that it acts to decrease the delayed phase of cell death observed after middle cerebral artery occlusion (MCAO). Furthermore, because ERs are pivotal to neuroprotection, we examined the temporal expression profiles of both ER subtypes, ERalpha and ERbeta, after MCAO and delineated potential roles for each receptor in estradiol-mediated neuroprotection. We quantified cell death in brains at various times after MCAO and analyzed ER expression by RT-PCR, in situ hybridization, and immunohistochemistry. We found that during the first 24 h, the mechanisms of estradiol-induced neuroprotection after MCAO are limited to attenuation of delayed cell death and do not influence immediate cell death. Furthermore, we discovered that ERs exhibit distinctly divergent profiles of expression over the evolution of injury, with ERalpha induction occurring early and ERbeta modulation occurring later. Finally, we provide evidence for a new and functional role for ERalpha in estradiol-mediated protection of the injured brain. These findings indicate that physiological levels of estradiol protect against delayed cell death after stroke-like injury through mechanisms requiring ERalpha.

Estrogens as neuroprotectants against ischemic stroke

Estrogens have proven vasoprotective properties against atherosclerosis that depend on the direct effect on vascular smooth muscle and endothelium and on systemic actions that imply serum lipids, coagulation and fibrinolytic cascades, vasoactive proteins and antioxidant systems. They also have neuroprotective effects against cerebral ischemia that include antioxidant and anti-inflammatory effects, modulation of protein synthesis, inhibition of apoptosis and trophic effects and preservation of microvascular blood flow in the ischemic area. Estrogenic actions depend on activation of specific estrogen receptors that modulate gene expression and produce long-term effects on vascular endothelial and smooth muscle cells, neurons and glia, on interaction with plasma membrane sites that produce rapid non-genomic actions and also on receptor-independent mechanisms. This paper reviews what it is known about the mechanisms underlying the vaso- and neuroprotective effects of estrogens. Experimental and clinical evidences of such protective effects are also discussed. Therapeutical implications for stroke prevention and treatment derived from the available evidence are considered.

Mechanisms of neuroprotection by estrogen

Over the past decade our recognition that estrogens function as important neurotrophic and neuroprotective factors has grown rapidly. Accumulating evidence from basic science studies demonstrates that estrogens exert profound protective actions against various forms of neurodegenerative diseases and injury. Although a thorough understanding of the mechanisms underlying the protective effect of estrogens is far from complete, significant progress has been achieved through the use of in vivo as well as in vitro models. Here we review the results from our laboratory demonstrating that low physiological levels of estradiol therapy exert powerful protection against ischemic stroke-like injury. Using an animal model of cerebrovascular stroke and in vitro explant cultures, we have begun to decipher under what circumstances 17beta-estradiol protects against neuronal death and to uncover its mechanisms of action. In addition, we will review recent work demonstrating that estradiol may additionally enhance the ability of the adult brain to undergo repair by influencing the production of new neurons under neuropathological conditions, as well as by promoting an anti-inflammatory response. As we uncover the important protective roles of ovarian steroid hormones in brain disease and injury, we increasingly appreciate that the mechanisms by which estrogens achieve these effects are diverse and complex.

Age-related changes in neuroprotection: is estrogen pro-inflammatory for the reproductive senescent brain?

Estrogen replacement therapy (ERT) is widely prescribed to postmenopausal women for relief from the adverse vasomotor effects of menopause, to reduce bone loss, to improve cardiovascular health, and to protect against metabolic disorders. However, there is now greater awareness of the increased risk to benefit ratio from the recently concluded Women's Health Initiative Memory Study (WHIMS), which reported that ERT increased the risk of cognitive impairment and dementia in elderly women. Studies from the experimental literature indicate that while estrogen is neuroprotective in many instances, estrogen replacement can be deleterious in some cases. These differences may be partly due to the age and species of the experimental model. The majority of the experimental data comes from studies where the age or endocrine status of the animal model is not comparable to that of menopausal or postmenopausal women, such as those in the WHIMS study. In this review, we will focus on age-related changes in estrogen's neuroprotective effects and evidence that reproductive senescence-related changes in the blood-brain barrier and the immune system may result in deleterious consequences for ERT.

Resuscitation from experimental heatstroke by estrogen therapy

OBJECTIVE

We investigated the effect of estrogen therapy on inflammatory responses, cardiovascular functions, and survival in a rat model of heatstroke.

DESIGN

Controlled, prospective study.

SETTING

Hospital medical research laboratory.

SUBJECTS

Sprague-Dawley rats (280-312 g of body weight, males and females).

INTERVENTIONS

Four major groups of anesthetized rats were designated for experiments: a) vehicle-treated male rats; b) vehicle- or premarin-treated estrus female rats; c) vehicle- or premarin-treated ovariectomized rats; and d) vehicle- or premarin-treated leuprolide-treated rats. All animals were exposed to heat stress (ambient temperature 43 degrees C for 70 mins) and then allowed to recover at room temperature (24 degrees C). Their survival time (interval between the onset of heatstroke and animal death) and physiologic and biochemical variables were monitored. Vehicle (normal saline 1 mL/kg of body weight, intravenously) or premarin (1 mg/mL/kg of body weight, intravenously) was administered 70 mins after initiation of heat stress. Ovariectomy or leuprolide (100 mug/kg/day, subcutaneously) injection was conducted 4 wks before the start of heat stress experiments. Another group of rats were exposed to 24 degrees C and used as normothermic controls.

MEASUREMENTS AND MAIN RESULTS

Compared with the estrus female rats, the ovariectomized rats, the leuprolide-treated rats, and male rats all had lower levels of plasma estradiol and lower survival time values. However, after an intravenous dose of premarin, both the plasma estradiol and survival time values were significantly increased. Compared with the normothermic controls, the vehicle-treated male and ovariectomized rats all displayed higher levels of serum tumor necrosis factor-alpha, which could be suppressed by premarin therapy. In contrast, the serum levels of IL-10 in these groups were significantly elevated by premarin during heatstroke. Furthermore, the heatstroke-induced hyperthermia, arterial hypotension, intracranial hypertension, and cerebral hypoperfusion, hypoxia, and ischemia were significantly attenuated by premarin therapy in ovariectomized rats.

CONCLUSIONS

We successfully demonstrated that estrogen replacement may improve survival during heatstroke by ameliorating inflammatory responses and cardiovascular dysfunction.

Neuroprotection by ovarian hormones in animal models of neurological disease

Ovarian hormones can protect against brain injury, neurodegeneration, and cognitive decline. Most attention has focused on estrogens and accumulating data demonstrate that estrogen seems to specifically protect cortical and hippocampal neurons from ischemic injury and from damage due to severe seizures. Although multiple studies demonstrate protection by estrogen, in only a few instances is the issue of how the steroid confers protection known. Here, we first review data evaluating the neuroprotective effects of estrogens, a selective estrogen receptor modulator (SERM), and estrogen receptor alpha- and beta-selective ligands in animal models of focal and global ischemia. Using focal ischemia in ovariectomized ERalphaKO, ERbetaKO, and wild-type mice, we clearly established that the ERalpha subtype is the critical ER mediating neuroprotection in mouse focal ischemia. In rats and mice, the middle cerebral artery occlusion (MCAO) model was used to represent cerebrovascular stroke, while in gerbils the two-vessel occlusion model, representing global ischemia, was used. The gerbil global ischemia model was used to evaluate the neuroprotective effects of estrogen, SERMs, and ERalpha- and ERbeta-selective compounds in the hippocampus. Analysis of neurogranin mRNA, a marker of viability of hippocampal neurons, with in situ hybridization, revealed that estrogen treatment protected the dorsal CA1 regions not only when administered before, but also when given 1 h after occlusion. Estrogen rarely is secreted alone and studies of neuroprotection have been less extensive for a second key ovarian hormone progesterone. In the second half of this review, we present data on neuroprotection by estrogen and progesterone in animal model of epilepsy followed by exploration into ovarian steroid effects on neuronal damage in models of multiple sclerosis and traumatic brain injury.

Estradiol negatively regulates HIV-LTR promoter activity in glial cells

HIV-associated dementia results from neuronal loss and an alteration of neuronal function due to a loss of synapses. While HIV infection in astrocytes is limited, astrocytes exhibit a chronic nonproductive infection that can lead to the release of neurotoxic proteins. Additionally, infection can disrupt the normal neurotrophic role of astrocytes that results in neuronal death. Gonadal steroid hormones are known to act as trophic and protective factors in the brain under a variety of normal and pathological conditions. In the present study, to determine if estrogen plays a role in the ability of Tat to function as a transcriptional activator within astrocytes, we examined the effect of estrogen on regulation of viral transcription. We utilized an immortalized human astrocyte cell line (SVGA) stably transfected with a reporter plasmid containing the HIV-1IIIB LTR driving the chloramphenicol acetyltransferase (CAT) gene. The amount of transcriptional activity was measured by quantifying the amount of CAT produced. We determined that 17beta-estradiol treatment (1 nM) had no effect on basal LTR activity. Following transfection with a Tat-expressing plasmid, there was a 100-fold increase in CAT production. This induction was reduced by 40% in cells pretreated with 17beta-estradiol. 17beta- Estradiol only suppressed transcription stimulated by Tat. Furthermore, we determined that this effect was specific to 17beta-estradiol and estrogen receptor agonists. This activity was limited to astrocytes as no effect was observed in a monocytic cell line. Finally, the mechanism of action did not involve an alteration in levels of Cdk9 or Cyclin T1 proteins necessary for Tat activation of the HIV-1 LTR. This study demonstrates a novel activity of 17beta-estradiol in glial cells that could play a role in the maintenance of neuronal health during HIV infection of the central nervous system.

Chronic Treatment with Physiological Doses of Estradiol Affects the GH-IGF-1 Axis and Fat Metabolism in Young and Middle-aged Ovariectomized Rats

Estrogen deficiency may be partly responsible for the metabolic syndrome and the condition may be reversible with hormone replacement therapy. However, after the heart and estrogen/progestin replacement study and the women's health initiative study the prospect of HRT on CVD has changed dramatically. As the pituitary and the liver are targets for estrogen action we assessed the effect of ovariectomy (OVX) and long-term (3 months) estradiol (E2) treatment by means of subcutaneously (s.c.) implanted silastic capsules on pituitary and liver function in young and middle-aged female rats. Our results demonstrate that triglyceride serum levels increased, whereas insulin-like growth factor-1 (IGF-1), high density lipoprotein cholesterol (HDL), and glucose levels decreased during the transition from young to middle-age. E2 treatment increased dose-independently growth hormone (GH) secretion. IGF-1 levels were increased upon OVX in middle-aged rats and the high dose of E2 decreased IGF-1 concentrations in both age groups. Cholesterol concentrations increased after OVX and were attenuated by E2 administration in middle-aged rats. Both, low density lipoprotein cholesterol (LDL) and HDL levels raised after castration and both parameters decreased in response to E2 in young and middle-aged rats. Glucose serum concentrations decreased after E2 treatment in all animals. Taken together, our results clearly demonstrate that the pituitary and the liver metabolism of middle-aged rats remain susceptible to the influence of OVX and E2 treatment.

Estradiol reduces nonclassical transcription at cyclic adenosine 3',5'-monophosphate response elements in glioma cells expressing estrogen receptor alpha

Estradiol can protect the brain from a variety of insults by activating membrane-initiated signaling pathways, and thereby modulate gene expression and lead to functional changes in neurons. These direct neuronal effects of the hormone have been well documented; however, it is less understood what effects estradiol may have on nonneuronal cells of the central nervous system. There is evidence that estradiol levels can induce the release of glial-derived growth factors and other cytokines, suggesting that estradiol may both directly and indirectly protect neurons. To determine whether 17beta-estradiol (E2) can activate rapid signaling and modulate nonclassical transcription in astrocytes, we stably transfected the C6 rat glioblastoma cell line with human estrogen receptor (ER) alpha (C6ERalpha) or rat ERbeta (C6ERbeta). Introduction of a cAMP response element-luciferase reporter gene into C6, C6ERalpha, and C6ERbeta cells leads to the observation that E2 treatment reduced isoproterenol-stimulated luciferase activity by 35% in C6ERalpha but had no effect on reporter gene expression in C6ERbeta or untransfected C6 cells. A similar effect was seen with a membrane-impermeable estrogen (E2-BSA), suggesting the modulation of nonclassical transcription by estradiol treatment is mediated by the activation of a membrane-initiated signaling pathway. Furthermore, pretreatment with wortmannin (phosphatidylinsositol 3-kinase) or U73122 (phospholipase C) attenuated the E2-induced reduction in nonclassical transcription. We conclude that E2 treatment reduces cAMP response element-mediated transcription in glioma cells expressing ERalpha and that this reduction is dependent on the activation of membrane-initiated signaling. These findings suggest a novel model of estrogen rapid signaling in astrocytes that leads to modulation of nonclassical transcription.

Acute effects of 17beta-estradiol on the extracellular concentration of excitatory amino acids and energy metabolites during transient cerebral ischemia in male rats

Elevation of extracellular levels of amino acids has been implicated in the pathogenesis of stroke. The failure of brain energy metabolism due to the lack of oxygen and glucose contributes also to cell loss. Estrogen has been shown to protect brain cells against ischemia by a still unclear mechanism. We used intracerebral microdialysis to monitor the effects of acute 17beta-estradiol treatment on the release of glutamate and aspartate and on the levels of the energy metabolites glucose and lactate. In male rats subjected to 90 min of transient middle cerebral artery occlusion followed by 24-h reperfusion, acute treatment with 17beta-estradiol (0.8 mg/kg, i.v.) at the time of occlusion reduced the ischemic infarct by about 50%. In these treated rats, the ischemia-induced increases of extracellular levels of glutamate and aspartate were significantly and rapidly reduced. The reduction of glucose level during occlusion was not affected by 17beta-estradiol treatment; however, the increase of extracellular lactate was reduced during occlusion and reperfusion, probably due to the reduced glutamate-driven astrocytic glycolysis. These data suggest that acute treatment with 17beta-estradiol at the onset of occlusion significantly reduces the ischemia-induced excitotoxicity in the cortex, a mechanism that may participate in the neuroprotective effect on cellular survival.