Modulation of Bcl-2 expression: a potential component of estrogen protection in NT2 neurons

Neurodevelopmental liabilities of substance abuse

The perinate is particularly risk-prone to chemical species which have the potential of inducing neuronal apoptosis or necrosis and thereby adversely altering development of the brain, to produce life-long functional and behavioral deficits. This paper is an overview for many substances of abuse, but the purview is much more broadened by the realization that even elevated levels of estrogens and corticosteroids in the pregnant mother can act as neuroteratogens, by passing via the placenta and altering neural development or inducing apoptosis in the perinate. Finally, therapeutic risks of anesthetics are highlighted, as these too induce neuronal apoptosis in the neonate by either blocking N-methyl-D-aspartate receptors or by acting as gamma-aminobutyric acid agonists. By understanding the mechanisms involved it may ultimately be possible to interrupt the mechanistic scheme and thereby prevent neuroteratological processes.

Estrogen and brain vulnerability

Accumulated clinical and basic evidence suggests that gonadal steroids affect the onset and progression of several neurodegenerative diseases and schizophrenia, and the recovery from traumatic neurological injury such as stroke. Thus, our view on gonadal hormones in neural function must be broadened to include not only their function in neuroendocrine regulation and reproductive behaviors, but also to include a direct participation in response to degenerative disease or injury. Recent findings indicate that the brain up-regulates both estrogen synthesis and estrogen receptor expression at sites of injury. Genetic or pharmacological inactivation of aromatase, the enzyme involved in estrogen synthesis, indicates that the induction of this enzyme in the brain after injury has a neuroprotective role. Some of the mechanisms underlying the neuroprotective effects of estrogen may be independent of the classically defined nuclear estrogen receptors (ERs). Other neuroprotective effects of estrogen do depend on the classical nuclear ERs, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that non-classical ERs in the membrane or cytoplasm alter phosphorylation cascades, such as those involved in the signaling of insulin-like growth factor-1 (IGF-1). Indeed, ERs and IGF-1 receptor interact in the activation of PI3K and MAPK signaling cascades and in the promotion of neuroprotection. The decrease in estrogen and IGF-1 levels with aging may thus result in an increased risk for neuronal pathological alterations after different forms of brain injury.

The estrogen receptor is not essential for all estrogen neuroprotection: new evidence from a new analog

We synthesized an estrogen analog, ZYC-5, lacking activity at the classical estrogen receptor and examined its neuroprotective potential against necrosis induced by N-methyl-d-aspartate (NMDA) and apoptosis/necrosis induced by the NMDA receptor antagonist (+)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP). ZYC-5 protected cortical neurons in a dose-dependent manner, and the neuroprotection was more robust than with 17beta-estradiol. The effect of ZYC-5 was not mediated by the classical estrogen receptor, because it was unaffected by the antagonists 4-hydroxytamoxifen and ICI 182,780. The ZYC-5 protection against excitotoxicity was not directly mediated through the NMDA receptor, because there was no effect of ZYC-5 on NMDA current or the intracellular calcium increase induced by NMDA. Results obtained with the free-radical-sensitive dye, dihydroethidium, suggested that the neuroprotection of ZYC-5 was partly related to its radical scavenging properties. Although some of estrogen's neuroprotective effects may depend upon the estrogen receptor, our results suggest the possibility of neuroprotection without hormonal side effects.

Bcl-2 expression in thalamus, brainstem, cerebellum and visual cortex of adult primate

Due to the functional importance of Bcl-2, which acts as an anti-apoptotic protein that also affects neural differentiation and adult neurogenesis, we undertook a detailed immunohistochemical study of the distribution of this protein in the brain of squirrel monkeys. The present study describes findings obtained at thalamic, brainstem, cerebellum and visual cortex levels, and the data are compared with our previous results gathered in the same species. At thalamic level, Bcl-2-positive neurons occur in anterior, rostral intralaminar, midline and lateral habenular nuclei. The protein is also expressed in several structures associated with the ventricular system, including the subventricular zone (SVZ), the subcommissural organ, and the periventricular grey at rostral and caudal tips of the fourth ventricle. At brainstem and cerebellar levels, Bcl-2-positive neurons occur in the dorsal raphe nucleus, inferior olivary complex, and in molecular and granular layers of the cerebellum. Finally, neurons of layer IV of the striate cortex display a very strong Bcl-2 immunoreactivity that contrasts with the poor labeling of neurons in adjacent parastriate and peristriate cortices. These finding suggests that Bcl-2 plays a role in the plasticity and structural maintenance of various structures in the primate brain and indicate that the mitotically active SVZ might be more extended along the rostrocaudal axis in primates than in rodents.

Estradiol is a protective factor in the adult and aging brain: understanding of mechanisms derived from in vivo and in vitro studies

We have shown that 17beta-estradiol exerts profound protective effects against stroke-like ischemic injury in female rats. These effects are evident using physiological levels of estradiol replacement in ovariectomized rats and require hormone treatment prior to the time of injury. The protective actions of estradiol appear to be most prominent in the cerebral cortex, where cell death is not apparent until at least 4 h after the initiation of ischemic injury and where cell death is thought to be apoptotic in nature. Middle-aged rats remain equally responsive to the protective actions of estradiol. The maintenance of responsiveness of the cerebral cortex to the neuroprotective actions of estradiol was unexpected since responsiveness of the hypothalamus to estradiol decreases dramatically by the time animals are middle-aged. We believe that the protective actions of estradiol require the estrogen receptor-alpha, since estradiol does not protect in estrogen receptor-alpha knockout mice. We have also implemented a method of culturing cerebral cortical explants to assess the protective effects of estradiol in vitro. This model exhibits remarkable parallelisms with our in vivo model of brain injury. We have found that 17beta-estradiol decreases the extent of cell death and that this protective effect requires hormone pretreatment. Finally, 17alpha-estradiol, which does not interact effectively with the estrogen receptor, does not protect; and addition of ICI 182,780, an estrogen receptor antagonist, blocks the protective actions of estradiol. We have begun to explore the molecular and cellular mechanisms of estradiol-mediated protection. In summary, our findings demonstrate that estradiol exerts powerful protective effects both in vivo and in vitro and suggest that these actions are mediated by estrogen receptors.

Estrogen and Bcl-2: gene induction and effect of transgene in experimental stroke

Female rodents producing endogenous estrogens are protected from stroke damage in comparison with male counterparts. This natural protection is lost after ovariectomy or reproductive senescence. The aim of this study is to determine whether estrogen reduces early neuronal injury and cell loss after ischemia by increasing the expression of Bcl-2. Male, intact female, ovariectomized, and estrogen-repleted ovariectomized rats were subjected to middle cerebral artery occlusion, and 22 hr later the level and localization of Bcl-2 mRNA and protein were determined. The levels of post-ischemic bcl-2 mRNA and protein were increased exclusively in neurons within the peri-infarct region. Intact females and estrogen-treated castrates demonstrated increased bcl-2 mRNA and protein expression compared with males and estrogen-deficient females, accompanied by a decrease in infarct size. To test the hypothesis that the neuroprotective mechanism of estrogen functions via Bcl-2, we compared ischemic outcome in male, female, and ovariectomized wild-type mice and mice overexpressing Bcl-2 exclusively in neurons. Wild-type female mice sustained smaller infarcts compared with males. Bcl-2 overexpression reduced infarct size in males, but provided no added protection in the female. Moreover, ovariectomy exacerbated infarction in wild-type females, but had no effect in Bcl-2 overexpressors. These data indicate that overexpression of Bcl-2 simulates the protection against ischemic injury conferred by endogenous female sex steroids. We concluded that estrogen rescues neurons after focal cerebral ischemia by increasing the level of Bcl-2 in peri-infarct regions and that estrogen-induced bcl-2 gene expression is an important downstream component of neuronal protection in female stroke.

Estrogen regulates bcl-x expression in rat hippocampus

In this study, we examined whether experimental alterations of circulating estrogen levels are associated with changes in the expression of bcl-x, an inhibitor of apoptosis. We report that bcl-x mRNA expression in rat hippocampus significantly decreases after reduction of estrogen levels resulting from ovariectomy. Exposure of ovariectomized rats to 17beta-estradiol for either 5 or 28 days restored bcl-x mRNA expression to levels at or above those observed in sham-ovariectomized control animals. These data demonstrate that physiological levels of estrogen regulate hippocampal expression of bcl-x, an important modulator of neuronal apoptosis. Estrogen-mediated regulation of bcl-x may be relevant to the maintenance of neuronal viability and may contribute to the mechanism of estrogen neuroprotection.

Estrogen attenuates cell death induced by carboxy-terminal fragment of amyloid precursor protein in PC12 through a receptor-dependent pathway

In the present study, we investigated effects of estrogen on cell death induced by carboxy-terminal fragment of amyloid precursor protein (CT), a candidate causative substance in the pathogenesis of Alzheimer's disease. 17 beta-Estradiol attenuated CT-induced cell death in PC12 cells, whereas 17 alpha-estradiol, nonestrogenic stereoisomer, did not exert any significant protective effect on CT-induced cell death. These results suggest that protective effects of estrogen may be mediated by estrogen receptor (ER) in PC12 cells. To confirm the results, we determined the effects of tamoxifen, an estrogen receptor antagonist. Tamoxifen blocked the protective effects of 17 beta-estradiol, although it did not affect those of 17 alpha-estradiol. Overall, it might be thought that the protective effect of estradiol on CT-induced cell death is achieved by hormonal properties mediated through the estrogen receptor rather than the structural properties as a reducing agent.

17beta-Estradiol reduces cortical lesion size in the glutamate excitotoxicity model by enhancing extracellular lactate: a new neuroprotective pathway

Estrogens play an important role in neuronal function and in protecting neurones in the cerebral cortex against pathological conditions. An in vivo model of glutamate excitotoxicity in which glutamate is applied to the cortex of rats through a microdialysis probe has been used to investigate the neuroprotective processes initiated by 17beta-estradiol. Rats were pre-treated with 17beta-estradiol (i.v.) before local application of 100 mM glutamate into the cortex through a microdialysis probe. Pre-treatment with 17beta-estradiol significantly reduced the size of the glutamate-induced cortical lesion. In the cortical microdialysates collected from the probe, a peak of lactate was observed immediately after glutamate application. After 17beta-estradiol pre-treatment this peak of lactate was significantly higher with estradiol than without 120 min after glutamate application, reaching 700% basal level at the end of measurement. The level of extracellular glucose was markedly decreased with and without 17beta-estradiol pre-treatment. Local blockage of neuronal lactate transporters with alpha-cyano-4-hydroxycinnamate (4-CIN) completely abolished the neuroprotective effect of 17beta-estradiol and induced a larger cortical lesion. An accumulation of extracellular lactate was observed after inhibition of the lactate transporters suggesting that transport of lactate into neurones is necessary for the neuroprotective effect of 17beta-estradiol. The anti-estrogen tamoxifen also abolished the neuroprotective effect of 17beta-estradiol on the lesion size and inhibited the production of lactate. These results suggest a new neuroprotective mechanism of 17beta-estradiol by activating glutamate-stimulated lactate production, which is estrogen receptor-dependent.

Neurotrophic and neuroprotective actions of estrogens and their therapeutic implications

Originally known for its regulation of reproductive functions, estradiol, a lipophilic hormone that can easily cross plasma membranes as well as the blood-brain barrier, maintains brain systems subserving arousal, attention, mood, and cognition. In addition, both synthetic and natural estrogens exert neurotrophic and neuroprotective effects. There is increasing evidence that estrogen actions are mediated by nongenomic as well as direct and indirect genomic pathways. Although in vitro models have provided the most extensive evidence for neurotrophic and neuroprotective actions to date, there are also in vivo studies that support these actions.

Mechanisms of estrogenic protection against gp120-induced neurotoxicity

gp120, an HIV coat glycoprotein that may play a role in AIDS-related dementia complex (ADC), induces neuronal toxicity characterized by NMDA receptor activation, accumulation of intracellular calcium, and downstream degenerative events including generation of reactive oxygen species and lipid peroxidation. We have previously demonstrated estrogenic protection against gp120 neurotoxicity in primary hippocampal cultures. We here characterize the mechanism of protection by blocking the classical cytosolic estrogen receptors and by measuring oxidative end points including accumulation of extracellular superoxide and lipid peroxidation. Despite blocking ERalpha and ERbeta with 1 microM tamoxifen, we do not see a decrease in the protection afforded by 100 nM 17 beta-estradiol against 200 pM gp120. Additionally, 17alpha-estradiol, which does not activate estrogen receptors, protects to the same extent as 17beta-estradiol. 17beta-Estradiol does, however, decrease gp120-induced lipid peroxidation and accumulation of superoxide. Together the data suggest an antioxidant mechanism of estrogen protection that is independent of receptor binding.

Apoptosis in cultured hNT neurons

Programmed cell death (apoptosis) is an important mechanism shaping the size of different cell populations within the developing nervous system. In our study we used the NT2/D1 clone originally established from the Ntera 2 cell line to investigate the baseline levels of apoptosis in cultured postmitotic hNT (NT2-N) neurons previously treated for 3, 4 or 5 weeks with retinoic acid (RA) and compared it with apoptosis in NT2 precursors unexposed to RA. First, we examined whether different lengths of exposure to RA might affect baseline apoptotic rate in differentiating hNT neurons. Second, we investigated whether cultured hNT neurons, previously shown to possess dopaminergic characteristics, would be preferentially affected by apoptosis. Using the terminal deoxynucleotidyl transferase (tdt)-labeling technique we found that the postmitotic hNT neuronal cells exposed to RA demonstrated significantly higher numbers of apoptotic cells (12.5-15.8%) in comparison to rapidly dividing NT2 precursor cell line (3.6-4.4%) at both studied (1 and 5 days in vitro, DIV) time points. Similar apoptotic nuclear morphology, including a variable extent of nuclear fragmentation was observed in all examined hNT cultures. On the other hand, the incidence of apoptotic nuclei was rare in cultures of NT2 precursors not subjected to RA treatment. Combined immunocytochemistry for tyrosine hydroxylase (TH) and Hoechst staining revealed dopaminergic hNT neurons destined to die. Our double-labeling studies have demonstrated that only a subset of TH-positive hNT cells had condensed chromatin after 1 (approx. 15%) and 5 (approx. 20%) DIV. NT2 precursors were not TH-positive. Collectively, our results demonstrated that exposure to differentiating agent RA triggers an apoptotic commitment in a subset of postmitotic hNT neurons. These results suggest that this cell line may serve as a model of neuronal development to test various pathogenic factors implicated in the etiology of Parkinson's disease (PD), as well as to screen numerous pharmacological treatments that may slow or prevent dopaminergic deterioration.

Estrogens and estrogen-like non-feminizing compounds. Their role in the prevention and treatment of Alzheimer's disease

The present position paper is intended to provide evidence that estrogen deprivation contributes to the occurrence and course of Alzheimer's disease (AD) and that currently available estrogen preparations may be useful in the prevention and treatment of AD in women. Additionally, there is now substantial preclinical evidence to support the development of novel non-feminizing estrogens for use in male and female subjects for the protection of neurons from damage and death that underlies the neuropathology of AD. Estrogens and non-feminizing estrogen-like compounds may exert their beneficial effects in AD through a variety of mechanisms, directly through their neuroprotective actions and indirectly through their neurotrophic effects. Inasmuch as estrogens are comparatively free of both acute and chronic toxicities, and non-feminizing estrogens are expected to be even safer, their use for years to decades for the prevention or treatment of AD is possible.

17-beta estradiol can reduce secondary ischemic damage and mortality of subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is a unique disorder commonly occurring when an aneurysm ruptures, leading to bleeding and clot formation, with a higher incidence in females. To evaluate the influence of 17-beta estradiol (E2) in the outcome of subarachnoid hemorrhage, SAH was induced by endovascular puncture of the intracranial segment of internal carotid artery in 15 intact females (INT), 19 ovariectomized females (OVX), and 13 ovariectomized female rats with E2 replacement (OVX + E2). Cerebral blood flow was recorded before and after SAH. All animals were decapitated immediately after death or 24 hours after SAH for clot area analysis. Brains were sliced and stained with 2,3,5-triphenyltetrazolium chloride (TTC) for secondary ischemic lesion analysis. The cortical cerebral blood flow (CBF), which was measured by a laser-Doppler flowmeter, decreased to 29.6%+/-17.7%, 22.8%+/-8.3%, and 43.5%+/-22.9% on the ipsilateral side (P = 0.01), and decreased to 63.4%+/-14.1%, 57.4%+/-11.0%, and 66.6%+/-17.9% on the contralateral side (P = 0.26) in INT, OVX, and OVX + E2, respectively. The subcortical CBF, which were measured by the H2 clearance method, were 7.77+/-12.03, 7.80+/-8.65, and 20.58+/-8.96 mL 100 g(-1) min(-1) on the ipsilateral side (P < 0.01), and 21.53+/-2.94, 25.13+/-3.01, and 25.30+/-3.23 mL 100 g(-1) min(-1) on the contralateral side in INT, OVX, and OVX + E2, respectively. The mortality was 53.3%, 68.4%, and 15.4% in INT, OVX, and OVX + E2, respectively (P = 0.01), whereas no significant difference in clot area was noted among the groups. The secondary ischemic lesion volume was 9.3%+/-8.4%, 24.3%+/-16.3%. and 7.0%+/-6.4% in INT, OVX, and OVX + E2, respectively (P < 0.01). This study demonstrated that E2 can reduce the mortality and secondary ischemic damage in a SAH model without affecting the clot volume.

Estrogens: trophic and protective factors in the adult brain

Our appreciation that estrogens are important neurotrophic and neuroprotective factors has grown rapidly. Although a thorough understanding of the molecular and cellular mechanisms that underlie this effect requires further investigation, significant progress has been made due to the availability of animal models in which we can test potential candidates. It appears that estradiol can act via mechanisms that require classical intracellular receptors (estrogen receptor alpha or beta) that affect transcription, via mechanisms that include cross-talk between estrogen receptors and second messenger pathways, and/or via mechanisms that may involve membrane receptors or channels. This area of research demands attention since estradiol may be an important therapeutic agent in the maintenance of normal neural function during aging and after injury.

Neuroprotection by estradiol

This review highlights recent evidence from clinical and basic science studies supporting a role for estrogen in neuroprotection. Accumulated clinical evidence suggests that estrogen exposure decreases the risk and delays the onset and progression of Alzheimer's disease and schizophrenia, and may also enhance recovery from traumatic neurological injury such as stroke. Recent basic science studies show that not only does exogenous estradiol decrease the response to various forms of insult, but the brain itself upregulates both estrogen synthesis and estrogen receptor expression at sites of injury. Thus, our view of the role of estrogen in neural function must be broadened to include not only its function in neuroendocrine regulation and reproductive behaviors, but also to include a direct protective role in response to degenerative disease or injury. Estrogen may play this protective role through several routes. Key among these are estrogen dependent alterations in cell survival, axonal sprouting, regenerative responses, enhanced synaptic transmission and enhanced neurogenesis. Some of the mechanisms underlying these effects are independent of the classically defined nuclear estrogen receptors and involve unidentified membrane receptors, direct modulation of neurotransmitter receptor function, or the known anti-oxidant activities of estrogen. Other neuroprotective effects of estrogen do depend on the classical nuclear estrogen receptor, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that estrogen receptors in the membrane or cytoplasm alter phosphorylation cascades through direct interactions with protein kinases or that estrogen receptor signaling may converge with signaling by other trophic molecules to confer resistance to injury. Although there is clear evidence that estradiol exposure can be deleterious to some neuronal populations, the potential clinical benefits of estrogen treatment for enhancing cognitive function may outweigh the associated central and peripheral risks. Exciting and important avenues for future investigation into the protective effects of estrogen include the optimal ligand and doses that can be used clinically to confer benefit without undue risk, modulation of neurotrophin and neurotrophin receptor expression, interaction of estrogen with regulated cofactors and coactivators that couple estrogen receptors to basal transcriptional machinery, interactions of estrogen with other survival and regeneration promoting factors, potential estrogenic effects on neuronal replenishment, and modulation of phenotypic choices by neural stem cells.

Estradiol protects against ischemic brain injury in middle-aged rats

Several clinical studies suggest that estradiol acts as a potent growth and protective factor in the adult brain. Postmenopausal women experience permanent hypoestrogenicity and suffer from increased risk of brain injury associated with neurodegenerative diseases such as stroke and Alzheimer's disease. Estrogen replacement therapy appears to decrease the risk and severity of these neurodegenerative conditions. Studies using animal models have shown that estradiol exerts similar effects in rodents and can enhance cell survival and induce synaptic plasticity. Therefore, we undertook studies to assess whether estradiol treatment can decrease brain injury and cell death induced by an experimental model of ischemia and whether aging animals remain responsive to the protective effects of estradiol. We will review results from recent studies that demonstrate that 1) in young animals, estrogens exert profound protective effects against ischemic brain injury induced by cerebral artery occlusion and 2) the response of aging animals has been tested with varying results. We will discuss and compare our experimental findings that utilize a permanent cerebral artery occlusion model and physiological levels of estradiol replacement therapy in young and middle-aged rats with those of previous studies. These observations provide important insights into the potential protective actions of estrogen replacement therapy on age- and disease-related processes in the brain.

Estradiol protects against injury-induced cell death in cortical explant cultures: a role for estrogen receptors

Estradiol has been shown to exert trophic and protective actions in the brain. Our laboratory has shown that in vivo, low physiological levels of estradiol protect the female rat brain against ischemic injury. In the present study, we used organotypic cortical explant cultures to begin to decipher the mechanisms of estradiol's actions. Injury was induced by exposure to kainic acid or potassium cyanide/2-deoxyglucose (KCN/2-DG) for varying lengths of time, and cell death was monitored by LDH release at 2, 6, 12, 24, 48, 72 and 96 h after injury. We found that exposure to 1 mM KCN/2 mM 2-DG for 2 h produced consistent delayed cell death that was detectable by 24 h. The presence of 17beta-estradiol (E2) during the 7 days prior to injury significantly reduced the extent of cell death; whereas, administration of E2 at the time of injury did not protect. The protective effects of estradiol were dose dependent. Low doses of E2 (1, 10, and 30 nM) significantly reduced cell death; however, higher concentrations of E2 (>60 nM) had no protective effect. The observations that low levels of E2 protect against cell death, and that pretreatment is required suggest that the protective actions of estradiol may involve estrogen receptors. Therefore, we examined the ability of 17alpha-estradiol, which does not efficiently activate the estrogen receptor, and the addition of the estrogen receptor antagonist, ICI 182,780, to influence the extent of cell death induced by KCN/2-DG. 17alpha-Estradiol failed to protect, and ICI 182,780 prevented E2 from protecting against cell death. Furthermore, E2 pretreatment is required for more than 24 h to be neuroprotective. Our results clearly show that in cortical explant cultures, estradiol protects cells against ischemic injury, and suggest that these protective actions involve estrogen receptors.

Neuroprotective effects of estrogens: potential mechanisms of action

Epidemiological studies associate post-menopausal estrogen use with a reduction in risk of Alzheimer's disease, a reduction in risk of Parkinson's disease, and death from stroke. The neuroprotective efficacy of estrogens have been well described and may contribute to these clinical effects. Estrogen-mediated neuroprotection has been described in several neuronal culture model systems with toxicities including serum-deprivation, beta-amyloid-induced toxicity, excitotoxicity, and oxidative stress. In animal models, estrogens have been shown to attenuate neuronal death in rodent models of cerebral ischemia, traumatic injury, and Parkinson's disease. Although estrogens are known to exert several direct effects on neurons, the cellular mechanisms behind the neuroprotective efficacy of the steroid are only beginning to be elucidated. In this review, we summarize the data supporting a neuroprotective role for estrogens in both culture and animal models and discuss neuronal effects of estrogens that may contribute to the neuroprotective effects. These effects include activation of the nuclear estrogen receptor, altered expression of bcl-2 and related proteins, activation of the mitogen activated kinase pathway, activation of cAMP signal transduction pathways, modulation of intracellular calcium homeostasis, and direct antioxidant activity.

Mechanisms of antiapoptotic effects of estrogens in nigral dopaminergic neurons

Parkinson's disease is characterized by the mesencephalic dopaminergic neuronal loss, possibly by apoptosis, and the prevalence is higher in males than in females. The estrogen receptor (ER) subtype in the mesencephalon is exclusively ER beta, a recently cloned novel subtype. Bound with estradiol, it enhances gene transcription through the estrogen response element (ERE) or inhibits it through the activator protein-1 (AP-1) site. We demonstrated that 17beta-estradiol provided protection against nigral neuronal apoptosis caused by exposure to either bleomycin sulfate (BLM) or buthionine sulfoximine (BSO). BLM and BSO-induced nigral apoptosis was blocked by inhibitors for caspase-3 or c-Jun/AP-1. The antiapoptotic effect by estradiol was blocked by ICI 182,780, an antagonist for ER, but not by a synthesized peptide that inhibits binding of the ER to the ERE. Estradiol had no effects on caspase-3 activation and c-Jun NH(2)-terminal kinase (JNK), which were activated by BLM. It also suppressed apoptosis by serum deprivation, which was independent of caspase-3 activation. Therefore, the antiapoptotic neuroprotection by estradiol is mediated by transcription through AP-1 site downstream from JNK and caspase-3 activation. Furthermore, 17alpha-estradiol, a stereoisomer without female hormone activity, also provided an antiapoptotic effect. Therefore, the antiapoptotic effect is independent of female hormone activity.

Phosphatidylinositol 3-kinase mediates neuroprotection by estrogen in cultured cortical neurons

It has been shown that estrogen replacement in menopausal women is effective in slowing down the progression of cognitive impairment in Alzheimer's disease. Although recent studies have demonstrated the neuroprotective effects of estrogen, the precise mechanism of neuroprotection has not been elucidated. In the present study, we show that the phosphatidylinositol 3-kinase (PI3-K) cascade is involved in the neuroprotective mechanism stimulated by estrogen. Exposure to glutamate reduced the viability of rat primary cortical neurons. Pretreatment with 10 nM 17beta-estradiol significantly attenuated the glutamate-induced toxicity. This neuroprotective effect of 17beta-estradiol was blocked by co-administration with LY294002, a selective PI3-K inhibitor, but not by co-administration with PD98059, a selective mitogen activated protein kinase kinase inhibitor. Pretreatment with ICI182780, a specific estrogen receptor antagonist, also blocked the neuroprotection. Immunoblotting assay revealed that treatment with 17beta-estradiol induced the phosphorylation of Akt/PKB, an effector immediately downstream of PI3-K. These results suggest that PI3-K mediates the neuroprotective effect of 17beta-estradiol against glutamate-induced neurotoxicity.

Gender differences in acute CNS trauma and stroke: neuroprotective effects of estrogen and progesterone

Increasing evidence has demonstrated striking sex differences in the pathophysiology of and outcome after acute neurological injury. Lesser susceptibility to postischemic and posttraumatic brain injury in females has been observed in experimental models. Additional evidence suggests this sex difference extends to humans as well. The greater neuroprotection afforded to females is likely due to the effects of circulating estrogens and progestins. In fact, exogenous administration of both hormones has been shown to improve outcome after cerebral ischemia and traumatic brain injury in experimental models. The neuroprotection provided by periinjury administration of these hormones extends to males as well. The mechanisms by which estrogen and progesterone provide such neuroprotection are likely multifactorial, and probably depend on the type and severity of injury as well as the type and concentration of hormone present. Both genomic and nongenomic mechanisms may be involved. Estrogen's putative effects include preservation of autoregulatory function, an antioxidant effect, reduction of A beta production and neurotoxicity, reduced excitotoxicity, increased expression of the antiapoptotic factor bcl-2, and activation of mitogen activated protein kinase pathways. It is hypothesized that several of these neuroprotective mechanisms can be linked back to estrogen's ability to act as a potent chemical (i.e., electron-donating) antioxidant. Progesterone, on the other hand, has a membrane stabilizing effect that also serves to reduce the damage caused by lipid peroxidation. In addition, it may also provide neuroprotection by suppressing neuronal hyperexcitability. The following review will discuss experimental and clinical evidence for sex differences in outcome after acute brain trauma and stroke, review the evidence implicating estrogens and progestins as mediators of this neuroprotection following acute neurological injury, and finally, address the specific mechanisms by which these hormones may protect the brain following acute neurological injury.

Estrogen as a neuroprotectant in stroke

Recent evidence suggests that reproductive steroids are important players in shaping stroke outcome and cerebrovascular pathophysiologic features. Although women are at lower risk for stroke than men, this native protection is lost in the postmenopausal years. Therefore, aging women sustain a large burden for stroke, contrary to a popular misconception that cancer is the main killer of women. Further, the value of hormone replacement therapy in stroke prevention or in improving outcome remains controversial. Estrogen has been the best studied of the sex steroids in both laboratory and clinical settings and is considered increasingly to be an endogenous neuroprotective agent. A growing number of studies demonstrate that exogenous estradiol reduces tissue damage resulting from experimental ischemic stroke in both sexes. This new concept suggests that dissecting interactions between estrogen and cerebral ischemia will yield novel insights into generalized cellular mechanisms of injury. Less is known about estrogen's undesirable effects in brain, for example, the potential for increasing seizure susceptibility and migraine. This review summarizes gender-specific aspects of clinical and experimental stroke and results of estrogen treatment on outcome in animal models of cerebral ischemia, and briefly discusses potential vascular and parenchymal mechanisms by which estrogen salvages brain.

Neuroprotective effects of chronic estradiol benzoate treatment on hippocampal cell loss induced by status epilepticus in the female rat

Neuroprotective properties of estrogen are supported by extensive experimental evidence. In this study, the effects of estrogen were examined on the neurodegeneration secondary to status epilepticus induced by kainic acid in the rat. Chronic supplementation of ovariectomized rats with estradiol benzoate (20 microg/day) did not modify the expression of seizures monitored by electroencephalography, but significantly reduced cellular loss in the hippocampus. This neuroprotection was in particular observed in the dentate hilus and CA3 pyramidal layer when treatment with estradiol benzoate was started five days before status epilepticus induction. These findings suggest that estrogen can exert neuroprotective effects in a model of status epilepticus, in the absence of anti-epileptic properties.

Estradiol exerts neuroprotective effects when administered after ischemic insult

BACKGROUND AND PURPOSE

17beta-Estradiol (E2) has been reported to exert neuroprotective effects when administered before an ischemic insult. This study was designed to determine whether E2 treatment after ischemia exerts the same effects and, if so, how long this therapeutic window remains open, and whether the effects are related to changes in cerebral blood flow (CBF).

METHODS

Female Sprague-Dawley rats were subjected to permanent middle cerebral artery occlusion (MCAO). In protocol 1, E2 was administered (100 microg/kg IV followed immediately by subcutaneous implantation of crystalline E2 in a silicone elastomer tube) to ovariectomized females (OVX+E2) at 0.5 (n=8), 1 (n=6), 2 (n=7), 3 (n=6), or 4 (n=9) hours after MCAO. Intact (INT; n=6) and ovariectomized females (OVX; n=12) were subjected to MCAO and received vehicle instead of E2. Two days after MCAO the animals were killed, and ischemic lesion volume was determined by 2,3,5-triphenyltetrazolium chloride staining. In protocol 2, CBF was monitored before and at 1, 24, and 48 hours in a group of animals receiving E2 or vehicle 0.5 hour after ischemia induction (INT, n=6; OVX, n=8; OVX+E2, n=6).

RESULTS

Lesion volume was 20.9+/-2.2% and 21.8+/-1.2% in the INT and OVX groups, respectively. E2 was found to decrease lesion volume significantly when administered within 3 hours after MCAO. The lesion volumes were 6.3+/-0.5%, 10.3+/-2.1%, 11.8+/-1.8%, 13.5+/-1.6%, and 17.9+/-2.8% when E2 was administered at 0.5, 1, 2, 3, or 4 hours after MCAO, respectively. CBF decreased to 43.1+/-2.2% and 25.4+/-1.0% in the INT and OVX animals, respectively, at 5 minutes after MCAO. In comparison to OVX rats, CBF was not different at 1 hour after E2 administration but was increased significantly in the OVX+E2 group 1 and 2 days after E2 administration.

CONCLUSIONS

E2 exerts neuroprotective effects when administered after ischemia, with a therapeutic window in a permanent focal cerebral ischemia model of approximately 3 hours. This effect of estradiol was associated with no immediate change in blood flow but with a delayed increase in CBF.

Stroke in estrogen receptor-alpha-deficient mice

BACKGROUND AND PURPOSE

Recent evidence suggests that endogenous estrogens or hormone replacement therapy can ameliorate brain damage from experimental stroke. Protective mechanisms involve enhanced cerebral vasodilation during ischemic stress as well as direct preservation of neuronal viability. We hypothesized that if the intracellular estrogen receptor subtype-alpha (ERalpha) is important to estrogen's signaling in the ischemic brain, then ERalpha-deficient (knockout) (ERalphaKO) female mice would sustain exaggerated cerebral infarction damage after middle cerebral artery occlusion.

METHODS

The histopathology of cresyl violet-stained tissues was evaluated after reversible middle cerebral artery occlusion (2 hours, followed by 22 hours of reperfusion) in ERalphaKO transgenic and wild-type (WT) mice (C57BL/6J background strain). End-ischemic cerebral blood flow mapping was obtained from additional female murine cohorts by using [(14)C]iodoantipyrine autoradiography.

RESULTS

Total hemispheric tissue damage was not altered by ERalpha deficiency in female mice: 51.9+/-10.6 mm(3) in ERalphaKO versus 60.5+/-5.0 mm(3) in WT. Striatal infarction was equivalent, 12.2+/-1.7 mm(3) in ERalphaKO and 13.4+/-1.0 mm(3) in WT mice, but cortical infarction was paradoxically smaller relative to that of the WT (20.7+/-4.5 mm(3) in ERalphaKO versus 30.6+/-4.1 mm(3) in WT). Intraocclusion blood flow to the parietal cortex was higher in ERalphaKO than in WT mice, likely accounting for the reduced infarction in this anatomic area. There were no differences in stroke outcomes by region or genotype in male animals.

CONCLUSIONS

Loss of ERalpha does not enhance tissue damage in the female animal, suggesting that estrogen inhibits brain injury by mechanisms that do not depend on activation of this receptor subtype.

Neuroprotective effects of female gonadal steroids in reproductively senescent female rats

BACKGROUND AND PURPOSE

Young adult female rats sustain smaller infarcts after experimental stroke than age-matched males. This sex difference in ischemic brain injury in young animals disappears after surgical ovariectomy and can be restored by estrogen replacement. We sought to determine whether ischemic brain injury continues to be smaller in middle-aged, reproductively senescent female rats compared with age-matched males and to test the effect of ovarian steroids on brain injury after experimental stroke in females.

METHODS

Four groups of 16-month old Wistar rats (males [n=9], untreated females [n=9], and females pretreated with 17beta-estradiol [25-microgram pellets administered subcutaneously for 7 days; n=9] or progesterone [10-mg pellets administered subcutaneously for 7 days; n=9] were subjected to 2 hours of middle cerebral artery occlusion with the intraluminal filament technique, followed by 22 hours of reperfusion. Physiological variables and laser-Doppler cerebral cortical perfusion were monitored throughout ischemia and early reperfusion. In a separate cohort of males (n=3), untreated females (n=3), females pretreated with 17beta-estradiol (n=3), and females pretreated with progesterone (n=3), end-ischemic regional cerebral blood flow was measured by [(14)C]iodoantipyrine autoradiography.

RESULTS

As predicted, infarct size was not different between middle-aged male and female rats. Cortical infarcts were 21+/-5% and 31+/-6% of ipsilateral cerebral cortex, and striatal infarcts were 44+/-7% and 43+/-5% of ipsilateral striatum in males and females, respectively. Both estrogen and progesterone reduced cortical infarct in reproductively senescent females (5+/-2% and 16+/-4% in estrogen- and progesterone-treated groups, respectively, compared with 31+/-6% in untreated group). Striatal infarct was smaller in the estrogen- but not in the progesterone-treated group. Relative change in laser-Doppler cerebral cortical perfusion from preischemic baseline and absolute end-ischemic regional cerebral blood flow were not affected by hormonal treatments.

CONCLUSIONS

We conclude that the protection against ischemic brain injury found in young adult female rats disappears after reproductive senescence in middle-aged females and that ovarian hormones alleviate stroke injury in reproductively senescent female rats by a blood flow-independent mechanism. These findings support a role for hormone replacement therapy in stroke injury prevention in postmenopausal women.

Neuroprotective strategies in Parkinson's disease: protection against progressive nigral damage induced by free radicals

Brain undergoes neurodegeneration when excess free radicals overwhelm antioxidative defense systems during senescence, head trauma and/or neurotoxic insults. A site-specific accumulation of ferrous citrate-iron complexes in the substantia nigra dopaminergic neurons could lead to exaggerated dopamine turnover, dopamine auto-oxidation, free radical generation, and oxidant stress. Eventually, this iron-catalyzed dopamine auto-oxidation results in the accumulation of neuromelanin, a progressive loss of nigral neurons, and the development of Parkinson's disease when brain dopamine depletion is greater than 80%. Emerging evidence indicates that free radicals such as hydroxyl radicals ((.-)OH) and nitric oxide ((.-)NO) may play opposite role in cell and animal models of parkinsonism. (.-)OH is a cytotoxic oxidant whereas oNO is an atypical neuroprotective antioxidant. (.-)NO and S-nitrosoglutathione (GSNO) protect nigral neurons against oxidative stress caused by 1-methyl-4-phenylpyridinium (MPP(+)), dopamine, ferrous citrate, hemoglobin, sodium nitroprusside and peroxynitrite. MPP(+), the toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), increases the nigral uptake of iron complexes and dopamine overflow leading to the generation of (.-)OH, protein oxidation, lipid peroxidation, and associated retrograde degeneration. In addition to GSNO, MPP(+)-induced oxidative neurotoxicity can be prevented by antioxidants including selegiline, 7-nitroindazole, 17beta-estradiol, melatonin, alpha-phenyl-tert-butylnitrone and U78517F. Similar to selegiline, 7-nitroindazole is a MAO-B inhibitor, which blocks the bio-activation of MPTP and oxidative stress. Freshly prepared but not light exposed, (.-)NO-exhausted GSNO is about 100 times more potent than the classic antioxidant glutathione. Via S-nitrosylation, GSNO also inhibits proteolysis and cytotoxicity caused by caspases and HIV-1 protease. Furthermore, in addition to protection against serum deprivation stress, the induction of neuronal NOS1 in human cells increases tolerance to MPP(+)-induced neuro-toxicity since newly synthesized (.-)NO prevents apoptosis possibly through up-regulation of bcl-2 and down regulation of p66(shc). In conclusion, reactive oxygen species are unavoidable by-products of iron-catalyzed dopamine auto-oxidation, which can initiate lipid peroxidation, protein oxidation, DNA damage, and nigral loss, all of which can be prevented by endogenous and exogenous (.-)NO. Natural and man-made antioxidants can be employed as part of preventative or neuroprotective treatments in Parkinson's disease and perhaps dementia complexes as well. For achieving neuroprotection and neuro-rescue in early clinical parkinsonian stages, a cocktail therapy of multiple neuroprotective agents may be more effective than the current treatment with extremely high doses of a single antioxidative agent.

Estradiol modulates bcl-2 in cerebral ischemia: a potential role for estrogen receptors

We have shown that physiological levels of estradiol exert profound protective effects on the cerebral cortex in ischemia induced by permanent middle cerebral artery occlusion. The major goal of this study was to begin to elucidate potential mechanisms of estradiol action in injury. Bcl-2 is a proto-oncogene that promotes cell survival in a variety of tissues including the brain. Because estradiol is known to promote cell survival via Bcl-2 in non-neural tissues, we tested the hypothesis that estradiol decreases cell death by influencing bcl-2 expression in ischemic brain injury. Furthermore, because estradiol may protect the brain through estrogen receptor-mediated mechanisms, we examined expression of both receptor subtypes ERalpha and ERbeta in the normal and injured brain. We analyzed gene expression by RT-PCR in microdissected regions of the cerebral cortex obtained from injured and sham female rats treated with estradiol or oil. We found that estradiol prevented the injury-induced downregulation of bcl-2 expression. This effect was specific to bcl-2, as expression of other members of the bcl-2 family (bax, bcl-x(L), bcl-x(S), and bad) was unaffected by estradiol treatment. We also found that estrogen receptors were differentially modulated in injury, with ERbeta expression paralleling bcl-2 expression. Finally, we provide the first evidence of functional ERbeta protein that is capable of binding ligand within the region of the cortex where estradiol-mediated neuroprotection was observed in cerebral ischemia. These findings indicate that estradiol modulates the expression of bcl-2 in ischemic injury. Furthermore, our data suggest that estrogen receptors may be involved in hormone-mediated neuroprotection.

The mitogen-activated protein kinase pathway mediates estrogen neuroprotection after glutamate toxicity in primary cortical neurons

Pharmacological and biochemical approaches were used to elucidate the involvement of growth factor signaling pathways mediating estrogen neuroprotection in primary cortical neurons after glutamate excitotoxicity. We addressed the activation of mitogen-activated protein kinase (MAPK) signaling pathways, which are activated by growth factors such as nerve growth factor (NGF). Inhibition of MAPK signaling with the MAPK kinase inhibitor PD98059 blocks both NGF and estrogen neuroprotection in these neurons. These results correlate with a rapid and sustained increase in MAPK activity within 30 min of estrogen exposure. The involvement of signaling molecules upstream from MAPK was also examined to determine whether activation of MAPK by estrogen is mediated by tyrosine kinase activity. Estrogen produces a rapid, transient activation of src-family tyrosine kinases and tyrosine phosphorylation of p21(ras)-guanine nucleotide activating protein. Effects of estrogen on neuroprotection, as well as rapid activation of tyrosine kinase and MAPK activity, are blocked by the anti-estrogen ICI 182,780. This provides evidence that activation of the MAPK pathway by estrogen participates in mediating neuroprotection via an estrogen receptor. These results describe a novel mechanism by which cytoplasmic actions of the estrogen receptor may activate the MAPK pathway, thus broadening the understanding of effects of estrogen in neurons.