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

Estrogens and the cognitive symptoms of schizophrenia: Possible neuroprotective mechanisms

Schizophrenia is a complex neuropsychiatric illness with marked sex differences. Women have later onset and lesser symptoms, which has led to the hypothesis that estrogens are protective in schizophrenia. Cognitive dysfunction is a hallmark of the disease and the symptom most correlated with functional outcome. Here we describe a number of mechanisms by which estrogens may be therapeutic in schizophrenia, with a focus on cognitive symptoms. We review the relationship between estrogens and brain derived neurotrophic factor, neuroinflammation, NMDA receptors, GABA receptors, and luteinizing hormone. Exploring these pathways may enable novel treatments for schizophrenia and a greater understanding of this devastating disease.

Antioxidant Treatment in Male Mice Prevents Mitochondrial and Synaptic Changes in an NMDA Receptor Dysfunction Model of Schizophrenia

Glutamate theories of schizophrenia suggest that the disease is associated with a loss of NMDA receptors, specifically on GABAergic parvalbumin-expressing interneurons (PVIs), leading to changes in the excitation-inhibition balance in the prefrontal cortex (PFC). Oxidative stress contributes to the loss of PVI and the development of schizophrenia. Here, we investigated whether the glutathione precursor N-acetyl cysteine (NAC) can prevent changes in synaptic transmission at pyramidal cells and PVIs that result from developmental NMDAR blockade and how these changes are related to mitochondrial dysfunction in the PFCs of mice. Perinatal treatment with ketamine induced persistent changes in the reduced glutathione/oxidized glutathione (glutathione disulfide) ratio in the medial PFC, indicating long-lasting increases in oxidative stress. Perinatal ketamine treatment also reduced parvalbumin expression, and it induced a decline in mitochondrial membrane potential, as well as elevations in mitochondrial superoxide levels. At the level of synaptic function ketamine reduced inhibition onto layer 2/3 pyramidal cells and increased excitatory drive onto PVI, indicating long-lasting disruptions in the excitation-inhibition balance. These changes were accompanied by layer-specific alterations in NMDAR function in PVIs. All of these changes were mitigated by coadministration of NAC. In addition, NAC given only during late adolescence was also able to restore normal mitochondria function and inhibition at pyramidal cells. These results show that ketamine-induced alterations in PFC physiology correlate with cell type-specific changes in mitochondria function. The ability of NAC to prevent or restore these changes supports the usefulness of antioxidant supplementation in the treatment of schizophrenia.

Antioxidant Treatment with N-acetyl Cysteine Prevents the Development of Cognitive and Social Behavioral Deficits that Result from Perinatal Ketamine Treatment

Alterations of the normal redox state can be found in all stages of schizophrenia, suggesting a key role for oxidative stress in the etiology and maintenance of the disease. Pharmacological blockade of N-methyl-D-aspartic acid (NMDA) receptors can disrupt natural antioxidant defense systems and induce schizophrenia-like behaviors in animals and healthy human subjects. Perinatal administration of the NMDA receptor (NMDAR) antagonist ketamine produces persistent behavioral deficits in adult mice which mimic a range of positive, negative, and cognitive symptoms that characterize schizophrenia. Here we tested whether antioxidant treatment with the glutathione (GSH) precursor N-acetyl-cysteine (NAC) can prevent the development of these behavioral deficits. On postnatal days (PND) 7, 9 and 11, we treated mice with subanesthetic doses (30 mg/kg) of ketamine or saline. Two groups (either ketamine or saline treated) also received NAC throughout development. In adult animals (PND 70-120) we then assessed behavioral alterations in a battery of cognitive and psychomotor tasks. Ketamine-treated animals showed deficits in a task of cognitive flexibility, abnormal patterns of spontaneous alternation, deficits in novel-object recognition, as well as social interaction. Developmental ketamine treatment also induced behavioral stereotypy in response to an acute amphetamine challenge, and it impaired sensorimotor gating, measured as reduced prepulse inhibition (PPI) of the startle response. All of these behavioral abnormalities were either prevented or strongly ameliorated by NAC co-treatment. These results suggest that oxidative stress is a major factor for the development of the ketamine-induced behavioral dysfunctions, and that restoring oxidative balance during the prodromal stage of schizophrenia might be able to ameliorate the development of several major symptoms of the disease.

Differences in Retinal Structure and Function between Aging Male and Female Sprague-Dawley Rats are Strongly Influenced by the Estrus Cycle

Purpose

Biological sex and age are considered as two important factors that may influence the function and structure of the retina, an effect that might be governed by sexual hormones such as estrogen. The purpose of this study was to delineate the influence that biological sex and age exert on the retinal function and structure of rodents and also clarify the effect that the estrus cycle might exert on the retinal function of female rats.

Method

The retinal function of 50 normal male and female albino Sprague-Dawley (SD) rats was investigated with the electroretinogram (ERG) at postnatal day (P) 30, 60, 100, 200, and 300 (n = 5–6 male and female rats/age). Following the ERG recording sessions, retinal histology was performed in both sexes. In parallel, the retinal function of premenopausal and menopausal female rats aged P540 were also compared.

Results

Sex and age-related changes in retinal structure and function were observed in our animal model. However, irrespective of age, no significant difference was observed in ERG and retinal histology obtained from male and female rats. Notwithstanding the above we did however notice that between P60 and P200 there was a gradual increase in ERG amplitudes of female rats compared to males. Furthermore, the ERG of premenopausal female rats aged 18 months old (P540) was larger compared to age-matched menopausal female rats as well as that of male rats.

Conclusion

Our results showed that biological sex and age can influence the retinal function and structure of albino SD rats. Furthermore, we showed that cycled female rats have better retinal function compared to the menopausal female rats suggesting a beneficial effect of the estrus cycle on the retinal function.

ESTROGEN REPLACEMENT THERAPY FOR STROKE

Stroke is the third most common cause of death and severe disability among Western populations. Overall, the incidence of stroke is uniformly higher in men than in women. Stroke is rare in women during the reproductive years, and rapidly increases after menopause, strongly suggesting that estrogen (E2) plays an important role in the prevention of stroke. Ongoing studies are currently evaluating both the benefits and risks associated with E2 replacement therapy and hormone replacement therapy in stroke. Equally important is the role of E2 receptor (ER), as studies indicate that ER populations in several tissue sites may significantly change during stress and aging. Such changes may affect the patient's susceptibility to neurological disorders including stroke, and greatly affect the response to selective E2 receptor modulators (SERMs). Replacement therapies may be inefficient with low ER levels. The goal of this review paper is to discuss an animal model that will allow investigations of the potential therapeutic effects of E2 and its derivatives in stroke. We hypothesize that E2 neuroprotection is, in part, receptor mediated. This hypothesis is a proof of principle approach to demonstrate a role for specific ER subtypes in E2 neuroprotection. To accomplish this, we use a retroviral mediated gene transfer strategy that express subtypes of the ER gene in regions of the rat brain most susceptible to neuronal damage, namely the striatum and cortex. The animal model is exposed to experimental stroke conditions involving middle cerebral artery occlusion (MCAo) method, and eventually the extent of neuronal damage will be evaluated. A reduction in neuronal damage is expected when E2 is administered with specific ER subtypes. From this animal model, an optimal E2 dose and treatment regimen can be determined. The animal model can help identify potential E2-like therapeutics in stroke, and screen for beneficial or toxic additives present in commercial E2 preparations that are currently available. Such studies will be informative in designing drug therapies for stroke.

How Nox2-containing NADPH oxidase affects cortical circuits in the NMDA receptor antagonist model of schizophrenia

SIGNIFICANCE

Schizophrenia is a complex neuropsychiatric disorder affecting around 1% of the population worldwide. Its mode of inheritance suggests a multigenic neurodevelopmental disorder with symptoms appearing during late adolescence/early adulthood, with its onset strongly influenced by environmental stimuli. Many neurotransmitter systems, including dopamine, glutamate, and gamma-aminobutyric acid, show alterations in affected individuals, and the behavioral and physiological characteristics of the disease can be mimicked by drugs that produce blockade of N-methyl-d-aspartate glutamate receptors (NMDARs).

RECENT ADVANCES

Mounting evidence suggests that drugs that block NMDARs specifically impair the inhibitory capacity of parvalbumin-expressing (PV+) fast-spiking neurons in adult and developing rodents, and alterations in these inhibitory neurons is one of the most consistent findings in the schizophrenic postmortem brain. Disruption of the inhibitory capacity of PV+ inhibitory neurons will alter the functional balance between excitation and inhibition in prefrontal cortical circuits producing impairment of working memory processes such as those observed in schizophrenia.

CRITICAL ISSUES

Mechanistically, the effect of NMDAR antagonists can be attributed to the activation of the Nox2-dependent reduced form of nicotinamide adenine dinucleotide phosphate oxidase pathway in cortical neurons, which is consistent with the emerging role of oxidative stress in the pathogenesis of mental disorders, specifically schizophrenia. Here we review the mechanisms by which NMDAR antagonists produce lasting impairment of the cortical PV+ neuronal system and the roles played by Nox2-dependent oxidative stress mechanisms.

FUTURE DIRECTIONS

The discovery of the pathways by which oxidative stress leads to unbalanced excitation and inhibition in cortical neural circuits opens a new perspective toward understanding the biological underpinnings of schizophrenia.

Paradox of schizophrenia genetics: is a paradigm shift occurring?

BACKGROUND

Genetic research of schizophrenia (SCZ) based on the nuclear genome model (NGM) has been one of the most active areas in psychiatry for the past two decades. Although this effort is ongoing, the current situation of the molecular genetics of SCZ seems disappointing or rather perplexing. Furthermore, a prominent discrepancy between persistence of the disease at a relatively high prevalence and a low reproductive fitness of patients creates a paradox. Heterozygote advantage works to sustain the frequency of a putative susceptibility gene in the mitochondrial genome model (MGM) but not in the NGM.

METHODS

We deduced a criterion that every nuclear susceptibility gene for SCZ should fulfill for the persistence of the disease under general assumptions of the multifactorial threshold model. SCZ-associated variants listed in the top 45 in the SZGene Database (the version of the 23rd December, 2011) were selected, and the distribution of the genes that could meet or do not meet the criterion was surveyed.

RESULTS

19 SCZ-associated variants that do not meet the criterion are located outside the regions where the SCZ-associated variants that could meet the criterion are located. Since a SCZ-associated variant that does not meet the criterion cannot be a susceptibility gene, but instead must be a protective gene, it should be linked to a susceptibility gene in the NGM, which is contrary to these results. On the other hand, every protective gene on any chromosome can be associated with SCZ in the MGM. Based on the MGM we propose a new hypothesis that assumes brain-specific antioxidant defenses in which trans-synaptic activations of dopamine- and N-methyl-d-aspartate-receptors are involved. Most of the ten predictions of this hypothesis seem to accord with the major epidemiological facts and the results of association studies to date.

CONCLUSION

The central paradox of SCZ genetics and the results of association studies to date argue against the NGM, and in its place the MGM is emerging as a viable option to account for genomic and pathophysiological research findings involving SCZ.

Divergent mechanisms for trophic actions of estrogens in the brain and peripheral tissues

17β-estradiol (E(2)) can enhance reproductive, cognitive, and affective functions; however, the mechanisms by which E(2) has these effects need to be better understood. Pleiotrophic effects of E(2) can occur via traditional and novel actions at various forms of estrogen receptors (ERs). In the central nervous system, trophic effects of E(2) may be related to beneficial effects of hormone replacement therapy (HRT). However, in peripheral reproductive tissues, E(2)'s capacity to evoke growth can increase risk of cancers. This review focuses on investigations aimed at elucidating divergent mechanisms of steroids to promote trophic effects in the brain, independent of effects on peripheral reproductive tissues. First, actions of estrogens via ERα or ERβ for peripheral growth (carcinogen-induced tumors, uterine growth) and hippocampus-dependent behaviors (affect, cognition) are described. Second, factors that influence these effects of estrogens are described (e.g. experience, timing/critical windows, non-ER mechanisms). Third, effects of estrogens at ERβ related to actions of progestogens, such as 5α-pregnan-3α-ol-20-one (3α,5α-THP) are described. In summary, effects of E(2) may occur via multiple mechanisms, which may underlie favorable effects in the brain with minimal peripheral trophic effects.

Estrogen receptor alpha inhibits the estrogen-mediated suppression of HIV transcription in astrocytes: implications for estrogen neuroprotection in HIV dementia

Many human immunodeficiency virus (HIV) proteins including Tat are produced by HIV-infected astrocytes and secreted into the brain resulting in extensive neuronal damage that contributes to the pathogenesis of HIV dementia. The neuroprotective hormone 17beta-estradiol (E2) is known to negatively regulate the HIV transcriptional promoter in human fetal astrocytes (SVGA cell line) in a Tat-dependent manner. In the present study we extended our investigation in HIV-infected SVGA cells and found a reduction in HIV p24 levels following E2 treatment in comparison to control. Although many E2-mediated events occur through estrogen receptor alpha (ERalpha), we found low levels of ERalpha mRNA and failed to detect ERalpha protein in SVGA cells. Paradoxically, when ERalpha was overexpressed the E2-mediated decrease in Tat transactivation of the promotor was prevented. To determine whether ERalpha expression is altered in the human brain following HIV infection, postmortum hippocampal tissue was obtained from cognitively normal HIV- and HIV+ patients, patients diagnosed with either mild cognitive/motor disorder (MCMD) or HIV-associated dementia (HAD). Immunohistochemistry and quantitative real-time PCR (qRT-PCR) for ERalpha and glial fibrillary acidic protein (GFAP) showed that ERalpha mRNA levels were not significantly different between groups, while GFAP increased in the hippocampus in the HIV+ compared to the HIV- group and was decreased in the MCMD and HAD subgroups compared to HIV+ controls. Notably the ratio of ERalpha-positive reactive astrocytes to total reactive astrocytes increased and significantly correlated with the severity of cognitive impairment following HIV infection. The data suggest that E2 would have the most dramatic effect in reducing HIV transcription early in the disease process when the subpopulation of astrocytes expressing ERalpha is low.

Does schizophrenia arise from oxidative dysregulation of parvalbumin-interneurons in the developing cortex?

An imbalance in the redox-state of the brain may be part of the underlying pathophysiology in schizophrenia. Inflammatory mediators, such as IL-6, which can tip the redox balance into a pro-oxidant state, have been consistently found to be altered in schizophrenia patients. However, the relationship of altered redox-state to altered brain functions observed in the disease has been unclear. Recent data from a pharmacological model of schizophrenia suggest that redox and inflammatory imbalances may be directly linked to the pathophysiology of the disease by alterations in fast-spiking interneurons. Repetitive adult exposure to the NMDA-R antagonist ketamine increases the levels of the proinflammatory cytokine interleukin-6 in brain which, through activation of the superoxide-producing enzyme NADPH oxidase (Nox2), leads to the loss of the GABAergic phenotype of PV-interneurons and to decreased inhibitory activity in prefrontal cortex. This effect is not observed after a single exposure to ketamine, suggesting that the first exposure to the NMDA-R antagonist primes the brain such that deleterious effects on PV-interneurons appear upon repetitive exposures. The effects of activation of the IL-6/Nox2 pathway on the PV-interneuronal system are reversible in the adult brain, but permanent in the developing cortex. The slow development of PV-interneurons, although essential for shaping of neuronal circuits during postnatal brain development, increases their vulnerability to deleterious insults that can permanently affect their maturational process. Thus, in individuals with genetic predisposition, the persistent activation of the IL-6/Nox2 pathway may be an environmental factor that tips the redox balance leading to schizophrenia symptoms in late adolescence and early adulthood.

Ketamine-induced loss of phenotype of fast-spiking interneurons is mediated by NADPH-oxidase

Abuse of the dissociative anesthetic ketamine can lead to a syndrome indistinguishable from schizophrenia. In animals, repetitive exposure to this N-methyl-d-aspartate-receptor antagonist induces the dysfunction of a subset of cortical fast-spiking inhibitory interneurons, with loss of expression of parvalbumin and the gamma-aminobutyric acid-producing enzyme GAD67. We show here that exposure of mice to ketamine induced a persistent increase in brain superoxide due to activation in neurons of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Decreasing superoxide production prevented the effects of ketamine on inhibitory interneurons in the prefrontal cortex. These results suggest that NADPH oxidase may represent a novel target for the treatment of ketamine-induced psychosis.

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.

The enantiomer of progesterone acts as a molecular neuroprotectant after traumatic brain injury

Previous work shows that neurosteroid enantiomers activate specific molecular receptors that relay neuroprotection. However, the actions of the enantiomer of progesterone (ent-PROG) at the PROG receptor (PR) are unknown. PR binding and transcriptional assays were performed to determine the actions of ent-PROG at the classical PR. Additionally, the neuroprotective effects of ent-PROG in traumatic brain injury (TBI) were investigated and compared to the actions of PROG and its metabolite allopregnanolone (ALLO), both of which have been shown to have neuroprotective properties when given after TBI. Binding studies performed in COS cells over-expressing the PR showed that ent-PROG inhibited PROG binding to the PR. In contrast, ent-PROG did not activate PR-mediated transcription. Rats received bilateral medial frontal cortex injury followed by treatments at 1, 6, 24 and 48h with PROG, ALLO or ent-PROG. Brains were processed for edema, protein and enzyme activity. ent-PROG treatment in vivo decreased cerebral edema, cell death mediators, inflammatory cytokines, and reactive gliosis, and increased antioxidant activity. These findings suggest that the progestin-mediated pro-survival response seen with TBI is regulated either independently of the classical PR or via nongenomic PR-regulated actions.

A nonfeminizing estrogen analog protects against ethanol withdrawal toxicity in immortalized hippocampal cells

We have shown that 17beta-estradiol protects against ethanol withdrawal toxicity in rats. Here, we investigated whether a cellular model of ethanol withdrawal could be developed in a cultured hippocampal cell line (HT22) and whether an adamantyl-containing nonfeminizing estrogen analog, ZYC26 [(3-hydroxy-2-adamantyl(1)-4-methyl-estra-1,3,5(10)-17-one], protects against ethanol withdrawal toxicity. HT22 cells were exposed to ethanol (0-500 mM) for 24 h in the presence or absence of ZYC26 or 17beta-estradiol. The ethanol solution was then removed from the cells for 4 h to create ethanol withdrawal. Samples were collected at the end of a 24-h ethanol exposure or at 4 h of ethanol withdrawal to assess cell viability using a calcein assay, lipid peroxidation by measuring malondialdehyde, and protein oxidation by measuring carbonyl contents. When tested, ethanol concentrations were constantly maintained during a 24-h ethanol exposure and eliminated at 4 h of ethanol withdrawal. Ethanol withdrawal decreased cell viability and increased the levels of malondialdehyde and carbonyls more than ethanol exposure. ZYC26 reduced the cell death and malondialdehyde levels at a lower dose (1 microM) than 17beta-estradiol (10 microM). The increased carbonyl contents were reduced only by ZYC26 treatment. These data suggest that ethanol withdrawal can be created in HT22 cells in a manner that is more toxic than ethanol exposure and that ZYC26 is a more potent cytoprotectant than 17beta-estradiol against cell death and oxidative damage induced by ethanol withdrawal. Therefore, ZYC26 can be a potential alternative estrogen therapy for a cellular and oxidative imbalance associated with ethanol withdrawal.

Estrogen attenuates gp120- and tat1-72-induced oxidative stress and prevents loss of dopamine transporter function

Postmenopausal women who are infected with HIV are at risk for experiencing dementia and Parkinson's-like symptoms associated with low levels of estrogen. Neurotoxic damage leading to these symptoms may involve HIV-associated proteins gp120 and/or tat(1-72) (tat). Our hypothesis is that 17beta-Estradiol (E(2)) is an effective agent for protection against gp120/tat-induced damage associated with increased oxidative stress, with particular focus on peroxynitrite-induced oxidative stress. We used SK-N-SH cells and striatal synaptosomes from Sprague-Dawley rats as model systems to assess neuroprotection by E(2). Cells coincubated with SIN-1(3-morpholinosydnonimine) or tat and gp120, together or separately, significantly increased oxidative stress on the SK-N-SH cells, as indicated by the increase in the levels of dichlorofluorescein (DCFH) fluorescence. These data suggest that a component of tat and gp120 neurotoxicity may be due to increased oxidative stress. Coincubation with E(2) attenuated tat- and gp120-induced increase in fluorescence. Coincubation with progesterone had no effect on tat-induced fluorescence, whereas coincubation with the E(2) antagonist ICI 182,780 and E(2) completely prevented the effects observed with E(2) alone. Both gp120 and tat decreased [(3)H] dopamine uptake into striatal synaptosomes by decreasing the V(max) of the dopamine transporter (DAT). Pretreatment of synaptosomes with E(2) (100 nM) partially reversed this reduction. In conclusion, E(2) appears to be effective for preventing the oxidative stress and loss of DAT function associated with gp120/tat neurotoxicity.

Long-term melatonin or 17beta-estradiol supplementation alleviates oxidative stress in ovariectomized adult rats

Melatonin is an endogenously generated potent antioxidant. Our previous results indicated that melatonin improved learning and memory deficits in the transgenic mouse model of Alzheimer's disease (AD) and ovariectomized (OVX) rats by improving cholinergic nerve system dysfunction, preventing apoptosis. In this study we aim to investigate the antioxidative effects of melatonin or estradiol in the brains of ovariectomized rats. OVX Sprague-Dawley rats received daily injections of melatonin (5, 10, or 20 mg/kg), 17beta-estradiol (80 microg/kg), or sesame oil for 16 weeks. We found an increase in brain mitochondrial thiobarbituric acid-reactive substances (TBARS) levels, a decrease in mitochondrial glutathione (GSH) content as well as mitochondrial superoxide dismutase (SOD) activity and upregulation of the apoptotic-related factors, such as Bax, Caspase-3, and Prostate apoptosis response-4 (Par-4) in the frontal cortex of OVX rats. In addition to oxidative stress, OVX also caused decreased activities of mitochondrial respiration complex I and complex IV, which implicated mitochondrial dysfunction. Melatonin or 17beta-estradiol antagonized the detrimental effects induced by OVX. Furthermore, immunohistochemistry results revealed that the abnormal upregulation of the apoptotic related factor such as Bax, Caspase-3, and (Par-4) greatly reduced expression after melatonin or 17beta-estradiol supplement action. These findings demonstrate the important effects of melatonin or 17beta-estradiol on postmenopausal neuropathy and support the potential application of melatonin in the treatment of dementia in postmenopausal women. Early, long-term melatonin application is a promising strategy which could potentially be applied in a clinical setting.

Neuroprotective effects of an estratriene analog are estrogen receptor independent in vitro and in vivo

Estrogens are potent neuroprotectants both in vitro and in vivo. In the present study, we compared the potency and efficacy of a non-feminizing estrogen, 2-(1-adamantyl)-4-methylestrone (ZYC-26), with its parent estrogen, estrone, and an expected non-neuroprotective 3-O-methyl analog of (17beta)-2-(1-adamantyl)estradiol (ZYC-23). These estratriene derivatives were tested for their ability to protect in an in vitro lipid peroxidation model, to neuroprotect against oxidative stress in cell culture models, to bind the estrogen receptors (ERalpha and ERbeta), to elicit uterotrophic effects, and to affect brain damage from transient middle cerebral artery occlusion. We observed that in contrast to estrone, neither ZYC-26 nor ZYC-23 bound to either estrogen receptors (ER) and both failed to elicit a uterotrophic response. In vitro, the active estrogen analogue ZYC-26 was more potent that estrogen in its ability to inhibit lipid peroxidation and to protect HT-22 cells from either glutamate or iodoacetic acid (IAA) toxicity. Further, ZYC-26 was as active in preventing brain damage from transient middle cerebral artery occlusion (MCAO) as was estrone. Collectively, these studies suggest that the antioxidant activity, rather than ER binding of non-feminizing estrogens such as ZYC-26, mediates their potent neuroprotective activity. Further, in view of the now known toxicities of chronic feminizing estrogen use in older women, non-feminizing estrogens may be a useful alternative for estrogen-induced brain protection.

Photoreceptor preservation in the S334ter model of retinitis pigmentosa by a novel estradiol analog

The cytoprotective activity of MITO-4565, a novel, non-hormonal, estradiol derivative, was evaluated in the S334ter transgenic model of retinitis pigmentosa (RP). Progressive blindness in RP is due to apoptotic death of the photoreceptors, a process mimicked by the animal models [Portera-Cailliau C, Sung C-H, Nathans J, Adler R. Apoptotic photoreceptor cell death in mouse models of retinitis pigmentosa. Proc Natl Acad Sci USA 1994;91:974-8]. On postnatal day 9, 10 transgenic S334ter rats received a single intraocular injection of MITO-4565 in the left eye, and vehicle in the right eye. By postnatal day 20, the thickness of the outer nuclear layer (ONL) in the superior retina of the untreated eyes was 5.76 +/- 1.12 microm (N = 10), versus 10.72 +/- 1.52 microm (N = 10) for eyes treated with MITO-4565 (P < 0.0001, ANOVA F = 1671). Comparable cytoprotection was also observed for the inferior retina. Cytoprotection by MITO-4565 was also observed in primary cultures of rat retinal ganglion cells against NMDA excitotoxicity. Data from studies of hexose monophosphate shunt flux, mitochondrial stability, and in vitro lipid peroxidation, are in accord with previous reports [Green PS, Gridley KE, Simpkins JW. Nuclear estrogen receptor independent neuroprotection by estratrienes: a novel interaction with glutathione. Neuroscience 1997;84:7-10]; a likely mechanism of action entails moderation of membrane lipid peroxidation in a redox couple with glutathione. Such preservation of membrane integrity is particularly crucial to mitochondria, where collapse of membrane potential precipitates cell death, and where GSH is maintained at mM concentrations. Indeed, exposure to MITO-4565, but not a methoxy substituted negative control, allowed mitochondria to retain membrane potential (DeltaPsi(m)) under conditions of Ca(2+) overload that would normally induce complete mitochondrial failure. Mitochondrial interventions offer a novel therapeutic approach for RP, and other degenerative diseases of the retina.

Estradiol and raloxifene protect cultured SN4741 neurons against oxidative stress

A large body of research has documented neuroprotective effects of estrogen against oxidative stress. Some neurodegenerative diseases such as Parkinson's disease, in which oxidative stress has been implicated as a contributing factor, affect more males than females, suggesting a possible protective effect of estrogen. We used the clonal substantia nigra cell line SN4741 to compare the neuroprotective properties of estrogen and raloxifene against oxidative stress, and to determine whether raloxifene acted as an estrogen agonist or antagonist in this system. We pretreated SN4741 cultures with alpha-estradiol, beta-estradiol, and raloxifene, and exposed them to hydrogen peroxide. Low nanomolar levels of raloxifene, beta-estradiol, and alpha-estradiol all significantly reduced cell death caused by oxidative stress. The estrogen receptor (ER) antagonist ICI 182,780 failed to reverse the neuroprotection by beta-estradiol, suggesting that the effect is not mediated by a classical ER. Western blotting using an antibody to the C-terminus region of ER-alpha revealed two bands, one at approximately 67 kDa (corresponding to ER-alpha) and a more prominent band at approximately 55-56 kDa. These results suggest that, in this cell line, both raloxifene and estrogen may be acting via a non-classical estrogen receptor.

beta-estradiol, dehydroepiandrosterone, and dehydroepiandrosterone sulfate protect against N-methyl-D-aspartate-induced neurotoxicity in rat hippocampal neurons by different mechanisms

We examined neuroprotective effects of beta-estradiol, dehydroepiandrosterone (DHEA), and dehydroepiandrosterone sulfate (DHEA-S) against N-methyl-D-aspartate (NMDA)-induced neurotoxicity in primary cultured rat hippocampal neurons. All three steroids demonstrated neuroprotective effects. Time-course studies revealed that steroid cotreatment for only 15 min at the same time as exposure to NMDA, but neither pretreatment nor addition of steroids for 24 h after NMDA-mediated neuroprotective effects. This indicates that short-term actions of these steroids are critical for this process. Acute treatment with beta-estradiol dose dependently inhibited NMDA-induced intracellular Ca(2+) increases, which strongly correlated with its neuroprotective effect via L-type voltage-gated calcium channels. Acute treatment with DHEA, but not with DHEA-S, significantly inhibited nitric oxide (NO) production and Ca(2+)-sensitive NO synthase (NOS) activity caused by NMDA stimulation. An NOS inhibitor, N(G)-monomethyl-L-arginine acetate was also protective against NMDA-induced neurotoxicity. These data indicate that beta-estradiol may exert neuroprotective effects mainly by reducing Ca(2+) increases but that DHEA may act by inhibiting NOS activity. Treatment with the sigma-1 receptor (Sig-1R) antagonists rimcazole or BD1063 (1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine dihydrochloride) partially, but significantly, reversed the neuroprotective effect of DHEA-S against NMDA-induced neurotoxicity, whereas muscimol, a GABA-A-receptor agonist, did not. This suggests that the neuroprotective effect of DHEA-S may be mediated via Sig-1R, at least in part. Together, our data suggest that the neurosteroid family members beta-estradiol, DHEA, and DHEA-S exert neuroprotective effects through different nongenomic mechanisms.

17beta-estradiol protects oligodendrocytes from cytotoxicity induced cell death

During pregnancy, changes in circulating levels of hormones, including estrogens, correlates with a significant decrease in the relapse incidence in women with Multiple Sclerosis (MS). In the present study, we demonstrate that both primary and cell line cultures of rat oligodendrocytes express the estrogen receptor (ER)-alpha and ERbeta estrogen receptors in the cytosol and nucleus, and that nuclear compartmentalization becomes more pronounced as the cells mature. Moreover, 17beta-estradiol significantly decreases the cytotoxic effects of the peroxynitrite generator 3-(4-morpholinyl)-sydnonimine (SIN-1) in both immature and mature oligodendrocytes in a dose dependent manner. This protective mechanism requires pretreatment with 17beta-estradiol and is blocked by ICI 182,780, a selective ERalpha/ERbeta antagonist. These results strongly suggest that 17beta-estradiol protects oligodendrocytes against SIN-1 mediated cytotoxicity through the activation of the estrogen receptors and provides new insights into the roles of the estrogen signaling pathways in myelin forming cells that are lost in demyelinating disorders.

Estradiol exacerbates hippocampal damage in a model of preterm infant brain injury

We have developed a model for prenatal hypoxia-ischemia in which muscimol, a selective gamma-aminobutyric acid A (GABA(A)) receptor agonist, administered to newborn rats, induces hippocampal damage. In the neonatal rat brain, activation of GABA(A) receptors leads to membrane depolarization and neuronal excitation. Because of our previous detection of sex differences in this model and the considerable interest in the neuroprotective effects of estradiol in the adult brain, we now investigate the effect of pretreatment with high physiological levels of estradiol in our model of prenatal hypoxia-ischemia. We used unbiased stereology to assess neuron number in the hippocampal formation of control, muscimol-treated, and estradiol- plus muscimol-treated animals. Muscimol decreased neuron number in the hippocampus, with damage exacerbated by pretreatment with estradiol. A hippocampal culture paradigm was developed to mirror the in vivo investigation. We observed elevated cytotoxicity (using the lactate dehydrogenase assay) by 48 h after treatment with estradiol plus muscimol, but decreased cytotoxicity between 2 and 24 h after treatment. To determine whether the actions of estradiol on muscimol-induced damage were via the estrogen receptor, hippocampal cultures were pretreated with ICI 182,780, a selective estrogen receptor antagonist. Treatment with ICI 182,780 blocked the potentiating effect of estradiol on the late period of cytotoxicity, but had no effect on the protective actions of estradiol during the early period of cytotoxicity. There appears to be a biphasic action of estradiol in our model of neonatal brain injury that involves early nongenomic, nonreceptor-mediated protection, followed by late deleterious receptor-mediated effects.

17beta-estradiol activates ICI 182,780-sensitive estrogen receptors and cyclic GMP-dependent thioredoxin expression for neuroprotection

Clinical studies suggest that estrogen may improve cognition in Alzheimer's patients. Basic experiments demonstrate that 17beta-estradiol protects against neurodegeneration in both cell and animal models. In the present study, a human SH-SY5Y cell model was used to investigate molecular mechanisms underlying the receptor-mediated neuroprotection of physiological concentrations of 17beta-estradiol. 17beta-estradiol (<10 nM) concomitantly increased neuronal nitric oxide synthase (NOS1) expression and cell viability. 17beta-estradiol-induced neuroprotection was blocked by the receptor antagonist ICI 182,780, also prevented by inhibitors of NOS1 (7-nitroindazole), guanylyl cyclase (LY 83,583), and cGMP-dependent protein kinase (PKG) (Rp-8-pCPT-cGMPs). In addition to the expression of NOS1 and MnSOD, 17beta-estradiol increased the expression of the redox protein thioredoxin (Trx), which was blocked by the inhibition of either cGMP formation or PKG activity. The expression of heme oxygenase 2 and brain-derived neurotrophic factor was not altered. Estrogen receptor-enhanced cell viability against oxidative stress may be linked to Trx expression because the Trx reductase inhibitor, 5,5'-dithio-bis(2-nitrobenzoic acid) significantly reduced the cytoprotective effect of 17beta-estradiol. Furthermore, Trx (1 microM) inhibited lipid peroxidation, proapoptotic caspase-3, and cell death during oxidative stress caused by serum deprivation. We conclude that cGMP-dependent expression of Trx--the redox protein with potent antioxidative and antiapoptotic properties--may play a pivotal role in estrogen-induced neuroprotection.