Estradiol reduces cytochrome c translocation and minimizes hippocampal damage caused by transient global ischemia in rat

Ranolazine Attenuates Brain Inflammation in a Rat Model of Type 2 Diabetes

Recent studies suggest a pathogenetic association between metabolic disturbances, including type 2 diabetes (T2DM), and cognitive decline and indicate that T2DM may represent a risk factor for Alzheimer's disease (AD). There are a number of experimental studies presenting evidence that ranolazine, an antianginal drug, acts as a neuroprotective drug. The aim of the present study was to evaluate the effects of ranolazine on hippocampal neurodegeneration and astrocytes activation in a T2DM rat model. Diabetes was induced by a high fat diet (HFD) and streptozotocin (STZ) injection. Animals were divided into the following groups: HFD/STZ + Ranolazine, HFD/STZ + Metformin, HFD/STZ + Vehicle, NCD + Vehicle, NCD + Ranolazine and NCD + Metformin. The presence of neurodegeneration was evaluated in the hippocampal cornus ammonis 1 (CA1) region by cresyl violet staining histological methods, while astrocyte activation was assessed by western blot analysis. Staining with cresyl violet highlighted a decrease in neuronal density and cell volume in the hippocampal CA1 area in diabetic HFD/STZ + Vehicle rats, while ranolazine and metformin both improved T2DM-induced neuronal loss and neuronal damage. Moreover, there was an increased expression of GFAP in the HFD/STZ + Vehicle group compared to the treated diabetic groups. In conclusion, in the present study, we obtained additional evidence supporting the potential use of ranolazine to counteract T2DM-associated cognitive decline.

Hydrogen Sulfide Produced by Gut Bacteria May Induce Parkinson's Disease

Several bacterial species can generate hydrogen sulfide (H2S). Study evidence favors the view that the microbiome of the colon harbors increased amounts of H2S producing bacteria in Parkinson's disease. Additionally, H2S can easily penetrate cell membranes and enter the cell interior. In the cells, excessive amounts of H2S can potentially release cytochrome c protein from the mitochondria, increase the iron content of the cytosolic iron pool, and increase the amount of reactive oxygen species. These events can lead to the formation of alpha-synuclein oligomers and fibrils in cells containing the alpha-synuclein protein. In addition, bacterially produced H2S can interfere with the body urate metabolism and affect the blood erythrocytes and lymphocytes. Gut bacteria responsible for increased H2S production, especially the mucus-associated species of the bacterial genera belonging to the Desulfovibrionaceae and Enterobacteriaceae families, are likely play a role in the pathogenesis of Parkinson's disease. Special attention should be devoted to changes not only in the colonic but also in the duodenal microbiome composition with regard to the pathogenesis of Parkinson's disease. Influenza infections may increase the risk of Parkinson's disease by causing the overgrowth of H2S-producing bacteria both in the colon and duodenum.

Liraglutide prevents cognitive decline in a rat model of streptozotocin-induced diabetes independently from its peripheral metabolic effects

Diabetes has been identified as a risk factor for cognitive dysfunctions. Glucagone like peptide 1 (GLP-1) receptor agonists have neuroprotective effects in preclinical animal models. We evaluated the effects of GLP-1 receptor agonist, liraglutide (LIR), on cognitive decline associated with diabetes. Furthermore, we studied LIR effects against hippocampal neurodegeneration induced by streptozotocin (STZ), a well-validated animal model of diabetes and neurodegeneration associated with cognitive decline. Diabetes and/or cognitive decline were induced in Wistar rats by intraperitoneal or intracerebroventricular injection of STZ and then rats were treated with LIR (300μg/kg daily subcutaneously) for 6 weeks. Rats underwent behavioral tests: Morris water maze, passive avoidance, forced swimming (FST), open field, elevated plus maze, rotarod tests. Furthermore, LIR effects on hippocampal neurodegeneration and mTOR pathway (AKT, AMPK, ERK and p70S6K) were assessed. LIR improved learning and memory only in STZ-treated animals. Anxiolytic effects were observed in all LIR-treated groups but pro-depressant effects in CTRL rats were observed. At a cellular/molecular level, intracerebroventricular STZ induced hippocampal neurodegeneration accompanied by decreased phosphorylation of AMPK, AKT, ERK and p70S6K. LIR reduced hippocampal neuronal death and prevented the decreased phosphorylation of AKT and p70S6K; AMPK was hyper-phosphorylated in comparison to CTRL group, while LIR had no effects on ERK. LIR reduced animal endurance in the rotarod test and this effect might be also linked to a reduction in locomotor activity during only the last two minutes of the FST. LIR had protective effects on cognitive functions in addition to its effects on blood glucose levels. LIR effects in the brain also comprised anxiolytic and pro-depressant actions (although influenced by reduced endurance). Finally, LIR protected from diabetes-dependent hippocampal neurodegeneration likely through an effect on mTOR pathway.

Estrogen therapy increases BDNF expression and improves post-stroke depression in ovariectomy-treated rats

The present study investigated the effect of exogenous estrogen on post-stroke depression. Rats were exposed to chronic mild stress following middle cerebral artery occlusion. The occurrence of post-stroke depression was evaluated according to the changes in preference for sucrose and performance in a forced swimming test. Estrogen therapy significantly improved these neurological symptoms, indicating that estrogen is effective in treating post-stroke depression. Increased brain-derived neurotrophic factor (BDNF) expression was reported in the hippocampus of rats that had been treated with estrogen for two weeks, suggesting that BDNF expression may be an important contributor to the improvement of post-stroke depression that is observed following estrogen therapy.

Effects of Estrogen and Phytoestrogen Treatment on an In Vitro Model of Recurrent Stroke on HT22 Neuronal Cell Line

An increase of stroke incidence occurs in women with the decline of estrogen levels following menopause. This ischemic damage may recur, especially soon after the first insult has occurred. We evaluated the effects of estrogen and phytoestrogen treatment on an in vitro recurrent stroke model using the HT22 neuronal cell line. HT22 cells were treated with 17β-estradiol or genistein 1 h after the beginning of the first of two oxygen and glucose deprivation/reoxygenation (OGD/R) cycles. During the second OGD, there was a deterioration of some components of the electron transport chain, such as cytochrome c oxidase subunit 1 with a subsequent increase of reactive oxygen species (ROS) production. Accordingly, there was also an increase of apoptotic phenomena demonstrated by poly(ADP-ribose) polymerase 1 cleavage, Caspase-3 activity, and Annexin V levels. The recurrent ischemic injury also raised the hypoxia-inducible factor 1α and glucose transporter 1 levels, as well as the ratio between the lipidated and cytosolic forms of microtubule-associated protein 1A/1B-light chain 3 (LC3-II/LC3-I). We found a positive effect of estradiol and genistein treatment by partially preserving the impaired cell viability after the recurrent ischemic injury; however, this positive effect does not seem to be mediated neither by blocking apoptosis processes nor by decreasing ROS production. This work contribute to the better understanding of the molecular mechanisms triggered by recurrent ischemic damage in neuronal cells and, therefore, could help with the development of an effective treatment to minimize the consequences of this pathology.

Does being female provide a neuroprotective advantage following spinal cord injury?

It has been controversial whether gender has any effect on recovery following spinal cord injury (SCI). Past experimental and clinical research aimed at addressing this subject has led to constrasting findings on whether females hold any advantage in locomotor recovery. Additionally, for studies supporting the notion of a female gender related advantage, a definite cause has not been explained. In a recent study, using large sample sizes for comparative male and female spinal cord injury cohorts, we reported that a significant gender advantage favoring females existed in both tissue preservation and functional recovery after taking into consideration discrepancies in age and weight of the animals across sexes. Prior animal research frequently used sample sizes that were too small to determine significance with certainty and also did not account for two other factors that influence locomotor performance: age and weight. Our finding is important in light of controversy surrounding the effect of gender on outcome and the fact that SCI affects more than ten thousand new individuals annually, a population that is disproportionately male. By deepening our understanding of why a gender advantage exists, potential new therapeutics can be designed to improve recovery for the male population following the initial trauma or putatively augment the neuroprotective privilege in females for enhanced outcomes.

Female Rats Demonstrate Improved Locomotor Recovery and Greater Preservation of White and Gray Matter after Traumatic Spinal Cord Injury Compared to Males

The possibility of a gender-related difference in recovery after spinal cord injury (SCI) remains a controversial subject. Current empirical animal research lacks sizable test groups to definitively determine whether significant differences exist. Evaluating locomotor recovery variances between sexes following a precise, clinically relevant spinal cord contusion model can provide valuable insight into a possible gender-related advantage in outcome post-SCI. In the current study, we hypothesized that by employing larger sample sizes in a reproducible contusive SCI paradigm, subtle distinctions in locomotor recovery between sexes, if they exist, would be elucidated through a broad range of behavioral tests. During 13 weeks of functional assessment after a thoracic (T8) contusive SCI in rat, significant differences owing to gender existed for the Basso, Beattie, and Bresnahan score and CatWalk hindlimb swing, support four, and single stance analyses. Significant differences in locomotor performance were noticeable as early as 4 weeks post-SCI. Stereological tissue-volume analysis determined that females, more so than males, also exhibited greater volumes of preserved gray and white matter within the injured cord segment as well as more spared ventral white matter area at the center of the lesion. The stereological tissue analysis differences favoring females directly correlated with the female rats' greater functional improvement observed at endpoint.

Evaluation of survival and neurological deficit in rats in the new model of global transient cerebral ischemia

We propose a modification to rat model of transient global cerebral ischemia with four-vessel occlusion avoiding pneumothorax and minimizing the consequences of surgery. Survival and neurological deficit in rats in this model was studied over 5 days.

Coumestrol treatment prevents Na+, K+ -ATPase inhibition and affords histological neuroprotection to male rats receiving cerebral global ischemia

OBJECTIVE

In this study, we investigated the possible mechanisms underlying the neuroprotective effects of coumestrol, a potent isoflavonoid with antioxidant activities and binding affinities for both estrogen receptors (ER) ER-alpha and ER-beta that are comparable to those of 17beta-estradiol, in a model of global ischemia in male subjects.

METHODS

Wistar rats underwent global ischemia (10 minutes) or sham surgery and received a single intracerebroventricular (icv) infusion of 20 μg of coumestrol or vehicle 1 hour before ischemia or 0, 3, 6, or 24 hours after reperfusion.

RESULTS

The data analysis revealed an extensive neuronal death in the CA1 hippocampal subfield at 7 days, and a significant decrease in the Na+, K+ -ATPase activity at 1 and 24 hours after ischemia, and both injuries were attenuated by coumestrol administration.

CONCLUSIONS

Coumestrol treatment was effective in preventing neuronal loss in all times of administration as well as able to rescue the Na+, K+ -ATPase activity, suggesting its potential benefits for either prevention or therapeutics use against cerebral ischemia in males.

Effects of hormone therapy on cognition and mood

OBJECTIVE

Results of the Women's Health Initiative (WHI) and Women's Health Initiative Memory Study (WHIMS) suggested that hormone therapy (HT) may be detrimental to cognitive health. This article reviews clinical studies that address issues relevant to those results.

DESIGN

Literature review.

INTERVENTION(S)

A search of Pubmed and Web of Science was conducted using the search terms HT and cognition, HT and mood. Clinical and observational studies were selected if they were published after the year 2000. Theories of HT mechanisms of action, pharmacology, biology, and observational and clinical trials are discussed.

RESULT(S)

Although observational and clinical trials show conflicting findings, methodologic considerations must be acknowledged. HT formulation and dose, route of administration, timing of initiation, length of treatment, and health of participants all contribute to inconsistencies in results. Transdermal estradiol and micronized progesterone administered at time of menopause are generally associated with cognitive and affective benefit.

CONCLUSION(S)

At the present time, results from existing studies are equivocal regarding the benefits of HT on cognition and affect. Future studies, such as the Kronos Early Estrogen Prevention Study (KEEPS), should address methodologic inconsistencies to provide clearer answers to this important question.

Periodic 17β-estradiol pretreatment protects rat brain from cerebral ischemic damage via estrogen receptor-β

Although chronic 17β-estradiol (E2) has been shown to be a cognition-preserving and neuroprotective agent in animal brain injury models, concern regarding its safety was raised by the failed translation of this phenomenon to the clinic. Previously, we demonstrated that a single bolus of E2 48 hr prior to ischemia protected the hippocampus from damage in ovariectomized rats via phosphorylation of cyclic-AMP response element binding protein, which requires activation of estrogen receptor subtype beta (ER-β). The current study tests the hypothesis that long-term periodic E2-treatment improves cognition and reduces post-ischemic hippocampal injury by means of ER-β activation. Ovariectomized rats were given ten injections of E2 at 48 hr intervals for 21 days. Hippocampal-dependent learning, memory and ischemic neuronal loss were monitored. Results demonstrated that periodic E2 treatments improved spatial learning, memory and ischemic neuronal survival in ovariectomized rats. Additionally, periodic ER-β agonist treatments every 48 hr improved post-ischemic cognition. Silencing of hippocampal ER-β attenuated E2-mediated ischemic protection suggesting that ER-β plays a key role in mediating the beneficial effects of periodic E2 treatments. This study emphasizes the need to investigate a periodic estrogen replacement regimen to reduce cognitive decline and cerebral ischemia incidents/impact in post-menopausal women.

Oral contraceptives and nicotine synergistically exacerbate cerebral ischemic injury in the female brain

Oral contraceptives (OC) and smoking-derived nicotine (N) are known to synergistically increase the risk and severity of cerebral ischemia in women. Although it has been known for some time that long-term use of OC and nicotine will have an increased risk of peripheral thrombus formation, little is known about how the combination of OC and nicotine increases severity of brain ischemia. Recent laboratory studies simulating the conditions of nicotine exposure produced by cigarette smoking and OC regimen of women in female rats confirms that the severity of ischemic hippocampal damage is far greater in female rats simultaneously exposed to OC than to nicotine alone. These studies also demonstrated that the concurrent exposure of OC and nicotine reduces endogenous 17β-estradiol levels and inhibits estrogen signaling in the brain of female rats. The endogenous 17β-estradiol plays a key role in cerebrovascular protection in women during their pre-menopausal life and loss of circulating estrogen at reproductive senescence increases both the incidence and severity of cerebrovascular diseases. Therefore, OC and nicotine induced severe post-ischemic damage might be a consequence of lack of estrogen signaling in the brain. In the present review we highlight possible mechanisms by which OC and nicotine inhibits estrogen signaling that could be responsible for severe ischemic damage in females.

The potential for estrogens in preventing Alzheimer's disease and vascular dementia

Estrogens are the best-studied class of drugs for potential use in the prevention of Alzheimer's disease (AD). These steroids have been shown to be potent neuroprotectants both in vitro and in vivo, and to exert effects that are consistent with their potential use in prevention of AD. These include the prevention of the processing of amyloid precursor protein (APP) into beta-amyloid (Aß), the reduction in tau hyperphosphorylation, and the elimination of catastrophic attempts at neuronal mitosis. Further, epidemiological data support the efficacy of early postmenopausal use of estrogens for the delay or prevention of AD. Collectively, this evidence supports the further development of estrogen-like compounds for prevention of AD. Several approaches to enhance brain specificity of estrogen action are now underway in an attempt to reduce the side effects of chronic estrogen therapy in AD.

Effects of 17beta-estradiol replacement on the apoptotic effects caused by ovariectomy in the rat hippocampus

AIMS

The aim of the present study was to investigate the effects of different periods of ovariectomy and 17beta-estradiol replacement on apoptotic cell death and expression of members of the Bcl-2 family in the rat hippocampus.

MAIN METHODS

Hippocampi were obtained from rats in proestrus, ovariectomized (15 days, 21 days and 36 days), ovariectomized for 15 days and then treated with 17beta-estradiol for 7 or 21 days, and rats ovariectomized and immediately treated with 17beta-estradiol for 21 days. The expression of Bcl-2 and Bax and the number of apoptotic cells were determined.

KEY FINDINGS

Ovariectomy decreased Bcl-2 expression and increased Bax expression and the number of apoptotic cells. Replacement with 17beta-estradiol (21 days) throughout the post-ovariectomy period reduced the number of apoptotic cells to the control levels, and prevented the effects of ovariectomy on Bax expression, but only partially restored the Bcl-2 expression. After 15 days of ovariectomy, the replacement with 17beta-estradiol for 21 days, but not for 7 days, restored the Bcl-2 and Bax expression and the percentage of apoptotic cells to the levels found in the proestrus control.

SIGNIFICANCE

The present results show that a physiological concentration of 17beta-estradiol may help maintain long-term neuronal viability by regulating the expression of members of the Bcl-2 family. Even after a period of hormonal deprivation, treatment with 17beta-estradiol is able to restore the expression of Bax and Bcl-2 to control levels, but the duration of the treatment is a key factor to obtain the desired effect. These data provide new understanding into the mechanisms contributing to the neuroprotective action of estrogen.

Trichlorfon-induced apoptosis in hepatocyte primary cultures of Carassius auratus gibelio

Trichlorfon, an organophosphorus pesticide, can disrupt metabolism, reproduction and immune functions of some aquatic animals. In the present study, the effect of trichlorfon on apoptosis and the underlying apoptotic mechanism were investigated in primary cultures of Carassius auratus gibelio hepatocytes. Analyses of cultures exposed to 0, 0.01, 0.1, and 1.0 mg L(-1) trichlorfon concentrations for 24h indicated that trichlorfon induced apoptosis and caused nuclear shrinkage, cell membrane rupture, cytoskeletal collapse, loss of cytoplasm, mitochondria vacuolization, and apoptotic body formation, as well as lipid droplet accumulation. Trichlorfon increased intracellular reactive oxygen species and malondialdehyde concentrations and caused cytochrome c release from mitochondria into the cytoplasm, leading to caspase-3 activation. These findings contributed to a better understanding of the mechanisms underlying trichlorfon-induced apoptosis via activation of mitochondrial pathways while clearly indicating that trichlorfon-induced cell death was via apoptosis accompanied by mitochondrial cytochrome c release and consequent caspase-3 activation.

Dose-related neuroprotective versus neurodamaging effects of estrogens in rat cerebral ischemia: a systematic analysis

Numerous studies of the effects of estrogens for stroke prevention have yielded conflicting results in human and animal studies alike. We present a systematical analysis of study design and methodological differences between 66 studies where estrogens' impact on ischemic brain damage in rat models has been investigated, providing evidence that the differences in results may be explained by high estrogen doses produced by slow-release pellets. These pellets have been used in all studies showing increased neurologic damage because of estrogens. Our data indicate that the increased neurologic damage is related to the pellets' plasma concentration profile with an early, prolonged, supraphysiological peak. Neither the method of inducing the ischemic brain lesions, the choice of variables for measuring outcome, the measured plasma concentrations of estrogens at the time of ischemia nor rat population attributes (sex, strain, age, and diseases) are factors contributing to the discrepancies in results. This suggests that the effects of estrogens for stroke prevention are concentration related with a complex dose-response curve, and underscores the importance of carefully validating the experimental methods used. Future studies of hormone-replacement therapy in women may have to take dosage and administration regimens into account.

Pretreatment with a single estradiol-17beta bolus activates cyclic-AMP response element binding protein and protects CA1 neurons against global cerebral ischemia

Estradiol-17beta is released from the ovaries in a cyclic manner during the normal estrous cycle in rats. During the transition from the diestrous to proestrous stage, the 17beta-estradiol increases in blood circulation. We hypothesized that a higher serum level of endogenous 17beta-estradiol would protect hippocampal pyramidal neurons against global cerebral ischemia via activation of the cyclic-AMP response element binding protein (CREB)-mediated signaling cascade. Furthermore, we asked if a single 17beta-estradiol bolus provides protection against ischemia in the absence of endogenous estradiol. To test these hypotheses, rats were subjected to global cerebral ischemia at different stages of the estrous cycle. Ischemia was produced by bilateral carotid occlusion and systemic hypotension. Brains were examined for histopathology at 7 days of reperfusion. Higher serum levels of 17beta-estradiol (at proestrus and estrus stages) correlated with increased immunoreactivity of pCREB in hippocampus and ischemic tolerance. At diestrus, when circulating gonadal hormone concentrations were lowest, the pCREB protein content of hippocampus was reduced and showed the least number of normal neurons after ischemia compared to other stages of the estrous cycle. A similar phosphorylation pattern was also observed for mitogen-activated protein kinase (MAPK) and calcium-calmodulin-dependent protein kinase (CaMKII) in hippocampus. The cyclic variation in ovarian hormones did not reflect phosphorylation of protein kinase B (Akt). To test the efficacy of a single bolus of 17beta-estradiol before ischemia, ovariectomized rats were treated with 17beta-estradiol (5/10/50 microg/kg) or vehicle (oil) and 48/72/96 h later rats were exposed to cerebral ischemia. A single 17beta-estradiol bolus treatment in ovariectomized rats significantly increased CREB mRNA activation and protected CA1 pyramidal neurons against ischemia. These results suggest that an exogenous bolus of 17beta-estradiol to ovariectomized rats protects hippocampus against ischemia via activation of the CREB pathway in a manner similar to the endogenous estrous cycle.

Chronic nicotine exposure inhibits 17beta-estradiol-mediated protection of the hippocampal CA1 region against cerebral ischemia in female rats

Nicotine addiction in women increases the risk of ischemic stroke. Importantly, women who smoke and use hormone replacement therapy/oral contraceptives greatly increase their risk of coronary heart disease and ischemic stroke as compared to nonsmoking women who use occasionally oral contraceptives. Nicotine addiction disturbs the normal periodicity of the menstrual cycle and induces early onset of menopause in women; however, the mechanism of the synergistic effects of nicotine and sex hormones on cerebrovascular health is not clearly understood. In the current study based on a rat model of global cerebral ischemia, our goals are (1) to determine whether chronic nicotine exposure abrogates beneficial effects of estrogen on hippocampal neurons subjected to ischemia, and (2) to determine whether nicotine exposure antagonizes estrogen signaling by reducing the availability of estrogen receptor(s). To test the effects of chronic nicotine exposure, normally cycling or ovariectomized rats were injected with nicotine daily for 15 days. To investigate the efficacy of estrogen treatment, nicotine-exposed ovariectomized rats were injected with a bolus of 17beta-estradiol and 48h later ischemia was induced. Our results demonstrated that chronic nicotine exposure followed by ischemic insult at the proestrus stage of the estrous cycle showed that only 14% of normal neurons remained compared to the non-nicotine-treated group (p<0.05). Similarly, a bolus of 17beta-estradiol to nicotine-treated ovariectomized rats showed only 26% of normal neurons remaining as against 47% in the non-nicotine-treated group. Nicotine exposure decreased ERbeta but not ERalpha protein levels in the hippocampus, suggesting a role for ERbeta in increased post-ischemic neurodegeneration from nicotine exposure.

17beta-estradiol attenuates hyperoxia-induced apoptosis in mouse C8-D1A cell line

In premature infants, oxygen free radicals generated following neonatal resuscitation are associated with subsequent diseases such as retinopathy of prematurity and bronchopulmonary dysplasia. Recent studies in brain tissue samples have shown that nonphysiologic oxygen levels play a key role in induction of apoptosis in the developing brain. Estrogen is a well-established agent in neuroprotection and, therefore, is thought to be neuroprotective even in the premature brain. Astrocytes appear to have a critical role in protection and survival of neurons in the brain. As one of the glial cell types, they have a great potential for possible involvement in the mediation of estrogen neuroprotective effects. The aim of our study was to analyze whether astrocytes in cell cultures are damaged by hyperoxia and whether 17beta-estradiol (E2) can protect them against apoptosis. Additionally, we investigated the mechanism of the protection by E2, hypothesizing that it is mediated through extracellular signal-regulated kinase (ERK1/2). Cells underwent eightfold more apoptosis when cultivated in hyperoxia compared with normoxia. Addition of E2 reduced apoptosis in hyperoxia by more than 50%. Levels of ERK1/2 and phosphorylated ERK1/2 were increased after hyperoxia compared with normoxia. Preincubation with E2 prior to exposure to hyperoxia resulted in decreased levels of ERK1/2 and pERK1/2. Hyperoxia induces apoptosis in C8-D1A cells, and E2 seems to be a protecting factor for astrocytes in hyperoxia. This effect is not mediated through up-regulation of pERK1/2.

Hypoxia-induced apoptotic cell death is prevented by oestradiol via oestrogen receptors in the developing central nervous system

The neuroprotective effects of oestrogens have been demonstrated against a variety of insults, including excitotoxicity, oxidative stress and cerebral ischemia under certain conditions. However, the molecular mechanisms underlying oestrogen neuroprotection are still unclear. We aimed to determine whether 17beta-oestradiol (E(2)) administration post-hypoxia (p-hx) was neuroprotective and whether these actions were mediated through oestrogen receptors (ER). For this purpose, 12-embyonic day-old chickens were subjected to acute hypoxia [8% (O(2)), 60 min], followed by different reoxygenation periods. To test the neuroprotective effect of E(2) and its mechanism, embryos were injected 30 min after the end of hypoxia with E(2) alone or with ICI 182 780, a competitive antagonist of ER. Cytochrome c (cyt c) release, an indicator of mitochondrial apoptotic pathway, was measured by western blot in optic lobe cytosolic extracts. DNA fragmentation by TUNEL fluorescence and caspase-3 fragmentation by immunofluorescence were detected on optic lobe sections. Acute hypoxia produces a significant increase in cyt c release from mitochondria at 4 h p-hx, followed by an increase in TUNEL positive cells 2 h later (6 h p-hx). Administration of E(2) (0.5 mg/egg) produced a significant decrease in cytosolic cyt c levels at 4 h p-hx, in caspase-3 activation and in TUNEL positive cells at 6 h p-hx compared to vehicle treated embryos. In the E(2)-ICI 182 780 treated embryos, cyt c release, caspase-3 fragmentation and TUNEL positive cells were similar to the hypoxic embryos, thus suggesting the requirement of an E(2)-ER interaction for E(2) mediated neuroprotective effects. In conclusion, E(2) prevents hypoxia-induced cyt c release and posterior cell death and these effects are mediated by oestrogen receptors.

Neuroprotective effect of diazoxide on brain injury induced by cerebral ischemia/reperfusion during deep hypothermia

OBJECT

The purpose of this study was to determine the effects of diazoxide on apoptosis and the relative mechanisms in a model of brain injury induced by cerebral ischemia/reperfusion (I/R) during deep hypothermia.

METHODS

Three-week-old Sprague-Dawley male rats were randomly and equitably divided into sham-operated group, placebo-treated group and diazoxide-treated group respectively. Specific examination of the regional cerebral blood flow (rCBF) was measured in the three groups continuously during the operation by laser Doppler flowmetry. Terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) was showed DNA fragmentation. The mRNA expressions of cytochrome c and full-length caspase-3 were determined by RT-PCR, while the protein expressions of cytochrome c and cleaved caspase-3 were determined by immunohistochemistry at 1 h, 6 h, 24 h, 72 h and 7 days after I/R, respectively. Cytosolic release of cytochrome c at 24 h after I/R was also confirmed by Western blot.

RESULTS

rCBF was significantly decreased in both of placebo-treated and diazoxide-treated group just after ischemia in the time interval 0-5 min, and had no obvious changes in all the time intervals during the operation. Diazoxide preconditioning significantly decreased the percentage of TUNEL-positive staining cells. The mRNA expressions of cytochrome c and full-length caspase-3 in diazoxide-treated group were significantly decreased. In addition, diazoxide provided a significant reduction in the protein expressions of cytochrome c and cleaved caspase-3.

CONCLUSION

These results suggested that the neuroprotective effects of diazoxide against cerebral I/R injury during deep hypothermia correlated with the reduction of DNA fragmentation, prevention of mitochondrial cytochrome c release and inhibition of caspase-3 activation.

Estradiol attenuates hyperoxia-induced cell death in the developing white matter

OBJECTIVE

Periventricular leukomalacia is the predominant type of brain injury in preterm infants underlying the development of cerebral palsy. Periventricular leukomalacia has its peak incidence at 23 to 32 weeks postconceptional age characterized by extensive oligodendrocyte migration and maturation. Oxygen toxicity has been identified as a possible contributing factor to the pathogenesis of cerebral palsy in survivors of preterm birth. 17beta-estradiol (E2) is important for the development and function of the central nervous system. Furthermore, neuroprotective properties have been attributed to estrogens. We examined the effect of E2 on hyperoxia-induced cell death in the developing white matter in the rat brain.

METHODS

Six-day-old (P6) rat pups, the immature oligodendroglial cell line (OLN-93), and primary oligodendrocyte cultures were subjected to 80% O(2) in the presence or absence of E2 (600 microg/kg intraperitoneally in vivo, 10(-6)-10(-10)M in vitro). Cell counts and lactate dehydrogenase assay were used to assess cell survival. Immunoblot analysis was used for detection of estrogen receptor expression and investigation of apoptotic signaling pathways. White matter injury was assessed by myelin basic protein immunocytochemistry at P11.

RESULTS

E2 produced significant dose-dependent protection against oxygen-induced apoptotic cell death in primary oligodendrocytes. Treatment with E2 prevented hyperoxia-induced proapoptotic Fas-upregulation and caspase-3 activation. Finally, E2 antagonized hyperoxia-induced inactivation of extracellular signal-regulated kinase 1 and 2 and Akt, key kinases of the mitogen-activated protein kinase and phosphatidylinositol 3-kinase cell survival promoting pathways, respectively. Loss of myelin basic protein labeling was seen in P11 pups after oxygen exposure, and E2 attenuated this injury.

INTERPRETATION

These results suggest a possible role for estrogens in the prevention of neonatal oxygen-induced white matter injury.

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.

Estrogen limits ischemic cell death by modulating caspase-12-mediated apoptotic pathways following middle cerebral artery occlusion

Estrogen has received considerable attention as a potential therapeutic agent against various forms of neurodegenerative diseases including stroke. Experimental data in animal models of stroke have provided exhaustive evidence of the neuroprotective properties of this steroid hormone. Our laboratory in particular has demonstrated that acute estrogen treatment in male rats significantly reduced (approximately 50%) ischemic cell death within 4 h following permanent occlusion of the middle cerebral artery occlusion (MCAO). However, the cellular and molecular mechanisms implicated in the protective actions of estrogen in this experimental model have yet to be elucidated. Accumulating evidence suggests that in various in vivo and in vitro models, estrogen can be pro-apoptotic and that this effect may be mediated by an estrogen-induced up-regulation of the Fas/FasL system and the subsequent activation of caspase-12. We therefore hypothesized that under ischemic conditions following MCAO, estrogen would up-regulate protective endoplasmic reticulum (ER) stress pathways leading to caspase-12 activation, thus limiting infarct volume. Our results showed that estrogen significantly increased activated caspase-12 at 2, 3 and 4 h post-MCAO. Immunostaining of brain sections showed a significantly higher number of terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling positive cells in estrogen-treated animals at 4 h, but not at 2 h, post-MCAO. These findings correlate with previous observations that differences in infarct volume between saline and estrogen-treated animals are not seen until 3 and 4 h post-MCAO. A decrease in m-calpain expression was observed in the infarct region only at 4 h post-MCAO following estrogen pre-treatment, suggesting m-calpain may not be involved in regulating estrogen-induced caspase-12 activation. Based on these cellular changes correlated to estrogen pretreatment, we conclude that estrogen may up-regulate ER-specific apoptotic pathways, thus limiting the extent of necrotic cell death which is responsible for the spreading depression and growth of the infarct volume following MCAO.

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.

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.

MAPK signaling is critical to estradiol protection of CA1 neurons in global ischemia

The importance of hormone therapy in affording protection against the sequelae of global ischemia in postmenopausal women remains controversial. Global ischemia arising during cardiac arrest or cardiac surgery causes highly selective, delayed death of hippocampal CA1 neurons. Exogenous estradiol ameliorates global ischemia-induced neuronal death and cognitive impairment in male and female rodents. However, the molecular mechanisms by which estrogens intervene in global ischemia-induced apoptotic cell death are unclear. Here we show that estradiol acts via the classical estrogen receptors, the IGF-I receptor, and the ERK/MAPK signaling cascade to protect CA1 neurons in ovariectomized female rats and gerbils. We demonstrate that global ischemia promotes early dephosphorylation and inactivation of ERK1 and the transcription factor cAMP-response element binding protein (CREB), subsequent down-regulation of the antiapoptotic protein Bcl-2, a known gene target of estradiol and CREB, and activation of caspase-3. Estradiol treatment increases basal phosphorylation of both ERK1 and ERK2 in hippocampal CA1 and prevents ischemia-induced dephosphorylation and inactivation of ERK1 and CREB, down-regulation of Bcl-2 and activation of the caspase death cascade. Whereas ERK/MAPK signaling is critical to CREB activation and neuronal survival, the impact of estradiol on Bcl-2 levels is ERK independent. These findings support a model whereby estradiol acts via the classical estrogen receptors and IGF-I receptors, which converge on activation of ERK/MAPK signaling and CREB to promote neuronal survival in the face of global ischemia.

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.

Inhibition of MLK3-MKK4/7-JNK1/2 pathway by Akt1 in exogenous estrogen-induced neuroprotection against transient global cerebral ischemia by a non-genomic mechanism in male rats

Numerous studies have demonstrated the neuroprotective effects of estrogen in experimental cerebral ischemia. To investigate molecular mechanisms of estrogen neuroprotection in global ischemia, immunoblotting, immunohistochemistry and Nissel-staining analysis were used. Our results showed that chronic pretreatment with beta-estradiol 3-benzoate (E2) enhanced Akt1 activation and reduced the activation of mixed-lineage kinase 3 (MLK3), mitogen-activated protein kinase kinase 4/7 (MKK4/7), and c-Jun N-terminal kinase 1/2 (JNK1/2) in the hippocampal CA1 subfield during reperfusion after 15 min of global ischemia. In addition, E2 reduced downstream JNK nuclear and non-nuclear components, c-Jun and Bcl-2 phosphorylation and Fas ligand protein expression induced by ischemia/reperfusion. Administration of phosphoinositide 3-kinase (PI3K) inhibitor LY 294,002 prevented both activation of Akt1 and inhibition of MLK3, MKK4/7 and JNK1/2. The interaction between ERalpha and the p85 subunit of PI3K was also examined. E2 and antiestrogen ICI 182,780 promoted and prevented this interaction, respectively. Furthermore, ICI 182,780 blocked both the activation of Akt1 and the inhibition of MLK3, MKK4/7 and JNK1/2. Photomicrographs of cresyl violet-stained brain sections showed that E2 reduced CA1 neuron loss after 5 days of reperfusion, which was abolished by ICI 182,780 and LY 294,002. Our data indicate that in response to estrogen, ERalpha interacts with PI3K to activate Akt1, which may inhibit the MLK3-MKK4/7-JNK1/2 pathway to protect hippocampal CA1 neurons against global cerebral ischemia in male rats.

Estradiol prevents release of cytochrome c from mitochondria and inhibits ischemia-induced apoptosis in perfused heart

The study investigated whether estradiol can prevent release of cytochrome c from mitochondria and induction of apoptosis after 30 and 60 min stop-flow heart ischemia in Langendorff-perfused female rat hearts. Pre-perfusion of hearts with 100 nM 17beta-estradiol prevented the loss of cytochrome c from mitochondria, its accumulation in cytosol, and inhibition of respiration during ischemia. Estradiol strongly reduced activation of caspase-3-like activity and decreased DNA strand breaks in the nuclei of cardiomyocytes (measured by TUNEL staining). The results show that 17beta-estradiol prevents the ischemia-induced release of cytochrome c from mitochondria, subsequent inhibition of mitochondrial respiration, and inhibits caspase activation and apoptosis. Therefore, inhibition of the intrinsic, mitochondria-mediated apoptotic pathway may be one of the mechanisms by which estrogens protect the heart against ischemic damage.

Estrogen preconditioning protects the hippocampal CA1 against ischemia

Estrogen is neuroprotective against ischemia in both in vivo and in vitro injury models. Because of the promising preclinical data on neuroprotection, the Women's Estrogen for Stroke Trial was initiated. The outcomes from this trial were, however, unsuccessful and questions emerged about the safety of chronic estrogen treatment in women. In contrast to the chronic estrogen treatment strategy, the present study aims to investigate: (1) the neuroprotective efficacy of single estrogen pretreatment/preconditioning; and (2) the existence of a similarity between estrogen- and ischemic preconditioning-induced neuroprotection against cerebral ischemia. The efficacy of estrogen was tested in an in vitro model of cerebral ischemia using hippocampal organotypic slice culture system. The hippocampal organotypic slice cultures were generated from female neonatal (9-11 days old) Sprague-Dawley rats. The slices were exposed to estradiol-17beta (0.5, 1, 5 nM) for various durations (1, 2 or 4 h) 48 h prior to ischemia (40 min of oxygen-glucose deprivation). For ischemic preconditioning, slices were exposed to sublethal oxygen-glucose deprivation (15 min), 48 h prior to lethal oxygen-glucose deprivation. Quantification of cell death in hippocampal CA1 region was conducted by using propidium iodide fluorescence staining technique. Results demonstrated that estrogen preconditioning significantly protects the hippocampal CA1 region against ischemia (P<0.001) and mimicked ischemic preconditioning-induced neuroprotection. The propidium iodide fluorescence values of estrogen preconditioning, ischemic preconditioning and ischemia groups were 21+/-2 (mean+/-S.E.M.) (1 nM; 2 h; n=15), 18+/-2 (5 nM; 4 h; n=12), 32+/-3 (n=8), 65+/-3 (n=27), respectively. Further, estrogen preconditioning initiated a calcium-mediated signaling pathway leading to protection of CA1 neurons against ischemia. Future investigations in estrogen preconditioning may suggest new estrogen regimens that avoid potential side effects of chronic estrogen treatment for stroke patients.

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.

Multiple pathways transmit neuroprotective effects of gonadal steroids

Numerous preclinical studies suggest that gonadal steroids, particularly estrogen, may be neuroprotective against insult or disease progression. This paper reviews the mechanisms contributing to estrogen-mediated neuroprotection. Rapid signaling pathways, such as MAPK, PI3K, Akt, and PKC, are required for estrogen's ability to provide neuroprotection. These rapid signaling pathways converge on genomic pathways to modulate transcription of E2-responsive genes via ERE-dependent and ERE-independent mechanisms. It is clear that both rapid signaling and transcription are important for estrogen's neuroprotective effects. A mechanistic understanding of estrogen-mediated neuroprotection is crucial for the development of therapeutic interventions that enhance quality of life without deleterious side effects.

Acute and chronic estradiol treatments reduce memory deficits induced by transient global ischemia in female rats

Transient global ischemia induces selective, delayed neuronal death in the hippocampal CA1 and delayed cognitive deficits. Estrogen treatment ameliorates hippocampal injury associated with global ischemia. Although much is known about the impact of estrogen on neuronal survival, relatively little is known about its impact on functional outcome assessed behaviorally. We investigated whether long-term estradiol (21-day pellets implanted 14 days prior to ischemia) or acute estradiol (50 microg infused into the lateral ventricles immediately after ischemia) attenuates ischemia-induced cell loss and improves visual and spatial working memory in ovariectomized female rats. Global ischemia significantly impaired visual and spatial memory, assessed by object recognition and object placement tests at 6-9 days. Global ischemia did not affect locomotion, exploration, or anxiety-related behaviors, assessed by an open-field test at 6 days. Long-term estradiol prevented the ischemia-induced deficit in visual working memory, maintaining normal performance in tests with retention intervals of up to 1 h. Long-term estradiol also prevented ischemia-induced deficits in spatial memory tests with short (1 and 7 min), but not longer (15 min), retention intervals. Acute estradiol significantly improved visual memory assessed with short retention intervals, but did not prevent deficits in spatial memory. Acute estradiol significantly increased the number of surviving CA1 neurons, assessed either at 7 days after ischemia or after the completion of behavioral testing 9 days after ischemia. In contrast, chronic estradiol did not reduce CA1 cell death 9 days after ischemia. Thus, long-term estradiol at near physiological levels and acute estradiol administered after ischemic insult improve functional recovery after global ischemia. These findings have important implications for intervention in the neurological sequellae associated with global ischemia.

17beta-estradiol protects SH-SY5Y Cells against HIV-1 gp120-induced cell death: evidence for a role of estrogen receptors

Despite the large body of experimental evidence demonstrating the neuroprotective properties of 17beta-estradiol (17beta-E2) both in vitro and in vivo experimental models of neuronal injury, the exact mechanisms implicated in neuroprotection have not been fully delineated. Some experimental evidence highlight a role for the antioxidant properties of 17beta-E2 in mediating protection against oxidative injury. Parallel to these, evidence also exist which point to alternative mechanisms involving estrogen receptors (ER). The HIV-1 coat protein, gp120, has been implicated in the progression of central nervous system damage caused by HIV-1 infection. The neurotoxic effects induced by gp120 are triggered via an excitotoxic mechanism of cell death which implicates alteration of calcium homeostasis, activation of calcium-dependent pathways, mitochondrial uncoupling and membrane lipid peroxidation. In the present study, we demonstrate that 17beta-E2 protects human SH-SY5Y neuroblastoma cells from cell death elicited by gp120. Tamoxifen and ICI 182,780, two ER antagonists, both antagonized 17beta-E2-mediated inhibition of cell death. Exposure of SH-SY5Y cells to gp120 for 30min caused a significant accumulation of intracellular reactive oxygen species (ROS) and this was abrogated by 17beta-E2; however, the ability of 17beta-E2 to counteract ROS generation induced by gp120 does not account for the reported prevention of cell death because ICI 182,780 failed to revert intracellular ROS reduction caused by 17beta-E2 though it was able to revert prevention of cell death. Furthermore, by using 17alpha-E2, the isomer unable to stimulate ER which, however, retains the antioxidant effects, we observed that a pre-treatment with 17alpha-E2 was effective in preventing gp120-induced accumulation of ROS but it failed to affect cell death caused by the viral protein. Collectively, these data demonstrate that neuroprotection afforded by 17beta-E2 is receptor-mediated and ROS scavenging effects may not be implicated.

From clinical evidence to molecular mechanisms underlying neuroprotection afforded by estrogens

Recent studies have highlighted that female sex hormones represent potential neuroprotective agents against damage produced by acute and chronic injuries in the adult brain. Clinical reports have documented the effectiveness of estrogens to attenuate symptoms associated with Parkinson's disease, and to reduce the risk of Alzheimer's disease and cerebrovascular stroke. This evidence is corroborated by numerous experimental studies documenting the protective role of female sex hormones both in vitro and in vivo. Accordingly, estrogens have been shown to promote survival and differentiation of several neuronal populations maintained in culture, and to reduce cell death associated with excitotoxicity, oxidative stress, serum deprivation or exposure to beta-amyloid. The neuroprotective effects of estrogens have been widely documented in animal models of neurological disorders, such as Alzheimer's and Parkinson's diseases, as well as cerebral ischemia. Although estrogens are known to exert several direct effects on neurones, the cellular and molecular mechanisms implicated in their protective actions on the brain are not completely understood. Thus, on the basis of clinical and experimental evidence, in this review, we discuss recent findings concerning the neuronal effects of estrogens that may contribute to their neuroprotective actions. Both estrogen receptor-dependent and -independent mechanisms will be described. These include modulation of cell death regulators, such as Bcl-2, Akt and calpain, as well as interaction with growth factors, such as BDNF, NGF, IGF-I and their receptors. The anti-inflammatory effects of estrogens will also be described, namely their ability to reduce brain levels of inflammatory mediators, cytokines and chemokines. Finally, a brief overview about receptor-independent mechanisms of neuroprotection will aim at describing the antioxidant effects of estrogens, as well as their ability to modulate neurotransmission.

Sex differences and the effect of hormonal therapy on ischemic brain injury

Epidemiological data emphasize the importance of sex differences in the mortality and morbidity of stroke and cardiovascular disease. The importance of hormonal influences on stroke outcome has pointed out the importance of gender, age, and presence of neural hormones. This clinical data has been substantiated by various experimental studies using clinically relevant models of cerebral ischemia and stroke. Published findings emphasize that male and female animals respond differently to periods of cerebral ischemia and that various combinations of hormonal treatments can provide protection, both histopathological and behavioral. Mechanisms underlying the hormonal effects on ischemic outcome are multifactorial. These include effects on vascular integrity and cerebral blood flow, excitotoxicity, oxidation pathways, inflammation, and apoptosis. Although many studies have shown positive results with hormonal treatments, negative findings have also been presented. Explanations for the limitations of hormonal treatment include uncertainties regarding therapeutic window, specific therapeutic dose range, as well as the specific pathophysiological processes being targeted. Additional studies are therefore required to clarify under what conditions hormonal therapy is most protective or not warranted. Experimental studies utilizing a variety of cerebral ischemia and stroke models are reviewed to indicate under what conditions sex differences and hormonal therapy are most important in terms of functional outcome.

Clinical Pharmacology and Differential Cognitive Efficacy of Estrogen Preparations

Menopause is associated with a significant decline in levels of estrogen, which reportedly leads to several distressing symptoms and adverse health effects on various target tissues including those on bones, heart, and brain. Although effective, the long-term safety and feasibility of therapy with both unopposed and opposed oral conjugated equine estrogen has been questioned by the recent findings of both the Women's Health Initiative (WHI) and the Women's Health Initiative Memory Study (WHIMS). The findings of both these studies have raised several critical issues related to hormone therapy that need to be systematically evaluated in clinical studies. Specifically, these issues relate to the differential efficacy and adverse-effects profile of various forms of estrogen and progestins, the importance of the route of administration of estrogen, the best timing to initiate postmenopausal hormone therapy, and the efficacy of cyclic versus continuous hormone therapy. This article focuses on estrogen and discusses issues related to selecting the best form and route of administration of the hormone. It includes information on basic clinical pharmacology of various forms of estrogen, neuroendocrinology of the menopause, neurobiology of estradiol and estrone, and results of selected basic science and human intervention studies with relevance to identifying the best form and route of administration of estrogen.

Glutamate-induced apoptosis in primary cortical neurons is inhibited by equine estrogens via down-regulation of caspase-3 and prevention of mitochondrial cytochrome c release

Background

Apoptosis plays a key role in cell death observed in neurodegenerative diseases marked by a progressive loss of neurons as seen in Alzheimer's disease. Although the exact cause of apoptosis is not known, a number of factors such as free radicals, insufficient levels of nerve growth factors and excessive levels of glutamate have been implicated. We and others, have previously reported that in a stable HT22 neuronal cell line, glutamate induces apoptosis as indicated by DNA fragmentation and up- and down-regulation of Bax (pro-apoptotic), and Bcl-2 (anti-apoptotic) genes respectively. Furthermore, these changes were reversed/inhibited by estrogens. Several lines of evidence also indicate that a family of cysteine proteases (caspases) appear to play a critical role in neuronal apoptosis. The purpose of the present study is to determine in primary cultures of cortical cells, if glutamate-induced neuronal apoptosis and its inhibition by estrogens involve changes in caspase-3 protease and whether this process is mediated by Fas receptor and/or mitochondrial signal transduction pathways involving release of cytochrome c.

Results

In primary cultures of rat cortical cells, glutamate induced apoptosis that was associated with enhanced DNA fragmentation, morphological changes, and up-regulation of pro-caspase-3. Exposure of cortical cells to glutamate resulted in a time-dependent cell death and an increase in caspase-3 protein levels. Although the increase in caspase-3 levels was evident after 3 h, cell death was only significantly increased after 6 h. Treatment of cells for 6 h with 1 to 20 mM glutamate resulted in a 35 to 45% cell death that was associated with a 45 to 65% increase in the expression of caspase-3 protein. Pretreatment with caspase-3-protease inhibitor z-DEVD or pan-caspase inhibitor z-VAD significantly decreased glutamate-induced cell death of cortical cells. Exposure of cells to glutamate for 6 h in the presence or absence of 17β-estradiol or Δ⁸, 17β-estradiol (10 nM-10 μM) resulted in the prevention of cell death and was associated with a significant dose-dependent decrease in caspase-3 protein levels, with Δ⁸, 17β-E₂ being more potent than 17β-E₂. Protein levels of Fas receptor remained unchanged in the presence of glutamate. In contrast, treatment with glutamate induced, in a time-dependent manner, the release of cytochrome c into the cytosol. Cytosolic cytochrome c increased as early as 1.5 h after glutamate treatment and these levels were 5 fold higher after 6 h, compared to levels in the untreated cells. Concomitant with these changes, the levels of cytochrome c in mitochondria decreased significantly. Both 17β-E₂ and Δ⁸, 17β-E₂ reduced the release of cytochrome c from mitochondria into the cytosol and this decrease in cytosolic cytochrome c was associated with inhibition of glutamate-induced cell death.

Conclusion

In the primary cortical cells, glutamate-induced apoptosis is accompanied by up-regulation of caspase-3 and its activity is blocked by caspase protease inhibitors. These effects of glutamate on caspase-3 appear to be independent of changes in Fas receptor, but are associated with the rapid release of mitochondrial cytochrome c, which precedes changes in caspase-3 protein levels leading to apoptotic cell death. This process was differentially inhibited by estrogens with the novel equine estrogen Δ⁸, 17β-E₂ being more potent than 17β-E₂. To our knowledge, this is the first study to demonstrate that equine estrogens can prevent glutamate-induced translocation of cytochrome c from mitochondria to cytosol in rat primary cortical cells.