Neuroprotection by estrogens in a mouse model of focal cerebral ischemia and in cultured neurons: evidence for a receptor-independent antioxidative mechanism

Perirhinal cortex to the nucleus accumbens circuit in novelty salience following methamphetamine self-administration

Methamphetamine (meth) use disorder is part of an overarching use disorder that encompasses continued drug seeking and an increased risk of returning to drug use following periods of abstaining. Chronic meth use results in drug-induced cortical plasticity in the perirhinal cortex (PRC) that mediates responses to novelty. PRH projection targets are numerous and include the nucleus accumbens core (NAc). Whereas the PRH-prefrontal cortex is involved in object recognition; we propose that the PRH-NAc is involved in novelty salience. Rats underwent short-access (ShA, 1 hr) or long-access (LgA, 6 hr) meth self-administration (SA). We then used a dual viral strategy to inhibit or activate PRH-NAc during a novel cue test in which rats were presented with meth-associated and novel levers. Response patterns on these levers differ depending on the meth access protocol: ShA meth SA results in equal responding on both novel- and meth-associated levers, whereas LgA meth results in perseverative responding on the meth-associated lever. Inactivation of the PRH-NAc increased responding on the meth lever relative to the novel lever, resulting in a LgA behavioral phenotype. In contrast, activation in LgA rats was without a behavioral effect. We also report that male LgA sucrose SA animals perseverated on the novel lever rather than the meth-associated lever, which contrast their meth SA counter parts and female specific patterns of behavior. These data open a new line of interest in the role of the PRH-NAc circuit in novelty salience through identification of the behavioral relevance of this circuit.

Sex differences in behavior, cognitive, and physiological recovery following methamphetamine administration

Intact executive functions are required for proper performance of cognitive tasks and relies on balance of excitatory and inhibitory (E/I) transmission in the medial prefrontal cortex (mPFC). Hypofrontality is a state of decreased activity in the mPFC and is seen in several neuropsychiatric conditions, including substance use disorders. People who chronically use methamphetamine (meth) develop hypofrontality and concurrent changes in cognitive processing across several domains. Despite the fact that there are sex difference in substance use disorders, few studies have considered sex as a biological variable regarding meth-mediated hypoactivity in mPFC and concurrent cognitive deficits. Hypofrontality along with changes in cognition are emulated in rodent models following repeated meth administration. Here, we used a meth sensitization regimen to study sex differences in a Temporal Order Memory (TOM) task following short (7 days) or prolonged (28 days) periods of abstinence. GABAergic transmission, GABAA receptor (GABAAR) and GABA Transporter (GAT) mRNA expression in the mPFC were evaluated with patch-clamp recordings and RT-qPCR, respectively. Both sexes sensitized to the locomotor activating effects of meth, with the effect persisting in females. After short abstinence, males and females had impaired TOM and increased GABAergic transmission. Female rats recovered from these changes after prolonged abstinence, whereas male rats showed enduring changes. In general, meth appears to elicit an overall decrease in GABAAR expression after short abstinence; whereas GABA transporters are decreased in meth female rats after prolonged abstinence. These results show sex differences in the long-term effects of repeated meth exposure and suggest that females have neuroprotective mechanisms that alleviate some of the meth-mediated cognitive deficits.

The Uncoupling Effect of 17β-Estradiol Underlies the Resilience of Female-Derived Mitochondria to Damage after Experimental TBI

Current literature finds females have improved outcomes over their male counterparts after severe traumatic brain injury (TBI), while the opposite seems to be true for mild TBI. This begs the question as to what may be driving these sex differences after TBI. Estrogen is thought to be neuroprotective in certain diseases, and its actions have been shown to influence mitochondrial function. Mitochondrial impairment is a major hallmark of TBI, and interestingly, this dysfunction has been shown to be more severe in males than females after brain injury. This suggests estrogen could be playing a role in promoting "mitoprotection" following TBI. Despite the existence of estrogen receptors in mitochondria, few studies have examined the direct role of estrogen on mitochondrial function, and no studies have explored this after TBI. We hypothesized ex vivo treatment of isolated mitochondria with 17β-estradiol (E2) would improve mitochondrial function after experimental TBI in mice. Total mitochondria from the ipsilateral (injured) and contralateral (control) cortices of male and female mice were isolated 24 h post-controlled severe cortical impact (CCI) and treated with vehicle, 2 nM E2, or 20 nM E2 immediately before measuring reactive oxygen species (ROS) production, bioenergetics, electron transport chain complex (ETC) activities, and β-oxidation of palmitoyl carnitine. Protein expression of oxidative phosphorylation (OXPHOS) complexes was also measured in these mitochondrial samples to determine whether this influenced functional outcomes with respect to sex or injury. While mitochondrial ROS production was affected by CCI in both sexes, there were other sex-specific patterns of mitochondrial injury 24 h following severe CCI. For instance, mitochondria from males were more susceptible to CCI-induced injury with respect to bioenergetics and ETC complex activities, whereas mitochondria from females showed only Complex II impairment and reduced β-oxidation after injury. Neither concentration of E2 influenced ETC complex activities themselves, but 20 nM E2 appeared to uncouple mitochondria isolated from the contralateral cortex in both sexes, as well as the injured ipsilateral cortex of females. These studies highlight the significance of measuring mitochondrial dysfunction in both sexes after TBI and also shed light on another potential neuroprotective mechanism in which E2 may attenuate mitochondrial dysfunction after TBI in vivo.

Neuroprotection or Sex Bias: A Protective Response to Traumatic Brain Injury in the Females

Traumatic brain injury (TBI) is a major healthcare problem and a common cause of mortality and morbidity. Clinical and preclinical research suggests sex-related differences in short- and longterm outcomes following TBI; however, males have been the main focus of TBI research. Females show a protective response against TBI. Female animals in preclinical studies and women in clinical trials have shown comparatively better outcomes against mild, moderate, or severe TBI. This reflects a favorable protective nature of the females compared to the males, primarily attributed to various protective mechanisms that provide better prognosis and recovery in the females after TBI. Understanding the sex difference in the TBI pathophysiology and the underlying mechanisms remains an elusive goal. In this review, we provide insights into various mechanisms related to the anatomical, physiological, hormonal, enzymatic, inflammatory, oxidative, genetic, or mitochondrial basis that support the protective nature of females compared to males. Furthermore, we sought to outline the evidence of multiple biomarkers that are highly potential in the investigation of TBI's prognosis, pathophysiology, and treatment and which can serve as objective measures and novel targets for individualized therapeutic interventions in TBI treatment. Implementations from this review are important for the understanding of the effect of sex on TBI outcomes and possible mechanisms behind the favorable response in females. It also emphasizes the critical need to include females as a biological variable and in sufficient numbers in future TBI studies.

Targeting Estrogen Signaling in the Radiation-induced Neurodegeneration: A Possible Role of Phytoestrogens

Radiation for medical use is a well-established therapeutic method with an excellent prognosis rate for various cancer treatments. Unfortunately, a high dose of radiation therapy comes with its own share of side effects, causing radiation-induced non-specific cellular toxicity; consequently, a large percentage of treated patients suffer from chronic effects during the treatment and even after the post-treatment. Accumulating data evidenced that radiation exposure to the brain can alter the diverse cognitive-related signaling and cause progressive neurodegeneration in patients because of elevated oxidative stress, neuroinflammation, and loss of neurogenesis. Epidemiological studies suggested the beneficial effect of hormonal therapy using estrogen in slowing down the progression of various neuropathologies. Despite its primary function as a sex hormone, estrogen is also renowned for its neuroprotective activity and could manage radiation-induced side effects as it regulates many hallmarks of neurodegenerations. Thus, treatment with estrogen and estrogen-like molecules or modulators, including phytoestrogens, might be a potential approach capable of neuroprotection in radiation-induced brain degeneration. This review summarized the molecular mechanisms of radiation effects and estrogen signaling in the manifestation of neurodegeneration and highlighted the current evidence on the phytoestrogen mediated protective effect against radiationinduced brain injury. This existing knowledge points towards a new area to expand to identify the possible alternative therapy that can be taken with radiation therapy as adjuvants to improve patients' quality of life with compromised cognitive function.

Cimicifuga racemosa Extract Ze 450 Re-Balances Energy Metabolism and Promotes Longevity

Recently, we reported that the Cimicifuga racemosa extract Ze 450 mediated protection from oxidative cell damage through a metabolic shift from oxidative phosphorylation to glycolysis. Here, we investigated the molecular mechanisms underlying the effects of Ze 450 against ferroptosis in neuronal cells, with a particular focus on mitochondria. The effects of Ze 450 on respiratory complex activity and hallmarks of ferroptosis were studied in isolated mitochondria and in cultured neuronal cells, respectively. In addition, Caenorhabditis elegans served as a model organism to study mitochondrial damage and longevity in vivo. We found that Ze 450 directly inhibited complex I activity in mitochondria and enhanced the metabolic shift towards glycolysis via cMyc and HIF1α regulation. The protective effects against ferroptosis were mediated independently of estrogen receptor activation and were distinct from effects exerted by metformin. In vivo, Ze 450 protected C. elegans from the mitochondrial toxin paraquat and promoted longevity in a dose-dependent manner. In conclusion, Ze 450 mediated a metabolic shift to glycolysis via direct effects on mitochondria and altered cell signaling, thereby promoting sustained cellular resilience to oxidative stress in vitro and in vivo.

Fenton Reaction-Induced Oxidative Damage to Membrane Lipids and Protective Effects of 17β-Estradiol in Porcine Ovary and Thyroid Homogenates

The Fenton reaction (Fe²⁺+H₂O₂→Fe³⁺+^•OH+OH^-) results in strong oxidative damage to macromolecules when iron (Fe) or hydrogen peroxide (H₂O₂) are in excess. This study aims at comparing Fe²⁺+H₂O₂-induced oxidative damage to membrane lipids (lipid peroxidation, LPO) and protective effects of 17β-estradiol (a potential antioxidant) in porcine ovary and thyroid homogenates. Iron, as one of the Fenton reaction substrates, was used in the highest achievable concentrations. Thyroid or ovary homogenates were incubated in the presence of: (1st) FeSO₄+H₂O₂ with/without 17β-estradiol (1 mM; 100, 10.0, 1.0 µM; 100, 10.0, 1.0 nM; 100, 10.0, 1.0 pM); five experiments were performed with different FeSO₄ concentrations (2400, 1200, 600, 300, 150 µM); (2nd) FeSO₄ (2400, 1200, 600, 300, 150 µM)+H₂O₂ with/without 17β-estradiol; three experiments were performed with three highest 17β-estradiol concentrations; (3rd) FeSO₄ (2400, 1200, 1100, 1000, 900, 800, 700, 600, 300, 150, 75 µM)+H₂O₂ (5 mM). LPO level [MDA+4-HDA/mg protein] was measured spectrophotometrically. The basal LPO level is lower in ovary than in thyroid homogenates. However, experimentally-induced LPO was higher in the former tissue, which was confirmed for the three highest Fe²⁺ concentrations (2400, 1200, 1100 µM). Exogenous 17β-estradiol (1 mM, 100, and 10 µM) reduced experimentally-induced LPO independently of iron concentration and that protective effect did not differ between tissues. The ovary, compared to the thyroid, reveals higher sensitivity to prooxidative effects of iron, however, it showed similar responsivity to protective 17β-estradiol activity. The therapeutic effect of 17β-estradiol against iron overload consequences should be considered with relation to both tissues.

4-Hydroxyestrone, an Endogenous Estrogen Metabolite, Can Strongly Protect Neuronal Cells Against Oxidative Damage

Earlier studies showed that endogenous estrogens have neuroprotective effect against oxidative damage. The present study seeks to investigate the protective effect of various endogenous estrogen metabolites against oxidative neurotoxicity in vitro and in vivo. Using immortalized mouse hippocampal neuronal cells as an in vitro model, 4-hydroxyestrone, an estrone metabolite with little estrogenic activity, is found to have the strongest neuroprotective effect against oxidative neurotoxicity among 25 endogenous estrogen metabolites tested, and its protective effect is stronger than 17β-estradiol. Similarly, 4-Hydroxyestrone also exerts a stronger protective effect than 17β-estradiol against kanic acid-induced hippocampal oxidative damage in rats. Neuroprotection by 4-hydroxyestrone involves increased cytoplasmic translocation of p53 resulting from SIRT1-mediated deacetylation of p53. Analysis of brain microsomal enzymes shows that estrogen 4-hydroxylation is the main metabolic pathway in the central nervous system. Together, these results show that 4-hydroxyestrone is an endogenous neuroestrogen that can strongly protect against oxidative neuronal damage.

Estrogen Formation and Inactivation Following TBI: What we Know and Where we Could go

Traumatic brain injury (TBI) is responsible for various neuronal and cognitive deficits as well as psychosocial dysfunction. Characterized by damage inducing neuroinflammation, this response can cause an acute secondary injury that leads to widespread neurodegeneration and loss of neurological function. Estrogens decrease injury induced neuroinflammation and increase cell survival and neuroprotection and thus are a potential target for use following TBI. While much is known about the role of estrogens as a neuroprotective agent following TBI, less is known regarding their formation and inactivation following damage to the brain. Specifically, very little is known surrounding the majority of enzymes responsible for the production of estrogens. These estrogen metabolizing enzymes (EME) include aromatase, steroid sulfatase (STS), estrogen sulfotransferase (EST/SULT1E1), and some forms of 17β-hydroxysteroid dehydrogenase (HSD17B) and are involved in both the initial conversion and interconversion of estrogens from precursors. This article will review and offer new prospective and ideas on the expression of EMEs following TBI.

Novel behavioural characteristics of the superoxide dismutase 1 G93A (SOD1G93A ) mouse model of amyotrophic lateral sclerosis include sex-dependent phenotypes

Amyotrophic lateral sclerosis (ALS) involves the rapid degeneration of upper and lower motor neurons leading to weakening and paralysis of voluntary movements. Mutations in copper-zinc superoxide dismutase 1 (SOD1) are a known genetic cause of ALS, and the SOD1 ^G93A mouse has been used extensively to investigate molecular mechanisms in ALS. In recent years, evidence suggests that ALS and frontotemporal dementia form a spectrum disorder ranging from motor to cognitive dysfunctions. Thus, we tested male and female SOD1 ^G93A mice for the first time before the onset of debilitating motor impairments in behavioural domains relevant to both ALS and frontotemporal dementia. SOD1 ^G93A males displayed reduced locomotion, exploration and increased anxiety-like behaviours compared with control males. Intermediate-term spatial memory was impaired in SOD1 ^G93A females, whereas long-term spatial memory deficits as well as lower acoustic startle response, and prepulse inhibition were identified in SOD1 ^G93A mice of both sexes compared with respective controls. Interestingly, SOD1 ^G93A males exhibited an increased conditioned cue freezing response. Nosing behaviours were also elevated in both male and female SOD1 ^G93A when assessed in social paradigms. In conclusion, SOD1 ^G93A mice exhibit a variety of sex-specific behavioural deficits beyond motor impairments supporting the notion of an ALS-frontotemporal spectrum disorder. Thus, SOD1 ^G93A mice may represent a useful model to test the efficacy of therapeutic interventions on clinical symptoms in addition to declining motor abilities.

The role of inflammation and potential use of sex steroids in intracranial aneurysms and subarachnoid hemorrhage

Background

Aneurysmal subarachnoid hemorrhage (aSAH) continues to be a devastating neurological condition with a high risk of associated morbidity and mortality. Inflammation has been shown to increase the risk of complications associated with aSAH such as vasospasm and brain injury in animal models and humans. The goal of this review is to discuss the inflammatory mechanisms of aneurysm formation, rupture and vasospasm and explore the role of sex hormones in the inflammatory response to aSAH.

Methods

A literature review was performed using PubMed using the following search terms: "intracranial aneurysm," "cerebral aneurysm," "dihydroepiandrosterone sulfate" "estrogen," "hormone replacement therapy," "inflammation," "oral contraceptive," "progesterone," "sex steroids," "sex hormones" "subarachnoid hemorrhage," "testosterone." Only studies published in English language were included in the review.

Results

Studies have shown that administration of sex hormones such as progesterone and estrogen at early stages in the inflammatory cascade can lower the risk and magnitude of subsequent complications. The exact mechanism by which these hormones act on the brain, as well as their role in the inflammatory cascade is not fully understood. Moreover, conflicting results have been published on the effect of hormone replacement therapy in humans. This review will scrutinize the variations in these studies to provide a more detailed understanding of sex hormones as potential therapeutic agents for intracranial aneurysms and aSAH.

Conclusion

Inflammation may play a role in the pathogenesis of intracranial aneurysm formation and subarachnoid hemorrhage, and administration of sex hormones as anti-inflammatory agents has been associated with improved functional outcome in experimental models. Further studies are needed to determine the therapeutic role of these hormones in the intracranial aneurysms and aSAH.

Effects of Estradiol and Progesterone-Induced Intracellular Calcium Fluxes on Toxoplasma gondii Gliding, Microneme Secretion, and Egress

Research has shown that estrogen is present and plays a critical role in vertebrate reproduction and metabolism, but the influence of steroids on Toxoplasma gondii has received less attention. Our data showed that estradiol and progesterone induced parasitic cytosolic Ca²⁺ fluxes. This process required estrogen to enter the cytoplasm of T. gondii, and cGMP-dependent protein kinase G (PKG) and phosphoinositide-phospholipase C (PI-PLC) emerged as important factors controlling parasitic intracellular (IC) Ca²⁺ signals. Cytosolic Ca²⁺, which is regulated by estradiol, was mostly mobilized from acidic organelles. Moreover, cytosolic Ca²⁺ slightly increased MIC2 protein secretion and promoted the gliding motility and egress of parasites, thus enhancing the pathogenicity of T. gondii, as shown in our previous research. We subsequently determined that the main source of Ca²⁺ regulated by progesterone was a neutral store. In contrast to the findings of estradiol, progesterone reduced MIC2 protein secretion and inhibited the gliding motility of parasites, which may decrease their pathogenicity. Additionally, unlike in mammals, estradiol and progesterone had no effect on nitric oxide (NO) or reactive oxygen species (ROS) production in T. gondii.

Modeling Environmentally-Induced Motor Neuron Degeneration in Zebrafish

Zebrafish have been used to investigate motor neuron degeneration, including as a model system to examine the pathogenesis of amyotrophic lateral sclerosis (ALS). The use of zebrafish for this purpose has some advantages over other in vivo model systems. In the current paper, we show that bisphenol A (BPA) exposure in zebrafish embryos results in motor neuron degeneration with affected motor function, reduced motor axon length and branching, reduced neuromuscular junction integrity, motor neuron cell death and the presence of activated microglia. In zebrafish, motor axon length is the conventional method for estimating motor neuron degeneration, yet this measurement has not been confirmed as a valid surrogate marker. We also show that reduced motor axon length as measured from the sagittal plane is correlated with increased motor neuron cell death. Our preliminary timeline studies suggest that axonopathy precedes motor cell death. This outcome may have implications for early phase treatments of motor neuron degeneration.

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.

Gender differences measured by the MATRICS consensus cognitive battery in chronic schizophrenia patients

Using Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), previous study showed significant gender differences for cognitive deficits in immediate and delayed memory in schizophrenia patients. However, RBANS does not include reasoning and problem solving, and social cognition. These cognitive functions can significantly affect the outcomes and daily life in patients. This study examined the gender differences of cognition using the measurement and treatment research to improve cognition in schizophrenia (MATRICS) consensus cognitive battery (MCCB), especially focusing on reasoning and problem solving, and social cognition in schizophrenia patients. The results showed that healthy controls exemplified better cognition than patients in both genders in all examined MCCB scores. Male healthy controls had better reasoning and problem solving and working memory than females, but these gender differences were not presented in schizophrenia patients. Also, male schizophrenia patients showed worse cognition than females on social cognition, processing speed, verbal learning and visual learning. Our results support that male schizophrenia patients had more cognitive impairment than females on reasoning and problem solving, social cognition, processing speed, working memory, verbal learning and visual learning.

Neuroprotective effects of estrogen in CNS injuries: insights from animal models

Among the estrogens that are biosynthesized in the human body, 17β-estradiol (estradiol or E2) is the most common and the best estrogen for neuroprotection in animal models of the central nervous system (CNS) injuries such as spinal cord injury (SCI), traumatic brain injury (TBI), and ischemic brain injury (IBI). These CNS injuries are not only serious health problems, but also enormous economic burden on the patients, their families, and the society at large. Studies from animal models of these CNS injuries provide insights into the multiple neuroprotective mechanisms of E2 and also suggest the possibility of translating the therapeutic efficacy of E2 in the treatment SCI, TBI, and IBI in humans in the near future. The pathophysiology of these injuries includes loss of motor function in the limbs, arms and their extremities, cognitive deficit, and many other serious consequences including life-threatening paralysis, infection, and even death. The potential application of E2 therapy to treat the CNS injuries may become a trend as the results are showing significant therapeutic benefits of E2 for neuroprotection when administered into the animal models of SCI, TBI, and IBI. This article describes the plausible mechanisms how E2 works with or without the involvement of estrogen receptors and provides an overview of the known neuroprotective effects of E2 in these three CNS injuries in different animal models. Because activation of estrogen receptors has profound implications in maintaining and also affecting normal physiology, there are notable impediments in translating E2 therapy to the clinics for neuroprotection in CNS injuries in humans. While E2 may not yet be the sole molecule for the treatment of CNS injuries due to the controversies surrounding it, the neuroprotective effects of its metabolite and derivative or combination of E2 with another therapeutic agent are showing significant impacts in animal models that can potentially shape the new treatment strategies for these CNS injuries in humans.

A novel mechanism of non-feminizing estrogens in neuroprotection

Estrogens are potent and efficacious neuroprotectants both in vitro and in vivo in a variety of models of neurotoxicity. We determined the structural requirements for neuroprotection in an in vitro assay using a panel of >70 novel estratrienes, synthesized to reduce or eliminate estrogen receptor (ER) binding. We observed that neuroprotection could be enhanced by as much as 200-fold through modifications that positioned a large bulky group at the C2 or C4 position of the phenolic A ring of the estratriene. Further, substitutions on the B, C or D rings either reduced or did not markedly change neuroprotection. Collectively, there was a negative correlation between binding to ERs and neuroprotection with the more potent compounds showing no ER binding. In an in vivo model for neuroprotection, transient cerebral ischemia, efficacious compounds were active in protection of brain tissue from this pro-oxidant insult. We demonstrated that these non-feminizing estrogens engage in a redox cycle with glutathione, using the hexose monophosphate shunt to apply cytosolic reducing potential to cellular membranes. Together, these results demonstrate that non-feminizing estrogens are neuroprotective and protect brain from the induction of ischemic- and Alzheimer's disease (AD)-like neuropathology in an animal model. These features of non-feminizing estrogens make them attractive compounds for assessment of efficacy in AD and stroke, as they are not expected to show the side effects of chronic estrogen therapy that are mediated by ER actions in the liver, uterus and breast.

Structure-activity studies of non-steroid analogues structurally-related to neuroprotective estrogens

Estrone and 17β-estradiol are phenolic steroids that are known to be neuroprotective in multiple models of neuronal injury. Previous studies have identified the importance of their phenolic steroid A-ring for neuroprotection and have identified ortho substituents at the C-2 and C-4 positions on the phenol ring that enhance this activity. To investigate the importance of the steroid ring system for neuroprotective activity, phenolic compounds having the cyclopent[b]anthracene, cyclopenta[b]phenanthrene, benz[f]indene, benz[e]indene, indenes linked to a phenol, and a phenolic spiro ring system were prepared. New synthetic methods were developed to make some of the cyclopent[b]anthracene analogues as well as the spiro ring system. Compounds were evaluated for their ability to protect HT-22 hippocampal neurons from glutamate neurotoxicity and their activity relative to a potent neuroprotective analogue of 17β-estradiol was determined. An adamantyl substituent placed ortho to the phenolic hydroxyl group gave neuroprotective analogues in all ring systems studied.

Protection against brain tissues oxidative damage as a possible mechanism for improving effects of low doses of estradiol on scopolamine-induced learning and memory impairments in ovariectomized rats

Background:

Regarding the anti-oxidative effects on the central nervous system, the possible protection against brain tissues oxidative damage as a possible mechanism for improving effects of low doses of estradiol on scopolamine-induced learning and memory impairments was investigated in ovariectomized (OVX) rats.

Materials and Methods:

The OVX rats treated by (1) vehicle, (2) scopolamine, and (3–4) scopolamine plus estradiol (20 or 20 or 60 μg/kg). Estradiol was administered (20 or 60 μg/kg, intraperitoneally) daily for 6 weeks after ovariectomy. The rats were examined for learning and memory using passive avoidance test. Scopolamine (2 mg/kg) was injected 30 min after training in the test. The brains were then removed to determine malondialdehyde (MDA) and thiol contents.

Results:

Scopolamine shortened the time latency to enter the dark compartment in (P < 0.01). Compared to scopolamine, pretreatment by both doses of estradiol prolonged the latency to enter the dark compartment (P < 0.01). The brain tissues MDA concentration as an index of lipid peroxidation was decreased (P < 0.05). Pretreatment by estradiol lowered the concentration of MDA, while it increased thiol content compared to scopolamine (P < 0.05 and P < 0.01).

Conclusions:

These results allow us to suggest a protection against brain tissues oxidative damage as a possible mechanism for improving effects of low doses of estradiol on scopolamine-induced learning and memory impairments in OVX rats.

Effects of Gender and Estrogen Receptors on Iron-Induced Brain Edema Formation

Our previous studies have shown that female mice have less brain edema and better recovery in neurological deficits after intracerebral hemorrhage (ICH) and that 17β-estradiol treatment in male mice markedly reduces ICH-induced brain edema. In this study, we investigated the role of gender and the estrogen receptors (ERs) in iron-induced brain edema. There were three parts in this study: (1) either male or female mice received an injection of 10 μL FeCl2 (1 mM) into the right caudate; (2) females received an intracaudate injection of FeCl2 or saline with 1 μg of ICI 182,780 (antagonists of ERs) or vehicle; and (3) males were treated with the ER regulator tamoxifen (5 mg/kg subcutaneously) or vehicle 1 h after FeCl2 injection. Mice were euthanized 24 h later for brain edema determination. FeCl2 induced lower brain edema in females than in males. Co-injection of ICI 182,780 with FeCl2 aggravated iron-induced brain edema in female mice. ICI 182,780 itself did not induce brain edema at the dose of 1 μg. Tamoxifen treatment reduced FeCl2-induced brain edema in male mice. In conclusion, iron induced less brain edema in female mice than in males. ER modification can affect iron-induced brain edema.

Estrogen Receptor (ER)-α36 Is Involved in Estrogen- and Tamoxifen-Induced Neuroprotective Effects in Ischemic Stroke Models

The neuroprotection by estrogen (E2) and tamoxifen is well documented in experimental stroke models; however, the exact mechanism is unclear. A membrane-based estrogen receptor, ER-α36, has been identified. Postmenopausal-levels of E2 act through ER-α36 to induce osteoclast apoptosis due to a prolonged activation of the mitogen-activated protein kinase (MAPK)/extracellular signal-related kinase (ERK) signaling. We hypothesized that ER-α36 may play a role in the neuroprotective activities of estrogen and tamoxifen. Here, we studied ER-α36 expression in the brain, as well as its neuroprotective effects against oxygen and glucose deprivation (OGD) in PC12 cells. We found that ER-α36 was expressed in both rat and human brain. In addition, OGD-induced cell death was prevented by l nmol/L 17β-estradiol (E2β). E2β activates the MAPK/ERK signaling pathway in PC12 cells under basal and OGD conditions by interacting with ER-α36 and also induces ER-α36 expression. Low-dose of tamoxifen up-regulated ER-α36 expression and enhanced neuronal survival in an ovariectomized ischemic stroke model. Furthermore, low-dose of tamoxifen enhanced neuroprotective effects by modulating activates or suppress ER-α36. Our results thus demonstrated that ER-α36 is involved in neuroprotective activities mediated by both estrogen and tamoxifen.

Estradiol attenuates down-regulation of PEA-15 and its two phosphorylated forms in ischemic brain injury

Estradiol exerts a neuroprotective effect against focal cerebral ischemic injury through the inhibition of apoptotic signals. Phosphoprotein enriched in astrocytes 15 (PEA-15) is mainly expressed in brain that perform anti-apoptotic functions. This study investigated whether estradiol modulates the expression of PEA-15 and two phosphorylated forms of PEA-15 (Ser 104 and Ser 116) in middle cerebral artery occlusion (MCAO)-induced injury and glutamate exposure-induced neuronal cell death. Adult female rats were ovariectomized to remove endogenous estradiol and treated with vehicle or estradiol prior to MCAO. Focal cerebral ischemia was induced by MCAO and cerebral cortices were collected 24 h after MCAO. Western blot analysis indicated that estradiol prevents the MCAO-induced decrease in PEA-15, phospho-PEA-15 (Ser 104), phospho-PEA-15 (Ser 116). Glutamate exposure induced a reduction in PEA-15, phospho-PEA-15 (Ser 104), phospho-PEA-15 (Ser 116) in cultured neurons, whereas estradiol treatment attenuated the glutamate toxicity-induced decrease in the expression of these proteins. It has been known that phosphorylation of PEA-15 is an important step in carrying out its anti-apoptotic function. Thus, these findings suggest that the regulation of PEA-15 phosphorylation by estradiol contributes to the neuroprotective function of estradiol in ischemic brain injury.

The neuroprotective effect of 17β-estradiol is independent of its antioxidative properties

INTRODUCTION

17β-Estradiol (E2) is neuroprotective in experimental models of stroke. While some postulate a mainly antioxidative action due to E2׳s free C3 hydroxyl group at its A-ring, others suggest that neuroprotection is mediated by a hormonal, receptor mediated effect. The aim of the current study was to clarify this issue by testing whether E2 analogues lacking hormonal activity are also neuroprotective following cerebral ischemia.

MATERIAL & METHODS

Focal cerebral ischemia was induced in male C57/BL6 mice by laser-Doppler-controlled endovascular occlusion of the middle cerebral artery for 40min. Mice received either 1) memantine, a NMDA-receptor antagonist, as a positive control, 2) E2 (1400µg/kg b.w.), or 3) 2,4,6-trimethylphenol (TMP), an E2 analogue without hormonal activity (1400, 140, or 14µg/kg b.w.). Motor function was tested 3h and 24h after ischemia. Thereafter mice were sacrificed and brain damage was quantified by histomorphometry.

RESULTS

Treatment with memantine or E2 significantly reduced infarct volume by >40% and significantly improved neurological function while treatment with TMP had no effect.

CONCLUSION

E2 is equally neuroprotective as antagonization of NMDA receptors while E2 analogues without hormonal activity are not neuroprotective. Therefore the current data suggest that the neuroprotection activity of E2 is independent of its free-radical scavenging properties.

Sex differences in cognitive impairment and Alzheimer's disease

Studies have shown differences in specific cognitive ability domains and risk of Alzheimer's disease between the men and women at later age. However it is important to know that sex differences in cognitive function during adulthood may have their basis in both organizational effects, i.e., occurring as early as during the neuronal development period, as well as in activational effects, where the influence of the sex steroids influence brain function in adulthood. Further, the rate of cognitive decline with aging is also different between the sexes. Understanding the biology of sex differences in cognitive function will not only provide insight into Alzheimer's disease prevention, but also is integral to the development of personalized, gender-specific medicine. This review draws on epidemiological, translational, clinical, and basic science studies to assess the impact of sex differences in cognitive function from young to old, and examines the effects of sex hormone treatments on Alzheimer's disease in men and women.

Therapeutic implications of estrogen for cerebral vasospasm and delayed cerebral ischemia induced by aneurysmal subarachnoid hemorrhage

Cerebral vasospasm (CV) remains the leading cause of delayed morbidity and mortality following aneurysmal subarachnoid hemorrhage (SAH). However, increasing evidence supports etiologies of delayed cerebral ischemia (DCI) other than CV. Estrogen, specifically 17 β -estradiol (E2), has potential therapeutic implications for ameliorating the delayed neurological deterioration which follows aneurysmal SAH. We review the causes of CV and DCI and examine the evidence for E2-mediated vasodilation and neuroprotection. E2 potentiates vasodilation by activating endothelial nitric oxide synthase (eNOS), preventing increased inducible NOS (iNOS) activity caused by SAH, and decreasing endothelin-1 production. E2 provides neuroprotection by increasing thioredoxin expression, decreasing c-Jun N-terminal kinase activity, increasing neuroglobin levels, preventing SAH-induced suppression of the Akt signaling pathway, and upregulating the expression of adenosine A2a receptor. The net effect of E2 modulation of these various effectors is the promotion of neuronal survival, inhibition of apoptosis, and decreased oxidative damage and inflammation. E2 is a potentially potent therapeutic tool for improving outcomes related to post-SAH CV and DCI. However, clinical evidence supporting its benefits remains lacking. Given the promising preclinical data available, further studies utilizing E2 for the treatment of patients with ruptured intracranial aneurysms appear warranted.

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.

Stroke neuroprotection: oestrogen and insulin-like growth factor-1 interactions and the role of microglia

Oestrogen has been shown to be neuroprotective for stroke and other neural injury models. Oestrogen promotes a neuroprotective phenotype through myriad actions, including stimulating neurogenesis, promoting neuronal differentiation and survival, suppressing neuroinflammation and maintaining the integrity of the blood-brain barrier. At the molecular level, oestrogen directly modulates genes that are beneficial for repair and regeneration via the canonical oestrogen receptor. Increasingly, evidence indicates that oestrogen acts in concert with growth factors to initiate neuroprotection. Oestrogen and insulin-like growth factor (IGF)-1 act cooperatively to influence cell survival, and combined steroid hormone/growth factor interaction has been well documented in the context of neurones and astrocytes. Here, we summarise the evidence that oestrogen-mediated neuroprotection is critically dependent on IGF-1 signalling, and specifically focus on microglia as the source of IGF-1 and the locus of oestrogen-IGF-1 interactions in stroke neuroprotection.

Interactive effects of hypoxia with estradiol-17β on tryptophan hydroxylase activity and serotonin levels in the Atlantic croaker hypothalamus

Hypoxia causes a marked decline in reproductive neuroendocrine function in Atlantic croaker due to decreases in the hypothalamic expression and activities of tryptophan hydroxylase (TPH, the rate limiting enzyme in serotonin synthesis) and aromatase. In the present study, the influence of the estrogen status on hypothalamic TPH and serotonin (5-HT) regulation by hypoxia (dissolved oxygen: 1.7 mg/L for 4 weeks) was investigated in croaker. Treatment in vivo with the aromatase inhibitor, ATD (1,4,6-androstatrien-3,17-dione), significantly decreased TPH activity, TPHs (TPH-1 and TPH-2) mRNAs expression, and 5-hydroxytryptophan (5-HTP, an immediate precursor of 5-HT) and 5-HT contents in croaker hypothalamus. Treatment with estradiol-17β partially restored hypothalamic TPH activity, TPHs mRNA expression, and 5-HTP and 5-HT contents in hypoxia-exposed fish. These results suggest that the hypoxia-induced inhibition of TPH and 5-HT synthesis is dependent on the estrogen status. To our knowledge, this is the first report of a role for estrogens in modulating neural TPH and 5-HT responses to hypoxia in aquatic vertebrates.

17Beta-estradiol suppresses AMPA-induced increases in regional cerebral O2consumption

We tested the hypothesis that 17 beta-estradiol would reduce the cerebral O2 consumption response resulting from glutamate receptor stimulation by alpha amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA). Fourteen ovariectomized rats were separated into 17 beta-estradiol (0.5 mg 21 day release pellet) and control (placebo pellet) groups to determine cerebral blood flow (14C-iodoantipyrine) and O2 consumption (microspectrophotometry). After topical cortical stimulation with 10(-3) M and 10(-4) M AMPA, cerebral blood flow increased significantly in both groups in a concentration-dependent manner. Cerebral O2 extraction was not significantly different in any region of the 17 beta-estradiol treated group. In the placebo treated group, the O2 extraction in the saline treated cortex and in the 10(-3) M AMPA treated cortex was significantly higher when compared to the 10(-4) M AMPA treated cortex. Cerebral O2 consumption in the control group increased by 20%, from 5.2 +/- 0.6 to 6.1 +/- 0.7, with 10(-4) M AMPA and significantly increased by 64% to 8.5 +/- 0.8 ml O2 min-1 100 g-1 with 10(-3) M AMPA. The 17 beta-estradiol group demonstrated no statistically significant difference in O2 consumption between the saline treated and AMPA treated cortex. Thus, 17 beta-estradiol reduced the effects of AMPA in increasing cerebral O2 consumption.

Sexual dimorphism in cerebral ischemia injury

Stroke is a leading cause of permanent disability and death. A complex series of biochemical and molecular mechanisms (e.g. the release of ROS/NOS, proapoptotic proteins and proinflammatory cytokine; neuronal depolarization, Ca2+ accumulation and so on) impair the neurologic functions of cerebral ischemia and stroke. We have known for some time that the epidemiology of human stroke is sexually dimorphic until late in life, well beyond the years of reproductive senescence and menopause. The principal mammalian estrogen (17β estradiol or E2) is neuroprotective in many types of brain injury and has been the major focus of investigation over the past several decades. However the incidence of stroke in women is lower than in men until decades past menopause, suggesting that factors beyond sex hormone contribute to these epidemiological sex differences. So a new concept is emerging: both sex steroids and biologic sex are important factors in clinical and experimental strokes. In this review, we will address sex steroids and gender differences in influencing the mechanisms and outcomes of brain ischemia stroke. These sex differences need to be identified which could help future translation to human neuroprotection.

Brain tissue oxidative damage as a possible mechanism for the deleterious effect of a chronic high dose of estradiol on learning and memory in ovariectomized rats

In addition to antioxidative effects, estrogens also exert pro-oxidative actions. The effect of chronic administration of a high dose of estradiol valerate on Morris water maze tasks and brain tissues oxidative damage was investigated. The Sham-Est and OVX-Est groups were treated with estradiol valerate (4 mg/kg) for 12 weeks. Escape latency and traveled path in the Sham-Est and OVX-Est groups were significantly higher than in the Sham and OVX groups (p<0.01 and p<0.001). In the probe trial, the animals of the Sham-Est and OVX-Est groups spent lower time in Q1 compared to Sham and OVX groups (p<0.05 and p<0.001). In Sham-Est and OVX-Est groups, the brain tissue total thiol concentration was significantly lower, and malondialdehyde (MDA) concentrations were higher than in the Sham and OVX groups (p<0.05 and p<0.001). It is concluded that administration of high exogenous levels of estradiol impairs performance and enhances oxidative stress.

17β-Estradiol attenuates secondary injury through activation of Akt signaling via estrogen receptor alpha in rat brain following subarachnoid hemorrhage

BACKGROUND

Apoptosis is implicated in vasospasm and the long-term sequelae of subarachnoid hemorrhage (SAH). This study tested the hypothesis that attenuation of SAH-induced apoptosis after 17β-estradiol (E2) treatment is associated with an increase in phosphorylation of Akt via estrogen receptor-α (ER-α) in rats.

MATERIALS AND METHODS

We examined the expression of phospho-Akt, ERα and ERβ, and apoptosis in cerebral cortex, hippocampus, and dentate gyrus in a two-hemorrhage SAH model in rats. We subcutaneously implanted other rats with a silicone rubber tube containing E2; they received daily injections of nonselective estrogen receptor antagonist (ICI 182,780), selective ERα-selective antagonist (methyl-piperidino-pyrazole), or ERβ-selective antagonist (R,R-tetrahydrochrysene) after the first hemorrhage.

RESULTS

At 7 d after the first SAH, protein levels of phospho-Akt and ERα were significantly decreased and caspase-3 was significantly increased in the dentate gyrus. The cell death assay revealed that DNA fragmentation was significantly increased in the dentate gyrus. Those actions were reversed by E2 and blocked by ICI 182,780 and methyl-piperidino-pyrazole, but not R,R-tetrahydrochrysene. However, there were no significant changes in the expression of the protein levels of phospho-Akt, ERα, ERβ, and caspase-3, and DNA fragmentation after SAH.

CONCLUSIONS

The present study shows that a beneficial effect of E2 in attenuating SAH-induced apoptosis is associated with activation of the expression of phospho-Akt and ERα, and alteration in caspase-3 protein expression via an ERα-dependent mechanism in the dentate gyrus. These data support further the investigation of E2 in the treatment of SAH in humans.

Window of opportunity: estrogen as a treatment for ischemic stroke

The neuroprotection research in the last 2 decades has witnessed a growing interest in the functions of estrogens as neuroprotectants against neurodegenerative diseases including stroke. The neuroprotective action of estrogens has been well demonstrated in both in vitro and in vivo models of ischemic stroke. However, the major conducted clinical trials so far have raised concern for the protective effect of estrogen replacement therapy in postmenopausal women. The discrepancy could be partly due to the mistranslation between the experimental stroke research and clinical trials. While predominant experimental studies tested the protective action of estrogens on ischemic stroke using acute treatment paradigm, the clinical trials have mainly focused on the effect of estrogen replacement therapy on the primary and secondary stroke prevention which has not been adequately addressed in the experimental stroke study. Although the major conducted clinical trials have indicated that estrogen replacement therapy has an adverse effect and raise concern for long term estrogen replacement therapy for stroke prevention, these are not appropriate for assessing the potential effects of acute estrogen treatment on stroke protection. The well established action of estrogen in the neurovascular unit and its potential interaction with recombinant tissue Plasminogen Activator (rtPA) makes it a candidate for the combined therapy with rtPA for the acute treatment of ischemic stroke. On the other hand, the "critical period" and newly emerged "biomarkers window" hypotheses have indicated that many clinical relevant factors have been underestimated in the experimental ischemic stroke research. The development and application of ischemic stroke models that replicate the clinical condition is essential for further evaluation of acute estrogen treatment on ischemic stroke which might provide critical information for future clinical trials. This article is part of a Special Issue entitled Hormone Therapy.

The cerebellum as a target for estrogen action

This review focuses on the effects of estrogens upon the cerebellum, a brain region long ignored as a site of estrogen action. Highlighted are the diverse effects of estradiol within the cerebellum, emphasizing the importance of estradiol signaling in cerebellar development, modulation of synaptic neurotransmission in the adult, and the potential influence of estrogens on various health and disease states. We also provide new data, consistent with previous studies, in which locally synthesized estradiol modulates cerebellar glutamatergic neurotransmission, providing one underlying mechanism by which the actions of estradiol can affect this brain region.

Neuroprotection and estrogen receptors

This review is intended to assess the state of current knowledge on the role of estrogen receptors (ERs) in the neuroprotective effects of estrogens in models for acute neuronal injury and death. We evaluate the overall evidence that estrogens are neuroprotective in acute injury and critically assess the role of ERα, ERβ, GPR 30, and nonreceptor-mediated mechanisms in these robust neuroprotective effects of this ovarian steroid hormone. We conclude that all three receptors, as well as nonreceptor-mediated mechanisms can be involved in neuroprotection, depending on the model used, the level of estrogen administrated, and the mode of administration of the steroid. Also, the signaling pathways used by both ER-dependent and ER-independent mechanisms to exert neuroprotection are considered. Finally, further studies that are needed to parse out the relative contribution of receptor versus nonreceptor-mediated signaling are discussed.

Estrogen and neuroprotection: from clinical observations to molecular mechanisms

We now appreciate that estrogen is a pleiotropic gonadal steroid that exerts profound effects on the plasticity and cell survival of the adult brain. Over the past century, the life span of women has increased, but the age of the menopause remains constant. This means that women may now live over one third of their lives in a hypoestrogenic, postmenopausal state. The impact of prolonged hypoestrogenicity on the brain is now a critical health concern as we realize that these women may suffer an increased risk of cognitive dysfunction and neurodegeneration due to a variety of diseases. Accumulating evidence from both clinical and basic science studies indicates that estrogen exerts critical protective actions against neurodegenerative conditions such as Alzheimer's disease and stroke. Here, we review the discoveries that comprise our current understanding of estrogen action against neurodegeneration. These findings carry far-reaching possibilities for improving the quality of life in our aging population.

17β-Estradiol inhibits apoptotic cell death of oligodendrocytes by inhibiting RhoA-JNK3 activation after spinal cord injury

A delayed oligodendrocyte cell death after spinal cord injury (SCI) contributes to chronic demyelination of spared axons, leading to a permanent neurological deficit. Therefore, therapeutic approaches to prevent oligodendrocyte cell death after SCI should be considered. Estrogens are well known to have a broad neuroprotective effect, but the protective effect of estrogens on oligodendrocytes after injury is largely unknown. Here, we demonstrated that 17β-estradiol attenuates apoptosis of oligodendrocytes by inhibiting RhoA and c-Jun-N-terminal kinase activation after SCI. Estrogen receptor (ER)-α and -β were expressed in oligodendrocytes of the spinal cord, and 17β-estradiol treatment significantly inhibited oligodendrocyte cell death at 7 d after injury as compared with vehicle (cyclodextrin) control. 17β-Estradiol also attenuated caspase-3 and -9 activation at 7 d and reduced the loss of axons from progressive degeneration. In addition, 17β-estradiol inhibited RhoA and JNK3 activation, which were activated and peaked at 3 and/or 5 d after injury. Furthermore, administration of Rho inhibitor, PEP-1-C3 exoenzyme, inhibited RhoA and JNK3 activation, and decreased phosphorylated c-Jun level at 5 d after injury. Additionally, the attenuation of RhoA and JNK3 activation as well as oligodendrocyte cell death by 17β-estradiol was reversed by ER antagonist, ICI182780. Our results thus indicate that 17β-estradiol treatment improves functional recovery after SCI in part by reducing oligodendrocyte cell death via inhibition of RhoA and JNK3 activation, which were ER dependent. Furthermore, improvement of hindlimb motor function by posttreatment of 17β-estradiol suggests its potential as a therapeutic agent for SCI patients.

Worsened outcome from middle cerebral artery occlusion in aged rats receiving 17β-estradiol

Although estrogens are neuroprotective in young adult animal models of stroke, clinical trials demonstrate that estrogens increase the incidence and severity of stroke in aged women. We have previously shown that experimental stroke pathophysiology differs between young adult and aged rats. The aim of this study was to determine the effects of 17β-estradiol after middle cerebral artery occlusion and reperfusion in young adult and aged female rats. Focal embolic stroke was performed by middle cerebral artery occlusion with fibrin clot followed by reperfusion with i.v. human recombinant tissue plasminogen activator. Histological and functional outcomes were measured at 24 h after middle cerebral artery occlusion with fibrin clot. Aged rats treated with 17β-estradiol had significantly increased infarct volumes compared with placebo-treated aged rats. Young adult rats treated with 17β-estradiol had significantly decreased infarct volumes and improved functional outcome compared with ovariectomized young adult rats. Our results suggest that 17β-estradiol may act in an age-dependent manner in the postischemic rat brain. In young adult rats, it is neuroprotective; chronic treatment with 17β-estradiol during aging leads to worsened ischemic brain injury in aged female rats.

17α-estradiol attenuates neuron loss in ovariectomized Dtg AβPP/PS1 mice

Quantitative microanalysis of brains from patients with Alzheimer's disease (AD) find neuronal loss and neuroinflammation in structures that control cognitive function. Though historically difficult to recapitulate in experimental models, several groups have recently reported that by middle-age, transgenic mice that co-express high levels of two AD-associated mutations, amyloid-β protein precursor (AβPP(swe)) and presenilin 1 (PS1(ΔE9)), undergo significant AD-type neuron loss in sub-cortical nuclei with heavy catecholaminergic projections to the hippocampal formation. Here we report that by 13 months of age these dtg AβPP(swe)/PS1(ΔE9) mice also show significant loss of pyramidal neuron in a critical region for learning and memory, the CA1 subregion of hippocampus, as a direct function of amyloid-β (Aβ) aggregation. We used these mice to test whether 17α-estradiol (17αE2), a less feminizing and non-carcinogenic enantiomer of 17β-estradiol, protects against this CA1 neuron loss. Female dtg AβPP(swe)/PS1(ΔE9) mice were ovariectomized at 8-9 months of age and treated for 60 days with either 17αE2 or placebo via subcutaneous pellets. Computerized stereology revealed that 17αE2 ameliorated the loss of neurons in CA1 and reduced microglial activation in the hippocampus. These findings support the view that 17αE2, which may act through non-genomic mechanisms independent of traditional estrogen receptors, could prevent or delay the progression of AD in older men and women.

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.

Potential age-dependent effects of estrogen on neural injury

In 2000, approximately 10 million women were receiving hormone replacement therapy (HRT) for alleviation of menopausal symptoms. A number of prior animal studies suggested that HRT may be neuroprotective and cardioprotective. Then, in 2003, reports from the Women's Health Initiative (WHI) indicated that long-term estrogen/progestin supplementation led to increased incidence of stroke. A second branch of the WHI in women with prior hysterectomy found an even stronger correlation between estrogen supplementation alone and stroke incidence. Follow-up analyses of the data, as well as data from other smaller clinical trials, have also demonstrated increased stroke severity in women receiving HRT or estrogen alone. This review examines the studies indicating that estrogen is neuroprotectant in animal models and explores potential reasons why this may not be true in postmenopausal women. Specifically, age-related differences in estrogen receptors and estrogenic actions in the brain are discussed, with the conclusion that animal models of disease must closely mimic human disease to produce clinically relevant results.

The impact of clinical and demographic variables on cognitive performance in methamphetamine-dependent individuals in rural South Carolina

Inconsistencies in reports on methamphetamine (METH) associated cognitive dysfunction may be attributed, at least in part, to the diversity of study sample features (eg, clinical and demographic characteristics). The current study assessed cognitive function in a METH-dependent population from rural South Carolina, and the impact of demographic and clinical characteristics on performance. Seventy-one male (28.2%) and female (71.8%) METH-dependent subjects were administered a battery of neurocognitive tests including the Test of Memory Malingering (TOMM), Shipley Institute of Living Scale, Paced Auditory Serial Addition Test (PASAT), Symbol Digit Modalities Test (SDMT), Grooved Pegboard Test, California Verbal Learning Test (CVLT), and Wisconsin Card Sorting Test (WCST). Demographic and clinical characteristics (eg, gender, frequency of METH use) were examined as predictors of performance. Subjects scored significantly lower than expected on one test of attention and one of fine motor function, but performed adequately on all other tests. There were no predictors of performance on attention; however, more frequent METH use was associated with better performance for males and worse for females on fine motor skills. The METH-dependent individuals in this population exhibit very limited cognitive impairment. The marked differences in education, Intellectual Quotient (IQ), and gender in our sample when compared to the published literature may contribute to these findings. Characterization of the impact of clinical and/or demographic features on cognitive deficits could be important in guiding the development of treatment interventions.

An early treatment with 17-β-estradiol is neuroprotective against the long-term effects of neonatal ionizing radiation exposure

Ionizing radiations can induce oxidative stress on target tissues, acting mainly through reactive oxygen species (ROS). The aim of this work was to investigate if 17-β-estradiol (βE) was able to prevent hippocampal-related behavioral and biochemical changes induced by neonatal ionizing radiation exposure and to elucidate a potential neuroprotective mechanism. Male Wistar rats were irradiated with 5 Gy of X-rays between 24 and 48 h after birth. A subset of rats was subcutaneously administered with successive injections of βE or 17-α-estradiol (αE), prior and after irradiation. Rats were subjected to different behavioral tasks to evaluate habituation and associative memory as well as anxiety levels. Hippocampal ROS levels and protein kinase C (PKC) activity were also assessed. Results show that although βE was unable to prevent radiation-induced hippocampal PKC activity changes, most behavioral abnormalities were reversed. Moreover, hippocampal ROS levels in βE-treated irradiated rats approached control values. In addition, αE administered to irradiated animals was effective in preventing radiation-induced alterations. In conclusion, βE was able to counteract behavioral and biochemical changes induced in irradiated animals, probably acting through an antioxidant mechanism.

Mechanisms of estrogens' dose-dependent neuroprotective and neurodamaging effects in experimental models of cerebral ischemia

Ever since the hypothesis was put forward that estrogens could protect against cerebral ischemia, numerous studies have investigated the mechanisms of their effects. Despite initial studies showing ameliorating effects, later trials in both humans and animals have yielded contrasting results regarding the fundamental issue of whether estrogens are neuroprotective or neurodamaging. Therefore, investigations of the possible mechanisms of estrogen actions in brain ischemia have been difficult to assess. A recently published systematic review from our laboratory indicates that the dichotomy in experimental rat studies may be caused by the use of insufficiently validated estrogen administration methods resulting in serum hormone concentrations far from those intended, and that physiological estrogen concentrations are neuroprotective while supraphysiological concentrations augment the damage from cerebral ischemia. This evidence offers a new perspective on the mechanisms of estrogens’ actions in cerebral ischemia, and also has a direct bearing on the hormone replacement therapy debate. Estrogens affect their target organs by several different pathways and receptors, and the mechanisms proposed for their effects on stroke probably prevail in different concentration ranges. In the current article, previously suggested neuroprotective and neurodamaging mechanisms are reviewed in a hormone concentration perspective in an effort to provide a mechanistic framework for the dose-dependent paradoxical effects of estrogens in stroke. It is concluded that five protective mechanisms, namely decreased apoptosis, growth factor regulation, vascular modulation, indirect antioxidant properties and decreased inflammation, and the proposed damaging mechanism of increased inflammation, are currently supported by experiments performed in optimal biological settings.