Estrogen attenuates the MPTP-induced loss of dopamine neurons from the mouse SNc despite a lack of estrogen receptors (ERalpha and ERbeta)

Neuroactive steroids and Parkinson's disease: Review of human and animal studies

The greater prevalence and incidence of Parkinson's disease (PD) in men suggest a beneficial effect of sex hormones. Neuroactive steroids have neuroprotective activities thus offering interesting option for disease-modifying therapy for PD. Neuroactive steroids are also neuromodulators of neurotransmitter systems and may thus help to control PD symptoms and side effect of dopamine medication. Here, we review the effect on sex hormones (estrogen, androgen, progesterone and its metabolites) as well as androstenediol, pregnenolone and dehydroepiandrosterone) in human studies and in animal models of PD. The effect of neuroactive steroids is reviewed by considering sex and hormonal status to help identify specifically for women and men with PD what might be a preventive approach or a symptomatic treatment. PD is a complex disease and the pathogenesis likely involves multiple cellular processes. Thus it might be useful to target different cellular mechanisms that contribute to neuronal loss and neuroactive steroids provide therapeutics options as they have multiple mechanisms of action.

MPTP induces neurodegeneration by modulating dopaminergic activity in catfish brain

Tyrosine hydroxylase (Th) is an allosteric rate-limiting enzyme in catecholamine (CA) biosynthesis. The CAs, dopamine (DA), norepinephrine (NE), and epinephrine are important neurotransmitters wherein DA contributes a key role in the central nervous system of vertebrates. The present study evaluated DA and Th's significance in DA-ergic activity and neurodegeneration upon 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) exposure in catfish. Further, the expression of certain brain-and ovary-related genes measured through qPCR were downregulated upon MPTP treatment which is in accordance with the decreased levels of L-Dopa, DA, and NE levels estimated through HPLC-ECD. Additionally, TEM analysis depicted structural disarray of brain upon MPTP exposure and also decreased serum levels of testosterone, 11-ketotestosterone, and estradiol-17β. MPTP treatment, in vitro, using primary brain cell culture resulted in diminished cell viability and increased ROS levels leading to elevated apoptotic cells significantly. Consequently, the study highlights the MPTP-induced neurodegeneration of the Th and DA-ergic activity in corroboration with female brain-related genes downregulation, also gonadal function as evidenced by depleted sex steroids level and low expression of ovary-related genes.

The Role of Sex and Sex Hormones in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a wide class of disorders of the central nervous system (CNS) with unknown etiology. Several factors were hypothesized to be involved in the pathogenesis of these diseases, including genetic and environmental factors. Many of these diseases show a sex prevalence and sex steroids were shown to have a role in the progression of specific forms of neurodegeneration. Estrogens were reported to be neuroprotective through their action on cognate nuclear and membrane receptors, while adverse effects of male hormones have been described on neuronal cells, although some data also suggest neuroprotective activities. The response of the CNS to sex steroids is a complex and integrated process that depends on (i) the type and amount of the cognate steroid receptor and (ii) the target cell type-either neurons, glia, or microglia. Moreover, the levels of sex steroids in the CNS fluctuate due to gonadal activities and to local metabolism and synthesis. Importantly, biochemical processes involved in the pathogenesis of NDs are increasingly being recognized as different between the two sexes and as influenced by sex steroids. The aim of this review is to present current state-of-the-art understanding on the potential role of sex steroids and their receptors on the onset and progression of major neurodegenerative disorders, namely, Alzheimer's disease, Parkinson's diseases, amyotrophic lateral sclerosis, and the peculiar motoneuron disease spinal and bulbar muscular atrophy, in which hormonal therapy is potentially useful as disease modifier.

Influence of Estrogen Modulation on Glia Activation in a Murine Model of Parkinson's Disease

Epidemiological data suggest a sexual dimorphism in Parkinson disease (PD), with women showing lower risk of developing PD. Vulnerability of the nigrostriatal pathway may be influenced by exposure to estrogenic stimulation throughout fertile life. To further address this issue, we analyzed the progression of nigrostriatal damage, microglia and astrocyte activation and microglia polarization triggered by intrastriatal injection of dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA) in male, female and ovariectomized (OVX) mice, as well as in OVX mice supplemented with 17βestradiol (OVX+E). Animals were sacrificed at different time points following 6-OHDA injection and brain sections containing striatum and substantia nigra pars compacta (SNc) underwent immunohistochemistry for tyrosine hydroxylase (TH) (dopaminergic marker), immunofluorescence for IBA1 and GFAP (markers of microglia and astrocyte activation, respectively) and triple immunoflorescent to identify polarization of microglia toward the cytotoxic M1 (DAPI/IBA1/TNFα) or cytoprotective M2 (DAPI/IBA1/CD206) phenotype. SNc damage induced by 6-OHDA was significantly higher in OVX mice, as compared to all other experimental groups, at 7 and 14 days after surgery. Astrocyte activation was higher in OVX mice with respect the other experimental groups, at all time points. Microglial activation in the SNc was detected at earlier time points in male, female and OVX+E, while in OVX mice was detected at all time-points. Microglia polarization toward the M2, but not the M1, phenotype was detected in female and OVX+E mice, while the M1 phenotype was observed only in male and OVX mice. Our results support the protective effects of estrogens against nigrostriatal degeneration, suggesting that such effects may be mediated by an interaction with microglia, which tend to polarize preferentially toward an M2, cytoprotective phenotype in the presence of intense estrogenic stimulation.

Genes, Gender, Environment, and Novel Functions of Estrogen Receptor Beta in the Susceptibility to Neurodevelopmental Disorders

Many neurological disorders affect men and women differently regarding prevalence, progression, and severity. It is clear that many of these disorders may originate from defective signaling during fetal or perinatal brain development, which may affect males and females differently. Such sex-specific differences may originate from chromosomal or sex-hormone specific effects. This short review will focus on the estrogen receptor beta (ERβ) signaling during perinatal brain development and put it in the context of sex-specific differences in neurodevelopmental disorders. We will discuss ERβ’s recent discovery in directing DNA de-methylation to specific sites, of which one such site may bear consequences for the susceptibility to the neurological reading disorder dyslexia. We will also discuss how dysregulations in sex-hormone signaling, like those evoked by endocrine disruptive chemicals, may affect this and other neurodevelopmental disorders in a sex-specific manner through ERβ.

Alteration of nuclear factor-kappaB pathway promote neuroinflammation depending on the functions of estrogen receptors in substantia nigra after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment

The simultaneous role of neuroprotective estrogen and neurodegenerative inflammation during the progression of Parkinson's disease (PD) is still remaining elusive. The novel importance of the present study in MPTP mediated mouse model of Parkinson's disease (PD) is-to investigate the status of neuronal and glial cells in a time chase experiment; to explore which pathway of NF-kappaB exist to proceed the neuroinflammation; to investigate the status of estrogen and the activation pattern of nuclear or cytosolic estrogen receptors in either sexes of Swiss albino mice during MPTP mediated progressive neurodegeneration in the substantia nigra. After MPTP intoxication, the nigral molecular anatomy was changed differently in separate time interval during the progression of neurodegeneration with/without association of glial cells and functional (via its nuclear and cytosolic receptors) estrogen level. Both the canonical and/or non-canonical pathways of NF-kappaB exist in the substantia nigra of both the sexes after MPTP treatment that is why inspite of presence of estrogen, neuroinflammation progresses. The homodimeric or heterodimeric form of ER-beta binds with NF-kappaB molecules p65 and RelB differently, but the canonical or non-canonical pathways of NF-kappaB molecules could not be stopped or may be promoted.

Alteration in Nuclear Factor-KappaB Pathway and Functionality of Estrogen via Receptors Promote Neuroinflammation in Frontal Cortex after 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Treatment

The MPTP mediated neurodegeneration in substantia nigra has been well studied, but not the status of frontal cortex. The novelty of the present study is to explore the sex difference of frontal cortex during MPTP intoxication and to investigate the role of estrogen and its receptors in presence of glial cells in a time chase experiment; to identify which pathway of NF-kappaB exist to proceed the neuroinflammation; to investigate the estrogen binding with its nuclear or cytosolic receptors and whether any direct relation exists between estrogen receptor (ER) -beta and NF-kappaB molecules p65 and RelB. The progression of neurodegeneration occurred with the association of glial cells and functional (via its nuclear and cytosolic receptors) estrogen level. Both the canonical and/or non canonical pathways of NF-kappaB exist in frontal cortex of both the sexes after MPTP treatment. The homodimeric or heterodimeric form of ER-beta binds with NF-kappaB molecules p65 and RelB differently, but the canonical or non canonical pathways of NF-kappaB molecules could not be stopped or may be promoted. The changes in the molecular and cellular pattern in frontal cortex of both sexes during MPTP intoxication depends on the estrogen function via its nuclear or cytosolic estrogen receptors.

Sex differences in Parkinson's disease

Parkinson's disease (PD) displays a greater prevalence and earlier age at onset in men. This review addresses the concept that sex differences in PD are determined, largely, by biological sex differences in the NSDA system which, in turn, arise from hormonal, genetic and environmental influences. Current therapies for PD rely on dopamine replacement strategies to treat symptoms, and there is an urgent, unmet need for disease modifying agents. As a significant degree of neuroprotection against the early stages of clinical or experimental PD is seen, respectively, in human and rodent females compared with males, a better understanding of brain sex dimorphisms in the intact and injured NSDA system will shed light on mechanisms which have the potential to delay, or even halt, the progression of PD. Available evidence suggests that sex-specific, hormone-based therapeutic agents hold particular promise for developing treatments with optimal efficacy in men and women.

Estetrol attenuates neonatal hypoxic-ischemic brain injury

Estetrol (E4) is a recently described natural estrogen with four hydroxyl-groups that is synthesized exclusively during pregnancy by the human fetal liver. It has important antioxidative activity. The aim of the present study was to define the importance of E4 in the attenuation of neonatal hypoxic-ischemic encephalopathy. Antioxidative effect of 650μM, 3.25mM and 6.5mM E4 on primary hippocampal cell cultures was studied before/after H202-induced oxidative stress. To examine oxidative stress and cell viability, lactate dehydrogenase activity and cell proliferation colorimetric assays were performed. To study the neuroprotective and therapeutic effects of E4 in vivo neonatal hypoxic-ischemic encephalopathy model of 7-day-old newborn rat pups was used. The neuroprotective and therapeutic effects of estetrol before/after hypoxic-ischemic insult was studied in 1mg/kg/day, 5mg/kg/day, 10mg/kg/day, 50mg/kg/day E4 pretreated/treated groups and compared with the sham and the vehicle treated groups. The body temperature of the rat pups was examined along with their body and brain weights. Brains were studied at the level of the hippocampus and cortex. Intact cell counting and expressions of microtubule-associated protein-2, doublecortin and vascular-endothelial growth factor were evaluated by histo- and immunohistochemistry. ELISAs were performed on blood samples to detect concentrations of S100B and glial fibrillary acidic protein as brain damage markers. This work reveals for the first time that E4 significantly decreases LDH activity and enhances cell proliferation in primary hippocampal neuronal cell cultures in vitro, and decreases the early gray matter loss and promotes neuro- and angiogenesis in vivo.

Sex differences in Parkinson's disease and other movement disorders

Movement disorders including Parkinson's disease (PD), Huntington's disease (HD), chorea, tics, and Tourette's syndrome (TS) display sex differences in disease susceptibility, disease pathogenesis, and clinical presentation. PD is more common in males than in females. Epidemiologic studies suggest that exposure to endogenous and exogenous estrogen contributes to these sex differences. There is extensive evidence that estrogen prevents dopaminergic neuron depletion induced by neurotoxins in PD animal models and therefore is neuroprotective. Estrogen may also decrease the efficacy of other neuroprotective substances such as caffeine in females but not males. Sex chromosomes can exert effects independent of sex steroid hormones on the development and maintenance of the dopamine system. As a result of hormone, chromosome and other unknown effects, there are sexual dimorphisms in the basal ganglia, and at the molecular levels in dopaminergic neurons that may lead to distinct mechanisms of pathogenesis in males and females. In this review, we summarize the evidence that estrogen and selective estrogen receptor modulators are neuroprotective in PD and discuss potential mechanisms of action. We also briefly review how sex differences in basal ganglia function and dopaminergic pathways may impact HD, chorea, and tics/Tourette's syndrome. Further understanding of these sex differences may lead to novel therapeutic strategies for prevention and treatment of these diseases.

Dietary administration of paraquat for 13 weeks does not result in a loss of dopaminergic neurons in the substantia nigra of C57BL/6J mice

Several investigations have reported that mice administered paraquat dichloride (PQ·Cl2) by intraperitoneal injection exhibit a loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). In this study, male and female C57BL/6J mice were administered PQ·Cl2 in the diet at concentrations of 0 (control), 10, and 50ppm for a duration of 13weeks. A separate group of mice were administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) during week 12 as positive controls to produce a loss of dopaminergic neurons in the SNpc. The comparative effects of PQ and MPTP on the SNpc and/or striatum were assessed using neurochemical, neuropathological, and stereological endpoints. Morphological and stereological assessments were performed by investigators 'blinded' to the origin of the tissue. Neither dose of PQ·Cl2 (10 or 50 ppm in the diet) caused a loss of striatal dopamine or dopamine metabolite concentrations in the brains of mice. Pathological assessments of the SNpc and striatum showed no evidence of neuronal degeneration or astrocytic/microglial activation. Furthermore, the number of tyrosine hydroxylase-positive (TH(+)) neurons in the SNpc was not reduced in PQ-treated mice. In contrast, MPTP caused a decrease in striatal dopamine concentration, a reduction in TH(+) neurons in the SNpc, and significant pathological changes including astrocytic and microglial activation in the striatum and SNpc. The MPTP-induced effects were greater in males than in females. It is concluded that 13weeks of continuous dietary exposure of C57BL/6J mice to 50ppm PQ·Cl2 (equivalent to 10.2 and 15.6mg PQ ion/kg body weight/day for males and females, respectively) does not result in the loss of, or damage to, dopaminergic neurons in the SNpc.

Estrogen: a master regulator of bioenergetic systems in the brain and body

Estrogen is a fundamental regulator of the metabolic system of the female brain and body. Within the brain, estrogen regulates glucose transport, aerobic glycolysis, and mitochondrial function to generate ATP. In the body, estrogen protects against adiposity, insulin resistance, and type II diabetes, and regulates energy intake and expenditure. During menopause, decline in circulating estrogen is coincident with decline in brain bioenergetics and shift towards a metabolically compromised phenotype. Compensatory bioenergetic adaptations, or lack thereof, to estrogen loss could determine risk of late-onset Alzheimer's disease. Estrogen coordinates brain and body metabolism, such that peripheral metabolic state can indicate bioenergetic status of the brain. By generating biomarker profiles that encompass peripheral metabolic changes occurring with menopause, individual risk profiles for decreased brain bioenergetics and cognitive decline can be created. Biomarker profiles could identify women at risk while also serving as indicators of efficacy of hormone therapy or other preventative interventions.

NAP (davunetide) modifies disease progression in a mouse model of severe neurodegeneration: protection against impairments in axonal transport

NAP (davunetide) is a novel neuroprotective compound with mechanism of action that appears to involve microtubule (MT) stabilization and repair. To evaluate, for the first time, the impact of NAP on axonal transport in vivo and to translate it to neuroprotection in a severe neurodegeneration, the SOD1-G93A mouse model for amyotrophic lateral sclerosis (ALS) was used. Manganese-enhanced magnetic resonance imaging (MRI), estimating axonal transport rates, revealed a significant reduction of the anterograde axonal transport in the ALS mice compared to healthy control mice. Acute NAP treatment normalized axonal transport rates in these ALS mice. Tau hyperphosphorylation, associated with MT dysfunction and defective axonal transport, was discovered in the brains of the ALS mice and was significantly reduced by chronic NAP treatment. Furthermore, in healthy wild type (WT) mice, NAP reversed axonal transport disruption by colchicine, suggesting drug-dependent protection against axonal transport impairment through stabilization of the neuronal MT network. Histochemical analysis showed that chronic NAP treatment significantly protected spinal cord motor neurons against ALS-like pathology. Sequential MRI measurements, correlating brain structure with ALS disease progression, revealed a significant damage to the ventral tegmental area (VTA), indicative of impairments to the dopaminergic pathways relative to healthy controls. Chronic daily NAP treatment of the SOD1-G93A mice, initiated close to disease onset, delayed degeneration of the trigeminal, facial and hypoglossal motor nuclei as was significantly apparent at days 90-100 and further protected the VTA throughout life. Importantly, protection of the VTA was significantly correlated with longevity and overall, NAP treatment significantly prolonged life span in the ALS mice.

Hormone replacement therapy and risk for neurodegenerative diseases

Over the past two decades, there has been a significant amount of research investigating the risks and benefits of hormone replacement therapy (HRT) with regards to neurodegenerative disease. Here, we review basic science studies, randomized clinical trials, and epidemiological studies, and discuss the putative neuroprotective effects of HRT in the context of Alzheimer's disease, Parkinson's disease, frontotemporal dementia, and HIV-associated neurocognitive disorder. Findings to date suggest a reduced risk of Alzheimer's disease and improved cognitive functioning of postmenopausal women who use 17β-estradiol. With regards to Parkinson's disease, there is consistent evidence from basic science studies for a neuroprotective effect of 17β-estradiol; however, results of clinical and epidemiological studies are inconclusive at this time, and there is a paucity of research examining the association between HRT and Parkinson's-related neurocognitive impairment. Even less understood are the effects of HRT on risk for frontotemporal dementia and HIV-associated neurocognitive disorder. Limits to the existing research are discussed, along with proposed future directions for the investigation of HRT and neurodegenerative diseases.

Signaling pathways mediating the neuroprotective effects of sex steroids and SERMs in Parkinson's disease

Studies with the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) animal model of Parkinson's disease have shown the ability of 17β-estradiol to protect the nigrostriatal dopaminergic system. This paper reviews the signaling pathways mediating the neuroprotective effect of 17β-estradiol against MPTP-induced toxicity. The mechanisms of 17β-estradiol action implicate activation of signaling pathways such as the phosphatidylinositol-3 kinase/Akt and the mitogen-activated protein kinase pathways. 17β-estradiol signaling is complex and integrates multiple interactions with signaling molecules that act to potentiate a protective effect. 17β-estradiol signaling is mediated via estrogen receptors, including GPER1, but others receptors, such as the IGF-1 receptor, are implicated in the neuroprotective effect. Glial and neuronal crosstalk is a critical factor in the maintenance of dopamine neuronal survival and in the neuroprotective action of 17β-estradiol. Compounds that stimulate GPER1 such as selective estrogen receptor modulators and phytoestrogens show neuroprotective activity and are alternatives to 17β-estradiol.

Neuroprotective effects of estrogens and androgens in CNS inflammation and neurodegeneration

Multiple sclerosis (MS) is a disease characterized by inflammation and demyelination. Currently, the cause of MS is unknown. Experimental autoimmune encephalomyelitis (EAE) is the most common mouse model of MS. Treatments with the sex hormones, estrogens and androgens, are capable of offering disease protection during EAE and are currently being used in clinical trials of MS. Beyond endogenous estrogens and androgens, treatments with selective estrogen receptor modulators (SERMs) for estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) are also capable of providing disease protection. This protection includes, but is not limited to, prevention of clinical disease, reduction of CNS inflammation, protection against demyelination, and protection against axonal loss. In EAE, current efforts are focused on using conditional cell specific knockouts of sex hormone receptors to identify the in vivo targets of these estrogens and androgens as well as downstream molecules responsible for disease protection.

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.

MPTP Neurotoxicity and Testosterone Induce Dendritic Remodeling of Striatal Medium Spiny Neurons in the C57Bl/6 Mouse

Nigrostriatal damage is increased in males relative to females. While estrogen is neuroprotective in females, less is known about potential protective effects of testosterone in males. We determined if castration enhances neuronal injury to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Castrates or sham-castrated mice were sacrificed 1 week following injection of MPTP (4 × 20 mg/kg) or saline (n = 11-12/group). The right striatum was immunostained for tyrosine hydroxylase (TH). The left hemisphere was stained by Golgi Cox to quantify neuronal morphology in medium spiny neurons (MSNs) of the dorsolateral striatum. MPTP reduced TH, but there was no effect of castration and no interaction. For MSN dendritic morphology, MPTP decreased the highest branch order and increased spine density on 2nd-order dendrites. Castrated males had shorter 5th-order dendrites. However, there was no interaction between gonadal status and MPTP. Thus, castration and MPTP exert nonoverlapping effects on MSN morphology with castration acting on distal dendrites and MPTP acting proximally.

The possible role of estrogen and selective estrogen receptor modulators in a rat model of Parkinson's disease

AIM

The aim of the present study was to assess and compare the effect of 17β-estradiol and two different selective estrogen receptor modulators (SERMs), tamoxifen and raloxifene, as well as a selective estrogen receptor alpha agonist, propyl-pyrazole-triol (PPT) and a selective estrogen receptor beta agonist, diarylpropionitrile (DPN), on behavioral and biochemical alterations in 6-hydroxydopamine (6-OHDA)-induced nigral dopaminergic cell death in rats.

MAIN METHODS

80 female Wister rats were used. Animals were divided into eight equal groups: Group I; Sham operated, Group II; subjected to ovariectomy (OVX), Group III; OVX rats received striatal injection of 6-OHDA, Groups IV-VIII; OVX rats received striatal injection of 6-OHDA and were injected daily with 17β-estradiol, tamoxifen, raloxifene, PPT and DPN respectively for 5days before 6-OHDA and continued for further 2weeks.

KEY FINDINGS

Results showed that striatal injection of 6-OHDA produced significant behavioral alteration suggestive of PD, together with significant decrease in striatal dopamine, homovanillic acid (HVA) and 3,4-dihydroxyphenyl acetic acid (DOPAC) concentrations. 6-OHDA-induced nigral dopaminergic cell death was characterized by oxidative stress, evidenced by significant decrease in striatal glutathione peroxidase activity, as well as apoptosis, evidenced by significant increase in nigral caspase-3 activity. Treatment with 17β-estradiol, raloxifene, PPT, but neither tamoxifen nor DPN, resulted in significant amelioration of the behavioral and biochemical alterations induced by 6-OHDA.

SIGNIFICANCE

These findings suggest that estrogen and some SERMs having estrogenic agonist activity in the brain, like raloxifene, might exert beneficial effect in PD.

Male/Female differences in neuroprotection and neuromodulation of brain dopamine

The existence of a sex difference in Parkinson's disease (PD) is observed as related to several variables, including susceptibility of the disease, age at onset, and symptoms. These differences between men and women represent a significant characteristic of PD, which suggest that estrogens may exert beneficial effects against the development and the progression of the disease. This paper reviews the neuroprotective and neuromodulator effects of 17β-estradiol and progesterone as compared to androgens in the nigrostriatal dopaminergic (NSDA) system of both female and male rodents. The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mice model of PD and methamphetamine toxicity faithfully reproduce the sex differences of PD in that endogenous estrogen levels appear to influence the vulnerability to toxins targeting the NSDA system. Exogenous 17β-estradiol and/or progesterone treatments show neuroprotective properties against NSDA toxins while androgens fail to induce any beneficial effect. Sex steroid treatments show male and female differences in their neuroprotective action against methamphetamine toxicity. NSDA structure and function, as well as the distribution of estrogen receptors, show sex differences and may influence the susceptibility to the toxins and the response to sex steroids. Genomic and non-genomic actions of 17β-estradiol converge to promote survival factors and the presence of both estrogen receptors α and β are critical to 17β-estradiol neuroprotective action against MPTP toxicity.

Peripheral vs. Central Sex Steroid Hormones in Experimental Parkinson's Disease

The nigrostriatal dopaminergic (NSDA) pathway degenerates in Parkinson's disease (PD), which occurs with approximately twice the incidence in men than women. Studies of the influence of systemic estrogens in females suggest sex hormones contribute to these differences. In this review we analyze the evidence revealing great complexity in the response of the healthy and injured NSDA system to hormonal influences, and emphasize the importance of centrally generated estrogens. At physiological levels, circulating estrogen (in females) or estrogen precursors (testosterone in males, aromatized to estrogen centrally) have negligible effects on dopaminergic neuron survival in experimental PD, but can modify striatal dopamine levels via actions on the activity or adaptive responses of surviving cells. However, these effects are sexually dimorphic. In females, estradiol promotes adaptive responses in the partially injured NSDA pathway, preserving striatal dopamine, whereas in males gonadal steroids and exogenous estradiol have a negligible or even suppressive effect, effectively exacerbating dopamine loss. On balance, the different effects of gonadal factors in males and females contribute to sex differences in experimental PD. Fundamental sex differences in brain organization, including the sexually dimorphic networks regulating NSDA activity are likely to underpin these responses. In contrast, estrogen generated locally appears to preserve striatal dopamine in both sexes. The available data therefore highlight the need to understand the biological basis of sex-specific responses of the NSDA system to peripheral hormones, so as to realize the potential for sex-specific, hormone-based therapies in PD. Furthermore, they suggest that targeting central steroid generation could be equally effective in preserving striatal dopamine in both sexes. Clarification of the relative roles of peripheral and central sex steroid hormones is thus an important challenge for future studies.

Estrogen actions in the brain and the basis for differential action in men and women: a case for sex-specific medicines

The classic view of estrogen actions in the brain was confined to regulation of ovulation and reproductive behavior in the female of all mammalian species studied, including humans. Burgeoning evidence now documents profound effects of estrogens on learning, memory, and mood as well as neurodevelopmental and neurodegenerative processes. Most data derive from studies in females, but there is mounting recognition that estrogens play important roles in the male brain, where they can be generated from circulating testosterone by local aromatase enzymes or synthesized de novo by neurons and glia. Estrogen-based therapy therefore holds considerable promise for brain disorders that affect both men and women. However, as investigations are beginning to consider the role of estrogens in the male brain more carefully, it emerges that they have different, even opposite, effects as well as similar effects in male and female brains. This review focuses on these differences, including sex dimorphisms in the ability of estradiol to influence synaptic plasticity, neurotransmission, neurodegeneration, and cognition, which, we argue, are due in a large part to sex differences in the organization of the underlying circuitry. There are notable sex differences in the incidence and manifestations of virtually all central nervous system disorders, including neurodegenerative disease (Parkinson's and Alzheimer's), drug abuse, anxiety, and depression. Understanding the cellular and molecular basis of sex differences in brain physiology and responses to estrogen and estrogen mimics is, therefore, vitally important for understanding the nature and origins of sex-specific pathological conditions and for designing novel hormone-based therapeutic agents that will have optimal effectiveness in men or women.

Independent influences of sex steroids of systemic and central origin in a rat model of Parkinson's disease: A contribution to sex-specific neuroprotection by estrogens

This review considers evidence which reveals considerable complexity and sex differences in the response of the nigrostriatal dopaminergic (NSDA) system to hormonal influences. This pathway degenerates in Parkinson's disease (PD) and sex hormones contribute to sex differences in PD, where men fare worse than women. Here we discuss evidence from animal studies which allows us to hypothesize that, contrary to expectations, the acclaimed neuroprotective property of physiological concentrations of estradiol arises not by promoting NSDA neuron survival, but by targeting powerful adaptive responses in the surviving neurons, which restore striatal DA functionality until over 60% of neurons are lost. Estrogen generated locally in the NSDA region appears to promote these adaptive mechanisms in females and males to preserve striatal DA levels in the partially injured NSDA pathway. However, responses to systemic steroids differ between the sexes. In females there is general agreement that gonadal steroids and exogenous estradiol promote striatal adaptation in the partially injured NSDA pathway to protect against striatal DA loss. In contrast, the balance of evidence suggests that in males gonadal factors and exogenous estradiol have negligible or even harmful effects. Sex differences in the organization of NSDA-related circuitry may well account for these differences. Compensatory mechanisms and sexually dimorphic hard-wiring are therefore likely to represent important biological substrates for sex dimorphisms. As these processes may be targeted differentially by systemic steroids in males and females, further understanding of the underlying processes would provide valuable insights into the potential for hormone-based therapies in PD, which would need to be sex-specific. Alternatively, evidence that estrogen generated locally is protective in the injured male NSDA pathway indicates the great therapeutic potential of harnessing central steroid synthesis to ameliorate neurodegenerative disorders. A clearer understanding of the relative contributions and inter-relationships of central and systemic steroids within the NSDA system is an important goal for future studies.

Mitochondrial mechanisms of estrogen neuroprotection

Mitochondria have become a primary focus in our search not only for the mechanism(s) of neuronal death but also for neuroprotective drugs and therapies that can delay or prevent Alzheimer's disease and other chronic neurodegenerative conditions. This is because mitochrondria play a central role in regulating viability and death of neurons, and mitochondrial dysfunction has been shown to contribute to neuronal death seen in neurodegenerative diseases. In this article, we review the evidence for the role of mitochondria in cell death and neurodegeneration and provide evidence that estrogens have multiple effects on mitochondria that enhance or preserve mitochondrial function during pathologic circumstances such as excitotoxicity, oxidative stress, and others. As such, estrogens and novel non-hormonal analogs have come to figure prominently in our efforts to protect neurons against both acute brain injury and chronic neurodegeneration.

17beta-estradiol protects against hypoxic/ischemic white matter damage in the neonatal rat brain

Developing oligodendrocytes (pre-OLs) are highly vulnerable to hypoxic-ischemic injury and associated excitotoxicity and oxidative stress. 17beta-Estradiol plays an important role in the development and function of the CNS and is neuroprotective. The sudden drop in circulating estrogen after birth may enhance the susceptibility of developing OLs to injury. Estrogen receptor (ER)-alpha and ER-beta are both expressed in OLs. We examined the effect of 17beta-estradiol on oxygen-glucose deprivation and oxidative stress-induced cell death in rat pre-OLs in vitro and on hypoxic-ischemic brain injury in vivo. Pre-OLs in culture were subjected to oxygen-glucose deprivation (OGD) or glutathione depletion in the presence or absence of 17beta-estradiol. LDH release, the Alamar blue assay, and phase-contrast microscopy were used to assess cell viability. Hypoxic-ischemic injury was generated in 6-day-old rats (P6) by unilateral carotid ligation and hypoxia (6% O(2) for 1 hr). Rat pups received one intraperitoneal injection of 300 or 600 microg/kg 17beta-estradiol or vehicle 12 hr prior to the surgical procedure. Injury was assessed by myelin basic protein (MBP) immunocytochemistry at P10. 17beta-Estradiol produced significant protection against OGD-induced cell death in primary OLs (EC(50) = 1.3 +/- 0.46 x 10(-9) M) and against oxidative stress. Moreover, 17beta-estradiol attenuated the loss of MBP labeling in P10 pups ipsilateral to the carotid ligation. These results suggest a potential role for estrogens in attenuation of hypoxic-ischemic and oxidative injury to developing OLs and in the prevention of periventricular leukomalacia.

Neuroprotective actions of sex steroids in Parkinson's disease

The sex difference in Parkinson's disease, with a higher susceptibility in men, suggests a modulatory effect of sex steroids in the brain. Numerous studies highlight that sex steroids have neuroprotective properties against various brain injuries. This paper reviews the protective effects of sex hormones, particularly estradiol, progesterone and androgens, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) animal model of Parkinson's disease as compared to methamphetamine toxicity. The molecular mechanisms underlying beneficial actions of sex steroids on the brain have been investigated showing steroid, dose, timing and duration specificities and presently focus is on the dopamine signaling pathways, the next frontier. Both genomic and non-genomic actions of estrogen converge to promote survival factors and show sex differences. Neuroprotection by estrogen involves activation of signaling molecules such as the phosphatidylinositol-3 kinase/Akt and the mitogen-activated protein kinase pathways. Interaction with growth factors, such as insulin-like growth factor 1, also contributes to protective actions of estrogen.

Differential expression of oestrogen receptor alpha following reproductive experience in young and middle-aged female rats

Reproductive experience (i.e. pregnancy and lactation) alters a number of physiological and behavioural endpoints, many of which are related to reproductive function and are regulated by oestrogen. For example, reproductive experience significantly attenuates the oestradiol-induced prolactin surge on the afternoon of pro-oestrous and circulating oestradiol levels are reduced at this time. Although parity-related effects on oestrogen receptor (ER) alpha have been observed within the anterior pituitary, there are currently no data regarding possible parity-induced alterations in ERalpha in the brain. Thus, the present study aimed to examine the effect of parity on the expression of ERalpha in reproductively relevant brain regions. Moreover, because previous findings have demonstrated that the long-term effects of reproductive experience are often oestrous cycle-dependent, ERalpha was examined at two stages of the oestrous cycle (i.e. dioestrous and pro-oestrous). Finally, because the expression of ERalpha is significantly influenced by age, both young and middle-aged females were included in the present study. ERalpha status was determined using immunohistochemistry in select brain regions involved in the regulation of reproductive behaviour in age-matched, cycling primiparous (i.e. one pregnancy and lactation) and nulliparous females as well as in age-matched, noncycling (i.e. persistent oestrous) 12 month-old primiparous and nulliparous females. Significant shifts in ERalpha cell numbers were observed in the medial preoptic area and medial amygdala as a consequence of reproductive experience in an oestrous-dependent manner. These findings indicate that significant changes in ERalpha activity occur in the brain as a function of reproductive experience.

Estrogen and SERM neuroprotection in animal models of Parkinson's disease

A higher prevalence and incidence of Parkinson disease (PD) is observed in men and beneficial motor effects of estrogens are observed in parkinsonian women. Lesion of the dopamine (DA) nigrostriatal pathway in animals with 1-methyl 4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) provides a model of PD and this is based on its use in humans as side-product of a drug abuse. Presently treatment of PD is mainly symptomatic. The MPTP mouse is used to study the neuroprotective roles of estrogenic drugs on the DA system. Estrogens, but not androgens, are active neuroprotectants as well as progesterone and dehydroepiandrosterone. An estrogen receptor agonist PPT and the selective estrogen receptor modulator raloxifene are also neuroprotective. Striatal DA neurons of estrogen receptor alpha knockout mice are more susceptible to MPTP toxicity than wild-type mice and neuroprotection by estradiol is associated with the activation of the PI3-K pathway involving Akt, GSK3beta, Bcl2 and BAD.

Circuit-based framework for understanding neurotransmitter and risk gene interactions in schizophrenia

Many risk genes interact synergistically to produce schizophrenia and many neurotransmitter interactions have been implicated. We have developed a circuit-based framework for understanding gene and neurotransmitter interactions. NMDAR hypofunction has been implicated in schizophrenia because NMDAR antagonists reproduce symptoms of the disease. One action of antagonists is to reduce the excitation of fast-spiking interneurons, resulting in disinhibition of pyramidal cells. Overactive pyramidal cells, notably those in the hippocampus, can drive a hyperdopaminergic state that produces psychosis. Additional aspects of interneuron function can be understood in this framework, as follows. (i) In animal models, NMDAR antagonists reduce parvalbumin and GAD67, as found in schizophrenia. These changes produce further disinhibition and can be viewed as the aberrant response of a homeostatic system having a faulty activity sensor (the NMDAR). (ii) Disinhibition decreases the power of gamma oscillation and might thereby produce negative and cognitive symptoms. (iii) Nicotine enhances the output of interneurons, and might thereby contribute to its therapeutic effect in schizophrenia.

Contribution of estrogen receptors alpha and beta to the effects of estradiol in the brain

Clinical and experimental studies show a modulatory role of estrogens in the brain and suggest their beneficial action in mental and neurodegenerative diseases. The estrogen receptors ERalpha and ERbeta are present in the brain and their targeting could bring selectivity and reduced risk of cancer. Implication of ERs in the effect of estradiol on dopamine, opiate and glutamate neurotransmission is reviewed. The ERalpha agonist, PPT, is shown as estradiol to modulate hippocampal NMDA receptors and AMPA receptors in cortex and striatum of ovariectomized rats whereas the ERbeta agonist DPN is inactive. Striatal DPN activity suggests implication of ERbeta in estradiol modulation of D2 receptors and transporters in ovariectomized rats and is supported by the lack of effect of estradiol in ERbeta knockout (ERKObeta) mice. Both ERalpha and ERbeta agonists modulate striatal preproenkephalin (PPE) gene expression in ovariectomized rats. In male mice PPT protects against MPTP toxicity to striatal dopamine; this implicates Akt/GSK3beta signaling and the apoptotic regulators Bcl2 and Bad. This suggests a role for ERalpha in striatal dopamine neuroprotection. ERKOalpha mice are more susceptible to MPTP toxicity and not protected by estradiol; differences in ERKObeta mice are subtler. These results suggest therapeutic potential for the brain of ER specific agonists.

More than a decade of estrogen neuroprotection

Considerable evidence has emerged through more than a decade of research supporting the neuroprotective and cognition-preserving effects of estrogens. Such basic research coupled with various epidemiological studies led quickly to the assessment of Premarin for the treatment of mild to moderate Alzheimer's disease (AD), initiated by the Alzheimer's Disease Cooperative Study Group and headed by Dr. Leon Thal. While this and subsequent trials with Premarin (Wyeth Research, Monmouth Junction, New Jersey) and PremPro (Wyeth Research), a conjugated equine estrogen preparation plus medoxyprogresterone acetate, have not supported the use of estrogens in treating advanced AD, considerable inferences have been made from these placebo controlled trials of estrogens. Here, we aimed to put these AD trials of estrogens in perspective by considering the potential mechanisms of these potent neuroprotective estrogens, the role of estrogens in other neurodegenerative conditions, such as cerebral ischemia, and based on our current understanding of estrogen neurobiology, offer insight into the design of future clinical trails of estrogens for neuronal protection.

OESTROGEN AND NIGROSTRIATAL DOPAMINERGIC NEURODEGENERATION: ANIMAL MODELS AND CLINICAL REPORTS OF PARKINSON'S DISEASE

1. The exact nature of oestrogen (positive, negative or no effect) in the dopaminergic neurodegenerative disorder Parkinson's disease is controversial. 2. In the present review, we summarize the data on oestrogen and nigrostriatal dopaminergic neurodegeneration in animal models and clinical reports of Parkinson's disease. 3. Most animal studies support the ability of oestrogen to function as a neuroprotectant against neurotoxins that target the nigrostriatal dopaminergic system. 4. Retrospective and prospective clinical studies generally support the findings from animal studies that oestrogen exerts a positive, or, at worst, no effect, in Parkinson's disease. 5. Oestrogen was chosen as one of the 12 neuroprotective compounds to be attractive candidates for further clinical trials (Phase II or III) in 2003.

Role of estrogen receptors in neuroprotection by estradiol against MPTP toxicity

Estradiol protects against striatal dopamine terminal loss caused by the neurotoxin MPTP in mice. This effect of estradiol is thought to be mediated by an interaction with estrogen receptors (ER), of which there are two: ERalpha and ERbeta. In the present study, the role of these two ERs in MPTP toxicity and its neuroprotection by estradiol was investigated using ER knock out mice (ERKO). MPTP (7, 9, or 11 mg/kg administered four times at 2h intervals) caused a dose-dependent decrease in striatal dopamine and dopamine metabolite DOPAC concentrations in wild type (WT) mice. The degree of dopamine and DOPAC depletion after MPTP was greater in the ERKOalpha mice than WT mice, whereas the ERKObeta mice exhibited no change in MPTP sensitivity. ERKObeta mice showed a lower DA turnover than WT and ERKOalpha mice. WT, ERKOalpha and ERKObeta mice were also treated for 10 days with exogenous estradiol and on day 5 of treatment were challenged with MPTP (9 mg/kg administered four times at 2h intervals). In the WT mice, estradiol partially prevented the MPTP-induced decrease in striatal dopamine and DOPAC concentrations. However, estradiol treatment was without significant neuroprotective effects in the ERKOalpha and ERKObeta mice. These results show a greater susceptibility to MPTP toxicity of ERKOalpha mice compared to WT and ERKObeta mice and a role for both ER receptors in striatal DA neuroprotection.

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.

Striatal susceptibility to a dopaminergic neurotoxin is independent of sex hormone effects on cell survival and DAT expression but is exacerbated by central aromatase inhibition

The aim of this study was to investigate further the hormone-dependent processes underlying sex differences in neurotoxic responses within the rat nigrostriatal dopaminergic (NSDA) pathway after partial lesioning with 6-OHDA, a state thought to mimic the early stages of Parkinson's disease where, in humans and animal models alike, males appear to be more susceptible. Contrary to our hypotheses, hormone manipulations (gonadectomy +/- oestrogen or androgen treatment) failed to alter survival of tyrosine hydroxylase immunoreactive cells in the substantia nigra pars compacta (SNc) after lesioning; this indicates that, unlike inherent sex differences in toxin-induced striatal dopamine depletion, sex differences in cell loss were not hormonally generated, and that hormone-dependent changes in dopamine depletion can occur independently of cell survival. In addition, hormonally induced changes in striatal expression of the dopamine transporter (DAT), an important factor for 6-OHDA toxicity, did not correlate with hormonal influences on striatal dopamine loss and, in males, central inhibition of aromatase prior to 6-OHDA infusion exacerbated striatal dopamine loss with no effect on SNc tyrosine hydroxylase-immunoreactive survival, suggesting locally generated oestrogen is neuroprotective. These results support the novel view that sex steroid hormones produced peripherally and centrally play a significant, sex-specific role within the sexually dimorphic NSDA pathway to modulate plastic, compensatory responses aimed at restoring striatal dopamine functionality, without affecting cell loss.

Estrogen receptor beta expression in the embryonic brain regulates development of calretinin-immunoreactive GABAergic interneurons

Our previous studies with estrogen receptor beta knockout (ERbeta(-/-)) mice demonstrated that ERbeta is necessary for embryonic development of the brain as early as embryonic day 14.5 (E14.5) and is involved in neuronal migration. Such early effects of ER were unexpected because estradiol synthesis and action in the brain occur at E18.5. In the present study, we examined the distribution of ERbeta in the developing brain and identified a population of ERbeta-regulated interneurons. ERbeta appears in the brain at E12.5, mainly localized in the wall of the midbrain, neuromere, hypothalamus, thalamus, and basal plate of pons. At E15.5 and E16.5, ERbeta expression increased in the hypothalamus, thalamus, and midbrain and appeared in the limbic forebrain. At E18.5, ERbeta expression was strongly expressed throughout the brain, including the cerebellum and striatum, whereas there were very few positive cells in the ventricular region. In the paraventricular thalamic nucleus and parafascicular nucleus, most of the calretinin-immunopositive interneurons expressed ERbeta. In ERbeta(-/-) mice, calretinin expression was markedly lower than in WT mice in the hippocampus, thalamus, and amygdala both at E16.5 and at E18.5. Epidermal growth factor receptor expression was lower in the cortex of ERbeta(-/-) than in WT mice at E15.5 and, unlike WT mice, was absent from the superficial marginal zone. These findings suggest that ERbeta in the embryonic brain is necessary for the development of calretinin-immunoreactive GABAergic interneurons and for neuronal migration in the cortex through modulating epidermal growth factor receptor expression at middle and later embryonic stages.

Estrogen counteracts ozone-induced oxidative stress and nigral neuronal death

Oxidative stress is implicated in the premature death of dopamine neurons in substantia nigra in Parkinson's disease. The incidence of Parkinson's disease is higher in men than in women, and estrogen may provide neuroprotection against oxidative damage. We examined the protective effects of estrogen on rat nigral death after chronic ozone inhalation. Ozone inhalation produced impaired nigral cell morphology and loss of dopamine neurons in ovariectomized rats. This was counteracted after 60 days of 17beta-estradiol treatment, when blood levels were highest. These results indicate that ozone exposure may be a useful Parkinson's disease model and neuroprotection afforded by 17beta-estradiol is dependent on the high levels achieved after its prolonged administration.

Development of 17alpha-estradiol as a neuroprotective therapeutic agent: rationale and results from a phase I clinical study

17alpha-estradiol (17alpha-E2) differs from its isomer, the potent feminizing hormone 17beta-estradiol (17beta-E2), only in the stereochemistry at one carbon, but this is sufficient to render it at least 200-fold less active as a transactivating hormone. Despite its meager hormonal activity, 17alpha-E2 is as potent as 17beta-E2 in protecting a wide variety of cell types, including primary neurons, from a diverse array of lethal and etiologically relevant stressors, including amyloid toxicity, serum withdrawal, oxidative stress, excitotoxicity, and mitochondrial inhibition, among others. Moreover, both estradiol isomers have shown efficacy in animal models of stroke, Alzheimer's disease (AD), and Parkinson's disease (PD). Data from many labs have yielded a mechanistic model in which 17alpha-E2 intercalates into cell membranes, where it terminates lipid peroxidation chain reactions, thereby preserving membrane integrity, and where it in turn is redox cycled by glutathione or by NADPH through enzymatic coupling. Maintaining membrane integrity is critical to mitochondrial function, where loss of impermeability of the inner membrane initiates both necrotic and apoptotic pathways. Thus, by serving as a mitoprotectant, 17alpha-E2 forestalls cell death and could correspondingly provide therapeutic benefit in a host of degenerative diseases, including AD, PD, Friedreich's ataxia, and amyotrophic lateral sclerosis, while at the same time circumventing the common adverse effects elicited by more hormonally active analogues. Positive safety and pharmacokinetic data from a successful phase I clinical study with oral 17alpha-E2 (sodium sulfate conjugate) are presented here, and several options for its future clinical assessment are discussed.

Implication of the phosphatidylinositol-3 kinase/protein kinase B signaling pathway in the neuroprotective effect of estradiol in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mice

The present experiments sought to determine the implication of estrogen receptors (ERalpha and ERbeta) and their interaction with insulin-like growth factor receptor (IGF-IR) signaling pathways in neuroprotection by estradiol against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity. C57BL/6 male mice were pretreated for 5 days with 17beta-estradiol, an estrogen receptor alpha (ERalpha) agonist, 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)tris-phenol (PPT), or an estrogen receptor beta (ERbeta) agonist, 5-androsten-3beta, 17beta-diol (Delta5-diol). On day 5, mice received MPTP (9 mg/kg) or saline injections, and estrogenic treatments were continued for 5 more days. MPTP decreased striatal dopamine, measured by high-performance liquid chromatography, to 59% of control values; 17beta-estradiol and PPT but not Delta5-diol protected against this depletion. MPTP increased IGF-IR measured by Western blot, which was prevented by PPT. The phosphorylation of protein kinase B (Akt) (at serine 473), an essential mediator of IGF-I neuroprotective actions, increased after 17beta-estradiol and tended to increase with PPT but not with Delta5-diol treatments in MPTP mice. Glycogen synthase kinase 3beta (GSK3beta) phosphorylation (at serine 9) was greatly reduced in MPTP mice; this was completely prevented by PPT, whereas 17beta-estradiol and Delta5-diol treatments were less effective. The ratio between the levels of striatal Bcl-2 and BAD proteins, two apoptotic regulators, decreased after MPTP treatment. This effect was effectively prevented only in the animals treated with PPT. In nonlesioned mice, 17beta-estradiol and PPT increased phosphorylation of striatal Akt and GSK3beta, whereas the other markers measured remained unchanged. Delta5-Diol increased GSK3beta phosphorylation less than the PPT treatment. These results suggest that a pretreatment with estradiol promoted dopamine neuron survival by activating ERalpha and increasing Akt and GSK3beta phosphorylation.

Influence of oestrogenic compounds on monoamine transporters in rat striatum

Oestrogens have been reported to modulate rat membrane (DAT) and vesicular (VMAT(2)) dopamine transporters. A recent pilot study of postmenopausal women showed that chronic oestrogen replacement therapy increases striatal DAT. In the present study, we first investigated whether the oestrogen receptors alpha and beta mediate the effects of oestradiol on DAT and VMAT(2). Two days after ovariectomy, Sprague-Dawley rats were treated for 2 weeks with oestradiol or specific ligands for oestrogen receptor alpha, 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) or oestrogen receptor beta, 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN). Ovariectomy caused a decrease in [(125)I]-3beta-(4-iodophenyl)-tropane-2beta-carboxylic acid isopropyl ester ([(125)I] RTI-121) specific binding to DAT transporters in the middle striatum compared to values for intact rats, and this was reversed by oestradiol replacement therapy. DPN, but not PPT, mimicked the effect of oestradiol. [(125)I] RTI-121 specific binding in the anterior and posterior striatum was not affected by ovariectomy or any of the drug treatments. Second, we investigated whether oestradiol increased DAT specific binding after a longer period of hormonal withdrawal (a model of hormonal withdrawal at menopause) and whether the selective oestrogen receptor modulators (SERMs), tamoxifen and raloxifene, could reproduce the oestradiol-induced increase of [(125)I] RTI-121 specific binding in long-term ovariectomised rats. Four months after ovariectomy, Sprague-Dawley rats were treated for 2 weeks with oestradiol, tamoxifen or raloxifene, and then killed. Ovariectomy decreased [(3)H] RTI-121 specific binding to DAT transporters in the middle striatum compared to values for intact rats. Treatment with oestradiol, tamoxifen and raloxifene reversed this effect. [(125)I] RTI-121 specific binding in anterior and posterior striatum was not affected by ovariectomy or treatment with oestrogen receptor ligands. In both experiments, neither ovariectomy nor the oestrogenic treatments modulated striatal [(3)H] tetrahydrobenazine specific binding to VMAT(2). Overall, these results suggest that oestrogen receptor beta mediates the oestradiol-induced increase of striatal DAT and that oestradiol can increase DAT density even after long-term steroid withdrawal. The results also support the premise that the SERMs tamoxifen and raloxifene exert oestrogenic agonist effects in the brain.

Oestrogens prevent loss of dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2) in substantia nigra of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mice

Previous results from our laboratory have shown that 17beta-oestradiol prevents 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) striatal dopamine depletion. 17beta-oestradiol, oestriol and oestrone are the naturally occurring oestogens in humans. Using various dopamine markers, the present study investigated whether oestrone and oestriol such as 17beta-oestradiol have neuroprotective activity in MPTP-treated mice. Male mice were treated with 17beta-oestradiol, oestriol or oestrone for 5 days before and after MPTP administration, and were compared with nonlesioned mice receiving the same treatment. Striatal concentrations of dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were assayed by high-performance liquid chromatography. Dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2) specific binding were measured by autoradiography. DAT, VMAT2 and tyrosine hydroxylase mRNA levels were measured by in situ hybridisation. MPTP induced a loss of DAT and VMAT2 specific binding in the striatum and substantia nigra, as well as a decrease of VMAT2 mRNA in the substantia nigra. 17beta-oestradiol treatment prevented the loss of these dopaminergic markers, as well as striatal concentrations of dopamine, DOPAC and HVA. Mice receiving oestriol and oestrone showed catecholamine concentrations comparable to MPTP mice. Oestriol treatment had no effect on dopaminergic markers in MPTP mice whereas oestrone prevented striatal DAT loss and the decrease of VMAT2 mRNA in the substantia nigra. In nonlesioned mice, 17beta-oestradiol, oestriol or oestrone had no effect on all the dopaminergic markers investigated. In conclusion, a weak or a lack of effect of oestriol and oestrone was observed compared to 17beta-oestradiol in MPTP mice and none of these steroids had an effect in nonlesioned mice. A DAT and VMAT2 specific binding decrease after MPTP in the striatum and substantia nigra, as well as a decrease of substantia nigra VMAT2 mRNA, was observed and could be prevented by oestradiol.