The effects of 17beta-estradiol on ischemia-induced neuronal damage in the gerbil hippocampus

The impact of estradiol on serotonin, glutamate, and dopamine systems

Estradiol, the most potent and prevalent member of the estrogen class of steroid hormones and is expressed in both sexes. Functioning as a neuroactive steroid, it plays a crucial role in modulating neurotransmitter systems affecting neuronal circuits and brain functions including learning and memory, reward and sexual behaviors. These neurotransmitter systems encompass the serotonergic, dopaminergic, and glutamatergic signaling pathways. Consequently, this review examines the pivotal role of estradiol and its receptors in the regulation of these neurotransmitter systems in the brain. Through a comprehensive analysis of current literature, we investigate the multifaceted effects of estradiol on key neurotransmitter signaling systems, namely serotonin, dopamine, and glutamate. Findings from rodent models illuminate the impact of hormone manipulations, such as gonadectomy, on the regulation of neuronal brain circuits, providing valuable insights into the connection between hormonal fluctuations and neurotransmitter regulation. Estradiol exerts its effects by binding to three estrogen receptors: estrogen receptor alpha (ERα), estrogen receptor beta (ERβ), and G protein-coupled receptor (GPER). Thus, this review explores the promising outcomes observed with estradiol and estrogen receptor agonists administration in both gonadectomized and/or genetically knockout rodents, suggesting potential therapeutic avenues. Despite limited human studies on this topic, the findings underscore the significance of translational research in bridging the gap between preclinical findings and clinical applications. This approach offers valuable insights into the complex relationship between estradiol and neurotransmitter systems. The integration of evidence from neurotransmitter systems and receptor-specific effects not only enhances our understanding of the neurobiological basis of physiological brain functioning but also provides a comprehensive framework for the understanding of possible pathophysiological mechanisms resulting to disease states. By unraveling the complexities of estradiol's impact on neurotransmitter regulation, this review contributes to advancing the field and lays the groundwork for future research aimed at refining understanding of the relationship between estradiol and neuronal circuits as well as their involvement in brain disorders.

The Critical Period for Neuroprotection by Estrogen Replacement Therapy and the Potential Underlying Mechanisms

17β-Estradiol (estradiol or E2) is a steroid hormone that has been broadly applied as a neuroprotective therapy for a variety of neurodegenerative and cerebrovascular disorders such as ischemic stroke, Alzheimer's disease, and Parkinson's disease. Several laboratory and clinical studies have reported that Estrogen Replacement Therapy (ERT) had no effect against these diseases in elderly postmenopausal women, and at worst, increased their risk of onset and mortality. This review focuses on the growing body of data from in vitro and animal models characterizing the potential underlying mechanisms and signaling pathways that govern successful neuroprotection by ERT, including the roles of E2 receptors in mediating neuroprotection, E2 genomic regulation of apoptosis- related pathways, membrane-bound receptor-mediated non-genomic signaling pathways, and the antioxidant mechanisms of E2. Also discussed is the current evidence for a critical period of effective treatment with estrogen following natural or surgical menopause and the outcomes of E2 administration within an advantageous time period. The known mechanisms governing the duration of the critical period include depletion of E2 receptors, the switch to a ketogenic metabolic profile by neuronal mitochondria, and a decrease in acetylcholine that accompanies E2 deficiency. Also the major clinical trials and observational studies concerning postmenopausal Hormone Therapy (HT) are summarized to compare their outcomes with respect to neurological disease and discuss their relevance to the critical period hypothesis. Finally, potential controversies and future directions for this field are discussed throughout the review.

Best time window for the use of calcium-modulating agents to improve functional recovery in injured peripheral nerves-An experiment in rats

Peripheral nerve injury can have a devastating effect on daily life. Calcium concentrations in nerve fibers drastically increase after nerve injury, and this activates downstream processes leading to neuron death. Our previous studies showed that calcium-modulating agents decrease calcium accumulation, which aids in regeneration of injured peripheral nerves; however, the optimal therapeutic window for this application has not yet been identified. In this study, we show that calcium clearance after nerve injury is positively correlated with functional recovery in rats suffering from a crushed sciatic nerve injury. After the nerve injury, calcium accumulation increased. Peak volume is from 2 to 8 weeks post injury; calcium accumulation then gradually decreased over the following 24-week period. The compound muscle action potential (CMAP) measurement from the extensor digitorum longus muscle recovered to nearly normal levels in 24 weeks. Simultaneously, real-time polymerase chain reaction results showed that upregulation of calcium-ATPase (a membrane protein that transports calcium out of nerve fibers) mRNA peaked at 12 weeks. These results suggest that without intervention, the peak in calcium-ATPase mRNA expression in the injured nerve occurs after the peak in calcium accumulation, and CMAP recovery continues beyond 24 weeks. Immediately using calcium-modulating agents after crushed nerve injury improved functional recovery. These studies suggest that a crucial time frame in which to initiate effective clinical approaches to accelerate calcium clearance and nerve regeneration would be prior to 2 weeks post injury. © 2017 Wiley Periodicals, Inc.

Pharmacology of conjugated equine estrogens: efficacy, safety and mechanism of action

Oral conjugated equine estrogens (CEE) are the most used estrogen formulation for postmenopausal hormone therapy either alone or in combination with a progestin. CEE is most commonly used for the management of early menopausal symptoms such as hot flashes, vaginitis, insomnia, and mood disturbances. Additionally, if used at the start of the menopausal phase (age 50-59 years), CEE prevents osteoporosis and may in some women reduce the risk of cardiovascular disease (CVD) and Alzheimer's disease (AD). There appears to be a common mechanism through which estrogens can protect against CVD and AD. CEE is a natural formulation of an extract prepared from pregnant mares' urine. The product monogram lists the presence of only 10 estrogens consisting of the classical estrogens, estrone and 17β-estradiol, and a group of unique ring B unsaturated estrogens such as equilin and equilenin. The ring B unsaturated estrogens are formed by an alternate steroidogenic pathway in which cholesterol is not an obligatory intermediate. Both the route of administration and structure of these estrogens play a role in the overall pharmacology of CEE. In contrast to 17β-estradiol, ring B unsaturated estrogens express their biological effects mainly mediated by the estrogen receptor β and not the estrogen receptor α. All estrogen components of CEE are antioxidants, and some ring B unsaturated estrogens have several fold greater antioxidant activity than estrone and 17β-estradiol. The cardioprotective and neuroprotective effects of CEE appear to be, to some extent, due to its ability to prevent the formation of oxidized LDL and HDL, and by inhibiting or modulating some of the key proteases involved in programmed cell death (apoptosis) induced by the excess neurotransmitter glutamate and other neurotoxins. Selective combinations of ring B unsaturated estrogens have the potential of being developed as novel therapeutic agents for the prevention of cardiovascular disease and Alzheimer's disease in both aging women and men. This article is part of a Special Issue entitled 'Menopause'.

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.

Neuroprotective action of acute estrogens: animal models of brain ischemia and clinical implications

The ovarian hormone 17β-estradiol (E2) exerts profound neuroprotective actions against ischemia-induced brain damage in rodent models of global and focal ischemia. This review focuses on the neuroprotective efficacy of post-ischemic administration of E2 and non-feminizing estrogen analogs in the aging brain, with an emphasis on studies in animals subjected to a long-term loss of circulating E2. Clinical findings from the Women's Health Initiative study as well as data from animal studies that used long-term, physiological levels of E2 treatment are discussed in this context. We summarize major published findings that highlight the effective doses and timing of E2 treatment relative to onset of ischemia. We then discuss recent findings from our laboratory showing that under some conditions the aging hippocampus remains responsive to E2 and some neuroprotective non-feminizing estrogen analogs even after prolonged periods of hormone withdrawal. Possible membrane-initiated signaling mechanisms that may underlie the neuroprotective actions of acutely administered E2 are also discussed. Based on these findings, we suggest that post-ischemic treatment with high doses of E2 or certain non-feminizing estrogen analogs may have great therapeutic potential for treatment of brain damage and neurodegeneration associated with ischemia.

Neuroprotective actions of estradiol and novel estrogen analogs in ischemia: translational implications

This review highlights our investigations into the neuroprotective efficacy of estradiol and other estrogenic agents in a clinically relevant animal model of transient global ischemia, which causes selective, delayed death of hippocampal CA1 neurons and associated cognitive deficits. We find that estradiol rescues a significant number of CA1 pyramidal neurons that would otherwise die in response to global ischemia, and this is true when hormone is provided as a long-term pretreatment at physiological doses or as an acute treatment at the time of reperfusion. In addition to enhancing neuronal survival, both forms of estradiol treatment induce measurable cognitive benefit in young animals. Moreover, estradiol and estrogen analogs that do not bind classical nuclear estrogen receptors retain their neuroprotective efficacy in middle-aged females deprived of ovarian hormones for a prolonged duration (8weeks). Thus, non-feminizing estrogens may represent a new therapeutic approach for treating the neuronal damage associated with global ischemia.

Functional interactions between steroid hormones and neurotrophin BDNF

Brain-derived neurotrophic factor (BDNF), a critical neurotrophin, regulates many neuronal aspects including cell differentiation, cell survival, neurotransmission, and synaptic plasticity in the central nervous system (CNS). Though BDNF has two types of receptors, high affinity tropomyosin-related kinase (Trk)B and low affinity p75 receptors, BDNF positively exerts its biological effects on neurons via activation of TrkB and of resultant intracellular signaling cascades including mitogen-activated protein kinase/extracellular signal-regulated protein kinase, phospholipase Cγ, and phosphoinositide 3-kinase pathways. Notably, it is possible that alteration in the expression and/or function of BDNF in the CNS is involved in the pathophysiology of various brain diseases such as stroke, Parkinson’s disease, Alzheimer’s disease, and mental disorders. On the other hand, glucocorticoids, stress-induced steroid hormones, also putatively contribute to the pathophysiology of depression. Interestingly, in addition to the reduction in BDNF levels due to increased glucocorticoid exposure, current reports demonstrate possible interactions between glucocorticoids and BDNF-mediated neuronal functions. Other steroid hormones, such as estrogen, are involved in not only sexual differentiation in the brain, but also numerous neuronal events including cell survival and synaptic plasticity. Furthermore, it is well known that estrogen plays a role in the pathophysiology of Parkinson’s disease, Alzheimer’s disease, and mental illness, while serving to regulate BDNF expression and/or function. Here, we present a broad overview of the current knowledge concerning the association between BDNF expression/function and steroid hormones (glucocorticoids and estrogen).

Effects of hypoxia on testosterone release in rat Leydig cells

The aim of this study was to explore the effect and action mechanisms of intermittent hypoxia on the production of testosterone both in vivo and in vitro. Male rats were housed in a hypoxic chamber (12% O(2) + 88% N(2), 1.5 l/ml) 8 h/day for 4 days. Normoxic rats were used as control. In an in vivo experiment, hypoxic and normoxic rats were euthanized and the blood samples collected. In the in vitro experiment, the enzymatically dispersed rat Leydig cells were prepared and challenged with forskolin (an adenylyl cyclase activator, 10(-4) M), 8-Br-cAMP (a membrane-permeable analog of cAMP, 10(-4) M), hCG (0.05 IU), the precursors of the biosynthesis testosterone, including 25-OH-C (10(-5) M), pregnenolone (10(-7) M), progesterone (10(-7) M), 17-OH-progesterone (10(-7) M), and androstendione (10(-7)-10(-5) M), nifedipine (L-type Ca(2+) channel blocker, 10(-6)-10(-4) M), nimodipine (L-type Ca(2+) channel blocker, 10(-5) M), tetrandrine (L-type Ca(2+) channel blocker, 10(-5) M), and NAADP (calcium-signaling messenger causing release of calcium from intracellular stores, 10(-6)-10(-4) M). The concentrations of testosterone in plasma and medium were measured by radioimmunoassay. The level of plasma testosterone in hypoxic rats was higher than that in normoxic rats. Enhanced testosterone production was observed in rat Leydig cells treated with hCG, 8-Br-cAMP, or forskolin in both normoxic and hypoxic conditions. Intermittent hypoxia resulted in a further increase of testosterone production in response to the testosterone precursors. The activity of 17β-hydroxysteroid dehydrogenase was stimulated by the treatment of intermittent hypoxia in vitro. The intermittent hypoxia-induced higher production of testosterone was accompanied with the influx of calcium via L-type calcium channel and the increase of intracellular calcium via the mechanism of calcium mobilization. These results suggested that the intermittent hypoxia stimulated the secretion of testosterone at least in part via stimulatory actions on the activities of adenylyl cyclase, cAMP, L-type calcium channel, and steroidogenic enzymes.

Chronic estradiol treatment increases CA1 cell survival but does not improve visual or spatial recognition memory after global ischemia in middle-aged female rats

Transient global ischemia induces selective, delayed neuronal death in the hippocampal CA1 and cognitive deficits. Physiological levels of 17beta-estradiol ameliorate ischemia-induced neuronal death and cognitive impairments in young animals. In view of concerns regarding hormone therapy in postmenopausal women, we investigated whether chronic estradiol treatment initiated 14 days prior to ischemia attenuates ischemia-induced CA1 cell loss and impairments in visual and spatial memory, in ovariohysterectomized (OVX), middle-aged (9-11 months) female rats. To determine whether the duration of hormone withdrawal affects the efficacy of estradiol treatment, hormone treatment was initiated immediately (0 week), 1 week, or 8 weeks after OVX. Age-matched, OVX and gonadally intact females were studied at each OVX interval. Ischemia was induced 1 week after animals were pretested on a variety of behavioral tasks. Global ischemia produced significant neuronal loss in the CA1 and impaired performance on visual and spatial recognition. Chronic estradiol modestly but significantly increased the number of surviving CA1 neurons in animals at all OVX durations. However, in contrast with previous results in young females, estradiol did not preserve visual or spatial memory performance in middle-aged females. All animals displayed normal locomotion, spontaneous alternation and social preference, indicating the absence of global behavioral impairments. Therefore, the neuroprotective effects of estradiol are different in middle-aged than in young rats. These findings highlight the importance of using older animals in studies assessing potential treatments for focal and global ischemia.

17beta-estradiol affects GABAergic transmission in developing hippocampus

Estrogens are potent modulators of the nervous system. In particular, 17beta-estradiol was shown to affect GABAergic synaptic transmission in hippocampus of adult animals in vivo but much less is known on the impact of this hormone on the GABAergic system in the developing brains. We have recently shown that phasic and tonic GABAergic transmissions are strongly modulated upon long-term treatment with exogenous 17beta-estradiol in hippocampal neurons developing in vitro. To check for the long-term estrogen effect in a more physiological developmental model, we have investigated the GABAergic transmission in developing brains of P7-P40 animals, injected daily with 17beta-estradiol. We have found that such a treatment clearly increased GABAergic mIPSC frequency and amplitude while the onset and decay of mIPSCs were shortened. These effects were statistically significant in the youngest considered age group (P7-P13) with a tendency to disappear in older animals. Long-term treatment with estradiol did not change the susceptibility of mIPSC amplitude to upregulation by flurazepam while mIPSC decay was prolonged by this drug to a larger extent in 17beta-estradiol-treated animals. 17beta-estradiol strongly upregulated GABAergic tonic current but again this effect was restricted to the youngest group of animals. We conclude that 17beta-estradiol strongly modulates the GABAergic synaptic transmission but this effect critically depends on the animal age being the most prominent in youngest animals.

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.

Acute treatment with 17beta-estradiol attenuates astrocyte-astrocyte and astrocyte-neuron communication

Astrocytes are now recognized as dynamic signaling elements in the brain. Bidirectional communication between neurons and astrocytes involves integration of neuronal inputs by astrocytes and release of gliotransmitters that modulate neuronal excitability and synaptic transmission. The ovarian steroid hormone, 17beta-estradiol, in addition to its rapid actions on neuronal electrical activity can rapidly alter astrocyte intracellular calcium concentration ([Ca2+]i) through a membrane-associated estrogen receptor. Using calcium imaging and electrophysiological techniques, we investigated the functional consequences of acute treatment with estradiol on astrocyte-astrocyte and astrocyte-neuron communication in mixed hippocampal cultures. Mechanical stimulation of an astrocyte evoked a [Ca2+]i rise in the stimulated astrocyte, which propagated to the surrounding astrocytes as a [Ca2+]i wave. Following acute treatment with estradiol, the amplitude of the [Ca2+]i elevation in astrocytes around the stimulated astrocyte was attenuated. Further, estradiol inhibited the [Ca2+]i rise in individual astrocytes in response to the metabotropic glutamate receptor agonist, trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid. Mechanical stimulation of astrocytes induced [Ca2+]i elevations and electrophysiological responses in adjacent neurons. Estradiol rapidly attenuated the astrocyte-evoked glutamate-mediated [Ca2+]i rise and slow inward current in neurons. Also, the incidence of astrocyte-induced increase in spontaneous postsynaptic current frequency was reduced in the presence of estradiol. The effects of estradiol were stereo-specific and reversible following washout. These findings may indicate that the regulation of neuronal excitability and synaptic transmission by astrocytes is sensitive to rapid estradiol-mediated hormonal control.

17beta-estradiol protects the neonatal brain from hypoxia-ischemia

Hypoxia-ischemia is relatively common in human infants. Hypoxia-ischemia can occur as a result of complications associated with prematurity or birth, frequently leading to altered brain development and cognitive and behavioral deficits that persist throughout life. Despite the relative frequency of neonatal hypoxic-ischemic encephalopathy, the immature brain sustains relatively less damage than an adult who experiences a similar crisis of oxygen and nutrient deprivation. Therefore, factors may be present that protect the developing brain. During late gestation, the infant brain encounters high levels of the steroid hormone 17beta-estradiol. This observation, combined with evidence supporting 17beta-estradiol as a neuroprotective agent, led us to hypothesize that increasing the basal level of 17beta-estradiol would reduce the amount of hypoxia-ischemia induced injury to the neonatal brain. To test that hypothesis we administered 17beta-estradiol using either a repeated dosing paradigm or a single dose paradigm to immature male and female rats. Here we show that the repeated dosing paradigm (three doses of 17beta-estradiol) provided approximately 70% protection of the hippocampus, basal ganglia, and amygdala. By contrast, a single administration of 17beta-estradiol 24 h prior to hypoxia-ischemia conferred little protection. The only exception was the pyramidal layer of the female hippocampus, which was modestly protected (16% reduction in damage). The protection afforded by the multiple administrations of 17beta-estradiol was similar for females and males, with the only exception being the male amygdala, which displayed less damage than the female amgydala. We conclude that 17beta-estradiol acts as a potent neuroprotective agent against hypoxia-ischemia induced damage to the developing brain, and that pretreating infants at risk for hypoxic-ischemic injury may be advisable.

Calcium homeostasis and modulation of synaptic plasticity in the aged brain

The level of intracellular Ca2+ plays a central role in normal and pathological signaling within and between neurons. These processes involve a cascade of events for locally raising and lowering cytosolic Ca2+. As the mechanisms for age-related alteration in Ca2+ dysregulation have been illuminated, hypotheses concerning Ca2+ homeostasis and brain aging have been modified. The idea that senescence is due to pervasive cell loss associated with elevated resting Ca2+ has been replaced by concepts concerning changes in local Ca2+ levels associated with neural activity. This article reviews evidence for a shift in the sources of intracellular Ca2+ characterized by a diminished role for N-methyl-D-aspartate receptors and an increased role for intracellular stores and voltage-dependent Ca2+ channels. Physiological and biological models are outlined, which relate a shift in Ca2+ regulation with changes in cell excitability and synaptic plasticity, resulting in a functional lesion of the hippocampus.

Failure of estradiol to ameliorate global ischemia-induced CA1 sector injury in middle-aged female gerbils

Global forebrain ischemia arising from brief occlusion of the carotid arteries in gerbils produces selective hippocampal CA1 neuronal loss. Pre-treatment with 17beta-estradiol ameliorates, in part, ischemia-induced damage in young animals. Because stroke and cardiac arrest are more likely to occur among elderly individuals, neuroprotective studies in older animals have compelling clinical relevance. We investigated whether estradiol would attenuate ischemia-induced hippocampal neuronal injury in middle-aged (12-14 months) male, intact female, ovariectomized (OVX) female and OVX females treated for 14 days with estradiol. Core temperature telemetry probes were also implanted at the time that estradiol was initiated. Ischemia was induced by bilateral occlusion of the common carotid arteries (5 min), during which time skull temperature was maintained under normothermic conditions. Estradiol blocked the modest spontaneous hyperthermia that normally follows ischemia. However, all four groups exhibited substantial neuronal cell loss in the CA1, assessed at 7 after ischemia. These findings indicate that estradiol pre-treatment under conditions that produce neuroprotection in young animals does not protect against ischemia-induced CA1 cell loss in middle-aged female gerbils.

Estrogen and brain-derived neurotrophic factor (BDNF) in hippocampus: complexity of steroid hormone-growth factor interactions in the adult CNS

In the CNS, there are widespread and diverse interactions between growth factors and estrogen. Here we examine the interactions of estrogen and brain-derived neurotrophic factor (BDNF), two molecules that have historically been studied separately, despite the fact that they seem to share common targets, effects, and mechanisms of action. The demonstration of an estrogen-sensitive response element on the BDNF gene provided an impetus to explore a direct relationship between estrogen and BDNF, and predicted that the effects of estrogen, at least in part, might be due to the induction of BDNF. This hypothesis is discussed with respect to the hippocampus, where substantial evidence has accumulated in favor of it, but alternate hypotheses are also raised. It is suggested that some of the interactions between estrogen and BDNF, as well as the controversies and implications associated with their respective actions, may be best appreciated in light of the ability of BDNF to induce neuropeptide Y (NPY) synthesis in hippocampal neurons. Taken together, this tri-molecular cascade, estrogen-BDNF-NPY, may be important in understanding the hormonal regulation of hippocampal function. It may also be relevant to other regions of the CNS where estrogen is known to exert profound effects, such as amygdala and hypothalamus; and may provide greater insight into neurological disorders and psychiatric illness, including Alzheimer's disease, depression and epilepsy.

Effects of 17beta-estradiol on intracellular calcium changes and neuronal survival after mechanical strain injury in neuronal-glial cultures

The neuroprotective effects of 17beta-estradiol (E2) were investigated using an in vitro model of traumatic brain injury in which cortical neuronal cultures were subjected to mechanical strain-injury. The rise in intracellular calcium ([Ca(2+)](i)) following neuronal injury was reduced by addition of 10 or 100 nM E2 to the cultures immediately following injury. Neuronal damage was measured 24 h after injury by propidium iodide uptake and cell viability by carboxyfluorescein diacetate uptake. Addition of 1, 10, or 100 nM E2 to cell cultures immediately following injury decreased neuronal damage and increased neuronal viability compared to vehicle-treated neurons. These results demonstrate the neuroprotective activity of E2 in an in vitro model of neuronal injury, and suggest that such effects may be related to the ability of E2 to modulate [Ca(2+)](i).

Chronic 17beta-estradiol pretreatment and ischemia-induced hippocampal degeneration and memory impairments: a 6-month survival study

Exogenous administration of estrogen has been shown to significantly reduce ischemia-induced neuronal degeneration. However, the long-term impact of such treatment on neuronal protection and functional recovery remain largely unknown. The present study assessed the effects of a 15-day pretreatment with 17beta-estradiol on memory deficits and neuronal damage up to 6 months following a 10-min global ischemia in rats. Four groups of ovariectomized female rats [sham-operated and ischemic rats receiving a 15-day pretreatment of either the vehicle or 17beta-estradiol (100 microg/kg)] were tested. The 8-arm radial maze and object recognition tests served to evaluate the impact of 17beta-estradiol treatment on ischemia-induced spatial and recognition memory impairments, respectively. Testing in the radial maze was initiated at two distinct time intervals following reperfusion (7 and 120 days) to evaluate changes in memory functions over time. Our findings revealed long-lasting neuroprotective effects of 17beta-estradiol treatment on hippocampal CA1 pyramidal cells in ovariectomized ischemic rats (43.5% greater neuronal survival than observed in vehicle-treated ischemic animals). Importantly, this neuronal protection translated into significant improvements of recognition and spatial memory functions in estradiol-treated ischemic rats.

Estrogens as neuroprotectants against ischemic stroke

Estrogens have proven vasoprotective properties against atherosclerosis that depend on the direct effect on vascular smooth muscle and endothelium and on systemic actions that imply serum lipids, coagulation and fibrinolytic cascades, vasoactive proteins and antioxidant systems. They also have neuroprotective effects against cerebral ischemia that include antioxidant and anti-inflammatory effects, modulation of protein synthesis, inhibition of apoptosis and trophic effects and preservation of microvascular blood flow in the ischemic area. Estrogenic actions depend on activation of specific estrogen receptors that modulate gene expression and produce long-term effects on vascular endothelial and smooth muscle cells, neurons and glia, on interaction with plasma membrane sites that produce rapid non-genomic actions and also on receptor-independent mechanisms. This paper reviews what it is known about the mechanisms underlying the vaso- and neuroprotective effects of estrogens. Experimental and clinical evidences of such protective effects are also discussed. Therapeutical implications for stroke prevention and treatment derived from the available evidence are considered.

Estradiol reduces cumulative mercury and associated disturbances in the hypothalamus-pituitary axis of ovariectomized rats

The aim of this research was to verify the incidence of endocrine dysfunction associated with mercury intoxication in the hypothalamus-pituitary reproductive system of normally cycling or castrated female rats and the possible protective action of estrogen replacement therapy. We found no differences in the frequency of estrus cycle stages (diestrus I, diestrus II, proestrus, and estrus) in normally cycling female rats during 54 days of daily oral administration of 0.004, 0.02, and 1 mg/kg MeHgCl. Conversely, the higher dose (1 mg/kg) induced a significant decrease in content of luteinizing hormone releasing hormone (LHRH) into the medial hypothalamus when administered daily during 3 days in ovariectomized rats. This effect was associated with increased levels of mercury found in the anterior pituitary gland and medial hypothalamus, rather than the anterior and posterior hypothalamus, striatum or cerebellum. A decrease in plasma levels of luteinizing hormone (LH) was also detected after administration of 7.5 mg/kg MeHgCl. These disturbances in LHRH and LH secretion induced by mercury were abolished or superimposed (respectively) by estrogenic replacement therapy (0.025 mg/kg 17beta estradiol cypionate, intramuscular). These effects were associated with a significant reduction in mercury content of the anterior pituitary gland and medial hypothalamus, suggesting a protective estrogenic effect.

Activation of membrane estrogen receptors induce pro-survival kinases

Experimental and epidemiological data suggest a neuroprotective role for estrogen (E(2)). We have recently shown that, in PC12 cells, non-permeable estradiol conjugated to bovine serum albumin (BSA) prevent serum-deprivation induced apoptosis through activation of specific membrane estrogen receptors (mER). In the present study, we explored in detail the early signaling events involved in this anti-apoptotic action, downstream to activation of mER. Our findings suggest that mER is associated to G-proteins, and its activation with non-permeable E(2)-BSA results in the activation of the following downstream pro-survival kinases pathways: (1) the PKB/Akt pathway, (2) the Src-->MEK-->ERK kinases and finally (3) the MAPK-->ERK kinases. Activation of these pro-survival signals leads to CREB phosphorylation and NFkappaB nuclear translocation, two transcription factors controlling the expression of anti-apoptotic Bcl-2 proteins. These data suggest that major pro-survival kinases are involved in the mER-mediated anti-apoptotic effects of estrogen. This is further supported by experiments with specific kinases inhibitors, which partially but significantly reversed the mER-mediated anti-apoptotic effect of E(2)-BSA. Our findings suggest that estrogen act via mER as potent cytoprotective factors, downstream activating pro-survival kinases, assuring thus an efficient and multipotent activation of the anti-apoptotic machinery.

Estrogen prevents glutamate-induced apoptosis in C6 glioma cells by a receptor-mediated mechanism

Estrogen-mediated neuroprotection is well established; however, no single mechanism of action for this effect has yet been established. As glial cells are integral for both the intact and injured nervous system, we hypothesized that estrogen-mediated neuroprotection may partly be attributed to attenuation of glial cell apoptosis, allowing them to protect neurons following injury. To assess the protective effects of estrogen on glia, C6 rat glioma cells were treated for 24 h with 500 microM glutamate. Cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, and apoptosis was confirmed by cell morphology and DNA fragmentation. Pretreatment with 10 nM 17beta-estradiol (estrogen) increased cell viability and attenuated apoptosis. Treatment with the stereoisomer 17alpha-estradiol, or estrogen plus estrogen receptor antagonist ICI 182,780, was significantly less effective, indicating that cytoprotection was receptor-mediated. Estrogen treatment upregulated expression of estrogen receptor alpha. Cell impermeable bovine serum albumin-conjugated estrogen was also protective, indicating activation of estrogen receptors on the cell membrane. Intracellular free [Ca2+] was increased after glutamate treatment. This increase was attenuated in cells pretreated with estrogen. Glutamate increased the activity of pro-apoptotic proteases, such as calpain and caspase-3, and these protease activities were significantly attenuated by estrogen. The mechanism by which estrogen decreased intracellular Ca2+ was examined by assaying cell viability after using inhibitors that either blocked extracellular Ca2+ influx or prevented the release of intracellular Ca2+ stores. While several inhibitors increased cell viability in glutamate-treated cells, none were as protective as estrogen, and estrogen co-treatment significantly increased cell viability. These findings indicate that estrogen-mediated cytoprotection may be related to effects on Ca2+ entry but that these effects are not limited to any one of these Ca2+ entry points alone.

From clinical evidence to molecular mechanisms underlying neuroprotection afforded by estrogens

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

Non-nuclear estrogen receptor beta and alpha in the hippocampus of male and female rats

Estrogens play important roles in the brain, acting through two receptor types, ERalpha and ERbeta, both recognized as transcription factors. In this study, we investigated the ERbeta mRNA and protein expression in the male and female rat brain, focusing on the hippocampus, and comparing with well-known ERalpha expression patterns. Extranuclear ERbeta localization, as shown by light microscopic immunocytochemistry and tissue fractionation experiments, was noted in the hippocampus, whereas nuclear ERbeta was present in the amygdala. Despite these marked differences in subcellular localizations, similar expression levels of ERbeta proteins as well as the profile of ERbeta mRNA isoforms were observed in the two brain structures. ERalpha was localized to the nucleus more so than ERbeta, yet not without an extranuclear component. Our results suggest that cytoplasmic estrogen receptors may play an important role in hippocampal physiology.

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

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

Sex hormones and brain aging

Sex steroids exert pleiotropic effects in the nervous system, preserving neural function and promoting neuronal survival. Therefore, the age-related decrease in sex steroids may have a negative impact on neural function. Progesterone, testosterone and estradiol prevent neuronal loss in the central nervous system in different experimental animal models of neurodegeneration. Furthermore, progesterone and its reduced derivatives dihydroprogesterone and tetrahydroprogesterone reduce aging-associated morphological abnormalities of myelin and aging-associated myelin fiber loss in rat peripheral nerves. However, the results from hormone replacement studies in humans are thus far inconclusive. A possible alternative to hormonal replacement therapy is to increase local steroidogenesis by neural tissues, which express enzymes for steroid synthesis and metabolism. Proteins involved in the intramitochondrial trafficking of cholesterol, the first step in steroidogenesis, such as the peripheral-type benzodiazepine receptor and the steroidogenic acute regulatory protein, are up-regulated in the nervous system after injury. Furthermore, steroidogenic acute regulatory protein expression is increased in the brain of 24-month-old rats compared with young adult rats. This suggests that brain steroidogenesis may be modified in adaptation to neurodegenerative conditions and to the brain aging process. Furthermore, recent studies have shown that local formation of estradiol in the brain, by the enzyme aromatase, is neuroprotective. Therefore, steroidogenic acute regulatory protein, peripheral-type benzodiazepine receptor and aromatase are attractive pharmacological targets to promote neuroprotection in the aged brain.

Neuroprotective Anti-Apoptosis Effect of Estrogens in Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading cause of death and functional disability in western countries, affecting mostly young patients. Despite intense and sustained efforts deployed for the development of new therapeutic strategies, no clinical benefit has been shown by any of the investigated compounds. Increasing attention has been drawn during the past two decades to the neuroprotective effects of estrogens, although most of the available data relate to ischemic brain injury. The purpose of the present study was to investigate the potential neuroprotective value of estrogens in TBI as a therapeutic modality. For this purpose, a contusion was created in the parietal cortex by dynamic cortical deformation in two groups of 10 Sprague-Dawley male rats. Following the injury, treated animals received conjugated estrogens for 3 days, using a subcutaneously implanted osmotic pump. Animals were then sacrificed, and TUNEL, anti-active Caspase 3, bcl-2, and bax labeling were performed in paraffin-embedded brain sections, allowing for comparative and quantitative analysis. In estrogen-treated animals, there was a marked and significant reduction of apoptosis in comparison with non-treated animals. The reduction in TUNEL and active Caspase 3 staining was similar and close to 50%. Optical analysis of histological slides prepared by bcl-2 labeling showed a significant increase in bcl-2 expression in estrogen-treated animals compared to non-treated animals. On the contrary, bax expression was not influenced by hormonal treatment, and no difference could be noticed between the two groups. These results support the potential therapeutic value of estrogens in TBI and further clarify their mode of action.

Effects of estrogen and progesterone treatment on rat hippocampal NMDA receptors: relationship to Morris water maze performance

Estrogen modulates NMDA receptors function in the brain. It increases both dendritic spine density and synapse number in the hippocampus, an effect that can be blocked by NMDA antagonist. In this study, we investigated the effect of 17beta-estradiol and progesterone treatment on NMDA receptors in ovariectomized rats. Two different doses were used for 10 weeks. Receptor autoradiography was done on brain sections using [(3)H] MK-801 as a ligand. Our results showed a significant increase in [(3)H] MK-801 binding in the dentate gyrus, CA3 and CA4 areas of the hippocampus of ovariectomized compared to sham operated rats. In addition, we observed similar changes in CA1. 17beta-estradiol treatment in both doses reduced the binding back to the normal level while progesterone treatment did not show any effect. Spatial reference memory was tested on Morris water maze task. Ovariectomy severely impaired spatial reference memory. Estradiol but not progesterone treatment significantly improved the memory performance of the ovariectomized rats. Low dose treatment showed better learning than high dose estrogen treatment. The decrease in the antagonist sites by estradiol treatment could result in an increase in the sensitivity of the hippocampus to the excitatory stimulation by glutamate system and hence the effect of estradiol on learning and memory. The changes of NMDA receptors in the hippocampus support the concept that estrogen-enhancing effect on spatial reference memory could be through the enhancing of NMDA function.

Estrogen attenuates hypoxic-ischemic brain injury in neonatal rats

Estrogen is neuroprotective in adult animals. We wished to determine if estrogen protects against brain injury in the newborn. Four-day-old rat pups were treated with subcutaneously implanted pellets containing 0.05 mg (2.4 microg/day) of 17beta-estradiol or vehicle, designed to release the estrogen over 21 days. At 7 days old the pups had the right carotid artery ligated followed by 2.5 h of 8% oxygen. Brain damage was evaluated by weight deficit of the right hemisphere at 22 days following hypoxia. Estradiol treatments reduced brain weight loss from -17.4+/-2.8% S.E.M. in the vehicle group (n=32) to -9.3+/-2.7% in the treated group (n=32, P<0.05). Brain cortex thiobarbituric acid reacting substances and caspase activities were assessed 24 h after reoxygenation. Estradiol significantly reduced a hypoxia-induced increase in brain thiobarbituric acid reactive substances (P<0.05). Levels of caspase-3, -8 and -9 activity increased due to hypoxia-ischemia. Estradiol had no effect on caspase activity. Estradiol reduced brain injury in the neonatal rat.

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

It is well-established that 17beta-estradiol (17beta-E(2)) confers neuroprotection to male and female rats exposed to focal cerebral ischemia, while less is known about the effects of the hormone under conditions of transient global ischemia. Since translocation of cytochrome c from the mitochondria to the cytosol is a critical step in apoptotic cell death after cerebral ischemia, we have investigated whether 17beta-E(2) interferes with such mechanism to exert neuroprotection. Global ischemia, induced in male Wistar rats by 5-min 4 vessel occlusion (4VO), resulted in a significant increase of cytosolic cytochrome c (cyt-c) levels as detected by Western blotting at 6h after reperfusion. 17beta-E(2) (0.2mg/kg, i.p.) given 1h before ischemia minimized cytochrome c translocation and the latter effect was partially reversed by tamoxifen (0.25mg/kg, i.p.). Bilateral cell counting revealed that delayed hippocampal damage typically caused by 4VO was abolished by 17beta-E(2) and this was partially reversed by tamoxifen in the CA3 subregion, but not in CA1/CA2 or CA4. These findings provide the original observation that 17beta-E(2) reduces delayed hippocampal damage caused by 4VO in male rats and blocks cytochrome c translocation during the early stages of neuronal death, thus providing an important mechanism involved in estrogen-mediated neuroprotection.

Decreased apoptosis in the forebrain of adult male medaka (Oryzias latipes) after aqueous exposure to ethinylestradiol

Endocrine disrupting compounds (EDCs), especially those that are estrogenic, are an issue of growing concern because they may ultimately adversely affect wildlife survival. 17-beta-Estradiol and its synthetic counterpart, 17-alpha-ethinylestradiol, two common EDCs, are associated with intersex conditions and impaired male reproductive behavior in fish. Male and female Japanese medaka (Oryzias latipes) were exposed to 10 ng/l ethinylestradiol for 6 months. Using terminal dideoxynucleotidyl-mediated dUTP nick end-labeling (TUNEL) to quantitate cell death, we found that ethinylestradiol-exposed males had significantly fewer apoptotic cells in the forebrain compared to untreated males and exposed females. Our results show that the effects of ethinylestradiol exposure are highly variable among individuals of the same species and even within tissues of the same individual. Thus, when examining the effects of EDCs on natural populations, data from a variety of tissues should be examined and the interpretation of any effects should include consideration of tissue-specific processes.

The neuroprotective effects of estrogen in SK-N-SH neuroblastoma cell cultures

Estrogen has been considered to be a neuroprotectant and a neuromodulator in many neuronal cell lines and tissue preparations. The protective effects of estrogen may be mediated through classical estrogen receptors (ERs), or may be due to its anti-oxidant properties which are independent of receptors. The current studies show that 17beta-estradiol (E2) is neuroprotective against beta-amyloid protein 25-35 (Abeta)-, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-, high density culture condition-, and serum deprivation-induced neuronal death in SK-N-SH human neuroblastoma cells. SK-N-SH cells express ERbeta, but not ERalpha, as detected by Western blot analysis. Among all the insults, MPTP, high density culture and serum deprivation induce apoptotic cell death in this cell system as detected by ELISA determination of mono/oligonucleosomes and DNA laddering, while Abeta induces necrotic cell death. The protective effects of E2 are abolished by the addition of tamoxifen and ICI 182,780 in the MPTP treated cells, but not in the other models, suggesting that the effect of E2 in the MPTP model is probably associated with activation of ERbeta. The addition of ICI 182,780 shows a mitogenic effect in SK-N-SH cells in the presence of E2 in control culture or in the Abeta treated groups. Also, ICI 182,780 induced expression of ERalpha. Collectively, the current studies suggest that E2 is neuroprotective in apoptotic and necrotic death induced by multiple insults in SK-N-SH human neuroblastoma cells. Involvement of ER is insult type dependent. ICI 182,780 is able to influence the expression of ERs, probably through upregulation of ERalpha when ERbeta is totally antagonized.

Neuroprotection by estrogen in animal models of global and focal ischemia

Estrogen has been demonstrated to protect against brain injury, neurodegeneration, and cognitive decline. Furthermore, estrogen seems to specifically protect cortical and hippocampal neurons from ischemic injury. Here our data evaluating the neuroprotective effects of estrogens, the selective estrogen receptor modulators (SERMs), and estrogen receptor alpha- and beta-selective ligands in animal models of ischemic injury are discussed. In rats and mice, the middle cerebral artery occlusion (MCAO) model was used as models representing cerebrovascular stroke, while in gerbils the two-vessel occlusion model, resenting acute heart attack, was used. Using focal ischemia in ovariectomized ERalphaKO, ERbetaKO, and wild-type mice, we clearly established that the ERalpha subtype is the critical ER-mediating neuroprotection in mouse focal ischemia. Because of the characteristic blood supply of the gerbil, the gerbil global ischemia model was used to evaluate the neuroprotective effects of estrogen, SERMs, and ERalpha- and ERbeta-selective compounds in the hippocampus. Analysis of neurogranin mRNA, a marker of viability of hippocampal neurons, with in situ hybridization, revealed that estrogen treatment resulted in a complete protection in the CA1 regions not only when administered before, but also when given 1 hour after occlusion. Our in vivo binding studies with (125)I-estrogen in gerbils revealed the presence of nuclear estrogen binding sites primarily in CA1 neurons, but not in the CA3 region, as we saw in rats and mice. Together, these observations demonstrate that estrogen protects from ischemic injury in both the focal and global ischemia models by acting primarily via classical nuclear receptors.

The use of estrogens and related compounds in the treatment of damage from cerebral ischemia

There are 750,000 new cases of stroke each year in the United States, and brain damage from stroke leads to high health care costs and disabilities. Needed, but currently not available, are therapies that can be administered prior to, during, or after cerebral ischemia that reduce or eliminate neuronal damage from stroke. To address this issue, we began to assess the neuroprotective effects of estrogens and related compounds in stroke neuroprotection to determine whether these compounds had potential for clinical application. First, we demonstrated that 17 beta-estradiol (E2) pretreatment exerted potent neuroprotection of the cerebral cortex over a wide dose range and pretreatment interval. Thereafter, we assessed the ability of a variety of non-feminizing estrogens to protect brain tissue from stroke. We observed that pretreatment with 17 alpha-estradiol, the complete enantiomer of E2 (ENT-E2), 2-adamantylestrone, and the enantiomer of 17-desoxyestradiol, were as effective as E2 in pretreatment protection from stroke damage. These data suggest that non-estrogen receptor mechanisms are involved in brain neuroprotection under our treatment conditions. We then determined whether the observed E2 protection could be extended to times after the onset of the cerebral ischemic event. Using a formulation of E2 that rapidly delivers the steroid, a necessary condition for acute therapy of an ongoing stroke, we demonstrated that 100 mg E2/kg could protect brain tissue for up to 3 h after the onset of the stroke. To determine whether this therapeutic window could be extended with higher doses of the steroid, we conducted a dose-response assessment of E2 when administered at 6 h after the onset of the ischemic event. While the effectiveness of the 100 micro g E2/kg was reduced at this time interval, higher doses of E2 were effective. E2, at doses of 500 and 1000 micro g/kg, reduced infarct volume by more than 50%, even with this 6-h delay in treatment. Collectively, these data indicate that estrogens could prove to be useful therapies in preventing brain damage from strokes.

17 Beta-oestradiol attenuates dexamethasone-induced lethal and sublethal neuronal damage in the striatum and hippocampus

Abnormal corticosteroid release is extensively associated with mood disorders. This association may result from the toxic actions of endogenous corticosteroids which can induce apoptosis of hippocampal neurons. Similarly, dexamethasone, a synthetic corticosteroid, can induce lethal and sublethal damage to rat hippocampal and striatal neurons and can result in steroid-induced psychoses in humans. The experiments reported here tested the hypothesis that pre-treatment with oestrogen would also attenuate dexamethasone-induced neuronal damage as oestrogens have neuroprotective actions against a variety of insults and falling levels of oestrogen are associated with increased vulnerability to mood disorders. Male Sprague-Dawley rats received three systemic injections which were a combination of vehicle, 17-beta-oestradiol (0.2 mg/kg, s.c.), the oestrogen receptor antagonist tamoxifen (10 mg/kg, s.c.) and dexamethasone (0.7 mg/kg, i.p.) and were killed 24 h after the final injection. Injections of dexamethasone (when preceded by vehicle injections) resulted in elevated levels of apoptosis and sub-lethal damage, as demonstrated by reduced levels of microtubule-associated protein-2-immunopositive neurons, in the striatum and hippocampus. This damage was regional with the dorsomedial caudate putamen and the dentate gyrus and CA1 and CA3 hippocampal sub-fields being particularly affected. Pretreatment with oestrogen substantially attenuated the dexamethasone-induced neuronal damage. This oestrogen-induced neuronal protection was in turn virtually eliminated by giving an initial injection of tamoxifen. These results suggest, therefore, that oestrogens can protect from corticosteroid-induced neuronal damage via an oestrogen receptor-mediated process.

7-Hydroxylated epiandrosterone (7-OH-EPIA) reduces ischaemia-induced neuronal damage both in vivo and in vitro

Recent evidence suggests that steroids such as oestradiol reduce ischaemia-induced neurodegeneration in both in vitro and in vivo models. A cytochrome P450 enzyme termed cyp7b that 7-hydroxylates many steroids is expressed at high levels in brain, although the role of 7-hydroxylated steroids is unknown. We have tested the hypothesis that the steroid-mediated neuroprotection is dependent on the formation of 7-hydroxy metabolites. Organotypic hippocampal slice cultures were prepared from Wistar rat pups and maintained in vitro for 14 days. Cultures were then exposed to 3 h hypoxia and neuronal damage assessed 24 h later using propidium iodide fluorescence as a marker of cell damage. Neurodegeneration occurred primarily in the CA1 pyramidal cell layer. The steroids oestradiol, dehydroepiandrosterone and epiandrosterone (EPIA) were devoid of neuroprotective efficacy when present at 100 nM pre-, during and post-hypoxia. The 7-hydroxy metabolites of EPIA, 7alpha-OH-EPIA and 7beta-OH-EPIA significantly reduced neurotoxicity at 100 nM and 10 nM. 7beta-OH-EPIA was also neuroprotective in two in vivo rat models of cerebral ischaemia: 0.1 mg/kg 7beta-OH-EPIA significantly reduced hippocampal cell loss in a model of global forebrain ischaemia, whereas 0.03 mg/kg was neuroprotective in a model of focal ischaemia even when administration was delayed until 6 h after the onset of ischaemia. Taken together, these data demonstrate that 7-hydroxylation of steroids confers neuroprotective efficacy, and that 7beta-OH-epiandrosterone represents a novel class of neuroprotective compounds with potential for use in acute neurodegenerative diseases.

Estrogen and selective estrogen receptor modulators: neuroprotection in the Women's Health Initiative era

Estrogen has been comprehensively studied as a neuroprotective agent in women, animals, and a variety of in vitro models of neural injury and degeneration. Most data suggest that estrogen can benefit the ischemic brain and reduce cell death. However, recent data from the Women's Health Initiative have raised concerns about the utility and safety of chronic estrogen use in women. While estrogen is a potent and reproducible neuroprotectant in animals and in vitro, its current administration in women has had unanticipated and paradoxical effects. Nonetheless, estrogen's diverse actions make it an ideal prototype for developing new neuroprotectants such as selective estrogen receptor modulators (SERMs). SERMs represent a class of drugs with mixed estrogen agonistic and antagonistic activity. Experimental and clinical data suggest a neuroprotective role for SERMs in normal and injured brain. The discrepancy among observational studies, preclinical data, and clinical trials emphasizes the need for further study of the mechanisms leading to the increased incidence of stroke observed in postmenopausal women. Research is still needed to optimize combined or estrogen alone hormone replacement therapy options as well as the prevention/management of cerebrovascular/ central nervous system disorders. This review critiques estrogen and SERMs' neuroprotective potential in experimental and clinical studies of stroke and cerebrovascular disease.

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

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

Estrogen prevents the loss of CA1 hippocampal neurons in gerbils after ischemic injury

Estrogen replacement therapy is thought to attenuate the incidence of Alzheimer's disease in women and enhance cognitive functions. In rodents, estrogen protects cerebral cortical neurons from ischemic injury and cultured neurons from a variety of perturbations. Because few nuclear estrogen receptors have been detected in the dorsal hippocampus, the present studies used a global ischemia model to evaluate the neuroprotective actions of estrogen in this region. Ovariectomized gerbils were treated with placebo, 0.5 mg or 1 mg pellets of estradiol for 13 days. On day 7, the common carotid arteries were occluded for 5 min and on day 13 the animals were killed. Analysis of neurogranin mRNA, a marker of hippocampal neurons, with in situ hybridization revealed a dramatic and selective loss of CA1 neurons in the placebo-treated ovariectomized gerbils, whereas both 0.5 mg and 1 mg pellets of 17beta-estradiol prevented cell loss. Subsequent studies showed that a variety of estrogens, including diethylstilbestrol, estrone and 17alpha-estradiol as well as vitamin E, also protected CA1 neurons from ischemic injury in ovariectomized gerbils, whereas treatment with the estrogen antagonist tamoxifen was ineffective. The results of in vivo binding studies with 17alpha-iodovinyl-11beta-methoxyestradiol revealed a concentration of nuclear estrogen binding sites in the CA1 region of the ovariectomized gerbil brain, whereas binding in other hippocampal regions was limited. Moreover, the binding studies revealed that the regional pattern of binding was not altered after ischemic injury, with the exception of the hippocampus, where the binding sites were attenuated in ovariectomized animals with time after ischemic injury. Together, these data demonstrate that a variety of steroidal and non-steroidal estrogens are potent neuroprotective agents in an animal model of global ischemia, agents that protect neurons critical for learning and memory and susceptible to neurodegenerative diseases.

Estrogen is neuroprotective via an apolipoprotein E-dependent mechanism in a mouse model of global ischemia

Estrogen can ameliorate brain damage in experimental models of focal cerebral ischemia., estrogen increases levels of apolipoprotein E (apoE), which also has neuroprotective effects in brain injury. The authors tested the hypotheses that physiologically relevant levels of 17beta-estradiol are neuroprotective in global cerebral ischemia and that neuroprotection is mediated via apoE. In the first study, subcutaneous implants of 17beta-estradiol were tested in female C57Bl/6J mice (ovariectomized and nonovariectomized) and plasma levels measured by radioimmunoassay to validate that physiologically relevant levels could be achieved. In the second study, female C57Bl/6J and apoE-deficient mice were ovariectomized and implanted with 17beta-estradiol or placebo pellet. Two weeks later, transient global ischemia was induced by bilateral carotid artery occlusion and the mice killed after 72 hours. Ischemic and normal neurons were counted in the caudate nucleus and CA1 pyramidal cell layer and the percentage of neuronal damage was compared between the treated groups. In C57Bl/6J mice, there was less neuronal damage in the 17beta-estradiol-treated group compared with placebo group in the caudate nucleus (15 +/- 20% versus 39 +/- 27%, = 0.02) and in the CA1 pyramidal cell layer (1.8 +/- 2% versus 10 +/- 14%, = 0.08). In contrast, neuronal damage was not significantly different between the 17beta-estradiol and placebo groups in apoE-deficient mice in the caudate nucleus (47 +/- 35% versus 53 +/- 29%, = 0.7) or in the CA1 pyramidal cell layer (24 +/- 19% versus 24 +/- 19%, = 1.0). The data indicate a neuroprotective role for estrogen in global ischemia, the mechanism of which is apoE-dependent.

Estrogen and brain vulnerability

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

Estrogen protects against global ischemia-induced neuronal death and prevents activation of apoptotic signaling cascades in the hippocampal CA1

The importance of postmenopausal estrogen replacement therapy in affording protection against the selective and delayed neuronal death associated with cardiac arrest or cardiac surgery in women remains controversial. Here we report that exogenous estrogen at levels that are physiological for hormone replacement in postmenopausal women affords protection against global ischemia-induced neuronal death and prevents activation of apoptotic signaling cascades in the hippocampal CA1 of male gerbils. Global ischemia induced a marked increase in activated caspase-3 in CA1, evident at 6 hr after ischemia. Global ischemia induced a marked upregulation of the proapoptotic neurotrophin receptor p75(NTR) in CA1, evident at 48 hr. p75(NTR) expression was induced primarily in terminal deoxynucleotidyl transferase-mediated UTP nick-end labeling-positive cells, indicating expression in neurons undergoing apoptosis. Global ischemia also induced a marked downregulation of mRNA encoding the AMPA receptor GluR2 subunit in CA1. Caspase-3, p75(NTR), and GluR2 were not significantly changed in CA3 and dentate gyrus, indicating that the ischemia-induced changes in gene expression were cell specific. Exogenous estrogen attenuated the ischemia-induced increases in activated caspase-3 and blocked the increase in p75(NTR) in post-ischemic CA1 neurons but did not prevent ischemia-induced downregulation of GluR2. These findings demonstrate that long-term estrogen at physiological levels ameliorates ischemia-induced hippocampal injury and indicate that estrogen intervenes at the level of apoptotic signaling cascades to prevent onset of death in neurons otherwise "destined to die."