Meharvan Singh

Ovarian steroid deprivation results in a reversible learning impairment and compromised cholinergic function in female Sprague-Dawley rats

We hypothesized that estradiol (E2) serves as a neurotrophomodulatory substance for basal forebrain cholinergic neurons thought to be involved in learning and memory. Learning/memory was assessed using the two-way active avoidance paradigm and the Morris water task. Female Sprague-Dawley rats were either ovariectomized (OVX) or OVX for 3 weeks, followed by s.c. implantation of a Silastic pellet containing 17-beta E2 (E2 pellet), resulting in a replacement of E2 to physiological levels. Ovary-intact (INTACT) animals served as our positive control. Active avoidance behavior and choline acetyltransferase (ChAT) activity in the frontal cortex and hippocampus were assessed at 5 and 28 weeks postovariectomy while performance on the Morris water task and high-affinity choline uptake (HACU) were measured only at the 5-week time point. At the 5-week time point, E2 replacement caused a significant elevation in the level of active avoidance performance relative to OVX animals. At the 28-week time point, OVX animals demonstrated a significantly lower number of avoidances relative to controls (61%) whereas E2-pellet animals not only demonstrated superior performance relative to OVX animals but also showed an accelerated rate of learning. Morris water task performance, on the other hand, was not significantly affected by estrogenic milieu despite a trend towards better performance in the E2-pellet group. Neurochemical analyses revealed that 5 weeks of ovariectomy was sufficient to reduce HACU in both the frontal cortex and hippocampus by 24 and 34%, respectively, while E2 replacement was successful in elevating HACU relative to OVX animals in both regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel Mechanisms of Estrogen Action in the Brain: New Players in an Old Story

Estrogen elicits a selective enhancement of the growth and differentiation of axons and dendrites (neurites) in the developing brain. Widespread colocalization of estrogen and neurotrophin receptors (trk) within estrogen and neurotrophin targets, including neurons of the cerebral cortex, sensory ganglia, and PC12 cells, has been shown to result in differential and reciprocal transcriptional regulation of these receptors by their ligands. In addition, estrogen and neurotrophin receptor coexpression leads to convergence or cross-coupling of their signaling pathways, particularly at the level of the mitogen-activated protein (MAP) kinase cascade. 17beta-Estradiol elicits rapid (within 5-15 min) and sustained (at least 2 h) tyrosine phosphorylation and activation of the MAP kinases, extracellular-signal regulated kinase (ERK)1, and ERK2, which is successfully inhibited by the MAP kinase/ERK kinase 1 inhibitor PD98059, but not by the estrogen receptor (ER) antagonist ICI 182,780 and also does not appear to result from estradiol-induced activation of trk. Furthermore, the ability of estradiol to phosphorylate ERK persists even in ER-alpha knockout mice, implicating other estrogen receptors such as ER-beta in these actions of estradiol. The existence of an estrogen receptor-containing, multimeric complex consisting of hsp90, src, and B-Raf also suggests a direct link between the estrogen receptor and the MAP kinase signaling cascade. Collectively, these novel findings, coupled with our growing understanding of additional signaling substrates utilized by estrogen, provide alternative mechanisms for estrogen action in the developing brain which could explain not only some of the very rapid effects of estrogen, but also the ability of estrogen and neurotrophins to regulate the same broad array of cytoskeletal and growth-associated genes involved in neurite growth and differentiation. This review expands the usually restrictive view of estrogen action in the brain beyond the confines of sexual differentiation and reproductive neuroendocrine function. It considers the much broader question of estrogen as a neural growth factor with important influences on the development, survival, plasticity, regeneration, and aging of the mammalian brain and supports the view that the estrogen receptor is not only a ligand-induced transcriptional enhancer but also a mediator of rapid, nongenomic events.

Role of Estrogen Replacement Therapy in Memory Enhancement and the Prevention of Neuronal Loss Associated With Alzheimer's Disease

Recent evidence supports a role for estrogens in both normal neural development and neuronal maintenance throughout life. Women spend 25-33% of their life in an estrogen-deprived state and retrospective studies have shown an inverse correlation between dose and duration of estrogen replacement therapy (ERT) and incidence of Alzheimers disease (AD), suggesting a role for estrogen in the prevention and/or treatment of neurodegenerative diseases. To explore these observations further, an animal model was developed using ovariectomy (OVX) and ovariectomy with estradiol replacement (E2) in female Sprague-Dawley rats to mimic postmenopausal changes. Using an active-avoidance paradigm and a spatial memory task, the effects of estrogen deprivation were tested on memory-related behaviors. OVX caused a decline in avoidance behavior, and estrogen replacement normalized the response. In the Morris water task of spatial memory, OVX animals showed normal spatial learning but were deficient in spatial memory, an effect that was prevented by estrogen treatment. Together these data indicate that OVX in rats results in an estrogen-reversible impairment of learning/memory behavior. Because a plethora of information has been generated that links decline in memory-related behavior to dysfunction of cholinergic neurons, the effects of estrogens on cholinergic neurons were tested. We demonstrated that OVX causes a decrease in high affinity choline uptake and choline acetyltransferase activity in the hippocampus and frontal cortex; ERT reverses this effect. Further, we showed that estrogens promote the expression of mRNA for brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), 2 neurotrophic substances that have been shown to ameliorate the effects of age and injury on cholinergic neurons. Tissue culture models were used to evaluate whether estrogen treatment increases the survival of neurons when exposed to a variety of insults. 17-beta-Estradiol (beta-E2) protects cells from the neurotoxic effects of serum deprivation and hypoglycemia in human neuroblastoma cell lines. We have also observed that 17-alpha-estradiol (alpha-E2), a weak estrogen, shows neuroprotective efficacy in the SK-N-SH cell line at concentrations equivalent to beta-E2. Finally, we have observed that tamoxifen, a classic estrogen antagonist, blocks only one-third of the neuroprotective effects of either alpha-E2 or beta-E2. Collectively, these results indicate that estrogen is behaviorally active in tests of learning/ memory; activates basal forebrain cholinergic neurons and neurotrophin expression; and is neuroprotective for human neuronal cultures. We conclude that estrogen may be a useful therapy for AD and other neurodegenerative diseases.

Ovarian hormones elicit phosphorylation of Akt and extracellular-signal regulated kinase in explants of the cerebral cortex.

Estradiol and progesterone both have been demonstrated to afford neuroprotection against various insults. In an attempt to identify potential mechanisms underlying these neuroprotective effects, two key elements within signal transduction pathways linked to neuroprotection were evaluated. In mouse cerebral cortical explants, both estradiol and progesterone elicited the phosphorylation of Akt, a downstream effector of the phosphoinositide-3 (PI-3) kinase pathway. Progesterone also elicited the phosphorylation of extracellular-signal regulated kinase (ERK), a component of the mitogen-activated protein kinase (MAPK) pathway. These effects were not inhibited by the progesterone receptor antagonist, RU486. However, inhibition of either MAPK/ERK kinase with PD98059 or PI-3 kinase with LY294002 successfully inhibited progesterones actions on ERK and Akt, respectively. Collectively, the data offer novel mechanisms for both progesterone and estrogen action in the central nervous system, demonstrating the functional and mechanistic diversity of gonadal hormones and supporting their neuroprotective potential for such neurodegenerative disorders as Alzheimer disease.

Matrix metalloproteinase-9 in cerebral aneurysms

OBJECTIVE Generalized disruption of arterial wall morphological changes in patients harboring cerebral aneurysms has been documented; however, little is known regarding the pathogenesis of these changes. To explore the role of the elastolytic gelatinase, matrix metalloproteinase-9 (MMP-9), levels of this enzyme in the wall of intracranial aneurysms were compared with those in both intracranial and extracranial arteries. The tissue levels of its major inhibitor, tissue inhibitor of metalloproteinase (TIMP), were measured in these tissues as well. The activity of MMP-9 in plasma was also evaluated. METHODS The aneurysm wall was excised from three of six patients undergoing craniotomies for aneurysm clipping. A 1-cm segment of superficial temporal artery (STA) was obtained from each of six patients. Additional STAs were obtained from six patients in the control group who were undergoing craniotomies for nonvascular disease. An intracranial artery was also obtained from the anterior temporal neocortical resection of a patient undergoing a craniotomy for mesial temporal sclerosis. MMP-9 and TIMP levels were determined via Western blot analysis. Using substrate gel Zymography, MMP-9 plasma activity was determined for a separate cohort of patients with aneurysms (n = 6) and patients in the control group (n = 6). RESULTS MMP-9 and TIMP levels in the aneurysm wall were markedly increased beyond levels in both extracranial arteries (STAs from patients with aneurysms and patients in the control group) and the intracranial artery. There were no differences in the levels of MMP-9 in the STAs of patients harboring aneurysms when compared with patients in the control group. Also, no differences were noted in plasma MMP-9 activity. CONCLUSION Local rather than systemic perturbations in MMP-9 levels may contribute to the matrix disruption associated with cerebral aneurysms. This local up-regulation is not the result of TIMP down-regulation. The lack of increased systemic metalloproteinase activity precludes the use of plasma MMP-9 activity as a screening tool for presymptomatic aneurysms. However, local therapeutic modulation of MMP-9 activity may help arrest aneurysm progression.

Progesterone increases brain-derived neuroptrophic factor expression and protects against glutamate toxicity in a mitogen-activated protein kinase- and phosphoinositide-3 kinase-dependent manner in cerebral cortical explants.

The higher prevalence and risk for Alzheimers disease in women relative to men has been partially attributed to the precipitous decline in gonadal hormone levels that occurs in women following the menopause. Although considerable attention has been focused on the consequence of estrogen loss, and thus estrogens neuroprotective potential, it is important to recognize that the menopause results in a precipitous decline in progesterone levels as well. In fact, progesterone is neuroprotective, although the precise mechanisms involved remain unclear. Based on our previous observation that progesterone elicits the phosphorylation of ERK and Akt, key effectors of the neuroprotective mitogen‐activated protein kinase (MAPK) and phosphoinositide‐3 kinase (PI3‐K) pathways, respectively, we determined whether activation of either of these pathways was necessary for progesterone‐induced protection. With organotypic explants (slice culture) of the cerebral cortex, we found that progesterone protected against glutamate‐induced toxicity. Furthermore, these protective effects were inhibited by either the MEK1/2 inhibitor UO126 or the PI3‐K inhibitor LY294002, supporting the requirement for both the MAPK and PI3‐K pathways in progesterone‐induced protection. In addition, at a concentration and duration of treatment consistent with our neuroprotection data, progesterone also increased the expression of brain‐derived neurotrophic factor (BDNF), at the level of both protein and mRNA. This induction of BDNF may be relevant to the protective effects of progesterone, in that inhibition of Trk signaling, with K252a, inhibited the protective effects of progesterone. Collectively, these data suggest that progesterone is protective via multiple and potentially related mechanisms.

Sex Differences in Cognitive Impairment and Alzheimer’s Disease

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

Novel Mechanisms for Estrogen-Induced Neuroprotection

Estrogens are gonadal steroid hormones that are present in the circulation of both males and females and that can no longer be considered within the strict confines of reproductive function. In fact, the bone, the cardiovascular system, and extrahypothalamic regions of the brain are now well-established targets of estrogens. Among the numerous aspects of brain function regulated by estrogens are their effects on mood, cognitive function, and neuronal viability. Here, we review the supporting evidence for estrogens as neuroprotective agents and summarize the various mechanisms that may be involved in this effect, focusing particularly on the mitochondria as an important target. On the basis of this evidence, we discuss the clinical applicability of estrogens in treating various age-related disorders, including Alzheimer disease and stroke, and identify the caveats that must be considered.

Estrogens and progesterone as neuroprotectants: what animal models teach us.

Estradiol and progesterone are two steroid hormones that target a variety of organ systems, including the heart, the bone and the brain. With respect to the latter, a large volume of basic science studies support the neuroprotective role of estradiol and/or progesterone. In fact, the results of such studies prompted the assessment of these hormones as protective agents against such disorders as Alzheimers disease, stroke and traumatic brain injury. Interestingly, results from the Womens Health Initiative (WHI) yielded results that appeared to be inconsistent with the data derived from in vitro and in vivo models. However, we argue that the results from the basic science studies were not inconsistent with the clinical trials, but rather, are consistent with, and may even have predicted, the results from the WHI. To illustrate this point, we review here certain in vivo paradigms that have been used to assess the protective effects of estrogens and progesterone, and describe how the results from these animal models point to the importance of the type of hormone, the age of the subjects and the method of hormone administration, in determining whether or not hormones are neuroprotective.

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 Alzheimers disease (AD), initiated by the Alzheimers 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.