Organotypic culture of the developing cerebral cortex and hypothalamus: relevance to sexual differentiation

Affective Behavior Shows Sex Differences in Mid-adulthood Rats Following Postnatal Immune Stimulation

Mid-adulthood represents the critical window period usually associated with the development of age-related diseases. Despite several attempts to delineate the pathological mechanisms underlying postnatal immune challenge and altered brain functions, the role of sex-dependent changes in affective behaviors of middle-aged animals requires more attention. In this study, we sought to investigate behavioral and molecular response patterns at mid-adulthood linked to early-life immune activation. Using affective behavioral test batteries, we showed that lipopolysaccharide (LPS)-induced postnatal immune challenge caused anxiety-like behaviors in both male and female Wistar rats at mid-adulthood, whereas only female rats exhibited depression-like behaviors. Our data further demonstrated a significant increase in microglial complexity and increased levels of tumor necrosis factor (TNFα), nitric oxide (NOx), and lipid peroxidation in the prefrontal cortex of female rats compared to their male counterparts and phosphate-buffered saline (PBS) littermate controls. With these results, we established significant interaction between sex differences and LPS-induced alterations in behavior and associated oxidative and immunohistochemical changes. These findings may provide an insight to better understand the neuroimmunological mechanisms of sex-dependent brain pathological manifestations occurring at mid-adulthood.

Genetic and Molecular Approaches to Study Neuronal Migration in the Developing Cerebral Cortex

The migration of neuronal cells in the developing cerebral cortex is essential for proper development of the brain and brain networks. Disturbances in this process, due to genetic abnormalities or exogenous factors, leads to aberrant brain formation, brain network formation, and brain function. In the last decade, there has been extensive research in the field of neuronal migration. In this review, we describe different methods and approaches to assess and study neuronal migration in the developing cerebral cortex. First, we discuss several genetic methods, techniques and genetic models that have been used to study neuronal migration in the developing cortex. Second, we describe several molecular approaches to study aberrant neuronal migration in the cortex which can be used to elucidate the underlying mechanisms of neuronal migration. Finally, we describe model systems to investigate and assess the potential toxicity effect of prenatal exposure to environmental chemicals on proper brain formation and neuronal migration.

Baby babbling at five months linked to sex hormone levels in early infancy

Gender-dependent differentiation of the brain at morphological, neurochemical and functional levels of organization have been shown to be primarily controlled by sex differences in gonadal hormone concentrations during pre- and early postnatal development. Indeed, previous studies have reported that pre- and perinatal hormonal environments influence brain development and, consequently, affect sex specific long-term language outcomes. Herein, we investigated whether postnatal surges of estrogen (estradiol) and androgen (testosterone) may predict properties of pre-speech babbling at five months. This study is the first attempt to investigate a possible correlation between sex hormones and infants' articulatory skills during the typical postnatal period of extended hormonal activity known as 'mini-puberty.' A hierarchical, multiple regression approach revealed a significant, robust positive relationship between 4-week concentrations of estradiol and individual articulatory skills. In contrast, testosterone concentrations at five months negatively correlated with articulatory skills at the same age in both boys and girls. Our findings reinforce the assumption of the importance of sex hormones for auditory-vocal development towards language in human infants.

Effect of Testosterone on Neuronal Morphology and Neuritic Growth of Fetal Lamb Hypothalamus-Preoptic Area and Cerebral Cortex in Primary Culture

Testosterone plays an essential role in sexual differentiation of the male sheep brain. The ovine sexually dimorphic nucleus (oSDN), is 2 to 3 times larger in males than in females, and this sex difference is under the control of testosterone. The effect of testosterone on oSDN volume may result from enhanced expansion of soma areas and/or dendritic fields. To test this hypothesis, cells derived from the hypothalamus-preoptic area (HPOA) and cerebral cortex (CTX) of lamb fetuses were grown in primary culture to examine the direct morphological effects of testosterone on these cellular components. We found that within two days of plating, neurons derived from both the HPOA and CTX extend neuritic processes and express androgen receptors and aromatase immunoreactivity. Both treated and control neurites continue to grow and branch with increasing time in culture. Treatment with testosterone (10 nM) for 3 days significantly (P < 0.05) increased both total neurite outgrowth (35%) and soma size (8%) in the HPOA and outgrowth (21%) and number of branch points (33%) in the CTX. These findings indicate that testosterone-induced somal enlargement and neurite outgrowth in fetal lamb neurons may contribute to the development of a fully masculine sheep brain.

Alzheimer's disease: review of hormone therapy trials and implications for treatment and prevention after menopause

Hormonal changes associated with the menopausal transition and postmenopause have the potential to influence processes linked to Alzheimer's disease symptoms and pathogenesis, but effects of menopause on Alzheimer risk can be addressed only indirectly. Nine randomized clinical trials of estrogen-containing hormone therapy in Alzheimer's disease patients were identified by a systematic literature search. Findings suggest that hormone therapy does not improve cognitive symptoms of women with Alzheimer's disease. No clinical trials of hormone therapy address Alzheimer prevention, but one clinical trial provides moderate evidence that continuous, combined estrogen plus progestogen initiated at age 65 years or older increases the risk of dementia. The timing, or critical window, hypothesis suggests that hormone therapy initiated at a younger age in closer temporal proximity to menopause may reduce the risk of Alzheimer's disease. This hypothesis is supported by observational research but is not addressed by clinical trial data. Unrecognized confounding is of concern in interpreting observational results, and research that helps resolve this issue will have important public health implications. Well-designed cohort studies, convergent evidence from appropriate laboratory models, and long-term clinical trials using surrogate biomarkers of brain function and neural pathology could provide relevant answers. Other estrogenic compounds are of theoretical interest with respect to Alzheimer treatment and risk. Effects of selective estrogen receptor modulators such as raloxifene may differ from those of estrogens; potential effects of phytoestrogens are not well studied. This article is part of a Special Issue entitled 'Menopause'.

White matter aberrations in prepubertal estrogen-naive girls with monosomic Turner syndrome

Turner syndrome (TS) offers a unique opportunity to investigate associations among genes, the brain, and cognitive phenotypes. In this study, we used 3 complementary analyses of diffusion tensor imaging (DTI) data (whole brain, region of interest, and fiber tractography) and a whole brain volumetric imaging technique to investigate white matter (WM) structure in prepubertal, nonmosaic, estrogen-naive girls with TS compared with age and sex matched typically developing controls. The TS group demonstrated significant WM aberrations in brain regions implicated in visuospatial abilities, face processing, and sensorimotor and social abilities compared with controls. Extensive spatial overlap between regions of aberrant WM structure (from DTI) and regions of aberrant WM volume were observed in TS. Our findings indicate that complete absence of an X chromosome in young females (prior to receiving exogenous estrogen) is associated with WM aberrations in specific regions implicated in characteristic cognitive features of TS.

Gonadectomy and hormone replacement affects in vivo basal extracellular dopamine levels in the prefrontal cortex but not motor cortex of adult male rats

Gonadectomy in adult male rats is known to impair performance on dopamine (DA)-dependent prefrontal cortical tasks and selectively dysregulate end points in the mesoprefrontal DA system including axon density. In this study, in vivo microdialysis and high-pressure liquid chromatography were used to determine whether short (4 day)- and/or long-term (28 day) gonadectomy and hormone replacement might also influence the more functionally relevant metric of basal extracellular DA level/tone. Assessments in medial prefrontal cortex revealed that DA levels were significantly lower than control in 4-day gonadectomized rats and similar to control in 4-day gonadectomized animals supplemented with both testosterone and estradiol. Among the long-term treatment groups, DA levels were significantly higher than control in gonadectomized rats and gonadectomized rats given estradiol but were similar to control in rats given testosterone. In contrast, extracellular DA levels measured in motor cortex were unaffected by long- or short-term gonadectomy. The effects of gonadectomy and hormone replacement on prefrontal cortical DA levels observed here parallel previously identified effects on prefrontal DA axon density and could represent hormone actions relevant to the modulation of DA-dependent prefrontal cortical function and perhaps its dysfunction in disorders such as schizophrenia, attention deficit hyperactivity disorder, and autism where males are disproportionately affected relative to females.

Estradiol is a potent protective, restorative, and trophic factor after brain injury

Estrogens are a group of pleiotropic steroid hormones that exhibit diverse mechanisms of action in multiple physiologic systems. Over the past 30 years, biomedical science has begun to appreciate that endogenous estrogens and their receptors display important roles beyond the reproductive system. Our growing appreciation of novel, nonreproductive functions for estrogens has fundamentally contributed to our knowledge of their role in human health and disease. Recent findings from the Women's Health Initiative have caused clinicians and scientists to question whether estrogens are protective factors or risk factors. In light of the dichotomy between basic science and clinical studies, this review will attempt to reconcile differences between them. We will focus on studies from our laboratory and others highlighting the beneficial properties of the most abundant endogenous estrogen, 17beta-estradiol, using in vivo and in vitro models of cerebral ischemia and neuronal injury. These studies demonstrate that 17beta-estradiol powerfully protects the brain using multiple molecular mechanisms that promote: (1) decreased cell death, (2) increased neurogenesis, (3) an enhancement of neurotrophic support, and (4) the suppression of proinflammatory pathways.

Estrogen receptor protein interaction with phosphatidylinositol 3-kinase leads to activation of phosphorylated Akt and extracellular signal-regulated kinase 1/2 in the same population of cortical neurons: a unified mechanism of estrogen action

17Beta-estradiol (E2)-induced neuroprotection is dependent on mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI3K) signaling cascades. We sought to determine whether E2 neuroprotective mechanisms are mediated by a unified signaling cascade activated by estrogen receptor (ER)-PI3K interaction within the same population of neurons or whether E2 activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt are independent signaling events in different neuronal populations. Immunoprecipitation of E2-treated cortical neurons was conducted to determine a protein-protein interaction between ER and the PI3K regulatory subunit p85. Subsequently, cortical neurons were treated with E2 alone or in presence of MAPK inhibitors or PI3K inhibitors. Results of these analyses indicated a protein-protein interaction between ER and p85 that was time-dependent and consistent with the temporal profile for generation of Akt (pAkt) and ERK1/2 phosphorylation (pERK1/2). E2-induced phosphorylation of Akt, was first apparent at 10 min and maximal at 30 min. Simultaneously, E2-induced pERK1/2 was first apparent at 5-10 min and maximal at 30 min. Inhibition of PI3K completely blocked E2 activation of pAkt at 10 and 30 min and blocked E2 activation of ERK1/2 at 10 min, which revealed a PI3K-independent activation of ERK at 30 min. Double immunocytochemical labeling for pERK1/2 and pAkt demonstrated that E2 induced both signaling pathways in the same neurons. These results indicate a unified signaling mechanism for rapid E2 action that leads to the coordinated activation of both pERK1/2 and pAkt in the same population of neurons. Implications of these results for understanding estrogen mechanism of action in neurons and therapeutic development are considered.

The neurology of menopause

BACKGROUND

Menopause is a normal milestone experienced annually by 2 million American women each year, and many women are concerned about the relation between menopause and health. Associated hormonal changes have the potential to influence neurologic disease, as do hormonal therapies prescribed for menopausal symptoms or other conditions. The objective of this article is to increase neurologists' awareness of the relation between menopause and neurologic illness.

REVIEW SUMMARY

This was a focused review of 4 common neurologic disorders potentially influenced by menopause or by estrogen-containing hormone therapy: stroke, epilepsy, Parkinson disease, and Alzheimer disease. Hormonal effects are germane to each illness, although clinical implications are clearer for stroke and Alzheimer disease than for epilepsy and Parkinson disease. For women with epilepsy, few clinical data directly address the role of menopause or estrogen-containing hormone therapy on seizure frequency. Relevant clinical research findings on Parkinson disease are inconsistent and provide an inadequate basis for practice guidelines. There is clinical trial evidence that hormone therapy does not reduce stroke incidence and may increase risk of ischemic stroke; hormone therapy cannot be recommended for stroke prevention. The natural menopausal transition is not characterized by objective memory loss. There is clinical trial evidence that hormone therapy should not be used for the postmenopausal woman age 65 years or older for the preservation of cognitive skills, prevention of dementia, or treatment of dementia due to Alzheimer disease. Long-term cognitive consequences of short-term hormone therapy used by younger women for menopausal symptoms remains an important area of uncertainty.

CONCLUSIONS

Increased awareness of hormonal influences on neurologic illness is important for the practicing neurologist.

Selective alterations of white matter associated with visuospatial and sensorimotor dysfunction in turner syndrome

Turner syndrome (TS) is a neurogenetic disorder characterized by impaired spatial, numerical, and motor functioning but relatively spared verbal ability. Results from previous neuroimaging studies suggest that gray matter alterations in parietal and frontal regions may contribute to atypical visuospatial and executive functioning in TS. Recent findings in TS also indicate variations in the shape of parietal gyri and white matter microstructural anomalies of the temporal lobe. Diffusion tensor imaging and structural imaging methods were used to determine whether 10 females with TS and 10 age- and gender-matched control subjects exhibited differences in fractional anisotropy, white matter density, and local brain shape. Relative to controls, females with TS had lower fractional anisotropy (FA) values in the deep white matter of the left parietal-occipital region extending anteriorly along the superior longitudinal fasciculus into the deep white matter of the frontal lobe. In addition, decreased FA values were located bilaterally in the internal capsule extending into the globus pallidus and in the right prefrontal region. Voxel-based morphometry (VBM) analysis showed corresponding white matter density differences in the internal capsules and left centrum semiovale. Tensor-based morphometry analysis indicated that the FA and VBM results were not attributable to differences in the local shape of brain structures. Compared with controls, females with TS had increases in FA values and white matter density in language-related areas of the inferior parietal and temporal lobes. These complementary analyses provide evidence for alterations in white matter pathways that subserve affected and preserved cognitive functions in TS.

Estrogen therapy: does it help or hurt the adult and aging brain? Insights derived from animal models

Hormone therapy and estrogen therapy in postmenopausal women have been thought to ameliorate cognitive dysfunction and decrease the risk and/or progress of neurodegenerative conditions such as Alzheimer's disease and stroke. Furthermore, estrogens have been shown to exert neuroprotective actions in a variety of in vitro and in vivo models of brain injury. However, the findings of the Women's Health Initiative have made us re-evaluate these assumptions. Our laboratory has shown that physiological levels of estradiol attenuate ischemic brain injury in young and middle-aged female rats. We have begun to probe the cellular and molecular mechanisms that underlie these novel non-reproductive actions of this steroid. Our findings demonstrate that in both young and aging rats, treatment with physiological concentrations of estradiol decreases ischemic injury by almost 50%, compared with oil-treated controls. Additionally, our data suggest that estradiol acts by altering the expression of genes that suppress apoptosis and enhance survival in the penumbral region of the infarct. These observations demonstrate that estrogen therapy protects against stroke-related injury in young and aging female rats and strongly suggest that middle-aged animals remain responsive to the protective actions of estradiol. Furthermore, they suggest that estrogen therapy protects against cell death by influencing the expression of genes that suppress apoptotic cell death pathways.

Hormone therapy and Alzheimer’s disease: benefit or harm?

Alzheimer's disease (AD) is the most common cause of dementia. After menopause, circulating levels of oestrogens decline markedly and oestrogen influences several brain processes predicted to modify AD risk. For example, oestrogen reduces the formation of beta-amyloid, a biochemical hallmark of AD. Oestrogen effects on oxidative stress and some effects on inflammation and the cerebral vasculature might also be expected to ameliorate risk. However, AD pathogenesis is incompletely understood and other oestrogen actions could be deleterious. Limited clinical trial evidence suggests that oestrogen therapy, begun after the onset of AD symptoms, is without substantial benefit or harm. Observational studies have associated oestrogen-containing hormone therapy with reduced AD risk. However, in the Women's Health Initiative Memory Study - a randomised, placebo-controlled trial of women 65 - 79 years of age - oral oestrogen plus progestin doubled the rate of dementia, with heightened risk appearing soon after treatment was initiated. Based on current evidence, hormone therapy is thus not indicated for the prevention of AD. Discrepancies between observational studies and the Women's Health Initiative clinical trial may reflect biases and unrecognised confounding factors in observational reports. Other explanations for divergent findings should be considered in future research, including effects of unopposed oestrogen or different hormone therapy preparations and the intriguing theoretical possibility that effects of hormone therapy on AD risk may be modified by the timing of use (e.g., initiation during the menopausal transition or early postmenopause versus initiation during the late postmenopause).

Sex differences in the stress-induced release of acetylcholine in the hippocampus and corticosterone from the adrenal cortex in rats

To assess sex differences in stress-induced acetylcholine (ACh) release in the hippocampus and corticosterone (CS) release from the adrenal cortex, an in vivo microdialysis study was performed in intact male (n = 6) and cycling female (diestrus 1, n = 5; proestrus, n = 6) rats. Dialysates and blood samples were taken from 11.00 to 16.00 h in freely moving rats, but restraint stress was applied from 12.00 to 13.00 h. Basal ACh release in the hippocampus was low without significant differences between groups. Although ACh release promptly increased with the onset of restraint stress in both sexes, the magnitude of the increase in males was significantly greater than in female rats during diestrus and proestrus (p < 0.01). Basal serum CS concentrations were significantly greater in proestrus than in diestrus (p < 0.01) or in male rats (p < 0.01). Serum CS concentrations significantly increased at the onset of restraint stress in both sexes (p < 0.01), but the magnitude of the increase was significantly greater in female than in male rats (p < 0.01). These results indicate that sex and/or gonadal steroid environment can affect the stress-induced ACh release in the hippocampus and CS release from the adrenal cortex.

Estrogen-receptor-? distribution in the human hypothalamus: Similarities and differences with ER? distribution

This study reports the first systematic rostrocaudal distribution of estrogen receptor beta immunoreactivity (ER beta-ir) in the human hypothalamus and adjacent areas in five males and five females between 20-39 years of age and compares its distribution to previously reported ER alpha in the same patients. ER beta-ir was generally observed more frequently in the cytoplasm than in the nucleus and appeared to be stronger in women. Basket-like fiber stainings, suggestive for ER beta-ir in synaptic terminals, were additionally observed in various areas. Men showed more robust nuclear ER beta-ir than women in the medial part of the bed nucleus of the stria terminalis, paraventricular and paratenial nucleus of the thalamus, while less intense, but more nuclear, ER beta-ir appeared to be present in, e.g., the BSTc, sexually dimorphic nucleus of the medial preoptic area, diagonal band of Broca and ventromedial nucleus. Women revealed more nuclear ER beta-ir than men of a low to intermediate level, e.g., in the suprachiasmatic, supraoptic, paraventricular, infundibular, and medial mamillary nucleus. These data indicate potential sex differences in ER beta expression. ER beta-ir expression patterns in subjects with abnormal hormone levels suggests that there may be sex differences in ER beta-ir that are "activational" rather than "organizational" in nature. Similarities, differences, potential functional, and clinical implications of the observed ER alpha and ER beta distributions are discussed in relation to reproduction, autonomic-function, mood, cognition, and neuroprotection in health and disease.

Estrogen receptor-alpha distribution in the human hypothalamus in relation to sex and endocrine status

The present study reports the first systematic rostrocaudal distribution of estrogen receptor-alpha immunoreactivity (ERalpha-ir) in the human hypothalamus and its adjacent areas in young adults. Postmortem material taken from 10 subjects (five male and five female), between 20 and 39 years of age, was investigated. In addition, three age-matched subjects with abnormal levels of estrogens were studied: a castrated, estrogen-treated 50-year-old male-to-female transsexual (T1), a 31-year-old man with an estrogen-producing tumor (S2), and an ovariectomized 46-year-old woman (S8). A strong sex difference, with more nuclear ERalpha-ir in women, was observed rostrally in the diagonal band of Broca and caudally in the medial mamillary nucleus. Less robust sex differences were observed in other brain areas, with more intense nuclear ERalpha-ir in men, e.g., in the sexually dimorphic nucleus of the medial preoptic area, paraventricular nucleus, and lateral hypothalamic area, whereas women had more nuclear ERalpha-ir in the suprachiasmatic nucleus and ventromedial nucleus. No nuclear sex differences in ERalpha were found, e.g., in the central part of the bed nucleus of the stria terminalis. In addition to nuclear staining, ERalpha-ir appeared to be sex-dependently present in the cytoplasm of neurons and was observed in astrocytes, plexus choroideus, and other non-neuronal cells. ERalpha-ir in T1, S2, and S8 suggested that most of the observed sex differences in ERalpha-ir are "activational" (e.g., ventromedial nucleus/medial mamillary nucleus) rather than "organizational." Species similarities and differences in ERalpha-ir distribution and possible functional implications are discussed.

Estradiol enhances Akt activation in cortical explant cultures following neuronal injury

We have previously demonstrated that estradiol reduces cell death in cortical explant cultures following injury induced by metabolic inhibition in a receptor-dependent fashion. In this study, we examined whether cell death involves apoptosis and assessed the potential mediators of estradiol's actions. Cortical explant cultures were generated from postnatal day 3 rat pups. On day 7 in vitro, explants were injured by exposure to 1 mM 2-DG/2 mM KCN for 2 h to model the metabolic inhibition observed during ischemia. Explants were fixed in 4% paraformaldehyde at 2, 6, 10 and 24 h following the injury period and 18-microm thick sections were cut on a cryostat and stained with cresyl violet to assess cell death. The same sections were also labeled by TUNEL to determine whether cell death occurred by apoptosis. Other sections were used for immunohistochemistry to determine whether cells that stained positive for activated caspase 3 were also immunopositive for NeuN, a neuronal marker, or GFAP, an astrocyte marker. Protein was extracted for Western blot analysis from a separate set of explants collected at 0, 0.5, 1, 2 and 4 h following the conclusion of the injury. Estradiol treatment significantly reduced the number of cells undergoing apoptotic cell death as indicated by nuclear condensation visualized by cresyl violet staining (P<0.05). TUNEL staining revealed that the majority of pyknotic and fragmented nuclei were also TUNEL positive. Furthermore, caspase 3 activation appeared to be restricted to neurons. To examine a possible mechanism by which estradiol prevents apoptosis, we examined the level of activation of Akt kinase, which mediates antiapoptotic signals. Potential activation was measured by phosphorylation of Akt at Ser473 by Western blot analysis. In the absence of estradiol, pAkt levels were significantly increased at 2 h following the termination of injury. Explants that were pretreated with estradiol exhibited elevated levels of pAkt at 1 h following injury. Treatment with ICI 182,780 prevented the effect of estradiol. These studies suggest that estradiol prevents injury-induced apoptosis and that Akt activation may mediate these protective effects.

Cognitive and behavioral characteristics of turner syndrome: exploring a role for ovarian hormones in female sexual differentiation

To better understand factors contributing to behavioral development, we studied patients with Turner syndrome (TS), a disorder typically marked by prenatal onset of ovarian dysfunction. We compared girls and women (ages 12 and up) with TS (n = 21) to matched controls (n = 21) in cognitive and motor skills, as well as sex-typed personality characteristics and activity preferences. Measures were categorized (based on prior studies) as showing an average male advantage (male-superior measures), female advantage (female-superior measures), or no sex difference (sex-neutral measures). It was hypothesized that, if gonadal function contributes to behavioral development, effects of this deficiency would be more prominent on sexually differentiated than sex-neutral measures and thus that patient-control differences would be most marked for measures that show sex differences. Our findings indicated that TS patients and controls differed more on cognitive and motor domains that show sex differences than on sex-neutral domains. Patients also had more "undifferentiated" personalities and showed reduced sex-typed interests and activities. Differing experiences, as indexed by interests and activities, did not explain the observed cognitive and motor differences. These results are consistent with a role for ovarian hormones acting on the brain to influence cognitive and behavioral development, although they do not rule out other possible interpretations.

Neurotrophic and neuroprotective actions of estrogens and their therapeutic implications

Originally known for its regulation of reproductive functions, estradiol, a lipophilic hormone that can easily cross plasma membranes as well as the blood-brain barrier, maintains brain systems subserving arousal, attention, mood, and cognition. In addition, both synthetic and natural estrogens exert neurotrophic and neuroprotective effects. There is increasing evidence that estrogen actions are mediated by nongenomic as well as direct and indirect genomic pathways. Although in vitro models have provided the most extensive evidence for neurotrophic and neuroprotective actions to date, there are also in vivo studies that support these actions.

Neuroendocrine regulation of sexually dimorphic brain structure and associated sexual behavior in male rats is genetically controlled

Steroid hormones, particularly 17beta-estradiol (E2), regulate the development and expression of neural structures and sexual behavior. Recently, we demonstrated that E2-regulated responses are controlled by quantitative trait loci. In this study, we quantified 1) volume of the sexually dimorphic nucleus (SDN) of the preoptic area (POA); 2) medial basal hypothalamic (MBH)-POA aromatase and 5alpha-reductase enzyme activities during prenatal development and in adults; 3) serum LH, testosterone, FSH, E2, prolactin (PRL), and corticosterone levels; 4) reproductive organ (i.e., testis and ventral prostate) weights; and 5) male mating behavior in Noble (NB/Cr) and Wistar-Furth (WF/NCr) rat strains to determine the genetic influence on the measured parameters. Maximal phenotypic divergence in male SDN-POA volumes was seen between NB/Cr versus WF/NCr and BDIX/Cr rats (among nine rat strains initially examined), with the average SDN-POA volume of NB/Cr male rats being significantly greater ( approximately 30%) than that of either WF/NCr or BDIX/Cr males. Subsequent experiments investigated WF/NCr versus NB/Cr male rats in further detail. Significantly higher MBH-POA aromatase activity was seen in adult WF/NCr versus NB/Cr males, while MBH-POA 5alpha-reductase rates were not significantly different (within or between sex) for the two rat strains assayed. Serum LH levels were significantly higher (by greater than sixfold) in WF/NCr versus NB/Cr males, whereas testis organ:body weight and ventral prostate:body weight ratios in WF/NCr versus NB/Cr males were significantly smaller (by approximately 6-fold for testis and approximately 1.5-fold for prostate values). Serum FSH levels were significantly higher (by twofold) in WF/NCr versus NB/Cr males. However, serum testosterone levels were not significantly different, whereas E2 levels were approximately twofold higher (but not significantly different) in WF/NCr versus NB/Cr animals. No significant differences were found in basal (i.e., nonstress) serum PRL or corticosterone levels between the WF/NCr and NB/Cr males. In male copulatory tests, NB/Cr males exhibited significantly more aggressive sexual behavior (e.g., in mounting, intromission, and ejaculation parameters) compared with WF/NCr males. Taken together, these findings indicate that WF/NCr males are, in general, low responders, whereas NB/Cr males are high responders to hormonal signals. The obtained data suggest that the correlative, phenotypic variation in SDN-POA volume (i.e., structure) and reproductive hormone patterns and mating behavior (i.e., function) of WF/NCr versus NB/Cr males is regulated by potentially E2-mediated mechanisms that are genetically controlled.

Estrogen synthesis in the male brain triggers development of the avian song control pathway in vitro

Sexual differentiation of the brain is determined in part by steroids such as estrogen, which are generally assumed to arise from the gonads. Here we show that estrogens are produced autonomously in cultured juvenile male zebra finch brain slices, and this brain-derived estrogen is both necessary and sufficient to trigger formation in vitro of a key male-specific synaptic connection in the telencephalic song control circuit. Male-like development was stimulated in female slices cultured with male slices or exposed to estrogen, and estrogen antagonists inhibited song circuit development in slices of either sex. These results reveal a new mode of sex-specific neural development, induced not by differential exposure to gonadal steroids, but rather by differential synthesis of steroids in the brain.

Estrogens: trophic and protective factors in the adult brain

Our appreciation that estrogens are important neurotrophic and neuroprotective factors has grown rapidly. Although a thorough understanding of the molecular and cellular mechanisms that underlie this effect requires further investigation, significant progress has been made due to the availability of animal models in which we can test potential candidates. It appears that estradiol can act via mechanisms that require classical intracellular receptors (estrogen receptor alpha or beta) that affect transcription, via mechanisms that include cross-talk between estrogen receptors and second messenger pathways, and/or via mechanisms that may involve membrane receptors or channels. This area of research demands attention since estradiol may be an important therapeutic agent in the maintenance of normal neural function during aging and after injury.

Migrating neurons cross a reelin-rich territory to form an organized tissue out of embryonic cortical slices

In this study we show that the radial migration of neuronal precursors out of cerebral cortex of embryonic brain slices cultured for 4-7 days gives rise to an organized tissue that forms de novo off developing slices. In our in vitro preparations, migrating neuronal precursors overshot the marginal zone, as did the elongation of radial glial processes out of the slices. These cells detached from radial glia at a distance from the cortex and differentiated into pyramidal and nonpyramidal profiles that expressed different neuronal markers. Glial precursors were shown to proliferate in the slice and in the neotissue, and to differentiate into astrocytes. We show that cells expressing reelin in the marginal zone of embryonic cortical slices persist after a week in culture, which implies that neuronal migration is not necessarily hindered by the presumed stop signals provided by reelin in the marginal zone. Furthermore, our results provide a new model for in vitro studies of migration and differentiation during cortical development.

Estradiol protects against ischemic brain injury in middle-aged rats

Several clinical studies suggest that estradiol acts as a potent growth and protective factor in the adult brain. Postmenopausal women experience permanent hypoestrogenicity and suffer from increased risk of brain injury associated with neurodegenerative diseases such as stroke and Alzheimer's disease. Estrogen replacement therapy appears to decrease the risk and severity of these neurodegenerative conditions. Studies using animal models have shown that estradiol exerts similar effects in rodents and can enhance cell survival and induce synaptic plasticity. Therefore, we undertook studies to assess whether estradiol treatment can decrease brain injury and cell death induced by an experimental model of ischemia and whether aging animals remain responsive to the protective effects of estradiol. We will review results from recent studies that demonstrate that 1) in young animals, estrogens exert profound protective effects against ischemic brain injury induced by cerebral artery occlusion and 2) the response of aging animals has been tested with varying results. We will discuss and compare our experimental findings that utilize a permanent cerebral artery occlusion model and physiological levels of estradiol replacement therapy in young and middle-aged rats with those of previous studies. These observations provide important insights into the potential protective actions of estrogen replacement therapy on age- and disease-related processes in the brain.

Estradiol protects against injury-induced cell death in cortical explant cultures: a role for estrogen receptors

Estradiol has been shown to exert trophic and protective actions in the brain. Our laboratory has shown that in vivo, low physiological levels of estradiol protect the female rat brain against ischemic injury. In the present study, we used organotypic cortical explant cultures to begin to decipher the mechanisms of estradiol's actions. Injury was induced by exposure to kainic acid or potassium cyanide/2-deoxyglucose (KCN/2-DG) for varying lengths of time, and cell death was monitored by LDH release at 2, 6, 12, 24, 48, 72 and 96 h after injury. We found that exposure to 1 mM KCN/2 mM 2-DG for 2 h produced consistent delayed cell death that was detectable by 24 h. The presence of 17beta-estradiol (E2) during the 7 days prior to injury significantly reduced the extent of cell death; whereas, administration of E2 at the time of injury did not protect. The protective effects of estradiol were dose dependent. Low doses of E2 (1, 10, and 30 nM) significantly reduced cell death; however, higher concentrations of E2 (>60 nM) had no protective effect. The observations that low levels of E2 protect against cell death, and that pretreatment is required suggest that the protective actions of estradiol may involve estrogen receptors. Therefore, we examined the ability of 17alpha-estradiol, which does not efficiently activate the estrogen receptor, and the addition of the estrogen receptor antagonist, ICI 182,780, to influence the extent of cell death induced by KCN/2-DG. 17alpha-Estradiol failed to protect, and ICI 182,780 prevented E2 from protecting against cell death. Furthermore, E2 pretreatment is required for more than 24 h to be neuroprotective. Our results clearly show that in cortical explant cultures, estradiol protects cells against ischemic injury, and suggest that these protective actions involve estrogen receptors.

Oestrogens and Alzheimer's disease

In the last decade, several reports suggest that oestrogen replacement therapy (ORT=ERT=estrogen replacement therapy) might prevent or delay Alzheimer's disease. Oestrogens influence and modulate brain structure and brain function. There are substantial arguments that the postmenopausal oestrogen loss might, together with other factors, accelerate the appearance of Alzheimer's disease. The evidence is suggestive, but not compelling, that ORT can reduce the relative risk to suffer from Alzheimer's disease. Furthermore, recent findings are consistent with the hypothesis that oestrogens might ameliorate the symptomatology in early stages of Alzheimer's disease. However, it has to be remembered that in most clinical trials the number of oestrogen users was quite small, and, usually, oestrogen use was not randomised. The aim of the present review is to discuss the data available today in view of their clinical relevance.

The estrogen replacement therapy of the Women's Health Initiative promotes the cellular mechanisms of memory and neuronal survival in neurons vulnerable to Alzheimer's disease

OBJECTIVES

The current study investigated the neurotrophic and neuroprotective action of the complex formulation of conjugated equine estrogens (CEEs), the most frequently prescribed estrogen replacement therapy in the United States and the estrogen replacement therapy of the Women's Health Initiative.

METHODS

Videomicroscopic, morphologic and biochemical analyses were conducted in primary cultures of hippocampal neurons to determine the neurotrophic and neuroprotective properties of CEEs.

RESULTS

Results of these analyses demonstrated that CEEs significantly increased hippocampal neuronal outgrowth, a cellular marker of memory formation. Dose response analyses indicated that the lowest effective concentration of CEEs exerted the maximal neurotrophic effect. Results of neuroprotection studies demonstrated that CEES induced highly significant neuroprotection against beta amyloid(25-35), hydrogen peroxide and glutamate-induced toxicity.

CONCLUSIONS

CEEs induced cellular markers of memory function in neurons critical to memory and vulnerable to negative effects of aging and Alzheimer's disease. In addition, CEEs significantly and potently protected neurons against toxic insults associated with Alzheimer's disease. Because CEEs are the estrogen replacement therapy of the Women's Health Initiative, results of the current study could provide cellular mechanisms for effects of CEEs on cognitive function and risk of Alzheimer's disease derived from this prospective clinical trial.

Estrogen status affects sensitivity to focal cerebral ischemia in stroke-prone spontaneously hypertensive rats

Estrogen treatment has been shown to reduce ischemic brain damage. Because endogenous estrogen levels fluctuate markedly during the estrous cycle, we investigated the effect of stage of estrous cycle on ischemic brain damage. Halothane anesthetized 3- to 5-mo-old female Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) in proestrus (high estradiol levels) or metestrus (low estradiol levels) underwent permanent middle cerebral artery occlusion. In SHRSP, infarct volume at 24 h postocclusion was 24% smaller in proestrus compared with metestrus [208.6 +/- 9.5 mm(3) (n = 7) vs. 272.7 +/- 23.8 mm(3) (n = 7), respectively, means +/- SE; P = 0.0278, unpaired t-test]. In WKY, infarct volumes were similar in proestrus and metestrus [157.0 +/- 5.4 mm(3) (n = 5) and 131.5 +/- 16.5 mm(3) (n = 8), respectively; P = not significant (NS)]. Brain swelling (ipsilateral minus contralateral hemispheric volumes) was similar in proestrus and metestrus for SHRSP [138 +/- 9 mm(3) (n = 6) and 136 +/- 10 mm(3) (n = 7), respectively] and for WKY [103 +/- 15 mm(3) (n = 5) and 90 +/- 11 mm(3) (n = 8), respectively]. Thus the reduction in infarct size in SHRSP is caused by a true attenuation of the infarct volume and not simply by a reduction in brain edema.

Estradiol protects against ischemic injury

Clinical studies demonstrate that estrogen replacement therapy in postmenopausal women may enhance cognitive function and reduce neurodegeneration associated with Alzheimer's disease and stroke. This study assesses whether physiologic levels of estradiol prevent brain injury in an in vivo model of permanent focal ischemia. Sprague-Dawley rats were ovariectomized; they then were implanted, immediately or at the onset of ischemia, with capsules that produced physiologically low or physiologically high 17beta-estradiol levels in serum (10 or 60 pg/mL, respectively). One week after ovariectomy, ischemia was induced. Estradiol pretreatment significantly reduced overall infarct volume compared with oil-pretreated controls (mean+/-SD: oil = 241+/-88; low = 139+/-91; high = 132+/-88 mm3); this protective effect was regionally specific to the cortex, since no protection was observed in the striatum. Baseline and ischemic regional CBF did not differ between oil and estradiol pretreated rats, as measured by laser Doppler flowmetry. Acute estradiol treatment did not protect against ischemic injury. Our finding that estradiol pretreatment reduces injury demonstrates that physiologic levels of estradiol can protect against neurodegeneration.

Sex and the developing brain: suppression of neuronal estrogen sensitivity by developmental androgen exposure

The developmental effects of androgen play a central role in sexual differentiation of the mammalian central nervous system. The cellular mechanisms responsible for mediating these effects remain incompletely understood. A considerable amount of evidence has accumulated indicating that one of the earliest detectable events in the mechanism of sexual differentiation is a selective and permanent reduction in estrogen receptor concentrations in specific regions of the brain. Using quantitative autoradiographic methods, it has been possible to precisely map the regional distribution of estrogen receptors in the brains of male and female rats, as well as to study the development of sexual dimorphisms in receptor distribution. Despite previous data suggesting that the left and right sides of the brain may be differentially responsive to early androgen exposure, there is no significant right-left asymmetry in estrogen receptor distribution, in either sex. Significant sex differences in receptor density are, however, observed in several regions of the preoptic area, the bed nucleus of the stria terminalis and the ventromedial nucleus of the hypothalamus, particularly in its most rostral and caudal aspects. In the periventricular preoptic area of the female, highest estrogen receptor density occurs in the anteroventral periventricular region: binding in this region is reduced by approximately 50% in the male, as compared to the female. These data are consistent with the hypothesis that androgen-induced defeminization of feminine behavioral and neuroendocrine responses to estrogen may involve selective reductions in the estrogen sensitivity of critical components of the neural circuitry regulating these responses, mediated in part through a reduction in estrogen receptor biosynthesis.

17 beta-Estradiol enhances the outgrowth and survival of neocortical neurons in culture

Results of this investigation demonstrate that exposure to 17 beta-estradiol differentially and significantly regulates cortical nerve cell outgrowth depending on the cortical region. Parietal and occipital neurons treated with 1 nM 17 beta-estradiol showed a greater magnitude of neuronal outgrowth whereas outgrowth of temporal cortex neurons was decreased in the presence of 1 nM 17 beta-estradiol. Frontal cortex neurons showed a consistent enhancement of neuronal outgrowth that did not reach statistical significance. The dose response profile for 17 beta-estradiol regulation of the macromorphological features exhibited a bimodal dose response relationship whereas the dose response profile for 17 beta-estradiol regulation of the micromorphological features exhibited a dose response more characteristic of an inverted V-shaped function. An antagonist to the NMDA receptor antagonist, AP5, abolished the growth promoting effect of 17 beta-estradiol whereas the nuclear estrogen receptor antagonist ICI 182,780 did not. Lastly, neocortical neurons exposed to 17 beta-estradiol exhibited greater viability and survival than control neurons over a two week period. These data indicate that 17 beta-estradiol can enhance the growth and viability of select populations of neocortical neurons and that the growth promoting effects of 17 beta-estradiol can be blocked by an antagonist to the NMDA glutamate receptor and not by an antagonist to the estrogen nuclear receptor.

Equilin, a principal component of the estrogen replacement therapy premarin, increases the growth of cortical neurons via an NMDA receptor-dependent mechanism

Regulation of both the outgrowth and the survival of neurons involved in cognitive function can have a significant impact on the function of neural networks involved in memory and other cognitive processes. Results of this investigation demonstrated that 17beta-estradiol and the estrogenic steroids estrone, estriol, mestranol, and equilin induced significant increases in cortical nerve cell growth. Of the neurotrophic estrogenic steroids, equilin was most efficacious. We therefore conducted an extensive analysis of equilin-induced neurotrophism. Equilin induced highly significant increases in the growth of both the macro and micro features of cortical nerve cell morphology. The growth-promoting effects of equilin were present in both serum-containing and serum-free media, indicating that the growth-promoting effect of equilin is direct and not dependent upon factors present in serum. Analysis of the regional selectivity of equilin-induced neurotrophism in the cerebral cortex demonstrated that equilin significantly increased the growth of neurons from the frontal, temporal, and occipital regions, with neurons from the parietal region also influenced, though more modestly. We pursued the mechanism of equilin-induced neurotrophism and found that the growth-promoting effects of equilin were completely abolished in the presence of the glutamatergic NMDAreceptor antagonist AP5. Equilin is a major component of Premarin, the leading prescribed pharmaceutical for estrogen replacement therapy for postmenopausal women in the United States. Results of this investigation have the potential of influencing the application and design of therapeutic agents for the prevention of cognitive decline in estrogen deficient women and for the prevention of Alzheimer's disease in postmenopausal women, a group that comprises a large sector of the population, the size of which will continue to grow in the coming decades.

Estrogen, cognition, and a woman's risk of Alzheimer's disease

Alzheimer's disease affects women more often than men, and women with this form of dementia show greater naming (semantic memory) deficits during the course of their illness. Gonadal steroids exert organizational and activational effects on central nervous system neurons and influence brain function in other important ways. Several estrogenic actions are potentially relevant to Alzheimer's disease, and it is hypothesized that one consequence of estrogen deprivation after the menopause is a higher risk of this dementing disorder. In healthy women without dementia, estrogen may enhance cognitive performance, especially in the domain of verbal memory, although the magnitude of such effects is small. Several small treatment trials of estrogen replacement in women with Alzheimer's disease, however, suggest that estrogen's effects on cognition could be larger in this population and may be most apparent on tasks of semantic memory. Analyses in voluntary cohorts associate postmenopausal estrogen replacement therapy with a lower risk of subsequent Alzheimer's disease. In 3 recent epidemiologic studies, information on postmenopausal estrogen use was collected prospectively; while inconclusive, findings raise the possibility that postmenopausal estrogen replacement reduces a woman's risk of subsequent dementia. New information from basic research and from large randomized treatment studies, cohort studies, and case-control studies is needed to resolve important unanswered clinical issues.

Neonatal treatment with tamoxifen causes immediate alterations of the sexually dimorphic nucleus of the preoptic area and medial preoptic area in male rats

Tamoxifen is an antiestrogen widely used for the treatment of breast cancer. Current evolutions in preventive strategies to include healthy premenopausal women warrant the study of its developmental toxicity. Perinatal treatment of male rodents with tamoxifen caused reproductive tract lesions and sexual behavior deficits similar to those induced by diethylstilbestrol (DES). Those abnormalities could originate, at least in part, from lesions of the hypothalamic-pituitary axis. The initial alterations caused by tamoxifen in the hypothalamic medial preoptic area (MPOA) and sexually dimorphic nucleus of the preoptic area (SDN-POA) were studied in 6-day-old male rat pups treated with 100 micrograms tamoxifen (group 1), 1 microgram DES (group 2) or vehicle (group 3) from PN1 to 5. In situ hybridization was performed to analyze the expression of the GAP-43 gene, a marker of neuronal differentiation, and morphometry was used to study the neuronal density in the SDN-POA and MPOA and the volume and number of neurons in the SDN-POA. Tamoxifen reduced severely the volume and neuron numbers in the SDN-POA (46.1% and 47.8% of controls, respectively). The neuronal density of the MPOA was not modified. GAP-43 gene expression was decreased as demonstrated by a greater percentage of unlabeled neurons (grade 0) mirrored by a lesser percentage of intensely labeled ones (grade 2) in the SDN-POA and MPOA. In contrast to the effects of the antiestrogen, DES did not affect the above endpoints. These data indicated that developmental exposure of male rat pups to tamoxifen-induced immediate neuronal loss in one and altered differentiation in two hypothalamic areas crucial to reproduction. How those initial alterations contribute to the pathogenesis of the reproductive disorders observed in the adult male needs further investigation.

A PET study of Turner's syndrome: effects of sex steroids and the X chromosome on brain

Women with Turner's syndrome (TS) allow us to study the neurobiological associates of cognitive and behavioral abnormalities because they lack one/part of one X chromosome, and endogenous estrogen. We studied 13 healthy controls (mean age +/- SD, 28 +/- 6 years) and 16 TS subjects (mean age +/- SD, 26 +/- 6 years). We measured cognitive abilities using neuropsychological tests, and cerebral metabolic rates for glucose with positron emission tomography. Compared to controls, TS subjects had significant absolute hypermetabolism in most brain areas; however, normalized metabolism was significantly lower in TS subjects than controls in the insula and association neocortices bilaterally, and there were significant differences in functional metabolic associations of brain region pairs originating in occipital cortex bilaterally, and within the right hemisphere. There were significant correlations between right-left cognitive and metabolic asymmetries in the TS group. Also, within TS a preliminary analysis demonstrated "X chromosome dosage" effects in language ability and left temporal metabolism, asymmetry of right-left test scores, and parietal metabolism. We hypothesize that within TS: i) generalized brain hypermetabolism reflects global abnormalities in neuron packing; ii) neuronal abnormalities occur in association neocortex that differ in nature or extent from whole brain and are associated with significant differences in normalized metabolism; iii) cognitive deficits are related to brain metabolic abnormalities; and iv) social-behavioral problems may be related to abnormalities of brain metabolism. Moreover, in human brain the X chromosome involved in development of the association neocortices.

Correlations between nonverbal intelligence and peripheral nerve conduction velocity in right-handed subjects: sex-related differences

The speed hypothesis of intelligence was tested in relation to median nerve conduction velocity (NCV) in right-handed subjects. In total, NCV did not significantly correlate with IQ (Cattell's Culture Fair Intelligence test). The motor and sensory median NCVs correlated with IQ positively linearly in men and negatively linearly in women. These results supported the general, unqualified speed hypothesis of intelligence only for men. It was concluded that the conduction speed of the input-output channels of brain as an information processing unit may differentially contribute to nonverbal intelligence, depending upon sex. The male hormone, testosterone, was suggested to be the main factor responsible for sex-related differences in the IQ-NCV relationships.

The effects of sex steroids, and the X chromosome, on female brain function: a study of the neuropsychology of adult Turner syndrome

Sex steroids and the X chromosome can independently affect cognitive abilities. Because subjects with Turner syndrome (TS) have gonadal aplasia, and various alterations in X chromosome structure and number, they provide a model to study the effects of sex steroids and of the X chromosome on human brain function. We used neuropsychological tests to study the cognitive abilities of 18 TS adults and 19 age/sex matched healthy controls. Nine TS subjects were mosaic for the 45,X karyotype, and 9 were non-mosaic 45,X (full TS). The TS group as a whole, compared to controls, had (i) significantly lower scores in tests of language and visual-spatial function, attention and memory, and (ii) a significantly greater discrepancy between verbal and performance scores. Mosaic TS subjects were intermediate between full TS and controls in some tests of verbal ability, but did not differ significantly from controls in others. Visuospatial ability was significantly lower than controls in both mosaic and full TS. Within the mosaic TS group, there was a significant negative correlation between visuospatial ability and % lymphocytes having a 45,X karyotype. Memory test scores were reduced independently of "X chromosome dosage" in all TS subjects. We conclude that in humans, the X chromosome is involved in development of both cerebral hemispheres, but moreso of the association neocortices. Also, sex steroids may modulate this effect--particularly in hippocampus.

Reciprocal regulation of estrogen and NGF receptors by their ligands in PC12 cells

Recent work has shown that estrogen receptor mRNA and protein co-localize with neurotrophin receptor systems in the developing basal forebrain. In the present study we examined the potential for reciprocal regulation of estrogen and neurotrophin receptor systems by their ligands in a prototypical neurotrophin target, the PC12 cell. Using in situ hybridization histochemistry, RT-PCR and a modified nuclear exchange assay, we found both estrogen receptor mRNA and estrogen binding in PC12 cells. Moreover, while estrogen binding was relatively low in naive PC12 cells, long-term exposure to NGF enhanced estrogen binding in these cells by sixfold. Furthermore, concurrent exposure to estrogen and NGF differentially regulated the expression of the two NGF receptor mRNAs. The expression of trkA mRNA was up-regulated, while p75NGFR mRNA was down-regulated transiently. The present data indicate that NGF may increase neuronal sensitivity to estrogen, and that estrogen, by differentially regulating p75NGFR and trkA mRNA, may alter the ratio of the two NGF receptors, and, consequently, neurotrophin responsivity. In view of the widespread co-localization of estrogen and neurotrophin receptor systems in the developing CNS, the reciprocal regulation of these receptor systems by NGF and estrogen may have important implications for processes governing neural maturation and the maintainance of neural function.

Molecular aspects of sexual differentiation of the rodent brain

The sexual differentiation of the brain is orchestrated by gonadal steroids during a restricted developmental period and results in permanent changes in the neural substrate including the capacity to support ovulation and expression of sex-specific reproductive behaviors. Sry gene-induced development of testes constitutes a binary switch directing all subsequent differentiation. Androgen produced by the testes of the embryonic male differentiates secondary sex characteristics but also acts in the brain to "masculinize" the neural substrate, many of the latter are the result of aromatization of testosterone to estrogen. Molecular characteristics of aromatase and 5 alpha-reductase enzymes are reviewed. It is assumed that estrogen binds to its receptor which then binds to DNA, inducing transcription of specific genes. Mutations in steroid receptor genes can markedly alter hormone-mediated differentiation. Two questions are addressed: (1), is the assumption correct that estrogen's effects on sexual differentiation are via the classic genomic action of steroids?; and (2) if so, what genes are transcribed as a result of the activated estrogen receptor complex? We have used antisense oligodeoxynucleotides designed to hybridize with and block the translation of mRNA for the estrogen receptor. Administration of antisense into brain of 3-day-old pups had permanent effects on estrogen-induced differentiation as indicated by behavioral and brain morphology differences in adulthood. These results demonstrate the effectiveness of antisense oligonucleotides if administered during a critical period and further confirm the widely accepted tenet that estrogen acts on the brain via its receptor. Subsequent experiments can now address the question of what genes are being activated by the estrogen receptor during development.

Sites of female reproductive vulnerability: implications for testing and risk assessment

All reproductive toxicity testing, whether in vivo or in vitro, should be conducted with consideration of the ultimate use of the data for reproductive risk assessment and the protection of human reproductive health. In this review selected sites of vulnerability in the female reproductive system are identified. These sites of female reproductive vulnerability may be utilized for in vitro toxicity testing assays, and the data from assays may be employed in the hazard identification and hazard characterization steps necessary for reproductive risk assessment. Using biomarkers of female reproductive function derived from in vitro toxicity testing it is possible to define functions that characterize female fecundity. These characterizations of female fecundity may be used to quantitate reproductive risk in human populations.

Estrogen receptors colocalize with low-affinity nerve growth factor receptors in cholinergic neurons of the basal forebrain

The rodent and primate basal forebrain is a target of a family of endogenous peptide signaling molecules, the neurotrophins--nerve growth factor, brain-derived neurotrophic factor, and neurotrophin 3--and of the gonadal steroid hormone estrogen, both of which have been implicated in cholinergic function. To investigate whether or not these ligands may act on the same neurons in the developing and adult rodent basal forebrain, we combined autoradiography with 125I-labeled estrogen and either nonisotopic in situ hybridization histochemistry or immunohistochemistry. We now report colocalization of intranuclear estrogen binding sites with the mRNA and immunoreactive protein for the low-affinity nerve growth factor receptor, which binds all three neurotrophins, and for the cholinergic marker enzyme choline acetyltransferase (acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6). Colocalization of estrogen and low-affinity nerve growth factor receptors implies that their ligands may act on the same neuron, perhaps synergistically, to regulate the expression of specific genes or gene networks that may influence neuronal survival, differentiation, regeneration, and plasticity. That cholinergic neurons in brain regions subserving cognitive functions may be regulated not only by the neurotrophins but also by estrogen may have considerable relevance for the development and maintenance of neural substrates of cognition. If estrogen-neurotrophin interactions are important for survival of target neurons, then clinical conditions associated with estrogen deficiency could contribute to the atrophy or death of these neurons. These findings have implications for the subsequent decline in those differentiated neural functions associated with aging and Alzheimer disease.

Sex differences in "cognitive" regions of the rat brain

This review is centered on anatomical sex differences in neuronal organization in parts of the rat nervous system that are associated with "cognitive" rather than reproductive function: the hippocampus and the cerebral cortex, including the corpus callosum. All three of these structures exhibit sexual dimorphism at the cellular level. It is notable that there is a dissociation between the gross size of a structure and the underlying cellular dimorphism. For example, no sex differences were detected in the size of the splenium of the corpus callosum, but female rats had more axons in this area than did male rats. These "cognitive" regions of the brain are susceptible to the nature of the postweaning environment; the degree and even direction of sex differences was influenced by the environment. There is evidence that testosterone plays a role in the dimorphism of the hippocampus.