Testosterone and estrogen affect neuronal differentiation but not proliferation in early embryonic cortex of the rat: the possible roles of androgen and estrogen receptors

Contributions of Sex Chromosomes and Gonadal Hormones to the Male Bias in a Maternal Antibody-Induced Model of Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a group of neurodevelopmental conditions that is four times more commonly diagnosed in males than females. While susceptibility genes located in the sex chromosomes have been identified in ASD, it is unclear whether they are sufficient to explain the male bias or whether gonadal hormones also play a key role. We evaluated the sex chromosomal and hormonal influences on the male bias in a murine model of ASD, in which mice are exposed in utero to a maternal antibody reactive to contactin-associated protein-like 2 (Caspr2), which was originally cloned from a mother of a child with ASD (termed C6 mice henceforth). In this model, only male mice are affected. We used the four-core-genotypes (FCG) model in which the Sry gene is deleted from the Y chromosome (Y⁻) and inserted into autosome 3 (TgSry). Thus, by combining the C6 and FCG models, we were able to differentiate the contributions of sex chromosomes and gonadal hormones to the development of fetal brain and adult behavioral phenotypes. We show that the presence of the Y chromosome, or lack of two X chromosomes, irrespective of gonadal sex, increased the susceptibility to C6-induced phenotypes including the abnormal growth of the developing fetal cerebral cortex, as well as a behavioral pattern of decreased open-field exploration in adult mice. Our results indicate that sex chromosomes are the main determinant of the male bias in the maternal C6-induced model of ASD. The less dominant hormonal effect may be due to modulation by sex chromosome genes of factors involved in gonadal hormone pathways in the brain.

High content imaging quantification of multiple in vitro human neurogenesis events after neurotoxin exposure

Background

Our objective was to test neural active compounds in a human developmental neurotoxicity (DNT) model that represents neural tube stages of vulnerability. Previously we showed that 14 days in vitro (DIV 14) was sufficient to generate cryopreserved neuronal cells for post thaw neurite recovery assays. However, short exposure and assessment may not detect toxicants that affect an early neurogenesis continuum, from a mitotic human neural progenitor (hNP) cell population through the course of neurite outgrowth in differentiating neurons. Therefore, we continuously exposed differentiating hNP cells from DIV 0 through DIV 14 to known toxicants and endocrine active compounds in order to assess at DIV 14 effects of these compounds in a human DNT maturation model for neurogenesis.

Methods

The Human DNT continuum (DIV 0 to DIV 14) was determined using immunocytochemistry for SOX1+ (proliferating hNP) and HuC/D+ (post mitotic neurons). The cumulative effects of five compounds was observed on neurite outgrowth in (βIII-tubulin+) and (HuC/D+) cells using high content imaging. All data were analyzed using a one-way ANOVA with a significance threshold of p < 0.05.

Results

During maturation in vitro, the neural cultures transitioned from uniform hNP cells (DIV 0) to predominantly mature post mitotic neuronal neurons (HuC/D+, 65%; DIV14) but also maintained a smaller population of hNP cells (SOX1+). Using this DNT maturation model system, Bis-1, testosterone, and β-estradiol inhibited neuronal maturation at micromolar levels but were unaffected by acetaminophen. β-estradiol also disrupted neurite extension at 10 μM. Treating cells in this window with Bisphenol A (BPA) significantly inhibited neurite outgrowth and branching in these continuum cultures but only at the highest concentrations tested (10 μM).

Conclusions

Cumulative effects of neurotoxicant exposure during a maturation continuum altered human neurogenesis at lower exposure levels than observed in acute exposure of static cryopreserved neurite recovery neurons cultures. Unlike prior acute studies, β-estradiol was highly toxic when present throughout the continuum and cytotoxicity was manifested starting early in the continuum via a non-estrogen receptor α (ER α) mechanism. Therefore, the effect of neural developmental neurotoxins can and should be determined during the dynamic process of human neural maturation.

Electronic supplementary material

The online version of this article (doi:10.1186/s40360-016-0107-4) contains supplementary material, which is available to authorized users.

The role of testosterone in colorectal carcinoma: pathomechanisms and open questions

Colorectal cancer (CRC) is the fourth commonest type of malignancy after breast, lung and prostate in the USA and accounts for approximately 49,190 deaths annually in USA alone. The 5-year survival rate of CRC has increased over the past decades, in part, due to greater awareness and the widespread implementation of national screening programmes. Recently, a number of studies reported that males have a higher risk of developing CRC due to the action of testosterone. Testosterone is an androgen that is responsible for the development of male secondary sex characteristics and for spermatogenesis. Studies on rats with mutated Apc tumour-suppressor gene subjected to either ovariectomy or orchidectomy exhibit different risks of CRC. Female rats subjected to ovariectomy are at higher risk of CRC, whereas orchidectomised male rats exhibit a lower risk of developing CRC. Sex hormones, in particular estrogen and testosterone, play a significant role in the development of CRC since the anti-neoplastic effect of estrogen lost during ovariectomy increases the risk of females developing CRC. Male mice exposed to testosterone after orchidectomy were also at greater risk than those who were orchidectomised but administered placebo only. Moreover, the recently established role of membrane androgen receptors in regression of CRC via non-genomic androgen-dependent action sets these receptors apart from intracellular androgen receptors (iARs) which themselves promote CRC development. In addition, testosterone-albumin conjugates are selective to membrane androgen receptors (mARs) and lead to apoptosis via caspase-3 activation. Akt kinases promote invasion of colon cancer cells when phosphorylated. These kinases are dephosphorylated upon activation of mARs, thereby reducing colon cancer cell motility and invasiveness. Testosterone similarly plays important roles in human CRC. Long cytosine-adenine-guanine (CAG) repeats in the gene for the androgen receptors have been associated with a poor 5-year survival compared to shorter CAG repeats. Very recently, the measurement of serum unbound testosterone has been suggested as a novel biomarker along with carcinoembryonic antigen in CRC. In conclusion, testosterone may promote the development of CRC via a number of pathways, which may place males at greater risk. Testosterone holds promise as a potential biomarker in CRC risk prediction; however, further studies are required to better define its role in colorectal neoplasia.

Gene array analysis of embryonic- versus adult-derived hypothalamic NPY-expressing cell lines

Few studies have utilized microarray analysis to understand the genome wide changes involved in the development of the hypothalamus despite its overall importance to basic physiology. Gene expression profiling of immortalized, clonal hypothalamic neurons, embryonic-derived mHypoE-46 and adult-derived mHypoA-2/12, reveals that the expression of 1225 probes was significantly changed between the two neuronal models. Further comparison of the gene expression profiles identified two categories of genes that were confirmed with qRT-PCR: (i) genes implicated in the Wnt signaling pathway; and (ii) transcription factors previously implicated in the development of the central nervous system. Yet, functional analysis of the two cell lines, including hormonal responses and secretion, indicate that they are comparable despite their developmental origin. This study provides a comprehensive analysis of embryonic- and adult-derived hypothalamic neuronal cell models that both express neuropeptide Y, and identifies novel genes as candidates for mediating the development of specific hypothalamic neurons.

Effects of genistein and daidzein on hippocampus neuronal cell proliferation and BDNF expression in H19-7 neural cell line

OBJECTIVES

Estrogen replacement therapy (ERT) reduces the risk of Alzheimer's disease and symptoms in postmenopausal and elderly women. However, ERT is associated with increased risk of uterine and breast cancer. Dietary phytoestrogens have been suggested as a potential alternative to ERT, while little information is available regarding the effects and the underlying mechanisms of such treatment on central neuron function. The present study aimed to determine the effects of phytoestrogens including genistein and daidzein on the proliferation and survival of the hippocampus neural cells, which are of importance in learning and memory function.

MEASUREMENTS

H19-7/IGF-IR neural cell line was cultured in DMEM absented of serum for 72 h, and treated with various concentrations of genistein, daidzein or 17β-estradiol. Neuronal cell viability and proliferation were determined by MTT and BrdU assay, respectively Cell cycle analysis was performed using flow cytometry. The effects of genistein and daidzein on brain-derived neurotrophic factor (BDNF) mRNA and protein expression were determined by RT-PCR and ELISA, respectively. The effect of Trk receptors inhibitor on genistein and daidzein - induced hippocampus neuronal cell proliferation was also examined.

RESULTS

17β-estradiol, genistein and daidzein ranged from 20 nM to 2000 nM significantly promoted hippocampus neuronal cell viability and proliferation. Similar to the effect of 17β-estradiol, genistein and daidzein induced an increase in the percentage of cells in S phase. Genistein and daidzein significantly increased the expression of BDNF mRNA and protein levels. The effect of genistien and daidzein on hippocampus neuronal proliferation was blocked by K252a, a selective Trk receptors inhibitor.

CONCLUSION

This study concluded that genistein and daidzein improved hippocampus neuronal cell viability and proliferation in vitro. These neuroprotective effects might be mediated by BDNF-Trk pathway.

Regulation of estrogen receptor alpha gene expression in the mouse prefrontal cortex during early postnatal development

Estrogens have many functions in the developing rodent brain, and most of these depend on the presence of estrogen receptors. Understanding how expression of these receptors are regulated is crucial for understanding the roles of estradiol in the male and female brain during development In rodents, the prefrontal cortex (PFC) has been shown to be involved in working memory, attention, and behavioral inhibition. Many studies have demonstrated an effect of estradiol on sex difference in these functions attributed to differences in the PFC. We have previously demonstrated that estrogen receptor alpha (ERα) expression decreases in the isocortex across early postnatal development. This decrease corresponds with an increase in methylation of many sites along the ERα promoter. Here we have examined both ERα and ERβ mRNA expression in the PFC to determine if methylation also plays a role in this important brain region. We investigated expression of alternate promoters for ERα and methylation of CpG sites along two of these promoters. We found that the pattern of ERα mRNA expression in PFC was similar to the pattern of ERα expression in the isocortex and that there were no sex differences in the level of expression across development. We did, however, find subtle differences in promoter expression and methylation that may indicate a sex-specific difference in PFC during development resulting in a difference in adult response.

Androgens selectively protect against apoptosis in hippocampal neurones

Androgens can protect neurones from injury, although androgen neuroprotection is not well characterised in terms of either specificity or mechanism. In the present study, we compared the ability of androgens to protect neurones against a panel of insults, empirically determined to induce cell death by apoptotic or non-apoptotic mechanisms. Three criteria defining but not inclusive of apoptosis are: protection by caspase inhibition, protection by protein synthesis inhibition and the presence of pyknotic nuclei. According to these criteria, beta-amyloid, staurosporine, and Apoptosis Activator II induced cell death involving apoptosis, whereas hydrogen peroxide (H(2)O(2)), iron, calcium ionophore and 3-nitropropionic acid induced cell death featuring non-apoptotic characteristics. Pretreatment of hippocampal neurones with testosterone or dihydrotestosterone attenuated cell death induced by beta-amyloid, staurosporine and Apoptosis Activator II, but none of the other insults. The anti-oxidant Trolox did not reduce cell death induced by beta-amyloid, staurosporine and Apoptosis Activator II, but did protect against H(2)O(2) and iron. Similarly, a supra-physiological concentration of oestrogen reduced cell death induced by H(2)O(2) and iron, an effect not observed with androgens. We also show that activation of oestrogen pathways was not necessary for androgen neuroprotection. These data suggest that androgens directly activate a neuroprotective mechanism specific to inhibition of cell death involving apoptosis. Determining the specificity of androgen neuroprotection may enable the development of androgen compounds for the treatment of neurodegenerative disorders.

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.

Dihydrotestosterone activates CREB signaling in cultured hippocampal neurons

Although androgens induce numerous actions in brain, relatively little is known about which cell signaling pathways androgens activate in neurons. Recent work in our laboratory showed that the androgens testosterone and dihydrotestosterone (DHT) activate androgen receptor (AR)-dependent mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling. Since the transcription factor cyclic AMP response element binding protein (CREB) is a downstream effector of MAPK/ERK and androgens activate CREB in non-neuronal cells, we investigated whether androgens activate CREB signaling in neurons. First, we observed that DHT rapidly activates CREB in cultured hippocampal neurons, as evidenced by CREB phosphorylation. Further, we observed that DHT-induced CREB phosphorylation is AR-dependent, as it occurs in PC12 cells stably transfected with AR but in neither wild-type nor empty vector-transfected cells. Next, we sought to identify the signal transduction pathways upstream of CREB phosphorylation using pharmacological inhibitors. DHT-induced CREB phosphorylation in neurons was found to be dependent upon protein kinase C (PKC) signaling but independent of MAPK/ERK, phosphatidylinositol 3-kinase, protein kinase A, and Ca(2+)/calmodulin-dependent protein kinase IV. These results demonstrate that DHT induces PKC-dependent CREB signaling, which may contribute to androgen-mediated neural functions.

Distribution patterns of estrogen receptor alpha and beta in the human cortex and hippocampus during development and adulthood

The expression of estrogen receptors (ERs) in the developing and adult human brain has not been clearly established, although estrogens are crucial for neuronal differentiation, synapse formation, and cognitive functions. By using immunohistochemistry, we have studied the distribution of ER alpha and ER beta in human cerebral cortex and hippocampus from early prenatal stages to adult life. ER alpha was detected in the cortex at 9 gestational weeks (GW), with a high expression in proliferating zones and the cortical plate. The staining intensity decreased gradually during prenatal development but increased again from birth to adulthood. In contrast, ER beta was first detected at 15 GW in proliferating zones, and at 16/17 GW, numerous ER beta immunopositive cells were also observed in the cortical plate. ER beta expression persisted in the adult cortex, being widely distributed throughout cortical layers II-VI. In addition, from around 15 GW to adulthood, ER alpha and ER beta were expressed in human hippocampus mainly in pyramidal cells of Ammon's horn and in the dentate gyrus. Western blotting and immunohistochemistry in the adult cerebral cortex and hippocampus revealed lower protein expression of ER alpha compared with ER beta. Double immunostaining showed that during fetal life both ERs are expressed in neurons as well as in radial glia, although only ER alpha is expressed in the Cajal-Retzius neurons of the marginal zone. These observations demonstrate that the expression of ER alpha and ER beta displays different spatial-temporal patterns during human cortical and hippocampal development and suggest that both ERs may play distinct roles in several processes related to prenatal brain development.

Flutamide and cyproterone acetate exert agonist effects: induction of androgen receptor-dependent neuroprotection

Androgens can exert profound effects on the organization, development, and function of the nervous system through activation of androgen receptors (ARs). Nonsteroidal and steroidal antiandrogens antagonize AR-mediated, classic genomic actions of androgens. However, emerging studies in nonneuronal cells indicate that antiandrogens can act as partial agonists for the AR. Here we investigated the effects of the antiandrogens flutamide and cyproterone acetate on neuroprotection induced by dihydrotestosterone (DHT). We observed that, although flutamide and cyproterone acetate blocked androgen-induced gene expression, they failed to inhibit DHT protection against apoptotic insults in cultured hippocampal neurons. Interestingly, flutamide and cyproterone acetate alone, like DHT, significantly reduced apoptosis. Furthermore, the protective actions of flutamide and cyproterone acetate were observed specifically in AR-expressing cell lines, suggesting a role for AR in the agonist effects of antiandrogens. Our results indicate that, in contrast to the classic antiandrogen properties of flutamide and cyproterone acetate, these AR modulators display agonist activities at the level of neuroprotection. These findings provide new insight into the agonist vs. antagonist properties of antiandrogens, information that will be crucial to understanding the neural implications of clinically used AR-modulating drugs.

Estrogen stimulates the neuronal differentiation of human umbilical cord blood mesenchymal stem cells (CD34-)

This study evaluated the effects of estrogen on the neuronal differentiation of human umbilical cord blood mesenchymal stem cells. Human umbilical cord blood mesenchymal stem cells cultured in a neuronal differentiation medium containing dimethylsulfoxide and butylated hydroxyanisole showed the expression of the neuronal cell-specific protein marker, beta-tubulin III. The estrogen treatment increased the proportion of neurons and neurite branching but reduced the mean neurite length. The relative expression of neurotropic factors such as brain-derived neurotropic factor, glial cell derived neurotropic factor, nerve growth factor, neurotrophin-3, and growth-associated protein 43 were higher in the estrogen-treated group than in the nontreated and estrogen receptor antagonist (ICI-182,780)-treated groups. These results suggest that estrogen stimulates the differentiation of neurons derived from human umbilical cord blood mesenchymal stem cells through the gene expression of neurotrophic factors.

Early post-hatching sex differences in cell proliferation and survival in the quail telencephalic ventricular zone and intermediate medial mesopallium

Previous studies indicated that avian telencephalic areas related to learned behavior, such as song perception and production, are sexually dimorphic. Our study focused on the eventual occurrence of dimorphism in the intermediate medial mesopallium, an area associated with learning in non-singing birds. During early post-hatching life (days 1 and 5) cell proliferation and survival of newborn cells were studied by means of 5-bromo-2-deoxy-uridine immunocytochemistry. Programmed cell death (apoptosis) was investigated at post-hatching day 10. The ventricular zone, intermediate medial part of mesopallium and lateral septal area was analyzed using stereological methods for cell counts. Short-term experiments revealed significantly higher numbers of newborn cells in male ventricular zone of mesopallium compared to female at post-hatching day 1. Long-term survival until post-hatching day 20 showed significantly higher numbers of labeled cells in the male compared to female intermediate medial part of mesopallium, which is the final destination of migrating cells born in the overlying ventricular zone. The vast majority of these early post-hatching newborn cells residing in the intermediate medial part of mesopallium expressed a neuronal phenotype. In addition to neurogenesis, higher numbers of apoptotic figures were found in the male intermediate medial part of mesopallium at post-hatching day 10, suggesting that cell death plays a role in the control of telencephalic regional cell density in males. Our findings indicate that sex-specific mechanisms possibly stimulate increased cell genesis and survival, as well as the counteracting event of increased apoptotic cell death that characterized the male intermediate medial part of mesopallium.

Androgens, aging, and Alzheimer's disease

Testosterone depletion is a normal consequence of aging in men that is associated with senescent effects in androgen- responsive tissues. We discuss new evidence that one consequence of testosterone depletion in men is an increased risk for the development of Alzheimer's disease (AD). Furthermore, we discuss two candidate mechanisms by which testosterone may affect AD pathogenesis. First, testosterone has been identified as an endogenous regulator of beta-amyloid, a protein that abnormally accumulates in AD brain and is implicated as a causal factor in the disease. Second, findings from several different paradigms indicate that testosterone has both neurotrophic and neuroprotective functions. These new findings support the clinical evaluation of androgen-based therapies for the prevention and treatment of AD.

Novel cellular phenotypes and subcellular sites for androgen action in the forebrain

Historically, morphological studies of the distribution of androgen receptors in the brain led to conclusions that the major regional targets of androgen action are involved in reproduction, that the primary cellular targets are neurons, and that functional androgen receptors are exclusively nuclear, consistent with the classical view of steroid receptors as ligand-dependent transcription factors. In this review, we discuss three separate but interrelated recent studies highlighting observations made with newer methodologies while assessing the regional, cellular or subcellular distribution of androgen receptors containing cells in the forebrain. Regional studies demonstrated that the largest forebrain target for androgen action in terms of the number of androgen receptor expressing cells is the cerebral cortex, rather than the main hypothalamic and limbic centers for reproductive function. Cellular studies to determine the phenotype of androgen receptor expressing cells confirmed that most of these cells are neurons but also revealed that small subpopulations are astrocytes. The expression of androgen receptors in astrocytes is both region and age dependent. In contrast, reactive astrocytes in the lesioned adult rat brain do not express androgen receptors whereas reactive microglia do. Finally, androgen receptor immunoreactive axons were identified in the cerebral cortex of the rat and human. These observations do not overturn classical views of the cellular and subcellular locus of steroid action in the nervous system, but rather broaden our view of the potential direct impact of gonadal steroid hormones on cellular function and emphasize the regional and developmental specificity of these effects on the nervous system.

Estradiol modulation of kainic acid-induced calcium elevation in neonatal hippocampal neurons

The developing hippocampus of both males and females is exposed to high levels of the gonadal steroid estradiol. The impact of this estradiol exposure on developing hippocampal neurons is essentially unknown. In the rat, the newborn hippocampus is relatively insensitive to excitotoxic brain injury, which in adults is associated with the release of amino acids, in particular glutamate, resulting in a significant increase in intracellular calcium and eventual cell death. We have shown previously in the rat that administration of the glutamate agonist, kainic acid (KA), on the day of birth results in limited hippocampal damage, which is ameliorated by treatment with the gonadal steroid, estradiol. We now show that KA induces an increase in intracellular calcium through L-type voltage-sensitive calcium channels early in development and, later in development, through polyamine-sensitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors with a modest increase through N-methyl-D-aspartate receptors. Pretreatment with the gonadal steroid, estradiol, decreases the percentage of neurons responding to KA and decreases the peak amplitude of the calcium transient early in development but has no effect later in development. Taken together, these data suggest that there is a developmental shift in the route of KA-induced intracellular calcium and estradiol modulates KA-induced intracellular calcium to a time restricted to early development, but whether this is the basis of the neuroprotective effect of estradiol remains to be determined.

Macrocephaly and the control of brain growth in autistic disorders

Autism is a childhood-onset neuropsychiatric disorder characterized by marked impairments in social interactions and communication, with restricted stereotypic and repetitive patterns of behavior, interests, and activities. Genetic epidemiology studies indicate that a strong genetic component exists to this disease, but these same studies also implicate significant environmental influence. The disorder also displays symptomatologic heterogeneity, with broad individual differences and severity on a graded continuum. In the search for phenotypes to resolve heterogeneity and better grasp autism's underlying biology, investigators have noted a statistical overrepresentation of macrocephaly, an indicator of enlarged brain volume. This feature is one of the most widely replicated biological findings in autism. What then does brain enlargement signify? One hypothesis invoked for the origin of macrocephaly is a reduction in neuronal pruning and consolidation of synapses during development resulting in an overabundance of neurites. An increase in generation of cells is an additional mechanism for macrocephaly, though it is less frequently discussed in the literature. Here, we review neurodevelopmental mechanisms regulating brain growth and highlight one underconsidered potential causal mechanism for autism and macrocephaly--an increase in neurogenesis and/or gliogenesis. We review factors known to control these processes with an emphasis on nuclear receptor activation as one signaling control that may be abnormal and contribute to increased brain volume in autistic disorders.

Regional, Laminar and Cellular Distribution of Immunoreactivity for ERβ in the Cerebral Cortex of Hormonally Intact, Postnatally Developing Male and Female Rats

Estrogen influences cerebral cortical development. Among the receptors involved are classical (ERalpha) and beta (ERbeta) intracellular estrogen receptors. In the first 2 weeks of postnatal life, cortical ERalpha is transiently expressed at much higher levels than in adulthood. In this study, development of ERbeta was examined by mapping ERbeta immunoreactivity in relation to major cortical regions, layers and cell types in postnatal male and female rats that were 1-28 postnatal days (PND) old. These studies revealed that ERbeta-immunoreactive nuclei were present in the allocortices on PND 1 but were not detected in isocortex until PND 7. Allocortical labeling was also higher on PND 1 than at all later ages, while in isocortical areas low numbers of ERbeta nuclei were seen on PND 7 that rose to higher, near adult densities by PND 21. Finally, double labeling showed that ERalpha was expressed mainly in neurons immunopositive for calretinin, while ERbeta was localized predominantly in parvalbumin-immunoreactive cells. Thus, the postnatal cortical developments of ERbeta and ERalpha occur according to different timetables, different patterns and in association with different cortical cells. It thus seems it likely that the two also make distinct contributions to postnatal cortical development and/or sexual differentiation.

Developmental neurotoxicity of PHAHs: Endocrine-mediated and general behavioral endpoints in adult male rats

During development, gonadal steroids exert effects on the nervous system which are long-lasting or organizational, in contrast to the transient activational actions in adulthood. Therefore, disturbance of neuroendocrine functions by developmental exposure to polyhalogenated aromatic hydrocarbons (PHAHs) is likely to affect sex-dependent behavior in adults. Our previous data revealed effects of maternal PCB exposure on sexual differentiation of the brain and subsequent sweet preference as sexually dimorphic behavior in adult offspring. Present research is focused on brominated flame retardants because of their wide-spread use and accumulation in human breast milk. Pregnant Long Evans rats were SC injected with PBDE 99 (2,2',4,4',5-PBDE) daily from gestational day 10 to 18. For comparison, an additional group was exposed to Aroclor 1254. Preliminary results indicate a dose-related increase in sweet preference in adult male offspring exposed to PBDE. Exposure also led to decreases in testosterone and estradiol serum levels. Additional decreases were detected in male anogenital distance. There were no changes of locomotor activity in the open field. On haloperidol-induced catalepsy, latencies were prolonged in all exposed males. In summary, PBDE induced endocrine effects and concomitant changes of sex-dependent behavior similar to PCBs. Outcome of general behavior suggests an involvement of dopaminergic processes in developmental PBDE exposure.

Effects of male and female sex steroids on the development of normal and the transient Froriep's dorsal root ganglia of the chick embryo

Sex steroids can influence developmental processes and support the survival of neurons in the embryonic central nervous system. Recent studies have shown that estrogen receptors are also expressed in the peripheral nervous system, in the dorsal root ganglia (DRG) of chick embryos. However, no studies have examined the effects of sex steroids on development of embryonic DRG. In the present study, 0.2 microg, 1.0 microg, 5.0 microg 10 microg, 20 microg, 25 microg, and 40 microg doses of testosterone or estradiol were delivered to chick embryos at Hamburger and Hamilton stage 18 (E3). The actions of these doses of sex steroids on the development of the C5DRG (fifth cervical ganglion, a "normal" DRG) and C2DRG (a transient ganglion known as a "Froriep's DRG") were then evaluated by quantifying ganglionic volumes, cell number, proliferation, and apoptosis after 1 day of growth to stage 23. We found that both testosterone and estradiol promoted proliferation of cells in both normal DRG and the Froriep's ganglia. By contrast, estradiol significantly increased the number of apoptotic cells, while testosterone strongly inhibited apoptosis. These actions of sex steroids on DRG development were dose-dependent, and C5DRG and C2DRG showed different sensitivities to the applied sex steroids. In addition, the present results demonstrated that specific ER and AR inhibitors (tamoxifen and flutamide) did not influence the effects of 5 microg E2 and 5 microg T on C2 and C5DRG significantly. These results demonstrate that male and female sex steroids can modulate DRG development through an epigenetic mechanism, as had been shown for the central nervous system.

Direct binding of estradiol enhances Slack (sequence like a calcium-activated potassium channel) channels’ activity

17Beta-estradiol (E2) is a major neuroregulator, exerting both genomic and non-genomic actions. E2 regulation of Slack (sequence like a calcium-activated potassium channel) potassium channels has not been identified in the CNS. We demonstrate E2-induced activation of Slack channels, which display a unitary conductance of about 60 pS, are inhibited by intracellular calcium, and are abundantly expressed in the nervous system. In lipid bilayers derived from rat cortical neuronal membranes, E2 increases Slack open probability and appears to decrease channel inactivation. Additionally, E2 binds to the Slack channel and activates outward currents in human embryonic kidney-293 cells that express Slack channels but not classical estrogen receptors (i.e. ERalpha or ERbeta). Neither E2-induced activation nor the binding intensity of E2 to the Slack channel is blocked by tamoxifen, an ER antagonist/agonist. Thus, E2 activates a potassium channel, Slack, through a non-traditional membrane binding site, adding to known non-genomic mechanisms by which E2 exerts pharmacological and toxicological effects in the CNS.

β-estradiol influences differentiation of hippocampal neurons in vitro through an estrogen receptor-mediated process

We utilized morphometric analysis of 3 day cultures of hippocampal neurons to determine the effects of both estradiol and the synthetic estrogen receptor modulator raloxifene on several parameters of neuronal growth and differentiation. These measurements included survival, neurite production, dendrite number, and axon and dendrite length and branching. 17 beta-Estradiol (10 nM) selectively stimulated dendrite branching; this effect was neither mimicked by alpha-estradiol, nor blocked by the estrogen receptor antagonist ICI 182780. The selective estrogen receptor modulator raloxifene (100 nM) neither mimicked nor reversed the effects of estradiol on dendritic branching. Western immunoblotting for the alpha and beta subtypes of estrogen receptor revealed the presence of alpha, but not beta, estrogen receptors in our hippocampal cultures. There is growing recognition of the effects of 17 beta-estradiol on neuronal development and physiology, with implications for brain sexual dimorphism, plasticity, cognition, and the maintenance of cognitive function during aging. The role of estradiol in hippocampal neuronal differentiation and function has particular implications for learning and memory. These data support the hypothesis that 17 beta-estradiol is acting via alpha estrogen receptors in influencing hippocampal development in vitro. Raloxifene, prescribed to combat osteoporosis in post-menopausal women, is a selective estrogen receptor modulator with tissue-specific agonist/antagonist properties. Because raloxifene had no effect on dendritic branching, we hypothesize that it does not interact with the alpha estrogen receptor in this experimental paradigm.

Enhancement of dendritic branching in cultured hippocampal neurons by 17beta-estradiol is mediated by nitric oxide

Both 17beta-estradiol (E2) and nitric oxide (NO) are important in neuronal development, learning and memory, and age-related memory changes. There is growing evidence that a number of estrogen receptor-mediated effects of estradiol utilize nitric oxide as an intermediary. The role of estradiol in hippocampal neuronal differentiation and function has particular implications for learning and memory. Low levels of estradiol (10nM) significantly increase dendritic branching in cultured embryonic rat hippocampal neurons (158% of control). This study investigates the hypothesis that the estrogen-stimulated increase in dendritic branching is mediated by nitric oxide. We found that nitric oxide donors also produce significantly increased dendritic branching S-nitroso-N-acetylpenicillamine (SNAP: 119%; 2,2'-(hydroxynitrosohydrazino)bis-ethanamine (NOC-18): 128% of control). We then determined that the increases in dendritic branching stimulated by estradiol or by a nitric oxide donor were both blocked by an inhibitor of guanylyl cyclase. Dendritic branching was also stimulated by a cell permeable analog of cyclic guanosine monophosphate (dibutyryl-cGMP: 173% of control). Estradiol-stimulated dendritic branching was reversed by the nitric oxide scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl imidazoline-1-oxyl 3-oxide (carboxy-PTIO). This study provides evidence that estradiol influences the development of embryonic hippocampal neurons in culture by increasing the production of nitric oxide or by increasing the sensitivity of the neurons to nitric oxide. Nitric oxide in turn stimulates dendritic branching via activation of guanylyl cyclase.

Estrogen receptor (ER)beta knockout mice reveal a role for ERbeta in migration of cortical neurons in the developing brain

The present study stems from our previous observations that the brains of adult estrogen receptor beta knockout (ERbeta-/-) mice show regional neuronal hypocellularity especially in the cerebral cortex. We now show that ERbeta is necessary for late embryonic development of the brain and is involved in both neuronal migration and apoptosis. At embryonic day (E)18.5, ERbeta-/- mouse brains were smaller than those of the wild-type (WT) littermates, and there were fewer neurons in the cortex. There were no differences in size or cellularity at E14.5. When proliferating cells were labeled with 5'-bromodeoxyuridine (BrdUrd) on E12.5, a time when cortical neurogenesis in mice begins, and examined on E14.5, there was no difference between WT and ERbeta-/- mice in the number of labeled cells in the cortex. However, when BrdUrd was administered between E14.5 and E16.5, a time when postmitotic neurons migrate to layers of the cortex, there were fewer BrdUrd-labeled cells in the superficial cortical layers by E18.5 and postnatal day 14 in mice lacking ERbeta. At E18.5, there were more apoptotic cells in the ventricular zone of mice lacking ERbeta. In addition, the processes of the cortical radial glia, which are essential for guiding the migrating neurons, were fragmented. These findings suggest that by influencing migration and neuronal survival, ERbeta has an important role in brain development.

Expression of androgen receptor mRNA in the late embryonic and early posthatch zebra finch brain

Zebra finch males sing and females do not, and the underlying neural circuitry in males is more developed than that in females. Sex steroid hormones influence the development of sex differences in this circuitry, including differences in androgen receptor (AR) expression, although the role of androgens has been controversial. We isolated a cDNA encoding a portion of the zebra finch AR and used in situ hybridization to examine the spatiotemporal pattern of AR mRNA expression in the brain during late embryonic development and at hatching. We detected AR mRNA in all the major subdivisions of the brain as early as embryonic day 10. No qualitative sex differences in AR mRNA expression patterns were observed. Cells lining the ventral arm of the lateral telencephalic ventricles expressed AR mRNA on embryonic day 11 and posthatching day 1, as did cells lining the third ventricle at all three developmental stages examined, suggesting that androgens may play a role in early stages of cellular proliferation, migration, or differentiation. AR mRNA was also detected in the hippocampus, neostriatum, septum, ventromedial archistriatum, hypothalamic regions, dorsal mesencephalon, and in and around the brainstem nucleus tracheosyringealis. Our results suggested that androgens act early in neural development and therefore may contribute to the process of sexual differentiation.

Ovarian hormones after postnatal day 20 reduce neuron number in the rat primary visual cortex

Previous work from our lab has documented a sex difference in neuron number in the binocular region of the adult rat primary visual cortex (Oc1B), with males having 19% more neurons than females. In the present study, the role of developmental steroid hormones in the formation of this difference was explored. Male and female rats underwent neonatal hormone manipulation (female + testosterone or dihydrotestosterone; male + flutamide) followed by gonadectomy on postnatal day 20. Animals that did not undergo hormone manipulation were either gonadectomized or sham operated at day 20. Neuron number was quantified in the monocular (Oc1M) and binocular (Oc1B) subfields of the adult rat primary visual cortex using the optical disector technique. As adults, day 20 gonadectomized females, as well as females + testosterone and females + dihydrotestosterone, had significantly more neurons than intact females. There was no difference in neuron number between postnatal day 20 gonadectomized males, males + flutamide, and intact males. Also, intact males had significantly more neurons than intact females in both in Oc1M and Oc1B. It appears that ovarian steroids after day 20 are the primary cause of the lower number of neurons in the primary visual cortex of the female rat.

Estrogen protects against beta-amyloid-induced neurotoxicity in rat hippocampal neurons by activation of Akt

The cellular mechanisms underlying the neuroprotective effects of estrogen are only beginning to be elucidated. Here we examined the role of protein kinase B (Akt) activation in 17beta-estradiol (E2) inhibition of beta-amyloid peptide (31-35) (Abeta31-35)-induced neurotoxicity in cultured rat hippocampal neurons. Abeta31-35 (25-30 betaM) significantly decreased the total number of microtubule associated protein-2 positive cells (MAP2+). This decrease was significantly reversed by pre-treatment with 100 nM E2. Further, 100 nM E2 alone significantly increased the total number of protein kinase B and microtubule associated protein-2 positive cells compared with controls. Such E2-induced increases were inhibited by LY294002 (20 microM), a specific PI3-K inhibitor, as well as by tamoxifen, an estrogen receptor antagonist/selective estrogen receptor modulator. These results indicate that the neuroprotective effects of E2 may be mediated at least in part via estrogen receptor-mediated protein kinase B activation.

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.

Differential mechanisms of neuroprotection by 17 beta-estradiol in apoptotic versus necrotic neurodegeneration

The major goal of this study was to compare mechanisms of the neuroprotective potential of 17 beta-estradiol in two models for oxidative stress-independent apoptotic neuronal cell death with that in necrotic neuronal cell death in primary neuronal cultures derived from rat hippocampus, septum, or cortex. Neuronal apoptosis was induced either by staurosporine or ethylcholine aziridinium (AF64A), as models for necrotic cell death glutamate exposure or oxygen-glucose deprivation (OGD) were applied. Long-term (20 hr) pretreatment (0.1 microm 17 beta-estradiol) was neuroprotective in apoptotic neuronal cell death induced by AF64A (40 microm) only in hippocampal and septal neuronal cultures and not in cortical cultures. The neuroprotective effect was blocked by the estrogen antagonists ICI 182,780 and tamoxifen and the phosphatidylinositol 3-kinase (PI3-K) inhibitor LY294002. In glutamate and OGD-induced neuronal damage, long-term pretreatment was not effective. In contrast, short-term (1 hr) pretreatment with 17 beta-estradiol in the dose range of 0.5-1.0 microm significantly reduced the release of lactate dehydrogenase and improved morphology of cortical cultures exposed to glutamate or OGD but was not effective in the AF64A model. Staurosporine-induced apoptosis was not prevented by either long- or short-term pretreatment. The strong expression of the estrogen receptor-alpha and the modulation of Bcl proteins by 17 beta-estradiol in hippocampal and septal but not in cortical cultures indicates that the prevention of apoptotic, but not of necrotic, neuronal cell death by 17 beta-estradiol possibly depends on the induction of Bcl proteins and the density of estrogen receptor-alpha.