Cellular localization of estrogen receptors on neurones in various regions of cultured rat CNS: coexistence with cholinergic and galanin receptors

Acetylcholine, Another Factor in Breast Cancer

Acetylcholine (ACh) is a neurotransmitter that regulates multiple functions in the nervous system, and emerging evidence indicates that it could play a role in cancer progression. However, this function is controversial. Previously, we showed that organophosphorus pesticides decreased the levels of the enzyme acetylcholinesterase in vivo, increasing ACh serum levels and the formation of tumors in the mammary glands of rats. Furthermore, we showed that ACh exposure in breast cancer cell lines induced overexpression of estrogen receptor alpha (ERα), a key protein described as the master regulator in breast cancer. Therefore, here, we hypothesize that ACh alters the ERα activity through a ligand-independent mechanism. The results here reveal that the physiological concentration of ACh leads to the release of Ca+2 and the activity of MAPK/ERK and PI3K/Akt pathways. These changes are associated with an induction of p-ERα and its recruitment to the nucleus. However, ACh fails to induce overexpression of estrogen-responsive genes, suggesting a different activation mechanism than that of 17ß-estradiol. Finally, ACh promotes the viability of breast cancer cell lines in an ERα-dependent manner and induces the overexpression of some EMT markers. In summary, our results show that ACh promotes breast cancer cell proliferation and ERα activity, possibly in a ligand-independent manner, suggesting its putative role in breast cancer progression.

Modulation of Muscarinic Signalling in the Central Nervous System by Steroid Hormones and Neurosteroids

Muscarinic acetylcholine receptors expressed in the central nervous system mediate various functions, including cognition, memory, or reward. Therefore, muscarinic receptors represent potential pharmacological targets for various diseases and conditions, such as Alzheimer's disease, schizophrenia, addiction, epilepsy, or depression. Muscarinic receptors are allosterically modulated by neurosteroids and steroid hormones at physiologically relevant concentrations. In this review, we focus on the modulation of muscarinic receptors by neurosteroids and steroid hormones in the context of diseases and disorders of the central nervous system. Further, we propose the potential use of neuroactive steroids in the development of pharmacotherapeutics for these diseases and conditions.

Astrocyte-Derived Thrombospondin Induces Cortical Synaptogenesis in a Sex-Specific Manner

The regulation of synaptic connectivity in the brain is vital to proper functioning and development of the CNS. Formation of neural networks in the CNS has been shown to be heavily influenced by astrocytes, which secrete factors, including thrombospondin (TSP) family proteins, that promote synaptogenesis. However, whether this process is different between males and females has not been thoroughly investigated. In this study, we found that cortical neurons purified from newborn male rats showed a significantly more robust synaptogenic response compared with female-derived cells when exposed to factors secreted from astrocytes. This difference was driven largely by the neuronal response to TSP2, which increased synapses in male neurons while showing no effect on female neurons. Blockade of endogenous 17β-estradiol (E2) production with letrozole normalized the TSP response between male and female cells, indicating a level of regulation by estrogen signaling. Our results suggest that male and female neurons show a divergent response to TSP synaptogenic signaling, contributing to sex differences in astrocyte-mediated synaptic connectivity.

Interactions between reproductive transitions during aging and addiction: promoting translational crosstalk between different fields of research

Discovery of neural mechanisms underlying neuropsychiatric disorders within the aging and addiction fields has been a main focus of the National Institutes of Health. However, there is a dearth of knowledge regarding the biological interactions of aging and addiction, which may have important influences on progression of disease and treatment outcomes in aging individuals with a history of chronic drug use. Thus, there is a large gap in these fields of research, which has slowed progress in understanding and treating substance use disorders (SUDs) as well as age-related diseases, specifically in women who experience precipitous reproductive cycle transitions during aging. The goal of this review is to highlight overlap of SUDs and age-related processes with a specific focus on menopause and smoking, and identify critical gaps. We have narrowed the focus of the review to smoking, as the majority of findings on hormonal and aging influences on drug use have come from this area of research. Further, we highlight female-specific issues such as transitional menopause and exogenous estrogen use. These issues may impact drug use cessation as well as outcomes with aging and age-related neurodegenerative diseases in women. We first review clinical studies for smoking, normal aging, and pathological aging, and discuss the few aging-related studies taking smoking history into account. Conversely, we highlight the dearth of clinical smoking studies taking age as a biological variable into account. Preclinical and clinical literature show that aging, age-related pathological brain disease, and addiction engage overlapping neural mechanisms. We hypothesize that these putative drivers interact in meaningful ways that may exacerbate disease and hinder successful treatment outcomes in such comorbid populations. We highlight areas where preclinical studies are needed to uncover neural mechanisms in aging and addiction processes. Collectively, this review highlights the need for crosstalk between different fields of research to address medical complexities of older adults, and specifically women, who smoke.

Sexually diergic hypothalamic-pituitary-adrenal axis responses to selective and non-selective muscarinic antagonists prior to cholinergic stimulation by physostigmine in rats

Central cholinergic systems regulate the hypothalamic-pituitary-adrenal (HPA) axis differentially in males and females (sexual diergism). We previously investigated the role of muscarinic receptors in this regulation by administering physostigmine (PHYSO), an acetylcholinesterase inhibitor, to male and female rats pretreated with scopolamine (SCOP), a nonselective muscarinic antagonist. SCOP pretreatment enhanced adrenocorticotropic hormone (ACTH) and corticosterone (CORT) responses in both sexes, but males had greater ACTH responses while females had greater CORT responses. In the present study, we further explored the role of muscarinic receptor subtypes in HPA axis regulation by administering PHYSO to male and female rats following SCOP or various doses of either the M1 or the M2 selective muscarinic receptor antagonists, pirenzepine (PIREN) or methoctramine (METHO). Blood was sampled before and at multiple times after PHYSO. ACTH and CORT were determined by highly specific immunoassays. M1 antagonism by PIREN prior to PHYSO resulted in sustained, dose-dependent increases in ACTH and CORT: ACTH responses were similar in both sexes, and CORT responses were greater in females. M2 antagonism by METHO prior to PHYSO resulted in overall decreases in ACTH and CORT: ACTH and CORT responses were higher in females but lower in both sexes than the hormone responses following PIREN or SCOP pretreatment. Area under the curve analyses supported these findings. These results suggest that specific muscarinic receptor subtypes differentially influence the HPA axis in a sexually diergic manner.

Concerted Gene Expression of Hippocampal Steroid Receptors during Spatial Learning in Male Wistar Rats: A Correlation Analysis

Adrenal and gonadal steroid receptor activities are significantly involved and interact in the regulation of learning, memory and stress. Thus, a coordinated expression of steroid receptor genes during a learning task can be expected. Although coexpression of steroid receptors in response to behavioral tasks has been reported the correlative connection is unclear. According to the inverted U-shape model of the impact of stress upon learning and memory we hypothesized that glucocorticoid (GR) receptor expression should be correlated to corticosterone levels in a linear or higher order manner. Other cognition modulating steroid receptors like estrogen receptors (ER) should be correlated to GR receptors in a quadratic manner, which describes a parabola and thus a U-shaped connection. Therefore, we performed a correlational meta-analyis of data of a previous study (Meyer and Korz, 2013a) of steroid receptor gene expressions during spatial learning, which provides a sufficient data basis in order to perform such correlational connections. In that study male rats of different ages were trained in a spatial holeboard or remained untrained and the hippocampal gene expression of different steroid receptors as well as serum corticosterone levels were measured. Expressions of mineralocorticoid (MR) and GR receptors were positively and linearly correlated with blood serum corticosterone levels in spatially trained but not in untrained animals. Training induced a cubic (best fit) relationship between mRNA levels of estrogen receptor α (ERα) and androgen receptor (AR) with MR mRNA. GR gene expression was linearly correlated with MR expression under both conditions. ERα m RNA levels were negatively and linearily and MR and GR gene expressions were cubicely correlated with reference memory errors (RME). Due to only three age classes correlations with age could not be performed. The findings support the U-shape theory of steroid receptor interaction, however the cubic fit suggest a more complex situation, which mechanisms may be revealed in further studies.

Tamoxifen Promotes Axonal Preservation and Gait Locomotion Recovery after Spinal Cord Injury in Cats

We performed experiments in cats with a spinal cord penetrating hemisection at T13-L1 level, with and without tamoxifen treatment. The results showed that the numbers of the ipsilateral and contralateral ventral horn neurons were reduced to less than half in the nontreated animals compared with the treated ones. Also, axons myelin sheet was preserved to almost normal values in treated cats. On the contrary, in the untreated animals, their myelin sheet was reduced to 28% at 30 days after injury (DAI), in both the ipsilateral and contralateral regions of the spinal cord. Additionally, we made hindlimb kinematics experiments to study the effects of tamoxifen on cat locomotion after the injury: at 4, 16, and 30 DAI. We observed that the ipsilateral hindlimb angular displacement (AD) of the pendulum-like movements (PLM) during gait locomotion was recovered to almost normal values in treated cats. Contralateral PLM acquired similar values to those obtained in intact cats. At 4 DAI, untreated animals showed a compensatory increment of PLM occurring in the contralateral hindlimb, which was partially recovered at 30 DAI. Our findings indicate that tamoxifen exerts a neuroprotective effect and preserves or produces myelinated axons, which could benefit the locomotion recovery in injured cats.

Estrogen-regulated synaptogenesis in the hippocampus: sexual dimorphism in vivo but not in vitro

Hippocampal neurons are capable of synthesizing estradiol de novo. Estradiol synthesis can be suppressed by aromatase inhibitors and by knock-down of steroid acute regulatory protein (StAR), whereas elevated levels of substrates of steroidogenesis enhance estradiol synthesis. In rat hippocampal cultures, the expression of estrogen receptors (ERs) and synaptic proteins, as well as synapse density, correlated positively with aromatase activity, regardless of whether the cultures originated from males or females. All effects induced by the inhibition of aromatase activity were rescued by application of estradiol to the cultures. In vivo, however, systemic application of letrozole, an aromatase inhibitor, induced synapse loss in female rats, but not in males. Furthermore, in the female hippocampus, density of spines and spine synapses varied with the estrus cycle. In addressing this in vivo-in vitro discrepancy, we found that gonadotropin-releasing hormone (GnRH) regulated estradiol synthesis via an aromatase-mediated mechanism and consistently regulated spine synapse density and the expression of synaptic proteins. Along these lines, GnRH receptor density was higher in the hippocampus than in the cortex and hypothalamus, and estrus cyclicity of spinogenesis was found in the hippocampus, but not in the cortex. Since GnRH receptor expression also varies with the estrus cycle, the sexual dimorphism in estrogen-regulated spine synapse density in the hippocampus very likely results from differences in the GnRH responsiveness of the male and the female hippocampus. This article is part of a Special Issue entitled 'Neurosteroids'.

Estradiol and GABAergic transmission in the hippocampus

Estradiol is synthesized in the hippocampus and is known to increase intrinsic hippocampal excitability and capacity for synaptic plasticity. A picture emerges that at least part of these effects are due to a complex modulation of GABAergic system in developing and adult hippocampus. During development, GABAergic system undergoes profound alterations and is particularly prone to modulation. During this period, estradiol could modulate both phasic and tonic GABAergic currents and promote excitatory GABA actions. In contrast, in adult hippocampus, estradiol-induced formation of new dendritic spines in pyramidal cells is paralleled with a reduction in GABAergic drive to these neurons. Such estradiol actions could be mediated primarily through interneurons expressing estrogen receptors. In this chapter, we provide an overview of the in vitro and in vivo studies addressing the role of estradiol in regulating the GABAergic system in the hippocampal formation during development and in the adulthood. Although the mechanisms underlying such a regulation remain largely unknown, we make an attempt to present the major hypotheses and concepts related to this issue.

The opposing roles of estradiol on synaptic protein expression in hippocampal cultures

Estrogen-induced synaptic plasticity was frequently shown by an increase of spines at apical dendrites of CA1 pyramidal neurons after systemic application of estradiol to ovariectomized rats. Surprisingly, exogenous application of estradiol to hippocampal cultures had no effect on spines and on spine synapses, although quantitative immunohistochemistry revealed an upregulation of spinophilin and of synaptophysin, in these cultures. The role of synaptophysin as a presynaptic marker and of spinophilin as a postsynaptic marker, appears questionable from these discrepancies. In contrast, synaptopodin, a marker protein of "mature" mushroom-shaped spines, was downregulated after treatment of hippocampal cultures with estradiol. Synaptopodin is strongly associated to the spine apparatus, a spine-specific cell organelle, which is present in 80% of all mushroom-shaped spines. Consistently, we found a reduction in the number of spines, containing a spine apparatus in response to estradiol, suggesting that the presence of a spine apparatus in many but not all spines is very likely a result of their dynamic character. In summary, synaptic proteins appear to be regulated by estradiol, independent of its function on spine and spine synapse formation.

17beta-estradiol affects GABAergic transmission in developing hippocampus

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

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

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

Distribution of galanin receptor-2 immunoreactive neurones in the ovine hypothalamus: no evidence for involvement in the control of gonadotrophin-releasing hormone secretion

Galanin is a small neuropeptide that mediates its effects via three receptor isoforms: galanin receptor-1, galanin receptor-2 and galanin receptor-3 (Gal-R1, Gal-R2 and Gal-R3). Galanin is thought to be an important intermediate in signalling in the hypothalamic-pituitary-gonadal axis and has been widely detected in the ovine hypothalamus. The expression of galanin and Gal-R1 has been reported to fluctuate during the reproductive cycle. Although the distribution of Gal-R1 has been determined in the ovine hypothalamus, the distribution of Gal-R2 was hitherto unknown. Using immunohistological and immunofluorescence techniques, we have mapped the distribution of Gal-R2 in the ovine hypothalamus, collected during the follicular phase of the oestrous cycle and examined colocalisation of Gal-R2 with oestrogen receptor alpha (ERalpha) and gonadotrophin-releasing hormone (GnRH). Gal-R2 was expressed in several regions of the hypothalamus (supraoptic nucleus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus) but not as widely expressed as Gal-R1. Areas of Gal-R2 expression overlapped with those reported for Gal-R1. We observed that, in certain defined regions of the hypothalamus, up to 50% of neurones that express Gal-R2 also express ERalpha. No neurones coexpressed Gal-R2 and GnRH. Thus, we conclude that, in follicular phase animals, this receptor plays little or no role in direct intermediary signal transmission in GnRH-mediated control of the reproductive cycle.

The galaninergic enteric nervous system of pleuronectiformes (Pisces, Osteichthyes): an immunohistochemical and confocal laser scanning immunofluorescence study

The galaninergic enteric nervous system of three flatfishes was studied using immunohistochemical and immunofluorescence methods. Galanin-like immunoreactivity was mainly detected within the enteric intramural neurons of the stomachs and the proximal intestines. The sole, Solea solea L. and the flounder, Platichthys flesus L. showed a similar occurrence and relative distribution of galaninergic intramural neurons. Rare nervous fibre immunoreactive to the anti-galanin serum were observed in the muscular layers of the oesophagus of the turbot, Psetta maxima L. The presence and relative abundance of galanin-like immunoreactive neurons in the remaining organs of the alimentary canal of the turbot showed a different pattern in comparison to those observed in the sole and the flounder. A galanin-like peptide was detected in nerve fibres running through the exocrine parenchyma of the pancreas of all three species. It is conceivable that the galaninergic system in these species plays a role in regulating gut muscle activity, and in controlling pancreatic secretion. Galanin and choline acetyltransferase were co-localized within the same neurons of the stomach and intestine. The result confirms the hypothesis that galanin in the gut of fish functions as a cholinergic modulator. Differently from other fish species, immunoreactive endocrine cells were not detected in the studied pleuronectiformes.

Astrocytes in 17beta-estradiol treated mixed hippocampal cultures show attenuated calcium response to neuronal activity

Glial cells in the brain are capable of responding to hormonal signals. The ovarian steroid hormone 17beta-estradiol, in addition to its actions on neurons, can directly affect glial cells. Estrogen receptors have been described on both neurons and astrocytes, suggesting a complex interplay between these two in mediating the effects of the hormone. Astrocytes sense and respond to neuronal activity with a rise in intracellular calcium concentration ([Ca(2+)](i)). Using simultaneous electrophysiology and calcium imaging techniques, we monitored neuronal activity evoked astrocyte ([Ca(2+)](i)) changes in mixed hippocampal cultures loaded with fluo-3 AM. Action potential firing in neurons, elicited by injecting depolarizing current pulses, was associated with ([Ca(2+)](i)) elevations in astrocytes, which could be blocked by 200 microM MCPG and also 1 microM TTX. We compared astrocytic ([Ca(2+)](i)) transients in control and 24-hour estradiol treated cultures. The amplitude of the ([Ca(2+)](i)) transient, the number of responsive astrocytes, and the ([Ca(2+)](i)) wave velocity were all significantly reduced in estradiol treated cultures. ([Ca(2+)](i)) rise in astrocytes in response to local application of the metabotropic glutamate receptor (mGluR) agonist t-ACPD was attenuated in estradiol treated cultures, suggesting functional changes in the astrocyte mGluR following 24-h treatment with estradiol. Since astrocytes can modulate synaptic transmission by release of glutamate, the attenuated ([Ca(2+)](i)) response seen following estradiol treatment could have functional consequences on astrocyte-neuron signaling.

Direct and indirect effects of estrogen on rat hippocampus

Estrogen-induced synaptic plasticity was frequently shown by an increase of spines at apical dendrites of CA1 pyramidal neurons after systemic application of estradiol to ovariectomized rats. Recent findings question this direct endocrine regulation of synaptogenesis by estradiol. We have shown, for the first time, that estrogens are synthesized de novo in rat hippocampal neurons. By using letrozole, an inhibitor of aromatase, estradiol levels in hippocampal dispersion cultures as well as in hippocampal slice cultures were significantly suppressed. Letrozole treatment resulted in a significant decrease in the density of spines and spine synapses and in the number of presynaptic boutons. Quantitative immunohistochemistry revealed a dose-dependent downregulation of spinophilin, a spine marker, and of synaptophysin, a presynaptic marker, in the hippocampus. Surprisingly, exogenous application of estradiol to the cultures had no effect. Indirect effects of estrogens, mediated via subcortical nuclei, may help to explain this phenomenon. Implantation of estrogen-filled cannulae into the median raphe, which projects to the hippocampus, resulted in a significant increase in spine density in the hippocampus after seven days of treatment. This increase was paralleled by a decrease in the density of serotonergic innervation of the strata lacunosum moleculare and radiatum of the CA1 region. Apart from direct endocrine mechanisms our findings suggest that estradiol-induced spinogenesis in the hippocampus is also mediated by indirect mechanisms and is furthermore regulated endogenously, in a paracrine manner.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Neurosteroid synthesis in the hippocampus: Role in synaptic plasticity

Neurosteroids are still found in the brain after steroidogenic glands were removed, indicating that they are synthesized either de novo or from endogenous precursors by enzymes present in the CNS. In fact, steroidogenic acute regulatory protein, and aromatase, two molecules essential for estrogen synthesis, are expressed in the hippocampus. We recently showed, for the first time, that estrogens are synthesized de novo in hippocampal neurons and that these hippocampus-derived estrogens are essential for synaptic plasticity. Both estrogen receptor isoforms, estrogen receptor alpha and estrogen receptor beta, are expressed in the hippocampus, and estradiol treatment of the cultures leads to an upregulation of estrogen receptor alpha. This finding confirmed the presence of functional estrogen receptors in hippocampal neurons and showed the responsiveness of the cultured hippocampal neurons to estradiol. By using letrozole, an inhibitor of aromatase, estradiol levels in hippocampal dispersion cultures as well as in hippocampal slice cultures were significantly suppressed which in turn led to a downregulation of estrogen receptor alpha. Letrozole treatment was followed by a significant decrease in the density of spines and spine synapses and in the number of presynaptic boutons. Quantitative immunohistochemistry revealed a dose-dependent downregulation of spinophilin, a spine marker, and of synaptophysin, a presynaptic marker, and of growth-associated protein 43 after letrozole treatment. Our data provide strong evidence for estrogens being potent modulators of structural synaptic plasticity and point to a paracrine rather than endocrine mechanism of estrogen action in the hippocampus.

Hippocampal synapses depend on hippocampal estrogen synthesis

Estrogens have been described to induce synaptogenesis in principal neurons of the hippocampus and have been shown to be synthesized and released by exactly these neurons. Here, we have focused on the significance of local estrogen synthesis on spine synapse formation and the synthesis of synaptic proteins. To this end, we reduced hippocampal estrogen synthesis in vitro with letrozole, a reversible nonsteroidal aromatase inhibitor. In hippocampal slice cultures, letrozole treatment resulted in a dose-dependent decrease of 17beta-estradiol as quantified by RIA. This was accompanied by a significant decrease in the density of spine synapses and in the number of presynaptic boutons. Quantitative immunohistochemistry revealed a downregulation of spinophilin, a marker of dendritic spines, and synaptophysin, a protein of presynaptic vesicles, in response to letrozole. Surprisingly, no increase in the density of spines, boutons, and synapses and in spinophilin expression was seen after application of estradiol to the medium of cultures that had not been treated with letrozole. However, synaptophysin expression was upregulated under these conditions. Our results point to an essential role of endogenous hippocampal estrogen synthesis in the maintenance of hippocampal spine synapses.

Estrogen induces phosphorylation of cyclic AMP response element binding (pCREB) in primary hippocampal cells in a time-dependent manner

Using hippocampal primary cell cultures at 14 days in vitro (div), we have investigated actions of 17-beta estradiol (E; 10 nM) on the phosphorylation of CREB and on signaling pathways that regulate CREB phosphorylation. After demonstrating that 14 div is optimal for these studies, we examined the time course of E induction of CREB phosphorylation (pCREB) at serine residue 133. The induction of pCREB occurs as early as 1 h following E treatment, presumably via a mechanism involving an E-stimulated signal transduction system, which is sustained for at least 24 h but inhibited by 48 h. The early activity may represent an initial signal required for events leading to phosphorylation of CREB while the sustained signal may lead to CREB-mediated gene expression for cell survival and synapse formation. Furthermore, we examined the pathways for E action preceding pCREB induction by blocking three major kinases (protein kinase; mitogen activated protein kinase, MAPK; and calcium-calmodulin kinase II, CaMKII) upstream of pCREB. We found that E stimulates each pathway at 24 h and that phosphorylation of CREB is dependent on both MAPK and CaMK activities, but less dependent on the Akt pathway. Because CREB has been linked to E induction of excitatory spine synapses, we used a spine marker, spinophilin, to establish E effects on spine formation. Spinophilin expression was up-regulated in response to E and this effect was blocked by an inhibitor of (CaMKII). These studies demonstrate the central role played by CaMKII pathway in the actions of E on both transcriptional regulation and structural reorganization in neurons.

Morphological effects of estrogen on cholinergic neurons in vitro involves activation of extracellular signal-regulated kinases

In the present study, we examined the ability of estrogen to enhance cholinergic neurite arborization in vitro and identified the signal transduction cascade associated with this effect. Basal forebrain primordia collected from rat pups on postnatal day 1 were cultured for 2 weeks and then treated with 5 nm 17beta-estradiol for 24 hr. Cholinergic neurons were identified immunocytochemically with an antibody against the vesicular acetylcholine transporter and digitally photographed. Morphological analysis indicated that female cultures respond to estrogen treatment with an increase in total neurite length per neuron (4.5-fold over untreated controls) and in total branch segment number per neuron (2.3-fold over controls). In contrast, there was no change in total neurite length per neuron in male cultures, and we also observed a decrease in total branch segment number per neuron (0.5-fold below controls). Detailed histograms indicated that estrogen increases primary and secondary branch length and number and also increases terminal neuritic branches to the seventh order in female cultures. In a second set of experiments, we investigated the signal transduction cascade involved in this response, and found that an upstream extracellular signal-regulated kinase (ERK) inhibitor blocked the ability of estrogen to enhance outgrowth in female cultures. Our study provides strong evidence in support of the fact that the ERK pathway is required for estrogen-induced structural plasticity in the cholinergic system of female rats. Understanding the intracellular processes that underlie the response of cholinergic neurons to estrogen provides a necessary step in elucidating how cholinergic neurons can be particularly susceptible to degeneration in postmenopausal women.

Colocalization and alteration of estrogen receptor-α and -β in the hippocampus in Alzheimer’s disease

The human hippocampus is severely affected in Alzheimer's disease (AD). Because postmenopausal estrogen use may decrease the risk and delay the onset and progression of AD, possibly by a direct action on the hippocampal neurons, we used fluorescence immunocytochemistry to examine the colocalization of estrogen receptor-alpha (ERalpha) and estrogen receptor-beta (ERbeta) in the hippocampus of elderly human controls and AD patients. Double-labeling cells (DLCs) of ERalpha and ERbeta can be divided into 3 types: double-cytoplasm-staining cells (DCCs), double-nucleus-staining cells (DNCs), and ERalpha nucleus-staining and ERbeta cytoplasm-staining cells (NCCs). There was no difference in the percentage of DLCs in total ERalpha-positive cells or in total ERbeta-positive cells in the CA1 to CA4 subfields of the hippocampus between controls and AD patients. Interestingly, the ratio of DNCs to the total ERalpha-positive cells (2.6% +/- 0.5%) or to the total ERbeta-positive cells (1.8% +/- 0.3%) in the CA1 subfield of the AD hippocampus was significantly decreased in comparison with controls (5.0% +/- 0.7% and 3.9% +/- 0.6%, respectively; P<0.001), suggesting that changes in the compartmentalization of these receptors could play a role in the pathogenesis of AD.

Effects of estrogen on muscarinic acetylcholine receptors in the rat hippocampus

The aim of the present study was to investigate whether different estrogen manipulations have effects on the expression of muscarinic acetylcholine receptors (mAChRs) in the adult female rat hippocampus. Hippocampus was obtained from rats in proestrus (control), ovariectomized for 2, 10 and 15 days, ovariectomized for 15 days and treated with 17beta-estradiol for 7 days, and treated with 17beta-estradiol immediately after ovariectomy for 21 days. Rats' estrogen status was monitored by measuring estradiol plasma levels and uterus relative weight. [3H]quinuclidinyl benzilate ([3H]QNB) binding studies indicated that ovariectomy time-dependently increases the number of mAChRs in hippocampus when compared to those obtained from control rats. Estradiol treatments for 21 days avoid the effect of ovariectomy. However, the estradiol treatments for 7 days after 15 days of ovariectomy slightly change the number of mAChRs. In conclusion, these results showed that ovariectomy time-dependently increases mAChRs number in the rat hippocampus. In addition, these data suggest that treatment with estradiol initiated within a specific period of time after the loss of ovarian function may be effective at preventing specific effects of hormone deprivation on hippocampus.

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.

Para/autocrine regulation of estrogen receptors in hippocampal neurons

Previous studies have shown that estrogens, originating from ovaries, have a wide variety of estrogen receptor (ER)-mediated effects in the hippocampus. In the present study, we have investigated whether estrogens, which are synthesized in the hippocampus, could induce these effects as well. As a parameter, we used ER expression in response to estrogen synthesis, because estrogen receptors are ligand-inducible transcription factors. The experiments were carried out with cultures of isolated adult rat hippocampal cells, which contained about 95% neurons and about 5% oligodendrocytes in serum-free and steroid-free medium. Hippocampal neurons express both estrogen receptor isoforms (ERalpha and ERbeta), as shown by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization. The release of estrogens by hippocampal neurons was quantified by radioimmunoassay (RIA). The ER isoforms (alpha and beta) were studied by semiquantitative immunocytochemical image analysis. Hippocampal cells precultured for 4 days were found to synthesize 17beta-estradiol for the next 8 days. This synthesis was completely inhibited by letrozol, an aromatase inhibitor. Inhibition of estrogen synthesis by letrozol induced a significant decrease in ERalpha expression, but an increase in ERbeta. As a control, supplementation of the medium with 17beta-estradiol resulted in a significant increase of ERalpha expression, whereas ERbeta was downregulated. Our findings provide evidence for a de novo synthesis of estrogens in the hippocampus, differential regulation of estrogen receptor isoforms by estrogen and consequently for a para/autocrine loop of estrogen action in the hippocampus.

Estrogen up-regulates estrogen receptor alpha and synaptophysin in slice cultures of rat hippocampus

Previous studies have shown that estrogen application increases the density of synaptic input and the number of spines on CA1 pyramidal neurons. Here, we have investigated whether Schaffer collaterals to CA1 pyramidal cells are involved in this estrogen-induced synaptogenesis on CA1 pyramidal neurons. To this end, we studied estrogen-induced expression of both estrogen receptor (ER) subtypes (ERalpha and ERbeta) together with the presynaptic marker synaptophysin in the rat hippocampus. In tissue sections as well as in slice cultures mRNA expression of ERalpha, ERbeta and synaptophysin was higher in CA3 than in CA1, and mRNA expression and immunoreactivity for both ER subtypes were found in both principal cells and interneurons. By using quantitative image analysis we found stronger nuclear immunoreactivity for ERalpha in CA3 than in CA1. In slice cultures, supplementation of the medium with 10(-8) M estradiol led to an increase of nuclear immunoreactivity for ERalpha, but not for ERbeta, which was accompanied by a dramatic up-regulation of synaptophysin immunoreactivity in stratum radiatum of CA1. Together these findings indicate that estrogen effects on hippocampal neurons are more pronounced in CA3 than in CA1 and that ER activation in CA3 neurons leads to an up-regulation of a presynaptic marker protein in the axons of these cells, the Schaffer collaterals. We conclude that estradiol-induced spine formation on CA1 pyramidal cells may be mediated presynaptically, very likely by activation of ERalpha in CA3 pyramidal cells, followed by an increase in Schaffer collateral synapses.

Galanin: neurobiologic mechanisms and therapeutic potential for Alzheimer's disease

The neuropeptide galanin (GAL) is widely distributed in the mammalian CNS. Several lines of evidence suggest that GAL may play a critical role in cognitive processes such as memory and attention through an inhibitory modulation of cholinergic basal forebrain activity. Furthermore, GAL fibers hyperinnervate remaining cholinergic basal forebrain neurons in Alzheimer's disease (AD). This suggests that GAL activity impacts cholinergic dysfunction in advanced AD. Pharmacological and in vitro autoradiographic studies indicate the presence of heterogeneous populations of GAL receptor (GALR) sites in the basal forebrain which bind GAL with both high and low affinity. Interestingly, we have recently observed that GALR binding sites increase in the anterior basal forebrain in late-stage AD. Three G protein-coupled GALRs have been identified to date that signal through a diverse array of effector pathways in vitro, including adenylyl cyclase inhibition and phospholipase C activation. The repertoire and distribution of GALR expression in the basal forebrain remains unknown, as does the nature of GAL and GALR plasticity in the AD basal forebrain. Recently, GAL knockout and overexpressing transgenic mice have been generated to facilitate our understanding of GAL activity in basal forebrain function. GAL knockout mice result in fewer cholinergic basal forebrain neurons and memory deficits. On the other hand, mice overexpressing GAL display hyperinnervation of basal forebrain and memory deficits. These data highlight the need to explore further the putative mechanisms by which GAL signaling might be beneficial or deleterious for cholinergic cell survival and activity within basal forebrain. This information will be critical to understanding whether pharmacological manipulation of GALRs would be effective for the amelioration of cognitive deficits in AD.

Divergent effects of ovarian steroids on neuronal survival during experimental allergic encephalitis in Lewis rats

Experimental allergic encephalitis, (EAE) a Th1-cell-dependent autoimmune disease of the central nervous system (CNS) used to study immune responses relevant to multiple sclerosis (MS) displays gender susceptibility. The underlying basis of the sexual dimorphism may reflect multiple factors including gender-specific hormones. To study the relationship between ovarian hormones and CNS inflammation, we induced EAE in susceptible female Lewis rats ovariectomized (OVX) 7 days earlier and implanted with blank capsules or capsules containing estradiol (E), progesterone (P), or both (EP). Rats were immunized with complete Freunds' adjuvant alone or combined with guinea pig myelin basic protein. Motor function was scored 0-5 on standard criteria (days 7-11 postimmunization). On day 11, the rats were euthanized and the lumbar spinal cord was analyzed for Nissl, neuron nuclear antigen, and DNA fragmentation with a TUNEL assay. Inflammation was judged qualitatively on a scale of 0-4. Our immunization protocol induced limited sensorimotor deficits in OVX rats (2.3 +/- 0.6, mean +/- SEM) with moderate inflammation (2.5 +/- 0.4). E limited both behavioral impairments (1.0 +/- 0.4) and inflammation (0.5 +/- 0.2). P-treated rats had more severe sensorimotor deficits (3.1 +/- 0.5) with increased inflammatory infiltrates (3.6 +/- 0.4) and markedly increased numbers of TUNEL(+) neurons. Neuron counts of the outer two Rexed lamina (L3-L5) showed a 20% neuron loss (P < 0.02) in P-treated rats with EAE in comparison to other groups. Coadministration of E with P prevented the consequences of P, including neuronal apoptosis (behavioral score, 0.6 +/- 0.6; inflammation, 1.4 +/- 0.5). Our results suggest a potential and novel function of P that increases the vulnerability of neurons to apoptotic injury in EAE and may have pathophysiologic implications in the progression of disability in women with MS.

Hippocampal estrogen beta-receptor immunoreactivity is increased in Alzheimer's disease

Post-menopausal estrogen use reduces the risk and severity of Alzheimer's disease (AD). The present study investigates the distribution of both estrogen receptors ER alpha and ER beta in the human hippocampus in aged controls and in AD cases with immunohistochemistry. No ER alpha immunoreactivity was observed both in controls and in AD cases. On the other hand, ER beta was observed in some neuronal cells in the hippocampal subfields CA1--4, in astrocytes and in extracellular deposits both in controls and AD cases. The ER beta immunoreactivity was distinctly increased in all AD cases in cellular and extracellular localizations indicating a role for ER beta-mediated estrogen effects in AD-related neuropathology. This study provides the first demonstration of ER beta in human hippocampus in aged controls compared to AD cases.

Sex differences in the cerebral blood flow response after brief hypercapnia in the rat

Hypercapnia primarily affects cerebral blood flow (CBF) and not cerebral metabolism. We compared the CBF responses due to electrical forepaw stimulation before and after brief hypercapnia in male, non-ovarectomized female, and ovarectomized female rats. Prior to hypercapnia the CBF responses were similar for all three groups. Seven minutes after brief hypercapnic exposure the CBF responses to forepaw stimulation were augmented in all groups. However, both 30 and 60 min after hypercapnia, the magnitude of the CBF responses to forepaw stimulation remained elevated for males and ovarectomized females, but not for non-ovarectomized females. These results suggest that estrogen may modulate the upregulation of the CBF response observed after transient hypercapnia.

Steroidogenic factor-1 expression in marmoset and rat hippocampus: co-localization with StAR and aromatase

Steroidogenic factor-1 (SF-1), an orphan nuclear receptor, was studied with respect to the expression of steroidogenic enzymes in the hippocampus of rat and marmoset, since SF-1 is a regulator of steroid biosynthesis in the gonads. We used the steroidogenic acute regulatory protein (StAR) as a marker of the first step in the cascade of oestrogen synthesis and aromatase as a marker of the last. StAR transports cholesterol to the inner mitochondrial membrane where it is converted by the cytochrome P-450 enzyme complex. This is the rate-limiting step in steroid biosynthesis. Aromatase metabolizes testosterone to oestrogen. Using an anti-SF-1 antibody we show that SF-1 is highly expressed in neuronal cells of the pyramidal layer (CA1--CA3) and in the dentate gyrus of rat and marmoset hippocampi. Binding of the antibody was seen in more than 60% of all cells in the pyramidal layer and in the fascia dentata. In situ hybridization studies revealed the same expression pattern for StAR and aromatase. StAR and aromatase-positive cells were strictly correlated with SF-1 as shown by computer-assisted confocal microscopy in double labelling experiments (immunohistochemistry and in situ hybridization). This coexpression may imply SF-1 as a possible regulator of steroidogenesis in the hippocampus. However, a few interneurones express solely SF-1 and aromatase but are negative for StAR. Since the expression of StAR represents the first step in steroidogenesis its expression is suggestive for a de novo synthesis of steroids. A small population of interneurones must import precursors for oestrogen synthesis from other sources. Responsive cells, as evidenced by the presence of oestrogen receptor transcripts, were also found in the pyramidal layer and dentate gyrus. In conclusion, (1) SF-1 could play a regulatory role in steroidogenesis in the hippocampus of marmoset and rat and (2) with respect to the capacity of steroidogenesis two populations of hippocampal neurones coexist.

Colocalization of androgen, estrogen and cholinergic receptors on cultured astrocytes of rat central nervous system

By means of immunohistochemical and electrophysiological methods, we have investigated the presence of androgen receptors on astrocytes in explant and primary cultures from various regions of rat central nervous system. Our studies have shown that a great number of astrocytes and neurones express androgen receptors as recognized by a specific monoclonal antibody. Immunoreactivity was mainly distributed over the soma of the astrocytes, the nuclei being intensely stained. In contrast, glial processes were only faintly stained or not stained. Double-immunostaining studies have provided evidence for a colocalization of androgen and estrogen alpha- and beta-receptors on many astrocytes. Furthermore, there was also a coexistence of glial androgen receptors with cholinergic muscarinic and nicotinic sites. Our immunohistochemical findings are supported by electrophysiological investigations demonstrating that 5alpha-androstan, 17beta-estradiol as well as the cholinergic agonists muscarine and nicotine caused hyperpolarizations on the same astrocytes. Our studies suggest that there is a coexistence of functional receptors for androgen, estrogen as well as for the cholinergic agonists on glial cells. Further investigations are needed to elucidate the physiological role of glial androgen, estrogen and cholinergic receptors and to define their function in neurodegenerative diseases.

Galanin receptor plasticity within the nucleus basalis in early and late Alzheimer's disease: an in vitro autoradiographic analysis

Hypertrophy of fibers containing galanin (GAL), the inhibitory neurotransmitter of acetylcholine, occur on remaining cholinergic nucleus basalis neurons in late stage Alzheimer's disease (AD). The present investigation evaluated whether changes in the number of GAL receptors (GALR) were detectable within the nucleus basalis in the early or late stage of AD when compared to age-matched controls. Postmortem neuropathological specimens were obtained at autopsy from three groups: late AD, early (possible) AD, and normal (age-matched controls) human subjects. Autoradiography of GALR binding was performed on human brain sections from each of the three groups. Analysis of autoradiographic images show no change in the distribution of ([125])hGAL binding sites in early AD cases throughout the nucleus basalis. In contrast, the number of ([125])hGAL binding sites was increased over the anterior nucleus basalis subfield in late stage AD. A region-of-interest densitometric analysis of the anterior nucleus basalis in the late stage AD cases depict an increase in the number of ([125])hGAL binding sites by approximately two-three-fold when compared to normal (age-matched controls). Quantitative measures of ([125])hGAL binding densities were not significantly different in the anterolateral, intermediate or posterior nucleus basalis subsectors of early or late stage AD when compared to age-matched controls. These observations show that the occurrence of overexpression of GALRs coincide with earlier reports showing galaninergic fibers hyperinnervating surviving cholinergic basal forebrain neurons in late stage AD.

Colocalization of neurotransmitter receptors on astrocytes in explant cultures of rat CNS

In recent years evidence has accumulated that astrocytes express functional receptors for a variety of neurotransmitters/neuromodulators. By means of electrophysiological and combined autoradiographic and immunohistochemical methods we have demonstrated the colocalization of cholinergic, adrenergic and peptidergic receptors on astrocytes in explant cultures from various regions of rat central nervous system. A great number of biochemical and electrophysiological studies from other laboratories have shown that most of the neurotransmitters exert their effects on second messenger systems and on Ca2+-activated K+-channels. Furthermore, certain neurotransmitters are involved in the regulation of energy metabolism by stimulating enzymatic breakdown of glycogen in astrocytes. It was suggested that there is a cross-talk between the various neurotransmitter receptors on the glial membrane and that these receptors act in a synergistic or antagonistic way. The coexistence of cholinergic and peptidergic receptors on astrocytes is of great interest since both neurotransmitter systems are involved in cognitive functions and are impaired in patients with Alzheimer's dementia. The question is therefore raised whether not only neurones but also astrocytes might be involved in neurodegenerative disorders such as Alzheimer's disease.

Histochemical and electrophysiological evidence for estrogen receptors on cultured astrocytes: colocalization with cholinergic receptors

By means of autoradiographic and immunohistochemical methods it was demonstrated that astrocytes in explant and primary cultures of rat neocortex, hippocampus, preoptic area and spinal cord express estrogen alpha- and beta-receptors. Immunoreactivity was mainly distributed over the soma, the nuclei being more intensely stained. Combined autoradiographic and immunohistochemical studies as well as double-immunostaining revealed a colocalization of estrogen alpha- and beta-receptors on many astrocytes. There was also a coexistence of estrogen receptors and cholinergic muscarinic and nicotinic sites. Electrophysiological investigations have shown that 17beta-estradiol induced hyperpolarizations on the majority of astrocytes in explant cultures of hippocampus and spinal cord, providing evidence for the existence of functional estrogen receptors on these cells. Furthermore, on the same astrocytes, 17beta-estradiol, muscarine and nicotine caused hyperpolarizations, suggesting a coexistence of receptors for estrogen and the cholinergic agonists on glial cells. The presence of glial estrogen receptors and their colocalization with cholinergic receptors is discussed with respect to the effects of these neurotransmitters/neuromodulators in development and maturation of the central nervous system, as well as to neurodegenerative events such as Alzheimer's disease.