Rapid signaling of estrogen in hypothalamic neurons involves a novel G-protein-coupled estrogen receptor that activates protein kinase C

Rapid action of oestrogen in luteinising hormone-releasing hormone neurones: the role of GPR30

Previously, we have shown that 17beta-oestradiol (E(2)) induces an increase in firing activity and modifies the pattern of intracellular calcium ([Ca(2+)](i)) oscillations with a latency < 1 min in primate luteinising hormone-releasing hormone (LHRH) neurones. A recent study also indicates that E(2), the nuclear membrane impermeable oestrogen, oestrogen-dendrimer conjugate, and the plasma membrane impermeable oestrogen, E(2)-BSA conjugate, all similarly stimulated LHRH release within 10 min of exposure in primate LHRH neurones, indicating that the rapid action of E(2) is caused by membrane signalling. The results from a series of studies further suggest that the rapid action of E(2) in primate LHRH neurones appears to be mediated by GPR30. Although the oestrogen receptor antagonist, ICI 182, 780, neither blocked the E(2)-induced LHRH release nor the E(2)-induced changes in [Ca(2+)](i) oscillations, E(2) application to cells treated with pertussis toxin failed to result in these changes in primate LHRH neurones. Moreover, knockdown of GPR30 in primate LHRH neurones by transfection with human small interference RNA for GPR30 completely abrogated the E(2)-induced changes in [Ca(2+)](i) oscillations, whereas transfection with control siRNA did not. Finally, the GPR30 agonist, G1, resulted in changes in [Ca(2+)](i) oscillations similar to those observed with E(2). In this review, we discuss the possible role of G-protein coupled receptors in the rapid action of oestrogen in neuronal cells.

Membrane-localised oestrogen receptor alpha and beta influence neuronal activity through activation of metabotropic glutamate receptors

Until recently, the idea that oestradiol could affect cellular processes independent of nuclear oestrogen receptors (ERs) was controversial. This was despite the large number of carefully controlled studies performed both within and outside the nervous system demonstrating that oestrogens regulate various intracellular signalling pathways by acting at the membrane surface of cells and/or at biological rates incompatible with the time course of genomic-initiated events. At present, it is far less controversial that oestradiol acts at surface membrane receptors to regulate nervous system function. Recent studies have demonstrated that the classical intracellular ERs, ERalpha and ERbeta, are major players in mediating the actions of oestradiol on the membrane surface. This review focuses on one potential mechanism by which surface-localised ERalpha and ERbeta stimulate intracellular signalling events in cells of the nervous system. After oestradiol treatment, both ERalpha and ERbeta are capable of activating different classes of metabotropic glutamate receptors (mGluRs). Oestradiol activation of mGluRs is independent of glutamate, but requires expression of several different caveolin proteins to compartmentalise the different ERs with mGluRs into functional signalling microdomains. ER/mGluR signalling is a potential means by which oestrogens can both rapidly and for extended periods, influence a variety of intracellular signalling processes and behaviours.

Cross-talk between membrane-initiated and nuclear-initiated oestrogen signalling in the hypothalamus

It is increasingly evident that 17beta-oestradiol (E(2)), via a distinct membrane oestrogen receptor (Gq-mER), can rapidly activate kinase pathways to have multiple downstream actions in central nervous system (CNS) neurones. We have found that E(2) can rapidly reduce the potency of the GABA(B) receptor agonist baclofen and mu-opioid receptor agonist DAMGO to activate G-protein-coupled, inwardly rectifying K(+) (GIRK) channels in hypothalamic neurones, thereby increasing the excitability (firing activity) of pro-opiomelanocortin (POMC) and dopamine neurones. These effects are mimicked by the membrane impermeant E(2)-BSA and a new ligand (STX) that is selective for the Gq-mER that does not bind to ERalpha or ERbeta. Both E(2) and STX are fully efficacious in attenuating the GABA(B) response in ERalpha, ERbeta and GPR 30 knockout mice in an ICI 182 780 reversible manner. These findings are further proof that E(2) signals through a unique plasma membrane ER. We have characterised the coupling of this Gq-mER to a Gq-mediated activation of phospholipase C leading to the up-regulation of protein kinase Cdelta and protein kinase A activity in these neurones, which ultimately alters gene transcription. Finally, as proof of principle, we have found that STX, similar to E(2), reduces food intake and body weight gain in ovariectomised females. STX, presumably via the Gq-mER, also regulates gene expression of a number of relevant targets including cation channels and signalling molecules that are critical for regulating (as a prime example) POMC neuronal excitability. Therefore, E(2) can activate multiple receptor-mediated pathways to modulate excitability and gene transcription in CNS neurones that are critical for controlling homeostasis and motivated behaviors.

Involvement of G protein-coupled receptor 30 (GPR30) in rapid action of estrogen in primate LHRH neurons

Previously, we have reported that 17beta-estradiol (E(2)) induces an increase in firing activity of primate LH-releasing hormone (LHRH) neurons. The present study investigates whether E(2) alters LHRH release as well as the pattern of intracellular calcium ([Ca(2+)](i)) oscillations and whether G protein-coupled receptor 30 (GPR30) plays a role in mediating the rapid E(2) action in primate LHRH neurons. Results are summarized: 1) E(2), the nuclear membrane-impermeable estrogen, estrogen-dendrimer conjugate, and the plasma membrane-impermeable estrogen, E(2)-BSA conjugate, all stimulated LHRH release within 10 min of exposure; 2) whereas the estrogen receptor antagonist, ICI 182,780, did not block the E(2)-induced LHRH release, E(2) application to cells treated with pertussis toxin failed to induce LHRH release; 3) GPR30 mRNA was expressed in olfactory placode cultures, and GPR30 protein was expressed in a subset of LHRH neurons; 4) pertussis toxin treatment blocked the E(2)-induced increase in [Ca(2+)](i) oscillations; 5) knockdown of GPR30 in primate LHRH neurons by transfection with small interfering RNA (siRNA) for GPR30 completely abrogated the E(2)-induced changes in [Ca(2+)](i) oscillations, whereas transfection with control siRNA did not; 6) the estrogen-dendrimer conjugate-induced increase in [Ca(2+)](i) oscillations also did not occur in LHRH neurons transfected with GPR30 siRNA; and 7) G1, a GPR30 agonist, resulted in changes in [Ca(2+)](i) oscillations, similar to those observed with E(2). Collectively, E(2) induces a rapid excitatory effect on primate LHRH neurons, and this rapid action of E(2) appears to be mediated, in part, through GPR30.

Neurobiological mechanisms underlying oestradiol negative and positive feedback regulation of gonadotrophin-releasing hormone neurones

The feedback actions of ovarian oestradiol during the female reproductive cycle are among the most unique in physiology. During most of the cycle, oestradiol exerts homeostatic, negative feedback upon the release of gonadotrophin-releasing hormone (GnRH). Upon exposure to sustained elevated oestradiol levels, however, there is a switch in the feedback effects of this hormone to positive, resulting in induction of a surge in the release of GnRH that serves as a neuroendocrine signal to initiate the ovulatory cascade. We review recent developments stemming from studies in an animal model exhibiting daily switches between positive and negative feedback that have probed the neurobiological mechanisms, including changes in neural networks and intrinsic properties of GnRH neurones, underlying this switch in oestradiol action.

Oestrogen modulates hypothalamic control of energy homeostasis through multiple mechanisms

The control of energy homeostasis in women is correlated with the anorectic effects of oestrogen, which can attenuate body weight gain and reduce food intake in rodent models. This review investigates the multiple signalling pathways and cellular targets that oestrogen utilises to control energy homeostasis in the hypothalamus. Oestrogen affects all of the hypothalamic nuclei that control energy homeostasis. Oestrogen controls the activity of hypothalamic neurones through gene regulation and neuronal excitability. Oestrogen's primary cellular pathway is the control of gene transcription through the classical oestrogen receptors (ERs) (ERalpha and ERbeta) with ERalpha having the primary role in energy homeostasis. Oestrogen also controls energy homeostasis through membrane-mediated events via membrane-associated ERs or a novel, putative membrane ER that is coupled to G-proteins. Therefore, oestrogen is coupled to at least two receptors with multiple signalling and transcriptional pathways to mediate immediate and long-term anorectic effects. Ultimately, it is the interactions of all the receptor-mediated processes in hypothalamus and other areas of the central nervous system that will determine the anorectic effects of oestrogen and its control of energy homeostasis.

Glycine receptor in rat hippocampal and spinal cord neurons as a molecular target for rapid actions of 17-beta-estradiol

Glycine receptors (GlyRs) play important roles in regulating hippocampal neural network activity and spinal nociception. Here we show that, in cultured rat hippocampal (HIP) and spinal dorsal horn (SDH) neurons, 17-β-estradiol (E₂) rapidly and reversibly reduced the peak amplitude of whole-cell glycine-activated currents (I_Gly). In outside-out membrane patches from HIP neurons devoid of nuclei, E₂ similarly inhibited I_Gly, suggesting a non-genomic characteristic. Moreover, the E₂ effect on I_Gly persisted in the presence of the calcium chelator BAPTA, the protein kinase inhibitor staurosporine, the classical ER (i.e. ERα and ERβ) antagonist tamoxifen, or the G-protein modulators, favoring a direct action of E₂ on GlyRs. In HEK293 cells expressing various combinations of GlyR subunits, E₂ only affected the I_Gly in cells expressing α2, α2β or α3β subunits, suggesting that either α2-containing or α3β-GlyRs mediate the E₂ effect observed in neurons. Furthermore, E₂ inhibited the GlyR-mediated tonic current in pyramidal neurons of HIP CA1 region, where abundant GlyR α2 subunit is expressed. We suggest that the neuronal GlyR is a novel molecular target of E₂ which directly inhibits the function of GlyRs in the HIP and SDH regions. This finding may shed new light on premenstrual dysphoric disorder and the gender differences in pain sensation at the CNS level.

Medial preoptic area interactions with dopamine neural systems in the control of the onset and maintenance of maternal behavior in rats

The medial preoptic area (MPOA) and dopamine (DA) neural systems interact to regulate maternal behavior in rats. Two DA systems are involved: the mesolimbic DA system and the incerto-hypothalamic DA system. The hormonally primed MPOA regulates the appetitive aspects of maternal behavior by activating mesolimbic DA input to the shell region of the nucleus accumbens (NAs). DA action on MPOA via the incerto-hypothalamic system may interact with steroid and peptide hormone effects so that MPOA output to the mesolimbic DA system is facilitated. Neural oxytocin facilitates the onset of maternal behavior by actions at critical nodes in this circuitry. DA-D1 receptor agonist action on either the MPOA or NAs can substitute for the effects of estradiol in stimulating the onset of maternal behavior, suggesting an overlap in underlying cellular mechanisms between estradiol and DA. Maternal memory involves the neural plasticity effects of mesolimbic DA activity. Finally, early life stressors may affect the development of MPOA-DA interactions and maternal behavior.

Tibolone rapidly attenuates the GABAB response in hypothalamic neurones

Tibolone is primarily used for the treatment of climacteric symptoms. Tibolone is rapidly converted into three major metabolites: 3 alpha- and 3beta-hydroxy (OH)-tibolone, which have oestrogenic effects, and the Delta 4-isomer (Delta 4-tibolone), which has progestogenic and androgenic effects. Because tibolone is effective in treating climacteric symptoms, the effects on the brain may be explained by the oestrogenic activity of tibolone. Using whole-cell patch clamp recording, we found previously that 17beta-oestradiol (E(2)) rapidly altered gamma-aminobutyric acid (GABA) neurotransmission in hypothalamic neurones through a membrane oestrogen receptor (mER). E(2) reduced the potency of the GABA(B) receptor agonist baclofen to activate G-protein-coupled, inwardly rectifying K(+) (GIRK) channels in hypothalamic neurones. Therefore, we hypothesised that tibolone may have some rapid effects through the mER and sought to elucidate the signalling pathway of tibolone's action using selective inhibitors and whole cell recording in ovariectomised female guinea pigs and mice. A sub-population of neurones was identified post hoc as pro-opiomelanocortin (POMC) neurones by immunocytochemical staining. Similar to E(2), we have found that tibolone and its active metabolite 3 beta OH-tibolone rapidly reduced the potency of the GABA(B) receptor agonist baclofen to activate GIRK channels in POMC neurones. The effects were blocked by the ER antagonist ICI 182 780. Other metabolites of tibolone (3 alpha OH-tibolone and Delta 4-tibolone) had no effect. Furthermore, tibolone (and 3 beta OH-tibolone) was fully efficacious in ER alpha knockout (KO) and ER beta KO mice to attenuate GABA(B) responses. The effects of tibolone were blocked by phospholipase C inhibitor U73122. However, in contrast to E(2), the effects of tibolone were not blocked by protein kinase C inhibitors or protein kinase A inhibitors. It appears that tibolone (and 3 beta OH-tibolone) activates phospholipase C leading to phosphatidylinositol bisphosphate metabolism and direct alteration of GIRK channel function. Therefore, tibolone may enhance synaptic efficacy through the G(q) signalling pathways of mER in brain circuits that are critical for maintaining homeostatic functions.

Genes associated with membrane-initiated signaling of estrogen and energy homeostasis

During the reproductive cycle, fluctuations in circulating estrogens affect multiple homeostatic systems controlled by hypothalamic neurons. Two of these neuronal populations are arcuate proopiomelanocortin and neuropeptide Y neurons, which control energy homeostasis and feeding. Estradiol modulates these neurons either through the classical estrogen receptors (ERs) to control gene transcription or through a G protein-coupled receptor (mER) activating multiple signaling pathways. To differentiate between these two divergent ER-mediated mechanisms and their effects on homeostasis, female guinea pigs were ovariectomized and treated systemically with vehicle, estradiol benzoate (EB) or STX, a selective mER agonist, for 4 wk, starting 7 d after ovariectomy. Individual body weights were measured after each injection day for 28 d, at which time the animals were euthanized, and the arcuate nucleus was microdissected. As predicted, the body weight gain was significantly lower for EB-treated females after d 5 and for STX-treated females after d 12 compared with vehicle-treated females. Total arcuate RNA was extracted from all groups, but only the vehicle and STX-treated samples were prepared for gene microarray analysis using a custom guinea pig gene microarray. In the arcuate nucleus, 241 identified genes were significantly regulated by STX, several of which were confirmed by quantitative real-time PCR and compared with EB-treated groups. The lower weight gain of EB-treated and STX-treated females suggests that estradiol controls energy homeostasis through both ERalpha and mER-mediated mechanisms. Genes regulated by STX indicate that not only does it control neuronal excitability but also alters gene transcription via signal transduction cascades initiated from mER activation.

The role of estrogen receptors in the control of energy and glucose homeostasis

Estrogens have been related to energy balance and glucose metabolism for a long time; however, the mechanisms involved in their actions are now being unveiled. The development of ERalpha and ERbeta knockout mice has demonstrated the participation of these receptors in the regulation of many processes related to the control of energy homeostasis. These include food intake and energy expenditure, insulin sensitivity in the liver and muscle, adipocyte growth and its body distribution as well as the pancreatic beta-cell function. In addition, other membrane receptors unrelated to ERalpha and ERbeta function in key tissues involved in energy balance and glucose homeostasis, i.e. the islet of Langerhans and the hypothalamus. Along with naturally occurring estrogens, there are endocrine disrupters that act as environmental estrogens and can impair the physiological action of ERalpha, ERbeta and other membrane ERs. New research is revealing a link between environmental estrogenic pollutants and the metabolic syndrome.

Rapid signaling by steroid receptors

Steroid receptors transcribe genes that lead to important biological processes, including normal organ development and function, tissue differentiation, and promotion of oncogenic transformation. These actions mainly result from nuclear steroid receptor action. However, for 50 years, it has been known that rapid effects of steroid hormones occur and could result from rapid signal transduction. Examples of these effects include stress responses to secreted glucocorticoids, rapid actions of thyroid hormones in the heart, and acute uterine/vaginal responses to injected estrogen. These types of responses have increasingly been attributed to rapid signaling by steroid hormones, upon engaging binding proteins most often at the cell surface of target organs. It is clear that rapid signal transduction serves an integrated role to modify existing proteins, altering their structure and activity, and to modulate gene transcription, often through collaboration with the nuclear pool of steroid receptors. The biological outcomes of steroid hormone actions thus reflect input from various cellular pools, cocoordinating the necessary events that are restrained in temporal and kinetic fashion. Here I describe the current understanding of rapid steroid signaling that is now appreciated to extend to virtually all members of this family of hormones and their receptors.

Membrane-initiated estrogen signaling in hypothalamic neurons

It is well known that many of the actions of 17beta-estradiol (E2) in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane steroid receptors for estrogen in hypothalamic and other brain neurons. But it is not well understood how estrogen signals via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for sometime that E2 can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by E2 in the hypothalamus, the nature of receptors involved and how they contribute to homeostatic functions.

Estradiol regulation of progesterone synthesis in the brain

Steroidogenesis is now recognized as a global phenomenon in the brain, but how it is regulated and its relationship to circulating steroids of peripheral origin have remained more elusive issues. Neurosteroids, steroids synthesized de novo in nervous tissue, have a large range of actions in the brain, but it is only recently that the role of neuroprogesterone in the regulation of arguably the quintessential steroid-dependent neural activity, regulation of the reproduction has been appreciated. Circuits involved in controlling the LH surge and sexual behaviors were thought to be influenced by estradiol and progesterone synthesized in the ovary and perhaps the adrenal. It is now apparent that estradiol of ovarian origin regulates the synthesis of neuroprogesterone, and it is the locally produced neuroprogesterone that is involved in the initiation of the LH surge and subsequent ovulation. In this model, estradiol induces the transcription of progesterone receptors while stimulating synthesis of neuroprogesterone. Although the complete signaling cascade has not been elucidated, many of the features have been characterized. The synthesis of neuroprogesterone occurs primarily in astrocytes and requires the interaction of membrane-associated estrogen receptor-alpha with metabotropic glutamate receptor-1a. This G protein-coupled receptor activates a phospholipase C that in turn increases inositol trisphosphate (IP3) levels mediating the release of intracellular stores of Ca2+ via an IP3 receptor gated Ca2+ channel. The large increase in free cytoplasmic Ca2+ ([Ca2+]i) stimulates the synthesis of progesterone, which can then diffuse out of the astrocyte and activate estradiol-induced progesterone receptors in local neurons to trigger the neural cascade to produce the LH surge. Thus, it is a cooperative action of astrocytes and neurons that is needed for estrogen positive feedback and stimulation of the LH surge.

Human myeloblastic leukemia cells (HL-60) express a membrane receptor for estrogen that signals and modulates retinoic acid-induced cell differentiation

Estrogen receptors are historically perceived as nuclear ligand activated transcription factors. An estrogen receptor has now been found localized to the plasma membrane of human myeloblastic leukemia cells (HL-60). Its expression occurs throughout the cell cycle, progressively increasing as cells mature from G(1) to S to G(2)/M. To ascertain that the receptor functioned, the effect of ligands, including a non-internalizable estradiol-BSA conjugate and tamoxifen, an antagonist of nuclear estrogen receptor function, were tested. The ligands caused activation of the ERK MAPK pathway. They also modulated the effect of retinoic acid, an inducer of MAPK dependent terminal differentiation along the myeloid lineage in these cells. In particular the ligands inhibited retinoic acid-induced inducible oxidative metabolism, a functional marker of terminal myeloid cell differentiation. To a lesser degree they also diminished retinoic acid-induced earlier markers of cell differentiation, namely CD38 and CD11b. However, they did not regulate retinoic acid-induced G(0) cell cycle arrest. There is thus a membrane localized estrogen receptor in HL-60 myeloblastic leukemia cells that can cause ERK activation and modulates the response of these cells to retinoic acid, indicating crosstalk between the membrane estrogen and retinoic acid evoked pathways relevant to propulsion of cell differentiation.

Estrogen signaling characteristics of Atlantic croaker G protein-coupled receptor 30 (GPR30) and evidence it is involved in maintenance of oocyte meiotic arrest

Human G protein-coupled receptor 30 (GPR30) mediates estradiol-17beta (E2) activation of adenylyl cyclase in breast cancer cells and displays E2 binding typical of membrane estrogen receptors (mERs). We identified a mER in Atlantic croaker ovaries with characteristics similar to those of human GPR30. To confirm the proposed role of GPR30 as a mER in this distantly related vertebrate group, we cloned GPR30 from croaker ovaries and examined its distribution, steroid binding, and signaling characteristics. Western blot analysis showed the GPR30 protein (approximately 40 kDa) is expressed on the plasma membranes of croaker oocytes and HEK293 cells stably transfected with GPR30 cDNA. Plasma membranes prepared from croaker GPR30-transfected cells displayed high-affinity, limited-capacity, and displaceable binding specific for estrogens, characteristic of mERs. Consistent with previous findings with human GPR30, estrogen treatment of plasma membranes from both croaker ovaries and GPR30-transfected cells caused activation of a stimulatory G protein (Gs) resulting in increased cAMP production. Treatment with E2 as well as G-1, a specific GPR30 ligand, significantly reduced both spontaneous and progestin-induced maturation of both croaker and zebrafish oocytes in vitro, suggesting a possible involvement of GPR30 in maintaining oocyte meiotic arrest in these species. Injection of antisense oligonucleotides to GPR30 into zebrafish oocytes blocked the inhibitory effects of estrogen on oocyte maturation, confirming a role for GPR30 in the control of meiotic arrest. These findings further support our previous suggestion that GPR30 is a vertebrate mER. In addition, the results suggest GRP30 may play a critical role in regulating reentry into the meiotic cell cycle in fish oocytes.

Nonestrogen-based hormonal therapies for Alzheimer’s disease

Alzheimer’s disease (AD) is becoming increasingly more prevalent worldwide and is a costly, devastating disease. The fact that females are more likely to be diagnosed with AD than men has contributed to the large amount of research and literature on estrogen’s ability to rescue cognitive decline in aging females. However, recent results from the Women’s Health Initiative Memory Study (WHIMS) and the Research into Memory, Brain Function and Estrogen Replacement (REMEMBER) study, in which estrogen-replacement therapy is shown not to be beneficial to cognition in postmenopausal women, especially in women of more advanced ages, have opened the study of hormones and hormone-based therapies beyond that of sex steroids. As such, recent findings, and the focus of this review, indicate that gonadotropins such as luteinizing hormone have a role on cognitive function and AD. It is likely that the interplay of age, and the timing of estrogen and these other less studied hormones will allow us to gain a better understanding of hypothalamic–pituitary–gonadal axis regulation after menopause and how it relates to AD, and will hopefully lead to new avenues of treatment for AD.

Non-genomic actions of estrogens and their interaction with genomic actions in the brain

Ligands for the nuclear receptor superfamily have at least two mechanisms of action: (a) classical transcriptional regulation of target genes (genomic mechanisms); and (b) non-genomic actions, which are initiated at the cell membrane, which could also impact transcription. Though transcriptional mechanisms are increasingly well understood, membrane-initiated actions of these ligands are incompletely understood. This has led to considerable debate over the physiological relevance of membrane-initiated actions of hormones versus genomic actions of hormones, with genomic actions predominating in the endocrine field. There is good evidence that the membrane-limited actions of hormones, particularly estrogens, involve the rapid activation of kinases and the release of calcium and that these are linked to physiologically relevant scenarios in the brain. We show evidence in this review, that membrane actions of estrogens, which activate these rapid signaling cascades, can also potentiate nuclear transcription in both the central nervous system and in non-neuronal cell lines. We present a theoretical scenario which can be used to understand this phenomenon. These signaling cascades may occur in parallel or in series but subsequently, converge at the modification of transcriptionally relevant molecules such as nuclear receptors and/or coactivators. In addition, other non-cognate hormones or neurotransmitters may also activate cascades to crosstalk with estrogen receptor-mediated transcription, though the relevance of this is less clear. The idea that coupling between membrane-initiated and genomic actions of hormones is a novel idea in neuroendocrinology and provides us with a unified view of hormone action in the central nervous system.

PI3K signaling effects in hypothalamic neurons mediated by estrogen

Multiple mechanisms mediate the effects of estrogen in the central nervous system, including signal transduction pathways such as protein kinase A, protein kinase C, and phosphatidylinositol 3-kinase (PI3K) pathways. Previously we demonstrated that estrogen regulates a number of PI3K-related genes in the hypothalamus, including the PI3K p55gamma regulatory subunit. We hypothesized that PI3K activation is critical for the effects of estrogen and that the p55gamma subunit may be more prevalent than the p85alpha regulatory subunit in the hypothalamus. Therefore, in the present study, we compared the mRNA distribution of the p55gamma and p85alpha regulatory subunits by using in situ hybridization in guinea pig. Expression level of p55gamma mRNA was greater than p85alpha in most hypothalamic nuclei. Twenty-four hours of estrogen treatment increased p55gamma mRNA expression in the paraventricular, suprachiasmatic, arcuate, and ventromedial nuclei, and little or no change was observed for p85alpha mRNA. Quantitative real-time PCR confirmed the in situ hybridization results. Next, we investigated the general role of PI3K signaling in the estrogen-mediated changes of arcuate proopiomelanocortin (POMC) neuronal excitability by using whole-cell recording. One cellular mechanism by which estrogen increases neuronal excitability is to desensitize (uncouple) gamma-aminobutyric acid type B (GABA(B)) receptors from their G-protein-gated inwardly rectifying K(+) channels in hypothalamic neurons. We found that the PI3K inhibitors wortmannin and LY294002 significantly reduced the estrogen-mediated GABA(B) receptor desensitization in POMC arcuate neurons, suggesting that PI3K signaling is a critical downstream mediator of the estrogen-mediated rapid effects. Collectively, these data suggest that the interplay between estrogen and PI3K occurs at multiple levels, including transcriptional and membrane-initiated signaling events that ultimately lead to changes in homeostatic function.

Axogenic effect of estrogen in male rat hypothalamic neurons involves Ca(2+), protein kinase C, and extracellular signal-regulated kinase signaling

17-beta-Estradiol (E2) stimulates the growth of axons in male-derived hypothalamic neurons in vitro. This effect is not exerted through the classical intracellular estrogen receptor (ER) but depends on a membrane mechanism involving TrkB. In the present study, we investigate the intracellular signaling cascade that mediates the axogenic effect of E2. Treatment with an intracellular Ca(2+) chelator, a Ca(2+)-dependent protein kinase C (PKC) inhibitor, or two specific inhibitors of extracellular signal-regulated kinases (ERK) mitogen-activated protein kinases (MAPK) completely inhibited the E2-induced axogenesis. E2 and the membrane-impermeant construct E2BSA rapidly induced phosphorylation of ERK, which was blocked by the specific inhibitor of the ERK pathway UO126 but not by the ER antagonist ICI 182,780. Decrease of intracellular free Ca(2+) or disruption of PKC activation by Ro 32-0432 attenuated ERK activation, indicating the confluence of signals in the MAPK pathway. Subcellular analysis of ERK demonstrated that the phospho-ERK signal is augmented in the nucleus after 15 min of E2 stimulation. We have also shown that E2 increased phosphorylation of CREB via ERK signaling. In summary, this study demonstrates that E2, probably via a membrane-associated receptor, induces axonal growth by activating CREB phosphorylation through ERK signaling by a mechanism involving Ca(2+) and PKC activation.

Unexpected suppression of neuronal G protein-activated, inwardly rectifying K+ current by common phospholipase C inhibitor

Stabilization of the binding of phosphatidylinositol bisphosphate (PIP(2)) to G protein-coupled inward rectifier K+ (GIRK) channels is essential for their activation, whereas hydrolysis of PIP(2) by phospholipase C (PLC) inhibits channel activity. Apparently inconsistent with this mechanism, we found that the commonly used PLC inhibitor, U73122 (1 microM), produced a significant reduction in the amplitude of baclofen (20 microM)-evoked GIRK currents in whole-cell recordings from acutely isolated rat neocortical pyramidal cells. Also, U73122 reduced the percentage of baclofen-responsive neurons from 100% (n=40) to 56% (n=25). Since NCDC (100 microM), a PLC inhibitor of another molecular class, displayed no effect on GIRK current amplitude or responsiveness (100%, n=6), inhibition of PLC is unlikely to account for the effects of U73122 in our preparation. Lending further support to this notion, the structurally closely related compound, U73343, which does not inhibit PLC, proved to be even more efficient in suppressing GIRK current as compared to U73122. In neurons, in which GIRK channels were irreversibly activated by GTPgammaS (n=10), the depressant action of U71322 was fully preserved. These findings hint at a direct interaction of U73122 with the GIRK channel or a closely associated protein. Caution is therefore warranted when employing this compound to examine the role of PLC and PIP(2) in the regulation of GIRK channel activity.

Rapid action of estrogens on intracellular calcium oscillations in primate luteinizing hormone-releasing hormone-1 neurons

Feedback controls of estrogen in LHRH-1 neurons play a pivotal role in reproductive function. However, the mechanism of estrogen action in LHRH-1 neurons is still unclear. In the present study, the effect of estrogens on intracellular calcium ([Ca(2+)](i)) oscillations in primate LHRH-1 neurons was examined. Application of 17beta-estradiol (E(2), 1 nm) for 10 min increased the frequency of [Ca(2+)](i) oscillations within a few minutes. E(2) also increased the frequency of [Ca(2+)](i) synchronization among LHRH-1 neurons. Similar E(2) effects on the frequency of [Ca(2+)](i) oscillations were observed under the presence of tetrodotoxin, indicating that estrogen appears to cause direct action on LHRH-1 neurons. Moreover, application of a nuclear membrane-impermeable estrogen dendrimer conjugate, not control dendrimer, resulted in a robust increase in the frequencies of [Ca(2+)](i) oscillations and synchronizations, indicating that effects estrogens on [Ca(2+)](i) oscillations and their synchronizations do not require their entry into the cell nucleus. Exposure of cells to E(2) in the presence of the estrogen receptor antagonist ICI 182,780 did not change the E(2)-induced increase in the frequency of [Ca(2+)](i) oscillations or the E(2)-induced increase in the synchronization frequency. Collectively, estrogens induce rapid, direct stimulatory actions through receptors located in the cell membrane/cytoplasm of primate LHRH-1 neurons, and this action of estrogens is mediated by an ICI 182,780-insensitive mechanism yet to be identified.

Estrogen induces rapid translocation of estrogen receptor beta, but not estrogen receptor alpha, to the neuronal plasma membrane

Estrogen receptors can activate transcription in the nucleus, and activate rapid signal transduction cascades in the cytosol. Multiple reports identify estrogen receptors at the plasma membrane, while others document the dynamic responses of estrogen receptor to ligand binding. However, the function and identity of membrane estrogen receptors remain controversial. We have used confocal microscopy and cell fractionation on the murine hippocampus-derived HT22 cell line and rat primary cortical neurons transfected with estrogen receptor-green fluorescent protein constructs to address the membrane localization of these receptors. We observe translocation of estrogen receptor beta (beta) to the plasma membrane 5 min after exposure to 17beta-estradiol, whereas estrogen receptor alpha (alpha) localization remains unchanged. Membrane localization of estrogen receptor beta is transient, selective for 17beta-estradiol, and is not blocked by ICI182,780. Inhibition of the mitogen-activated protein kinase pathway does not block estrogen-mediated estrogen receptor beta membrane translocation, and in fact prolongs membrane localization. These data suggest that while both estrogen receptor alpha and estrogen receptor beta can be present at the neuronal membrane, their presence is differentially regulated.

Estrogen alters spinal NMDA receptor activity via a PKA signaling pathway in a visceral pain model in the rat

Pain symptoms in several chronic pain disorders in women, including irritable bowel syndrome, fluctuate with the menstrual cycle suggesting a gonadal hormone component. In female rats, estrogens modulate visceral sensitivity although the underlying mechanism(s) are unknown. In the present study the effects of 17-beta estradiol on N-methyl-D-aspartate (NMDA) receptor signaling of colorectal nociceptive processing in the spinal cord were examined. Estrogen receptor alpha and the NR1 subunit of the NMDA receptor are co-expressed in dorsal horn neurons, supporting a direct action of estradiol on NMDA receptors. Intrathecal administration of the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (APV) dose-dependently attenuated the visceromotor response with greater potency in ovariectomized (OVx) rats compared to OVx with estradiol replacement (E2) rats. Estradiol significantly increased protein expression of NR1 in the lumbosacral spinal cord compared to OVx rats. Colorectal distention significantly increased phosphorylation of NR1ser-897, a PKA phosphorylation site on the NR1 subunit in E2, but not OVx rats. Intrathecal administration of a PKA inhibitor significantly attenuated the visceromotor response, decreased NR1 phosphorylation and increased the potency of APV to attenuate the visceromotor response compared to vehicle-treated E2 rats. These data suggest that estradiol increases spinal processing of visceral nociception by increasing NMDA receptor NR1 subunit expression and increasing site-specific receptor phosphorylation on the NR1 subunit contributing to an increase in NMDA receptor activity.

Estrogen signaling through the transmembrane G protein-coupled receptor GPR30

Steroids play an important role in the regulation of normal physiology and the treatment of disease. Steroid receptors have classically been described as ligand-activated transcription factors mediating long-term genomic effects in hormonally regulated tissues. It is now clear that steroids also mediate rapid signaling events traditionally associated with growth factor receptors and G protein-coupled receptors. Although evidence suggests that the classical steroid receptors are capable of mediating many of these events, more recent discoveries reveal the existence of transmembrane receptors capable of responding to steroids with cellular activation. One such receptor, GPR30, is a member of the G protein-coupled receptor superfamily and mediates estrogen-dependent kinase activation as well as transcriptional responses. In this review, we provide an overview of the evidence for the cellular and physiological actions of GPR30 in estrogen-dependent processes and discuss the relationship of GPR30 with classical estrogen receptors.

Estrogens promote human testicular germ cell cancer through a membrane-mediated activation of extracellular regulated kinase and protein kinase A

Clinical and experimental studies have suggested that estrogens, the archetype of female hormones, participate in the control of male germ cell proliferation and that fetal exposure to environmental estrogens may contribute to hypofertility and/or to testicular germ cell cancer. However, the underlying mechanisms remain to be elucidated. 17beta-Estradiol (E2) conjugated to BSA was able to stimulate human testicular seminoma cell proliferation by triggering a rapid, nongenomic, membrane-mediated activation of ERK1/2 and cAMP-dependent protein kinase A (PKA). Both ERK1/2 and PKA participated in this promoting effect. This activation was associated with phosphorylation of the transcription factor cAMP response element-binding protein and the nuclear factor retinoblastoma protein. Enhanced proliferation together with ERK activation could be reversed by pertussis toxin, a G protein inhibitor. Estrogen receptors (ERs) in JKT-1 were characterized by immunofluorescence, subcellular fractioning, and Western blot. JKT-1 cells did not express ERalpha but ERbeta, which localized to the mitochondria and the nucleus but not to the membrane. Moreover, neither ICI-182,780, a classical ER antagonist, nor tamoxifen, a selective ER modulator, could reverse the 17beta-estradiol-BSA-induced promoting effect. Estrogens contribute to human testicular germ cell cancer proliferation by rapid activation of ERK1/2 and PKA through a membrane nonclassical ER. This nongenomic effect represents a new basis for understanding the estrogenic control of spermatogenesis and evaluating the role of fetal exposure to xenoestrogens during malignant transformation of testicular germ stem cells.

Oestradiol rapidly enhances spontaneous glycinergic synaptic inhibition of hypoglossal motoneurones

Whereas oestradiol is well-known to facilitate excitatory glutamatergic synaptic transmission, its effects on fast inhibitory neurotransmission are not as well established. Possible acute modulation of the spontaneous glycinergic synaptic activity by oestradiol was investigated in voltage-clamped hypoglossal motoneurones by whole-cell patch-clamp recording in rat brainstem slices. The spontaneous glycinergic synaptic activity was continuously recorded in each neurone under control conditions, during 12-20 min of perfusion with 17beta-oestradiol and during washing. When oestradiol was diluted in ethanol, the control solution contained the same amount of ethanol. At 100 nM, oestradiol markedly increased the frequency of the total spontaneous glycinergic activity. Similar experiments were performed after blockade of action potentials by tetrodotoxin, aiming to isolate miniature glycinergic synaptic currents. Oestradiol increased the frequency of glycinergic miniatures in most slices, in some cases within less than 1 min. In some slices, oestradiol also favoured the occurrence of glycinergic miniatures of large amplitude. These effects were slowly reversible during washing. At 1 nm, oestradiol still increased the frequency of glycinergic miniatures. The results were confirmed in the absence of ethanol by using water-soluble cyclodextrin-encapsulated oestradiol. In these experiments, the control solution contained the same amount of (2-hydroxypropyl)-beta-cyclodextrin as the oestradiol-containing solution. In addition, prolonged control recordings were performed without applying oestradiol to check the stability of the glycinergic synaptic activity during prolonged whole-cell recordings. The results show, for the first time, that, within a few minutes, oestradiol can enhance the spontaneous synaptic release of a major inhibitory transmitter, glycine.

Estrogen genomic and membrane actions at an intersection

Estradiol is a primary hormonal signal that synchronizes central neuronal activity changes during the female reproductive cycle. The cellular and molecular mechanisms for many of these changes are still not well understood. Exciting new findings of Roepke and colleagues indicate that estradiol regulates expression of key potassium channels as well as modulatory intracellular signaling molecules. This regulation might contribute to region-specific changes in neuronal excitability in the hypothalamic arcuate nucleus and to the integration of neuronal responses that influence homeostatic functions.

Ecdysteroids, juvenile hormone and insect neuropeptides: Recent successes and remaining major challenges

In the recent decade, tremendous progress has been realized in insect endocrinology as the result of the application of a variety of advanced methods in neuropeptidome- and receptor research. Hormones of which the existence had been shown by bioassays four decades ago, e.g. bursicon (a member of the glycoprotein hormone family) and pupariation factor (Neb-pyrokinin 2, a myotropin), could be identified, along with their respective receptors. In control of diurnal rhythms, clock genes got company from the neuropeptide Pigment Dispersing Factor (PDF), of which the receptor could also be identified. The discovery of Inka cells and their function in metamorphosis was a true hallmark. Analysis of the genomes of Caenorhabditis elegans, Drosophila melanogaster and Apis mellifera yielded about 75, 100 and 200 genes coding for putative signaling peptides, respectively, corresponding to approximately 57, 100 and 100 peptides of which the expression could already be proven by means of mass spectrometry. The comparative approach invertebrates-vertebrates recently yielded indications for the existence of counterparts in insects for prolactin, atrial natriuretic hormone and Growth Hormone Releasing Hormone (GRH). Substantial progress has been realized in identifying the Halloween genes, a membrane receptor(s) for ecdysteroids, a nuclear receptor for methylfarnesoate, and dozens of GPCRs for insect neuropeptides. The major remaining challenges concern the making match numerous orphan GPCRs with orphan peptidic ligands, and elucidating their functions. Furthermore, the endocrine control of growth, feeding-digestion, and of sexual differentiation, in particular of males, is still poorly understood. The finding that the prothoracic glands produce an autocrine factor with growth factor-like properties and secrete proteins necessitates a reevaluation of their role in development.

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.

Extranuclear steroid receptors: nature and actions

Rapid effects of steroid hormones result from the actions of specific receptors localized most often to the plasma membrane. Fast-acting membrane-initiated steroid signaling (MISS) leads to the modification of existing proteins and cell behaviors. Rapid steroid-triggered signaling through calcium, amine release, and kinase activation also impacts the regulation of gene expression by steroids, sometimes requiring integration with nuclear steroid receptor function. In this and other ways, the integration of all steroid actions in the cell coordinates outcomes such as cell fate, proliferation, differentiation, and migration. The nature of the receptors is of intense interest, and significant data suggest that extranuclear and nuclear steroid receptor pools are the same proteins. Insights regarding the structural determinants for membrane localization and function, as well as the nature of interactions with G proteins and other signaling molecules in confined areas of the membrane, have led to a fuller understanding of how steroid receptors effect rapid actions. Increasingly, the relevance of rapid signaling for the in vivo functions of steroid hormones has been established. Examples include steroid effects on reproductive organ development and function, cardiovascular responsiveness, and cancer biology. However, although great strides have been made, much remains to be understood concerning the integration of extranuclear and nuclear receptor functions to organ biology. In this review, we highlight the significant progress that has been made in these areas.

A contactin-receptor-like protein tyrosine phosphatase beta complex mediates adhesive communication between astroglial cells and gonadotrophin-releasing hormone neurones

Although it is well established that gonadotrophin-releasing hormone (GnRH) neurones and astrocytes maintain an intimate contact throughout development and adult life, the cell-surface molecules that may contribute to this adhesiveness remain largely unknown. In the peripheral nervous system, the glycosylphosphatidyl inositol (GPI)-anchored protein contactin is a cell-surface neuronal protein required for axonal-glial adhesiveness. A glial transmembrane protein recognised by neuronal contactin is receptor-like protein tyrosine phosphatase beta (RPTP beta), a phosphatase with structural similarities to cell adhesion molecules. In the present study, we show that contactin, and its preferred in cis partner Caspr1, are expressed in GnRH neurones. We also show that the RPTP beta mRNA predominantly expressed in hypothalamic astrocytes encodes an RPTP beta isoform (short RPTP beta) that uses its carbonic anhydrase (CAH) extracellular subdomain to interact with neuronal contactin. Immunoreactive contactin is most abundant in GnRH nerve terminals projecting to both the organum vasculosum of the lamina terminalis and median eminence, implying GnRH axons as an important site of contactin-dependent cell adhesiveness. GT1-7 immortalised GnRH neurones adhere to the CAH domain of RPTPbeta, and this adhesiveness is blocked when contactin GPI anchoring is disrupted or contactin binding capacity is immunoneutralised, suggesting that astrocytic RPTP beta interacts with neuronal contactin to mediate glial-GnRH neurone adhesiveness. Because the abundance of short RPTP beta mRNA increases in the female mouse hypothalamus (but not in the cerebral cortex) before puberty, it appears that an increased interaction between GnRH axons and astrocytes mediated by RPTP beta-contactin is a dynamic mechanism of neurone-glia communication during female sexual development.

Rapid estradiol-17beta modulation of opioid actions on the electrical and secretory activity of rat oxytocin neurons in vivo

During pregnancy, emergence of endogenous opioid inhibition of oxytocin neurons is revealed by increased oxytocin secretion after administration of the opioid receptor antagonist, naloxone. Here we show that prolonged estradiol-17beta and progesterone treatment (mimicking pregnancy levels) potentiates naloxone-induced oxytocin secretion in urethane-anesthetized virgin female rats. We further show that estradiol-17beta alone rapidly modifies opioid interactions with oxytocin neurons, by recording their firing rate in anesthetized rats sensitized to naloxone by morphine dependence. Naloxone-induced morphine withdrawal strongly increased the firing rate of oxytocin neurons in morphine dependent rats. Estradiol-17beta did not alter basal oxytocin neuron firing rate over 30 min, but amplified naloxone-induced increases in firing rate. Firing pattern analysis indicated that acute estradiol-17beta increased oxytocin secretion in dependent rats by increasing action potential clustering without an overall increase in firing rate. Hence, rapid estradiol-17beta actions might underpin enhanced oxytocin neuron responses to naloxone in pregnancy.

Estrogen regulation of genes important for K+ channel signaling in the arcuate nucleus

Estrogen affects the electrophysiological properties of a number of hypothalamic neurons by modulating K(+) channels via rapid membrane actions and/or changes in gene expression. The interaction between these pathways (membrane vs. transcription) ultimately determines the effects of estrogen on hypothalamic functions. Using suppression subtractive hybridization, we produced a cDNA library of estrogen-regulated, brain-specific guinea pig genes, which included subunits from three prominent K+ channels (KCNQ5, Kir2.4, Kv4.1, and Kvbeta(1)) and signaling molecules that impact channel function including phosphatidylinositol 3-kinase (PI3K), protein kinase Cepsilon (PKCepsilon), cAMP-dependent protein kinase (PKA), A-kinase anchor protein (AKAP), phospholipase C (PLC), and calmodulin. Based on these findings, we dissected the arcuate nucleus from ovariectomized guinea pigs treated with estradiol benzoate (EB) or vehicle and analyzed mRNA expression using quantitative real-time PCR. We found that EB significantly increased the expression of KCNQ5 and Kv4.1 and decreased expression of KCNQ3 and AKAP in the rostral arcuate. In the caudal arcuate, EB increased KCNQ5, Kir2.4, Kv4.1, calmodulin, PKCepsilon, PLCbeta(4), and PI3Kp55gamma expression and decreased Kvbeta(1). The effects of estrogen could be mediated by estrogen receptor-alpha, which we found to be highly expressed in the guinea pig arcuate nucleus and, in particular, proopiomelanocortin neurons. In addition, single-cell RT-PCR analysis revealed that about 50% of proopiomelanocortin and neuropeptide Y neurons expressed KCNQ5, about 40% expressed Kir2.4, and about 60% expressed Kv4.1. Therefore, it is evident that the diverse effects of estrogen on arcuate neurons are mediated in part by regulation of K(+) channel expression, which has the potential to affect profoundly neuronal excitability and homeostatic functions, especially when coupled with the rapid effects of estrogen on K(+) channel function.

Expression of G protein-coupled receptor 30 in the hamster ovary: differential regulation by gonadotropins and steroid hormones

The nongenomic actions of estradiol-17beta are mediated by transmembrane estrogen receptors. Recently, G protein-coupled receptor 30 (GPR30) has been suggested to be a transmembrane estrogen receptor that can mediate rapid and transcription-independent estradiol-17beta signaling in different cell types. However, the expression, regulation, or biological relevance of GPR30 in the ovary remains unknown. We examined the expression and hormonal regulation of GPR30 mRNA and protein in hamster ovarian cells during the estrous cycle and after hypophysectomy and hormone replacement. GPR30 protein expression was high in the theca, appreciable in the granulosa, but low in luteal cells. GPR30 protein levels in granulosa and theca cells increased steadily with the development of preantral and antral follicles, respectively. GPR30 mRNA and protein levels increased significantly on diestrous (d 3 of the estrous cycle), but decreased on d 4 at 1600 h after the LH surge. GPR30 mRNA levels increased significantly after hypophysectomy. Although steroid treatment failed to alter ovarian GPR30 mRNA levels, either FSH or LH effectively reduced the levels. Interestingly, the decrease in GPR30 mRNA corresponded to a marked increase in the receptor protein levels. FSH treatment, either alone or together with LH, resulted in a marked increase in GPR30 immunostaining in granulosa cells. LH alone significantly increased immunostaining in theca cells. These results suggest that GPR30 is expressed in the membrane of hamster granulosa and theca cells, and the expression is regulated by gonadotropins. The unique pattern of GPR30 expression suggests that gonadotropin-regulated follicular cell functions may involve GPR30 activity.

Tearful relationships? Sex, hormones, the lacrimal gland, and aqueous-deficient dry eye

Sex and the endocrine system exert a significant influence on the physiology and pathophysiology of the lacrimal gland. The purpose of this article is to briefly review the nature and magnitude of these interactions between sex, hormones and lacrimal tissue, and to address how they may relate to the pathogenesis of aqueous-deficient dry eye. Towards this end, this article has a 3-fold approach: first, to summarize the influence of androgens, estrogens, glucocorticoids, mineralocorticoids, retinoic acid, prolactin, alpha-melanocyte stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, growth hormone, thyroid-stimulating hormone, arginine vasopressin, oxytocin, thyroxine, parathyroid hormone, insulin, glucagon, melatonin, human chorionic gonadotropin and cholecystokinin on the structure and function of the lacrimal gland; second, to discuss the mechanism of action of each hormone on lacrimal tissue; and third, to discuss the clinical relevance of the endocrine-lacrimal gland interrelationship, with a particular focus on each hormone's role (i.e. if relevant) in the development of aqueous-tear deficiency.

Serotonin 5-hydroxytryptamine2C receptor signaling in hypothalamic proopiomelanocortin neurons: role in energy homeostasis in females

Hypothalamic proopiomelanocortin (POMC) neurons play a critical role in the regulation of energy balance, and there is a convergence of critical synaptic input including GABA and serotonin on POMC neurons to regulate their output. We found previously that 17beta-estradiol (E(2)) reduced the potency of the GABA(B) receptor agonist baclofen to activate G protein-coupled inwardly rectifying potassium (GIRK) channels in hypothalamic POMC neurons through a membrane estrogen receptor (mER) via a Galpha(q) phospholipase C (PLC)-protein kinase Cdelta-protein kinase A pathway. We hypothesized that the mER and neurotransmitter receptor signaling pathways converge to control energy homeostasis. Because 5-HT(2C) receptors mediate many of the effects of serotonin in POMC neurons, we elucidated the common signaling pathways of E(2) and 5-HT in guinea pigs using single-cell reverse transcription-polymerase chain reaction (RT-PCR), real time RT-PCR, and whole-cell patch recording. Both 5-hydroxytryptamine(2C) (5-HT(2C)) and 5-HT(2A) receptors were coexpressed in POMC neurons. The 5-HT(2A/C) agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) desensitized the GABA(B) response in a dose-dependent manner, which was antagonized by the selective 5-HT(2C) receptor antagonists 8-[5-(2,4-dimethoxy-5-(4-trifluoromethylphenylsulphonamido) phenyl-5-oxopentyl]1,3,8-triazaspiro[4.5] decane-2,4-dione hydrochloride (RS102221) and 1,2,3, 4,10,14b-hexahydro-2-methyldibenzo [c,f]pyrazino[1,2-a]-azepine hydrochloride (ORG 3363). The 5-HT(2C) receptor was Galpha(q)-coupled to PLC activation and hydrolysis of plasma membrane phosphatidylinositol bisphosphate to directly inhibit GIRK channel activity. Coapplication of the two agonists at their EC(50) concentrations (DOI, 20 muM, and E(2), 50 nM) produced additive effects. Although there was a significant gender difference in the effects of E(2) on baclofen responses, there was no gender difference in 5-HT(2C) receptor-mediated effects. Finally, both DOI and estrogen (intracerebroventricular) inhibited feeding in ovariectomized female mice. Therefore, the Galpha(q) signaling pathways of the mER and 5-HT(2C) receptors may converge to enhance synaptic efficacy in brain circuits that are critical for maintaining homeostatic functions.

Estrogen evokes a rapid effect on intracellular calcium in neurons characterized by calcium oscillations in the arcuate nucleus

Rapid estrogen effects became an interesting topic to explain estrogen effects not associated with the classical nuclear pathway. The rapid estrogen effect on intracellular calcium oscillations was characterized in neurons of the arcuate nucleus. Ratiometric calcium imaging (fura-2AM) was used to measure intracellular calcium in brain slices of female Swiss Webster mice (median of age 27 days p.n.). Calcium oscillations were dependent on intracellular calcium and also on calcium influx from the extracellular space. The perfusion of slices with calcium-free solution inhibited spontaneous calcium oscillations. The metabotropic glutamate receptor agonist t-ACPD (5 microM) and low concentrated ryanodine (100 nM) induced intracellular calcium release when slices were perfused with calcium-free solution. 17beta-estradiol (10 nM) also induced intracellular calcium release in calcium-free ACSF. This effect was inhibited by the preceding administration of thapsigargin (2 microM) indicating the association of the rapid estrogen effect with intracellular calcium stores. The administration of the non-selective phospholipase C-inhibitor ET-18 (30 microM), but not U73122 (10 microM), and the inhibition of protein kinase A by H-89 (0.25 microM) suppressed the rapid estrogen effect. Analyses indicated a qualitative, but not quantitatively significant effect of 17beta-estradiol on calcium oscillations.

Acute effects of estrogen on neuronal physiology

It has been known for more than 30 years that estrogen can alter the intrinsic and synaptic physiology of neurons within minutes. The physiological significance of these acute effects has been unclear, however, because some effects require higher concentrations of estrogen than are detected in plasma, and because estrogen secreted by the ovary rises and falls over a time course of days, not minutes. These concerns may be answered by new research demonstrating that estrogen is produced at high levels within the brain itself, and that production of estrogen in the brain may be regulated by neuronal activity. Additionally, recent studies indicate that classical estrogen receptor proteins are found not only in the nucleus where they regulate gene expression but also at extranuclear sites, including at synapses. These findings, together with evidence for new types of extranuclear estrogen receptors, suggest that estrogen might act directly at synapses to activate second messenger signaling, thereby rapidly altering neuronal excitability, synaptic transmission, and/or synaptic plasticity.

Potential role of female sex hormones in the pathophysiology of migraine

Clinical evidence indicates that female sex steroids may contribute to the high prevalence of migraine in women, as well as changes in the frequency or severity of migraine attacks that are in tandem with various reproductive milestones in women's life. While female sex steroids do not seem to be involved in the pathogenesis of migraine per se, they may modulate several mediators and/or receptor systems via both genomic and non-genomic mechanisms; these actions may be perpetuated at the central nervous system, as well as at the peripheral (neuro)vascular level. For example, female sex steroids have been shown to enhance: (i) neuronal excitability by elevating Ca(2+) and decreasing Mg(2+) concentrations, an action that may occur with other mechanisms triggering migraine; (ii) the synthesis and release of nitric oxide (NO) and neuropeptides, such as calcitonin gene-related peptide CGRP, a mechanism that reinforces vasodilatation and activates trigeminal sensory afferents with a subsequent stimulation of pain centres; and (iii) the function of receptors mediating vasodilatation, while the responses of receptors inducing vasoconstriction are attenuated. The serotonergic, adrenergic and gamma-aminobutyric acid (GABA)-ergic systems are also modulated by sex steroids, albeit to a varying degree and with potentially contrasting effects on migraine outcome. Taken together, female sex steroids seem to be involved in an array of components implicated in migraine pathogenesis. Future studies will further delineate the extent and the clinical relevance of each of these mechanisms, and will thus expand the knowledge on the femininity of migraine.

Membrane-initiated actions of estrogens in neuroendocrinology: emerging principles

Hormonal ligands for the nuclear receptor superfamily have at least two interacting mechanisms of action: 1) classical transcriptional regulation of target genes (genomic mechanisms); and 2) nongenomic actions that are initiated at the cell membrane, which could impact transcription. Although transcriptional mechanisms are increasingly well understood, membrane-initiated actions of these ligands are incompletely understood. Historically, this has led to a considerable divergence of thought in the molecular endocrine field. We have attempted to uncover principles of hormone action that are relevant to membrane-initiated actions of estrogens. There is evidence that the membrane-limited actions of hormones, particularly estrogens, involve the rapid activation of kinases and the release of calcium. Membrane actions of estrogens, which activate these rapid signaling cascades, can also potentiate nuclear transcription. These signaling cascades may occur in parallel or in series but subsequently converge at the level of modification of transcriptionally relevant molecules such as nuclear receptors and/or coactivators. In addition, other hormones or neurotransmitters may also activate cascades to crosstalk with estrogen receptor-mediated transcription. The idea of synergistic coupling between membrane-initiated and genomic actions of hormones fundamentally revises the paradigms of cell signaling in neuroendocrinology.

17-Beta-estradiol-mediated activation of extracellular-signal regulated kinase, phosphatidylinositol 3-kinase/protein kinase B-Akt and N-methyl-D-aspartate receptor phosphorylation in cortical synaptoneurosomes

In addition to its well-known activational mechanism, the steroid hormone 17-beta-estradiol (E2) has been shown to rapidly activate various signal transduction pathways that could participate in estrogen-mediated regulation of synaptic plasticity. Although the mechanisms underlying these effects are not clearly understood, it has been repeatedly suggested that they involve a plasma membrane receptor which has direct links to several intracellular signaling cascades. To further address the question of whether E2 acts directly at the synapse and through membrane-bound receptors, we studied the effects of E2 and of ligands of estrogen receptors on various signaling pathways in cortical synaptoneurosomes. Our results demonstrate that E2 elicits N-methyl-D-aspartate receptor phosphorylation and activates the extracellular signal-regulated kinase and the phosphatidylinositol 3-kinase/Akt signal transduction pathways in this cortical membrane preparation. Furthermore, we provide evidence for the presence of a membrane-bound estrogen receptor responsible for these effects in cortical synaptoneurosomes. Our study demonstrates that E2 directly acts at cortical synapses, and that synaptoneurosomes provide a useful system to investigate the mechanisms by which E2 regulates synaptic transmission and plasticity.

Functional significance of the rapid regulation of brain estrogen action: where do the estrogens come from?

Estrogens exert a wide variety of actions on reproductive and non-reproductive functions. These effects are mediated by slow and long lasting genomic as well as rapid and transient non-genomic mechanisms. Besides the host of studies demonstrating the role of genomic actions at the physiological and behavioral level, mounting evidence highlights the functional significance of non-genomic effects. However, the source of the rapid changes in estrogen availability that are necessary to sustain their fast actions is rarely questioned. For example, the rise of plasma estrogens at pro-estrus that represents one of the fastest documented changes in plasma estrogen concentration appears too slow to explain these actions. Alternatively, estrogen can be synthesized in the brain by the enzyme aromatase providing a source of locally high concentrations of the steroid. Furthermore, recent studies demonstrate that brain aromatase can be rapidly modulated by afferent inputs, including glutamatergic afferents. A role for rapid changes in estrogen production in the central nervous system is supported by experiments showing that acute aromatase inhibition affects nociception as well as male sexual behavior and that preoptic aromatase activity is rapidly (within min) modulated following mating. Such mechanisms thus fulfill the gap existing between the fast actions of estrogen and their mode of production and open new avenues for the understanding of estrogenic effects on the brain.

Minireview: rapid glucocorticoid signaling via membrane-associated receptors

Glucocorticoids are secreted into the systemic circulation from the adrenal cortex and initiate a broad range of actions throughout the organism that regulate the function of multiple organ systems, including the liver, muscle, the immune system, the pancreas, fat tissue, and the brain. Delayed glucocorticoid effects are mediated by classical steroid mechanisms involving transcriptional regulation. Relatively rapid effects of glucocorticoids also occur that are incompatible with genomic regulation and invoke a noncanonical mode of steroid action. Studies conducted in several labs and on different species suggest that the rapid effects of glucocorticoids are mediated by the activation of one or more membrane-associated receptors. Here, we provide a brief review focused on multiple lines of evidence suggesting that rapid glucocorticoid actions are triggered by, or at least dependent on, membrane-associated G protein-coupled receptors and activation of downstream signaling cascades. We also discuss the possibility that membrane-initiated actions of glucocorticoids may provide an additional mechanism for the regulation of gene transcription.

Synthesis and biological evaluation of SERMs with potent nongenomic estrogenic activity

We have synthesized novel SERMs that activate a rapid response in CNS neurons, but which lack the ability to bind to the nuclear estrogen receptors (ERalpha and ERbeta). These compounds are analogues of 4-hydroxytamoxifen, but unlike 4-hydroxytamoxifen, they do not exist as a mixture of E/Z isomers. They contain a carboxamide insertion between the olefin and basic phenyl side chain, which results in more stable geometric isomers. The amide insertion also eliminates their ability to bind to the nuclear estrogen receptors, and hence, they are unable to modulate ER-mediated gene transcription as do classical estrogens and SERMs. We show that one of these analogues, ST-X, elicits a potent nongenomic estrogen response in the CNS by rapidly inhibiting GIRK activation in hypothalamic gamma-aminobutyric acid (GABA) and proopiomelanocortin (POMC) neurons. To our knowledge, ST-X is the only SERM that modulates rapid estrogen responses, but which lacks nuclear ER activity.

Steroid requirements for regulation of the alpha4 subunit of the GABA(A) receptor in an in vitro model

The alpha4 subunit of the GABA(A) receptor (GABAR) has relatively low expression in the CNS, but is increased in vivo following 48 h administration of the GABA-modulatory steroid 3alpha-OH-5alpha[beta]-pregnan-20-one (THP or [allo]pregnanolone) to female rats. The purpose of the following study was to determine the optimal conditions for steroid-induced upregulation of alpha4 expression in an in vitro model. To this end, we used the IMR-32 cell, a neuroblastoma cell line, which normally expresses alpha4 mRNA at low levels. In undifferentiated IMR-32 cells, 48 h administration of THP increased alpha4 expression when ambient THP levels were reduced by the 5alpha-reductase blocker 4MA, suggesting that the background steroid milieu affects steroid regulation of this subunit. Following neuronal differentiation in serum-free medium, 48 h THP treatment significantly increased alpha4 expression two-fold following application of nerve growth factor (NGF) suggesting that development of neuronal processes facilitates this effect of the steroid. In the absence of NGF treatment, combined administration of 17beta-estradiol (E2) plus THP also increased alpha4 expression to a similar extent as THP following NGF treatment. In addition, E2 alone effectively increased alpha4 expression to maximal levels following NGF treatment. In contrast, neuronal differentiation in the absence of serum deprivation did not increase alpha4 levels. These results suggest that both THP and E2 can increase expression of the GABAR alpha4 subunit, but that this effect is dependent upon the background steroid milieu as well as the degree of neuronal development. These findings demonstrate optimal conditions for steroid-induced upregulation of the alpha4 subunit in an in vitro system.

Rapid inhibition of Ca2+ influx by neurosteroids in murine embryonic sensory neurones

The non-genomic role of neuroactive steroids on [Ca2+]i transients induced by GABA receptor activation was investigated in cultured dorsal root ganglia (DRG) neurones at embryonic stage E13. [Ca2+]i measurements were performed with Fura-2 fast fluorescence microfluorimetry. Application of the GABAA receptor agonist muscimol (Musci) evoked an increase in [Ca2+]i, confirming the excitatory effect of GABA at this embryonic stage. The muscimol-induced [Ca2+]i response was inhibited by progesterone (Proges) and its primary metabolite allopregnanolone (Allo) in a rapid, reversible and dose-dependent manner. These calcium transients were suppressed in the absence of external Ca2+ or in the presence of Ni2+ + Cd2+ suggesting an involvement of voltage-activated Ca2+ channels. In contrast, none of these steroids affected the resting [Ca2+]i nor exhibited any inhibitory effect on 50 mM KCl-induced [Ca2+]i increases. In view of the well-established potentiation of GABAA receptor by direct binding of neurosteroids, the inhibitory effects described in this study seem to involve distinct mechanisms. This new inhibitory effect of progesterone is observed at low and physiological concentrations, is rapid and independent of RU38486, an antagonist of the classic progesterone receptor, probably involving a membrane receptor. Using RT-PCR, we demonstrated the expression of progesterone receptor membrane component 1 (Pgrmc1), encoding 25-Dx, a membrane-associated progesterone binding protein in DRG neurones at different stages of development. In conclusion, we describe for the first time a rapid effect of progestins on embryonic DRG neurones involving an antagonistic effect of progesterone and allopregnanolone on GABAA receptors.