Acute estradiol application increases inward and decreases outward whole-cell currents of neurons in rat hypothalamic ventromedial nucleus

Estrogenic Modulation of Ionic Channels, Pumps and Exchangers in Airway Smooth Muscle

To preserve ionic homeostasis (primarily Ca²⁺, K⁺, Na⁺, and Cl^-), in the airway smooth muscle (ASM) numerous transporters (channels, exchangers, and pumps) regulate the influx and efflux of these ions. Many of intracellular processes depend on continuous ionic permeation, including exocytosis, contraction, metabolism, transcription, fecundation, proliferation, and apoptosis. These mechanisms are precisely regulated, for instance, through hormonal activity. The lipophilic nature of steroidal hormones allows their free transit into the cell where, in most cases, they occupy their cognate receptor to generate genomic actions. In the sense, estrogens can stimulate development, proliferation, migration, and survival of target cells, including in lung physiology. Non-genomic actions on the other hand do not imply estrogen's intracellular receptor occupation, nor do they initiate transcription and are mostly immediate to the stimulus. Among estrogen's non genomic responses regulation of calcium homeostasis and contraction and relaxation processes play paramount roles in ASM. On the other hand, disruption of calcium homeostasis has been closely associated with some ASM pathological mechanism. Thus, this paper intends to summarize the effects of estrogen on ionic handling proteins in ASM. The considerable diversity, range and power of estrogens regulates ionic homeostasis through genomic and non-genomic mechanisms.

Estrogens rapidly shape synaptic and intrinsic properties to regulate the temporal precision of songbird auditory neurons

Sensory neurons parse millisecond-variant sound streams like birdsong and speech with exquisite precision. The auditory pallial cortex of vocal learners like humans and songbirds contains an unconventional neuromodulatory system: neuronal expression of the estrogen synthesis enzyme aromatase. Local forebrain neuroestrogens fluctuate when songbirds hear a song, and subsequently modulate bursting, gain, and temporal coding properties of auditory neurons. However, the way neuroestrogens shape intrinsic and synaptic properties of sensory neurons remains unknown. Here, using a combination of whole-cell patch clamp electrophysiology and calcium imaging, we investigate estrogenic neuromodulation of auditory neurons in a region resembling mammalian auditory association cortex. We found that estradiol rapidly enhances the temporal precision of neuronal firing via a membrane-bound G-protein coupled receptor and that estradiol rapidly suppresses inhibitory synaptic currents while sparing excitation. Notably, the rapid suppression of intrinsic excitability by estradiol was predicted by membrane input resistance and was observed in both males and females. These findings were corroborated by analysis of in vivo electrophysiology recordings, in which local estrogen synthesis blockade caused acute disruption of the temporal correlation of song-evoked firing patterns. Therefore, on a modulatory timescale, neuroestrogens alter intrinsic cellular properties and inhibitory neurotransmitter release to regulate the temporal precision of higher-order sensory neurons.

Can distinctly different rapid estrogen actions share a common mechanistic step?

Contribution to Special Issue on Fast effects of steroids. This paper reviews early evidence for the existence of rapid, non-genomic effects of estrogens on neurons, and, further, proposes that these rapid effects are often synergistic with later, genomic effects. Finally, suggestions about potential molecular mechanisms underlying the rapid effects of estrogens are offered. A mechanistic step we propose to be common among rapid estrogenic actions includes membrane ER's binding to histamine, and NMDA receptors and subsequent dimerization, and clustering (respectively) in a manner that enhances histamine and NMDA actions.

Inhibition of blood-brain barrier efflux transporters promotes seizure in pregnant rats: Role of circulating factors

Seizure-provoking factors circulate late in gestation during normal pregnancy, but do not readily gain access to the brain due to the protective nature of the blood-brain barrier. In particular, efflux transporters are powerful ATP-driven pumps that actively prevent unwanted compounds from entering the brain. We hypothesized that acute inhibition of efflux transporters at the blood-brain barrier would result in spontaneous seizures in pregnant rats. We further hypothesized that the blood-brain barrier protects the maternal brain from seizure by increasing expression and/or activity of p-glycoprotein (P-gp), a major efflux transporter. Main blood-brain barrier efflux transporters were inhibited in-vivo in nonpregnant (Nonpreg) and pregnant (Preg; d19) Sprague Dawley rats (n=8/group). Seizures were monitored in conscious animals for 8h via chronically implanted electroencephalography (EEG) electrodes in the hippocampus and motor cortex and time-synced video. P-gp activity was measured via a calcein accumulation assay in freshly isolated cortical and hippocampal capillaries from Preg (d20) and Nonpreg rats (n=8-16/group), to assess regional susceptibility to transporter inhibition. P-gp expression, capillary density, and microglial activation as a measure of neuroinflammation were quantified using immunohistochemistry (n=4-6/group). Efflux transporter inhibition elicited hippocampal seizures within 1h in 100% of Preg rats that was not associated with neuroinflammation or elevated tumor necrosis factor alpha (TNFα) or vascular endothelial growth factor (VEGF), but negatively correlated with levels of estradiol. Hippocampal seizures were considerably less prevalent in Nonpreg rats. However, behavioral seizures in the motor cortex developed of similar severity in both groups of rats, demonstrating regional heterogeneity in response to efflux transporter inhibition. Basal P-gp activity was similar between groups, however, exposure to serum from Preg rats significantly decreased P-gp activity in the hippocampus, but not cortex, compared to serum from Nonpreg rats (0.29±0.1units/s in Preg vs. 0.06±0.02units/s in Nonpreg rats; p<0.05) that was not associated with elevated TNFα or VEGF. Thus, pregnancy differentially increased the susceptibility of the hippocampus to seizures in response to blood-brain barrier efflux transporter inhibition that may be due to the inhibitory effect of circulating factors in pregnancy on P-gp activity in the hippocampus.

Ventromedial Hypothalamus and the Generation of Aggression

Aggression is a costly behavior, sometimes with severe consequences including death. Yet aggression is prevalent across animal species ranging from insects to humans, demonstrating its essential role in the survival of individuals and groups. The question of how the brain decides when to generate this costly behavior has intrigued neuroscientists for over a century and has led to the identification of relevant neural substrates. Various lesion and electric stimulation experiments have revealed that the hypothalamus, an ancient structure situated deep in the brain, is essential for expressing aggressive behaviors. More recently, studies using precise circuit manipulation tools have identified a small subnucleus in the medial hypothalamus, the ventrolateral part of the ventromedial hypothalamus (VMHvl), as a key structure for driving both aggression and aggression-seeking behaviors. Here, we provide an updated summary of the evidence that supports a role of the VMHvl in aggressive behaviors. We will consider our recent findings detailing the physiological response properties of populations of VMHvl cells during aggressive behaviors and provide new understanding regarding the role of the VMHvl embedded within the larger whole-brain circuit for social sensation and action.

Development of Electrophysiological Properties of Nucleus Gigantocellularis Neurons Correlated with Increased CNS Arousal

Many types of data have suggested that neurons in the nucleus gigantocellularis (NGC) in the medullary reticular formation are critically important for CNS arousal and behavioral responsiveness. To extend this topic to a developmental framework, whole-cell patch-recorded characteristics of NGC neurons in brainstem slices and measures of arousal-dependent locomotion of postnatal day 3 (P3) to P6 mouse pups were measured and compared. These neuronal characteristics developed in an orderly, statistically significant monotonic manner over the course of P3-P6: (1) proportion of neurons capable of firing action potential (AP) trains, (2) AP amplitude, (3) AP threshold, (4) amplitude of inward and outward currents, (5) amplitude of negative peak currents, and (6) steady state currents (in I-V plot). These measurements reflect the maturation of sodium and certain potassium channels. Similarly, all measures of locomotion, latency to first movement, total locomotion duration, net locomotion distance, and total quiescence time also developed monotonically over P3-P6. Most importantly, electrophysiological and behavioral measures were significantly correlated. Interestingly, the behavioral measures were not correlated with frequency of excitatory postsynaptic currents or the proportion of neurons showing these currents, responses to a battery of neurotransmitter agents, or rapid activating potassium currents (including IA). Considering the results here in the context of a large body of literature on NGC, we hypothesize that the developmental increase in NGC neuronal excitability participates in causing the increased behavioral responsivity during the postnatal period from P3 to P6.

Analyses of rapid estrogen actions on rat ventromedial hypothalamic neurons

Rapid estrogen actions are widely diverse across many cell types. We conducted a series of electrophysiological studies on single rat hypothalamic neurons and found that estradiol (E2) could rapidly and independently potentiate neuronal excitation/depolarizations induced by histamine (HA) and N-Methyl-d-Aspartate (NMDA). Now, the present whole-cell patch study was designed to determine whether E2 potentiates HA and NMDA depolarizations - mediated by distinctly different types of receptors - by the same or by different mechanisms. For this, the actions of HA, NMDA, as well as E2, were investigated first using various ion channel blockers and then by analyzing and comparing their channel activating characteristics. Results indicate that: first, both HA and NMDA depolarize neurons by inhibiting K(+) currents. Second, E2 potentiates both HA and NMDA depolarizations by enhancing the inhibition of K(+) currents, an inhibition caused by the two transmitters. Third, E2 employs the very same mechanism, the enhancement of K(+) current inhibition, thus to rapidly potentiate HA and NMDA depolarizations. These data are of behavioral importance, since the rapid E2 potentiation of depolarization synergizes with nuclear genomic actions of E2 to facilitate lordosis behavior, the primary female-typical reproductive behavior.

Estrogen regulation of the transient outward K(+) current involves estrogen receptor α in mouse heart

BACKGROUND AND OBJECTIVE

We have previously shown that androgens upregulate cardiac K(+) channels and shorten repolarization. However, the effects that estrogens (E2) and estrogen receptors (ER) might have on the various repolarizing K(+) currents and underlying ion channels remain incompletely understood. Accordingly, our objective was to verify whether and how E2 and its ERs subtypes influence these K(+) currents.

METHODS AND RESULTS

In order to examine the influence of E2 and ERs on K(+) currents we drastically lowered the E2 level through ovariectomy (OVX; 74% reduction vs CTL) and in parallel, we used female mice lacking either ERα (ERαKO) or ERβ (ERβKO). In OVX mice, results showed a specific increase of 35% in the density of the Ca(2+)-independent transient outward K(+) current (Ito) compared to CTL. Western blots showed increase in Kv4.2 and Kv4.3 sarcolemmal protein expression while qPCR revealed higher mRNA expression of only Kv4.3 in OVX mice. This upregulation of Ito was correlated with a shorter ventricular action potential duration and QTc interval. In ERαKO but not ERβKO mice, the mRNA of Kv4.3 was selectively increased. Furthermore, when ventricular myocytes obtained from ERαKO and ERβKO were cultured in the presence of E2, results showed that E2 reduced Ito density only in ERβKO myocytes confirming the repressive role of E2-ERα in regulating Ito.

CONCLUSION

Altogether, these results suggest that E2 negatively regulates the density of Ito through ERα, this highlights a potential role for this female hormone and its α-subtype receptor in modulating cardiac electrical activity.

Behavioral effects and mechanisms of migraine pathogenesis following estradiol exposure in a multibehavioral model of migraine in rat

Migraine is one of the most common neurological disorders, leading to more than 1% of total disability reported and over 68 million visits to emergency rooms or physician's offices each year in the United States. Three times as many women as men have migraine, and while the mechanism behind this is not well understood, 17β-estradiol (estradiol) has been implicated to play a role. Studies have demonstrated that exposure to estrogen can lead to activation of inflammatory pathways, changes in sodium gated channel activity, as well as enhanced vasodilation and allodynia. Estradiol receptors are found in trigeminal nociceptors, which are involved in signaling during a migraine attack. The purpose of this study was to investigate the role of estradiol in migraine pathogenesis utilizing a multibehavioral model of migraine in rat. Animals were surgically implanted with a cannula system to induce migraine and behavior was assessed following exposure to a proestrus level of estradiol for total locomotor activity, light and noise sensitivity, evoked grooming patterns, and enhanced acoustic startle response. Results demonstrated decreased locomotor activity, increased light and noise sensitivity, altered facial grooming indicative of allodynia and enhanced acoustic startle. Further examination of tissue samples revealed increased expression of genes associated with inflammation and vasodilation. Overall, this study demonstrates exacerbation of migraine-like behaviors following exposure to estradiol and helps further explain the underlying mechanisms behind sex differences found in this common neurological disorder.

The antidepressant-like effects of topiramate alone or combined with 17β-estradiol in ovariectomized Wistar rats submitted to the forced swimming test

RATIONALE

There is a significant delay in the clinical response of antidepressant drugs, and antidepressant treatments produce side effects.

OBJECTIVE

We examined the relationship between 17β-estradiol and topiramate in ovariectomized Wistar rats submitted to the forced swimming test (FST).

METHODS

Topiramate was administered alone or combined with 17β-estradiol to ovariectomized rats submitted to the FST.

RESULTS

Topiramate (20 mg/kg, P < 0.05; 30 mg/kg, P < 0.05) reduced immobility by increasing swimming; these effects were antagonized by finasteride (50 mg/kg). In interaction experiments, topiramate (10 mg/kg) plus 17β-estradiol (5 micrograms per rat; P < 0.05) reduced immobility by increasing swimming behavior. Besides, 17β-estradiol (2.5 micrograms per rat) shortened the onset of the antidepressant-like effects of topiramate (P < 0.05). In the open field test, topiramate alone or combined with 17β-estradiol (P < 0.05) reduced locomotion.

CONCLUSIONS

Topiramate alone or combined with 17β-estradiol produced antidepressant-like actions; and 17β-estradiol shortened the onset of the antidepressant-like effects of topiramate.

Regulation of voltage-gated sodium channel expression in cancer: hormones, growth factors and auto-regulation

Although ion channels are increasingly being discovered in cancer cells in vitro and in vivo, and shown to contribute to different aspects and stages of the cancer process, much less is known about the mechanisms controlling their expression. Here, we focus on voltage-gated Na⁺ channels (VGSCs) which are upregulated in many types of carcinomas where their activity potentiates cell behaviours integral to the metastatic cascade. Regulation of VGSCs occurs at a hierarchy of levels from transcription to post-translation. Importantly, mainstream cancer mechanisms, especially hormones and growth factors, play a significant role in the regulation. On the whole, in major hormone-sensitive cancers, such as breast and prostate cancer, there is a negative association between genomic steroid hormone sensitivity and functional VGSC expression. Activity-dependent regulation by positive feedback has been demonstrated in strongly metastatic cells whereby the VGSC is self-sustaining, with its activity promoting further functional channel expression. Such auto-regulation is unlike normal cells in which activity-dependent regulation occurs mostly via negative feedback. Throughout, we highlight the possible clinical implications of functional VGSC expression and regulation in cancer.

G protein-coupled estrogen receptor in energy homeostasis and obesity pathogenesis

Obesity and its related metabolic diseases have reached a pandemic level worldwide. There are sex differences in the prevalence of obesity and its related metabolic diseases, with men being more vulnerable than women; however, the prevalence of these disorders increases dramatically in women after menopause, suggesting that sex steroid hormone estrogens play key protective roles against development of obesity and metabolic diseases. Estrogens are important regulators of several aspects of metabolism, including body weight and body fat, caloric intake and energy expenditure, and glucose and lipid metabolism in both males and females. Estrogens act in complex ways on their nuclear estrogen receptors (ERs) ERα and ERβ and transmembrane ERs such as G protein-coupled estrogen receptor. Genetic tools, such as different lines of knockout mouse models, and pharmacological agents, such as selective agonists and antagonists, are available to study function and signaling mechanisms of ERs. We provide an overview of the evidence for the physiological and cellular actions of ERs in estrogen-dependent processes in the context of energy homeostasis and body fat regulation and discuss its pathology that leads to obesity and related metabolic states.

17β-Estradiol regulates the gene expression of voltage-gated sodium channels: role of estrogen receptor α and estrogen receptor β

Estradiol (E2) plays a key role in pain modulation, and the biological effects of E2 are transduced by binding estrogen receptors (ERs). Voltage-gated sodium (Nav) channels are responsible for the generation and propagation of action potentials in the membranes of most neurons and excitable cells. Adult dorsal root ganglion (DRG) neurons can express the ERs (ERα and ERβ), and Nav channels (TTX-S: Nav1.1, Nav1.6, and Nav1.7; and TTX-R: Nav1.8, and Nav1.9). Although E2 modulates Nav channel currents, little is known about the molecular mechanisms involved. In this study, we investigate the mRNA expressions of Nav channel subtypes mediated differentially by the ERs in the DRGs of wild-type (WT) and estrogen receptor knockout (αERKO and βERKO) mice. By means of quantitative real-time PCR, we found that the expressions of Nav1.1, Nav1.7, Nav1.8, and Nav1.9 subtypes were elevated in αERKO and βERKO mice, whereas Nav1.6 mRNA decreased in αERKO, but not in βERKO mice. The mRNA expressions of Nav subtypes were increased in E2-treated WT ovariectomized animals. We also found that E2-regulation of Nav1.1 and Nav1.9 mRNA expressions is dependent on ERα, ERβ, and another ER, whereas E2-regulation of Nav1.8 appears to be in an ERβ-dependent manner.

Voltage-gated potassium currents are targets of diurnal changes in estradiol feedback regulation and kisspeptin action on gonadotropin-releasing hormone neurons in mice

Estradiol has both negative and positive feedback actions upon gonadotropin-releasing hormone (GnRH) release; the latter actions trigger the preovulatory GnRH surge. Although neurobiological mechanisms of the transitions between feedback modes are becoming better understood, the roles of voltage-gated potassium currents, major contributors to neuronal excitability, are unknown. Estradiol alters two components of potassium currents in these cells: a transient current, I(A), and a sustained current, I(K). Kisspeptin is a potential mediator between estradiol and GnRH neurons and can act directly on GnRH neurons. We examined how estradiol, time of day, and kisspeptin interact to regulate these conductances in a mouse model exhibiting daily switches between estradiol negative (morning) and positive feedback (evening). Whole-cell voltage clamp recordings were made from GnRH neurons in brain slices from ovariectomized (OVX) mice and from OVX mice treated with estradiol (OVX+E). There were no diurnal changes in either I(A) or I(K) in GnRH neurons from OVX mice. In contrast, in GnRH neurons from OVX+E mice, I(A) and I(K) were greater during the morning when GnRH neuron activity is low and smaller in the evening when GnRH neuron activity is high. Estradiol increased I(A) in the morning and decreased it in the evening, relative to that in cells from OVX mice. Exogenously applied kisspeptin reduced I(A) regardless of time of day or estradiol status. Estradiol, interacting with time of day, and kisspeptin both depolarized I(A) activation. These findings extend our understanding of both the neurobiological mechanisms of estradiol negative vs. positive regulation of GnRH neurons and of kisspeptin action on these cells.

Histaminergic responses by hypothalamic neurons that regulate lordosis and their modulation by estradiol

How do fluctuations in the level of generalized arousal of the brain affect the performance of specific motivated behaviors, such as sexual behaviors that depend on sexual arousal? A great deal of previous work has provided us with two important starting points in answering this question: (i) that histamine (HA) serves generalized CNS arousal and (ii) that heightened electrical activity of neurons in the ventromedial nucleus of the hypothalamus (VMN) is necessary and sufficient for facilitating the primary female sex behavior in laboratory animals, lordosis behavior. Here we used patch clamp recording technology to analyze HA effects on VMN neuronal activity. The results show that HA acting through H1 receptors (H1R) depolarizes these neurons. Further, acute administration of estradiol, an estrogen necessary for lordosis behavior to occur, heightens this effect. Hyperpolarization, which tends to decrease excitability and enhance inhibition, was not affected by acute estradiol or mediated by H1R but was mediated by other HA receptor subtypes, H2 and H3. Sampling of mRNA from individual VMN neurons showed colocalization of expression of H1 receptor mRNA with estrogen receptor (ER)-alpha mRNA but also revealed ER colocalization with the other HA receptor subtypes and colocalization of different subtypes with each other. The latter finding provides the molecular basis for complex "push-pull" regulation of VMN neuronal excitability by HA. Thus, in the simplest causal route, HA, acting on VMN neurons through H1R provides a mechanism by which elevated states of generalized CNS arousal can foster a specific estrogen-dependent, aroused behavior, sexual behavior.

Is the role of estrogens and estrogen receptors in epilepsy still underestimated?

The etiology of epilepsy still represents an open subject of discussions and research. Contrary to the majority of diseases for which drugs are developed following the origin of disease, epilepsy is treated symptomatically because it is perceived to have diverse causes. Recent results of oncological, neurological, developmental and biochemical studies suggest that the reproductive dysfunction in men and women, as a side effect related with antiepileptic therapy, points to the single origin of this disease. It seems that contrary to the present definition of estrogen as a compound affecting seizure susceptibility, based on causal chains: of increased estrogen levels (alcohol intake) and seizure, fact that all antiepileptic drugs are aromatase inhibitors or have estrogen binding properties, described cases of seizures in epileptic patients taking quinine as preventive therapy against malaria, impact of photic activation and sleep on estrogen level, it can be assumed that estrogen plays the leading role in the mutual origin of different types of epilepsy.

Estradiol shapes auditory processing in the adult brain by regulating inhibitory transmission and plasticity-associated gene expression

Estradiol impacts a wide variety of brain processes, including sex differentiation, mood, and learning. Here we show that estradiol regulates auditory processing of acoustic signals in the vertebrate brain, more specifically in the caudomedial nidopallium (NCM), the songbird analog of the mammalian auditory association cortex. Multielectrode recordings coupled with local pharmacological manipulations in awake animals reveal that both exogenous and locally generated estradiol increase auditory-evoked activity in NCM. This enhancement in neuronal responses is mediated by suppression of local inhibitory transmission. Surprisingly, we also found that estradiol is both necessary and sufficient for the induction of multiple mitogen-activated protein kinase (MAPK)-dependent genes thought to be required for synaptic plasticity and memorization of birdsong. Specifically, we show that local blockade of estrogen receptors or aromatase activity in awake birds decrease song-induced MAPK-dependent gene expression. Infusions of estradiol in acoustically isolated birds induce transcriptional activation of these genes to levels comparable with song-stimulated animals. Our results reveal acute and rapid nongenomic functions for estradiol in central auditory physiology and suggest that such roles may be ubiquitously expressed across sensory systems.

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.

Extra-nuclear signaling of estrogen receptors

Estrogen controls multiple biological functions through binding to estrogen receptors (ERs). Traditionally, ERs have been regarded as transcription factors regulating the expression of target genes. However, growing evidence of rapid estrogen's actions in a number of tissues has been accumulating and alternative mechanisms of signal transduction have been proposed. These so called "extra-nuclear actions" do not require gene expression or protein synthesis and are independent of the nuclear localization of ERs. Indeed, some of these actions are elicited by ERs residing at or near the plasma membrane. Membrane-associated molecules such as ion channels, G proteins, the tyrosine kinase c-Src as well as growth factor receptors are modulated by liganded ERs within the membrane, leading to the activation of downstream cascades such as mitogen-activated protein kinase, phosphatidylinositol 3-OH kinase, protein kinase A, and protein kinase C. These cascades mediate some important rapid actions of estrogen, such as the activation of nitric oxide synthesis or the remodeling of actin cytoskeleton. In addition, these pathways are critical for the regulation of the expression of a number of target proteins implicated in cell proliferation, apoptosis, differentiation, movement, and homeostasis. In this manner, the extra-nuclear pathways are tightly integrated with the genomic pathways to orchestrate the full spectrum of estrogen's biological functions. The recent advancements in the characterization of the molecular basis of the extra-nuclear signaling of estrogen helps to understand the role of estrogen on human cells, and may in future turn out to be of relevance for clinical purposes.

Reverse engineering the lordosis behavior circuit

Reverse engineering takes the facts we know about a device or a process and reasons backwards to infer the principles underlying the structure-function relations. The goal of this review is to apply this approach to a well-studied hormone-controlled behavior, namely the reproductive stance of female rodents, lordosis. We first provide a brief overview on the considerable amount of progress in the analysis of female reproductive behavior. Then, we propose an analysis of the mechanisms of this behavior from a reverse-engineering perspective with the goal of generating novel hypotheses about the properties of the circuitry elements. In particular, the previously proposed neuronal circuit modules, feedback signals, and genomic mechanisms are considered to make predictions in this manner. The lordosis behavior itself appears to proceed ballistically once initiated, but negative and positive hormonal feedback relations are evident in its endocrine controls. Both rapid membrane-initiated and slow genomic hormone effects contribute to the behavior's control. We propose that the value of the reverse-engineering approach is based on its ability to provide testable, mechanistic hypotheses that do not emerge from either traditional evolutionary or simple reductionistic perspectives, and several are proposed in this review. These novel hypotheses may generalize to brain functions beyond female reproductive behavior. In this way, the reverse-engineering perspective can further develop our conceptual frameworks for behavioral and systems neuroscience.

Effects of 17beta-estradiol on neuronal cell excitability and neurotransmission in the suprachiasmatic nucleus of rat

17beta-Estradiol receptors have been found in several brain nuclei including the suprachiasmatic nucleus (SCN) of mammalian species. The SCN is believed to act as brain clock regulating circadian and circannual biological rhythms, such as body temperature, sleep, and mood. Here, we examined whether 17beta-estradiol (E2) could affect cell excitability and synaptic transmission in the SCN. Bath application of E2 (0.03-3 microM) increased the spontaneous firing frequency and depolarized cell membrane of the SCN neurons significantly. Furthermore, E2 (0.03-3 microM) increased (by about 25-150% of control) frequency of the miniature excitatory postsynaptic currents. Amplitude of the evoked excitatory postsynaptic currents was enhanced (by about 32% of control) after exposure to 1 microM E2. The paired-pulse ratio was reduced by E2. These effects were prevented by the estrogen receptor antagonist, ICI 182780. Exposure to the biologically inactive 17alpha-estradiol did not cause any significant changes in the parameters mentioned above. These findings are in favor of an implication of estrogen in modulation of neuronal activity in SCN and possibly regulating circadian rhythms.

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.

Site-specific regulation of gene expression by estrogen in the hypothalamus of adult female rats

Estrogen plays critical roles in the neuroendocrine system of adult female rats through separate actions, respectively, in the preoptic area (POA) and the ventromedial nucleus of the hypothalamus (VMH). Seven-week-old rats were treated with/without estrogen after they were ovariectomized, and four estrogen-responsive, neuronal system-related genes, encoding alpha4 neuronal nicotinic acetylcholine receptor (Chrna4), GABA(A) receptor delta (Gabrd), serotonin receptor 6 (Htr6), and GABA transporter 2 (Slc6a13), were investigated by real-time RT-PCR and Western blot analyses to examine their differential regulation by estrogen between the anterior part containing POA and the posterior part containing VMH. We further examined Bax, Bcl2, and Prkce, the former two genes to be involved in the gene expression network of Chrna4 and the latter gene, that of Gabrd. The regulation of Bax and Bcl2 by estrogen differed between the anterior and posterior parts. The results demonstrated differential regulation of these neuronal system-related genes by estrogen between the anterior and posterior parts of the hypothalamus and suggested the roles of gene expression networks for the respective genes in the neuroendocrine system of adult female rats.

17beta-estradiol at physiological concentrations augments Ca(2+) -activated K+ currents via estrogen receptor beta in the gonadotropin-releasing hormone neuronal cell line GT1-7

Estrogens play essential roles in the neuroendocrine control of reproduction. In the present study, we focused on the effects of 17beta-estradiol (E2) on the K(+) currents that regulate neuronal cell excitability and carried out perforated patch-clamp experiments with the GnRH-secreting neuronal cell line GT1-7. We revealed that a 3-d incubation with E2 at physiological concentrations (100 pm to 1 nm) augmented Ca(2+)-activated K(+) [K(Ca)] currents without influencing Ca(2+)-insensitive voltage-gated K(+) currents in GT1-7 cells. Acute application of E2 (1 nm) had no effect on the either type of K(+) current. The augmentation was completely blocked by an estrogen receptor (ER) antagonist, ICI-182,780. An ERbeta-selective agonist, 2,3-bis(4-hydroxyphenyl)-propionitrile, augmented the K(Ca) currents, although an ERalpha-selective agonist, 4,4',4''-[4-propyl-(1H)-pyrazole-1,3,5-triyl]tris-phenol, had no effect. Knockdown of ERbeta by means of RNA interference blocked the effect of E2 on the K(Ca) currents. Furthermore, semiquantitative RT-PCR analysis revealed that the levels of BK channel subunit mRNAs for alpha and beta4 were significantly increased by incubating cells with 300 pm E2 for 3 d. In conclusion, E2 at physiological concentrations augments K(Ca) currents through ERbeta in the GT1-7 GnRH neuronal cell line and increases the expression of the BK channel subunit mRNAs, alpha and beta4.