Cyclic estradiol replacement attenuates stress-induced c-Fos expression in the PVN of ovariectomized rats

Estrogens, age, and, neonatal stress: panic disorders and novel views on the contribution of non-medullary structures to respiratory control and CO2 responses

CO₂ is a fundamental component of living matter. This chemical signal requires close monitoring to ensure proper match between metabolic production and elimination by lung ventilation. Besides ventilatory adjustments, CO₂ can also trigger innate behavioral and physiological responses associated with fear and escape but the changes in brain CO₂/pH required to induce ventilatory adjustments are generally lower than those evoking fear and escape. However, for patients suffering from panic disorder (PD), the thresholds for CO₂-evoked hyperventilation, fear and escape are reduced and the magnitude of those reactions are excessive. To explain these clinical observations, Klein proposed the false suffocation alarm hypothesis which states that many spontaneous panics occur when the brain's suffocation monitor erroneously signals a lack of useful air, thereby maladaptively triggering an evolved suffocation alarm system. After 30 years of basic and clinical research, it is now well established that anomalies in respiratory control (including the CO₂ sensing system) are key to PD. Here, we explore how a stress-related affective disorder such as PD can disrupt respiratory control. We discuss rodent models of PD as the concepts emerging from this research has influenced our comprehension of the CO₂ chemosensitivity network, especially structure that are not located in the medulla, and how factors such as stress and biological sex modulate its functionality. Thus, elucidating why hormonal fluctuations can lead to excessive responsiveness to CO₂ offers a unique opportunity to gain insights into the neuroendocrine mechanisms regulating this key aspect of respiratory control and the pathophysiology of respiratory manifestations of PD.

Estradiol regulates voltage-gated potassium currents in corticotropin-releasing hormone neurons

Corticotropin-releasing hormone (CRH) neurons are the primary neural population controlling the hypothalamic-pituitary-adrenal (HPA) axis and the secretion of adrenal stress hormones. Previous work has demonstrated that stress hormone secretion can be regulated by circulating levels of estradiol. However, the effect of estradiol on CRH neuron excitability is less clear. Here, we show that chronic estradiol replacement following ovariectomy increases two types of potassium channel currents in CRH neurons: fast inactivating voltage-gated A-type K+ channel currents (IA) and non-inactivating M-type K+ channel currents (IM). Despite the increase in K+ currents following estradiol replacement, there was no overall change in CRH neuron spiking excitability assessed with either frequency-current curves or current ramps. Together, these data reveal a complex picture whereby ovariectomy and estradiol replacement differentially modulate distinct aspects of CRH neuron and HPA axis function.

Plasticity of intrinsic excitability across the estrous cycle in hypothalamic CRH neurons

Stress responses are highly plastic and vary across physiological states. The female estrous cycle is associated with a number of physiological changes including changes in stress responses, however, the mechanisms driving these changes are poorly understood. Corticotropin-releasing hormone (CRH) neurons are the primary neural population controlling the hypothalamic-pituitary-adrenal (HPA) axis and stress-evoked corticosterone secretion. Here we show that CRH neuron intrinsic excitability is regulated over the estrous cycle with a peak in proestrus and a nadir in estrus. Fast inactivating voltage-gated potassium channel (I_A) currents showed the opposite relationship, with current density being lowest in proestrus compared to other cycle stages. Blocking I_A currents equalized excitability across cycle stages revealing a role for I_A in mediating plasticity in stress circuit function over the female estrous cycle.

Sex differences in the hypothalamic-pituitary-adrenal axis' response to stress: an important role for gonadal hormones

The hypothalamic-pituitary-adrenal (HPA) axis, a neuroendocrine network that controls hormonal responses to internal and external challenges in an organism's environment, exhibits strikingly sex-biased activity. In adult female rodents, acute HPA function following a stressor is markedly greater than it is in males, and this difference has largely been attributed to modulation by the gonadal hormones testosterone and estradiol. These gonadal hormones are produced by the hypothalamic-pituitary-gonadal (HPG) axis and have been shown to determine sex differences in adult HPA function after acute stress via their activational and organizational effects. Although these actions of gonadal hormones are well supported, the possibility that sex chromosomes similarly influence HPA activity is unexplored. Moreover, questions remain regarding sex differences in the activity of the HPA axis following chronic stress and the underlying contributions of gonadal hormones and sex chromosomes. The present review examines what is currently known about sex differences in the neuroendocrine response to stress, as well as outstanding questions regarding this sex bias. Although it primarily focuses on the rodent literature, a brief discussion of sex differences in the human HPA axis is also included.

Estrogen signaling in the medial amygdala decreases emotional stress responses and obesity in ovariectomized rats

Declining estradiol (E2), as occurs during menopause, increases risk for obesity and psychopathology (i.e., depression, anxiety). E2 modulates mood and energy homeostasis via binding to estrogen receptors (ER) in the brain. The often comorbid and bidirectional relationship between mood and metabolic disorders suggests shared hormonal and/or brain networks. The medial amygdala (MeA) is abundant in ERs and regulates mood, endocrine, and metabolic stress responses; therefore we tested the hypothesis that E2 in the MeA mitigates emotional and metabolic dysfunction in a rodent model of surgical menopause. Adult female rats were ovariectomized (OVX) and received bilateral implants of E2 or cholesterol micropellets aimed at the MeA. E2-MeA decreased anxiety-like (center entries, center time) and depression-like (immobility) behaviors in the open field and forced swim tests (FST), respectively in ovariectomized rats. E2-MeA also prevented hyperphagia, body weight gain, increased visceral adiposity, and glucose intolerance in ovariectomized rats. E2-MeA decreased caloric efficiency, suggestive of increased energy expenditure. E2-MeA also modulated c-Fos neural activity in amygdalar (central and medial) and hypothalamic (paraventricular and arcuate) brain regions that regulate mood and energy homeostasis in response to the FST, a physically demanding task. Given the shared neural circuitry between mood and body weight regulation, c-Fos expression in discrete brain regions in response to the FST may be due to the psychologically stressful and/or metabolic demands of the task. Together, these findings suggest that the MeA is a critical node for mediating estrogenic effects on mood and energy homeostasis.

Naturally Occurring Changes in Estradiol Concentrations in the Menopause Transition Predict Morning Cortisol and Negative Mood in Perimenopausal Depression

Risk of depression increases considerably during the menopause transition (or perimenopause) - the 5-6 years surrounding the last menstrual period. While the mechanisms underlying this increased risk are unknown, we have hypothesized that excessive estradiol (E2) fluctuation, which accompanies the perimenopause, may be implicated. We have furthermore proposed that dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis may underlie E2 fluctuation's effect on mood. This study examined the relationship between weekly changes in salivary E2, salivary cortisol levels and weekly mood in 30 perimenopausal women recruited to achieve equal numbers of women with current depression, past depression, and no history of depression. Greater weekly increases in E2 were associated with increased cortisol among past and currently depressed women; greater E2 increases were also associated with negative mood among currently depressed women. These findings provide evidence that HPA axis dysregulation, correlated with E2 fluctuation, may be implicated in the pathophysiology of perimenopausal depression.

Effects of CB1 receptor agonism and antagonism on behavioral fear and physiological stress responses in adult intact, ovariectomized, and estradiol-replaced female rats

There is growing interest in the development of cannabis-based therapies for the treatment of fear and anxiety disorders. There are a few studies, but none in females, of the effects of the highly selective cannabinoid receptor type 1 (CB1) agonist, arachidonyl 2'-chlorethylamide (ACEA), on behavioral fear. In experiment 1 involving gonadally-intact females, ACEA (either 0.1 or 0.01 mg/kg) was without effect in the elevated plus maze (EPM), and the lower dose decreased anxiety in the open field test (OFT). AM251 increased anxiety in the EPM and decreased locomotor activity in the OFT. Twenty-four hours after fear conditioning, neither ACEA nor AM251 affected generalized fear or conditioned fear recall. AM251 and 0.1 mg/kg ACEA impaired, and 0.01 mg/kg ACEA enhanced, within-session fear extinction. AM251 increased plasma corticosterone concentrations after the fear extinction session, whereas ACEA was without effect. Based on evidence that estradiol may moderate the effects of CB1 receptor signaling in females, experiment 2 involved ovariectomized (OVX) rats provided with 10-μg 17β-Estradiol and compared with OVX rats without hormone replacement (oil vehicle). Irrespective of hormone treatment, AM251 increased anxiety in the EPM, whereas ACEA (0.01 mg/kg) was without effect. Neither hormone nor drug altered anxiety in the OFT, but estradiol increased and AM251 decreased distance traveled. After fear conditioning, AM251 decreased generalized fear. Neither hormone nor drug had any effect on recall or extinction of conditioned fear, however, ACEA and AM251 increased fear-induced plasma corticosterone concentrations. Further, when results with intact rats were compared with those from OVX rats, gonadal status did not moderate the effects of either AM251 or ACEA, although OVX displayed greater anxiety and fear than did intact rats. Thus, the effects of CB1 receptor antagonism and agonism in adult female rats do not depend on ovarian estradiol.

Sex differences in the HPA axis

The hypothalamic-pituitary-adrenal (HPA) axis is a major component of the systems that respond to stress, by coordinating the neuroendocrine and autonomic responses. Tightly controlled regulation of HPA responses is critical for maintaining mental and physical health, as hyper- and hypo-activity have been linked to disease states. A long history of research has revealed sex differences in numerous components of the HPA stress system and its responses, which may partially form the basis for sex disparities in disease development. Despite this, many studies use male subjects exclusively, while fewer reports involve females or provide direct sex comparisons. The purpose of this article is to present sex comparisons in the functional and molecular aspects of the HPA axis, through various phases of activity, including basal, acute stress, and chronic stress conditions. The HPA axis in females initiates more rapidly and produces a greater output of stress hormones. This review focuses on the interactions between the gonadal hormone system and the HPA axis as the key mediators of these sex differences, whereby androgens increase and estrogens decrease HPA activity in adulthood. In addition to the effects of gonadal hormones on the adult response, morphological impacts of hormone exposure during development are also involved in mediating sex differences. Additional systems impinging on the HPA axis that contribute to sex differences include the monoamine neurotransmitters norepinephrine and serotonin. Diverse signals originating from the brain and periphery are integrated to determine the level of HPA axis activity, and these signals are, in many cases, sex-specific.

17β-estradiol differentially regulates stress circuitry activity in healthy and depressed women

Many regions within stress neurocircuitry, including the anterior hypothalamus, amygdala, hippocampus, and medial prefrontal cortex, are densely populated with sex steroid receptors. Substantial evidence from animal studies indicates that the gonadal hormone 17β-estradiol (E₂) impacts the structure and function of these regions, but human studies are limited. Characterizing estradiol's role in stress circuitry in vivo in humans may have important clinical implications given the comorbidity between major depressive disorder (MDD), stress circuitry dysfunction and endocrine dysregulation. In this study, we determined estradiol's role in modulating activity within cortical and subcortical stress circuitry regions in healthy and MDD women. Subjects were part of a population-based birth cohort, the New England Family Study. Capitalizing on the endogenous fluctuation in E₂ during the menstrual cycle, we conducted a within-person repeated-measures functional neuroimaging study in which 15 women with recurrent MDD, in remission, and 15 healthy control women underwent hormonal evaluations, behavioral testing, and fMRI scanning on two occasions, under low and high E₂ conditions. Subjects completed an fMRI scan while undergoing a mild visual stress challenge that reliably activated stress neural circuitry. Results demonstrate that E₂ modulates activity across key stress circuitry regions, including bilateral amygdala, hippocampus, and hypothalamus. In healthy women, robust task-evoked BOLD signal changes observed under low E₂ conditions were attenuated under high E₂ conditions. This hormonal capacity to regulate activity in stress circuitry was not observed in MDD women, despite their remitted status, suggesting that dysregulation of gonadal hormone function may be a characteristic trait of the disease. These findings serve to deepen our understanding of estradiol's actions in the healthy brain and the neurobiological mechanisms that may underlie the pronounced sex difference in MDD risk.

Effects of the estrous cycle and ovarian hormones on central expression of interleukin-1 evoked by stress in female rats

Exposure to stressors such as foot shock (FS) leads to increased expression of multiple inflammatory factors, including the proinflammatory cytokine interleukin-1 (IL-1) in the brain. Studies have indicated that there are sex differences in stress reactivity, suggesting that the fluctuations in gonadal steroid levels across the estrous cycle may play a regulatory role in the stress-induced cytokine expression. The present studies were designed to investigate the role of 17-β-estradiol (E2) and progesterone (Pg) in regulating the cytokine response within the paraventricular nucleus (PVN) of the hypothalamus through analysis of gene expression with real-time RT-PCR. Regularly cycling female rats showed a stress-induced increase in PVN IL-1 levels during the diestrous, proestrous, and estrous stages. During the metestrous stage, no change in IL-1 levels was seen following FS; however, estrogen receptor (ER)-β levels did increase. Ovariectomy resulted in an increase in PVN IL-1 levels, which was attenuated by treatment with estradiol benzoate (10 or 50 µg), indicating an E2-mediated anti-inflammatory effect. Ovariectomized rats treated with Pg (500 or 1,250 µg) showed no alteration in IL-1 levels, but Pg did up-regulate ER-β gene expression. The results from the current study implicate a potential mechanism through which high availability of endogenous Pg during the metestrous stage increases ER-β sensitivity, which in turn attenuates the PVN IL-1 response to stress. Thus, the interaction between gonadal steroid hormones and their central receptors may exert a powerful inhibitory effect on neuroimmune consequences of stress throughout the estrous cycle.

Anxiolytic effects and neuroanatomical targets of estrogen receptor-β (ERβ) activation by a selective ERβ agonist in female mice

The dichotomous anxiogenic and anxiolytic properties of estrogens have been reported to be mediated by two distinct neural estrogen receptors (ER), ERα and ERβ, respectively. Using a combination of pharmacological and genetic approaches, we confirmed that the anxiolytic actions of estradiol are mediated by ERβ and extended and these observations to demonstrate the neuroanatomical targets involved in ERβ activation in these behavioral responses. We examined the effects of the biologically active S-enantiomer of diarylpropionitrile (S-DPN) on anxiety-related behavioral measures, the corresponding stress hormonal response to hypothalamo-pituitary-adrenal axis reactivity, and potential sites of neuronal activation in mutant female mice carrying a null mutation for ERβ gene (βERKO). S-DPN administration significantly reduced anxiety-like behaviors in the open field, light-dark exploration, and the elevated plus maze (EPM) in ovariectomized wild-type (WT) mice, but not in their βERKO littermates. Stress-induced corticosterone (CORT) and ACTH were also attenuated by S-DPN in the WT mice but not in the βERKO mice. Using c-fos induction after elevated plus maze, as a marker of stress-induced neuronal activation, we identified the anterodorsal medial amygdala and bed nucleus of the stria terminalis as the neuronal targets of S-DPN action. Both areas showed elevated c-fos mRNA expression with S-DPN treatment in the WT but not βERKO females. These studies provide compelling evidence for anxiolytic effects mediated by ERβ, and its neuroanatomical targets, that send or receive projections to/from the paraventricular nucleus, providing potential indirect mode of action for the control of hypothalamo-pituitary-adrenal axis function and behaviors.

Hormones, heart disease, and health: individualized medicine versus throwing the baby out with the bathwater

It is increasingly axiomatic that depression has widespread adverse physiological effects and, conversely, that a variety of physiological systems impact the risk for developing depression. This convergence of depression and altered physiology is particularly dramatic during midlife--a time during which reproductive failure presages dramatic increases in prevalence of both heart disease and depression. The potentially meaningful and illuminating links between estrogen deficiency, cardiovascular disease (CVD), and depression have largely been obscured, first by assertions, subsequently repudiated, that the perimenopause was not a time of increased risk of depression, and more recently by the denegration of hormone replacement therapy by initial reports of the Women's Health Initiative. Increasingly, however, research has led to unavoidable conclusions that CVD and depression share common, mediating pathogenic processes and that these same processes are dramatically altered by the presence or absence of estrogen (E2). This review summarizes data supporting these contentions with the intent of placing depression and estrogen therapy in their proper physiologic context.

Pretreatment with CP-154526 blocks the modifying effects of alarm pheromone on components of sexual behavior in male, but not in female, rats

We previously demonstrated that an alarm pheromone released from male donor Wistar rats evoked several physiological and behavioral responses in recipient rats. However, the pheromone effects on social behavior were not analyzed. In the present study, we examined whether the alarm pheromone affects sexual behavior in male or female rats. When a pair of male and female subjects was exposed to the alarm pheromone during sexual behavior, the ejaculation latency was elongated, the number of mounts was increased, and the hit rate (number of intromissions/number of mounts and intromissions) was decreased in the male subject. In contrast, female sexual behavior was not affected by the alarm pheromone. When we exposed only the male or female subject of the pair to the pheromone just before sexual behavior, the results were similar: the pheromone effects were evident in male, but not in female, subjects. In addition, when we pretreated with corticotropin-releasing factor (CRF) antagonist (CP-154526) before exposing the male subject to the alarm pheromone, the pheromone effects were attenuated in a dose-dependent manner. These results indicate that the alarm pheromone modifies male, but not female, components of sexual behavior and that CRF participates in the effects.

Hormones, heart disease, and health: individualized medicine versus throwing the baby out with the bathwater

It is increasingly axiomatic that depression has widespread adverse physiological effects, and conversely that a variety of physiological systems impact the risk for developing depression. This convergence of depression and altered physiology is particularly dramatic during midlife-a time during which reproductive failure presages dramatic increases in prevalence of both heart disease and depression. The potentially meaningful and illuminating links between estrogen (E2) deficiency, cardiovascular disease (CVD), and depression have largely been obscured, first by assertions, subsequently repudiated that the perimenopause was not a time of increased risk of depression, and more recently by the denegration of hormone replacement therapy by initial reports of the Women's Health Initiative. Increasingly, however, research has led to unavoidable conclusions that CVD and depression share common and mediating pathogenic processes and that these same processes are dramatically altered by the presence or absence of E2. This review summarizes data supporting this contention with the intent of placing depression and E2 therapy in their proper physiologic context.

The organizational role of testicular hormones and the androgen receptor in anxiety-related behaviors and sensorimotor gating in rats

Perinatal exposure to testosterone (T), which can act upon both the androgen receptor (AR) and, via aromatization of T into estrogens, upon estrogen receptors, organizes many adult behaviors in rodents. We compared behaviors in wild-type (WT) male rats and AR-deficient rats with the testicular feminization mutation (Tfm), which on the day of birth were either gonadectomized (Neo-Gdx) or sham operated. In adulthood, all rats were either gonadectomized or sham operated and implanted with T capsules to equilibrate circulating androgens. In each of four tests of behavior related to anxiety (open field, novel object exposure, light/dark box, and elevated plus maze), Neo-Gdx rats showed decreased indices of anxiety and increased activity compared with rats sham operated on the day of birth, with no differences between WT or Tfm males within treatment groups. These results indicate that testicular hormones act in development to increase adult indices of anxiety and decrease activity in males and that functional ARs are not required for this effect. Acoustic startle response was also reduced by Neo-Gdx, suggesting that postnatal testicular secretions potentiate this behavior as well. Adult corticosterone levels and sensorimotor gating, as measured by prepulse inhibition of the acoustic startle response, were increased by neonatal castration in both WT and Tfm rats. These findings indicate a role of T before adulthood in the organization of anxiety-related behaviors, activity, the hypothalamic-pituitary-adrenal axis, and sensorimotor gating in rats, all of which appears to be AR independent.

Antidepressant effects of estrogens: a basic approximation

The use of estrogenic compounds as antidepressants or as coadjuvants to facilitate the effect of antidepressants has reported controversial results, suggesting that many factors could influence their actions. This review analyzes, from a basic research perspective, the possible factors that may underlie the antidepressant action of estrogens alone or in combination. The possible mechanisms of action of estrogens alone and in combination with the selective serotonin reuptake inhibitor, fluoxetine, the selective noradrenaline reuptake inhibitor, desipramine, and the mixed serotonin/noradrenaline reuptake inhibitor, venlafaxine are reviewed, focusing on monoaminergic systems and estrogen receptors as main targets. The antidepressant effect of estrogens depends on the type of estrogen, treatment duration, doses, sex, time after ovariectomy, and age. Estrogens potentiate the antidepressant-like action of fluoxetine, venlafaxine, and desipramine and drastically shorten their latency of action. The antidepressant-like effect of estrogens alone or in combination with antidepressants seems to be mediated by monoaminergic and classic estrogen receptors, as WAY100635, an antagonist to the serotonin 1A receptor, idaxozan, an antagonist to alpha2 adrenergic receptors, and RU 58668, an estrogen receptor antagonist, blocked their antidepressant-like effect. In conclusion, estrogens produce antidepressant-like actions by themselves and importantly facilitate the action of clinically used antidepressants.

Gender-related qualitative differences in baseline and post-stress anxiety responses are not reflected in the incidence of criterion-based PTSD-like behaviour patterns

BACKGROUND

Most epidemiological studies report higher prevalence rates of stress-related disorders such as acute stress disorder, post-traumatic stress disorder (PTSD) and major depressive disorder in women than in men. Few animal models of PTSD have taken gender differences into account and have typically used male subjects. In order to explore gender-related PTSD-like stress-responses more thoroughly, we applied an animal model that focuses selectively on individual patterns of behavioural responses.

METHODS

Prevalence rates of individual behavioural response to a single exposure to predator scent stress (PSS) were assessed by both elevated plus-maze and startle response paradigms. Prevalence rates of extreme behavioural disruption (EBR) on both tests were assessed, correlated to corticosterone levels, and compared to global population response data. In addition, we assessed learning and memory in the Morris water-maze (MWM).

RESULTS

There were no significant differences between the behavioural responses related to oestrous cycle phase in terms of global data for the groups or in terms of prevalence rates of EBR. The overall patterns of response of males and females were affected, yet females demonstrated greater levels of baseline anxiety-like behaviour and lower peak levels of post-exposure anxiety-like behaviour than males. However, the prevalence rates of individual subjects who responded with PTSD-like behaviour were equal for female and male subjects. PSS-exposed female subjects demonstrated significantly compromised performance in the MWM compared to males.

CONCLUSIONS

In this animal model, the results clarified that the assumption that females are more vulnerable is true for the magnitude of the response, but not for the prevalence of pathological response patterns in rat populations.

The gad2 promoter is a transcriptional target of estrogen receptor (ER)alpha and ER beta: a unifying hypothesis to explain diverse effects of estradiol

Estradiol (E(2)) regulates a wide range of neural functions, many of which require activation of estrogen receptor alpha (ERalpha) and/or ERbeta, ligand-gated transcriptional regulators. Surprisingly, very few neural gene targets of ERs have been identified, and these cannot easily explain the myriad effects of E(2). GABA regulates most of the same neural functions as E(2), and GABAergic neurons throughout the brain contain ER. Therefore, we examined whether E(2) directly regulates expression of glutamic acid decarboxylase 2 (gad2), the enzyme primarily responsible for GABA synthesis for synaptic release. Using dual luciferase assays, we found that E(2), but not other gonadal steroids, stimulated the activity of a 2691 bp rat gad2 promoter reporter construct. Activation required either ERalpha or ERbeta, and ERbeta did not repress ERalpha-mediated transactivation. Site-directed mutagenesis studies identified three estrogen response elements (EREs) with cell-specific functions. An ERE at -711 upstream of the gad2 translational start site was essential for transactivation in both MCF-7 breast cancer cells and SN56.B5.G4 neural cells, but an ERE at -546 enhanced transcription only in neural cells. A third ERE at -1958 was inactive in neural cells but exerted potent transcriptional repression in E(2)-treated MCF-7 cells. Chromatin immunoprecipitation assays in mouse GABAergic N42 cells confirmed that E(2) induced ERalpha binding to a DNA fragment containing sequences corresponding to the -546 and -711 EREs of the rat promoter. Based on these data, we propose that direct transcriptional regulation of gad2 may explain, at least in part, the ability of E(2) to impact such a diverse array of neural functions.

Sex differences in psychopathology: of gonads, adrenals and mental illness

Stress-related disorders such as anxiety and depression are disproportionately prevalent in women. Women are more likely to experience depression and anxiety disorders during periods of marked hormonal fluctuations, suggesting that gonadal hormones are involved in stress pathology. Depression and anxiety are both associated with aberrant secretion of glucocorticoids, which also show marked fluctuations across the reproductive cycle and in response to gonadal steroids. Thus, interactions between gonadal and stress hormones may play a major role in predisposing females to stress-related disease. The purpose of this brief review is to highlight preclinical data regarding the role of estrogens in depression and anxiety-like behaviors. While it is evident the exogenous estrogens modulate affective behavior in rodents, there is some disagreement in the literature, perhaps related to experimental designs that vary with respect to administration parameters and stress. Beneficial effects of estrogens on mood are most likely due to estrogen receptor (ER)beta signaling. The antidepressant and anxiolytic effects of ERbeta are consistent with its role in attenuating glucocorticoid responses to stress, suggesting that estrogens, acting at ERbeta, may improve mood by suppressing glucocorticoid hyperactivity. However, additional studies demonstrate that ERbeta signaling in the hippocampus is sufficient to induce antidepressant and anxiolytic behaviors. Thus, ERbeta may improve mood via primary actions on hypothalamic (i.e., paraventricular nucleus) and/or extra-hypothalamic sites. Overall, the preclinical research suggests that selective ER modulators targeting ERbeta may be an attractive alternative or adjunct treatment to currently prescribed antidepressants or anxiolytics.

Sex differences in stress responses: focus on ovarian hormones

Women in the reproductive age are more vulnerable to develop affective disorders than men. This difference may attribute to anatomical differences, hormonal influences and environmental factors such as stress. However, the higher prevalence in women normalizes once menopause is established, suggesting that ovarian hormones may play an important role in the development of depression in women. Ovarian hormones such as estrogen can pass the brain-blood barrier and bind to cytoplasmatic estrogen receptor (ER)-alpha and ER-beta in different areas of the limbic system. During stress, estrogen can modulate the behavioral and neurobiological response depending on the concentrations of estrogen. In this review we present evidence for disparate effects of chronic stress on neuroplasticity and brain activity in male and female rats. Furthermore, we will demonstrate that effects of social support on coping with stress can be mimicked by social housing of rats and that this model can be used for identification of underlying neurobiological mechanisms, including behavior, phosphorylation of CREB and ERK1/2, and brain activity changes as measured with fos expression. Using cyclic administration of estrogen in ovariectomized female rats we could specifically address effects of different plasma estrogen levels and antidepressants on stress-induced neuroplasticity and activity changes. In this model we also studied effects of estrogen on recovery after chronic stress. We conclude that the female brain has a different innate strategy to handle stress than the male brain and that female animal models are necessary for studying the underlying mechanisms and options for treatment.

Activation of ERbeta increases levels of phosphorylated nNOS and NO production through a Src/PI3K/Akt-dependent pathway in hypothalamic neurons

Estrogen plays a role in restoring homeostatic balance during the stress response by altering hypothalamic function and NO production in the brain. While we know that estrogen acts on the hypothalamus to stimulate the NO system through an ERbeta-dependent mechanism in neurons, the molecular mechanisms responsible for these effects are unknown. Because phosphorylation of nNOS at Ser(1412) increases nNOS activity which leads to increased NO production, we investigated the effects of ERbeta activation on nNOS phosphorylation at Ser(1412) and NO production in primary hypothalamic neurons. Using the selective ERbeta agonist, DPN (10nM), we show that activation of ERbeta rapidly increases phosphorylation levels of nNOS at Ser(1412) and NO production. We also show that the PI3K pathway, but not the MAPK pathway, mediates the increases in levels of Ser(1412) phosphorylation and NO production induced by ERbeta activation, as the selective PI3K inhibitor, LY294002 (10microM), blocked the effects of ERbeta activation. Finally, we demonstrate that Src kinase acts upstream of the PI3K/Akt pathway based on our finding that the selective Src inhibitor, PP2 (10microM), blocked the increases in nNOS phosphorylation levels, NO production, and PI3K/Akt activity induced by ERbeta activation. Together, our results show that Src kinase mediates ERbeta-induced increases in phosphorylation levels of nNOS at Ser(1412) and NO production by activating the PI3K/Akt pathway. These findings provide novel insight into the signaling mechanisms through which E2 stimulates the NO system in hypothalamic neurons.

Estrogen potentiates adrenocortical responses to stress in female rats

It is well established that estrogens markedly enhance the glucocorticoid response to acute stress in females. However, the precise mechanism responsible for this regulation is poorly understood. Here, we tested whether estrogens enhance the activation of the paraventricular nucleus (PVN) of the hypothalamus by measuring stress-induced c-fos mRNA expression in the PVN of restraint-stressed ovariectomized (OVX) rats treated with physiologically relevant doses of estradiol (E(2)), the major female estrogen. As expected, E(2) enhanced plasma corticosterone responses to restraint in OVX females. However, E(2) markedly attenuated the stress-induced c-fos gene expression in the PVN and inhibited plasma ACTH responses in these animals. Furthermore, E(2)-inhibitory effects were mimicked by progesterone (P) alone or in combination with E(2). Interestingly, the suppressive central effects of both E(2) and P were apparently independent of basal paraventricular corticotropin-releasing hormone (CRH) transcription, since these ovarian steroids did not significantly affect PVN CRH mRNA expression in unstressed rats. These unexpected findings suggested that E(2) promotes glucocorticoid hypersecretion in females by additional peripheral (i.e., adrenal) mechanisms. Indeed, E(2) markedly enhanced plasma corticosterone responses and adrenal corticosterone content in dexamethasone-blocked OVX rats challenged with varying doses of exogenous ACTH. These results suggest that enhanced adrenal sensitive to ACTH is an important physiological mechanism mediating E(2)-related glucocorticoid hypersecretion in stressed females.

Increased limbic phosphorylated extracellular-regulated kinase 1 and 2 expression after chronic stress is reduced by cyclic 17β-estradiol administration

Chronic stress induced neuronal changes that may have consequences for subsequent stress responses. For example, chronic stress in rats rearranges dendritic branching patterns and disturbs the phosphorylation of extracellular-regulated kinase 1 and 2 (ERK) 1/2 throughout the limbic system. Stress-induced psychopathology occurs more often in women, however, most of studies have been done in male rats. Therefore, we studied the effect of stress in female rats. Other studies show that estradiol can modulate neuronal plasticity and might protect against stress-induced aberrations. To investigate the role of estradiol in stress responses we manipulated the hormone levels. Ovariectomized rats were cyclically treated with vehicle or with 17beta-estradiol-benzoate (1x in 4 days, 10 microg/250 g, s.c.) and subjected to either acute (3 days) or chronic (22 days) stress. In ovariectomized rats, the number of c-Fos positive cells in the infralimbic and prelimbic cortex of the prefrontal cortex and in the medial and basolateral amygdala was increased after acute stress. Moreover, acute stress reduced the number of phosphorylated ERK1/2 positive neurons in the prefrontal cortex of ovariectomized rats. Chronic stress, on the other hand, abolished normal patterns of c-Fos immunoreactivity in the prefrontal cortex and amygdala and increased the prefrontocortical phosphorylation of ERK1/2 in ovariectomized rats. Cyclic estradiol treatment preserved the neuronal reactivity in the infralimbic cortex after chronic stress and prevented sustained accumulation of phosphorylated ERK1/2. Therefore, cyclic estradiol administration apparently preserves the integrity of signal transduction cascades in limbic structures, which may protect against the harmful consequences of recurrent stress.

Estrogen in the paraventricular nucleus attenuates L-glutamate-induced increases in mean arterial pressure through estrogen receptor beta and NO

Estrogen (E2) acts in the brain to decrease blood pressure (BP) responses to psychological stress. A likely site for the effects of E2 is the hypothalamic paraventricular nucleus (PVN), an important regulator of autonomic functions. We studied the effects of E2 in the PVN on BP and heart rate (HR) responses to l-glutamate injections into the PVN of male urethane-anesthetized rats. Microinjections of l-glutamate (50 nmol) into the PVN increased BP by 14+/-2.5 mm Hg and HR by 30+/-5.6 bpm. Microinjections of E2 (0.1, 1, and 10 pmol) into the PVN 30 minutes before l-glutamate dose-dependently attenuated the pressor response by 25%, 34%, and 59%, respectively, but did not affect HR. We determined that E2 receptor (ER) beta mediates the effect of E2, because activation of ERbeta with diarylpropionitrile (50 pmol) attenuated the response by 57%, whereas activation of ERalpha with propyl-pyrazole-triol (20 pmol) had no effect. Furthermore, inhibition of ERbeta with R,R-tetrahydrochrysene (50 pmol) blocked the effect of E2, but inhibition of ERalpha with methyl-piperidino-pyrazole (1 nmol) did not. Finally, we found that the effect of E2 is mediated by NO, because the NO synthase (NOS) inhibitor, N(G)-nitro-l-arginine methyl ester (2 nmol), the neuronal NOS inhibitor, 7-nitroindazole sodium salt (0.1 pmol), and the endothelial NOS inhibitor, N5-(1-iminoethyl)-l-ornithine (200 pmol) blocked the effect of E2. The effect was partially blocked with the gamma-aminobutyric acid(A) receptor inhibitor bicuculline. Our results demonstrate that E2 in the PVN attenuates the l-glutamate-induced pressor response and that this effect is mediated by ERbeta, NO produced by neuronal NO synthase and eNOS, and partly by gamma-aminobutyric acid.

Stress-induced sensitization of the limbic system in ovariectomized rats is partly restored by cyclic 17beta-estradiol administration

Chronic stress induces neurobiological alterations which have consequences for subsequent stress handling. In the current experiment, ovariectomized rats were subjected daily to a stressor for 21 days. Thereafter, the rats were treated for 21 days with 17beta-estradiol benzoate (10 microg/250 g, once every 4 days) or mirtazapine (10 mg/kg, daily). In this way, we were able to evaluate the ability of these compounds to reverse chronic stress-induced changes in the activity of the limbic system. After 21 days of recovery and treatment, the rats were re-exposed to the adverse environment of the initial stressor and perfused 2 h later. Ovariectomized rats displayed increased numbers of c-Fos-positive nuclei, after re-exposure to the stressor, in the paraventricular nucleus of the hypothalamus, dentate gyrus, medial prefrontal cortex and central and medial amygdala. Cyclic estradiol treatment attenuated the sensitization of the paraventricular nucleus and central amygdala. Mirtazapine increased the number of c-Fos-positive nuclei in the central amygdala and dentate gyrus. Long-term transcriptional changes induced by chronic stress were determined with DeltaFosB immunoreactivity. The medial prefrontal cortex showed an increased number of DeltaFosB-positive nuclei after chronic stress and this was not affected by estradiol or mirtazapine administration during recovery. In conclusion, cyclic estradiol administration reversed chronic stress-induced sensitization in the limbic system, the paraventricular nucleus and central amygdala of female rats, output regions of the limbic system involved in fear responses. Mirtazapine did not achieve this reversal of stress-induced aberrations in the limbic system after 21 days of treatment.

Estrogen receptor-beta: recent lessons from in vivo studies

The unexpected discovery of a second form of the estrogen receptor (ER), designated ERbeta, surprised and energized the field of estrogen research. In the 9 yr since its identification, the remarkable efforts from academic and industrial scientists of many disciplines have made significant progress in elucidating its biology. A powerful battery of tools, including knockout mice as well as a panel of receptor-selective agonists, has allowed an investigation into the role of ERbeta. To date, in vivo efficacy studies are limited to rodents. Current data indicate that ERbeta plays a minor role in mediating estrogen action in the uterus, on the hypothalamus/pituitary, the skeleton, and other classic estrogen target tissues. However, a clear role for ERbeta has been established in the ovary, cardiovascular system, and brain as well as in several animal models of inflammation including arthritis, endometriosis, inflammatory bowel disease, and sepsis. The next phase of research will focus on elucidating, at a molecular level, how ERbeta exerts these diverse effects and exploring the clinical utility of ERbeta-selective agonists.