Uncoupling of 5-HT1A receptors in the brain by estrogens: regional variations in antagonism by ICI 182,780

The impact of estradiol on serotonin, glutamate, and dopamine systems

Estradiol, the most potent and prevalent member of the estrogen class of steroid hormones and is expressed in both sexes. Functioning as a neuroactive steroid, it plays a crucial role in modulating neurotransmitter systems affecting neuronal circuits and brain functions including learning and memory, reward and sexual behaviors. These neurotransmitter systems encompass the serotonergic, dopaminergic, and glutamatergic signaling pathways. Consequently, this review examines the pivotal role of estradiol and its receptors in the regulation of these neurotransmitter systems in the brain. Through a comprehensive analysis of current literature, we investigate the multifaceted effects of estradiol on key neurotransmitter signaling systems, namely serotonin, dopamine, and glutamate. Findings from rodent models illuminate the impact of hormone manipulations, such as gonadectomy, on the regulation of neuronal brain circuits, providing valuable insights into the connection between hormonal fluctuations and neurotransmitter regulation. Estradiol exerts its effects by binding to three estrogen receptors: estrogen receptor alpha (ERα), estrogen receptor beta (ERβ), and G protein-coupled receptor (GPER). Thus, this review explores the promising outcomes observed with estradiol and estrogen receptor agonists administration in both gonadectomized and/or genetically knockout rodents, suggesting potential therapeutic avenues. Despite limited human studies on this topic, the findings underscore the significance of translational research in bridging the gap between preclinical findings and clinical applications. This approach offers valuable insights into the complex relationship between estradiol and neurotransmitter systems. The integration of evidence from neurotransmitter systems and receptor-specific effects not only enhances our understanding of the neurobiological basis of physiological brain functioning but also provides a comprehensive framework for the understanding of possible pathophysiological mechanisms resulting to disease states. By unraveling the complexities of estradiol's impact on neurotransmitter regulation, this review contributes to advancing the field and lays the groundwork for future research aimed at refining understanding of the relationship between estradiol and neuronal circuits as well as their involvement in brain disorders.

Sex Differences in Serotonin 5-HT 1A Receptor Responses to Repeated Restraint Stress in Adult Male and Female Rats

BACKGROUND

Male and female rats were exposed to repeated restraint to determine how changes in serotonin (5-hydroxytryptamine; 5-HT) 1A receptors associate with stress hypothalamic-pituitary-adrenal (HPA) axis habituation.

METHODS

In response to 2-hour episodes of restraint, repeated daily for 5 consecutive days, males and females displayed reliable declines in HPA output, indicated by diminished adrenocorticotropin and corticosterone secretory responses. Using the 5-HT 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) as a pharmacological challenge for inducing hypothermia and elevations in plasma corticosterone, males displayed sensitized hypothermal responses after repeated restraint, whereas corticosterone responses to 8-OH-DPAT were enhanced in both sexes following single or repeated exposure.

RESULTS

Only males showed elevations in 5-HT 1A receptor G-protein coupling responses in the dorsal raphe after repeated restraint, whereas only females showed an increase in 5-HT 1A receptor responses in the hippocampus following single or repeated exposure. G-protein coupling responses within both regions correlated positively with 5-HT 1A receptor binding capacity. Thus, despite expressing similar capacities for stress HPA axis habituation, males and females emerged from repeated restraint to show region-specific changes in 5-HT 1A receptor function that may be explained, at least in part, by changes in receptor availability.

CONCLUSIONS

Based on the hypothermal and corticosteroid responses to 8-OH-DPAT, the present data suggest that stress habituation is met by an increase in the sensitivity of presynaptic 5-HT 1A receptors in males and by an increase in the sensitivity of a population of postsynaptic receptors in both sexes.

Regulation of Serotonin 1A Receptor SUMOylation by SENP2 and PIASxα

Serotonin 1A receptors (5-HT1ARs) are implicated in the control of mood, cognition, and memory and in various neuropsychiatric disorders such as depression and anxiety. As such, understanding the regulation of 5-HT1ARs will inform the development of better treatment approaches. We previously demonstrated 5-HT1ARs are SUMOylated by SUMO1 in the rat brain. Agonist stimulation increased SUMOylation and was further enhanced when combined with 17β-estradiol-3-benzoate (EB), which are treatments that cause the transient and prolonged desensitization of 5-HT1AR signaling, respectively. In the current study, we identified the protein inhibitor of activated STAT (PIAS)xα as the enzyme that facilitates SUMOylation, and SENP2 as the protein that catalyzes the deSUMOylation of 5-HT1ARs. We demonstrated that PIASxα significantly increased in the membrane fraction of rats co-treated with EB and an agonist, compared to either the EB-treated or vehicle-treated groups. The acute treatment with an agonist alone shifted the location of SENP2 from the membrane to the cytoplasmic fraction, but it has little effect on PIASxα. Hence, two separate mechanisms regulate SUMOylation and the activity of 5-HT1ARs by an agonist and EB. The effects of EB on 5-HT1AR SUMOylation and signaling may be related to the higher incidence of mood disorders in women during times with large fluctuations in estrogens. Targeting the SUMOylation of 5-HT1ARs could have important clinical relevance for the therapy for several neuropsychiatric disorders in which 5-HT1ARs are implicated.

Targeting Nuclear Receptors in Neurodegeneration and Neuroinflammation

Nuclear receptors, also known as ligand-activated transcription factors, regulate gene expression upon ligand signals and present as attractive therapeutic targets especially in chronic diseases. Despite the therapeutic relevance of some nuclear receptors in various pathologies, their potential in neurodegeneration and neuroinflammation is insufficiently established. This perspective gathers preclinical and clinical data for a potential role of individual nuclear receptors as future targets in Alzheimer's disease, Parkinson's disease, and multiple sclerosis, and concomitantly evaluates the level of medicinal chemistry targeting these proteins. Considerable evidence suggests the high promise of ligand-activated transcription factors to counteract neurodegenerative diseases with a particularly high potential of several orphan nuclear receptors. However, potent tools are lacking for orphan receptors, and limited central nervous system exposure or insufficient selectivity also compromises the suitability of well-studied nuclear receptor ligands for functional studies. Medicinal chemistry efforts are needed to develop dedicated high-quality tool compounds for the therapeutic validation of nuclear receptors in neurodegenerative pathologies.

The combination of mirtazapine plus venlafaxine reduces immobility in the forced swim test and does not inhibit female sexual behavior

INTRODUCTION

Depression is a psychiatric disorder with higher incidence in women. Among the most common and less investigated adverse effects of antidepressants are the female sexual dysfunctions. Up to one third of the patients fail to respond to antidepressants; therefore, more treatment alternatives are necessary. The combination of mirtazapine plus venlafaxine, known as "California Rocket Fuel" has shown to be an option for treatment-resistant depression. However, there are no reports of the effects of this combination in animal models and its action on female sexual behavior is unknown.

AIM

To analyze the effect of mirtazapine and venlafaxine alone or combined -given at doses with actions on the forced swim test- on female rat sexual behavior.

METHODS

Mirtazapine (10, 20 or 40 mg/kg) and venlafaxine (15, 30 or 60 mg/kg) or their combinations (2.5/3.75, 5/7.5, 10/15 and 20/30 mg/kg mirtazapine and venlafaxine, respectively) were injected to sexually receptive female rats. We evaluated their effect on the forced swim test (FST). The doses that reduced immobility were tested on proceptivity and receptivity.

RESULTS

Mirtazapine (40 mg/kg) and venlafaxine (60 mg/kg), administered alone, or combined (mirtazapine, 5, 10 and 20 mg/kg plus venlafaxine, 7.5, 15 and 30 mg/kg) reduced immobility, but affected motor activity. However, the reduced locomotion after the lowest combination (5/7.5 mg/kg) was smaller. Mirtazapine at 40 mg/kg reduced proceptivity and receptivity, while 60 mg/kg venlafaxine only decreased proceptivity. The combination of 5/7.5 mg/kg mirtazapine and venlafaxine did not affect female sexual behavior.

CONCLUSIONS

Mirtazapine and venlafaxine exerted an effect in the FST, which was also evident when sub-effective doses of both antidepressants were combined. This combination also lacked adverse effects on female sexual behavior. The results suggest that "California Rocket Fuel" could be an effective antidepressant therapy with no adverse sexual effects in women.

Serotonin-estrogen interactions: What can we learn from pregnancy?

We have reviewed the scientific literature related to four diseases in which to serotonin (5-HT) is involved in the etiology, herein named 5-HT-linked diseases, and whose prevalence is influenced by estrogenic status: depression, migraine, irritable bowel syndrome and eating disorders. These diseases all have in common a sex-dimorphic prevalence, with women more frequently affected than men. The co-occurrence between these 5-HT-linked diseases suggests that they have common physiopathological mechanisms. In most 5-HT-linked diseases (except for anorexia nervosa and irritable bowel syndrome), a decrease in the serotonergic tone is observed and estrogens are thought to contribute to the improvement of symptoms by stimulating the serotonergic system. Human pregnancy is characterized by a unique 5-HT and estrogen synthesis by the placenta. Pregnancy-specific disorders, such as hyperemesis gravidarum, gestational diabetes mellitus and pre-eclampsia, are associated with a hyperserotonergic state and decreased estrogen levels. Fetal programming of 5-HT-linked diseases is a complex phenomenon that involves notably fetal-sex differences, which suggest the implication of sex steroids. From a mechanistic point of view, we hypothesize that estrogens regulate the serotonergic system, resulting in a protective effect against 5-HT-linked diseases, but that, in turn, 5-HT affects estrogen synthesis in an attempt to retrieve homeostasis. These two processes (5-HT and estrogen biosynthesis) are crucial for successful pregnancy outcomes, and thus, a disruption of this 5-HT-estrogen relationship may explain pregnancy-specific pathologies or pregnancy complications associated with 5-HT-linked diseases.

The Effects and Action Mechanisms of Phytoestrogens on Vasomotor Symptoms During Menopausal Transition: Thermoregulatory Mechanism

Background:

Phytoestrogens have recently been claimed to positively influence menopausal discomforts, including hot flashes. However, little is known about the influence of phytoestrogens on core body temperature during oestrogen fluctuation at menopause.

Objective:

Previously published findings showed that phytoestrogens could relieve menopausal complaints, thus, the present review was aimed at assessing the effects of phytoestrogens on thermoregulatory mechanism during menopausal transition.

Results:

The molecular mechanisms underlying hot flashes are complex. Oestrogen fluctuations cause hypothalamic thermoregulatory centre dysfunction, which leads to hot flashes during menopause. The phytoestrogens of interest, in relation to human health, include isoflavones, lignans, coumestans, and stilbenes, which are widely distributed in nature. The phytoestrogens are capable of reducing hot flashes via their oestrogen-like hormone actions. The potential effects of phytoestrogens on hot flashes and their molecular mechanisms of action on thermoregulatory centre are discussed in this review.

Conclusion:

The effects of phytoestrogens on these mechanisms may help explain their beneficial effects in alleviating hot flashes and other menopausal discomforts.

Socially Housed Female Macaques: a Translational Model for the Interaction of Chronic Stress and Estrogen in Aging

PURPOSE OF REVIEW

Estrogen's role in cognitive aging remains unclear. Despite evidence implicating stress in pathological aging, the interaction of stress with estrogen on cognition in older women has received little attention, and few animal models exist with which to examine this interaction.

RECENT FINDINGS

We present evidence that aging socially subordinate female macaques that experience chronic psychosocial stress constitute a suitable model to investigate this. First, we review studies showing that estrogen modulates cognition in animal models, as well as studies demonstrating that estrogen's action on certain types of cognition is impaired by stress. Next, we discuss data showing that middle-aged socially subordinate female macaques exhibit distinct stress-induced phenotypes, and review our investigations indicating that estrogen modulates behavior and physiology differently in subordinate female monkeys. We conclude that socially housed female macaques represent a translational animal model for investigating the interplay of chronic stress and estrogen on cognitive aging in women.

Influence of nonylphenol and octylphenol exposure on 5-HT, 5-HT transporter, and 5-HT2A receptor

Nonylphenol (NP) and octylphenol (OP) are priority environmental contaminants that have a potential role as endocrine disruptors. They can be biomagnified in the food chain and pose an estrogenic health risk to human health. A 28-day oral toxicity study was performed to observe the impact of single and combined exposure to NP and OP on 5-HT transporter (SERT) as well as 5-HT_2A receptor. Results showed that the 5-HT levels in rat plasma increased with exposure to middle-dose and high-dose NP, to high-dose OP, and to low, middle, and high doses of combined NP and OP (P < 0.05), while the 5-HT levels in rat platelets increased when exposed to NP/OP or combined NP and OP of middle or high dose (P < 0.05). The expression levels of SERT in rat platelets decreased when exposed to high-dose NP/OP or high dose of combined NP and OP (P < 0.05). Meanwhile, the expression levels of 5-HT_2A in rat platelets decreased when exposed to high-dose NP/OP as well as combined NP and OP (P < 0.05). These findings suggested that exposure to NP and OP could influence the metabolic network of 5-hydroxytryptamine via transportation and receptor binding pathways.

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.

Pharmacology of serotonin and female sexual behavior

In this review, first a historical perspective of serotonin's (5-HT) involvement in female sexual behavior is presented. Then an overview of studies implicating 5-HT is presented. The effect of drugs that increase or decrease CNS levels of 5-HT is reviewed. Evidence is presented that drugs which increase 5-HT have negative effects on female sexual behavior while a decrease in 5-HT is associated with facilitation of sexual behavior. Studies with compounds that act on 5-HT₁, 5-HT₂ or 5-HT₃ receptors are discussed. Most evidence indicates that 5-HT₁A receptor agonists inhibit sexual behavior while 5-HT₂ or 5-HT₃ receptors may exert a positive influence. There is substantial evidence to support a role for 5-HT in the modulation of female consummatory sexual behavior, but studies on the role of 5-HT in other elements of female sexual behavior (e.g. desire, motivation, sexual appetite) are few. Future studies should be directed at determining if these additional components of female sexual behavior are also modulated by 5-HT.

Estradiol potentiates 8-OH-DPAT-induced sumoylation of 5-HT₁A receptor: characterization and subcellular distribution of sumoylated 5-HT₁A receptors

Sumoylation is a recently described post-translational modification and only a few sumoylated neurotransmitter receptors are known. Through the present studies, we discovered that serotonin1A receptors (5-HT1A-Rs) can be sumoylated by SUMO1 (small-ubiquitin-related modifier 1) protein. The SUMO1-5-HT1A-R is ∼55kDa, is located in the membrane fraction, but not the cytosol, and is distributed in all of the brain regions expressing 5-HT1A-Rs examined. Acute stimulation of 5-HT1A-Rs significantly increased SUMO1-5-HT1A-R in rat hypothalamus. Pre-treatment with estradiol for 2 days, which causes a partial desensitization of 5-HT1A-R signaling, potentiated agonist-induced increases in SUMO1-5-HT1A-Rs in the hypothalamus of ovariectomized rats. Using discontinuous gradient centrifugation followed by digitonin treatment, we found that the majority of SUMO1-5-HT1A-Rs is co-localized with endoplasmic-reticulum and trans-Golgi-network markers. Although a small proportion of SUMO1-5-HT1A-Rs are located in the detergent resistant microdomain (DRM) that contain active G-protein coupled receptors, their distribution was different from that of the Gαz protein that couples to the receptors. These data suggest that the SUMO1-5-HT1A-Rs are an inactive form of 5-HT1A-Rs, a finding further supported by results showing minimal 5-HT1A-R agonist binding to SUMO1-5-HT1A-Rs. Furthermore, SUMO1-5-HT1A-Rs in the DRM were increased by treatment with a 5-HT1A-R agonist, 8-OH-DPAT ((+)8-hydroxy-2-dipropylaminotetralin). Together, these data suggest that sumoylation of 5-HT1A-Rs may be related to 5-HT1A-R trafficking and internalization, which may contribute to 5-HT1A-R desensitization. Since 5-HT1A-Rs play an important role in mood regulation, the present results significantly impact on the understanding of the pathogenesis of affective disorders and development of better therapeutic approaches for these diseases.

Estradiol accelerates the effects of fluoxetine on serotonin 1A receptor signaling

A major problem with current anti-depressant therapy is that it takes on average 6-7 weeks for remission. Since desensitization of serotonin (5-HT)1A receptor signaling contributes to the anti-depressive response, acceleration of the desensitization may reduce this delay in response to antidepressants. The purpose of the present study was to test the hypothesis that estradiol accelerates fluoxetine-induced desensitization of 5-HT1A receptor signaling in the paraventricular nucleus of the hypothalamus (PVN) of rats, via alterations in components of the 5-HT1A receptor signaling pathway. Ovariectomized rats were injected with estradiol and/or fluoxetine, then adrenocorticotropic hormone (ACTH) and oxytocin responses to a 5-HT1A receptor agonist (+)-8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) were examined to assess the function of 5-HT1A receptors in the PVN. Treatment with estradiol for either 2 or 7 days or fluoxetine for 2 days produced at most a partial desensitization of 5-HT1A receptor signaling, whereas 7 days of fluoxetine produced full desensitization. Combined treatment with estradiol and fluoxetine for 2 days produced nearly a full desensitization, demonstrating an accelerated response compared to either treatment alone. With two days of combined treatments, estradiol prevented the fluoxetine-induced increase in 5-HT1A receptor protein, which could contribute to the more rapid desensitization. Furthermore, EB treatment for 2 days decreased the abundance of the 35 kD Gαz protein which could contribute to the desensitization response. We found two isoforms of Gαz proteins with molecular mass of 35 and 33 kD, which differentially distributed in the detergent resistant microdomain (DRM) and in Triton X-100 soluble membrane region, respectively. The 35 kD Gαz proteins in the DRM can be sumoylated by SUMO1. Stimulation of 5-HT1A receptors with 8-OH-DPAT increases the sumoylation of Gαz proteins and reduces the 33 kD Gαz proteins, suggesting that these responses may be related to the desensitization of 5-HT1A receptors. Treatment with estradiol for 2 days also reduced the levels of the G-protein coupled estrogen receptor GPR30, possibly limiting to the ability of estradiol to produce only a partial desensitization response. These data provide evidence that estradiol may be effective as a short-term adjuvant to SSRIs to accelerate the onset of therapeutic effects.

Protective actions of sex steroid hormones in Alzheimer's disease

Risk for Alzheimer's disease (AD) is associated with age-related loss of sex steroid hormones in both women and men. In post-menopausal women, the precipitous depletion of estrogens and progestogens is hypothesized to increase susceptibility to AD pathogenesis, a concept largely supported by epidemiological evidence but refuted by some clinical findings. Experimental evidence suggests that estrogens have numerous neuroprotective actions relevant to prevention of AD, in particular promotion of neuron viability and reduction of beta-amyloid accumulation, a critical factor in the initiation and progression of AD. Recent findings suggest neural responsiveness to estrogen can diminish with age, reducing neuroprotective actions of estrogen and, consequently, potentially limiting the utility of hormone therapies in aged women. In addition, estrogen neuroprotective actions are also modulated by progestogens. Specifically, continuous progestogen exposure is associated with inhibition of estrogen actions whereas cyclic delivery of progestogens may enhance neural benefits of estrogen. In recent years, emerging literature has begun to elucidate a parallel relationship of sex steroid hormones and AD risk in men. Normal age-related testosterone loss in men is associated with increased risk to several diseases including AD. Like estrogen, testosterone has been established as an endogenous neuroprotective factor that not only increases neuronal resilience against AD-related insults, but also reduces beta-amyloid accumulation. Androgen neuroprotective effects are mediated both directly by activation of androgen pathways and indirectly by aromatization to estradiol and initiation of protective estrogen signaling mechanisms. The successful use of hormone therapies in aging men and women to delay, prevent, and or treat AD will require additional research to optimize key parameters of hormone therapy and may benefit from the continuing development of selective estrogen and androgen receptor modulators.

Progesterone reduces the effect of the serotonin 1B/1D receptor antagonist, GR 127935, on lordosis behavior

Ovariectomized rats were hormonally primed with 10 microg estradiol benzoate or with estradiol benzoate plus 500 microg progesterone. Rats received a bilateral infusion with 200 ng of the 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-1-1'-biphenyl-4-carboxamide hydrochloride (GR 127935), into the ventromedial nucleus of the hypothalamus (VMN), followed by a 5 min restraint or home cage experience. In estrogen-primed females that had experienced minimal handling between ovariectomy and use in the experiment, infusion with the water vehicle transiently inhibited lordosis behavior, and the 5-HT(1B/1D) receptor antagonist amplified this inhibition. There were no effects in rats hormonally primed with estrogen and progesterone. Handling for two days before the experiment reduced the effects of the infusions in estrogen-primed rats. However, when a 5 min restraint experience followed infusion with GR 127935, there was a significant decline in lordosis behavior that persisted for 10 to 15 min after the experience. Regardless of the prior experience or type of infusion, the addition of progesterone to the hormonal priming completely prevented the lordosis inhibition. These findings are consistent with prior evidence that progesterone protects against the inhibitory effects of a 5 min restraint experience on lordosis behavior. Moreover, these are the first experiments to demonstrate an inhibitory effect of a selective 5-HT(1B/1D) receptor antagonist in the VMN on lordosis behavior of estrogen primed, but not estrogen and progesterone primed, ovariectomized rats.

Estradiol-sertraline synergy in ovariectomized rats

This study investigated estradiol (E(2)) modulation of the antidepressant effects of a selective serotonin (5-HT) reuptake inhibitor (SSRI; sertraline) and a tricyclic antidepressant (imipramine) as measured by the forced swim test (FST) followed by assessment of gene and protein expression for the 5-HT transporter (SERT) and multiple 5-HT receptors. Female Sprague-Dawley rats were ovariectomized (OVX) and two-thirds of the rats received E(2) implants (OVE). 4 weeks later, implants were withdrawn in half of the OVE rats (OVW) to capture a time point when E(2) levels were rapidly declining. Rats in each hormone group were treated with vehicle, sertraline (10 mg/kg) or imipramine (10 mg/kg), 24, 5 and 1h before the FST. Immediately after the FST, midbrain, hippocampus and prefrontal cortex tissue was removed and frozen for analysis of gene expression via quantitative real-time PCR (midbrain tissue) and protein expression via Western blot (prefrontal cortex and hippocampal tissue). In the FST, sertraline decreased immobility and increased swimming in OVE rats, as well as increased swimming in OVW rats. In contrast, no sertraline effect was observed in OVX rats. Rats treated with imipramine showed increased climbing but no changes in immobility or swimming. No changes in protein expression were detected in any treatment group. However, in vehicle-treated rats, E(2) increased midbrain SERT mRNA expression, with no effect on midbrain mRNA for the 5-HT receptors. In sertraline-treated rats, E(2) decreased 5-HT(2A) receptor mRNA, and E(2)-withdrawal increased 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptor mRNA. In imipramine-treated rats, E(2) (and E(2)-withdrawal) did not affect mRNA expression for any of the target genes. Thus, E(2) synergized behaviorally and neurochemically with an SSRI but not a tricyclic antidepressant.

Black Cohosh: Insights into its Mechanism(s) of Action

The Women's Health Initiative found that combination estrogen and progesterone hormone replacement therapy increases breast cancer and cardiovascular disease risk, which compelled many women to seek herbal alternatives such as black cohosh extract (BCE) to relieve their menopausal symptoms. While several clinical trials document the efficacy of BCE in alleviating menopausal symptoms, preclinical studies to determine how BCE works have yielded conflicting results. Part of this is because there is not a universally accepted method to standardize the dose of black cohosh triterpenes, the presumed active ingredients in the extract. Although the mechanism by which BCE relieves symptoms is unknown, several hypotheses have been proposed: it acts 1) as a selective estrogen receptor modulator, 2) through serotonergic pathways, 3) as an antioxidant, or 4) on inflammatory pathways. We found that while the most prominent triterpene in BCE, 23-epi-26-deoxyactein, suppresses cytokine-induced nitric oxide production in brain microglial cells, the whole BCE extract actually enhanced this pathway. A variety of activities have been reported for black cohosh and its compounds, but the absorption and tissue distribution of these compounds is unknown.

The role of steroid hormones in the NF1 phenotype: focus on pregnancy

The Neurofibromatosis Type 1 (NF1) gene functions as a tumor suppressor gene. Loss of its protein, neurofibromin, in the autosomal dominant disorder NF1 is associated with peripheral nervous system tumors, particularly neurofibromas, benign lesions in which the major cell type is the Schwann Cell (SC). Benign and malignant human tumors found in NF1 patients are heterogeneous with respect to their cellular composition. The number and size of neurofibromas in NF1 patients has been shown to increase during pregnancy, with, in some cases, post-partum regression, which suggests hormonal involvement in this increase. However, in this review, we consider evidence from the literature that both direct hormonal influence on tumor growth and on angiogenesis may contribute to these effects.

From menarche to menopause: exploring the underlying biology of depression in women experiencing hormonal changes

Epidemiologic data consistently report an elevated prevalence of major depressive disorder (MDD) in women. This increase begins during adolescence and continues through the menopausal transition. Population-based clinical studies report an increase in the incidence of MDD during perimenopause compared to either the premenopausal or postmenopausal period. Evidence suggests that fluctuations and decline of hormonal levels are correlated with this observed increase in risk for MDD. A strong predictor of depression in the perimenopausal period is a previous history of MDD. However, recent studies revealed an increased risk of new onset depression in perimenopausal women without a history of MDD. Additionally, recent reports have indicated that the presence of vasomotor symptoms may be associated with an increased the risk for MDD. The objective of this paper is to review evidence that would support our hypothesis that neurotransmitter systems are affected by changes in hormonal status over the course of a woman's life, leading to increase vulnerability to perimenopausal depression. Relevant data from nonclinical experiments will be discussed in the context of observed clinical evidence of the risk for MDD before, during, and after the menopausal transition. A testable hypothesis will be proposed to advance our understanding of hormonal effects on mood.

The importance of baseline in identifying 8-OH-DPAT-induced effects on prepulse inhibition in rats

BACKGROUND AND PURPOSE

Prepulse inhibition (PPI) of the acoustic startle response is a model of sensorimotor gating which is disrupted in schizophrenia and other mental illnesses. We and others have shown that treatment with the 5-hydroxytryptamine-1A (5-HT(1A)) receptor agonist, 8-OH-DPAT, disrupts PPI in rats. In the present study, we highlight the importance of baseline levels on the effect of 8-OH-DPAT on PPI.

EXPERIMENTAL APPROACH

Adult male and female Sprague-Dawley rats were gonadectomised. These rats were treated with saline, 0.02 and 0.5 mg kg(-1) of 8-OH-DPAT using a random-sequence, repeated-measures protocol. The rats were allocated into high and low baseline groups depending on their baseline PPI observed after saline treatment.

KEY RESULTS

Treatment with 0.5 mg kg(-1) of 8-OH-DPAT significantly disrupted PPI in both male and female rats. In male rats only, 0.02 mg kg(-1) 8-OH-DPAT caused a small, but significant, increase in PPI. When these male rats were allocated to either a high or low baseline PPI group, 0.5 mg kg(-1) 8-OH-DPAT disrupted PPI in the high baseline group only. In contrast, treatment with 0.02 mg kg(-1) 8-OH-DPAT increased PPI only in the low baseline PPI group. There were no changes in the effect of 8-OH-DPAT administration in female rats when they were divided into high and low baseline PPI groups.

CONCLUSIONS AND IMPLICATIONS

The level of baseline PPI is an important variable that can influence the direction of drug effects induced by 8-OH-DPAT. The explanation for this phenomenon could be differential activation of pre- and postsynaptic 5-HT(1A) receptors.

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.

Oestrogen modulation of the effect of 8-OH-DPAT on prepulse inhibition: effects of aromatase deficiency and castration in mice

OBJECTIVE

The aim of this study was to investigate the interaction of sex steroid hormones, particularly oestrogen, in the regulation of prepulse inhibition (PPI) by serotonin-1A (5-HT1A) receptors.

MATERIALS AND METHODS

We studied aromatase knockout (ArKO) mice, which are unable to produce oestrogen but have high levels of testosterone, and the effects of castration.

RESULTS AND DISCUSSION

Treatment of male ArKO mice with the 5-HT1A receptor agonist, 8-hydroxy-dipropyl-aminotetralin (8-OH-DPAT), caused an increase in PPI that was significantly greater than in male wild-type controls. Castration of male mice caused a significant enhancement of the effect of 8-OH-DPAT in control mice; however, there was no change in the effect of this drug in ArKO mice. There was no significant effect of 8-OH-DPAT on PPI in either female ArKO or wild-type controls. In all experiments, the effects of 8-OH-DPAT on startle were not different between the groups. [3H]8-OH-DPAT autoradiography showed no differences in 5-HT1A receptor binding densities in areas of the forebrain, hippocampus or raphe region that could explain the PPI results. These data show that the absence of oestrogen in male ArKO mice leads to a greater effect of 5-HT1A receptor stimulation on PPI. This effect can be mimicked in male control mice by castration. The differential involvement of oestrogen and testosterone in these animal models is discussed.

An overlooked connection: serotonergic mediation of estrogen-related physiology and pathology

BACKGROUND

In humans, serotonin has typically been investigated as a neurotransmitter. However, serotonin also functions as a hormone across animal phyla, including those lacking an organized central nervous system. This hormonal action allows serotonin to have physiological consequences in systems outside the central nervous system. Fluctuations in estrogen levels over the lifespan and during ovarian cycles cause predictable changes in serotonin systems in female mammals.

DISCUSSION

We hypothesize that some of the physiological effects attributed to estrogen may be a consequence of estrogen-related changes in serotonin efficacy and receptor distribution. Here, we integrate data from endocrinology, molecular biology, neuroscience, and epidemiology to propose that serotonin may mediate the effects of estrogen. In the central nervous system, estrogen influences pain transmission, headache, dizziness, nausea, and depression, all of which are known to be a consequence of serotonergic signaling. Outside of the central nervous system, estrogen produces changes in bone density, vascular function, and immune cell self-recognition and activation that are consistent with serotonin's effects. For breast cancer risk, our hypothesis predicts heretofore unexplained observations of the opposing effects of obesity pre- and post-menopause and the increase following treatment with hormone replacement therapy using medroxyprogesterone.

SUMMARY

Serotonergic mediation of estrogen has important clinical implications and warrants further evaluation.

Estradiol rapidly activates male sexual behavior and affects brain monoamine levels in the quail brain

Steroids are generally viewed as transcription factors binding to intracellular receptors and activating gene transcription. Rapid cellular effects mediated via non-genomic mechanisms have however been identified and one report showed that injections of estradiol rapidly stimulate chemoinvestigation and mounting behavior in castrated male rats. It is not known whether such effects take place in other species and what are the cellular underlying mechanisms. We show here that a single injection of estradiol (500 microg/kg) rapidly and transiently activates copulatory behavior in castrated male quail pre-treated with a dose of testosterone behaviorally ineffective by itself. The maximal behavioral effect was observed after 15 min. In a second experiment, the brain of all subjects was immediately collected after behavioral tests performed 15 min after injection. The preoptic area--hypothalamus (HPOA), hindbrain, telencephalon and cerebellum were isolated and monoamines measured by HPLC-ED. Estradiol increased levels of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) and 5-HIAA/serotonin ratios in the telencephalon and hindbrain independently of whether animals had mated or not. Estradiol also affected these measures in HPOA and cerebellum but this effect was correlated with the level of sexual activity so that significant effects of the treatment only appeared when sexual activity was used as a covariate. Interactions between estradiol effects and sexual activity were also observed for dopamine in the HPOA and for serotonin in the hindbrain and cerebellum. Together, these data demonstrate that a single estradiol injection rapidly activates male sexual behavior in quail and that this behavioral effect is correlated with changes in monoaminergic activity.

In vivo occupancy of female rat brain estrogen receptors by 17β-estradiol and tamoxifen

Estrogens or antiestrogens are currently used by millions of women, but the interaction of these hormonal agents with brain estrogen receptors (ER) in vivo has not been characterized to date. Our goal was to assess, in vivo, the extent and regional distribution of brain ER occupancy in rats chronically exposed to 17beta-estradiol (E(2)) or tamoxifen (TAM). For that purpose, female ovariectomized Sprague-Dawley rats were implanted with subcutaneous pellets containing either placebo (OVX), E(2), or TAM for 3 weeks. ER occupancy in grossly dissected regions was quantified with 16alpha-[(18)F]fluoroestradiol ([(18)F]FES). Both E(2) and TAM produced significant decreases in radioligand uptake in the brain although the effect of E(2) was larger and more widespread than the effect of TAM. Detailed regional analysis of the interaction was then undertaken using a radioiodinated ligand, 11beta-methoxy-16alpha-[(125)I]iodo-estradiol ([(125)I]MIE(2)), and quantitative ex vivo autoradiography. E(2) treatment resulted in near-complete (86.6 +/- 17.5%) inhibition of radioligand accumulation throughout the brain, while ER occupancy in the TAM group showed a marked regional distribution such that percentage inhibition ranged from 40.5 +/- 15.6 in the ventrolateral part of the ventromedial hypothalamic nucleus to 84.6 +/- 4.5 in the cortical amygdala. These results show that exposure to pharmacologically relevant levels of TAM produces a variable, region-specific pattern of brain ER occupancy, which may be influenced by the regional proportion of ER receptor subtypes. These findings may partially explain the highly variable and region-specific effects observed in neurochemical, metabolic, and functional studies of the effects of TAM in the brain of experimental animals as well as human subjects.

WAY100635 and female rat lordosis behavior

Sexually receptive proestrous rats with bilateral cannulae in the ventromedial nucleus of the hypothalamus (VMN) were infused with 200 ng of (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) or with 8-OH-DPAT plus varying concentrations (200 to 2000 ng) of the 5-HT1A receptor antagonist, N-[2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY100635). 8-OH-DPAT inhibited lordosis behavior within 15 min of the infusion and every dose of WAY100635 prevented the inhibition. When non-sexually receptive, ovariectomized rats, hormonally primed with 0.5 microg estradiol benzoate and 500 microg progesterone, were infused with WAY100635 (400 to 2000 ng), the 5-HT1A receptor antagonist did not facilitate lordosis responding. These findings support earlier findings that activation of 5-HT1A receptors in the mediobasal hypothalamus inhibits lordosis behavior. However, they further demonstrate that tonic activation of 5-HT1A receptors is not responsible for the absence of sexual receptivity in suboptimally hormonally primed ovariectomized rats.

Estrogen reduces serotonin-1A receptor-mediated oxytocin release and Galpha(i/o/z) proteins in the hypothalamus of ovariectomized rats

The present study examined the effect of estradiol on hypothalamic serotonin-1A (5-HT(1A)) receptor signaling in female rats. We first examined the time-course effects of a single injection of the 5-HT(1A) receptor agonist (+/-)8-OH-DPAT (5, 15 or 30 min prior to decapitation), and dose response of (+)8-OH-DPAT (50, 100, 200 or 500 microg/kg, s.c.) on plasma hormones in ovariectomized rats that received a daily injection of beta-estradiol 3-benzoate (10 microg/day, s.c.) or vehicle (sesame oil) for 2 days. In vehicle- and estrogen-treated rats, the peak response of hormones occurred at 15 min after injection and the time-course of oxytocin and adrenocorticotropic hormone (ACTH) responses to an injection of 8-OH-DPAT were comparable. However, only the oxytocin response was reduced by estrogen treatment. A second experiment compared the ACTH and oxytocin responses with doses of 50 or 200 microg/kg, s.c. of (+)8-OH-DPAT vs. (+/-)8-OH-DPAT in ovariectomized rats that were treated with oil or beta-estradiol 3-benzoate (10 microg/day, s.c.) for 2 days. (+)8-OH-DPAT and (+/-)8-OH-DPAT produced a similar magnitude of increase in plasma levels of ACTH and oxytocin. Treatment with beta-estradiol 3-benzoate produced a significant and comparable reduction in the oxytocin response to the highest dose (200 microg/kg, s.c.) of both (+)8-OH-DPAT and (+/-)8-OH-DPAT but did not alter the ACTH response to either (+)8-OH-DPAT or (+/-)8-OH-DPAT. In the dose-response experiment, a dose of 50 microg/kg of (+)8-OH-DPAT produced a maximal increase in plasma levels of ACTH, while the maximal oxytocin response was achieved with a dose of 200 microg/kg, s.c. Treatment with beta-estradiol 3-benzoate reduced the maximal oxytocin response to (+)8-OH-DPAT (by 29%) but did not alter the ACTH response to any doses of (+)8-OH-DPAT. To examine potential mechanisms mediating the effects of estrogen on 5-HT(1A) receptor signaling, we measured the levels of Galpha(i), Galpha(o) and Galpha(z) proteins, which couple 5-HT(1A) receptors to their effector enzymes, in two subregions of the hypothalamus. The levels of Galpha(z) protein were reduced in the mediobasal hypothalamus (containing the ventromedial and arcuate nuclei), which mainly expresses estrogen receptor-alpha, but not in the paraventricular hypothalamus, which mainly expresses estrogen receptor-beta. Estradiol reduced the levels of Galpha(i2) and Galpha(i3 )proteins in both hypothalamic regions but did not affect Galpha(i1) levels in either area. Combined, the data suggest that racemic and stereoselective 8-OH-DPAT have similar neuroendocrine effects and that both estrogen receptor-alpha and estrogen receptor-beta mediate the reduction in levels of Galpha(i2,3) proteins.