Regulation of estrogen receptor concentrations in the rat brain: effects of sustained androgen and estrogen exposure

Aromatase Inhibition Eliminates Sexual Receptivity Without Enhancing Weight Gain in Ovariectomized Marmoset Monkeys

Context

Ovarian estradiol supports female sexual behavior and metabolic function. While ovariectomy (OVX) in rodents abolishes sexual behavior and enables obesity, OVX in nonhuman primates decreases, but does not abolish, sexual behavior, and inconsistently alters weight gain.

Objective

We hypothesize that extra-ovarian estradiol provides key support for both functions, and to test this idea, we employed aromatase inhibition to eliminate extra-ovarian estradiol biosynthesis and diet-induced obesity to enhance weight gain.

Methods

Thirteen adult female marmosets were OVX and received (1) estradiol-containing capsules and daily oral treatments of vehicle (E2; n = 5); empty capsules and daily oral treatments of either (2) vehicle (VEH, 1 mL/kg, n = 4), or (3) letrozole (LET, 1 mg/kg, n = 4).

Results

After 7 months, we observed robust sexual receptivity in E2, intermediate frequencies in VEH, and virtually none in LET females (P = .04). By contrast, few rejections of male mounts were observed in E2, intermediate frequencies in VEH, and high frequencies in LET females (P = .04). Receptive head turns were consistently observed in E2, but not in VEH and LET females. LET females, alone, exhibited robust aggressive rejection of males. VEH and LET females demonstrated increased % body weight gain (P = .01). Relative estradiol levels in peripheral serum were E2 >>> VEH > LET, while those in hypothalamus ranked E2 = VEH > LET, confirming inhibition of local hypothalamic estradiol synthesis by letrozole.

Conclusion

Our findings provide the first evidence for extra-ovarian estradiol contributing to female sexual behavior in a nonhuman primate, and prompt speculation that extra-ovarian estradiol, and in particular neuroestrogens, may similarly regulate sexual motivation in other primates, including humans.

Impact of pubertal onset on region-specific Esr2 expression

In female rats, pubertal onset is associated with maturation of the medial prefrontal cortex (mPFC) and mPFC-mediated behaviours. These behavioural and anatomical changes are likely a result of the effects of oestrogens at the nuclear oestrogen receptor (ER)β, which is expressed at higher levels than the ERα isoform in the adult mPFC. Researchers have previously quantified ERβ protein and Esr2 RNA in rodents during early postnatal development and adulthood, although an adolescent-specific trajectory of this receptor in the mPFC has not been documented. Given that Esr2 expression can fluctuate in the presence or absence of oestrogens, puberty and the subsequent rise in gonadal hormones could influence levels of ERβ in the adolescent brain. To further explore this, we used RNAscope® technology to quantify the amount of Esr2 mRNA in pre-pubertal adolescent, recently post-pubertal adolescent and adult female rats. We show that Esr2 expression decreases significantly in the mPFC, striatum and motor cortex between pre-pubertal adolescence and adulthood. In the mPFC, this decrease occurs rapidly at pubertal onset, with no significant decrease in Esr2 levels between the recently post-pubertal and adult cohort. By contrast, the striatum and motor cortex had no significant differences in the amount of Esr2 mRNA between pre- and post-pubertal females. Insofar as the amount of Esr2 expression is proportional to functional ERβ, these results suggest ERβ decreases in a region-specific pattern in response to pubertal onset and highlight a role for this receptor in the maturational events that occur in the female rat mPFC at puberty.

A long-term cyclic plus tonic regimen of 17β-estradiol improves the ability to handle a high spatial working memory load in ovariectomized middle-aged female rats

The influence of estrogens on modifying cognition has been extensively studied, revealing that a wide array of factors can significantly impact cognition, including, but not limited to, subject age, estrogen exposure duration, administration mode, estrogen formulation, stress history, and progestogen presence. Less known is whether long-term, extended exposure to estrogens would benefit or otherwise impact cognition. The present study examined the effects of 17β-estradiol (E2) exposure for seven months, beginning in late adulthood and continuing into middle age, using a regimen of cyclic exposure (bi-monthly subcutaneous injection of 10 μg E2), or Cyclic+Tonic exposure (bi-monthly subcutaneous injection of 10 μg E2 + Silastic capsules of E2) in ovariectomized female Fischer-344-CDF rats. Subjects were tested on a battery of learning and memory tasks. All groups learned the water radial-arm maze (WRAM) and Morris water maze tasks in a similar fashion, regardless of hormone treatment regimen. In the asymptotic phase of the WRAM, rats administered a Cyclic+Tonic E2 regimen showed enhanced performance when working memory was taxed compared to Vehicle and Cyclic E2 groups. Assessment of spatial memory on object placement and object recognition was not possible due to insufficient exploration of objects; however, the Cyclic+Tonic group showed increased total time spent exploring all objects compared to Vehicle-treated animals. Overall, these data demonstrate that long-term Cyclic+Tonic E2 exposure can result in some long-term cognitive benefits, at least in the spatial working memory domain, in a surgically menopausal rat model.

A transcriptomics model of estrogen action in the ovine fetal hypothalamus: evidence for estrogenic effects of ICI 182,780

Estradiol plays a critical role in stimulating the fetal hypothalamus-pituitary-adrenal axis at the end of gestation. Estradiol action is mediated through nuclear and membrane receptors that can be modulated by ICI 182,780, a pure antiestrogen compound. The objective of this study was to evaluate the transcriptomic profile of estradiol and ICI 182,780, testing the hypothesis that ICI 182,780 antagonizes the action of estradiol in the fetal hypothalamus. Chronically catheterized ovine fetuses were infused for 48 h with: vehicle (Control, n = 6), 17β-estradiol 500 μg/kg/day (Estradiol, n = 4), ICI 182,780 5 μg/kg/day (ICI 5 μg, n = 4) and ICI 182,780 5 mg/kg/day (ICI 5 mg, n = 5). Fetal hypothalami were collected afterward, and gene expression was measured through microarray. Statistical analysis of transcriptomic data was performed with Bioconductor-R and Cytoscape software. Unexpectedly, 35% and 15.5% of the upregulated differentially expressed genes (DEG) by Estradiol significantly overlapped (P < 0.05) with upregulated DEG by ICI 5 mg and ICI 5 μg, respectively. For the downregulated DEG, these percentages were 29.9% and 15.5%, respectively. There was almost no overlap for DEG following opposite directions between Estradiol and ICI ICI 5 mg or ICI 5 μg. Furthermore, most of the genes in the estrogen signaling pathway - after activation of the epidermal growth factor receptor - followed the same direction in Estradiol, ICI 5 μg or ICI 5 mg compared to Control. In conclusion, estradiol and ICI 182,780 have estrogenic genomic effects in the developing brain, suggesting the possibility that the major action of estradiol on the fetal hypothalamus involves another receptor system rather than estrogen receptors.

Estrogens as neuroprotectants: Estrogenic actions in the context of cognitive aging and brain injury

There is ample empirical evidence to support the notion that the biological impacts of estrogen extend beyond the gonads to other bodily systems, including the brain and behavior. Converging preclinical findings have indicated a neuroprotective role for estrogen in a variety of experimental models of cognitive function and brain insult. However, the surprising null or even detrimental findings of several large clinical trials evaluating the ability of estrogen-containing hormone treatments to protect against age-related brain changes and insults, including cognitive aging and brain injury, led to hesitation by both clinicians and patients in the use of exogenous estrogenic treatments for nervous system outcomes. That estrogen-containing therapies are used by tens of millions of women for a variety of health-related applications across the lifespan has made identifying conditions under which benefits with estrogen treatment will be realized an important public health issue. Here we provide a summary of the biological actions of estrogen and estrogen-containing formulations in the context of aging, cognition, stroke, and traumatic brain injury. We have devoted special attention to highlighting the notion that estrogen appears to be a conditional neuroprotectant whose efficacy is modulated by several interacting factors. By developing criteria standards for desired beneficial peripheral and neuroprotective outcomes among unique patient populations, we can optimize estrogen treatments for attenuating the consequences of, and perhaps even preventing, cognitive aging and brain injury.

Androgenic mechanisms of sexual differentiation of the nervous system and behavior

Testicular androgens are the major endocrine factor promoting masculine phenotypes in vertebrates, but androgen signaling is complex and operates via multiple signaling pathways and sites of action. Recently, selective androgen receptor mutants have been engineered to study androgenic mechanisms of sexual differentiation of the nervous system and behavior. The focus of these studies has been to evaluate androgenic mechanisms within the nervous system by manipulating androgen receptor conditionally in neural tissues. Here we review both the effects of neural loss of AR function as well as the effects of neural overexpression of AR in relation to global AR mutants. Although some studies have conformed to our expectations, others have proved challenging to assumptions underlying the dominant hypotheses. Notably, these studies have called into question both the primacy of direct, neural mechanisms and also the linearity of the relationship between androgenic dose and sexual differentiation of brain and behavior.

The effects of long-term estradiol treatment on social behavior and gene expression in adult female rats

This study tested the effects of long-term estradiol (E₂) replacement on social behavior and gene expression in brain nuclei involved in the regulation of these social behaviors in adult female rats. We developed an ultrasonic vocalization (USV) test and a sociability test to examine communications, social interactions, and social preference, using young adult female cagemates. All rats were ovariectomized (OVX) and implanted with a Silastic capsule containing E₂ or vehicle, and housed in same-treatment pairs for a 3-month period. Then, rats were behaviorally tested, euthanized, and 5 nuclei in the brain's social decision-making circuit were selected for neuromolecular profiling by a multiplex qPCR method. Our novel USV test proved to be a robust tool to measure numbers and types of calls emitted by cagemates that had been reintroduced after a 1-week separation. Results also showed that E₂-treated OVX rats had profoundly decreased numbers of USV calls compared to vehicle-treated OVX rats. In a test of sociability, in which a female was allowed to choose between her cagemate or a same-treatment novel rat, we found few effects of E₂ compared to vehicle, although interestingly, rats chose the cagemate over an unfamiliar conspecific. Gene expression results revealed that the supraoptic nucleus had the greatest number of gene changes caused by E₂: Oxt, Oxtr and Avp were increased, and Drd2, Htr1a, Grin2b, and Gabbr1 were decreased, by E₂. No genes were affected in the prefrontal cortex, and 1-4 genes were changed in paraventricular nucleus (Pgr), bed nucleus of the stria terminalis (Oxtr, Esr2, Dnmt3a), and medial amygdala (Oxtr, Ar, Foxp1, Tac3). Thus, E₂ changes communicative interactions between adult female rats, together with selected expression of genes in the brain, especially in the supraoptic nucleus.

Sex hormones and adult hippocampal neurogenesis: Regulation, implications, and potential mechanisms

Neurogenesis within the adult hippocampus is modulated by endogenous and exogenous factors. Here, we review the role of sex hormones in the regulation of adult hippocampal neurogenesis in males and females. The review is framed around the potential functional implications of sex hormone regulation of adult hippocampal neurogenesis, with a focus on cognitive function and mood regulation, which may be related to sex differences in incidence and severity of dementia and depression. We present findings from preclinical studies of endogenous fluctuations in sex hormones relating to reproductive function and ageing, and from studies of exogenous hormone manipulations. In addition, we discuss the modulating roles of sex, age, and reproductive history on the relationship between sex hormones and neurogenesis. Because sex hormones have diverse targets in the central nervous system, we overview potential mechanisms through which sex hormones may influence hippocampal neurogenesis. Lastly, we advocate for a more systematic consideration of sex and sex hormones in studying the functional implications of adult hippocampal neurogenesis.

In vitro Autoradiographic Analysis of Regional Changes in Estrogen Receptor Alpha in the Brains of Cycling Female Rats

BACKGROUND/AIMS

The contributions of the three principal ovarian steroid hormones (estradiol, progesterone and testosterone) to the regulation of estrogen receptor alpha (ERα) levels in the rat brain were examined during the estrous cycle.

METHODS

Receptor concentrations were measured using an in vitro autoradiographic technique designed to separately quantify free, unoccupied receptors and receptors 'occupied' by (bound to) endogenous hormone.

RESULTS

ERα occupation increased at proestrus and declined at estrus, reflecting changes in circulating estradiol and testosterone levels. Total ERα content followed a pattern that was the inverse of the occupation data, falling over the night of proestrus. Between 2.00 and 10.00 a.m. on the day of estrus, total ERα concentrations recovered in all brain regions except the ventromedial nucleus (VMN), in which ERα binding remained depressed at estrus. Administration of the progesterone antagonist mifepristone on the afternoon of proestrus resulted in recovery of ERα levels in the VMN by the morning of estrus, consistent with the hypothesis that the preovulatory progesterone surge selectively inhibits VMN ERα expression. Residual ERα occupation observed at estrus, when estradiol is not detectable in the serum, likely reflects intracranial aromatization of circulating androgens, since the pattern of receptor occupation observed at this stage of the cycle could be reproduced in ovariectomized rats by replacement with testosterone.

CONCLUSION

These findings indicate that ERα binding in the brain fluctuates during the rat estrous cycle in a region-specific manner and suggest that local aromatization of testosterone may contribute significantly to ERα occupation when circulating estradiol levels are low.

Estrogen Effects on Cognitive and Synaptic Health Over the Lifecourse

Estrogen facilitates higher cognitive functions by exerting effects on brain regions such as the prefrontal cortex and hippocampus. Estrogen induces spinogenesis and synaptogenesis in these two brain regions and also initiates a complex set of signal transduction pathways via estrogen receptors (ERs). Along with the classical genomic effects mediated by activation of ER α and ER β, there are membrane-bound ER α, ER β, and G protein-coupled estrogen receptor 1 (GPER1) that can mediate rapid nongenomic effects. All key ERs present throughout the body are also present in synapses of the hippocampus and prefrontal cortex. This review summarizes estrogen actions in the brain from the standpoint of their effects on synapse structure and function, noting also the synergistic role of progesterone. We first begin with a review of ER subtypes in the brain and how their abundance and distributions are altered with aging and estrogen loss (e.g., ovariectomy or menopause) in the rodent, monkey, and human brain. As there is much evidence that estrogen loss induced by menopause can exacerbate the effects of aging on cognitive functions, we then review the clinical trials of hormone replacement therapies and their effectiveness on cognitive symptoms experienced by women. Finally, we summarize studies carried out in nonhuman primate models of age- and menopause-related cognitive decline that are highly relevant for developing effective interventions for menopausal women. Together, we highlight a new understanding of how estrogen affects higher cognitive functions and synaptic health that go well beyond its effects on reproduction.

Neurocognitive effects of estrogens across the adult lifespan in nonhuman primates: State of knowledge and new perspectives

This article is part of a Special Issue "Estradiol and cognition". This review discusses the unique contribution of nonhuman primate research to our understanding of the neurocognitive effects of estrogens throughout the adult lifespan in females. Mounting evidence indicates that estrogens affect many aspects of hippocampal, prefrontal and cholinergic function in the primate brain and the underlying mechanisms are beginning to be elucidated. In addition, estrogens may also influence cognitive function indirectly, via the modulation of other systems that impact cognition. We will focus on the effects of estrogens on sleep and emphasize the need for primate models to better understand these complex interactions. Continued research with nonhuman primates is essential for the development of therapies that are optimal for the maintenance of women's cognitive health throughout the lifespan.

Trajectories and phenotypes with estrogen exposures across the lifespan: What does Goldilocks have to do with it?

This article is part of a Special Issue "Estradiol and cognition". Estrogens impact the organization and activation of the mammalian brain in both sexes, with sex-specific critical windows. Throughout the female lifespan estrogens activate brain substrates previously organized by estrogens, and estrogens can induce non-transient brain and behavior changes into adulthood. Therefore, from early life through the transition to reproductive senescence and beyond, estrogens are potent modulators of the brain and behavior. Organizational, reorganizational, and activational hormone events likely impact the trajectory of brain profiles during aging. A "brain profile," or quantitative brain measurement for research purposes, is typically a snapshot in time, but in life a brain profile is anything but static--it is in flux, variable, and dynamic. Akin to this, the only thing continuous and consistent about hormone exposures across a female's lifespan is that they are noncontinuous and inconsistent, building and rebuilding on past exposures to create a present brain and behavioral landscape. Thus, hormone variation is especially rich in females, and is likely the destiny for maximal responsiveness in the female brain. The magnitude and direction of estrogenic effects on the brain and its functions depend on a myriad of factors; a "Goldilocks" phenomenon exists for estrogens, whereby if the timing, dose, and regimen for an individual are just right, markedly efficacious effects present. Data indicate that exogenously-administered estrogens can bestow beneficial cognitive effects in some circumstances, especially when initiated in a window of opportunity such as the menopause transition. Could it be that the age-related reduction in efficacy of estrogens reflects the closure of a late-in-life critical window occurring around the menopause transition? Information from classic and contemporary works studying organizational/activational estrogen actions, in combination with acknowledging the tendency for maximal responsiveness to cyclicity, will elucidate ways to extend sensitivity and efficacy into post-menopause.

Rapid Effects of Estradiol on Aggression in Birds and Mice: The Fast and the Furious

Across invertebrates and vertebrates, steroids are potent signaling molecules that affect nearly every cell in the organism, including cells of the nervous system. Historically, researchers have focused on the genomic (or "nuclear-initiated") effects of steroids. However, all classes of steroids also have rapid non-genomic (or "membrane-initiated") effects, although there is far less basic knowledge of these non-genomic effects. In particular, steroids synthesized in the brain ("neurosteroids") have genomic and non-genomic effects on behavior. Here, we review evidence that estradiol has rapid effects on aggression, an important social behavior, and on intracellular signaling cascades in relevant regions of the brain. In particular, we focus on studies of song sparrows (Melospiza melodia) and Peromyscus mice, in which estradiol has rapid behavioral effects under short photoperiods only. Furthermore, in captive Peromyscus, estrogenic compounds (THF-diols) in corncob bedding profoundly alter the rapid effects of estradiol. Environmental factors in the laboratory, such as photoperiod, diet, and bedding, are critical variables to consider in experimental design. These studies are consistent with the hypothesis that locally-produced steroids are more likely than systemic steroids to act via non-genomic mechanisms. Furthermore, these studies illustrate the dynamic balance between genomic and non-genomic signaling for estradiol, which is likely to be relevant for other steroids, behaviors, and species.

Effects of sex steroids and estrogen receptor agonists on the expression of estrogen receptor alpha in the principal division of the bed nucleus of the stria terminalis of female rats

Estrogen actions on neurons of the principal division of the bed nucleus of the stria terminalis (BNSTpr) are essential for the regulation of female sexual behavior. However, little is known about the effects of estradiol and progesterone (P) on estrogen receptor alpha (ERα) expression in this nucleus. To study this subject, we used stereological methods to estimate the total number of ERα-immunoreactive (ERα-ir) neurons in the BNSTpr of female rats at each stage of the estrous cycle and of ovariectomized rats after administration of estradiol benzoate (EB) and/or P. To ascertain the percentage of ERα-positive neurons in the BNSTpr, the total number of neurons in this nucleus was also estimated. In order to identify the specific role played by the selective activation of each ER in the expression of ERα, ovariectomized rats were injected with the ERα agonist, propyl-pyrazole triol (PPT), or the ERβ agonist, diaryl-propionitrile (DPN). Data show that ERα is expressed in 40-60% of the BNSTpr neurons and that the number of ERα-ir neurons is lowest at proestrus. This value is paralleled by the administration of EB. The number of ERα-ir neurons was not modified by P. PPT induced no changes in the number of ERα-ir neurons. Contrariwise, DPN induced a decrease in the total number of ERα-ir neurons to values similar to those of EB-treated rats. These results show that P has no effect in the modulation of ERα expression and demonstrate that estradiol regulation of ERα in BNSTpr neurons is mediated by activation of ERβ.

A comparison of the pharmacokinetic profile of an ascending-dose, extended-regimen combined oral contraceptive to those of other extended regimens

Quartette (levonorgestrel [LNG]/ethinyl estradiol [EE] and EE) is an ascending-dose, extended-regimen combined oral contraceptive (COC) that consists of a constant dose of LNG 150 µg on days 1 to 84 with EE 20 µg on days 1 to 42, 25 µg on days 43 to 63, 30 µg on days 64 to 84, and 10 µg of EE monotherapy on days 85 to 91. A population pharmacokinetic (PK) model for EE was developed using nonlinear mixed-effects modeling to characterize the PK profile of EE administered in Quartette and other extended-regimen LNG/EE COCs. Model-predicted plasma concentration-time profiles demonstrated a stepwise increase in systemic exposure to EE during the first 84 days of the cycle following each EE dose change. Lower concentrations of EE were noted during the final 7-day period of EE 10 µg. Gradual increases in EE seen with Quartette may decrease the incidence of unscheduled bleeding frequently observed during early cycles of extended-regimen COCs.

Continuously delivered ovarian steroids do not alter dendritic spine density or morphology in macaque dorsolateral prefrontal cortical neurons

Aged ovariectomized (OVX) female monkeys, a model for menopause in humans, show a decline in spine density in the dorsolateral prefrontal cortex (dlPFC) and diminished performance in cognitive tasks requiring this brain region. Previous studies in our laboratory have shown that long-term cyclic treatment with 17β-estradiol (E) produces an increase in spine density and in the proportion of thinner spines in layer III pyramidal neurons in the dlPFC of both young and aged OVX rhesus monkeys. Here we used 3D reconstruction of Lucifer yellow-loaded neurons to investigate whether clinically relevant schedules of hormone therapy would produce similar changes in prefrontal cortical neuronal morphology as long-term cyclic E treatment in young female monkeys. We found that continuously delivered E, with or without a cyclic progesterone treatment, did not alter spine density or morphology in the dlPFC of young adult OVX rhesus monkeys. We also found that the increased density of thinner spines evident in the dlPFC 24h after E administration in the context of long-term cyclic E therapy is no longer detectable 20days after E treatment. When compared with the results of our previously published investigations, our results suggest that cyclic fluctuations in serum E levels may cause corresponding fluctuations in the density of thin spines in the dlPFC. By contrast, continuous administration of E does not support sustained increases in thin spine density. Physiological fluctuations in E concentration may be necessary to maintain the morphological sensitivity of the dlPFC to E.

Factors influencing the cognitive and neural effects of hormone treatment during aging in a rodent model

Whether hormone treatment alters brain structure or has beneficial effects on cognition during aging has recently become a topic of debate. Although previous research has indicated that hormone treatment benefits memory in menopausal women, several newer studies have shown no effect or detrimental effects. These inconsistencies emphasize the need to evaluate the role of hormones in protecting against age-related cognitive decline in an animal model. Importantly, many studies investigating the effects of estrogen and progesterone on cognition and related brain regions have used young adult animals, which respond differently than aged animals. However, when only the studies that have examined the effects of hormone treatment in an aging model are reviewed, there are still varied behavioral and neural outcomes. This article reviews some of the important factors that can influence the behavioral and neural outcomes of hormone treatment including the type of estrogen administered, whether or not estrogen is combined with progesterone and if so, the type of progesterone used, as well as the route, mode, and length of treatment. How these factors influence cognitive outcomes highlights the importance of study design and avoiding generalizations from a small number of studies. This article is part of a Special Issue entitled Hormone Therapy.

Chronic administration of tibolone modulates anxiety-like behavior and enhances cognitive performance in ovariectomized rats

Hormone replacement therapy (HRT) may be prescribed to prevent the symptoms of menopause. This therapy may include estrogenic and/or progestin components and may increase the incidence of endometrial and breast cancers. Tibolone (TIB), which is also made up of estrogen and progestin components, is often used to reduce the impact of HRT. However, the effect of TIB on the processes of learning, memory and anxiety has yet to be fully elucidated. The aim of this study was to evaluate the long-term effect on learning, memory processes and anxiety in ovariectomized rats caused by different doses of TIB (0 mg/kg, 0.01 mg/kg, 0.1 mg/kg 1.0 mg/kg and 10 mg/kg, administered daily via the oral route for 18 weeks). Two behavioral animal models, the autoshaping and T maze models were employed. The concentrations of acetyl choline transferase (ChAT) and tryptophan hydroxylase (TPH) in the hippocampus were directly measured by Western blot. No significant changes were observed in the autoshaping model and spontaneous activity test. In the T maze, increased latency was observed with TIB doses of 1 and 10 mg/kg compared to the vehicle. We observed that the ChAT content decreased with increasing doses of TIB, whereas TPH content increased with doses of 1 and 10 mg/kg of TIB. These data indicate that high doses of TIB improved emotional learning, which may be related to the modulation of the cholinergic and serotonergic systems by TIB.

Long-term replacement of estrogen in combination with medroxyprogesterone acetate improves acquisition of an alternation task in middle-aged female rats

Studies have shown that ovarian hormones protect against some of the cognitive deficits associated with aging. Although much of the literature in rodents has focused on hippocampal dependent tasks, studies suggest that tasks dependent on the prefrontal cortex are also influenced by ovarian hormones. The present study investigated the effects of ovarian hormone treatment during aging on a delayed alternation t-maze. Female Long Evans hooded rats were ovariectomized at middle age (11-12 months) and placed in 1 of 5 treatment groups: no replacement, chronic estradiol (E(2)), cyclic E(2), chronic E(2) and progesterone, or chronic E(2) and medroxyprogesterone acetate (MPA). Following 6 months of hormone treatment, animals were trained to alternate in a t-maze. After reaching criterion, a series of delays from 5 to 90 s were introduced in random order. Rats receiving E(2) with MPA reached criterion significantly faster than animals not receiving treatment and those who received chronic or cyclic E(2) only. There was a nonsignificant trend for animals receiving E(2) and progesterone to reach criterion in fewer sessions than animals receiving E(2) only. Mode of administration, cyclic or chronic, did not affect performance. Hormones did not affect performance on the delayed alternation. This study, in combination with previous research, indicates that hormone effects cannot be generalized across tasks, age, or duration, and long-term estrogen in combination with MPA can be beneficial for some tasks.

Androgen inhibition of sexual receptivity is modulated by estrogen

Sexual receptivity induced in ovariectomized rats by the long-term administration of estradiol benzoate (EB) can be inhibited by concurrent administration of androgens. Experiment 1 examined the role of time course and dose of androgens in the inhibition of estrogen-induced sexual receptivity. Ovariectomized rats were treated with EB (2.0 microg per rat per day) for 6 days and tested for sexual receptivity (Test Day I). EB treatment continued for 15 days concomitant with daily administration of one of three doses of dihydrotestosterone propionate (DHTP; 7.5, 0.75, 0.075 mg/kg) or 3α-androstanediol (3α-Adiol; 3.75, 1.0, 0.375 mg/kg). Four tests for sexual receptivity were conducted on days 3, 6, 14, and 15 of the androgen/vehicle treatment period (Test Days II-V). On Day 15 (Test Day V), the rats received progesterone (1.0 mg per rat) 4 h before testing. Using the same experimental design, Experiment 2 examined the effect of increasing the dose of estrogen on the androgenic inhibition of sexual receptivity. Ovariectomized rats were treated with one of two doses of EB (2.0 or 10.0 microg per rat per day) concomitant with daily administration of DHTP (7.5 mg/kg) or 3α-Adiol (3.75 mg/kg). In Experiment 1, the highest doses of both DHTP and 3α-Adiol significantly inhibited estrogen-induced sexual receptivity. Data from Experiment 2 indicate that the inhibitory effects of DHTP but not 3α-Adiol can be moderated by an increased dose of EB.

Copulation modifies AR and ERα mRNA expression in the male rat brain

BACKGROUND/AIMS

One day after male sexual behavior [one ejaculation or copulation to satiety (ad libitum copulation during 4h with the same female)] androgen receptor immunoreactivity (AR-ir) is decreased and estrogen receptor alpha immunoreactivity (ERα-ir) increased in various brain areas related with its control. Seven days after sexual satiety there was a limited recovery of sexual behavior accompanied by a partial recuperation in the AR-ir. In this study we evaluated if these changes in AR-ir and ERα-ir were paralleled by variations in their respective mRNA.

METHODS

Sexually experienced male rats were sacrificed at different intervals: immediately, 24h or seven days after sexual satiety or 24h after one ejaculation. The changes in AR and ERα mRNA were analyzed by in situ hybridization using digoxigenine-labeled oligonucleotide probes in the MPOA, LSV and the bed nucleus of the stria terminalis, medial division, anterior (BSTMA).

RESULTS

AR mRNA density was decreased in the MPOA and the LSV immediately and 24h after one ejaculation or sexual satiety. Seven days after copulating to satiety, there was a recovery of AR mRNA. In the BSTMA the different behavioral conditions did not modify the AR mRNA expression. In the MPOA, LSV and BSTMA the ERα mRNA increased after a single ejaculation and at all intervals after sexual satiety.

CONCLUSION

In some brain areas and after some intervals of sexual activity, the changes in steroid protein receptors expression seem to be consequence of parallel changes in the expression of the respective mRNA.

Tonic Premarin dose-dependently enhances memory, affects neurotrophin protein levels and alters gene expression in middle-aged rats

Premarin™ is the most commonly prescribed estrogenic component of hormone therapy, given since 1942. The current study is the first examining cognitive effects of tonic Premarin treatment in an animal model. Middle-aged ovariectomized (Ovx) rats received vehicle or one of three doses of Premarin (12, 24 or 36μg daily). Rats were tested on a spatial working and reference memory maze battery. Both medium- and high-dose Premarin enhanced memory retention, while low-dose Premarin impaired learning and memory retention. Correlations with serum hormone levels showed that as the ratio of estrone:17β-estradiol increased, animals tended to show better working memory performance. Taken together with the dissociation of dose-specific estrogenic profiles, results suggest that higher levels of estrone, in the presence of 17β-estradiol concentrations higher than that of Ovx levels, may be beneficial for memory. Moreover, Premarin exerted dose and brain-region specific effects on BDNF and NGF protein levels, with most marked changes in cingulate and perirhinal cortices. Hippocampal gene expression profiling demonstrated significant Premarin-induced transcriptional changes in genes linked to plasticity and cognition. These findings indicate that Premarin can impact memory and the brain, and that dosing should be recognized as a clinically relevant factor possibly affecting the direction and efficacy of cognitive outcome.

Blockade of estrogen action upregulates estrogen receptor-alpha mRNA in the fetal brain

BACKGROUND

Fetal neuroendocrine maturation in late gestation is critical for maintenance of fetal homeostasis, growth, and readiness for birth. Sheep express estrogen receptors (ERs) in various brain regions. However, little is known about the regulation of ER-alpha and ER-beta in the ovine brain prenatally.

OBJECTIVE

The present study was designed to test the hypothesis that the expression of ER is influenced by circulating estrogens in the late-gestation sheep fetus.

METHODS

Six chronically-catheterized twin fetal sheep were treated with vehicle or the ER blocker ICI 182,780 i.c.v. (0.25 microg/day). Fetuses were sacrificed 6-14 days after surgery and start of infusion. Brain regions were rapidly isolated and snap-frozen for later extraction of mRNA and protein. ER-alpha and ER-beta mRNA was measured using real-time PCR and protein was measured using Western blot.

RESULTS

Treatment with ICI 182,780 increased ER-alpha mRNA, especially in cerebellum and hippocampus. There were no changes in ER-alpha protein and no changes in ER-beta at either the mRNA or protein level.

CONCLUSION

Expression of ER-alpha is influenced by endogenous estrogens in the ovine fetal brain.

A recently identified hypothalamic nucleus expressing estrogen receptor alpha

We report evidence for the existence of a unique nucleus in the rat hypothalamus. This nerve cell group is situated in the interstitial area between the arcuate nucleus and ventromedial nucleus of the hypothalamus, and is primarily oriented sagittally, in a spindle shape. This nucleus was a well defined structure in Nissl-stained sections because of its location in an otherwise cell-poor zone. This sagittalis nucleus of the hypothalamus (SGN) exhibits significant sex differences in its volume and cell numbers, as defined by Nissl staining and estrogen receptor (ER) alpha immunoreactivity (ir), being significantly larger in males than in females. Treatment of neonatal females with testosterone eliminated these sex differences. It is noteworthy that adult female rats have estrous cycle-related variations in the ERalpha-ir cell distribution, decreasing during the proestrus phase of the cycle. Pharmacological experiments demonstrated that the single injection of estradiol benzoate had a significant effect on the ERalpha-ir cell count, suggesting the hormonal responsiveness of SGN neurons. This unique hypothalamic nucleus with its morphological sex differences and hormonal responsiveness is embedded in a region important for the regulation of endocrine functions and sexual behaviors.

Androgen and estrogen (alpha) receptor localization on periaqueductal gray neurons projecting to the rostral ventromedial medulla in the male and female rat

The periaqueductal gray (PAG) is involved in many gonadal steroid-sensitive behaviors, including responsiveness to pain. The PAG projects to the rostral ventromedial medulla (RVM), comprising the primary circuit driving pain inhibition. Morphine administered systemically or directly into the PAG produces greater analgesia in male compared to female rats, while manipulation of gonadal hormones alters morphine potency in both sexes. It is unknown if these alterations are due to steroidal actions on PAG neurons projecting to the RVM. The expression of androgen (AR) and estrogen (ERalpha) receptors in the PAG of female rats and within this descending inhibitory pathway in both sexes is unknown. The present study used immunohistochemical techniques (1) to map the distribution of AR and ERalpha across the rostrocaudal axis of the PAG; and (2) to determine whether AR and/or ERalpha were colocalized on PAG neurons projecting to the RVM in male and female rats. AR and ERalpha immunoreactive neurons (AR-IR, ERalpha-IR) were densely distributed within the caudal PAG of male rats, with the majority localized in the lateral/ventrolateral PAG. Females had significantly fewer AR-IR neurons, while the quantity of ERalpha was comparable between the sexes. In both sexes, approximately 25-50% of AR-IR neurons and 20-50% of ERalpha-IR neurons were retrogradely labeled. This study provides direct evidence of the expression of steroid receptors in the PAG and the descending pathway driving pain inhibition in both male and female rats and may provide a mechanism whereby gonadal steroids modulate pain and morphine potency.

Brain aging modulates the neuroprotective effects of estrogen on selective aspects of cognition in women: a critical review

Although there is now a substantial literature on the putative neuroprotective effects of estrogen on cognitive functioning in postmenopausal women, it is replete with inconsistencies. The critical period hypothesis, posited several years ago, attempts to account for the discrepancies in this literature by positing that estrogen treatment (ET) will protect aspects of cognition in older women only when treatment is initiated soon after the menopause. Indeed, evidence from basic neuroscience and from the animal and human literature reviewed herein provides compelling support for the critical period hypothesis. Although it is not known with certainty why estrogen does not protect cognition and may even cause harm when administered to women over the age of 65years, it is likely that the events that characterize brain aging, such as a reduction in brain volume and in neuronal size, alterations in neurotransmitter systems, and a decrease in dendritic spine numbers, form an unfavorable background that precludes a neuroprotective effects of exogenous estrogen on the brain. Other factors that have likely contributed to the discrepancies in the estrogen-cognition literature include differences in the estrogen compounds used, their route of administration, cyclic versus continuous regimens, and the concomitant use of progestins. This critical analysis attempts to define conditions under which ET may protect aspects of cognition in aging women while also considering the cost/benefit ratio for the treatment of women aged 50-59years. Suggestions for specific future research questions are also addressed.

Cerebral hypoperfusion increases estrogen receptor abundance in the ovine fetal brain and pituitary

BACKGROUND/AIMS

Estrogen is an important component of fetal neuroendocrine function in late-gestation fetal sheep; however, little is known about the regulation of estrogen receptor abundance in the brain and pituitary of fetuses. The present study was performed to test the hypotheses that estrogen receptor abundance in the fetal brain and pituitary are influenced by circulating estradiol concentrations and that they are acutely regulated after cerebral hypoperfusion.

METHODS

We studied 16 time-dated fetal sheep (124-128 days gestation) that were chronically catheterized and instrumented at least 5 days before study. Four groups (n = 4 each) were studied in which fetuses received estradiol (0.25 mg/day, producing physiological increases in fetal plasma estradiol concentrations) or placebo implants, and in which fetuses received a 10-min period of brachiocephalic occlusion (BCO) or sham-BCO. One hour after BCO or sham-BCO, fetuses were euthanized and tissues rapidly removed for analysis of estrogen receptors (ER)-alpha and -beta at the mRNA and protein levels.

RESULTS

Both BCO and estradiol treatment were effective in changing ER expression, although the effects were region-specific. BCO dramatically increased ER-alpha in the pituitary and both ER-alpha and ER-beta in the brainstem, while decreasing ER-alpha expression in the hypothalamus. Estradiol treatment decreased ER-alpha expression in the hypothalamus, whereas it increased ER-alpha expression in the brainstem, cerebral cortex and hippocampus.

CONCLUSIONS

We conclude that the expression of ER-alpha and ER-beta in the brain and pituitary of fetal sheep are influenced by circulating estrogen concentrations and acutely regulated in response to cerebral hypoperfusion.

Anabolic androgens restore mating after sexual satiety in male rats

Androgen receptors and estrogen receptors importantly participate in the neuroendocrine control of masculine mating behavior. Sexual satiety is the long term inhibition of masculine mating behavior after repeated ejaculations and is associated to changes in both androgen receptor and estrogen receptor-alpha expression. Androgen receptor expression is up-regulated by systemic chronic administration of anabolic androgens, 5alpha-dihydrotestosterone or estradiol benzoate. This study was carried out to investigate the effect of these treatments on sexual satiety development and recovery; additionally flutamide or tamoxifen treatments -- alone or together with anabolic androgens -- were also included. Chronic 15-day treatment with 5alpha-dihydrotestosterone (5 mg/kg) or tamoxifen (15 mg/kg) inhibited, whereas estradiol benzoate treatment (5 mg/kg) facilitated, mating behavior during sexual satiety development. The proportion of animals that ejaculated 48 h after sexual satiety was increased after 17-day treatment with a mixture of anabolic androgens containing 2 mg/kg testosterone propionate, 2 mg/kg nandrolone decanoate and 1 mg/kg boldenone undecylenate. This effect was only blocked by the combined administration of flutamide plus tamoxifen. The data suggest that anabolic androgens metabolites synergize to restore mating behavior after sexual satiety.

Estrogen replacement regimen and brain infusion of lipopolysaccharide differentially alter steroid receptor expression in the uterus and hypothalamus

The regimen of estrogen replacement can alter the consequences of estrogen therapy and stressors. To determine the long-term effects and interaction of these systems on the brain and periphery, adult female rats were infused with lipopolysaccharide (LPS) into the fourth ventricle of the brain for 4 weeks, and ovariectomized rats were administered either constant or pulsed regimens of estrogen replacement (17beta-estradiol) until sacrifice at 8 weeks. Constant, but not pulsed, estrogen replacement reduced ERalpha and increased HSP90, HSP70, and PR(B) uterine protein levels. Both estrogen regimens increased ERbeta, HSP27, and PR(A) uterine proteins. Both regimens reduced hypothalamic levels of ERalpha, but not ERbeta, HSP, or PR. No changes were observed in the hippocampus. Long-term brain infusion of LPS activated microglia and reduced body weight, but did not alter corticosterone or nitrotyrosine levels. LPS infusion into intact rats suppressed uterine weight, increased ERalpha and decreased HSP90 in the uterus. LPS did not alter uterine weight in ovariectomized rats treated with constant or pulsed estrogen. Together, these data suggest the timing of estrogen replacement and neuroinflammatory stressors can profoundly affect uterine and hypothalamic steroid receptor expression and may be important parameters to consider in the post-menopausal intervention with estrogen.

The influence of gonadal hormones on neuronal excitability, seizures, and epilepsy in the female

It is clear from both clinical observations of women, and research in laboratory animals, that gonadal hormones exert a profound influence on neuronal excitability, seizures, and epilepsy. These studies have led to a focus on two of the primary ovarian steroid hormones, estrogen and progesterone, to clarify how gonadal hormones influence seizures in women with epilepsy. The prevailing view is that estrogen is proconvulsant, whereas progesterone is anticonvulsant. However, estrogen and progesterone may not be the only reproductive hormones to consider in evaluating excitability, seizures, or epilepsy in the female. It seems unlikely that estrogen and progesterone would exert single, uniform actions given our current understanding of their complex pharmacological and physiological relationships. Their modulatory effects are likely to depend on endocrine state, relative concentration, metabolism, and many other factors. Despite the challenges these issues raise to future research, some recent advances have helped clarify past confusion in the literature. In addition, testable hypotheses have developed for complex clinical problems such as "catamenial epilepsy." Clinical and animal research, designed with the relevant endocrinological and neurobiological issues in mind, will help advance this field in the future.

Effects of continuous and intermittent estrogen treatments on memory in aging female mice

The manner in which hormone therapy is given to postmenopausal women may significantly influence its ability to reduce age-associated memory loss. To test the hypothesis that a regimen that approximates the timing of estrogen surges in the natural cycle is more beneficial for memory than a regimen that provides continuous levels of estrogen, we examined the effects of continuous and intermittent estrogen regimens on spatial and object memory in aging female mice. Mice (18 months) were treated with 0.2 mg/kg 17beta-estradiol (E(2)) or vehicle (VEH) for 3 months following ovariectomy. A fast-acting water-soluble cyclodextrin-encapsulated E(2) was used to ensure metabolism within 24 h. Vehicle-treated mice received daily injections of 2-hydroxypropyl-beta-cyclodextrin vehicle. The continuous estradiol group (Contin E(2)) was injected daily with estradiol. The intermittent group (Twice/wk E(2)) received estradiol every 4 days and vehicle on all other days. Mice (21 months) were tested in water-escape motivated 8-arm radial arm maze (WRAM) and object recognition tasks. During WRAM acquisition, the Twice/wk E(2) group committed significantly more reference memory errors than VEH and Contin E(2) groups, and tended to make more working memory errors than the VEH group. The Contin E(2) group did not differ from VEH on either WRAM measure. Additionally, the Twice/wk E(2) group tended to exhibit impaired object recognition. Thus, neither treatment improved spatial or object memory. Indeed, intermittent estradiol was detrimental to both types of memory. These results suggest that the timing of administration may play an important role in the mnemonic response of aging females to estrogen.

Sex hormones, central nervous system and pain

The aim of the present review, which highlights some relationships between sex hormones, the CNS and pain, is to provide reference points for discussion on one of the most intriguing aspects of pain pathophysiology: the presence of sex differences in the response threshold to phasic painful stimuli and in the incidence of chronic pain syndromes. The first part of the review deals with sex steroids and their mechanisms of action. In the second part, the connections between sex steroids, the CNS and pain are illustrated to introduce possible areas of discussion in the study of sex differences in experimental and clinical pain.

Oral contraceptives and mood in women with and without premenstrual dysphoria: a theoretical model

Despite numerous studies on the topic, there is no consensus to date on the effects of oral contraceptives on mood or the mechanism(s) by which they exert these effects. This review article presents a theoretical model to explain the way in which oral contraceptives may affect mood. Specifically, it is argued that progestins exert differential effects on endogenous levels of neurosteroids, thereby altering mood. After providing an overview of the effects of estrogen, progesterone, and progesterone's metabolites on cortical excitability and the role of neurosteroids in depression and premenstrual dysphoria, this article reviews the research that has been conducted on the relationship between oral contraceptives and neurosteroids. Finally, suggestions for future research are made with the dual aim of improving existing studies on the relationship between oral contraceptives and mood and further investigating the possibility that fluctuations in neurosteroid levels are responsible for the effects of oral contraceptives on mood.

Sex, Hormones, and Alzheimer's Disease

More women than men have Alzheimer's disease (AD). Retrospective studies suggested that hormone replacement therapy (HRT) might counteract this disparity by reducing the risk of developing dementia. However, a recent, large, prospective study revealed the puzzling result that HRT increased dementia risk. A review of the literature was conducted to generate hypotheses that might explain why more women than men have AD, and how HRT may increase dementia risk. Longer life span of women than men may be the largest factor in the preponderance of women with AD. Longer duration of disease, less vascular dementia, and less testosterone in women than men may also contribute somewhat. HRT might increase dementia risk by several mechanisms: greater risk of strokes, leading to dementia; use of medroxyprogesterone acetate and estrone, which might have somewhat different possible effects on neuronal and cerebrovascular function than may progesterone and estradiol; decrease of free testosterone which might protect against AD; a dose or delivery method perhaps producing drug levels that might lie outside a hypothetical beneficial range; and down-regulation of estrogen receptors on cholinergic neurons, possibly reducing cholinergic activity. Further study is required to discern by which of several possible mechanisms HRT increases dementia risk.

Hormone therapy and Alzheimer’s disease: benefit or harm?

Alzheimer's disease (AD) is the most common cause of dementia. After menopause, circulating levels of oestrogens decline markedly and oestrogen influences several brain processes predicted to modify AD risk. For example, oestrogen reduces the formation of beta-amyloid, a biochemical hallmark of AD. Oestrogen effects on oxidative stress and some effects on inflammation and the cerebral vasculature might also be expected to ameliorate risk. However, AD pathogenesis is incompletely understood and other oestrogen actions could be deleterious. Limited clinical trial evidence suggests that oestrogen therapy, begun after the onset of AD symptoms, is without substantial benefit or harm. Observational studies have associated oestrogen-containing hormone therapy with reduced AD risk. However, in the Women's Health Initiative Memory Study - a randomised, placebo-controlled trial of women 65 - 79 years of age - oral oestrogen plus progestin doubled the rate of dementia, with heightened risk appearing soon after treatment was initiated. Based on current evidence, hormone therapy is thus not indicated for the prevention of AD. Discrepancies between observational studies and the Women's Health Initiative clinical trial may reflect biases and unrecognised confounding factors in observational reports. Other explanations for divergent findings should be considered in future research, including effects of unopposed oestrogen or different hormone therapy preparations and the intriguing theoretical possibility that effects of hormone therapy on AD risk may be modified by the timing of use (e.g., initiation during the menopausal transition or early postmenopause versus initiation during the late postmenopause).

Chronic oral estrogen affects memory and neurochemistry in middle-aged female mice

This study tested whether chronic oral estrogen could improve memory and alter neural plasticity in the hippocampus and neocortex of middle-aged female mice. Ovariectomized C57BL/6 mice were administered 1,000, 1,500, or 2,500 nM 17beta-estradiol in drinking water for 5 weeks prior to and during spatial and object memory testing. Synaptophysin, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) levels were then measured in hippocampus and neocortex. The medium dose impaired spatial reference memory in the radial-arm maze, whereas all doses improved object recognition. The high dose increased hippocampal synaptophysin and NGF levels, whereas the medium dose decreased these neocortical levels. The high dose decreased neocortical BDNF levels. These data suggest that chronic oral estrogen selectively affects memory and neural function in middle-aged female mice.

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.

Expression of estrogen receptors (alpha, beta) and androgen receptor in serotonin neurons of the rat and mouse dorsal raphe nuclei; sex and species differences

Sex steroids have been inferred to be involved in the regulation of affective status at least partly through the serotonergic (5-HT) system, particularly in the dorsal raphe nucleus (DRN), which innervates enormous projections to the cerebral cortex and limbic system. In the present study, the expression of estrogen receptors-alpha and -beta (ERalpha, ERbeta), androgen receptor (AR) and 5-HT was examined immunohistochemically in the rat and mouse DRN in both sexes. The results showed that large numbers of ERalpha- and/or ERbeta-immunoreactive (ERalpha-I, ERbeta-I) cells were found in the DRN of both male and female mice, whereas only small numbers of ERalpha-I cells and no ERbeta-I cells were seen in the rat DRN of each sex. With respect to AR-immunoreactive (AR-I) cells, moderate numbers of such cells were present only in male rats and mice, and no or very few could be observed in female ones. The ERalpha-I, ERbeta-I, and AR-I cells were mainly distributed in the rostral DRN. In double-immunostaining, many 5-HT-I neurons were found to show ERalpha and/or ERbeta expression specifically in the rostral DRN (particularly dorsal, ventral and interfascicular parts) of mice of both sexes, but not in that of rats. In contrast, only a few 5-HT neurons were observed to show AR expression in the DRN of both rodents. The current results strongly suggest that sex steroids can modulate the affective regulation of the serotonergic system through ERalpha and/or ERbeta in 5-HT neurons of the mouse rostral DRN (but not so much through AR), and that such effects might be different depending on the sex and species, as shown by the prominent sex differences in AR expression and prominent species differences in ERalpha and ERbeta expression.

Sex steroid hormones and the neural control of breathing

We review evidence that sex steroid hormones including estrogen, progesterone and testosterone are involved in the central neural control of breathing. Sex hormones may exert their effects on respiratory motoneurons via neuromodulators, in particular, the serotonergic system. Recent studies have shown that levels of serotonin (5HT) in the hypoglossal and phrenic nuclei are greater in female than in male rats. Serotonin-dependent plasticity in hypoglossal and phrenic motor output also differs in male and female rats. Changing levels of gonadal hormones throughout the estrus cycle coincide with changing levels of 5HT in respiratory motor nuclei, and gonadectomy in male rats results in a decrease in 5HT-dependent plasticity in respiratory motor output. We speculate that sex steroid hormones are critically involved in adaptations in the neural control of breathing throughout life, and that decreasing levels of these hormones with increasing age may have a negative influence on the respiratory control system in response to challenge.

Gedächtnisleistung im Alter:

Zusammenfassung. Das höhere Lebensalter ist durch zahlreiche Veränderungen des Hormonsystems charakterisiert. Besonders markant ist die Abnahme der Sexualsteroidhormone (Östradiol, Progesteron, Testosteron, DHEA), während die basalen Spiegel des “Stresshormons“ Cortisol stabil bleiben oder leicht ansteigen. Die vorliegende Übersichtsarbeit diskutiert die Relevanz dieser hormonellen Veränderungen für Funktion und Struktur des Gehirns am Beispiel der Gedächtnisleistung im höheren Lebensalter. Bei älteren Frauen wurden wiederholt gedächtnisverbessernde und neuroprotektive Effekte von Östradiol berichtet. Inwieweit und in welche Richtung Progesteron die Östrogeneffekte moduliert, ist noch unklar, da sowohl synergistische als auch antagonistische Effekte berichtet wurden. Die Rolle des Testosterons für die Gedächtnisleistung des alternden Mannes ist bisher kaum untersucht. Mehrere Studien haben hingegen gezeigt, dass DHEA bei gesunden älteren Männern und Frauen keine positiven Effekte auf die Gedächtnisleistung ausübt. Das Nebennierenrindenhormon Cortisol verschlechtert akut Leistungen des Arbeitsgedächtnisses und des deklarativen Gedächtnisses. Darüber hinaus gibt es vermehrt Hinweise darauf, dass erhöhte basale Cortisolspiegel im Alter sowohl zu einer Verschlechterung der Gedächtnisleistung als auch zu einer Verringerung des Hippocampusvolumens führen. Zusammengenommen verdeutlichen diese Befunde, dass Steroidhormone die Struktur und Funktion des menschlichen zentralen Nervensystems nachhaltig beeinflussen.

Distribution of estrogen receptor beta mRNA-containing cells in ovariectomized and estrogen-treated female rat brain

Estrogen receptor (ER)-beta is a member of the nuclear receptor superfamily and mediates various estrogenic actions. Changes in ER-alpha mRNA expression induced by estrogen have been well documented, whereas those with regard to ER-beta have only been reported for a part of the hypothalamus. In the present study, we examined the effect of estrogen on ER-beta mRNA expression in the female rat brain. Detection of ER-beta mRNA using the in situ hybridization method with a digoxigenin-labeled RNA probe was performed in two groups of female rats: ovariectomized (OVX) and estrogen (E2)-treated. A wide distribution of ER-beta mRNA-containing cells was demonstrated in both groups. In the E2-treated group compared with the OVX group, the number of ER-beta mRNA-containing cells was significantly reduced in the external plexiform layer of the olfactory bulb, entorhinal cortex, intermediate part of the lateral septal nucleus, nucleus of the horizontal limb of the diagonal band, amygdala (lateral, medial and basolateral part), thalamus (anteroventral, laterodorsal and lateral posterior part), medial geniculate nucleus, suprachiasmatic nucleus and Purkinje cells in the cerebellum. These results reveal that ER-beta mRNA-containing cells were decreased by estrogen in several brain regions in the female rat brain, suggesting that ER-beta mRNA is downregulated by the physiological level of estrogen in a region-specific manner.

Changes in estrogen receptor-alpha expression in hypothalamic dopaminergic neurons during proestrous prolactin surge

A surge of prolactin (PRL) occurs in female rats during proestrus in response to elevated estradiol and progesterone levels. Dopamine is the primary hypothalamic inhibitor of PRL secretion from the pituitary. Using double-label immunocytochemistry, we investigated the pattern of estrogen receptor-alpha (ER-alpha) immunoreactivity in dopaminergic neurons in the arcuate nucleus (ARC) and the periventricular nucleus (PeVN) during the proestrous PRL surge and compared it to that during diestrus, when PRL levels are constantly low. Our results showed that during diestrus >80% of dopaminergic neurons in the ARC were also positive for ER-alpha, and this colocalization percentage decreased significantly during proestrus. By contrast, <15% of dopaminergic neurons in the PeVN expressed ER-alpha, and the low percentage of ER-alpha expression was unchanged throughout proestrus and diestrus. Results from estrogen plus progesterone treated ovariectomized rats showed similar patterns of ER-alpha expression within the ARC and the PeVN and, once again, compared with the control group, had a significant reduction in ER-alpha immunoreactivity in dopaminergic neurons in the ARC, but not in the PeVN. These results provide an anatomic basis that dopaminergic neurons in the ARC and the PeVN are functionally different regarding to ER-alpha expression. Our study also supports the hypothesis that dopaminergic neurons in the ARC are an important neuronal population responsive to estrogen by changing the expression of ER-alpha in those neurons. This modification of sensitivity of dopaminergic neurons in the ARC in response to ovarian steroids may be an important molecular mechanism involved in PRL regulation, including the regulation of the proestrous surge of PRL.

Sex differences in progesterone receptor expression: a potential mechanism for estradiol-mediated sexual differentiation

The differential exposure of males and females to testosterone (T) and its metabolite estradiol (E) contributes to the development of sex differences in the brain. However, the mechanisms by which T and E permanently alter neural development remain virtually unknown. Two regions of the rat preoptic area, the anteroventral periventricular nucleus (AVPv) and the medial preoptic nucleus (MPN), are sexually dimorphic and serve as models for studying the hormonal mechanisms of sexual differentiation. Around birth, these regions express dramatically higher levels of progesterone receptor immunoreactivity (PRir) in males than they do in females. The present study examined the possibility that sexually dimorphic induction of PR expression in these two regions constitutes a potential mechanism of E-mediated sexual differentiation. Prenatal exposure to either T propionate or the synthetic estrogen, diethylstilbestrol, but not dihydrotestosterone propionate, significantly increased PRir levels in the MPN and AVPv of fetal females compared with controls. Prenatal exposure to the aromatase inhibitor, 1,4,6-androstatriene-3,17-dione, significantly reduced PRir in the MPN and AVPv of fetal males, whereas the androgen receptor antagonist flutamide had no effect. This suggests that aromatization of T into E is crucial for the sex difference in PR expression in the MPN and AVPv during development.

Long-term estrogen therapy worsens the behavioral and neuropathological consequences of chronic brain inflammation

Alzheimer's disease (AD) is accompanied by chronic neuroinflammation and occurs with greater incidence in postmenopausal women. The increased incidence may be delayed by estrogen replacement therapy (ERT). The authors investigated the interaction of chronic ERT and lipopolysaccharide (LPS)-induced neuroinflammation in the female rat. Ovariectomy did not impair water maze performance; however, addition of chronic ERT or neuroinflammation resulted in an impairment that became exacerbated by the simultaneous occurrence of both conditions. Chronic LPS activated microglia, which was not reduced by ERT. Intact females receiving LPS infusion were not impaired in the water maze and had significantly fewer activated microglia. Results suggest that chronic ERT in postmenopausal women may exacerbate the memory impairment induced by the chronic neuroinflammation associated with AD.