Somatostatin-immunoreactive neurones in the hypothalamic ventromedial nucleus possess oestrogen receptors in the male and female rat

Peering into the Brain's Estrogen Receptors: PET Tracers for Visualization of Nuclear and Extranuclear Estrogen Receptors in Brain Disorders

Estrogen receptors (ERs) play a multitude of roles in brain function and are implicated in various brain disorders. The use of positron emission tomography (PET) tracers for the visualization of ERs' intricate landscape has shown promise in oncology but remains limited in the context of brain disorders. Despite recent progress in the identification and development of more selective ligands for various ERs subtypes, further optimization is necessary to enable the reliable and efficient imaging of these receptors. In this perspective, we briefly touch upon the significance of estrogen signaling in the brain and raise the setbacks associated with the development of PET tracers for identification of specific ERs subtypes in the brain. We then propose avenues for developing efficient PET tracers to non-invasively study the dynamics of ERs in the brain, as well as neuropsychiatric diseases associated with their malfunction in a longitudinal manner. This perspective puts several potential candidates on the table and highlights the unmet needs and areas requiring further research to unlock the full potential of PET tracers for ERs imaging, ultimately aiding in deepening our understanding of ERs and forging new avenues for potential therapeutic strategies.

Hypothalamic estrogen receptor alpha establishes a sexually dimorphic regulatory node of energy expenditure

Estrogen receptor a (ERa) signaling in the ventromedial hypothalamus (VMH) contributes to energy homeostasis by modulating physical activity and thermogenesis. However, the precise neuronal populations involved remain undefined. Here, we describe six neuronal populations in the mouse VMH by using single-cell RNA transcriptomics and in situ hybridization. ERa is enriched in populations showing sex biased expression of reprimo (Rprm), tachykinin 1 (Tac1), and prodynorphin (Pdyn). Female biased expression of Tac1 and Rprm is patterned by ERa-dependent repression during male development, whereas male biased expression of Pdyn is maintained by circulating testicular hormone in adulthood. Chemogenetic activation of ERa positive VMH neurons stimulates heat generation and movement in both sexes. However, silencing Rprm gene function increases core temperature selectively in females and ectopic Rprm expression in males is associated with reduced core temperature. Together these findings reveal a role for Rprm in temperature regulation and ERa in the masculinization of neuron populations that underlie energy expenditure.

Diverse actions of estradiol on anorexigenic and orexigenic hypothalamic arcuate neurons

Contribution to Special Issue on Fast effects of steroids. There is now compelling evidence for membrane-associated estrogen receptors in hypothalamic neurons that are critical for the hypothalamic control of homeostatic functions. It has been known for some time that estradiol (E2) can rapidly alter hypothalamic neuronal activity within seconds, indicating that some cellular effects can occur via membrane initiated events. However, our understanding of how E2 signals via membrane-associated receptors and how these signals impact physiological functions is only just emerging. Thus, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell excitability and even gene transcription in hypothalamic neurons. One population of hypothalamic neurons, the anorexigenic proopiomelanocortin (POMC) neurons, has long been considered to be a target of E2's actions based on gene (Pomc) expression studies. However, we now know that E2 can rapidly alter POMC neuronal activity within seconds and activate several intracellular signaling cascades that ultimately affect gene expression, actions which are critical for maintaining sensitivity to insulin in metabolically stressed states. E2 also affects the orexigenic Neuropeptide Y/Agouti-related Peptide (NPY/AgRP) neurons in similarly rapid but antagonistic manner. Therefore, this review will summarize our current state of knowledge of how E2 signals via rapid membrane-initiated and intracellular signaling cascades in POMC and NPY/AgRP neurons to regulate energy homeostasis.

Neuroanatomical connections between kisspeptin neurones and somatostatin neurones in female and male rat hypothalamus: a possible involvement of SSTR1 in kisspeptin release

Somatostatin (SST) a neuropeptide involved in the central modulation of several physiological functions, is co-distributed in the same hypothalamic areas as kisspeptin (KP), the most potent secretagogue of the gonadotropin-releasing hormone (GnRH) secretion known to date. As SST infused intracerebroventricularly (icv) evoked a potent inhibition of GnRH release, we explored neuroanatomical relationships between KP and SST populations in male and female rats. For that, intact males and ovariectomised oestradiol-replaced females were killed and their brains processed in order to simultaneously detect KP, SST and synapsin, a marker for synapses. We observed numerous appositions of KP on SST neurones both in female and male arcuate nucleus (ARC) and ventromedial hypothalamus. A large association between SST terminals and KP neurones at the level of the pre-optic area (POA) was also observed in female rats and in a more limited frame in males. Finally, most KP neurones from the ARC showed SST appositions in both sexes. To determine whether SST could affect KP cell activity, we assessed whether SST receptors (SSTR) were present on KP neurones in the ARC. We also looked for the presence of SSTR1 and SSTR2A in the brain of male rats. Brains were processed through a sequential double immunocytochemistry in order to detect KP and SSTR1 or KP and SSTR2A. We observed overlapping distributions of immunoreactive neurones for SSTR1 and KP and counted approximately one third of KP neurones with SSTR1. In contrast, neurones labelled for SSTR2A or KP were often juxtaposed in the ARC and the occurrence of double-labelled neurones was sporadic (<5%). These results suggest that SST action on KP neurones would pass mainly through SSTR1 at the level of the ARC. This article is protected by copyright. All rights reserved.

Kisspeptin and GnRH pulse generation

The reproductive neuropeptide gonadotropin-releasing hormone (GnRH) has two modes of secretion. Besides the surge mode, which induces ovulation in females, the pulse mode of GnRH release is essential to cause various reproductive events in both sexes, such as spermatogenesis, follicular development, and sex steroid synthesis. Some environmental cues control gonadal activities through modulating GnRH pulse frequency. Researchers have looked for the anatomical location of the mechanism generating GnRH pulses, the GnRH pulse generator, in the brain, because an artificial manipulation of GnRH pulse frequency is of therapeutic importance to stimulate or suppress gonadal activity. Discoveries of kisspeptin and, consequently, KNDy (kisspeptin/neurokinin B/dynorphin) neurons in the hypothalamus have provided a clue to the possible location of the GnRH pulse generator. Our analyses of hypothalamic multiple-unit activity revealed that KNDy neurons located in the hypothalamic arcuate nucleus might play a central role in the generation of GnRH pulses in goats, and perhaps other mammalian species. This chapter further discusses the possible mechanisms for GnRH pulse generation.

Membrane-initiated actions of estradiol that regulate reproduction, energy balance and body temperature

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

Neuroendocrine alterations in the fragile X mouse

The expression of GABA(A) receptors in the fragile X mouse brain is significantly downregulated. We additionally found that the expression of somatostatin and voltage-sensitive calcium channels (VSCCs) is also reduced. GABA(A) and the VSCCs, through a synergistic interaction, perform a critical role in mediating activity-dependent developmental processes. In the developing brain, GABA is excitatory and its actions are mediated through GABA(A) receptors. Subsequent to GABA-mediated depolarization, the VSCCs are activated and intracellular calcium is increased, which mediates gene transcription and other cellular events. GABAergic excitation mediated through GABA(A) receptors and the subsequent activation of the VSCCs are critically important for the establishment of neuronal connectivity within immature neuronal networks. Data from our laboratories suggest that there is a dysregulation of axonal pathfinding during development in the fragile X mouse brain and that this is likely due to a dysregulation of the synergistic interactions of GABA and VSCC. Thus, we hypothesize that the altered expression of these critical channels in the early stages of brain development leads to altered activity-dependent gene expression that may potentially lead to the developmental delay characteristic of the fragile X syndrome.

Sex differences in the neural circuit that mediates female sexual receptivity

Female sexual behavior in rodents, typified by the lordosis posture, is hormone-dependent and sex-specific. Ovarian hormones control this behavior via receptors in the hypothalamic ventromedial nucleus (VMH). This review considers the sex differences in the morphology, neurochemistry and neural circuitry of the VMH to gain insights into the mechanisms that control lordosis. The VMH is larger in males compared with females, due to more synaptic connections. Another sex difference is the responsiveness to estradiol, with males exhibiting muted, and in some cases reverse, effects compared with females. The lack of lordosis in males may be explained by differences in synaptic organization or estrogen responsiveness, or both, in the VMH. However, given that damage to other brain regions unmasks lordosis behavior in males, a male-typical VMH is unlikely the main factor that prevents lordosis. In females, key questions remain regarding the mechanisms whereby ovarian hormones modulate VMH function to promote lordosis.

Somatostatin in the rat periventricular nucleus: sex differences and effect of gonadal steroids

In the rat, the sexual dimorphism in growth hormone release is driven by sex steroids, and is suggested to result mainly from differences in somatostatin (SOM) release patterns from the median eminence. We studied the effect of gonadal steroids on SOM peptide-containing cells in the periventricular nucleus (PeVN) of ovariectomized (OVX) female rats, and compared these data with data from intact male rats. Adult female rats were treated with estradiol (E₂) and/or progesterone (P), 3 months (long-term) or 2 weeks (short-term) after ovariectomy (OVX). Perfusion-fixed brains were sliced and stained, and the number of SOM-immunoreactive (-ir) cells and total SOM-ir area (in μm²) were determined using computer assisted analysis. SOM-ir cells in the PeVN showed a very characteristic rostro-caudal distribution and localization in relation to the third ventricle. Both the number of SOM-ir cells and total SOM-ir area in the PeVN were higher in male compared to OVX female rats. Neither the number of SOM-ir cells, nor the total SOM-ir area in the PeVN was affected by E₂ or P treatment alone. Treatment with both gonadal steroids, however, did increase total SOM-immunoreactivity. This study is the first to describe SOM cell distribution within the rat PeVN in great detail. A clear sex difference exists in SOM peptide content in the rat PeVN. In addition, E₂ and P may act synergistically to affect SOM cells in the female PeVN, suggesting that both gonadal steroids may be involved in the generation of the typical feminine SOM release pattern.

Microarray profiling of gene expression patterns in adult male rat brain following acute progesterone treatment

Progesterone can influence various behaviors in adult male rats, however, little is known about which particular genes are regulated by progesterone in the male rat brain. Using focused microarray technology, we where able to define a subset of genes that are responsive to progesterone. Nylon membrane-based cDNA microarrays were used to profile gene expression patterns in the preoptic area/mediobasal hypothalamus (POA/MBH) of male rat brain 7 h following a single injection of progesterone. RNA was isolated from the brains of 6 male rats injected with progesterone and 6 male rats injected with sesame oil. Next, we hybridized the RNA from each animal to individual cDNA microarrays that contained more than 100 target genes, all of which are involved in cAMP and or calcium signaling pathways. Direct side-by-side comparison of all 12 arrays revealed differences in the expression patterns of 12 different genes. We confirmed the data gathered from the arrays on 4 different genes using Real-Time PCR. These data begin to outline the important role played by progesterone in mediating changes in gene expression within the male brain.

Age-related changes in growth hormone (GH)-releasing hormone and somatostatin neurons in the hypothalamus and in GH cells in the anterior pituitary of female mice

We have observed growth hormone-releasing hormone (GHRH)-immunoreactive (ir) neurons in the arcuate nucleus (ARC), somatostatin (SS)-ir neurons in the periventricular nucleus (PeN), and pituitary growth hormone (GH)-ir cells in female C57BL/6J mice at 2 months old (2 M), 4, 12 and 23 M, using immunocytochemical and morphometric methods. The number of GHRH-ir neurons decreased with age. The number of SS-ir neurons increased from 2 to 4 M, but decreased after 4 M. The volume of the anterior pituitary and the number of adenohypophysial parenchymal cells fell from 12 to 23 M. The proportion of GH-ir cells decreased significantly from 2 to 4 M and decreased in number from 12 to 23 M as well as in size from 2 to 4 M and from 12 to 23 M. Our results show that both GHRH-ir neurons and SS-ir neurons are fewer in old female mice, but the ratio of the number of SS-ir neurons to GHRH-ir neurons increases in old females. We suggest that the fall in the number and size of GH-ir cells in the pituitary gland with age may be involved in the increase in the ratio of the number of SS-ir neurons to GHRH-ir neurons in the hypothalamus in female mice, as well as in males.

Major sex differences in non-genomic estrogen actions on intracellular signaling in mouse brain in vivo

Rapid effects of estrogen have now been identified throughout the brain but the extent to which these actions may be different in males and females is unknown. Previous work has shown that estrogen rapidly phosphorylates Ser133 of cAMP responsive element binding protein (CREB) through a non-genomic mechanism. Using this indicator, we have examined here whether non-genomic estrogen actions occur in a sexually dimorphic manner within the adult brain. Male and female mice were gonadectomized and 3 weeks later treated with 17-beta-estradiol or vehicle for 1 h prior to perfusion fixation and subsequent CREB and phosphorylated CREB (pCREB) immunostaining of brain sections. The numbers of cells expressing CREB immunoreactivity were not altered by estrogen treatment or different in males and females in any of the brain regions examined. However, estrogen treatment significantly (P<0.05) increased pCREB-immunoreactive cell numbers in the medial preoptic area, ventrolateral division of the ventromedial nucleus, medial septum and CA1 region of the hippocampus of female mice. In contrast, estrogen increased pCREB levels in the medial septum and CA1 but not in the preoptic area or ventromedial nucleus of male mice. To evaluate the extent to which non-genomic estrogen actions may be sexually differentiated within a single neuronal phenotype, dual labeling immunocytochemistry was undertaken to evaluate the gonadotropin-releasing hormone (GnRH) neuronal phenotype. Estrogen significantly (P<0.05) increased the numbers of GnRH neurons expressing pCREB in female but not male mice. Together, these results demonstrate the existence of a marked sex difference in estrogen's non-genomic effects upon brain function in vivo.

Defense of body weight depends on dietary composition and palatability in rats with diet-induced obesity

Sprague-Dawley rats selectively bred for diet-induced obesity (DIO) or diet resistance (DR) were characterized on diets of differing energy content and palatability. Over 10 wk, DR rats on a high-energy (HE) diet (31% fat) gained weight similarly to DR rats fed chow (4.5% fat), but they became obese on a palatable liquid diet (Ensure). DIO rats gained 22% more weight on an HE diet and 50% more on Ensure than chow-fed DIO rats. DIO body weight gains plateaued when switched from HE diet to chow. But, Ensure-fed DIO rats switched to chow spontaneously reduced their intake and weight to that of rats switched from HE diet to chow. They also reduced their hypothalamic proopiomelanocortin and dynorphin but not neuropeptide Y mRNA expression by 17-40%. When reexposed to Ensure after 7 wk, they again overate and matched their body weights to rats maintained on Ensure throughout. All Ensure-fed rats had a selective reduction in dynorphin mRNA in the ventromedial hypothalamic nucleus. Thus genetic background, diet composition, and palatability interact to produce disparate levels of defended body weight and central neuropeptide expression.

Disruption of the gene encoding SF-1 alters the distribution of hypothalamic neuronal phenotypes

The ventromedial nucleus of the hypothalamus (VMH) in mice first emerges as a histologically distinct cell cluster around embryonic day 17 (E17). The earliest known marker for cells destined to form the VMH is the orphan nuclear receptor, steroidogenic factor 1 (SF-1), which can be detected in the hypothalamic primordium by E11. Strikingly, the VMH is absent in newborn SF-1 knockout mice, suggesting that SF-1 is essential for the development of VMH neurons. We reported previously that the VMH can be identified before it emerges as a histologically distinct nucleus (i.e., at E13) by the exclusion of cells that are immunoreactive for both gamma-aminobutyric acid (GABA) and the synthetic enzyme, glutamic acid decarboxylase (GAD67). Subsequently, by E15, the developing VMH is demarcated further by cells that are immunoreactive for neuropeptide Y, estrogen receptor alpha (ERalpha), and galanin. It is noteworthy that the normal exclusion of GABA from the developing VMH is not seen in SF-1 knockout mice, and cells that are immunoreactive for neuropeptide Y, ERalpha, and galanin also are distributed aberrantly in this region. Thus, the absence of SF-1 profoundly affects the cellular architecture of the VMH from early stages in its formation. These data suggest that, directly or indirectly, SF-1 plays important roles in determining the distribution of cells in the mediobasal hypothalamus.

Special relationship of gamma-aminobutyric acid to the ventromedial nucleus of the hypothalamus during embryonic development

The ventromedial nucleus of the hypothalamus (VMH) is a key nucleus for regulating homeostatic, neuroendocrine, and behavioral functions. We conducted immunocytochemical analyses by using antisera directed against gamma-aminobutyric acid (GABA), its synthetic enzyme glutamic acid decarboxylase (GAD67), GABA-A receptor subunits (alpha2, beta3, epsilon), estrogen receptor-alpha, and Neuropeptide Y (NPY) in the region of the VMH in embryonic mice to identify potential patterning elements for VMH formation. Cells and fibers containing GABA and GAD67 encircled the primordial VMH as early as embryonic day 13 (E13) when the cytoarchitecture of the VMH was not recognizable by Nissl stain. At E16-17 the cytoarchitecture of the VMH became recognizable by Nissl stain as GABAergic fibers invaded the nucleus, continued postnatally, and by adulthood the density of GABAergic fibers was greater inside than outside the VMH. GABA-A receptor subunit expression (beta3 by E13 and alpha2 by E15) within the primordial VMH suggested potential sensitivity to the surrounding GABA signal. Brain slices were used to test whether fibers from distal or proximal sites influenced VMH development. Coronal Vibratome slices were prepared and maintained in vitro for 0-3 days. Nissl stain analyses showed a uniform distribution of cells in the region of the VMH on the day of plating (E15). After 3 days in vitro, cellular aggregation suggesting VMH formation was seen. Nuclear formation in vitro suggests that key factors resided locally within the coronal plane of the slices. It is suggested that either GABA intrinsic to the region nearby the VMH directly influences the development and organization of the VMH, or along with other markers provides an early indicator of pattern determination that precedes the cellular organization of the VMH.

Correlation of hypothalamic somatostatin mRNA expression and peptide content with secretion: sexual dimorphism and differential regulation by gonadal factors

Sex differences in growth hormone (GH) secretion in the rat are thought to be determined, to a large extent, by gonadal steroid-dependent sex differences in somatostatin (SRIH) secretion from neurones in the periventricular nucleus (PeN) which project to the median eminence (ME). The present study aimed to obtain direct evidence for sex differences and gonadal regulation of SRIH release within this pathway and to determine the relationships between SRIH mRNA expression, SRIH peptide content and SRIH secretion in the adult rat. Somatostatin mRNA expression in the PeN and peptide content in both PeN and ME were higher in males than females (P<0.05). However, both basal and 56 mM K+-stimulated SRIH release in vitro from hypothalamic explants incorporating the PeN-ME pathway were higher (P<0.01) in females. The gonadectomy of female rats resulted in significantly reduced basal levels of SRIH release equivalent to that of males but had no effect on SRIH mRNA/peptide content or K+-stimulated release. In contrast, gonadectomy of male rats reduced SRIH mRNA and peptide contents and elevated K+-stimulated secretion (P<0.01) to levels similar to that seen in intact females, without affecting basal release. In summary, these results demonstrate that in the PeN-ME of the adult rat: (1) SRIH mRNA and peptide content is well correlated and sexually dimorphic but dependent on gonadal factors in the male only; (2) SRIH secretion is sexually dimorphic and dependent on gonadal factors; but (3) differences in mRNA/peptide content do not reflect secretory capacity; and (4) gonadal factors differentially modulate SRIH secretory dynamics in males and females.

Ontogeny and sexual differentiation of somatostatin biosynthesis and secretion in the hypothalamic periventricular-median eminence pathway

The biosynthesis of somatostatin (SRIH) in the hypothalamic periventricular nucleus (PeN) is sexually differentiated in neonatal and adult rats by virtue of the organizational and activational actions, respectively, of sex steroid hormones. Little information exists, however, on the normal pattern of maturation of these neurones or on how the sexually differentiated biosynthesis may relate to ontogenetic changes in somatostatin secretion during the neonatal and pubertal periods of development. Hence in the present study we determined the postnatal developmental profile of SRIH mRNA and peptide levels in the PeN-median eminence (ME) pathway as well as SRIH secretion, using an acute explant preparation, from the day of birth, through puberty and into adulthood in male and female rats. The results demonstrate that: (1) developmental sex differences in SRIH biosynthesis in PeN neurones occurred in an orderly cascade with differences observed for mRNA expression at postnatal day 5, for peptide content in the perikarya at postnatal day 10 and for peptide content in the nerve terminal (ME) by postnatal day 25; (2) sex differences in SRIH release were not evident prior to postnatal day 40; and (3) the developmental profile of SRIH biosynthesis in PeN neurones is unique compared with other hypothalamic (ventromedial nucleus) and extrahypothalamic (parietal cortex) populations. Specific developmental changes in the biosynthetic and secretory activity of the hypothalamic SRIH PeN-ME pathway may have a functional importance in the maturation of hypothalamic SRIH pathways involved in the regulation of GH secretion.

Localization of the preprosomatostatin-mRNA by in situ hybridization in the ewe hypothalamus

The peptidergic neurohormone somatostatin (SRIF) derives from a precursor called preprosomatostatin (PPS) by proteolysis. We have isolated by RT-PCR and sequenced a partial cDNA coding for the ovine PPS. It contains a 348 base pairs coding sequence that shares strong similarities with previously cloned mammalian cDNAs. The ovine cDNA was used to synthesize radiolabeled cRNA to probe the PPS mRNA in the ewe hypothalamus by in situ hybridization. The PPS mRNA-containing cells are widely distributed in the hypothalamus. According to the number of silver grains over a cell, they show various staining intensities. The distribution of the PPS mRNA is in good agreement with that of the peptide previously determined using immunohistochemistry. The strongest labeled areas include the periventricular region of the paraventricular nucleus and the lateral division of the ventromedial nucleus. The difference in labeling intensity observed in the diverse populations of labeled neurons could reflect various levels of neuronal activity.

Sexually dimorphic expression of androgen receptor immunoreactivity by somatostatin neurones in rat hypothalamic periventricular nucleus and bed nucleus of the stria terminalis

Gonadal steroids exert important regulatory actions on the hypothalamic neurones regulating growth hormone secretion and are believed to play a role in generating its sexually dimorphic pattern of secretion. Recent evidence indicates that estrogen actions on one of these neural populations, the periventricular somatostatin (SOM) neurones, are likely to be indirect as they do not possess nuclear estrogen receptors in either sex although androgen receptors (ARs) have been reported within these cells in male rats. The present study has used double-labelling immunocytochemistry procedures to examine whether sex differences exist in AR expression by SOM neurones located in the periventricular nucleus and bed nucleus of the stria terminalis (BNST). Within the hypothalamus, SOM-immunoreactive neurones were found concentrated in the periventricular nucleus while both anterior and posterior divisions of the BNST contained scattered populations of SOM cells. Cells immunoreactive for the AR were detected in all of these areas. Although the intensity of AR cell nuclei staining was equivalent in males and females in regions such as the lateral septum, the intensity of AR staining in many individual cells of the periventricular nucleus and posterior BNST of the female was reduced when compared with the male. Double-labelling experiments revealed that approximately 40% of periventricular SOM neurones expressed AR immunoreactivity in the male compared with significantly (P < 0.01) fewer cells in the female (approximately 7%). In the BNST, double-labelled cells were only detected within the principle encapsulated, interfascicular and transverse nuclei of its posterior division. Approximately 60% of SOM cells in these nuclei expressed AR immunoreactivity in the male while significantly (P < 0.01) fewer did so in the female (approximately 25%). These results indicate that substantial sex differences exist in AR expression by SOM neurones in both the periventricular nucleus and BNST. Such differences in AR expression by periventricular SOM cells may contribute to their sexuality dimorphic nature and, consequently, sex differences in growth hormone secretion.