Puberty enables oestradiol-induced progesterone synthesis in female mouse hypothalamic astrocytes

The development of oestrogen positive feedback is a hallmark of female puberty. Both oestrogen and progesterone signalling are required for the functioning of this neuroendocrine feedback loop but the physiological changes that underlie the emergence of positive feedback remain unknown. Only after puberty does oestradiol (E2) facilitate progesterone synthesis in the rat female hypothalamus (neuroP), an event critical for positive feedback and the LH surge. We hypothesize that prior to puberty, these astrocytes have low levels of membrane oestrogen receptor alpha (ERα), which is needed for facilitation of neuroP synthesis. Thus, we hypothesized that prepubertal astrocytes are unable to respond to E2 with increased neuroP synthesis due a lack of membrane ERα. To test this, hypothalamic tissues and enriched primary hypothalamic astrocyte cultures were acquired from prepubertal (postnatal week 3) and post-pubertal (week 8) female mice. E2-facilitated neuroP was measured in the hypothalamus pre- and post-puberty, and hypothalamic astrocyte responses were measured after treatment with E2. Prior to puberty, E2-facilitated neuroP synthesis did not occur in the hypothalamus, and mERα expression was low in hypothalamic astrocytes, but E2-facilitated neuroP synthesis in the rostral hypothalamus and mERα expression increased post-puberty. The increase in mERα expression in hypothalamic astrocytes corresponded with a post-pubertal increase in caveolin-1 protein, PKA phosphorylation, and a more rapid [Ca²⁺ ]_i flux in response to E2. Together, results from the present study indicate that E2-facilitated neuroP synthesis occurs in the rostral hypothalamus, develops during puberty, and corresponds to a post-pubertal increase in mERα levels in hypothalamic astrocytes.

Progesterone receptor-Src kinase signaling pathway mediates neuroprogesterone induction of the luteinizing hormone surge in female rats

Neural circuits in female rats are exposed to sequential estradiol and progesterone to regulate the release of luteinizing hormone (LH) and ultimately ovulation. Estradiol induces progesterone receptors (PGRs) in anteroventral periventricular nucleus (AVPV) kisspeptin neurons, and as estradiol reaches peak concentrations, neuroprogesterone (neuroP) synthesis is induced in hypothalamic astrocytes. This local neuroP signals to PGRs expressed in kisspeptin neurons to trigger the LH surge. We tested the hypothesis that neuroP-PGR signaling through Src family kinase (Src) underlies the LH surge. As observed in vitro, PGR and Src are co-expressed in AVPV neurons. Estradiol treatment increased the number of PGR immunopositive cells and PGR and Src colocalization. Furthermore, estradiol treatment increased the number of AVPV cells that had extranuclear PGR and Src in close proximity (< 40 nm). Infusion of the Src inhibitor (PP2) into the AVPV region of ovariectomized/adrenalectomized (ovx/adx) rats attenuated the LH surge in trunk blood collected 53 h post-estradiol (50 µg) injection that induced neuroP synthesis. Although PP2 reduced the LH surge in estradiol benzoate treated ovx/adx rats, activation of either AVPV PGR or Src in 2 µg estradiol-primed animals significantly elevated LH concentrations compared to dimethyl sulfoxide infused rats. Finally, antagonism of either AVPV PGR or Src blocked the ability of PGR or Src activation to induce an LH surge in estradiol-primed ovx/adx rats. These results indicate that neuroP, which triggers the LH surge, signals through an extranuclear PGR-Src signaling pathway.

Membrane estrogen signaling in female reproduction and motivation

Estrogen receptors were initially identified in the uterus, and later throughout the brain and body as intracellular, ligand-regulated transcription factors that affect genomic change upon ligand binding. However, rapid estrogen receptor signaling initiated outside of the nucleus was also known to occur via mechanisms that were less clear. Recent studies indicate that these traditional receptors, estrogen receptor-α and estrogen receptor-β, can also be trafficked to act at the surface membrane. Signaling cascades from these membrane-bound estrogen receptors (mERs) not only rapidly effect cellular excitability, but can and do ultimately affect gene expression, as seen through the phosphorylation of CREB. A principal mechanism of neuronal mER action is through glutamate-independent transactivation of metabotropic glutamate receptors (mGluRs), which elicits multiple signaling outcomes. The interaction of mERs with mGluRs has been shown to be important in many diverse functions in females, including, but not limited to, reproduction and motivation. Here we review membrane-initiated estrogen receptor signaling in females, with a focus on the interactions between these mERs and mGluRs.

Progesterone Receptors in AVPV Kisspeptin Neurons Are Sufficient for Positive Feedback Induction of the LH Surge

Kisspeptin, encoded by Kiss1, stimulates gonadotropin-releasing hormone neurons to govern reproduction. In female rodents, estrogen-sensitive kisspeptin neurons in the rostral anteroventral periventricular (AVPV) hypothalamus are thought to mediate estradiol (E2)-induced positive feedback induction of the preovulatory luteinizing hormone (LH) surge. AVPV kisspeptin neurons coexpress estrogen and progesterone receptors (PGRs) and are activated during the LH surge. While E2 effects on kisspeptin neurons have been well studied, progesterone's regulation of kisspeptin neurons is less understood. Using transgenic mice lacking PGR exclusively in kisspeptin cells (termed KissPRKOs), we previously demonstrated that progesterone action specifically in kisspeptin cells is essential for ovulation and normal fertility. Unlike control females, KissPRKO females did not generate proper LH surges, indicating that PGR signaling in kisspeptin cells is required for positive feedback. However, because PGR was knocked out from all kisspeptin neurons in the brain, that study was unable to determine the specific kisspeptin population mediating PGR action on the LH surge. Here, we used targeted Cre-mediated adeno-associated virus (AAV) technology to reintroduce PGR selectively into AVPV kisspeptin neurons of adult KissPRKO females, and tested whether this rescues occurrence of the LH surge. We found that targeted upregulation of PGR in kisspeptin neurons exclusively in the AVPV is sufficient to restore proper E2-induced LH surges in KissPRKO females, suggesting that this specific kisspeptin population is a key target of the necessary progesterone action for the surge. These findings further highlight the critical importance of progesterone signaling, along with E2 signaling, in the positive feedback induction of LH surges and ovulation.

RNA-sequencing of AVPV and ARH reveals vastly different temporal and transcriptomic responses to estradiol in the female rat hypothalamus

In females, estrogens have two main modes of action relating to gonadotropin secretion: positive feedback and negative feedback. Estrogen positive and negative feedback are controlled by different regions of the hypothalamus: the preoptic area/anterior portion (mainly the anteroventral periventricular nucleus, AVPV) of the hypothalamus is associated with estrogen positive feedback while the mediobasal hypothalamus (mainly the arcuate nucleus of the hypothalamus, ARH), is associated with estrogen negative feedback. In this study, we examined the temporal pattern of gene transcription in these two regions following estrogen treatment. Adult, ovariectomized, Long Evans rats received doses of estradiol benzoate (EB) or oil every 4 days for 3 cycles. On the last EB priming cycle, hypothalamic tissues were dissected into the AVPV+ and ARH+ at 0 hrs (baseline/oil control), 6 hrs, or 24 hrs after EB treatment. RNA was extracted and sequenced using bulk RNA sequencing. Differential gene analysis, gene ontology, and weighted correlation network analysis (WGCNA) was performed. Overall, we found that the AVPV+ and ARH+ respond differently to estradiol stimulation. In both regions, estradiol treatment resulted in more gene up-regulation than down-regulation. S100g was very strongly up-regulated by estradiol in both regions at 6 and 24 hrs after EB treatment. In the AVPV+ the highest number of differentially expressed genes occurred 24 hrs after EB. In the ARH+, the highest number of genes differentially expressed by EB occurred between 6 and 24 hrs after EB, while in the AVPV+, the fewest genes changed their expression between these time points, demonstrating a temporal difference in the way that EB regulates transcription these two areas. Several genes strongly implicated in gonadotropin release were differentially affected by estradiol including Esr1, encoding estrogen receptor-α and Kiss1, encoding kisspeptin. As an internal validation, Kiss1 was up-regulated in the AVPV+ and down-regulated in the ARH+. Gene network analysis revealed the vastly different clustering of genes modulated by estradiol in the AVPV+ compared with the ARH+. These results indicate that gene expression in these two hypothalamic regions have specific responses to estradiol in timing and direction.

Consensus Parameter: Research Methodologies to Evaluate Neurodevelopmental Effects of Pubertal Suppression in Transgender Youth

Purpose: Pubertal suppression is standard of care for early pubertal transgender youth to prevent the development of undesired and distressing secondary sex characteristics incongruent with gender identity. Preliminary evidence suggests pubertal suppression improves mental health functioning. Given the widespread changes in brain and cognition that occur during puberty, a critical question is whether this treatment impacts neurodevelopment.

Methods: A Delphi consensus procedure engaged 24 international experts in neurodevelopment, gender development, puberty/adolescence, neuroendocrinology, and statistics/psychometrics to identify priority research methodologies to address the empirical question: is pubertal suppression treatment associated with real-world neurocognitive sequelae? Recommended study approaches reaching 80% consensus were included in the consensus parameter.

Results: The Delphi procedure identified 160 initial expert recommendations, 44 of which ultimately achieved consensus. Consensus study design elements include the following: a minimum of three measurement time points, pubertal staging at baseline, statistical modeling of sex in analyses, use of analytic approaches that account for heterogeneity, and use of multiple comparison groups to minimize the limitations of any one group. Consensus study comparison groups include untreated transgender youth matched on pubertal stage, cisgender (i.e., gender congruent) youth matched on pubertal stage, and an independent sample from a large-scale youth development database. The consensus domains for assessment includes: mental health, executive function/cognitive control, and social awareness/functioning.

Conclusion: An international interdisciplinary team of experts achieved consensus around primary methods and domains for assessing neurodevelopmental effects (i.e., benefits and/or difficulties) of pubertal suppression treatment in transgender youth.

Hypothalamic Astrocyte Development and Physiology for Neuroprogesterone Induction of the Luteinizing Hormone Surge

Neural circuits in female rats sequentially exposed to estradiol and progesterone underlie so-called estrogen positive feedback that induce the surge release of pituitary luteinizing hormone (LH) leading to ovulation and luteinization of the corpus hemorrhagicum. It is now well-established that gonadotropin releasing hormone (GnRH) neurons express neither the reproductively critical estrogen receptor-α (ERα) nor classical progesterone receptor (PGR). Estradiol from developing ovarian follicles acts on ERα-expressing kisspeptin neurons in the rostral periventricular region of the third ventricle (RP3V) to induce PGR expression, and kisspeptin release. Circulating estradiol levels that induce positive feedback also induce neuroprogesterone (neuroP) synthesis in hypothalamic astrocytes. This local neuroP acts on kisspeptin neurons that express PGR to augment kisspeptin expression and release needed to stimulate GnRH release, triggering the LH surge. In vitro and in vivo studies demonstrate that neuroP signaling in kisspeptin neurons occurs through membrane PGR activation of Src family kinase (Src). This signaling cascade has been also implicated in PGR signaling in the arcuate nucleus of the hypothalamus, suggesting that Src may be a common mode of membrane PGR signaling. Sexual maturation requires that signaling between neuroP synthesizing astrocytes, kisspeptin and GnRH neurons be established. Prior to puberty, estradiol does not facilitate the synthesis of neuroP in hypothalamic astrocytes. During pubertal development, levels of membrane ERα increase in astrocytes coincident with an increase of PKA phosphorylation needed for neuroP synthesis. Currently, it is not clear whether these developmental changes occur in existing astrocytes or are due to a new population of astrocytes born during puberty. However, strong evidence suggests that it is the former. Blocking new cell addition during puberty attenuates the LH surge. Together these results demonstrate the importance of pubertal maturation involving hypothalamic astrocytes, estradiol-induced neuroP synthesis and membrane-initiated progesterone signaling for the CNS control of ovulation and reproduction.

Immunohistochemical amplification of mCherry fusion protein is necessary for proper visualization

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Virally expressed fluorescent reporter mCherry fusion protein does not adequately demonstrate its presence in fixed tissue.

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Immunohistochemical amplification of mCherry can rescue this deficit.

Fluorescent reporter proteins are a powerful tool being increasingly integrated into biological experiments. Their utility spans techniques such as live-cell imaging, validating transgene expression, and studying cell-type specific anatomy. As these reporters become more widely used, it is necessary to fully understand their benefits and limitations. One such recently developed red fluorescent protein, mCherry, has been well utilized due to its stability, brightness, and pH resistance. In the course of an experiment using the fluorescent reporter protein mCherry fused to a G-protein coupled receptor (mCherry fusion protein), our lab discovered a notable inability for the fusion protein to faithfully produce fluorescent signal representative of its expression in fixed tissue. Here, we demonstrate the importance of immunohistochemical amplification in tissue injected with various adeno-associated viruses (AAVs), containing mCherry fusion protein as a reporter. Our findings demonstrate that antibody amplification consistently provides a stronger signal when mCherry fusion protein is used as a reporter protein.

ERαΔ4, an ERα splice variant missing exon4, interacts with caveolin-3 and mGluR2/3

The two isoforms of the nuclear estrogen receptor, ERα and ERβ are widely expressed in the central nervous system. Although they were first described as nuclear receptors, both isoforms have also been found at the cell membrane where they mediate cell signaling. Surface biotinylation studies using neuronal and glial primary cultures label an alternatively spliced form of ERα. The 52 kDa protein, ERαΔ4, is missing exon 4 and is highly expressed in membrane fractions derived from cultured cells. In vivo, both full-length (66 kDa) ERα and ERαΔ4 are present in membrane fractions. In response to estradiol, full-length ERα and ERαΔ4 are initially trafficked to the membrane, and then internalized in parallel. Previous studies determined that only the full-length ERα associates with metabotropic glutamate receptor-1a (mGluR1a), initiating cellular signaling. The role of ERαΔ4, remained to be elucidated. Here, we report ERαΔ4 trafficking, association with mGluR2/3, and downstream signaling in female rat arcuate nucleus (ARH). Caveolin (CAV) proteins are needed for ER transport to the cell membrane, and using co-immunoprecipitation CAV-3 was shown to associate with ERαΔ4. CAV-3 was necessary for ERαΔ4 trafficking to the membrane: in the ARH, microinjection of CAV-3 siRNA reduced CAV-3 and ERαΔ4a in membrane fractions by 50%, and 60%, respectively. Moreover, co-immunoprecipitation revealed that ERαΔ4 associated with inhibitory mGluRs, mGluR2/3. Estrogen benzoate (EB) treatment (5 μg; s.c.; every 4 days; three cycles) reduced levels of cAMP, an effect attenuated by antagonizing mGluR2/3. Following EB treatment, membrane levels of ERαΔ4 and mGluR2/3 were reduced implying ligand-induced internalization. These results implicate ERαΔ4 in an estradiol-induced inhibitory cell signaling in the ARH.

Pubertal development of estradiol-induced hypothalamic progesterone synthesis

In females, a hallmark of puberty is the luteinizing hormone (LH) surge that triggers ovulation. Puberty initiates estrogen positive feedback onto hypothalamic circuits, which underlie the stimulation of gonadotropin releasing hormone (GnRH) neurons. In reproductively mature female rodents, both estradiol (E2) and progesterone (P4) signaling are necessary to stimulate the surge release of GnRH and LH. Estradiol membrane-initiated signaling facilitates progesterone (neuroP) synthesis in hypothalamic astrocytes, which act on E2-induced progesterone receptors (PGR) to stimulate kisspeptin release, thereby activating GnRH release. How the brain changes during puberty to allow estrogen positive feedback remains unknown. In the current study, we hypothesized that a critical step in estrogen positive feedback was the ability for estradiol-induced neuroP synthesis. To test this idea, hypothalamic neuroP levels were measured in groups of prepubertal, pubertal and young adult female Long Evans rats. Steroids were measured with liquid chromatography tandem mass spectrometry (LC-MS/MS). Hypothalamic neuroP increases from pre-puberty to young adulthood in both gonad-intact females and ovariectomized rats treated with E2. The pubertal development of hypothalamic E2-facilitated progesterone synthesis appears to be one of the neural switches facilitating reproductive maturation.

Estradiol Membrane-Initiated Signaling in the Brain Mediates Reproduction

Over the past few years our understanding of estrogen signaling in the brain has expanded rapidly. Estrogens are synthesized in the periphery and in the brain, acting on multiple receptors to regulate gene transcription, neural function, and behavior. Various estrogen-sensitive signaling pathways often operate in concert within the same cell, increasing the complexity of the system. In females, estrogen concentrations fluctuate over the estrous/menstrual cycle, dynamically modulating estrogen receptor (ER) expression, activity, and trafficking. These dynamic changes influence multiple behaviors but are particularly important for reproduction. Using the female rodent model, we review our current understanding of estradiol signaling in the regulation of sexual receptivity.

Integrating Neural Circuits Controlling Female Sexual Behavior

The hypothalamus is most often associated with innate behaviors such as is hunger, thirst and sex. While the expression of these behaviors important for survival of the individual or the species is nested within the hypothalamus, the desire (i.e., motivation) for them is centered within the mesolimbic reward circuitry. In this review, we will use female sexual behavior as a model to examine the interaction of these circuits. We will examine the evidence for a hypothalamic circuit that regulates consummatory aspects of reproductive behavior, i.e., lordosis behavior, a measure of sexual receptivity that involves estradiol membrane-initiated signaling in the arcuate nucleus (ARH), activating β-endorphin projections to the medial preoptic nucleus (MPN), which in turn modulate ventromedial hypothalamic nucleus (VMH) activity-the common output from the hypothalamus. Estradiol modulates not only a series of neuropeptides, transmitters and receptors but induces dendritic spines that are for estrogenic induction of lordosis behavior. Simultaneously, in the nucleus accumbens of the mesolimbic system, the mating experience produces long term changes in dopamine signaling and structure. Sexual experience sensitizes the response of nucleus accumbens neurons to dopamine signaling through the induction of a long lasting early immediate gene. While estrogen alone increases spines in the ARH, sexual experience increases dendritic spine density in the nucleus accumbens. These two circuits appear to converge onto the medial preoptic area where there is a reciprocal influence of motivational circuits on consummatory behavior and vice versa. While it has not been formally demonstrated in the human, such circuitry is generally highly conserved and thus, understanding the anatomy, neurochemistry and physiology can provide useful insight into the motivation for sexual behavior and other innate behaviors in humans.

Estrogen and Progesterone Integration in an in vitro Model of RP3V Kisspeptin Neurons

Positive feedback on gonadotropin release requires not only estrogen but also progesterone to activate neural circuits. In rodents, ovarian estradiol (E2) stimulates progesterone synthesis in hypothalamic astrocytes (neuroP), needed for the luteinizing hormone (LH) surge. Kisspeptin (kiss) neurons are the principal stimulators of gonadotropin-releasing hormone neurons, and disruption of kiss signaling abrogates the LH surge. Similarly, blocking steroid synthesis in the hypothalamus or deleting classical progesterone receptor (PGR) selectively in kiss neurons prevents the LH surge. These results suggest a synergistic action of E2 and progesterone in kiss neurons to affect gonadotropin release. The mHypoA51, immortalized kiss-expressing neuronal cell line derived from adult female mice, is a tractable model for examining integration of steroid signaling underlying estrogen positive feedback. Here, we report that kiss neurons in vitro integrate E2 and progesterone signaling to increase levels of kiss translation and release. mHypoA51 neurons expressed nonclassical membrane progesterone receptors (mPRα and mPRβ) and E2-inducible PGR, required for progesterone-augmentation of E2-induced kiss expression. With astrocyte-conditioned media or in mHypoA51-astrocyte co-culture, neuroP augmented stimulatory effects of E2 on kiss protein. Progesterone activation of classical, membrane-localized PGR led to activation of MAPK and Src kinases. Importantly, progesterone or Src activation induced release of kiss from E2-primed mHypoA51 neurons. Consistent with previous studies, the present results provide compelling evidence that the interaction of E2 and progesterone stimulates kiss expression and release. Further, these results demonstrate a mechanism though which peripheral E2 may prime kiss neurons to respond to neuroP, mediating estrogen positive feedback.

Rodent Models of Non-classical Progesterone Action Regulating Ovulation

It is becoming clear that steroid hormones act not only by binding to nuclear receptors that associate with specific response elements in the nucleus but also by binding to receptors on the cell membrane. In this newly discovered manner, steroid hormones can initiate intracellular signaling cascades which elicit rapid effects such as release of internal calcium stores and activation of kinases. We have learned much about the translocation and signaling of steroid hormone receptors from investigations into estrogen receptor α, which can be trafficked to, and signal from, the cell membrane. It is now clear that progesterone (P4) can also elicit effects that cannot be exclusively explained by transcriptional changes. Similar to E2 and its receptors, P4 can initiate signaling at the cell membrane, both through progesterone receptor and via a host of newly discovered membrane receptors (e.g., membrane progesterone receptors, progesterone receptor membrane components). This review discusses the parallels between neurotransmitter-like E2 action and the more recently investigated non-classical P4 signaling, in the context of reproductive behaviors in the rodent.

Peripheral and Central Mechanisms Involved in the Hormonal Control of Male and Female Reproduction

Reproduction involves the integration of hormonal signals acting across multiple systems to generate a synchronised physiological output. A critical component of reproduction is the luteinising hormone (LH) surge, which is mediated by oestradiol (E2 ) and neuroprogesterone interacting to stimulate kisspeptin release in the rostral periventricular nucleus of the third ventricle in rats. Recent evidence indicates the involvement of both classical and membrane E2 and progesterone signalling in this pathway. A metabolite of gonadotrophin-releasing hormone (GnRH), GnRH-(1-5), has been shown to stimulate GnRH expression and secretion, and has a role in the regulation of lordosis. Additionally, gonadotrophin release-inhibitory hormone (GnIH) projects to and influences the activity of GnRH neurones in birds. Stress-induced changes in GnIH have been shown to alter breeding behaviour in birds, demonstrating another mechanism for the molecular control of reproduction. Peripherally, paracrine and autocrine actions within the gonad have been suggested as therapeutic targets for infertility in both males and females. Dysfunction of testicular prostaglandin synthesis is a possible cause of idiopathic male infertility. Indeed, local production of melatonin and corticotrophin-releasing hormone could influence spermatogenesis via immune pathways in the gonad. In females, vascular endothelial growth factor A has been implicated in an angiogenic process that mediates development of the corpus luteum and thus fertility via the Notch signalling pathway. Age-induced decreases in fertility involve ovarian kisspeptin and its regulation of ovarian sympathetic innervation. Finally, morphological changes in the arcuate nucleus of the hypothalamus influence female sexual receptivity in rats. The processes mediating these morphological changes have been shown to involve the rapid effects of E2 controlling synaptogenesis in this hypothalamic nucleus. In summary, this review highlights new research in these areas, focusing on recent findings concerning the molecular mechanisms involved in the central and peripheral hormonal control of reproduction.

Estradiol Membrane-Initiated Signaling and Female Reproduction

The discoveries of rapid, membrane-initiated steroid actions and central nervous system steroidogenesis have changed our understanding of the neuroendocrinology of reproduction. Classical nuclear actions of estradiol and progesterone steroids affecting transcription are essential. However, with the discoveries of membrane-associated steroid receptors, it is becoming clear that estradiol and progesterone have neurotransmitter-like actions activating intracellular events. Ultimately, membrane-initiated actions can influence transcription. Estradiol membrane-initiated signaling (EMS) modulates female sexual receptivity and estrogen feedback regulating the luteinizing hormone (LH) surge. In the arcuate nucleus, EMS activates a lordosis-regulating circuit that extends to the medial preoptic nucleus and subsequently to the ventromedial nucleus (VMH)--the output from the limbic and hypothalamic regions. Here, we discuss how EMS leads to an active inhibition of lordosis behavior. To stimulate ovulation, EMS facilitates astrocyte synthesis of progesterone (neuroP) in the hypothalamus. Regulation of GnRH release driving the LH surge is dependent on estradiol-sensitive kisspeptin (Kiss1) expression in the rostral periventricular nucleus of the third ventricle (RP3V). NeuroP activation of the LH surge depends on Kiss1, but the specifics of signaling have not been well elucidated. RP3V Kiss1 neurons appear to integrate estradiol and progesterone information which feeds back onto GnRH neurons to stimulate the LH surge. In a second population of Kiss1 neurons, estradiol suppresses the surge but maintains tonic LH release, another critical component of the estrous cycle. Together, evidence suggests that regulation of reproduction involves membrane action of steroids, some of which are synthesized in the brain.

Classical and membrane-initiated estrogen signaling in an in vitro model of anterior hypothalamic kisspeptin neurons

The neuropeptide kisspeptin is essential for sexual maturation and reproductive function. In particular, kisspeptin-expressing neurons in the anterior rostral periventricular area of the third ventricle are generally recognized as mediators of estrogen positive feedback for the surge release of LH, which stimulates ovulation. Estradiol induces kisspeptin expression in the neurons of the rostral periventricular area of the third ventricle but suppresses kisspeptin expression in neurons of the arcuate nucleus that regulate estrogen-negative feedback. To focus on the intracellular signaling and response to estradiol underlying positive feedback, we used mHypoA51 cells, an immortalized line of kisspeptin neurons derived from adult female mouse hypothalamus. mHypoA51 neurons express estrogen receptor (ER)-α, classical progesterone receptor (PR), and kisspeptin, all key elements of estrogen-positive feedback. As with kisspeptin neurons in vivo, 17β-estradiol (E2) induced kisspeptin and PR in mHypoA51s. The ERα agonist, 1,3,5-Tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole, produced similar increases in expression, indicating that these events were mediated by ERα. However, E2-induced PR up-regulation required an intracellular ER, whereas kisspeptin expression was stimulated through a membrane ER activated by E2 coupled to BSA. These data suggest that anterior hypothalamic kisspeptin neurons integrate both membrane-initiated and classical nuclear estrogen signaling to up-regulate kisspeptin and PR, which are essential for the LH surge.

β-arrestin regulates estradiol membrane-initiated signaling in hypothalamic neurons

Estradiol (E2) action in the nervous system is the result of both direct nuclear and membrane-initiated signaling (EMS). E2 regulates membrane estrogen receptor-α (ERα) levels through opposing mechanisms of EMS-mediated trafficking and internalization. While ß-arrestin-mediated mERα internalization has been described in the cortex, a role of ß-arrestin in EMS, which underlies multiple physiological processes, remains undefined. In the arcuate nucleus of the hypothalamus (ARH), membrane-initiated E2 signaling modulates lordosis behavior, a measure of female sexually receptivity. To better understand EMS and regulation of ERα membrane levels, we examined the role of ß-arrestin, a molecule associated with internalization following agonist stimulation. In the present study, we used an immortalized neuronal cell line derived from embryonic hypothalamic neurons, the N-38 line, to examine whether ß-arrestins mediate internalization of mERα. β-arrestin-1 (Arrb1) was found in the ARH and in N-38 neurons. In vitro, E2 increased trafficking and internalization of full-length ERα and ERαΔ4, an alternatively spliced isoform of ERα, which predominates in the membrane. Treatment with E2 also increased phosphorylation of extracellular-signal regulated kinases 1/2 (ERK1/2) in N-38 neurons. Arrb1 siRNA knockdown prevented E2-induced ERαΔ4 internalization and ERK1/2 phosphorylation. In vivo, microinfusions of Arrb1 antisense oligodeoxynucleotides (ODN) into female rat ARH knocked down Arrb1 and prevented estradiol benzoate-induced lordosis behavior compared with nonsense scrambled ODN (lordosis quotient: 3 ± 2.1 vs. 85.0 ± 6.0; p < 0.0001). These results indicate a role for Arrb1 in both EMS and internalization of mERα, which are required for the E2-induction of female sexual receptivity.

Membrane estrogen receptor regulation of hypothalamic function

Over the decades, our understanding of estrogen receptor (ER) function has evolved. Today we are confronted by at least two nuclear ERs, ERα and ERβ, and a number of putative membrane ERs, including ERα, ERβ, ER-X, GPR30 and Gq-mER. These receptors all bind estrogens or at least estrogenic compounds and activate intracellular signaling pathways. In some cases, a well-defined pharmacology and physiology has been discovered. In other cases, the identity or the function remains to be elucidated. This mini-review attempts to synthesize our understanding of 17β-estradiol membrane signaling within hypothalamic circuits involved in homeostatic functions, focusing on reproduction and energy balance.

Membrane-initiated estradiol signaling regulating sexual receptivity

Estradiol has profound actions on the structure and function of the nervous system. In addition to nuclear actions that directly modulate gene expression, the idea that estradiol can rapidly activate cell signaling by binding to membrane estrogen receptors (mERs) has emerged. Even the regulation of sexual receptivity, an action previously thought to be completely regulated by nuclear ERs, has been shown to have a membrane-initiated estradiol signaling (MIES) component. This highlighted the question of the nature of mERs. Several candidates have been proposed, ERα, ERβ, ER-X, GPR30 (G protein coupled estrogen receptor), and a receptor activated by a diphenylacrylamide compound, STX. Although each of these receptors has been shown to be active in specific assays, we present evidence for and against their participation in sexual receptivity by acting in the lordosis-regulating circuit. The initial MIES that activates the circuit is in the arcuate nucleus of the hypothalamus (ARH). Using both activation of μ-opioid receptors (MOR) in the medial preoptic nucleus and lordosis behavior, we document that both ERα and the STX-receptor participate in the required MIES. ERα and the STX-receptor activation of cell signaling are dependent on the transactivation of type 1 metabotropic glutamate receptors (mGluR1a) that augment progesterone synthesis in astrocytes and protein kinase C (PKC) in ARH neurons. While estradiol-induced sexual receptivity does not depend on neuroprogesterone, proceptive behaviors do. Moreover, the ERα and the STX-receptor activation of medial preoptic MORs and augmentation of lordosis were sensitive to mGluR1a blockade. These observations suggest a common mechanism through which mERs are coupled to intracellular signaling cascades, not just in regulating reproduction, but in actions throughout the neuraxis including the cortex, hippocampus, striatum, and dorsal root ganglias.

Nervous system physiology regulated by membrane estrogen receptors

Our understanding of estrogen signaling in the nervous system has undergone a significant shift in recent years. For over three decades, the idea that all estradiol actions were explained by direct regulation of transcription held sway. Within the past decade, the idea that in addition to classical effects, membrane-initiated actions of estradiol are important has gained traction. While several novel putative membrane estrogen receptors (ERs) have been described, a large fraction of measured responses appear to be due to membrane-localized estrogen receptor-alpha (ER alpha) and estrogen receptor-beta (ER beta), the same proteins that regulate gene expression. These membrane-localized ERs participate in the regulation of the synthesis of neuroprogesterone, dorsal root ganglion (DRG) neuron excitation, and female sexual receptivity. This is achieved by the modulation of intracellular cell signaling pathways usually associated with the activation of G protein-coupled receptors (GPCRs). ER alpha and ER beta are themselves not GPCRs that directly activate G proteins to regulate physiological responses, but rather interact with traditional GPCRs to initiate cell signaling. This review presents results that support a direct protein-protein interaction between ER alpha and ER beta with metabotropic glutamate receptors (mGluRs), allowing estradiol to signal through mGluRs. This ER/mGluR hypothesis explains how estradiol can activate a wide-range of intracellular pathways and provides an underlying mechanism for the hitherto seemingly unrelated rapid membrane actions in the nervous system.

Membrane estrogen receptors acting through metabotropic glutamate receptors: an emerging mechanism of estrogen action in brain

It has been over 60 years since the first studies have been published describing the effects of steroid hormones on brain function. For over 30 years, estrogen has been presumed to directly affect gene expression and protein synthesis through a specific receptor. More than 20 years ago, the first estrogen receptor was cloned and identified as a transcription factor. Yet, throughout their course of study, estrogens have also been observed to affect nervous system function via mechanisms independent of intracellular receptor regulation of gene expression. Up until recently, the membrane estrogen receptors responsible for these rapid actions have remained elusive. Recent studies have demonstrated that a large number of these rapid, membrane-initiated actions of estradiol are due to surface expression of classical estrogen receptors. This review focuses on the importance of membrane estrogen receptor interactions with metabotropic glutamate receptors for understanding rapid estradiol signaling mechanisms and downstream effectors, as well as their significance in a variety of physiological processes.

PI3 kinase/Akt activation mediates estrogen and IGF-1 nigral DA neuronal neuroprotection against a unilateral rat model of Parkinson's disease

Recently, using the medial forebrain bundle (MFB) 6-hydroxydopmaine (6-OHDA) lesion rat model of Parkinson's disease (PD), we have demonstrated that blockade of central IGF-1 receptors (IGF-1R) attenuated estrogen neuroprotection of substantia nigra pars compacta (SNpc) DA neurons, but exacerbated 6-OHDA lesions in IGF-1 only treated rats (Quesada and Micevych [2004]: J Neurosci Res 75:107-116). This suggested that the IGF-1 system is a central mechanism through which estrogen acts to protect the nigrostriatal DA system. Moreover, these results also suggest that IGF-1R-induced intracellular signaling pathways are involved in the estrogen mechanism that promotes neuronal survival. In vitro, two convergent intracellular signaling pathways used by estrogen and IGF-1, the mitogen-activated protein kinase (MAPK/ERK), and phosphatidyl-inositol-3-kinase/Akt (PI3K/Akt), have been demonstrated to be neuroprotective. Continuous central infusions of MAPK/ERK and PI3K/Akt inhibitors were used to test the hypothesis that one or both of these signal transduction pathways mediates estrogen and/or IGF-1 neuroprotection of SNpc DA neurons after a unilateral administration of 6-OHDA into the MFB of rats. Motor behavior tests and tyrosine hydroxylase immunoreactivity revealed that the inhibitor of the PI3K/Akt pathway (LY294002) blocked the survival effects of both estrogen and IGF-1, while an inhibitor of the MAPK/ERK signaling (PD98059) was ineffective. Western blot analyses showed that estrogen and IGF-1 treatments increased PI3K/Akt activation in the SN; however, MAPK/ERK activation was decreased in the SN. Indeed, continuous infusions of inhibitors blocked phosphorylation of PI3K/Akt and MAPK/ERK. These findings indicate that estrogen and IGF-1-mediated SNpc DA neuronal protection is dependent on PI3K/Akt signaling, but not on the MAPK/ERK pathway.

Overexpression of hippocampal Ca2+/calmodulin-dependent protein kinase II improves spatial memory

Hippocampal alpha-calcium/calmodulin-dependent protein kinase II (alphaCaMKII) has been implicated in neuronal plasticity and spatial learning. In the present experiment, an adeno-associated virus (AAV) vector was designed to express alphaCaMKII driven by the U6 promotor. Microinfusion of this vector into the rat hippocampus increased alphaCaMKII immunoreactivity by approximately 73% (Western analysis) and improved performance in a water maze task. Locomotor activity and exploratory behavior in an open field task were not altered by the overexpression of alphaCaMKII. These data support a role for alphaCaMKII in spatial or explicit memory storage. The advantages of viral vectors for manipulating target proteins expression compared with genetically modified mouse models are discussed.

Estradiol stimulates progesterone synthesis in hypothalamic astrocyte cultures

The brain synthesizes steroids de novo, especially progesterone. Recently estradiol has been shown to stimulate progesterone synthesis in the hypothalamus and enriched astrocyte cultures derived from neonatal cortex. Estradiol-induced hypothalamic progesterone has been implicated in the control of the LH surge. The present studies were undertaken to determine whether hypothalamic astrocytes derived from female neonatal or female postpubertal rats increased production of progesterone in response to an estradiol challenge. Estradiol induced progesterone synthesis in postpubertal astrocytes but not neonatal astrocytes. This estradiol action was blocked by the estrogen receptor antagonist ICI 182,780. Previously we had demonstrated that estradiol stimulates a rapid increase in free cytosolic Ca(2+) ([Ca(2+)](i)) spikes in neonatal cortical astrocytes acting through a membrane estrogen receptor. We now report that estradiol also rapidly increased [Ca(2+)](i) spikes in hypothalamic astrocytes. The membrane-impermeable estradiol-BSA construct also induced [Ca(2+)](i) spikes. Both estradiol-BSA and estradiol were blocked by ICI 182,780. Depleting intracellular Ca(2+) stores prevented the estradiol-induced increased [Ca(2+)](i) spikes, whereas removing extracellular Ca(2+) did not prevent estradiol-induced [Ca(2+)](i) spikes. Together these results indicate that estradiol acts through a membrane-associated receptor to release intracellular stores of Ca(2+). Thapsigargin, used to mimicked the intracellular release of Ca(2+) by estradiol, increased progesterone synthesis, suggesting that estradiol-induced progesterone synthesis involves increases in [Ca(2+)](i). Estradiol treatment did not change levels of steroid acute regulatory protein, P450 side chain cleavage, 3beta-hydroxysteroid dehydrogenase, and sterol carrier protein-2 mRNAs as measured by quantitative RT-PCR, suggesting that in vitro, estradiol regulation of progesterone synthesis in astrocytes does not depend on transcription of new steroidogenic proteins. The present results are consistent with our hypothesis that estrogen-positive feedback regulating the LH surge involves stimulating local progesterone synthesis by hypothalamic astrocytes.

Site-specific estrogen and progestin regulation of orphanin FQ/nociceptin and nociceptin opioid receptor mRNA expression in the female rat limbic hypothalamic system

The distributions of orphanin FQ (OFQ/N; also known as nociceptin) and its cognate receptor, opioid receptor-like receptor-1 (NOP), overlap steroid-responsive regions throughout reproductive circuits of the limbic system and hypothalamus. For example, in the ventromedial nucleus of the hypothalamus (VMH), OFQ/N facilitates lordosis in female rats through estrogen and progesterone regulation of nociceptin activity. We studied estrogen and progesterone regulation of OFQ/N and NOP mRNA expression in limbic-hypothalamic reproductive circuits. Ovariectomized rats were treated with 17beta-estradiol-benzoate (2 microg) and 26 hours later with oil or progesterone (500 microg) and were killed 30 hours after initial treatment. Alternate brain sections were processed for OFQ/N or NOP mRNA in situ hybridization. High levels of hybridization for NOP and OFQ/N and overlapping distributions were observed throughout the limbic hypothalamic reproductive circuits; however, in VMH, only NOP expression was observed. Estrogen treatment increased NOP mRNA expression in anteroventral periventricular nucleus (AVPV), median preoptic nucleus, and VMH. Subsequent progesterone treatment did not alter estrogen-induced expression of NOP mRNA in VMH or median preoptic nucleus but reduced expression in the AVPV. OFQ/N mRNA levels were also regulated by steroids. In the caudal part of the posterodorsal medial amygdala, estrogen increased OFQ/N mRNA levels, and progesterone did not alter this increase, whereas, in the medial part of the medial preoptic nucleus, estrogen and progesterone were needed to increase OFQ/N mRNA levels. Steroid regulation of OFQ/N and NOP in the medial preoptic nucleus and VMH is consistent with emerging data indicating that this opioid system regulates female reproduction.

Direct Regulation of Adult Brain Function by the Male-Specific Factor SRY

The central dogma of mammalian brain sexual differentiation has contended that sex steroids of gonadal origin organize the neural circuits of the developing brain. Recent evidence has begun to challenge this idea and has suggested that, independent of the masculinizing effects of gonadal secretions, XY and XX brain cells have different patterns of gene expression that influence their differentiation and function. We have previously shown that specific differences in gene expression exist between male and female developing brains and that these differences precede the influences of gonadal hormones. Here we demonstrate that the Y chromosome-linked, male-determining gene Sry is specifically expressed in the substantia nigra of the adult male rodent in tyrosine hydroxylase-expressing neurons. Furthermore, using antisense oligodeoxynucleotides, we show that Sry downregulation in the substantia nigra causes a statistically significant decrease in tyrosine hydroxylase expression with no overall effect on neuronal numbers and that this decrease leads to motor deficits in male rats. Our studies suggest that Sry directly affects the biochemical properties of the dopaminergic neurons of the nigrostriatal system and the specific motor behaviors they control. These results demonstrate a direct male-specific effect on the brain by a gene encoded only in the male genome, without any mediation by gonadal hormones.

Estrogen receptor-alpha mediates estradiol attenuation of ATP-induced Ca2+ signaling in mouse dorsal root ganglion neurons

A mechanism underlying gender-related differences in pain perception may be estrogen modulation of nociceptive signaling in the peripheral nervous system. In rat, dorsal root ganglion (DRG) neurons express estrogen receptors (ERs) and estrogen rapidly attenuates ATP-induced Ca2+ signaling. To determine which estrogen receptor mediates rapid actions of estrogen, we showed ERalpha and ERbeta expression in DRG neurons from wild-type (WT) female mice by RT-PCR. To study whether ERalpha or ERbeta mediates this response, we compared estradiol action mediating Ca2+ signaling in DRG neurons from WT, ERalpha knockout (ERalphaKO), and ERbetaKO mice in vitro. ATP, an algesic agent, induced [Ca2+]i transients in 48% of small DRG neurons from WT mice. 17beta-Estradiol (E2) inhibited ATP-induced intracellular Ca2+ concentration ([Ca2+]i) with an IC50 of 27 nM. The effect of E2 was rapid (5-min exposure) and stereo specific; 17alpha-estradiol had no effect. E2 action was blocked by the ER antagonist ICI 182,780 (1 microM) in WT mouse. Estradiol coupled to bovine serum albumin (E-6-BSA), which does not penetrate the plasma membrane, had the same effect as E2 did, suggesting that a membrane-associated ER mediated the response. In DRG neurons from ERbetaKO mice, E2 attenuated the ATP-induced [Ca2+]i flux as it did in WT mice, but in DRG neurons from ERalphaKO mice, E2 failed to inhibit the ATP-induced [Ca2+]i increase. These results show that mouse DRG neurons express ERs and the rapid attenuation of ATP-induced [Ca2+]i signaling is mediated by membrane-associated ERalpha.

Neurosteroids and female reproduction: estrogen increases 3beta-HSD mRNA and activity in rat hypothalamus

A central event in mammalian reproduction is the LH surge that induces ovulation and corpus luteum formation. Typically, the LH surge is initiated in ovariectomized rats by sequential treatment with estrogen and progesterone (PROG). The traditional explanation for this paradigm is that estrogen induces PROG receptors (PR) that are activated by exogenous PROG. Recent evidence suggests that whereas exogenous estrogen is necessary, exogenous PROG is not. In ovariectomized-adrenalectomized rats, estrogen treatment increases hypothalamic PROG levels before an LH surge. This estrogen-induced LH surge was blocked by an inhibitor of 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase (3beta-HSD), the proximal enzyme for PROG synthesis. These data indicate that estrogen induces de novo synthesis of PROG from cholesterol in the hypothalamus, which initiates the LH surge. The mechanism(s) by which estrogen up-regulates neuro-PROG is unknown. We investigated whether estrogen increases 1) mRNA levels for several proteins involved in PROG synthesis and/or 2) activity of 3beta-HSD in the hypothalamus. In ovariectomized-adrenalectomized rats, estrogen treatment increased 3beta-HSD mRNA in the hypothalamus, as measured by relative quantitative RT-PCR. The mRNAs for other proteins involved in steroid synthesis (sterol carrier protein 2, steroidogenic acute regulatory protein, and P450 side chain cleavage) were detectable in hypothalamus but not affected by estrogen. In a biochemical assay, estrogen treatment also increased 3beta-HSD activity. These data support the hypothesis that PROG is a neurosteroid, produced locally in the hypothalamus from cholesterol, which functions in the estrogen positive-feedback mechanism driving the LH surge.

Medial preoptic area δ-opioid receptors inhibit lordosis

Endogenous opioid peptides that activate the delta-opioid receptor (DOR) are thought to facilitate female receptive behavior. This facilitation of lordosis has been demonstrated by intracerebroventricular infusions and injection of DOR-active ligands into the ventromedial hypothalamic nucleus, an area with robust DOR binding. However, DOR binding is distributed throughout the hypothalamus, and the role of DOR in other areas of the hypothalamus has not been examined. In the current study, we demonstrated DOR immunoreactivity in the medial preoptic area (MPO), in particular medial preoptic nucleus (MPN) of the preoptic area. DOR immunoreactive processes were sparsely distributed in the medial and lateral parts of the MPN. Larger DOR immunoreactive fibers were localized in the ventrolateral aspect of the lateral MPN. The MPN is involved in the modulation of female sexual receptivity and the distribution of DOR in this area suggested to us that DOR may regulate lordosis. Ovariectomized rats with unilateral cannulae aimed at the MPN were given 5microg 17beta-estradiol benzoate (EB), once every 4 days and tested for lordosis. [D-Pen(2), D-Pen(5)]-enkephalin (DPDPE), a DOR agonist, microinfused into the MPO, 52-54h after EB-priming, inhibited lordosis when compared with the aCSF (vehicle) control (P <== 0.05). The inhibitory effects of DPDPE were reversed by microinjection of naltrindole, a DOR antagonist (P <== 0.05). Interestingly, the DOR inhibition of lordosis is similar to the micro-opioid receptor inhibition of lordosis in the MPN. These results indicate that DOR in the MPO, particularly in the MPNm, plays an important role in the regulation of lordosis.

Estrogen-induced mu-opioid receptor internalization in the medial preoptic nucleus is mediated via neuropeptide Y-Y1 receptor activation in the arcuate nucleus of female rats

The endogenous peptides beta-endorphin (beta-END) and neuropeptide Y (NPY) have been implicated in regulating sexual receptivity. Both beta-END and NPY systems are activated by estrogen and inhibit female sexual receptivity. The initial estrogen-induced sexual nonreceptivity is correlated with the activation and internalization of mu-opioid receptors (MORs), in the medial preoptic nucleus (MPN). Progesterone reverses the estrogen-induced activation/internalization of MOR and induces the sexual receptive behavior lordosis. To determine whether NPY and endogenous opioids interact, we tested the hypothesis that estrogen-induced MOR activation is mediated through NPY-Y1 receptor (Y1R) activation. Retrograde tract tracing demonstrated Y1Ron beta-END neurons that projected to the MPN. Sex steroid modulation of MOR in the MPN acts through NPY and the Y1R. Estradiol administration or intracerebroventricular injection of NPY activated/internalized Y1R in the arcuate nucleus and MOR in the MPN of ovariectomized (OVX) rats. Moreover, the selective Y1R agonist [Leu31, Pro34]-Neuropeptide Y (LPNY) internalized MOR in the MPN of OVX rats. The Y1R antagonist (Cys31, Nva34)-Neuropeptide Y (27-36)2 prevented estrogen-induced Y1R and MOR activation/internalization. NPY reversed the progesterone blockade of estradiol-induced Y1R and MOR internalization in the arcuate nucleus and MPN, respectively. Behaviorally, LPNY inhibited estrogen plus progesterone-induced lordosis, and the MOR-selective antagonist D-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr amide reversed LPNY-induced inhibition of lordosis. These results suggest that a sequential sex steroid activation of NPY and MOR circuits regulates sexual receptivity.

Estrogen interacts with the IGF-1 system to protect nigrostriatal dopamine and maintain motoric behavior after 6-hydroxdopamine lesions

The most prominent neurochemical hallmark of Parkinson's disease (PD) is the loss of nigrostriatal dopamine (DA). Animal models of PD have concentrated on depleting DA and therapies have focused on maintaining or restoring DA. Within this context estrogen protects against 6-hydroxdopamine (6-OHDA) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesions of the nigrostriatal DA pathway. Present studies tested the hypothesis that neuroprotective estrogen actions involve activation of the insulin-like growth factor-1 (IGF-1) system. Ovariectomized rats were treated with either a single subcutaneous injection of 17beta-estradiol benzoate or centrally or peripherally IGF-1. All rats were infused unilaterally with 6-OHDA into the medial forebrain bundle (MFB) to lesion the nigrostriatal DA pathway. Tyrosine hydroxylase (TH) immunocytochemistry confirmed that rats injected with 6-OHDA had a massive loss of TH immunoreactivity in both the ipsilateral substantia nigra compacta (60% loss) and the striatum (>95% loss) compared to the contralateral side. Loss of TH immunoreactivity was correlated with loss of asymmetric forelimb movements, a behavioral assay for motor deficits. Pretreatment with estrogen or IGF-1 significantly prevented 6-OHDA-induced loss of substantia nigra compacta neurons (20% loss) and TH immunoreactivity in DA fibers in the striatum (<20% loss) and prevented the loss of asymmetric forelimb use. Blockage of IGF-1 receptors by intracerebroventricular JB-1, an IGF-1 receptor antagonist, attenuated both estrogen and IGF-1 neuroprotection of nigrostriatal DA neurons and motor behavior. These findings suggest that IGF-1 and estrogen acting through the IGF-1 system may be critical for neuroprotective effects of estrogen on nigrostriatal DA neurons in this model of PD.

Presynaptic N-methyl-D-aspartate receptor expression is increased by estrogen in an aromatase-rich area of the songbird hippocampus

The vertebrate hippocampus (HP) is sensitive to estrogens, in part via effects on N-methyl-D-aspartate (NMDA)-type glutamate receptors (NR). Although the precise mechanism of this interaction is unclear, it constitutes a key interface in the plasticity of the adult vertebrate HP. The songbird HP expresses high levels of aromatase (estrogen synthase), suggesting that locally generated steroid may affect excitatory pathways. By using light, confocal, and electron microscopy with antibodies that specifically recognize aromatase and NR, we have 1) mapped their distribution in the zebra finch brain, 2) documented their coexpression in HP neurons, 3) studied the ultrastructure of NR-expressing cells in the HP, and 4) tested the influence of estrogen on the cellular and subcellular characteristics of NR-positive HP neurons. Aromatase and NR are coexpressed in HP neurons. NRs are detectable in presynaptic boutons of the songbird HP in addition to postsynaptic loci. Treatment with estrogen increased the somal size and innervation of NR-positive neurons and the frequency of presynaptic NR. Autoreception of excitatory neurotransmission via presynaptic NR may promote the strengthening of activity-dependent, excitatory synapses, thereby enhancing learning. NR-mediated autoreception may underlie estrogenic enhancement of HP structural and functional plasticity.

Estradiol inhibits atp-induced intracellular calcium concentration increase in dorsal root ganglia neurons

Estrogen has been implicated in modulation of pain processing. Although this modulation occurs within the CNS, estrogen may also act on primary afferent neurons whose cell bodies are located within the dorsal root ganglia (DRG). Primary cultures of rat DRG neurons were loaded with Fura-2 and tested for ATP-induced changes in intracellular calcium concentration ([Ca(2+)](i)) by fluorescent ratio imaging. ATP, an algesic agent, induces [Ca(2+)](i) changes via activation of purinergic 2X (P2X) type receptors and voltage-gated Ca(2+) channels (VGCC). ATP (10 microM) caused increased [Ca(2+)](i) transients (226.6+/-16.7 nM, n = 42) in 53% of small to medium DRG neurons. A 5-min incubation with 17 beta-estradiol (100 nM) inhibited ATP-induced [Ca(2+)](i) (164+/-14.6 nM, P<0.05) in 85% of the ATP-responsive DRG neurons, whereas the inactive isomer 17 alpha-estradiol had no effect. Both the mixed agonist/antagonist tamoxifen (1 microM) and specific estrogen receptor antagonist ICI 182780 (1 microM) blocked the estradiol inhibition of ATP-induced [Ca(2+)](i) transients. Estradiol coupled to bovine serum albumin, which does not diffuse through the plasma membrane, blocked ATP-induced [Ca(2+)](i), suggesting that estradiol acts at a membrane-associated estrogen receptor. Attenuation of [Ca(2+)](i) transients was mediated by estrogen action on VGCC. Nifedipine (10 microM), an L-type VGCC antagonist mimicked the effect of estrogen and when co-administered did not increase the estradiol inhibition of ATP-induced [Ca(2+)](i) transients. N- and P-type VGCC antagonists omega-conotoxin GVIA (1 microM) and omega-agatoxin IVA (100 nM), attenuated the ATP-induced [Ca(2+)](i) transients. Co-administration of these blockers with estrogen induced a further decrease of the ATP-induced [Ca(2+)](i) flux. Together, these results suggest that although ATP stimulation of P2X receptors activates L-, N-, and P-type VGCC, estradiol primarily blocks L-type VGCC. The estradiol regulation of this ATP-induced [Ca(2+)](i) transients suggests a mechanism through which estradiol may modulate nociceptive signaling in the peripheral nervous system.

Estrogen receptor-alpha is required for estrogen-induced mu-opioid receptor internalization

Endogenous opioid circuits are pivotal for the regulation of sexual receptivity. Treatment of mice with morphine, a preferential mu-opioid receptor (MOR) agonist, severely attenuates lordosis. Estrogen induces internalization of MOR in cell groups of the limbic-hypothalamic lordosis-regulating circuit. Because rapid MOR internalization is mediated by estrogen release of endogenous opioid peptides, internalization has been used as a neurochemical signature of estrogen action in the central nervous system. Together these results indicate that estrogen induces a MOR mediated inhibition of sexual receptivity. To determine which estrogen receptor, estrogen receptor-alpha (ERalpha) or estrogen receptor-beta (ERbeta), mediates MOR internalization, ERalpha knockout (ERalphaKO), ERbeta knockout (ERbetaKO) and wild-type (WT) mice were used in the present study. WT, ERalphaKO and ERbetaKO mice had similar MOR distributions in the limbic-hypothalamic lordosis-regulating circuit. Estrogen treatment internalized MOR in the medial preoptic nucleus of ovariectomized WT and ERbetaKO, but not ERalphaKO mice. Treatment of ERalphaKO mice with the selective endogenous MOR ligand, endomorphin-1, induced levels of MOR internalization similar to WT mice suggesting that MOR in ERalphaKO mice could be activated and were probably functional. The results of the present experiments indicate that ERalpha is required for estrogen-induced MOR internalization and suggest that ERalpha can mediate rapid actions of estrogen.

Site-specific decrease of progesterone receptor mRNA expression in the hypothalamus of middle-aged persistently estrus rats

Middle-aged females gradually become acyclic and spontaneously develop a persistently estrus (PE) state. PE rats, acyclic for 30 days (early PE), are unresponsive to the positive feedback action of estrogen, but respond to a progesterone challenge with a luteinizing hormone (LH) surge and ovulation; unlike long-term PE rats, acyclic for 90 days, neither estrogen nor estrogen plus progesterone will elicit an LH surge [10th International Congress of Endocrinology, San Francisco, P3 (1996) 1061]. We hypothesize that the PE state may develop due to a diminished level of estrogen-induced progesterone receptor (PR) expression in the hypothalamus that prevents progesterone from stimulating LH regulating circuits. To test this hypothesis, PR mRNA levels were measured in hypothalamic regions of young, proestrus (2-3 months of age), early PE (10-12 months) and long-term PE (13-15 months) rats. The anteroventral periventricular nucleus (AVPV), an important regulatory site of the LH surge, had decreased PR mRNA levels in early and long-term PE rats compared with proestrus rats. However, PR mRNA levels were reduced only in long-term PE rats in the ventromedial nucleus (VMH) and arcuate nucleus (ARH). In the medial preoptic nucleus (MPN), levels of PR mRNA did not change. A previous report showed that exogenous progesterone stimulates an LH surge in young and early PE animals, indicating that the expression of PR mRNA demonstrated in this study is sufficient to mediate progesterone facilitation of the LH surge in early PE rats. In acyclic, long-term PE rats, diminished estrogen-induced expression of progesterone receptors is correlated with a previously shown inability to respond to exogenous progesterone.

The superior olivary complex of the hamster has multiple periods of cholinergic neuron development

Cholinergic neurons of the superior olivary complex share a common embryological and phylogenetic origin with brainstem motor neurons and serve as the major descending efferent pathway either to the cochlea as part of the olivocochlear system or to the cochlear nucleus. In this study, we investigated the developmental expression patterns of choline acetyltransferase (ChAT) and its co-localization with calcitonin gene-related peptide within the superior olivary complex and neighboring brainstem motor nuclei. At embryonic day 12, neurons in the ventral nucleus of the trapezoid body were first to express ChAT. The temporal expression pattern of both ChAT mRNA and immunoreactivity in this periolivary region mimicked motor neurons in the facial and trigeminal motor nuclei. Just before birth, shell neurons surrounding the lateral superior olive expressed ChAT. Neither ChAT-positive periolivary neurons nor shell neurons co-expressed calcitonin gene-related peptide during development or in the adult. Immediately following birth, intrinsic neurons within the lateral superior olive expressed ChAT but not calcitonin gene-related peptide. However, a transient increase in the number of ChAT-positive neurons in the lateral superior olive coincided with the onset of the calcitonin gene-related peptide co-expression within these neurons. We conclude that ChAT expression appears first in periolivary regions containing medial olivocochlear neurons, precedes the expression of calcitonin gene-related peptide in the superior olivary complex, and is co-expressed with calcitonin gene-related peptide within the lateral superior olive containing lateral olivocochlear neurons. These data suggest that the lateral olivocochlear system co-expresses ChAT and calcitonin gene-related peptide, whereas the medial olivocochlear system does not.

Progesterone blockade of estrogen activation of mu-opioid receptors regulates reproductive behavior

The mu-opioid receptor (MOR), a G-protein-coupled receptor, is internalized after endogenous agonist binding. Although receptor activation and internalization are separate events, internalization is a good assay for activation because endogenous opioid peptides all induce internalization. Estrogen treatment of ovariectomized rats induces MOR internalization, providing a neurochemical signature of estrogen activation of the medial preoptic nucleus. MOR activation appears to be the mechanism via which estrogen acts in the medial preoptic area to prevent the display of female reproductive behavior during the first 20-24 hr after estrogen treatment. Naltrexone, an alkaloid universal opioid receptor antagonist, prevented MOR internalization, suggesting that estrogen induces the release of endogenous opioid peptides that in turn activate the MOR. Enkephalins and beta-endorphin are nonselective endogenous MOR ligands. The most selective endogenous MOR ligands are the endomorphins. Infusions of selective MOR agonists, H-Tyr-d-Ala-Gly-N-Met-Phe-glycinol-enkephalin (DAMGO) or endomorphin-1, into the medial preoptic nucleus attenuated lordosis, and their effects were blocked with the MOR antagonist H-d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP). Infusion of endomorphin-1 internalized MOR. To determine whether progestin also acts via the MOR system to facilitate reproductive behavior, ovariectomized rats were primed with 17beta-estradiol and progesterone. Progestin facilitation of lordosis was correlated with a reduction of estrogen-induced MOR internalization. Progestin reversed estrogen-induced MOR internalization, suggesting that progesterone blocked estrogen-induced endogenous opioid release, relieving estrogen inhibition and facilitating lordosis. These results indicate a central role of MOR in the mediation of sex steroid activation of the CNS to regulate female reproductive behavior.

OSP/claudin-11 forms a complex with a novel member of the tetraspanin super family and beta1 integrin and regulates proliferation and migration of oligodendrocytes

Oligodendrocyte-specific protein (OSP)/claudin-11 is a major component of central nervous system myelin and forms tight junctions (TJs) within myelin sheaths. TJs are essential for forming a paracellular barrier and have been implicated in the regulation of growth and differentiation via signal transduction pathways. We have identified an OSP/claudin-11-associated protein (OAP)1, using a yeast two-hybrid screen. OAP-1 is a novel member of the tetraspanin superfamily, and it is widely expressed in several cell types, including oligodendrocytes. OAP-1, OSP/claudin-11, and beta1 integrin form a complex as indicated by coimmunoprecipitation and confocal immunocytochemistry. Overexpression of OSP/claudin-11 or OAP-1 induced proliferation in an oligodendrocyte cell line. Anti-OAP-1, anti-OSP/claudin-11, and anti-beta1 integrin antibodies inhibited migration of primary oligodendrocytes, and migration was impaired in OSP/claudin-11-deficient primary oligodendrocytes. These data suggest a role for OSP/claudin-11, OAP-1, and beta1 integrin complex in regulating proliferation and migration of oligodendrocytes, a process essential for normal myelination and repair.

Striatal ionotropic glutamate receptor ontogeny in the rat

Rat striatal N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate (KA) receptor staining were evaluated postnatally in the rat. Immunohistochemistry was used to detect subunit proteins of the three glutamate receptor subtypes. The glutamate receptors displayed distinct developmental expression patterns in the striatum. Morphological distributions for the NMDA R1 subunit (representative of NMDA receptors), Glu R1 and Glu R2/3 subunits (indicative of AMPA receptors), and Glu R5/6/7 subunits (demonstrating KA receptors) attained adult expression patterns and levels at different postnatal time points. The ontogenic maturation sequence of striatal glutamate receptor expression was KA, then AMPA and lastly NMDA. Staining patterns for NMDA and AMPA subunit proteins were detected initially as dense patches in the neuropil, which changed to a homogeneous stain of the striatum by the second week of life. Cellular staining for the three subtypes was intense within the highly reactive neuropil patches, but less intensely stained in neurons located outside these zones. The KA receptor subunit did not exhibit neuropil heterogeneity, but was distributed evenly at birth. All three glutamate receptor subtypes were visible within the striatal neuron populations. Populations of striatal neurons that expressed the three differential glutamate receptor subtypes overlap, exhibit different growth patterns and dendritic staining. These results support a functional emergence of different glutamate receptor activation within the striatum and provide a potential therapeutic means to isolate developmental disorders specifically associated with excitatory circuits of the basal ganglia.

Expression of BDNF and TrkB mRNAs in the crista neurosensory epithelium and vestibular ganglia following ototoxic damage

Following ototoxic lesion with the aminoglycoside gentamicin, the vestibular neurosensory epithelia undergo degeneration and then limited spontaneous regeneration. The spatio-temporal expression of brain-derived neurotrophic factor (BDNF) and of its high affinity receptor (trkB) mRNA was investigated in the vestibular end organs and ganglia of chinchillas following gentamicin ototoxicity. In the vestibular ganglia of untreated chinchillas, the level of expression of BDNF mRNA is low. At 1 and 2 weeks after intraotic treatment with gentamicin, BDNF mRNA levels in the vestibular ganglia were elevated significantly compared to untreated chinchillas and chinchillas 4 weeks after treatment. At 4 weeks after gentamicin treatment, BDNF mRNA levels were at intact levels of expression. In the crista ampullaris, high levels of BDNF transcripts were found in the untreated chinchillas. At 1 and 2 weeks after treatment, when only supporting cells are present in the crista, BDNF mRNA was undetectable. Four weeks after aminoglycoside treatment BDNF mRNA was present in the epithelium but at lower levels than in the intact epithelium. In contrast to its ligand, high levels of trkB mRNA hybridization were present in the vestibular ganglia of untreated chinchillas and trkB mRNA levels did not change following gentamicin treatment. In the vestibular epithelia, trkB mRNA was not detected either in the intact epithelium or after gentamicin ototoxicity. These data suggest that BDNF may be involved in the maintenance of the vestibular ganglia and contribute to neurite outgrowth to new and repaired hair cells following ototoxic damage.

Alterations in diaphragm contractility after nandrolone administration: an analysis of potential mechanisms

The aim of this study was to evaluate the potential mechanisms underlying the improved contractility of the diaphragm (Dia) in adult intact male hamsters after nandrolone (Nan) administration, given subcutaneously over 4 wk via a controlled-release capsule (initial dose: 4.5 mg. kg-1. day-1; with weight gain, final dose: 2.7 mg. kg-1. day-1). Control (Ctl) animals received blank capsules. Isometric contractile properties of the Dia were determined in vitro after 4 wk. The maximum velocity of unloaded shortening (Vo) was determined in vitro by means of the slack test. Dia fibers were classified histochemically on the basis of myofibrillar ATPase staining and fiber cross-sectional area (CSA), and the relative interstitial space was quantitated. Ca2+-activated myosin ATPase activity was determined by quantitative histochemistry in individual diaphragm fibers. Myosin heavy chain (MHC) isoforms were identified electrophoretically, and their proportions were determined by using scanning densitometry. Peak twitch and tetanic forces, as well as Vo, were significantly greater in Nan animals compared with Ctl. The proportion of type IIa Dia fibers was significantly increased in Nan animals. Nan increased the CSA of all fiber types (26-47%), whereas the relative interstitial space decreased. The relative contribution of fiber types to total costal Dia area was preserved between the groups. Proportions of MHC isoforms were similar between the groups. There was a tendency for increased expression of MHC2B with Nan. Ca2+-activated myosin ATPase activity was increased 35-39% in all fiber types in Nan animals. We conclude that, after Nan administration, the increase in Dia specific force results from the relatively greater Dia CSA occupied by hypertrophied muscle fibers, whereas the increased ATPase activity promotes a higher rate of cross-bridge turnover and thus increased Vo. We speculate that Nan in supraphysiological doses have the potential to offset or ameliorate conditions associated with enhanced proteolysis and disordered protein turnover.

The passerine hippocampus is a site of high aromatase: inter- and intraspecies comparisons

The vertebrate hippocampus (HP) plays a critical role in the organization of memory. Estrogens alter synaptic morphology and function in the mammalian HP and may potentiate memory performance. Previous studies suggest that the songbird HP itself is a site of significant aromatase expression, intimating that local estrogen synthesis may provide a source of this steroid to estrogen-sensitive neural circuits. To explore the potential role of local estrogen synthesis on HP structure and function, we have characterized aromatase message and activity in the zebra finch HP. Toward this end we have compared (a) HP aromatase mRNA with that at other neural loci, (b) HP aromatase activity between adults of both sexes, and (c) HP and hypothalamic preoptic area (HPOA) aromatase activity among songbirds and nonsongbirds. Finally we asked whether aromatase activity was intrinsic to the HP by maintaining it in culture, isolated from the rest of the telencephalon. The HP of every songbird studied expresses aromatase, with comparable levels across sexes. Notably, aromatase activity was found at higher levels in the songbird HP than in the HPOA. In both nonsongbird species investigated, however, HP aromatase activity was undetectable under identical assay conditions. Additionally, the developing songbird HP continues to express aromatase when cultured in isolation from the rest of the telencephalon. The data suggest that HP aromatase is characteristic of passeriformes and, as in the HPOA, may represent a mechanism whereby estrogen is made available to neural circuits. Passerines may prove invaluable animal models for investigations of the estrogenic modulation of HP structure and function.

Estrogen-induced alteration of mu-opioid receptor immunoreactivity in the medial preoptic nucleus and medial amygdala

The mu-opioid receptor (mu-OR), like most G-protein-coupled receptors, is rapidly internalized after agonist binding. Although opioid peptides induce internalization in vivo, there are no studies that demonstrate mu-OR internalization in response to natural stimuli. In this study, we used laser-scanning microscopy to demonstrate that estrogen treatment induces the translocation of mu-OR immunoreactivity (mu-ORi) from the membrane to an internal location in steroid-sensitive cell groups of the limbic system and hypothalamus. Estrogen-induced internalization was prevented by the opioid antagonist naltrexone, suggesting that translocation was largely dependent on release of endogenous agonists. Estrogen treatment also altered the pattern of mu-ORi at the bright-field light microscopic level. In the absence of stimulation, the majority of immunoreactivity is diffuse, with few definable mu-OR+ cell bodies or processes. After stimulation, the density of distinct processes filled with mu-ORi was significantly increased. We interpreted the increase in the number of mu-OR+ processes as indicating increased levels of internalization. Using this increase in the density of mu-OR+ fibers, we showed that treatment of ovariectomized rats with estradiol benzoate induced a rapid and reversible increase in the number of fibers. Significant internalization was noted within 30 min and lasted for >24 hr after estrogen treatment in the medial preoptic nucleus, the principal part of the bed nucleus, and the posterodorsal medial amygdala. Naltrexone prevented the increase of mu-OR+ processes. These data imply that estrogen treatment stimulates the release of endogenous opioids that activate mu-OR in the limbic system and hypothalamus providing a "neurochemical signature" of steroid activation of these circuits.

Peripubertal ontogeny and estrogen stimulation of cholecystokinin and preproenkephalin mRNA in the rat hypothalamus and limbic system

The neuropeptide cholecystokinin (CCK) is expressed in limbic system and hypothalamic nuclei that form a circuit that regulates the display of the female rodent reproductive behavior, lordosis. CCK mRNA and peptide levels fluctuate across the estrous cycle and have been shown to be modulated by estrogen exposure. The objective of these experiments was to examine the expression of CCK mRNA during postnatal development of this limbic-hypothalamic, lordosis regulating circuit, and to determine the age at which CCK mRNA expression becomes responsive to estrogen stimulation, by using quantitative in situ hybridization histochemistry. CCK mRNA levels were below the level of detectability within the circuit during the postnatal period, but increased during the peripubertal period. Rats were injected with either estradiol benzoate (EB), EB and progesterone, progesterone, or oil, and were killed 48 hours later on postnatal day (PND) 15, 20, and 25. Alternate brain sections were processed for CCK and preproenkephalin (PPE) mRNA in situ hybridization histochemistry. EB treatment induced CCK mRNA expression in the central portion of the medial preoptic nucleus and posterodorsal medial amygdala at PND 20 and 25, respectively. However, EB treatment increased PPE mRNA levels within the nuclei of the circuit at all ages examined. Progesterone had neither an independent nor additive effect on the EB induction of these neuropeptide messages. The estrogenic induction of CCK mRNA appears to be dependent on estrogen sensitive pathways of neurotransmission, or components of second messenger pathways which regulate CCK mRNA expression in the adult limbic-hypothalamic lordosis regulating circuit, which are not functional before PND 18-25.

Orphanin FQ/nociceptin in the ventromedial nucleus facilitates lordosis in female rats

The localization of opioid receptor-like (ORL-1) orphan receptor in the ventromedial nucleus of the hypothalamus (VMH) suggested a role for this opioid system in the regulation of lordosis behavior. Recently, the ligand for ORL-1, orphanin FQ/nociceptin (OFQ/N), has been characterized and also demonstrated to be present in the VMH. The present experiments examined whether OFQ/N in the VMH facilitates lordosis behavior in estrogen-primed, sexually unreceptive female rats, and whether estrogen regulates ORL-1 levels in the VMH. Estrogen was shown to increase ORL-1 immunoreactivity in the VMH, and microinfusions of OFQ/N into the VMH facilitated lordosis behavior in a dose-dependent manner.

Expression of brain-derived neurotrophic factor and its receptor mRNA in the vestibuloauditory system of the bullfrog

Brain-derived neurotrophic factor (BDNF) is a neurotrophin which has been suggested to play a crucial role in the development and maintenance of the inner ear. In the present study, we investigated the expression of mRNAs of BDNF and its high-affinity receptor trkB in the vestibuloauditory system of the adult bullfrog. In situ hybridization was performed using riboprobes transcribed from Xenopus BDNF and trkB cDNA clones. BDNF mRNA was expressed in the sensory epithelia of the ampullary cristae, utricular and saccular maculae, lagena, and amphibian and basilar papillae. Strong hybridization for BDNF mRNA was also found in neuron somata of the vestibuloauditory nuclear complex. trkB mRNA was detected in the sensory epithelia of all vestibular and auditory endorgans. High levels of both BDNF and trkB mRNAs were found in vestibuloauditory ganglion cells. These results support the hypothesis that BDNF participates in the maintenance of vestibuloauditory neurons and may be important for the trophic regulation of vestibular and auditory sensory epithelia in this animal model.

Oligodendrocyte-specific protein (OSP) is a major component of CNS myelin

An oligodendrocyte-specific protein (OSP) cDNA was recently identified and found to be expressed primarily in oligodendrocytes and has a deduced amino acid sequence similar to that of peripheral myelin protein 22 (PMP-22). We raised antibodies against a synthetic peptide corresponding to OSP amino acid residues 179-194 which reacted with a 22 kd protein in mouse CNS. OSP immunoreactivity localized to spinal cord white matter tracts using immunohistochemistry in a similar distribution to that of MBP. OSP localized to CNS myelin biochemically with more than a 30-fold enrichment measured in purified myelin. We further purified the proteolipid fraction of myelin and determined that OSP contributes approximately 7% of total myelin protein making it the third most abundant protein in CNS myelin. No binding was found to several agglutinins or a HNK1-specific antibody suggesting that OSP is not a glycoprotein.