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

The Hypothalamus and Pituitary Gland Regulate Reproduction and Are Involved in the Development of Polycystic Ovary Syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS) is a complex condition with unclear mechanisms, posing a challenge for prevention and treatment of PCOS. The role of the hypothalamus and pituitary gland in regulating female reproduction is critical. Abnormalities in the hypothalamus and pituitary can impair reproductive function. It is important to study hypothalamic and pituitary changes in patients with PCOS.

SUMMARY

This article reviews articles on the hypothalamus and PCOS with the goal of summarizing what abnormalities of the hypothalamic-pituitary-ovarian axis are present in patients with PCOS and to clarify the pathogenesis of PCOS. We find that the mechanisms by which the hypothalamus and pituitary regulate reproduction in girls are complex and are associated with altered sex hormone levels, obesity, and insulin resistance. Different animal models of PCOS are characterized by different alterations in the hypothalamus and pituitary and respond differently to different treatments, which correspond to the complex pathogenesis of patients with PCOS.

KEY MESSAGES

Arcuate nucleus (ARC) is associated with luteinizing hormone (LH) surges. Suprachiasmatic nucleus, ARC, and RP3V are associated with LH surges. Animal models of PCOS have different characteristics.

Cellular and molecular mechanisms of action of ovarian steroid hormones II: Regulation of sexual behavior in female rodents

Female sexual behaviors in rodents (lordosis and appetitive or "proceptive" behaviors) are induced through a genomic mechanism by the sequential actions of estradiol (E2) and progesterone (P), or E2 and testosterone (T) at their respective receptors. However, non-steroidal agents, such as gonadotropin-releasing hormone (GnRH), Prostaglandin E2 (PGE2), noradrenaline, dopamine, oxytocin, α-melanocyte stimulating hormone, nitric oxide, leptin, apelin, and others, facilitate different aspects of female sexual behavior through their cellular and intracellular effects at the membrane and genomic levels in ovariectomized rats primed with E2. These neurotransmitters often act as intermediaries of E2 and P (or T). The classical model of steroid hormone action through intracellular receptor binding has been complemented by an alternative scenario wherein the steroid functions as a transcription factor after binding the receptor protein to DNA. Another possible mechanism occurs through the activation of second messenger systems (cyclic AMP, cyclic GMP, calcium), which subsequently initiate phosphorylation events via diverse kinase systems (protein kinases A, G, or C). These kinases target the progesterone receptor (PR) or associated effector proteins that connect the PR to the trans-activation machinery. This may also happen to the androgen receptor (AR). In addition, other cellular mechanisms could be involved since the chemical structure of these non-steroidal agents causes a change in their lipophobicity that prevents them from penetrating the cell and exerting direct transcriptional effects; however, they can exert effects on different components of the cell membrane activating a cross-talk between the cell membrane and the regulation of the transcriptional mechanisms.

Cellular and molecular mechanisms of action of ovarian steroid hormones. I: Regulation of central nervous system function

The conventional way steroid hormones work through receptors inside cells is widely acknowledged. There are unanswered questions about what happens to the hormone in the end and why there isn't always a strong connection between how much tissue takes up and its biological effects through receptor binding. Steroid hormones can also have non-traditional effects that happen quickly but don't involve entering the cell. Several possible mechanisms for these non-traditional actions include (a) changes in membrane fluidity, (b) steroid hormones acting on receptors on the outer surface of cells, (c) steroid hormones regulating GABAA receptors on cell membranes, and (d) activation of steroid receptors by factors like EGF, IGF-1, and dopamine. Data also suggests that steroid hormones may be inserted into DNA through receptors, acting as transcription factors. These proposed new mechanisms of action should not be seen as challenging the conventional mechanism. Instead, they contribute to a more comprehensive understanding of how hormones work, allowing for rapid, short-term, and prolonged effects to meet the body's physiological needs.

Astrogenesis in the hypothalamus: A life-long process contributing to the development and plasticity of neuroendocrine networks

Astrocytes are now recognized as integral components of neural circuits, regulating their maturation, activity and plasticity. Neuroendocrinology has provided fertile ground for revealing the diverse strategies used by astrocytes to regulate the physiological and behavioural outcomes of neural circuit activity in response to internal and environmental inputs. However, the development of astrocytes in the hypothalamus has received much less attention than in other brain regions such as the cerebral cortex and spinal cord. In this review, we synthesize our current knowledge of astrogenesis in the hypothalamus across various life stages. A distinctive feature of hypothalamic astrogenesis is that it persists all throughout lifespan, and involves multiple cellular sources corresponding to radial glial cells during early development, followed by tanycytes, parenchymal progenitors and locally dividing astrocytes. Astrogenesis in the hypothalamus is closely coordinated with the maturation of hypothalamic neurons. This coordination is exemplified by recent findings in neurons producing gonadotropin-releasing hormone, which actively shape their astroglial environment during infancy to integrate functionally into their neural network and facilitate sexual maturation, a process vulnerable to endocrine disruption. While hypothalamic astrogenesis shares common principles with other brain regions, it also exhibits specific features in its dynamics and regulation, both at the inter- and intra-regional levels. These unique properties emphasize the importance of further exploration. Additionally, we discuss the experimental strategies used to assess astrogenesis in the hypothalamus and their potential bias and limitations. Understanding the mechanisms of hypothalamic astrogenesis throughout life will be crucial for comprehending the development and function of the hypothalamus under both physiological and pathological conditions.

Neuroactive steroids in the neuroendocrine control of food intake, metabolism, and reproduction

Neuroactive steroids are a type of steroid hormones produced within the nervous system or in peripheral glands and then transported to the brain to exert their neuromodulatory effects. Neuroactive steroids have pleiotropic effects, that include promoting myelination, neuroplasticity, and brain development. They also regulate important physiological functions, such as metabolism, feeding, reproduction, and stress response. The homoeostatic processes of metabolism and reproduction are closely linked and mutually dependent. Reproductive events, such as pregnancy, bring about significant changes in metabolism, and metabolic status may affect reproductive function in mammals. In females, the regulation of reproduction and energy balance is controlled by the fluctuations of oestradiol and progesterone throughout the menstrual cycle. Neurosteroids play a key role in the neuroendocrine control of reproduction. The synthesis of neuroestradiol and neuroprogesterone within the brain is a crucial process that facilitates the release of GnRH and LH, which in turn, regulate the transition from oestrogen-negative to oestrogen-positive feedback. In addition to their function in the reproductive system, oestrogen has a key role in the regulation of energy homoeostasis by acting at central and peripheral levels. The oestrogenic effects on body weight homoeostasis are primarily mediated by oestrogen receptors-α (ERα), which are abundantly expressed in multiple brain regions that are implicated in the regulation of food intake, basal metabolism, thermogenesis, and brown tissue distribution. The tight interplay between energy balance and reproductive physiology is facilitated by shared regulatory pathways, namely POMC, NPY and kisspeptin neurons, which are targets of oestrogen regulation and likely participate in different aspects of the joint control of energy balance and reproductive function. The aim of this review is to present a summary of the progress made in uncovering shared regulatory pathways that facilitate the tight coupling between energy balance and reproductive physiology, as well as their reciprocal interactions and the modulation induced by neurosteroids.

Kisspeptin control of hypothalamus-pituitary-ovarian functions

The discovery of Kisspeptin (Kiss) has opened a new direction in research on neuroendocrine control of reproduction in vertebrates. Belonging to the RF amide family of peptides, Kiss and its cognate receptor Gpr54 (Kissr) have a long and complex evolutionary history. Multiple forms of Kiss and Kissr are identified in non-mammalian vertebrates, with the exception of birds, and monotreme mammals. However, only a single form of the ligand (KISS1/Kiss1) and receptor (KISS1R/Kiss1r) is retained in higher mammals. Kiss1 is distributed in the hypothalamus-pituitary-gonadal (HPG) axis and its primary function is to stimulate gonadotropin-releasing hormone (GnRH) secretion. Kiss1 neurons are distributed in the rostral periventricular area of the third ventricle (RP3V) and arcuate/infundibular nucleus (ARN/IFN). The ARN/IFN is considered the GnRH pulse generator controlled by steroid negative feedback, and the RP3V neurons is concerned with GnRH surge induced by steroid positive feedback in females. The Kiss1-Kiss1r signaling is important in all aspects of reproduction: puberty onset, maintenance of adult gonadal functions and reproductive aging, and hence assumes therapeutic potentials in the treatment of reproductive dysfunctions and induction of artificial reproduction. This chapter reviews involvement of Kiss1 in the control of the HPG axis functions in female mammals.

Kisspeptin signaling in astrocytes modulates the reproductive axis

Reproduction is safeguarded by multiple, often cooperative, regulatory networks. Kisspeptin signaling, via KISS1R, plays a fundamental role in reproductive control, primarily by regulation of hypothalamic GnRH neurons. We disclose herein a pathway for direct kisspeptin actions in astrocytes that contributes to central reproductive modulation. Protein-protein interaction and ontology analyses of hypothalamic proteomic profiles after kisspeptin stimulation revealed that glial/astrocyte markers are regulated by kisspeptin in mice. This glial-kisspeptin pathway was validated by the demonstrated expression of Kiss1r in mouse astrocytes in vivo and astrocyte cultures from humans, rats, and mice, where kisspeptin activated canonical intracellular signaling-pathways. Cellular coexpression of Kiss1r with the astrocyte markers GFAP and S100-β occurred in different brain regions, with higher percentage in Kiss1- and GnRH-enriched areas. Conditional ablation of Kiss1r in GFAP-positive cells in the G-KiR-KO mouse altered gene expression of key factors in PGE2 synthesis in astrocytes and perturbed astrocyte-GnRH neuronal appositions, as well as LH responses to kisspeptin and LH pulsatility, as surrogate marker of GnRH secretion. G-KiR-KO mice also displayed changes in reproductive responses to metabolic stress induced by high-fat diet, affecting female pubertal onset, estrous cyclicity, and LH-secretory profiles. Our data unveil a nonneuronal pathway for kisspeptin actions in astrocytes, which cooperates in fine-tuning the reproductive axis and its responses to metabolic stress.

High-amplitude network co-fluctuations linked to variation in hormone concentrations over the menstrual cycle

Many studies have shown that the human endocrine system modulates brain function, reporting associations between fluctuations in hormone concentrations and brain connectivity. However, how hormonal fluctuations impact fast changes in brain network organization over short timescales remains unknown. Here, we leverage a recently proposed framework for modeling co-fluctuations between the activity of pairs of brain regions at a framewise timescale. In previous studies we showed that time points corresponding to high-amplitude co-fluctuations disproportionately contributed to the time-averaged functional connectivity pattern and that these co-fluctuation patterns could be clustered into a low-dimensional set of recurring "states." Here, we assessed the relationship between these network states and quotidian variation in hormone concentrations. Specifically, we were interested in whether the frequency with which network states occurred was related to hormone concentration. We addressed this question using a dense-sampling dataset (N = 1 brain). In this dataset, a single individual was sampled over the course of two endocrine states: a natural menstrual cycle and while the subject underwent selective progesterone suppression via oral hormonal contraceptives. During each cycle, the subject underwent 30 daily resting-state fMRI scans and blood draws. Our analysis of the imaging data revealed two repeating network states. We found that the frequency with which state 1 occurred in scan sessions was significantly correlated with follicle-stimulating and luteinizing hormone concentrations. We also constructed representative networks for each scan session using only "event frames"-those time points when an event was determined to have occurred. We found that the weights of specific subsets of functional connections were robustly correlated with fluctuations in the concentration of not only luteinizing and follicle-stimulating hormones, but also progesterone and estradiol.

The interaction of membrane estradiol receptors and metabotropic glutamate receptors in adaptive and maladaptive estradiol-mediated motivated behaviors in females

Estrogen receptors were initially identified as intracellular, ligand-regulated transcription factors that result in 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) can rapidly alter cellular excitability and gene expression, particularly through the phosphorylation of CREB. A principal mechanism of neuronal mER action has been shown to occur through glutamate-independent transactivation of metabotropic glutamate receptors (mGlu), which elicits multiple signaling outcomes. The interaction of mERs with mGlu has been shown to be important in many diverse functions in females, including driving motivated behaviors. Experimental evidence suggests that a large part of estradiol-induced neuroplasticity and motivated behaviors, both adaptive and maladaptive, occurs through estradiol-dependent mER activation of mGlu. Herein we will review signaling through estrogen receptors, both "classical" nuclear receptors and membrane-bound receptors, as well as estradiol signaling through mGlu. We will focus on how the interactions of these receptors and their downstream signaling cascades are involved in driving motivated behaviors in females, discussing a representative adaptive motivated behavior (reproduction) and maladaptive motivated behavior (addiction).

Circadian Regulation of Hormonal Timing and the Pathophysiology of Circadian Dysregulation

Circadian rhythms are endogenously generated, daily patterns of behavior and physiology that are essential for optimal health and disease prevention. Disruptions to circadian timing are associated with a host of maladies, including metabolic disease and obesity, diabetes, heart disease, cancer, and mental health disturbances. The circadian timing system is hierarchically organized, with a master circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks throughout the CNS and periphery. The SCN receives light information via a direct retinal pathway, synchronizing the master clock to environmental time. At the cellular level, circadian rhythms are ubiquitous, with rhythms generated by interlocking, autoregulatory transcription-translation feedback loops. At the level of the SCN, tight cellular coupling maintains rhythms even in the absence of environmental input. The SCN, in turn, communicates timing information via the autonomic nervous system and hormonal signaling. This signaling couples individual cellular oscillators at the tissue level in extra-SCN brain loci and the periphery and synchronizes subordinate clocks to external time. In the modern world, circadian disruption is widespread due to limited exposure to sunlight during the day, exposure to artificial light at night, and widespread use of light-emitting electronic devices, likely contributing to an increase in the prevalence, and the progression, of a host of disease states. The present overview focuses on the circadian control of endocrine secretions, the significance of rhythms within key endocrine axes for typical, homeostatic functioning, and implications for health and disease when dysregulated. © 2022 American Physiological Society. Compr Physiol 12: 1-30, 2022.

The great migration: how glial cells could regulate GnRH neuron development and shape adult reproductive life

In mammals, reproductive function is under the control of hypothalamic neurons named Gonadotropin-Releasing Hormone (GnRH) neurons. These neurons migrate from the olfactory placode to the brain, during embryonic development. For the past 40 years, these neurons have been considered an example of tangential migration, i.e., dependent on the olfactory/vomeronasal/terminal nerves. Numerous studies have highlighted the factors involved in the migration of these neurons but thus far overlooked the cellular microenvironment that produces them. Many of these factors are dysregulated in hypogonadotropic hypogonadism, resulting in subfertility/infertility. Nevertheless, over the past ten years, several papers have reported the influence of glial cells (named olfactory ensheathing cells [OECs]) in the migration and differentiation of GnRH neurons. This review will describe the atypical origins, migration, and differentiation of these neurons, focusing on the latest discoveries. There will be a more specific discussion on the involvement of OECs in the development of GnRH neurons, during embryonic and perinatal life; as well as on their potential implication in the development of congenital or idiopathic hypogonadotropic hypogonadism (such as Kallmann syndrome).

Neuroendocrine mechanisms underlying estrogen positive feedback and the LH surge

A fundamental principle in reproductive neuroendocrinology is sex steroid feedback: steroid hormones secreted by the gonads circulate back to the brain to regulate the neural circuits governing the reproductive neuroendocrine axis. These regulatory feedback loops ultimately act to modulate gonadotropin-releasing hormone (GnRH) secretion, thereby affecting gonadotropin secretion from the anterior pituitary. In females, rising estradiol (E₂) during the middle of the menstrual (or estrous) cycle paradoxically "switch" from being inhibitory on GnRH secretion ("negative feedback") to stimulating GnRH release ("positive feedback"), resulting in a surge in GnRH secretion and a downstream LH surge that triggers ovulation. While upstream neural afferents of GnRH neurons, including kisspeptin neurons in the rostral hypothalamus, are proposed as critical loci of E₂ feedback action, the underlying mechanisms governing the shift between E₂ negative and positive feedback are still poorly understood. Indeed, the precise cell targets, neural signaling factors and receptors, hormonal pathways, and molecular mechanisms by which ovarian-derived E₂ indirectly stimulates GnRH surge secretion remain incompletely known. In many species, there is also a circadian component to the LH surge, restricting its occurrence to specific times of day, but how the circadian clock interacts with endocrine signals to ultimately time LH surge generation also remains a major gap in knowledge. Here, we focus on classic and recent data from rodent models and discuss the consensus knowledge of the neural players, including kisspeptin, the suprachiasmatic nucleus, and glia, as well as endocrine players, including estradiol and progesterone, in the complex regulation and generation of E₂-induced LH surges in females.

Kisspeptin neuron electrophysiology: Intrinsic properties, hormonal modulation, and regulation of homeostatic circuits

The obligatory role of kisspeptin (KISS1) and its receptor (KISS1R) in regulating the hypothalamic-pituitary-gonadal axis, puberty and fertility was uncovered in 2003. In the few years that followed, an impressive body of work undertaken in many species established that neurons producing kisspeptin orchestrate gonadotropin-releasing hormone (GnRH) neuron activity and subsequent GnRH and gonadotropin hormone secretory patterns, through kisspeptin-KISS1R signaling, and mediate many aspects of gonadal steroid hormone feedback regulation of GnRH neurons. Here, we review knowledge accrued over the past decade, mainly in genetically modified mouse models, of the electrophysiological properties of kisspeptin neurons and their regulation by hormonal feedback. We also discuss recent progress in our understanding of the role of these cells within neuronal circuits that control GnRH neuron activity and GnRH secretion, energy balance and, potentially, other homeostatic and reproductive functions.

Signal pathway of LH-induced expression of nuclear progestin receptor in vertebrate ovulation

Nuclear progestin receptor (PGR), which is induced in the follicles destined to undergo ovulation, is believed to be obligatory for rupture of the follicles during ovulation in vertebrates. Studies in some mammals and teleost medaka have revealed the outline of the central signaling pathway that leads to the PGR expression in the preovulatory follicles at ovulation. In this review, we summarize the current knowledge on what signaling mediators are involved in the LH-induced follicular expression of PGR at ovulation in these animals. LH-inducibility of follicular PGR expression is conserved. In both group of animals, activation of the LH receptor on the granulosa cell surface with LH commonly results in the increase of intracellular cAMP levels, while the downstream signaling cascades activated by high level of cAMP are totally different between mice and medaka. PGR is currently presumed to be induced via PKA/CREB-mediated transactivation and ERK1/2-dependent signaling in mice, but the receptor is induced via EPAC/RAP and AKT/CREB pathways in the teleost medaka. The differences and similarities in the signaling pathways for PGR expression between them is discussed from comparative and evolutionary aspects. We also discussed questions concerning PGR expression and its regulation needed to be investigated in future.

Neuroendocrine control of gonadotropin-releasing hormone: Pulsatile and surge modes of secretion

The concept that different systems control episodic and surge secretion of gonadotropin-releasing hormone (GnRH) was well established by the time that GnRH was identified and formed the framework for studies of the physiological roles of GnRH, and later kisspeptin. Here, we focus on recent studies identifying the neural mechanisms underlying these two modes of secretion, with an emphasis on their core components. There is now compelling data that kisspeptin neurons in the arcuate nucleus that also contain neurokinin B (NKB) and dynorphin (i.e., KNDy cells) and their projections to GnRH dendrons constitute the GnRH pulse generator in mice and rats. There is also strong evidence for a similar role for KNDy neurons in sheep and goats, and weaker data in monkeys and humans. However, whether KNDy neurons act on GnRH dendrons and/or GnRH soma and dendrites that are found in the mediobasal hypothalamus (MBH) of these species remains unclear. The core components of the GnRH/luteinising hormone surge consist of an endocrine signal that initiates the process and a neural trigger that drives GnRH secretion during the surge. In all spontaneous ovulators, the core endocrine signal is a rise in estradiol secretion from the maturing follicle(s), with the site of estrogen positive feedback being the rostral periventricular kisspeptin neurons in rodents and neurons in the MBH of sheep and primates. There is considerable species variations in the neural trigger, with three major classes. First, in reflex ovulators, this trigger is initiated by coitus and carried to the hypothalamus by neural or vascular pathways. Second, in rodents, there is a time of day signal that originates in the suprachiasmatic nucleus and activates rostral periventricular kisspeptin neurons and GnRH soma and dendrites. Finally, in sheep nitric oxide-producing neurons in the ventromedial nucleus, KNDy neurons and rostral kisspeptin neurons all appear to participate in driving GnRH release during the surge.

Kisspeptin-neuron control of LH pulsatility and ovulation

Feedback from oestradiol (E2) plays a critical role in the regulation of major events in the physiological menstrual cycle including the release of gonadotrophins to stimulate follicular growth, and the mid-cycle luteinising hormone (LH) surge that leads to ovulation. E2 predominantly exerts its action via oestrogen receptor-alpha (ERα), however, as gonadotrophin releasing hormone (GnRH) neurons lack ERα, E2-feedback is posited to be indirectly mediated via upstream neurons. Kisspeptin (KP) is a neuropeptide expressed in hypothalamic KP-neurons that control GnRH secretion and plays a key role in the central mechanism regulating the hypothalamic-pituitary-gonadal (HPG) axis. In the rodent arcuate (ARC) nucleus, KP is co-expressed with Neurokinin B and Dynorphin; and thus, these neurons are termed 'Kisspeptin-Neurokinin B-Dynorphin' (KNDy) neurons. ARC KP-neurons function as the 'GnRH pulse generator' to regulate GnRH pulsatility, as well as mediating negative feedback from E2. A second KP neuronal population is present in the rostral periventricular area of the third ventricle (RP3V), which includes anteroventral periventricular (AVPV) nucleus and preoptic area neurons. These RP3V KP-neurons mediate positive feedback to induce the mid-cycle luteinising hormone (LH) surge and subsequent ovulation. Here, we describe the role of KP-neurons in these two regions in mediating this differential feedback from oestrogens. We conclude by considering reproductive diseases for which exploitation of these mechanisms could yield future therapies.

Membrane-initiated actions of sex steroids and reproductive behavior: A historical account

It was assumed for a long time that sex steroids are activating reproductive behaviors by the same mechanisms that produce their morphological and physiological effects in the periphery. However during the last few decades an increasing number of examples were identified where behavioral effects of steroids were just too fast to be mediated via changes in DNA transcription. This progressively forced behavioral neuroendocrinologists to recognize that part of the effects of steroids on behavior are mediated by membrane-initiated events. In this review we present a selection of these early data that changed the conceptual landscape and we provide a summary the different types of membrane-associated receptors (estrogens, androgens and progestagens receptors) that are playing the most important role in the control of reproductive behaviors. Then we finally describe in more detail three separate behavioral systems in which membrane-initiated events have clearly been established to contribute to behavior control.

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.

A role for glial fibrillary acidic protein (GFAP)-expressing cells in the regulation of gonadotropin-releasing hormone (GnRH) but not arcuate kisspeptin neuron output in male mice

GnRH neurons are the final central neural output regulating fertility. Kisspeptin neurons in the hypothalamic arcuate nucleus (KNDy neurons) are considered the main regulator of GnRH output. GnRH and KNDy neurons are surrounded by astrocytes, which can modulate neuronal activity and communicate over distances. Prostaglandin E2 (PGE2), synthesized primarily by astrocytes, increases GnRH neuron activity and downstream pituitary release of luteinizing hormone (LH). We hypothesized that glial fibrillary acidic protein (GFAP)-expressing astrocytes play a role in regulating GnRH and/or KNDy neuron activity and LH release. We used adeno-associated viruses to target designer receptors exclusively activated by designer drugs (DREADDs) to GFAP-expressing cells to activate Gq- or Gi-mediated signaling. Activating Gq signaling in the preoptic area, near GnRH neurons, but not in the arcuate, increases LH release in vivo and GnRH firing in vitro via a mechanism in part dependent upon PGE2. These data suggest that astrocytes can activate GnRH/LH release in a manner independent of KNDy neurons.

The KiNG of reproduction: Kisspeptin/ nNOS interactions shaping hypothalamic GnRH release

Gonadotropin-releasing hormone (GnRH) is the master regulator of the hypothalamic-pituitary-gonadal (HPG) axis, and therefore of fertility and reproduction. The release pattern of GnRH by the hypothalamus includes both pulses and surges. However, despite a considerable body of evidence in support of a determinant role for kisspeptin, the mechanisms regulating a GnRH pulse and surge remain a topic of debate. In this review we challenge the view of kisspeptin as an absolute "monarch", and instead present the idea of a Kisspeptin-nNOS-GnRH or "KiNG" network that is responsible for generating the "GnRH pulse" and "GnRH surge". In particular, the neuromodulator nitric oxide (NO) has opposite effects to kisspeptin on GnRH secretion in many respects, acting as the Yin to kisspeptin's Yang and creating a dynamic system in which kisspeptin provides the "ON" signal, promoting GnRH release, while NO mediates the "OFF" signal, acting as a tonic brake on GnRH secretion. This interplay between an activator and an inhibitor, which is in turn fine-tuned by the gonadal steroid environment, thus leads to the generation of GnRH pulses and surges and is crucial for the proper development and function of the reproductive axis.