A CRH Receptor Type 1 Agonist Increases GABA Transmission to GnRH Neurons in a Circulating-Estradiol-Dependent Manner

Suppression of Luteinizing Hormone Secretion in Female Mice by a Urocortin 2-CRHR2 Signaling Pathway

Physiologic stress elicits impairment of reproductive function, in part, by the suppression of luteinizing hormone (LH) secretion. Two populations of kisspeptin-synthesizing neurons in the hypothalamus play essential roles in controlling the pulsatile and surge modes of LH secretion and are potential direct targets of stress-activated neural circuits; however, the mechanism(s) for impairment of kisspeptin cells during stress remain unclear. Here, we conducted 4 experiments to test the hypothesis that corticotropin-releasing hormone receptor 2 (CRHR2) signaling contributes to impaired pulsatile and surge LH secretion via direct actions on kisspeptin cells. First, we observed that cells expressing a specific ligand of CRHR2, urocortin 2 (UCN2), show enhanced c-Fos in the paraventricular nucleus (PVN) following acute hypoglycemia, a metabolic stressor that rapidly suppresses LH pulses by impairing arcuate kisspeptin neuron activation. Second, we determined that central injection of UCN2 rapidly inhibits LH pulses. Furthermore, UCN2 disrupts evening expression of the estradiol-induced LH surge and reduces kisspeptin cell activation in the rostral periventricular hypothalamic region (RP3V). Next, we identified CRHR2 in a majority of both arcuate and RP3V kisspeptin cells. Finally, we observed that UCN2 cells in the PVN are activated following chemogenetic stimulation of catecholamine neurons in the nucleus of the solitary tract. Together these data demonstrate that UCN2-CRHR2 signaling disrupts the pulsatile and surge modes of LH secretion via direct suppression of kisspeptin cells. Furthermore, these findings suggest UCN2 cells in the PVN are regulated by metabolic stress and brainstem norepinephrine signaling pathways that convey stress cues to the hypothalamus.

Sex and Time of Day Alter the Interactions Between Hypothalamic Glia and the Neural Circuits Controlling Reproduction

An upstream network, including glia and arcuate nucleus (ARC) kisspeptin neurons, controls hormone secretion from preoptic area (POA) gonadotropin-releasing hormone (GnRH) neurons, which form the final common pathway for the central control of fertility. In males, chemogenetic activation of Gq-mediated signaling in POA glia activated GnRH neurons and downstream luteinizing hormone (LH) release, while chemogenetic activation of ARC glia had no effect on ARC kisspeptin neurons. We characterized sex differences and time-of-day effects in these critical circuits to understand their effects on reproduction. Chemogenetic activation of glial fibrillary acidic protein (GFAP)-expressing cells increased intracellular calcium concentrations regardless of sex, brain region, or time of day. Activation of POA glia or treatment with the gliotransmitter analog dimethyl prostaglandin E2 (dmPGE2) increased GnRH neuron firing rate, and these responses were dependent upon sex and time of day. In contrast, ARC kisspeptin neuron firing rate was unresponsive to ARC glia activation or dmPGE2 regardless of sex or time of day. POA glial activation increased LH levels in males and females but the response in males was more rapid. ARC glia activation had no effect on LH in females and the response in males was delayed compared to POA glia activation. A similar LH response persisted after ARC kisspeptin neuron ablation, suggesting it is not mediated by those neurons. GnRH neurons, rather than arcuate kisspeptin neurons, are thus the main target of glial regulation of reproductive neuroendocrine output and this regulation is dependent on sex and time of day.

Brainstem Noradrenergic Neuronal Populations: Dual Effects on Regulating GnRH and LH Secretion

Noradrenergic neurons are a brain network that integrate viscero-sensorial signals to modulate neural and neuroendocrine function. Although it has been known for decades that noradrenergic neural circuits influence neuroendocrine and reproductive function, the cellular and molecular players involved remain largely unknown. The objective of this review is to summarize past and current knowledge regarding the influence of brainstem noradrenergic systems on GnRH and gonadotrophin secretion. The main noradrenergic cell groups A1, A2, and A6, known as the ventrolateral medulla, nucleus of the solitary tract, and locus coeruleus, respectively, are involved in the control of reproductive neuroendocrine secretion. Current evidence suggests that brainstem noradrenergic circuits promote the generation and maintenance of the LH surge in both spontaneous (rats, sheep) and induced (rabbit, ferret) ovulators. In contrast, recent studies have established that LH pulsatile secretion is suppressed by specific activation of brainstem noradrenergic cell groups. The duality of the GnRH/LH response to noradrenaline reflects the inherent complexity of hindbrain noradrenaline neurons, which are responsive to stressors and gonadal steroids (ie, estradiol) and coexpress a variety of neurotransmitters and neuropeptides. Therefore, elucidating the organization and functionality of brainstem noradrenergic systems will provide targets for controlling reproduction and understanding the interconnection with stress.

Kisspeptin in functional hypothalamic amenorrhea: Pathophysiology and therapeutic potential

Functional hypothalamic amenorrhea (FHA) is one of the most common causes of secondary amenorrhea, resulting in anovulation and infertility, and is a low estrogen state that increases the risk of cardiovascular disease and impairs bone health. FHA is characterized by acquired suppression of physiological pulsatile gonadotropin-releasing hormone (GnRH) release by the hypothalamus in the absence of an identifiable structural cause, resulting in a functional hypogonadotropic hypogonadism. FHA results from either decreased energy intake and/or excessive exercise, leading to low energy availability and weight loss-often in combination with psychological stress on top of a background of genetic susceptibility. The hypothalamic neuropeptide kisspeptin is a key component of the GnRH pulse generator, tightly regulating pulsatile GnRH secretion and the downstream reproductive axis. Here, we review the physiological regulation of pulsatile GnRH secretion by hypothalamic kisspeptin neurons and how their activity is modulated by signals of energy status to affect reproductive function. We explore endocrine factors contributing to the suppression of GnRH pulsatility in the pathophysiology of FHA and how hypothalamic kisspeptin neurons likely represent a final common pathway through which these factors affect GnRH pulse generation. Finally, we discuss the therapeutic potential of kisspeptin as a novel treatment for women with FHA.

Estradiol action in the female hypothalamo-pituitary-gonadal axis

It has now been about a century since a flurry of discoveries identified first the pituitary, then more specifically the anterior pituitary and soon thereafter the central nervous system as components regulating gonadal and downstream reproductive functions. This was an era of ablation/replacement designs using at first rudimentary and then increasingly pure preparations of gonadal and pituitary "activities" or transplanting actual glands, whole or homogenized, among subjects. There was, of course, controversy as is typical of lively and productive scientific debates to this day. The goals of this commentary are to briefly review the history of this work and how the terms referring to interactions among the components of the hypothalamo (as the central neural component was soon associated with)-pituitary-gonadal (HPG) axis evolved, and then to question if the current terms used have kept up with our understanding of the system. The focus in this review will be the actions of estradiol primarily upon the hypothalamus. Important actions of progesterone on the hypothalamus as well as both steroids on the pituitary response to hypothalamic factors are both acknowledged and largely ignored in this document, as are any sex differences as we focus on females.

Oxytocin and corticotropin-releasing hormone exaggerate nucleus tractus solitarii neuronal and synaptic activity following chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) in rodents mimics the hypoxia-induced elevation of blood pressure seen in individuals experiencing episodic breathing. The brainstem nucleus tractus solitarii (nTS) is the first site of visceral sensory afferent integration, and thus is critical for cardiorespiratory homeostasis and its adaptation during a variety of stressors. In addition, the paraventricular nucleus of the hypothalamus (PVN), in part through its nTS projections that contain oxytocin (OT) and/or corticotropin-releasing hormone (CRH), contributes to cardiorespiratory regulation. Within the nTS, these PVN-derived neuropeptides alter nTS activity and the cardiorespiratory response to hypoxia. Nevertheless, their contribution to nTS activity after CIH is not fully understood. We hypothesized that OT and CRH would increase nTS activity to a greater extent following CIH, and co-activation of OT+CRH receptors would further magnify nTS activity. Our data show that compared to their normoxic controls, 10 days' CIH exaggerated nTS discharge, excitatory synaptic currents and Ca2+ influx in response to CRH, which were further enhanced by the addition of OT. CIH increased the tonic functional contribution of CRH receptors, which occurred with elevation of mRNA and protein. Together, our data demonstrate that intermittent hypoxia exaggerates the expression and function of neuropeptides on nTS activity. KEY POINTS: Episodic breathing and chronic intermittent hypoxia (CIH) are associated with autonomic dysregulation, including elevated sympathetic nervous system activity. Altered nucleus tractus solitarii (nTS) activity contributes to this response. Neurons originating in the paraventricular nucleus (PVN), including those containing oxytocin (OT) and corticotropin-releasing hormone (CRH), project to the nTS, and modulate the cardiorespiratory system. Their role in CIH is unknown. In this study, we focused on OT and CRH individually and together on nTS activity from rats exposed to either CIH or normoxia control. We show that after CIH, CRH alone and with OT increased to a greater extent overall nTS discharge, neuronal calcium influx, synaptic transmission to second-order nTS neurons, and OT and CRH receptor expression. These results provide insights into the underlying circuits and mechanisms contributing to autonomic dysfunction during periods of episodic breathing.

Allostasis in Neuroendocrine Systems Controlling Reproduction

Allostasis provides a supporting role to the homeostatic control of biological variables in mammalian species. While the concept of homeostasis is related to the control of variables within a set point or range that are essential to life, allostasis refers to systems that facilitate adaptation to challenges that the organism faces and the new requirements for survival. Essential for such adaptation is the role played by the brain in eliciting neural and neuroendocrine responses. Reproductive function is fundamental for the survival of species but is costly in energetic terms and requires a synchrony with an ever-changing environment. Thus, in many species reproductive function is blocked or delayed over immediate challenges. This review will cover the physiological systems and neuroendocrine pathways that supply allostatic control over reproductive neuroendocrine systems. Light, hypoxia, temperature, nutrition, psychosocial, and immune mediators influence the neuroendocrine control of reproductive functions through pathways that are confluent at the paraventricular nucleus; however, understanding of the integrative responses to these stimuli has not been clarified. Likely, the ultimate consequence of these allostatic mechanisms is the modification of kisspeptin and gonadotropin-releasing hormone neuronal activity, thus compromising reproduction function in the short term, while preserving species survivability.

Hypothalamic kisspeptin neurons as potential mediators of estradiol negative and positive feedback

Gonadal steroid feedback regulates the brain's patterned secretion of gonadotropin-releasing hormone (GnRH). Negative feedback, which occurs in males and during the majority of the female cycle, modulates the amplitude and frequency of GnRH pulses. Positive feedback occurs in females when high estradiol induces a surge pattern of GnRH release. These two forms of feedback and their corresponding patterns of GnRH secretion are thought to be mediated by kisspeptin-expressing neurons in two hypothalamic areas: the arcuate nucleus and the anteroventral periventricular area. In this review, we present evidence for this theory and remaining questions to be addressed.

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.

Regulation of the gonadotropin-releasing hormone neuron during stress

The effect of stress on reproduction and gonadal function has captivated investigators for almost 100 years. Following the identification of gonadotropin-releasing hormone (GnRH) 50 years ago, a niche research field emerged fixated on how stress impairs this central node controlling downstream pituitary and gonadal function. It is now clear that both episodic GnRH secretion in males and females and surge GnRH secretion in females are inhibited during a variety of stress types. There has been considerable advancement in our understanding of numerous stress-related signaling molecules and their ability to impair reproductive neuroendocrine activity during stress. Recently, much attention has turned to the effects of stress on two populations of kisspeptin neurons: the stimulatory afferents to GnRH neurons that regulate pulsatile and surge-type gonadotropin secretion. Indeed, future work is still required to fully construct the neuroanatomical framework underlying stress effects, directly or indirectly, on GnRH neuron function. The present review evaluates and synthesizes evidence related to stress-related signaling molecules acting directly on GnRH neurons. Here, we review the evidence for and against the action of a handful of signaling molecules as inhibitors of GnRH neuron function, including corticotropin-releasing hormone, urocortins, norepinephrine, cortisol/corticosterone, calcitonin gene-related peptide and arginine-phenylalanine-amide-related peptide-3.

Neuropeptides as regulators of the hypothalamus-pituitary-gonadal (HPG) axis activity and their putative roles in stress-induced fertility disorders

Neuropeptides being regulators of the hypothalamus-pituitary-adrenal (HPA) axis activity, also affect the function of the hypothalamus-pituitary-gonadal (HPG) axis by regulating gonadotrophin-releasing hormone (GnRH) secretion from hypothalamic neurons. Here, we review the available data on how neuropeptides affect HPG axis activity directly or indirectly via their influence on the HPA axis. The putative role of neuropeptides in stress-induced infertility, such as polycystic ovary syndrome, is also described. This review discusses both well-known neuropeptides (i.e., kisspeptin, Kp; oxytocin, OT; arginine-vasopressin, AVP) and more recently discovered peptides (i.e., relaxin-3, RLN-3; nesfatin-1, NEFA; phoenixin, PNX; spexin, SPX). For the first time, we present an up-to-date review of all published data regarding interactions between the aforementioned neuropeptide systems. The reviewed literature suggest new pathophysiological mechanisms leading to fertility disturbances that are induced by stress.

Neuroendocrine interactions of the stress and reproductive axes

Reproduction is controlled by a sequential regulation of the hypothalamo-pituitary-gonadal (HPG) axis. The HPG axis integrates multiple inputs to maintain proper reproductive functions. It has long been demonstrated that stress alters fertility. Nonetheless, the central mechanisms of how stress interacts with the reproductive system are not fully understood. One of the major pathways that is activated during the stress response is the hypothalamo-pituitary-adrenal (HPA) axis. In this review, we discuss several aspects of the interactions between these two neuroendocrine systems to offer insights to mechanisms of how the HPA and HPG axes interact. We have also included discussions of other systems, for example GABA-producing neurons, where they are informative to the overall picture of stress effects on reproduction.

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.

Characterization and evaluation of the tissue distribution of CRH, apelin, and GnRH2 reveal responses to feeding states in Schizothorax davidi

Schizothorax davidi is a rare fish in Southwest China and is considered a promising species for aquaculture. Compared with other teleosts, little is known about the endocrine regulation of feeding in this species. In this study, we identified the CRH, apelin, and GnRH2 genes in S. davidi and assessed the effects of different energy statuses on CRH, apelin, and GnRH2 expression. Our results showed that the full-length cDNA sequences of CRH, apelin, and GnRH2 of S. davidi were 995, 905, and 669 bp long, respectively. Furthermore, CRH was mainly expressed in the hypothalamus, telencephalon, and myelencephalon; apelin was highly expressed in the spleen and heart; and GnRH2 mRNA was widely distributed in all examined tissues, with the highest level in the hypothalamus. Notably, the levels of CRH and GnRH2 increased in the hypothalamus at 1 h and 3 h post-feeding, while hypothalamic apelin levels decreased. Conversely, CRH and GnRH2 expression in the hypothalamus significantly decreased after fasting for 7 days and returned to the control levels after re-feeding for 3 or 5 days. In contrast, fasting increased apelin levels in the hypothalamus. Overall, this study suggests that CRH, apelin, and GnRH2 play critical roles in appetite regulation in S. davidi. These results provide an essential groundwork to elucidate the appetite regulatory systems in S. davidi as well as in other teleosts.