Estradiol-Dependent Stimulation and Suppression of Gonadotropin-Releasing Hormone Neuron Firing Activity by Corticotropin-Releasing Hormone in Female Mice

Chemogenetic activation of PVN CRH neurons disrupts the estrous cycle and LH dynamics in female mice

Introduction

The impact of stress on reproductive function is significant. Hypothalamic paraventricular nucleus (PVN) corticotrophin-releasing hormone (CRH) plays a major role in regulating the stress response. Understanding how the hypothalamic-pituitary-adrenal (HPA) axis and the hypothalamic-pituitary-gonadal (HPG) axis interact is crucial for comprehending how stress can lead to reproductive dysfunction. However, whether stress influences reproductive function via modulating PVN CRH or HPA sequelae is not fully elucidated.

Methods

In this study, we investigated the impact of chemogenetic activation of PVN CRH neurons on reproductive function. We chronically and selectively stimulated PVN CRH neurons in female CRH-Cre mice using excitatory designer receptor exclusively activated by designer drugs (DREADDs) viral constructs, which were bilaterally injected into the PVN. The agonist compound-21 (C21) was delivered through the drinking water. We determined the effects of DREADDs activation of PVN CRH neurons on the estrous cycles, LH pulse frequency in diestrus and metestrus and LH surge in proestrus mice. The effect of long-term C21 administration on basal corticosterone secretion and the response to acute restraint stress during metestrus was also examined. Additionally, computer simulations of a mathematical model were used to determine the effects of DREADDs activation of PVN CRH neurons, simulating chronic stress, on the physiological parameters examined experimentally.

Results

As a result, and consistent with our mathematical model predictions, the length of the estrous cycle was extended, with an increase in the time spent in estrus and metestrus, and a decrease in proestrus and diestrus. Additionally, the frequency of LH pulses during metestrus was decreased, but unaffected during diestrus. The occurrence of the preovulatory LH surge during proestrus was disrupted. The basal level of corticosterone during metestrus was not affected, but the response to acute restraint stress was diminished after long-term C21 application.

Discussion

These data suggest that PVN CRH neurons play a functional role in disrupting ovarian cyclicity and the preovulatory LH surge, and that the activity of the GnRH pulse generator remains relatively robust during diestrus but not during metestrus under chronic stress exposure in accordance with our mathematical model predictions.

Acute restraint stress rapidly impacts reproductive neuroendocrinology and downstream gonad function in big brown bats (Eptesicus fuscus)

Animals face unpredictable challenges that require rapid, facultative physiological reactions to support survival but may compromise reproduction. Bats have a long-standing reputation for being highly sensitive to stressors, with sensitivity and resilience varying both within and among species, yet little is known about how stress affects the signaling that regulates reproductive physiology. Here, we provide the first description of the molecular response of the hypothalamic-pituitary-gonadal (HPG) axis of male big brown bats (Eptesicus fuscus) in response to short-term stress using a standardized restraint manipulation. This acute stressor was sufficient to upregulate plasma corticosterone and resulted in a rapid decrease in circulating testosterone. While we did not find differences in the mRNA expression of key steroidogenic enzymes (StAR, aromatase, 5-alpha reductase), seminiferous tubule diameter was reduced in stressed bats coupled with a 5-fold increase in glucocorticoid receptor (GR) mRNA expression in the testes. These changes, in part, may be mediated by RFamide-related peptide (RFRP) because fewer immunoreactive cell bodies were detected in the brains of stressed bats compared with controls - suggesting a possible increase in secretion - and increased RFRP expression locally in the gonads. The rapid sensitivity of the bat testes to stress may be connected to deleterious impacts on tissue health and function as supported by significant transcriptional upregulation of key pro-apoptotic signaling molecules (Bax, cytochrome c). Experiments like this broadly contribute to our understanding of the stronger ecological predictions regarding physiological responses of bats within the context of stress, which may impact decisions surrounding animal handling and conservation approaches.

BDNF Val66Met genotype and adolescent glucocorticoid treatment induce sex-specific disruptions to fear extinction and amygdala GABAergic interneuron expression in mice

BACKGROUND

The BDNF Val66Met single nucleotide polymorphism has been implicated in stress sensitivity and Post-Traumatic Stress Disorder (PTSD) risk. We previously reported that chronic young-adult stress hormone treatment enhanced fear memory in adult BDNF^Val66Met mice with the Met/Met genotype. This study aimed to extend this work to fear extinction learning, spontaneous recovery of fear, and neurobiological correlates in the amygdala.

METHODS

Male and female Val/Val and Met/Met mice received corticosterone in their drinking water during late adolescence to model chronic stress. Following a 2-week recovery period, the mice underwent fear conditioning and extinction training. Immunofluorescent labelling was used to assess density of three interneuron subtypes; somatostatin, parvalbumin and calretinin, within distinct amygdala nuclei.

RESULTS

No significant effects of genotype, treatment or sex were found for fear learning. However, adolescent CORT treatment selectively abolished fear extinction of female Met/Met mice. No effect of genotype, sex, or treatment was observed for spontaneous recovery of fear. Significant main effects of genotype and CORT emerged for somatostatin and calretinin cell density, again in females only, further supporting sex-specific effects of the Met/Met genotype and chronic CORT exposure.

CONCLUSION

BDNF Val66Met genotype interacts with chronic adolescent stress hormone exposure to abolish fear extinction in female Met/Met mice in adulthood. This effect was associated with female-specific interneuron dysfunction induced by either genotype or stress hormone exposure, depending on the interneuron subtype. These data provide biological insight into the role of BDNF in sex differences in sensitivity to stress and vulnerability to stress-related disorders in adulthood.

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.

The effects of neonatal maternal deprivation and chronic unpredictable stresses on migraine-like behaviors in adult rats

BACKGROUND

Stress is known to cause migraine. This study investigates the effects of neonatal maternal deprivation (MD) and chronic unpredictable stress (CUS) on migraine in rats.

METHODS

Seventy rats were randomly divided into ten groups (five groups of each sex, and seven rats/group). The groups included: untreated intact, nitroglycerin (NTG) only, NTG + MD, NTG + CUS (10 weeks after birth), and NTG + MD + CUS. For the induction of MD, pups were separated from their mothers from postnatal day 2 to day 14. The CUS was conducted by daily exposure to different stressors for 2 weeks. For the induction of migraine after stress, NTG (5 mg/kg/IP) was administered every second day for 9 days. Afterward, NTG-related symptoms, including climbing behavior, facial rubbing, body grooming, freezing behavior, and head-scratching, were recorded for 90 min. Statistical differences between the groups were analyzed by one-way and two-way ANOVA followed by the Newman-Keuls test.

RESULTS

Migraine symptoms, including increased head-scratching, facial rubbing, and decreased climbing behavior, were more significant in females than in males. Head scratching and facial rubbing increased in stressed females, but not in males as compared to NTG-treated rats. Body grooming was significantly decreased in MD males compared to the NTG group. The effects of NTG in MD + CUS on the rats did not differ from those in the MD or CUS groups.

CONCLUSIONS

MD and CUS had a sex-related aggravating effect on the development of migraine, while the combination of MD and CUS had no additive migraine-aggravating effect.

Regulation of stress response on the hypothalamic-pituitary-gonadal axis via gonadotropin-inhibitory hormone

Under stressful condition, reproductive function is impaired due to the activation of various components of the hypothalamic-pituitaryadrenal (HPA) axis, which can suppress the activity of the hypothalamic-pituitary-gonadal (HPG) axis at multiple levels. A hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH) is a key negative regulator of reproduction that governs the HPG axis. Converging lines of evidence have suggested that different stress types and their duration, such as physical or psychological, and acute or chronic, can modulate the GnIH system. To clarify the sensitivity and reactivity of the GnIH system in response to stress, we summarize and critically review the available studies that investigated the effects of various stressors, such as restraint, nutritional/metabolic and social stress, on GnIH expression and/or its neuronal activity leading to altered HPG action. In this review, we focus on GnIH as the potential novel mediator responsible for stress-induced reproductive dysfunction.

Urocortin3 in the Posterodorsal Medial Amygdala Mediates Stress-induced Suppression of LH Pulsatility in Female Mice

Psychosocial stress disrupts reproduction and interferes with pulsatile LH secretion. The posterodorsal medial amygdala (MePD) is an upstream modulator of the reproductive axis and stress. Corticotropin-releasing factor type 2 receptors (CRFR2s) are activated in the presence of psychosocial stress together with increased expression of the CRFR2 ligand Urocortin3 (Ucn3) in the MePD of rodents. We investigate whether Ucn3 signalling in the MePD is involved in mediating the suppressive effect of psychosocial stress on LH pulsatility. First, we administered Ucn3 into the MePD and monitored the effect on LH pulses in ovariectomized mice. Next, we delivered Astressin2B, a selective CRFR2 antagonist, intra-MePD in the presence of predator odor, 2,4,5-trimethylthiazole (TMT) and examined the effect on LH pulses. Subsequently, we virally infected Ucn3-cre-tdTomato mice with inhibitory designer receptor exclusively activated by designer drugs (DREADDs) targeting MePD Ucn3 neurons while exposing mice to TMT or restraint stress and examined the effect on LH pulsatility as well as corticosterone release. Administration of Ucn3 into the MePD dose-dependently inhibited LH pulses and administration of Astressin2B blocked the suppressive effect of TMT on LH pulsatility. Additionally, DREADDs inhibition of MePD Ucn3 neurons blocked TMT and restraint stress-induced inhibition of LH pulses and corticosterone release. These results demonstrate for the first time that Ucn3 neurons in the MePD mediate psychosocial stress-induced suppression of the GnRH pulse generator and corticosterone secretion. Ucn3 signalling in the MePD plays a role in modulating the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes, and this brain locus may represent a nodal center in the interaction between the reproductive and stress axes.

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.

Neuroendocrine regulation of pubertal suppression in the naked mole-rat: What we know and what comes next

Puberty is a key developmental milestone that marks an individual's maturation in several ways including, but not limited to, reproductive maturation, changes in behaviors and neural organization. The timing at which puberty occurs is variable both within individuals of the same species and between species. These variations can be aligned with ecological cues that delay or suppress puberty. Naked mole-rats are colony-living rodents where reproduction is restricted to a few animals; all other animals are pubertally-suppressed. Animals removed from suppressive colony cues can reproductively mature, presenting the unique opportunity to study adult-onset puberty. Recently, we found that RFRP-3 administration sustains pubertal delay in naked mole-rats removed from colony. In this review, we explore what is known about regulators that control puberty onset, the role of stress/social status in pubertal timing, the status of knowledge of pubertal suppression in naked mole-rats and what comes next.

Indirect Suppression of Pulsatile LH Secretion by CRH Neurons in the Female Mouse

Acute stress is a potent suppressor of pulsatile luteinizing hormone (LH) secretion, but the mechanisms through which corticotrophin-releasing hormone (CRH) neurons inhibit gonadotropin-releasing hormone (GnRH) release remain unclear. The activation of paraventricular nucleus (PVN) CRH neurons with Cre-dependent hM3Dq in Crh-Cre female mice resulted in the robust suppression of pulsatile LH secretion. Channelrhodopsin (ChR2)-assisted circuit mapping revealed that PVN CRH neuron projections existed around kisspeptin neurons in the arcuate nucleus (ARN) although many more fibers made close appositions with GnRH neuron distal dendrons in the ventral ARN. Acutely prepared brain slice electrophysiology experiments in GnRH- green fluorescent protein (GFP) mice showed a dose-dependent (30 and 300 nM CRH) activation of firing in ~20% of GnRH neurons in both intact diestrus and ovariectomized mice with inhibitory effects being uncommon (<8%). Confocal GCaMP6 imaging of GnRH neuron distal dendrons in acute para-horizontal brain slices from GnRH-Cre mice injected with Cre-dependent GCaMP6s adeno-associated viruses demonstrated no effects of 30 to 300 nM CRH on GnRH neuron dendron calcium concentrations. Electrophysiological recordings of ARN kisspeptin neurons in Crh-Cre,Kiss1-GFP mice revealed no effects of 30 -300 nM CRH on basal or neurokinin B-stimulated firing rate. Similarly, the optogenetic activation (2-20 Hz) of CRH nerve terminals in the ARN of Crh-Cre,Kiss1-GFP mice injected with Cre-dependent ChR2 had no effect on kisspeptin neuron firing. Together, these studies demonstrate that PVN CRH neurons potently suppress LH pulsatility but do not exert direct inhibitory control over GnRH neurons, at their cell body or dendron, or the ARN kisspeptin neuron pulse generator in the female mouse.

Gonadal Cycle-Dependent Expression of Genes Encoding Peptide-, Growth Factor-, and Orphan G-Protein-Coupled Receptors in Gonadotropin- Releasing Hormone Neurons of Mice

Rising serum estradiol triggers the surge release of gonadotropin-releasing hormone (GnRH) at late proestrus leading to ovulation. We hypothesized that proestrus evokes alterations in peptidergic signaling onto GnRH neurons inducing a differential expression of neuropeptide-, growth factor-, and orphan G-protein-coupled receptor (GPCR) genes. Thus, we analyzed the transcriptome of GnRH neurons collected from intact, proestrous and metestrous GnRH-green fluorescent protein (GnRH-GFP) transgenic mice using Affymetrix microarray technique. Proestrus resulted in a differential expression of genes coding for peptide/neuropeptide receptors including Adipor1, Prokr1, Ednrb, Rtn4r, Nmbr, Acvr2b, Sctr, Npr3, Nmur1, Mc3r, Cckbr, and Amhr2. In this gene cluster, Adipor1 mRNA expression was upregulated and the others were downregulated. Expression of growth factor receptors and their related proteins was also altered showing upregulation of Fgfr1, Igf1r, Grb2, Grb10, and Ngfrap1 and downregulation of Egfr and Tgfbr2 genes. Gpr107, an orphan GPCR, was upregulated during proestrus, while others were significantly downregulated (Gpr1, Gpr87, Gpr18, Gpr62, Gpr125, Gpr183, Gpr4, and Gpr88). Further affected receptors included vomeronasal receptors (Vmn1r172, Vmn2r-ps54, and Vmn1r148) and platelet-activating factor receptor (Ptafr), all with marked downregulation. Patch-clamp recordings from mouse GnRH-GFP neurons carried out at metestrus confirmed that the differentially expressed IGF-1, secretin, and GPR107 receptors were operational, as their activation by specific ligands evoked an increase in the frequency of miniature postsynaptic currents (mPSCs). These findings show the contribution of certain novel peptides, growth factors, and ligands of orphan GPCRs to regulation of GnRH neurons and their preparation for the surge release.

The impact of chronic restraint stress on the estrous cycle in NMRI female mice

Introduction: Stress represents a set of reactions in the organism activated by external factors. In order to maintain homeostasis and protect the organism, numerous mechanisms for adaptation to stress evolved. Stressors that act in short-term period cause acute stress reaction with generally positive effect on organism. When the stressor persists, and the organism fails to respond to the challenge, chronic stress develops, leading to pathological conditions, such as women's menstrual cycle disorders. Aim: To examine the impact of chronic restraint stress on the estrous cycle in NMRI female mice. Material and methods: A number of 12 mature female NMRI mice were randomly divided into control (n = 6) and experimental (n = 6) group. The induction of stress was performed for the experimental group by using restrain chambers 2 hours daily in 14 days. From the beginning of the experiment, vaginal lavages were taken from all mice for making smears that were analyzed to determine estrous cycle stages. For analysis of chronic stress effect, the frequency of estrus stages alternation observed in experimental group compared to control was examined. Results: The estrous cycle was observed and divided into proestrus, estrus, metestrus and diestrus. During the adaptation period, prolonged diestrus was dominantly present in both groups. When stress was induced, in the experimental group animals the absence of diestrus stage and oscillation to other stages was obseved, in contrast to the control group, where the diestus stage was frequently observed. Frequency of diestrus stage deviation in stressed animals was shown to be statistically significant (p < 0.01) when compared to control. Conclusion: The induction of chronic restraint stress in female NMRI mice leads to the alternation of the estrous cycle. Considering the fact that NMRI female mice share the similar hormonal regulation of the estrous cycle with women's menstrual cycle, they could present a solid model for studying women's reproductive disorders.

Dynamic Hormone Control of Stress and Fertility

Neuroendocrine axes display a remarkable diversity of dynamic signaling processes relaying information between the brain, endocrine glands, and peripheral target tissues. These dynamic processes include oscillations, elastic responses to perturbations, and plastic long term changes observed from the cellular to the systems level. While small transient dynamic changes can be considered physiological, larger and longer disruptions are common in pathological scenarios involving more than one neuroendocrine axes, suggesting that a robust control of hormone dynamics would require the coordination of multiple neuroendocrine clocks. The idea of apparently different axes being in fact exquisitely intertwined through neuroendocrine signals can be investigated in the regulation of stress and fertility. The stress response and the reproductive cycle are controlled by the Hypothalamic-Pituitary-Adrenal (HPA) axis and the Hypothalamic-Pituitary-Gonadal (HPG) axis, respectively. Despite the evidence surrounding the effects of stress on fertility, as well as of the reproductive cycle on stress hormone dynamics, there is a limited understanding on how perturbations in one neuroendocrine axis propagate to the other. We hypothesize that the links between stress and fertility can be better understood by considering the HPA and HPG axes as coupled systems. In this manuscript, we investigate neuroendocrine rhythms associated to the stress response and reproduction by mathematically modeling the HPA and HPG axes as a network of interlocked oscillators. We postulate a network architecture based on physiological data and use the model to predict responses to stress perturbations under different hormonal contexts: normal physiological, gonadectomy, hormone replacement with estradiol or corticosterone (CORT), and high excess CORT (hiCORT) similar to hypercortisolism in humans. We validate our model predictions against experiments in rodents, and show how the dynamic responses of these endocrine axes are consistent with our postulated network architecture. Importantly, our model also predicts the conditions that ensure robustness of fertility to stress perturbations, and how chronodisruptions in glucocorticoid hormones can affect the reproductive axis' ability to withstand stress. This insight is key to understand how chronodisruption leads to disease, and to design interventions to restore normal rhythmicity and health.

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

GnRH neurons are central regulators of reproduction and respond to factors affecting fertility, such as stress. Corticotropin-releasing hormone (CRH) is released during stress response. In brain slices from unstressed controls, CRH has opposite, estradiol-dependent effects on GnRH neuron firing depending on the CRH receptor activated; activating CRHR-1 stimulates whereas activating CRHR-2 suppresses activity. We investigated possible direct and indirect mechanisms. Mice were ovariectomized and either not treated further (OVX) or given a capsule producing high positive feedback (OVX + E) or low negative feedback (OVX + low E) physiologic circulating estradiol levels. We tested possible direct effects on GnRH neurons by altering voltage-gated potassium currents. Two types of voltage-gated potassium currents (transient IA and sustained IK) were measured; neither CRHR-1 nor CRHR-2 agonists altered potassium current density in GnRH neurons from OVX + E mice. Further, neither CRH nor receptor-specific agonists altered action potential generation in response to current injection in GnRH neurons from OVX + E mice. To test the possible indirect actions, GABAergic postsynaptic currents were monitored. A CRHR-1 agonist increased GABAergic transmission frequency to GnRH neurons from OVX + E, but not OVX, mice, whereas a CRHR-2 agonist had no effect. Finally, we tested if CRH alters the firing rate of arcuate kisspeptin neurons, which provide an important excitatory neuromodulatory input to GnRH neurons. CRH did not acutely alter firing activity of these neurons from OVX, OVX + E or OVX + low E mice. These results suggest CRH increases GnRH neuron activity in an estradiol-dependent manner in part by activating GABAergic afferents. Mechanisms underlying inhibitory effects of CRH remain unknown.

Firing patterns of gonadotropin-releasing hormone neurons are sculpted by their biologic state

Gonadotropin-releasing hormone (GnRH) neurons form the final pathway for the central neuronal control of fertility. GnRH is released in pulses that vary in frequency in females, helping drive hormonal changes of the reproductive cycle. In the common fertility disorder polycystic ovary syndrome (PCOS), persistent high-frequency hormone release is associated with disrupted cycles. We investigated long- and short-term action potential patterns of GnRH neurons in brain slices before and after puberty in female control and prenatally androgenized (PNA) mice, which mimic aspects of PCOS. A Monte Carlo (MC) approach was used to randomize action potential interval order. Dataset distributions were analysed to assess (i) if organization persists in GnRH neuron activity in vitro, and (ii) to determine if any organization changes with development and/or PNA treatment. GnRH neurons in adult control, but not PNA, mice produce long-term patterns different from MC distributions. Short-term patterns differ from MC distributions before puberty but become absorbed into the distributions with maturation, and the distributions narrow. These maturational changes are blunted by PNA treatment. Firing patterns of GnRH neurons in brain slices thus maintain organization dictated at least in part by the biologic status of the source and are disrupted in models of disease.

Neural and endocrine mechanisms underlying stress-induced suppression of pulsatile LH secretion

Stress is well-known to inhibit a variety of reproductive processes, including the suppression of episodic Gonadotropin releasing hormone (GnRH) secretion, typically measured via downstream luteinizing hormone (LH) secretion. Since pulsatile secretion of GnRH and LH are necessary for proper reproductive function in both males and females, and stress is common for both human and animals, understanding the fundamental mechanisms by which stress impairs LH pulses is of critical importance. Activation of the hypothalamic-pituitary-adrenal axis, and its corresponding endocrine factors, is a key feature of the stress response, so dissecting the role of stress hormones, including corticotrophin releasing hormone (CRH) and corticosterone, in the inhibition of LH secretion has been one key research focus. However, some evidence suggests that these stress hormones alone are not sufficient for the full inhibition of LH caused by stress, implicating the additional involvement of other hormonal or neural signaling pathways in this process (including inputs from the brainstem, amygdala, parabrachial nucleus, and dorsomedial nucleus). Moreover, different stress types, such as metabolic stress (hypoglycemia), immune stress, and psychosocial stress, appear to suppress LH secretion via partially unique neural and endocrine pathways. The mechanisms underlying the suppression of LH pulses in these models offer interesting comparisons and contrasts, including the specific roles of amygdaloid nuclei and CRH receptor types. This review focuses on the most recent and emerging insights into endocrine and neural mechanisms responsible for the suppression of pulsatile LH secretion in mammals, and offers insights in important gaps in knowledge.

History of Estrogen: Its Purification, Structure, Synthesis, Biologic Actions, and Clinical Implications

This mini-review summarizes key points from the Clark Sawin Memorial Lecture on the History of Estrogen delivered at Endo 2018 and focuses on the rationales and motivation leading to various discoveries and their clinical applications. During the classical period of antiquity, incisive clinical observations uncovered important findings; however, extensive anatomical dissections to solidify proof were generally lacking. Initiation of the experimental approach followed later, influenced by Claude Bernard's treatise "An Introduction to the Study of Experimental Medicine." With this approach, investigators began to explore the function of the ovaries and their "internal secretions" and, after intensive investigations for several years, purified various estrogens. Clinical therapies for hot flashes, osteoporosis, and dysmenorrhea were quickly developed and, later, methods of hormonal contraception. Sophisticated biochemical methods revealed the mechanisms of estrogen synthesis through the enzyme aromatase and, after discovery of the estrogen receptors, their specific biologic actions. Molecular techniques facilitated understanding of the specific transcriptional and translational events requiring estrogen. This body of knowledge led to methods to prevent and treat hormone-dependent neoplasms as well as a variety of other estrogen-related conditions. More recently, the role of estrogen in men was uncovered by prismatic examples of estrogen deficiency in male patients and by knockout of the estrogen receptor and aromatase in animals. As studies became more extensive, the effects of estrogen on nearly every organ were described. We conclude that the history of estrogen illustrates the role of intellectual reasoning, motivation, and serendipity in advancing knowledge about this important sex steroid.

Neuroanatomical Framework of the Metabolic Control of Reproduction

A minimum amount of energy is required for basic physiological processes, such as protein biosynthesis, thermoregulation, locomotion, cardiovascular function, and digestion. However, for reproductive function and survival of the species, extra energy stores are necessary. Production of sex hormones and gametes, pubertal development, pregnancy, lactation, and parental care all require energy reserves. Thus the physiological systems that control energy homeostasis and reproductive function coevolved in mammals to support both individual health and species subsistence. In this review, we aim to gather scientific knowledge produced by laboratories around the world on the role of the brain in integrating metabolism and reproduction. We describe essential neuronal networks, highlighting key nodes and potential downstream targets. Novel animal models and genetic tools have produced substantial advances, but critical gaps remain. In times of soaring worldwide obesity and metabolic dysfunction, understanding the mechanisms by which metabolic stress alters reproductive physiology has become crucial for human health.

Emerging insights into hypothalamic-pituitary-gonadal axis regulation and interaction with stress signalling

Reproduction and fertility are regulated via hormones of the hypothalamic-pituitary-gonadal (HPG) axis. Control of this reproductive axis occurs at all levels, including the brain and pituitary, and allows for the promotion or inhibition of gonadal sex steroid secretion and function. In addition to guiding proper gonadal development and function, gonadal sex steroids also act in negative- and positive-feedback loops to regulate reproductive circuitry in the brain, including kisspeptin neurones, thereby modulating overall HPG axis status. Additional regulation is also provided by sex steroids made within the brain, including neuroprogestins. Furthermore, because reproduction and survival need to be coordinated and balanced, the HPG axis is able to modulate (and be modulated by) stress hormone signalling, including cortiscosterone, from the hypothalamic-pituitary-adrenal (HPA) axis. This review covers recent data related to the neural, hormonal and stress regulation of the HPG axis and emerging interactions between the HPG and HPA axes, focusing on actions at the level of the brain and pituitary.

Deciphering the Contributions of CRH Receptors in the Brain and Pituitary to Stress-Induced Inhibition of the Reproductive Axis

Based on pharmacological studies, corticotropin-releasing hormone (CRH) and its receptors play a leading role in the inhibition of the hypothalamic–pituitary–gonadal (HPG) axis during acute stress. To further study the effects of CRH receptor signaling on the HPG axis, we generated and/or employed male mice lacking CRH receptor type 1 (CRHR1) or type 2 (CRHR2) in gonadotropin-releasing hormone neurons, GABAergic neurons, or in all central neurons and glia. The deletion of CRHRs revealed a preserved decrease of plasma luteinizing hormone (LH) in response to either psychophysical or immunological stress. However, under basal conditions, central infusion of CRH into mice lacking CRHR1 in all central neurons and glia, or application of CRH to pituitary cultures from mice lacking CRHR2, failed to suppress LH release, unlike in controls. Our results, taken together with those of the earlier pharmacological studies, suggest that inhibition of the male HPG axis during acute stress is mediated by other factors along with CRH, and that CRH suppresses the HPG axis at the central and pituitary levels via CRHR1 and CRHR2, respectively.

GnRH Neurons on LSD: A Year of Rejecting Hypotheses That May Have Made Karl Popper Proud

Gonadotropin-releasing hormone (GnRH) neurons are critical to many aspects of fertility regulation, from producing episodic release critical to both sexes, to providing a central signal to induce the ovulatory cascade in females. This year saw progress through the rejection, and occasional support, of hypotheses in understanding how GnRH neurons contribute to these processes. This brief review provides one laboratory's view of new insights into possible roles for these cells in development, adult reproductive function, and what may go wrong with GnRH neurons in some cases of infertility.