Exposure to Acute Psychosocial Stress Disrupts the Luteinizing Hormone Surge Independent of Estrous Cycle Alterations in Female Mice

The cycle of stress: A systematic review of the impact of chronic psychological stress models on the rodent estrous cycle

Stress is known to impair reproduction through interactions between the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes. However, while it is well accepted that stress can alter estrous cycle regularity, a key indicator of female's HPG axis function, effects of different types of psychological stress have been inconsistent. This systematic review evaluated the impact of rodent models of psychological stress on estrous cyclicity, while reporting biological parameters pertaining to HPA or HPG axis function assessed within these studies. We performed a systematic database search and included articles that implemented a psychological stress model in rodents and reported estrous cyclicity for at least two cycles after initiation of stress. Of the 32 studies included, 62.5% reported post-stress alterations to estrous cyclicity, with Chronic Mild Stress (CMS) models showing the most conclusive effects. Twenty-five studies measured HPG or HPA axis markers, with cycle disruptions being commonly observed in parallel with altered estradiol and increased corticosterone levels. Our review highlights gaps in reporting estrous cyclicity assessments and makes recommendations to improve comparability between studies.

Early-Life Resource Scarcity in Mice Does Not Alter Adult Corticosterone or Preovulatory Luteinizing Hormone Surge Responses to Acute Psychosocial Stress

Early-life stressors can affect reproductive development and change responses to adult stress. We tested if resource scarcity in the form of limited bedding and nesting (LBN) from postnatal days (PND) 4 to 11 delayed sexual maturation in male and female mice and/or altered the response to an acute, layered, psychosocial stress (ALPS) in adulthood. Contrary to the hypotheses, age and mass at puberty were unaffected by the present application of LBN. Under basal conditions and after ALPS, corticosterone concentrations in males, diestrous females, and proestrous females reared in standard (STD) or LBN environments were similar. ALPS disrupts the luteinizing hormone (LH) surge in most mice when applied on the morning of proestrus; this effect was not changed by resource scarcity. In this study, the paucity of effects in the offspring may relate to a milder response of CBA dams to the paradigm. While LBN dams exited the nest more often and their offspring were smaller than STD-reared offspring on PND11, dam corticosterone concentrations were similar on PND11. To test if ALPS disrupts the LH surge by blunting the increase in excitatory GABAergic input to gonadotropin-releasing hormone (GnRH) neurons on the afternoon of proestrus, we conducted whole-cell voltage-clamp recordings. The frequency of GABAergic postsynaptic currents in GnRH neurons was not altered by LBN, ALPS, or their interaction. It remains possible that ALPS acts at afferents of GnRH neurons, changes response of GnRH neurons to input, and/or alters pituitary responsiveness to GnRH and that a more pronounced resource scarcity would affect the parameters studied.

Gonads under stress: A systematic review and meta-analysis on the effects of acute psychosocial stress on gonadal steroids secretion in humans

Animal research has shown that the hypothalamus-pituitary-gonadal (HPG) axis is inhibited by (chronic and/or severe) stress, which can lead to impaired fertility and reproductive functioning, presumably caused by the inhibition of gonadal steroid secretion and in interactions with glucocorticoids. However, what has not been clarified is how acute psychosocial stress modulates gonadal steroid secretion in humans. Here we summarize the experimental research on the acute effects of stress on the secretion of gonadal steroids in humans. A systematic literature search revealed 21 studies (with N=881 individuals) measuring testosterone, progesterone or estradiol in response to a standardized acute laboratory stressor in healthy humans. Both our literature review and quantitative meta-analysis suggest that in humans, acute stress stimulates rather than inhibits HPG axis activity, although there is a considerable heterogeneity in the reported methods and results. Increased gonadal steroids in response to acute stress contrasts with many animal studies reporting the opposite pattern, at least regarding severe and/or chronic stressors. We discuss methodological issues and challenges for future research and hope to stimulate experimental studies within this area. A better understanding of these mechanisms is needed, and may have important implications for health and disease, as well as the modulation of various behaviors by acute stressors.

NK3R signalling in the posterodorsal medial amygdala is involved in stress-induced suppression of pulsatile LH secretion in female mice

Psychosocial stress negatively impacts reproductive function by inhibiting pulsatile luteinizing hormone (LH) secretion. The posterodorsal medial amygdala (MePD) is responsible in part for processing stress and modulating the reproductive axis. Activation of the neurokinin 3 receptor (NK3R) suppresses the gonadotropin-releasing hormone (GnRH) pulse generator, under hypoestrogenic conditions, and NK3R activity in the amygdala has been documented to play a role in stress and anxiety. We investigate whether NK3R activation in the MePD is involved in mediating the inhibitory effect of psychosocial stress on LH pulsatility in ovariectomised female mice. First, we administered senktide, an NK3R agonist, into the MePD and monitored the effect on pulsatile LH secretion. We then delivered SB222200, a selective NK3R antagonist, intra-MePD in the presence of predator odour, 2,4,5-trimethylthiazole (TMT) and examined the effect on LH pulses. Senktide administration into the MePD dose-dependently suppresses pulsatile LH secretion. Moreover, NK3R signalling in the MePD mediates TMT-induced suppression of the GnRH pulse generator, which we verified using a mathematical model. The model verifies our experimental findings: (i) predator odour exposure inhibits LH pulses, (ii) activation of NK3R in the MePD inhibits LH pulses and (iii) NK3R antagonism in the MePD blocks stressor-induced inhibition of LH pulse frequency in the absence of ovarian steroids. These results demonstrate for the first time that NK3R neurons in the MePD mediate psychosocial stress-induced suppression of the GnRH pulse generator.

A mouse model of oral contraceptive exposure: Depression, motivation, and the stress response

Hormonal contraceptives, including oral contraceptives (OCs), regulate hormonal cycles and broadly affect physiological processes, including stress responsivity. Whereas many users describe overall improved mood, up to 10 % of OC users experience adverse effects, including depression and anxiety. Given the link between regulation of hypothalamic-pituitary-adrenal (HPA) axis, stress exposure, and risk for depression, it is likely that OC-effects on stress mediate increased risk or increased resilience to these disorders. In this study, we developed and characterized a tractable mouse model of OC exposure with which to identify the mechanisms underlying OC modulation of brain, behavior, and mood. Specifically, we aimed to determine whether translationally relevant doses of OC-hormones in mice mimic changes in stress responsivity observed in humans taking OCs and describe behavioral changes during OC exposure. Young adult female C57Bl/6 N mice received daily ethinyl estradiol (EE) and levonorgestrel (LVNG) in 10 % sucrose, EE and drospirenone (DRSP) in 10 % sucrose, or 10 % sucrose alone. Translationally relevant doses of EE + LVNG-exposure, but not EE + DRSP, suppressed the acute stress response, consistent with effects observed in human OC users. EE + LVNG caused a specific anhedonia-like effect, without broad changes in stress-coping behavior, other depression-like behaviors, or anxiety-like behaviors. The suppression of regular estrous cycling, together with the blunting of the corticosterone response to acute stress, demonstrate the utility of this model for future studies to identify the mechanisms underlying OC interactions with stress, motivation, and risk for depression.

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.

Higher Depressive Symptoms in Irregular Menstrual Cycles: Converging Evidence from Cross-Sectional and Prospective Assessments

BACKGROUND

Menstrual cycle regularity is an important marker of reproductive health and associated with physiological and psychological illnesses, as well as experiencing stress. We hypothesized that individuals with irregular menstrual cycles report higher depressive symptom severity, after controlling for stress occurrence.

METHODS

The hypothesis was examined through two measurement approaches: a cross-sectional and a prospective, longitudinal study. In the cross-sectional study, participants (n = 394) reported depressive symptoms and their overall menstrual cycle regularity. In the longitudinal study, participants (n = 77) completed questionnaires on depressive symptoms and stress during the mid-follicular and periovulatory phase of one menstrual cycle. Depressive symptoms were compared between participants with regular and irregular cycles through a Welch t test and an ANCOVA.

RESULTS

Participants with irregular menstrual cycles reported more depressive symptoms in the cross-sectional analysis. Similarly, in the longitudinal analysis, the group with a current irregular menstrual cycle reported more depressive symptoms after controlling for stress occurrence. When including only complete data sets without multiple imputation (n = 52), the direction of the effects remained but did not reach statistical significance.

CONCLUSIONS

The results indicate an association between depressive symptoms and menstrual cycle irregularity. Limitations were that although we investigated the menstrual cycle prospectively, it would have been more precise to include two or more cycles and daily sex hormone measurements. Further limitations were the suboptimal statistical power and the data collection during the COVID pandemic. We give recommendations on how to incorporate the association of depressive symptoms and cycle irregularity in future study designs on women's mental health.

Effect of Bee Bread on Corticosterone Level in Rat Dams Exposed to Gestational Heat Stress

Environmental temperature rises are powerful stimuli that can alter both the sympathetic nervous system and the hypothalamic-pituitary-adrenocortical axis (HPA). Heat stress has been shown to harm pregnancy outcomes such as causing spontaneous abortion, low birth weight, growth retardation and stillbirth. Supplementation of bee bread in pregnant rats under heat stress exposure has been shown to improve the pregnancy outcomes. However, whether supplementation of bee bread during heat stress exposure may also reduce the level of the stress hormone, corticosterone has yet been reported. Therefore, this study aims to determine the effect of bee bread on corticosterone level, progesterone level, oestradiol level and zonation of the adrenal cortex of pregnant rats under heat stress exposure. Pregnant rats were randomly categorised into four groups (n = 6): Control (C: standard feeding), Treatment 1 (T1: 0.5 g bee bread/kg body weight/day), Treatment 2 (T2: standard feeding with heat exposure), and Treatment 3 (T3: 0.5 g bee bread/kg body weight/day with heat exposure). Bee bread (0.5 g/kg body weight/day) was force-fed to pregnant rats through oral gavage beginning on day 0 of pregnancy and continuing until delivery. Heat stress was generated experimentally by putting both T2 and T3 rats in an egg incubator for 45 min daily at a temperature of 43°C till delivery. On a postnatal Day 21, dams were euthanised to assess serum corticosterone, progesterone, oestradiol levels and adrenal gland histology. Rats in the T2 group had a significantly (P < 0.05) increase in the zona fasciculata thickness (94.95 ± 1.55 μm) and higher corticosterone levels (49.57 ± 1.57 ng/mL) compared with control. However, supplementation of bee bread during heat stress was able to show an improvement in adrenal zona fasciculata thickness by decreasing to 79.89 ± 3.08 μm and corticosterone level reduced to 35.31 ± 1.73 ng/mL significantly (P < 0.05). Therefore, these findings may imply that bee bread is effective as a neutralizer in lowering the production of stress hormone.

Menstrual cycle-related changes in HPA axis reactivity to acute psychosocial and physiological stressors - a systematic review and meta-analysis of longitudinal studies

Sex disparities are evident in the biological response to acute stressors, with a suggested influence of ovarian hormones on hypothalamic-pituitary-adrenal (HPA) axis functioning. This systematic review and meta-analysis investigates differences in HPA axis reactivity to acute psychosocial or physiological stressors between menstrual cycle phases. A systematic literature search of six databases resulted in 12 longitudinal studies (n = 182) examining HPA axis reactivity in healthy, naturally-cycling, non-breastfeeding participants aged between 18 and 45 years in at least two cycle phases. The quality of cortisol and menstrual cycle assessment was rated and a descriptive synthesis and meta-analysis of HPA axis reactivity between two broader and five more precise cycle phases was conducted. Three studies provided sufficient data for the meta-analysis and showed a significant, small-sized effect, indicating higher cortisol reactivity in the luteal than in the follicular cycle phase. More primary studies with high-quality menstrual cycle and cortisol assessment are needed. The review did not receive funding and was pre-registered (PROSPERO; CRD42020181632).

Continuous body temperature as a window into adolescent development

Continuous body temperature is a rich source of information on hormonal status, biological rhythms, and metabolism, all of which undergo stereotyped change across adolescence. Due to the direct actions of these dynamic systems on body temperature regulation, continuous temperature may be uniquely suited to monitoring adolescent development and the impacts of exogenous reproductive hormones or peptides (e.g., hormonal contraception, puberty blockers, gender affirming hormone treatment). This mini-review outlines how traditional methods for monitoring the timing and tempo of puberty may be augmented by markers derived from continuous body temperature. These features may provide greater temporal precision, scalability, and reduce reliance on self-report, particularly in females. Continuous body temperature data can now be gathered with ease across a variety of wearable form factors, providing the opportunity to develop tools that aid in individual, parental, clinical, and researcher awareness and education.

Understanding the Diurnal Oscillation of the Gut Microbiota Using Microbial Culture

The composition of the gut microbiota oscillates according to the light-dark cycle. However, the existing literature demonstrates these oscillations only by molecular methods. Microbial cultures are an interesting method for studying metabolically active microorganisms. In this work, we aimed to understand the diurnal oscillation of the intestinal microbiota in Wistar male rats through microbial culture analysis. Over a 24 h period, three animals were euthanized every 6 h. Intestinal segments were dissected immediately after euthanasia and diluted in phosphate-buffered saline (PBS) for plating in different culture media. The CFU/mL counts in feces samples cultured in the Brucella medium were significantly higher at ZT0, followed by ZT6, ZT18, and ZT12 (p = 0.0156), which demonstrated the diurnal oscillation of metabolically active anaerobic bacteria every 6 h using microbial culture. In addition, quantitative differences were demonstrated in anaerobic bacteria and fungi in different gastrointestinal tract tissues.

The Corticosterone–Glucocorticoid Receptor–AP1/CREB Axis Inhibits the Luteinizing Hormone Receptor Expression in Mouse Granulosa Cells

Under stress conditions, luteinizing hormone (LH)-mediated ovulation is inhibited, resulting in insufficient oocyte production and excretion during follicular development. When the body is stressed, a large amount of corticosterone (CORT) is generated, which will lead to a disorder of the body’s endocrine system and damage to the body. Our previous work showed that CORT can block follicular development in mice. Since LH acts through binding with the luteinizing hormone receptor (Lhcgr), the present study aimed to investigate whether and how corticosterone (CORT) influences Lhcgr expression in mouse ovarian granulosa cells (GCs). For this purpose, three-week-old ICR female mice were injected intraperitoneally with pregnant mare serum gonadotropin (PMSG). In addition, the treatment group was injected with CORT (1 mg/mouse) at intervals of 8 h and the control group was injected with the same volume of methyl sulfoxide (DMSO). GCs were collected at 24 h, 48 h, and 55 h after PMSG injection. For in vitro experiments, the mouse GCs obtained from healthy follicles were treated with CORT alone, or together with inhibitors against the glucocorticoid receptor (Nr3c1). The results showed that the CORT caused a downregulation of Lhcgr expression in GCs, which was accompanied by impaired cell viability. Moreover, the effect of the CORT was mediated by binding to its receptor (Nr3c1) in GCs. Further investigation revealed that Nr3c1 might regulate the transcription of Lhcgr through inhibiting the expression of Lhcgr transcription factors, including AP1 and Creb. Taken together, our findings suggested a possible mechanism of CORT-induced anovulation involving the inhibition of Lhcgr expression in GCs by the CORT–Nr3c1–AP1/Creb axis.

Pattern of gonadotropin secretion along the estrous cycle of C57BL/6 female mice

The pattern of gonadotropin secretion along the estrous cycle was elegantly described in rats. Less information exists about the pattern of gonadotropin secretion in gonad-intact mice, particularly regarding the follicle-stimulating hormone (FSH). Using serial blood collections from the tail-tip of gonad-intact C57BL/6 mice on the first day of cornification (transition from diestrus to estrus; hereafter called proestrus), we observed that the luteinizing hormone (LH) and FSH surge cannot be consistently detected since only one out of eight females (12%) showed increased LH levels. In contrast, a high percentage of mice (15 out of 21 animals; 71%) exhibited LH and FSH surges on the proestrus when a single serum sample was collected. Mice that exhibited LH and FSH surges on the proestrus showed c-Fos expression in gonadotropin-releasing hormone- (GnRH; 83.4% of co-localization) and kisspeptin-expressing neurons (42.3% of co-localization) of the anteroventral periventricular nucleus (AVPV). Noteworthy, mice perfused on proestrus, but that failed to exhibit LH surge, showed a smaller, but significant expression of c-Fos in GnRH (32.7%) and AVPVKisspeptin (14.0%) neurons. Finally, 96 serial blood samples were collected hourly in eight regular cycling C57BL/6 females to describe the pattern of LH and FSH secretion along the estrous cycle. Small elevations in LH and FSH levels were detected at the time expected for the LH surge. In summary, the present study improves our understanding of the pattern of gonadotropin secretion and the activation of central components of the hypothalamic-pituitary-gonadal axis along the estrous cycle of C57BL/6 female mice.

Elevated prolactin secretion during proestrus in mice: Absence of a defined surge

Throughout the reproductive cycle in rodents, prolactin levels are generally low. In some species, including rats, a prolactin surge occurs on proestrus with peak concentrations coinciding with the preovulatory luteinizing hormone (LH) surge. In mice, however, there are conflicting reports relating to the occurrence and timing of a proestrous prolactin surge. To gain further insight into the incidence and characteristics of this surge in mice, we have used serial tail tip blood sampling and trunk blood collection from both C57BL/6J (inbred) and Swiss Webster (outbred) mouse strains to build a profile of prolactin secretion during proestrus in individual mice. A clearly defined LH surge was detected in most animals, suggesting the blood sampling approach was suitable for detecting patterns of hormone secretion on proestrus. Despite this, levels of prolactin were quite variable between individuals. Overall both mouse strains showed a generalized rise in prolactin levels on the day of proestrus compared with levels seen in diestrus. This pattern is quite distinct from the discreet, circadian-entrained surge observed in rats.

Role of Posterodorsal Medial Amygdala Urocortin-3 in Pubertal Timing in Female Mice

Post-traumatic stress disorder impedes pubertal development and disrupts pulsatile LH secretion in humans and rodents. The posterodorsal sub-nucleus of the medial amygdala (MePD) is an upstream modulator of the hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator, pubertal timing, as well as emotional processing and anxiety. Psychosocial stress exposure alters neuronal activity within the MePD increasing the expression of Urocortin3 (Ucn3) and its receptor corticotropin-releasing factor type-2 receptor (CRFR2) while enhancing the inhibitory output from the MePD to key hypothalamic reproductive centres. We test the hypothesis that psychosocial stress, processed by the MePD, is relayed to the hypothalamic GnRH pulse generator to delay puberty in female mice. We exposed C57Bl6/J female mice to the predator odor, 2,4,5-Trimethylthiazole (TMT), during pubertal transition and examined the effect on pubertal timing, pre-pubertal LH pulses and anxiety-like behaviour. Subsequently, we virally infected Ucn3-cre-tdTomato female mice with stimulatory DREADDs targeting MePD Ucn3 neurons and determined the effect on pubertal timing and pre-pubertal LH pulse frequency. Exposure to TMT during pubertal development delayed puberty, suppressed pre-pubertal LH pulsatility and enhanced anxiety-like behaviour, while activation of MePD Ucn3 neurons reduced LH pulse frequency and delayed puberty. Early psychosocial stress exposure decreases GnRH pulse generator frequency delaying puberty while inducing anxiety-behaviour in female mice, an effect potentially involving Ucn3 neurons in the MePD.

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.

Perspective: Gestational Tryptophan Fluctuation Altering Neuroembryogenesis and Psychosocial Development

Tryptophan, as the sole precursor of serotonin, mainly derived from diets, is essential for neurodevelopment and immunomodulation. Gestational tryptophan fluctuation may account for the maternal-fetal transmission in determining neuroembryogenesis with long-lasting effects on psychological development. Personality disorders and social exclusion are related to psychosocial problems, leading to impaired social functioning. However, it is not clear how the fluctuation in mother-child transmission regulates the neuroendocrine development and gut microbiota composition in progeny due to that tryptophan metabolism in pregnant women is affected by multiple factors, such as diets (tryptophan-enriched or -depleted diet), emotional mental states (anxiety, depression), health status (hypertension, diabetes), and social support as well as stresses and management skills. Recently, we have developed a non-mammal model to rationalize those discrepancies without maternal effects. This perspective article outlines the possibility and verified the hypothesis in bully-victim research with this novel model: (1). Summarizes the effects of the maternal tryptophan administration on the neuroendocrine and microbial development in their offspring; (2). Highlights the inconsistency and limitations in studying the relationship between gestational tryptophan exposure and psychosocial development in humans and viviparous animals; and (3). Evidences that embryonic exposure to tryptophan and its metabolite modify bullying interactions in the chicken model. With the current pioneer researches on the biomechanisms underlying the bully-victim interaction, the perspective article provides novel insights for developing appropriate intervention strategies to prevent psychological disorders among individuals, especially those who experienced prenatal stress, by controlling dietary tryptophan and medication therapy during pregnancy.

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.

Effects of lighting patterns in pubertal development and metabolism of female wistar rats

Modern lifestyle is characterized by constant exposure to artificial light, which is associated with alterations in biological rhythms, abnormalities to reproductive cycles and metabolic changes. In this study, we aimed to evaluate the effects of four different lighting patterns on puberty timing and on possible metabolic changes in female Wistar rats. Additionally, we developed a machine learning algorithm to automatically classify the stages of the estrous cycle. Adult Wistar rats mated during a week at a photoperiod station where they were exposed to combined red-green-blue lights (RGB) during the photoperiod that varied its spectral composition (i.e., variable color temperature) during the day (RGB-v; N = 14), RGB during the photoperiod with a fixed light color temperature (RGB-f; N = 13) during the whole photoperiod; constant darkness (DD; N = 13) and constant fixed light (LL; N = 15). Experiments were performed only on female litters from postnatal day (PND) 22 to 50. Body weight, puberty onset, estrous cyclicity and serum metabolic parameters were measured. We also collected pictures of vaginal smears to create a dataset of 15,936 images to construct an automatic classifier based on convolutional neural networks. No significant differences were found in the age of vaginal opening; however, the RGB-v group showed a significantly lower number of complete and consecutives cycles. Also, the RGB-f group showed the first complete estrous cycle significantly earlier than the RGB-v group. Female rats housed in LL condition presented significantly lower mean body weight from PND 33 to PDN 47 compared to the other groups. Furthermore, higher levels of plasma triglycerides were found in the DD group compared to RGB-f and RGB-v. HDL levels were significantly lower in RGB-v compared to RGB-f and LL groups. Total cholesterol was significantly lower in RGB-v compared to all groups. Visceral fat was significantly higher in RGB-f compared to the LL group. These results suggest that both changes in photoperiod and lighting quality affect pubertal development and alter lipid profiles and visceral fat accumulation.

Short Daily Exposure to Environmental Enrichment, Fluoxetine, or Their Combination Reverses Deterioration of the Coat and Anhedonia Behaviors with Differential Effects on Hippocampal Neurogenesis in Chronically Stressed Mice

Depression is a neuropsychiatric disorder with a high impact on the worldwide population. To overcome depression, antidepressant drugs are the first line of treatment. However, pre-clinical studies have pointed out that antidepressants are not entirely efficacious and that the quality of the living environment after stress cessation may play a relevant role in increasing their efficacy. As it is unknown whether a short daily exposure to environmental enrichment during chronic stress and antidepressant treatment will be more effective than just the pharmacological treatment, this study analyzed the effects of fluoxetine, environmental enrichment, and their combination on depressive-associated behavior. Additionally, we investigated hippocampal neurogenesis in mice exposed to chronic mild stress. Our results indicate that fluoxetine reversed anhedonia. Besides, fluoxetine reversed the decrement of some events of the hippocampal neurogenic process caused by chronic mild stress. Conversely, short daily exposure to environmental enrichment changed the deterioration of the coat and anhedonia. Although, this environmental intervention did not produce significant changes in the neurogenic process affected by chronic mild stress, fluoxetine plus environmental enrichment showed similar effects to those caused by environmental enrichment to reverse depressive-like behaviors. Like fluoxetine, the combination reversed the declining number of Ki67, doublecortin, calretinin cells and mature newborn neurons. Finally, this study suggests that short daily exposure to environmental enrichment improves the effects of fluoxetine to reverse the deterioration of the coat and anhedonia in chronically stressed mice. In addition, the combination of fluoxetine with environmental enrichment produces more significant effects than those caused by fluoxetine alone on some events of the neurogenic process. Thus, environmental enrichment improves the benefits of pharmacological treatment by mechanisms that need to be clarified.

d-Leucine protects oocytes from chronic psychological stress in mice

PURPOSE

Psychological stress could negatively influence female reproductive ability. d-Leucine (d-Leu) is a d-type amino acid found in foods and mammalian tissues. We have examined the protective effects of d-Leu on oocyte abnormality induced by psychological stress.

METHODS

Female mice (6-week-old) were divided into three groups: control, restraint stress (RS), and RS/d-Leu. The RS and RS/d-Leu mice were holed for 3 hours daily during 14 days. RS/d-Leu mice were fed 0.3% d-Leu diet. The oocyte maturation failure was analyzed by shapes of spindles and chromosomes. In addition, levels of heme-oxygenase-1 (HO-1) and superoxide dismutase (SOD) expression in the ovaries were also examined. Whether d-Leu reduces the generation of reactive oxygen species (ROS) in cultured cells, K562 cells were treated with d-Leu, and then ROS in K562 were analyzed.

RESULTS

Oocyte maturation failure was increased in RS mice. d-Leu reduced abnormal oocytes to control level. The expression levels of HO-1 and SOD2 increased in RS/d-Leu mice compared to those of RS mice. ROS levels were decreased in K562 cells with d-Leu in a dose-dependent manner.

CONCLUSIONS

We concluded that d-Leu protects oocytes from psychological stress through the induction of HO-1 and SOD2 expression then by reducing oxidative 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.

Psychological Stress and Functional Endometrial Disorders: Update of Mechanism Insights

The human endometrium plays a vital role in providing the site for embryo implantation and maintaining the normal development and survival of the embryo. Recent studies have shown that stress is a common factor for the development of unexplained reproductive disorders. The nonreceptive endometrium and disturbed early maternal-fetal interaction might lead to infertility including the repeated embryo implantation failure and recurrent spontaneous abortion, or late pregnancy complications, thereby affecting the quality of life as well as the psychological status of the affected individuals. Additionally, psychological stress might also adversely affect female reproductive health. In recent years, several basic and clinical studies have tried to investigate the harm caused by psychological stress to reproductive health, however, the mechanism is still unclear. Here, we review the relationship between psychological stress and endometrial dysfunction, and its consequent effects on female infertility to provide new insights for clinical therapeutic interventions in the future.

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.

Androgen receptor positively regulates gonadotropin-releasing hormone receptor in pituitary gonadotropes

Within pituitary gonadotropes, the gonadotropin-releasing hormone receptor (GnRHR) receives hypothalamic input from GnRH neurons that is critical for reproduction. Previous studies have suggested that androgens may regulate GnRHR, although the mechanisms remain unknown. In this study, we demonstrated that androgens positively regulate Gnrhr mRNA in mice. We then investigated the effects of androgens and androgen receptor (AR) on Gnrhr promoter activity in immortalized mouse LβT2 cells, which represent mature gonadotropes. We found that AR positively regulates the Gnrhr proximal promoter, and that this effect requires a hormone response element (HRE) half site at -159/-153 relative to the transcription start site. We also identified nonconsensus, full-length HREs at -499/-484 and -159/-144, which are both positively regulated by androgens on a heterologous promoter. Furthermore, AR associates with the Gnrhr promoter in ChIP. Altogether, we report that GnRHR is positively regulated by androgens through recruitment of AR to the Gnrhr proximal promoter.

Impact of chronic variable stress on neuroendocrine hypothalamus and pituitary in male and female C57BL/6J mice

Chronic stress exerts multiple negative effects on the physiology and health of an individual. In the present study, we examined hypothalamic, pituitary and endocrine responses to 14 days of chronic variable stress (CVS) in male and female C57BL/6J mice. In both sexes, CVS induced a significant decrease in body weight and enhanced the acute corticosterone stress response, which was accompanied by a reduction in thymus weight only in females. However, single-point blood measurements of basal prolactin, thyroid-stimulating hormone, luteinising hormone, growth hormone and corticosterone levels taken at the end of the CVS were not different from those of controls. Similarly, pituitary mRNA expression of Fshb, Lhb, Prl and Gh was unchanged by CVS, although Pomc and Tsh were significantly elevated. Within the adrenal medulla, mRNA for Th, Vip and Gal were elevated following CVS. Avp transcript levels within the paraventricular nucleus of the hypothalamus were increased by CVS; however, levels of Gnrh1, Crh, Oxt, Sst, Trh, Ghrh, Th and Kiss1 remained unchanged. Oestrous cycles were lengthened slightly by CVS and ovarian histology revealed a reduction in the number of preovulatory follicles and corpora lutea. Taken together, these observations indicate that 14 days of CVS induces an up-regulation of the neuroendocrine stress axis and creates a mild disruption of female reproductive function. However, the lack of changes in other neuroendocrine axes controlling anterior and posterior pituitary secretion suggest that most neuroendocrine axes are relatively resilient to CVS.

Unexpected Role of Physiological Estrogen in Acute Stress-Induced Memory Deficits

Stress may promote emotional and cognitive disturbances, which differ by sex. Adverse outcomes, including memory disturbances, are typically observed following chronic stress, but are now being recognized also after short events, including mass shootings, assault, or natural disasters, events that consist of concurrent multiple acute stresses (MAS). Prior work has established profound and enduring effects of MAS on memory in males. Here we examined the effects of MAS on female mice and probed the role of hormonal fluctuations during the estrous cycle on MAS-induced memory problems and the underlying brain network and cellular mechanisms. Female mice were impacted by MAS in an estrous cycle-dependent manner: MAS impaired hippocampus-dependent spatial memory in early-proestrous mice, characterized by high levels of estradiol, whereas memory of mice stressed during estrus (low estradiol) was spared. As spatial memory requires an intact dorsal hippocampal CA1, we examined synaptic integrity in mice stressed at different cycle phases and found a congruence of dendritic spine density and spatial memory deficits, with reduced spine density only in mice stressed during high estradiol cycle phases. Assessing MAS-induced activation of brain networks interconnected with hippocampus, we identified differential estrous cycle-dependent activation of memory- and stress-related regions, including the amygdala. Network analyses of the cross-correlation of fos expression among these regions uncovered functional connectivity that differentiated impaired mice from those not impaired by MAS. In conclusion, the estrous cycle modulates the impact of MAS on spatial memory, and fluctuating physiological levels of sex hormones may contribute to this effect.SIGNIFICANCE STATEMENT: Effects of stress on brain functions, including memory, are profound and sex-dependent. Acute stressors occurring simultaneously result in spatial memory impairments in males, but effects on females are unknown. Here we identified estrous cycle-dependent effects of such stresses on memory in females. Surprisingly, females with higher physiological estradiol experienced stress-induced memory impairment and a loss of underlying synapses. Memory- and stress-responsive brain regions interconnected with hippocampus were differentially activated across high and low estradiol mice, and predicted memory impairment. Thus, at functional, network, and cellular levels, physiological estradiol influences the effects of stress on memory in females, providing insight into mechanisms of prominent sex differences in stress-related memory disorders, such as post-traumatic stress disorder.

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.

The reproductive stress hypothesis

In this paper, we propose the reproductive stress hypothesis that describes the pregnant females response to reproductive events based upon the activation of the hypothalamic–pituitary–adrenal axis and sympathetic adrenomedullary system. The main components of the reproductive stress hypothesis can be summarized as follows: (1) events unique to reproduction including empathema, pregnancy, parturition and lactation cause non-specific responses in females, called active reproductive stress; (2) the fetus is a special stressor for pregnant females where endocrine hormones, including corticotropin-releasing hormones and fetal glucocorticoids secreted by the fetus and placenta, enter the maternal circulatory system, leading to another stress response referred to as passive reproductive stress and (3) response to uterine tension and intrauterine infection is the third type of stress, called fetal intrauterine stress. Appropriate reproductive stress is a crucial prerequisite in normal reproductive processes. By contrast, excessive or inappropriate reproductive stress may result in dysfunctions of the reproductive system, such as compromised immune function, leading to susceptibility to disease. The novel insights of the reproductive stress hypothesis have important implications for deciphering the pathogenesis of certain diseases in pregnant animals, including humans, which in turn may be applied to preventing and treating their occurrence.

Central aspects of systemic oestradiol negative- and positive-feedback on the reproductive neuroendocrine system

The central nervous system regulates fertility via the release of gonadotrophin-releasing hormone (GnRH). This control revolves around the hypothalamic-pituitary-gonadal axis, which operates under traditional homeostatic feedback by sex steroids from the gonads in males and most of the time in females. An exception is the late follicular phase in females, when homeostatic feedback is suspended and a positive-feedback response to oestradiol initiates the preovulatory surges of GnRH and luteinising hormone. Here, we briefly review the history of how mechanisms underlying central control of ovulation by circulating steroids have been studied, discuss the relative merit of different model systems and integrate some of the more recent findings in this area into an overall picture of how this phenomenon occurs.

The role of prolactin in co-ordinating fertility and metabolic adaptations during reproduction

Mammalian pregnancy and lactation is accompanied by a period of infertility that takes place in the midst of a sustained increase in food intake. Indeed, successful reproduction in females is dependent on co-ordination of the distinct systems that regulate reproduction and metabolism. Rather than arising from different mechanisms during pregnancy and lactation, we propose that elevations in lactogenic hormones (predominant among these being prolactin and the placental lactogens), are ideally placed to influence both of these systems at the appropriate time. We review the literature examining the impacts of lactogens on fertility and energy homeostasis in the virgin state, during pregnancy and lactation and potential long-term impacts of reproductive experience. Taken together, the literature indicates that duration and pattern of lactogen exposure is a vital factor in the ability of these hormones to alter reproduction and food intake. Transient increases in prolactin, as typically seen in healthy virgin females and males, are unable to exert lasting impacts. Importantly, both suppression of fertility and increased food intake are only observed following exposure to chronically-elevated levels of lactogens. Physiologically, the only time this pattern of lactogenic secretion is maintained in the healthy female is during pregnancy and lactation, when co-ordination between these regulatory systems emerges. This article is part of the special issue on 'Neuropeptides'.

Reproductive monitoring of collared peccary females (Pecari tajacu) by analysis of fecal progesterone metabolites

The measurement of fecal progesterone metabolites (fPM) by enzyme immunoassay analysis is a non-invasive technique that permits gathering reproductive information from wildlife without the stress associated with restraint. In the collared peccary (Pecari tajacu), a high correlation between serum progesterone and fPM levels (r² = 0.783) suggests that fPM can be used to monitor their reproductive function. We monitored fPM during the estrous cycle of 15 collared peccary females. Estrous cycles averaged 27.9 ± 4.5 days (n = 28), ranging from 21 to 36 days. The luteal phase was 22.2 ± 4.8 days and the inter-luteal phase was 4.3 ± 1.4 days. Mean concentration of fPM across pregnancy were not different from those found during the luteal phase (1230 ± 718 and 1265 ± 584 ng/100 mg dried feces, respectively), however, significant differences were found when luteal phase concentrations were compared only against fPM concentrations during late pregnancy. In addition, late pregnancy fPM concentrations (1893 ± 551 ng/100 mg) were also significantly higher than those in the early (639 ± 339 ng/100 mg) and mid (1134 ± 449 ng/100 mg) pregnancy. For females during the early post-partum period, fPM concentrations were significantly increased (243 ± 118 ng/100 mg) than those of non-cycling females (103 ± 89 ng/100 mg). The analysis of fPM is a simple, non-invasive methodology to detect the ovarian activity in the collared peccary; moreover, it provides a husbandry tool, which may be used to help understand how social structure may impact reproduction.

Genetic dissection of the different roles of hypothalamic kisspeptin neurons in regulating female reproduction

The brain regulates fertility through gonadotropin-releasing hormone (GnRH) neurons. Estradiol induces negative feedback on pulsatile GnRH/luteinizing hormone (LH) release and positive feedback generating preovulatory GnRH/LH surges. Negative and positive feedbacks are postulated to be mediated by kisspeptin neurons in arcuate and anteroventral periventricular (AVPV) nuclei, respectively. Kisspeptin-specific ERα knockout mice exhibit disrupted LH pulses and surges. This knockout approach is neither location-specific nor temporally controlled. We utilized CRISPR-Cas9 to disrupt ERα in adulthood. Mice with ERα disruption in AVPV kisspeptin neurons have typical reproductive cycles but blunted LH surges, associated with decreased excitability of these neurons. Mice with ERα knocked down in arcuate kisspeptin neurons showed disrupted cyclicity, associated with increased glutamatergic transmission to these neurons. These observations suggest that activational effects of estradiol regulate surge generation and maintain cyclicity through AVPV and arcuate kisspeptin neurons, respectively, independent from its role in the development of hypothalamic kisspeptin neurons or puberty onset.

Noise-induced sleep disruption increases weight gain and decreases energy metabolism in female rats

Background/objectives:

Inadequate sleep increases obesity and environmental noise contributes to poor sleep. However, women may be more vulnerable to noise and hence more susceptible to sleep disruption-induced weight gain than men. In male rats, exposure to environmental (i.e. ambient) noise disrupts sleep and increases feeding and weight gain. However, the effects of environmental noise on sleep and weight gain in female rats are unknown. Thus, this study was designed to determine whether noise exposure would disturb sleep, increase feeding and weight gain and alter the length of the estrous cycle in female rats.

Subjects/methods:

Female rats (12-weeks old) were exposed to noise for 17d (8h/d during the light period) to determine the effects of noise on weight gain and food intake. In a separate set of females, estrous cycle phase and length, EEG, EMG, spontaneous physical activity and energy expenditure were recorded continuously for 27d during baseline (control, 9d), noise exposure (8h/d, 9d) and recovery (9d) from sleep disruption.

Results:

Noise exposure significantly increased weight gain and food intake compared to females that slept undisturbed. Noise also significantly increased wakefulness, reduced sleep and resulted in rebound sleep during the recovery period. Total energy expenditure was significantly lower during both noise exposure and recovery due to lower energy expenditure during spontaneous physical activity and sleep. Notably, noise did not alter the estrous cycle length.

Conclusions:

As previously observed in male rats, noise exposure disrupted sleep and increased weight gain in females but did not alter the length of the estrous cycle. This is the first demonstration of weight gain in female rats during sleep disruption. We conclude that the sleep disruption caused by exposure to environmental noise is a significant tool for determining how sleep loss contributes to obesity in females.

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.

Identification of Genes Enriched in GnRH Neurons by Translating Ribosome Affinity Purification and RNAseq in Mice

Gonadotropin-releasing hormone (GnRH) neurons are a nexus of fertility regulation. We used translating ribosome affinity purification coupled with RNA sequencing to examine messenger RNAs of GnRH neurons in adult intact and gonadectomized (GDX) male and female mice. GnRH neuron ribosomes were tagged with green fluorescent protein (GFP) and GFP-labeled polysomes isolated by immunoprecipitation, producing one RNA fraction enhanced for GnRH neuron transcripts and one RNA fraction depleted. Complementary DNA libraries were created from each fraction and 50-base, paired-end sequencing done and differential expression (enhanced fraction/depleted fraction) determined with a threshold of >1.5- or <0.66-fold (false discovery rate P ≤ 0.05). A core of ∼840 genes was differentially expressed in GnRH neurons in all treatments, including enrichment for Gnrh1 (∼40-fold), and genes critical for GnRH neuron and/or gonadotrope development. In contrast, non-neuronal transcripts were not enriched or were de-enriched. Several epithelial markers were also enriched, consistent with the olfactory epithelial origins of GnRH neurons. Interestingly, many synaptic transmission pathways were de-enriched, in accordance with relatively low innervation of GnRH neurons. The most striking difference between intact and GDX mice of both sexes was a marked downregulation of genes associated with oxidative phosphorylation and upregulation of glucose transporters in GnRH neurons from GDX mice. This may suggest that GnRH neurons switch to an alternate fuel to increase adenosine triphosphate production in the absence of negative feedback when GnRH release is elevated. Knowledge of the GnRH neuron translatome and its regulation can guide functional studies and can be extended to disease states, such as polycystic ovary syndrome.

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.

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

Gonadotropin-releasing hormone (GnRH) neurons are the final central regulators of reproduction, integrating various inputs that modulate fertility. Stress typically inhibits reproduction but can be stimulatory; stress effects can also be modulated by steroid milieu. Corticotropin-releasing hormone (CRH) released during the stress response may suppress reproduction independent of downstream glucocorticoids. We hypothesized CRH suppresses fertility by decreasing GnRH neuron firing activity. To test this, mice were ovariectomized (OVX) and either implanted with an estradiol capsule (OVX+E) or not treated further to examine the influence of estradiol on GnRH neuron response to CRH. Targeted extracellular recordings were used to record firing activity from green fluorescent protein-identified GnRH neurons in brain slices before and during CRH treatment; recordings were done in the afternoon when estradiol has a positive feedback effect to increase GnRH neuron firing. In OVX mice, CRH did not affect the firing rate of GnRH neurons. In contrast, CRH exhibited dose-dependent stimulatory (30 nM) or inhibitory (100 nM) effects on GnRH neuron firing activity in OVX+E mice; both effects were reversible. The dose-dependent effects of CRH appear to result from activation of different receptor populations; a CRH receptor type-1 agonist increased firing activity in GnRH neurons, whereas a CRH receptor type-2 agonist decreased firing activity. CRH and specific agonists also differentially regulated short-term burst frequency and burst properties, including burst duration, spikes/burst, and/or intraburst interval. These results indicate that CRH alters GnRH neuron activity and that estradiol is required for CRH to exert both stimulatory and inhibitory effects on GnRH neurons.

Acute Psychosocial Stress Inhibits LH Pulsatility and Kiss1 Neuronal Activation in Female Mice

Psychosocial stress, such as isolation and restraint, disrupts reproductive neuroendocrine activity. Here we investigate the impact of psychosocial stress on luteinizing hormone (LH) pulses and gene expression and neuronal activation within Rfrp and Kiss1 cells in female mice. Mice were ovariectomized (OVX) and handled daily to habituate to the tail-tip blood collection procedure. Blood was collected every 5 minutes for 180 minutes for measurement of LH. After 90 minutes, stress animals were placed into restraint devices and isolated to new cages. No-stress control animals remained in their home cages. LH pulses occurred at regular intervals during the entire 180-minute sampling period in controls. In contrast, stress induced a rapid and robust suppression of pulsatile LH secretion. Stress reduced the frequency of pulses by 60% and diminished basal LH levels by 40%; pulse amplitude was unaffected. In a separate cohort of OVX females, brains were collected after 45, 90, or 180 minutes of stress or in no-stress controls. At all time points, stress induced a potent decrease in arcuate Kiss1 neuronal activation, using cfos induction as a marker, with a 50% to 60% suppression vs control levels, whereas Rfrp and cfos coexpression in the dorsal-medial nucleus was elevated after 45 minutes of stress. Although arcuate Kiss1 gene expression remained stable, Rfrp expression was elevated 20% after 180 minutes of stress. These findings demonstrate rapid suppression of LH pulsatile secretion by psychosocial stress, associated with reduced cfos induction in Kiss1 neurons and time-dependent increases in Rfrp neuronal activation and messenger RNA.

Stress and the HPA Axis: Balancing Homeostasis and Fertility

An organism’s reproductive fitness is sensitive to the environment, integrating cues of resource availability, ecological factors, and hazards within its habitat. Events that challenge the environment of an organism activate the central stress response system, which is primarily mediated by the hypothalamic–pituitary–adrenal (HPA) axis. The regulatory functions of the HPA axis govern the cardiovascular and metabolic system, immune functions, behavior, and reproduction. Activation of the HPA axis by various stressors primarily inhibits reproductive function and is able to alter fetal development, imparting a biological record of stress experienced in utero. Clinical studies and experimental data indicate that stress signaling can mediate these effects through direct actions in the brain, gonads, and embryonic tissues. This review focuses on the mechanisms by which stress activation of the HPA axis impacts fertility and fetal development.