The effect of oestradiol and progesterone on hypoglycaemic stress-induced suppression of pulsatile luteinizing hormone release and on corticotropin-releasing hormone mRNA expression in the rat

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.

Posterodorsal Medial Amygdala Urocortin-3, GABA, and Glutamate Mediate Suppression of LH Pulsatility in Female Mice

The posterodorsal subnucleus of the medial amygdala (MePD) is an upstream modulator of the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-adrenal (HPA) axes. Inhibition of MePD urocortin-3 (Ucn3) neurons prevents psychological stress-induced suppression of luteinizing hormone (LH) pulsatility while blocking the stress-induced elevations in corticosterone (CORT) secretion in female mice. We explore the neurotransmission and neural circuitry suppressing the gonadotropin-releasing hormone (GnRH) pulse generator by MePD Ucn3 neurons and we further investigate whether MePD Ucn3 efferent projections to the hypothalamic paraventricular nucleus (PVN) control CORT secretion and LH pulsatility. Ucn3-cre-tdTomato female ovariectomized (OVX) mice were unilaterally injected with adeno-associated virus (AAV)-channelrhodopsin 2 (ChR2) and implanted with optofluid cannulae targeting the MePD. We optically activated Ucn3 neurons in the MePD with blue light at 10 Hz and monitored the effect on LH pulses. Next, we combined optogenetic stimulation of MePD Ucn3 neurons with pharmacological antagonism of GABAA or GABAB receptors with bicuculline or CGP-35348, respectively, as well as a combination of NMDA and AMPA receptor antagonists, AP5 and CNQX, respectively, and observed the effect on pulsatile LH secretion. A separate group of Ucn3-cre-tdTomato OVX mice with 17β-estradiol replacement were unilaterally injected with AAV-ChR2 in the MePD and implanted with fiber-optic cannulae targeting the PVN. We optically stimulated the MePD Ucn3 efferent projections in the PVN with blue light at 20 Hz and monitored the effect on CORT secretion and LH pulses. We reveal for the first time that activation of Ucn3 neurons in the MePD inhibits GnRH pulse generator frequency via GABA and glutamate signaling within the MePD, while MePD Ucn3 projections to the PVN modulate the HPG and HPA axes.

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

Psychological stress is linked to infertility by suppressing the hypothalamic GnRH pulse generator. The posterodorsal subnucleus of the medial amygdala (MePD) is an upstream regulator of GnRH pulse generator activity and displays increased neuronal activation during psychological stress. The MePD is primarily a GABAergic nucleus with a strong GABAergic projection to hypothalamic reproductive centers; however, their functional significance has not been determined. We hypothesize that MePD GABAergic signalling mediates psychological stress-induced suppression of pulsatile LH secretion. We selectively inhibited MePD GABA neurons during psychological stress in ovariectomized (OVX) Vgat-cre-tdTomato mice to determine the effect on stress-induced suppression of pulsatile LH secretion. MePD GABA neurons were virally infected with inhibitory hM4DGi-designer receptor exclusively activated by designer drugs (DREADDs) to selectively inhibit MePD GABA neurons. Furthermore, we optogenetically stimulated potential MePD GABAergic projection terminals in the hypothalamic arcuate nucleus (ARC) and determined the effect on pulsatile LH secretion. MePD GABA neurons in OVX female Vgat-cre-tdTomato mice were virally infected to express channelrhodopsin-2 and MePD GABAergic terminals in the ARC were selectively stimulated by blue light via an optic fiber implanted in the ARC. DREADD-mediated inhibition of MePD GABA neurons blocked predator odor and restraint stress-induced suppression of LH pulse frequency. Furthermore, sustained optogenetic stimulation at 10 and 20 Hz of MePD GABAergic terminals in the ARC suppressed pulsatile LH secretion. These results show for the first time that GABAergic signalling in the MePD mediates psychological stress-induced suppression of pulsatile LH secretion and suggest a functionally significant MePD GABAergic projection to the hypothalamic GnRH pulse generator.

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.

Hindlimb unloading-induced reproductive suppression via Downregulation of hypothalamic Kiss-1 expression in adult male rats

BACKGROUND

Spaceflights-induced microgravity can alter various physiological processes in human's body including the functional status of the reproductive system. Rodent model of tail-suspension hindlimb unloading is extensively used to stimulate the organs responses to the microgravity condition. This study explores the potential effects of hindlimb unloading on testicular functions and spermatogenesis in adult male rats and the underlying mechanism/s.

METHODS

Twenty Sprague-Dawley rats were allotted into two groups: normally loaded group (control; all arms were in touch with the grid floor) and hindlimb unloaded group (HU; only the forearms were in contact with the grid floor).

RESULTS

Following 30 days of exposure, the HU group saw a decline in body weight, testicular and epidydimal weights, and all semen parameters. The circulating concentrations of gonadotropin-releasing hormone (GnRH), follicle stimulating hormone (FSH), luteinizing hormone (LH) and testosterone significantly decreased, while levels of kisspeptin, corticosterone, inhibin, prolactin and estradiol (E2) increased in the HU group. Intratesticular levels of 5α-reductase enzyme and dihydrotestosterone (DHT) were suppressed, while the levels of aromatase and kisspeptin were significantly elevated in the HU group. Hypothalamic kisspeptin (Kiss1) mRNA expression levels were downregulated while its receptors (Kiss1R) were upregulated in the HU group. On the contrary, the mRNA expression levels of testicular Kiss1 were upregulated while Kiss1R were downregulated. The pituitary mRNA expression levels of FSHβ and LHβ decreased in the HU group. The levels of the antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and nitric oxide (NO) concentrations, and total antioxidant capacity (TAC) were elevated while malondialdehyde (MDA) concentrations declined in the testes of HU group. The testes of the HU rats showed positive immunostaining of caspase-3, heat shock protein 70 (HSP70) and Bcl2.

CONCLUSIONS

Altogether, these results revealed an inhibitory effect of hindlimb unloading on kisspeptin signaling in the hypothalamic-pituitary-testicular axis with impaired spermatogenesis and steroidogenesis.

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.

Neuroendocrine pathways underlying risk and resilience to PTSD in women

Women are twice as likely than men to suffer from posttraumatic stress disorder (PTSD). While women have increased exposure to traumatic events of many types and have greater prevalence of comorbid psychiatric disorders compared to men, these differences do not account for the overall sex difference in the prevalence of PTSD. The current review summarizes significant findings that implicate the role of estradiol, progesterone, and allopregnanolone in female risk for PTSD symptoms and dysregulation of fear psychophysiology that is cardinal to PTSD. We also discuss how these steroid hormones influence the stress axis and neural substrates critical for the regulation of fear responses. Understanding the role of ovarian steroid hormones in risk and resilience for trauma-related adverse mental health outcomes across the lifespan in women has important translational, clinical, and intergenerational implications for mitigating the consequences of trauma exposure.

Mathematical Modelling of Endocrine Systems

Hormone rhythms are ubiquitous and essential to sustain normal physiological functions. Combined mathematical modelling and experimental approaches have shown that these rhythms result from regulatory processes occurring at multiple levels of organisation and require continuous dynamic equilibration, particularly in response to stimuli. We review how such an interdisciplinary approach has been successfully applied to unravel complex regulatory mechanisms in the metabolic, stress, and reproductive axes. We discuss how this strategy is likely to be instrumental for making progress in emerging areas such as chronobiology and network physiology. Ultimately, we envisage that the insight provided by mathematical models could lead to novel experimental tools able to continuously adapt parameters to gradual physiological changes and the design of clinical interventions to restore normal endocrine function.

The Role of Kiss1 Neurons As Integrators of Endocrine, Metabolic, and Environmental Factors in the Hypothalamic-Pituitary-Gonadal Axis

Kisspeptin-GPR54 signaling in the hypothalamus is required for reproduction and fertility in mammals. Kiss1 neurons are key regulators of gonadotropin-releasing hormone (GnRH) release and modulation of the hypothalamic-pituitary-gonadal (HPG) axis. Arcuate Kiss1 neurons project to GnRH nerve terminals in the median eminence, orchestrating the pulsatile secretion of luteinizing hormone (LH) through the intricate interaction between GnRH pulse frequency and the pituitary gonadotrophs. Arcuate Kiss1 neurons, also known as KNDy neurons in rodents and ruminants because of their co-expression of neurokinin B and dynorphin represent an ideal hub to receive afferent inputs from other brain regions in response to physiological and environmental changes, which can regulate the HPG axis. This review will focus on studies performed primarily in rodent and ruminant species to explore potential afferent inputs to Kiss1 neurons with emphasis on the arcuate region but also considering the rostral periventricular region of the third ventricle (RP3V). Specifically, we will discuss how these inputs can be modulated by hormonal, metabolic, and environmental factors to control gonadotropin secretion and fertility. We also summarize the methods and techniques that can be used to study functional inputs into Kiss1 neurons.

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.

Kisspeptin in the medial amygdala and sexual behavior in male rats

The medial amygdala (MeA) is crucial for sexual behavior; kisspeptin (Kiss1) also plays a role in sexual function. Kisspeptin receptor (Kiss1r) knockout mice display no sexual behavior. Recently Kiss1 and Kiss1r have been discovered in the posterodorsal subnucleus of the medial amygdala (MePD). We hypothesised that Kiss1 in the MePD may have an influence on male sexual behavior. To test this we bilaterally cannulated the MePD and infused kisspeptin-10 in male rats. This caused the rats to have multiple erections, an effect specific to Kiss1 receptor activation, because Kiss1r antagonism blocked the erectile response. When Kiss1 was infused into the lateral cerebroventricle, there were no observed erections. We also measured the plasma levels of LH when Kiss1 is infused into the MePD or lateral cerebroventricle; Kiss1 increased plasma LH to comparable levels when infused into both sites. We conclude that Kiss1 has a role in male sexual behavior, which is specific to the MePD.

Associations of cortisol/testosterone and cortisol/sex hormone-binding globulin ratios with atherosclerosis in middle-age women

BACKGROUND AND AIMS

The cortisol/testosterone (C/T) ratio has been hypothesized to be a better predictor of atherosclerosis than cortisol alone. No study has assessed whether the C/T and C/sex hormone-binding globulin (SHBG) ratios are associated with atherosclerosis in a U.S. population sample.

METHODS

This substudy included 367 women who had both cortisol from year 15 and testosterone and SHBG at year 16 of the Coronary Artery Risk Development in Young Adults study, an ongoing observational cohort in the United States. Of these, intima-media thickness (IMT) was available at follow-up year 20 in 339 (n = 332 with measurement at carotid bulb), and 303 were free of prevalent coronary artery calcium (CAC) at year 15. Area under the curve (AUC) of salivary cortisol was available in 302 individuals. Ratios of AUCs of cortisol to total testosterone, free testosterone, and SHBG were categorized into tertiles. Associations with CAC and IMT were assessed by regression models adjusted for age, race, body mass index, systolic blood pressure, menopause, oral contraceptive use, diabetes, alcohol, and smoking.

RESULTS

Only the highest tertile of the AUC/free testosterone ratio was positively associated with carotid bulb IMT (β = 0.088, P = 0.006). This tertile was also positively associated with new onset CAC between year 15 and 25 (OR 3.45, 95% CI 1.18-10.06). Tertiles of cortisol or testosterone alone were not associated with new onset CAC.

CONCLUSION

AUC/Free testosterone ratio may be more associated with atherosclerosis in women than either indicator alone. The ratio may serve as a suitable biomarker of cortisol-linked stress.

Prenatal ethanol exposure induces an intrauterine programming of enhanced sensitivity of the hypothalamic–pituitary–adrenal axis in female offspring rats fed with post-weaning high-fat diet

“Intrauterine programming” involved in the intrauterine origin of prenatal ethanol exposure induced enhanced sensitivity of the HPA axis in female offspring rats fed with high-fat diet.

Purification of polysaccharides from Cordyceps militaris and their anti‑hypoxic effect

Acute mountain sickness, one of the most common altitude diseases, causes lung and brain injury. The present study aimed to investigate the anti‑hypoxic effect of purified polysaccharides extracted from Cordyceps militaris. The aqueous extract of Cordyceps militaris was purified progressively through a DEAE‑52 cellulose anion exchange column and a Sepharose G‑100 column. The fraction CMN1, with a molecular weight of 37842 Da, was the main fraction obtained and its chemical composition and structural characteristics were determined. CMN1 was found to have a monosaccharide composition of L‑rhamnose, L‑arabinose, D‑mannose, D‑galactose. The backbone of CMN1 comprised (1→2) and (1→3) linkages, with branched (1→6) and (1→4) linkages. The anti‑hypoxic effects of CMN1 were determined using a sodium nitrite toxicosis test, acute cerebral ischemic/hypoxic test and normobarie hypoxia test. CMN1 (0.5 g/kg) possessed a similar anti‑hypoxic effect to rhodiola oral liquid. Overall, the Cordyceps militaris polysaccharide, CMN1, was identified as an effective agent against hypoxia.

Neurokinin B signaling in the female rat: a novel link between stress and reproduction

Acute systemic stress disrupts reproductive function by inhibiting pulsatile gonadotropin secretion. The underlying mechanism involves stress-induced suppression of the GnRH pulse generator, the functional unit of which is considered to be the hypothalamic arcuate nucleus kisspeptin/neurokinin B/dynorphin A neurons. Agonists of the neurokinin B (NKB) receptor (NK3R) have been shown to suppress the GnRH pulse generator, in a dynorphin A (Dyn)-dependent fashion, under hypoestrogenic conditions, and Dyn has been well documented to mediate several stress-related central regulatory functions. We hypothesized that the NKB/Dyn signaling cascade is required for stress-induced suppression of the GnRH pulse generator. To investigate this ovariectomized rats, iv administered with Escherichia coli lipopolysaccharide (LPS) following intracerebroventricular pretreatment with NK3R or κ-opioid receptor (Dyn receptor) antagonists, were subjected to frequent blood sampling for hormone analysis. Antagonism of NK3R, but not κ-opioid receptor, blocked the suppressive effect of LPS challenge on LH pulse frequency. Neither antagonist affected LPS-induced corticosterone secretion. Hypothalamic arcuate nucleus NKB neurons project to the paraventricular nucleus, the major hypothalamic source of the stress-related neuropeptides CRH and arginine vasopressin (AVP), which have been implicated in the stress-induced suppression of the hypothalamic-pituitary-gonadal axis. A separate group of ovariectomized rats was, therefore, used to address the potential involvement of central CRH and/or AVP signaling in the suppression of LH pulsatility induced by intracerebroventricular administration of a selective NK3R agonist, senktide. Neither AVP nor CRH receptor antagonists affected the senktide-induced suppression of the LH pulse; however, antagonism of type 2 CRH receptors attenuated the accompanying elevation of corticosterone levels. These data indicate that the suppression of the GnRH pulse generator by acute systemic stress requires hypothalamic NKB/NK3R signaling and that any involvement of CRH therewith is functionally upstream of NKB.

The effect of long-term ovariectomy on midbrain stress systems in free ranging macaques

Communication between the serotonin system and the CRF system plays a pivotal role in the mediation of stress and stress reactivity. CRF appears to be inhibitory of serotonin neurotransmission through the CRF receptor type 1 (CRF-R1). Serotonin neurons also detect the urocortins, which are thought to be anxiolytic. Components of the CRF system in the serotonergic dorsal raphe region were examined in macaques that were ovary-intact or ovariectomized for 3 years living in a relatively natural environment. Female Japanese macaques (Macaca fuscata) were ovariectomized or tubal-ligated (n=5/group) and returned to their natal troop for 3 years. Quantitation of (1) CRF innervation of the serotonergic dorsal raphe, (2) CRF-Receptor type 1 (CRF-R1) in the dorsal raphe, (3) Urocortin 1 (UCN1) cells near the Edinger-Westfal nucleus and (4) UCN1 axons, was obtained with immunocytochemical staining and image analysis. There was no statistical difference in CRF axonal staining in the dorsal raphe, or in UCN1 axonal staining near the dorsal raphe. However, the average number of detectable UCN1 postive cells was significantly lower in the Ovx group than in the Intact group (p=0.003). Average CRF-R1 positive pixel number and positive cell number were significantly higher in the Ovx group than in the Intact group (p=0.005 and 0.02, respectivly). The higher expression of CRF-R1 and lower expression of UCN1 in the Ovx group indicates they may be more vulnerable to stress. The greater expression of CRF-R1 could cause a greater inhibition of serotonin upon a stress-induced increase in CRF as well.

Immunohistochemical analysis of the colocalization of corticotropin-releasing hormone receptor and glucocorticoid receptor in kisspeptin neurons in the hypothalamus of female rats

Kisspeptin, a neuropeptide encoded by Kiss1 gene, plays pivotal roles in the regulation of reproductive function. Recently various stressors and stress-induced molecules such as corticotropin-releasing hormone (CRH) and corticosterone have been shown to inhibit Kiss1 expression in rat hypothalamus. To determine whether CRH and glucocorticoids directly act on kisspeptin neurons, we examined the colocalization of CRH receptor (CRH-R) and glucocorticoid receptor (GR) in kisspeptin neurons in the female rat hypothalamus. Double-labeling immunohistochemistry revealed that most kisspeptin neurons in the anteroventral periventricular nucleus and periventricular nucleus continuum (AVPV/PeN), and arcuate nucleus (ARC) expressed CRH-R. We also observed a few close appositions of CRH immunoreactive fibers on some of kisspeptin neurons in AVPV/PeN and ARC. On the other hand, most kisspeptin neurons in AVPV/PeN expressed GR, whereas only a few of kisspeptin neurons in ARC expressed GR. Altogether, our study provides neuroanatomical evidence of the direct modulation of kisspeptin neurons by CRH and glucocorticoids and suggests that stress-induced CRH and glucocorticoids inhibit gonadotropin secretion via the kisspeptin system.

Suppression of the GnRH pulse generator by neurokinin B involves a κ-opioid receptor-dependent mechanism

Neurokinin B (NKB) and its receptor (NK3R) are coexpressed with kisspeptin, Dynorphin A (Dyn), and their receptors [G-protein-coupled receptor-54 (GPR54)] and κ-opioid receptor (KOR), respectively] within kisspeptin/NKB/Dyn (KNDy) neurons in the hypothalamic arcuate nucleus (ARC), the proposed site of the GnRH pulse generator. Much previous research has employed intracerebroventricular (icv) administration of KNDy agonists and antagonists to address the functions of KNDy neurons. We performed a series of in vivo neuropharmacological experiments aiming to determine the role of NKB/NK3R signaling in modulating the GnRH pulse generator and elucidate the interaction between KNDy neuropeptide signaling systems, targeting our interventions to ARC KNDy neurons. First, we investigated the effect of intra-ARC administration of the selective NK3R agonist, senktide, on pulsatile LH secretion using a frequent automated serial sampling method to obtain blood samples from freely moving ovariectomized 17β-estradiol-replaced rats. Our results show that senktide suppresses LH pulses in a dose-dependent manner. Intra-ARC administration of U50488, a selective KOR agonist, also caused a dose-dependent, albeit more modest, decrease in LH pulse frequency. Thus we tested the hypothesis that Dyn/KOR signaling localized to the ARC mediates the senktide-induced suppression of the LH pulse by profiling pulsatile LH secretion in response to senktide in rats pretreated with nor-binaltorphimine, a selective KOR antagonist. We show that nor-binaltorphimine blocks the senktide-induced suppression of pulsatile LH secretion but does not affect LH pulse frequency per se. In order to address the effects of acute activation of ARC NK3R, we quantified (using quantitative RT-PCR) changes in mRNA levels of KNDy-associated genes in hypothalamic micropunches following intra-ARC administration of senktide. Senktide down-regulated expression of genes encoding GnRH and GPR54 (GNRH1 and Kiss1r, respectively), but did not affect the expression of Kiss1 (which encodes kisspeptin). We conclude that NKB suppresses the GnRH pulse generator in a KOR-dependent fashion and regulates gene expression in GnRH neurons.

GPR54-dependent stimulation of luteinizing hormone secretion by neurokinin B in prepubertal rats

Kisspeptin, neurokinin B (NKB) and dynorphin A (Dyn) are coexpressed within KNDy neurons that project from the hypothalamic arcuate nucleus (ARC) to GnRH neurons and numerous other hypothalamic targets. Each of the KNDy neuropeptides has been implicated in regulating pulsatile GnRH/LH secretion. In isolation, kisspeptin is generally known to stimulate, and Dyn to inhibit LH secretion. However, the NKB analog, senktide, has variously been reported to inhibit, stimulate or have no effect on LH secretion. In prepubertal mice, rats and monkeys, senktide stimulates LH secretion. Furthermore, in the monkey this effect is dependent on kisspeptin signaling through its receptor, GPR54. The present study tested the hypotheses that the stimulatory effects of NKB on LH secretion in intact rats are mediated by kisspeptin/GPR54 signaling and are independent of a Dyn tone. To test this, ovarian-intact prepubertal rats were subjected to frequent automated blood sampling before and after intracerebroventricular injections of KNDy neuropeptide analogs. Senktide robustly induced single LH pulses, while neither the GPR54 antagonist, Kp-234, nor the Dyn agonist and antagonist (U50488 and nor-BNI, respectively) had an effect on basal LH levels. However, Kp-234 potently blocked the senktide-induced LH pulses. Modulation of the Dyn tone by U50488 or nor-BNI did not affect the senktide-induced LH pulses. These data demonstrate that the stimulatory effect of NKB on LH secretion in intact female rats is dependent upon kisspeptin/GPR54 signaling, but not on Dyn signaling.

Middle-aged female rats retain sensitivity to the anorexigenic effect of exogenous estradiol

It is well established that estradiol (E2) decreases food intake and body weight in young female rats. However, it is not clear if female rats retain responsiveness to the anorexigenic effect of E2 during middle age. Because middle-aged females exhibit reduced responsiveness to E2, manifesting as a delayed and attenuated luteinizing hormone surge, it is plausible that middle-aged rats are less responsive to the anorexigenic effect of E2. To test this we monitored food intake in ovariohysterectomized young and middle-aged rats following E2 treatment. E2 decreased food intake and body weight to a similar degree in both young and middle-aged rats. Next, we investigated whether genes that mediate the estrogenic inhibition of food intake are similarly responsive to E2 by measuring gene expression of the anorexigenic genes corticotropin-releasing hormone (CRH), proopiomelanocortin (POMC), the long form of the leptin receptor (Lepr) and serotonin 2C receptors (5HT2CR) and the orexigenic genes agouti-related peptide (AgRP), neuropeptide Y (NPY), prepromelanin-concentrating hormone (pMCH) and orexin in the hypothalamus of young and middle-aged OVX rats treated with E2. As expected, E2 increased expression of all anorexigenic genes while decreasing expression of all orexigenic genes in young rats. Although CRH, 5HT2CR, Lepr, AgRP, NPY and orexin were also sensitive to E2 treatment in middle-aged rats, POMC and pMCH expression were not influenced by E2 in middle-aged rats. These data demonstrate that young and middle-aged rats are similarly sensitive to the anorexigenic effect of E2 and that most, but not all feeding-related genes retain sensitivity to E2.

The role of GABAergic signalling in stress-induced suppression of gonadotrophin-releasing hormone pulse generator frequency in female rats

Stress exerts profound inhibitory effects on reproductive function by suppressing the pulsatile release of gonadotrophin-releasing hormone (GnRH) and therefore luteinising hormone (LH). This effect is mediated in part via the corticotrophin-releasing factor (CRF) system, although another potential mechanism is via GABAergic signalling within the medial preoptic area (mPOA) because this has known inhibitory influences on the GnRH pulse generator and shows increased activity during stress. In the present study, we investigated the role of the preoptic endogenous GABAergic system in stress-induced suppression of the GnRH pulse generator. Ovariectomised oestradiol-replaced rats were implanted with bilateral and unilateral cannulae targeting toward the mPOA and lateral cerebral ventricle, respectively; blood samples (25 μl) were taken via chronically implanted cardiac catheters every 5 min for 6 h for the measurement of LH pulses. Intra-mPOA administration of the specific GABA(A) receptor antagonist, bicuculline (0.2 pmol each side, three times at 20-min intervals) markedly attenuated the inhibitory effect of lipopolysaccharide (LPS; 25 μg/kg i.v.) but not restraint (1 h) stress on pulsatile LH secretion. By contrast, restraint but not LPS stress-induced suppression of LH pulse frequency was reversed by application of the selective GABA(B) receptor antagonist, CGP-35348, into the mPOA (1.5 nmol each side, three times at 20-min intervals). However, intra-mPOA application of either bicuculline or CGP-35348 attenuated the inhibitory effect of CRF (1 nmol i.c.v.) on the pulsatile LH secretion. These data indicate a pivotal and differential role of endogenous GABAergic signalling in the mPOA with respect to mediating psychological and immunological stress-induced suppression of the GnRH pulse generator.

Effects of chronic alcohol consumption and withdrawal on the response of the male and female hypothalamic-pituitary-adrenal axis to acute immune stress

The hypothalamic-pituitary-adrenal (HPA) axis plays a central role in the response to stress, and its activity is sexually dimorphic and modulated by sex steroids. Recent work indicates that HPA axis functioning is disturbed by chronic alcohol consumption and subsequent withdrawal in rats of both sexes, but particularly in females. To examine the influence of sex steroid hormones in HPA axis response to acute stress after ingestion of a 20% ethanol solution over 6months and subsequent withdrawal (2months), intact males, and estradiol- and oil-injected ovariectomized females received a single intraperitoneal injection of lipopolysaccharide (LPS). Six hours after LPS administration, corticosterone concentrations were increased in all male groups; however, in ethanol-treated rats they remained below those of control and withdrawn rats. mRNA levels of corticotrophin-releasing hormone (CRH) increased, and were identical in all groups after LPS stimulation, whereas those of vasopressin, although increased, remained below control levels. LPS stimulation elevated corticosterone concentrations in all oil-injected female groups, but did not alter those of estradiol-injected females. In oil- and estradiol-injected ethanol-treated females, CRH mRNA levels did not change in response to LPS stimulation, whereas those of vasopressin increased, but stayed below control levels. In withdrawn oil- and estradiol-injected females, CRH and vasopressin gene expression increased, but did not reach control levels. These data show that prolonged alcohol consumption produces long-lasting, possibly irreversible, changes in the neuroendocrine system that regulates the production of corticosteroids, and that these consequences are more profound in females, particularly when estrogen levels are low.

Reproductive experience modifies the effects of estrogen receptor alpha activity on anxiety-like behavior and corticotropin releasing hormone mRNA expression

Previous studies have demonstrated that prior reproductive experience can influence anxiety-like behaviors, although neural mechanisms underlying this shift remain unknown. Studies in virgin females suggest that activation of the two estrogen receptor subtypes, ERα and ERβ, have differing effects on anxiety. Specifically, ERβ activation has been shown to reduce anxiety-like behaviors, while ERα activation has no significant effect. The purpose of the present study was to examine the possible roles of ERα and ERβ subtypes in parity-induced alterations in anxiety-like behavior, as tested on the elevated plus maze (EPM). Groups of ovariectomized, age-matched, nulliparous and primiparous females were tested on the EPM following administration of the ERα agonist 4,4',4''-(4-Propyl-{1H}-pyrazole-1,3,5-tryl)trisphenol (PPT; 1 mg/kg), the ERβ agonist Diarylpropionitrile (DPN; 1 mg/kg) or vehicle (DMSO). All drugs were administered once daily for 4 days prior to testing as this dosing paradigm has previously been used to demonstrate anxiolytic effects of DPN in virgin rats. In addition, as exposure to the EPM is a psychological stressor, physiological markers of the stress response were measured in both plasma (corticosterone) and brain (corticotropin releasing hormone; CRH) post-EPM testing. Unexpectedly, the ERα agonist PPT selectively increased the time spent exploring the open arms of the EPM in non-lactating, primiparous females, with no significant effects of DPN observed in either nulliparous or primiparous subjects. All females administered PPT and tested on the EPM demonstrated significantly reduced corticosterone secretion when compared to vehicle-treated controls. In addition, significant effects of both reproductive experience and PPT administration on CRH mRNA expression were observed in both the paraventricular nucleus and amygdala using qPCR. These findings indicate that reproductive experience modulates the effects of ERα activation on both EPM behavior related to anxiety and CRH gene expression.

The inhibitory effects of neurokinin B on GnRH pulse generator frequency in the female rat

Neurokinin B (NKB) and its receptor (neurokinin-3 receptor) are coexpressed with kisspeptin and dynorphin A (Dyn) within neurons of the hypothalamic arcuate nucleus, the suggested site of the GnRH pulse generator. It is thought that these neuropeptides interact to regulate gonadotropin secretion. Using the ovariectomized (OVX) and OVX 17β-estradiol-replaced rat models, we have carried out a series of in vivo neuropharmacological and electrophysiological experiments to elucidate the hierarchy between the kisspeptin, NKB, and Dyn signaling systems. Rats were implanted with intracerebroventricular cannulae and cardiac catheters for frequent (every 5 min) automated serial blood sampling. Freely moving rats were bled for 6 h, with intracerebroventricular injections taking place after a 2-h control bleeding period. A further group of OVX rats was implanted with intra-arcuate electrodes for the recording of multiunit activity volleys, which coincide invariably with LH pulses. Intracerebroventricular administration of the selective neurokinin-3 receptor agonist, senktide (100-600 pmol), caused a dose-dependent suppression of LH pulses and multiunit activity volleys. The effects of senktide did not differ between OVX and 17β-estradiol-replaced OVX animals. Pretreatment with a selective Dyn receptor (κ opioid receptor) antagonist, norbinaltorphimine (6.8 nmol), blocked the senktide-induced inhibition of pulsatile LH secretion. Intracerebroventricular injection of senktide did not affect the rise in LH concentrations after administration of kisspeptin (1 nmol), and neither did kisspeptin preclude the senktide-induced suppression of LH pulses. These data show that NKB suppresses the frequency of the GnRH pulse generator in a Dyn/κ opioid receptor-dependent fashion.

Acute restraint stress increases intrahypothalamic oestradiol concentrations in conjunction with increased hypothalamic oestrogen receptor β and aromatase mRNA expression in female rats

Activation of the hypothalamic-pituitary-adrenal axis is considered to be one of the key physiological responses to stress and, interestingly, shows a marked sex difference. Oestradiol plays an important role in this sex difference. The present study investigated the systemic and intrahypothalamic oestradiol response to physical restraint stress in female rats. We used jugular catheterisation and intrahypothalamic microdialysis to simultaneously measure plasma oestradiol and local oestradiol concentrations in the paraventricular nucleus (PVN) of the hypothalamus. We also assessed corticotrophin-releasing hormone (CRH), aromatase, and oestrogen receptor (ER) α and β mRNA expression in the PVN by quantitative polymerase chain reaction immediately after the acute stress period. As expected, PVN CRH mRNA and plasma corticosterone were significantly increased after acute stress. Interestingly, the local oestradiol concentration in the PVN also increased during the 1-h stress period in pro-oestrus and in ovariectomised (OVX) animals. Aromatase mRNA expression in the PVN was increased markedly in pro-oestrus but only modestly in oestrus. PVN ERβ but not ERα mRNA expression was significantly elevated in pro-oestrous animals. In addition, plasma oestradiol levels increased 10 min after stress, both during pro-oestrus and oestrus but not in OVX animals. To conclude, we report an intra-hypothalamic oestradiol response to restraint stress. The rising hypothalamic oestradiol concentration together with increased ERβ gene expression indicates a positive feedback of hypothalamic oestradiol signalling during acute stress in rats.

The role of the medial and central amygdala in stress-induced suppression of pulsatile LH secretion in female rats

Stress exerts profound inhibitory effects on reproductive function by suppressing the pulsatile release of GnRH and therefore LH. Although the mechanisms by which stressors disrupt the hypothalamic GnRH pulse generator remain to be fully elucidated, numerous studies have implicated the amygdala, especially its medial (MeA) and central nuclei (CeA), as key modulators of the neuroendocrine response to stress. In the present study, we investigated the roles of the MeA and CeA in stress-induced suppression of LH pulses. Ovariectomized rats received bilateral ibotenic acid or sham lesions targeting the MeA or CeA; blood samples (25 μl) were taken via chronically implanted cardiac catheters every 5 min for 6 h for the measurement of LH pulses. After 2 h of baseline sampling, the rats were exposed to either: restraint (1 h), insulin-induced hypoglycemia (IIH) (0.3 U/kg, iv), or lipopolysaccharide (LPS) (25 μg/kg, iv) stress. The restraint but not IIH or LPS stress-induced suppression of LH pulses was markedly attenuated by the MeA lesions. In contrast, CeA lesioning attenuated LPS, but not restraint or IIH stress-induced suppression of LH pulses. Moreover, after restraint stress, the number of Fos-positive neurons and the percentage of glutamic acid decarboxylase(67) neurons expressing Fos was significantly greater in the GnRH-rich medial preoptic area (mPOA) of rats with intact, rather than lesioned, MeA. These data indicate that the MeA and CeA play key roles in psychogenic and immunological stress-induced suppression of the GnRH pulse generator, respectively, and the MeA-mediated effect may involve γ-aminobutyric acid ergic signaling within the mPOA.

The role of the bed nucleus of the stria terminalis in stress-induced inhibition of pulsatile luteinising hormone secretion in the female rat

The bed nucleus of the stria terminalis (BNST) occupies a central position in the neural circuitry regulating the hypothalamic-pituitary-adrenocortical axis response to stress. The potential role of the BNST in stress-induced suppression of the gondotrophin-releasing hormone (GnRH) pulse generator, the central regulator of the reproductive system, was assessed by examining the effects of micro-infusion of corticotrophin-releasing factor (CRF) or its antagonist into the BNST on pulsatile luteinising hormone (LH) secretion or stress-induced inhibition of LH pulses, respectively. Ovariectomised oestrogen-treated rats were implanted chronically with bilateral cannulae in the dorsolateral BNST and i.v. catheters. CRF (25, 50 or 100 pmol in 200 nl of artificial cerebrospinal fluid) administered bilaterally into the BNST resulted in a dose-dependent decrease in LH pulse frequency, and induced Fos expression in glutamic acid decarboxylase immunostained neurones in the medial preoptic area. These results suggest that the activation of hypothalamic GABAergic neurones in response to intra-BNST administration of CRF may be involved in the suppression of LH pulses. Furthermore, administration of CRF antagonist (280 pmol astressin-B, three times at 20-min intervals) into the BNST effectively blocked the suppression of pulsatile LH secretion in response to restraint (1 h) but not hypoglycaemic (0.25 U insulin/kg, i.v.) stress. These data suggest that CRF innervation of the dorsolateral BNST plays a key, but differential, role in stress-induced suppression of the GnRH pulse generator.

Ovarian steroid regulation of the midbrain corticotropin releasing factor and urocortin systems in macaques

A significant number of postmenopausal women report increased anxiety and vulnerability to stress, which has been linked to decreased secretion of ovarian steroids. Communication between the serotonin system and the corticotropin releasing factor (CRF) system determines stress sensitivity or resilience. This study examines the effects of the ovarian steroids, estradiol (E) and progesterone (P) on the CRF system components that impact serotonin neurons in the midbrain of nonhuman primates. Ovariectomized rhesus macaques were treated with placebo, E alone for 1 month, or E supplemented with P for the last 2 weeks. Quantitative (q)RT-PCR and immunocytochemistry were employed. E±P treatment decreased CRF-R1 and increased CRF-R2 gene expression in hemi-midbrain blocks and in laser captured serotonin neurons. Also in hemi-midbrains, E treatment increased urocortin 1 (UCN1) and CRFBP gene expression, but supplemental P treatment reversed these effects. E±P decreased CRF fiber density in the dorsal, interfascicular and median raphe nuclei and decreased CRF-R1 immunostaining in the dorsal raphe. E increased CRF-R2 immunostaining in the dorsal and median raphe. E±P increased UCN1 immunostaining in the cell bodies and increased UCN1 fiber density in the caudal linear nucleus. Estrogen receptor beta (ERβ), but not ERα was detected in the nucleus of UCN1-positive neurons. While the mechanism of ovarian hormone regulation of the midbrain CRF system requires further investigation, these studies clearly demonstrate another pathway by which ovarian hormones may have positive effects on anxiety and mood regulation.

Corticotrophin-releasing factor and stress-induced inhibition of the gonadotrophin-releasing hormone pulse generator in the female

It is well established that stress activates the hypothalamo-pituitary-adrenal (HPA) axis and suppresses the hypothalamo-pituitary-gonadal (HPG) axis. A large literature dealing with various stressors that regulate gonadotrophin-releasing hormone (GnRH) secretion in a variety of species (including nonhuman primates, sheep, and rats) provides evidence that stress modulates GnRH secretion by activating the corticotrophin-releasing factor (CRF) system and sympathoadrenal pathways, as well as the limbic brain. Different stressors may suppress the HPG axis by activating or inhibiting various pathways in the CNS. In addition to CRF being the principal hypophysiotropic factor driving the HPA axis, it is a potent inhibitor of the GnRH pulse generator. The suppression of the GnRH pulse generator by a variety of stressful stimuli can be blocked by CRF antagonists, suggesting a pivotal role for endogenous CRF. The differential roles for CRF receptor type 1 (CRF-R1) and CRF-R2 in stress-induced suppression of the GnRH pulse generator add to the complexity of CRF regulation of the HPG axis. Although the precise sites and mechanisms of action remain to be elucidated, noradrenergic and gamma-amino-butyric acid (GABA) neurones are implicated in the system's regulation, and opioids and kisspeptin in the medial preoptic area (mPOA) and hypothalamic arcuate nucleus (ARC) may operate downstream of the CRF neuronal system.

Baby on board: do responses to stress in the maternal brain mediate adverse pregnancy outcome?

Stress and adverse environmental surroundings result in suboptimal conditions in a pregnant mother such that she may experience poor pregnancy outcome including complete pregnancy failure and preterm labor. Furthermore her developing baby is at risk of adverse programming, which confers susceptibility to long term ill health. While some mechanisms at the feto-maternal interface underlying these conditions are understood, the underlying cause for their adverse adaptation is often not clear. Progesterone plays a key role at many levels, including control of neuroendocrine responses to stress, procuring the required immune balance and controlling placental and decidual function, and lack of progesterone can explain many of the unwanted consequences of stress. How stress that is perceived by the mother inhibits progesterone secretion and action is beginning to be investigated. This overview of maternal neuroendocrine responses to stress throughout pregnancy analyses how they interact to compromise progesterone secretion and precipitate undesirable effects in mother and offspring.

Corticotrophin-releasing factor alters the timing of puberty in the female rat

Puberty is a developmental process that is dependent upon activation of the hypothalamic gonadotrophin-releasing hormone (GnRH) pulse generator. It is well established that the stress neuropeptide, corticotrophin-releasing factor (CRF), has a profound inhibitory action on GnRH pulse generator frequency. Although stress is known to affect the timing of puberty, the role of CRF is unknown. The present study aimed to test the hypothesis that CRF plays a critical role in the timing of puberty. On postnatal day (pnd) 28, female rat pups were chronically implanted with i.c.v. cannulae and received 14 days of administration of either CRF, CRF receptor antagonist (astressin-B) or artificial cerebrospinal fluid via an osmotic mini-pump. A separate group of rats served as nonsurgical controls. As a marker of puberty, rats were monitored for vaginal opening and first vaginal oestrus. Levels of CRF, CRF receptor types 1 and 2 (CRF-R1, CRF-R2) mRNA expression in micropunches of the medial preoptic area (mPOA), hypothalamic paraventricular nucleus (PVN) and arcuate nucleus (ARC) were determined across pubertal development; brain tissue was collected from a naive group of rats on pnd 14, 32, on the day of vaginal opening, and pnd 77 (Adult). Administration of CRF resulted in a delay in the onset of puberty, whereas astressin-B advanced pubertal onset. Additionally, CRF and CRF-R1 mRNA expression was reduced in the mPOA, but not ARC, at puberty. In the PVN, expression of CRF, but not CRF-R1 mRNA, was reduced at the time of puberty. These data support the hypothesis that CRF signalling may play an important role in modulating the timing of puberty in the rat.

A glucocorticoid sensitive biphasic rhythm of testosterone secretion

Studies of the hypothalamic-pituitary-adrenal (HPA) axis and the hypothalamic-pituitary-testicular (HPT) axis have revealed a reciprocal relationship between these two endocrine pathways. In rats, for example, disruption of the HPT axis alters the circadian secretion of corticosterone. Stress, on the other hand, can have varying effects on testosterone secretion in both rats and humans. Furthermore, in contrast to humans, where several pulses of testosterone secretion can be detected across the 24-h period with the largest in the morning, rats appear to exhibit a diurnal rhythm of testosterone secretion. In the present study, we used an automated blood sampling system to investigate the true circadian pattern of testosterone secretion under basal conditions and investigated how this responds to changes in levels of circulating corticosteroids. Analysis of plasma testosterone revealed the expected bimodal pattern of basal testosterone secretion. The two secretory episodes were 12.59 h +/- 41 min apart and 4.04 h +/- 16 min long, with one in the light phase and the other in the dark phase of the cycle. Interestingly, when both testosterone and corticosterone diurnal profile were compared, we found that the circadian rise in plasma corticosterone levels falls neatly between the two testosterone secretory episodes. Treatment of rats with the synthetic glucocorticoid methylprednisolone in their drinking water abolished the normal bimodal profile of testosterone secretion. These rats show transient pulses of testosterone throughout the 24 h, but no circadian pattern. By contrast, adrenalectomised rats maintain their bimodal circadian pattern, suggesting that an intact HPA axis is not necessary for generation of the endogenous HPT rhythm. Thus, although the circadian rhythm of testosterone does not depend on normal HPA function, increased levels of glucocorticoids can abolish normal HPT rhythmicity.

Social subordination and polymorphisms in the gene encoding the serotonin transporter enhance estradiol inhibition of luteinizing hormone secretion in female rhesus monkeys

Psychosocial factors, particularly social stress, may compromise reproduction. However, some individuals may be more susceptible to socially induced infertility. The present study used group-housed, adult, ovariectomized rhesus monkeys to test the hypothesis that exposure to psychosocial stress, imposed by social subordination, would enhance estradiol (E2)-negative feedback inhibition of LH. Because polymorphisms in the gene encoding the serotonin transporter (SLC6A4) may contribute to individual differences in response to adverse environments, we determined whether subordinate females with the short-promoter-length allele (s-variant) would show greater suppression of LH. Subordinate females, particularly those with the s-variant SLC6A4 genotype, received significantly higher rates of noncontact aggression from more dominant cage mates and had consistently lower body weights. Serum LH was not influenced by social status in the absence of E2. In contrast, subordinate females were hypersensitive to E2-negative feedback inhibition of LH. Furthermore, serum LH in subordinate females with s-variant SLC6A4 genotype was maximally suppressed by Day 4 of treatment, whereas nadir concentrations were not reached until later in treatment in other females. Finally, pharmacological elevation of serum cortisol potentiated E2-negative feedback inhibition in all females. The current data suggest that infertility induced by psychosocial stressors may be mediated by hypersensitivity to E2-negative feedback and that polymorphisms in the SLC6A4 gene may contribute to differences in reproductive compromise in response to chronic stress.

A role for the medial preoptic area in CGRP-induced suppression of pulsatile LH secretion in the female rat

Calcitonin gene-related peptide (CGRP) is involved in a variety of stress responses and plays a pivotal role in stress-induced suppression of the GnRH pulse generator in the rat. Intracerebroventricular administration of CGRP suppresses luteinizing hormone (LH) pulses and increases Fos expression within the medial preoptic area (mPOA) and paraventricular nucleus (PVN). The aims of the present study were to investigate whether the mPOA or PVN are sites of action for CGRP-induced suppression of LH pulses and whether lipopolysaccharide (LPS), restraint or insulin-induced hypoglycaemia, stressors known to suppress LH pulses, affect mRNA expression for CGRP and its receptor subunits (calcitonin receptor-like receptor (CL) and RAMP-1) in the mPOA and PVN. Micro-infusion of CGRP (50, 250 or 500 pmol) into the mPOA, but not the PVN, dose-dependently suppressed LH pulse frequency. LPS, restraint and hypoglycaemia suppressed RAMP-1 mRNA, but not CL or CGRP mRNA expression in the mPOA. In the PVN, all three stressors suppressed CL mRNA expression, but only LPS or restraint suppressed RAMP-1 mRNA, and CGRP mRNA was unaffected. These results provide evidence that, unlike the PVN, the mPOA might play an important role in the inhibitory effect of CGRP on pulsatile LH secretion. Additionally, CGRP receptor function may be involved in this brain region in stress-induced suppression of the GnRH pulse generator.

Cortisol reduces gonadotropin-releasing hormone pulse frequency in follicular phase ewes: influence of ovarian steroids

Stress-like elevations in plasma glucocorticoids suppress gonadotropin secretion and can disrupt ovarian cyclicity. In sheep, cortisol acts at the pituitary to reduce responsiveness to GnRH but does not affect GnRH pulse frequency in the absence of ovarian hormones. However, in ewes during the follicular phase of the estrous cycle, cortisol reduces LH pulse frequency. To test the hypothesis that cortisol reduces GnRH pulse frequency in the presence of ovarian steroids, the effect of cortisol on GnRH secretion was monitored directly in pituitary portal blood of follicular phase sheep in the presence and absence of a cortisol treatment that elevated plasma cortisol to a level observed during stress. An acute (6 h) cortisol increase in the midfollicular phase did not lower GnRH pulse frequency. However, a more prolonged (27 h) increase in cortisol beginning just before the decrease in progesterone reduced GnRH pulse frequency by 45% and delayed the preovulatory LH surge by 10 h. To determine whether the gonadal steroid milieu of the follicular phase enables cortisol to reduce GnRH pulse frequency, GnRH was monitored in ovariectomized ewes treated with estradiol and progesterone to create an artificial follicular phase. A sustained increment in plasma cortisol reduced GnRH pulse frequency by 70% in this artificial follicular phase, in contrast to the lack of an effect in untreated ovariectomized ewes as seen previously. Thus, a sustained stress-like level of cortisol suppresses GnRH pulse frequency in follicular phase ewes, and this appears to be dependent upon the presence of ovarian steroids.

Down-regulation of hypothalamic kisspeptin and its receptor, Kiss1r, mRNA expression is associated with stress-induced suppression of luteinising hormone secretion in the female rat

Identification of kisspeptin (Kiss1) and its G protein-coupled receptor 54 (Kiss1r) as an essential component of the hypothalamic-pituitary-gonadal (HPG) axis controlling gonadotrophin secretion raises the possibility that kisspeptin-Kiss1r signalling may play a critical role in the transduction of stress-induced suppression of reproduction. We examined the effects of: (i) three different stressors, known to suppress pulsatile luteinising hormone (LH) secretion; (ii) corticotrophin-releasing factor (CRF); and (iii) corticosterone on Kiss1 and Kiss1r expression in key hypothalamic sites regulating gonadotrophin secretion: the medial preoptic area (mPOA) and arcuate nucleus (ARC). Ovariectomised oestrogen-replaced rats were implanted with i.v., subcutaneous or i.c.v. cannulae. Blood samples were collected at 5-min intervals for 5-6 h for detection of LH. Quantitative reverse transcriptase-polymerase chain reaction was used to determine Kiss1 and Kiss1r mRNA levels in brain punches of the mPOA and ARC collected 6 h after restraint, insulin-induced hypoglycaemia or lipopolysaccharide stress, or after i.c.v. administration of CRF, or acute or chronic subcutaneous administration of corticosterone. We observed down-regulation of at least one component of the kisspeptin-Kiss1r signalling system by each of the stress paradigms within the mPOA and ARC. CRF decreased Kiss1 and Kiss1r expression in both the mPOA and ARC. Both acute and chronic stress levels of corticosterone resulted in a concomitant decrease in Kiss1 and an increase in kiss1r mRNA expression in the mPOA and ARC. This differential regulation of Kiss1 and Kiss1r might account for the lack of effect corticosterone has on pulsatile LH secretion. Considering the pivotal role for kisspeptin-Kiss1r signalling in the control of the HPG axis, these results suggest that the reduced Kiss1-Kiss1r expression may be a contributing factor in stress-related suppression of LH secretion.

Continuous expression of corticotropin-releasing factor in the central nucleus of the amygdala emulates the dysregulation of the stress and reproductive axes

An increase in corticotropin-releasing factor (CRF) is a putative factor in the pathophysiology of stress-related disorders. As CRF expression in the central nucleus of the amygdala (CeA) is important in adaptation to chronic stress, we hypothesized that unrestrained synthesis of CRF in CeA would mimic the consequences of chronic stress exposure and cause dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, increase emotionality and disrupt reproduction. To test this hypothesis, we used a lentiviral vector to increase CRF-expression site specifically in CeA of female rats. Increased synthesis of CRF in CeA amplified CRF and arginine vasopressin peptide concentration in the paraventricular nucleus of the hypothalamus, and decreased glucocorticoid negative feedback, both markers associated with the pathophysiology of depression. In addition, continuous expression of CRF in CeA also increased the acoustic startle response and depressive-like behavior in the forced swim test. Protein levels of gonadotropin-releasing hormone in the medial preoptic area were significantly reduced by continuous expression of CRF in CeA and this was associated with a lengthening of estrous cycles. Finally, sexual motivation but not sexual receptivity was significantly attenuated by continuous CRF synthesis in ovariectomized estradiol-progesterone-primed females. These data indicate that unrestrained CRF synthesis in CeA produces a dysregulation of the HPA axis, as well as many of the behavioral, physiological and reproductive consequences associated with stress-related disorders.Molecular Psychiatry (2009) 14, 37-50; doi:10.1038/mp.2008.91; published online 12 August 2008.

The role of corticotrophin-releasing hormone receptors in the calcitonin gene-related peptide-induced suppression of pulsatile luteinising hormone secretion in the female rat

Corticotrophin-releasing hormone (CRH) plays a pivotal role in the suppression of the gonadotrophin-releasing hormone (GRH) pulse generator in response to stress and intracerebroventricular (i.c.v.) administration of calcitonin gene-related peptide (CGRP). We have previously shown both CRH receptor subtypes, CRH-R1 and CRH-R2, are involved in the stress-induced suppression of LH pulses. The aims of the present study were to examine the role of CRH-R1 and CRH-R2 in CGRP-induced suppression of LH pulses, and to investigate the effects of CGRP on CRH expression in the paraventricular nucleus (PVN) and central nucleus of the amygdala (CeA), which have prominent CRH neurone populations that receive dense CGRP innervations. The suppression of LH pulses by CGRP (1.5 microg i.c.v.) was completely prevented by intravenous administration of the CRH-R1 antagonist SSR125543Q (7.5 mg/rat i.v., 30 min before CGRP), but was not affected by the CRH-R2 antagonist, astressin(2)-B (100 microg i.c.v., 10 min before CGRP). CGRP increased the CRH mRNA expression in PVN and CeA. These results provide evidence of a role for CRH-R1 in mediating the suppressive effects of CGRP on pulsatile LH secretion in the female rat, and additionally raise the possibility of an involvement of PVN and CeA CRH neuronal populations in this suppression.

Ovarian steroid treatment decreases corticotropin-releasing hormone (CRH) mRNA and protein in the hypothalamic paraventricular nucleus of ovariectomized monkeys

Corticotropin-releasing hormone (CRH) gene and protein expression were examined in the paraventricular nucleus (PVN) of ovariectomized female macaques treated with placebo or hormone therapy (HT) consisting of either estrogen (E) for 28 days, or progesterone (P) for the last 14 of 28 days, or E for 28 days supplemented with P for the last 14 of 28 days using Silastic capsules implanted s.c. in the periscapular region (n=4/group). Perfusion fixed sections (25 microm) at five levels of the PVN (rostral to caudal at 250 microm intervals) were immunostained (ICC) with an antibody to human CRH or processed in an in situ hybridization (ISH) assay with a monkey specific CRH riboprobe. The immunostained CRH-positive area was quantified with a Marianas Stereology Workstation and Slidebook 4.2. There was a significant decrease in the immunological CRH signal with E, P, and E+P treatment as measured by total or average pixels and microns (analysis of variance (ANOVA), p<0.002; Student-Newman-Keul's post hoc test versus placebo control group, p<0.05). There was also a decrease in the number of detectable CRH neurons (ANOVA, p<0.03) with HT. The sections processed for ISH were exposed to autoradiographic films. The CRH mRNA signal was analyzed with NIH Image. The average optical density and positive pixel area of the CRH mRNA signal was significantly suppressed by ovarian HT (ANOVA p<0.002; Student-Newman-Keul's post hoc test versus placebo control group, p<0.05). In summary, 1 month of stable treatment with a moderate dose of E, P or E+P significantly reduced CRH mRNA and protein in the PVN of ovariectomized monkeys. These results suggest that this hormone treatment regimen may increase stress resilience in surgically menopausal primates.

Neonatal lipopolysaccharide exposure exacerbates stress-induced suppression of luteinizing hormone pulse frequency in adulthood

Early life exposure to immunological challenge has programming effects on the adult hypothalamo-pituitary-adrenocortical axis stress responsivity, and stress is known to suppress GnRH pulse generator activity, especially LH pulses. We investigated the effects of neonatal exposure to endotoxin on stress-induced suppression of pulsatile LH secretion and the involvement of corticotropin-releasing factor (CRF) receptor mechanisms in adult rats. Pups at 3 and 5 d of age were administered lipopolysaccharide (LPS, 50 microg/kg, ip). At 12 wk of age, they were ovariectomized and implanted with sc 17beta-estradiol capsules and i.v. cannulas. Blood samples (25 microl) were collected every 5 min for 5 h for LH measurement. After 2 h of sampling, rats were given LPS (25 microg/kg, iv). CRF and CRF-R1 and CRF-R2 receptor mRNA was determined by RT-PCR in medial preoptic area (mPOA) micropunches collected at 3 h after LPS administration. There was no difference in basal LH pulse frequency between neonatal LPS- and neonatal saline-treated controls. However, neonatal endotoxin-treated rats exhibited a significantly greater LPS stress-induced suppression of LH pulse frequency. Basal mPOA CRF-R1 expression was unchanged in neonatal LPS- and neonatal saline-treated rats. However, CRF-R1 expression was significantly increased in response to LPS stress in neonatal LPS-treated animals but not in neonatal saline-treated controls. CRF and CRF-R2 expression was unchanged in all treatment groups. These data demonstrate that exposure to bacterial endotoxin in early neonatal life programs long-term sensitization of the GnRH pulse generator to the inhibitory influence of stress in adulthood, an effect that might involve up-regulation of CRF-R1 expression in the mPOA.

Does cortisol acting via the type II glucocorticoid receptor mediate suppression of pulsatile luteinizing hormone secretion in response to psychosocial stress?

This study assessed the importance of cortisol in mediating inhibition of pulsatile LH secretion in sheep exposed to a psychosocial stress. First, we developed an acute psychosocial stress model that involves sequential layering of novel stressors over 3-4 h. This layered-stress paradigm robustly activated the hypothalamic-pituitary-adrenal axis and unambiguously inhibited pulsatile LH secretion. We next used this paradigm to test the hypothesis that cortisol, acting via the type II glucocorticoid receptor (GR), mediates stress-induced suppression of pulsatile LH secretion. Our approach was to determine whether an antagonist of the type II GR (RU486) reverses inhibition of LH pulsatility in response to the layered stress. We used two animal models to assess different aspects of LH pulse regulation. With the first model (ovariectomized ewe), LH pulse characteristics could vary as a function of both altered GnRH pulses and pituitary responsiveness to GnRH. In this case, antagonism of the type II GR did not prevent stress-induced inhibition of pulsatile LH secretion. With the second model (pituitary-clamped ovariectomized ewe), pulsatile GnRH input to the pituitary was fixed to enable assessment of stress effects specifically at the pituitary level. In this case, the layered stress inhibited pituitary responsiveness to GnRH and antagonism of the type II GR reversed the effect. Collectively, these findings indicate acute psychosocial stress inhibits pulsatile LH secretion, at least in part, by reducing pituitary responsiveness to GnRH. Cortisol, acting via the type II GR, is an obligatory mediator of this effect. However, under conditions in which GnRH input to the pituitary is not clamped, antagonism of the type II GR does not prevent stress-induced inhibition of LH pulsatility, implicating an additional pathway of suppression that is independent of cortisol acting via this receptor.

Corticotrophin-releasing factor type 2 receptor-mediated suppression of gonadotrophin-releasing hormone mRNA expression in GT1-7 cells

Corticotrophin-releasing factor (CRF) released during stress has been implicated in the suppression of the hypothalamo-pituitary-gonadal (HPG) axis, especially the gonadotrophin-releasing hormone (GnRH) pulse generator, the central neural regulator of pituitary LH and FSH secretion, resulting in reproductive dysfunction. The gonadal steroid 17beta-oestradiol (E2) has been shown to enhance CRF- and stress-induced suppression of pulsatile LH secretion. In the present study, we investigated the potential direct action of CRF on GnRH neurones by using GT1-7 cells, an established GnRH cell line. Furthermore, we investigated the modulatory influence of E2 on the effects of CRF and expression of CRF type 2 receptors (CRF-R2). Expression of CRF-R2 in the GT1-7 cells was detected by reverse transcription-polymerase chain reaction (RT-PCR). CRF produced a dose-dependent suppression of GnRH mRNA expression, an effect reversed by the selective CRF-R2 antagonist, astressin2-B (Ast2-B). E2 combined with CRF resulted in a greater suppression of GnRH expression compared with either treatment alone. E2 also increased CRF-R2 expression. These results demonstrate for the first time expression of CRF-R2 in the GT1-7 cells and suggest that CRF may directly regulate GnRH gene expression, an effect mediated, at least in part, by CRF-R2. They also raise the possibility that up-regulation of CRF-R2 may contribute to the sensitising influence of E2 on CRF- and stress-induced suppression of the GnRH pulse generator.

Differential role of corticotrophin-releasing factor receptor types 1 and 2 in stress-induced suppression of pulsatile luteinising hormone secretion in the female rat

Corticotrophin-releasing factor (CRF) plays a pivotal role in stress-induced suppression of the gonadotrophin-releasing hormone pulse generator. We have previously shown that type 2 CRF receptors (CRF(2)) mediate restraint stress-induced suppression of luteinising hormone (LH) pulses in the rat. The present study aimed: (i) to determine whether type 1 CRF receptors (CRF(1)) are also involved in this response to restraint and (ii) to investigate the differential involvement of CRF(1) and CRF(2) in the suppression of LH pulses in response to the metabolic perturbation of insulin-induced hypoglycemia and the innate immunological challenge of lipopolysaccharide (LPS). Ovariectomised rats with oestrogen replacement were implanted with intracerebroventricular (i.c.v.) and intravenous (i.v.) cannulae. Blood samples (25 microl) were collected every 5 min for 5 h for LH measurement. After 2 h of controlled blood sampling, rats were either exposed to restraint (1 h) or injected intravenously with insulin (0.25 IU/kg) or LPS (5 microg/kg). All three stressors suppressed LH pulses. The CRF(1) antagonist SSR125543Q (11.5 micromol/rat i.v., 30 min before stressor) blocked the inhibitory response to restraint, but not hypoglycaemia or LPS stress. In addition to its effect on restraint, the CRF(2) antagonist astressin(2)-B (28 nmol/rat i.c.v., 10 min before insulin or LPS) blocked hypoglycaemia or LPS stress-induced suppression of LH pulses. These results suggest that hypoglycaemia and LPS stress-induced LH suppression involves activation of CRF(2) while restraint stress-induced inhibition of LH pulses involves both CRF(1) and CRF(2).

Cortisol, Testosterone, and Coronary Heart Disease

Background— There is a popular belief that chronic stress causes heart disease through psychoneuroendocrine mechanisms. We have examined whether an elevated circulating cortisol-to-testosterone ratio increases the risk of ischemic heart disease.

Methods and Results— We undertook a prospective cohort study of 2512 men aged 45 to 59 years between 1979 and 1983 from Caerphilly, South Wales, with a mean follow-up of 16.5 years. Subjects underwent a clinical examination, and morning fasting blood samples were taken for analysis of cortisol levels, testosterone levels, and other cardiovascular risk factors. The ratio of cortisol to testosterone showed weak associations with potential confounding factors but strong positive associations with components of the insulin resistance syndrome ( P <0.001). A positive linear trend was seen across quintiles of cortisol:testosterone ratio for incident ischemic heart disease (age-adjusted OR per z score change in ratio 1.22, 95% CI 1.07 to 1.38, P =0.003). This was markedly attenuated after adjustment for components of the insulin resistance syndrome (age-adjusted OR per z score change in ratio 1.10, 95% CI 0.96 to 1.25, P =0.18). There was no association between the cortisol:testosterone ratio and other causes of death (age-adjusted hazard ratio 0.99, 95% CI 0.88 to 1.11, P =0.81).

Conclusions— This is the first population-based prospective study that has found a specific association between cortisol:testosterone ratio and incident ischemic heart disease, apparently mediated through the insulin resistance syndrome. Whether this reflects the effects of chronic stress, behavioral factors, or genetic influences remains to be determined.

Calcitonin gene-related peptide-induced suppression of luteinizing hormone pulses in the rat: the role of endogenous opioid peptides

Calcitonin gene-related peptide (CGRP) is involved in a variety of stress responses in the rat. Central administration of CGRP activates the hypothalamo-pituitary-adrenal axis resulting in increased corticosterone secretion. We have previously shown that central CGRP suppresses the gonadotrophin-releasing hormone (GnRH) pulse generator, specifically LH pulses. Endogenous opioid peptides (EOPs) have been shown to play an important role in stress-induced suppression of the reproductive axis. The aim of the present study was to test the hypothesis that EOPs mediate CGRP-induced suppression of pulsatile LH secretion. Ovariectomized rats were implanted with intracerebroventricular (i.c.v.) and i.v. cannulae. Intravenous administration of the opioid antagonist naloxone (250 microg) completely blocked the suppression of LH pulses induced by 1.5 microg i.c.v. CGRP and significantly attenuated the suppression of pulsatile LH secretion induced by 5 microg i.c.v. CGRP. Furthermore, intravenous administration of naloxone was found to immediately restore normal LH pulse frequency in animals treated 90 min earlier with 1.5 microg i.c.v. CGRP. Co-administration (i.c.v.) of CGRP (1.5 microg) with the mu and kappa opioid receptor-specific antagonists naloxone (10 microg) and norbinaltorphimine (5 microg), respectively, blocked the CGRP-induced suppression of LH pulses, whilst i.c.v. co-administration of CGRP (1.5 microg) with the delta opioid receptor-specific antagonist naltrindole (5 microg) did not. These data provide evidence that EOPs play a pivotal role in mediating the inhibitory effects of CGRP on pulsatile LH secretion in the rat. They also suggest that the mu and kappa, but not the delta, opioid receptors may be responsible for mediating the effects of CGRP on LH pulses.

Neuroendocrinology of nutritional infertility

Natural selection has linked the physiological controls of energy balance and fertility such that reproduction is deferred during lean times, particularly in female mammals. In this way, an energetically costly process is confined to periods when sufficient food is available to support pregnancy and lactation. Even in the face of abundance, nutritional infertility ensues if energy intake fails to keep pace with expenditure. A working hypothesis is proposed in which any activity or condition that limits the availability of oxidizable fuels (e.g., undereating, excessive energy expenditure, diabetes mellitus) can inhibit both gonadotropin-releasing hormone (GnRH)/luteinizing hormone secretion and female copulatory behaviors. Decreases in metabolic fuel availability appear to be detected by cells in the caudal hindbrain. Hindbrain neurons producing neuropeptide Y (NPY) and catecholamines (CA) then project to the forebrain where they contact GnRH neurons both directly and also indirectly via corticotropin-releasing hormone (CRH) neurons to inhibit GnRH secretion. In the case of estrous behavior, the best available evidence suggests that the inhibitory NPY/CA system acts primarily via CRH or urocortin projections to various forebrain loci that control sexual receptivity. Disruption of these signaling processes allows normal reproduction to proceed in the face of energetic deficits, indicating that the circuitry responds to energy deficits and that no signal is necessary to indicate that there is an adequate energy supply. While there is a large body of evidence to support this hypothesis, the data do not exclude nutritional inhibition of reproduction by other pathways and processes, and the full story will undoubtedly be more complex than this.

Gonadal steroid modulation of the limbic-hypothalamic- pituitary-adrenal (LHPA) axis is influenced by social status in female rhesus monkeys

Chronic stress can have a deleterious effect on the re-productive axis that, for females, is manifested in an increased incidence of infertility. However, gonadal steroids may, in turn, affect a female's response to stress as measured by activity within the limbic-hypothalamic-pituitary-adrenal (LHPA) axis. What is not clear is whether a history of exposure to stress modifies the effect of gonadal steroids on LHPA responsivity. Rhesus monkeys present a unique opportunity to assess LHPA responsivity when housed socially in groups. Under these situations, monkeys exhibit a rich network of affiliation and have established social status hierarchies. Previous work indicates that socially subordinate macaque females are hypercortisolemic due to diminished gluco-corticoid negative feedback. The present study tested the hypothesis that estradiol (E2) would decrease gluco-corticoid negative feedback, assessed from a dexamethasone (DEX) suppression test, and increase the response to corticotropin releasing factor (CRF) and that these effects would be attenuated by co-treatment with P4. In addition, we also determined whether E2 and P4 would differentially affect LHPA responsiveness to pharmacological challenge in socially dominant compared with subordinate females. Endogenous gonadal hormone secretion in female rhesus monkeys (n = 7) was suppressed by continuous treatment with a sustained release formulation of the GnRH analog leuprolide acetate (Lupron Depot). The response to a combined DEX suppression-CRF stimulation test was assessed using a counterbalanced design during a placebo (control) treatment condition and during E2, P4, and E2 + P4 re-placement therapy. Females who were members of a large breeding group of 140 adults and juveniles of both sexes, were classified as dominant (n = 4) or subordinate (n = 3) based on the relative social dominance positions within the group. Plasma levels of cortisol were significantly higher during E2 replacement compared to the other treatment conditions following DEX suppression and stimulation with CRF.