Systemic challenge with endotoxin stimulates corticotropin-releasing hormone and arginine vasopressin secretion into hypophyseal portal blood: coincidence with gonadotropin-releasing hormone suppression

The impact of inflammatory stress on hypothalamic kisspeptin neurons: Mechanisms underlying inflammation-associated infertility in humans and domestic animals

Inflammatory diseases attenuate reproductive functions in humans and domestic animals. Lipopolysaccharide (LPS), an endotoxin released by bacteria, is known to disrupt female reproductive functions in various inflammatory diseases. LPS administration has been used to elucidate the impact of pathophysiological activation of the immune system on reproduction. Hypothalamic kisspeptin neurons are the master regulators of mammalian reproduction, mediating direct stimulation of hypothalamic gonadotropin-releasing hormone (GnRH) release and consequent release of gonadotropins, such as luteinizing hormone (LH) and follicle-stimulating hormone from the pituitary. The discovery of kisspeptin neurons in the mammalian hypothalamus has drastically advanced our understanding of how inflammatory stress causes reproductive dysfunction in both humans and domestic animals. Inflammation-induced ovarian dysfunction could be caused, at least partly, by aberrant GnRH and LH secretion, which is regulated by kisspeptin signaling. In this review, we focus on the effects of LPS on hypothalamic kisspeptin neurons to outline the impact of inflammatory stress on neuroendocrine regulation of mammalian reproductive systems. First, we summarize the attenuation of female reproduction by LPS during inflammation and the effects of LPS on ovarian and pituitary function. Second, we outline the inhibitory effects of LPS on pulsatile- and surge-mode GnRH/LH release. Third, we discuss the LPS-responsive hypothalamic-pituitary-adrenal axis and hypothalamic neural systems in terms of the cytokine-mediated pathway and the possible direct action of LPS via its hypothalamic receptors. This article describes the impact of LPS on hypothalamic kisspeptin neurons and the possible mechanisms underlying LPS-mediated disruption of LH pulses/surge via kisspeptin neurons.

Magnetic Resonance Imaging, a New Tool for Neuroendocrine Research in Sheep

Magnetic resonance imaging (MRI) brain analysis is used in rodents and for clinical investigation in humans, and it becomes also possible now for large animal models studies. Specific facilities are available with clinical scanners and benefit to neuroendocrine investigations in sheep. Sheep has a large gyrencephalic brain and its organization is very similar to primates and human, and among physiological regulations, oestrous cycle of the ewes is similar to women. Therefore, this animal is a good model for preclinical researches using MRI, as illustrated with steroids impact on the brain. New data were obtained concerning the effect of sexual steroids on neuronal networks involved in the control of reproduction and in the influence of sexual steroids on cognition. In addition to the importance of such data for understanding the role of these hormones on brain functions, they give new insights to consider the sheep as a powerful model for preclinical studies in the field of neuroendocrinology. These points are discussed in this short review.

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.

Gonadotropin-Releasing Hormone (GnRH) Neuron Potassium Currents and Excitability in Both Sexes Exhibit Minimal Changes upon Removal of Negative Feedback

Gonadotropin-releasing hormone (GnRH) drives pituitary secretion of luteinizing hormone and follicle-stimulating hormone, which in turn regulate gonadal functions including steroidogenesis. The pattern of GnRH release and thus fertility depend on gonadal steroid feedback. Under homeostatic (negative) feedback conditions, removal of the gonads from either females or males increases the amplitude and frequency of GnRH release and alters the long-term firing pattern of these neurons in brain slices. The neurobiological mechanisms intrinsic to GnRH neurons that are altered by homeostatic feedback are not well studied and have not been compared between sexes. During estradiol-positive feedback, which is unique to females, there are correlated changes in voltage-gated potassium currents and neuronal excitability. We thus hypothesized that these same mechanisms would be engaged in homeostatic negative feedback. Voltage-gated potassium channels play a direct role in setting excitability and action potential properties. Whole-cell patch-clamp recordings of GFP-identified GnRH neurons in brain slices from sham-operated and castrated adult female and male mice were made to assess fast and slow inactivating potassium currents as well as action potential properties. Surprisingly, no changes were observed among groups in most potassium current properties, input resistance, or capacitance, and this was reflected in a lack of differences in excitability and specific action potential properties. These results support the concept that, in contrast to positive feedback, steroid-negative feedback regulation of GnRH neurons in both sexes is likely conveyed to GnRH neurons via mechanisms that do not induce major changes in the biophysical properties of these cells.

Understanding the trade-off between the environment and fertility in cows and ewes

The environment contributes to production diseases that in turn badly affect cow performance, fertility and culling. Oestrus intensity is lower in lame cows, and in all cows 26% potential oestrus events are not expressed (to avoid getting pregnant). To understand these trade-offs, we need to know how animals react to their environment and how the environment influences hypothalamus-pituitary-adrenal axis (HPA) interactions with the hypothalamus-pituitary-ovarian axis (HPO). Neurotransmitters control secretion of GnRH into hypophyseal portal blood. GnRH/LH pulse amplitude and frequency drive oestradiol production, culminating in oestrus behaviour and a precisely-timed GnRH/LH surge, all of which are disrupted by poor environments. Responses to peripheral neuronal agents give clues about mechanisms, but do these drugs alter perception of stimuli, or suppress consequent responses? In vitro studies confirm some neuronal interactions between the HPA and HPO; and immuno-histochemistry clarifies the location and sequence of inter-neurone activity within the brain. In both species, exogenous corticoids, ACTH and/or CRH act at the pituitary (reduce LH release by GnRH), and hypothalamus (lower GnRH pulse frequency and delay surge release). This requires inter-neurones as GnRH cells do not have receptors for HPA compounds. There are two (simultaneous, therefore fail-safe?) pathways for CRH suppression of GnRH release via CRH-Receptors: one being the regulation of kisspeptin/dynorphin and other cell types in the hypothalamus, and the other being the direct contact between CRH and GnRH cell terminals in the median eminence. When we domesticate animals, we must provide the best possible environment otherwise animals trade-off with lower production, less intense oestrus behaviour, and impaired fertility. Avoiding life-time peri-parturient problems by managing persistent lactations in cows may be a worthy trade-off on both welfare and economic terms – better than the camouflage use of drugs/hormones/feed additives/intricate technologies? In the long term, getting animals and environment in a more harmonious balance is the ultimate strategy.

Peripheral interleukin-1β inhibits arcuate kiss1 cells and LH pulses in female mice

Peripheral immune/inflammatory challenges rapidly disrupt reproductive neuroendocrine function. This inhibition is considered to be centrally mediated via suppression of gonadotropin-releasing hormone secretion, yet the neural pathway(s) for this effect remains unclear. We tested the hypothesis that interleukin-1β inhibits pulsatile luteinizing hormone secretion in female mice via inhibition of arcuate kisspeptin cell activation, a population of neurons considered to be the gonadotropin-releasing hormone pulse generator. In the first experiment, we determined that the inhibitory effect of peripheral interleukin-1β on luteinizing hormone secretion was enhanced by estradiol. We next utilized serial sampling and showed that interleukin-1β reduced the frequency of luteinizing hormone pulses in ovariectomized female mice treated with estradiol. The interleukin-1β-induced suppression of pulse frequency was associated with reduced kisspeptin cell activation, as determined by c-Fos coexpression, but not as a result of impaired responsiveness to kisspeptin challenge. Together, these data suggest an inhibitory action of interleukin-1β upstream of kisspeptin receptor activation. We next tested the hypothesis that estradiol enhances the activation of brainstem nuclei responding to interleukin-1β. We determined that the expression of interleukin-1 receptor was elevated within the brainstem following estradiol. Interleukin-1β induced c-Fos in the area postrema, ventrolateral medulla, and nucleus of the solitary tract; however, the response was not increased by estradiol. Collectively, these data support a neural mechanism whereby peripheral immune/inflammatory stress impairs reproductive neuroendocrine function via inhibition of kisspeptin cell activation and reduced pulsatile luteinizing hormone secretion. Furthermore, these findings implicate the influence of estradiol on peripherally mediated neural pathways such as those activated by peripheral cytokines.

Rhythmicity matters: Circadian and ultradian patterns of HPA axis activity

Oscillations are a fundamental feature of neural and endocrine systems. The hypothalamic-pituitary-adrenal (HPA) axis dynamically controls corticosteroid secretion in basal conditions and in response to stress. Across the 24-h day, HPA axis activity oscillates with both an ultradian and circadian rhythm. These rhythms have been shown to be important for regulating metabolism, inflammation, mood, cognition and stress responsiveness. Here we will discuss the neural and endocrine mechanisms driving these rhythms, the physiological importance of these rhythms and health consequences when they are disrupted.

Temporally Tuned Corticosteroid Feedback Regulation of the Stress Axis

Activity of the hypothalamic-pituitary-adrenal (HPA) axis is tuned by corticosteroid feedback. Corticosteroids regulate cellular function via genomic and nongenomic mechanisms, which operate over diverse time scales. This review summarizes recent advances in our understanding of how corticosteroid feedback regulates hypothalamic stress neuron function and output through synaptic plasticity, changes in intrinsic excitability, and modulation of neuropeptide production. The temporal kinetics of corticosteroid actions in the brain versus the pituitary have important implications for how organisms respond to stress. Furthermore, we will discuss, some of the technical limitations and missing links in the field, and the potential implications these may have on our interpretations of corticosteroid negative feedback experiments.

Effect of Central Injection of Neostigmine on the Bacterial Endotoxin Induced Suppression of GnRH/LH Secretion in Ewes during the Follicular Phase of the Estrous Cycle

Induced by a bacterial infection, an immune/inflammatory challenge is a potent negative regulator of the reproduction process in females. The reduction of the synthesis of pro-inflammatory cytokine is considered as an effective strategy in the treatment of inflammatory induced neuroendocrine disorders. Therefore, the effect of direct administration of acetylcholinesterase inhibitor—neostigmine—into the third ventricle of the brain on the gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretions under basal and immune stress conditions was evaluated in this study. In the study, 24 adult, 2-years-old Blackhead ewes during the follicular phase of their estrous cycle were used. Immune stress was induced by the intravenous injection of LPS Escherichia coli in a dose of 400 ng/kg. Animals received an intracerebroventricular injection of neostigmine (1 mg/animal) 0.5 h before LPS/saline treatment. It was shown that central administration of neostigmine might prevent the inflammatory-dependent decrease of GnRH/LH secretion in ewes and it had a stimulatory effect on LH release. This central action of neostigmine is connected with its inhibitory action on local pro-inflammatory cytokines, such as interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)α synthesis in the hypothalamus, which indicates the importance of this mediator in the inhibition of GnRH secretion during acute inflammation.

Involvement of prolactin in the meloxicam-dependent inflammatory response of the gonadotropic axis to prolonged lipopolysaccharide treatment in anoestrous ewes

An immune challenge can affect the reproductive process in females. Peripheral administration of bacterial endotoxin (lipopolysaccharide; LPS) decreases LH secretion and disrupts ovarian cyclicity. The aim of the present study was to determine the effects of a cyclo-oxygenase (COX)-2 inhibitor (meloxicam) on gonadotropin-releasing hormone (GnRH) and LH secretion in anoestrous ewes during systemic inflammation induced by LPS. LPS (400ngkg^-1 per day) suppressed LH release. In three individuals, meloxicam (500μgkg^-1, i.v.) abolished LPS-induced LH suppression. In another three ewes LH was ineffective. Similar changes were observed in hypothalamic GnRH expression. The effect of meloxicam depended on the circulating level of prolactin: meloxicam abolished inflammatory-dependent suppression of GnRH and LH secretion when plasma prolactin levels were similar to those in untreated animals, but was ineffective in those with elevated levels of prolactin. We conclude that COX-2 inhibitors minimise the negative effect of inflammation on the reproductive system but that this effect may be antagonised by prolactin.

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.

Peripheral Inhibitor of AChE, Neostigmine, Prevents the Inflammatory Dependent Suppression of GnRH/LH Secretion during the Follicular Phase of the Estrous Cycle

The study was designed to test the hypothesis that the inhibition of acetylcholinesterase (AChE) activity at the periphery by Neostigmine (0.5 mg/animal) will be sufficient to prevent inflammatory dependent suppression of the gonadotropin-releasing hormone (GnRH)/luteinising hormone (LH) secretion in ewes in the follicular phase of the estrous cycle, and this effect will be comparable with the systemic AChE inhibitor, Donepezil (2.5 mg/animal). An immune/inflammatory challenge was induced by peripheral administration of lipopolysaccharide (LPS; 400 ng/kg). Peripheral treatment with Donepezil and Neostigmine prevented the LPS-induced decrease (P < 0.05) in LHβ gene expression in the anterior pituitary gland (AP) and in LH release. Moreover, Donepezil completely abolished (P < 0.05) the suppressory effect of inflammation on GnRH synthesis in the preoptic area, when pretreatment with Neostigmine reduced (P < 0.05) the decrease in GnRH content in this hypothalamic structure. Moreover, administration of both AChE inhibitors diminished (P < 0.05) the inhibitory effect of LPS treatment on the expression of GnRH receptor in the AP. Our study shows that inflammatory dependent changes in the GnRH/LH secretion may be eliminated or reduced by AChE inhibitors suppressing inflammatory reaction only at the periphery such as Neostigmine, without the need for interfering in the central nervous system.

Review: Endotoxin and the hypothalamo-pituitary-adrenal (HPA) axis

Endotoxin is considered to be a systemic (immunological) stressor eliciting a prolonged activation of the hypothalamo-pituitary-adrenal (HPA) axis. The HPA-axis response after an endotoxin challenge is mainly due to released cytokines (IL-1, IL-6 and TNF-α) from stimulated peripheral immune cells, which in turn stimulate different levels of the HPA axis. Controversy exists regarding the main locus of action of endotoxin on glucocorticoid secretion, since the effect of endotoxin on this neuro-endocrine axis has been observed in intact animals and after ablation of the hypothalamus; however, a lack of LPS effect has been described at both pituitary and adrenocortical levels. The resulting increase in adrenal glucocorticoids has well-documented inhibitory effects on the inflammatory process and on inflammatory cytokine release. Therefore, immune activation of the adrenal gland by endotoxin is thought to occur by cytokine stimulation of corticosteroid-releasing hormone (CRH) production in the median eminence of the hypothalamus, which, in turn stimulates the secretion of ACTH from the pituitary. Acute administration of endotoxin stimulates ACTH and cortisol secretion and the release of CRH and vasopressin (AVP) in the hypophysial portal blood. During repeated endotoxemia, tolerance of both immune and HPA function develops, with a crucial role for glucocorticoids in the modulation of the HPA axis. A single exposure to a high dose of LPS can induce a long-lasting state of tolerance to a second exposure of LPS, affecting the response of plasma TNF-α and HPA hormones. Although there are gender differences in the HPA response to endotoxin and IL-1, these responses are enhanced by castration and attenuated by androgen and estrogen replacement. Estrogens attenuate the endotoxin-induced stimulation of IL-6, TNF-α and IL-1ra release and subsequent activation in postmenopausal women. There appears to be a temporal and functional relation between the HPA-axis response to endotoxin and nitric oxide formation in the neuro-endocrine hypothalamus, suggesting a stimulatory role for nitric oxide in modulating the HPA response to immune challenges.

Spontaneous and CRH-Induced Excitability and Calcium Signaling in Mice Corticotrophs Involves Sodium, Calcium, and Cation-Conducting Channels

Transgenic mice expressing the tdimer2(12) form of Discosoma red fluorescent protein under control of the proopiomelanocortin gene's regulatory elements are a useful model for studying corticotrophs. Using these mice, we studied the ion channels and mechanisms controlling corticotroph excitability. Corticotrophs were either quiescent or electrically active, with a 22-mV difference in the resting membrane potential (RMP) between the 2 groups. In quiescent cells, CRH depolarized the membrane, leading to initial single spiking and sustained bursting; in active cells, CRH further facilitated or inhibited electrical activity and calcium spiking, depending on the initial activity pattern and CRH concentration. The stimulatory but not inhibitory action of CRH on electrical activity was mimicked by cAMP independently of the presence or absence of arachidonic acid. Removal of bath sodium silenced spiking and hyperpolarized the majority of cells; in contrast, the removal of bath calcium did not affect RMP but reduced CRH-induced depolarization, which abolished bursting electrical activity and decreased the spiking frequency but not the amplitude of single spikes. Corticotrophs with inhibited voltage-gated sodium channels fired calcium-dependent action potentials, whereas cells with inhibited L-type calcium channels fired sodium-dependent spikes; blockade of both channels abolished spiking without affecting the RMP. These results indicate that the background voltage-insensitive sodium conductance influences RMP, the CRH-depolarization current is driven by a cationic conductance, and the interplay between voltage-gated sodium and calcium channels plays a critical role in determining the status and pattern of electrical activity and calcium signaling.

Kisspeptin, c-Fos and CRFR type 2 co-expression in the hypothalamus after insulin-induced hypoglycaemia

Normal reproductive function is dependent upon availability of glucose and insulin-induced hypoglycaemia is a metabolic stressor known to disrupt the ovine oestrous cycle. We have recently shown that IIH has the ability to delay the LH surge of intact ewes. In the present study, we examined brain tissue to determine: (i) which hypothalamic regions are activated with respect to IIH and (ii) the effect of IIH on kisspeptin cell activation and CRFR type 2 immunoreactivity, all of which may be involved in disruptive mechanisms. Follicular phases were synchronized with progesterone vaginal pessaries and at 28 h after progesterone withdrawal (PW), animals received saline (n = 6) or insulin (4 IU/kg; n = 5) and were subsequently killed at 31 h after PW (i.e., 3 h after insulin administration). Peripheral hormone concentrations were evaluated, and hypothalamic sections were immunostained for either kisspeptin and c-Fos (a marker of neuronal activation) or CRFR type 2. Within 3 h of treatment, cortisol concentrations had increased whereas plasma oestradiol concentrations decreased in peripheral plasma (p < 0.05 for both). In the arcuate nucleus (ARC), insulin-treated ewes had an increased expression of c-Fos. Furthermore, the percentage of kisspeptin cells co-expressing c-Fos increased in the ARC (from 11 to 51%; p < 0.05), but there was no change in the medial pre-optic area (mPOA; 14 vs 19%). CRFR type 2 expression in the lower part of the ARC and the median eminence was not altered by insulin treatment. Thus, disruption of the LH surge after IIH in the follicular phase is not associated with decreased kisspeptin cell activation or an increase in CRFR type 2 in the ARC but may involve other cell types located in the ARC nucleus which are activated in response to IIH.

Kisspeptin, c-Fos and CRFR type 2 expression in the preoptic area and mediobasal hypothalamus during the follicular phase of intact ewes, and alteration after LPS

Increasing estradiol concentrations during the late follicular phase stimulate sexual behavior and the GnRH/LH surge, and it is known that kisspeptin signaling is essential for the latter. Administration of LPS can block these events, but the mechanism involved is unclear. We examined brain tissue from intact ewes to determine: i) which regions are activated with respect to sexual behavior, the LH surge and LPS administration, ii) the location and activation pattern of kisspeptin cells in control and LPS treated animals, and iii) whether CRFR type 2 is involved in such disruptive mechanisms. Follicular phases were synchronized with progesterone vaginal pessaries and control animals were killed at 0 h, 16 h, 31 h or 40 h (n=4-6/group) after progesterone withdrawal (time zero). At 28 h, other animals received endotoxin (LPS; 100 ng/kg) and were subsequently killed at 31 h or 40 h (n=5/group). LH surges only occurred in control ewes, during which there was a marked increase in c-Fos expression within the ventromedial nucleus (VMN), arcuate nucleus (ARC), and medial preoptic area (mPOA), as well as an increase in the percentage of kisspeptin cells co-expressing c-Fos in the ARC and mPOA compared to animals sacrificed at all other times. Expression of c-Fos also increased in the bed nucleus of the stria terminalis (BNST) in animals just before the expected onset of sexual behavior. However, LPS treatment increased c-Fos expression within the VMN, ARC, mPOA and diagonal band of broca (dBb), along with CRFR type 2 immunoreactivity in the lower part of the ARC and median eminence (ME), compared to controls. Furthermore, the percentage of kisspeptin cells co-expressing c-Fos was lower in the ARC and mPOA. Thus, we hypothesize that in intact ewes, the BNST is involved in the initiation of sexual behavior while the VMN, ARC, and mPOA as well as kisspeptin cells located in the latter two areas are involved in estradiol positive feedback only during the LH surge. By contrast, disruption of sexual behavior and the LH surge after LPS involves cells located in the VMN, ARC, mPOA and dBb, as well as cells containing CRFR type 2 in the lower part of the ARC and ME, and is accompanied by inhibition of kisspeptin cell activation in both the ARC and mPOA.

Repeated oral administration of lipopolysaccharide from Escherichia coli 0111:B4 modulated humoral immune responses in periparturient dairy cows

The objective of this study was to evaluate the effects of repeated oral exposure to LPS on humoral immune responses of periparturient dairy cows. Sixteen Holstein cows were assigned to two treatment groups 2 wk before the expected day of parturition. Cows were administered orally, twice weekly at wk -2, -1 and +1 around parturition, with the following treatments: 3 ml saline; or 3 ml of saline containing LPS from Escherichia coli 0111:B4. The amount of LPS administered during wk -2, -1, and +1 was 0.01, 0.05, or 0.1 µg/kg body weight, respectively. Multiple blood samples were collected by jugular vein and various immune and clinical variables were measured. Results indicated that, on one hand, concentrations of plasma IgG anti-LPS Abs decreased (P < 0.01) and those of IgM anti-LPS Abs increased (P < 0.01) in cows treated with oral LPS. On the other hand, there were no overall differences (P > 0.05) in the concentrations of serum amyloid A, LPS-binding protein, haptoglobin, cortisol, IgA anti-LPS Abs in the plasma, feed intake, body temperature and rumen contractions rate between the control and treatment groups. To our knowledge, this study is the first to show that repeated oral administration with LPS from E. coli 0111:B4 has the potential to stimulate humoral immune responses in periparturient dairy cows.

Identification of gene networks and pathways associated with Guillain-Barré syndrome

BACKGROUND

The underlying change of gene network expression of Guillain-Barré syndrome (GBS) remains elusive. We sought to identify GBS-associated gene networks and signaling pathways by analyzing the transcriptional profile of leukocytes in the patients with GBS.

METHODS AND FINDINGS

Quantitative global gene expression microarray analysis of peripheral blood leukocytes was performed on 7 patients with GBS and 7 healthy controls. Gene expression profiles were compared between patients and controls after standardization. The set of genes that significantly correlated with GBS was further analyzed by Ingenuity Pathways Analyses. 256 genes and 18 gene networks were significantly associated with GBS (fold change ≥2, P<0.05). FOS, PTGS2, HMGB2 and MMP9 are the top four of 246 significantly up-regulated genes. The most significant disease and altered biological function genes associated with GBS were those involved in inflammatory response, infectious disease, and respiratory disease. Cell death, cellular development and cellular movement were the top significant molecular and cellular functions involved in GBS. Hematological system development and function, immune cell trafficking and organismal survival were the most significant GBS-associated function in physiological development and system category. Several hub genes, such as MMP9, PTGS2 and CREB1 were identified in the associated gene networks. Canonical pathway analysis showed that GnRH, corticotrophin-releasing hormone and ERK/MAPK signaling were the most significant pathways in the up-regulated gene set in GBS.

CONCLUSIONS

This study reveals the gene networks and canonical pathways associated with GBS. These data provide not only networks between the genes for understanding the pathogenic properties of GBS but also map significant pathways for the future development of novel therapeutic strategies.

Effect of LPS on reproductive system at the level of the pituitary of anestrous ewes

In our research we focused our attention on the effect of the immune stress induced by bacterial endotoxin-lipopolysaccharide (LPS) on the hypothalamic-pituitary-gonadal axis (HPG) at the pituitary level. We examined the effect of intravenous (i.v.) LPS injection on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release from the anterior pituitary gland (AP) in anestrous ewes. The effect of endotoxin on prolactin and cortisol circulating levels was also determined. We also researched the effect of immune challenge on the previously mentioned pituitary hormones and their receptors genes expression in the AP. Our results demonstrate that i.v. LPS injection decreased the plasma concentration of LH (23%; p < 0.05) and stimulates cortisol (245%; p < 0.05) and prolactin (60%; p < 0.05) release but has no significant effect on the FSH release assayed during 6 h after LPS treatment in comparison with the control levels. The LPS administration affected the genes expression of gonadotropins' β-subunits, prolactin and their receptors in the AP. Endotoxin injection significantly decreased the LHβ and LH receptor (LHR) gene expression (60%, 64%; p < 0.01 respectively), increased the amount of mRNA encoding FSHβ, FSH receptor (FSHR) (124%, 0.05; 166%, p < 0.01; respectively), prolactin and prolactin receptor (PRLR) (50%, 47%, p < 0.01; respectively). The presented, results suggest that immune stress is a powerful modulator of the HPG axis at the pituitary level. The changes in LH secretion could be an effect of the processes occurring in the hypothalamus. However, the direct effect of immune mediators, prolactin, cortisol and other components of the hypothalamic pituitary-adrenal (HPA) axis on the activity of gonadotropes has to be considered as well. Those molecules could affect LH synthesis directly through a modulation at all stages of LHβ secretion as well as indirectly influencing the GnRHR expression and leading to reduced pituitary responsiveness to GnRH stimulation.

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.

Neuroimmune regulation in immunocompetence, acute illness, and healing

Adaptive immunocompetence is maintained by growth hormone (GH), prolactin (PRL), and vasopressin (VP). Innate or natural immunocompetence depends on cytokines, hormones (especially of the hypothalamus-pituitary-adrenal axis), and catecholamines. The acute phase response (APR, or acute febrile illness) is an emergency defense reaction whereby the adaptive, T cell-dependent, immune reactions are suppressed and the innate immune function is dramatically amplified. Infection and various forms of injury induce APR. Cytokines [interleukin (IL)-1beta, tumor necrosis factor-alpha, and IL-6] stimulate corticotropin-releasing hormone (CRH) and VP secretion and cause a "sympathetic outflow." Colony-stimulating factors activate leukocytes. CRH is a powerful activator of the pituitary adrenocortical axis and elevates glucocorticoid (GC) levels. Cytokines, GCs, and catecholamines play fundamental roles in the amplification of natural immune defense mechanisms. VP supports the APR at this stage. However, VP remains active and is elevated for a longer period than is CRH. VP, but not CRH, is elevated during chronic inflammatory diseases. VP controls adaptive immune function and stimulates adrenocorticotropic hormone (ACTH) and PRL secretion. PRL maintains the function of the thymus and of the T cell-dependent adaptive immune system. The ACTH-adrenal axis stimulates natural immunity and of suppressor/regulatory T cells, which suppress the adaptive immune system. VP also has a direct effect on lymphoid cells, the significance of which remains to be elucidated. It is suggested that VP regulates the process of recovery from acute illness.

The parvocellular vasopressinergic system and responsiveness of the hypothalamic pituitary adrenal axis during chronic stress

Vasopressin (VP) secreted from parvocellular neurons of the hypothalamic paraventricular nucleus (PVN) stimulates pituitary adrenocorticotropic hormone (ACTH) secretion, through interaction with receptors of the V1b subtype (V1bR) in the pituitary corticotroph, mainly by potentiating the stimulatory effects of corticotrophin-releasing hormone (CRH). Chronic stress paradigms associated with corticotroph hyperresponsiveness lead to preferential expression of hypothalamic VP over CRH and upregulation of pituitary V1bR, suggesting that VP has a primary role during adaptation of the hypothalamic pituitary adrenal (HPA) axis to long-term stimulation. However, studies using pharmacological or genetic ablation of V1bR have shown that VP is required for full ACTH responses to some stressors, but not for the sensitization of ACTH responses to a novel stress observed during chronic stress. Studies using minipump infusion of a peptide V1 antagonist in long-term adrenalectomized rats have revealed that VP mediates proliferative responses in the pituitary. Nevertheless, only a minor proportion of cells undergoing mitogenesis co-express markers for differentiated corticotrophs or precursors, suggesting that new corticotrophs are recruited from yet undifferentiated cells. The overall evidence supports a limited role of VP regulating acute ACTH responses to some acute stressors and points to cell proliferation and pituitary remodelling as alternative roles for the marked increases in parvocellular vasopressinergic activity during prolonged activation of the HPA axis.

Psychosocial stress inhibits amplitude of gonadotropin-releasing hormone pulses independent of cortisol action on the type II glucocorticoid receptor

Our laboratory has developed a paradigm of psychosocial stress (sequential layering of isolation, blindfold, and predator cues) that robustly elevates cortisol secretion and decreases LH pulse amplitude in ovariectomized ewes. This decrease in LH pulse amplitude is due, at least in part, to a reduction in pituitary responsiveness to GnRH, caused by cortisol acting via the type II glucocorticoid receptor (GR). The first experiment of the current study aimed to determine whether this layered psychosocial stress also inhibits pulsatile GnRH release into pituitary portal blood. The stress paradigm significantly reduced GnRH pulse amplitude compared with nonstressed ovariectomized ewes. The second experiment tested if this stress-induced decrease in GnRH pulse amplitude is mediated by cortisol action on the type II GR. Ovariectomized ewes were allocated to three groups: nonstress control, stress, and stress plus the type II GR antagonist RU486. The layered psychosocial stress paradigm decreased GnRH and LH pulse amplitude compared with nonstress controls. Importantly, the stress also lowered GnRH pulse amplitude to a comparable extent in ewes in which cortisol action via the type II GR was antagonized. Therefore, we conclude that psychosocial stress reduces the amplitude of GnRH pulses independent of cortisol action on the type II GR. The present findings, combined with our recent observations, suggest that the mechanisms by which psychosocial stress inhibits reproductive neuroendocrine activity at the hypothalamic and pituitary levels are fundamentally different.

Homeless street children in Nepal: use of allostatic load to assess the burden of childhood adversity

As challenges to child well-being through economic disadvantage, family disruption, and migration or displacement escalate world wide, the need for cross-culturally robust understanding of childhood adversity proportionately increases. Toward this end, developmental risk was assessed in four contrasting groups of 107 Nepali children ages 10-14 years that represent distinctive, common conditions in which contemporary children grow up. Relative cumulative burden (allostatic load) indexed by multiple dimensions of physical and psychosocial stress was ascertained among homeless street boys and three family-based groups, from poor urban squatter settlements, urban middle class, and a remote rural village. Biomarkers of stress and vulnerability to stress included growth status, salivary cortisol, antibodies to Epstein-Barr virus, acute phase inflammatory responses (alpha1-antichymotrypsin), and cardiovascular fitness and reactivity (flex heart rate and pressor response). Individual biomarkers of risk and allostatic load differed markedly among groups, were highest in villagers, and varied by components of allostatic load. Such data suggest a need for critical appraisal of homelessness and migration as a risk factor to youth, given prevailing local conditions such as rural poverty, and represents the only multidimensional study of childhood allostatic load and developmental risk in non-Western settings.

Attenuated stress response to acute lipopolysaccharide challenge and ethanol administration in vasopressin V1b receptor knockout mice

The arginine vasopressin (Avp) 1b receptor (Avpr1b) present on anterior pituitary corticotrophs is involved in the stimulation of adrenocorticotrophic hormone (ACTH) secretion, especially during times of stress. Corticotrophin-releasing hormone (CRH) is considered the major ACTH secretagogue during acute stress whereas Avp appears to be the more dominant mediator of the hypothalamic-pituitary-adrenal (HPA) axis response during chronic stress situations. To investigate the role of the Avpr1b in the HPA axis response to acute stress, we measured ACTH and corticosterone (CORT) plasma levels in Avpr1b knockout (KO) mice and wild-type controls in response to bacterial lipopolysaccharide (LPS) challenge and ethanol (EtOH) administration. Mice deficient in Avpr1b had markedly compromised plasma ACTH and CORT responses to acute (30 min) LPS, but normal ACTH and CORT response to more extended exposure (4 h) to the immune system activator. The plasma ACTH and CORT levels stimulated by intoxicating, sedative doses of EtOH (3.2 and 4 g/kg) were significantly decreased in the Avpr1b KO mice compared to wild-type littermates. Significantly higher EtOH-induced plasma ACTH and CORT secretion was measured in female than in male Avpr1b wild-type mice. There were no differences in the blood alcohol levels following acute EtOH administration in Avpr1b KO or wild-type mice of either gender. Our results clearly suggest that Avpr1b plays a significant role in the HPA axis response to acute immune stress and EtOH intoxication.

Astressin B, a corticotropin-releasing hormone receptor antagonist, accelerates the return to normal luteal function after an inflammatory-like stress challenge in the rhesus monkey

Endogenous release of CRH in stress has been associated with a dysfunctional reproductive endocrine axis. In the rhesus monkey, an inflammatory-like stress challenge in the luteal phase decreases luteal secretory function. Here, we tested the effectiveness of astressin B, a nonspecific CRH receptor antagonist, in constraining the deleterious impact of a 10-d lipopolysaccharide (LPS) challenge on the menstrual cycle. Two protocols were carried out in nine animals. In the first, the animals, after showing two normal consecutive control cycles, were injected daily for 10 days with LPS (75-125 mug/d) during the luteal phase of the cycle. The animals were followed through the two postchallenge cycles. The second protocol, carried out in the following year, was identical with protocol 1, except that the animals were treated with astressin B (0.45 mg/kg) 1 h before each daily LPS challenge during the luteal phase. Blood samples were obtained daily to document cyclic hormones levels. The LPS challenge significantly decreased luteal progesterone and LH release during the challenge cycle. Inhibition of luteal progesterone extended to the two successive postchallenge cycles. Astressin B treatment prevented luteal LH but not luteal progesterone decrease during the treatment cycle and restored normal progesterone secretion during the two posttreatment cycles. We conclude that the deleterious impact of a short-term inflammatory stress challenge on luteal function is far longer than the stress period itself. Systemic administration of astressin B accelerates the return to normal luteal function, presumably by restoring normal neuroendocrine regulation of gonadotropin secretion.

Neuronal activity and CRF receptor gene transcription in the brains of rats with colitis

We aimed to characterize neuronal and corticotropin-releasing factor (CRF) pathways at the acute phase of a model of colitis in rats. Male rats received an intracolonic injection of either vehicle (controls) or trinitrobenzenesulfonic acid (TNBS) and were killed 1, 2, 3, 4, 6, 12, or 24 h later. Coronal frozen sections of the brain were cut and mRNAs encoding the rat c-fos, CRF1 receptor, and CRF2alpha,beta receptors were assayed by in situ hybridization histochemistry. Localization of these transcripts within CRF-immunoreactive (CRF-ir) neurons of the paraventricular nucleus (PVN) of the hypothalamus was also determined. Intracolonic TNBS induced c-fos mRNA expression in brain nuclei involved in the autonomic, behavioral, and neuroendocrine response to a stimulus (PVN, amygdala, locus coeruleus, parabrachial nucleus, nucleus of the solitary tract) and in circumventricular organs (lamina terminalis, subfornical organ, area postrema). CRF pathways, particularly in the PVN, were activated in this model as represented by a robust signal of c-fos and CRF1 receptor transcripts in the PVN and numerous CRF-ir neurons expressed c-fos or CRF1 receptor transcripts in the PVN of TNBS-treated animals. No expression of CRF2 receptor transcripts was observed in the PVN, either in basal conditions or after TNBS. These neuroanatomical data argue for an involvement of CRF pathways, through CRF1 receptor, within the PVN in TNBS-induced colitis.

Does the type II glucocorticoid receptor mediate cortisol-induced suppression in pituitary responsiveness to gonadotropin-releasing hormone?

Stress-like elevations in plasma cortisol suppress LH pulse amplitude in ovariectomized ewes by inhibiting pituitary responsiveness to GnRH. Here we sought to identify the receptor mediating this effect. In a preliminary experiment GnRH and LH pulses were monitored in ovariectomized ewes treated with cortisol plus spironolactone, which antagonizes the type I mineralocorticoid receptor (MR), or with cortisol plus RU486, which antagonizes both the type II glucocorticoid receptor (GR) and the progesterone receptor (PR). Cortisol alone reduced LH pulse amplitude, but not pulsatile GnRH secretion, indicating that it reduced pituitary responsiveness to endogenous GnRH. RU486, but not spironolactone, reversed this suppression. We next tested whether RU486 reverses the inhibitory effect of cortisol on pituitary responsiveness to exogenous GnRH pulses of fixed amplitude, frequency, and duration. Hourly GnRH pulses were delivered to ovariectomized ewes in which endogenous GnRH pulses were blocked by estradiol during seasonal anestrus. Cortisol alone reduced the amplitude of LH pulses driven by the exogenous GnRH pulses. RU486, but not an antagonist of PR (Organon 31710), prevented this suppression. Thus, the efficacy of RU486 in blocking the suppressive effect of cortisol is attributed to antagonism of GR, not PR. Together, these observations imply that the type II GR mediates cortisol-induced suppression of pituitary responsiveness to GnRH.

Hypothalamic interleukin-1 beta and tumor necrosis factor-alpha, but not interleukin-6, mediate the endotoxin-induced suppression of the reproductive axis in rats

It is well established that endotoxemia disrupts reproductive capability, and several proinflammatory cytokines, especially IL-1 beta, IL-6, and TNF-alpha in the brain, have been implicated in this endocrine aberration. However, no previous study has directly compared the effects of the three major proinflammatory cytokines (IL-1 beta, IL-6, and TNF-alpha) on the in vivo release of hypothalamic GnRH, a secretagogue of LH from the pituitary. Therefore, in this study, we addressed this issue with two complementary approaches involving push-pull perfusion in freely moving ovariectomized female rats. First, we examined the effects of systemic lipopolysaccharide (LPS) treatment on the release of plasma LH, and of GnRH, IL-1 beta, IL-6, and TNF-alpha in the hypothalamic medial preoptic area (MPOA), where the majority of GnRH neuronal perikarya are located. LPS inhibited the secretion of both LH and GnRH and concomitantly stimulated the release of all three cytokines. We next tested the effects of direct MPOA perfusion with the respective cytokines (at three different concentrations each) on the GnRH and LH secretion. IL-1 beta and TNF-alpha, at the concentrations that were observed in the MPOA after the LPS injection, were equipotent in inhibiting the GnRH-LH system, whereas IL-6 was ineffective (even at a supraphysiological concentration). These results strongly suggest that IL-1 beta and TNF-alpha may represent the major proinflammatory cytokines mediating the LPS-induced suppression of GnRH and LH release, whereas the role of IL-6 seems to be insignificant.

Does cortisol mediate endotoxin-induced inhibition of pulsatile luteinizing hormone and gonadotropin-releasing hormone secretion?

Bacterial endotoxin (lipopolysaccharide), a commonly used model of immune/inflammatory stress, inhibits reproductive neuroendocrine activity and concurrently induces a profound stimulation of the hypothalamo-pituitary-adrenal axis. We employed two approaches to test the hypothesis that enhanced secretion of cortisol mediates endotoxin-induced suppression of pulsatile GnRH and LH secretion in the ovariectomized ewe. First, we mimicked the endotoxin-induced increase in circulating cortisol by delivering the glucocorticoid in the absence of the endotoxin challenge. Within 1-2 h, experimentally produced increments in circulating cortisol suppressed pulsatile LH secretion in a dose-dependent fashion. Second, we blocked the endotoxin-induced stimulation of cortisol secretion using the drug metyrapone, which inhibits the 11-beta hydroxylase enzyme necessary for cortisol biosynthesis. In the absence of a marked stimulation of cortisol secretion, endotoxin still profoundly inhibited pulsatile GnRH and LH secretion. We conclude that, although enhanced cortisol secretion may contribute to endotoxin-induced suppression of reproductive neuroendocrine activity, the marked stimulation of the glucocorticoid is not necessary for this response. Our findings are consistent with the hypothesis that immune/inflammatory stress inhibits reproductive neuroendocrine activity via more than one inhibitory pathway, one involving enhanced secretion of cortisol and the other(s) being independent of this glucocorticoid.

Endocrine, metabolic and clinical effects of intravenous endotoxin injection after pre-treatment with meloxicam in heifers

Meloxicam (M), a non-steroid anti-inflammatory drug for use in animals, reduces prostaglandin (PG) synthesis by inhibiting cyclooxygenases-1 and -2. The aim of this study was to evaluate M's capability to prevent the inflammatory response elicited by endotoxin (ET). Furthermore, we wanted to evaluate a possible effect of M on delta13-reductase and 15-hydroxy prostanoate dehydrogenase, enzymes responsible for the initial metabolism of PGF2alpha. Four heifers acting as their own controls were used in the study. The heifers received an i.v. injection of either saline (S) or M (0.5 mg/kg) at 1.5 h before an i.v. injection of ET (50 ng/kg b.w. i.v.). The trial lasted 57 h after ET injection and blood samples were withdrawn for analyses of 15-ketodihydro-PGF2alpha (PG metabolite), cortisol, white blood cells (WBC), Fe, Zn and Ca. Clinical examinations were performed throughout the trial. In the S + ET trial, ET injection elicited a rapid increase of the PG metabolite, a prolonged cortisol release and reduced levels of WBC, Fe, Zn and Ca. General appearance and body temperature were affected. In the M + ET trial the PG release was totally abolished, the cortisol release was reduced and the clinical effect was milder, also effects on Fe, Zn and Ca were milder in the M + ET trial, but M did not prevent the pyrogenic effect of ET. In the next two trials, we injected PGF2alpha (500 ng/kg i.v.) with and without M pre-treatment. After PGF2alpha injection, plasma samples were collected for measurement of the PG metabolite. M had no effect on PGF2alpha metabolism. In conclusion, M effectively suppresses several of the inflammatory reactions seen after ET injections and has no major influence on the PGF2alpha metabolism.

The effect of endotoxin administration on the secretory dynamics of oxytocin in follicular phase mares: relationship to stress axis hormones

The primary aim of this study was to define the secretory dynamics of oxytocin and vasopressin in pituitary venous effluent from ambulatory horses during acute endotoxaemia, a stimulus that may release both hormones. Our secondary aim was to investigate the role of oxytocin in regulating adrenocorticotropic hormone (ACTH) secretion by comparing oxytocin, vasopressin, corticotropin-releasing hormone (CRH) and ACTH secretory profiles during endotoxaemia and by monitoring the ACTH response to oxytocin administration. Pituitary venous blood was collected nonsurgically continuously and divided into 1-min segments from eight follicular phase mares. Four mares were sampled for 30 min before and 3.5 h after receiving an i.v. infusion of bacterial endotoxin (TOX). Four control mares were sampled for 2.5 h without infusion of TOX. Another three follicular phase mares were given 5 U of oxytocin to replicate the peak response to TOX and pituitary blood collected every 1 min for 10 min before and 15 min after injection. Endotoxin raised the secretion rates of all hormones measured. All hormones were released episodically throughout the experiment, with TOX increasing the amplitude of peaks in each hormone. Peaks in oxytocin and vasopressin were coincident in each treated mare. Similarly, ACTH peaks were coincident with peaks of oxytocin and vasopressin in each treated mare, and with peaks of CRH in three mares. However, oxytocin administration did not affect ACTH secretion. We conclude that during endotoxaemia in horses: (i) oxytocin and vasopressin are secreted synchronously; (ii) oxytocin is unlikely to be acting as an ACTH secretagogue since inducing peak oxytocin concentrations observed during TOX does not raise ACTH; and therefore (iii) the close relationship between oxytocin and ACTH secretion is circumstantial and due to the fact that oxytocin secretion is concurrent with that of vasopressin, a proven ACTH secretagogue in horses.

Acute and chronic regulation of pituitary receptors for vasopressin and corticotropin releasing hormone

At least two hypothalamic peptides, corticotropin releasing hormone (CRH) and vasopressin (VP), are important in regulating adrenocorticotropin (ACTH) release from the anterior pituitary. Both are secreted in a pulsatile manner and stimulate ACTH secretion by interacting with G protein-coupled receptors (GPCRs), namely the type 1 CRH receptor and V1b receptor, respectively. Repeated or prolonged stimulation with either peptide can cause reduced ACTH responsiveness or desensitisation, both in vivo and in vitro. Desensitisation of perifused sheep anterior pituitary cells to VP was found to be rapid and occurred following treatment with 5 nM VP for 5 min. This is within the range of concentrations and durations of VP pulses seen in sheep portal blood during acute stress. In contrast, significant desensitisation of the ACTH response to CRH required pre-treatment for longer than 25 min with a CRH concentration of 1 nM, suggesting that endogenous pulses may not elicit desensitisation. Although rapid GPCR desensitisation involves uncoupling of receptors from their G proteins, commonly mediated by receptor phosphorylation, and internalisation of receptors, desensitisation of neither the CRH nor VP receptor was mediated by PKA or PKC, respectively. Desensitisation of the response to VP was found to be dependent upon receptor internalisation, and resensitisation could be delayed by treatment with a protein phosphatase 2B inhibitor. The rapid kinetics of desensitisation of the ACTH response to VP suggest that this process is important in regulating the response to acute rather than chronic stress. If, as has been suggested, CRH acts in a permissive way to set corticotrope gain, desensitisation to CRH could also be important in long term regulation of ACTH secretion.

Molecular integration of hypothalamo-pituitary-adrenal axis-related neurohormones on the GnRH neuron

Gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus is pivotal to the regulation of reproductive physiology in vertebrates. GnRH and the reproductive axis, in general, can be inhibited during periods of stress or injury. Stress, in the form of mechanical, psychological or immunological insult to an organism results in the activation of the hypothalamo-pituitary-adrenal (HPA) axis initiated by the hypothalamic release of corticotropin-releasing factor (CRF). Recent studies indicate that CRF may act either directly on the GnRH neuron to down-regulate GnRH synthesis, or indirectly via a β-endorphin-mediated pathway. Moreover, in vitro studies suggest that CRF-related peptides can increase the sensitivity of the GnRH neuron to prolactin by increasing the synthesis of the prolactin receptor.

Prostaglandins mediate the endotoxin-induced suppression of pulsatile gonadotropin-releasing hormone and luteinizing hormone secretion in the ewe

Five experiments were conducted to test the hypothesis that PGs mediate the endotoxin-induced inhibition of pulsatile GnRH and LH secretion in the ewe. Our approach was to test whether the PG synthesis inhibitor, flurbiprofen, could reverse the inhibitory effects of endotoxin on pulsatile LH and GnRH secretion in ovariectomized ewes. Exp 1-4 were cross-over experiments in which ewes received either flurbiprofen or vehicle 2 weeks apart. Jugular blood samples were taken for LH analysis throughout a 9-h experimental period. Depending on the specific purpose of the experiment, flurbiprofen or vehicle was administered after 3.5 h, followed by endotoxin, vehicle, or ovarian steroids (estradiol plus progesterone) at 4 h. In Exp 1, flurbiprofen reversed the endotoxin-induced suppression of mean serum LH concentrations and the elevation of body temperature. In Exp 2, flurbiprofen prevented the endotoxin-induced inhibition of pulsatile LH secretion and stimulation of fever, reduced the stimulation of plasma cortisol and progesterone, but did not affect the rise in circulating tumor necrosis factor-alpha. In Exp 3, flurbiprofen in the absence of endotoxin had no effect on pulsatile LH secretion. In Exp 4, flurbiprofen failed to prevent suppression of pulsatile LH secretion induced by luteal phase levels of the ovarian steroids progesterone and estradiol, which produce a nonimmune suppression of gonadotropin secretion. In Exp 5, flurbiprofen prevented the endotoxin-induced inhibition of pulsatile GnRH release into pituitary portal blood. Our finding that this PG synthesis inhibitor reverses the inhibitory effect of endotoxin leads to the conclusion that PGs mediate the suppressive effects of this immune/inflammatory challenge on pulsatile GnRH and LH secretion.

Genetic targeting of green fluorescent protein to gonadotropin-releasing hormone neurons: characterization of whole-cell electrophysiological properties and morphology

GnRH neurons form the final common pathway for central control of reproduction, with regulation achieved by changing the pattern of GnRH pulses. To help elucidate the neurobiological mechanisms underlying pulsatile GnRH release, we generated transgenic mice in which the green fluorescent protein (GFP) reporter was genetically targeted to GnRH neurons. The expression of GFP allowed identification of 84-94% of immunofluorescently-detected GnRH neurons. Conversely, over 99.5% of GFP-expressing neurons contained immunologically detectable GnRH peptide. In hypothalamic slices, GnRH neurons could be visualized with fluorescence, allowing for identification of individual GnRH neurons for patch-clamp recording and subsequent morphological analysis. Whole-cell current-clamp recordings revealed that all GnRH neurons studied (n = 23) fire spontaneous action potentials. Both spontaneous firing (n = 9) and action potentials induced by injection of depolarizing current (n = 17) were eliminated by tetrodotoxin, indicating that voltage-dependent sodium channels are involved in generating action potentials in these cells. Direct intracellular morphological assessment of GnRH dendritic morphology revealed GnRH neurons have slightly more extensive dendrites than previously reported. GnRH-GFP transgenic mice represent a new model for the study of GnRH neuron structure and function, and their use should greatly increase our understanding of this important neuroendocrine system.