Intrahypothalamic action of corticotrophin-releasing factor (CRF) to inhibit growth hormone and LH release in the rat

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

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

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

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Sexual Motivation in the Female and Its Opposition by Stress

A well worked-out motivational system in laboratory animals produces estrogen-dependent female sex behavior. Here, we review (a) the logical definition of sexual motivation and (b) the basic neuronal and molecular mechanisms that allow the behavior to occur. Importantly, reproductive mechanisms in the female can be inhibited by stress. This is interesting because, in terms of the specificity of neuroendocrine dynamics in space and time, the two families of phenomena, sex and stress, are the opposite of each other. We cover papers that document stress effects on the underlying processes of reproductive endocrinology in the female. Not all of the mechanisms for such inhibition have been clearly laid out. Finally, as a current topic of investigation, this system offers several avenues for new investigation which we briefly characterize.

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.

Stress and reproduction: controversies and challenges

Inhibition of reproductive function by the activation of the stress-response has been observed since times of antiquity, however delineating a molecular mechanism by which this occurs in vertebrates continues to present a major challenge. Because recent genome sequencing programs have identified the presence of numerous paralogous peptides and receptors, our understanding of the complexity of the interaction between the reproductive and stress axes has expanded. At the neuroendocrine level, numerous studies have focused on the interaction between the corticotropin-releasing factor (CRF) and gonadotropin-releasing hormone (GnRH) systems in vertebrates. Moreover, most of these studies have been performed using rodent models and may not be completely relevant for non-mammalian vertebrates. A further problem lies in the variation of the functional expression of paralogous genes in the different taxa. In particular, the urocortin 2 and GnRH-II systems have been lost in some lineages, where its function has been taken over by urocortin 3 and GnRH-I, respectively. Establishing an integrated model that incorporates all paralogous systems for both the stress and reproductive system remains to be achieved.

Influence of Stress and Nutrition on Cattle Immunity

Today, the scientific community readily embraces the fact that stress and nutrition impact every physiologic process in the body. At last, the specific mechanisms by which stress and nutrition affect the immune function are being elucidated. The debate among animal scientists concerning the definition and quantification of stress as it relates to animal productivity and well-being is ongoing. However, an increased appreciation and understanding of the effects of stress on livestock production has emerged throughout the scientific community and with livestock producers. The intent of this article is to provide an overview of the general concepts of stress and immunology, and to review the effects of stress and nutrition on the immune system of cattle.

Severity of the catabolic condition differentially modulates hypothalamic expression of growth hormone-releasing hormone in the fasted mouse: potential role of neuropeptide Y and corticotropin-releasing hormone

To determine whether the severity of the catabolic condition differentially regulates the GH axis, male mice were either fed ad libitum or fasted for 12, 24, and 48 h. Hypothalami, pituitaries, and stomachs were collected for assessment of mRNA levels by quantitative real-time RT-PCR, and blood collected for measurement of plasma hormone and metabolite levels by commercial assay kits. Overnight (12 h) fasting resulted in a significant suppression of circulating glucose, insulin, IGF-I, and leptin levels and an increase in corticosterone, free fatty acids, and n-octanoyl ghrelin levels, and these directional changes were maintained at the 24- and 48-h time points. Fasting (24 h) also increased circulating GH levels, which was associated with an increase in pituitary mRNA levels for GHRH receptor and ghrelin receptor and a decrease in mRNA levels for somatostatin (SST) receptor (SSTR) subtypes, SSTR2, SSTR3, and SSTR5, where the changes in ghrelin receptor and SSTR expression persisted after 48 h fasting. Hypothalamic SST mRNA levels were not altered by fasting, whereas there was a transient rise in stomach SST mRNA levels 24 h after food withdrawal. In contrast, there was a biphasic effect of fasting on GHRH expression. GHRH mRNA levels were significantly elevated at 12 and 24 h but fell to approximately 50% of fed controls 48 h after food withdrawal. A sequential rise in hypothalamic neuropeptide Y (NPY) and CRH mRNA levels preceded the fall in GHRH expression, where fasting-induced changes in CRH and GHRH mRNA levels were not observed in 48-h-fasted NPY knockout mice. These observations, in light of previous reports showing both NPY and CRH can inhibit GHRH expression and GH release, suggest that these neuronal systems may work in concert to control the ultimate impact of fasting on GH axis function.

Involvement of corticotropin-releasing factor and adrenocorticotropic hormone in the ovarian maturation, seawater acclimation, and induced spawning of Liza ramada

In the present study, we investigated the distribution and activities of corticotropin-releasing factor (CRF) and adrenocorticotropic hormone (ACTH) immunoreactive (ir) cells in the brain and pituitary of Liza ramada during ovarian maturation, seawater acclimation, and induction of spawning. Using immunohistochemistry, we detected that CRF-ir cell bodies exist in different brain regions: medulla oblongata (MO), midbrain tegmentum, habenula, nucleus preopticus (NPO), and in a ventral hypothalamic region corresponding to the nucleus lateralis tuberis (NLTP). In the pituitary gland, we detected some ACTH-producing cells in the rostral pars distalis (RPD) containing CRF immunoreactivity. The synthetic and secretory activity of CRF-ir cells in the NPO and MO as well as ACTH-ir cells in the pituitary were enhanced during ovarian maturation. During seawater acclimation, CRF-ir cells in the NPO and MO and ACTH-ir cells in the pituitary showed dramatic increases in their synthetic activity. These cells showed dramatic increase in their secretory activity during spawning induced by human chorionic gonadotropin (HCG) injection in L. ramada. Finally, hormonally induced ovulation was accompanied with elevation of plasma cortisol and depletion of CRF and ACTH immunoreactivity within the brain and the pituitary gland, respectively. Taken together, our findings suggest that mature breeders of L. ramada may respond to stress resulting from ovarian maturation, and seawater acclimation as well as induced spawning. Mechanisms include enhancement of the synthetic and/or secretory activity of CRF-ir cells in the NPO and MO as well as ACTH-ir cells in the pituitary gland along with a rise in plasma cortisol during ovulation, supporting the possible role of these hormones during stress and reproduction in L. ramada.

Three-dimensional representation of the neurotransmitter systems of the human hypothalamus: inputs of the gonadotrophin hormone-releasing hormone neuronal system

The gonadotrophin-releasing hormone (GnRH) represents the final common pathway of a neuronal network that integrates multiple external and internal factors to control fertility. Among the many inputs GnRH neurones receive, oestrogens play the most important role. In females, oestrogen, in addition to the negative feedback, also exhibits a positive feedback influence upon the activity and output of GnRH neurones to generate the preovulatory luteinising hormone surge and ovulation. Until recently, the belief has been that the GnRH neurones do not contain oestrogen receptors and that the action of oestrogen upon GnRH neurones is indirect, involving several, oestrogen-sensitive neurotransmitter and neuromodulator systems that trans-synaptically regulate the activity of the GnRH neurones. Although this concept still holds for humans, recent studies indicate that oestrogen receptor-beta is expressed in GnRH neurones of the rat. This review provides three dimensional stereoscopic images of GnRH-immunoreactive (IR) and some peptidergic (neuropeptide Y-, substance P-, beta-endorphin-, leu-enkaphalin-, corticotrophin hormone-releasing- and galanin-IR) and catecholaminergic neurones and the communication of these potential oestrogen-sensitive neuronal systems with GnRH neurones in the human hypothalamus. Because the post-mortem human tissue does not allow the electron microscopic identification of synapses on GnRH neurones, the data presented here are based on light microscopic immunocytochemical experiments using high magnification with oil immersion, semithin sections or confocal microscopy.

Inhibition by lipopolysaccharide of naloxone-induced luteinising hormone secretion is accompanied by increases in corticotropin-releasing factor immunoreactivity in hypothalamic paraventricular neurones in female rats

We have recently reported that lipopolysaccharide (LPS), a bacterial endotoxin, inhibits steroid-induced as well as naloxone-induced luteinising hormone (LH) secretion in ovariectomised oestrogen-primed rats. In the present study, we examined whether corticotropin-releasing factor (CRF) may be involved in the LPS-induced inhibition of LH secretion. Unanaesthetised rats were treated with an intravenous (i.v.) injection of LPS (10 microg) or saline, followed by an i.v. injection of naloxone (20 mg/kg). After sequential blood samples were collected for determination of serum LH concentrations, the brains were fixed and CRF-immunoreactivity was examined histochemically. In control rats receiving saline injections, only a small number of CRF-immunoreactive (ir) cells were found in the parvocellular portion of the hypothalamic paraventricular nucleus (PVN), and naloxone significantly increased serum LH concentrations within 10 min. By contrast, in LPS-treated rats, the number of CRF-ir cells was significantly greater than that in control rats, and the effect of naloxone was completely abolished. In a separate experiment, an intracerebroventricular injection of 5 microg CRF inhibited naloxone-induced LH release, mimicking the effect of LPS. These results suggest that LPS stimulates production of CRF in PVN neurones, which in turn inhibits LH secretion without opioidergic mediation.

Comparative study of the sources of neuronal projections to the site of gonadotrophin-releasing hormone perikarya and to the anteroventral periventricular nucleus in female rats

The rat ovulatory cycle is dependent on the preoptic region encompassing the gonadotrophin-releasing hormone (GnRH) perikarya and the anteroventral periventricular nucleus (AVPV). Retrograde tract tracing was used to identify and compare the sources of inputs to these sites in female rats. Within the telencephalon and diencephalon, the incidence of retrograde labelling from both sites was moderate to abundant in the ventral lateral septum, posteromedial bed nucleus of the stria terminalis, amygdalohippocampal area and the periventricular, medial preoptic, anterodorsal preoptic, dorsomedial suprachiasmatic, arcuate, and posterior ventrolateral ventromedial hypothalamic nuclei. In these regions, the incidence of retrograde labelling was either greater from the AVPV than from the GnRH perikarya site or similar from both sites. In the medial amygdaloid, parastrial, striohypothalamic, and ventral premammillary nuclei, the retrograde labelling from the AVPV greatly exceeded the sparse incidence from the GnRH perikarya site. In contrast, retrograde labelling from the GnRH perikarya site predominated in the median preoptic, lateroanterior and dorsomedial hypothalamic nuclei, subparaventricular zone, and retrochiasmatic area; it was abundant in the AVPV. Caudal to the diencephalon, retrograde labelling from either site was sparse, except in the lateral parabrachial nucleus, which displayed a particularly high incidence from the GnRH perikarya site. Other mesencephalic regions labelled from either site included the periaqueductal gray and dorsal and median raphe nuclei. The most caudal labelling was found in the ventrolateral medulla and region of the solitary tract nucleus; this was almost exclusively from the GnRH perikarya site. These findings further elucidate the neuroanatomical connections underlying the control of the ovulatory cycle.

Gonadotropin‐Releasing Hormone: Gene Evolution, Expression, and Regulation

The gonadotropin-releasing hormone (GnRH) gene is a superb example of the diverse regulation that is required to maintain the function of an evolutionarily conserved and fundamental gene. Because reproductive capacity is critical to the survival of the species, physiological homeostasis dictates optimal conditions for reproductive success, and any perturbation from this balance may affect GnRH expression. These disturbances may include alterations in signals dictated by stress, nutritional imbalance, body weight, and neurological problems; therefore, changes in other neuroendocrine systems may directly influence the hypothalamic-pituitary-gonadal axis through direct regulation of GnRH. Thus, to maintain optimal reproductive capacity, the regulation of the GnRH gene is tightly constrained by a number of diverse signaling pathways and neuromodulators. In this review, we summarize what is currently known of GnRH gene structure, the location and function of the two isoforms of the GnRH gene, some of the many hormones and neuromodulators found to affect GnRH expression, and the molecular mechanisms responsible for the regulation of the GnRH gene. We also discuss the latest models used to study the transcriptional regulation of the GnRH gene, from cell models to evolving in vivo technologies. Although we have come a long way in the last two decades toward uncovering the intricacies behind the control of the GnRH neuron, there remain vast distances to cover before direct therapeutic manipulation of the GnRH gene to control reproductive competence is possible.

Intracerebroventricular injection of corticotropin-releasing hormone receptor antagonist blocks the suppression of pulsatile luteinizing hormone secretion induced by neuromedin U in ovariectomized rats after 48hours of fasting

We recently reported that neuromedin U (NMU) and fasting synergistically suppressed the pulsatile LH secretion, even though NMU has been shown to act as a satiety factor. In the present study, we examined whether this synergistic effect on the pulsatile LH secretion was mediated via corticotropin-releasing hormone (CRH) neurons. Adult ovariectomized (OVX) rats were stereotaxically implanted with a guide cannula into the third ventricle. After 2 weeks of recovery, blood samples were taken under freely-moving conditions at 6-min intervals for 180 min from 09:00 to 12:00 h in OVX rats that had been fasted for 48 h. After first 60 min of blood sampling, astressin (2 nmol/3 microl), a CRH receptor antagonist, dissolved in artificial cerebrospinal fluid (aCSF) or aCSF (3 microl) was injected as a control into the third ventricle. Thirty minutes after the first injection, the rats were injected with NMU (1 nmol/3 microl) into the third ventricle. We found that pre-treatment with astressin completely blocked the prolongation of the interpulse interval, which should be induced by NMU. We confirmed that a single intracerebroventricular injection of astressin per se did not affect the pulsatile LH secretion. The present study suggests that synergistic inhibitory effect of NMU and fasting on the pulsatile LH secretion is at least in part mediated via CRH neurons.

The influence of 17beta-oestradiol on corticotrophin-releasing hormone induced suppression of luteinising hormone pulses and the role of CRH in hypoglycaemic stress-induced suppression of pulsatile LH secretion in the female rat

Corticotrophin-releasing hormone (CRH) released during stress has been implicated in the disruption of the reproductive neuroendocrine axis, and 17beta-oestradiol (E2) has been shown to enhance stress-induced suppression of pulsatile gonadotrophin-releasing hormone (GnRH) and luteinising hormone (LH) release. The aims of the present study were to examine the role of CRH in hypoglycaemic stress-induced suppression of LH pulses, and to investigate the influence of E2 on the inhibitory effect of CRH on pulsatile LH secretion in the female rat. Suppression of LH pulses by insulin-induced hypoglycaemic (IIH) stress was completely prevented by intracerebroventricular (icv) administration of a CRH antagonist. Central administration of CRH (5 microg) resulted in an interruption of LH pulses in E2 treated animals, but had little or no effect in the absence of this gonadal steroid. These results provide evidence of a pivotal role for CRH in mediating the suppressive effect of IIH stress on pulsatile LH secretion in the female rat, and highlight a sensitising role for E2 in CRH-induced suppression of LH pulses.

Evaluation of the effects of chronic mild stressors on hedonic and physiological responses: sex and strain compared

The chronic mild stress (CMS) paradigm was developed in order to simulate in animals the symptom of anhedonia, a major feature of depression. Typically, changes in hedonic status are interpreted from a decrease in either intake or preference for a mild sucrose solution. Although the incidence of clinical depression is significantly higher in women than in men, the study of this disorder in most animal models of depression has been based on the responses of male rodents. The purpose of this study was to compare the effects of 6 weeks of CMS administration among male and female rats of two rat strains, Sprague-Dawley (SD) and Long Evans (LE), with respect to physiological (body, adrenal gland, and spleen weight) and biochemical (plasma corticosterone levels) indices of stress as well as evaluations of 1 and 24 h sucrose intake and preference. Estrous cycle was tracked throughout the study. Overall, our results indicate a slower rate of weight gain in animals, greater in males, exposed to the chronic stressor regime. Furthermore, CMS is shown to disrupt estrous cycling, predominantly in the Long Evans strain of rats. The main behavioral finding was a significant reduction in 24 h sucrose intake in female treated groups, which was not accompanied by alterations in preference. Corticosterone levels were elevated in CMS-treated animals relative to the singly housed control groups, but exposure to a subsequent stressor was not influenced by the stress history. Taken together, the effects of chronic stressor exposure are evident, based on physiological and biochemical indices, although none of the measures distinguished any striking gender specific reactions. The usefulness of sucrose intake or preference as behavioral indices of CMS-induced anhedonia in males and females is modest at best.

Studies on the neuroanatomical basis for stress-induced oestrogen-potentiated suppression of reproductive function: evidence against direct corticotropin-releasing hormone projections to the vicinity of luteinizing hormone-releasing hormone cell bodies in female rats

Various studies implicate corticotropin-releasing hormone (CRH) as a mediator for the inhibitory effects of stress on reproduction. This study was designed to elucidate the underlying neuroanatomy. The retrograde tracer cholera toxin was picospritzed into the vicinity of the luteinizing hormone-releasing hormone (LHRH) perikarya. CRH neurones were examined for the tracer in the medial preoptic nucleus (MPO), bed nucleus of the stria terminalis (BST), paraventricular nucleus (PVN), central amygdaloid nucleus (CeM), parabrachial nucleus (PB) and additional locations. Retrograde label was not detected in CRH neurones at any of these sites; nevertheless, in the MPO and PB, abundant retrogradely-labelled perikarya intermingled with CRH neurones. In the BST, CeM and PVN, sites containing major CRH cell populations, retrogradely-labelled cells were scarce or absent; however, retrograde labelling was found in adjacent regions: lateral septum, medial amygdaloid nucleus and areas bordering the PVN. Double-label in situ hybridization for the mRNAs for LHRH and the CRH type-1 receptor (CRH-R1) identified the receptor transcript at sites rostral and lateral to the LHRH neurones (in the vertical and horizontal limbs of the diagonal band) but not in the LHRH neurones. Given the ability of oestrogen to potentiate stress-induced suppression of LH release, the identification of CRH neurones immunoreactive for oestrogen receptor (ER) alpha in the MPO and for ER beta in the caudal PVN may be significant. In this context, it is also noteworthy that CRH neurones within the MPO and PB which are, respectively, immunopositive and immunonegative for ER alpha, lie within the vicinity of retrogradely-labelled cells. The present findings suggest that the means by which CRH may mediate inhibitory effects of stressors on LH release do not involve direct CRH projections to LHRH neurones; the indirect means for such regulation, and the sites at which oestrogen may potentiate the inhibitory response, remain to be established.

Plasma corticosterone response to acute and chronic voluntary exercise in female house mice

Plasma levels of corticosterone (B) respond acutely to exercise in all mammals that have been studied, but the literature contains conflicting reports regarding how chronic activity alters this response. We measured acute and chronic effects of voluntary activity on B in a novel animal model, mice selectively bred for high voluntary wheel running. Female mice were housed with or without wheels for 8 wk beginning at 26 days of age. Wheel-access selection mice had significantly higher B at night 8, day 15, and night 29, compared with wheel-access controls. Elevation of B was an acute effect of voluntary exercise. When adjusted for running in the previous 20 min, no difference between wheel-access selection and control animals remained. No training effect on B response was observed. These results are among the strongest evidence that, in some animals, the acute B response is unaffected by chronic voluntary exercise. In mice without wheels, selection mice had significantly higher B than controls at day 15, night 29, and night 50, suggesting that selection resulted in a modulation of the hypothalamic-pituitary-adrenal axis. Growth over the first 4 wk of treatment was significantly and inversely related to average night B levels within each of the four treatment groups.

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.

Oreochromis mossambicus (tilapia) corticotropin-releasing hormone: cDNA sequence and bioactivity

Although hypothalamic corticotropin-releasing hormone (CRH) is involved in the stress response in all vertebrate groups, only a limited number of studies on this neuroendocrine peptide deals with non-mammalian neuroendocrine systems. We determined the cDNA sequence of the CRH precursor of the teleost Oreochromis mossambicus (tilapia) and studied the biological potency of the CRH peptide in a homologous teleost bioassay. Polymerase chain reaction (PCR) with degenerate and specific primers yielded fragments of tilapia CRH cDNA. Full-length CRH cDNA (988 nucleotides) was obtained by screening a tilapia hypothalamus cDNA library with the tilapia CRH PCR products. The precursor sequence (167 amino acids) contains a signal peptide, the CRH peptide and a motif conserved among all vertebrate CRH precursors. Tilapia CRH (41 aa) displays between 63% and 80% amino acid sequence identity to CRH from other vertebrates, whereas the degree of identity to members of the urotensin I/urocortin lineage is considerably lower. In a phylogenetic tree, based on alignment of all full CRH peptide precursors presently known, the three teleost CRH precursors (tilapia; sockeye salmon, Oncorhynchus nerka; white sucker, Catostomus commersoni) form a monophyletic group distinct from amphibian and mammalian precursors. Despite the differences between the primary structures of tilapia and rat CRH, maximally effective concentrations of tilapia and rat CRH were equally potent in stimulating adrenocorticotropic hormone (ACTH) and alpha-MSH release by tilapia pituitaries in vitro. The tilapia and salmon CRH sequences show that more variation exists between orthologous vertebrate CRH structures, and teleost CRHs in particular than previously recognized. Whether the structural differences reflect different mechanisms of action of this peptide in the stress response remains to be investigated.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.

Effect of corticotropin releasing factor injected into the median eminence on growth hormone secretion in male rats

We determined the dose-response relationship and examined the time-related effect of CRF (corticotropin releasing factor) injected directly into the Median Eminence (ME) on GH (growth hormone) secretion in conscious intact and castrated male rats. Doses of 0.25, 0.75, 1, and 1.5 nmol CRF dissolved in 1 microl of saline, or saline alone in the controls, were injected into the ME, and blood samples collected through indwelling catheters implanted in the jugular vein, 30, 60, 90, and 120 min post-injection to determine plasma GH levels by RIA. After 120 min the animals were decapitated. Trunk blood of decapitated animals was used to determine plasma testosterone and glucose levels. CRF at all the doses studied significantly decreased plasma GH in castrated and intact animals. The results suggest that in male as in female rats, CRF inhibits by itself GH secretion, at least in part, by a central action in the ME; all the doses of CRF studied suppressed GH secretion in castrated and intact males; finally, CRF at ME levels may participate in a variety of stress-related responses, including growth inhibition, through GH suppression.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Normal suppression of the reproductive axis following stress in corticotropin-releasing hormone-deficient mice

The hypothalamic neuropeptide CRH has been postulated to inhibit LH secretion by a central action within the brain. To characterize the physiological significance of CRH in stressor-induced inhibition of LH secretion, CRH-deficient and wild-type mice were subjected to restraint or food withdrawal, and plasma LH levels were determined. The proestrus LH surge of female mice was equally suppressed by restraint in both genotypes, and central administration of a CRH antagonist did not alleviate this suppression in either genotype. Male mice of both genotypes also demonstrated suppression of both LH and testosterone secretion following restraint. Furthermore, food withdrawal caused similar suppression of LH secretion in both female and male mice regardless of CRH status. These data demonstrate that CRH is not necessary to inhibit LH secretion following either restraint or food withdrawal and that other molecules are able to suppress LH secretion during the response to stress in the context of CRH deficiency.

An electrophysiological and morphological investigation of the projections of growth hormone-releasing peptide-6-responsive neurons in the rat arcuate nucleus to the median eminence and to the paraventricular nucleus

Growth hormone-releasing peptide-6 injection induces c-fos messenger RNA expression in many arcuate nucleus neurons, and sub-populations of neurons in this region project to the hypothalamic paraventricular nucleus. We examined electrophysiologically whether arcuate nucleus neurons that project to the paraventricular nucleus also project to the median eminence, and whether these neurons are activated by systemic injection of growth hormone-releasing peptide-6. Of 116 arcuate nucleus neurons tested, 43 were antidromically-identified as projecting to the paraventricular nucleus and a further 30 as projecting to the median eminence; these populations displayed distinct electrophysiological characteristics, and contrasting patterns of orthodromic response to stimulation of the median eminence and paraventricular nucleus, indicating that these two populations are functionally distinct with limited communication between them. Only one cell was antidromically-identified as projecting to both these regions. Three of 10 arcuate nucleus neurons that projected to the paraventricular nucleus were activated by injection of growth hormone-releasing peptide-6. In parallel experiments, we examined whether Fos protein expression is induced in arcuate nucleus neurons that project to the paraventricular nucleus, as identified by retrograde-labelling with FluoroGold. Immunocytochemical studies revealed that 20% of arcuate nucleus neurons that were retrogradely-labelled from the paraventricular nucleus were Fos-positive following growth hormone-releasing peptide-6 injection, although cells that were both Fos-positive and retrogradely-labelled accounted for less than 5% of the total number of Fos-positive arcuate nucleus neurons. These results confirm that there is a direct projection from the arcuate nucleus to the paraventricular nucleus and indicate that growth hormone-releasing peptide-6 activates some of these neurons.

Evaluation of hypothalamic-pituitary-adrenal axis in amenorrhoeic women with insulin-dependent diabetes

Diabetes is associated with a higher incidence of secondary hypogonadotrophic amenorrhoea. In amenorrhoeic women with insulin-dependent diabetes a derangement in hypothalamic-pituitary-ovary axis has been proposed. No data exist on hypothalamic-pituitary-adrenal function in these women. Gonadotrophin releasing hormone (GnRH), corticotrophin releasing hormone (CRH), metoclopramide and thyroid releasing hormone (TRH) tests were performed in 15 diabetic women, eight amenorrhoeic (AD) and seven eumenorrhoeic (ED). Frequent blood samples were taken during 24 h to evaluate cortisol plasma concentrations. There were no differences between the groups in body mass index, duration of diabetes, insulin dose and metabolic control. The AD women had lower plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin, oestradiol, androstenedione and 17-hydroxyprogesterone (17-OHP) than the ED women. The responses of pituitary gonadotrophins to GnRH, and of thyroid stimulating hormone (TSH) to TRH, were similar in both groups. The AD women had a lower prolactin response to TRH and metoclopramide, and lower ACTH and cortisol responses to CRH, than the ED women. Mean cortisol concentrations > 24 h were higher in the amenorrhoeic group. Significant differences in cortisol concentrations from 2400 to 1000 h were found between the two groups. Insulin-dependent diabetes may involve mild chronic hypercortisolism which may affect metabolic control. Stress-induced activation of the hypothalamic-pituitary-adrenal axis would increase hypothalamic secretion of CRH. This would lead directly and perhaps also indirectly by increasing dopaminergic tonus to inhibition of GnRH secretion and hence hypogonadotrophic amenorrhoea. Amenorrhoea associated with metabolically controlled insulin-dependent diabetes is a form of functional hypothalamic amenorrhoea that requires pharmacological and psychological management.

Inhibition of luteinizing hormone secretion and expression of c-fos and corticotrophin-releasing factor genes in the paraventricular nucleus during insulin-induced hypoglycaemia in sheep

Insulin can act within the brain to stimulate ovine luteinizing hormone (LH) secretion, but insulin-induced hypoglycaemia inhibits LH via unknown brain sites, possibly involving corticotrophin-releasing factor (CRF). Castrate male sheep, with (E+) or without (E-) subcutaneous oestradiol implants, were blood sampled every 12 min for 8 h. Insulin (0.25 or 0.5 IU/kg) was injected at 4 h via the carotid artery or jugular vein. All treatments reduced LH output with no differences between dose rate nor route of administration, but sensitivity was greater in E+ than E-sheep. There was no evidence for an effect of insulin on LH 0-1 h postinjection; however, 1-3 h after insulin, when hypoglycaemia was established, LH pulses were inhibited in both E+ and E- sheep (P<0.001). Additional intravenous (i.v.) glucose injections given 1 h (20 mmol) and 2 h (10 mmol) after insulin (0.5 IU/kg) were each followed by an LH pulse within 30 min (75% response in both E+ and E-sheep). In a separate experiment, sheep were killed 2 h after i.v. insulin (0.5 IU/kg) or saline. In-situ hybridization revealed c-fos mRNA in the paraventricular nucleus (PVN), but not in any other hypothalamic nuclei nor in the hindbrain; and this was linked with increased CRF gene expression in the PVN. Similar c-fos and CRF gene expression was seen in insulin-treated sheep given additional i.v. glucose (20 and 10 mmol, respectively, 40 and 20 min ante mortem), but not in saline-treated controls. Therefore, insulin-induced hypoglycaemia inhibited LH secretion, with oestradiol potentiating the effect, and was associated with gonadal steroid-independent c-fos gene expression and increased CRF gene expression in the PVN. The ovine PVN may be involved in mediating insulin-induced hypoglycaemic inhibition of LH by a mechanism which might involve CRF.

Effect of corticotropin releasing factor (CRF) injected into the median eminence on LH secretion in male rats

We determined the dose-response relationship and examined the time-related effect of CRF (corticotropin releasing factor) injected directly into the Median Eminence (ME) on LH secretion in conscious intact and castrated male rats. Doses of 0.25, 0.75, 1, and 1.5 nmol CRF dissolved in 1 microl of saline (or saline only in the controls) were injected into the ME and blood samples collected 30, 60, 90, and 120 min postinjection to determine by RIA serum LH. CRF at doses of 0.75, 1 and 1.5 nmol significantly decreased serum LH in castrated and intact animals. The lower dose of CRF did not decrease LH in the two groups studied. The results suggest that in males as in females, CRF inhibits by itself LH secretion, at least in part, by a central action in the ME; the inhibitory effect of CRF on LH is similar in castrated and intact males; the dose of 0.25 nmoles of CRF was ineffective in decreasing LH and finally that CRF at ME levels may participate in a variety of stress-related responses, including reproduction inhibition, through LH suppression.

Effect of CRF injected into the median eminence on GH secretion in female rats under different steroid status

To evaluate whether the median eminence (ME) is a site of action of CRF (corticotropin releasing factor) on GH secretion and to determine the possible role of estradiol and progesterone in modifying theses secretion, we injected CRF (0.25, 0.75, 1, and 1.5 nmol of peptide dissolved in 1 microliter of water) directly into the ME in three experimental groups of rats: Long-term ovariectomized (OVX); OVX primed by estradiol (OVX +/- E) and OVX primed by estradiol plus progesterone (OVX +/- EP). Blood was collected to determine GH (30, 60, 90, and 120 min postinjection). Serum T3, T4, and glucose levels were measured in OVX +/- E rats 30 min postinjection. CRF at all doses studied significantly decreased serum GH levels in the three experimental groups. Serum T3, T4, and glucose levels were unchanged after CRF administration. The results suggest that: CRF inhibits "per se" GH secretion, at least in part, by a central action in the ME. The inhibitory effect of CRF on GH is independent of the estrogen/progesterone status of the animal. CRF at ME levels may participate in a variety of stress-related responses, including growth inhibition, through GH suppression.

Hypercortisolism and obesity

Obesity is a multifactorial heterogenous condition. The location of excess fat on the body determines the risk of morbidity and mortality for significant disease. Visceral, or intraabdominal, fat is the fat depot most highly associated with illness and death from cardiocerebrovascular disease and diabetes. Visceral fat is also associated with a quartet of metabolic disturbances. Referred to as the metabolic syndrome, these abnormalities include hypertension, hyperlipidemia, hyperinsulinemia, and insulin resistance. The metabolic syndrome is also present in Cushing's syndrome, which is characterized by primary hypercortisolism as well as profound visceral adiposity and obesity. The interrelationship between hyperactivation or hypersensitivity of the stress axis and disease can be elucidated by an understanding of the effect of excess glucocorticoids upon energy storage and metabolism. The complex interactions of the stress axis upon the growth and reproductive axes, as well as upon the adipose tissue, suggest that chronic stress, whether psychological and/or physical, exerts an intense effect upon body composition, which, in turn, significantly affects the longevity and survival of the organism.

Effect of corticotropin-releasing factor on the pituitary-ovary axis in human luteal phase

In order to test the possible negative effect of corticotropin-releasing factor (CRF) on the pituitary-gonadal axis, two gonadotropin-releasing hormone (GnRH) tests, on two consecutive days, were performed in five normally cycling women in the midluteal phase of the menstrual cycle. The first GnRH test was performed after 2 h during which time basal blood samples were collected every 10 min. On the second day, the GnRH test was repeated after a 2-h CRF infusion which was continued throughout the GnRH test. Our results demonstrate that while CRF increases cortisol levels it affects neither the basal nor the GnRH-stimulated gonadotropin levels. Similarly, both basal and luteinizing hormone (LH)-stimulated progesterone concentrations were not modified by the CRF infusion thereby also excluding a possible direct effect of the peptide on corpus luteum steroidogenesis.

Permissive role of corticotropin-releasing factor in the acute stress-induced prolactin release in female rats

The present study examined the role of corticotropin-releasing factor (CRF) in the acute stress-induced release of prolactin (PRL) in ovariectomized estrogen-primed female rats. Acute immobilization stress induced a marked increase in serum PRL levels in animals treated with saline intraventricularly (i.c.v.). However, a prior icv injection of alpha-helical CRF(9-41), a CRF antagonist, completely eliminated the immobilization-induced PRL release in the majority of animals, providing evidence for involvement of CRF in the acute stress-induced PRL release. On the other hand, an i.c.v. injection of CRF did not affect basal PRL release at any dose in non-stressed animals, suggesting that the peptide plays a permissive role which enables other undefined stress mediator(s) to stimulate PRL release.

Regulation of adrenocorticotropin secretion in vitro by anterior pituitary corticotrophs from fallow deer (Dama dama)

The actions of corticotropin-releasing hormone (CRH), vasopressin (VP), the synthetic glucocorticoid dexamethasone (DEX), and mifepristone (RU 486), a glucocorticoid antagonist, on the secretion of adrenocorticotropin (ACTH) by cultured fallow deer corticotrophs were studied in vitro. On Day 5 of primary culture, corticotrophs were challenged for up to 4 hr with medium alone (Control), CRH, VP, DEX, forskolin (FSK), phorbol ester (TPA), cyclic AMP (cAMP), and/or RU 486 at various concentrations and combinations. CRH, VP, FSK and TPA each stimulated (P < 0.01) the secretion of ACTH in dose- and time-related manners. Relative to Control, CRH at 0.001 and 0.1 microM and VP at 0.01 and 1 microM increased (P < 0.01) medium concentration of ACTH by 7.3-, 13.5-, 3.7- and 9.0-fold, respectively. There was a treatment x incubation time interaction (P < 0.01) such that at 30-min posttreatment, CRH-induced ACTH secretion tended (P < 0.10) to be less than that obtained via VP treatment, whereas at 1, 3, and 4 hr posttreatment, medium concentration of ACTH from cells treated with 0.1 microM CRH was greater (P < 0.05) than that in cells treated with 1 microM VP. At equimolar doses of 0.01 and 0.1 microM, CRH was 3.4- and 3.0-fold more potent (treatment x dose, P < 0.05) than VP. Cotreatment with 1 microM DEX reduced (P < 0.001) the stimulatory effects of CRH (0.1 microM), VP (1 microM), FSK (10 microMs), TPA (0.1 microM), and cAMP (0.001 M). However, the coaddition of RU 486 (1 microM) to the CRH plus DEX- and the FSK plus DEX-treated wells partially negated the inhibitory effects of DEX. RU 486 completely negated the inhibitory effects of DEX on the VP-, TPA-, and cAMP-stimulated secretion of ACTH. These data indicate that CRH is a more potent stimulator of ACTH secretion than is VP in primary culture of fallow deer pituitary cells. This study also demonstrates the utility of an in vitro culture system to investigate stress-related hormonal interactions in cervids.

Corticotropin-releasing factor receptors: physiology, pharmacology, biochemistry and role in central nervous system and immune disorders

Corticotropin-releasing factor (CRF) plays a major role in coordinating the endocrine, autonomic, behavioral and immune responses to stress through actions in the brain and the periphery. CRF receptors identified in brain, pituitary and spleen have comparable kinetic and pharmacological characteristics, guanine nucleotide sensitivity and adenylate cyclase-stimulating activity. Differences were observed in the molecular mass of the CRF receptor complex between the brain (58,000 Da) and the pituitary and spleen (75,000 Da), which appeared to be due to differential glycosylation of the receptor proteins. The recently cloned CRF receptor in the pituitary and the brain (designated as CRF1) encodes a 415 amino acid protein comprising seven putative membrane-spanning domains and is structurally related to the calcitonin/vasoactive intestinal peptide/growth hormone-releasing hormone subfamily of G-protein-coupled receptors. A second member of the CRF receptor family encoding a 411 amino acid rat brain protein with approximately 70% homology to CRF1 has recently been identified (designated as CRF2); there exists an additional splice variant of the CRF2 receptor with a different N-terminal domain encoding a protein of 431 amino acids. In autoradiographic studies, CRF receptors were localized in highest densities in the anterior and intermediate lobes of the pituitary gland, olfactory bulb, cerebral cortex, amygdala, cerebellum and the macrophage-enriched zones and red pulp regions of the spleen. CRF can modulate the number of CRF receptors in a reciprocal manner. For example, stress and adrenalectomy increase hypothalamic CRF secretion which, in turn, down-regulates CRF receptors in the anterior pituitary. CRF receptors in the brain and pituitary are also altered as a consequence of the development and aging processes. In addition to a physiological role for CRF in integrating the responses of the brain, endocrine and immune systems to physiological, psychological and immunological stimuli, recent clinical data implicate CRF in the etiology and pathophysiology of various endocrine, psychiatric, neurologic and inflammatory illnesses. Hypersecretion of CRF in the brain may contribute to the symptomatology seen in neuropsychiatric disorders, such as depression, anxiety-related disorders and anorexia nervosa. Furthermore, overproduction of CRF at peripheral inflammatory sites, such as synovial joints may contribute to autoimmune diseases such as rheumatoid arthritis. In contrast, deficits in brain CRF are apparent in neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease and Huntington's disease, as they relate to dysfunction of CRF neurons in the brain areas affected in the particular disorder. Strategies directed at developing CRF-related agents may hold promise for novel therapies for the treatment of these various disorders.

Relationship of anxiety, stress, and psychosocial development to reproductive health

In this review of articles on prenatal stress, anxiety, development, and reproductive health outcomes in pregnancy, the focus is on recent research in which the relationships among the major types of stressors, anxiety, and development, on the one hand, and maternal, fetal, and neonatal problems or complications, on the other hand, were examined. Available and effective treatment measures, although limited, are introduced and discussed at length in a follow-up article. Recommendations for policy development based on current research and treatment methods and directions for future research and treatment studies are presented in a third article.

Differential Effects of Corticotropin-Releasing Hormone on Central Dopaminergic and Noradrenergic Neurons

Corticotropin-releasing hormone (CRH) has been shown to be a central mediator for most, if not all, stress-induced responses. Since stressful stimuli may decrease hypothalamic tuberoinfundibular and tuberohypophysial dopaminergic neuronal activities, we aimed to determine whether CRH is involved. Using central administration of various doses of ovine CRH (oCRH; 1, 3 and 10 &mgr;g/rat) into the lateral cerebroventricle of either male or female rats, the neurochemical changes in various parts of the central nervous system, including the hypothalamus, were determined by high-performance liquid chromatography at various times after the injection (30, 60, 120 and 240 min). The concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG), two major metabolites of dopamine and norepinephrine, respectively, in discrete brain regions were used as indices for catecholaminergic neuron activity. Plasma corticosterone levels increased significantly after all doses of oCRH and at all time points studied. oCRH also exerted significant stimulatory effects on noradrenergic neuron terminals in the frontal cortex, and on dopaminergic neuron terminals in the nucleus accumbens, hypothalamic paraventricular and periventricular nuclei, and intermediate pituitary lobe. Dopaminergic neuron terminals in the median eminence and the neural lobe of the pituitary, however, were not affected. There was no major difference in the responses between male and female rats. We conclude that CRH has a differential effect on central catecholaminergic neurons. Copyright 1995 S. Karger AG, Basel

Effect of corticotropin releasing factor (CRF) in the median eminence on gonadotropins in ovariectomized rats with or without steroid priming: dose-response study

We determined the dose-response relationship and examined the time related effect of CRF (corticotropin releasing factor) injected directly into the median eminence (ME) on LH and FSH secretion in conscious female rats of different steroid status. Doses of 0.25, 0.75, 1, and 1.5 nM CRF dissolved in 1 microliter of water were injected into the ME in 5 experimental groups of rats: Short-term (2 days) ovariectomized (sOVX); long-term (3-4 weeks) ovariectomized (lOVX); lOVX primed by estradiol benzoate (EB) 4 h before the experiment (lOVX+E); lOVX primed by EB 36 h before the experiment (lOVXE) and lOVX primed by EB 72 h and progesterone 6 h before experiment (lOVXP). Blood was collected at 30, 60, 90, and 120 min postinjection to determine LH and FSH by RIA. CRF at the doses of 0.75, 1, and 1.5 nM significantly decreased serum LH levels in all groups. The dose of 0.25 nM CRF was ineffective. The highest dose (1.5 nM) of CRF had no effect on serum FSH levels. The results suggest that CRF inhibits LH secretion, at least in part, by a central action of GnRH release in the ME, and that this effect is independent of the estrogen/progesterone status of the animal.

Adrenalectomy reverses stress-induced suppression of luteinizing hormone secretion in long-term ovariectomized rats

An inhibitory effect of stress on reproductive function is well established. This inhibition involves activation of the hypothalamic-pituitary-adrenal (HPA) axis, which leads to a suppression of LH secretion. It has been proposed that this suppression is mediated by a direct effect of CRF that is independent of glucocorticoid actions. We tested this proposition by examining plasma LH levels in adult rats that were both ovariectomized (OVX) and adrenalectomized (ADX). Each animal was surgically implanted with an indwelling atrial cannula and exposed to intermittent foot shock for 100 min. Blood samples were drawn just prior to putting the animals into the test cage and then at 20-min intervals. Results revealed normal castrate levels of plasma LH in both ADX and ADX/OVX animals prior to shock. A significant shock-induced suppression of LH was observed in OVX animals within 20 min after the onset of shock and remained throughout the duration of the session. In contrast, no evidence was obtained for a suppression of LH in OVX/ADX animals at any time point. Additional studies demonstrated a marked suppression of LH in experimentally naive OVX/ADX females implanted with corticosterone capsules for 2 weeks prior to blood sampling. Overall, these results support a primary role for glucocorticoid actions in the stress-induced inhibition of reproductive function.

Corticotropin-releasing factor and interleukin-1 receptors in the brain-endocrine-immune axis. Role in stress response and infection

CRF and IL-1 receptors were identified, characterized, and localized in brain, endocrine, and immune tissues. CRF receptors with comparable kinetic and pharmacological characteristics were localized in the anterior and intermediate lobes of the pituitary, in brain areas involved in mediating stress responses, and in the macrophage-enriched marginal zones of the spleen. The discrete localization of IL-1 receptors in neurons of the hippocampus provides further support for the role of IL-1 as a neurotransmitter/neuromodulator/growth factor in the CNS. The neuroendocrine effects of IL-1 may be mediated through actions of the cytokine in brain. However, given the high densities of IL-1 receptors in the anterior pituitary and testis, direct effects of the cytokine at the pituitary or gonadal levels seem highly likely. Overall, these data support a role for IL-1 and CRF in coordinating and integrating the brain-endocrine-immune responses to physiological, pharmacological, and pathological stimuli.

Reciprocal synaptic relations between CRF-immunoreactive- and TRH-immunoreactive neurons in the paraventricular nucleus of the rat hypothalamus

By double immunoelectron microscopy, we studied synaptic relations between corticotropin-releasing factor (CRF)-immunoreactive (ir) and thyrotropin-releasing hormone (TRH)-ir neurons in the paraventricular nucleus (PVN) of the rat hypothalamus. CRF-ir and TRH-ir neurons made reciprocal synaptic connections in the medial and periventricular parvocellular regions. These results may suggest that both the parvocellular neurons interplay on their hypophysiotropic functions within the PVN.

Centrally injected interleukin-1 beta inhibits the hypothalamic LHRH secretion and circulating LH levels via prostaglandins in rats

Intracerebroventricular (icv) infusion of interleukin-1 beta (IL-1 beta) significantly lowers plasma LH levels in castrated male rats, and interferes with LHRH release into the median eminence of proestrus female rats. We have investigated the potential role of arachidonic acid metabolites in mediating these inhibitory effects, by administering indomethacin (INDO, a cyclooxygenase inhibitor) or nor- dihydroguaiaretic acid (NDGA, a lipoxygenase inhibitor) 15 min prior to injection of the cytokine. While not measurably altering basal LH or LHRH secretion in castrated or proestrus rats, respectively, INDO completely reversed the action of IL-1 beta on the secretion of these 2 hormones. In contrast, NDGA did not alter IL-1-induced decreases in LH release. The peripheral administration of endotoxin (LPS) also interferes with LH release. Because the iv injection of IL-1 does not alter LH secretion, this effect is believed to be at least in part mediated by increased synthesis of cytokines within the CNS. We observed that in contrast to results obtained in rats injected with IL-1 icv, INDO did not reverse the inhibitory action of LPS. Our results thus suggest either that central IL-1 is not the primary modulator of LPS-induced decrease in LH values, or that pathways other than those involving arachidonic acid are important.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of intratesticular administration of anti-corticotropin-releasing factor antiserum (a-CRF) on testicular function in neonatal rats

The possible physiological role of testicular corticotropin-releasing factor (CRF) in the regulation of testicular functions was studied in neonatal rats. Two microlitres of anti-CRF-antiserum (dilution: 1:10 or 1:100) was injected intratesticularly to 5 d-old rats with two testes and to hemicastrates. Five days after hemicastration and treatment of the remaining testis with the antiserum, serum testosterone concentration and basal testosterone secretion in vitro decreased significantly. Unilateral testicular injection of a-CRF in rats with two testes resulted in a significant drop in serum testosterone level with no change in basal testosterone production. Data indicate that in neonatal rats testicular CRF might be a local stimulator of steroidogenesis.

Increase of the proportion of corticosteroidogenic activity of ACTH versus immunoreactive ACTH in rat plasma extracts during stress

The relationship between bioactive and immunoreactive adrenocorticotrophin (ACTH) was studied under various conditions. Rats were stressed by ether and the immunoreactive (I-ACTH) and bioactive (B-ACTH) ACTH levels were compared to the normal values obtained from "handled" rats. The bioactivity was determined by measuring the corticosteroid production in an isolated rat adrenal cell system. Stress exposure enhanced both the I-ACTH and B-ACTH levels. I-ACTH rose from 86.9 +/- 37.9 to 363 +/- 148.7 pg/ml, whereas the B-ACTH levels rose from 34.2 +/- 11.9 to 424.2 +/- 170.9 pg/ml. The bioactive to immunoreactive ratio (B/I ratio) was calculated as 0.47 +/- 0.13 in the control group and rose to the significantly higher value of 1.10 +/- 0.22 in the stressed rats. The conclusion is that under normal circumstances only about 50% of plasma immunoreactive ACTH shows corticosteroidogenic activity and that the B/I ACTH ratio might be a useful tool to estimate the physiological phase of the pituitary releasing activity.

Immobilization stress rapidly decreases hypothalamic corticotropin-releasing hormone secretion in vitro in the male 344/N Fischer rat

Corticotropin-Releasing-Hormone (CRH) is the principal secretagogue for plasma ACTH and corticosterone secretion and plays an important role in coordinating a variety of physiological and behavioral responses to stress. To explore whether there is a rapid change in the secretory response of the hypothalamic CRH neuron during acute stress, we report here a study of the effects of KCl and norepinephrine (NE) on CRH release in vitro from rat hypothalami explanted after 5, 30, 60, and 120 minutes of immobilization. We also measured the plasma levels of ACTH, beta-endorphin, corticosterone, prolactin, GH, and TSH at these intervals. As the duration of immobilization increased, KCl and NE-induced CRH release in vitro progressively fell. After reaching a maximal rise after 30 minutes of immobilization, plasma ACTH, beta-endorphin, and prolactin progressively fell in plasma, whereas corticosterone remained elevated up to 120 minutes; TSH and GH secretion rapidly declined and remained suppressed. Taken together, these data suggest that during immobilization stress, the responsiveness of the hypothalamic CRH neuron rapidly falls, owing either to CRH depletion and/or desensitization to NE, and this is paralleled by a concomitant decrease in pituitary-adrenal responsiveness.