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

Neuroendocrine interactions of the stress and reproductive axes

Reproduction is controlled by a sequential regulation of the hypothalamo-pituitary-gonadal (HPG) axis. The HPG axis integrates multiple inputs to maintain proper reproductive functions. It has long been demonstrated that stress alters fertility. Nonetheless, the central mechanisms of how stress interacts with the reproductive system are not fully understood. One of the major pathways that is activated during the stress response is the hypothalamo-pituitary-adrenal (HPA) axis. In this review, we discuss several aspects of the interactions between these two neuroendocrine systems to offer insights to mechanisms of how the HPA and HPG axes interact. We have also included discussions of other systems, for example GABA-producing neurons, where they are informative to the overall picture of stress effects on reproduction.

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

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

An Inhibitory Circuit From Brainstem to GnRH Neurons in Male Mice: A New Role for the RFRP Receptor

RFamide-related peptides (RFRPs, mammalian orthologs of gonadotropin-inhibitory hormone) convey circadian, seasonal, and social cues to the reproductive system. They regulate gonadotropin secretion by modulating gonadotropin-releasing hormone (GnRH) neurons via the RFRP receptor. Mice lacking this receptor are fertile but exhibit abnormal gonadotropin responses during metabolic challenges, such as acute fasting, when the normal drop in gonadotropin levels is delayed. Although it is known that these food intake signals to the reproductive circuit originate in the nucleus tractus solitarius (NTS) in the brainstem, the phenotype of the neurons conveying the signal remains unknown. Given that neuropeptide FF (NPFF), another RFamide peptide, resides in the NTS and can bind to the RFRP receptor, we hypothesized that NPFF may regulate GnRH neurons. To address this question, we used a combination of techniques: cell-attached electrophysiology on GnRH-driven green fluorescent protein-tagged neurons in acute brain slices; calcium imaging on cultured GnRH neurons; and immunostaining on adult brain tissue. We found (1) NPFF inhibits GnRH neuron excitability via the RFRP receptor and its canonical signaling pathway (Gi/o protein and G protein-coupled inwardly rectifying potassium channels), (2) NPFF-like fibers in the vicinity of GnRH neurons coexpress neuropeptide Y, (3) the majority of NPFF-like cell bodies in the NTS also coexpress neuropeptide Y, and (4) acute fasting increased NPFF-like immunoreactivity in the NTS. Together these data indicate that NPFF neurons within the NTS inhibit GnRH neurons, and thus reproduction, during fasting but prior to the energy deficit.

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

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

Acute restraint stress modifies the heart rate biorhythm in the poststress period

We studied the changes in the heart and the activity biorhythms in mice exposed to acute (one 120-minute session) and repeated (7 two-hour sessions) restraint stress in 129J1/CF1 mice (WT) and in mice without M₂ muscarinic receptors (M₂KO) during the prestress period, during stress (STR) and for five days after the last stress session (POST). There were changes in the mesor (a midline based on the distribution of values across the circadian cycles; decreased in M₂KO by 6% over all POST), day means (inactive period of diurnal rhythm in mice; higher in M₂KO and further increased on STR and on the second to the fifth POST) and night means (active period; lower by 13% in M₂KO and remained decreased in STR and in POST). The total area under the curve was decreased both in the WT and M₂KO on STR and in all POST. Repeated stress caused changes over all days of STR, but the initial values were restored in POST. The average night values were decreased, and the day means were increased by 16% over all STR in M₂KO. The day means decreased by 14% in the 4 POST in WT. The activity biorhythm parameters were almost unchanged. We show here that stress can specifically affect heart biorhythm in M₂KO mice, especially when the stress is acute. This implies the role of M₂ muscarinic receptor in stress response.

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.

Hypothalamic-Pituitary-Adrenal Axis Modulation of Glucocorticoids in the Cardiovascular System

The collective of endocrine organs acting in homeostatic regulation—known as the hypothalamic-pituitary-adrenal (HPA) axis—comprises an integration of the central nervous system as well as peripheral tissues. These organs respond to imminent or perceived threats that elicit a stress response, primarily culminating in the release of glucocorticoids into the systemic circulation by the adrenal glands. Although the secretion of glucocorticoids serves to protect and maintain homeostasis in the typical operation at baseline levels, inadequate regulation can lead to physiologic and psychologic pathologies. The cardiovascular system is especially susceptible to prolonged dysregulation of the HPA axis and glucocorticoid production. There is debate about whether cardiovascular health risks arise from the direct detrimental effects of stress axis activation or whether pathologies develop secondary to the accompanying metabolic strain of excess glucocorticoids. In this review, we will explore the emerging research that indicates stress does have direct effects on the cardiovascular system via the HPA axis activation, with emphasis on the latest research on the impact of glucocorticoids signaling in the vasculature and the heart.

GABAergic regulation of the HPA and HPG axes and the impact of stress on reproductive function

The hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes are regulated by GABAergic signaling at the level of corticotropin-releasing hormone (CRH) and gonadotropin-releasing hormone (GnRH) neurons, respectively. Under basal conditions, activity of CRH and GnRH neurons are controlled in part by both phasic and tonic GABAergic inhibition, mediated by synaptic and extrasynaptic GABAA receptors (GABAARs), respectively. For CRH neurons, this tonic GABAergic inhibition is mediated by extrasynaptic, δ subunit-containing GABAARs. Similarly, a THIP-sensitive tonic GABAergic current has been shown to regulate GnRH neurons, suggesting a role for δ subunit-containing GABAARs; however, this remains to be explicitly demonstrated. GABAARs incorporating the δ subunit confer neurosteroid sensitivity, suggesting a potential role for neurosteroid modulation in the regulation of the HPA and HPG axes. Thus, stress-derived neurosteroids may contribute to the impact of stress on reproductive function. Interestingly, excitatory actions of GABA have been demonstrated in both CRH neurons at the apex of control of the HPA axis and in GnRH neurons which mediate the HPG axis, adding to the complexity for the role of GABAergic signaling in the regulation of these systems. Here we review the effects that stress has on GnRH neurons and HPG axis function alongside evidence supporting GABAARs as a major interface between the stress and reproductive axes.

Schizothorax prenanti corticotropin-releasing hormone (CRH): molecular cloning, tissue expression, and the function of feeding regulation

Corticotropin-releasing hormone (CRH) is a potent mediator of endocrine, autonomic, behavioral, and immune responses to stress. For a better understanding of the structure and function of the CRH gene and to study its effect on feeding regulation in cyprinid fish, the cDNA of the CRH gene from the brain of Schizothorax prenanti was cloned and sequenced. The full-length CRH cDNA consisted of 1,046 bp with an open reading frame of 489 bp encoding a protein of 162 amino acids. Real-time quantitative PCR analyses revealed that CRH was widely expressed in central and peripheral tissues. In particular, high expression level of CRH was detected in brain. Furthermore, CRH mRNA expression was examined in different brain regions, especially high in hypothalamus. In addition, there was no significant change in CRH mRNA expression in fed group compared with the fasted group in the S. prenanti hypothalamus during short-term fasting. However, CRH gene expression presented significant decrease in the hypothalamus in fasted group compared with the fed group (P < 0.05) on day 7; thereafter, re-feeding could lead to a significant increase in CRH mRNA expression in fasted group on day 9. The results suggest that the CRH may play a critical role in feeding regulation in S. prenanti.

Sexual dimorphism in stress-induced changes in adrenergic and muscarinic receptor densities in the lung of wild type and corticotropin-releasing hormone-knockout mice

We tested the hypothesis that single and repeated immobilization stress affect densities of alpha(1)-adrenoceptor (alpha(1)-AR) and beta-AR subtypes, muscarinic receptors (MR), adenylyl cyclase activity (AC) and phospholipase C activity (PLC) in lungs of male and female wild type (WT) and corticotropin-releasing hormone gene (CRH-knockout (KO)) disrupted mice. We found sex differences in the basal levels of alpha(1)-AR subtypes (females had 2-3 times higher density of receptors than males) and MR (males had twice the density found in females). In marked contrast, beta-AR subtype densities did not differ between sexes. CRH gene disruption decreased all three studied receptors in intact mice (to 20-50% of WT) in both sexes (except beta(1)-AR in females). Stress induced sexually dimorphic responses, while all alpha(1)-AR subtypes decreased in females (to 30% of control approximately), only alpha(1A)-AR level diminished (about 50%) in males. beta(1)-AR decreased in males (to about 40%) but remained stable in females. beta(2)-AR diminished in females (to about 20-60%) and also in males (to about 30-60%). MR decreased in both sexes (approximately to 50%). AC activity diminished in males (to < 50%) while PLC activity was not changed. In CRH-KO mice, the stress response was severely diminished. Paradoxically, the receptor response to stress was less affected by CRH-KO in males than in females. AC activity did not change in CRH-KO mice. In conclusion, in mice the stress reaction is sexually dimorphic and an intact hypothalamo-pituitary-adrenocortical system is required for the normal reaction of pulmonary adrenergic and MR to stress.

Enduring influences of peripubertal/adolescent stressors on behavioral response to estradiol and progesterone in adult female mice

Exposure to stressors during particular stages of development leads to acute and long-term physiological and behavioral changes. We have reported that shipping mice during the peripubertal/adolescent period results in decreased induction of feminine sexual behavior by estradiol and progesterone in adult female mice. To study further the factors involved in this decreased behavioral response, female mice were exposed to a variety of experimental stressors when 6 wk old. Effects of peripubertal/adolescent exposure to these stressors on acute plasma corticosterone levels and changes in body weight and adult behavioral response to estradiol and progesterone were assessed. Although restraint for three daily 3-h periods, 36-h food deprivation, or a multiple stressor regimen acutely increased plasma corticosterone levels and reduced body weight, only exposure to particular doses of the bacterial endotoxin lipopolysaccharide (LPS; 1-1.5 mg/kg body weight, doses that induced moderate levels of sickness behavior in these studies) resulted in reduced behavioral response to estradiol and progesterone in adulthood. Like the effects of shipping, the effects of LPS on adult feminine sexual behavior appear most robust when injected at 6 wk old and are limited to exposure during a vulnerable period at approximately 4-6 wk of age. Therefore, an immune stressor during the peripubertal/adolescent period, but not restraint, food restriction, or a combined stressor, has an enduring influence on behavioral response to estradiol and progesterone. This demonstrates that the decreased response to estradiol and progesterone is not a general response to all stressors during this developmental stage.

Expression of a gonadotropin-releasing hormone receptor-simian virus 40 T-antigen transgene has sex-specific effects on the reproductive axis

The GnRH receptor (GnRHR) responds to pulsatile GnRH signals to coordinate pituitary gonadotropin synthesis and secretion. Previously, a 1.2-kb fragment of the 5'-flanking region isolated from the mouse GnRHR gene was shown to target expression to pituitary gonadotropes in vivo. The 1.2-kb gene promoter fused to the simian virus 40 large T antigen (TAg) was used to generate transgenic mice that form gonadotrope-derived pituitary tumors at 4-5 months of age. Transgenic female mice have hypogonadotropic hypogonadism, infantile gonads, and are infertile throughout their life span, whereas males remain reproductively intact until their tumors become large. We hypothesized that the targeted TAg expression causes a sex-specific disruption of the reproductive axis at the level of the pituitary gland. To test this hypothesis, we characterized the pituitary gonadotropin beta-subunit and TAg expression patterns, and measured plasma gonadotropin and gonadal steroid levels in female and male mice before and after pituitary tumor development. TAg expression was observed in transgenic females and males 15 d of age, before tumor development. Interestingly, and in contrast to the transgenic males, pituitary LH beta and FSH beta subunit protein levels, and plasma LH and FSH levels, were reduced in transgenic females. Reproductive organs in transgenic female mice remained underdeveloped but were normal in transgenic males. We conclude that the expression of the TAg transgene driven by the GnRHR gene promoter results in female-specific infertility due to disruption of gonadotropin production and secretion even before tumor development.

Neurobiology of stress-induced reproductive dysfunction in female macaques

It is now well accepted that stress can precipitate mental and physical illness. However, it is becoming clear that given the same stress, some individuals are very vulnerable and will succumb to illness while others are more resilient and cope effectively, rather than becoming ill. This difference between individuals is called stress sensitivity. Stress sensitivity of an individual appears to be influenced by genetically inherited factors, early life (even prenatal) stress, and by the presence or absence of factors that provide protection from stress. In comparison to other stress-related diseases, the concept of sensitivity versus resilience to stress-induced reproductive dysfunction has received relatively little attention. The studies presented herein were undertaken to begin to identify stable characteristics and the neural underpinnings of individuals with sensitivity to stress-induced reproductive dysfunction. Female cynomolgus macaques with normal menstrual cycles either stop ovulating (stress sensitive) or to continue to ovulate (stress resilient) upon exposure to a combined metabolic and psychosocial stress. However, even in the absence of stress, the stress-sensitive animals have lower secretion of the ovarian steroids, estrogen and progesterone, have higher heart rates, have lower serotonin function, have fewer serotonin neurons and lower expression of pivotal serotonin-related genes, have lower expression of 5HT2A and 2C genes in the hypothalamus, have higher gene expression of GAD67 and CRH in the hypothalamus, and have reduced gonadotropin-releasing hormone transport to the anterior pituitary. Altogether, the results suggest that the neurobiology of reproductive circuits in stress-sensitive individuals is compromised. We speculate that with the application of stress, the dysfunction of these neural systems becomes exacerbated and reproductive function ceases.

Hyperglycaemic hyperosmolar nonketotic state as a cause of low gonadotrophin levels in postmenopausal diabetic women: a role for severe hypernatraemia

Hypogonadotrophic hypogonadism is associated with uncontrolled diabetes mellitus. Hyperglycaemia is a unique metabolic abnormality of the hyperglycaemic hyperosmolar nonketotic state (HHNKS) and, as glucose availability regulates gonadotrophin release, we investigated whether gonadotrophin release is inhibited in diabetic women with HHNKS, and whether hyperglycaemia, hypernatraemia or both inhibit in vitro gonadotrophin-releasing hormone (GnRH) expression in GT1-7 neurones. Three groups of postmenopausal women were studied: nine diabetics with HHNKS, nine hospitalised ill nondiabetics and 15 healthy women. In addition, the effects of glucose (5.55, 33.3, 66.6 mmol/l) and sodium chloride (150 and 170 mmol/l) on GnRH expression were investigated using GT1-7 neurones. Postmenopausal diabetics with HHNKS showed a decrease in serum levels of luteinising hormone (diabetic HHNKS 2.2 +/- 0.9 IU/l versus ill nondiabetic 21.0 +/- 2.3 IU/l and healthy controls 20.9 +/- 2.8 IU/l, P < 0.01), follicle-stimulating hormone (diabetic HHNKS 8.2 +/- 2.1 IU/l versus ill nondiabetic 50.4 +/- 9.1 IU/l and controls 60.2 +/- 6.9 IU/l, P < 0.01) and free 3,5,3'-triiodothyronine (diabetic HHNKS 1.48 +/- 0.57 pmol/l versus ill nondiabetic 4.28 +/- 0.26 pmol/l and controls 3.88 +/- 0.11 pmol/l, P < 0.01). The plasma cortisol level was higher in both diabetic (985 +/- 130 nmol/l) and ill nondiabetic (726 +/- 52 nmol/l) women than in healthy women (512 +/- 47 nmol/l), but no differences were observed in plasma oestradiol, thyroid-stimulating hormone or free thyroxine. In vitro GT1-7 neurones expressed three-fold less GnRH at 170 mmol/l than at 150 mmol/l NaCl, whereas changing glucose concentrations in the culture medium did not affect GnRH expression. In conclusion, postmenopausal diabetic women with HHNKS show decreased serum gonadotrophin levels, and severe hypernatraemia may participate in the hypogonadotropism observed in HHNKS.

Improvement in reproductive parameters in hypogonadal female mice by regulated gene replacement therapy in the central nervous system

One of the challenges of gene targeting is to achieve regulated transgene expression in specific target cells. The hypogonadal (hpg) mice are genetically deficient in hypothalamic gonadotropin-releasing hormone (GnRH) production due to a deletion in the GnRH gene, resulting in hypogonadotropic hypogonadism. Here we show an improvement in reproductive parameters of adult female homozygous hpg mice by direct infusion into the hypothalamic preoptic area (POA) of a herpes simplex virus (HSV)-based amplicon vector containing a 13.5 kb genomic fragment encoding the GnRH gene together with its cognate promoter and regulatory elements. Following vector injection, GnRH-expressing neurons were detected in the POA, and pituitary and plasma gonadotropin levels as well as ovarian and uterine weights increased. In addition, a subset of injected hpg mice demonstrated cyclic estrous changes, consistent with regulated control of GnRH production. Administration of kisspeptin-10 resulted in an increase in plasma luteinizing hormone levels, further supporting appropriate regulation of the introduced GnRH transgene. These findings indicate that delivery of the GnRH gene resulted in selective neuronal expression of GnRH and regulated hypothalamic GnRH release. To our knowledge, this is the first example of the correct targeting of a gene under its cognate promoter to neurons resulting in selective and regulated synthesis of a biologically active peptide, and thus may have a wide range of applications in the treatment of human disorders.

Functions of corticotropin-releasing hormone in anthropoid primates: From brain to placenta

Corticotropin-releasing hormone (CRH) is an ancient regulatory molecule. The CRH hormone family has at least four ligands, two receptors, and a binding protein. Its well-known role in the hypothalamic-pituitary-adrenal (HPA) axis is only one of many. The expression of CRH and its related peptides is widespread in peripheral tissue, with important functions in the immune system, energy metabolism, and female reproduction. For example, CRH is involved in the implantation of fertilized ova and in maternal tolerance to the fetus. An apparently unique adaptation has evolved in anthropoid primates: placental expression of CRH. Placental CRH stimulates the fetal adrenal zone, an adrenal structure unique to primates, to produce dehydroepiandrosterone sulfate (DHEAS), which is converted to estrogen by the placenta. Cortisol induced from the fetal and maternal adrenal glands by placental CRH induces further placental CRH expression, forming a positive feedback system that results in increasing placental production of estrogen. In humans, abnormally high placental expression of CRH is associated with pregnancy complications (e.g., preterm labor, intrauterine growth restriction (IUGR), and preeclampsia). Within anthropoid primates, there are at least two patterns of placental CRH expression over gestation: monkeys differ from great apes (and humans) by having a midgestational peak in CRH expression. The functional significance of these differences between monkeys and apes is not yet understood, but it supports the hypothesis that placental CRH performs multiple roles during gestation. A clearer understanding of the diversity of patterns of placental CRH expression among anthropoid primates would aid our understanding of its role in human pregnancy.

Expression of type 1 corticotropin-releasing hormone (CRH) receptor mRNA in the hypothalamic paraventricular nucleus following restraint stress in CRH-deficient mice

Previous studies have demonstrated that various types of stress increase type 1 corticotropin-releasing hormone (CRH) receptor (currently abbreviated to CRF1 receptor) mRNA in the hypothalamic paraventricular nucleus (PVN) of rats, but not mice. This study investigated whether different sensitivities of glucocorticoid-mediated negative feedback effects can explain this species difference in stress-induced PVN CRF1 receptor mRNA expression. First, the CRF1 receptor mRNA level in the PVN of CRH knockout (KO) mice during acute restraint stress was compared with that in wild-type (WT) mice. Consistent with previous findings, WT mice showed no induction of CRF1 receptor mRNA in the PVN following acute restraint, regardless of normal hypothalamic-pituitary-adrenocortical responses. In contrast, CRF1 receptor mRNA in the PVN of CRH KO mice was increased following 2 h of restraint. Since the response of tyrosine hydroxylase (TH) mRNA in the locus coeruleus (LC) to restraint was similar between CRH KO and WT mice, it is unlikely that enhanced noradrenergic input into the PVN was responsible for the CRF1 receptor mRNA induction in CRH KO mice. Second, to determine whether CRH KO per se or a low corticosterone response to stress is required to induce CRF1 receptor mRNA expression in the PVN in mice, the response of adrenalectomized WT mice was examined. Acute restraint increased the CRF1 receptor mRNA level in the PVN of adrenalectomized WT mice, similar to the case for CRH KO mice. TH mRNA in the LC showed similar increases in sham and adrenalectomized WT mice. These results indicate that PVN CRF1 receptor mRNA is much more sensitive to glucocorticoid-mediated negative feedback in mice than in rats, such that a normal increase in plasma corticosterone during stress can mask CRF1 receptor mRNA induction in the PVN of mice.

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.

Impaired leptin expression and abnormal response to fasting in corticotropin-releasing hormone-deficient mice

Leptin has been postulated to comprise part of an adipostat, whereby during states of excessive energy storage, elevated levels of the hormone prevent further weight gain by inhibiting appetite. A physiological role for leptin in this regard remains unclear because the presence of excessive food, and therefore the need to restrain overeating under natural conditions, is doubtful. We have previously shown that CRH-deficient (Crh(-/-)) mice have glucocorticoid insufficiency and lack the fasting-induced increase in glucocorticoid, a hormone important in stimulating leptin synthesis and secretion. We hypothesized that these mice might have low circulating leptin. Indeed, Crh(-/-) mice exhibited no diurnal variation of leptin, whereas normal littermates showed a clear rhythm, and their leptin levels were lower than their counterparts. A continuous peripheral CRH infusion to Crh(-/-) mice not only restored corticosterone levels, but it also increased leptin expression to normal. Surprisingly, 36 h of fasting elevated leptin levels in Crh(-/-) mice, rather than falling as in normal mice. This abnormal leptin change during fasting in Crh(-/-) mice was corrected by corticosterone replacement. Furthermore, Crh(-/-) mice lost less body weight during 24 h of fasting and ate less food during refeeding than normal littermates. Taken together, we conclude that glucocorticoid insufficiency in Crh(-/-) mice results in impaired leptin production as well as an abnormal increase in leptin during fasting, and propose that the fast-induced physiological reduction in leptin may play an important role to stimulate food intake during the recovery from fasting.

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.

Lessons from CRH knockout mice

Corticotropin-releasing hormone (CRH), the major regulator of hypothalamic-pituitary-adrenal (HPA) axis, has a wide spectrum of actions within the central nervous system and the periphery. The development and use of Crh knockout mice (Crh-/-) has been an important tool for addressing the physiologic and pathologic roles of CRH. This review describes the generation and characterization ofCrh -deficient mice as well as the use of these mice to study the role of CRH in maternal and fetal HPA axes development and in the regulation of the adult HPA axis and behavior. The review concludes with information about recently discovered CRH-related peptides and their possible roles in some of the functions thought initially to be mediated by CRH.

Glucose relays information regarding nutritional status to the neural circuits that control the somatotropic, corticotropic, and gonadotropic axes in adult male rhesus macaques

In male mammals, the neuroendocrine responses to fasting include increased GH and cortisol secretion and suppressed LH and T levels. Because blood glucose levels fall during fasting, we hypothesized that this modest, but consistent, change in blood glucose was a metabolic signal for the neuroendocrine adjustments of reproductive and metabolic hormones. Glucose (D-dextrose, 480 kcal/d) was infused into fasted (48 h) adult male rhesus macaques; and LH, cortisol, and GH were measured in plasma from samples collected at 15-min intervals for the last 15 h of the fast. We analyzed hormone secretion by deconvolution analysis, and the orderliness of release patterns by the approximate entropy statistic. Circulating blood glucose was 76 +/- 7 mg/dl in the fed control group, significantly higher (P < 0.01) than the level of 56 +/- 3 mg/dl in the fasted group. The increase in GH pulsatility and the 2-fold elevation in cortisol levels observed in the fasted male macaques were prevented by parenteral glucose delivery. The suppression of LH in fasted animals was not relieved by glucose infusions but seemed to be partially prevented in three of the animals. These findings are consistent with the hypothesis that glucose serves as a signal of nutritional status controlling adaptive neuroendocrine responses to fasting in the primate.

Corticotropin-releasing hormone regulates IL-6 expression during inflammation

Stimulation of the hypothalamic-pituitary-adrenal (HPA) axis by proinflammatory cytokines results in increased release of glucocorticoid that restrains further development of the inflammatory process. IL-6 has been suggested to stimulate the HPA axis during immune activation independent of the input of hypothalamic corticotropin-releasing hormone (CRH). We used the corticotropin-releasing hormone-deficient (Crh(-/-)) mouse to elucidate the effect of CRH deficiency on IL-6 expression and IL-6-induced HPA axis activation during turpentine-induced inflammation. We demonstrate that during inflammation CRH is required for a normal adrenocorticotropin hormone (ACTH) increase but not for adrenal corticosterone rise. The paradoxical increase of plasma IL-6 associated with CRH deficiency suggests that IL-6 release during inflammation is CRH-dependent. We also demonstrate that adrenal IL-6 expression is CRH-dependent, as its basal and inflammation-induced expression is blocked by CRH deficiency. Our findings suggest that during inflammation, IL-6 most likely compensates for the effects of CRH deficiency on food intake. Finally, we confirm that the HPA axis response is defective in Crh(-/-)/IL-6(-/-) mice. These findings, along with the regulation of IL-6 by CRH, support the importance of the interaction between the immune system and the HPA axis in the pathophysiology of inflammatory diseases.

Leptin regulates pulsatile luteinizing hormone and growth hormone secretion in the sheep

Administration of leptin during reduced nutrition improves reproductive activity in several monogastric species and reverses GH suppression in rodents. Whether leptin is a nutritional signal regulating neuroendocrine control of pituitary function in ruminant species is unclear. The present study examined the control of pulsatile LH and GH secretion in sheep. We determined whether exogenous leptin could prevent either the suppression of pulsatile LH secretion or the enhancement of GH secretion that occur during fasting. Recombinant human met-leptin (rhmet-leptin; 50 microg/kg BW; n = 8) or vehicle (n = 7) was administered s.c. every 8 h during a 78-h fast to estrogen-treated, castrated yearling males. LH and GH were measured in blood samples collected every 15 min for 6 h before fasting and during the last 6 h of fasting. Leptin was measured both by a universal leptin assay and by an assay specific for ovine leptin. During the fast, endogenous plasma leptin fell from 1.49 +/- 0.16 to 1.03 +/- 0.13 ng/ml. The average concentration of rhmet-leptin 8 h after leptin administration was 18.0 ng/ml. During fasting, plasma insulin, glucose, and insulin-like growth factor I levels declined, and nonesterified fatty acid concentrations increased similarly in vehicle-treated and leptin-treated animals. In vehicle-treated animals, LH pulse frequency declined markedly during fasting (5.6 +/- 0.5 vs. 1.1 +/- 0.5 pulses/6 h; fed vs. fasting; P < 0.0001). Leptin treatment prevented the fall in LH pulse frequency (5.0 +/- 0.4 vs. 4.9 +/- 0.4 pulses/6 h; P = 0.6). Neither fasting nor leptin administration altered GH pulse frequency. Fasting produced a modest increase in mean concentrations of circulating GH in control animals (2.4 +/- 0.5 vs. 3.4 +/- 0.6 ng/ml; P = 0.04), whereas there was a much greater increase in GH during leptin treatment (2.7 +/- 0.6 vs. 8.6 +/- 1.6 ng/ml; P = 0.0001). GH pulse amplitudes were also increased by fasting in control (P = 0.04) and leptin-treated sheep (P = 0.007). The finding that exogenous rhmet-leptin regulates LH and GH secretion in sheep indicates that this fat-derived hormone conveys information about nutrition to mechanisms controlling neuroendocrine function in ruminants.

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.

Urocortin expression in the Edinger-Westphal nucleus is up-regulated by stress and corticotropin-releasing hormone deficiency

Urocortin is a 40-amino acid mammalian peptide related to CRH and urotensin. The physiological role of urocortin is unknown, but it has been postulated to serve some of the functions previously attributed to CRH. We had earlier found that urocortin messenger RNA (mRNA) expression within the mouse brain is confined to the region of the Edinger-Westphal (EW) nucleus of the midbrain. To further characterize the regulation of the urocortin gene, we first cloned and sequenced the mouse gene, confirming the presence of a single gene in the murine genome. A general survey of mouse tissues using Northern blot analysis revealed the presence of urocortin mRNA only within the midbrain. By in situ hybridization analysis, we found that urocortin mRNA expression in the EW nucleus is responsive to stress, as mRNA levels increased approximately 3-fold after 3 h of restraint. Chronic glucocorticoid treatment, although not affecting basal levels, blocked the stress-induced rise in urocortin mRNA. Using CRH-deficient [knockout (KO)] mice, we examined the effect of combined CRH and glucocorticoid deficiency upon urocortin mRNA expression. As in wild-type (WT) mice, we had previously found that urocortin expression in CRHKO mouse brain was not detected outside of the EW nucleus. However, we found that urocortin expression within the EW of CRHKO mice is up-regulated 2- to 3-fold compared with that in WT mice. This up-regulation is not due to a lack of inhibition by glucocorticoids, as urocortin mRNA levels in the EW nucleus of CRHKO mice did not change after glucocorticoid supplementation. As the EW does not project to any brain regions known to be involved in the behavioral responses to stress, urocortin expressed in this site is unlikely to mediate stress-induced behaviors. On the other hand, as the EW nucleus may play a role in the regulation of the autonomic nervous system and projects to various brain stem nuclei that express the CRH receptor, urocortin originating in the EW may play a role in the regulation of the autonomic nervous system during stress.

Stress-induced behaviors require the corticotropin-releasing hormone (CRH) receptor, but not CRH

Corticotropin-releasing hormone (CRH) is a central regulator of the hormonal stress response, causing stimulation of corticotropin and glucocorticoid secretion. CRH is also widely believed to mediate stress-induced behaviors, implying a broader, integrative role for the hormone in the psychological stress response. Mice lacking the CRH gene exhibit normal stress-induced behavior that is specifically blocked by a CRH type 1 receptor antagonist. The other known mammalian ligand for CRH receptors is urocortin. Normal and CRH-deficient mice have an identical distribution of urocortin mRNA, which is confined to the region of the Edinger-Westphal nucleus, and is absent from regions known to mediate stress-related behaviors. Since the Edinger-Westphal nucleus is not known to project to any brain regions believed to play a role in anxiety-like behavior, an entirely different pathway must be postulated for urocortin in the Edinger-Westphal nucleus to mediate these behaviors in CRH-deficient mice. Alternatively, an unidentified CRH-like molecule other than CRH or urocortin, acting through the CRH receptors in brain regions believed to mediate stress-induced behaviors, may mediate the behavioral response to stress, either alone or in concert with CRH.

Animal models of CRH deficiency

Corticotropin-releasing hormone (CRH), the major regulator of hypothalamic-pituitary-adrenal (HPA) axis, was first isolated due to its ability to stimulate the release of adrenocorticotropic hormone from the anterior pituitary. Later, it was also found to have also a wide spectrum of actions within the central nervous system and the periphery. Studies with pharmacological administration of this peptide and/or antagonists and antibody neutralization techniques have yielded important information concerning the physiological relevance of CRH. The development of CRH knockout mice (CRH KO) has been an important tool for addressing the physiologic and pathologic roles of CRH. This review describes the phenotype of CRH-deficient mice, as well as the use of this model to study the roles of CRH on fetal development and postnatal life. The role of CRH in prenatal development and postnatal regulation of the HPA axis, in activation of the reproductive system during stress, and in modulation of the immune function will be discussed. The review concludes with a comparison of CRH KO mice with other models of CRH deficiency.