The KiSS-1 receptor GPR54 is essential for the development of the murine reproductive system

Isolated gonadotropic deficiency with and without anosmia: a developmental defect or a neuroendocrine regulation abnormality of the gonadotropic axis

Hypogonadotropic hypogonadism has been described in several human genetic diseases. Congenital isolated hypogonadotropic hypogonadism is classified into two categories: one that is associated with anosmia (Kallmann syndrome) and one that is apparently isolated. Mutations and deletions of the KAL1 gene, which encodes for a protein involved in cell adhesion, have been observed in many cases of the X-linked form of Kallmann syndrome. Recently, loss-of-function mutations of fibroblast growth factor receptor-1 (FGFR1) were associated with an autosomal dominant form of Kallmann syndrome. Genotype-phenotype correlations confirm the large spectrum of the phenotype due to FGFR1 mutations. Cases of isolated hypogonadotropic hypogonadism were considered to be idiopathic until the description of mutations of the gonadotropin releasing hormone receptor, luteinizing hormone and follicle stimulating hormone genes. However, defects in these genes only account for a small percentage of familial cases, which suggests that other proteins may be involved in regulation of the gonadotropic axis. We recently described GPR54 as one of these proteins by genome mapping in a very informative family. In vivo studies and genotype-phenotype correlations indicate that gonadotropic axis regulation by GPR54 occurs mainly at the level of the hypothalamus.

[GnRH resistance and the GPR54 gene]

Current data make it possible to relate idiopathic hypogonadotrophic hypogonadism to mutations affecting the GnRH I-1 receptor and also to new "loss-of-function" mutations concerning another receptor, GPR54. It now seems that mutations of the pituitary GnRH receptor are not the only explanation of most cases of sporadic isolated hypogonadotrophic hypogonadism, and, on the contrary, there are certain familial forms, where no mutation has been demonstrated, suggesting the potential involvement of other genes. The role of another G protein-coupled glycoprotein membrane receptor, GRP54, already known for its involvement as a metastasis suppressor, has been demonstrated. Bioclinical studies of families affected with the disorder by pheno/genotypic correlation demonstrated that "loss-of-function" mutations affecting the GPR54 gene coding for GRP54 are the cause of hypogonadotrophic hypogonadism. GPR54 therefore appears to be involved at hypothalamic and pituitary level. It is not involved in sexual differentiation, but may modulate GnRH secretion or affect its pituitary response. Further investigations are required to determine the levels of action of this receptor which may provide a new pharmacological target in the future.

KiSS-1 Expression in Human Breast Cancer

The KiSS-1 gene encodes a 145 amino acid residue peptide that is further processed to a final peptide, metastin, a ligand to a G-coupled orphan receptor (OT7T175/AXOR12). KiSS-1 has been identified as a putative human metastasis suppressor gene in melanomas and in breast cancer cell lines. This study aimed to determine the expression and distribution of KiSS-1 and its receptor in human breast cancer tissues and to identify a possible link between expression levels and patient prognosis. Frozen sections from breast cancer primary tumours (matched tumour 124 and background 33) were immuno-stained with KiSS-1 antibody. RNA was reverse transcribed and analyzed by Q-PCR (standardized using beta-actin, and normalized with cytokeratin-19 levels). Levels of expression of KiSS-1 were higher in tumour compared to background tissues (3,124+/-1,262 vs 2,397+/-1,181) and significantly increased in node positive tumours compared to node negative (3,637+/-1,719 vs 2,653+/-1,994, P = 0.02). KiSS-1 expression was also increased with increasing grade and TNM status. There were no such trends with the KiSS-1 receptor. Expression of KiSS-1 was higher in patients who had died from breast cancer than those who had remained healthy (4,631+/-3,024 vs 2,280+/-1,403) whereas expression of the receptor was reduced (480+/-162 vs 195+/-134). Immunohistochemical staining showed increased expression of KiSS-1 in tumour sections. Insertion of the KiSS-1 gene into the human breast cancer cell line MDA-MB-231, resulted in cells that were significantly more motile and invasive in behaviour, with reduced adhesion to matrix, using respective assays. In conclusion, KiSS-1 expression is increased in human breast cancer, particularly in patients with aggressive tumours and with mortality. Over-expression of KiSS-1 in breast cancer cells result in more aggressive phenotype. Together, it suggests that KiSS-1 plays a role beyond the initial metastasis repressor in this cancer type.

Design and synthesis of downsized metastin (45-54) analogs with maintenance of high GPR54 agonistic activity

Metastin has been identified as a metastasis suppressor gene product that mediates its function through a G protein coupled receptor, GPR54. To refine insight into the critical pharmacophore for the activation of GPR54, we have conducted alanine and d-amino acid scanning on a biologically active metastin fragment (45-54). Based on these data and structures of peptides previously reported to activate GPR54, a series of shortened metastin (45-54) derivatives were synthesized and tested for the ability to induce GPR54 signaling. These biological experiments were performed in yeast containing human GPR54 that was coupled to the pheromone response pathway and a pheromone responsive lacZ reporter gene. Compounds 32, 33, and 39, which possess an N-terminal basic group and a C-terminal RW-amide motif, were strong agonists, similar to the level of metastin. This may provide an approach to reverse the pro-metastatic effect of metastin deletion in multiple malignant tumors.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Metastin and its G protein-coupled receptor, GPR54: critical pathway modulating GnRH secretion

Photoperiod, food availability, temperature, stress, and hormonal cues are some of the varied signals used by mammalian species to activate or suppress their timing of sexual maturation. All ultimately converge upon gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus. Through its stimulation of LH and FSH from the pituitary, GnRH represents a critical step in the reproductive cascade. While few dispute this central role of GnRH, little is understood of the mechanisms influencing the developmental fate and physiologic controls of GnRH neurons. Identification of the signals which modulate pulsatile GnRH secretion is critical to advancing understanding of normal puberty and reproductive competency. The recent identification of loss-of-function mutations in GPR54, a receptor for kisspeptin-1, has highlighted a new pathway for the timing of puberty and reproductive control.

Regulation of Kiss1 gene expression in the brain of the female mouse

The Kiss1 gene encodes a family of neuropeptides called kisspeptins, which activate the receptor G protein-coupled receptor-54 and play a role in the neuroendocrine regulation of GnRH secretion. We examined whether estradiol (E2) regulates KiSS-1 in the forebrain of the female mouse by comparing KiSS-1 mRNA expression among groups of ovary-intact (diestrus), ovariectomized (OVX), and OVX plus E2-treated mice. In the arcuate nucleus (Arc), KiSS-1 expression increased after ovariectomy and decreased with E2 treatment. Conversely, in the anteroventral periventricular nucleus (AVPV), KiSS-1 expression was reduced after ovariectomy and increased with E2 treatment. To determine whether the effects of E2 on KiSS-1 are mediated through estrogen receptor (ER)alpha or ERbeta, we evaluated the effects of E2 in OVX mice that lacked functional ERalpha or ERbeta. In OVX mice that lacked functional ERalpha, KiSS-1 mRNA did not respond to E2 in either the Arc or AVPV, suggesting that ERalpha is essential for mediating the inhibitory and stimulatory effects of E2. In contrast, KiSS-1 mRNA in OVX mice that lacked functional ERbeta responded to E2 exactly as wild-type animals. Double-label in situ hybridization revealed that virtually all KiSS-1-expressing neurons in the Arc and AVPV coexpress ERalpha, suggesting that the effects of E2 are mediated directly through KiSS-1 neurons. We conclude that KiSS-1 neurons in the Arc, which are inhibited by E2, may play a role in the negative feedback regulation of GnRH secretion, whereas KiSS-1 neurons in the AVPV, which are stimulated by E2, may participate in the positive feedback regulation of GnRH secretion.

A KiSS to remember

The Kiss1 gene encodes a family of neuropeptides named kisspeptins, which bind to a (former orphan) G-protein-coupled receptor called GPR54. Recent investigations suggest that kisspeptins play a vital role in regulating the secretion of gonadotropin-releasing hormone (GnRH). New evidence confirms that kisspeptins act through GPR54 to stimulate GnRH secretion. Kisspeptins and GPR54 are crucial for pubertal maturation in the primate. Kiss1 mRNA is differentially regulated by sex steroids in distinct hypothalamic nuclei.

Kisspeptins: regulators of metastasis and the hypothalamic-pituitary-gonadal axis

The kisspeptins are the peptide products of the KiSS-1 gene and the endogenous agonists for the GPR54 receptor. Although KiSS-1 was initially discovered as a metastasis suppressor gene, recent evidence suggests the kisspeptin/GPR54 system is a key regulator of the reproductive system. Disrupted GPR54 signalling causes hypogonadotrophic hypogonadism in rodents and man. Central or peripheral administration of kisspeptin potently stimulates the hypothalamic-pituitary-gonadal axis, increasing circulating gonadotrophin concentrations in a number of animal models. These effects appear likely to be mediated via the hypothalamic gonadotrophin-releasing hormone system, although kisspeptins may have direct effects on the anterior pituitary gland. Hypothalamic KiSS-1 expression is regulated by circulating sex steroids. The precise physiological role of the kisspeptin system in the regulation of reproductive function remains to be elucidated.

KiSS-1 and GPR54 as new players in gonadotropin regulation and puberty

The recent identification of loss-of-function mutations in the gene encoding GPR54, the receptor for the KiSS-1-derived peptides, kisspeptins, has highlighted a previously unrecognized pathway in the physiologic regulation of puberty and reproduction. Patients with loss-of-function mutations in GPR54 have idiopathic hypogonadotropic hypogonadism, and mice lacking GPR54 similarly fail to undergo puberty and have immature reproductive organs and low levels of sex steroids and gonadotropins. These observations have led to the hypothesis that kisspeptins activate hypothalamic GnRH release, thereby serving as a pivotal factor in the pubertal activation of the reproductive cascade. This hypothesis is supported by subsequent studies in rodent and primate models that have demonstrated localization of KiSS-1 mRNA in the hypothalamus, colocalization of GPR54 in GnRH neurons, GnRH-dependent activation of LH and FSH release by intracerebroventricular or peripheral administration of kisspeptin, and increased hypothalamic KiSS-1 and GPR54 mRNA levels at the onset of puberty. Taken together, these findings weave a compelling case for a role of the kisspeptin-GPR54 system in the activation of GnRH neurons at the time of pubertal awakening of the reproductive axis.

Effects of KiSS-1 peptide, the natural ligand of GPR54, on follicle-stimulating hormone secretion in the rat

KiSS-1 was originally identified as a metastasis suppressor gene encoding an array of structurally related peptides, namely kisspeptins, which acting through the G protein-coupled receptor GPR54 are able to inhibit tumor progression. Unexpectedly, a reproductive facet of this newly discovered system has recently arisen, and characterization of the role of the KiSS-1/GPR54 system in the neuroendocrine control of gonadotropin secretion has been initiated. However, such studies have been so far mostly restricted to LH, and very little is known about the actual contribution of this system in the regulation of FSH release. To address this issue, the effects of KiSS-1 peptide on FSH secretion were monitored in vivo and in vitro under different experimental conditions. Intracerebroventricular administration of KiSS-1 peptide significantly stimulated FSH secretion in prepubertal and adult rats. Yet, dose-response analyses in vivo demonstrated an ED(50) value for the FSH-releasing effects of KiSS-1 of 400 pmol, i.e. approximately 100-fold higher than that of LH. In addition, systemic (ip and iv) injection of KiSS-1 significantly stimulated FSH secretion in vivo. However, KiSS-1 failed to elicit basal FSH release directly at the pituitary level, although it moderately enhanced GnRH-stimulated FSH secretion in vitro. Finally, mechanistic studies revealed that the ability of KiSS-1 to elicit FSH secretion was abolished by the blockade of endogenous GnRH actions, but it was persistently observed in different models of leptin insufficiency and after blockade of endogenous excitatory amino acid and nitric oxide pathways, i.e. relevant signals in the neuroendocrine control of gonadotropin secretion. In summary, our results extend previous recent observations on the role of KiSS-1 in the control of LH secretion and provide solid evidence for a stimulatory effect of KiSS-1 on FSH release, acting at central level. Overall, it is proposed that the KiSS-1/GPR54 system is a novel, pivotal downstream element in the neuroendocrine network governing gonadotropin secretion.

Increased hypothalamic GPR54 signaling: a potential mechanism for initiation of puberty in primates

To further study the role of GPR54 signaling in the onset of primate puberty, we used the monkey to examine the ability of kisspeptin-10 to elicit the release of gonadotropin-releasing hormone (GnRH) precociously, and we describe the expression of GPR54 and KiSS-1 in the hypothalamus during the peripubertal period. Agonadal juvenile male monkeys were implanted with a lateral cerebroventricular cannula and a jugular vein catheter. The responsiveness of the juvenile pituitary to endogenous GnRH release was heightened with a chronic pulsatile i.v. infusion of synthetic GnRH before kisspeptin-10 (112-121) injection. Intracerebroventricular (30 microg or 100 microg) or i.v. (100 microg) bolus injections of kisspeptin-10 elicited a robust GnRH discharge, as reflected by luteinizing hormone secretion, which was abolished by pretreatment with a GnRH-receptor antagonist. RNA was isolated from the hypothalamus of agonadal males before (juvenile) and after (pubertal) the pubertal resurgence of pulsatile GnRH release and from juvenile, early pubertal, and midpubertal ovary-intact females. KiSS-1 mRNA levels detected by real-time PCR increased with puberty in both male and female monkeys. In intact females, but not in agonadal males, GPR54 mRNA levels in the hypothalamus increased approximately 3-fold from the juvenile to midpubertal stage. Hybridization histochemistry indicated robust KiSS-1 and GPR54 mRNA expression in the region of the arcuate nucleus. These findings are consistent with the hypothesis that GPR54 signaling by its cognate ligand in the primate hypothalamus may be activated at the end of the juvenile phase of development and may contribute to the pubertal resurgence of pulsatile GnRH release, the central drive for puberty.

Kisspeptin directly stimulates gonadotropin-releasing hormone release via G protein-coupled receptor 54

We have recently described a molecular gatekeeper of the hypothalamic-pituitary-gonadal axis with the observation that G protein-coupled receptor 54 (GPR54) is required in mice and men for the pubertal onset of pulsatile luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion to occur. In the present study, we investigate the possible central mode of action of GPR54 and kisspeptin ligand. First, we show that GPR54 transcripts are colocalized with gonadotropin-releasing hormone (GnRH) neurons in the mouse hypothalamus, suggesting that kisspeptin, the GPR54 ligand, may act directly on these neurons. Next, we show that GnRH neurons seem anatomically normal in gpr54-/- mice, and that they show projections to the median eminence, which demonstrates that the hypogonadism in gpr54-/- mice is not due to an abnormal migration of GnRH neurons (as occurs with KAL1 mutations), but that it is more likely due to a lack of GnRH release or absence of GnRH neuron stimulation. We also show that levels of kisspeptin injected i.p., which stimulate robust LH and FSH release in wild-type mice, have no effect in gpr54-/- mice, and therefore that kisspeptin acts directly and uniquely by means of GPR54 signaling for this function. Finally, we demonstrate by direct measurement, that the central administration of kisspeptin intracerebroventricularly in sheep produces a dramatic release of GnRH into the cerebrospinal fluid, with a parallel rise in serum LH, demonstrating that a key action of kisspeptin on the hypothalamo-pituitary-gonadal axis occurs directly at the level of GnRH release. The localization and GnRH release effects of kisspeptin thus define GPR54 as a major control point in the reproductive axis and suggest kisspeptin to be a neurohormonal effector.

Characterization of the potent luteinizing hormone-releasing activity of KiSS-1 peptide, the natural ligand of GPR54

Loss-of-function mutations of the gene encoding GPR54, the putative receptor for the KiSS-1-derived peptide metastin, have been recently associated with hypogonadotropic hypogonadism, in both rodents and humans. Yet the actual role of the KiSS-1/GPR54 system in the neuroendocrine control of gonadotropin secretion remains largely unexplored. To initiate such analysis, the effects of KiSS-1 peptide on LH secretion were monitored using in vivo and in vitro settings under different experimental conditions. Central intracerebroventricular administration of KiSS-1 peptide potently elicited LH secretion in vivo over a range of doses from 10 pmol to 1 nmol. The effect of centrally injected KiSS-1 appeared to be mediated via the hypothalamic LHRH. However, no effect of central administration of KiSS-1 was detected on relative LHRH mRNA levels. Likewise, systemic (i.p. and i.v.) injection of KiSS-1 markedly stimulated LH secretion. This effect was similar in terms of maximum response to that of central administration of KiSS-1 and might be partially attributed to its ability to stimulate LH secretion directly at the pituitary. Finally, the LH-releasing activity of KiSS-1 was persistently observed after blockade of endogenous excitatory amino acid and nitric oxide pathways, i.e. relevant neurotransmitters in the neuroendocrine control of LH secretion. In summary, our results provide solid evidence for a potent stimulatory effect of KiSS-1 on LH release, acting at central levels (likely the hypothalamus) and eventually at the pituitary, and further document a novel role of the KiSS-1/GPR54 system as a relevant downstream element in the neuroendocrine network governing LH secretion.

Mutant G-protein-coupled receptors as a cause of human diseases

G-protein-coupled receptors (GPCR) are involved in directly and indirectly controlling an extraordinary variety of physiological functions. Their key roles in cellular communication have made them the target for more than 60% of all currently prescribed drugs. Mutations in GPCR can cause acquired and inherited diseases such as retinitis pigmentosa (RP), hypo- and hyperthyroidism, nephrogenic diabetes insipidus, several fertility disorders, and even carcinomas. To date, over 600 inactivating and almost 100 activating mutations in GPCR have been identified which are responsible for more than 30 different human diseases. The number of human disorders is expected to increase given the fact that over 160 GPCR have been targeted in mice. Herein, we summarize the current knowledge relevant to understanding the molecular basis of GPCR function, with primary emphasis on the mechanisms underlying GPCR malfunction responsible for different human diseases.

GPR54 and puberty

At puberty, pulsatile secretion of hormones initiates sexual maturation of the gonads. The G-protein-coupled receptor GPR54 is crucially involved in the initiation of puberty, along with its ligand metastin. Mice lacking GPR54 fail to undergo puberty and have immature reproductive organs and low levels of sex steroids and gonadotrophic hormones, but have normal levels of gonadotrophin-releasing hormone in the hypothalamus. In humans, several cases of hypogonadism have been ascribed to mutations in GPR54. Production of metastin and, to a lesser extent, GPR54 are negatively regulated by testosterone and oestrogen, and injecting GPR54 ligands can increase hormone secretion in rodents. Thus, GPR54 is required for normal functioning of the hypothalamic-pituitary-gonadal axis, probably at the level of gonadotrophin-releasing-hormone secretion.

Advanced vaginal opening and precocious activation of the reproductive axis by KiSS-1 peptide, the endogenous ligand of GPR54

The awakening of the gonadotrophic axis at puberty is the end-point of a complex cascade of sex developmental events that leads to the attainment of reproductive capacity. Recently, loss-of-function mutations of the gene encoding GPR54, the putative receptor for the KiSS-1-derived peptide metastin, have been linked to hypogonadotrophic hypogonadism, both in rodents and humans. However, the actual role of the KiSS-1/GPR54 system in the timing of puberty onset remains unexplored. We report herein that chronic central administration of KiSS-1 peptide to immature female rats induced the precocious activation of the gonadotrophic axis, as estimated by advanced vaginal opening, elevated uterus weight, and increased serum levels of luteinizing hormone (LH) and oestrogen. The central effect of KiSS-1 upon LH release appeared to be mediated via the hypothalamic LH-releasing hormone. In contrast, despite the well-documented permissive role of body fat stores and the adipocyte-derived hormone leptin in puberty maturation, acute activation of the gonadotrophic axis by KiSS-1 was persistently observed in pubertal animals under food deprivation, after central immunoneutralization of leptin, and in a model of leptin resistance. Overall, the present results, together with our recent data on maximum expression of KiSS-1 and GPR54 genes in the hypothalamus at puberty, provide novel evidence for a role of the KiSS-1 system as a downstream element in the hypothalamic network triggering the onset of puberty.

Developmental and hormonally regulated messenger ribonucleic acid expression of KiSS-1 and its putative receptor, GPR54, in rat hypothalamus and potent luteinizing hormone-releasing activity of KiSS-1 peptide

The gonadotropic axis is centrally controlled by a complex regulatory network of excitatory and inhibitory signals that is activated at puberty. Recently, loss of function mutations of the gene encoding G protein-coupled receptor 54 (GPR54), the putative receptor for the KiSS-1-derived peptide metastin, have been associated with lack of puberty onset and hypogonadotropic hypogonadism. Yet the pattern of expression and functional role of the KiSS-1/GPR54 system in the rat hypothalamus remain unexplored to date. In the present work, expression analyses of KiSS-1 and GPR54 genes were conducted in different physiological and experimental settings, and the effects of central administration of KiSS-1 peptide on LH release were assessed in vivo. Persistent expression of KiSS-1 and GPR54 mRNAs was detected in rat hypothalamus throughout postnatal development, with maximum expression levels at puberty in both male and female rats. Hypothalamic expression of KiSS-1 and GPR54 genes changed throughout the estrous cycle and was significantly increased after gonadectomy, a rise that was prevented by sex steroid replacement both in males and females. Moreover, hypothalamic expression of the KiSS-1 gene was sensitive to neonatal imprinting by estrogen. From a functional standpoint, intracerebroventricular administration of KiSS-1 peptide induced a dramatic increase in serum LH levels in prepubertal male and female rats as well as in adult animals. In conclusion, we provide novel evidence of the developmental and hormonally regulated expression of KiSS-1 and GPR54 mRNAs in rat hypothalamus and the ability of KiSS-1 peptide to potently stimulate LH secretion in vivo. Our current data support the contention that the hypothalamic KiSS-1/GPR54 system is a pivotal factor in central regulation of the gonadotropic axis at puberty and in adulthood.

Orphan G protein-coupled receptors and obesity

The use of orphan G protein-coupled receptors (GPCRs) as targets to identify new transmitters has led over the last decade to the discovery of 12 novel neuropeptide families. Each one of these new neuropeptides has opened its own field of research, has brought new insights in distinct pathophysiological conditions and has offered new potentials for therapeutic applications. Interestingly, several of these novel peptides have seen their roles converge on one physiological response: the regulation of food intake and energy expenditure. In this manuscript, we discuss four deorphanized GPCR systems, the ghrelin, orexins/hypocretins, melanin-concentrating hormone (MCH) and neuropeptide B/neuropeptide W (NPB/NPW) systems, and review our knowledge of their role in the regulation of energy balance and of their potential use in therapies directed at feeding disorders.

A role for kisspeptins in the regulation of gonadotropin secretion in the mouse

Kisspeptins are products of the KiSS-1 gene, which bind to a G protein-coupled receptor known as GPR54. Mutations or targeted disruptions in the GPR54 gene cause hypogonadotropic hypogonadism in humans and mice, suggesting that kisspeptin signaling may be important for the regulation of gonadotropin secretion. To examine the effects of kisspeptin-54 (metastin) and kisspeptin-10 (the biologically active C-terminal decapeptide) on gonadotropin secretion in the mouse, we administered the kisspeptins directly into the lateral cerebral ventricle of the brain and demonstrated that both peptides stimulate LH secretion. Further characterization of kisspeptin-54 demonstrated that it stimulated both LH and FSH secretion, at doses as low as 1 fmol; moreover, this effect was shown to be blocked by pretreatment with acyline, a potent GnRH antagonist. To learn more about the functional anatomy of kisspeptins, we mapped the distribution of KiSS-1 mRNA in the hypothalamus. We observed that KiSS-1 mRNA is expressed in areas of the hypothalamus implicated in the neuroendocrine regulation of gonadotropin secretion, including the anteroventral periventricular nucleus, the periventricular nucleus, and the arcuate nucleus. We conclude that kisspeptin-GPR54 signaling may be part of the hypothalamic circuitry that governs the hypothalamic secretion of GnRH.

Peripheral administration of metastin induces marked gonadotropin release and ovulation in the rat

Metastin is a novel peptide that has been isolated from the human placenta as the cognate ligand of the G-protein-coupled receptor OT7T175 (or GPR54). However, its physiological functions have not yet been fully investigated. In the present study, we show that subcutaneous administration of metastin increased the plasma levels of gonadotropins (follicle-stimulating hormone and luteinizing hormone) and induced ovulation in prepubertal female rats that had been pretreated with pregnant mare serum gonadotropin to induce follicle maturation. Furthermore, metastin administration drastically increased the plasma levels of gonadotropins in male rats. This action was abolished by pretreatment with a GnRH antagonist, and was accompanied by induction of c-Fos immunoreactivity in GnRH neurons. These results suggest that s.c. administered metastin induces the release of gonadotropin via activation of the hypothalamic GnRH neurons.

The expanding family of -RFamide peptides and their effects on feeding behaviour

Neuropeptides terminating in -Arg-Phe-NH(2) (-RFamide) were first discovered in molluscan nervous systems, but were soon recognized to occur widely throughout the invertebrates. Progress in characterizing members of the family in vertebrates has been slower. In mammals, however, it is now clear that there are at least five genes encoding members of the family, and at least five G-protein-coupled receptors at which they act. The tissue distribution of the peptides and their receptors is wide and there are likely to be many different functions. One of the emerging themes from recent research is that these peptides are involved in control of feeding behaviour both in invertebrates and in vertebrates. This would seem to be a remarkable example of conservation of chemical structure and biological function throughout nervous system evolution.

Kisspeptin activation of gonadotropin releasing hormone neurons and regulation of KiSS-1 mRNA in the male rat

The KiSS-1 gene codes for a family of neuropeptides called kisspeptins which bind to the G-protein-coupled receptor GPR54. To assess the possible effects of kisspeptins on gonadotropin secretion, we injected kisspeptin-52 into the lateral cerebral ventricles of adult male rats and found that kisspeptin-52 increased the serum levels of luteinizing hormone (p < 0.05). To determine whether the kisspeptin-52-induced stimulation of luteinizing hormone secretion was mediated by gonadotropin-releasing hormone (GnRH), we pretreated adult male rats with a GnRH antagonist (acyline), then challenged the animals with intracerebroventricularly administered kisspeptin-52. The GnRH antagonist blocked the kisspeptin-52-induced increase in luteinizing hormone. To examine whether kisspeptins stimulate transcriptional activity in GnRH neurons, we administered kisspeptin-52 intracerebroventricularly and found by immunocytochemistry that 86% of the GnRH neurons coexpressed Fos 2 h after the kisspeptin-52 challenge, whereas fewer than 1% of the GnRH neurons expressed Fos following injection of the vehicle alone (p < 0.001). To assess whether kisspeptins can directly act on GnRH neurons, we used double-label in situ hybridization and found that 77% of the GnRH neurons coexpress GPR54 mRNA. Finally, to determine whether KiSS-1 gene expression is regulated by gonadal hormones, we measured KiSS-1 mRNA levels by single-label in situ hybridization in intact and castrated males and found significantly higher levels in the arcuate nucleus of castrates. These results demonstrate that GnRH neurons are direct targets for regulation by kisspeptins and that KiSS-1 mRNA is regulated by gonadal hormones, suggesting that KiSS-1 neurons play an important role in the feedback regulation of gonadotropin secretion.