Hypertrophy and increased kisspeptin gene expression in the hypothalamic infundibular nucleus of postmenopausal women and ovariectomized monkeys

Sex steroid control of hypothalamic Kiss1 expression in sheep and rodents: comparative aspects

In recent years, the Kiss1 gene has been cast into the reproductive spotlight. In the short period since the discovered link between kisspeptins, the encoded peptides of Kiss1, and fertility, these peptides are now known to be critical for the neuroendocrine control of reproduction. Kisspeptin producing cells in the hypothalamus are poised to become the 'missing link' in the sex steroid feedback control of GnRH secretion. These cells contain all the necessary components to relay information of the sex steroid environment to GnRH neurons, which possess the kisspeptin receptor, GPR54. Sex steroids regulate Kiss1 mRNA, and kisspeptin expression in the hypothalamus, in a manner consistent with both negative and positive feedback control of GnRH. The precise nature of sex steroid effects, in particular those of estrogen, on Kiss1 expression have been extensively studied in the female rodent and ewe. In the arcuate nucleus (ARC) of both species, kisspeptin cells appear to forward signals pertinent to negative feedback regulation of GnRH, although in the ewe it appears this population of Kiss1 cell is also responsible for positive feedback regulation of GnRH at the time of the preovulatory GnRH/LH surge. In rodents, these positive feedback signals appear to be mediated by kisspeptin cells exclusively within the anteroventral periventricular nucleus (AVPV). There are no Kiss1 cells in the ovine AVPV, but there is a population in the preoptic area. The role these preoptic area cells play in the sex steroid feedback regulation of GnRH secretion, if any, is yet to be revealed.

The neuroanatomy of the kisspeptin system in the mammalian brain

The kisspeptin precursor is the protein transcribed from the Kiss-1 gene and the kisspeptins are the peptides that are posttranslationally processed from the precursor. The kisspeptins activate the G-protein coupled receptor GPR54 and are strongly implicated in puberty onset and in regulation of the hypothalamo-pituitary gonadal axis in mammals. Physiological studies have indicated that these effects occur via a direct activation of the GnRH neurons, and at an unknown site in the median eminence or directly on the gonadotropes. Paradoxically, while the function of kisspeptin is relatively well understood, little data are available about the localization of kisspeptin neurons in the brain, and in particular the projection patterns of kisspeptin containing axons implicated in regulation of the hypothalamo-pituitary gonadal axis. This review covers the current information about the localization of kisspeptin neurons in the mammalian brain and discusses the facts and artifacts of the methods of their detection. The available data suggest that kisspeptins are synthesized in neurons in the anteroventral periventricular nucleus and the arcuate nucleus. Both populations are considered to be involved in control of gonadotropes. In addition, kisspeptin nerve terminals and receptors are found in other hypothalamic area suggesting that kisspeptins are involved in regulation of other yet unknown homeostatic or neuroendocrine functions.

Postmenopausal increase in KiSS-1, GPR54, and luteinizing hormone releasing hormone (LHRH-1) mRNA in the basal hypothalamus of female rhesus monkeys

The G-protein coupled receptor, GPR54, and its ligand, kisspeptin-54 (a KiSS-1 derived peptide) have been reported to be important players in control of LHRH-1 release. However, the role of the GPR54 signaling in primate reproductive senescence is still unclear. In the present study we investigated whether KiSS-1, GPR54, and LHRH-1 mRNA in the brain change after menopause in female rhesus monkeys using quantitative real-time PCR. Results indicate that KiSS-1, GPR54, and LHRH-1 mRNA levels in the medial basal hypothalamus (MBH) in postmenopausal females (28.3+/-1.1 years of age, n=5) were all significantly higher than that in eugonadal adult females (14.7+/-2.1 years of age, n=9), whereas KiSS-1, GPR54, and LHRH-1 mRNA levels in the preoptic area (POA) did not have any significant changes between the two age groups. To further determine the potential contribution by the absence of ovarian steroids, we compared the changes in KiSS-1, GPR54, and LHRH-1 mRNA levels in young adult ovarian intact vs. young ovariectomized females. Results indicate that KiSS-1 and LHRH-1 mRNA levels in the MBH, not POA, in ovariectomized females were significantly higher than those in ovarian intact females, whereas GPR54 mRNA levels in ovariectomized females had a tendency to be elevated in the MBH, although the values were not quite statistically significant. Collectively, in the primate the reduction in the negative feedback control by ovarian steroids appears to be responsible for the aging changes in kisspeptin-GPR54 signaling and the elevated state of the LHRH-1 neuronal system.

Kisspeptin and the regulation of the hypothalamic-pituitary-gonadal axis in the rhesus monkey (Macaca mulatta)

The present article reviews recent studies of monkeys and, in some cases, humans that have been conducted to examine the role of kisspeptin-GPR54 signaling in the regulation of the hypothalamic-pituitary-gonadal axis in higher primates. This area of peptide biology was initiated in 2003 by the discovery that loss of function mutations of GPR54 in man were associated with hypogonadotropic hypogonadism and absent or delayed puberty. Puberty in the monkey, an experimental model commonly used to study this fundamental developmental stage, is first described. This is followed by a review of the role of kisspeptin in the regulation of the postnatal ontogeny of GnRH pulsatility. The roles of kisspeptin in GnRH pulse generation and in the feedback loops governing gonadotropin secretion in primates are then discussed. A brief section on kisspeptin-GPR54 signaling at the pituitary and gonadal levels is also included. The review concludes with a discussion of the phenomenon of GPR54 downregulation by continuous exposure to kisspeptin and its therapeutic implications.

Menopause and the human hypothalamus: evidence for the role of kisspeptin/neurokinin B neurons in the regulation of estrogen negative feedback

Menopause is characterized by depletion of ovarian follicles, a reduction of ovarian hormones to castrate levels and elevated levels of serum gonadotropins. Rather than degenerating, the reproductive neuroendocrine axis in postmenopausal women is intact and responds robustly to the removal of ovarian hormones. Studies in both human and non-human primates provide evidence that the gonadotropin hypersecretion in postmenopausal women is secondary to increased gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus. In addition, menopause is accompanied by hypertrophy of neurons in the infundibular (arcuate) nucleus expressing KiSS-1, neurokinin B (NKB), substance P, dynorphin and estrogen receptor alpha (ERalpha) mRNA. Ovariectomy in experimental animals induces nearly identical findings, providing evidence that these changes are a compensatory response to ovarian failure. The anatomical site of the hypertrophied neurons, as well as the extensive data implicating kisspeptin, NKB and dynorphin in the regulation of GnRH secretion, provide compelling evidence that these neurons are part of the neural network responsible for the increased levels of serum gonadotropins in postmenopausal women. We propose that neurons expressing KiSS-1, NKB, substance P, dynorphin and ERalpha mRNA in the infundibular nucleus play an important role in sex-steroid feedback on gonadotropin secretion in the human.

Sexual differentiation and the Kiss1 system: hormonal and developmental considerations

The nervous system (both central and peripheral) is anatomically and physiologically differentiated between the sexes, ranging from gender-based differences in the cerebral cortex to motoneuron number in the spinal cord. Although genetic factors may play a role in the development of some sexually differentiated traits, most identified sex differences in the brain and behavior are produced under the influence of perinatal sex steroid signaling. In many species, the ability to display an estrogen-induced luteinizing hormone (LH) surge is sexually differentiated, yet the specific neural population(s) that allows females but not males to display such estrogen-mediated "positive feedback" has remained elusive. Recently, the Kiss1/kisspeptin system has been implicated in generating the sexually dimorphic circuitry underlying the LH surge. Specifically, Kiss1 gene expression and kisspeptin protein levels in the anteroventral periventricular (AVPV) nucleus of the hypothalamus are sexually differentiated, with females displaying higher levels than males, even under identical hormonal conditions as adults. These findings, in conjunction with accumulating evidence implicating kisspeptins as potent secretagogues of gonadotropin-releasing hormone (GnRH), suggest that the sex-specific display of the LH surge (positive feedback) reflects sexual differentiation of AVPV Kiss1 neurons. In addition, developmental kisspeptin signaling via its receptor GPR54 appears to be critical in males for the proper sexual differentiation of a variety of sexually dimorphic traits, ranging from complex social behavior to specific forebrain and spinal cord neuronal populations. This review discusses the recent data, and their implications, regarding the bi-directional relationship between the Kiss1 system and the process of sexual differentiation.

The role of kisspeptin in the control of gonadotrophin secretion

BACKGROUND

Kisspeptins, and their cognate receptor gpr-54, were first found to regulate the hypothalamic-pituitary-gonadal (HPG) axis in 2003, when two groups demonstrated that mutations in gpr-54 cause idiopathic hypogonadotropic hypogonadism characterized by delayed or absent puberty. This review aims to highlight discoveries in the KiSS-1/gpr-54 system, focusing on their regulation of the HPG axis in male and female reproductive systems of both mammalian and non-mammalian vertebrates.

METHODS

A search of PUBMED and the authors' files was done without limitations by language or species for citations relevant to kisspeptin, reproduction and signal transduction.

RESULTS

Kisspeptins and gpr-54 are critical for puberty and the regulation of reproduction. Kisspeptins have been implicated in mediating many of the important signals relayed to the gonadotrophin-releasing hormone (GnRH) neuron such as positive and negative feedback, metabolic input and photoperiod. The ability of kisspeptin neurons to co-ordinate different signals impinging on the HPG axis makes it one of the most important regulators of GnRH and the reproductive axis.

CONCLUSIONS

Kisspeptins are pivotal regulators of the HPG axis and reproduction, with the ability to integrate signals from both internal and external sources. Knowledge about the signalling mechanisms involved in kisspeptin stimulation of GnRH would help improve the understanding of the importance of this critical pathway in reproduction.

Changes in prodynorphin gene expression and neuronal morphology in the hypothalamus of postmenopausal women

Human menopause is characterised by ovarian failure, gonadotrophin hypersecretion and hypertrophy of neurones expressing neurokinin B (NKB), kisspeptin (KiSS)-1 and oestrogen receptor (ER) alpha gene transcripts within the hypothalamic infundibular (arcuate) nucleus. In the arcuate nucleus of experimental animals, dynorphin, an opioid peptide, is colocalised with NKB, kisspeptin, ER alpha and progesterone receptors. Moreover, ovariectomy decreases the expression of prodynorphin gene transcripts in the arcuate nucleus of the ewe. Therefore, we hypothesised that the hypertrophied neurones in the infundibular nucleus of postmenopausal women would express prodynorphin mRNA and that menopause would be accompanied by changes in prodynorphin gene transcripts. In the present study, in situ hybridisation was performed on hypothalamic sections from premenopausal and postmenopausal women using a radiolabelled cDNA probe targeted to prodynorphin mRNA. Autoradiography and computer-assisted microscopy were used to map and count labelled neurones, measure neurone size and compare prodynorphin gene expression between premenopausal and postmenopausal groups. Neurones expressing dynorphin mRNA in the infundibular nucleus of the postmenopausal women were larger and exhibited hypertrophied morphological features. Moreover, there were fewer neurones labelled with the prodynorphin probe in the infundibular nucleus of the postmenopausal group compared to the premenopausal group. The number of dynorphin mRNA-expressing neurones was also reduced in the medial preoptic/anterior hypothalamic area of postmenopausal women without changes in cell size. No differences in cell number or size of dynorphin mRNA-expressing neurones were observed in any other hypothalamic region. Previous studies using animal models provide strong evidence that the changes in prodynorphin neuronal size and gene expression in postmenopausal women are secondary to the ovarian failure of menopause. Given the inhibitory effect of dynorphin on the reproductive axis, decreased dynorphin gene expression could play a role in the elevation in luteinising hormone secretion that occurs in postmenopausal women.

Kisspeptin and KISS1R: a critical pathway in the reproductive system

In 2003, three groups around the world simultaneously discovered that KISS1R (GPR54) is a key gatekeeper of sexual maturation in both mice and men. Developmental changes in the expression of the ligand for KISS1R, kisspeptin, support its critical role in the pubertal transition. In addition, kisspeptin, a powerful stimulus of GNRH-induced gonadotropin secretion and may modulate both positive and negative sex steroid feedback effects at the hypothalamic level. Genetic studies in humans have revealed both loss-of-function and gainof-function mutations in patients with idiopathic hypogonadotropic hypogonadism and precocious puberty respectively. This review examines the kisspeptin/KISS1R pathway in the reproductive system.

Structural interactions between kisspeptin and GnRH neurons in the mediobasal hypothalamus of the male rhesus monkey (Macaca mulatta) as revealed by double immunofluorescence and confocal microscopy

Kisspeptin is recognized to play a critical role in eliciting the pubertal resurgence of pulsatile GnRH release, the proximal trigger of puberty in higher primates. Expression of the kisspeptin receptor (GPR54) by GnRH neurons indicates a direct action of kisspeptin on the GnRH neuronal network. The purpose of the present study was to examine the distribution of kisspeptin cell bodies in the monkey hypothalamus and to assess the structural basis for the stimulatory action of kisspeptin on the GnRH neuronal network. Three castrated male rhesus monkeys, 39-51 months of age, were deeply anesthetized and their brains perfused transcardially with 4% paraformaldehyde in PBS. Serial 25-microm coronal sections throughout the hypothalamus were prepared, and immunopositive neurons identified using a cocktail of specific primary antibodies (sheep anti-kisspeptin at 1:120,000, and rabbit anti-GnRH at 1:100,000) detected with fluorescently tagged secondary antibodies (antisheep, Alexa Fluor 488; antirabbit, Cy3) in combination with confocal microscopy. Kisspeptin perikarya were found only in the mediobasal hypothalamus (MBH) almost exclusively in the posterior two-thirds of the arcuate nucleus. Surprisingly, kisspeptin-beaded axons made only infrequent contacts with GnRH neurons (kisspeptin and GnRH profiles abutting in a 0.5- to 1.0-mum optical section) in the MBH. In the median eminence, kisspeptin and GnRH axons were found in extensive and intimate association. GnRH contacts on kisspeptin perikarya and dendrites were observed. These findings indicate that nonsynaptic pathways of communication in the median eminence should be considered as a possible mechanism of kisspeptin regulation of GnRH release, and provide an anatomical basis for reciprocal control of kisspeptin neuronal activity by GnRH.

Kisspeptin immunoreactive neurons in the equine hypothalamus Interactions with GnRH neuronal system

To determine if kisspeptin could be implicated in the control of reproduction in equine species, we studied the distribution of kisspeptin neurons and their anatomical interactions with GnRH neurons in the hypothalamus of pony mares. Brains were collected in three pony mares between 2 and 4h after ovulation. One major population of kisspeptin immunoreactive cell bodies was found in the arcuate nucleus (ARC), where they extended from the middle of the nucleus to the premammillary recess. Kisspeptin immunoreactive varicose fibers extended from the preoptic area to the mammillary nuclei, with important densities especially in the anterior periventricular area and the median eminence (ME). Rare close appositions of kisspeptin fibres on GnRH cell bodies were observed in the ARC. Close appositions between kisspeptin and GnRH fibres were also confirmed at a low incidence in the anterior basal periventricular area and at a high incidence in the ME. This work provides neuroanatomical bases for further investigations into the role of kisspeptin in equine reproduction.

[18F]2-fluoro-2-deoxy-D-glucose positron emission tomography demonstration of estrogen negative and positive feedback on luteinizing hormone secretion in women

CONTEXT

Precise regulation of the neuroendocrine components of the female reproductive axis involves both negative and positive feedback of estrogen on gonadotropin secretion.

OBJECTIVE

Our objective was to determine the hypothalamic and/or pituitary sites of estrogen negative and positive feedback using neuroimaging techniques.

DESIGN AND SETTING

A graded estrogen infusion protocol was administered at a General Clinical Research Center in an academic medical center.

SUBJECTS

Healthy postmenopausal women (n = 11) were recruited for study.

INTERVENTIONS

Serum samples were measured every 4 h. A structural magnetic resonance imaging was performed at baseline, and [(18)F]2-fluoro-2-deoxy-d-glucose ((18)FDG) positron emission tomography was performed at baseline and 24 and 72 h. FDG positron emission tomography was co-registered with magnetic resonance imaging scans, and region of interest analysis was performed.

MAIN OUTCOME MEASURES

Serum LH and estradiol were assessed. Normalized values for glucose uptake were extracted from each region of interest for each subject at each time point.

RESULTS

A decrease in normalized (18)FDG uptake was apparent in the hypothalamus at 24 h (P < 0.02) associated with decreased LH (P < 0.0005). The increase in LH at 72 h (P < 0.0005) was associated with increased pituitary (18)FDG uptake (P < 0.02) but no change in hypothalamic uptake.

CONCLUSIONS

Changes in (18)FDG uptake as a measure of metabolic activity can be demonstrated in the hypothalamus and pituitary in association with discrete hormonal events. Results are consistent with mediation of estrogen negative feedback on LH at the hypothalamus, whereas estrogen positive feedback occurs at the pituitary with no evidence of increased hypothalamic activity in women.

Kisspeptin: a novel regulator of reproductive function

The UK and international neuroendocrine community was deeply shocked and saddened the unbelievably premature death of Michael Harbuz in Bristol in 2006. Mick was a superb friend and colleague, and played a huge part in the development and activities of the British Neuroendocrine Group/British Society for Neuroendocrinology (BSN), serving as both Membership Secretary and Treasurer between 1999 and 2004. Mick was a leader in the field of neuroendocrine-immune interactions, and brought a great deal of charisma, humour and ability to meetings and conferences. He was also a passionate and committed supporter of the progress of young researchers and of their participation in neuroendocrine events. He recognised that today's postgraduate students and postdoctoral research fellows are tomorrow's neuroendocrine researchers, be it in academia, the health services or industry. To recognise Mick's great commitment to and enthusiasm for postgraduate education both in the University of Bristol and in the BSN, we decided to honour and remember him by instituting the 'Michael Harbuz Young Investigator Prize Lecture' to be delivered annually. Dr Waljit Dhillo from Imperial College London was the inaugural recipient of this award, and presented his lecture at the Annual Meeting of the BSN in Nottingham in September 2007, upon which this review is based. Recent evidence demonstrates that the neuropeptide kisspeptin and its receptor, GPR54, have a fundamental role in initiating the onset of puberty and are important in regulating reproductive function. This review discusses the evidence available from animals and humans demonstrating that kisspeptin potently stimulates the release of gonadotrophins by stimulating the release of gonadotrophin-releasing hormone and that a lack of kisspeptin or GPR54 results in reproductive failure.

The kisspeptin (KiSS-1)/GPR54 system in cancer biology

Kisspeptin (KiSS-1) gene, initially described as a melanoma metastasis suppressor gene, encodes a number of peptides (kp-54, kp-14, kp-13, kp-10), which are endogenous ligands to a G protein-coupled receptor, referred as hOT7T175 or AXOR12 or GPR54. So far intensive investigation has provided substantiate evidence supporting the role of KiSS-1/GPR54 system in cancer biology as well as in the regulation of the reproductive function and trophoblast invasion. The precise mechanism by which KiSS-1/GPR54 system is affecting cancer cell growth and metastasis includes complex endocrine, paracrine and autocrine actions. Nevertheless, the detail mechanism of such actions is still under intensive investigation. Herein we review the evidence which support the role of KiSS-1/GPR54 system in cancer biology.

Hypothalamic control of the pituitary-gonadal axis in higher primates: key advances over the last two decades

This review provides a brief historical background to the foundation of primate reproductive neuroendocrinology that was laid by Ernst Knobil during the late 1960s and early 1970s. This is followed by a discussion of studies conducted over the last two decades that I view as having contributed to the current understanding of the field of primate reproductive neuroendocrinology. The review concludes with a short summary of key questions that remain to be addressed.

The role of kisspeptins and GPR54 in the neuroendocrine regulation of reproduction

Neurons that produce gonadotropin-releasing hormone (GnRH) reside in the basal forebrain and drive reproductive function in mammals. Understanding the circuitry that regulates GnRH neurons is fundamental to comprehending the neuroendocrine control of puberty and reproduction in the adult. This review focuses on a family of neuropeptides encoded by the Kiss1 gene, the kisspeptins, and their cognate receptor, GPR54, which have been implicated in the regulation of GnRH secretion. Kisspeptins are potent secretagogues for GnRH, and the Kiss1 gene is a target for regulation by gonadal steroids (e.g., estradiol and testosterone), metabolic factors (e.g., leptin), photoperiod, and season. Kiss1 neurons in the arcuate nucleus may regulate the negative feedback effect of gonadal steroids on GnRH and gonadotropin secretion in both sexes. The expression of Kiss1 in the anteroventral periventricular nucleus (AVPV) is sexually dimorphic, and Kiss1 neurons in the AVPV may participate in the generation of the preovulatory GnRH/luteinizing hormone (LH) surge in the female rodent. Kiss1 neurons have emerged as primary transducers of internal and environmental cues to regulate the neuroendocrine reproductive axis.

GPR54 and rGnRH I gene expression during the onset of puberty in Nile tilapia

The Kiss1/GPR54 system has recently been shown to play a key role in the onset of puberty in mammals. Growing evidence suggests that this system is also conserved across vertebrates although very few studies so far have been performed in lower vertebrates. The aims of this study were firstly in the teleost Nile tilapia to screen tissues for GPR54 expression levels, secondly to measure the expression patterns of GPR54 and GnRH I receptor (rGnRH I) in whole brains during the onset of puberty and finally to determine the effects of continuous illumination (LL) on receptor expression levels. Results confirmed that GPR54 was predominantly expressed in the brain and pituitary of adult tilapia. Furthermore, a significant increase of GPR54 gene expression was found in tilapia brains at 11 weeks post hatch (wph) followed by rGnRH I at 13 wph just prior to the histological observation of vitellogenic oocytes and active spermatogenesis in ova and testes at 17 wph. These results suggest a correlation between the increase of GPR54 expression in the brain and the onset of puberty. Finally, a significant effect of LL was observed on GPR54 expression levels which were characterized by a delayed surge with significantly lower levels than those of control fish. The current study not only suggests a link between the Kiss1/GPR54 system and the onset of puberty in a tropical batch spawning teleost that would be a highly conserved feature across vertebrates but also that the transcriptional mechanisms regulating GPR54 expression could be directly or indirectly influenced by light.

Opposite roles of estrogen receptor (ER)-alpha and ERbeta in the modulation of luteinizing hormone responses to kisspeptin in the female rat: implications for the generation of the preovulatory surge

Ovulation is triggered by the preovulatory rise of gonadotropins, which is in turn elicited by the preceding increase in circulating estrogen. Kisspeptins, ligands of G protein-coupled receptor 54 encoded by the KiSS-1 gene, have emerged as potent stimulators of GnRH/LH secretion, and KiSS-1 neurons at the anteroventral periventricular nucleus have been shown to be involved in the generation of preovulatory LH surge, estrogen being a potent elicitor of KiSS-1 gene expression selectively at the anteroventral periventricular nucleus. Whether, in addition to transcriptional effects, estrogen influences other aspects of kisspeptin-induced GnRH/LH release in the female remains unexplored. We provide herein evidence for the specific roles of estrogen receptor (ER)-alpha and ERbeta in the modulation of LH responses to kisspeptin and the generation of the preovulatory surge. Selective blockade of ERalpha in cyclic females blunted LH responses to kisspeptin, eliminated the endogenous preovulatory rise of LH, and blocked ovulation. In contrast, antagonism of ERbeta failed to cause major changes in terms of LH surge and ovulatory rate but significantly augmented acute LH responses to kisspeptin. Notably, defective LH secretion and ovulation after ERalpha blockade were not observed after GnRH stimulation, which elicited maximal acute (<2 h) LH responses regardless of ERalpha/ERbeta signaling. In addition, net LH secretion in response to kisspeptin was decreased by ovariectomy and increased after selective activation of ERalpha but not ERbeta. Altogether, our data document the prominent positive role of ERalpha in the regulation of GnRH/LH responsiveness to kisspeptin and, thereby, ovulation. In addition, our results disclose the putative function of ERbeta as negative modifier of GnRH/LH response to kisspeptin, a phenomenon that might contribute to partially restraining LH secretion at certain physiological states.

New frontiers in kisspeptin/GPR54 physiology as fundamental gatekeepers of reproductive function

Identification, in late 2003, of inactivating mutations of the G protein-coupled receptor GPR54 as causative factor for absence of puberty and hypogonadotropic hypogonadism in humans and mice was a major breakthrough in modern Neuroendocrinology, and drew considerable interest on the characterization of the roles of this receptor and its ligands (kisspeptins, encoded by the KiSS-1 gene) in the physiological control of essential facets of reproduction. After 3 years of intense research activity, kisspeptins are universally recognized as essential activators of the gonadotropic axis, with key roles in puberty onset and the control of gonadotropin secretion. While these fundamental functions are now well settled, novel aspects of kisspeptin/GPR54 physiology have emerged, including their involvement in the neuroendocrine control of ovulation and the metabolic gating of reproductive function. In addition, the 'comparative endocrinology' of this system has begun to be explored recently. These facets of kisspeptin/GPR54 function, as fundamental gatekeepers of reproduction, will be comprehensively reviewed herein.

GPR54 and kisspeptins

The G-protein coupled receptor GPR54 has an essential role in the initiation and maintenance of mammalian fertility. Humans and mice with mutations in GPR54 have hypogonadotropic hypogonadism characterized by absence of sexual maturation and low levels of gonadotropic hormones (LH and FSH). The ligand for GPR54 is encoded by the KISS1 gene, which produces a 54-amino-acid peptide (metastin or kisspeptin-54) that can be cleaved into shorter peptides (kisspeptins 14, 13 and 10) with similar potencies. Kisspeptin administration stimulates gonadotropin release in several species by inducing GnRH secretion from hypothalamic GnRH neurons expressing GPR54. Kisspeptins are produced by neurons located in the AVPV and ARC regions of the hypothalamus. Expression of Kiss1 in these neurons is differentially regulated by sex steroids providing a mechanism by which testosterone or estrogen can regulate GnRH release. The AVPV region is sexually dimorphic with highest expression of kisspeptin in females. Positive feedback by estrogen on expression of Kiss1 in the AVPV region may be responsible for the pre-ovulatory LH surge during the estrus cycle. Central administration of kisspeptin to immature female rats can induce precocious activation of the gonadotropic axis, causing advanced vaginal opening, elevated uterus weight, increased serum levels of LH and estrogen and induce ovulation. Kisspeptins/GPR54 have also been implicated in regulating the estrus cycle of seasonal breeders and in the control of lactational amenorrhea. Expression of Gpr54 and Kiss1 have also been reported in several peripheral tissues including the pituitary, ovary, testes and the placenta raising the possibility that these genes may have additional functions in these tissues. Regulation of kisspeptin expression by peripheral factors such as leptin may be involved in coordinating metabolic status with the reproductive axis.

Kisspeptin neurons in the arcuate nucleus of the ewe express both dynorphin A and neurokinin B

Kisspeptin is a potent stimulator of GnRH secretion that has been implicated in the feedback actions of ovarian steroids. In ewes, the majority of hypothalamic kisspeptin neurons are found in the arcuate nucleus (ARC), with a smaller population located in the preoptic area. Most arcuate kisspeptin neurons express estrogen receptor-alpha, as do a set of arcuate neurons that contain both dynorphin and neurokinin B (NKB), suggesting that all three neuropeptides are colocalized in the same cells. In this study we tested this hypothesis using dual immunocytochemistry and also determined if kisspeptin neurons contain MSH or agouti-related peptide. To assess colocalization of kisspeptin and dynorphin, we used paraformaldehyde-fixed tissue from estrogen-treated ovariectomized ewes in the breeding season (n = 5). Almost all ARC, but no preoptic area, kisspeptin neurons contained dynorphin. Similarly, almost all ARC dynorphin neurons contained kisspeptin. In experiment 2 we examined colocalization of kisspeptin and NKB in picric-acid fixed tissue collected from ovary intact ewes (n = 9). Over three quarters of ARC kisspeptin neurons also expressed NKB, and a similar percentage of NKB neurons contained kisspeptin. In contrast, no kisspeptin neurons stained for MSH or agouti-related peptide. These data demonstrate that, in the ewe, a high percentage of ARC kisspeptin neurons also produce dynorphin and NKB, and we propose that a single subpopulation of ARC neurons contains all three neuropeptides. Because virtually all of these neurons express estrogen and progesterone re-ceptors, they are likely to relay the feedback effects of these steroids to GnRH neurons to regulate reproductive function.

Kisspeptin-54 stimulates gonadotropin release most potently during the preovulatory phase of the menstrual cycle in women

CONTEXT

Kisspeptin, the endogenous ligand of the G protein-coupled receptor 54, is a key regulator of the hypothalamo-pituitary-gonadal (HPG) axis. GPR54-null mice exhibit reproductive dysfunction, and exogenous kisspeptin potently stimulates the HPG axis in rodents, primates, and human males. The effects of kisspeptin administration to human females are unknown.

OBJECTIVE

Our objective was to investigate the effects of kisspeptin on LH release during the menstrual cycle in female volunteers.

DESIGN

Bolus sc kisspeptin-54 was administered to female volunteers, and plasma gonadotropins were measured.

SETTING

The study took place at a hospital clinical research facility.

VOLUNTEERS

Subjects were healthy female volunteers with regular menstrual cycles.

INTERVENTION

1) Volunteers received a sc bolus injection of kisspeptin-54 (0, 0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 nmol/kg; n = 3-4 per dose) in the follicular phase; and 2) volunteers (n = 8) received a sc bolus injection of either kisspeptin-54 (0.4 nmol/kg) or saline in random order during each phase of the menstrual cycle.

MAIN OUTCOME MEASURES

Plasma gonadotropins were measured.

RESULTS

1) Kisspeptin-54 caused a dose-dependent increase in mean LH over time at doses from 0.2-6.4 nmol/kg. 2) Kisspeptin-54 increased plasma LH compared with saline injection in all phases of the cycle. The effect of kisspeptin was greatest in the preovulatory phase and least in the follicular phase of the cycle [mean increase in LH over baseline (IU/liter) +/- sem for follicular phase was 0.12 +/- 0.17; preovulatory phase, 20.64 +/- 2.91 (P < 0.001 vs. follicular phase); luteal phase, 2.17 +/- 0.79 (P < 0.01 vs. follicular phase)].

CONCLUSION

Elevation of plasma kisspeptin in human females potently stimulates LH release in the preovulatory phase and provides a novel mechanism for manipulation of the HPG axis in women.

Emerging ideas about kisspeptin- GPR54 signaling in the neuroendocrine regulation of reproduction

Neurons that produce gonadotropin-releasing hormone (GnRH) drive the reproductive axis, but the molecular and cellular mechanisms by which hormonal and environmental signals regulate GnRH secretion remain poorly understood. Kisspeptins are products of the Kiss1 gene, and the interaction of kisspeptin and its receptor GPR54 plays a crucial role in governing the onset of puberty and adult reproductive function. This review discusses the latest ideas about kisspeptin-GPR54 signaling in the neuroendocrine regulation of reproduction, with special emphasis on the role of Kiss1 and kisspeptin in the negative and positive feedback control of gonadotropin secretion by sex steroids, timing of puberty onset, sexual differentiation of the brain and photoperiodic regulation of seasonal reproduction.