Kisspeptin immunoreactive cells of the ovine preoptic area and arcuate nucleus co-express estrogen receptor alpha

Kisspeptin can stimulate gonadotropin-releasing hormone (GnRH) release by a direct action at GnRH nerve terminals

The G protein-coupled receptor GPR54, and its peptide ligand kisspeptin (Kp), are crucial for the induction and maintenance of mammalian reproductive function. GPR54 is expressed by GnRH neurons and is directly activated by Kp to stimulate GnRH release. We hypothesized that Kp may be able to act at the GnRH nerve terminals located in the mediobasal hypothalamus (MBH) region. To test this hypothesis, we used organotypic culture of MBH explants challenged with Kp, followed by RIA to detect GnRH released into the cultured medium. Kp stimulation for 1 h induced GnRH release from wild-type male MBH in a dose-dependent manner, whereas this did not occur in MBH explants isolated from Gpr54 null mice. Continuous Kp stimulation caused a sustained GnRH release for 4 h, followed by a decrease of GnRH release, suggesting a desensitization of GPR54 activity. Tetrodotoxin did not alter the Kp-induced GnRH release, indicating that Kp can act directly at the GnRH nerve terminals. To localize Gpr54 expression within the MBH, we used transgenic mice, in which Gpr54 expression is tagged with an IRES-LacZ reporter gene and can be visualized by beta-galactosidase staining. Gpr54 expression was detected outside of the median eminence, in the pars tuberalis. In conclusion, our results provide evidence for a potent stimulating effect of Kp at GnRH nerve terminals in the MBH of the mouse. This study suggests a new point at which Kp can act on GnRH neurons.

Photoperiodic signalling through the melatonin receptor turns full circle

Photoperiod exerts profound influence on the physiology of mammals through the action of melatonin on the neuroendocrine system. Over the last 20 years, studies have moved away from a melatonin receptor-focused approach to understanding how photoperiod regulates neuroendocrine activity through studies of downstream effects on gene expression. This paper reviews the recent progress made in our understanding of the effects of photoperiod on gene expression in the hypothalamus, and considers how this new information can be reconciled with the species-specific location of melatonin receptors.

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.

Absence of gonadotropin-releasing hormone 1 and Kiss1 activation in alpha-fetoprotein knockout mice: prenatal estrogens defeminize the potential to show preovulatory luteinizing hormone surges

Sex differences in gonadal function are driven by either cyclical (females) or tonic (males) hypothalamic GnRH1 release and, subsequently, gonadotrophin (LH and FSH) secretion from the pituitary. This sex difference seems to depend on the perinatal actions of gonadal hormones on the hypothalamus. We used alpha-fetoprotein (AFP) knockout mice (Afp(-/-)) to study the mechanisms by which estrogens affect the sexual differentiation of the GnRH1 system. Afp(-/-) mice lack the protective actions of AFP against estrogens circulating during embryonic development, leading to infertility probably due to a hypothalamic dysfunction. Therefore, we first determined whether Afp(-/-) females are capable of showing a steroid-induced preovulatory LH surge by FOS/GnRH1 immunohistochemistry and RIA of plasma LH levels. Because the KISS1/GPR54 system is a key upstream regulator of the GnRH1 system as well as being sexually dimorphic, we also analyzed whether Kisspeptin-10 neurons were activated in Afp(-/-) mice after treatment with estradiol and progesterone. We found that the GnRH1 and Kisspeptin-10 neuronal systems are defeminized in Afp(-/-) females because they did not show either steroid-induced LH surges or significant FOS/GnRH1 double labeling. Furthermore, Kisspeptin-10 immunoreactivity and neural activation, measured by the number of double-labeled FOS/Kisspeptin-10 cells, were lower in Afp(-/-) females, suggesting a down-regulation of GnRH1 function. Thus, the sex difference in the ability to show preovulatory LH surges depends on the prenatal actions of estrogens in the male hypothalamus and, thus, is lost in Afp(-/-) females because they lack AFP to protect them against the defeminizing effects of estrogens during prenatal development.

The effect of testosterone and season on prodynorphin messenger RNA expression in the preoptic area-hypothalamus of the ram

Testosterone and season influence mRNA expression for the opioid, enkephalin, in the preoptic area and hypothalamus of rams. Dynorphin is another opioid which has been shown to play a role in the control of reproductive function in females. We now report effects of season and testosterone on the expression of prodynorphin mRNA in the hypothalamus of the ram. Castrated adult Romney Marsh rams (5/group) received vehicle or testosterone propionate (i.m.) during either the 'breeding' season or 'non-breeding' season. Prodynorphin mRNA expression was quantified in the hypothalami by in situ hybridisation. Testosterone treatment increased prodynorphin mRNA expression in the supraoptic nucleus and the bed nucleus of the stria terminalis in the breeding season but not during the non-breeding season. Prodynorphin mRNA expression was also higher in the breeding season than in the non-breeding season in the caudal preoptic area, paraventricular nucleus and accessory supraoptic nucleus, irrespective of treatment. No effects of treatment were observed in any other regions of the hypothalamus. We conclude that testosterone and season regulate prodynorphin mRNA expression in a region-specific manner, which may influence seasonal changes in reproductive function.

Identification of KiSS-1 product kisspeptin and steroid-sensitive sexually dimorphic kisspeptin neurons in medaka (oryzias latipes)

Recently, a novel physiologically active peptide, kisspeptin (metastin), has been reported to facilitate sexual maturation and ovulation by directly stimulating GnRH neurons in several mammalian species. Despite its importance in the neuroendocrine regulation of reproduction, kisspeptin neurons have only been studied in mammals, and there has been no report on the kisspeptin or kisspeptin neuronal systems in nonmammalian vertebrates. We used medaka for the initial identification of the KiSS-1 gene and the anatomical distribution of KiSS-1 mRNA expressing neurons (KiSS-1 neurons) in the brain of nonmammalian species. In situ hybridization for the medaka KiSS-1 gene cloned here proved that two kisspeptin neuronal populations are localized in the hypothalamic nuclei, the nucleus posterioris periventricularis and the nucleus ventral tuberis (NVT). Furthermore, NVT KiSS-1 neurons were sexually dimorphic in number (male neurons >> female neurons) under the breeding conditions. We also found that the number of KiSS-1 neurons in the NVT but not that in the nucleus posterioris periventricularis was positively regulated by ovarian estrogens. The fact that there were clear differences in the number of NVT KiSS-1 neurons between the fish under the breeding and nonbreeding conditions strongly suggests that the steroid-sensitive changes in the KiSS-1 mRNA expression in the NVT occur physiologically, according to the changes in the reproductive state. From the present results, we conclude that the medaka KiSS-1 neuronal system is involved in the central regulation of reproductive functions, and, given many experimental advantages, the medaka brain may serve as a good model system to study its physiology.

Kisspeptin is present in ovine hypophysial portal blood but does not increase during the preovulatory luteinizing hormone surge: evidence that gonadotropes are not direct targets of kisspeptin in vivo

There is strong evidence that kisspeptin acts to regulate GnRH secretion, but whether there is also a component of action on the gonadotropes is not clear. Using quantitative RT-PCR, we found that G protein-coupled receptor-54 mRNA is expressed in ovine pituitary cell fractions enriched for gonadotropes as well as in somatotropes and lactotropes. To test whether kisspeptin acts directly on the pituitary gonadotropes, we first examined LH release from primary ovine pituitary cell cultures treated with kisspeptin. We found that kisspeptin treatment increased the concentration of LH in culture media by 80%, compared with control, but only in pituitary cultures from ewes during the follicular phase of the estrous cycle. After this, we determined whether kisspeptin acts on the pituitary gland in vivo. Using GnRH-replaced ovariectomized hypothalamo-pituitary-disconnected ewes, we were not able to achieve any effect of kisspeptin on LH under steady-state conditions or during the period of an estrogen-induced LH surge. Finally, we collected hypophysial portal blood samples from ovariectomized ewes and measured kisspeptin levels. Low but detectable amounts of kisspeptin were found in portal plasma, but levels were similar in ovariectomized ewes that were untreated or given estrogen to elicit an LH surge. Thus, although we observed an effect of kisspeptin on LH release in vitro in some situations, similar findings were not obtained in vivo. Moreover, the low concentrations of kisspeptin in hypophysial portal blood and the lack of any change during the period of an estrogen-induced GnRH/LH surge suggest that action on the pituitary gland is not of major consequence in terms of LH release.

Kisspeptin signalling in the brain: Steroid regulation in the rodent and ewe

The Kiss1 gene encodes a family of peptides called kisspeptins, which are the natural ligands for the receptor GPR54. In humans and mice, inactivating mutations of GPR54 results in hypogonadotropic hypogonadism, indicating that kisspeptins play a vital role in the regulation of GnRH secretion. In many species, centrally administered kisspeptins stimulate gonadotrophin secretion in a GnRH-dependant manner. Moreover, virtually all GnRH neurons coexpress GPR54. In the hypothalamus, the vast majority of kisspeptin producing cells also express sex steroid receptors, particularly estrogen receptor alpha. Thus, sex steroids are able to directly regulate the expression of Kiss1 mRNA, implicating kisspeptins as the 'missing link' between sex steroid feedback and GnRH secretion. Kiss1-expressing cells are localised to various regions of the forebrain in rodents, primates and sheep. In the arcuate nucleus (ARC) of the rodent and the ewe, sex steroids inhibit the expression of Kiss1 mRNA, suggesting that the kisspeptin secreting neurons here are the conduit for the negative feedback regulation of GnRH secretion. However, in the rodent anteroventral periventricular nucleus (AVPV), sex steroids induce the expression of Kiss1, implying that these kisspeptin neurons play a role in the positive feedback regulation of GnRH secretion. In sheep, there are no Kiss1 neurons in the AVPV and Kiss1 mRNA expression in the ARC is stimulated immediately prior to the preovulatory GnRH/luteinising hormone surge. Thus, kisspeptin neurons in the ARC of the ewe appear well placed to play a role in the negative and positive feedback regulation of GnRH exerted by sex steroids.

Pituitary and/or peripheral estrogen-receptor alpha regulates follicle-stimulating hormone secretion, whereas central estrogenic pathways direct growth hormone and prolactin secretion in postmenopausal women

BACKGROUND

Estradiol (E(2)) stimulates GH and prolactin secretion and suppresses FSH secretion in postmenopausal women. Whether central nervous system (CNS) or pituitary mechanisms (or both) mediate such actions is not known.

OBJECTIVE

Our objective was to distinguish between hypothalamic and pituitary or peripheral (hepatic) actions of E2.

SETTING

This study was performed in an academic medical center.

DESIGN

This was a double-blind, prospectively randomized, placebo (Pl)-controlled study.

METHODS

The capability of a selective, noncompetitive, non-CNS permeant estrogen receptor (ER)-alpha antagonist, fulvestrant (FUL) to antagonize the effects of transdermal E2 and Pl on GH, prolactin, and FSH secretion was assessed in 43 women (ages 50-80 yr) in a four parallel-cohort study. Each woman received four secretagogue infusions to stimulate GH secretion. IGF-I and its binding proteins were measured secondarily.

RESULTS

Administration of Pl/E2 increased GH and prolactin concentrations by 100%, and suppressed FSH concentrations by more than 50% (each P<or=0.004 compared with Pl/Pl). Treatment with FUL/E2 compared with Pl/E2 partially relieved estrogen's inhibition of FSH secretion (P=0.041), without altering E2's stimulation of prolactin secretion. ANOVA further revealed that: 1) estrogen milieu (P=0.014) and secretagogue type (P<0.001) each determined GH concentrations; 2) FUL/Pl suppressed IGF-I concentrations (P<0.001); 3) FUL abrogated estrogen's elevation of IGF binding protein-1 concentrations (P<0.001); and 4) FUL did not oppose estrogen's suppression of IGF binding protein-3 concentrations (P<0.001).

SUMMARY AND CONCLUSIONS

Responses to a non-CNS permeant ERalpha antagonist indicate that E2 inhibits FSH secretion in part via pituitary/peripheral ERalpha, drives prolactin output via nonpituitary/nonperipheral-ERalpha effects, and directs GH secretion and IGF-I-binding proteins by complex mechanisms.

Effects of kisspeptin-10 on the electrophysiological manifestation of gonadotropin-releasing hormone pulse generator activity in the female rat

Kisspeptins are extraordinarily potent in stimulating gonadotropic hormone secretion via an action on the hypothalamic GnRH neural system. Because the physiological frequency of the GnRH pulse generator is a critical component of the control system that governs reproductive processes, the aim of this study was to examine the effect of kisspeptin-10 on pulsatile LH secretion and on the electrophysiological manifestation of GnRH pulse generator activity to determine frequency modulatory effects. Adult Sprague Dawley rats were ovariectomized and chronically implanted with electrodes in the arcuate nucleus to record the characteristic increases in hypothalamic multiunit electrical activity volleys coincident with the initiation of each LH pulse measured in peripheral blood and/or indwelling cardiac catheters for the collection of blood samples (25 microl) every 5 min for 6-7 h for the measurement of LH. Intravenous infusion of kisspeptin-10 (7.5, 35, and 100 nmol) induced a dose-dependent increase in LH secretion. The stimulatory effect of kisspeptin-10 (100 nmol) on LH secretion was blocked by the GnRH antagonist cetrorelix, precluding a singular action on gonadotropes. Unexpectedly, however, the marked increase in LH release in response to kisspeptin-10 (100 nmol) administration was not accompanied by any change in multiunit electrical activity volley frequency. It seem unlikely, therefore, that kisspeptin-10 has an appreciable frequency modulatory effect on GnRH pulse generator activity in the female rat.

RFamide-related peptide gene is a melatonin-driven photoperiodic gene

In seasonal species, various physiological processes including reproduction are organized by photoperiod via melatonin, but the mechanisms of melatonin action are still unknown. In birds, the peptide gonadotropin-inhibiting hormone (GnIH) has been shown to have inhibitory effects on reproductive activity and displays seasonal changes of expression. Here we present evidence in mammals that the gene orthologous to GnIH, the RFamide-related peptide (RFRP) gene, expressed in the mediobasal hypothalamus, is strongly regulated by the length of the photoperiod, via melatonin. The level of RFRP mRNA and the number of RFRP-immunoreactive cell bodies were reduced in sexually quiescent Syrian and Siberian hamsters acclimated to short-day photoperiod (SD) compared with sexually active animals maintained under long-day photoperiod (LD). This was contrasted in the laboratory Wistar rat, a non-photoperiodic breeder, in which no evidence for RFRP photoperiodic modulation was seen. In Syrian hamsters, the reduction of RFRP expression in SD was independent from secondary changes in gonadal steroids. By contrast, the photoperiodic variation of RFRP expression was abolished in pinealectomized hamsters, and injections of LD hamsters with melatonin for 60 d provoked inhibition of RFRP expression down to SD levels, indicating that the regulation is dependent on melatonin. Altogether, these results demonstrate that in these hamster species, the RFRP neurons are photoperiodically modulated via a melatonin-dependent process. These observations raise questions on the role of RFRP as a general inhibitor of reproduction and evoke new perspectives for understanding how melatonin controls seasonal processes via hypothalamic targets.

KiSS-1 and GPR54 genes are co-expressed in rat gonadotrophs and differentially regulated in vivo by oestradiol and gonadotrophin-releasing hormone

Kisspeptin, the product derived from KiSS-1, and its cognate receptor, GPR54, both exert a role in the neuroendocrine control of reproduction by regulating gonadotrophin-releasing hormone (GnRH) secretion. In the present study, we demonstrate, using dual immunofluorescence with specific antibodies, that the KiSS-1 and GPR54 genes are both expressed in rat gonadotrophs. All luteinising hormone beta-immunoreactive (LH beta-ir) cells were stained by the KiSS-1 antibody but some kisspeptin-ir cells were not LH beta positive; thus, we cannot exclude the possibility that kisspeptins are expressed in other pituitary cells. All GPR54-ir are co-localised with LH beta cells, but only a subset of LH beta cells are stained with the GPR54 antibody. Using the real-time reverse transcription-polymerase chain reaction (RT-PCR), we found that the expression of KiSS-1 and GPR54 is differentially regulated by steroids. In the female, KiSS-1 mRNA levels dramatically decreased following ovariectomy (OVX), and this decrease was prevented by administration of 17beta-oestradiol (E(2)), but not by administration of GnRH antagonist or agonist. Administration of E(2) in OVX rats receiving either GnRH antagonist or agonist clearly shows that E(2) acts directly on the pituitary to positively control KiSS-1 expression. In OVX rats, administration of the selective oestrogen receptor (ER)alpha ligand propylpyrazoletriol, but not the selective ER beta ligand diarylpropionitrile, mimics this effect. By contrast, our study shows that GPR54 expression is positively regulated by GnRH and negatively controlled by chronic exposure to E(2). In summary, our data document for the first time that, in the female rat pituitary, KiSS-1 expression is up-regulated by oestradiol, similarly to that seen in the anteroventral periventricular nucleus of the hypothalamus. Conversely, GPR54 is up-regulated by GnRH, which exclusively targets gonadotrophs.

Neural connectivity in the mediobasal hypothalamus of the sheep brain

The ventromedial nucleus of the hypothalamus (VMN) and the arcuate nucleus (ARC) are two centres regulating energy balance and food intake, but inter-connectivity of these nuclei is not well defined in non-rodent species. In this study, we performed retrograde tracing and immunohistochemistry in the ovine brain with ewes receiving FluoroGold (FG) injections into either ARC or VMN for the mapping of retrogradely labelled cells. Strong reciprocal connections were found between the two regions. The distribution of the FG labelled neurons in other regions of the hypothalamus and brain stem was also mapped. Some of the cells projecting from ARC to VMN were immunopositive for neuropeptide Y, galanin, adrenocorticotropin (marker of pro-opiomelanocortin cells) or tyrosine hydroxylase (marker of dopaminergic cells). Melanin-concentrating hormone and orexin neurons in the lateral hypothalamic area were also found to provide input to the VMN and ARC. This observed interconnectivity between regions important for metabolic regulation and other neuroendocrine functions presumably allows coordinated functions. Input to both the ARC and VMN from other brain regions, such as brain stem cell groups, provides a further level of regulation. These data provide a substrate upon which further understanding of appetite regulation and neuroendocrine function can be derived in this species.

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.

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.

Kiss1-/- mice exhibit more variable hypogonadism than Gpr54-/- mice

The G protein-coupled receptor Gpr54 and its ligand metastin (derived from the Kiss1 gene product kisspeptin) are key gatekeepers of sexual maturation. Gpr54 knockout mice demonstrate hypogonadotropic hypogonadism, but until recently, the phenotype of Kiss1 knockout mice was unknown. This report describes the reproductive phenotypes of mice carrying targeted deletions of Kiss1 or Gpr54 on the same genetic background. Both Kiss1 and Gpr54 knockout mice are viable but infertile and have abnormal sexual maturation; the majority of males lack preputial separation, and females have delayed vaginal opening and absence of estrous cycling. Kiss1 and Gpr54 knockout males have significantly smaller testes compared with controls. Gpr54 knockout females have smaller ovaries and uteri than wild-type females. However, Kiss1 knockout females demonstrate two distinct phenotypes: half have markedly reduced gonadal weights similar to those of Gpr54 knockout mice, whereas half exhibit persistent vaginal cornification and have gonadal weights comparable with those of wild-type females. FSH levels in both Kiss1 and Gpr54 knockout males and females are significantly lower than in controls. When injected with mouse metastin 43-52, a Gpr54 agonist, Gpr54 knockout mice fail to increase gonadotropins, whereas Kiss1 knockout mice respond with increased gonadotropin levels. In summary, both Kiss1 and Gpr54 knockout mice have abnormal sexual maturation consistent with hypogonadotropic hypogonadism, although Kiss1 knockout mice appear to be less severely affected than their receptor counterparts. Kiss1 knockout females demonstrate a bimodal phenotypic variability, with some animals having higher gonadal weight, larger vaginal opening, and persistent vaginal cornification.

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

CONTEXT

Human menopause is characterized by ovarian failure, gonadotropin hypersecretion, and neuronal hypertrophy in the hypothalamic infundibular (arcuate) nucleus. Recent studies have demonstrated a critical role for kisspeptins in reproductive regulation, but it is not known whether menopause is accompanied by changes in hypothalamic kisspeptin neurons.

OBJECTIVES

Our objective was to map the location of neurons expressing kisspeptin gene (KiSS-1) transcripts in the human hypothalamus and determine whether menopause is associated with changes in the size and gene expression of kisspeptin neurons. In monkeys, our objective was to evaluate the effects of ovariectomy and hormone replacement on neurons expressing KiSS-1 mRNA in the infundibular nucleus.

SUBJECTS

Hypothalamic tissues were collected at autopsy from eight premenopausal and nine postmenopausal women and from 42 young cynomolgus monkeys in various endocrine states.

METHODS

We used hybridization histochemistry, quantitative autoradiography, and computer-assisted microscopy.

RESULTS

Examination of human hypothalamic sections revealed that KiSS-1 neurons were located predominantly in the infundibular nucleus. In the infundibular nucleus of postmenopausal women, there was a significant increase in the size of neurons expressing KiSS-1 mRNA and the number of labeled cells and autoradiographic grains per neuron. Similar to postmenopausal women, ovariectomy induced neuronal hypertrophy and increased KiSS-1 gene expression in the monkey infundibular nucleus. Conversely, in ovariectomized monkeys, estrogen replacement markedly reduced KiSS-1 gene expression.

CONCLUSIONS

The cynomolgus monkey experiments provide strong evidence that the increase in KiSS-1 neuronal size and gene expression in postmenopausal women is secondary to ovarian failure. These studies suggest that kisspeptin neurons regulate estrogen negative feedback in the human.

Hypogonadotropic hypogonadism in mice lacking a functional Kiss1 gene

The G protein-coupled receptor GPR54 (AXOR12, OT7T175) is central to acquisition of reproductive competency in mammals. Peptide ligands (kisspeptins) for this receptor are encoded by the Kiss1 gene, and administration of exogenous kisspeptins stimulates hypothalamic gonadotropin-releasing hormone (GnRH) release in several species, including humans. To establish that kisspeptins are the authentic agonists of GPR54 in vivo and to determine whether these ligands have additional physiological functions we have generated mice with a targeted disruption of the Kiss1 gene. Kiss1-null mice are viable and healthy with no apparent abnormalities but fail to undergo sexual maturation. Mutant female mice do not progress through the estrous cycle, have thread-like uteri and small ovaries, and do not produce mature Graffian follicles. Mutant males have small testes, and spermatogenesis arrests mainly at the early haploid spermatid stage. Both sexes have low circulating gonadotropin (luteinizing hormone and follicle-stimulating hormone) and sex steroid (beta-estradiol or testosterone) hormone levels. Migration of GnRH neurons into the hypothalamus appears normal with appropriate axonal connections to the median eminence and total GnRH content. The hypothalamic-pituitary axis is functional in these mice as shown by robust luteinizing hormone secretion after peripheral administration of kisspeptin. The virtually identical phenotype of Gpr54- and Kiss1-null mice provides direct proof that kisspeptins are the true physiological ligand for the GPR54 receptor in vivo. Kiss1 also does not seem to play a vital role in any other physiological processes other than activation of the hypothalamic-pituitary-gonadal axis, and loss of Kiss1 cannot be overcome by compensatory mechanisms.

Evidence that down regulation of hypothalamic KiSS-1 expression is involved in the negative feedback action of testosterone to regulate luteinising hormone secretion in the adult male rhesus monkey (Macaca mulatta)

In the male monkey, luteinising hormone (LH) secretion is regulated by a negative feedback action of testicular testosterone that is exerted indirectly at the hypothalamic level to decelerate pulsatile gonadotrophin-releasing hormone release (GnRH). The purpose of the present experiment was to investigate whether the kisspeptin-G protein-coupled receptor 54 (GPR54) signalling pathway is involved in mediating the action of testosterone to suppress GnRH release in the monkey, as has been indicated by studies of nonprimates. To this end, 12 castrated adult male rhesus monkeys were implanted with either testosterone containing or empty Silastic capsules. Testosterone treatment produced a square wave increment in circulating testosterone levels within the physiologic range. After suppression of LH and follicle-stimulating hormone secretion was established at 5-6 weeks of testosterone exposure, the animals were killed and expression of the genes encoding for kisspeptin, GPR54 and GnRH determined in the mediobasal hypothalamus and preoptic area of both treated and control animals using RNase protection assays. The suppression in pituitary gonadotrophin secretion was associated with a reduction in kisspeptin mRNA levels in the mediobasal hypothalamus, but not the preoptic area. GPR54 mRNA levels, on the other hand, were not influenced by testosterone treatment. These results are consistent with those previously reported for the rodent, and suggest that the neurobiology of the negative feedback action of testicular testosterone on LH secretion in the monkey, a representative higher primate, may be mediated by kisspeptinergic neurones upstream to the GnRH network.

Sexual differentiation of Kiss1 gene expression in the brain of the rat

The Kiss1 gene codes for kisspeptins, which have been implicated in the neuroendocrine regulation of reproduction. In the brain, Kiss1 mRNA-expressing neurons are located in the arcuate (ARC) and anteroventral periventricular (AVPV) nuclei. Kiss1 neurons in the AVPV appear to play a role in generating the preovulatory GnRH/LH surge, which occurs only in females and is organized perinatally by gonadal steroids. Because Kiss1 is involved in the sexually dimorphic GnRH/LH surge, we hypothesized that Kiss1 expression is sexually differentiated, with females having more Kiss1 neurons than either males or neonatally androgenized females. To test this, male and female rats were neonatally treated with androgen or vehicle; then, as adults, they were left intact or gonadectomized and implanted with capsules containing sex steroids or nothing. Kiss1 mRNA levels in the AVPV and ARC were determined by in situ hybridization. Normal females expressed significantly more Kiss1 mRNA in the AVPV than normal males, even under identical adult hormonal conditions. This Kiss1 sex difference was organized perinatally, as demonstrated by the observation that neonatally androgenized females displayed a male-like pattern of adulthood Kiss1 expression in the AVPV. In contrast, there was neither a sex difference nor an influence of neonatal treatment on Kiss1 expression in the ARC. Using double-labeling techniques, we determined that the sexually differentiated Kiss1 neurons in the AVPV are distinct from the sexually differentiated population of tyrosine hydroxylase (dopaminergic) neurons in this region. Our findings suggest that sex differences in kisspeptin signaling from the AVPV subserve the cellular mechanisms controlling the sexually differentiated GnRH/LH surge.

Kisspeptin: a key link to seasonal breeding

In seasonal species, photoperiod (i.e. daylength) tightly regulates reproduction to ensure that birth occurs at the most favorable time of year. In mammals, a distinct photoneuroendocrine circuit controls this process via the pineal hormone melatonin. This hormone is responsible for the seasonal regulation of reproduction, but the anatomical substrate and the cellular mechanism through which melatonin modulates sexual activity is far from understood. The Syrian hamster is widely used to explore the photoneuroendocrine system, because it is a seasonal model in which sexual activity is promoted by long summer days (LD) and inhibited by short winter days (SD). Recent evidences indicate that the products of the KiSS-1 gene, kisspeptins, and their specific receptor GPR54, represent potent stimulators of the sexual axis. We have shown that melatonin impacts on KiSS-1 expression to control reproduction in the Syrian hamster. In this species, KiSS-1 is expressed in the antero-ventral-periventricular and arcuate nuclei of the hypothalamus at significantly higher levels in hamsters kept in LD as compared to SD. In the arcuate nucleus, the downregulation of KiSS-1 expression in SD appears to be mediated by melatonin and not by secondary changes in gonadal hormones. Remarkably, a chronic administration of kisspeptin restores testicular activity in SD hamsters, despite persisting photoinhibitory conditions. Overall, these findings are consistent with a role of KiSS-1/GPR54 in the seasonal control of reproduction. We propose that the photoperiod, via melatonin, modulates KiSS-1 neurons to drive the reproductive axis.

Kisspeptin expression in the brain: catalyst for the initiation of puberty

In 2003, two independent groups of researchers discovered almost simultaneously that inactivating mutations of the G protein coupled receptor, GPR54, cause hypogonadotropic hypogonadism in mice and men. Since this discovery, kisspeptins, the natural ligands for GPR54, have been thrust into the reproductive neuroendocrine spotlight, as major regulators of GnRH function. Kisspeptins are the peptide products of the KiSS-1 gene, and potently stimulate gonadotrophin secretion when administered either centrally or peripherally. Expression of KiSS-1 has been localised to specific regions of the hypothalamus in many species and is regulated by gonadal steroids and across the estrous cycle. It appears that kisspeptin transmits steroid feedback signals to GnRH cells, especially the positive feedback effect of estrogen that causes the preovulatory GnRH/LH surge. Importantly, kisspeptin function appears to be fundamental to the initiation of puberty.

Postnatal development of kisspeptin neurons in mouse hypothalamus; sexual dimorphism and projections to gonadotropin-releasing hormone neurons

The neuropeptide kisspeptin has recently been implicated as having a critical role in the activation of the GnRH neurons to bring about puberty. We examined here the postnatal development of kisspeptin neuronal populations and their projections to GnRH neurons in the mouse. Three populations of kisspeptin neurons located in the 1) anteroventral periventricular nucleus (AVPV) and the preoptic periventricular nucleus (PeN), 2) dorsomedial hypothalamus, and 3) arcuate nucleus were identified using an antisera raised against mouse kisspeptin-10. A marked 10-fold (P<0.01), female-dominant sex difference in the numbers of kisspeptin neurons existed in the AVPV/PeN but not elsewhere. Kisspeptin neurons in the AVPV/PeN of both sexes displayed a similar pattern of postnatal development with no cells detected at postnatal day (P) 10, followed by increases from P25 to reach adult levels by puberty onset (P<0.01; P31 females and P45 males). This pattern was not found in the dorsomedial hypothalamus or arcuate nucleus. Dual immunofluorescence experiments demonstrated close appositions between kisspeptin fibers and GnRH neuron cell bodies that were first apparent at P25 and increased across postnatal development in both sexes. These studies demonstrate kisspeptin peptide expression in the mouse hypothalamus and reveal the postnatal development of a sexually dimorphic continuum of kisspeptin neurons within the AVPV and PeN. This periventricular population of kisspeptin neurons reaches adult-like proportions at the time of puberty onset and is the likely source of the kisspeptin inputs to GnRH neurons.

Elevated KiSS-1 expression in the arcuate nucleus prior to the cyclic preovulatory gonadotrophin-releasing hormone/lutenising hormone surge in the ewe suggests a stimulatory role for kisspeptin in oestrogen-positive feedback

Kisspeptins are encoded by the gene KiSS-1 and regulate gonadotrophin-releasing hormone (GnRH) and gonadotrophin secretion in various species, including humans. Here, we quantify gene expression of KiSS-1 in the arcuate nucleus (ARC) across the ovine oestrous cycle and demonstrate an increase in the caudal division of the ARC during the preovulatory period. These data strongly suggest that kisspeptins are involved in the generation of the preovulatory GnRH and luteinising hormone surge.

Kisspeptin Mediates the Photoperiodic Control of Reproduction in Hamsters

The KiSS-1 gene encodes kisspeptin, the endogenous ligand of the G-protein-coupled receptor GPR54. Recent data indicate that the KiSS-1/GPR54 system is critical for the regulation of reproduction and is required for puberty onset. In seasonal breeders, reproduction is tightly controlled by photoperiod (i.e., day length). The Syrian hamster is a seasonal model in which reproductive activity is promoted by long summer days (LD) and inhibited by short winter days (SD). Using in situ hybridization and immunohistochemistry, we show that KiSS-1 is expressed in the arcuate nucleus of LD hamsters. Importantly, the KiSS-1 mRNA level was lower in SD animals but not in SD-refractory animals, which spontaneously reactivated their sexual activity after several months in SD. These changes of expression are not secondary to the photoperiodic variations of gonadal steroids. In contrast, melatonin appears to be necessary for these seasonal changes because pineal-gland ablation prevented the SD-induced downregulation of KiSS-1 expression. Remarkably, a chronic administration of kisspeptin-10 restored the testicular activity of SD hamsters despite persisting photoinhibitory conditions. Overall, these findings are consistent with a role of KiSS-1/GPR54 in the seasonal control of reproduction. We propose that photoperiod, via melatonin, modulates KiSS-1 signaling to drive the reproductive axis.