Kisspeptin signaling is indispensable for neurokinin B, but not glutamate, stimulation of gonadotropin secretion in mice

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Kisspeptin, neurokinin B, and dynorphin act in the arcuate nucleus to control activity of the GnRH pulse generator in ewes

Recent work has led to the hypothesis that kisspeptin/neurokinin B/dynorphin (KNDy) neurons in the arcuate nucleus play a key role in GnRH pulse generation, with kisspeptin driving GnRH release and neurokinin B (NKB) and dynorphin acting as start and stop signals, respectively. In this study, we tested this hypothesis by determining the actions, if any, of four neurotransmitters found in KNDy neurons (kisspeptin, NKB, dynorphin, and glutamate) on episodic LH secretion using local administration of agonists and antagonists to receptors for these transmitters in ovariectomized ewes. We also obtained evidence that GnRH-containing afferents contact KNDy neurons, so we tested the role of two components of these afferents: GnRH and orphanin-FQ. Microimplants of a Kiss1r antagonist briefly inhibited LH pulses and microinjections of 2 nmol of this antagonist produced a modest transitory decrease in LH pulse frequency. An antagonist to the NKB receptor also decreased LH pulse frequency, whereas NKB and an antagonist to the receptor for dynorphin both increased pulse frequency. In contrast, antagonists to GnRH receptors, orphanin-FQ receptors, and the N-methyl-D-aspartate glutamate receptor had no effect on episodic LH secretion. We thus conclude that the KNDy neuropeptides act in the arcuate nucleus to control episodic GnRH secretion in the ewe, but afferent input from GnRH neurons to this area does not. These data support the proposed roles for NKB and dynorphin within the KNDy neural network and raise the possibility that kisspeptin contributes to the control of GnRH pulse frequency in addition to its established role as an output signal from KNDy neurons that drives GnRH pulses.

Activation of neurokinin 3 receptors stimulates GnRH release in a location-dependent but kisspeptin-independent manner in adult mice

GnRH neurons form the final common pathway for the central control of reproduction. GnRH release occurs from terminals in the external layer of the median eminence (ME) for neuroendocrine control of the pituitary, and near GnRH-GnRH fiber appositions within the preoptic area (POA). Whether or not control of GnRH secretion by neuromodulators is different in these 2 areas is unknown. Mutations in neurokinin B (NKB) or the neurokinin-3 receptor (NK3R) are linked to hypogonadotropic hypogonadism in humans, suggesting that NKB may regulate GnRH secretion. Using fast scan cyclic voltammetry through carbon-fiber microelectrodes, we examined real-time GnRH release in response to the NK3R agonist senktide in the ME and POA. Coronal brain slices were acutely prepared from adult gonad-intact GnRH-green fluorescent protein male mice, and carbon-fiber microelectrodes were placed either within green fluorescent protein-positive terminal fields of the ME or near GnRH-GnRH fiber appositions in the POA. Senktide induced GnRH release consistently in the ME but not the POA, indicating that GnRH release is differentially regulated by NKB in a location-dependent manner. Senktide also induced GnRH secretion in the ME of kisspeptin-knockout (Kiss1 knockout) mice. Interestingly, release amplitude was lower compared with wild-type mice. These data indicate regulation of GnRH release by NK3R agonists is site specific and suggest that kisspeptin is not a required mediator between NK3R activation and GnRH secretion in the ME. This information will be useful for informing future models of afferent regulation of GnRH release.

Kisspeptin and GnRH pulse generation

The reproductive neuropeptide gonadotropin-releasing hormone (GnRH) has two modes of secretion. Besides the surge mode, which induces ovulation in females, the pulse mode of GnRH release is essential to cause various reproductive events in both sexes, such as spermatogenesis, follicular development, and sex steroid synthesis. Some environmental cues control gonadal activities through modulating GnRH pulse frequency. Researchers have looked for the anatomical location of the mechanism generating GnRH pulses, the GnRH pulse generator, in the brain, because an artificial manipulation of GnRH pulse frequency is of therapeutic importance to stimulate or suppress gonadal activity. Discoveries of kisspeptin and, consequently, KNDy (kisspeptin/neurokinin B/dynorphin) neurons in the hypothalamus have provided a clue to the possible location of the GnRH pulse generator. Our analyses of hypothalamic multiple-unit activity revealed that KNDy neurons located in the hypothalamic arcuate nucleus might play a central role in the generation of GnRH pulses in goats, and perhaps other mammalian species. This chapter further discusses the possible mechanisms for GnRH pulse generation.

Redundancy in Kiss1 expression safeguards reproduction in the mouse

Kisspeptin (Kiss1) signaling to GnRH neurons is widely acknowledged to be a prerequisite for puberty and reproduction. Animals lacking functional genes for either kisspeptin or its receptor exhibit low gonadotropin secretion and infertility. Paradoxically, a recent study reported that genetic ablation of nearly all Kiss1-expressing neurons (Kiss1 neurons) does not impair reproduction, arguing that neither Kiss1 neurons nor their products are essential for sexual maturation. We posited that only minute quantities of kisspeptin are sufficient to support reproduction. If this were the case, animals having dramatically reduced Kiss1 expression might retain fertility, testifying to the redundancy of Kiss1 neurons and their products. To test this hypothesis and to determine whether males and females differ in the required amount of kisspeptin needed for reproduction, we used a mouse (Kiss1-CreGFP) that has a severe reduction in Kiss1 expression. Mice that are heterozygous and homozygous for this allele (Kiss1(Cre/+) and Kiss1(Cre/Cre)) have ∼50% and 95% reductions in Kiss1 transcript, respectively. We found that although male Kiss1(Cre/Cre) mice sire normal-sized litters, female Kiss1(Cre/Cre) mice exhibit significantly impaired fertility and ovulation. These observations suggest that males require only 5% of normal Kiss1 expression to be reproductively competent, whereas females require higher levels for reproductive success.

PRR repeats in the intracellular domain of KISS1R are important for its export to cell membrane

Inactivating mutations of KISS-1 receptor (KISS1R) have been recently described as a rare cause of isolated hypogonadotropic hypogonadism transmitted as a recessive trait. Few mutations have been described, and the structure-function relationship of KISS1R remains poorly understood. Here, we have taken advantage of the discovery of a novel mutation of KISS1R to characterize the structure and function of an uncommon protein motif composed of 3 proline-arginine-arginine (PRR) repeats located within the intracellular domain. A heterozygous insertion of 1 PRR repeat in-frame with 3 PRR repeats leading to synthesis of a receptor bearing 4 PRR repeats (PRR-KISS1R) was found in the index case. Functional analysis of PRR-KISS1R showed a decrease of the maximal response to kisspeptin stimulation, associated to a lower cell surface expression without modification of total expression. PRR-KISS1R exerts a dominant negative effect on the synthesis of the wild-type (WT)-KISS1R. This effect was due to the nature of inserted residues but also to the difference of the length of the intracellular domain between PRR-KISS1R and WT-KISS1R. A molecular dynamic analysis showed that the additional PRR constrained this arginine-rich region into a polyproline type II helix. Altogether, this study shows that a heterozygous insertion in KISS1R may lead to hypogonadotropic hypogonadism by a dominant negative effect on the WT receptor. An additional PRR repeat into a proline-arginine-rich motif can dramatically changed the conformation of the intracellular domain of KISS1R and its probable interaction with partner proteins.

Distinct expression patterns predict differential roles of the miRNA-binding proteins, Lin28 and Lin28b, in the mouse testis: studies during postnatal development and in a model of hypogonadotropic hypogonadism

Lin28 (also termed Lin28a) and Lin28b are related RNA-binding proteins, involved in the control of microRNA synthesis, especially of the let-7 family, with putative functions in early (embryo) development. However, their roles during postnatal maturation remain ill defined. Despite the general assumption that Lin28 and Lin28b share similar targets and functions, conclusive demonstration of such redundancy is still missing. In addition, recent observations suggest a role of Lin28 proteins in mammalian reproduction, which is yet to be defined. We document herein the patterns of RNA expression and protein distribution of Lin28 and Lin28b in mouse testis during postnatal development and in a model of hypogonadotropic hypogonadism as a result of inactivation of the kisspeptin receptor, Gpr54. Lin28 and Lin28b mRNAs were expressed in mouse testis across postnatal maturation, but their levels disparately varied between neonatal and pubertal periods, with peak Lin28 levels in infantile testes and sustained elevation of Lin28b mRNA in young adult male gonads, where relative levels of let-7a and let-7b miRNAs were significantly suppressed. In addition, Lin28 peptides displayed totally different patterns of cellular distribution in mouse testis: Lin28 was located in undifferentiated and type-A1 spermatogonia, whereas Lin28b was confined to spermatids and interstitial Leydig cells. These profiles were perturbed in Gpr54 null mouse testis, which showed preserved but irregular Lin28 signal and absence of Lin28b peptide, which was rescued by administration of gonadotropins, mainly hCG (as super-agonist of LH). In addition, increased relative levels of Lin28, but not Lin28b, mRNA and of let-7a/let-7b miRNAs were observed in Gpr54 KO mouse testes. Altogether, our data are the first to document the divergent patterns of cellular distribution and mRNA expression of Lin28 and Lin28b in the mouse testis along postnatal maturation and their alteration in a model of congenital hypogonadotropic hypogonadism. Our findings suggest distinct functional roles of these two related, but not overlapping, miRNA-binding proteins in the male gonad.

Kisspeptins and RFRP-3 Act in Concert to Synchronize Rodent Reproduction with Seasons

Seasonal mammals use the photoperiodic variation in the nocturnal production of the pineal hormone melatonin to synchronize their reproductive activity with seasons. In rodents, the (SD) short day profile of melatonin secretion has long been proven to inhibit reproductive activity. Lately, we demonstrated that melatonin regulates the expression of the hypothalamic peptides kisspeptins (Kp) and RFamide-related peptide-3 (RFRP-3), recently discovered as potent regulators of gonadotropin-releasing hormone (GnRH) neuron activity. In the male Syrian hamster, Kp expression in the arcuate nucleus is down-regulated by melatonin independently of the inhibitory feedback of testosterone. A central or peripheral administration of Kp induces an increase in pituitary gonadotropins and gonadal hormone secretion, but most importantly a chronic infusion of the peptide reactivates the photo-inhibited reproductive axis of Syrian hamsters kept in SD conditions. RFRP-3 expression in the dorsomedial hypothalamus is also strongly inhibited by melatonin in a SD photoperiod. Although RFRP-3 is usually considered as an inhibitory component of the gonadotropic axis, a central acute administration of RFRP-3 in the male Syrian hamster induces a marked increase in gonadotropin secretion and testosterone production. Furthermore, a chronic central infusion of RFRP-3 in SD-adapted hamsters reactivates the reproductive axis, in the same manner as Kp. Both Kp and RFRP-3 neurons project onto GnRH neurons and both neuropeptides regulate GnRH neuron activity. In addition, central RFRP-3 infusion was associated with a significant increase in arcuate Kp expression. However, the actual sites of action of both peptides in the Syrian hamster brain are still unknown. Altogether our findings indicate that Kp and RFRP neurons are pivotal relays for the seasonal regulation of reproduction, and also suggest that RFRP neurons might be the primary target of the melatoninergic message.

Neurokinin B causes acute GnRH secretion and repression of GnRH transcription in GT1-7 GnRH neurons

Genetic studies in human patients with idiopathic hypogonadotropic hypogonadism (IHH) identified mutations in the genes that encode neurokinin B (NKB) and the neurokinin 3 receptor (NK3R). However, determining the mechanism whereby NKB regulates gonadotropin secretion has been difficult because of conflicting results from in vivo studies investigating the luteinizing hormone (LH) response to senktide, a NK3R agonist. NK3R is expressed in a subset of GnRH neurons and in kisspeptin neurons that are known to regulate GnRH secretion. Thus, one potential source of inconsistency is that NKB could produce opposing direct and indirect effects on GnRH secretion. Here, we employ the GT1-7 cell model to elucidate the direct effects of NKB on GnRH neuron function. We find that GT1-7 cells express NK3R and respond to acute senktide treatment with c-Fos induction and increased GnRH secretion. In contrast, long-term senktide treatment decreased GnRH secretion. Next, we focus on the examination of the mechanism underlying the long-term decrease in secretion and determine that senktide treatment represses transcription of GnRH. We further show that this repression of GnRH transcription may involve enhanced c-Fos protein binding at novel activator protein-1 (AP-1) half-sites identified in enhancer 1 and the promoter, as well as chromatin remodeling at the promoter of the GnRH gene. These data indicate that NKB could directly regulate secretion from NK3R-expressing GnRH neurons. Furthermore, whether the response is inhibitory or stimulatory toward GnRH secretion could depend on the history or length of exposure to NKB because of a repressive effect on GnRH transcription.

Model systems for studying kisspeptin signalling: mice and cells

Kisspeptins are a family of overlapping neuropeptides, encoded by the Kiss1 gene, that are required for activation and maintenance of the mammalian reproductive axis. Kisspeptins act within the hypothalamus to stimulate release of gonadotrophic releasing hormone and activation of the pituitary-gonadal axis. Robust model systems are required to dissect the regulatory mechanisms that control Kiss1 neuronal activity and to examine the molecular consequences of kisspeptin signalling. While studies in normal animals have been important in this, transgenic mice with targeted mutations affecting the kisspeptin signalling pathway have played a significant role in extending our understanding of kisspeptin physiology. Knock-out mice recapitulate the reproductive defects associated with mutations in humans and provide an experimentally tractable model system to interrogate regulatory feedback mechanisms. In addition, transgenic mice with cell-specific expression of modulator proteins such as the CRE recombinase or fluorescent reporter proteins such as GFP allow more sophisticated analyses such as cell or gene ablation or electrophysiological profiling. At a less complex level, immortalized cell lines have been useful for studying the role of kisspeptin in cell migration and metastasis and examining the intracellular signalling events associated with kisspeptin signalling.

Sex steroid regulation of kisspeptin circuits

Kisspeptin cells appear to be the "missing link," bridging the divide between levels of gonadal steroids and feedback control of gonadotropin-releasing hormone (GnRH) secretion. Kisspeptin neurons are important in the generation of both sex steroid negative and estrogen positive feedback signals to GnRH neurons, the former being involved in the tonic regulation of GnRH secretion in males and females and the latter governing the preovulatory GnRH/luteinizing hormone (LH) surge in females. In rodents, kisspeptin-producing cells populate the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus (ARC), and estrogen regulation of kisspeptin has been extensively studied in these regions. Kisspeptin cells in the ARC appear to receive and forward signals applicable to negative feedback regulation of GnRH. In the female rodent AVPV, kisspeptin cells are important for positive feedback regulation of GnRH and the preovulatory LH surge. In sheep and primates, a rostral population of kisspeptin cells is located in the dorsolateral preoptic area (POA) as well as the ARC. Initial studies showed kisspeptin cells in the latter were involved in both the positive and negative feedback regulation of GnRH. Interestingly, further studies now suggest that kisspeptin cells in the ovine POA may also play an important role in generating estrogen positive feedback. This chapter discusses the current consensus knowledge regarding the interaction between sex steroids and kisspeptin neurons in mammals.

Development and Aging of the Kisspeptin-GPR54 System in the Mammalian Brain: What are the Impacts on Female Reproductive Function?

The prominent role of the G protein coupled receptor GPR54 and its peptide ligand kisspeptin in the progression of puberty has been extensively documented in many mammalian species including humans. Kisspeptins are very potent gonadotropin-releasing hormone secretagogues produced by two main populations of neurons located in two ventral forebrain regions, the preoptic area and the arcuate nucleus. Within the last 2 years a substantial amount of data has accumulated concerning the development of these neuronal populations and their timely regulation by central and peripheral factors during fetal, neonatal, and peripubertal stages of development. This review focuses on the development of the kisspeptin-GPR54 system in the brain of female mice, rats, sheep, monkeys, and humans. We will also discuss the notion that this system represents a major target through which signals from the environment early in life can reprogram reproductive function.

Interactions between kisspeptins and neurokinin B

Reproductive function is tightly regulated by an intricate network of central and peripheral factors; however, the precise mechanism triggering critical reproductive events, such as puberty onset, remains largely unknown. Recently, the neuropeptides kisspeptin (encoded by Kiss1) and neurokinin B (NKB, encoded by TAC3 in humans and Tac2 in rodents) have been placed as essential gatekeepers of puberty. Studies in humans and rodents have revealed that loss-of-function mutations in the genes encoding either kisspeptin and NKB or their receptors, Kiss1r and neurokinin 3 receptor (NK3R), lead to impaired sexual maturation and infertility. Kisspeptin, NKB, and dynorphin A are co-expressed in neurons of the arcuate nucleus (ARC), so-called Kisspeptin/NKB/Dyn (KNDy) neurons. Importantly, these neurons also co-express NK3R. Compelling evidence suggests a stimulatory role of NKB (or the NK3R agonist, senktide) on LH release in a number of species. This effect is likely mediated by autosynaptic inputs of NKB on KNDy neurons to induce the secretion of gonadotropin-releasing hormone (GnRH) in a kisspeptin--dependent manner, with the coordinated actions of other neuroendocrine factors, such as dynorphin, glutamate, or GABA. Thus, we have proposed a model in which NKB feeds back to the KNDy neuron to shape the pulsatile release of kisspeptin, and hence GnRH, in a mechanism also dependent on the sex steroid level. Additionally, NKB may contribute to the regulation of the reproductive function by metabolic cues. Investigating how NKB and kisspeptin interact to regulate the gonadotropic axis will offer new insights into the control of GnRH release during puberty onset and the maintenance of the reproductive function in adulthood, offering a platform for the understanding and treatment of a number of reproductive disorders.

Deciphering puberty: novel partners, novel mechanisms

Puberty is a fascinating developmental phase that involves the attainment of reproductive capacity and the completion of sexual and somatic maturation. As a life-changing event, puberty onset is precisely controlled by interconnected regulatory pathways that are sensitive to numerous endogenous signals and environmental cues. The mechanisms of normal puberty and its potential deviations have been thoroughly studied in humans and model species. Yet, characterization of the neurobiological basis of puberty is still incomplete. Progress on this front is not only relevant from a physiological perspective but would also help to unravel the underlying causes for the observed changes in the timing of puberty in humans, with a trend for earlier puberty onset, especially in girls. In this review, we will provide a synoptic overview of some recent developments in the field that have deepened our understanding of the neuroendocrine and molecular basis for the control of puberty onset. These include not only the demonstration of the involvement of the hypothalamic Kiss1 system in the control of puberty and its modulation by metabolic cues but also the identification of the roles of other neuropeptide pathways and molecular mediators in the regulation of puberty. In addition, the potential contribution of novel regulatory mechanisms, such as epigenetics, in the central control of puberty will be briefly discussed. Characterization of these novel players and regulatory mechanisms will improve our understanding of the basis of normal puberty and its eventual alterations in various pathological conditions.

Multiple kisspeptin receptors in early osteichthyans provide new insights into the evolution of this receptor family

Deorphanization of GPR54 receptor a decade ago led to the characterization of the kisspeptin receptor (Kissr) in mammals and the discovery of its major role in the brain control of reproduction. While a single gene encodes for Kissr in eutherian mammals including human, other vertebrates present a variable number of Kissr genes, from none in birds, one or two in teleosts, to three in an amphibian, xenopus. In order to get more insight into the evolution of Kissr gene family, we investigated the presence of Kissr in osteichthyans of key-phylogenetical positions: the coelacanth, a representative of early sarcopterygians, the spotted gar, a non-teleost actinopterygian, and the European eel, a member of an early group of teleosts (elopomorphs). We report the occurrence of three Kissr for the first time in a teleost, the eel. As measured by quantitative RT-PCR, the three eel Kissr were differentially expressed in the brain-pituitary-gonadal axis, and differentially regulated in experimentally matured eels, as compared to prepubertal controls. Subfunctionalisation, as shown by these differences in tissue distribution and regulation, may have represented significant evolutionary constraints for the conservation of multiple Kissr paralogs in this species. Furthermore, we identified four Kissr in both coelacanth and spotted gar genomes, providing the first evidence for the presence of four Kissr in vertebrates. Phylogenetic and syntenic analyses supported the existence of four Kissr paralogs in osteichthyans and allowed to propose a clarified nomenclature of Kissr (Kissr-1 to -4) based on these paralogs. Syntenic analysis suggested that the four Kissr paralogs arose through the two rounds of whole genome duplication (1R and 2R) in early vertebrates, followed by multiple gene loss events in the actinopterygian and sarcopterygian lineages. Due to gene loss there was no impact of the teleost-specific whole genome duplication (3R) on the number of Kissr paralogs in current teleosts.

Differential modulation of gonadotropin responses to kisspeptin by aminoacidergic, peptidergic, and nitric oxide neurotransmission

Kisspeptins (Kp), products of the Kiss1 gene, have emerged as essential elements in the control of GnRH neurons and gonadotropic secretion. However, despite considerable progress in the field, to date limited attention has been paid to elucidate the potential interactions of Kp with other neurotransmitters known to centrally regulate the gonadotropic axis. We characterize herein the impact of manipulations of key aminoacidergic (glutamate and GABA), peptidergic (NKB, Dyn, and MCH), and gaseous [nitric oxide (NO)] neurotransmission on gonadotropin responses to Kp-10 in male rats. Blockade of ionotropic glutamate receptors (of the NMDA and non-NMDA type) variably decreased LH responses to Kp-10, whereas activation of both ionotropic and metabotropic receptors, which enhanced LH and FSH release per se, failed to further increase gonadotropin responses to Kp-10. In fact, coactivation of metabotropic receptors attenuated LH and FSH responses to Kp-10. Selective activation of GABA(A) receptors decreased Kp-induced gonadotropin secretion, whereas their blockade elicited robust LH and FSH bursts and protracted responses to Kp-10 when combined with GABA(B) receptor inhibition. Blockade of Dyn signaling (at κ-opioid receptors) enhanced LH responses to Kp-10, whereas activation of Dyn and NKB signaling modestly reduced Kp-induced LH and FSH release. Finally, MCH decreased basal LH secretion and modestly reduced FSH responses to Kp-10, whereas LH responses to Kp-10 were protracted after inhibition of NO synthesis. In summary, we present herein evidence for the putative roles of glutamate, GABA, Dyn, NKB, MCH, and NO in modulating gonadotropic responses to Kp in male rats. Our pharmacological data will help to characterize the central interactions and putative hierarchy of key neuroendocrine pathways involved in the control of the gonadotropic axis.

Kisspeptin neurons from mice to men: similarities and differences

The discovery that kisspeptin was critical for normal fertility in humans ushered in a new chapter in our understanding of the control of GnRH secretion. In this paper, we will review recent data on the similarities and differences across several mammalian species in the role of kisspeptin in reproductive neuroendocrinology. In all mammals examined to date, there is strong evidence that kisspeptin plays a key role in the onset of puberty and is necessary for both tonic and surge secretion of GnRH in adults, although kisspeptin-independent systems are also apparent in these studies. Similarly, two groups of kisspeptin neurons, one in the arcuate nucleus (ARC) and the other more rostrally, have been identified in all mammals, although the latter is concentrated in a limited area in rodents and more scattered in other species. Estrogen has divergent actions on kisspeptin expression in these two regions across these species, stimulating it the latter and inhibiting expression in the former. There is also strong evidence that the rostral population participates in the GnRH surge, whereas the ARC population contributes to steroid-negative feedback. There may be species differences in the role of these two populations in puberty, with the ARC cells important in rats, sheep, and monkeys, whereas both have been implicated in mice. ARC kisspeptin neurons also appear to participate in the GnRH surge in sheep and guinea pigs, whereas the data on this possibility in rodents are contradictory. Similarly, both populations are sexually dimorphic in sheep and humans, whereas most data in rodents indicate that this occurs only in the rostral population. The functional consequences of these species differences remain to be fully elucidated but are likely to have significance for understanding normal neuroendocrine control of reproduction as well as for use of kisspeptin agonists/antagonists as a therapeutic tool.

Kiss of the mutant mouse: how genetically altered mice advanced our understanding of kisspeptin's role in reproductive physiology

The kisspeptin system has emerged as one of the most important circuits within the central network governing reproduction. Although kisspeptin physiology has been examined in many species, much of our understanding of this system has come from mice. Recently, the study of several innovative strains of genetically engineered mouse models has revealed intriguing and unexpected insights into the functions of kisspeptin signaling in the hypothalamus. Here, we review the advancements in our knowledge of the central kisspeptin system through the use of mutant mice.

Suppression of the GnRH pulse generator by neurokinin B involves a κ-opioid receptor-dependent mechanism

Neurokinin B (NKB) and its receptor (NK3R) are coexpressed with kisspeptin, Dynorphin A (Dyn), and their receptors [G-protein-coupled receptor-54 (GPR54)] and κ-opioid receptor (KOR), respectively] within kisspeptin/NKB/Dyn (KNDy) neurons in the hypothalamic arcuate nucleus (ARC), the proposed site of the GnRH pulse generator. Much previous research has employed intracerebroventricular (icv) administration of KNDy agonists and antagonists to address the functions of KNDy neurons. We performed a series of in vivo neuropharmacological experiments aiming to determine the role of NKB/NK3R signaling in modulating the GnRH pulse generator and elucidate the interaction between KNDy neuropeptide signaling systems, targeting our interventions to ARC KNDy neurons. First, we investigated the effect of intra-ARC administration of the selective NK3R agonist, senktide, on pulsatile LH secretion using a frequent automated serial sampling method to obtain blood samples from freely moving ovariectomized 17β-estradiol-replaced rats. Our results show that senktide suppresses LH pulses in a dose-dependent manner. Intra-ARC administration of U50488, a selective KOR agonist, also caused a dose-dependent, albeit more modest, decrease in LH pulse frequency. Thus we tested the hypothesis that Dyn/KOR signaling localized to the ARC mediates the senktide-induced suppression of the LH pulse by profiling pulsatile LH secretion in response to senktide in rats pretreated with nor-binaltorphimine, a selective KOR antagonist. We show that nor-binaltorphimine blocks the senktide-induced suppression of pulsatile LH secretion but does not affect LH pulse frequency per se. In order to address the effects of acute activation of ARC NK3R, we quantified (using quantitative RT-PCR) changes in mRNA levels of KNDy-associated genes in hypothalamic micropunches following intra-ARC administration of senktide. Senktide down-regulated expression of genes encoding GnRH and GPR54 (GNRH1 and Kiss1r, respectively), but did not affect the expression of Kiss1 (which encodes kisspeptin). We conclude that NKB suppresses the GnRH pulse generator in a KOR-dependent fashion and regulates gene expression in GnRH neurons.

Low degree of overlap between kisspeptin, neurokinin B, and dynorphin immunoreactivities in the infundibular nucleus of young male human subjects challenges the KNDy neuron concept

Previous immunohistochemical and in situ hybridization studies of sheep, goats, and rodents indicated that kisspeptin (KP), neurokinin B (NKB), and dynorphin A (DYN) are extensively colocalized in the hypothalamic arcuate nucleus, thus providing a basis for the KP/NKB/DYN (KNDy) neuron concept; in both sexes, KNDy neuropeptides have been implicated in the generation of GnRH neurosecretory pulses and in the negative feedback effects of sexual steroids to the reproductive axis. To test the validity and limitations of the KNDy neuron concept in the human, we carried out the comparative immunohistochemical analysis of the three neuropeptides in the infundibular nucleus (Inf; also known as arcuate nucleus) and stalk of young male human individuals (<37 yr). Results of quantitative immunohistochemical experiments established that the regional densities of NKB immunoreactive (IR) perikarya and fibers, and the incidence of afferent contacts they formed onto GnRH neurons, were about 5 times as high as those of the KP-IR elements. Dual-immunofluorescent studies confirmed that considerable subsets of the NKB-IR and KP-IR cell bodies and fibers are separate, and only about 33% of NKB-IR perikarya and 75% of KP-IR perikarya were dual labeled. Furthermore, very few DYN-IR cell bodies could be visualized in the Inf. DYN-IR fibers were also rare and, with few exceptions, distinct from the KP-IR fibers. The abundance and colocalization patterns of the three immunoreactivities showed similar trends in the infundibular stalk around portal blood vessels. Together these results indicate that most NKB neurons in the Inf do not synthesize detectable amounts of KP and DYN in young male human individuals. These data call for a critical use of the KNDy neuron terminology when referring to the putative pulse generator system of the mediobasal hypothalamus. We conclude that the functional importance of these three neuropeptides in reproductive regulation considerably varies among species, between sexes, and at different ages.

Neurokinin B and the control of the gonadotropic axis in the rat: developmental changes, sexual dimorphism, and regulation by gonadal steroids

Neurokinin B (NKB), encoded by Tac2 in rodents, and its receptor, NK3R, have recently emerged as important regulators of reproduction; NKB has been proposed to stimulate kisspeptin output onto GnRH neurons. Accordingly, NKB has been shown to induce gonadotropin release in several species; yet, null or even inhibitory effects of NKB have been also reported. The basis for these discrepant findings, as well as other key aspects of NKB function, remains unknown. We report here that in the rat, LH responses to the NK3R agonist, senktide, display a salient sexual dimorphism, with persistent stimulation in females, regardless of the stage of postnatal development, and lack of LH responses in males from puberty onward. Such dimorphism was independent of the predominant sex steroid after puberty, because testosterone administration to adult females failed to prevent LH responses to senktide, and LH responsiveness was not restored in adult males treated with estradiol or the nonaromatizable androgen, dihydrotestosterone. Yet, removal of sex steroids by gonadectomy switched senktide effects to inhibitory, both in adult male and female rats. Sexual dimorphism was also evident in the numbers of NKB-positive neurons in the arcuate nucleus (ARC), which were higher in adult female rats. This is likely the result of differences in sex steroid milieu during early periods of brain differentiation, because neonatal exposures to high doses of estrogen decreased ARC NKB neurons at later developmental stages. Likewise, neonatal estrogenization resulted in lower serum LH levels that were normalized by senktide administration. Finally, we document that the ability of estrogen to inhibit hypothalamic Tac2 expression seems region specific, because estrogen administration decreased Tac2 levels in the ARC but increased them in the lateral hypothalamus. Altogether, our data provide a deeper insight into relevant aspects of NKB function as major regulator of the gonadotropic axis in the rat, including maturational changes, sexual dimorphism, and differential regulation by sex steroids.

Increased neurokinin B (Tac2) expression in the mouse arcuate nucleus is an early marker of pubertal onset with differential sensitivity to sex steroid-negative feedback than Kiss1

At puberty, neurokinin B (NKB) and kisspeptin (Kiss1) may help to amplify GnRH secretion, but their precise roles remain ambiguous. We tested the hypothesis that NKB and Kiss1 are induced as a function of pubertal development, independently of the prevailing sex steroid milieu. We found that levels of Kiss1 mRNA in the arcuate nucleus (ARC) are increased prior to the age of puberty in GnRH/sex steroid-deficient hpg mice, yet levels of Kiss1 mRNA in wild-type mice remained constant, suggesting that sex steroids exert a negative feedback effect on Kiss1 expression early in development and across puberty. In contrast, levels of Tac2 mRNA, encoding NKB, and its receptor (NK3R; encoded by Tacr3) increased as a function of puberty in both wild-type and hpg mice, suggesting that during development Tac2 is less sensitive to sex steroid-dependent negative feedback than Kiss1. To compare the relative responsiveness of Tac2 and Kiss1 to the negative feedback effects of gonadal steroids, we examined the effect of estradiol (E(2)) on Tac2 and Kiss1 mRNA and found that Kiss1 gene expression was more sensitive than Tac2 to E(2)-induced inhibition at both juvenile and adult ages. This differential estrogen sensitivity was tested in vivo by the administration of E(2). Low levels of E(2) significantly suppressed Kiss1 expression in the ARC, whereas Tac2 suppression required higher E(2) levels, supporting differential sensitivity to E(2). Finally, to determine whether inhibition of NKB/NK3R signaling would block the onset of puberty, we administered an NK3R antagonist to prepubertal (before postnatal d 30) females and found no effect on markers of pubertal onset in either WT or hpg mice. These results indicate that the expression of Tac2 and Tacr3 in the ARC are markers of pubertal activation but that increased NKB/NK3R signaling alone is insufficient to trigger the onset of puberty in the mouse.

GPR54-dependent stimulation of luteinizing hormone secretion by neurokinin B in prepubertal rats

Kisspeptin, neurokinin B (NKB) and dynorphin A (Dyn) are coexpressed within KNDy neurons that project from the hypothalamic arcuate nucleus (ARC) to GnRH neurons and numerous other hypothalamic targets. Each of the KNDy neuropeptides has been implicated in regulating pulsatile GnRH/LH secretion. In isolation, kisspeptin is generally known to stimulate, and Dyn to inhibit LH secretion. However, the NKB analog, senktide, has variously been reported to inhibit, stimulate or have no effect on LH secretion. In prepubertal mice, rats and monkeys, senktide stimulates LH secretion. Furthermore, in the monkey this effect is dependent on kisspeptin signaling through its receptor, GPR54. The present study tested the hypotheses that the stimulatory effects of NKB on LH secretion in intact rats are mediated by kisspeptin/GPR54 signaling and are independent of a Dyn tone. To test this, ovarian-intact prepubertal rats were subjected to frequent automated blood sampling before and after intracerebroventricular injections of KNDy neuropeptide analogs. Senktide robustly induced single LH pulses, while neither the GPR54 antagonist, Kp-234, nor the Dyn agonist and antagonist (U50488 and nor-BNI, respectively) had an effect on basal LH levels. However, Kp-234 potently blocked the senktide-induced LH pulses. Modulation of the Dyn tone by U50488 or nor-BNI did not affect the senktide-induced LH pulses. These data demonstrate that the stimulatory effect of NKB on LH secretion in intact female rats is dependent upon kisspeptin/GPR54 signaling, but not on Dyn signaling.

Kisspeptins and the reproductive axis: potential applications to manage reproduction in farm animals

Kisspeptins (Kp) are a family of neuropeptides produced mainly by two hypothalamic neuronal cell populations. They have recently emerged as a major regulator of the gonadotropin axis and their action is located upstream of the gonadotropin-releasing hormone (GnRH) cell population. In less than 10 yr a growing body of literature has demonstrated the involvement of these peptides in most, if not all, aspects of reproductive axis maturation and function. In contrast to these abundant basic research studies, few experiments have evaluated the potential application of Kp as tools to manipulate reproduction in domestic animals. In mammals, exogenous Kp administration potently stimulates gonadotropin secretion. This action is exerted mainly, if not exclusively, through the stimulation of GnRH release. Intravenous, intraperitoneal, or subcutaneous administration of Kp induced a robust and rapid increase in plasma gonadotropins (luteinizing hormone [LH] and follicle-stimulating hormone [FSH]). However, this stimulatory effect is of short duration. Prolonged LH and FSH release over several hours can be achieved only when Kp are given as repeated multiple bolus or as an infusion. Kp administration was used in two experimental models, ewe and pony mare, with the aim of inducing well-timed and synchronized ovulations. During the breeding season, progesterone-synchronized ewes were given an intravenous infusion of Kp starting 30 h after the removal of progesterone implants. An LH surge was induced in all Kp-treated animals within 2 h of infusion onset. In contrast, in pony mares a constant infusion of Kp for 3 d in the the late follicular phase was unable to induce synchronized ovulation. Another set of studies showed that Kp could be used to activate reproductive function in acyclic animals. Pulsatile administration of Kp in prepubertal ewe lambs was shown to activate ovarian function, leading to enhanced ovarian steroidogenesis, stimulation of LH preovulatory surge, and ovulation. In anestrous ewes, an intravenous infusion of a low dose of Kp induced an immediate and sustained release of gonadotropins, followed a few hours later by an LH surge. This hormonal pattern mimicked hormonal changes normally observed during the estrous cycle follicular phase and was associated with a high percentage of ovulating animals (80%). In summary, exogenous administration of Kp appears to be a new tool to manipulate reproduction. However, optimal doses and periods of treatment should be defined for each species, and the development of powerful analogs or long-term release formulations is necessary before large-scale applications in domestic animals could be envisaged.

Kisspeptins and Reproduction: Physiological Roles and Regulatory Mechanisms

Procreation is essential for survival of species. Not surprisingly, complex neuronal networks have evolved to mediate the diverse internal and external environmental inputs that regulate reproduction in vertebrates. Ultimately, these regulatory factors impinge, directly or indirectly, on a final common pathway, the neurons producing the gonadotropin-releasing hormone (GnRH), which stimulates pituitary gonadotropin secretion and thereby gonadal function. Compelling evidence, accumulated in the last few years, has revealed that kisspeptins, a family of neuropeptides encoded by the Kiss1 gene and produced mainly by neuronal clusters at discrete hypothalamic nuclei, are pivotal upstream regulators of GnRH neurons. As such, kisspeptins have emerged as important gatekeepers of key aspects of reproductive maturation and function, from sexual differentiation of the brain and puberty onset to adult regulation of gonadotropin secretion and the metabolic control of fertility. This review aims to provide a comprehensive account of the state-of-the-art in the field of kisspeptin physiology by covering in-depth the consensus knowledge on the major molecular features, biological effects, and mechanisms of action of kisspeptins in mammals and, to a lesser extent, in nonmammalian vertebrates. This review will also address unsolved and contentious issues to set the scene for future research challenges in the area. By doing so, we aim to endow the reader with a critical and updated view of the physiological roles and potential translational relevance of kisspeptins in the integral control of reproductive function.

Neurokinin Bs and neurokinin B receptors in zebrafish-potential role in controlling fish reproduction

The endocrine regulation of vertebrate reproduction is achieved by the coordinated actions of several peptide neurohormones, tachykinin among them. To study the evolutionary conservation and physiological functions of neurokinin B (NKB), we identified tachykinin (tac) and tac receptor (NKBR) genes from many fish species, and cloned two cDNA forms from zebrafish. Phylogenetic analysis showed that piscine Tac3s and mammalian neurokinin genes arise from one lineage. High identity was found among different fish species in the region encoding the NKB; all shared the common C-terminal sequence. Although the piscine Tac3 gene encodes for two putative tachykinin peptides, the mammalian ortholog encodes for only one. The second fish putative peptide, referred to as neurokinin F (NKF), is unique and found to be conserved among the fish species when tested in silico. tac3a was expressed asymmetrically in the habenula of embryos, whereas in adults zebrafish tac3a-expressing neurons were localized in specific brain nuclei that are known to be involved in reproduction. Zebrafish tac3a mRNA levels gradually increased during the first few weeks of life and peaked at pubescence. Estrogen treatment of prepubertal fish elicited increases in tac3a, kiss1, kiss2, and kiss1ra expression. The synthetic zebrafish peptides (NKBa, NKBb, and NKF) activated Tac3 receptors via both PKC/Ca(2+) and PKA/cAMP signal-transduction pathways in vitro. Moreover, a single intraperitoneal injection of NKBa and NKF significantly increased leuteinizing hormone levels in mature female zebrafish. These results suggest that the NKB/NKBR system may participate in neuroendocrine control of fish reproduction.

Evidence of a role for kisspeptin and neurokinin B in puberty of female sheep

Puberty onset in female sheep is marked by a decrease in estradiol-negative feedback, allowing for the increase in GnRH and LH pulses that heralds the first ovulation. Based on recent genetic studies in humans, two possible neuropeptides that could promote puberty onset are kisspeptin and neurokinin B (NKB). Our first experiment determined whether the NKB agonist, senktide, could stimulate LH secretion in prepubertal ewes. A second study used prepubertal and postpubertal ewes that were intact or ovariectomized (OVX) to test the hypothesis that expression of kisspeptin and NKB in the arcuate nucleus increased postpubertally. For comparison, kisspeptin and NKB expression in age-matched intact, and castrated males were also examined. In experiment 1, the percentage of ewes showing an LH pulse immediately after injection of senktide (100 μg, 60%; 500 μg, 100%) was greater than that for water-injected controls (experiment 1a, 25%; experiment 1b, 20%). In experiment 2, kisspeptin-positive cell numbers in the arcuate nucleus increased after puberty in intact females and were increased by OVX in prepubertal but not postpubertal ewes. Changes in kisspeptin cell numbers were paralleled by changes in kisspeptin-close contacts onto GnRH neurons in the medial preoptic area. NKB cell numbers did not differ significantly between intact prepubertal and postpubertal ewes but increased with OVX in both age groups. NKB fiber immunoreactivity was greater in postpubertal than in prepubertal intact ewes. In age-matched males, kisspeptin and NKB cell numbers increased with castration, but decreased with age. These results support the hypothesis that kisspeptin is a gatekeeper to female ovine puberty and raise the possibility that NKB may also play a role, albeit through different means.

Role of neurokinin B in the control of female puberty and its modulation by metabolic status

Human genetic studies have revealed that neurokinin B (NKB) and its receptor, neurokinin-3 receptor (NK3R), are essential elements for normal reproduction; however, the precise role of NKB-NK3R signaling in the initiation of puberty remains unknown. We investigated here the regulation of Tac2 and Tacr3 mRNAs (encoding NKB and NK3R, respectively) in female rats and demonstrated that their hypothalamic expression is increased along postnatal maturation. At puberty, both genes were widely expressed throughout the brain, including the lateral hypothalamic area and the arcuate nucleus (ARC)/medial basal hypothalamus, where the expression of Tacr3 increased across pubertal transition. We showed that central administration of senktide (NK3R agonist) induced luteinizing hormone (LH) secretion in prepubertal and peripubertal females. Conversely, chronic infusion of an NK3R antagonist during puberty moderately delayed the timing of vaginal opening (VO) and tended to decrease LH levels. The expression of NKB and its receptor was sensitive to changes in metabolic status during puberty, as reflected by a reduction in Tacr3 (and, to a lesser extent, Tac2) expression in the ARC after a 48 h fast. Yet, acute LH responses to senktide in pubertal females were preserved, if not augmented, under fasting conditions, suggesting sensitization of the NKB-NK3R-gonadotropin-releasing hormone signaling pathway under metabolic distress. Moreover, repeated administration of senktide to female rats with pubertal arrest due to chronic undernutrition rescued VO (in ∼50% of animals) and potently elicited LH release. Altogether, our observations suggest that NKB-NK3R signaling plays a role in pubertal maturation and that its alterations may contribute to pubertal disorders linked to metabolic stress and negative energy balance.

New insights into the control of pulsatile GnRH release: the role of Kiss1/neurokinin B neurons

Gonadotropin-releasing hormone (GnRH) is the ultimate output signal of an intricate network of neuroendocrine factors that, acting on the pituitary, trigger gonadotropin release. In turn, gonadotropins exert their trophic action on the gonads to stimulate the synthesis of sex steroids thus completing the gonadotropic axis through feedback regulatory mechanisms of GnRH release. These feedback loops are predominantly inhibitory in both sexes, leading to tonic pulsatile release of GnRH from puberty onward. However, in the female, rising levels of estradiol along the estrous cycle evoke an additional positive feedback that prompts a surge-like pattern of GnRH release prior to ovulation. Kisspeptins, secreted from hypothalamic Kiss1 neurons, are poised as major conduits to regulate this dual secretory pathway. Kiss1 neurons are diverse in origin, nature, and function, convening distinct neuronal populations in two main hypothalamic nuclei: the arcuate nucleus (ARC) and the anteroventral periventricular nucleus. Recent studies from our group and others point out Kiss1 neurons in the ARC as the plausible generator of GnRH pulses through a system of pulsatile kisspeptin release shaped by the coordinated action of neurokinin B (NKB) and dynorphin A (Dyn) that are co-expressed in Kiss1 neurons (so-called KNDy neurons). In this review, we aim to document the recent findings and working models directed toward the identification of the Kiss1-dependent mechanisms of GnRH release through a synoptic overview of the state-of-the-art in the field.