The role of neurokinin B signalling in reproductive neuroendocrinology

Estrogen promotes gonadotropin-releasing hormone expression by regulating tachykinin 3 and prodynorphin systems in chicken

The "KNDy neurons" located in the hypothalamic arcuate nucleus (ARC) of mammals are known to co-express kisspeptin, neurokinin B (NKB), and dynorphin (DYN), and have been identified as key mediators of the feedback regulation of steroid hormones on gonadotropin-releasing hormone (GnRH). However, in birds, the genes encoding kisspeptin and its receptor GPR54 are genomic lost, leaving unclear mechanisms for feedback regulation of GnRH by steroid hormones. Here, the genes tachykinin 3 (TAC3) and prodynorphin (PDYN) encoding chicken NKB and DYN neuropeptides were successfully cloned. Temporal expression profiling indicated that TAC3, PDYN and their receptor genes (TACR3, OPRK1) were mainly expressed in the hypothalamus, with significantly higher expression at 30W than at 15W. Furthermore, overexpression or interference of TAC3 and PDYN can regulate the GnRH mRNA expression. In addition, in vivo and in vitro assays showed that estrogen (E2) could promote the mRNA expression of TAC3, PDYN, and GnRH, as well as the secretion of GnRH/LH. Mechanistically, E2 could dimerize the nuclear estrogen receptor 1 (ESR1) to regulate the expression of TAC3 and PDYN, which promoted the mRNA and protein expression of GnRH gene as well as the secretion of GnRH. In conclusion, these results revealed that E2 could regulate the GnRH expression through TAC3 and PDYN systems, providing novel insights for reproductive regulation in chickens.

GHRH Neurons from the Ventromedial Hypothalamic Nucleus Provide Dynamic and Sex-Specific Input to the Brain Glucose-Regulatory Network

The ventromedial hypothalamic nucleus (VMN) controls glucose counter-regulation, including pituitary growth hormone (GH) secretion. VMN neurons that express the transcription factor steroidogenic factor-1/NR5A1 (SF-1) participate in glucose homeostasis. Research utilized in vivo gene knockdown tools to determine if VMN growth hormone-releasing hormone (Ghrh) regulates hypoglycemic patterns of glucagon, corticosterone, and GH outflow according to sex. Intra-VMN Ghrh siRNA administration blunted hypoglycemic hypercorticosteronemia in each sex, but abolished elevated GH release in males only. Single-cell multiplex qPCR showed that dorsomedial VMN (VMNdm) Ghrh neurons express mRNAs encoding Ghrh, SF-1, and protein markers for glucose-inhibitory (γ-aminobutyric acid) or -stimulatory (nitric oxide; glutamate) neurotransmitters. Hypoglycemia decreased glutamate decarboxylase67 (GAD67) transcripts in male, not female VMNdm Ghrh/SF-1 neurons, a response that was refractory to Ghrh siRNA. Ghrh gene knockdown prevented, in each sex, hypoglycemic down-regulation of Ghrh/SF-1 nerve cell GAD65 transcription. Ghrh siRNA amplified hypoglycemia-associated up-regulation of Ghrh/SF-1 neuron nitric oxide synthase mRNA in male and female, without affecting glutaminase gene expression. Ghrh gene knockdown altered Ghrh/SF-1 neuron estrogen receptor-alpha (ERα) and ER-beta transcripts in hypoglycemic male, not female rats, but up-regulated GPR81 lactate receptor mRNA in both sexes. Outcomes infer that VMNdm Ghrh/SF-1 neurons may be an effector of SF-1 control of counter-regulation, and document Ghrh modulation of hypoglycemic patterns of glucose-regulatory neurotransmitter along with estradiol and lactate receptor gene transcription in these cells. Co-transmission of glucose-inhibitory and -stimulatory neurochemicals of diverse chemical structure, spatial, and temporal profiles may enable VMNdm Ghrh neurons to provide complex dynamic, sex-specific input to the brain glucose-regulatory network.

Serum kisspeptin, neurokinin B and inhibin B levels can be used as alternative parameters to distinguish idiopathic CPP from premature thelarche in the early stages of puberty

OBJECTIVE

There is controversial results about serum kisspeptin, neurokinin-B (NKB), anti-Müllerian hormone (AMH) and inhibin B (INHB) levels in girls with central precocious puberty (CPP). Aim of this study is to evaluate serum levels of these four peptides in patients presented with early pubertal signs, and to evaluate their diagnostic validity in the diagnosis of CPP.

DESIGN

Cross-sectional study.

PATIENTS

Study included 99 girls (51 CPP, 48 premature thelarche [PT]) whose breast development started before 8 years and 42 age-matched healthy prepubertal girls. Clinical findings, antropometric measurements, laboratory and radiological findings were recorded. Gonadotropin-releasing hormone (GnRH) stimulation test was performed in all cases with early breast development.

MEASUREMENTS

Kisspeptin, NKB, INHB and AMH levels were measured in fasting serum samples using enzyme-linked immunosorbent assay method.

RESULTS

There was no statistically significant difference between mean ages of girls with CPP (7.1 ± 1.2 years), PT (7.2 ± 1.3 years) and prepubertal controls (7.0 ± 1.0 years). Serum kisspeptin, NKB and INHB levels were higher in CPP group compared to PT and control groups, while serum AMH level was lower in CPP group. Serum kisspeptin, NKB, and INHB were all positively correlated with bone age (BA) advancement, and peak luteinizing hormone in GnRH test. Multiple stepwise regression analysis revealed that the most important factors used to differentiate CPP from PT were advanced BA, serum kisspeptin, NKB and INHB levels (AUC: 0.819, p < .001).

CONCLUSIONS

We, first showed in the same patients' group that serum kisspeptin, NKB and INHB were higher in patients with CPP and can be used as alternative parameters to distinguish CPP from PT.

Tachykinins, new players in the control of reproduction and food intake: A comparative review in mammals and teleosts

In vertebrates, the tachykinin system includes tachykinin genes, which encode one or two peptides each, and tachykinin receptors. The complexity of this system is reinforced by the massive conservation of gene duplicates after the whole-genome duplication events that occurred in vertebrates and furthermore in teleosts. Added to this, the expression of the tachykinin system is more widespread than first thought, being found beyond the brain and gut. The discovery of the co-expression of neurokinin B, encoded by the tachykinin 3 gene, and kisspeptin/dynorphin in neurons involved in the generation of GnRH pulse, in mammals, put a spotlight on the tachykinin system in vertebrate reproductive physiology. As food intake and reproduction are linked processes, and considering that hypothalamic hormones classically involved in the control of reproduction are reported to regulate also appetite and energy homeostasis, it is of interest to look at the potential involvement of tachykinins in these two major physiological functions. The purpose of this review is thus to provide first a general overview of the tachykinin system in mammals and teleosts, before giving a state of the art on the different levels of action of tachykinins in the control of reproduction and food intake. This work has been conducted with a comparative point of view, highlighting the major similarities and differences of tachykinin systems and actions between mammals and teleosts.

Developmental aspects of the hypothalamic-pituitary network related to reproduction in teleost fish

The hypothalamic-pituitary-gonadal axis is the main system that regulates reproduction in vertebrates through a complex network that involves different neuropeptides, neurotransmitters, and pituitary hormones. Considering that this axis is established early on life, the main goal of the present work is to gather information on its development and the actions of its components during early life stages. This review focuses on fish because their neuroanatomical characteristics make them excellent models to study neuroendocrine systems. The following points are discussed: i) developmental functions of the neuroendocrine components of this network, and ii) developmental disruptions that may impact adult reproduction. The importance of the components of this network and their susceptibility to external/internal signals that can alter their specific early functions and/or even the establishment of the reproductive axis, indicate that more studies are necessary to understand this complex and dynamic network.

Interplay of KNDy and nNOS neurons: A new possible mechanism of GnRH secretion in the adult brain

Reproduction in mammals is favoured when there is sufficient energy available to permit the survival of offspring. Neuronal nitric oxide synthase expressing neurons produce nitric oxide in the proximity of the gonadotropin-releasing hormone neurons in the preoptic region. nNOS neurons are an integral part of the neuronal network controlling ovarian cyclicity and ovulation. Nitric oxide can directly regulate the activity of the GnRH neurons and play a vital role neuroendocrine axis. Kisspeptin neurons are essential for the GnRH pulse and surge generation. The anteroventral periventricular nucleus (AVPV), kisspeptin neurons are essential for GnRH surge generation. KNDy neurons are present in the hypothalamus's arcuate nucleus (ARC), co-express NKB and dynorphin, essential for GnRH pulse generation. Kisspeptin-neurokinin B-dynorphin (KNDy) neuroendocrine molecules of the hypothalamus are key components in the central control of GnRH secretion. The hypothalamic neurons kisspeptin, KNDy, nitric oxide synthase (NOS), and other mediators such as leptin, adiponectin, and ghrelin, play an active role in attaining puberty. Kisspeptin signalling is mediated by NOS, which further results in the secretion of GnRH. Neuronal nitric oxide is critical for attaining puberty, but its direct role in adult GnRH secretion is poorly understood. This review mainly focuses on the role of nNOS and its interplay with KNDy neurons in the hormonal regulation of reproduction.

Egg-laying and brooding stage-specific hormonal response and transcriptional regulation in pituitary of Muscovy duck (Cairina moschata)

Broodiness is an interesting topic in reproductive biology for its reduced egg production. The strong brooding trait of Muscovy duck has become a major factor restricting the development of its industry. Broody phenotype and environmental factors influencing broodiness in poultry have been extensively studied, but the molecular regulation mechanism of broodiness remains unclear. In this research, the Muscovy duck reproductive endocrine hormones and pituitary transcriptome profiles during egg-laying phases (LP) and brooding phases (BP) were studied. During BP (n = 19), prolactin (PRL) levels was higher, while progesterone (P4) and estradiol (E2) were lower as compared to ducks during their LP (n = 20) (P < 0.01). We then examined the pituitary transcriptome of Muscovy duck at the 2 reproductive stages. A total of 398 differentially expressed genes included 20 transcription factors were identified (fold change ≥ 1.5, P < 0.01). There were 109 upregulated and 289 downregulated genes at brooding phases (n = 6) compared with egg-laying phases (n = 6). Real-time quantitative PCR analysis was carried out to verify the transcriptome results. The present study suggested that neuroactive ligand-receptor interaction pathway, calcium signaling pathway, and response to steroid hormones biological process are critical for controlling broodiness in the ducks. Further analysis revealed that SHH, PTGS2, RLN3, and transcription factor AP-1 may act as central signal modulators of hormonal and behavioral regulation mechanism associated with broodiness.

Effects of fasting on the central expression of appetite-regulating and reproductive hormones in wild-type and Casper zebrafish (Danio rerio)

Appetite and reproduction are closely related functions that are both regulated by brain hormones. Appetite stimulators include orexin and neuropeptide Y (NPY), and reproductive hormones include gonadotropin-releasing hormone (GnRH), gonadotropin-inhibitory hormone (GnIH), kisspeptin, and neurokinin B (NKB). GnRH stimulates the secretion of pituitary gonadotropes, and kisspeptin and GnIH modulate this action. Kisspeptin secretion is further controlled by neurokinin B (NKB) and dynorphin A (Dyn). To better understand the mechanisms regulating appetite and reproduction in fish, we examined the effects of fasting, reproductive stage, gender, and strain on the brain mRNA expression of appetite (orexin and NPY) and reproductive (GnRH, kisspeptin, GnIH, and NKB) hormones in zebrafish. In order to compare strains, we used both wild-type and transparent Casper zebrafish. In female wild-type zebrafish, fasting increased the expression of all hormones investigated, with the exception of Kiss2. Only NPY and Kiss2 were increased in male wild-type zebrafish during fasting. In Casper zebrafish, only GnIH and NKB in males were affected by fasting, suggesting that Casper fish may be more resistant to fasting than wild fish. Fasting increased expressions of orexin, GnRH2, Kiss1, GnIH and NKB in wild-type females with more eggs or larger eggs relative to body weight, compared to those with fewer or smaller eggs, suggesting that more mature females are more affected by fasting. No significant interactions of fasting and reproductive stage were noted in female Casper fish. To investigate whether differences between Casper and wild-type fish were due to genes involved in pigmentation, we compared the brain mRNA expressions of enzymes involved in melanin synthesis (tyrosinase and tyrosine hydroxylase - TH), melanocortin receptors (MC3R and MC4R), and the melanocortin precursor (proopiomelanocortin - POMC) between the two strains. Casper zebrafish had lower levels of MC3R, tyrosinase, TH1, TH2, and POMC than wild-type fish. Overall, our results suggest the existence of gender- and reproductive stage-specific, as well as strain-specific variations in the mechanisms regulating feeding and reproduction in zebrafish, and that the melanocortin system and melanin pathways may be in part responsible for these differences between strains.

The effects of supraphysiological levels of testosterone on neural networks upstream of gonadotropin-releasing hormone neurons

OBJECTIVES

Several pathological conditions are associated with hyper-production of testosterone; however, its impacts are not well understood. Hence, we evaluated the effects of supraphysiological levels of testosterone on gonadotropin-releasing hormone (GnRH) system in the hypothalamus of male rats. Also, we assessed the expression of two excitatory (kisspeptin and neurokinin-B) and two inhibitory (dynorphin and RFamide-related-peptide) neuropeptides upstream of GnRH neurons as possible routes to relay androgen information.

MATERIALS AND METHODS

Gonadectomized (GDX) male rats received single injection of 100, 250 or 500 mg/kg testosterone undecanoate and three weeks later, posterior (PH) and anterior (AH) hypothalamus was dissected for evaluation of target genes using quantitative RT-PCR.

RESULTS

We found that GnRH mRNA in the PH was high in GDX rats and 500 mg/kg testosterone reduced GnRH level expression. Finding revealed extremely high level of Kiss1 mRNA in the PH of GDX rats. However, in GDX rats treated with different levels of testosterone, Kiss1 expression was not significantly different than control. We also found that testosterone replacement increased the Kiss1 mRNA level in the AH. Moreover, neurokinin-B mRNA level in PH of GDX rats was similar to control. However, excess testosterone levels were effective in significantly inducing the down-regulation of neurokinin-B expression. The basal level of dynorphin mRNA was increased following testosterone treatments in the AH, where we found no significant difference in the level of RFamide-related-peptide mRNA between the experimental groups.

CONCLUSION

Excess levels of testosterone could act differently from its physiological concentration to regulate hypothalamic androgen sensitive neurons to control GnRH cell.

Neurokinin B signaling in hermaphroditic species, a study of the orange-spotted grouper (Epinephelus coioides)

Neurokinin B (NKB) plays important roles in the mammalian reproductive axis by modulating the release of gonadotropin-releasing hormone (GnRH) and gonadotropins. In the present study, the tac3 cDNA was cloned from a hermaphroditic species, the orange-spotted grouper. Sequence analysis showed that the grouper Tac3 precursor encoded two tachykinin peptides, NKB and NKB-related peptide (NKBRP). Expression analysis in different tissues revealed that tac3 mRNA was highly expressed in the brain of the orange-spotted grouper. In situ hybridization further revealed that it was localized in some hypothalamic nuclei associated with reproductive regulation. During ovarian development, an increase of tac3 expression in the hypothalamus was observed at vitellogenesis stage. Intraperitoneal administration of NKB could increase the gnrh1 and lhβ mRNA levels, and enhance the serum estrogen levels, but did not significantly influence lhβ expression in cultured pituitary cells, indicating that NKB does not directly exert its actions on the pituitary gland. However, it was found that NKBRP had no effect on the expression of two gnrhs and two gths in vivo and in vitro. Effects of sex steroids on tac3 expression were further investigated. During the 17-methyltestosterone-induced sex change in the orange-spotted grouper, hypothalamic tac3 expression showed no significant change. Interestingly, ovariectomy greatly stimulated tac3 expression, while the 17β-estradiol treatment reversed this effect. In general, our data highly indicated that NKB signaling could activate the reproductive axis in the orange-spotted grouper. Our study is the first description of the NKB signaling in the hermaphroditic species.

The 3rd World Conference on Kisspeptin, "Kisspeptin 2017: Brain and Beyond":Unresolved questions, challenges and future directions for the field

The 3rd World Conference on Kisspeptin, "Kisspeptin 2017: Brain and Beyond" was held March 30-31 at the Rosen Centre Hotel in Orlando, Florida, providing an international forum for multidisciplinary scientists to meet and share cutting-edge research on kisspeptin biology and its relevance to human health and disease. The meeting built upon previous world conferences focused on the role of kisspeptin and associated peptides in the control of gonadotropin-releasing hormone (GnRH) secretion and reproduction. Based on recent discoveries, the scope of this meeting was expanded to include functions of kisspeptin and related peptides in other physiological systems including energy homeostasis, pregnancy, ovarian and uterine function, and thermoregulation. In addition, discussions addressed the translation of basic knowledge of kisspeptin biology to the treatment of disease, with the goal of seeking consensus about the best approaches to improve human health. The two-day meeting featured a non-traditional structure, with each day starting with poster sessions followed by lunch discussions and facilitated large-group sessions with short presentations to maximize the exchange of new, unpublished data. Topics were identified by a survey prior to the meeting, and focused on major unresolved questions, important controversies, and future directions in the field. Finally, career development activities provided mentoring for trainees and junior investigators, and networking opportunities for those individuals with established researchers in the field. Overall, the meeting was rated as a success by attendees and covered a wide range of lively and provocative discussion topics on the changing nature of the field of "kisspeptinology" and its future. This article is protected by copyright. All rights reserved.

Sex- and brain region-specific patterns of gene expression associated with socially-mediated puberty in a eusocial mammal

The social environment can alter pubertal timing through neuroendocrine mechanisms that are not fully understood; it is thought that stress hormones (e.g., glucocorticoids or corticotropin-releasing hormone) influence the hypothalamic-pituitary-gonadal axis to inhibit puberty. Here, we use the eusocial naked mole-rat, a unique species in which social interactions in a colony (i.e. dominance of a breeding female) suppress puberty in subordinate animals. Removing subordinate naked mole-rats from this social context initiates puberty, allowing for experimental control of pubertal timing. The present study quantified gene expression for reproduction- and stress-relevant genes acting upstream of gonadotropin-releasing hormone in brain regions with reproductive and social functions in pre-pubertal, post-pubertal, and opposite sex-paired animals (which are in various stages of pubertal transition). Results indicate sex differences in patterns of neural gene expression. Known functions of genes in brain suggest stress as a key contributing factor in regulating male pubertal delay. Network analysis implicates neurokinin B (Tac3) in the arcuate nucleus of the hypothalamus as a key node in this pathway. Results also suggest an unappreciated role for the nucleus accumbens in regulating puberty.

Tachykinin-3 Genes and Peptides Characterized in a Basal Teleost, the European Eel: Evolutionary Perspective and Pituitary Role

In mammals, neurokinin B (NKB) is a short peptide encoded by the gene tac3. It is involved in the brain control of reproduction by stimulating gonadotropin-releasing hormone (GnRH) neurons, mainly via kisspeptin. We investigated tac3 genes and peptides in a basal teleost, the European eel, which shows an atypical blockade of the sexual maturation at a prepubertal stage. Two tac3 paralogous genes (tac3a and tac3b) were identified in the eel genome, each encoding two peptides (NKBa or b and NKB-related peptide NKB-RPa or b). Amino acid sequence of eel NKBa is identical to human NKB, and the three others are novel peptide sequences. The four eel peptides present the characteristic C-terminal tachykinin sequence, as well as a similar alpha helix 3D structure. Tac3 genes were identified in silico in 52 species of vertebrates, and a phylogeny analysis was performed on the predicted TAC3 pre-pro-peptide sequences. A synteny analysis was also done to further assess the evolutionary history of tac3 genes. Duplicated tac3 genes in teleosts likely result from the teleost-specific whole genome duplication (3R). Among teleosts, TAC3b precursor sequences are more divergent than TAC3a, and a loss of tac3b gene would have even occurred in some teleost lineages. NKB-RP peptide, encoded beside NKB by tac3 gene in actinopterygians and basal sarcopterygians, would have been lost in ancestral amniotes. Tissue distribution of eel tac3a and tac3b mRNAs showed major expression of both transcripts in the brain especially in the diencephalon, as analyzed by specific qPCRs. Human NKB has been tested in vitro on primary culture of eel pituitary cells. Human NKB dose-dependently inhibited the expression of lhβ, while having no effect on other glycoprotein hormone subunits (fshβ, tshβ, and gpα) nor on gh. Human NKB also dose-dependently inhibited the expression of GnRH receptor (gnrh-r2). The four eel peptides have been synthesized and also tested in vitro. They all inhibited the expression of both lhβ and of gnrh-r2. This reveals a potential dual inhibitory role of the four peptides encoded by the two tac3 genes in eel reproduction, exerted at the pituitary level on both luteinizing hormone and GnRH receptor.

A high salt diet inhibits obesity and delays puberty in the female rat

BACKGROUND/OBJECTIVES

Processed foods are considered major contributors to the worldwide obesity epidemic. In addition to high sugar and fat contents, processed foods contain large amounts of salt. Owing to the correlations with rising adiposity, salt has recently been proposed to be obesogenic. This study investigated three hypotheses: (i) high salt contributes to weight gain and adiposity in juvenile female rats, (ii) puberty onset would be altered because salt is known to affect neuronal systems involved in activating the reproductive system, and (iii) enhanced adiposity will act synergistically with salt to drive early puberty onset.

DESIGN

Female weanling rats (post-natal day 21, n=105) were fed a low fat/low salt diet, low fat/high salt diet, high fat/low salt diet or a high salt/high fat diet for 24 days. Metabolic measures, including weight gain, food intake, fecal output, activity and temperature were recorded in subsets of animals.

RESULTS

Body weight, retroperitoneal and perirenal fat pad weight, and adipocyte size were all lower in animals fed high fat/high salt compared with animals fed high fat alone. Leptin levels were reduced in high fat/high salt fed animals compared with high fat/low salt-fed animals. Daily calorie intake was higher initially but declined with adjusted food intake and was not different among groups after 5 days. Osmolality and corticosterone were not different among groups. Fecal analysis showed excess fat excretion and a decreased digestive efficiency in animals fed high fat/low salt but not in animals fed high fat/high salt. Although respiratory exchange ratio was reduced by high dietary fat or salt, aerobic-resting metabolic rate was not affected by the diet. High salt delayed puberty onset, regardless of dietary fat content.

CONCLUSIONS

Salt delays puberty and prevents the obesogenic effect of a high fat diet. The reduced weight gain evident in high salt-fed animals is not due to differences in food intake or digestive efficiency.

Withdrawn: Discovering Genes Essential to the Hypothalamic Regulation of Human Reproduction Using a Human Disease Model: Adjusting to Life in the "-Omics" Era

The neuroendocrine regulation of reproduction is an intricate process requiring the exquisite coordination of an assortment of cellular networks, all converging on the GnRH neurons. These neurons have a complex life history, migrating mainly from the olfactory placode into the hypothalamus, where GnRH is secreted and acts as the master regulator of the hypothalamic-pituitary-gonadal axis. Much of what we know about the biology of the GnRH neurons has been aided by discoveries made using the human disease model of isolated GnRH deficiency (IGD), a family of rare Mendelian disorders that share a common failure of secretion and/or action of GnRH causing hypogonadotropic hypogonadism. Over the last 30 years, research groups around the world have been investigating the genetic basis of IGD using different strategies based on complex cases that harbor structural abnormalities or single pleiotropic genes, endogamous pedigrees, candidate gene approaches as well as pathway gene analyses. Although such traditional approaches, based on well-validated tools, have been critical to establish the field, new strategies, such as next-generation sequencing, are now providing speed and robustness, but also revealing a surprising number of variants in known IGD genes in both patients and healthy controls. Thus, before the field moves forward with new genetic tools and continues discovery efforts, we must reassess what we know about IGD genetics and prepare to hold our work to a different standard. The purpose of this review is to: 1) look back at the strategies used to discover the "known" genes implicated in the rare forms of IGD; 2) examine the strengths and weaknesses of the methodologies used to validate genetic variation; 3)substantiate the role of known genes in the pathophysiology of the disease; and 4) project forward as we embark upon a widening use of these new and powerful technologies for gene discovery. (Endocrine Reviews 36: 603-621, 2015).

Kisspeptin as a promising oocyte maturation trigger for in vitro fertilisation in humans

The aim of this review is to analyse the effectiveness of exogenous kisspeptin administration as a novel alternative of triggering oocyte maturation, instead of currently used triggers such as human chorionic gonadotropin (hCG) or gonadotropin releasing hormone (GnRH) agonist, in women undergoing in vitro fertilisation (IVF) treatment. Kisspeptin has been considered a master regulator of two modes of GnRH and hence gonadotropin secretion, pulses and surges. Administration of kisspeptin-10 and kisspeptin-54 induces the luteinising hormone (LH) surge required for egg maturation and ovulation in animal investigations and LH release during the preovulatory phase of the menstrual cycle and hypothalamic amenorrhoea in humans. Exogenous kisspeptin-54 has been successfully administered as a promising method of triggering oocyte maturation, following ovarian stimulation with gonadotropins and GnRH antagonists in women undergoing IVF, due to its efficacy considering achieved pregnancy rates compared to hCG and GnRH agonists. Also, its safety in patients at high risk of developing ovarian hyperstimulation syndrome is noteworthy. Nevertheless, further studies would be desirable to establish the optimal trigger of egg maturation and to improve the reproductive outcome for women undergoing IVF treatment.

Neurokinin B regulates reproduction via inhibition of kisspeptin in a teleost, the striped bass

Kisspeptin and neurokinin B (NKB) are neuropeptides co-expressed in the mammalian hypothalamus and coordinately control GnRH signaling. We have found that Nkb and kisspeptin neurons are distinct in the teleost, striped bass (STB) and capitalized on this phenomenon to study the mode of action of Nkb and its related neuropeptide-F (Nkf), both of which are encoded by the tac3 gene. In vitro brain slices and in vivo administration studies revealed that Nkb/f consistently downregulated kiss2, whereas antagonist (AntD) administration restored this effect. Overall, a minor effect was noted on gnrh1 expression, whereas Gnrh1 content in the pituitaries was reduced after Nkb/f treatment and increased with AntD. Concomitantly, immunostaining demonstrated that hypothalamic Nkb neurons border and densely innervate the largest kiss2 neuronal population in the hypothalamus, which also coexpresses Nkb receptor. No expression of Nkb receptor or Nkb neuronal projections was detected near/in Gnrh1 soma in the preoptic area. At the level of the pituitary, however, the picture was more complex: both Nkb/f and AntD upregulated lhb and fshb expression and Lh secretion in vivo Together with the stimulatory effect of Nkb/f on Lh/Fsh secretion from pituitary cells, in vitro, this may indicate an additional independent action of Nkb/f within the pituitary, in which the hypothalamic pathway is more dominant. The current study demonstrates that Nkb/f utilizes multiple pathways to regulate reproduction in the STB and that in the brain, Nkb mainly acts as a negative modulator of kiss2 to regulate the release of Gnrh1.

Design, Synthesis, and Evaluation of a Neurokinin-1 Receptor-Targeted Near-IR Dye for Fluorescence-Guided Surgery of Neuroendocrine Cancers

The neurokinin-1 receptor (NK1R) is implicated in the growth and metastasis of many tumors, including cancers of the brain (e.g., gliomas, glioblastomas, and astrocytomas), skin (e.g., melanomas), and neuroendocrine tissues (cancers of the breast, stomach, pancreas, larynx, and colon). Because overexpression of NK1R has been reported in most of these malignancies, we have undertaken designing an NK1R-targeted near-infrared (NIR) fluorescent dye for fluorescence-guided surgeries of these cancers. We demonstrate here that an NK1R-binding ligand linked to the NIR dye LS288 selectively accumulates in NK1R-expressing tumor xenografts with high affinity (Kd = 13 nM), allowing intraoperative imaging of these cancers in live mice. Because tumor accumulation is nearly quantitatively blocked by excess unlabeled ligand, and because NK1R-negative tumors and normal tissues display virtually no uptake, we conclude that the observed tumor retention is NK1R-mediated. Results on the synthesis, in vitro characterization, and animal testing of NK1R-targeted NIR dye are presented.

Investigating the KNDy Hypothesis in Humans by Coadministration of Kisspeptin, Neurokinin B, and Naltrexone in Men

CONTEXT

A subpopulation of hypothalamic neurons colocalize three neuropeptides, namely kisspeptin, neurokinin B (NKB), and dynorphin, collectively termed KNDy neurons. Animal studies suggest they interact to affect pulsatile GnRH release (KNDy hypothesis); kisspeptin stimulates, NKB modulates, and dynorphin (an opioid) inhibits.

OBJECTIVE

To investigate the KNDy hypothesis in humans, we assessed for the first time the effects of the coadministration of kisspeptin-54, NKB, and an opioid receptor antagonist, naltrexone, on LH pulsatility (surrogate marker for GnRH pulsatility) and gonadotropin release.

DESIGN, SETTING, AND PARTICIPANTS

This was an ethically approved prospective, single-blinded, placebo-controlled study. Healthy male volunteers (n = 5/group) attended our research facility for eight study visits.

INTERVENTION AND MAIN OUTCOME MEASURE

After 1 hour of baseline blood sampling, participants received a different intervention at each visit: oral 50 mg naltrexone, 8-hour iv infusions of vehicle, 2.56 nmol/kg · h NKB, 0.1 nmol/kg · h kissspeptin-54 (KP) alone and in combination. Frequent blood sampling to measure plasma gonadotropins and sex steroids was conducted and LH pulsatility was determined using blinded deconvolution analysis.

RESULTS

All kisspeptin and naltrexone containing groups potently increased LH and LH pulsatility (P < .001 vs vehicle). NKB alone did not affect gonadotropins. NKB+KP had significantly lower increases in gonadotropins compared with kisspeptin alone (P < .01). Naltrexone+KP was the only group to significantly increase LH pulse amplitude (P < .001 vs vehicle).

CONCLUSIONS

Our results suggest significant interactions between the KNDy neuropeptides on LH pulsatility and gonadotropin release in humans. This has important implications for improving our understanding of GnRH pulse generation in humans.

Control of the onset of puberty

PURPOSE OF REVIEW

The mechanism of puberty initiation remains an enigma, despite extensive research in the field. Pulsatile pituitary gonadotropin secretion under the guidance of hypothalamic gonadotropin-releasing hormone (GnRH) constitutes a sine qua non for pubertal onset. In turn, the secretion of GnRH in the human hypothalamus is regulated by kisspeptin and its receptor as well as by permissive or opposing signals mediated by neurokinin B and dynorphin acting on their respective receptors. These three supra-GnRH regulators compose the Kisspeptin, Neurokinin B and Dynorhin neurons (KNDy) system, a key player in pubertal onset and progression.

RECENT FINDINGS

The recent discovery that makorin ring finger protein 3 is also involved in puberty initiation provided further insights into the regulation of the KNDy pathway. In fact, the inhibitory (γ-amino butyric acid, neuropeptide Y, and RFamide-related peptide-3) and stimulatory signals (glutamate) acting upstream of KNDy called into question the role of makorin ring finger protein 3 as the gatekeeper of puberty. Meanwhile, the findings that 'neuroestradiol' produced locally and endocrine disruptors from the environment may influence GnRH secretion is intriguing. Finally, epigenetic mechanisms have been implicated in pubertal onset through recently discovered mechanisms.

SUMMARY

The exact molecular machinery underlying puberty initiation in humans is under intensive investigation. In this review, we summarize research evidence in the field, while emphasizing the areas of uncertainty and underlining the impact of current information on the evolving theory regarding this fascinating phenomenon.

Metabolic control of female puberty: potential therapeutic targets

INTRODUCTION

The onset of puberty in females is highly sensitive to the nutritional status and the amount of energy reserves of the organism. This metabolic information is sensed and transmitted to hypothalamic GnRH neurons, considered to be ultimately responsible for triggering puberty through the coordinated action of different peripheral hormones, central neurotransmitters, and molecular mediators.

AREAS COVERED

This article will review and discuss (i) the relevant actions of the adipose hormone leptin, as a stimulatory/permissive signal, and the gut hormone ghrelin, as an inhibitory factor, in the metabolic control of female puberty; (ii) the crucial role of the hypothalamic kisspeptin neurons, recently emerged as essential gatekeepers of puberty, in transmitting this metabolic information to GnRH neurons; and (iii) the potential involvement of key cellular energy sensors, such as mTOR, as molecular mediators in this setting.

EXPERT OPINION

The thorough characterization of the physiological roles of the above elements in the metabolic control of female puberty, along with the discovery of novel factors, pathways, and mechanisms involved, will promote our understanding of the complex networks connecting metabolism and puberty and, ultimately, will aid in the design of target-specific treatments for female pubertal disorders linked to conditions of metabolic stress.

Surge-Like Luteinising Hormone Secretion Induced by Retrochiasmatic Area NK3R Activation is Mediated Primarily by Arcuate Kisspeptin Neurones in the Ewe

The neuropeptides neurokinin B (NKB) and kisspeptin are potent stimulators of gonadotrophin-releasing hormone (GnRH)/luteinsing hormone (LH) secretion and are essential for human fertility. We have recently demonstrated that selective activation of NKB receptors (NK3R) within the retrochiasmatic area (RCh) and the preoptic area (POA) triggers surge-like LH secretion in ovary-intact ewes, whereas blockade of RCh NK3R suppresses oestradiol-induced LH surges in ovariectomised ewes. Although these data suggest that NKB signalling within these regions of the hypothalamus mediates the positive-feedback effects of oestradiol on LH secretion, the pathway through which it stimulates GnRH/LH secretion remains unclear. We proposed that the action of NKB on RCh neurones drives the LH surge by stimulating kisspeptin-induced GnRH secretion. To test this hypothesis, we quantified the activation of the preoptic/hypothalamic populations of kisspeptin neurones in response to POA or RCh administration of senktide by dual-label immunohistochemical detection of kisspeptin and c-Fos (i.e. marker of neuronal activation). We then administered the NK3R agonist, senktide, into the RCh of ewes in the follicular phase of the oestrous cycle and conducted frequent blood sampling during intracerebroventricular infusion of the kisspeptin receptor antagonist Kp-271 or saline. Our results show that the surge-like secretion of LH induced by RCh senktide administration coincided with a dramatic increase in c-Fos expression within arcuate nucleus (ARC) kisspeptin neurones, and was completely blocked by Kp-271 infusion. We substantiate these data with evidence of direct projections of RCh neurones to ARC kisspeptin neurones. Thus, NKB-responsive neurones in the RCh act to stimulate GnRH secretion by inducing kisspeptin release from KNDy neurones.

Withdrawn: Discovering Genes Essential to the Hypothalamic Regulation of Human Reproduction Using a Human Disease Model: Adjusting to Life in the "-Omics" Era

The neuroendocrine regulation of reproduction is an intricate process requiring the exquisite coordination of an assortment of cellular networks, all converging on the GnRH neurons. These neurons have a complex life history, migrating mainly from the olfactory placode into the hypothalamus, where GnRH is secreted and acts as the master regulator of the hypothalamic-pituitary-gonadal axis. Much of what we know about the biology of the GnRH neurons has been aided by discoveries made using the human disease model of isolated GnRH deficiency (IGD), a family of rare Mendelian disorders that share a common failure of secretion and/or action of GnRH causing hypogonadotropic hypogonadism. Over the last 30 years, research groups around the world have been investigating the genetic basis of IGD using different strategies based on complex cases that harbor structural abnormalities or single pleiotropic genes, endogamous pedigrees, candidate gene approaches as well as pathway gene analyses. Although such traditional approaches, based on well-validated tools, have been critical to establish the field, new strategies, such as next-generation sequencing, are now providing speed and robustness, but also revealing a surprising number of variants in known IGD genes in both patients and healthy controls. Thus, before the field moves forward with new genetic tools and continues discovery efforts, we must reassess what we know about IGD genetics and prepare to hold our work to a different standard. The purpose of this review is to: 1) look back at the strategies used to discover the "known" genes implicated in the rare forms of IGD; 2) examine the strengths and weaknesses of the methodologies used to validate genetic variation; 3)substantiate the role of known genes in the pathophysiology of the disease; and 4) project forward as we embark upon a widening use of these new and powerful technologies for gene discovery. (Endocrine Reviews 36: 603-621, 2015).

Discovering Genes Essential to the Hypothalamic Regulation of Human Reproduction Using a Human Disease Model: Adjusting to Life in the "-Omics" Era

The neuroendocrine regulation of reproduction is an intricate process requiring the exquisite coordination of an assortment of cellular networks, all converging on the GnRH neurons. These neurons have a complex life history, migrating mainly from the olfactory placode into the hypothalamus, where GnRH is secreted and acts as the master regulator of the hypothalamic-pituitary-gonadal axis. Much of what we know about the biology of the GnRH neurons has been aided by discoveries made using the human disease model of isolated GnRH deficiency (IGD), a family of rare Mendelian disorders that share a common failure of secretion and/or action of GnRH causing hypogonadotropic hypogonadism. Over the last 30 years, research groups around the world have been investigating the genetic basis of IGD using different strategies based on complex cases that harbor structural abnormalities or single pleiotropic genes, endogamous pedigrees, candidate gene approaches as well as pathway gene analyses. Although such traditional approaches, based on well-validated tools, have been critical to establish the field, new strategies, such as next-generation sequencing, are now providing speed and robustness, but also revealing a surprising number of variants in known IGD genes in both patients and healthy controls. Thus, before the field moves forward with new genetic tools and continues discovery efforts, we must reassess what we know about IGD genetics and prepare to hold our work to a different standard. The purpose of this review is to: 1) look back at the strategies used to discover the "known" genes implicated in the rare forms of IGD; 2) examine the strengths and weaknesses of the methodologies used to validate genetic variation; 3) substantiate the role of known genes in the pathophysiology of the disease; and 4) project forward as we embark upon a widening use of these new and powerful technologies for gene discovery.

The NK3 Receptor Antagonist ESN364 Interrupts Pulsatile LH Secretion and Moderates Levels of Ovarian Hormones Throughout the Menstrual Cycle

Women's health disorders such as uterine fibroids and endometriosis are currently treated by GnRH modulators that effectively suppress the hypothalamic-pituitary-gonadal axis. The neurokinin-3 receptor (NK3R) is an alternative target with an important role in the modulation of this axis. In this report, we demonstrate that systemic administration of an NK3R antagonist (ESN364) prolongs the LH interpulse interval in ovarectomized ewes and significantly lowers plasma LH and FSH concentrations in castrated nonhuman primates (Macaca fascicularis). Moreover, daily oral dosing of ESN364 throughout the menstrual cycle in M fascicularis lowered plasma estradiol levels in a dose-dependent manner, although nadir levels of estradiol were maintained well above menopausal levels. Nevertheless, estradiol levels during the follicular phase were sufficiently inhibited at all doses to preclude the triggering of ovulation as evidenced by the absence of the LH surge and failure of a subsequent luteal phase rise in plasma progesterone concentrations, consistent with the absence of normal cycle changes in the uterus. Apart from the point at surge, FSH levels were not altered over the course of the menstrual cycle. These effects of ESN364 were reversible upon cessation of drug treatment. Together these data support the proposed role of neurokinin B-NK3R signaling in the control of pulsatile GnRH secretion. Furthermore, in contrast to GnRH antagonists, NK3R antagonists induce a partial suppression of estradiol and thereby offer a viable therapeutic approach to the treatment of ovarian sex hormone disorders with a mitigated risk of menopausal-like adverse events in response to long-term drug exposure.

Does salt have a permissive role in the induction of puberty?

Puberty is starting earlier than ever before and there are serious physiological and sociological implications as a result of this development. Current research has focused on the potential role of high caloric, and commensurate high adiposity, contributions to early puberty. However, girls with normal BMI also appear to be initiating puberty earlier. Westernized diets, in addition to being high in fat and sugar, are also high in salt. To date, no research has investigated a link between elevated salt and the reproductive axis. We hypothesize that a high salt diet can result in an earlier onset of puberty through three mechanisms that are not mutually exclusive. (1) High salt activates neurokinin B, a hormone that is involved in both the reproductive axis and salt regulation, and this induces kisspeptin release and ultimate activation of the reproductive axis. (2) Vasopressin released in response to high salt acts on vasopressin receptors expressed on kisspeptin neurons in the anteroventral periventricular nucleus, thereby stimulating gonadotropin releasing hormone and subsequently luteinizing hormone secretion. (3) Salt induces metabolic changes that affect the reproductive axis. Specifically, salt acts indirectly to modulate adiposity, ties in with the obesity epidemic, and further compounds the pathologic effects of obesity. Our overall hypothesis offers an additional cause behind the induction of puberty and provides testable postulates to determine the mechanism of potential salt-mediated affects on puberty.

Comprehensive Review on Kisspeptin and Its Role in Reproductive Disorders

Kisspeptin has recently emerged as a key regulator of the mammalian reproductive axis. It is known that kisspeptin, acting centrally via the kisspeptin receptor, stimulates secretion of gonadotrophin releasing hormone (GnRH). Loss of kisspeptin signaling causes hypogonadotrophic hypogonadism in humans and other mammals. Kisspeptin interacts with other neuropeptides such as neurokinin B and dynorphin, to regulate GnRH pulse generation. In addition, a growing body of evidence suggests that kisspeptin signaling be regulated by nutritional status and stress. Kisspeptin may also represent a novel potential therapeutic target in the treatment of fertility disorders. Early human studies suggest that peripheral exogenous kisspeptin administration stimulates gonadotrophin release in healthy adults and in patients with certain forms of infertility. This review aims to concisely summarize what is known about kisspeptin as a regulator of reproductive function, and provide an update on recent advances within this field.

Discovery and optimization of novel antagonists to the human neurokinin-3 receptor for the treatment of sex-hormone disorders (Part I)

Neurokinin-3 receptor (NK3R) has recently emerged as important in modulating the tonic pulsatile gonadotropin-releasing hormone (GnRH) release. We therefore decided to explore NK3R antagonists as therapeutics for sex-hormone disorders that can potentially benefit from lowering GnRH pulsatility with consequent diminished levels of plasma luteinizing hormone (LH) and correspondingly attenuated levels of circulating androgens and estrogens. The discovery and lead optimization of a novel N-acyl-triazolopiperazine NK3R antagonist chemotype achieved through bioisosteric lead change from the high-throughput screening (HTS) hit is described. A concomitant improvement in the antagonist bioactivity and ligand lipophilic efficiency (LLE) parameter were the principal guidelines in the lead optimization efforts. Examples of advanced lead analogues to demonstrate the amenability of this chemotype to achieving a suitable pharmacokinetic (PK) profile are provided as well as pharmacokinetic-pharmacodynamic (PKPD) correlations to analyze the trends observed for LH inhibition in castrated rats and monkeys that served as preliminary in vivo efficacy models.

Significance of serum neurokinin B and kisspeptin levels in the differential diagnosis of premature thelarche and idiopathic central precocious puberty

The aim of the present study was to investigate the diagnostic role of serum neurokinin B level and its relationship with kisspeptin and leptin, which are known to be involved in the initiation of pubertal process. Girls who presented with breast development (<8 years) were included in the study. All patients underwent bone age assessment. Basal levels of serum follicle stimulating hormone and luteinizing hormone were measured and gonadotropin releasing hormone stimulation test was performed. Patients with a bone age/chronological age ratio >1 and a peak luteinizing hormone response in gonadotropin releasing hormone stimulation test >5mIU/L were included in the central precocious puberty group, while patients who did not meet these criteria were included in the premature thelarche group. Patients with organic pathologies were excluded. Healthy prepubertal girls with similar age were included as the control group. Leptin, kisspeptin and neurokinin B levels were measured by ELISA method. The study included 20 girls with idiopathic central precocious puberty 22 girls with premature thelarche and 24 prepubertal controls. While serum kisspeptin, leptin and neurokinin B levels were significantly higher in central precocious puberty and premature thelarche groups compared to controls, no significant difference was found between central precocious puberty and premature thelarche groups. Increased serum levels of leptin, kisspeptin and neurokinin B in patients with premature thelarche and central precocious puberty suggest that they take part during the initiation of pubertal process, however, these markers are not able to differentiate patients with central precocious puberty from premature thelarche.

Effects and interactions of tachykinins and dynorphin on FSH and LH secretion in developing and adult rats

Kisspeptin/neurokinin B/dynorphin (KNDy) neurons, which coexpress kisspeptins (Kps), neurokinin B (NKB), and dynorphin (Dyn), regulate gonadotropin secretion. The KNDy model proposes that NKB (a stimulator, through NK3R) and Dyn (an inhibitor, through κ-opioid receptor) shape Kp secretion onto GnRH neurons. However, some aspects of this paradigm remain ill defined. Here we aimed to characterize the following: 1) the effects of NKB signaling on FSH secretion and 2) the role of Dyn in gonadotropin secretion after NK3R activation; 3) additionally, we explored the roles of other tachykinin receptors, NK1R and NK2R, on gonadotropin release. Thus, the effects of the NK3R agonist, senktide, on FSH release were explored across postnatal development in male and female rats; gonadotropin responses to agonists of NK1R substance P and NK2R [neurokinin A (NKA)] were also monitored. Moreover, the effects of senktide on gonadotropin secretion were assessed after antagonizing Dyn actions by nor-binaltorphimine didydrochloride. Before puberty, rats of both sexes showed increased FSH secretion to senktide (and Kp-10). Conversely, adult female rats were irresponsive to senktide in terms of FSH, despite proven LH responses, whereas the adult males did not display FSH or LH responses to senktide, even at high doses. In turn, substance P and NKA stimulated gonadotropin secretion in prepubertal rats, whereas in adults modest gonadotropin responses to NKA were detected. By pretreatment with a Dyn antagonist, adult males became responsive to senktide in terms of LH secretion and displayed elevated basal LH and FSH levels; nor-binaltorphimine didydrochloride treatment uncovered FSH responses to senktide in adult females. Furthermore, the expression of Pdyn and Opkr1 (encoding Dyn and κ-opioid receptor, respectively) in the mediobasal hypothalamus was greater in males than in females at prepubertal ages. Overall, our data contribute to refining our understanding on how the elements of the KNDy node and related factors (ie, other tachykinins) differentially participate in the control of gonadotropins at different stages of rat postnatal maturation.

Mutations in KISS1 are not responsible for idiopathic hypogonadotropic hypogonadism in Chinese patients

PURPOSE

To investigate whether mutations in the KISS1 gene are present in 170 Chinese patients with idiopathic hypogonadotropic hypogonadism (IHH).

METHODS

Mutational screening of the KISS1 gene was performed in 170 Chinese patients with IHH (133 male cases and 37 female cases) and 187 matched controls (94 males and 93 females).

RESULTS

Two known single-nucleotide polymorphisms (SNP), c. 58G > A in exon 1 and c. 242C > G in exon 2, were identified. However, no difference of genotype and allelic frequencies between cases and controls was observed.

CONCLUSIONS

The results suggest that mutations in the coding sequence of KISS1 are not common in patients with IHH in this Chinese population.

Real-Time GnRH Gene Transcription in GnRH Promoter-Driven Luciferase-Expressing Transgenic Mice: Effect of Kisspeptin

Pulsatile secretion of hypothalamic gonadotropin-releasing hormone (GnRH) is indispensable for controlling proper pituitary gonadotrope functions; however, the mechanism underlying GnRH pulse generation remains largely unknown. It is important to understand the cellular oscillator in individual GnRH neurons and temporal synchronization among GnRH neurons. In this brief review, we summarize our recent findings on episodic GnRH gene transcription at the single GnRH neuron level and in synchronized multicellular burst in relation to the temporal pattern of GnRH secretion. We also detail the effects of kisspeptin on ultradian rhythmic GnRH gene transcription and secretion. We extend our discussion to the hierarchical interaction between circadian and ultradian rhythms. Taken together, the current review elucidates the genomic control of GnRH pulse generation in hypothalamic neurons.

KISS1R: Hallmarks of an Effective Regulator of the Neuroendocrine Axis

Kisspeptin (KP) is now well recognized as a potent stimulator of gonadotropin-releasing hormone (GnRH) secretion and thereby a major regulator of the neuroendocrine-reproductive axis. KP signals via KISS1R, a G protein-coupled receptor (GPCR) that activates the G proteins Gαq/11. Modulation of the interaction of KP with KISS1R is therefore a potential new therapeutic target for stimulating (in infertility) or inhibiting (in hormone-dependent diseases) the reproductive hormone cascade. Major efforts are underway to target KISS1R in the treatment of sex steroid hormone-dependent disorders and to stimulate endogenous hormonal responses along the neuroendocrine axis as part of in vitro fertilization protocols. The development of analogs modulating KISS1R signaling will be aided by an understanding of the intracellular pathways and dynamics of KISS1R signaling under normal and pathological conditions. This review focuses on KISS1R recruitment of intracellular signaling (Gαq/11- and β-arrestin-dependent) pathways that mediate GnRH secretion and the respective roles of rapid desensitization, internalization, and recycling of resensitized receptors in maintaining an active population of KISS1R at the cell surface to facilitate prolonged KP signaling. Additionally, this review summarizes and discusses the major findings of an array of studies examining the desensitization of KP signaling in man, domestic and laboratory animals. This discussion highlights the major effects of ligand efficacy and concentration and the physiological, developmental, and metabolic status of the organism on KP signaling. Finally, the potential for the utilization of KP and analogs in stimulating and inhibiting the reproductive hormone cascade as an alternative to targeting the downstream GnRH receptor is discussed.

Neurokinin B and reproductive functions: "KNDy neuron" model in mammals and the emerging story in fish

In mammals, neurokinin B (NKB), the gene product of the tachykinin family member TAC3, is known to be a key regulator for episodic release of luteinizing hormone (LH). Its regulatory actions are mediated by a subpopulation of kisspeptin neurons within the arcuate nucleus with co-expression of NKB and dynorphin A (commonly called the "KNDy neurons"). By forming an "autosynaptic feedback loop" within the hypothalamus, the KNDy neurons can modulate gonadotropin-releasing hormone (GnRH) pulsatility and subsequent LH release in the pituitary. NKB regulation of LH secretion has been recently demonstrated in zebrafish, suggesting that the reproductive functions of NKB may be conserved from fish to mammals. Interestingly, the TAC3 genes in fish not only encode the mature peptide of NKB but also a novel tachykinin-like peptide, namely NKB-related peptide (or neurokinin F). Recent studies in zebrafish also reveal that the neuroanatomy of TAC3/kisspeptin system within the fish brain is quite different from that of mammals. In this article, the current ideas of "KNDy neuron" model for GnRH regulation and steroid feedback, other reproductive functions of NKB including its local actions in the gonad and placenta, the revised model of tachykinin evolution from invertebrates to vertebrates, as well as the emerging story of the two TAC3 gene products in fish, NKB and NKB-related peptide, will be reviewed with stress on the areas with interesting questions for future investigations.

Effects and therapeutic potentials of kisspeptin analogs: regulation of the hypothalamic-pituitary-gonadal axis

The hypothalamic peptide kisspeptin (metastin), the endogenous ligand of the G protein-coupled receptor KISS1R, plays a critical role in controlling GnRH release from hypothalamic GnRH neurons and thereby regulates hypothalamic-pituitary-gonadal functions. Although the therapeutic potential of kisspeptin is attractive, its susceptibility to proteolytic degradation limits its utility. To overcome this, KISS1R agonists or antagonists as peptide analogs or small molecules have been investigated. Kisspeptin analogs have been most extensively studied by reducing the length of the peptide from the original 54 amino acids to 10 amino acids or less and by substituting key amino acid residues. Also, 2 investigational kisspeptin agonist analogs have been evaluated in clinical studies in men; in agreement with animal studies, abrupt elevations in gonadotropin and testosterone levels were observed as an acute effect, followed by rapid reductions in these hormones as a chronic effect. Some studies of small-molecule KISS1R antagonists have also been published. In this review, we present a brief overview on kisspeptin/KISS1R physiology in reproductive functions and summarize the available knowledge of both agonists and antagonists. We also focus on the kisspeptin agonist analogs by summarizing key pharmacological findings from both clinical and preclinical studies, and discuss their potential therapeutic utility.