Neurokinin B acts via the neurokinin-3 receptor in the retrochiasmatic area to stimulate luteinizing hormone secretion in sheep

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.

Immunoelectron microscopic observation of the subcellular localization of kisspeptin, neurokinin B and dynorphin A in KNDy neurons in the arcuate nucleus of the female rat

KNDy neurons are named for their co-expression of three neuropeptides, kisspeptin, neurokinin B (NKB) and dynorphin A (DynA). These cells, located in the hypothalamic arcuate nucleus (ARC), are associated with generation of gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) pulses to control follicular growth and steroidogenesis. However, subcellular sorting mechanisms for secretory vesicles containing these neuropeptides have not been elucidated. In this study, we analyzed the localization pattern of kisspeptin, NKB and DynA in the ARC of the ovariectomized rat immediately treated with estrogen using immunoelectron microscopy. First, we identified neuropeptides by dual-labeled fluorescence immunohistochemistry, with results indicating all three neuropeptides co-express within individual ARC cells in female rats. Next, we investigated the subcellular localization pattern of kisspeptin, NKB, and/or DynA using post-embedding double immunoelectron microscopy, indicating that each type of neuropeptide is contained within separate and individual neurosecretory vesicles. This suggests sorting and packaging of kisspeptin, NKB and DynA is differentially regulated within KNDy neurons. Our findings facilitate understanding of regulatory mechanisms underlying kisspeptin secretion in KNDy neurons, and generation of GnRH/LH pulses induced by kisspeptin in the ARC.

Ovarian and hormonal responses to single or continuous peripheral administration of senktide, a neurokinin 3 receptor agonist, during the follicular phase in goats

The present study aimed to investigate the effects of single or continuous administration of a neurokinin 3 receptor agonist, senktide, on hormonal and follicular dynamics in follicular phase goats. Goats were injected with PGF2α in the luteal phase and treated with an intravaginal progesterone device for 10 d. At 12 h after the cessation of progesterone treatment, the goats received a single intravenous injection of senktide (200 nmol, n = 4) or vehicle (n = 4), or continuous intravenous infusion of senktide (20 nmol/min, n = 6) or vehicle (n = 6) for 6 h. Blood sampling and ovarian ultrasonography were performed during the experiment. A single injection of senktide did not influence the number of luteinizing hormone (LH) pulses and mean LH concentration. On the other hand, continuous injection of senktide caused a sustained increase in LH secretion, and mean LH concentration in samples collected at 10-min intervals for 6 h after the start of infusion was higher than that of vehicle-treated goats (2.8 ± 1.3 vs 1.0 ± 0.6 ng/mL, P < 0.01). In 4 of 6 goats, LH concentrations reached their peaks during the 6-h senktide infusion, and ovulation was observed at 48 h after the start of infusion without estrous behavior. The remaining 2 senktide-treated goats and all vehicle-treated goats showed estrus and ovulated at 72 or 96 h after treatment. These results suggest that continuous infusion of senktide in follicular phase goats can cause a sustained increase in LH and advance the time of ovulation.

Goldfish neurokinin B: Cloning, tissue distribution, and potential role in regulating reproduction

Neurokinin B (NKB) is a member of the tackykinin (TAC) family known to play a critical role in the neuroendocrine regulation of reproduction in mammals. However, its biological functions in teleosts are less clear. The aim of this study was to determine the role of NKB in fish reproduction using goldfish as a model. Two transcripts, TAC3a and TAC3b, which encode several NKBs, including NKBa-13, NKBa-10, NKBb-13, and NKBb-11, were cloned. Phylogenetic analysis revealed that NKBa-10 and NKBb-11 are closely related to mammalian NKB, while NKB-13s are more conserved in teleosts. Quantitative real-time PCR analyses in various tissues showed that TAC3a and TAC3b mRNAs were mainly expressed in the brain. In situ hybridization further detected TAC3a and TAC3b mRNAs in several regions of the brain known to be involved in the regulation of reproduction and metabolism, as well as in the neurohypophysis of the pituitary. To investigate the potential role of NKBs in reproduction, goldfish were injected intraperitoneally with synthetic NKBa-13, -10, NKBb-13, or -11 peptides and the mRNA levels of hypothalamic gonadotropin-releasing hormone (GnRH) and pituitary gonadotropin subunits were measured. NKBa-13, -10, or NKBb-13, but not -11, significantly increased hypothalamic salmon GnRH and pituitary FSHβ and LHβ mRNA levels in both female and male goldfish. Finally, ovariectomy increased, while estradiol replacement reduced, TAC3a mRNA levels without affecting TAC3b expression in the hypothalamus. These data suggest that NKBa-13, -10, and NKBb-13 play a role in mediating the estrogen negative feedback regulation of gonadotropins.

Prenatal Testosterone Treatment Leads to Changes in the Morphology of KNDy Neurons, Their Inputs, and Projections to GnRH Cells in Female Sheep

Prenatal testosterone (T)-treated ewes display a constellation of reproductive defects that closely mirror those seen in PCOS women, including altered hormonal feedback control of GnRH. Kisspeptin/neurokinin B/dynorphin (KNDy) neurons of the arcuate nucleus (ARC) play a key role in steroid feedback control of GnRH secretion, and prenatal T treatment in sheep causes an imbalance of KNDy peptide expression within the ARC. In the present study, we tested the hypothesis that prenatal T exposure, in addition to altering KNDy peptides, leads to changes in the morphology and synaptic inputs of this population, kisspeptin cells of the preoptic area (POA), and GnRH cells. Prenatal T treatment significantly increased the size of KNDy cell somas, whereas POA kisspeptin, GnRH, agouti-related peptide, and proopiomelanocortin neurons were each unchanged in size. Prenatal T treatment also significantly reduced the total number of synaptic inputs onto KNDy neurons and POA kisspeptin neurons; for KNDy neurons, the decrease was partly due to a decrease in KNDy-KNDy synapses, whereas KNDy inputs to POA kisspeptin cells were unaltered. Finally, prenatal T reduced the total number of inputs to GnRH cells in both the POA and medial basal hypothalamus, and this change was in part due to a decreased number of inputs from KNDy neurons. The hypertrophy of KNDy cells in prenatal T sheep resembles that seen in ARC kisspeptin cells of postmenopausal women, and together with changes in their synaptic inputs and projections to GnRH neurons, may contribute to defects in steroidal control of GnRH observed in this animal model.

Daily successive changes in reproductive gene expression and neuronal activation in the brains of pubertal female mice

Puberty is governed by the secretion of gonadotropin releasing hormone (GnRH), but the roles and identities of upstream neuropeptides that control and time puberty remain poorly understood. Indeed, how various reproductive neural gene systems change before and during puberty, and in relation to one another, is not well-characterized. We detailed the daily pubertal profile (from postnatal day [PND] 15 to PND 30) of neural Kiss1 (encoding kisspeptin), Kiss1r (kisspeptin receptor), Tac2 (neurokinin B), and Rfrp (RFRP-3, mammalian GnIH) gene expression and day-to-day c-fos induction in each of these cell types in developing female mice. Kiss1 expression in the AVPV/PeN increased substantially over the pubertal transition, reaching adult levels around vaginal opening (PND 27.5), a pubertal marker. However, AVPV/PeN Kiss1 neurons were not highly activated, as measured by c-fos co-expression, at any pubertal age. In the ARC, Kiss1 and Tac2 cell numbers showed moderate increases across the pubertal period, and neuronal activation of Tac2/Kiss1 cells was moderately elevated at all pubertal ages. Additionally, Kiss1r expression specifically in GnRH neurons was already maximal by PND 15 and did not change with puberty. Conversely, both Rfrp expression and Rfrp/c-fos co-expression in the DMN decreased markedly in the early pre-pubertal stage. This robust decrease of the inhibitory RFRP-3 population may diminish inhibition of GnRH neurons during early puberty. Collectively, our data identify the precise timing of important developmental changes - and in some cases, lack thereof - in gene expression and neuronal activation of key reproductive neuropeptides during puberty, with several changes occurring well before vaginal opening.

Prenatal testosterone excess decreases neurokinin 3 receptor immunoreactivity within the arcuate nucleus KNDy cell population

Prenatal exposure of the female ovine foetus to excess testosterone leads to neuroendocrine disruptions in adulthood, as demonstrated by defects in responsiveness with respect to the ability of gonadal steroids to regulate gonadotrophin-releasing hormone (GnRH) secretion. In the ewe, neurones of the arcuate nucleus (ARC), which co-expresses kisspeptin, neurokinin B (NKB) and dynorphin (termed KNDy cells), play a key role in steroid feedback control of GnRH and show altered peptide expression after prenatal testosterone treatment. KNDy cells also co-localise NKB receptors (NK3R), and it has been proposed that NKB may act as an autoregulatory transmitter in KNDy cells where it participates in the mechanisms underlying steroid negative-feedback. In addition, recent evidence suggests that NKB/NK3R signalling may be involved in the positive-feedback actions of oestradiol leading to the GnRH/luteinising hormone (LH) surge in the ewe. Thus, we hypothesise that decreased expression of NK3R in KNDy cells may be present in the brains of prenatal testosterone-treated animals, potentially contributing to reproductive defects. Using single- and dual-label immunohistochemistry we found NK3R-positive cells in diverse areas of the hypothalamus; however, after prenatal testosterone treatment, decreased numbers of NK3R immunoreactive (-IR) cells were seen only in the ARC. Moreover, dual-label confocal analyses revealed a significant decrease in the percentage of KNDy cells (using kisspeptin as a marker) that co-localised NK3R. To investigate how NKB ultimately affects GnRH secretion in the ewe, we examined GnRH neurones in the preoptic area (POA) and mediobasal hypothalamus (MBH) for the presence of NK3R. Although, consistent with earlier findings, we found no instances of NK3R co-localisation in GnRH neurones in either the POA or MBH; in addition, > 70% GnRH neurones in both areas were contacted by NK3R-IR presynaptic terminals suggesting that, in addition to its role at KNDy cell bodies, NKB may regulate GnRH neurones by presynaptic actions. In summary, the finding of decreased NK3R within KNDy cells in prenatal testosterone-treated sheep complements previous observations of decreased NKB and dynorphin in the same population, and may contribute to deficits in the feedback control of GnRH/LH secretion in this animal model.

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.

Leap of Faith: Does Serum Luteinizing Hormone Always Accurately Reflect Central Reproductive Neuroendocrine Activity?

The function of the central aspects of the hypothalamic-pituitary-gonadal axis has been assessed in a number of ways including direct measurements of the hypothalamic output and indirect measures using gonadotropin release from the pituitary as a bioassay for reproductive neuroendocrine activity. Here, methods for monitoring these various parameters are briefly reviewed and then examples presented of both concordance and discrepancy between central and peripheral measurements, with a focus on situations in which elevated gonadotropin-releasing hormone neurosecretion is not reflected accurately by pituitary luteinizing hormone release. Implications for the interpretation of gonadotropin data are discussed.

Evidence that Neurokinin B Controls Basal Gonadotropin-Releasing Hormone Secretion but Is Not Critical for Estrogen-Positive Feedback in Sheep

BACKGROUND

Loss-of-function mutations in genes encoding kisspeptin or neurokinin B (NKB) or their receptors cause infertility. NKB is coproduced in kisspeptin neurons in the arcuate nucleus (ARC), and these neurons also produce the NKB receptor (NK3R), allowing autosynaptic function. We tested the hypothesis that NKB action in ARC kisspeptin neurons is aligned with increased pulsatile secretion of luteinizing hormone (LH) and/or activation of the estrogen-induced LH surge in ewes.

METHODS

Using in situ hybridization and immunohistochemistry, we examined NKB expression in kisspeptin neurons during the ovine estrous cycle. We infused kisspeptin, senktide (an NK3R agonist), or dynorphin into the lateral ventricle during the luteal phase of the estrous cycle to determine effects on pulsatile LH secretion. Finally, we examined the effect of an NK3R antagonist (MRK-08) in ovariectomized ewes.

RESULTS

NKB (Tac3) mRNA expression in mid-ARC kisspeptin neurons was elevated during the mid-to-late follicular phase of the estrous cycle. The number of NKB-immunoreactive cells and NKB/kisspeptin terminals in the median eminence was similar during the estrous cycle. Kisspeptin and senktide increased LH pulse frequency and mean LH levels. Central MRK-08 infusion eliminated the LH pulses but did not prevent an estrogen-positive feedback on LH secretion.

CONCLUSIONS

NKB expression in ARC kisspeptin neurons is upregulated during the late follicular phase of the estrous cycle, when the pulsatile secretion of gonadotropin-releasing hormone (GnRH)/LH is maximal. When GnRH/LH secretion is minimal, central senktide infusion induces LH secretion, similar to the response to kisspeptin. Although the increase in LH in response to senktide appeared surge-like, we did not observe any change in the surge following NK3R antagonist treatment. We conclude that NKB plays a role in increasing basal GnRH/LH pulsatility in the follicular phase of the cycle but is not essential for estrogen-induced positive feedback.

Absence of GPR54 and TACR3 mutations in sporadic cases of idiopathic central precocious puberty

BACKGROUND/AIMS

Kisspeptin (KISS1)/GPR54 (KISSR) signaling complex and neurokinin B (NKB)/NKB receptor (TACR3) signaling have been proposed as an integral part of the network coordinating GnRH release. GPR54 (KISS1R) and TACR3 gene mutations have been described in cases of idiopathic hypogonadotrophic hypogonadism, while limited data exist on gain-of-function mutation in GPR54 (KISS1R) gene causing idiopathic central precocious puberty (ICPP). No data on TACR3 mutations in ICPP have been described so far. The aim of this study was to elucidate the possible impact of GPR54 (KISS1R) and TACR3 mutations in ICPP.

METHODS

PCR-amplified genomic DNA of 38 girls with ICPP was analyzed for GPR54 and TACR3 gene mutations.

RESULTS

No GPR54 or TACR3 mutations were found. The A/G coding sequence single nucleotide polymorphism (SNP) on the GPR54 gene (dbSNP ID: rs10407968) was found in 2 patients with ICPP.

CONCLUSION

Our data indicate that GPR54 and TACR3 gene mutations are not a frequent cause of ICPP. The identified A/G synonymous SNP (dbSNP ID: rs10407968) located in exon 1 of the gene is not likely to have a pathogenic role in exon splicing and therefore in the premature initiation of puberty.

Direct regulation of gonadotropin release by neurokinin B in tilapia (Oreochromis niloticus)

Neurokinin B (NKB) was recently identified as a key regulator of reproduction in mammals and fish. Fish were found to possess a specific novel neurokinin termed NKF. To study the role of NKB/NKF in the regulation of fish reproduction and to investigate the role of NKB/NKF and their receptors in the piscine pituitary, we have identified the NKB/tachikinin 3 receptor (tac3r) system in tilapia. Bioinformatics and phylogenetic analyses have demonstrated that the tilapia holds 1 putative tac3 gene and 2 NKB receptor genes (tac3ra and tac3rb) that clustered with other piscine Tac3 and NKB receptor lineages. Furthermore, we found that in African cichlids, NKB peptides differ from other vertebrate NKBs in their C-terminal sequence, possessing isoleucine instead of valine as the X in the NKB FXGLM-NH2-terminal consensus sequence. Signal transduction analysis demonstrated that tilapia NKB (tiNKB), tiNKF, and human NKB activated both CRE-luc and SRE-luc transcriptional activity of both tilapia and human NKB receptors. Two hours after ip injection of tiNKB, the plasma levels of both FSH and LH were increased, whereas tiNKF was more effective in increasing LH levels. However, tiNKB was more effective than tiNKF in increasing both FSH and LH from tilapia pituitary dispersed cells. Using in situ hybridization and fluorescent immunohistochemistry, we have shown that LH cells possess tac3, tac3ra, and tac3rb mRNAs, whereas FSH cells possess mainly tac3rb and tac3ra and tac3 to a much lesser extent. These results suggest that the members of the NKB/tac3r system may serve as paracrine/autocrine regulators of gonadotropin release in fish pituitary.

Effects of senktide, a neurokinin 3 receptor agonist, on luteinizing hormone secretion and follicular development in anestrous Shiba goats: a pilot study

Background

Recent studies suggest that neurokinin B and its receptor, neurokinin 3 receptor, have an essential role in the regulation of gonadotropin-releasing hormone and luteinizing hormone (LH) release in several mammalian species. As the first trial, this pilot study reports the effect of intravenous treatment with senktide, a selective agonist of neurokinin 3 receptor, on LH secretion, follicular development in female goats that were clinically diagnosed with anestrus.

Findings

Anestrous goats were intravenously administered 200 nmol senktide at 4-h intervals for 24 h. Most of them examined (5/6 cases) showed a pulsatile increase in LH secretion after each injection of senktide, whereas the remaining one case showed a surge-like increase of LH secretion. Ovulation was confirmed in 5/6 cases at the range of 48–96 h after the beginning of treatment.

Conclusions

This pilot study demonstrated that intravenous treatment with senktide has therapeutic action in goats with anestrus by inducing LH release, which could promote follicular development and ovulation.

Neurokinin-3 receptor activation in the retrochiasmatic area is essential for the full pre-ovulatory luteinising hormone surge in ewes

Neurokinin B (NKB) is essential for human reproduction and has been shown to stimulate luteinising hormone (LH) secretion in several species, including sheep. Ewes express the neurokinin-3 receptor (NK3R) in the retrochiasmatic area (RCh) and there is one report that placement of senktide, an NK3R agonist, therein stimulates LH secretion that resembles an LH surge in ewes. In the present study, we first confirmed that local administration of senktide to the RCh produced a surge-like increase in LH secretion, and then tested the effects of this agonist in two other areas implicated in the control of LH secretion and where NK3R is found in high abundance: the preoptic area (POA) and arcuate nucleus (ARC). Bilateral microimplants containing senktide induced a dramatic surge-like increase in LH when given in the POA similar to that seen with RCh treatment. By contrast, senktide treatment in the ARC resulted in a much smaller but significant increase in LH concentrations suggestive of an effect on tonic secretion. The possible role of POA and RCh NK3R activation in the LH surge was next tested by treating ewes with SB222200, an NK3R antagonist, in each area during an oestradiol-induced LH surge. SB222200 in the RCh, but not in the POA, reduced the LH surge amplitude by approximately 40% compared to controls, indicating that NK3R activation in the former region is essential for full expression of the pre-ovulatory LH surge. Based on these data, we propose that the actions of NKB in the RCh are an important component of the pre-ovulatory LH surge in ewes.

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.

Disparate changes in kisspeptin and neurokinin B expression in the arcuate nucleus after sex steroid manipulation reveal differential regulation of the two KNDy peptides in rats

Kisspeptin, neurokinin B (NKB) and dynorphin A are coexpressed in a population of neurons in the arcuate nucleus (ARC), termed KNDy neurons, which were recently recognized as important elements for the generation of GnRH pulses. However, the topographic distribution of these peptides and their regulated expression by sex steroids are still not well understood. In this study, detailed examination of NKB and kisspeptin immunoreactivity in the rat ARC was carried out, including comparison between sexes, with and without sex steroid replacement. Neurons expressing kisspeptin and NKB were more prominent in the caudal ARC of females, whereas neurons expressing NKB, but not kisspeptin, were the most abundant in the male. Sex steroid manipulation revealed differential regulation of kisspeptin and NKB; although kisspeptin immunoreactive (ir) cells increased in response to gonadectomy, NKB remained unchanged. Furthermore, the number of NKB-ir cells increased upon sex steroid replacement compared with gonadectomy, whereas kisspeptin did not, suggesting that sex steroids differently regulate these peptides. In addition, only in females did the density of kisspeptin- and NKB-ir fibers in the ARC increase upon sex steroid replacement in relation to sham and ovariectomy, respectively, suggesting sex-specific regulation of release. In conclusion, our observations reveal sex differences in the number of kisspeptin- and NKB-ir cells, which are more prominent in the caudal ARC. The divergent regulation of kisspeptin and NKB peptide contents in the ARC as a function of sex and steroid milieu enlarge our understanding on how these neuropeptides are posttranscriptionally regulated in KNDy neurons.

Photoperiodic co-regulation of kisseptin, neurokinin B and dynorphin in the hypothalamus of a seasonal rodent

In many species, sexual activity varies on a seasonal basis. Kisspeptin (Kp), a hypothalamic neuropeptide acting as a strong activator of gonadotrophin-releasing hormone neurones, plays a critical role in this adaptive process. Recent studies report that two other neuropeptides, namely neurokinin B (NKB) and dynorphin (DYN), are co-expressed with Kp (and therefore termed KNDy neurones) in the arcuate nucleus and that these peptides are also considered to influence GnRH secretion. The present study aimed to establish whether hypothalamic NKB and DYN expression is photoperiod-dependent in a seasonal rodent, the Syrian hamster, which exhibits robust seasonal rhythms in reproductive activity. The majority of Kp neurones in the arcuate nucleus co-express NKB and DYN and the expression of all three peptides is decreased under a short (compared to long) photoperiod, leading to a 60% decrease in the number of KNDy neurones under photo-inhibitory conditions. In seasonal rodents, RFamide-related peptide (RFRP) neurones of the dorsomedial hypothalamus are also critical for seasonal reproduction. Interestingly, NKB and DYN are also expressed in the dorsomedial hypothalamus but do not co-localise with RFRP-immunoreactive neurones, and the expression of both NKB and DYN is higher under a short photoperiod, which is opposite to the short-day inhibition of RFRP expression. In conclusion, the present study shows that NKB and DYN display different photoperiodic variations in the Syrian hamster hypothalamus. In the arcuate nucleus, NKB and DYN, together with Kp, are down-regulated under a short photoperiod, whereas, in the dorsomedial hypothalamus, NKB and DYN are up-regulated under a short photoperiod.

Prenatal androgen excess enhances stimulation of the GNRH pulse in pubertal female rats

In adolescent girls with polycystic ovary syndrome (PCOS), neuroendocrine derangements manifest after the onset of puberty, characterized by rapid LH pulse frequency. The early mechanism underlying the pubertal regulation of the GNRH/LH pulsatile release in adolescents with PCOS remains uncertain. To determine the effects of prenatal androgen exposure on the activation of GNRH neurons and generation of LH pulse at puberty, we administrated 5α-dihydrotestosterone to pregnant rats and observed serum LH levels and expression of hypothalamic genes in female offspring from postnatal 4 to 8 weeks. The 6-week-old prenatally androgenized (PNA) female rats exhibited an increase in LH pulse frequency. The hypothalamic expression of neurokinin B (Nkb (Tac2)) and Lepr mRNA levels in PNA rats increased remarkably before puberty and remained high during puberty, whereas elevated Kiss1 mRNA levels were detected only after the onset of puberty. Exogenous kisspeptin, NK3R agonist, and leptin triggered tonic stimulation of GNRH neurons and increased LH secretion in 6-week-old PNA rats. Leptin upregulated Kiss1 mRNA levels in the hypothalamus of pubertal PNA rats; however, pretreatment with a kisspeptin antagonist failed to suppress the elevated serum LH stimulated by leptin, indicating that the stimulatory effects of leptin may be conveyed indirectly to GNRH neurons via other neural components within the GNRH neuronal network, rather than through the kisspeptin-GPR54 pathway. These findings validate the hypotheses that NKB and leptin play an essential role in the activation of GNRH neurons and initiation of increased LH pulse frequency in PNA female rats at puberty and that kisspeptin may coordinate their stimulatory effects on LH release.

Neurokinin B signaling in the female rat: a novel link between stress and reproduction

Acute systemic stress disrupts reproductive function by inhibiting pulsatile gonadotropin secretion. The underlying mechanism involves stress-induced suppression of the GnRH pulse generator, the functional unit of which is considered to be the hypothalamic arcuate nucleus kisspeptin/neurokinin B/dynorphin A neurons. Agonists of the neurokinin B (NKB) receptor (NK3R) have been shown to suppress the GnRH pulse generator, in a dynorphin A (Dyn)-dependent fashion, under hypoestrogenic conditions, and Dyn has been well documented to mediate several stress-related central regulatory functions. We hypothesized that the NKB/Dyn signaling cascade is required for stress-induced suppression of the GnRH pulse generator. To investigate this ovariectomized rats, iv administered with Escherichia coli lipopolysaccharide (LPS) following intracerebroventricular pretreatment with NK3R or κ-opioid receptor (Dyn receptor) antagonists, were subjected to frequent blood sampling for hormone analysis. Antagonism of NK3R, but not κ-opioid receptor, blocked the suppressive effect of LPS challenge on LH pulse frequency. Neither antagonist affected LPS-induced corticosterone secretion. Hypothalamic arcuate nucleus NKB neurons project to the paraventricular nucleus, the major hypothalamic source of the stress-related neuropeptides CRH and arginine vasopressin (AVP), which have been implicated in the stress-induced suppression of the hypothalamic-pituitary-gonadal axis. A separate group of ovariectomized rats was, therefore, used to address the potential involvement of central CRH and/or AVP signaling in the suppression of LH pulsatility induced by intracerebroventricular administration of a selective NK3R agonist, senktide. Neither AVP nor CRH receptor antagonists affected the senktide-induced suppression of the LH pulse; however, antagonism of type 2 CRH receptors attenuated the accompanying elevation of corticosterone levels. These data indicate that the suppression of the GnRH pulse generator by acute systemic stress requires hypothalamic NKB/NK3R signaling and that any involvement of CRH therewith is functionally upstream of NKB.

The kisspeptin-GnRH pathway in human reproductive health and disease

BACKGROUND

The discovery of kisspeptin as key central regulator of GnRH secretion has led to a new level of understanding of the neuroendocrine regulation of human reproduction. The related discovery of the kisspeptin-neurokinin B-dynorphin (KNDy) pathway in the last decade has further strengthened our understanding of the modulation of GnRH secretion by endocrine, metabolic and environmental inputs. In this review, we summarize current understanding of the physiological roles of these novel neuropeptides, and discuss the clinical relevance of these discoveries and their potential translational applications.

METHODS

A systematic literature search was performed using PUBMED for all English language articles up to January 2014. In addition, the reference lists of all relevant original research articles and reviews were examined. This review focuses mainly on published human studies but also draws on relevant animal data.

RESULTS

Kisspeptin is a principal regulator of the secretion of gonadotrophins, and through this key role it is critical for the onset of puberty, the regulation of sex steroid-mediated feedback and the control of adult fertility. Although there is some sexual dimorphism, both neuroanatomically and functionally, these functions are apparent in both men and women. Kisspeptin acts upstream of GnRH and, following paracrine stimulatory and inhibitory inputs from neurokinin B and dynorphin (KNDy neuropeptides), signals directly to GnRH neurones to control pulsatile GnRH release. When administered to humans in different isoforms, routes and doses, kisspeptin robustly stimulates LH secretion and LH pulse frequency. Manipulation of the KNDy system is currently the focus of translational research with the possibility of future clinical application to regulate LH pulsatility, increasing gonadal sex steroid secretion in reproductive disorders characterized by decreased LH pulsatility, including hypothalamic amenorrhoea and hypogonadotropic hypogonadism. Conversely there may be scope to reduce the activity of the KNDy system to reduce LH secretion where hypersecretion of LH adds to the phenotype, such as in polycystic ovary syndrome.

CONCLUSIONS

Kisspeptin is a recently discovered neuromodulator that controls GnRH secretion mediating endocrine and metabolic inputs to the regulation of human reproduction. Manipulation of kisspeptin signalling has the potential for novel therapies in patients with pathologically low or high LH pulsatility.

Effects of neurokinin B administration on reproductive hormone secretion in healthy men and women

BACKGROUND

Neurokinin B (NKB) is a member of the tachykinin family of peptides. Inactivating mutations in the tachykinin 3 or tachykinin 3 receptor gene are associated with pubertal failure and congenital hypogonadotrophic hypogonadism in humans. This suggests that NKB may have a critical role in human reproduction. The effects of NKB administration have not been investigated previously in humans.

AIM

The aim of this study was to determine the effects of iv administration of NKB on gonadotrophin secretion in healthy male and female volunteers.

METHODS

A total of 23 healthy men and 11 healthy women participated in the study. After an initial dose-finding study (study 1), men received a 4-hour infusion of vehicle (gelofusin) followed by a 4-hour infusion of NKB (2.56 or 5.12 nmol/kg/h) (study 2), and an 8-hour infusion of vehicle or NKB during different visits (study 3). Healthy women underwent a dose-finding study consisting of a 3-hour NKB administration during the follicular phase of the menstrual cycle, and the maximum dose of NKB was also tested during the preovulatory and midluteal phases of menstrual cycle (study 4).

RESULTS

Mean LH, FSH, and T secretion were not significantly altered during a 90-minute infusion of NKB (0.4-5.12 nmol/kg/h), or a 4-hour infusion of NKB (5.12 nmol/kg/h). No alterations in gonadotrophin secretion or LH pulsatility were observed during an 8-hour infusion of NKB when compared with vehicle. Doses of 0.64-5.12 nmol/kg/h NKB did not significantly alter LH, FSH, or estradiol secretion in healthy women during the follicular phase of the menstrual cycle. Finally, 5.12 nmol/kg/h did not significantly alter reproductive hormone secretion during the preovulatory or midluteal phases of the menstrual cycle.

CONCLUSIONS

This is the first clinical study of NKB administration. None of the doses of NKB tested were associated with significant alterations in reproductive hormone secretion in healthy male or female volunteers. These novel data add to our understanding of the physiological actions of NKB in human reproduction.

A role for neurokinin B in pulsatile GnRH secretion in the ewe

The recent description of infertility in humans with loss-of-function mutations in genes for neurokinin B (NKB) or its receptor (NK3R) has focused attention on the importance of this tachykinin in the control of GnRH secretion. In a number of species, NKB neurons in the arcuate nucleus also produce two other neuropeptides implicated in the control of GnRH secretion: (1) kisspeptin, which is also essential for fertility in humans, and (2) dynorphin, an inhibitory endogenous opioid peptide. A number of characteristics of this neuronal population led to the hypothesis that they may be responsible for driving synchronous release of GnRH during episodic secretion of this hormone, and there is now considerable evidence to support this hypothesis in sheep and goats. In this article, we briefly review the history of work on the NKB system in sheep and then review the anatomy of NKB signaling in the ewe. We next describe evidence from a number of species that led to development of a model for the role of these neurons in episodic GnRH secretion. Finally, we discuss recent experiments in sheep and goats that tested this hypothesis and led to a modified version of the model, and then broaden our focus to briefly consider the possible roles of NKB in other species and systems.

The role of neurokinin B signalling in reproductive neuroendocrinology

The KNDy neuropeptides, kisspeptin, neurokinin B (NKB) and dynorphin A (Dyn), have been implicated in regulating pulsatile luteinising hormone (LH) secretion. Studies of the interactions between KNDy signalling systems, however, are currently few. Although the stimulatory effect of kisspeptin and the inhibitory effect of Dyn on the gonadotropin-releasing hormone pulse generator are widely accepted, the effects of NKB in rodents are variable and sometimes controversial. Literature describing increased LH secretion in response to NKB receptor agonism predominates and is in line with human physiology, as well as the pathophysiology of pubertal failure associated with disruption of NKB signalling. However, the robust suppression of the LH pulse, induced by the same treatment under hypoestrogenic conditions, may hold clues as to the mechanisms of reproductive inhibition under pathological conditions. This review discusses the recent evidence for this paradox and outlines a revised working model incorporating the mechanisms by which KNDy neuropeptides modulate the reproductive axis.

TACR3 mutations disrupt NK3R function through distinct mechanisms in GnRH-deficient patients

Neurokinin B (NKB) and its G-protein-coupled receptor, NK3R, have been implicated in the neuroendocrine control of GnRH release; however, little is known about the structure-function relationship of this ligand-receptor pair. Moreover, loss-of-function NK3R mutations cause GnRH deficiency in humans. Using missense mutations in NK3R we previously identified in patients with GnRH deficiency, we demonstrate that Y256H and Y315C NK3R mutations in the fifth and sixth transmembrane domains (TM5 and TM6), resulted in reduced whole-cell (79.3±7.2%) or plasma membrane (67.3±7.3%) levels, respectively, compared with wild-type (WT) NK3R, with near complete loss of inositol phosphate (IP) signaling, implicating these domains in receptor trafficking, processing, and/or stability. We further demonstrate in a FRET-based assay that R295S NK3R, in the third intracellular loop (IL3), bound NKB but impaired dissociation of Gq-protein subunits from the receptor compared with WT NK3R, which showed a 10.0 ± 1.3% reduction in FRET ratios following ligand binding, indicating activation of Gq-protein signaling. Interestingly, R295S NK3R, identified in the heterozygous state in a GnRH-deficient patient, also interfered with dissociation of G proteins and IP signaling from wild-type NK3R, indicative of dominant-negative effects. Collectively, our data illustrate roles for TM5 and TM6 in NK3R trafficking and ligand binding and for IL3 in NK3R signaling.

A population of kisspeptin/neurokinin B neurons in the arcuate nucleus may be the central target of the male effect phenomenon in goats

Exposure of females to a male pheromone accelerates pulsatile gonadotropin-releasing hormone (GnRH) secretion in goats. Recent evidence has suggested that neurons in the arcuate nucleus (ARC) containing kisspeptin and neurokinin B (NKB) play a pivotal role in the control of GnRH secretion. Therefore, we hypothesized that these neurons may be the central target of the male pheromone. To test this hypothesis, we examined whether NKB signaling is involved in the pheromone action, and whether ARC kisspeptin/NKB neurons receive input from the medial nucleus of the amygdala (MeA)—the nucleus suggested to relay pheromone signals. Ovariectomized goats were implanted with a recording electrode aimed at a population of ARC kisspeptin/NKB neurons, and GnRH pulse generator activity, represented by characteristic increases in multiple-unit activity (MUA) volleys, was measured. Pheromone exposure induced an MUA volley and luteinizing hormone (LH) pulse in control animals, whereas the MUA and LH responses to the pheromone were completely suppressed by the treatment with an NKB receptor antagonist. These results indicate that NKB signaling is a prerequisite for pheromone action. In ovariectomized goats, an anterograde tracer was injected into the MeA, and possible connections between the MeA and ARC kisspeptin/NKB neurons were examined. Histochemical observations demonstrated that a subset of ARC kisspeptin/NKB neurons receive efferent projections from the MeA. These results suggest that the male pheromone signal is conveyed via the MeA to ARC kisspeptin neurons, wherein the signal stimulates GnRH pulse generator activity through an NKB signaling-mediated mechanism in goats.

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.

Regulation of arcuate neurons coexpressing kisspeptin, neurokinin B, and dynorphin by modulators of neurokinin 3 and κ-opioid receptors in adult male mice

Pulsatile GnRH release is essential to fertility and is modulated by gonadal steroids, most likely via steroid-sensitive afferents. Arcuate neurons coexpressing kisspeptin, neurokinin B (NKB), and dynorphin (KNDy neurons) are steroid-sensitive and have been postulated to both generate GnRH pulses and mediate steroid feedback on pulse frequency. KNDy neurons are proposed to interact with one another via NKB and dynorphin to activate and inhibit the KNDy network, respectively, and thus alter kisspeptin output to GnRH neurons. To test the roles of NKB and dynorphin on KNDy neurons and the steroid sensitivity of these actions, targeted extracellular recordings were made of Tac2(NKB)-GFP-identified neurons from castrate and intact male mice. Single-cell PCR confirmed most of these cells had a KNDy phenotype. The neurokinin 3 receptor (NK3R) agonist senktide increased action potential firing activity of KNDy neurons. Dynorphin reduced spontaneous KNDy neuron activity, but antagonism of κ-opioid receptors (KOR) failed to induce firing activity in quiescent KNDy neurons. Senktide-induced activation was greater in KNDy neurons from castrate mice, whereas dynorphin-induced suppression was greater in KNDy neurons from intact mice. Interactions of dynorphin with senktide-induced activity were more complex; dynorphin treatment after senktide had no consistent inhibitory effect, whereas pretreatment with dynorphin decreased senktide-induced activity only in KNDy neurons from intact but not castrate mice. These data suggest dynorphin-mediated inhibition of senktide-induced activity requires gonadal steroid feedback. Together, these observations support the hypotheses that activation of NK3R and KOR, respectively, excites and inhibits KNDy neurons and that gonadal steroids modulate these effects.

Current and future applications of GnRH, kisspeptin and neurokinin B analogues

Reproductive hormones affect all stages of life from gamete production, fertilization, fetal development and parturition, neonatal development and puberty through to adulthood and senescence. The reproductive hormone cascade has, therefore, been the target for the development of numerous drugs that modulate its activity at many levels. As the central regulator of the cascade, gonadotropin-releasing hormone (GnRH) agonists and antagonists have found extensive applications in treating a wide range of hormone-dependent diseases, such as precocious puberty, prostate cancer, benign prostatic hyperplasia, endometriosis and uterine fibroids, as well as being an essential component of in vitro fertilization protocols. The neuroendocrine peptides that regulate GnRH neurons, kisspeptin and neurokinin B, have also been identified as therapeutic targets, and novel agonists and antagonists are being developed as modulators of the cascade upstream of GnRH. Here, we review the development and applications of analogues of the major neuroendocrine peptide regulators of the reproductive hormone cascade: GnRH, kisspeptin and neurokinin B.

Kisspeptin signaling is required for the luteinizing hormone response in anestrous ewes following the introduction of males

The introduction of a novel male stimulates the hypothalamic-pituitary-gonadal axis of female sheep during seasonal anestrus, leading to the resumption of follicle maturation and ovulation. How this pheromone cue activates pulsatile secretion of gonadotropin releasing hormone (GnRH)/luteinizing hormone (LH) is unknown. We hypothesised that pheromones activate kisspeptin neurons, the product of which is critical for the stimulation of GnRH neurons and fertility. During the non-breeding season, female sheep were exposed to novel males and blood samples collected for analysis of plasma LH profiles. Females without exposure to males served as controls. In addition, one hour before male exposure, a kisspeptin antagonist (P-271) or vehicle was infused into the lateral ventricle and continued for the entire period of male exposure. Introduction of a male led to elevated mean LH levels, due to increased LH pulse amplitude and pulse frequency in females, when compared to females not exposed to a male. Infusion of P-271 abolished this effect of male exposure. Brains were collected after the male effect stimulus and we observed an increase in the percentage of kisspeptin neurons co-expressing Fos, by immunohistochemistry. In addition, the per-cell expression of Kiss1 mRNA was increased in the rostral and mid (but not the caudal) arcuate nucleus (ARC) after male exposure in both aCSF and P-271 treated ewes, but the per-cell content of neurokinin B mRNA was decreased. There was also a generalized increase in Fos positive cells in the rostral and mid ARC as well as the ventromedial hypothalamus of females exposed to males. We conclude that introduction of male sheep to seasonally anestrous female sheep activates kisspeptin neurons and other cells in the hypothalamus, leading to increased GnRH/LH secretion.

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.

Kisspeptin restores pulsatile LH secretion in patients with neurokinin B signaling deficiencies: physiological, pathophysiological and therapeutic implications

Pulsatile gonadotropin-releasing hormone (GnRH) is crucial to normal reproductive function and abnormalities in pulse frequency give rise to reproductive dysfunction. Kisspeptin and neurokinin B (NKB), neuropeptides secreted by the same neuronal population in the ventral hypothalamus, have emerged recently as critical central regulators of GnRH and thus gonadotropin secretion. Patients with mutations resulting in loss of signaling by either of these neuroendocrine peptides fail to advance through puberty but the mechanisms mediating this remain unresolved. We report here that continuous kisspeptin infusion restores gonadotropin pulsatility in patients with loss-of-function mutations in NKB (TAC3) or its receptor (TAC3R), indicating that kisspeptin on its own is sufficient to stimulate pulsatile GnRH secretion. Moreover, our findings suggest that NKB action is proximal to kisspeptin in the reproductive neuroendocrine cascade regulating GnRH secretion, and may act as an autocrine modulator of kisspeptin secretion. The ability of continuous kisspeptin infusion to induce pulsatile gonadotropin secretion further indicates that GnRH neurons are able to set up pulsatile secretion in the absence of pulsatile exogenous kisspeptin.

Kisspeptin and seasonality of reproduction

Wild and domesticated species display seasonality in reproductive function, controlled predominantly by photoperiod. Seasonal alterations in breeding status are caused by changes in the secretion of gonadotropin-releasing hormone (GnRH) that are mediated by upstream neuronal afferents that regulate the GnRH cells. In particular, kisspeptin appears to play a major role in seasonality of reproduction, transducing the feedback effect of gonadal steroids as well as having an independent (nonsteroid dependent) circannual rhythm. A substantial body of data on this issue has been obtained from studies in sheep and hamsters and this is reviewed here in detail. Kisspeptin function is upregulated during the breeding season in sheep, stimulating reproductive function, but contradictory data are found in Siberian and Syrian hamsters. The relative quiescence of kisspeptin cells in the nonbreeding season can be counteracted by administration of the peptide, leading to activation of reproductive function. Although there is a major role for melatonin in the transduction of photoperiod to the reproductive system, kisspeptin cells do not appear to express the melatonin receptor, so the means by which seasonality changes the level of kisspeptin activity remains unknown.

Negative effects of high glucose exposure in human gonadotropin-releasing hormone neurons

Metabolic disorders are often associated with male hypogonadotropic hypogonadism, suggesting that hypothalamic defects involving GnRH neurons may impair the reproductive function. Among metabolic factors hyperglycemia has been implicated in the control of the reproductive axis at central level, both in humans and in animal models. To date, little is known about the direct effects of pathological high glucose concentrations on human GnRH neurons. In this study, we investigated the high glucose effects in the human GnRH-secreting FNC-B4 cells. Gene expression profiling by qRT-PCR, confirmed that FNC-B4 cells express GnRH and several genes relevant for GnRH neuron function (KISS1R, KISS1, sex steroid and leptin receptors, FGFR1, neuropilin 2, and semaphorins), along with glucose transporters (GLUT1, GLUT3, and GLUT4). High glucose exposure (22 mM; 40 mM) significantly reduced gene and protein expression of GnRH, KISS1R, KISS1, and leptin receptor, as compared to normal glucose (5 mM). Consistent with previous studies, leptin treatment significantly induced GnRH mRNA expression at 5 mM glucose, but not in the presence of high glucose concentrations. In conclusion, our findings demonstrate a deleterious direct contribution of high glucose on human GnRH neurons, thus providing new insights into pathogenic mechanisms linking metabolic disorders to reproductive dysfunctions.

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.

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.

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.

Central administration of neurokinin B activates kisspeptin/NKB neurons in the arcuate nucleus and stimulates luteinizing hormone secretion in ewes during the non-breeding season

Human genetic studies have suggested that kisspeptin and neurokinin B (NKB) play pivotal roles in the control of gonadotropin-releasing hormone (GnRH) secretion. However, the role of NKB in this context is less clear compared with that of kisspeptin. In the present study, we investigated the ratio of colocalization of kisspeptin and NKB in neurons in the arcuate nucleus (ARC), the effects of intracerebroventricular infusion of NKB on luteinizing hormone (LH) secretion and whether the treatment activates ARC kisspeptin/NKB neurons in seasonally anestrous ewes. Double-labeling immunohistochemistry revealed that the majority of kisspeptin neurons coexpressed NKB in the ARC. Infusion of NKB for 2 h into the lateral ventricle elicited a discharge of LH, which resulted in significant increases in LH concentrations between 20 and 50 min after the start of infusion compared with a saline-infused control. Animals were sacrificed immediately after the end of infusion, and Fos expression in ARC kisspeptin neurons was immunohistochemically examined. The NKB treatment activated kisspeptin neurons throughout the ARC, and approximately 70% of kisspeptin neurons expressed Fos immunoreactivity at the caudal portion of the nucleus. The present study demonstrated that a central infusion of NKB elicited a discharge of LH, which was associated with the activation of a large population of ARC kisspeptin/NKB neurons in seasonally anestrous ewes. The results suggest that NKB plays a stimulatory role in the control of pulsatile GnRH secretion and that the population of ARC kisspeptin/NKB neurons is one of sites of the NKB action in sheep.

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.