Neurokinin B is critical for normal timing of sexual maturation but dispensable for adult reproductive function in female mice

The effect of NK3-Saporin injection within the arcuate nucleus on puberty, the LH surge, and the response to Senktide in female sheep†

The timing of puberty onset is reliant on increased gonadotropin-releasing hormone (GnRH). This elicits a corresponding increase in luteinizing hormone (LH) due to a lessening of sensitivity to the inhibitory actions of estradiol (E2). The mechanisms underlying the increase in GnRH release likely involve a subset of neurons within the arcuate (ARC) nucleus of the hypothalamus that contain kisspeptin, neurokinin B (NKB), and dynorphin (KNDy neurons). We aimed to determine if KNDy neurons in female sheep are critical for: timely puberty onset; the LH surge; and the response to an intravenous injection of the neurokinin-3 receptor (NK3R) agonist, senktide. Prepubertal ewes received injections aimed at the ARC containing blank-saporin (control, n = 5) or NK3-saporin (NK3-SAP, n = 6) to ablate neurons expressing NK3R. Blood samples taken 3/week for 65 days following surgery were assessed for progesterone to determine onset of puberty. Control ewes exhibited onset of puberty at 33.2 ± 3.9 days post sampling initiation, whereas 5/6 NK3-SAP treated ewes didn't display an increase in progesterone. After an artificial LH surge protocol, surge amplitude was lower in NK3-SAP ewes. Finally, ewes were treated with senktide to determine if an LH response was elicited. LH pulses were evident in both groups in the absence of injections, but the response to senktide vs saline was similar between groups. These results show that KNDy cells are necessary for timely puberty onset and for full expresson of the LH surge. The occurrence of LH pulses in NK3-SAP treated ewes may indicate a recovery from an apulsatile state.

TAC3 regulates GnRH/gonadotropin synthesis in female chickens

Neurokinin B (NKB), a peptide encoded by the tachykinin 3 (TAC3), is critical for reproduction in all studied species. However, its potential roles in birds are less clear. Using the female chicken (c-) as a model, we showed that cTAC3 is composed of five exons with a full-length cDNA of 787 bp, which was predicted to generate the mature NKB peptide containing 10 amino acids. Using cell-based luciferase reporter assays, we demonstrated that cNKB could effectively and specifically activate tachykinin receptor 3 (TACR3) in HEK293 cells, suggesting its physiological function is likely achieved via activating cTACR3 signaling. Notably, cTAC3 and cTACR3 were predominantly and abundantly expressed in the hypothalamus of hens and meanwhile the mRNA expression of cTAC3 was continuously increased during development, suggesting that NKB-TACR3 may emerge as important components of the neuroendocrine reproductive axis. In support, intraperitoneal injection of cNKB could significantly promote hypothalamic cGnRH-Ι, and pituitary cFSHβ and cLHβ expression in female chickens. Surprisingly, cTAC3 and cTACR3 were also expressed in the pituitary gland, and cNKB treatment significantly increased cLHβ and cFSHβ expression in cultured primary pituitary cells, suggesting cNKB can also act directly at the pituitary level to stimulate gonadotropin synthesis. Collectively, our results reveal that cNKB functionally regulate GnRH/gonadotropin synthesis in female chickens.

Neurokinin B Administration Induces Dose Dependent Proliferation of Seminal Vesicles in Adult Rats

BACKGROUND

Neurokinin B; an endogenous decapeptide, mediates its reproductive physiological actions through gonadotropin releasing hormone. Despite the potential role of Neurokinin B on seminal vesicles, its effects on seminal vesicles in adult male mammals remain elusive. We aimed to investigate the potentials of variable doses of Neurokinin B, its agonist and antagonist on histomorphology and expression of NK3R on seminal vesicles, and secretory activity of seminal vesicles in adult male rats.

METHODS

Adult male Sprague Dawley rats (n=10 in each group) were administered intraperitoneally with Neurokinin B in three variable doses: 1 μg, 1 ηg and 10 ρg while, Senktide (Neurokinin B agonist) and SB222200 (Neurokinin B antagonist) in 1 μg doses consecutively for 12 days. After 12 days of peptide treatment, half of the animals (n=05) in each group were sacrificed while remaining half (n=05) were kept for another 12 days without any treatment to investigate treatment reversal. Seminal vesicles were dissected and excised tissue was processed for light microscopy, immunohistochemistry and estimation of seminal fructose levels.

RESULTS

Treatment with Neurokinin B and Senktide significantly increased while SB222200 slightly decrease the seminal vesicles weight, epithelial height and seminal fructose levels as compared to control. Light microscopy revealed increased epithelial height and epithelial folding as compared to control in all Neurokinin B and Senktide treated groups while decreased in SB222200. Effects of various doses of Neurokinin B, Senktide and SB222200 on seminal vesicles weight, epithelial height, seminal fructose levels and histomorphology were reversed when rats were maintained without treatments. Immuno-expression of Neurokinin B shows no change in treatment and reversal groups.

CONCLUSION

Continuous administration of Neurokinin B and Senktide effect positively while SB222200 have detrimental effects on cellular morphology, epithelial height and seminal fructose levels in seminal vesicles. Effects of peptide treatments depicted a reversal towards control group when rats were kept without any treatment.

Maternal cafeteria diet influences kisspeptin (Kiss1), kisspeptin receptor(Gpr54), and sirtuin (Sirt1) genes, hormonal and metabolic profiles, and reproductive functions in rat offspring in a sex-specific manner†

Kisspeptin (KP, encoded by Kiss1, binding to the Gpr54 receptor) is a neuropeptide conveying information on the metabolic status to the hypothalamic-pituitary-gonadal axis. KP acts together with dynorphin A (encoded by Pdyn) and neurokinin B (encoded by Tac2) to regulate reproduction. KP is crucial for the onset of puberty and is under the control of sirtuin (encoded by Sirt1). We hypothesize that the maternal cafeteria (CAF) diet has adverse effects on the offspring's hormonal, metabolic, and reproductive functions due to sex-specific alterations in the expression of Kiss1, Gpr54, Pdyn, Tac2, and Sirt1 in the hypothalamus, and Kiss1, Gpr54, and Sirt1 in the liver. Rats were fed a CAF diet before pregnancy, during pregnancy, and during lactation. The vaginal opening was monitored. Offspring were sacrificed in three age points: PND 30, PND 35, and PND 60 (females) and PND 40, PND 45, and PND 60 (males). Their metabolic and hormonal status was assessed. mRNA for Kiss1, Gpr54, Pdyn, Tac2, and Sirt1 were measured by real-time PCR in the hypothalamus and/or livers. We found that CAF offspring had lower weight and altered body composition; increased cholesterol and triglyceride levels, sex-specific changes in glucose and insulin levels; sex-dependent changes in Sirt1/Kiss1 mRNA ratio in the hypothalamus; sex-specific alterations in Kiss1 and Sirt1 mRNA in the liver with more diversity in males; and a delayed puberty onset in females. We concluded that the mother's CAF diet leads to sex-specific alterations in metabolic and reproductive outcomes via Kiss1/Gpr54 and Sirt1 systems in offspring.

Mechanism of Arcuate Kisspeptin Neuron Synchronization in Acute Brain Slices From Female Mice

The mechanism by which arcuate kisspeptin (ARNKISS) neurons co-expressing glutamate, neurokinin B, and dynorphin intermittently synchronize their activity to drive pulsatile hormone secretion remains unclear in females. In order to study spontaneous synchronization within the ARNKISS neuron network, acute brain slices were prepared from adult female Kiss1-GCaMP6 mice. Analysis of both spontaneous synchronizations and those driven by high frequency stimulation of individual ARNKISS neurons revealed that the network exhibits semi-random emergent excitation dependent upon glutamate signaling through AMPA receptors. No role for NMDA receptors was identified. In contrast to male mice, ongoing tachykinin receptor tone within the slice operated to promote spontaneous synchronizations in females. As previously observed in males, we found that ongoing dynorphin transmission in the slice did not contribute to synchronization events. These observations indicate that a very similar AMPA receptor-dependent mechanism underlies ARNKISS neuron synchronizations in the female mouse supporting the "glutamate two-transition" model for kisspeptin neuron synchronization. However, a potentially important sex difference appears to exist with a more prominent facilitatory role for tachykinin transmission in the female.

Mechanism of kisspeptin neuron synchronization for pulsatile hormone secretion in male mice

The mechanism by which arcuate nucleus kisspeptin (ARN^KISS) neurons co-expressing glutamate, neurokinin B, and dynorphin intermittently synchronize their activity to generate pulsatile hormone secretion remains unknown. An acute brain slice preparation maintaining synchronized ARN^KISS neuron burst firing was used alongside in vivo GCaMP GRIN lens microendoscope and fiber photometry imaging coupled with intra-ARN microinfusion. Studies in intact and gonadectomized male mice revealed that ARN^KISS neuron synchronizations result from near-random emergent network activity within the population and that this was critically dependent on local glutamate-AMPA signaling. Whereas neurokinin B operated to potentiate glutamate-generated synchronizations, dynorphin-kappa opioid tone within the network served as a gate for synchronization initiation. These observations force a departure from the existing "KNDy hypothesis" for ARN^KISS neuron synchronization. A "glutamate two-transition" mechanism is proposed to underlie synchronizations in this key hypothalamic central pattern generator driving mammalian fertility.

Neuropeptides and hormones in hypothalamus-pituitary axis of Chinese sturgeon (Acipenser sinensis)

The hypothalamus and pituitary serve as important neuroendocrine center, which is able to secrete a variety of neuropeptides and hormones to participate in the regulation of reproduction, growth, stress and feeding in fish. Chinese sturgeon is a basal vertebrate lineage fish with a special evolutionary status, but the information on its neuroendocrine system is relatively scarce. Using the transcriptome data on the hypothalamus-pituitary axis of Chinese sturgeon as reference, we found out 46 hypothalamus neuropeptide genes, which were involved in regulation of reproduction, growth, stress and feeding. The results of sequence alignment showed that the neuroendocrine system of Chinese sturgeon evolves slowly, which confirms that Chinese sturgeon is a species with a slow phenotypic evolution rate. In addition, we also isolated six pituitary hormones genes from Chinese sturgeon, including reproductive hormones: follicle-stimulating homone (FSH) and luteinizing hormone (LH), growth-related hormones: growth hormone (GH)/prolactin (PRL)/somatolactin (SL), and stress-related hormone gene: proopiomelanocortin (POMC). Similar to teleost, immunostaining localization analysis in Chinese sturgeon pituitary showed that LH and FSH were located in the pituitary proximal pars distalis, SL was located in the pituitary rostral pars distalis, and POMC was located in the pituitary pars intermedia and pituitary rostral pars distalis. This study will give a contribution to enrich our information on the neuroendocrine system in Chinese sturgeon.

Hypothalamic Overexpression of Makorin Ring Finger Protein 3 Results in Delayed Puberty in Female Mice

Makorin ring finger protein 3 (MKRN3) is an important neuroendocrine player in the control of pubertal timing and upstream inhibitor of gonadotropin-releasing hormone secretion. In mice, expression of Mkrn3 in the hypothalamic arcuate and anteroventral periventricular nucleus is high early in life and declines before the onset of puberty. Therefore, we aimed to explore if the persistence of hypothalamic Mkrn3 expression peripubertally would result in delayed puberty. Female mice that received neonatal bilateral intracerebroventricular injections of a recombinant adeno-associated virus expressing Mkrn3 had delayed vaginal opening and first estrus compared with animals injected with control virus. Subsequent estrous cycles and fertility were normal. Interestingly, male mice treated similarly did not exhibit delayed puberty onset. Kiss1, Tac2, and Pdyn mRNA levels were increased in the mediobasal hypothalamus in females at postnatal day 28, whereas kisspeptin and neurokinin B protein levels in the arcuate nucleus were decreased, following Mkrn3 overexpression, compared to controls. Cumulatively, these data suggest that Mkrn3 may directly or indirectly target neuropeptides of Kiss1 neurons to degradation pathways. This mouse model suggests that MKRN3 may be a potential contributor to delayed onset of puberty, in addition to its well-established roles in central precocious puberty and the timing of menarche.

Neuroendocrine control of gonadotropin-releasing hormone: Pulsatile and surge modes of secretion

The concept that different systems control episodic and surge secretion of gonadotropin-releasing hormone (GnRH) was well established by the time that GnRH was identified and formed the framework for studies of the physiological roles of GnRH, and later kisspeptin. Here, we focus on recent studies identifying the neural mechanisms underlying these two modes of secretion, with an emphasis on their core components. There is now compelling data that kisspeptin neurons in the arcuate nucleus that also contain neurokinin B (NKB) and dynorphin (i.e., KNDy cells) and their projections to GnRH dendrons constitute the GnRH pulse generator in mice and rats. There is also strong evidence for a similar role for KNDy neurons in sheep and goats, and weaker data in monkeys and humans. However, whether KNDy neurons act on GnRH dendrons and/or GnRH soma and dendrites that are found in the mediobasal hypothalamus (MBH) of these species remains unclear. The core components of the GnRH/luteinising hormone surge consist of an endocrine signal that initiates the process and a neural trigger that drives GnRH secretion during the surge. In all spontaneous ovulators, the core endocrine signal is a rise in estradiol secretion from the maturing follicle(s), with the site of estrogen positive feedback being the rostral periventricular kisspeptin neurons in rodents and neurons in the MBH of sheep and primates. There is considerable species variations in the neural trigger, with three major classes. First, in reflex ovulators, this trigger is initiated by coitus and carried to the hypothalamus by neural or vascular pathways. Second, in rodents, there is a time of day signal that originates in the suprachiasmatic nucleus and activates rostral periventricular kisspeptin neurons and GnRH soma and dendrites. Finally, in sheep nitric oxide-producing neurons in the ventromedial nucleus, KNDy neurons and rostral kisspeptin neurons all appear to participate in driving GnRH release during the surge.

Kisspeptins and the neuroendocrine control of reproduction: Recent progress and new frontiers in kisspeptin research

In late 2003, a major breakthrough in our understanding of the mechanisms that govern reproduction occurred with the identification of the reproductive roles of kisspeptins, encoded by the Kiss1 gene, and their receptor, Gpr54 (aka, Kiss1R). The discovery of this unsuspected reproductive facet attracted an extraordinary interest and boosted an intense research activity, in human and model species, that, in a relatively short period, established a series of basic concepts on the physiological roles of kisspeptins. Such fundamental knowledge, gathered in these early years of kisspeptin research, set the scene for the more recent in-depth dissection of the intimacies of the neuronal networks involving Kiss1 neurons, their precise mechanisms of regulation and the molecular underpinnings of the function of kisspeptins as pivotal regulators of all key aspects of reproductive function, from puberty onset to pulsatile gonadotropin secretion and the metabolic control of fertility. While no clear temporal boundaries between these two periods can be defined, in this review we will summarize the most prominent advances in kisspeptin research occurred in the last ten years, as a means to provide an up-dated view of the state of the art and potential paths of future progress in this dynamic, and ever growing domain of Neuroendocrinology.

Anti-Müllerian Hormone Regulation of Synaptic Transmission in the Hippocampus Requires MAPK Signaling and Kv4.2 Potassium Channel Activity

Anti-Müllerian hormone (AMH) is a paracrine factor generated peripherally by the gonads to regulate gonadal function in adult mammals. We recently reported that AMH and AMH-specific receptor Anti-Müllerian hormone receptor 2 (AMHR2) are expressed in the hippocampus, and exogenous AMH protein rapidly increased synaptic transmission and long-term synaptic plasticity at the CA3-CA1 synapses. Here we examined the cell-specific expression of AMHR2 and the cellular mechanism of rapid boosting effect of AMH on synaptic transmission in mouse hippocampus. Immunofluorescence staining showed that AMHR2 was specifically expressed in the soma and dendrites of hippocampal pyramidal neurons, but not glial cells. Electrophysiological recordings on acute hippocampal slices showed that AMH did not affect AMPAR-mediated or N-Methyl-D-aspartic acid receptor (NMDAR)-mediated excitatory postsynaptic currents at the CA3-CA1 synapses. The small-conductance Ca²⁺-activated K⁺ channel (SK2) and A-type K⁺ channel (Kv4.2) contribute to shaping excitatory postsynaptic potentials (EPSPs) at the CA3-CA1 synapses. Bath application of apamin to block SK2 did not alter AMH effect on increasing EPSPs, whereas blocking Kv4.2 channel with 4-aminopyridine, or chelating internal Ca²⁺ with BAPTA occluded the action of AMH on boosting EPSPs. Kv4.2 activity is regulated by p38 mitogen-activated kinase (MAPK). Blocking p38 MAPK with SB203580 occluded the effect of AMH on increasing EPSPs. These results show that Kv4.2 channel contributes to the rapid action of AMH on boosting synaptic transmission in a Ca²⁺- and p38 MAPK-dependent manner. Our findings provide functional evidence that AMH enhances synaptic transmission through Kv4.2 channel in the hippocampus, suggesting a possible role of Kv4.2 channel in AMH-regulated neuronal process underlying learning and memory.

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.

The Role of Kisspeptin in the Control of the Hypothalamic-Pituitary-Gonadal Axis and Reproduction

The discovery of kisspeptin as a critical central regulatory factor of GnRH release has given people a novel understanding of the neuroendocrine regulation in human reproduction. Kisspeptin activates the signaling pathway by binding to its receptor kisspeptin receptor (KISS1R) to promote GnRH secretion, thereby regulating the hypothalamic-pituitary-gonadal axis (HPG) axis. Recent studies have shown that kisspeptin neurons located in arcuate nucleus (ARC) co-express neurokinin B (NKB) and dynorphin (Dyn). Such neurons are called KNDy neurons. KNDy neurons participate in the positive and negative feedback of estrogen to GnRH secretion. In addition, kisspeptin is a key factor in the initiation of puberty, and also regulates the processes of female follicle development, oocyte maturation, and ovulation through the HPG axis. In male reproduction, kisspeptin also plays an important role, getting involved in the regulation of Leydig cells, spermatogenesis, sperm functions and reproductive behaviors. Mutations in the KISS1 gene or disorders of the kisspeptin/KISS1R system may lead to clinical symptoms such as idiopathic hypogonadotropic hypogonadism (iHH), central precocious puberty (CPP) and female infertility. Understanding the influence of kisspeptin on the reproductive axis and related mechanisms will help the future application of kisspeptin in disease diagnosis and treatment. In this review, we critically appraise the role of kisspeptin in the HPG axis, including its signaling pathways, negative and positive feedback mechanisms, and its control on female and male reproduction.

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.

The Differential Roles for Neurodevelopmental and Neuroendocrine Genes in Shaping GnRH Neuron Physiology and Deficiency

Gonadotropin releasing hormone (GnRH) neurons are hypothalamic neuroendocrine cells that control sexual reproduction. During embryonic development, GnRH neurons migrate from the nose to the hypothalamus, where they receive inputs from several afferent neurons, following the axonal scaffold patterned by nasal nerves. Each step of GnRH neuron development depends on the orchestrated action of several molecules exerting specific biological functions. Mutations in genes encoding for these essential molecules may cause Congenital Hypogonadotropic Hypogonadism (CHH), a rare disorder characterized by GnRH deficiency, delayed puberty and infertility. Depending on their action in the GnRH neuronal system, CHH causative genes can be divided into neurodevelopmental and neuroendocrine genes. The CHH genetic complexity, combined with multiple inheritance patterns, results in an extreme phenotypic variability of CHH patients. In this review, we aim at providing a comprehensive and updated description of the genes thus far associated with CHH, by dissecting their biological relevance in the GnRH system and their functional relevance underlying CHH pathogenesis.

Genetics of hypogonadotropic Hypogonadism-Human and mouse genes, inheritance, oligogenicity, and genetic counseling

Hypogonadotropic hypogonadism, which may be normosmic (nHH) or anosmic/hyposmic, known as Kallmann syndrome (KS), is due to gonadotropin-releasing hormone deficiency, which results in absent puberty and infertility. Investigation of the genetic basis of nHH/KS over the past 35 years has yielded a substantial increase in our understanding, as variants in 44 genes in OMIM account for ~50% of cases. The first genes for KS (ANOS1) and nHH (GNRHR) were followed by the discovery that FGFR1 variants may cause either nHH or KS. Associated anomalies include midline facial defects, neurologic deficits, cardiac anomalies, and renal agenesis, among others. Mouse models for all but one gene (ANOS1) generally support findings in humans. About half of the known genes implicated in nHH/KS are inherited as autosomal dominant and half are autosomal recessive, whereas only 7% are X-linked recessive. Digenic and oligogenic inheritance has been reported in 2-20% of patients, most commonly with variants in genes that may result in either nHH or KS inherited in an autosomal dominant fashion. In vitro analyses have only been conducted for both gene variants in eight cases and for one gene variant in 20 cases. Rigorous confirmation that two gene variants in the same individual cause the nHH/KS phenotype is lacking for most. Clinical diagnosis is probably best accomplished by targeted next generation sequencing of the known candidate genes with confirmation by Sanger sequencing. Elucidation of the genetic basis of nHH/KS has resulted in an enhanced understanding of this disorder, as well as normal puberty, which makes genetic diagnosis clinically relevant.

Neuroendocrine regulation of pubertal suppression in the naked mole-rat: What we know and what comes next

Puberty is a key developmental milestone that marks an individual's maturation in several ways including, but not limited to, reproductive maturation, changes in behaviors and neural organization. The timing at which puberty occurs is variable both within individuals of the same species and between species. These variations can be aligned with ecological cues that delay or suppress puberty. Naked mole-rats are colony-living rodents where reproduction is restricted to a few animals; all other animals are pubertally-suppressed. Animals removed from suppressive colony cues can reproductively mature, presenting the unique opportunity to study adult-onset puberty. Recently, we found that RFRP-3 administration sustains pubertal delay in naked mole-rats removed from colony. In this review, we explore what is known about regulators that control puberty onset, the role of stress/social status in pubertal timing, the status of knowledge of pubertal suppression in naked mole-rats and what comes next.

Characterization of the Action of Tachykinin Signaling on Pulsatile LH Secretion in Male Mice

The alternation of the stimulatory action of the tachykinin neurokinin B (NKB) and the inhibitory action of dynorphin within arcuate (ARH) Kiss1 neurons has been proposed as the mechanism behind the generation of gonadotropin-releasing hormone (GnRH) pulses through the pulsatile release of kisspeptin. However, we have recently documented that GnRH pulses still exist in gonadectomized mice in the absence of tachykinin signaling. Here, we document an increase in basal frequency and amplitude of luteinizing hormone (LH) pulses in intact male mice deficient in substance P, neurokinin A (NKA) signaling (Tac1KO), and NKB signaling (Tac2KO and Tacr3KO). Moreover, we offer evidence that a single bolus of the NKB receptor agonist senktide to gonad-intact wild-type males increases the basal release of LH without changing its frequency. Altogether, these data support the dispensable role of the individual tachykinin systems in the generation of LH pulses. Moreover, the increased activity of the GnRH pulse generator in intact KO male mice suggests the existence of compensation by additional mechanisms in the generation of kisspeptin/GnRH pulses.

Congenital ablation of Tacr2 reveals overlapping and redundant roles of NK2R signaling in the control of reproductive axis

Tachykinin (TAC) signaling is an important element in the central control of reproduction. TAC family is mainly composed of substance P (SP), neurokinin A (NKA), and NKB, which bind preferentially to NK1, NK2, and NK3 receptors, respectively. While most studies have focused on the reproductive functions of NKB/NK3R, and to a lesser extent SP/NK1R, the relevance of NK2R, encoded by Tacr2, remains poorly characterized. Here, we address the physiological roles of NK2R in regulating the reproductive axis by characterizing a novel mouse line with congenital ablation of Tacr2. Activation of NK2R evoked acute luteinizing hormone (LH) responses in control mice, similar to those of agonists of NK1R and NK3R. Despite the absence of NK2R, Tacr2^-/- mice displayed only partially reduced LH responses to an NK2R agonist, which, nonetheless, were abrogated after blockade of NK3R in Tacr2^-/- males. While Tacr2^-/- mice displayed normal pubertal timing, LH pulsatility was partially altered in Tacr2^-/- females in adulthood, with suppression of basal LH levels, but no changes in the number of LH pulses. In addition, trends for increase in breeding intervals were detected in Tacr2^-/- mice. However, null animals of both sexes were fertile, with no changes in estrous cyclicity or sex preference in social behavioral tests. In conclusion, stimulation of NK2R elicited LH responses in mice, while congenital ablation of Tacr2 partially suppressed basal and stimulated LH secretion, with moderate reproductive impact. Our data support a modest, albeit detectable, role of NK2R in the control of the gonadotropic axis, with partially overlapping and redundant functions with other tachykinin receptors.NEW & NOTEWORTHY We have explored here the impact of congenital ablation of the gene (Tacr2) encoding the tachykinin receptor, NK2R, in terms of neuroendocrine control of the reproductive axis, using a novel Tacr2 KO mouse line. Our data support a modest, albeit detectable, role of NK2R in the control of the gonadotropic axis, with partially overlapping and redundant functions with other tachykinin receptors.

Puberty, A Sensitive Window of Hypothalamic Development and Plasticity

Puberty is a developmental period characterized by a broad range of physiologic changes necessary for the acquisition of adult sexual and reproductive maturity. These changes mirror complex modifications within the central nervous system, including within the hypothalamus. These modifications result in the maturation of a fully active hypothalamic-pituitary-gonadal (HPG) axis, the neuroendocrine cascade ensuring gonadal activation, sex steroid secretion, and gametogenesis. A complex and finely regulated neural network overseeing the HPG axis, particularly the pubertal reactivation of gonadotropin-releasing hormone (GnRH) secretion, has been progressively unveiled in the last 3 decades. This network includes kisspeptin, neurokinin B, GABAergic, and glutamatergic neurons as well as glial cells. In addition to substantial modifications in the expression of key targets, several changes in neuronal morphology, neural connections, and synapse organization occur to establish mature and coordinated neurohormonal secretion, leading to puberty initiation. The aim of this review is to outline the current knowledge of the major changes that neurons secreting GnRH and their neuronal and glial partners undergo before and after puberty. Emerging mediators upstream of GnRH, uncovered in recent years, are also addressed herein. In addition, the effects of sex steroids, particularly estradiol, on changes in hypothalamic neurodevelopment and plasticity are discussed.

Tachykinin Signaling Is Required for Induction of the Preovulatory Luteinizing Hormone Surge and Normal Luteinizing Hormone Pulses

Tachykinins (neurokinin A [NKA], neurokinin B [NKB], and substance P [SP]) are important components of the neuroendocrine control of reproduction by direct stimulation of Kiss1 neurons to control GnRH pulsatility, which is essential for reproduction. Despite this role of tachykinins in successful reproduction, knockout (KO) mice for Tac1 (NKA/SP) and Tac2 (NKB) genes are fertile, resembling the phenotype of human patients bearing NKB signaling mutations, who often reverse their hypogonadal phenotype. This suggests the existence of compensatory mechanisms among the different tachykinin ligand-receptor systems to maintain reproduction in the absence of one of them. In order to test this hypothesis, we generated complete tachykinin-deficient mice (Tac1/Tac2KO). Male mice displayed delayed puberty onset and decreased luteinizing hormone (LH) pulsatility (frequency and amplitude of LH pulses) but preserved fertility. However, females did not show signs of puberty onset (first estrus) within 45 days after vaginal opening, they displayed a low frequency (but normal amplitude) of LH pulses, and 80% of them remained infertile. Further evaluation identified a complete absence of the preovulatory LH surge in Tac1/Tac2KO females as well as in wild-type females treated with NKB or SP receptor antagonists. These data confirmed a fundamental role of tachykinins in the timing of puberty onset and LH pulsatility and uncovered a role of tachykinin signaling in facilitation of the preovulatory LH surge. Overall, these findings indicate that tachykinin signaling plays a dominant role in the control of ovulation, with potential implications as a pathogenic mechanism and a therapeutic target to improve reproductive outcomes in women with ovulation impairments.

Tachykinin signaling in the control of puberty onset

The tachykinin family of peptides has emerged as a critical component of the central control of the reproductive axis. Mounting evidence suggests that neurokinin B (NKB) plays an essential role in sexual maturation and fertility by directly stimulating the release of kisspeptin, with the contribution of additional tachykinins (neurokinin A [NKA] and substance P [SP]) in the fine tuning of the activity of Kiss1 neurons. The expression of tachykinins increases in the hypothalamus before puberty and, therefore, they are considered as initiators of pubertal development by stimulating the awakening of Kiss1 neurons. This is supported by studies showing delayed or absent puberty onset in humans and mice devoid of tachykinin signaling, and the advancement of puberty onset in rodents subjected to chronic activation of tachykinin receptors. This review compiles the current knowledge on the role of tachykinins in the control of puberty onset.

Tachykinins and Kisspeptins in the Regulation of Human Male Fertility

Infertility is a global disease affecting one out of six couples of reproductive age in the world, with a male factor involved in half the cases. There is still much to know about the regulation of human male fertility and thus we decided to focus on two peptide families that seem to play a key role in this function: tachykinins and kisspeptins. With this aim, we conducted an exhaustive review in order to describe the role of tachykinins and kisspeptins in human fertility and their possible implications in infertility etiopathogenesis. Many advances have been made to elucidate the roles of these two families in infertility, and multiple animal species have been studied, including humans. All of this knowledge could lead to new advances in male infertility diagnosis and treatment, but further research is needed to clarify all the implications of tachykinins and kisspeptins in fertility.

Hypothalamic Reproductive Endocrine Pulse Generator Activity Independent of Neurokinin B and Dynorphin Signaling

CONTEXT

Kisspeptin-neurokinin B (NKB)-dynorphin neurons are critical regulators of the hypothalamic-pituitary-gonadal axis. NKB and dynorphin are hypothesized to influence the frequency of GnRH pulses, whereas kisspeptin is hypothesized to be a generator of the GnRH pulse. How these neuropeptides interact remains unclear.

OBJECTIVE

To probe the role of NKB in GnRH pulse generation and to determine the interactions between NKB, kisspeptin, and dynorphin in humans and mice with a complete absence of NKB.

DESIGN

Case/control.

SETTING

Academic medical center.

PARTICIPANTS

Members of a consanguineous family bearing biallelic loss-of-function mutations in the gene encoding NKB and NKB-deficient mice.

INTERVENTIONS

Frequent blood sampling to characterize neuroendocrine profile and administration of kisspeptin, GnRH, and naloxone, a nonspecific opioid receptor antagonist used to block dynorphin.

MAIN OUTCOME MEASURES

LH pulse characteristics.

RESULTS

Humans lacking NKB demonstrate slow LH pulse frequency, which can be increased by opioid antagonism. Mice lacking NKB also demonstrate impaired LH secretion, which can be augmented with an identical pharmacologic manipulation. Both mice and humans with NKB deficiency respond to exogenous kisspeptin.

CONCLUSION

The preservation of LH pulses in the absence of NKB and dynorphin signaling suggests that both peptides are dispensable for GnRH pulse generation and kisspeptin responsiveness. However, NKB and dynorphin appear to have opposing roles in the modulation of GnRH pulse frequency.

Characterization of the Role of NKA in the Control of Puberty Onset and Gonadotropin Release in the Female Mouse

The tachykinin neurokinin B (NKB, Tac2) is critical for proper GnRH release in mammals, however, the role of the other tachykinins, such as substance P (SP) and neurokinin A (NKA) in reproduction, is still not well understood. In this study, we demonstrate that NKA controls the timing of puberty onset (similar to NKB and SP) and stimulates LH release in adulthood through NKB-independent (but kisspeptin-dependent) mechanisms in the presence of sex steroids. Furthermore, this is achieved, at least in part, through the autosynaptic activation of Tac1 neurons, which express NK2R (Tacr2), the receptor for NKA. Conversely, in the absence of sex steroids, as observed in ovariectomy, NKA inhibits LH through a mechanism that requires the presence of functional receptors for NKB and dynorphin (NK3R and KOR, respectively). Moreover, the ability of NKA to modulate LH secretion is absent in Kiss1KO mice, suggesting that its action occurs upstream of Kiss1 neurons. Overall, we demonstrate that NKA signaling is a critical component in the central control of reproduction, by contributing to the indirect regulation of kisspeptin release.

NKB signaling in the posterodorsal medial amygdala stimulates gonadotropin release in a kisspeptin-independent manner in female mice

Neurokinin B (NKB) signaling is critical for reproduction in all studied species. The existing consensus is that NKB induces GnRH release via kisspeptin (Kiss1) stimulation in the arcuate nucleus. However, the stimulatory action of NKB is dependent on circulating estrogen (E2) levels, without which, NKB inhibits luteinizing hormone (LH) release. Importantly, the evidence supporting the kisspeptin-dependent role of NKB, derives from models of persistent hypogonadal state [e.g. Kiss1r knock-out (KO) mice], with reduced E2 levels. Here, we demonstrate that in the presence of E2, NKB signaling induces LH release in a kisspeptin-independent manner through the activation of NK3R (NKB receptor) neurons in the posterodorsal medial amygdala (MePD). Importantly, we show that chemogenetic activation of MePD Kiss1 neurons induces LH release, however, the stimulatory action of NKB in this area is Kiss1 neuron-independent. These results document the existence of two independent neuronal circuitries within the MePD that regulate reproductive function in females.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

KNDy Cells Revisited

In the past decade since kisspeptin/neurokinin B/dynorphin (KNDy) cells were first identified in the mammalian hypothalamus, a plethora of new research has emerged adding insights into the role of this neuronal population in reproductive neuroendocrine function, including the basis for GnRH pulse generation and the mechanisms underlying the steroid feedback control of GnRH secretion. In this mini-review, we provide an update of evidence regarding the roles of KNDy peptides and their postsynaptic receptors in producing episodic GnRH release and assess the relative contribution of KNDy neurons to the "GnRH pulse generator." In addition, we examine recent work investigating the role of KNDy neurons as mediators of steroid hormone negative feedback and review evidence for their involvement in the preovulatory GnRH/LH surge, taking into account species differences that exist among rodents, ruminants, and primates. Finally, we summarize emerging roles of KNDy neurons in other aspects of reproductive function and in nonreproductive functions and discuss critical unresolved questions in our understanding of KNDy neurobiology.

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.

Disentangling puberty: novel neuroendocrine pathways and mechanisms for the control of mammalian puberty

BACKGROUND

Puberty is a complex developmental event, controlled by sophisticated regulatory networks that integrate peripheral and internal cues and impinge at the brain centers driving the reproductive axis. The tempo of puberty is genetically determined but is also sensitive to numerous modifiers, from metabolic and sex steroid signals to environmental factors. Recent epidemiological evidence suggests that the onset of puberty is advancing in humans, through as yet unknown mechanisms. In fact, while much knowledge has been gleaned recently on the mechanisms responsible for the control of mammalian puberty, fundamental questions regarding the intimate molecular and neuroendocrine pathways responsible for the precise timing of puberty and its deviations remain unsolved.

OBJECTIVE AND RATIONALE

By combining data from suitable model species and humans, we aim to provide a comprehensive summary of our current understanding of the neuroendocrine mechanisms governing puberty, with particular focus on its central regulatory pathways, underlying molecular basis and mechanisms for metabolic control.

SEARCH METHODS

A comprehensive MEDLINE search of articles published mostly from 2003 to 2017 has been carried out. Data from cellular and animal models (including our own results) as well as clinical studies focusing on the pathophysiology of puberty in mammals were considered and cross-referenced with terms related with central neuroendocrine mechanisms, metabolic control and epigenetic/miRNA regulation.

OUTCOMES

Studies conducted during the last decade have revealed the essential role of novel central neuroendocrine pathways in the control of puberty, with a prominent role of kisspeptins in the precise regulation of the pubertal activation of GnRH neurosecretory activity. In addition, different transmitters, including neurokinin-B (NKB) and, possibly, melanocortins, have been shown to interplay with kisspeptins in tuning puberty onset. Alike, recent studies have documented the role of epigenetic mechanisms, involving mainly modulation of repressors that target kisspeptins and NKB pathways, as well as microRNAs and the related binding protein, Lin28B, in the central control of puberty. These novel pathways provide the molecular and neuroendocrine basis for the modulation of puberty by different endogenous and environmental cues, including nutritional and metabolic factors, such as leptin, ghrelin and insulin, which are known to play an important role in pubertal timing.

WIDER IMPLICATIONS

Despite recent advancements, our understanding of the basis of mammalian puberty remains incomplete. Complete elucidation of the novel neuropeptidergic and molecular mechanisms summarized in this review will not only expand our knowledge of the intimate mechanisms responsible for puberty onset in humans, but might also provide new tools and targets for better prevention and management of pubertal deviations in the clinical setting.

The Roles of Neurokinins and Endogenous Opioid Peptides in Control of Pulsatile LH Secretion

Work over the last 15 years on the control of pulsatile LH secretion has focused largely on a set of neurons in the arcuate nucleus (ARC) that contains two stimulatory neuropeptides, critical for fertility in humans (kisspeptin and neurokinin B (NKB)) and the inhibitory endogenous opioid peptide (EOP), dynorphin, and are now known as KNDy (kisspeptin-NKB-dynorphin) neurons. In this review, we consider the role of each of the KNDy peptides in the generation of GnRH pulses and the negative feedback actions of ovarian steroids, with an emphasis on NKB and dynorphin. With regard to negative feedback, there appear to be important species differences. In sheep, progesterone inhibits GnRH pulse frequency by stimulating dynorphin release, and estradiol inhibits pulse amplitude by suppressing kisspeptin. In rodents, the role of KNDy neurons in estrogen negative feedback remains controversial, progesterone may inhibit GnRH via dynorphin, but the physiological significance of this action is unclear. In primates, an EOP, probably dynorphin, mediates progesterone negative feedback, and estrogen inhibits kisspeptin expression. In contrast, there is now compelling evidence from several species that kisspeptin is the output signal from KNDy neurons that drives GnRH release during a pulse and may also act within the KNDy network to affect pulse frequency. NKB is thought to act within this network to initiate each pulse, although there is some redundancy in tachykinin signaling in rodents. In ruminants, dynorphin terminates GnRH secretion at the end of pulse, most likely acting on both KNDy and GnRH neurons, but the data on the role of this EOP in rodents are conflicting.

Role of Kisspeptin and Neurokinin B in Puberty in Female Non-Human Primates

In human patients, loss-of-function mutations in the genes encoding kisspeptin (KISS1) and neurokinin B (NKB) and their receptors (KISS1R and NK3R, respectively) result in an abnormal timing of puberty or the absence of puberty. To understand the neuroendocrine mechanism of puberty, we investigated the contribution of kisspeptin and NKB signaling to the pubertal increase in GnRH release using rhesus monkeys as a model. Direct measurements of GnRH and kisspeptin in the median eminence of the hypothalamus with infusion of agonists and antagonists for kisspeptin and NKB reveal that kisspeptin and NKB signaling stimulate GnRH release independently or collaboratively by forming kisspeptin and NKB neuronal networks depending on the developmental age. For example, while in prepubertal females, kisspeptin and NKB signaling independently stimulate GnRH release, in pubertal females, the formation of a collaborative kisspeptin and NKB network further accelerates the pubertal increase in GnRH release. It is speculated that the collaborative mechanism between kisspeptin and NKB signaling to GnRH neurons is necessary for the complex reproductive function in females.

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.

Short-Term High-Fat Diet Increases Leptin Activation of CART Neurons and Advances Puberty in Female Mice

Leptin is a permissive factor for puberty initiation, participating as a metabolic cue in the activation of the kisspeptin (Kiss1)-gonadotropin-releasing hormone neuronal circuitry; however, it has no direct effect on Kiss1 neurons. Leptin acts on hypothalamic cocaine- and amphetamine-regulated transcript (CART) neurons, participating in the regulation of energy homeostasis. We investigated the influence of a short-term high-fat diet (HFD) on the effect of leptin on puberty timing. Kiss1-hrGFP female mice received a HFD or regular diet (RD) after weaning at postnatal day (PN)21 and were studied at PN28 and PN32. The HFD increased body weight and plasma leptin concentrations and decreased the age at vaginal opening (HFD, 32 ± 0.53 days; RD, 38 ± 0.67 days). Similar colocalization of neurokinin B and dynorphin in Kiss1-hrGFP neurons of the arcuate nucleus (ARC) was observed between the HFD and RD groups. The HFD increased CART expression in the ARC and Kiss1 messenger RNA expression in the anteroventral periventricular (AVPV)/anterior periventricular (Pe). The HFD also increased the number of ARC CART neurons expressing leptin-induced phosphorylated STAT3 (signal transducer and activator of transcription 3) at PN32. Close apposition of CART fibers to Kiss1-hrGFP neurons was observed in the ARC of both RD- and HFD-fed mice. In conclusion, these data reinforce the notion that a HFD increases kisspeptin expression in the AVPV/Pe and advances puberty initiation. Furthermore, we have demonstrated that the HFD-induced earlier puberty is associated with an increase in CART expression in the ARC. Therefore, these data indicate that CART neurons in the ARC can mediate the effect of leptin on Kiss1 neurons in early puberty induced by a HFD.

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).

Tac1 Signaling Is Required for Sexual Maturation and Responsiveness of GnRH Neurons to Kisspeptin in the Male Mouse

The tachykinins substance P (SP) and neurokinin A (Tac1) have emerged as novel regulators of kisspeptin/GnRH release. Recently, we documented that SP modulates reproductive function in the female mouse. Here, we extended this characterization to the male mouse. Tac1-/- male mice showed delayed puberty onset. They also presented significantly decreased expression levels of Pdyn (dynorphin) and Nos1 (nitric oxide synthase) in the mediobasal hypothalamus and elevated Gnrh1 levels. Unexpectedly, the response of Tac1-/- mice to central kisspeptin or senktide (neurokinin B receptor-agonist) administration was significantly decreased compared with controls, despite the preserved ability of GnRH neurons to stimulate luteinizing hormone release as demonstrated by central N-methyl-D-aspartate receptor administration, suggesting a deficit at the GnRH neuron level. Importantly, we demonstrated that kisspeptin receptor and SP receptor (NK1R) heterodimerize, indicating that changes in the SP tone could alter the responsiveness of GnRH neurons to kisspeptin. Finally, electrophysiological recordings from arcuate Kiss1 neurons showed that, although virtually all Kiss1 neurons responded to NKB and senktide, only half responded to an NK1R agonist and none to the neurokinin A receptor agonist at a 1-μM dose. In summary, we provide compelling evidence for a role of Tac1 in the control of reproductive function in the male mouse, suggesting a predominant central action that may involve a change in the balance of neural factors that control GnRH expression.

Basal Serum Neurokinin B Levels in Differentiating Idiopathic Central Precocious Puberty from Premature Thelarche

OBJECTIVE

To find out the diagnostic role of kisspeptin and neurokinin B in idiopathic central precocious puberty (ICPP) and premature thelarche (PT).

METHODS

The girls who presented with early breast development before the age of 8 years were evaluated. Patients with intracranial pathologies were excluded. Basal and stimulated follicle-stimulating hormone/luteinizing hormone (LH) levels and basal neurokinin B/kisspeptin levels were measured. Patients who had peak value of LH >5 mIU/mL and a bone age (BA)/chronological age (CA) ratio >1.1 were diagnosed as central precocious puberty (CPP), while cases who did not meet these criteria were diagnosed as PT. Healthy age-matched prepubertal girls were included as the control group.

RESULTS

The study group contained 25 girls with ICPP (7±0.8 years), 35 girls with PT (6.8±0.7 years), and 30 controls (6.7±0.7 years). Basal serum kisspeptin and neurokinin B levels were 2.36±0.47 ng/mL and 2.61±0.32 ng/mL, respectively in the ICPP group, 2.23±0.43 ng/mL and 2.24±0.23 ng/mL, respectively in the PT group, and 1.92±0.33 ng/mL and 2.03±0.24 ng/mL, respectively in the controls. Both kisspeptin and neurokinin B levels were higher in the ICPP and PT groups compared to controls (p<0.05). Moreover, basal neurokinin B level was different between ICPP and PT groups (p<0.01). A serum neurokinin B level of 2.42 ng/mL provided the most appropriate level to differentiate ICPP from PT, with a sensitivity of 84% and specificity of 77.1%.

CONCLUSION

Differentiation of CPP from PT is sometime difficult, and there is a need for a simple method for the differential diagnosis. Our results suggest that basal serum neurokinin B level can be used as an adjunctive parameter to differentiate ICCP from PT.

Do Substance P and Neurokinin A Play Important Roles in the Control of LH Secretion in Ewes?

There is now general agreement that neurokinin B (NKB) acts via neurokinin-3-receptor (NK3R) to stimulate secretion of GnRH and LH in several species, including rats, mice, sheep, and humans. However, the roles of two other tachykinins, substance P (SP) and neurokinin A, which act primarily via NK1R and NK2R, respectively, are less clear. In rodents, these signaling pathways can stimulate LH release and substitute for NKB signaling; in humans, SP is colocalized with kisspeptin and NKB in the mediobasal hypothalamus. In this study, we examined the possible role of these tachykinins in control of the reproductive axis in sheep. Immunohistochemistry was used to describe the expression of SP and NK1R in the ovine diencephalon and determine whether these proteins are colocalized in kisspeptin or GnRH neurons. SP-containing cell bodies were largely confined to the arcuate nucleus, but NK1R-immunoreactivity was more widespread. However, there was very low coexpression of SP or NK1R in kisspeptin cells and none in GnRH neurons. We next determined the minimal effective dose of these three tachykinins that would stimulate LH secretion when administered into the third ventricle of ovary-intact anestrous sheep. A much lower dose of NKB (0.2 nmol) than of neurokinin A (2 nmol) or SP (10 nmol) consistently stimulated LH secretion. Moreover, the relative potency of these three neuropeptides parallels the relative selectivity of NK3R. Based on these anatomical and pharmacological data, we conclude that NKB-NK3R signaling is the primary pathway for the control of GnRH secretion by tachykinins in ewes.

Expanding the Role of Tachykinins in the Neuroendocrine Control of Reproduction

Reproductive function is driven by the hormonal interplay between the gonads and brain-pituitary axis. Gonadotropin-releasing hormone (GnRH) is released in a pulsatile manner, which is critical for the attainment and maintenance of fertility, however, GnRH neurons lack the ability to directly respond to most regulatory factors, and a hierarchical upstream neuronal network governs its secretion. We and others proposed a model in which Kiss1 neurons in the arcuate nucleus (ARC), so called KNDy neurons, release kisspeptin (a potent GnRH secretagogue) in a pulsatile manner to drive GnRH pulses under the coordinated autosynaptic action of its cotransmitters, the tachykinin neurokinin B (NKB, stimulatory) and dynorphin (inhibitory). Numerous genetic and pharmacological studies support this model; however, additional regulatory mechanisms (upstream of KNDy neurons) and alternative pathways of GnRH secretion (kisspeptin-independent) exist, but remain ill defined. In this aspect, attention to other members of the tachykinin family, namely substance P (SP) and neurokinin A (NKA), has recently been rekindled. Even though there are still major gaps in our knowledge about the functional significance of these systems, substantial evidence, as discussed below, is placing tachykinin signaling as an important pathway for the awakening of the reproductive axis and the onset of puberty to physiological GnRH secretion and maintenance of fertility in adulthood.

Ablation of KNDy Neurons Results in Hypogonadotropic Hypogonadism and Amplifies the Steroid-Induced LH Surge in Female Rats

In the human infundibular (arcuate) nucleus, a subpopulation of neurons coexpress kisspeptin and neurokinin B (NKB), 2 peptides required for normal reproductive function. A homologous group of neurons exists in the arcuate nucleus of rodents, termed KNDy neurons based on the coexpression of kisspeptin, NKB, and dynorphin. To study their function, we recently developed a method to selectively ablate KNDy neurons using NK3-SAP, a neurokinin 3 receptor agonist conjugated to saporin (SAP). Here, we ablated KNDy neurons in female rats to determine whether these neurons are required for estrous cyclicity and the steroid induced LH surge. NK3-SAP or Blank-SAP (control) was microinjected into the arcuate nucleus using stereotaxic surgery. After monitoring vaginal smears for 3-4 weeks, rats were ovariectomized and given 17β-estradiol and progesterone in a regimen that induced an afternoon LH surge. Rats were killed at the time of peak LH levels, and brains were harvested for NKB and dual labeled GnRH/Fos immunohistochemistry. In ovary-intact rats, ablation of KNDy neurons resulted in hypogonadotropic hypogonadism, characterized by low levels of serum LH, constant diestrus, ovarian atrophy with increased follicular atresia, and uterine atrophy. Surprisingly, the 17β-estradiol and progesterone-induced LH surge was 3 times higher in KNDy-ablated rats. Despite the marked increase in the magnitude of the LH surge, the number of GnRH or anterior ventral periventricular nucleus neurons expressing Fos was not significantly different between groups. Our studies show that KNDy neurons are essential for tonic levels of serum LH and estrous cyclicity and may play a role in limiting the magnitude of the LH surge.

Leptin receptor null mice with reexpression of LepR in GnRHR expressing cells display elevated FSH levels but remain in a prepubertal state

Leptin signals energy sufficiency to the reproductive hypothalamic-pituitary-gonadal (HPG) axis. Studies using genetic models have demonstrated that hypothalamic neurons are major players mediating these effects. Leptin receptor (LepR) is also expressed in the pituitary gland and in the gonads, but the physiological effects of leptin in these sites are still unclear. Female mice with selective deletion of LepR in a subset of gonadotropes show normal pubertal development but impaired fertility. Conditional deletion approaches, however, often result in redundancy or developmental adaptations, which may compromise the assessment of leptin's action in gonadotropes for pubertal maturation. To circumvent these issues, we adopted a complementary genetic approach and assessed if selective reexpression of LepR only in gonadotropes is sufficient to enable puberty and improve fertility of LepR null female mice. We initially assessed the colocalization of gonadotropin-releasing hormone receptor (GnRHR) and LepR in the HPG axis using GnRHR-IRES-Cre (GRIC) and LepR-Cre reporter (tdTomato or enhanced green fluorescent protein) mice. We found that GRIC and leptin-induced phosphorylation of STAT3 are expressed in distinct hypothalamic neurons. Whereas LepR-Cre was observed in theca cells, GRIC expression was rarely found in the ovarian parenchyma. In contrast, a subpopulation of gonadotropes expressed the LepR-Cre reporter gene (tdTomato). We then crossed the GRIC mice with the LepR null reactivable (LepR(loxTB)) mice. These mice showed an increase in FSH levels, but they remained in a prepubertal state. Together with previous findings, our data indicate that leptin-selective action in gonadotropes serves a role in adult reproductive physiology but is not sufficient to allow pubertal maturation in mice.

Sex-specific regulation of weight and puberty by the Lin28/let-7 axis

Growth and pubertal timing differ in boys and girls. Variants in/near LIN28B associate with age at menarche (AAM) in genome-wide association studies and some AAM-related variants associate with growth in a sex-specific manner. Sex-specific growth patterns in response to Lin28b perturbation have been detected in mice, and overexpression of Lin28a has been shown to alter pubertal timing in female mice. To investigate further how Lin28a and Lin28b affect growth and puberty in both males and females, we evaluated Lin28b loss-of-function (LOF) mice and Lin28a gain-of-function (GOF) mice. Because both Lin28a and Lin28b can act via the conserved microRNA let-7, we also examined let-7 GOF mice. As reported previously, Lin28b LOF led to lighter body weights only in male mice while Lin28a GOF yielded heavier mice of both sexes. Let-7 GOF mice weighed less than controls, and males were more affected than females. Timing of puberty was assessed by vaginal opening (VO) and preputial separation (PS). Male Lin28b LOF and male let-7 GOF, but not female, mice displayed alteration of pubertal timing, with later PS than controls. In contrast, both male and female Lin28a GOF mice displayed late onset of puberty. Together, these data point toward a complex system of regulation by Lin28a, Lin28b, and let-7, in which Lin28b and let-7 can impact both puberty and growth in a sex-specific manner, raising the possibility that this pathway may contribute to differential regulation of male and female growth and puberty in humans.

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.

Substance p regulates puberty onset and fertility in the female mouse

Puberty is a tightly regulated process that leads to reproductive capacity. Kiss1 neurons are crucial in this process by stimulating GnRH, yet how Kiss1 neurons are regulated remains unknown. Substance P (SP), an important neuropeptide in pain perception, induces gonadotropin release in adult mice in a kisspeptin-dependent manner. Here, we assessed whether SP, through binding to its receptor NK1R (neurokinin 1 receptor), participates in the timing of puberty onset and fertility in the mouse. We observed that 1) selective NK1R agonists induce gonadotropin release in prepubertal females; 2) the expression of Tac1 (encoding SP) and Tacr1 (NK1R) in the arcuate nucleus is maximal before puberty, suggesting increased SP tone; 3) repeated exposure to NK1R agonists prepubertally advances puberty onset; and 4) female Tac1(-/-) mice display delayed puberty; moreover, 5) SP deficiency leads to subfertility in females, showing fewer corpora lutea and antral follicles and leading to decreased litter size. Thus, our findings support a role for SP in the stimulation of gonadotropins before puberty, acting via Kiss1 neurons to stimulate GnRH release, and its involvement in the attainment of full reproductive capabilities in female mice.