Role of Kisspeptin and Neurokinin B Signaling in Male Rhesus Monkey Puberty

The role of neuroestrogens in the estrogen-induced gonadotropin surge in male monkeys

Neuroestrogens locally synthesized in the brain are known to play a role in sexual behaviors. However, the question of whether neuroestrogens are involved in the regulation of the gonadotropin-releasing hormone (GnRH) release is just emerging. Because previous studies in this lab indicate that neuroestradiol is also important for the pulsatile release as well as the surge release of GnRH in female rhesus monkeys, in the present study, we examined whether neuroestradiol plays a role in the estrogen-induced LH surge in orchidectomized (ORX) male rhesus monkeys. Unlike in rodents, it is known that a high dose of estrogen treatment can result in the LH surge in ORX male rhesus monkeys. Results that the administration of the aromatase inhibitor, letrozole, failed to attenuate the estrogen-induced LH surge, suggest that unlike in ovariectomized females, neuroestrogens do not play a role in the LH surge experimentally induced by the exogenous estrogen treatment in ORX male monkeys.

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.

The role of puberty on physical and brain development: A longitudinal study in male Rhesus Macaques

This study examined the role of male pubertal maturation on physical growth and development of neurocircuits that regulate stress, emotional and cognitive control using a translational nonhuman primate model. We collected longitudinal data from male macaques between pre- and peri-puberty, including measures of physical growth, pubertal maturation (testicular volume, blood testosterone -T- concentrations) and brain structural and resting-state functional MRI scans to examine developmental changes in amygdala (AMY), hippocampus (HIPPO), prefrontal cortex (PFC), as well as functional connectivity (FC) between those regions. Physical growth and pubertal measures increased from pre- to peri-puberty. The indexes of pubertal maturation -testicular size and T- were correlated at peri-puberty, but not at pre-puberty (23 months). Our findings also showed ICV, AMY, HIPPO and total PFC volumetric growth, but with region-specific changes in PFC. Surprisingly, FC in these neural circuits only showed developmental changes from pre- to peri-puberty for HIPPO-orbitofrontal FC. Finally, testicular size was a better predictor of brain structural maturation than T levels -suggesting gonadal hormones-independent mechanisms-, whereas T was a strong predictor of functional connectivity development. We expect that these neural circuits will show more drastic pubertal-dependent maturation, including stronger associations with pubertal measures later, during and after male puberty.

Variation in Hypothalamic GnIH Expression and Its Association with GnRH and Kiss1 during Pubertal Progression in Male Rhesus Monkeys (Macaca mulatta)

Modulation of pulsatile gonadotropin-releasing hormone (GnRH) secretion across postnatal development in higher primates is not fully understood. While gonadotropin-inhibitory hormone (GnIH) is reported to suppress reproductive axis activity in birds and rodents, little is known about the developmental trajectory of GnIH expression in rhesus monkeys throughout the pubertal transition. This study was aimed at examining the variation in GnIH immunoreactivity (-ir) and associated changes among GnIH, GnRH, and Kiss1 mRNA expression in the hypothalamus of infant, juvenile, prepubertal, and adult male rhesus monkeys. The brains from rhesus macaques were collected from infancy until adulthood and were examined using immunofluorescence and RT-qPCR. The mean GnIH-ir was found to be significantly higher in prepubertal animals (p < 0.01) compared to infants, and significantly reduced in adults (p < 0.001). Significantly higher (p < 0.001) GnRH and Kiss1 mRNA expression was noted in adults while GnIH mRNA expression was the highest at the prepubertal stage (p < 0.001). Significant negative correlations were seen between GnIH-GnRH (p < 0.01) and GnIH-Kiss1 (p < 0.001) expression. Our findings suggest a role for GnIH in the prepubertal suppression of the reproductive axis, with disinhibition of the adult reproductive axis occurring through decreases in GnIH. This pattern of expression suggests that GnIH may be a viable target for the development of novel therapeutics and contraceptives for humans.

The mechanism underlying the pubertal increase in pulsatile GnRH release in primates

In primates, the gonatotropin-releasing hormone (GnRH) neurosecretory system, consisting of GnRH, kisspeptin, and neurokinin B neurons, is active during the neonatal/early infantile period. During the late infantile period, however, activity of the GnRH neurosecretory system becomes minimal as a result of gonadal steroid independent central inhibition, and this suppressed GnRH neurosecretory state continues throughout the prepubertal period. At the initiation of puberty, the GnRH neurosecretory system becomes active again because of the decrease in central inhibition. During the progress of puberty, kisspeptin and neurokinin B signaling to GnRH neurons further increases, resulting in the release of gonadotropins and subsequent gonadal maturation, and hence puberty. This review further discusses potential substrates of central inhibition and subsequent pubertal modification of the GnRH neurosecretory system by the pubertal increase in steroid hormones, which ensures the regulation of adult reproductive function.

Inverse age-related changes between hypothalamic NPY and KISS1 gene expression during pubertal initiation in male rhesus monkey

The neuroendocrine mechanism underlying the sinusoidal wave nature of gonadotropin-releasing hormone pulse generator activity from infantile to adult age still needs to be meticulously defined. Direct inhibition of kisspeptin neurons by neuropeptide Y (NPY) and close intimacy between the two rekindle the importance of these two neuropeptides controlling reproductive axis activity. Thus, the present study was undertaken to decipher simultaneous fluctuations and to profile correlative changes in the relative expression of KISS1, NPY, and their receptor genes from the mediobasal hypothalamus of infant (n = 3), juvenile, pre-pubertal, and adult (n = 4 in each stage) male rhesus monkey (Macaca mulatta) by RT-qPCR. Significant elevation (p < 0.05-0.01) in KISS1 and KISS1R and low (p < 0.05) expression in NPY and NPY1R mRNA in the adult group as compared to the pre-pubertal group was observed. Moreover, significantly high (p < 0.05) expression of NPY and NPY1R mRNA with non-significant (p> 0.05) decline in KISS1 and KISS1R in pre-pubertal animals in comparison to infants describe inverse correlative age-associated changes during pubertal development. Current findings imply that NPY may contribute as a neurobiological brake for the dormancy of kisspeptin neurons before pubertal onset, while dwindling of this brake is likely to occasion kisspeptin dependent hypothalamic-pituitary-gonadal axis activation at puberty. These findings may help in the development of clinical and therapeutic strategies to regulate fertility in humans.

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.

Kobayashi Award 2019: The neuroendocrine regulation of the mammalian reproduction

Mammalian reproductive function is a complex system of many players orchestrated by the hypothalamus-pituitary-gonadal (HPG) axis. The hypothalamic gonadotropin-releasing hormone (GnRH) and the consequent pituitary gonadotropin release show two modes of secretory patterns, namely the surge and pulse modes. The surge mode is triggered by the positive feedback action of estrogen secreted from the mature ovarian follicle to induce ovulation in females of most mammalian species. The pulse mode of GnRH release is required for stimulating tonic gonadotropin secretion to drive folliculogenesis, spermatogenesis and steroidogenesis and is negatively fine-tuned by the sex steroids. Accumulating evidence suggests that hypothalamic kisspeptin neurons are the master regulator for animal reproduction to govern the HPG axis. Specifically, kisspeptin neurons located in the anterior hypothalamus, such as the anteroventral periventricular nucleus (AVPV) in rodents and preoptic nucleus (POA) in ruminants, primates and others, and the neurons located in the arcuate nucleus (ARC) in posterior hypothalamus in most mammals are considered to play a key role in generating the surge and pulse modes of GnRH release, respectively. The present article focuses on the role of AVPV (or POA) kisspeptin neurons as a center for GnRH surge generation and of the ARC kisspeptin neurons as a center for GnRH pulse generation to mediate estrogen positive and negative feedback mechanisms, respectively, and discusses how the estrogen epigenetically regulates kisspeptin gene expression in these two populations of neurons. This article also provides the mechanism how malnutrition and lactation suppress GnRH/gonadotropin pulses through an inhibition of the ARC kisspeptin neurons. Further, the article discusses the programming effect of estrogen on kisspeptin neurons in the developmental brain to uncover the mechanism underlying the sex difference in GnRH/gonadotropin release as well as an irreversible infertility induced by supra-physiological estrogen exposure in rodent models.

Physiological Characterization and Transcriptomic Properties of GnRH Neurons Derived From Human Stem Cells

Gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus play a key role in the regulation of reproductive function. In this study, we sought an efficient method for generating GnRH neurons from human embryonic and induced pluripotent stem cells (hESC and hiPSC, respectively). First, we found that exposure of primitive neuroepithelial cells, rather than neuroprogenitor cells, to fibroblast growth factor 8 (FGF8), was more effective in generating GnRH neurons. Second, addition of kisspeptin to FGF8 further increased the efficiency rates of GnRH neurogeneration. Third, we generated a fluorescent marker mCherry labeled human embryonic GnRH cell line (mCh-hESC) using a CRISPR-Cas9 targeting approach. Fourth, we examined physiological characteristics of GnRH (mCh-hESC) neurons: similar to GnRH neurons in vivo, they released the GnRH peptide in a pulsatile manner at ~60 min intervals; GnRH release increased in response to high potassium, kisspeptin, estradiol, and neurokinin B challenges; and injection of depolarizing current induced action potentials. Finally, we characterized developmental changes in transcriptomes of GnRH neurons using hESC, hiPSC, and mCh-hESC. The developmental pattern of transcriptomes was remarkably similar among the 3 cell lines. Collectively, human stem cell-derived GnRH neurons will be an important tool for establishing disease models to understand diseases, such as idiopathic hypothalamic hypogonadism, and testing contraceptive drugs.

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.

Hypothalamic neurokinin signalling and its application in reproductive medicine

The discovery of the essential requirement for kisspeptin and subsequently neurokinin B signalling for human reproductive function has sparked renewed interest in the neuroendocrinology of reproduction. A key discovery has been a population of cells co-expressing both these neuropeptides and dynorphin in the hypothalamus, directly regulating gonadotropin hormone releasing hormone (GnRH) secretion and thus pituitary secretion of gonadotropins. These neurons also project to the vasomotor centre, and their overactivity in estrogen deficiency results in the common and debilitating hot flushes of the menopause. Several antagonists to the neurokinin 3 receptor, for which neurokinin B is the endogenous ligand, have been developed, and are entering clinical studies in human reproductive function and clinical trials. Even single doses can elicit marked declines in testosterone levels in men, and their use has elicited evidence of the regulation of ovarian follicle growth in women. The most advanced indication is the treatment of menopausal vasomotor symptoms, where these drugs show remarkable results in both the degree and speed of symptom control. A range of other reproductive indications are starting to be explored, notably in polycystic ovary syndrome, the most common endocrinopathy in women.

Expression of genes for Kisspeptin (KISS1), Neurokinin B (TAC3), Prodynorphin (PDYN), and gonadotropin inhibitory hormone (RFRP) across natural puberty in ewes

Expression of particular genes in hypothami of ewes was measured across the natural pubertal transition by in situ hybridization. The ewes were allocated to three groups (n = 4); prepubertal, postpubertal and postpubertally gonadectomized (GDX). Prepubertal sheep were euthanized at 20 weeks of age and postpubertal animals at 32 weeks. GDX sheep were also euthanized at 32 weeks, 1 week after surgery. Expression of KISS1, TAC3, PDYN in the arcuate nucleus (ARC), RFRP in the dorsomedial hypothalamus and GNRH1 in the preoptic area was quantified on a cellular basis. KISS1R expression by GNRH1 cells was quantified by double-label in situ hybridization. Across puberty, detectable KISS1 cell number increased in the caudal ARC and whilst PDYN cell numbers were low, numbers increased in the rostral ARC. TAC3 expression did not change but RFRP expression/cell was reduced across puberty. There was no change across puberty in the number of GNRH1 cells that expressed the kisspeptin receptor (KISS1R). GDX shortly after puberty did not increase expression of any of the genes of interest. We conclude that KISS1 expression in the ARC increases during puberty in ewes and this may be a causative factor in the pubertal activation of the reproductive axis. A reduction in expression of RFRP may be a factor in the onset of puberty, removing negative tone on GNRH1 cells. The lack of changes in expression of genes following GDX suggest that the effects of gonadal hormones may differ in young and mature animals.

Mating-induced increase in Kiss1 mRNA expression in the anteroventral periventricular nucleus prior to an increase in LH and testosterone release in male rats

Kisspeptin has an indispensable role in gonadotropin-releasing hormone/gonadotropin secretion in mammals. In rodents, kisspeptin neurons are located in distinct brain regions, namely the anteroventral periventricular nucleus-periventricular nucleus continuum (AVPV/PeN), arcuate nucleus (ARC), and medial amygdala (MeA). Among them, the physiological role of AVPV/PeN kisspeptin neurons in males has not been clarified yet. The present study aims to investigate the acute effects of the olfactory and/or mating stimulus with a female rat on hypothalamic and MeA Kiss1 mRNA expression, plasma luteinizing hormone (LH) and testosterone levels in male rats. Intact male rats were exposed to the following stimuli: exposure to clean bedding; exposure to female-soiled bedding as a female-olfactory stimulus; exposure to female-soiled bedding and mating stimulus with a female rat. The mating stimulus significantly increased the number of the AVPV/PeN Kiss1 mRNA-expressing cells in males within 5 minutes after the exposure, and significantly increased LH and testosterone levels, followed by an increase in male sexual behavior. Whereas, the males exposed to female-soiled bedding showed a moderate increase in LH levels and no significant change in testosterone levels and the number of the AVPV/PeN Kiss1 mRNA-expressing cells. Importantly, none of the stimuli affected the number of Kiss1 mRNA-expressing cells in the ARC and MeA. These results suggest that the mating-induced increase in AVPV/PeN Kiss1 mRNA expression may be, at least partly, involved in stimulating LH and testosterone release, and might consequently ensure male mating behavior. This study would be the first report suggesting that the AVPV/PeN kisspeptin neurons in males may play a physiological role in ensuring male reproductive performance.

Neuroendocrine mechanisms of puberty in non-human primates

An increase in pulsatile release of gonadotropin releasing hormone (GnRH) initiates puberty in mammalian species. While mutations in KISS1 and TAC3 and their receptors, KISS1R and NK3R, respectively, result in the absence or abnormal timing of puberty, the neurocircuitry and precise role of kisspeptin and neurokinin B (NKB) in regulation of the GnRH neurosecretory system in primate puberty remain elusive. This review discusses how kisspeptin and NKB signaling contributes to the pubertal increase in GnRH release in non-human primates and how remodeling of the NKB and kisspeptin signaling circuitry controlling GnRH neurons takes place during the progress of puberty. Importantly, the pubertal remodeling of kisspeptin and NKB signaling ensures efficient functions of the GnRH neurosecretory system that regulates sex-specific reproduction in primates.

Role of Kisspeptin and NKB in Puberty in Nonhuman Primates: Sex Differences

To understand the roles of kisspeptin and neurokinin B (NKB) in puberty and sex differences in their involvement, we conducted a series of experiments measuring the release of gonadotropin-releasing hormone (GnRH) and kisspeptin in the median eminence of the hypothalamus in male and female monkeys throughout sexual development. Results indicate that kisspeptin-10 and the NKB agonist, senktide, stimulated GnRH release in males and females at the prepubertal and pubertal stages, but females are much more sensitive to kisspeptin signaling than males. Moreover, throughout the progress of puberty, major remodeling of kisspeptin and NKB signaling pathways for the regulation of GnRH release takes place. In females during puberty, reciprocal pathways (i.e., kisspeptin signaling mediated through NKB neurons and NKB signaling mediated through kisspeptin neurons) are established, to provide powerful and flexible mechanisms for GnRH neurosecretory activity necessary for complex female reproductive function in adulthood. By contrast, during puberty in males, reciprocal pathways are consolidated to a simpler kisspeptin-dominant signaling pathway. Nevertheless, in primates, both kisspeptin and NKB signaling are contributing factors for the pubertal increase in GnRH release, rather than initiating puberty.

Mechanism of pulsatile GnRH release in primates: Unresolved questions

The pulsatility of GnRH release is essential for reproductive function. The key events in reproductive function, such as puberty onset and ovulatory cycles, are regulated by the frequency and amplitude modulation of pulsatile GnRH release. Abnormal patterns of GnRH pulsatility are seen in association with disease states, such as polycystic ovarian syndrome and anorexia nervosa. Recent studies with physiological, track-tracing, optogenetic and electrophysiological recording experiments indicate that a group of kisspeptin neurons in the arcuate nucleus (ARC) of the hypothalamus are responsible for pulsatile GnRH release. Thus, the kisspeptin neuron in the ARC has been called the "GnRH pulse-generator." However, a few pieces of evidence do not quite fit into this concept. This article reviews some old works and discusses unresolved issues on the mechanism of GnRH pulse generation.

Central Mechanism Controlling Pubertal Onset in Mammals: A Triggering Role of Kisspeptin

Pubertal onset is thought to be timed by an increase in pulsatile gonadotropin-releasing hormone (GnRH)/gonadotropin secretion in mammals. The underlying mechanism of pubertal onset in mammals is still an open question. Evidence accumulated in the last 15 years suggests that kisspeptin/neurokinin B/dynorphin A (KNDy) neurons in the hypothalamic arcuate nucleus play a key role in pubertal onset by triggering pulsatile GnRH/gonadotropin secretin in mammals. Specifically, KNDy neurons are now considered a part of GnRH pulse generator, in which neurokinin B facilitates and dynorphin A inhibits, the synchronized discharge of KNDy neurons in autocrine and/or paracrine manners. Kisspeptin serves as a potent secretagogue of GnRH secretion and thus its release is fundamental to pubertal increase in GnRH/gonadotropin secretion in mammals. Proposed mechanisms inhibiting Kiss1 (kisspeptin gene) expression during childhood to juvenile varies from species to species: we envisage that negative feedback action of estrogen plays a key role in the inhibition of Kiss1 expression in KNDy neurons in rodents and sheep, whereas estrogen-independent inhibition of kisspeptin secretion by γ-amino butyric acid or neuropeptide Y are suggested to be responsible for the pre-pubertal suppression of GnRH/gonadotropin secretion in primates. Taken together, the timing of pubertal onset is postulated to be controlled by upstream regulators for kisspeptin biosynthesis and secretion in mammals.