The effect of short fasting on the hypothalamic neuronal system of kisspeptin in peripubertal female lambs

KNDy neurons as the GnRH pulse generator: Recent studies in ruminants

This review considers three aspects of recent work on the role of KNDy neurons in GnRH pulse generation in ruminants. First, work on basic mechanisms of pulse generation includes several tests of this hypothesis, all of which support it, and evidence that Kiss1r-containing neurons form a positive feedback circuit with the KNDy neural network that strengthen the activity of this network. The second section on pathways mediating external inputs focuses on the influence of nutrition and photoperiod, and describes the evidence supporting roles for proopiomelanocortin (POMC) and agouti-related peptide (AgRP) afferents to KNDy cells in each of these. Finally, we review studies exploring the potential applications of manipulating signaling by kisspeptin, and the other KNDy peptides, to control reproductive function in domestic animals and conclude that, although these approaches show some promise, they do not have major advantages over current practices at this time.

Food deprivation differentially modulates gene expression of LPXRFa and kisspeptin systems in the brain-pituitary axis of half-smooth tongue sole (Cynoglossus semilaevis)

LPXRFa, also known as gonadotropin-inhibitory hormone (GnIH), and kisspeptin (Kiss) are two major hypothalamic peptides that modulate the reproductive axis of vertebrates, including teleosts. However, little information is available regarding the actions of nutritional status on the regulation of these two neuroendocrine systems in fish. Herein, we assessed the effects of starvation and refeeding on the expression of lpxrfa, kiss2 and their receptors (lpxrfa-r and kiss2r respectively) at the brain-pituitary level of half-smooth tongue sole (Cynoglossus semilaevis). Food deprivation for 4 weeks induced a rise in brain lpxrfa as well as brain and pituitary lpxrfa-r mRNA levels, and refeeding restored brain lpxrfa and lpxrfa-r expression back to normal. However, pituitary lpxrfa-r mRNA levels still remained high after 1 week of refeeding. Neither lpxrfa nor kiss2 transcripts in the pituitary were altered by fasting, but their mRNA levels increased significantly after 1 week of refeeding, and declined back to the control levels after 2 weeks of refeeding. None of brain kiss2 and kiss2r along with pituitary kiss2r transcripts were modified by the nutritional status. In summary, our results revealed an interaction between energy status and the elements of LPXRFa and Kiss systems in the brain-pituitary axis of half-smooth tongue sole. Food deprivation and refeeding differentially regulated the two systems, which provided additional evidence for the involvement of the LPXRFa and Kiss systems in the regulation of reproduction by energy balance in non-mammalian species.

Impact of food restriction on the medio-basal hypothalamus of intact ewes as revealed by a large-scale transcriptomics study

In mammals, the medio-basal hypothalamus (MBH) integrates photoperiodic and food-related cues to ensure timely phasing of physiological functions, including seasonal reproduction. The current human epidemics of obesity and associated reproductive disorders exemplifies the tight link between metabolism and reproduction. Yet, how food-related cues impact breeding at the level of the MBH remains unclear. In this respect, the sheep, which is a large diurnal mammal with a marked dual photoperiodic/metabolic control of seasonal breeding, is a relevant model. Here, we present a large-scale study in ewes (n = 120), which investigated the impact of food restriction (FRes) on the MBH transcriptome using unbiased RNAseq, followed by RT-qPCR. Few genes (~100) were impacted by FRes and the transcriptional impact was very modest (<2-fold increase or < 50% decrease for most genes). As anticipated, FRes increased expression of Npy/AgRP/LepR and decreased expression of Pomc/Cartpt, while Kiss1 expression was not impacted. Of particular interest, Eya3, Nmu and Dio2, genes involved in photoperiodic decoding within the MBH, were also affected by FRes. Finally, we also identified a handful of genes not known to be regulated by food-related cues (e.g., RNase6, HspA6, Arrdc2). In conclusion, our transcriptomics study provides insights into the impact of metabolism on the MBH in sheep, which may be relevant to human, and identifies possible molecular links between metabolism and (seasonal) reproduction.

Effects of low energy availability on female reproductive function

Background

It is known that metabolic and nutritional disturbances induce reproductive dysfunction in females. The main cause of these alterations is reduced gonadotrophin‐releasing hormone (GnRH) secretion from the hypothalamus, and the underlying mechanisms have gradually been elucidated.

Methods

The present review summarizes current knowledge about the effects of nutrition/metabolism on reproductive functions, especially focusing on the GnRH regulation system.

Main findings

Various central and peripheral factors are involved in the regulation of GnRH secretion, and alterations in their activity combine to affect GnRH neurons. Satiety‐related factors, i.e., leptin, insulin, and alpha‐melanocyte‐stimulating hormone, directly and indirectly stimulate GnRH secretion, whereas orexigenic factors, i.e., neuropeptide Y, Agouti‐related protein, orexin, and ghrelin, attenuate GnRH secretion. In addition, kisspeptin, which is a potent positive regulator of GnRH, expression is reduced by metabolic and nutritional disturbances.

Conclusion

These neuroendocrine systems may be defensive mechanisms, which help organisms to survive adverse conditions by temporarily suppressing reproduction.

Intermittent fasting ameliorates di-(2-ethylhexyl) phthalate-induced precocious puberty in female rats: A study of the hypothalamic-pituitary-gonadal axis

Di-(2-ethylhexyl) phthalate has been reported to interfere with the development and function of animal reproductive systems. However, hardly any studies provide methods to minimize or prevent the adverse effects of DEHP on reproduction. The energy balance state of mammals is closely related to reproductive activities, and the reproductive axis can regulate reproductive activities according to changes in the body's energy balance state. In this study, the effects of every other day fasting (EODF), as a way of intermittent fasting, on preventing the precocious puberty induced by DEHP in female rats was studied. EODF significantly improved the advancement of vaginal opening age (as the markers of puberty onset) and elevated serum levels of luteinizing hormone and estradiol (detected by ELISA) induced by 5 mg kg^-1 DEHP exposure (D5). The mRNA and western blot results showed that the EODF could minimized the increase of gonadotropin-releasing hormone expression induced by DEHP exposure. The administration of DEHP could elevate the levels of kisspeptin protein and the number of kisspeptin-immunoreactive neurons in anteroventral periventricular nucleu, and this increase was diminished considerably by EODF treatment. In contrast, the D5 and D0 groups showed no remarkable difference in the level of Kiss1 expression in arcuate nucleus, whereas the D5 + EODF group had a remarkable decrease in kisspeptin expression as compared with the other two groups. Our results indicated that EODF might inhibit the acceleration of puberty onset induced by DEHP exposure via HPG axis.

Evidence That Agouti-Related Peptide May Directly Regulate Kisspeptin Neurons in Male Sheep

Agouti-related peptide (AgRP) neurons, which relay information from peripheral metabolic signals, may constitute a key central regulator of reproduction. Given that AgRP inhibits luteinizing hormone (LH) secretion and that nutritional suppression of LH elicits an increase in AgRP while suppressing kisspeptin expression in the arcuate nucleus (ARC) of the hypothalamus, we sought to examine the degree to which AgRP could directly regulate ARC kisspeptin neurons. Hypothalamic tissue was collected from four castrated male sheep (10 months of age) and processed for the detection of protein (AgRP input to kisspeptin neurons) using immunohistochemistry and mRNA for melanocortin 3 and 4 receptors (MC3R; MC4R) in kisspeptin neurons using RNAscope. Immunohistochemical analysis revealed that the majority of ARC kisspeptin neurons are contacted by presumptive AgRP terminals. RNAscope analysis revealed that nearly two thirds of the ARC kisspeptin neurons express mRNA for MC3R, while a small percentage (<10%) colocalize MC4R. Taken together, this data provides neuroanatomical evidence for a direct link between orexigenic AgRP neurons and reproductively critical kisspeptin neurons in the sheep, and builds upon our current understanding of the central link between energy balance and reproduction.

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.

Neuronal networks that regulate gonadotropin-releasing hormone/luteinizing hormone secretion during undernutrition: evidence from sheep

Gonadotropin-releasing hormone (GnRH) neurons are the final common conduit from the central nervous system in the reproductive axis, controlling luteinizing hormone (LH) secretion from the gonadotropes of the anterior pituitary. Although it is generally accepted that undernutrition inhibits GnRH/LH secretion, the central mechanisms that underlie the link between energy balance and reproduction remain to be fully elucidated. Sheep have been a longstanding and invaluable animal model for examination of the nutritional regulation of GnRH/LH secretion, given their ability to serve a biomedical and agricultural purpose. In this review, we summarize work that has used the ovine model to examine the central mechanisms whereby undernutrition regulates GnRH/LH secretion. Specifically, we focus our attention to the arcuate nucleus of the hypothalamus and on neurons that express kisspeptin, neurokinin B, dynorphin, proopiomelanocortin, and neuropeptide y/agouti-related peptide (NPY/AgRP). We examine their roles in mediating the effects of leptin and insulin and their effects on LH during undernutrition, as well as their regulation under conditions of undernutrition. This review will also highlight the interactions between the aforementioned neuronal networks themselves, which may be important for our understanding of the roles each play in relaying information regarding energy status during times of undernutrition to ultimately regulate GnRH/LH secretion.

Undernutrition reduces kisspeptin and neurokinin B expression in castrated male sheep

Undernutrition impairs reproductive success through suppression of gonadotropin-releasing hormone (GnRH), and subsequently luteinizing hormone (LH), secretion. Given that kisspeptin and neurokinin B (NKB) neurons in the arcuate nucleus (ARC) of the hypothalamus are thought to play key stimulatory roles in the generation of GnRH/LH pulses, we hypothesized that feed restriction would reduce the ARC mRNA abundance and protein expression of kisspeptin and NKB in young, male sheep. Fourteen wethers (castrated male sheep five months of age) were either fed to maintain (FM; n = 6) pre-study body weight or feed-restricted (FR; n = 8) to lose 20% of pre-study body weight over 13 weeks. Throughout the study, weekly blood samples were collected and assessed for LH concentration using RIA. At Week 13 of the experiment, animals were killed, heads were perfused with 4% paraformaldehyde, and brain tissue containing the hypothalamus was collected, sectioned, and processed for detection of mRNA (RNAscope) and protein (immunohistochemistry) for kisspeptin and NKB. Mean LH was significantly lower and LH inter-pulse interval was significantly higher in FR wethers compared to FM wethers at the end of the experiment (Week 13). RNAscope analysis revealed significantly fewer cells expressing mRNA for kisspeptin and NKB in FR wethers compared to FM controls, and immunohistochemical analysis revealed significantly fewer immunopositive kisspeptin and NKB cells in FR wethers compared to FM wethers. Taken together, this data supports the idea that long-term feed restriction regulates GnRH/LH secretion through central suppression of kisspeptin and NKB in male sheep.

LAY SUMMARY

While undernutrition is known to impair reproduction at the level of the brain, the components responsible for this in the brain remain to be fully understood. Using male sheep we examined the effect of undernutrition on two stimulatory molecules in the brain critical for reproduction: kisspeptin and neurokinin B. Feed restriction for several weeks resulted in decreased luteinizing hormone in the blood indicating reproductive function was suppressed. In addition, undernutrition also reduced both kisspeptin and neurokinin B levels within a region of the brain involved in reproduction, the hypothalamus. Given that they have stimulatory roles in reproduction, we believe that undernutrition acts in the brain to reduce kisspeptin and neurokinin B levels leading to the reduction in luteinizing hormone secretion. In summary, long-term undernutrition inhibits reproductive function in sheep through suppression of kisspeptin and neurokinin B within the brain.

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

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

Expression of kisspeptin protein in hypothalamus and LH profile of growing female lambs

Kisspeptin (kp) is considered to be one of the major regulators of the induction of pubertal events via the activation of the gonadotrophin-releasing hormone-LH system. The aim of the present study was to analyse expression of immunoreactive (ir) kp in the hypothalamic neurons of female lambs from the neonatal to the peripubertal period (5 days to 32 weeks) in relation to the plasma LH pattern using immunohistochemistry and image analysis. Hypothalami were collected from female lambs (n=33) from the infantile, juvenile, prepubertal and peripubertal periods. The population of kp-ir perikarya was detected mainly in the arcuate nucleus and their number increased gradually from 5 to 16 weeks of age and was maintained at a high level up to the peripubertal stage. This was reflected by the significant (P<0.05) gradual increase in the percentage of hypothalamic area occupied by kp-ir neurons and increase in the number of kp-ir perikarya within the arcuate nucleus. The same pattern of kp immunoreactivity was observed in the median eminence. Plasma LH concentration increased from Week 5 to Weeks 12-16 and further increased at Week 32. LH pulse frequency increased from Week 5 to 32 (P<0.05). Thus, changes in kp expression reflected changes in the LH pattern during lamb growth. The data obtained provide evidence about the participation of kp in the mechanisms of ontogenic development of ovine reproductive processes.

Effects of Low Energy Availability on Reproductive Functions and Their Underlying Neuroendocrine Mechanisms

It is known that metabolic disturbances suppress reproductive functions in females. The mechanisms underlying metabolic and nutritional effects on reproductive functions have been established based on a large body of clinical and experimental data. From the 1980s to 1990s, it was revealed that disrupted gonadotropin-releasing hormone (GnRH) secretion is the main cause of reproductive impairments in metabolic and nutritional disorders. From the late 1990s to early 2000s, it was demonstrated that, in addition to their primary functions, some appetite- or metabolism-regulating factors affect GnRH secretion. Furthermore, in the early 2000s, kisspeptin, which is a potent positive regulator of GnRH secretion, was newly discovered, and it has been revealed that kisspeptin integrates the effects of metabolic status on GnRH neurons. Recent studies have shown that kisspeptin mediates at least some of the effects of appetite- and metabolism-regulating factors on GnRH neurons. Thus, kisspeptin might be a useful clinical target for treatments aimed at restoring reproductive functions in individuals with metabolic or nutritional disturbances, such as those who exercise excessively, experience marked weight loss, or suffer from eating disorders. This paper presents a review of what is currently known about the effects of metabolic status on reproductive functions and their underlying mechanisms by summarizing the available evidence.

Regulation of GnRH pulsatility in ewes

Early work in ewes provided a wealth of information on the physiological regulation of pulsatile gonadotropin-releasing hormone (GnRH) secretion by internal and external inputs. Identification of the neural systems involved, however, was limited by the lack of information on neural mechanisms underlying generation of GnRH pulses. Over the last decade, considerable evidence supported the hypothesis that a group of neurons in the arcuate nucleus that contain kisspeptin, neurokinin B and dynorphin (KNDy neurons) are responsible for synchronizing secretion of GnRH during each pulse in ewes. In this review, we describe our current understanding of the neural systems mediating the actions of ovarian steroids and three external inputs on GnRH pulsatility in light of the hypothesis that KNDy neurons play a key role in GnRH pulse generation. In breeding season adults, estradiol (E₂) and progesterone decrease GnRH pulse amplitude and frequency, respectively, by actions on KNDy neurons, with E₂ decreasing kisspeptin and progesterone increasing dynorphin release onto GnRH neurons. In pre-pubertal lambs, E₂ inhibits GnRH pulse frequency by decreasing kisspeptin and increasing dynorphin release, actions that wane as the lamb matures to allow increased pulsatile GnRH secretion at puberty. Less is known about mediators of undernutrition and stress, although some evidence implicates kisspeptin and dynorphin, respectively, in the inhibition of GnRH pulse frequency by these factors. During the anoestrus, inhibitory photoperiod acting via melatonin activates A15 dopaminergic neurons that innervate KNDy neurons; E₂ increases dopamine release from these neurons to inhibit KNDy neurons and suppress the frequency of kisspeptin and GnRH release.

The interaction of fasting, caloric restriction, and diet-induced obesity with 17β-estradiol on the expression of KNDy neuropeptides and their receptors in the female mouse

Arcuate neurons that coexpress kisspeptin (Kiss1), neurokinin B (Tac2), and dynorphin (Pdyn) mediate negative feedback of 17β-estradiol (E2) on the HPG axis. Previous studies report that fasting and caloric restriction reduce arcuate Kiss1 expression. The objective of this study was to determine the interactions of E2 with fasting, caloric restriction, and diet-induced obesity on KNDy gene and receptor expression. Ovariectomized female mice were separated into control and estradiol benzoate (E2B)-treated groups. E2B decreased Kiss1 and the tachykinin 2 receptor, Tac3r, in ARC tissue and Tac2 in Tac2 neurons. Diet-induced obesity decreased Kiss1 in oil-treated animals and the kisspeptin receptor, Kiss1r and Tac3r in the ARC of E2B-treated animals. Chronic caloric (30%) restriction reduced all three neuropeptides in oil-treated females and Kiss1r by E2B in CR animals. Taken together, our experiments suggest that steroidal environment and energy state negatively regulate KNDy gene expression in both ARC and Tac2 neurons.