Coordinated seasonal regulation of metabolic and reproductive hypothalamic peptides in the desert jerboa

Melatonin and Seasonal Synchrony in Mammals

In mammals, seasonal opportunities and challenges are anticipated through programmed changes in physiology and behavior. Appropriate anticipatory timing depends on synchronization to the external solar year, achieved through the use of day length (photoperiod) as a synchronizing signal. In mammals, nocturnal production of melatonin by the pineal gland is the key hormonal mediator of photoperiodic change, exerting its effects via the hypothalamopituitary axis. In this review/perspective, we consider the key developments during the history of research into the seasonal synchronizer effect of melatonin, highlighting the role that the pars tuberalis-tanycyte module plays in this process. We go on to consider downstream pathways, which include discrete hypothalamic neuronal populations. Neurons that express the neuropeptides kisspeptin and (Arg)(Phe)-related peptide-3 (RFRP-3) govern seasonal reproductive function while neurons that express somatostatin may be involved in seasonal metabolic adaptations. Finally, we identify several outstanding questions, which need to be addressed to provide a much thorough understanding of the deep impact of melatonin upon seasonal synchronization.

GnRH and the photoperiodic control of seasonal reproduction: Delegating the task to kisspeptin and RFRP-3

Synchronization of mammalian breeding activity to the annual change of photoperiod and environmental conditions is of the utmost importance for individual survival and species perpetuation. Subsequent to the early 1960s, when the central role of melatonin in this adaptive process was demonstrated, our comprehension of the mechanisms through which light regulates gonadal activity has increased considerably. The current model for the photoperiodic neuroendocrine system points to pivotal roles for the melatonin-sensitive pars tuberalis (PT) and its seasonally-regulated production of thyroid-stimulating hormone (TSH), as well as for TSH-sensitive hypothalamic tanycytes, radial glia-like cells located in the basal part of the third ventricle. Tanycytes respond to TSH through increased expression of thyroid hormone (TH) deiodinase 2 (Dio2), which leads to heightened production of intrahypothalamic triiodothyronine (T3) during longer days of spring and summer. There is strong evidence that this local, long-day driven, increase in T3 links melatonin input at the PT to gonadotropin-releasing hormone (GnRH) output, to align breeding with the seasons. The mechanism(s) through which T3 impinges upon GnRH remain(s) unclear. However, two distinct neuronal populations of the medio-basal hypothalamus, which express the (Arg)(Phe)-amide peptides kisspeptin and RFamide-related peptide-3, appear to be well-positioned to relay this seasonal T3 message towards GnRH neurons. Here, we summarize our current understanding of the cellular, molecular and neuroendocrine players, which keep track of photoperiod and ultimately govern GnRH output and seasonal breeding.

RFamide peptides, the novel regulators of mammalian HPG axis: A review

The RFamide-related peptides (RFRPs) are the group of neuropeptides synthesized predominantly from the hypothalamus that negatively affects the hypothalamo-hypophyseal-gonadal (hypothalamic–pituitary–gonadal [HPG]) axis. These peptides are first identified in quail brains and emerged as the mammalian orthologs of avian gonadotropin inhibitory hormones. The RFRP-3 neurons in the hypothalamus are present in several mammalian species. The action of RFRP-3 is mediated through a G-protein-coupled receptor called OT7T022. The predominant role of RFRP-3 is the inhibition of HPG axis with several other effects such as the regulation of metabolic activity, stress regulation, controlling of non-sexual motivated behavior, and sexual photoperiodicity in concert with other neuropeptides such as kisspeptin, neuropeptide-Y (NPY), pro-opiomelanocortin, orexin, and melanin. RFamide peptides synthesized in the granulosa cells, interstitial cells, and seminiferous tubule regulate steroidogenesis and gametogenesis in the gonads. The present review is intended to provide the recent findings that explore the role of RFRP-3 in regulating HPG axis and its potential applications in the synchronization of reproduction and its therapeutic interventions to prevent stress-induced amenorrhea.

An Inhibitory Circuit From Brainstem to GnRH Neurons in Male Mice: A New Role for the RFRP Receptor

RFamide-related peptides (RFRPs, mammalian orthologs of gonadotropin-inhibitory hormone) convey circadian, seasonal, and social cues to the reproductive system. They regulate gonadotropin secretion by modulating gonadotropin-releasing hormone (GnRH) neurons via the RFRP receptor. Mice lacking this receptor are fertile but exhibit abnormal gonadotropin responses during metabolic challenges, such as acute fasting, when the normal drop in gonadotropin levels is delayed. Although it is known that these food intake signals to the reproductive circuit originate in the nucleus tractus solitarius (NTS) in the brainstem, the phenotype of the neurons conveying the signal remains unknown. Given that neuropeptide FF (NPFF), another RFamide peptide, resides in the NTS and can bind to the RFRP receptor, we hypothesized that NPFF may regulate GnRH neurons. To address this question, we used a combination of techniques: cell-attached electrophysiology on GnRH-driven green fluorescent protein-tagged neurons in acute brain slices; calcium imaging on cultured GnRH neurons; and immunostaining on adult brain tissue. We found (1) NPFF inhibits GnRH neuron excitability via the RFRP receptor and its canonical signaling pathway (Gi/o protein and G protein-coupled inwardly rectifying potassium channels), (2) NPFF-like fibers in the vicinity of GnRH neurons coexpress neuropeptide Y, (3) the majority of NPFF-like cell bodies in the NTS also coexpress neuropeptide Y, and (4) acute fasting increased NPFF-like immunoreactivity in the NTS. Together these data indicate that NPFF neurons within the NTS inhibit GnRH neurons, and thus reproduction, during fasting but prior to the energy deficit.

Neuropeptidergic and Neuroendocrine Systems Underlying Eusociality and the Concomitant Social Regulation of Reproduction in Naked Mole-Rats: A Comparative Approach

The African mole-rat family (Bathyergidae) includes the first mammalian species identified as eusocial: naked mole-rats. Comparative studies of eusocial and solitary mole-rat species have identified differences in neuropeptidergic systems that may underlie the phenomenon of eusociality. These differences are found in the oxytocin, vasopressin and corticotrophin-releasing factor (CRF) systems within the nucleus accumbens, amygdala, bed nucleus of the stria terminalis and lateral septal nucleus. As a corollary of their eusociality, most naked mole-rats remain pre-pubertal throughout life because of the presence of the colony's only reproductive female, the queen. To elucidate the neuroendocrine mechanisms that mediate this social regulation of reproduction, research on the hypothalamo-pituitary-gonadal axis in naked mole-rats has identified differences between the many individuals that are reproductively suppressed and the few that are reproductively mature: the queen and her male consorts. These differences involve gonadal steroids, gonadotrophin-releasing hormone-1 (GnRH-1), kisspeptin, gonadotrophin-inhibitory hormone/RFamide-related peptide-3 (GnIH/RFRP-3) and prolactin. The comparative findings in eusocial and solitary mole-rat species are assessed with reference to a broad range of studies on other mammals.

Pleiotropic effects of proopiomelanocortin and VGF nerve growth factor inducible neuropeptides for the long-term regulation of energy balance

Seasonal rhythms in energy balance are well documented across temperate and equatorial zones animals. The long-term regulated changes in seasonal physiology consists of a rheostatic system that is essential to successful time annual cycles in reproduction, hibernation, torpor, and migration. Most animals use the annual change in photoperiod as a reliable and robust environmental cue to entrain endogenous (i.e. circannual) rhythms. Research over the past few decades has predominantly examined the role of first order neuroendocrine peptides for the rheostatic changes in energy balance. These anorexigenic and orexigenic neuropeptides in the arcuate nucleus include neuropeptide y (Npy), agouti-related peptide (Agrp), cocaine and amphetamine related transcript (Cart) and pro-opiomelanocortin (Pomc). Recent studies also indicate that VGF nerve growth factor inducible (Vgf) in the arcuate nucleus is involved in the seasonal regulation of energy balance. In situ hybridization, qPCR and RNA-sequencing studies have identified that Pomc expression across fish, avian and mammalian species, is a neuroendocrine marker that reflects seasonal energetic states. Here we highlight that long-term changes in arcuate Pomc and Vgf expression is conserved across species and may provide rheostatic regulation of seasonal energy balance.

RFRP3 increases food intake in a sex-dependent manner in the seasonal hamster Phodopus sungorus

In addition to its regulatory role in luteinising hormone secretion, Rfamide-related peptide 3 (RFRP3) has also been reported to modulate food intake in several mammalian species. Djungarian hamsters (Phodopus sungorus), similar to other seasonal mammals, display a remarkable inhibition of RFRP3 expression in winter short-day conditions, associated with decreased food intake and bodyweight. This species is therefore a valuable model for assessing whether RFRP3 might be involved in the seasonal control of feeding behaviour and investigating its possible brain targets. We found that, although both male and female animals exhibit the same robust reduction in Rfrp expression in short- (SD) compared to long-day (LD) conditions, acute central administration of RFRP3 displays sex-dependent effects on food intake. RFRP3 increased food intake in female hamsters in SD or in LD dioestrus, but not in LD pro-oestrus, indicating that the orexigenic effect of RFRP3 is observed in conditions of low circulating oestradiol levels. In male hamsters, food intake was not changed by acute injections of RFRP3, regardless of whether animals were in SD or LD conditions. Analysing the gene expression of various metabolic neuropeptides in the brain of RFRP3-injected Djungarian hamsters revealed that Npy expression was increased in female but not in male animals. The present study suggests that, in Djungarian hamsters, RFRP3 exhibits a sex-dependent orexigenic effect possibly by inducing increased Npy expression.

An integrative view of mammalian seasonal neuroendocrinology

Seasonal neuroendocrine cycles that govern annual changes in reproductive activity, energy metabolism and hair growth are almost ubiquitous in mammals that have evolved at temperate and polar latitudes. Changes in nocturnal melatonin secretion regulating gene expression in the pars tuberalis (PT) of the pituitary stalk are a critical common feature in seasonal mammals. The PT sends signal(s) to the pars distalis of the pituitary to regulate prolactin secretion and thus the annual moult cycle. The PT also signals in a retrograde manner via thyroid-stimulating hormone to tanycytes, which line the ventral wall of the third ventricle in the hypothalamus. Tanycytes show seasonal plasticity in gene expression and play a pivotal role in regulating local thyroid hormone (TH) availability. Within the mediobasal hypothalamus, the cellular and molecular targets of TH remain elusive. However, two populations of hypothalamic neurones, which produce the RF-amide neuropeptides kisspeptin and RFRP3 (RF-amide related peptide 3), are plausible relays between TH and the gonadotrophin-releasing hormone-pituitary-gonadal axis. By contrast, the ways by which TH also impinges on hypothalamic systems regulating energy intake and expenditure remain unknown. Here, we review the neuroendocrine underpinnings of seasonality and identify several areas that warrant further research.

Kisspeptin and RFRP3 modulate body mass in Phodopus sungorus via two different neuroendocrine pathways

Many animals exhibit remarkable metabolic and reproductive adaptations to seasonal changes in their environment. When day length shortens, Djungarian hamsters (Phodopus sungorus) reduce their body weight and inhibit their reproductive activity, whereas the opposite occurs in springtime. These physiological adaptations are considered to depend on photoperiodic changes in hypothalamic genes encoding the peptides kisspeptin (Kp) and RFamide-related peptide 3 (RFRP3) for the control of reproduction, as well as pro-opiomelanocortin and somatostatin for metabolic regulation. The present study investigates the effect of Kp and RFRP3 on long-term body weight regulation, aiming to establish whether metabolic and reproductive hypothalamic networks may interact during adaptation to seasonal physiology. We found that chronic central administration of both Kp and RFRP3 in short photoperiod-adapted male Djungarian hamsters increased body weight, although via different pathways. The effect of Kp was dependent on testicular activity because castration prevented the body weight increase and was associated with an increase in pro-opiomelanocortin and neuropeptide Y expression. On the other hand, the orexigenic effect of RFRP3 was associated with an increase in circulating insulin and leptin levels, although it had no effect on any of the hypothalamic metabolic genes investigated, and did not change circulating levels of sex steroids. Notably, neither Kp, nor RFRP3 altered female hamster metabolic parameters. Thus, using a rodent model exhibiting seasonal changes in reproduction and metabolism, the present study demonstrates that, in addition to its role in the central control of reproduction, Kp also participates in body weight control in a sex-dependent manner via an anabolic action of testosterone. Conversely, RFRP3 affects body weight control in males mostly by acting on adiposity, with no overt effect on the reproductive system in both sexes.

Functional Implications of RFRP-3 in the Central Control of Daily and Seasonal Rhythms in Reproduction

Adaptation of reproductive activity to environmental changes is essential for breeding success and offspring survival. In mammals, the reproductive system displays regular cycles of activation and inactivation which are synchronized with seasonal and/or daily rhythms in environmental factors, notably light intensity and duration. Thus, most species adapt their breeding activity along the year to ensure that birth and weaning of the offspring occur at a time when resources are optimal. Additionally, female reproductive activity is highest at the beginning of the active phase during the period of full oocyte maturation, in order to improve breeding success. In reproductive physiology, it is therefore fundamental to delineate how geophysical signals are integrated in the hypothalamo-pituitary-gonadal axis, notably by the neurons expressing gonadotropin releasing hormone (GnRH). Several neurochemicals have been reported to regulate GnRH neuronal activity, but recently two hypothalamic neuropeptides belonging to the superfamily of (Arg)(Phe)-amide peptides, RFRP-3 and kisspeptin, have emerged as critical for the integration of environmental cues within the reproductive axis. The goal of this review is to survey the current understanding of the role played by RFRP-3 in the temporal regulation of reproduction, and consider how its effect might combine with that of kisspeptin to improve the synchronization of reproduction to environmental challenges.

The roles of RFamide-related peptides (RFRPs), mammalian gonadotropin-inhibitory hormone (GnIH) orthologues in female reproduction

OBJECTIVES

To benefit from reproduction and deal with challenges in the environmental conditions, animals must adapt internal physiology to maximize the reproduction rate. Maladaptive variations in the neurochemical systems and reproductive system can lead to manifestation of several significant mammalian reprocesses, including mammalian ovarian lifespan. RFamide-related peptide (RFRP, Rfrp), mammalian orthologues of gonadotropin-inhibitory hormone (GnIH), which is a regulator to prevent the gonadotropin-releasing hormone (GnRH) neural activity, is known to be related to reproductive traits. This review aimed to summarize recent five-year observations to outline historic insights and novel perspectives into the functions of RFRPs in coding the mammalian reproductive physiology, especially highlight recent advances in the impact on RFRPs in regulating mammalian ovary lifespan.

MATERIALS AND METHODS

We reviewed the recent five-year important findings of RFRP system involved in mammalian ovary development. Data for this review were collected from Google Scholar and PubMed using the RFRP keyword combined with the keywords related to physiological or pathological reproductive functions.

RESULTS

Recent discoveries are focused on three major fronts in research on RFRP role in female reproduction including reproductive functions, energy balance, and stress regulation. The roles of RFRPs in various development phases of mammal reproduction including prepuberty, puberty, estrous cycle, pregnancy, milking, menopause, and/or ovarian diseases have been shown.

CONCLUSION

Overall, these recent advances demonstrate that RFRPs serve as critical mediators in mammalian ovarian development.

A suprachiasmatic-independent circadian clock(s) in the habenula is affected by Per gene mutations and housing light conditions in mice

For many years, the suprachiasmatic nucleus (SCN) was considered as the unique circadian pacemaker in the mammalian brain. Currently, it is known that other brain areas are able to oscillate in a circadian manner. However, many of them are dependent on, or synchronized by, the SCN. The Habenula (Hb), localized in the epithalamus, is a key nucleus for the regulation of monoamine activity (dopamine, serotonin) and presents circadian features; nonetheless, the clock properties of the Hb are not fully described. Here, we report, first, circadian expression of clock genes in the lateral habenula (LHb) under constant darkness (DD) condition in wild-type mice which is disturbed in double Per1^-/--Per2^Brdm1 clock-mutant mice. Second, using Per2::luciferase transgenic mice, we observed a self-sustained oscillatory ability (PER2::LUCIFERASE bioluminescence rhythmicity) in the rostral and caudal part of the Hb of arrhythmic SCN-ablated animals. Finally, in Per2::luciferase mice exposed to different lighting conditions (light-dark, constant darkness or constant light), the period or amplitude of PER2 oscillations, in both the rostral and caudal Hb, were similar. However, under DD condition or from SCN-lesioned mice, these two Hb regions were out of phase, suggesting an uncoupling of two putative Hb oscillators. Altogether, these results suggest that an autonomous clock in the rostral and caudal part of the Hb requires integrity of circadian genes to tick, and light information or SCN innervation to keep synchrony. The relevance of the Hb timing might reside in the regulation of circadian functions linked to motivational (reward) and emotional (mood) processes.

Analysis of gonadotrophin-releasing hormone-1 and kisspeptin neuronal systems in the nonphotoregulated seasonally breeding eastern rock elephant-shrew (Elephantulus myurus)

Of the 18 sub-Saharan elephant-shrew species, only eastern rock elephant-shrews reproduce seasonally throughout their distribution, a process seemingly independent of photoperiod. The present study characterizes gonadal status and location/intensity of gonadotrophin-releasing hormone-1 (GnRH-1) and kisspeptin immunoreactivities in this polyovulating species in the breeding and nonbreeding seasons. GnRH-1-immunoreactive (ir) cell bodies are predominantly in the medial septum, diagonal band, and medial preoptic area; processes are generally sparse except in the external median eminence. Kisspeptin-ir cell bodies are detected only within the arcuate nucleus; the density of processes is generally low, except in the septohypothalamic nucleus, ventromedial bed nucleus of the stria terminalis, arcuate nucleus, and internal and external median eminence. Kisspeptin-ir processes are negligible at locations containing GnRH-1-ir cell bodies. The external median eminence is the only site with conspicuously overlapping distributions of the respective immunoreactivities and, accordingly, a putative site for kisspeptin's regulation of GnRH-1 release in this species. In the nonbreeding season in males, there is an increase in the rostral population of GnRH-1-ir cell bodies and density of GnRH-1-ir processes in the median eminence. In both sexes, the breeding season is associated with increased kisspeptin-ir process density in the rostral periventricular area of the third ventricle and arcuate nucleus; at the latter site, this is positively correlated with gonadal mass. Cross-species comparisons lead us to hypothesize differential mechanisms within these peptidergic systems: that increased GnRH-1 immunoreactivity during the nonbreeding season reflects increased accumulation with reduced release; that increased kisspeptin immunoreactivity during the breeding season reflects increased synthesis with increased release.

Identification, Quantitation, and Imaging of the Crustacean Peptidome

Crustaceans serve as a useful, simplified model for studying peptides and neuromodulation, as they contain numerous neuropeptide homologs to mammals and enable electrophysiological studies at the single-cell and neural circuit levels. In particular, crustaceans contain well-defined neural networks, including the stomatogastric ganglion, esophageal ganglion, commissural ganglia, and several neuropeptide-rich organs, such as the brain, pericardial organs, and sinus glands. Due to the lack of a genomic database for crustacean peptides, an important step of crustacean peptidomics involves the discovery and identification of novel peptides and the construction of a database, more recently with the aid of mass spectrometry (MS). Herein, we present a general workflow and detailed methods for MS-based peptidomic analysis of crustacean tissue samples and circulating fluids. In conjunction with profiling, quantitation can also be performed with isotopic or isobaric labeling. Information regarding the localization patterns and changes of peptides can be studied via mass spectrometry imaging. Combining these sample preparation strategies and MS analytical techniques allows for a multifaceted approach to obtaining deep knowledge of crustacean peptidergic signaling pathways.

Differential response patterns of kisspeptin and RFamide-related peptide to photoperiod and sex steroid feedback in the Djungarian hamster (Phodopus sungorus)

Many animals synchronise their reproductive activity with the seasons to optimise the survival of their offspring. This synchronisation involves switching on and off their gonadotrophic axis. Ever since their discovery as key regulators of gonadotrophin-releasing hormone (GnRH) neurones, the hypothalamic RF-amide peptides kisspeptin and RFamide-related peptide (RFRP) have been a major focus of research on the seasonal regulation of the gonadotrophic axis. In the present study, we investigated the regulation of both neuropeptides in the Djungarian hamster, a major animal model for the study of seasonal reproduction. During the long-day breeding period, kisspeptin neurones in the anteroventral periventricular area are solely controlled by a positive sex steroid feedback and, in the arcuate nucleus, they are subject to a very strong negative sex steroid feedback associated with a minor photoperiodic effect. During short-day sexual quiescence, the disappearance of this hormonal feedback leads to high levels of kisspeptin in arcuate neurones. Notably, chronic central administration of kisspeptin is able to over-ride the photoperiodic inhibition of the gonadotrophic axis and reactivate the reproductive function. Therefore, our data suggest that kisspeptin secretion by arcuate neurones during sexual quiescence is inhibited by mechanisms upstream of kisspeptin neurones. RFRP expression is solely controlled by photoperiod, being strongly reduced in short days independently of the sex steroid feedback. Thus, kisspeptin and RFRP display contrasting patterns of expression and regulation. Upstream mechanisms controlling these neurones should be the focus of further studies on the roles of these RFamide neuropeptides in the seasonal control of reproduction.

Kisspeptin and RFRP-3 differentially regulate food intake and metabolic neuropeptides in the female desert jerboa

Jerboas are wild rodents exhibiting exceptional adaptation to their desert environment. Under harsh autumn conditions, they shut down reproduction, increase body weight and hibernate, while during spring they become sexually active even under negative energy-balance. We recently reported that these rhythms are associated with synchronized changes in genes expressing reproductive (Kiss1, Rfrp) and metabolic (Npy and Pomc) peptides, raising the hypothesis of coordinated seasonal regulation of both functions. Here we analyzed whether kisspeptin and RFRP-3 regulate food-intake in parallel to their established reproductive functions. Intracerebroventricular administration of kisspeptin inhibited food intake by 1.5-fold in fasted, but not ad-libitum fed, female jerboas captured in spring, an effect associated with an increase in Pomc and decrease in Rfrp mRNA levels. By contrast, intracerebroventricular injection of RFRP-3 induced a 4-fold increase in food-intake in ad-libitum female jerboas, together with a decrease in Pomc and increase in Npy mRNA levels. This orexigenic effect of RFRP-3 was observed in both spring and autumn, whereas kisspeptin's anorexigenic effect was only observed in spring. Altogether, this study reports opposite metabolic effects of kisspeptin and RFRP-3 in the female jerboa and strengthens our hypothesis of a coordinated, season-dependent, regulation of reproductive activity and food intake through interactions of these hypothalamic peptides.