Effect of RF-amide-related peptide-3 on luteinizing hormone and follicle-stimulating hormone synthesis and secretion in ovine pituitary gonadotropes

Neural Versus Gonadal GnIH: Are they Independent Systems? A Mini-Review

Based on research in protochordates and basal vertebrates, we know that communication across the first endocrine axes likely relied on diffusion. Because diffusion is relatively slow, rapid responses to some cues, including stress-related cues, may have required further local control of axis outputs (e.g., steroid hormone production by the gonads). Despite the evolution of much more efficient circulatory systems and complex nervous systems in vertebrates, production of many "neuro"transmitters has been identified outside of the hypothalamus across the vertebrate phylogeny and these neurotransmitters are known to locally regulate endocrine function. Our understanding of tissue-specific neuropeptide expression and their role coordinating physiological/behavioral responses of the whole organism remains limited, in part, due to nomenclature and historic dogma that ignores local regulation of axis output. Here, we review regulation of gonadotropin-inhibitory hormone (GnIH) across the reproductive axis in birds and mammals to bring further attention to context-dependent disparities and similarities in neuropeptide production by the brain and gonads. We find that GnIH responsiveness to cues of stress appears conserved across species, but that the response of specific tissues and the direction of GnIH regulation varies. The implications of differential regulation across tissues remain unclear in most studies, but further work that manipulates and contrasts function in different tissues has the potential to inform us about both organism-specific function and endocrine axis evolution.

The gonadotropin-inhibitory hormone (Lpxrfa) system's regulation of reproduction in the brain-pituitary axis of the zebrafish (Danio rerio)

Gonadotropin-inhibitory hormone (GNIH) was discovered in quail with the ability to reduce gonadotropin expression/secretion in the pituitary. There have been few studies on GNIH orthologs in teleosts (LPXRFamide (Lpxrfa) peptides), which have provided inconsistent results. Therefore, the goal of this study was to determine the roles and modes of action by which Lpxrfa exerts its functions in the brain-pituitary axis of zebrafish (Danio rerio). We localized Lpxrfa soma to the ventral hypothalamus, with fibers extending throughout the brain and to the pituitary. In the preoptic area, Lpxrfa fibers interact with gonadotropin-releasing hormone 3 (Gnrh3) soma. In pituitary explants, zebrafish peptide Lpxrfa-3 downregulated luteinizing hormone beta subunit and common alpha subunit expression. In addition, Lpxrfa-3 reduced gnrh3 expression in brain slices, offering another pathway for Lpxrfa to exert its effects on reproduction. Receptor activation studies, in a heterologous cell-based system, revealed that all three zebrafish Lpxrfa peptides activate Lpxrf-R2 and Lpxrf-R3 via the PKA/cAMP pathway. Receptor activation studies demonstrated that, in addition to activating Lpxrf receptors, zebrafish Lpxrfa-2 and Lpxrfa-3 antagonize Kisspeptin-2 (Kiss2) activation of Kisspeptin receptor-1a (Kiss1ra). The fact that kiss1ra-expressing neurons in the preoptic area are innervated by Lpxrfa-ir fibers suggests an additional pathway for Lpxrfa action. Therefore, our results suggest that Lpxrfa may act as a reproductive inhibitory neuropeptide in the zebrafish that interacts with Gnrh3 neurons in the brain and with gonadotropes in the pituitary, while also potentially utilizing the Kiss2/Kiss1ra pathway.

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 roles of kisspeptin and gonadotropin inhibitory hormone in stress-induced reproductive disorders

Several kinds of stress suppress the hypothalamic-pituitary-gonadal (HPG) axis and reproductive behavior in humans and animals. These changes can eventually cause diseases and disorders, such as amenorrhea and infertility. In previous studies, it has been shown that stress-related factors, e.g., corticotropin-releasing hormone, cortisol, and pro-inflammatory cytokines, promote the stress-induced suppression of the HPG axis. However, these mechanisms are not sufficient to explain how stress suppresses HPG axis activity, and it has been suggested that some other factors might also be involved. In the early 21st century, novel neuroendocrine peptides, kisspeptin and gonadotropin inhibitory hormone (GnIH)/RFamide-related peptide 3 (RFRP-3), which directly regulate GnRH/gonadotropin synthesis and secretion, were newly discovered. Growing evidence indicates that kisspeptin and GnIH/RFRP-3 play pivotal roles in the stress-induced disruption of the HPG axis and reproductive behavior in addition to their physiological functions. This review summarizes what is currently known about the roles of kisspeptin and GnIH/RFRP-3 in stress-induced reproductive disorders.

Discovery of GnIH and Its Role in Hypothyroidism-Induced Delayed Puberty

It is known that hypothyroidism delays puberty in mammals. Interaction between the hypothalamo-pituitary-thyroid (HPT) and hypothalamo-pituitary-gonadal (HPG) axes may be important processes in delayed puberty. Gonadotropin-inhibitory hormone (GnIH) is a newly discovered hypothalamic neuropeptide that inhibits gonadotropin synthesis and release in quail. It now appears that GnIH is conserved across various mammals and primates, including humans, and inhibits reproduction. We have further demonstrated that GnIH is involved in pubertal delay induced by thyroid dysfunction in female mice. Hypothyroidism delays pubertal onset with the increase in hypothalamic GnIH expression and the decrease in circulating gonadotropin and estradiol levels. Thyroid status regulates GnIH expression by epigenetic modification of the GnIH promoter region. Furthermore, knockout of GnIH gene abolishes the effect of hypothyroidism on delayed pubertal onset. Accordingly, it is considered that GnIH is a mediator of pubertal disorder induced by thyroid dysfunction. This is a novel function of GnIH that interacts between the HPT-HPG axes in pubertal onset delay. This mini-review summarizes the structure, expression, and function of GnIH and highlights the action of GnIH in pubertal disorder induced by thyroid dysfunction.

The influence of dopaminergic system inhibition on biosynthesis of gonadotrophin-releasing hormone (GnRH) and GnRH receptor in anoestrous sheep; hierarchical role of kisspeptin and RFamide-related peptide-3 (RFRP-3)

This study aimed to explain how prolonged inhibition of central dopaminergic activity affects the cellular processes governing gonadotrophin-releasing hormone (GnRH) and LH secretion in anoestrous sheep. For this purpose, the study included two experimental approaches: first, we investigated the effect of infusion of sulpiride, a dopaminergic D2 receptor antagonist (D2R), on GnRH and GnRH receptor (GnRHR) biosynthesis in the hypothalamus and on GnRHR in the anterior pituitary using an immunoassay. This analysis was supplemented by analysis of plasma LH levels by radioimmunoassay. Second, we used real-time polymerase chain reaction to analyse the influence of sulpiride on the levels of kisspeptin (Kiss1) mRNA in the preoptic area and ventromedial hypothalamus including arcuate nucleus (VMH/ARC), and RFamide-related peptide-3 (RFRP-3) mRNA in the paraventricular nucleus (PVN) and dorsomedial hypothalamic nucleus. Sulpiride significantly increased plasma LH concentration and the levels of GnRH and GnRHR in the hypothalamic–pituitary unit. The abolition of dopaminergic activity resulted in a significant increase in transcript level of Kiss1 in VMH/ARC and a decrease of RFRP-3 in PVN. The study demonstrates that dopaminergic neurotransmission through D2R is involved in the regulatory pathways of GnRH and GnRHR biosynthesis in the hypothalamic–pituitary unit of anoestrous sheep, conceivably via mechanisms in which Kiss1 and RFRP-3 participate.

Astrocyte-Specific Deletion of Peroxisome-Proliferator Activated Receptor-γ Impairs Glucose Metabolism and Estrous Cycling in Female Mice

Mice lacking peroxisome-proliferator activated receptor-γ (PPARγ) in neurons do not become leptin resistant when placed on a high-fat diet (HFD). In male mice, this results in decreased food intake and increased energy expenditure, causing reduced body weight, but this difference in body weight is not observed in female mice. In addition, estrous cycles are disturbed and the ovaries present with hemorrhagic follicles. We observed that PPARγ was more highly expressed in astrocytes than neurons, so we created an inducible, conditional knockout of PPARγ in astrocytes (AKO). The AKO mice had impaired glucose tolerance and hepatic steatosis that did not worsen with HFD. Expression of gluconeogenic genes was elevated in the mouse livers, as was expression of several genes involved in lipogenesis, lipid transport, and storage. The AKO mice also had a reproductive phenotype with fewer estrous cycles, elevated plasma testosterone levels, reduced corpora lutea formation, and alterations in hypothalamic and ovarian gene expression. Thus, the phenotypes of the AKO mice were very different from those seen in the neuronal knockout mice, suggesting distinct roles for PPARγ in these two cell types.

Central RFRP-3 Stimulates LH Secretion in Male Mice and Has Cycle Stage-Dependent Inhibitory Effects in Females

RFamide-related peptide (RFRP)-3 is a neuropeptide thought to play an inhibitory role in the regulation of reproduction in various mammalian species, although some stimulatory effects have been reported. To date, the effects of RFRP-3 on gonadotropin secretion have been scarcely studied in mice. The aim of the current study was to characterize the effect of RFRP-3 administration on gonadotropin secretion in male and female mice. Adult intact and castrated male mice received acute central injections of 0.5 to 5 nmol of RFRP-3, and luteinizing hormone (LH) concentration was assayed in tail-tip blood samples. RFRP-3 had a dose-dependent stimulatory effect on LH secretion when administered centrally to both intact and castrated mice, and this effect was diminished when RFRP-3 was administered to kisspeptin receptor knockout mice. In female mice, central RFRP-3 had an inhibitory effect on LH secretion when administered at the time of the preovulatory LH surge in intact mice, or of an estradiol-induced LH surge in ovariectomized mice. Conversely, central RFRP-3 administration had no effect on LH levels in intact diestrus or ovariectomized, low-dose estradiol-implanted mice. Finally, peripheral administration of RFRP-3 to intact males and to females at the time of the preovulatory LH surge or during diestrus had no effect on LH secretion. Taken together, these results provide a detailed description of sex- and cycle stage-dependent effects of RFRP-3 on gonadotrophin secretion in mice. Moreover, it appears that the stimulatory effects are mediated in part by the receptor for kisspeptin, a potent stimulator of the reproductive axis.

Variants in the 3' untranslated region of the ovine acetyl-coenzyme A acyltransferase 2 gene are associated with dairy traits and exhibit differential allelic expression

The acetyl-CoA acyltransferase 2 (ACAA2) gene encodes an enzyme of the thiolase family that is involved in mitochondrial fatty acid elongation and degradation by catalyzing the last step of the respective β-oxidation pathway. The increased energy needs for gluconeogenesis and triglyceride synthesis during lactation are met primarily by increased fatty acid oxidation. Therefore, the ACAA2 enzyme plays an important role in the supply of energy and carbon substrates for lactation and may thus affect milk production traits. This study investigated the association of the ACAA2 gene with important sheep traits and the putative functional involvement of this gene in dairy traits. A single nucleotide substitution, a T to C transition located in the 3' untranslated region of the ACAA2 gene, was used in mixed model association analysis with milk yield, milk protein yield and percentage, milk fat yield and percentage, and litter size at birth. The single nucleotide polymorphism was significantly associated with total lactation production and milk protein percentage, with respective additive effects of 6.81 ± 2.95 kg and -0.05 ± 0.02%. Additionally, a significant dominance effect of 0.46 ± 0.21 kg was detected for milk fat yield. Homozygous TT and heterozygous CT animals exhibited higher milk yield compared with homozygous CC animals, whereas the latter exhibited increased milk protein percentage. Expression analysis from age-, lactation-, and parity-matched female sheep showed that mRNA expression of the ACAA2 gene from TT animals was 2.8- and 11.8-fold higher in liver and mammary gland, respectively. In addition, by developing an allelic expression imbalance assay, it was estimated that the T allele was expressed at an average of 18% more compared with the C allele in the udder of randomly selected ewes. We demonstrated for the first time that the variants in the 3' untranslated region of the ovine ACAA2 gene are differentially expressed in homozygous ewes of each allele and exhibit allelic expression imbalance within heterozygotes in a tissue-specific manner, supporting the existence of cis-regulatory DNA variation in the ovine ACAA2 gene. This is the first study reporting differential allelic imbalance expression of a candidate gene associated with milk production traits in dairy sheep.

Intracerebroventricular injection of RFRP-3 delays puberty onset and stimulates growth hormone secretion in female rats

BACKGROUND

Puberty onset is a complex, organized biological process with multilevel regulation, and its physiopathological mechanisms are yet to be elucidated. RFRP-3, the mammalian ortholog to gonadotropin-inhibitory hormone, is implicated in inhibiting the synthesis and release of gonadotropin in mammals. However, it is unclear whether RFRP-3 participates in regulating pubertal development.

METHODS

This study investigated the functional significance and regulatory mechanism of hypothalamic RFRP-3 neuropeptide in the onset of puberty in young female rats. On postnatal day 22, we implanted cannulas into the lateral ventricles of female rat pups. From postnatal day 28 to postnatal day 36, the intracerebroventricular injection of RFRP-3, or vehicle, was conducted twice a day. To investigate whether puberty onset was affected, we examined the body weight, age of vaginal opening, serum hormone levels, uterus and ovary development, and hypothalamic Kiss-1 mRNA expression.

RESULTS

Intracerebroventricular injection of RFRP-3 significantly decreased the serum concentrations of luteinizing hormone and estradiol, delayed uterine maturation, and postponed the time of vaginal opening. This study suggests that RFRP-3 can delay the onset of puberty in young female rats; the expression of Kiss-1 mRNA is potently inhibited in the RFRP-3 group. Moreover, our data show that RFRP-3 elevates serum growth hormone levels.

CONCLUSIONS

These data suggest that intracerebroventricular injection of RFRP-3 significantly delays the onset of puberty in female rats. Additionally, RFRP-3 may be associated with prepubertal rise in the secretion of growth hormone.

Effect of gonadotropin-inhibitory hormone on luteinizing hormone secretion in humans

BACKGROUND

Gonadotropin-inhibitory hormone (GnIH, human homologue of RFRP-3) suppresses gonadotropin secretion in animal models, but its effects have not been studied in the human.

OBJECTIVE

We tested the hypotheses that exogenous GnIH inhibits LH secretion (i) in postmenopausal women and (ii) in men concurrently administered exogenous kisspeptin.

DESIGN

Following in vitro and in vivo preclinical studies to functionally characterize the GnIH peptide, a dose-finding study (human GnIH: 1·5-150 μg/kg/h, iv for 3 h) was undertaken, and 50 μg/kg/h selected for further evaluation. Five postmenopausal women were administered 50 μg/kg/h iv infusion for 3 h or vehicle on two separate days. Four men were administered kisspeptin-10 (0·3 μg/kg iv bolus) with simultaneous infusion of GnIH (50 μg/kg/h, iv for 3 h) or vehicle.

PARTICIPANTS

Healthy postmenopausal women (mean age 58 ± 2 years, LH: 30·8 ± 2·9 IU/l, FSH: 78·7 ± 6·4 IU/l, oestradiol: <50 pmol/l) and men (39·8 ± 2·1 years, mean total testosterone 12·1 ± 1·8 nmol/l, LH 2·2 ± 0·2 IU/l).

PRIMARY OUTCOME

Change in area under curve (AUC) of LH during GnIHvs vehicle.

RESULTS

During GnIH administration in postmenopausal women, LH secretion decreased (ΔAUC: -9·9 ± 1·8 IU/3 h) vs vehicle (ΔAUC: -0·5 ± 1·7 IU/3 h; P = 0·02). Kisspeptin-10-stimulated LH responses in men were not affected by GnIH co-administration (60-min AUC of LH 6·2 ± 0·8 IU/h with kisspeptin-10 alone, 6·3 ± 1·0 IU/h, kisspeptin-10 with GnIH, P = 0·72). Exogenous GnIH was well tolerated, with no adverse events reported.

CONCLUSIONS

Gonadotropin-inhibitory hormone decreased LH secretion in postmenopausal women in this first-in-human study. Kisspeptin-stimulated LH secretion in men was not inhibited during concomitant administration of GnIH.

RFamide-related peptide-3 (RFRP-3) suppresses sexual maturation in a eusocial mammal

Neuroendocrine mechanisms underlying social inhibition of puberty are not well understood. Here, we use a model exhibiting the most profound case of pubertal suppression among mammals to explore a role for RFamide-related peptide-3 [RFRP-3; mammalian ortholog to gonadotropin-inhibitory hormone (GnIH)] in neuroendocrine control of reproductive development. Naked mole rats (NMRs) live in sizable colonies where breeding is monopolized by two to four dominant animals, and no other members exhibit signs of puberty throughout their lives unless they are removed from the colony. Because of its inhibitory action on the reproductive axis in other vertebrates, we investigated the role of RFRP-3 in social reproductive suppression in NMRs. We report that RFRP-3 immunofluorescence expression patterns and RFRP-3/GnRH cross-talk are largely conserved in the NMR brain, with the exception of the unique presence of RFRP-3 cell bodies in the arcuate nucleus (Arc). Immunofluorescence comparisons revealed that central expression of RFRP-3 is altered by reproductive status, with RFRP-3 immunoreactivity enhanced in the paraventricular nucleus, dorsomedial nucleus, and Arc of reproductively quiescent NMRs. We further observed that exogenous RFRP-3 suppresses gonadal steroidogenesis and mating behavior in NMRs given the opportunity to undergo puberty. Together, our findings establish a role for RFRP-3 in preserving reproductive immaturity, and challenge the view that stimulatory peptides are the ultimate gatekeepers of puberty.

Dual Actions of Mammalian and Piscine Gonadotropin-Inhibitory Hormones, RFamide-Related Peptides and LPXRFamide Peptides, in the Hypothalamic-Pituitary-Gonadal Axis

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that decreases gonadotropin synthesis and release by directly acting on the gonadotrope or by decreasing the activity of gonadotropin-releasing hormone (GnRH) neurons. GnIH is also called RFamide-related peptide in mammals or LPXRFamide peptide in fishes due to its characteristic C-terminal structure. The primary receptor for GnIH is GPR147 that inhibits cAMP production in target cells. Although most of the studies in mammals, birds, and fish have shown the inhibitory action of GnIH in the hypothalamic-pituitary-gonadal (HPG) axis, several in vivo studies in mammals and many in vivo and in vitro studies in fish have shown its stimulatory action. In mouse, although the firing rate of the majority of GnRH neurons is decreased, a small population of GnRH neurons is stimulated by GnIH. In hamsters, GnIH inhibits luteinizing hormone (LH) release in the breeding season when their endogenous LH level is high but stimulates LH release in non-breeding season when their LH level is basal. Besides different effects of GnIH on the HPG axis depending on the reproductive stages in fish, higher concentration or longer duration of GnIH administration can stimulate their HPG axis. These results suggest that GnIH action in the HPG axis is modulated by sex-steroid concentration, the action of neuroestrogen synthesized by the activity of aromatase stimulated by GnIH, estrogen membrane receptor, heteromerization and internalization of GnIH, GnRH, and estrogen membrane receptors. The inhibitory and stimulatory action of GnIH in the HPG axis may have a physiological role to maintain reproductive homeostasis according to developmental and reproductive stages.

A Journey through the Gonadotropin-Inhibitory Hormone System of Fish

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that belongs to the RFamide peptide family and was first identified in the quail brain. From the discovery of avian GnIH, orthologous GnIH peptides have been reported in a variety of vertebrates, including mammals, amphibians, teleosts and agnathans, but also in protochordates. It has been clearly established that GnIH suppresses reproduction in avian and mammalian species through its inhibitory actions on brain GnRH and pituitary gonadotropins. In addition, GnIH also appears to be involved in the regulation of feeding, growth, stress response, heart function and social behavior. These actions are mediated via G protein-coupled GnIH receptors (GnIH-Rs), of which two different subtypes, GPR147 and GPR74, have been described to date. With around 30,000 species, fish represent more than one-half of the total number of recognized living vertebrate species. In addition to this impressive biological diversity, fish are relevant because they include model species with scientific and clinical interest as well as many exploited species with economic importance. In spite of this, the study of GnIH and its physiological effects on reproduction and other physiological processes has only been approached in a few fish species, and results obtained are in some cases conflicting. In this review, we summarize the information available in the literature on GnIH sequences identified in fish, the distribution of GnIH and GnIH-Rs in central and peripheral tissues, the physiological actions of GnIH on the reproductive brain-pituitary-gonadal axis, as well as other reported effects of this neuropeptide, and existing knowledge on the regulatory mechanisms of GnIH in fish.

Testicular Steroidogenesis and Locomotor Activity Are Regulated by Gonadotropin-Inhibitory Hormone in Male European Sea Bass

Gonadotropin-inhibitory hormone (GnIH) is a neurohormone that suppresses reproduction by acting at both the brain and pituitary levels. In addition to the brain, GnIH may also be produced in gonads and can regulate steroidogenesis and gametogenesis. However, the function of GnIH in gonadal physiology has received little attention in fish. The main objective of this study was to evaluate the effects of peripheral sbGnih-1 and sbGnih-2 implants on gonadal development and steroidogenesis during the reproductive cycle of male sea bass (Dicentrarchus labrax). Both Gnihs decreased testosterone (T) and 11-ketotestosterone (11-KT) plasma levels in November and December (early- and mid-spermatogenesis) but did not affect plasma levels of the progestin 17,20β-dihydroxy-4-pregnen-3-one (DHP). In February (spermiation), fish treated with sbGnih-1 and sbGnih-2 exhibited testicles with abundant type A spermatogonia and partial spermatogenesis. In addition, we determined the effects of peripheral Gnih implants on plasma follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) levels, as well as on brain and pituitary expression of the main reproductive hormone genes and their receptors during the spermiation period (February). Treatment with sbGnih-2 increased brain gnrh2, gnih, kiss1r and gnihr transcript levels. Whereas, both Gnihs decreased lhbeta expression and plasma Lh levels, and sbGnih-1 reduced plasmatic Fsh. Finally, through behavioral recording we showed that Gnih implanted animals exhibited a significant increase in diurnal activity from late spermatogenic to early spermiogenic stages. Our results indicate that Gnih may regulate the reproductive axis of sea bass acting not only on brain and pituitary hormones but also on gonadal physiology and behavior.

Patterns of hypothalamic GnIH change over the reproductive period in starlings and rats

Gonadotropin inhibitory hormone (GnIH) exerts powerful inhibitory effects on various levels of the vertebrate hypothalamic-pituitary-gonadal (reproductive) axis, yet little is known of how it might change naturally over the course of reproduction. We characterized patterns of hypothalamic GnIH cell abundance over the reproductive period in two popular models used for the study of reproductive endocrinology: European starlings (Sturnus vulgaris) and Sprague-Dawley rats (Rattus norvegicus). We also examined the effects on an unpredictable change in the environment on GnIH cell abundance during the reproductive period, specifically during the period of parental care, by simulating a nest predation event and removing eggs/pups. In both species, we report changes in GnIH cell abundance are occurring at similar reproductive time points but are not always directionally parallel; this may be due to a difference in life histories and physiology mediating parental care. We discovered that cells immunoreactive for the GnIH peptide in male and female starlings are most highly abundant on the first day of incubation and the first day after the first chick hatches. Conversely in rats, GnIH cell abundance decreases in dams on the first day after pups are born. In both male and female starlings and female rats, GnIH cell abundance increases in response to egg/pup loss, indicating that GnIH responds to an unpredictable change in the environment in a potentially conserved fashion. These changes in GnIH cell abundance during the reproductive period inspire further investigation of its adaptive role in reproductive physiological events and behaviors, especially parental care.

No Evidence That RFamide-Related Peptide 3 Directly Modulates LH Secretion in the Ewe

The neuropeptide RFamide-related peptide 3 (RFRP-3) has been implicated in the control of gonadotropin secretion in both birds and mammals. However, in mammals, depending on species, sex and photoperiod, inhibitory, excitatory, or no effect of RFRP-3 on the plasma concentration of LH has been reported. In the ewe, treatment with RFRP-3 either reduced LH concentration or had no effect, and treatment with an RFRP-3 receptor antagonist (ie, RF9) resulted in increased concentration of plasma LH. To clarify these conflicting results in the present study, a set of experiments was performed in ewes. Multiple iv injections of RFRP-3 (6 × 50 μg) in ovariectomized ewes had no effect on plasma LH pulsatility. In intact ewes a bolus injection (500 μg) or an injection (250, 500, or 1000 μg) followed by a 4-hour perfusion (250, 500, or 1000 μg · h(-1)) of RFRP-3 had no effect on the LH pulse induced by kisspeptin (6.5 μg). In ovariectomized, estrogen-replaced ewes, the LH surge induced by estradiol benzoate was not modified by a 24-hour perfusion of RFRP-3 (500 μg h(-1)). Finally, although treatment with RF9 induced a robust release of LH, treatment with a more selective RFRP-3 receptor antagonist, GJ14, resulted in no evident increase of LH. In contrast to the inhibitory effect previously suggested, our data are more consistent with the concept that RFRP-3 has no direct effect on LH secretion in ewes and that RF9 effect on LH release is likely not RFRP-3 receptor mediated. Hence, RFRP-3 probably has a minor role on the control of LH secretion in the ewe.

Gonadotropin Inhibitory Hormone Down-Regulates the Brain-Pituitary Reproductive Axis of Male European Sea Bass (Dicentrarchus labrax)

Gonadotropin-inhibitory hormone (GnIH) inhibits gonadotropin synthesis and release from the pituitary of birds and mammals. However, the physiological role of orthologous GnIH peptides on the reproductive axis of fish is still uncertain, and their actions on the main neuroendocrine systems controlling reproduction (i.e., GnRHs, kisspeptins) have received little attention. In a recent study performed in the European sea bass, we cloned a cDNA encoding a precursor polypeptide that contained C-terminal MPMRFamide (sbGnIH-1) and MPQRFamide (sbGnIH-2) peptide sequences, developed a specific antiserum against sbGnIH-2, and characterized its central and pituitary GnIH projections in this species. In this study, we analyzed the effects of intracerebroventricular injection of sbGnIH-1 and sbGnIH-2 on brain and pituitary expression of reproductive hormone genes (gnrh1, gnrh2, gnrh3, kiss1, kiss2, gnih, lhbeta, fshbeta), and their receptors (gnrhr II-1a, gnrhr II-2b, kiss1r, kiss2r, and gnihr) as well as on plasma Fsh and Lh levels. In addition, we determined the effects of GnIH on pituitary somatotropin (Gh) expression. The results obtained revealed the inhibitory role of sbGnIH-2 on brain gnrh2, kiss1, kiss2, kiss1r, gnih, and gnihr transcripts and on pituitary fshbeta, lhbeta, gh, and gnrhr-II-1a expression, whereas sbGnIH-1 only down-regulated brain gnrh1 expression. However, at different doses, central administration of both sbGnIH-1 and sbGnIH-2 decreased Lh plasma levels. Our work represents the first study reporting the effects of centrally administered GnIH in fish and provides evidence of the differential actions of sbGnIH-1 and sbGnIH-2 on the reproductive axis of sea bass, the main inhibitory role being exerted by the sbGnIH-2 peptide.

Molecular Plasticity of Male and Female Murine Gonadotropes Revealed by mRNA Sequencing

Gonadotropes in the anterior pituitary gland are of particular importance within the hypothalamic-pituitary-gonadal axis because they provide a means of communication and thus a functional link between the brain and the gonads. Recent results indicate that female gonadotropes may be organized in the form of a network that shows plasticity and adapts to the altered endocrine conditions of different physiological states. However, little is known about functional changes on the molecular level within gonadotropes during these different conditions. In this study we capitalize on a binary genetic strategy in order to fluorescently label murine gonadotrope cells. Using this mouse model allows to produce an enriched gonadotrope population using fluorescence activated cell sorting to perform mRNA sequencing. By using this strategy, we analyze and compare the expression profile of murine gonadotropes in different genders and developmental and hormonal stages. We find that gonadotropes taken from juvenile males and females, from cycling females at diestrus and at proestrus, from lactating females, and from adult males each have unique gene expression patterns with approximately 100 to approximately 500 genes expressed only in one particular stage. We also demonstrate extensive gene-expression profile changes with up to approximately 2200 differentially expressed genes when comparing female and male development, juveniles and adults, and cycling females. Differentially expressed genes were significantly enriched in the GnRH signaling, calcium signaling, and MAPK signaling pathways by Kyoto Encyclopedia of Genes and Genomes analysis. Our data provide an unprecedented molecular view of the primary gonadotropes and reveal a high degree of molecular plasticity within the gonadotrope population.

RF-amide neuropeptides and their receptors in Mammals: Pharmacological properties, drug development and main physiological functions

RF-amide neuropeptides, with their typical Arg-Phe-NH2 signature at their carboxyl C-termini, belong to a lineage of peptides that spans almost the entire life tree. Throughout evolution, RF-amide peptides and their receptors preserved fundamental roles in reproduction and feeding, both in Vertebrates and Invertebrates. The scope of this review is to summarize the current knowledge on the RF-amide systems in Mammals from historical aspects to therapeutic opportunities. Taking advantage of the most recent findings in the field, special focus will be given on molecular and pharmacological properties of RF-amide peptides and their receptors as well as on their implication in the control of different physiological functions including feeding, reproduction and pain. Recent progress on the development of drugs that target RF-amide receptors will also be addressed.

How to contribute to the progress of neuroendocrinology: New insights from discovering novel neuropeptides and neurosteroids regulating pituitary and brain functions

Obtaining new insights by discovering novel neuropeptides and neurosteroids regulating pituitary and brain functions is essential for the progress of neuroendocrinology. At the beginning of 1970s, gonadotropin-releasing hormone (GnRH) was discovered in mammals. Since then, it was generally accepted that GnRH is the only hypothalamic neuropeptide regulating gonadotropin release in vertebrates. In 2000, however, gonadotropin-inhibitory hormone (GnIH), a novel hypothalamic neuropeptide that actively inhibits gonadotropin release, was discovered in quail. The follow-up studies demonstrated that GnIH acts as a new key player for regulation of reproduction across vertebrates. It now appears that GnIH acts on the pituitary and the brain to serve a number of behavioral and physiological functions. On the other hand, a new concept has been established that the brain synthesizes steroids, called neurosteroids. The formation of neurosteroids in the brain was originally demonstrated in mammals and subsequently in other vertebrates. Recently, 7α-hydroxypregnenolone was discovered as a novel bioactive neurosteroid inducing locomotor behavior of vertebrates, indicating that neurosteroidogenesis in the brain is still incompletely elucidated in vertebrates. At the beginning of 2010s, it was further found that the pineal gland actively produces neurosteroids. Pineal neurosteroids act on the brain to regulate locomotor rhythms and neuronal survival. Furthermore, the interaction of neuropeptides and neurosteroids is becoming clear. GnIH decreases aggressive behavior by regulating neuroestrogen synthesis in the brain. This review summarizes these new insights by discovering novel neuropeptides and neurosteroids in the field of neuroendocrinology.

Identification and localization of gonadotropin-inhibitory hormone (GnIH) orthologs in the hypothalamus of the red-eared slider turtle, Trachemys scripta elegans

Gonadotropin-inhibitory hormone (GnIH) was discovered in 2000 as a novel hypothalamic neuropeptide that inhibited gonadotropin release in the Japanese quail. GnIH and its orthologs have a common C-terminal LPXRFamide (X=L or Q) motif, and have been identified in vertebrates from agnathans to humans, apart from reptiles. In the present study, we characterized a cDNA encoding GnIH orthologs in the brain of the red-eared slider turtle. The deduced precursor protein consisted of 205 amino-acid residues, encoding three putative peptide sequences that included the LPXRFamide motif at their C-termini. In addition, the precursor sequence was most similar to those of avian species. Immunoaffinity purification combined with mass spectrometry confirmed that three mature peptides were produced in the brain. In situ hybridization and immunohistochemistry showed that turtle GnIH-containing cells were restricted to the periventricular hypothalamic nucleus. Immunoreactive fibers were densely distributed in the median eminence. Thus, GnIH and related peptides may act on the pituitary to regulate pituitary hormone release in turtles as well as other vertebrates.

Differential response of GnIH in the brain and gonads following acute stress in a songbird

Gonadotropin-inhibitory hormone (GnIH) acts to inhibit reproduction at all levels of the hypothalamo-pituitary-gonad axis. GnIH expression and/or immunoreactivity in the hypothalamus increase with acute stress in some birds and mammals, and thus may be involved in stress-induced reproductive inhibition. Much is known about GnIH and stress in seasonal and continuous breeders, but far less is known about these interactions in opportunistic breeders. For opportunistically breeding animals, reproductive readiness is closely associated with unpredictable environmental cues, and thus the GnIH system may be more sensitive to stress. To test this, we collected tissues from zebra finches immediately following capture or after 60 min of restraint. Restraint significantly increased plasma corticosterone in males and females but, contrary to studies on other species, restrained birds had significantly fewer GnIH immunoreactive (GnIH-ir) cell bodies than control birds. GnIH-ir cell number did not differ between the sexes. Stressed females had lower mRNA expression of the beta subunit of follicle stimulating hormone (FSHβ) in the pituitary, suggesting that the reduction in observed GnIH immunoreactivity in females may have been due to increased GnIH release in response to acute stress. GnIH expression increased in the testes, but not the ovaries, of restrained animals. Our data suggest that although GnIH responsiveness to stress appears to be conserved across species, specific tissue response and direction of GnIH regulation is not. Variation in the GnIH response to stress between species might be the result of ecological adaptations or other species differences in the response of the GnIH system to stress.

Molecular, cellular, morphological, physiological and behavioral aspects of gonadotropin-inhibitory hormone

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that was isolated from the brains of Japanese quail in 2000, which inhibited luteinizing hormone release from the anterior pituitary gland. Here, we summarize the following fifteen years of researches that investigated on the mechanism of GnIH actions at molecular, cellular, morphological, physiological, and behavioral levels. The unique molecular structure of GnIH peptide is in its LPXRFamide (X=L or Q) motif at its C-terminal. The primary receptor for GnIH is GPR147. The cell signaling pathway triggered by GnIH is initiated by inhibiting adenylate cyclase and decreasing cAMP production in the target cell. GnIH neurons regulate not only gonadotropin synthesis and release in the pituitary, but also regulate various neurons in the brain, such as GnRH1, GnRH2, dopamine, POMC, NPY, orexin, MCH, CRH, oxytocin, and kisspeptin neurons. GnIH and GPR147 are also expressed in gonads and they may regulate steroidogenesis and germ cell maturation in an autocrine/paracrine manner. GnIH regulates reproductive development and activity. In female mammals, GnIH may regulate estrous or menstrual cycle. GnIH is also involved in the regulation of seasonal reproduction, but GnIH may finely tune reproductive activities in the breeding seasons. It is involved in stress responses not only in the brain but also in gonads. GnIH may inhibit male socio-sexual behavior by stimulating the activity of cytochrome P450 aromatase in the brain and stimulates feeding behavior by modulating the activities of hypothalamic and central amygdala neurons.

RFRP Neurons - The Doorway to Understanding Seasonal Reproduction in Mammals

Seasonal control of reproduction is critical for the perpetuation of species living in temperate zones that display major changes in climatic environment and availability of food resources. In mammals, seasonal cues are mainly provided by the annual change in the 24-h light/dark ratio (i.e., photoperiod), which is translated into the nocturnal production of the pineal hormone melatonin. The annual rhythm in this melatonin signal acts as a synchronizer ensuring that breeding occurs when environmental conditions favor survival of the offspring. Although specific mechanisms might vary among seasonal species, the hypothalamic RF (Arg-Phe) amide-related peptides (RFRP-1 and -3) are believed to play a critical role in the central control of seasonal reproduction and in all seasonal species investigated, the RFRP system is persistently inhibited in short photoperiod. Central chronic administration of RFRP-3 in short day-adapted male Syrian hamsters fully reactivates the reproductive axis despite photoinhibitory conditions, which highlights the importance of the seasonal changes in RFRP expression for proper regulation of the reproductive axis. The acute effects of RFRP peptides, however, depend on species and photoperiod, and recent studies point toward a different role of RFRP in regulating female reproductive activity. In this review, we summarize the recent advances made to understand the role and underlying mechanisms of RFRP in the seasonal control of reproduction, primarily focusing on mammalian species.

Endocrine and neuroendocrine regulation of fathering behavior in birds

This article is part of a Special Issue "Parental Care". Although paternal care is generally rare among vertebrates, care of eggs and young by male birds is extremely common and may take on a variety of forms across species. Thus, birds provide ample opportunities for investigating both the evolution of and the proximate mechanisms underpinning diverse aspects of fathering behavior. However, significant gaps remain in our understanding of the endocrine and neuroendocrine influences on paternal care in this vertebrate group. In this review, I focus on proximate mechanisms of paternal care in birds. I place an emphasis on specific hormones that vary predictably and/or unpredictably during the parental phase in both captive and wild birds: prolactin and progesterone are generally assumed to enhance paternal care, whereas testosterone and corticosterone are commonly-though not always correctly-assumed to inhibit paternal care. In addition, because endocrine secretions are not the sole mechanistic influence on paternal behavior, I also explore potential roles for certain neuropeptide systems (specifically the oxytocin-vasopressin nonapeptides and gonadotropin inhibitory hormone) and social and experiential factors in influencing paternal behavior in birds. Ultimately, mechanistic control of fathering behavior in birds is complex, and I suggest specific avenues for future research with the goal of narrowing gaps in our understanding of this complexity. Such avenues include (1) experimental studies that carefully consider not only endocrine and neuroendocrine mechanisms of paternal behavior, but also the ecology, phylogenetic history, and social context of focal species; (2) investigations that focus on individual variation in both hormonal and behavioral responses during the parental phase; (3) studies that investigate mechanisms of maternal and paternal care independently, rather than assuming that the mechanistic foundations of care are similar between the sexes; (4) expansion of work on interactions of the neuroendocrine system and fathering behavior to a wider array of paternal behaviors and taxa (e.g., currently, studies of the interactions of testosterone and paternal care largely focus on songbirds, whereas studies of the interactions of corticosterone, prolactin, and paternal care in times of stress focus primarily on seabirds); and (5) more deliberate study of exceptions to commonly held assumptions about hormone-paternal behavior interactions (such as the prevailing assumptions that elevations in androgens and glucocorticoids are universally disruptive to paternal care). Ultimately, investigations that take an intentionally integrative approach to understanding the social, evolutionary, and physiological influences on fathering behavior will make great strides toward refining our understanding of the complex nature by which paternal behavior in birds is regulated.

GnIH Control of Feeding and Reproductive Behaviors

In 2000, Tsutsui and colleagues discovered a neuropeptide gonadotropin-inhibitory hormone (GnIH) that inhibits gonadotropin release in birds. Subsequently, extensive studies during the last 15 years have demonstrated that GnIH is a key neurohormone that regulates reproduction in vertebrates, acting in the brain and on the pituitary to modulate reproduction and reproductive behavior. On the other hand, deprivation of food and other metabolic challenges inhibit the reproductive axis as well as sexual motivation. Interestingly, recent studies have further indicated that GnIH controls feeding behavior in vertebrates, such as in birds and mammals. This review summarizes the discovery of GnIH and its conservation in vertebrates and the neuroendocrine control of feeding behavior and reproductive behavior by GnIH.

The Role of Arginine-Phenylalanine-Amide-Related Peptides in Mammalian Reproduction

Until 2000 it was believed that gonadotropin-releasing hormone (GnRH) was the sole regulator of hypophyseal gonadotropes. In 2000, the discovery of a gonadotropin inhibitory hormone (GnIH) initiated a revolution in the field of reproductive physiology. Identification of GnIH homologues in mammals, the arginine-phenylalanine-amide (RFamide)-related peptides (RFRPs), indicated a similar function. Subsequently, further works conducted in various laboratories worldwide have shown that these neuropeptides inhibit the hypothalamic-hypophyseal axis. This review discusses the role of RFRPs in mammalian reproductive processes.

Sex differences in the photoperiodic regulation of RF-Amide related peptide (RFRP) and its receptor GPR147 in the syrian hamster

RF-(Arg-Phe) related peptides (RFRP-1 and -3) are considered to play a role in the seasonal regulation of reproduction; however, the effect of the peptides depends on species and gender. This study aimed at comparing the RFRP system in male and female Syrian hamsters over long and short photoperiods to investigate the neuroanatomical basis of these differential effects. The neuroanatomical distribution of RFRP neurons and fibers, revealed using an antiserum recognizing RFRP-1 and -3, as well as GPR147 mRNA, are similar in male and female Syrian hamsters. RFRP neurons are mainly found in the medial hypothalamus, whereas RFRP projections and GPR147 mRNA are observed in the preoptic area, anteroventral-periventricular nucleus, suprachiasmatic nucleus, paraventricular nucleus, bed nucleus of the stria terminalis, ventromedial hypothalamus, habenular nucleus, and arcuate nucleus. The number of RFRP neurons is higher in females than in males, and in both sexes, the number of RFRP neurons is reduced in short photoperiods. GPR147 mRNA levels are higher in females than in males and are downregulated in short photoperiods, particularly in females. Interestingly, the number of RFRP-positive fibers in the anteroventral-periventricular nucleus is higher only in females adjusted to a short photoperiod. Our results suggest that the RFRP system, which is strongly regulated by photoperiod in both male and female Syrian hamsters, is particularly important in females, with a distinct role in the anteroventral-periventricular nucleus, possibly in the regulation of the preovulatory luteinizing hormone surge via kisspeptin neurons.

RF9 Acts as a KISS1R Agonist In Vivo and In Vitro

RF9, a reported antagonist of the mammalian gonadotropin-inhibitory hormone receptor, stimulates gonadotropin secretion in mammals. Recent studies have suggested that the stimulatory effect of RF9 on gonadotropin secretion relies on intact kisspeptin receptor (KISS1R) signaling, but the underlying mechanisms remain to be elucidated. Using Chinese Hamster Ovary cells stably transfected with KISS1R, we show that RF9 binds specifically to KISS1R, with a Kd of 1.6 × 10(-5)M, and stimulates an increase in intracellular calcium and inositol phosphate accumulation in a KISS1R-dependent manner, with EC50 values of 3.0 × 10(-6)M and 1.6 × 10(-7)M, respectively. RF9 also stimulated ERK phosphorylation, with a time course similar to that of kisspeptin-10. RFRP-3, the putative endogenous ligand for NPFFR1, did not stimulate inositol phosphate accumulation or pERK, nor did it alter responses to of kisspeptin-10 or RF9. In agreement with these in vitro data, we found that RF9 stimulated a robust LH increase in Npffr1(-/-) mice, similar to that in wild-type littermates, whereas the stimulatory effect of RF9 was markedly reduced in Kiss1r(-/-) and double Kiss1r(-/-)/Npfrr1(-/-) mice. The stimulatory effect of RF9 on LH secretion was restored by the selective rescue of Kiss1r expression in GnRH neurons, in Kiss1r(-/-T) mice. Taken together, our study demonstrates that RF9 acts primarily as a KISS1R agonist, but not as an allosteric modulator, to stimulate LH secretion. Our findings raise questions regarding the utility of RF9 for assessing NPFF1R function and de-emphasize a predominant role of this signaling system in central regulation of reproduction.

Neuronal Gonadotrophin-Releasing Hormone (GnRH) and Astrocytic Gonadotrophin Inhibitory Hormone (GnIH) Immunoreactivity in the Adult Rat Hippocampus

Gonadotrophin-releasing hormone (GnRH) and gonadotrophin inhibitory hormone (GnIH) are neuropeptides secreted by the hypothalamus that regulate reproduction. GnRH receptors are not only present in the anterior pituitary, but also are abundantly expressed in the hippocampus of rats, suggesting that GnRH regulates hippocampal function. GnIH inhibits pituitary gonadotrophin secretion and is also expressed in the hippocampus of a songbird; its role outside of the reproductive axis is not well established. In the present study, we employed immunohistochemistry to examine three forms of GnRH [mammalian GnRH-I (mGnRH-I), chicken GnRH-II (cGnRH-II) and lamprey GnRH-III (lGnRH-III)] and GnIH in the adult rat hippocampus. No mGnRH-I and cGnRH-II+ cell bodies were present in the hippocampus. Sparse mGnRH-I and cGnRH-II+ fibres were present within the CA1 and CA3 fields of the hippocampus, along the hippocampal fissure, and within the hilus of the dentate gyrus. No lGnRH-III was present in the rodent hippocampus. GnIH-immunoreactivity was present in the hippocampus in cell bodies that resembled astrocytes. Males had more GnIH+ cells in the hilus of the dentate gyrus than females. To confirm the GnIH+ cell body phenotype, we performed double-label immunofluorescence against GnIH, glial fibrillary acidic protein (GFAP) and NeuN. Immunofluorescence revealed that all GnIH+ cell bodies in the hippocampus also contained GFAP, a marker of astrocytes. Taken together, these data suggest that GnRH does not reach GnRH receptors in the rat hippocampus primarily via synaptic release. By contrast, GnIH might be synthesised locally in the rat hippocampus by astrocytes. These data shed light on the sites of action and possible functions of GnRH and GnIH outside of the hypothalamic-pituitary-gonadal axis.

Hypothalamus as an endocrine organ

The endocrine hypothalamus constitutes those cells which project to the median eminence and secrete neurohormones into the hypophysial portal blood to act on cells of the anterior pituitary gland. The entire endocrine system is controlled by these peptides. In turn, the hypothalamic neuroendocrine cells are regulated by feedback signals from the endocrine glands and other circulating factors. The neuroendocrine cells are found in specific regions of the hypothalamus and are regulated by afferents from higher brain centers. Integrated function is clearly complex and the networks between and amongst the neuroendocrine cells allows fine control to achieve homeostasis. The entry of hormones and other factors into the brain, either via the cerebrospinal fluid or through fenestrated capillaries (in the basal hypothalamus) is important because it influences the extent to which feedback regulation may be imposed. Recent evidence of the passage of factors from the pars tuberalis and the median eminence casts a new layer in our understanding of neuroendocrine regulation. The function of neuroendocrine cells and the means by which pulsatile secretion is achieved is best understood for the close relationship between gonadotropin releasing hormone and luteinizing hormone, which is reviewed in detail. The secretion of other neurohormones is less rigid, so the relationship between hypothalamic secretion and the relevant pituitary hormones is more complex.

The Inhibitory Effects of RFamide-Related Peptide 3 on Luteinizing Hormone Release Involves an Estradiol-Dependent Manner in Prepubertal but Not in Adult Female Mice

The mammalian gonadotropin-inhibitory hormone (GnIH) ortholog, RFamide-related peptide (RFRP), is considered to act on gonadotropin-releasing hormone (GnRH) neurons and the pituitary to inhibit gonadotropin synthesis and release. However, there is little evidence documenting whether RFamide-related peptide 3 (RFRP-3) plays a primary role in inhibition of the hypothalamo-pituitary-gonadal (HPG) axis prior to the onset of puberty. The present study aimed to understand the functional significance of the neuropeptide on pubertal development. The developmental changes in reproductive-related gene expression at the mRNA level were investigated in the hypothalamus of female mice. The results indicated that RFRP-3 may be an endogenous inhibitory factor for the activation of the HPG axis prior to the onset of puberty. In addition, centrally administered RFRP-3 significantly suppressed plasma luteinizing hormone (LH) levels in prepubertal female mice. Surprisingly, centrally administered RFRP-3 had no effects on plasma LH levels in ovariectomized (OVX) prepubescent female mice. In contrast, RFRP-3 also inhibited plasma LH levels in OVX prepubescent female mice that were treated with 17beta-estradiol replacement. Our study also examined the effects of RFRP-3 on plasma LH release in adult female mice that were ovariectomized at dioestrus, with or without estradiol (E2). Our results showed that the inhibitory effects of RFRP-3 were independent of E2 status. Quantitative real-time PCR and immunohistochemistry analyses showed that RFRP-3 inhibited GnRH expression at both the mRNA and protein levels in the hypothalamus. These data demonstrated that RFRP-3 could effectively suppress pituitary LH release, via the inhibition of GnRH transcription and translation in prepubescent female mice, which is associated with estrogen signaling pathway and developmental stages.

An evolutionary scenario for gonadotrophin-inhibitory hormone in chordates

In 2000, we discovered a novel hypothalamic neuropeptide that actively inhibits gonadotrophin release in quail and termed it gonadotrophin-inhibitory hormone (GnIH). GnIH peptides have subsequently been identified in most representative species of gnathostomes. They all share a C-terminal LPXRFamide (X = L or Q) motif. GnIH can inhibit gonadotrophin synthesis and release by decreasing the activity of GnRH neuroes, as well as by directly inhibiting pituitary gonadotrophin secretion in birds and mammals. To investigate the evolutionary origin of GnIH and its ancestral function, we identified a GnIH precursor gene encoding GnIHs from the brain of sea lamprey, the most ancient lineage of vertebrates. Lamprey GnIHs possess a C-terminal PQRFamide motif. In vivo administration of one of lamprey GnIHs stimulated the expression of lamprey GnRH in the hypothalamus and gonadotophin β mRNA in the pituitary. Thus, GnIH may have emerged in agnathans as a stimulatory neuropeptide that subsequently diverged to an inhibitory neuropeptide during the course of evolution from basal vertebrates to later-evolved vertebrates, such as birds and mammals. From a structural point of view, pain modulatory neuropeptides, such as neuropeptide FF (NPFF) and neuropeptide AF, share a C-terminal PQRFamide motif. Because agnathans possess both GnIH and NPFF genes, the origin of GnIH and NPFF genes may date back before the emergence of agnathans. More recently, we identified a novel gene encoding RFamide peptides in the amphioxus. Molecular phylogenetic analysis and synteny analysis indicated that this gene is closely related to the genes of GnIH and NPFF of vertebrates. The results suggest that the identified protochordate gene is similar to the common ancestor of GnIH and NPFF genes, indicating that the origin of GnIH and NPFF may date back to the time of the emergence of early chordates. The GnIH and NPFF genes may have diverged by whole-genome duplication during the course of vertebrate evolution.

Molecular identification of GnIH/GnIHR signal and its reproductive function in protogynous hermaphroditic orange-spotted grouper (Epinephelus coioides)

Gonadotropin-inhibitory hormone (GnIH) and its receptor (GnIHR) play an important role in reproduction regulation in birds, mammals and some teleost species. In protogynous hermaphroditic orange-spotted grouper (Epinephelus coioides), the GnIH/GnIHR signaling pathway and its reproductive function have not been addressed yet. In this study, GnIH and GnIHR in orange-spotted grouper were characterized. gGnIH possessed three putative peptides (gGnIH-I, -II, -III), while gGnIHR showed the characteristics of G protein-coupled receptor and was clustered with GPR147. Functional assays demonstrated that three synthetic gGnIH peptides significantly decreased the forskolin-induced CRE promoter activity, but only gGnIH-I could significantly decrease SRE promoter activity in COS-7 cells transfected with gGnIHR. During the process of ovarian differentiation and development, gGnIH mRNA level in hypothalamus was low at the gonadal primordium stage with gonia, then increased significantly at the early differentiated gonad with primary growth oocytes, while decreased significantly at the developing gonads with cortical-alveolus and vitellogenic stage oocytes. During MT-induced sex reversal, gGnIH mRNA level in hypothalamus increased significantly when the fish completely reversed from female to male. However, gGnIHR mRNA level in pituitary decreased significantly in intersex and completely reversed male fish. Intraperitoneal injection (i.p.) of three gGnIH peptides significantly decreased GnRH1 mRNA levels in hypothalamus, and gGnIH-II significantly inhibited synthesis of LHβ in pituitary. In summary, we firstly identified the GnIH/GnIHR signal in protogynous orange-spotted grouper, which might be involved in the regulation of the reproductive function of sex differentiation, gonadal development and sex reversal via regulating the synthesis of both GnRH and GtH.

The involvement of gonadotropin inhibitory hormone and kisspeptin in the metabolic regulation of reproduction

Recently, kisspeptin (KP) and gonadotropin inhibitory hormone (GnIH), two counteracting neuropeptides, have been acknowledged as significant regulators of reproductive function. KP stimulates reproduction while GnIH inhibits it. These two neuropeptides seem to be pivotal for the modulation of reproductive activity in response to internal and external cues. It is well-documented that the current metabolic status of the body is closely linked to its reproductive output. However, how reproductive function is regulated by the body's energy status is less clear. Recent studies have suggested an active participation of hypothalamic KP and GnIH in the modulation of reproductive function according to available metabolic cues. Expression of KISS1, the KP encoding gene, is decreased while expression of RFRP (NPVF), the gene encoding GnIH, is increased in metabolic deficiency conditions. The lower levels of KP, as suggested by a decrease in KISS1 gene mRNA expression, during metabolic deficiency can be corrected by administration of exogenous KP, which leads to an increase in reproductive hormone levels. Likewise, administration of RF9, a GnIH receptor antagonist, can reverse the inhibitory effect of fasting on testosterone in monkeys. Together, it is likely that the integrated function of both these hypothalamic neuropeptides works as a reproductive output regulator in response to a change in metabolic status. In this review, we have summarized literature from nonprimate and primate studies that demonstrate the involvement of KP and GnIH in the metabolic regulation of reproduction.

Food, stress, and circulating testosterone: Cue integration by the testes, not the brain, in male zebra finches (Taeniopygia guttata)

Food abundance is closely associated with reproductive readiness in vertebrates. Food scarcity can activate the hypothalamo-pituitary-adrenal axis, decrease sex steroid secretion, and dampen reproductive behavior. However, the mechanisms underlying these transient effects are unclear. Gonadotropin inhibitory hormone (GnIH), a neuropeptide present in the brain and gonads, is also influenced by glucocorticoids and fasting in some species. We investigated whether fasting stress activated the GnIH system in zebra finches (Taeniopygia guttata), with the potential for downstream effects on reproductive physiology and behavior. We fasted or fed males ad libitum for 10h. Fasting increased corticosterone and decreased testosterone in circulation. To assess whether the decrease in testosterone was mediated by changes in the hypothalamus and/or the gonads, we (1) quantified GnRH- and GnIH-positive neurons in the hypothalamus, (2) assessed hypothalamic gene expression for GnRH and GnIH, and (3) examined gene expression for proteins involved in testosterone synthesis in fasted and control birds. No measure of hypothalamic neuropeptides was related to treatment or circulating steroids. However, birds with higher corticosterone had higher testicular GnIH expression and lower testosterone. StAR and LHR expression were lower in the testes of fasted birds than controls. Thus, the decrease in testosterone was not likely mediated by hypothalamic GnIH, but rather by direct actions of fasting and/or corticosterone on the testes, indicating that the testes can integrate and respond to cues of stress directly. Such local inhibition of testosterone synthesis may allow for rapid and reversible changes in physiology and behavior when conditions are inappropriate for breeding.

Seasonal control of gonadotropin-inhibitory hormone (GnIH) in birds and mammals

Animals inhabiting temperate and boreal latitudes experience marked seasonal changes in the quality of their environments and maximize reproductive success by phasing breeding activities with the most favorable time of year. Whereas the specific mechanisms driving seasonal changes in reproductive function vary across species, converging lines of evidence suggest gonadotropin-inhibitory hormone (GnIH) serves as a key component of the neuroendocrine circuitry driving seasonal changes in reproduction and sexual motivation in some species. In addition to anticipating environmental change through transduction of photoperiodic information and modifying reproductive state accordingly, GnIH is also positioned to regulate acute changes in reproductive status should unpredictable conditions manifest throughout the year. The present overview summarizes the role of GnIH in avian and mammalian seasonal breeding while considering the similarities and disparities that have emerged from broad investigations across reproductively photoperiodic species.

Knockdown of hypothalamic RFRP3 prevents chronic stress-induced infertility and embryo resorption

Whereas it is well established that chronic stress induces female reproductive dysfunction, whether stress negatively impacts fertility and fecundity when applied prior to mating and pregnancy has not been explored. In this study, we show that stress that concludes 4 days prior to mating results in persistent and marked reproductive dysfunction, with fewer successful copulation events, fewer pregnancies in those that successfully mated, and increased embryo resorption. Chronic stress exposure led to elevated expression of the hypothalamic inhibitory peptide, RFamide-related peptide-3 (RFRP3), in regularly cycling females. Remarkably, genetic silencing of RFRP3 during stress using an inducible-targeted shRNA completely alleviates stress-induced infertility in female rats, resulting in mating and pregnancy success rates indistinguishable from non-stress controls. We show that chronic stress has long-term effects on pregnancy success, even post-stressor, that are mediated by RFRP3. This points to RFRP3 as a potential clinically relevant single target for stress-induced infertility.

DOI:http://dx.doi.org/10.7554/eLife.04316.001

Infertility has become alarmingly common in otherwise healthy women and around 15% of healthy couples younger than 30 years old are unable to conceive within the first year of trying. High-stress levels are known to decrease short-term fertility in humans and other animals, which may serve to prevent pregnancy during times when food or other resources are in short supply.

However, it is not clear if exposure to stress has lasting effects on fertility. Previous studies have found that when male rats experience stress, they release a protein called RFRP3. This protein inhibits brain activity, leading to a reduction in the release of reproductive hormones.

Geraghty et al. took a closer look at how stress may cause lasting fertility problems in female rats. The researchers exposed female rats to stress by restricting their movements for 3 hr each day over the course of 18 days, which increased the levels of stress hormones in the animals. They allowed the rats to recover for one full reproductive cycle—equivalent to a month in humans—and found that while their stress hormone levels returned to normal, RFRP3 levels in the brain remained high. Even after the recovery period, the females were less likely to mate. Also, the females that did mate were less likely to become pregnant, and the ones that did were more likely to lose some of the embryos. Overall, the level of reproductive success in these rats was only 21%, down from 76% in the control group (who were not exposed to the stress).

Next, Geraghty et al. injected a genetically engineered virus into the brain of the stressed rats to switch off the gene that makes RFRP3 during the stress period. This reduced the levels of the RFRP3 protein and restored the mating, pregnancy, and embryo survival rates to the normal levels seen in unstressed rats.

These results suggest that increased levels of RFRP3 during stress can have lasting negative effects on fertility. In the future, developing therapies that lower RFRP3 levels may help individuals who experience fertility problems.

DOI:http://dx.doi.org/10.7554/eLife.04316.002

Gonadotropin surge-inhibiting/attenuating factors: a review of current evidence, potential applications, and future directions for research

Animal studies in the 1980s suggested the existence of an ovarian hormone, termed gonadotropin surge-inhibiting/attenuating factor (GnSIF/AF), that modulates pituitary secretion of luteinizing hormone (LH). Given the importance of identifying regulatory factors of the hypothalamic-pituitary-ovarian axis and the accumulating data suggesting its existence, we conducted a comprehensive literature search using PubMed, Web of Science, Scopus, and Embase to identify articles related to GnSIF/AF. The search generated 161 publications, of which 97 were included in this study. Several attempts have been made to identify and characterize this hormone and several candidates have been identified, but the protein sequences of these putative GnSIF/AF factors differ widely from one study to another. In addition, while the RF-amide RFRP-3 is known foremost as a neuropeptide, some research supports an ovarian origin for this non-steroidal hormone, thereby suggesting a role for RFRP-3 either as a co-modulator of GnSIF/AF or as a gonadotropin-inhibiting factor in the hypothalamus (GnIH). Discovery of the KNDy neurons that modulate GnRH secretion, on the other hand, further encourages the search for substance(s) that modulate their activity and that indirectly affect LH secretion and the hypothalamic-pituitary-ovarian axis. While it has remained an elusive hormone, GnSIF/AF holds many potential applications for contraception, in vitro fertilization, and/or cancer as well as for understanding polycystic ovary syndrome, metabolic diseases, and/or pubertal development. In this review, we rigorously examine the available evidence regarding the existence of GnSIF/AF, previous attempts at its identification, limitations to its discovery, future directions of research, and potential clinical applications.

Contribution of GnIH Research to the Progress of Reproductive Neuroendocrinology

Since the discovery of gonadotropin-releasing hormone (GnRH) in mammals at the beginning of the 1970s, it was generally accepted that GnRH is the only hypothalamic neuropeptide regulating gonadotropin release in mammals and other vertebrates. In 2000, however, gonadotropin-inhibitory hormone (GnIH), a novel hypothalamic neuropeptide that actively inhibits gonadotropin release, was discovered in quail. Numerous studies over the past decade and a half have demonstrated that GnIH serves as a key player regulating reproduction across vertebrates, acting on the brain and pituitary to modulate reproductive physiology and behavior. In the latter case, recent evidence indicates that GnIH can regulate reproductive behavior through changes in neurosteroid, such as neuroestrogen, biosynthesis in the brain. This review summarizes the discovery of GnIH, and the contributions to GnIH research focused on its mode of action, regulation of biosynthesis, and how these findings advance our understanding of reproductive neuroendocrinology.

Glucocorticoid Regulation of Reproduction

It is well accepted that stress, measured by increased glucocorticoid secretion, leads to profound reproductive dysfunction. In times of stress, glucocorticoids activate many parts of the fight or flight response, mobilizing energy and enhancing survival, while inhibiting metabolic processes that are not necessary for survival in the moment. This includes reproduction, an energetically costly procedure that is very finely regulated. In the short term, this is meant to be beneficial, so that the organism does not waste precious energy needed for survival. However, long-term inhibition can lead to persistent reproductive dysfunction, even if no longer stressed. This response is mediated by the increased levels of circulating glucocorticoids, which orchestrate complex inhibition of the entire reproductive axis. Stress and glucocorticoids exhibits both central and peripheral inhibition of the reproductive hormonal axis. While this has long been recognized as an issue, understanding the complex signaling mechanism behind this inhibition remains somewhat of a mystery. What makes this especially difficult is attempting to differentiate the many parts of both of these hormonal axes, and new neuropeptide discoveries in the last decade in the reproductive field have added even more complexity to an already complicated system. Glucocorticoids (GCs) and other hormones within the hypothalamic-pituitary-adrenal (HPA) axis (as well as contributors in the sympathetic system) can modulate the hypothalamic-pituitary-gonadal (HPG) axis at all levels-GCs can inhibit release of GnRH from the hypothalamus, inhibit gonadotropin synthesis and release in the pituitary, and inhibit testosterone synthesis and release from the gonads, while also influencing gametogenesis and sexual behavior. This chapter is not an exhaustive review of all the known literature, however is aimed at giving a brief look at both the central and peripheral effects of glucocorticoids on the reproductive function.

Seasonal effects of GnIH on basal and GnRH-induced goldfish somatotrope functions

To understand how gonadotropin-inhibitory hormone (GnIH) regulates goldfish GH cell functions, we monitored GH release and expression during early, mid-, and/or late gonadal recrudescence. In vivo and in vitro responses to goldfish (g) GnIH were different, indicating direct action at the level of pituitary, as well as interactions with other neuroendocrine factors involved in GH regulation. Injection of gGnIH consistently reduced basal serum GH levels but elevated pituitary gh mRNA levels, indicating potential dissociation of GH release and synthesis. Goldfish GnRH (sGnRH and cGnRHII) injection differentially stimulated serum GH and pituitary gh mRNA levels with some seasonal differences; these responses were reduced by gGnIH. In contrast, in vitro application of gGnIH during 24-h static incubation of goldfish pituitary cells generally elevated basal GH release and attenuated sGnRH-induced changes in gh mRNA, while suppressing basal gh mRNA levels at mid- and late recrudescence but elevating them at early recrudescence. gGnIH attenuated the GH release responses to sGnRH during static incubation at early, but not at mid- and late recrudescence. In cell column perifusion experiments examining short-term GH release, gGnIH reduced the cGnRHII- and sGnRH-stimulated secretion at late recrudescence but inhibited tha action of cGnRHII only during mid-recrudescence. Interestingly, a reduction of basal GH release upon perifusion with gGnIH during late recrudescence was followed by a rebound increase in GH release upon gGnIH removal. These results indicate that gGnIH exerts complex effects on basal and GnRH-stimulated goldfish GH cell functions and can differentially affect GH release and mRNA expression in a seasonal reproductive manner.

Breakthrough in neuroendocrinology by discovering novel neuropeptides and neurosteroids: 1. Discovery of gonadotropin-inhibitory hormone (GnIH) across vertebrates

Bargmann-Scharrer's discovery of "neurosecretion" in the first half of the 20th century has since matured into the scientific discipline of neuroendocrinology. Identification of novel neurohormones, such as neuropeptides and neurosteroids, is essential for the progress of neuroendocrinology. Our studies over the past two decades have significantly broadened the horizons of this field of research by identifying novel neuropeptides and neurosteroids in vertebrates that have opened new lines of scientific investigation in neuroendocrinology. Since the discovery of gonadotropin-releasing hormone (GnRH) in mammals at the beginning of 1970s, it was generally believed that GnRH is the only hypothalamic neuropeptide regulating gonadotropin release in vertebrates. In 2000, however, we discovered a novel hypothalamic neuropeptide that actively inhibits gonadotropin release in quail and termed it gonadotropin-inhibitory hormone (GnIH). It now appears that GnIH is highly conserved across vertebrates, including humans, and serves a number of behavioral and physiological functions other than regulation of reproduction, providing enormous opportunity for investigators from a wide array of disciplines to study this neuropeptide. This review summarizes the discovery of GnIH and its contribution to the progress of neuroendocrinology.

Review: evolution of GnIH and related peptides structure and function in the chordates

Discovery of gonadotropin-inhibitory hormone (GnIH) in the Japanese quail in 2000 was the first to demonstrate the existence of a hypothalamic neuropeptide inhibiting gonadotropin release. We now know that GnIH regulates reproduction by inhibiting gonadotropin synthesis and release via action on the gonadotropin-releasing hormone (GnRH) system and the gonadotrope in various vertebrates. GnIH peptides identified in birds and mammals have a common LPXRF-amide (X = L or Q) motif at the C-terminus and inhibit pituitary gonadotropin secretion. However, the function and structure of GnIH peptides are diverse in fish. Goldfish GnIHs possessing a C-terminal LPXRF-amide motif have both stimulatory and inhibitory effects on gonadotropin synthesis or release. The C-terminal sequence of grass puffer and medaka GnIHs are MPQRF-amide. To investigate the evolutionary origin of GnIH and its ancestral structure and function, we searched for GnIH in agnathans, the most ancient lineage of vertebrates. We identified GnIH precursor gene and mature GnIH peptides with C-terminal QPQRF-amide or RPQRF-amide from the brain of sea lamprey. Lamprey GnIH fibers were in close proximity to GnRH-III neurons. Further, one of lamprey GnIHs stimulated the expression of lamprey GnRH-III peptide in the hypothalamus and gonadotropic hormone β mRNA expression in the pituitary. We further identified the ancestral form of GnIH, which had a C-terminal RPQRF-amide, and its receptors in amphioxus, the most basal chordate species. The amphioxus GnIH inhibited cAMP signaling in vitro. In sum, the original forms of GnIH may date back to the time of the emergence of early chordates. GnIH peptides may have had various C-terminal structures slightly different from LPXRF-amide in basal chordates, which had stimulatory and/or inhibitory functions on reproduction. The C-terminal LPXRF-amide structure and its inhibitory function on reproduction may be selected in later-evolved vertebrates, such as birds and mammals.

Physiological roles of gonadotropin-inhibitory hormone signaling in the control of mammalian reproductive axis: studies in the NPFF1 receptor null mouse

RF-amide-related peptide-3 (RFRP-3), the mammalian ortholog of the avian gonadotropin-inhibiting hormone (GnIH), operates via the NPFF1 receptor (NPFF1R) to repress the reproductive axis, therefore acting as counterpart of the excitatory RF-amide peptide, kisspeptin (ligand of Gpr54). In addition, RFRP-3 modulates feeding and might contribute to the integrative control of energy homeostasis and reproduction. Yet, the experimental evidence supporting these putative functions is mostly indirect, and the physiological roles of RFRP-3 remain debatable and obscured by the lack of proper analytical tools and models. To circumvent these limitations, we characterize herein the first mouse line with constitutive inactivation of NPFF1R. Ablation of NPFF1R did not compromise fertility; rather, litters from NPFF1R null mice were larger than those from wild-type animals. Pubertal timing was not altered in NPFF1R deficient mice; yet, pre-pubertal knockout (KO) males displayed elevated LH levels, which normalized after puberty. Adult NPFF1R null male mice showed increased Kiss1 expression in the hypothalamic arcuate nucleus, higher serum FSH levels, and enhanced LH responses to GnRH. However, genetic elimination of NPFF1R was unable to reverse the state of hypogonadism caused by the lack of kisspeptin signaling, as revealed by double NPFF1R/Gpr54 KO mice. NPFF1R null mice displayed altered feedback responses to gonadal hormone withdrawal. In addition, metabolic challenges causing gonadotropin suppression, such as short-term fasting and high-fat diet, were less effective in dampening LH secretion in NPFF1R-deficient male mice, suggesting that absence of this inhibitory pathway partially prevented gonadotropin suppression by metabolic stress. Our data are the first to document the impact of elimination of GnIH signaling on reproductive parameters and their modulation by metabolic challenges. Whereas, in keeping with its inhibitory role, the NPFF1R pathway seems dispensable for preserved puberty and fertility, our results surface different alterations due to the lack of GnIH signaling that prominently include changes in the sensitivity to fasting- and obesity-associated hypogonadotropism.