Hypothalamic LPXRF-amide peptides in vertebrates: identification, localization and hypophysiotropic activity

Gonadotropin-inhibitory hormone action in the brain and pituitary

Gonadotropin-inhibitory hormone (GnIH) was first identified in the Japanese quail as a hypothalamic neuropeptide inhibitor of gonadotropin secretion. Subsequent studies have shown that GnIH is present in the brains of birds including songbirds, and mammals including humans. The identified avian and mammalian GnIH peptides universally possess an LPXRFamide (X = L or Q) motif at their C-termini. Mammalian GnIH peptides are also designated as RFamide-related peptides from their structures. The receptor for GnIH is the G protein-coupled receptor 147 (GPR147), which is thought to be coupled to G(αi) protein. Cell bodies of GnIH neurons are located in the paraventricular nucleus (PVN) in birds and the dorsomedial hypothalamic area (DMH) in mammals. GnIH neurons in the PVN or DMH project to the median eminence to control anterior pituitary function. GPR147 is expressed in the gonadotropes and GnIH suppresses synthesis and release of gonadotropins. It was further shown in immortalized mouse gonadotrope cell line (LβT2 cells) that GnIH inhibits gonadotropin-releasing hormone (GnRH) induced gonadotropin subunit gene transcriptions by inhibiting adenylate cyclase/cAMP/PKA-dependent ERK pathway. GnIH neurons also project to GnRH neurons in the preoptic area, and GnRH neurons express GPR147 in birds and mammals. Accordingly, GnIH may inhibit gonadotropin synthesis and release by decreasing the activity of GnRH neurons as well as directly acting on the gonadotropes. GnIH also inhibits reproductive behavior possibly by acting within the brain. GnIH expression is regulated by a nocturnal hormone melatonin and stress in birds and mammals. Accordingly, GnIH may play a role in translating environmental information to inhibit reproductive physiology and behavior of birds and mammals. Finally, GnIH has therapeutic potential in the treatment of reproductive cycle and hormone-dependent diseases, such as precocious puberty, endometriosis, uterine fibroids, and prostatic and breast cancers.

Characterization of novel RFamide peptides in the central nervous system of the brown hagfish: isolation, localization, and functional analysis

RFamide (RFa) peptides play various important roles in the central nervous system in both invertebrates and vertebrates. However, there is no evidence of the existence of any RFamide peptide in the brain of hagfish, one of the oldest lineages of vertebrates. In this study, we sought to identify novel RFamide peptides from the brains of hagfish (Paramyxine atami). We identified four novel RFamide peptides, which had the C-terminal Pro-Gln-Arg-Phe-NH2 structure. cDNA cloning revealed that the identified RFamide peptides are encoded in two types of cDNA. Molecular phylogenetic analysis of the two precursors indicated that the hagfish RFamide peptides belong to the PQRFamide peptide group that includes mammalian neuropeptide FF and AF. Based on immunohistochemistry and in situ hybridization, hagfish PQRFamide peptide precursor mRNA and its translated peptides were localized in the infundibular nucleus of the hypothalamus. Immunoreactive fibers were terminated on blood vessels in the infundibular nucleus. Dense immunoreactive fibers were also observed in other brain regions. We further showed that one of the hagfish PQRFamide peptides significantly stimulated the expression of gonadotropin-β mRNA in the cultured hagfish pituitary. These results indicate that the control mechanism of gonadotropin expression by a hypothalamic neuropeptide evolved in the agnathan brain. This is the first evidence describing the identification of RFamide peptides in the hagfish brain. This is also the first report showing the regulation of gonadotropin expression by a homolog of neuropeptide FF that belongs to the PQRFamide peptide group in any vertebrate.

Effects of RFamide-related peptide (RFRP)-1 and RFRP-3 on oxytocin release and anxiety-related behaviour in rats

RFamide-related peptides (RFRP-1 and RFRP-3) are localised in neurones of the dorsomedial hypothalamus in rats. The dorsomedial hypothalamus plays an essential role in neuroendocrine and behavioural stress responses. In the present study, we examined the role of RFRP in the control of neuroendocrine and behavioural responses in rats. Stressful stimuli increased expression of Fos protein in RFRP-immunoreactive neurones of the dorsomedial hypothalamus, suggesting that stressful stimuli activate RFRP neurones. Intracerebroventricular injection of RFRPs increased the expression of Fos protein in oxytocin neurones in the hypothalamus and plasma concentrations of adrenocorticotrophic hormone and oxytocin. The hypothalamic paraventricular and supraoptic nuclei expressed mRNA of GPR147, the putative RFRP receptor, and application of RFRPs to isolated supraoptic nuclei facilitated oxytocin release, suggesting that RFRPs activate oxytocin neurones directly. Furthermore, the administration of RFRPs induced anxiety-related behaviour in rats in open-field tests. All these data taken together suggest that RFRPs play a role in the control of neuroendocrine and behavioural stress responses in rats.

Age-dependent variation in the RFRP-3 neurons is inversely correlated with gonadal activity of mice

The present study analyzed changes in the expression of RFamide-related peptide-3 (RFRP-3; a mammalian ortholog of avian gonadotropin-inhibitory hormone), in the brain and correlated it with testicular activity of mice of different age groups (day-old, 1-, 3-, 5-, 7-, 9-, 11-, 13-week and 1.5-year-old). Testicular activity after a progressive increase up to 13-week of age declined in the old mice. On the other hand, while immunoreactive (ir) RFRP-3 neurons were not seen in the day-old mice, few appeared in 1-week-old mice, their number and size increased drastically at 3-week of age. This condition remained unaltered until 7-week of age followed by a progressive decline up to the age of 13-week and thereafter increased again in the old age. The present findings indicate that hyperactivity of the ir-RFRP-3 neurons of dorsomedial nucleus of hypothalamus (DMH) observed in prepubertal mice declines in reproductively active mice and increases again in the old mice having declined reproductive performance. It is concluded that aging mice exhibits inverse correlation of RFRP-3 neurons and gonadal activity suggesting that function of RFRP-3 is not initiated until 1-week of age and thereafter it could participate in the regulation of gonadal development.

Neuropeptides in the gonads: from evolution to pharmacology

Vertebrate gonads are the sites of synthesis and binding of many peptides that were initially classified as neuropeptides. These gonadal neuropeptide systems are neither well understood in isolation, nor in their interactions with other neuropeptide systems. Further, our knowledge of the control of these gonadal neuropeptides by peripheral hormones that bind to the gonads, and which themselves are under regulation by true neuropeptide systems from the hypothalamus, is relatively meager. This review discusses the existence of a variety of neuropeptides and their receptors which have been discovered in vertebrate gonads, and the possible way in which such systems could have evolved. We then focus on two key neuropeptides for regulation of the hypothalamo-pituitary-gonadal axis: gonadotropin-releasing hormone (GnRH) and gonadotropin-inhibitory hormone (GnIH). Comparative studies have provided us with a degree of understanding as to how a gonadal GnRH system might have evolved, and they have been responsible for the discovery of GnIH and its gonadal counterpart. We attempt to highlight what is known about these two key gonadal neuropeptides, how their actions differ from their hypothalamic counterparts, and how we might learn from comparative studies of them and other gonadal neuropeptides in terms of pharmacology, reproductive physiology and evolutionary biology.

Phylogenetic aspects of gonadotropin-inhibitory hormone and its homologs in vertebrates

The decapeptide gonadotropin-releasing hormone (GnRH) is the primary factor responsible for the hypothalamic control of gonadotropin secretion in vertebrates, but a hypothalamic neuropeptide inhibiting gonadotropin secretion was, until recently, unknown in vertebrates. In 2000, we discovered a novel hypothalamic dodecapeptide that inhibits gonadotropin release in quail and termed it gonadotropin-inhibitory hormone (GnIH). GnIH acts on the pituitary and GnRH neurons in the hypothalamus via a novel G protein-coupled receptor for GnIH to inhibit gonadal development and maintenance by decreasing gonadotropin release and synthesis. The pineal hormone melatonin is a key factor controlling GnIH neural function. Because GnIH exists and functions in several avian species, GnIH is considered to be a new key neuropeptide controlling avian reproduction. After the discovery of GnIH in birds, the presence of GnIH homologs has been demonstrated in other vertebrates from fish to humans. Interestingly, mammalian GnIH homologs also act to inhibit reproduction by decreasing gonadotropin release in several mammalian species. It is concluded that GnIH and GnIH homologs act to inhibit gonadotropin release in higher vertebrates.

Molecular genetics of the developing neuroendocrine hypothalamus

Formation of the mammalian endocrine system and neuroendocrine organs involves complex regulatory networks resulting in a highly specialized cell system able to secrete a diverse array of peptide hormones. The hypothalamus is located in the mediobasal region of the brain and acts as a gateway between the endocrine and nervous systems. From an endocrinology perspective, the parvicellular neurons of the hypothalamus are of particular interest as they function as a control centre for several critical physiological processes including growth, metabolism and reproduction by regulating hormonal signaling from target cognate cell types in the anterior pituitary. Delineating the genetic program that controls hypothalamic development is essential for complete understanding of parvicellular neuronal function and the etiology of congenital disorders that result from hypothalamic-pituitary axis dysfunction. In recent years, studies have shed light on the interactions between signaling molecules and activation of transcription factors that regulate hypothalamic cell fate commitment and terminal differentiation. The aim of this review is to summarize the recent molecular and genetic findings that have advanced our understanding of the emergence of the known important hypophysiotropic signaling molecules in the hypothalamus. We have focused on reviewing the literature that provides evidence of the dependence on expression of specific genes for the normal development and function of the cells that secrete these neuroendocrine factors, as well as studies of the elaboration of the spatial or temporal patterns of changes in gene expression that drive this development.

Discovery and evolutionary history of gonadotrophin-inhibitory hormone and kisspeptin: new key neuropeptides controlling reproduction

Gonadotrophin-releasing hormone (GnRH) is the primary hypothalamic factor responsible for the control of gonadotrophin secretion in vertebrates. However, within the last decade, two other hypothalamic neuropeptides have been found to play key roles in the control of reproductive functions: gonadotrophin-inhibitory hormone (GnIH) and kisspeptin. In 2000, we discovered GnIH in the quail hypothalamus. GnIH inhibits gonadotrophin synthesis and release in birds through actions on GnRH neurones and gonadotrophs, mediated via GPR147. Subsequently, GnIH orthologues were identified in other vertebrate species from fish to humans. As in birds, mammalian and fish GnIH orthologues inhibit gonadotrophin release, indicating a conserved role for this neuropeptide in the control of the hypothalamic-pituitary-gonadal axis across species. Subsequent to the discovery of GnIH, kisspeptin, encoded by the KiSS-1 gene, was discovered in mammals. By contrast to GnIH, kisspeptin has a direct stimulatory effect on GnRH neurones via GPR54. GPR54 is also expressed in pituitary cells, but whether gonadotrophs are targets for kisspeptin remains unresolved. The KiSS-1 gene is also highly conserved and has been identified in mammals, amphibians and fish. We have recently found a second isoform of KiSS-1, designated KiSS-2, in several vertebrates, but not birds, rodents or primates. In this review, we highlight the discovery, mechanisms of action, and functional significance of these two chief regulators of the reproductive axis.

Gonadotropin-inhibitory hormone (GnIH) and its control of central and peripheral reproductive function

Identification of novel neurohormones that regulate the reproductive axis is essential for the progress of neuroendocrinology. The decapeptide gonadotropin-releasing hormone (GnRH) is the primary factor responsible for the hypothalamic control of gonadotropin secretion. Gonadal sex steroids and inhibin modulate gonadotropin secretion via feedback from the gonads, but a neuropeptide that directly inhibits gonadotropin secretion was unknown in vertebrates until 2000 when a hypothalamic dodecapeptide serving this function was discovered in quail. Because of its action on cultured pituitary in quail, it was named gonadotropin-inhibitory hormone (GnIH). GnIH acts on the pituitary and on GnRH neurons in the hypothalamus via a novel G protein-coupled receptor (GPR147). GPR74 may also be a possible candidate GnIH receptor. GnIH decreases gonadotropin synthesis and release, inhibiting gonadal development and maintenance. Melatonin stimulates the expression and release of GnIH via melatonin receptors expressed by GnIH neurons. GnIH actions and interactions with GnRH seem common not only to several avian species, but also to mammals. Thus, GnIH is considered to have an evolutionarily conserved role in controlling vertebrate reproduction, and GnIH homologs have also been identified in the hypothalamus of mammals. As in birds, mammalian GnIH homologs act to inhibit gonadotropin release in several species. More recent evidence in birds and mammals indicates that GnIH may operate at the level of the gonads as an autocrine/paracrine regulator of steroidogenesis and gametogenesis. Importantly, GnIH in birds and mammals appears to act at all levels of the hypothalamo-pituitary-gonadal (HPG) axis, and possibly over different time-frames (minutes-days). Thus, GnIH and its homologs appear to act as key neurohormones controlling vertebrate reproduction. The discovery of GnIH has enabled us to understand and manipulate vertebrate reproduction from an entirely new perspective.

Effects of gonadotropin-inhibitory hormone on folliculogenesis and steroidogenesis of cyclic mice

OBJECTIVE

To evaluate the effects of gonadotropin-inhibitory hormone (GnIH) treatment on ovarian activity of mice.

DESIGN

Animal study.

SETTING

Reproductive physiology laboratory of university department of zoology.

ANIMAL(S)

Twelve-week-old female mice of inbred Parkes strain.

INTERVENTION(S)

Mice treated with different doses of GnIH (control, 100 ng, 500 ng, and 2 μg per day) for 8 days were studied. For in vitro study, the ovaries of proestrus mice were cultured with different doses of GnIH for 24 hours at 37 °C.

MAIN OUTCOME MEASURE(S)

Folliculogenesis, steroidogenesis, luteogenesis, and apoptosis in the ovaries of control and GnIH-treated mice.

RESULT(S)

GnIH treatment produced significant changes in body mass, circulating steroid levels, and ovarian activity in the mice. GnIH also caused dose-dependent histologic changes in follicular development and luteinization. The antral follicles showed abnormal changes. The mice treated with increasing dose of GnIH showed significant changes in steroid synthesis owing to inhibitory effects of GnIH on ovarian expression of LH receptor, steroidogenic acute regulatory, and 3β-hydroxysteroid dehydrogenase proteins.

CONCLUSION(S)

GnIH inhibited follicular development and steroidogenesis in the ovary of mice. This study thus suggests biologic significance of this neuropeptide in regulating ovarian activity.

Identification, localization, and function of a novel avian hypothalamic neuropeptide, 26RFa, and its cognate receptor, G protein-coupled receptor-103

Several neuropeptides with the C-terminal RFamide sequence have been identified in the hypothalamus of a variety of vertebrates. Among the RFamide peptide groups, however, only LPXRFamide peptides, including gonadotropin-inhibitory hormone, have been characterized in the avian brain. In the present study, we sought for the presence of other RFamide peptides in the avian hypothalamus. We identified a cDNA encoding an RFamide peptide orthologous to 26RFa (also referred to as QRFP) in the hypothalamus of the Japanese quail. The deduced quail 26RFa precursor consisted of 120-amino-acid residues, encoding one RFamide peptide with 27 amino acids. This RFamide peptide was flanked at the N terminus by a dibasic amino acid cleavage site and at the C terminus by a glycine amidation signal. Quantitative RT-PCR analysis demonstrated specific expression of quail 26RFa mRNA in the diencephalon including the hypothalamus. Furthermore, mass spectrometry analysis revealed the presence of a peptide exhibiting the mass of mature 26RFa, indicating that the peptide is actually produced from the precursor in the diencephalon. 26RFa-producing cell bodies were localized in the anterior hypothalamic nucleus in the brain. Synthetic 26RFa increased intracellular Ca(2+) concentration in HEK293T cells transfected with the chicken G protein-coupled receptor GPR103. Intracerebroventricular injection of 26RFa in broiler chicks stimulated feeding behavior. These data provide the first evidence for the occurrence of the peptide 26RFa in the avian hypothalamus and indicate that this peptide exerts orexigenic activity.

A review of FMRFamide- and RFamide-like peptides in metazoa

Neuropeptides are a diverse class of signalling molecules that are widely employed as neurotransmitters and neuromodulators in animals, both invertebrate and vertebrate. However, despite their fundamental importance to animal physiology and behaviour, they are much less well understood than the small molecule neurotransmitters. The neuropeptides are classified into families according to similarities in their peptide sequence; and on this basis, the FMRFamide and RFamide-like peptides, first discovered in molluscs, are an example of a family that is conserved throughout the animal phyla. In this review, the literature on these neuropeptides has been consolidated with a particular emphasis on allowing a comparison between data sets in phyla as diverse as coelenterates and mammals. The intention is that this focus on the structure and functional aspects of FMRFamide and RFamide-like neuropeptides will inform understanding of conserved principles and distinct properties of signalling across the animal phyla.

The role of the terminal nerve and GnRH in olfactory system neuromodulation

Animals must regulate their sensory responsiveness appropriately with respect to their internal and external environments, which is accomplished in part via centrifugal modulatory pathways. In the olfactory sensory system, responsiveness is regulated by neuromodulators released from centrifugal fibers into the olfactory epithelium and bulb. Among the modulators known to modulate neural activity of the olfactory system, one of the best understood is gonadotropin-releasing hormone (GnRH). This is because GnRH derives mainly from the terminal nerve (TN), and the TN-GnRH system has been suggested to function as a neuromodulator in wide areas of the brain, including the olfactory bulb. In the present article we examine the modulatory roles of the TN and GnRH in the olfactory epithelium and bulb as a model for understanding the ways in which olfactory responses can be tuned to the internal and external environments.

Neuroendocrinology of reproduction in teleost fish

This review aims at synthesizing the most relevant information regarding the neuroendocrine circuits controlling reproduction, mainly gonadotropin release, in teleost fish. In teleosts, the pituitary receives a more or less direct innervation by neurons sending projections to the vicinity of the pituitary gonadotrophs. Among the neurotransmitters and neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRH) and dopamine, acting as stimulatory and inhibitory factors (in many but not all fish) on the liberation of LH and to a lesser extent that of FSH. The activity of the corresponding neurons depends on a complex interplay between external and internal factors that will ultimately influence the triggering of puberty and sexual maturation. Among these factors are sex steroids and other peripheral hormones and growth factors, but little is known regarding their targets. However, very recently a new actor has entered the field of reproductive physiology. KiSS1, first known as a tumor suppressor called metastin, and its receptor GPR54, are now central to the regulation of GnRH, and consequently LH and FSH secretion in mammals. The KiSS system is notably viewed as instrumental in integrating both environmental cues and metabolic signals and passing this information onto the reproductive axis. In fish, there are two KiSS genes, KiSS1 and KiSS2, expressed in neurons of the preoptic area and mediobasal hypothalamus. Pionneer studies indicate that KiSS and GPR54 expression seem to be activated at puberty. Although precise information as to the physiological effects of KiSS1 in fish, notably on GnRH neurons and gonadotropin release, is still limited, KiSS neurons may emerge as the "gatekeeper" of puberty and reproduction in fish as in mammals.

Melatonin stimulates the release of gonadotropin-inhibitory hormone by the avian hypothalamus

Gonadotropin-inhibitory hormone (GnIH), a neuropeptide that inhibits gonadotropin synthesis and release, was first identified in quail hypothalamus. GnIH acts on the pituitary and GnRH neurons in the hypothalamus via GnIH receptor to inhibit gonadal development and maintenance. In addition, GnIH neurons express melatonin receptor and melatonin induces GnIH expression in the quail brain. Thus, it seems that melatonin is a key factor controlling GnIH neural function. In the present study, we investigated the role of melatonin in the regulation of GnIH release and the correlation of GnIH release with LH release in quail. Melatonin administration dose-dependently increased GnIH release from hypothalamic explants in vitro. GnIH release was photoperiodically controlled. A clear diurnal change in GnIH release was observed in quail, and this change was negatively correlated with changes in plasma LH concentrations. GnIH release during the dark period was greater than that during the light period in explants from quail exposed to long-day photoperiods. Conversely, plasma LH concentrations decreased during the dark period. In contrast to LD, GnIH release increased under short-day photoperiods, when the duration of nocturnal secretion of melatonin increases. These results indicate that melatonin may play a role in stimulating not only GnIH expression but also GnIH release, thus inhibiting plasma LH concentrations in quail. This is the first report describing the effect of melatonin on neuropeptide release.

Identification of human GnIH homologs, RFRP-1 and RFRP-3, and the cognate receptor, GPR147 in the human hypothalamic pituitary axis

The existence of a hypothalamic gonadotropin-inhibiting system has been elusive. A neuropeptide named gonadotropin-inhibitory hormone (GnIH, SIKPSAYLPLRF-NH(2)) which directly inhibits gonadotropin synthesis and release from the pituitary was recently identified in quail hypothalamus. Here we identify GnIH homologs in the human hypothalamus and characterize their distribution and biological activity. GnIH homologs were isolated from the human hypothalamus by immunoaffinity purification, and then identified as MPHSFANLPLRF-NH(2) (human RFRP-1) and VPNLPQRF-NH(2) (human RFRP-3) by mass spectrometry. Immunocytochemistry revealed GnIH-immunoreactive neuronal cell bodies in the dorsomedial region of the hypothalamus with axonal projections to GnRH neurons in the preoptic area as well as to the median eminence. RT-PCR and subsequent DNA sequencing of the PCR products identified human GnIH receptor (GPR147) mRNA expression in the hypothalamus as well as in the pituitary. In situ hybridization further identified the expression of GPR147 mRNA in luteinizing hormone producing cells (gonadotropes). Human RFRP-3 has recently been shown to be a potent inhibitor of gonadotropin secretion in cultured sheep pituitary cells by inhibiting Ca(2+) mobilization. It also directly modulates GnRH neuron firing. The identification of two forms of GnIH (RFRP-1 and RFRP-3) in the human hypothalamus which targets human GnRH neurons and gonadotropes and potently inhibit gonadotropin in sheep models provides a new paradigm for the regulation of hypothalamic-pituitary-gonadal axis in man and a novel means for manipulating reproductive functions.

Gonadotropin-inhibitory hormone receptor signaling and its impact on reproduction in chickens

In birds, as in other vertebrates, reproduction is controlled by the hypothalamo-pituitary-gonadal axis with each component secreting specific neuropeptides or hormones. Until recently, it was believed this axis is exclusively under the stimulatory control of hypothalamic gonadotropin-releasing hormone I (GnRH-I) which in turn, stimulates luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretion from the pituitary gland. However, the discovery of a novel inhibitory hypothalamic peptide able to reduce LH secretion (gonadotropin-inhibitory hormone: GnIH) challenged this dogma. Furthermore, with the characterization of its specific receptor (GnIHR), progress has been made to clarify the physiological relevance of GnIH in birds. This short review discusses the recent advances in GnIHR signaling at the level of the pituitary gland and the gonads. GnIHR is a member of the G-protein coupled receptor (GPCR) family which couples to G(alphai) and, upon activation inhibits adenylyl cyclase (AC) activity, thus reducing intracellular cAMP levels. This implies that GnIH interferes with signaling of any GPCR coupled to G(alphas), including GnRH, LH and FSH receptors. In the chicken pituitary gland, the GnRHR-II/GnIHR ratio changes during sexual maturation in favor of GnRHR-II that appears to result in hypothalamic control of gonadotropin secretion shifting from inhibitory to stimulatory, with corresponding changes in GnRH-induced cAMP levels. Within the gonads, GnIH and its receptor may act in an autocrine/paracrine manner and may interfere with LH and FSH signaling to influence ovarian follicular maturation and recruitment, as well as spermatogenesis.

Evidence that RF-amide related peptides are inhibitors of reproduction in mammals

Gonadotropin releasing hormone (GnRH) secretion represents the final common pathway in the control of the reproductive axis. Dogma has been that GnRH is solely responsible for the control of gonadotropin secretion, but emerging data presents a strong case for the existence of a gonadotropin inhibitory hormone in mammals. This evidence arose from initial work in avian species to isolate and identify a factor that inhibited gonadotropin release, which is known as gonadotropin inhibitory hormone (GnIH). The mammalian ortholog of avian GnIH is named RF-amide related peptide (RFRP). There are two forms of RFRP in mammals, RFRP-1 and RFRP-3 encoded by a single gene, but there has been skepticism and controversy as to whether these peptides play a significant role in the regulation of gonadotropin secretion. There is now a significant body of evidence that one or more RFRP exists in mammals and acts as an inhibitor of GnRH and/or gonadotropin secretion. Moreover, RFRP-producing neurons have been shown to transmit information to GnRH cells and/or gonadotropes in relation to seasonal status and to coordinate events around the preovulatory luteinizing hormone surge. This review will focus on the significant advances in RFRP research in mammalian species.

Bovine C-terminal octapeptide of RFamide-related peptide-3 suppresses luteinizing hormone (LH) secretion from the pituitary as well as pulsatile LH secretion in bovines

Gonadotropin-inhibiting hormone (GnIH), observed in quail as a member of the RFamide neuropeptide family, suppresses luteinizing hormone (LH) secretion from the avian pituitary. Rats and cattle have an active gene of another member of the RFamide neuropeptide family, termed RFamide-related peptide-3 (RFRP-3), although bovine RFRP-3 is different from that of rats in both length and amino-acid sequence. A single injection of GnIH or RFRP-3 inhibited LH secretion in rodents, which continued for various periods. This study was conducted to evaluate the effects of bovine C-terminal octapeptide of RFRP-3 (RFRP-3-8) on LH secretion from cultured anterior pituitary (AP) cells of cattle, and the effects of RFRP-3-8 injections on pulsatile LH secretion in castrated male calves. The suppressive effect of RFRP-3-8 on LH secretion from AP cells was observed in the presence of gonadotropin-releasing hormone (GnRH), but not in the absence of GnRH in culture media. In another experiment collecting blood samples serially from castrated male calves with repeated intravenous injections of RFRP-3-8 (n=6) or saline (n=6), the RFRP-3-8 group showed suppressed LH pulse frequency during the injection period (P<0.05); however, the RFRP-3-8 group showed no difference from the saline group in all measures of LH secretion in the postinjection period. In conclusion, our results suggested that RFRP-3-8 suppresses LH secretion from cultured AP cells, as well as LH pulse frequency in cattle.

A new key neurohormone controlling reproduction, gonadotrophin-inhibitory hormone in birds: discovery, progress and prospects

In vertebrates, the neuropeptide control of gonadotrophin secretion is primarily through the stimulatory action of the hypothalamic decapeptide, gonadotrophin-releasing hormone (GnRH). Gonadal sex steroids and inhibin inhibit gonadotrophin secretion via feedback from the gonads, but a hypothalamic neuropeptide inhibiting gonadotrophin secretion was, until recently, unknown in vertebrates. In 2000, we discovered a novel hypothalamic dodecapeptide that directly inhibits gonadotrophin release in quail and termed it gonadotrophin-inhibitory hormone (GnIH). GnIH acts on the pituitary and GnRH neurones in the hypothalamus via a novel G-protein-coupled receptor for GnIH to inhibit gonadal development and maintenance by decreasing gonadotrophin release and synthesis. The pineal hormone melatonin is a key factor controlling GnIH neural function. GnIH occurs in the hypothalamus of several avian species and is considered to be a new key neurohormone inhibiting avian reproduction. Thus, the discovery of GnIH provides novel directions to investigate neuropeptide regulation of reproduction. This review summarises the discovery, progress and prospects of GnIH, a new key neurohormone controlling reproduction.

Evolutionary origin and divergence of GnIH and its homologous peptides

Probing undiscovered hypothalamic neuropeptides that play important roles in the regulation of pituitary function in vertebrates is essential for the progress of neuroendocrinology. In 2000, we discovered a novel hypothalamic dodecapeptide inhibiting gonadotropin release in quail and termed it gonadotropin-inhibitory hormone (GnIH). GnIH acts on the pituitary and gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus via a novel G protein-coupled receptor for GnIH to inhibit gonadal development and maintenance by decreasing gonadotropin release and synthesis. Similar findings were observed in other avian species. Thus, GnIH is a key factor controlling avian reproduction. To give our findings a broader perspective, we also found GnIH homologous peptides in the hypothalamus of other vertebrates, such as mammals, reptiles, amphibians and teleosts. GnIH and its homologs share a common C-terminal LPXRFamide (X=L or Q) motif. A mammalian GnIH homolog also inhibited gonadotropin release in mammals like the GnIH action in birds. In contrast to higher vertebrates, hypophysiotropic activities of GnIH homologs were different in lower vertebrates. To clarify the evolutionary origin of GnIH and its homologs, we further sought to identify novel LPXRFamide peptides from the brain of sea lamprey and hagfish, two extant groups of the oldest lineage of vertebrates, Agnatha. In these agnathans, LPXRFamide peptide and its cDNA were identified only from the brain of hagfish. Based on these findings over the past decade, this paper summarizes the evolutionary origin and divergence of GnIH and its homologous peptides.

A new key neurohormone controlling reproduction, gonadotropin-inhibitory hormone (GnIH): Biosynthesis, mode of action and functional significance

Identification of novel neurohormones that play important roles in the regulation of pituitary function is essential for the progress of neurobiology. The decapeptide gonadotropin-releasing hormone (GnRH) is the primary factor responsible for the hypothalamic control of gonadotropin secretion. Gonadal sex steroids and inhibin inhibit gonadotropin secretion via feedback from the gonads, but a neuropeptide inhibitor of gonadotropin secretion was, until recently, unknown in vertebrates. In 2000, a novel hypothalamic dodecapeptide that inhibits gonadotropin release was identified in quail and termed gonadotropin-inhibitory hormone (GnIH). This was the first demonstration of a hypothalamic neuropeptide inhibiting gonadotropin release in any vertebrate. GnIH acts on the pituitary and GnRH neurons in the hypothalamus via a novel G protein-coupled receptor for GnIH to inhibit gonadal development and maintenance by decreasing gonadotropin release and synthesis. GnIH neurons express the melatonin receptor and melatonin stimulates the expression of GnIH. Because GnIH exists and functions in several avian species, GnIH is considered to be a new key neurohormone controlling avian reproduction. From a broader perspective, subsequently the presence of GnIH homologous peptides has been demonstrated in other vertebrates. Mammalian GnIH homologous peptides also act to inhibit reproduction by decreasing gonadotropin release in several mammalian species. Thus, the discovery of GnIH has opened the door to a new research field in reproductive neurobiology. This review summarizes the advances made in our understanding of the biosynthesis, mode of action and functional significance of GnIH, a newly discovered key neurohormone, and its homologous peptides.

RFamide-related peptide and its cognate receptor in the sheep: cDNA cloning, mRNA distribution in the hypothalamus and the effect of photoperiod

Photoperiodic responses enable animals to adapt their physiology to predictable patterns of seasonal environmental change. In mammals, this depends on pineal melatonin secretion and effects in the hypothalamus, but the cellular and molecular substrates of its action are poorly understood. The recent identification of a mammalian orthologue of the avian gonadotrophin-inhibitory hormone gene has led to interest in its possible involvement in seasonal breeding. In long-day breeding Syrian hamsters, hypothalamic RFamide-related peptide (RFRP) expression is increased by exposure to long photoperiod. Because, opposite to hamsters, sheep are short-day breeders, we predicted that a conserved role in mammalian reproductive activation would decrease RFRP expression in sheep under a long photoperiod. We cloned the ovine RFRP cDNA and examined its expression pattern in Soay sheep acclimated to a 16 : 8 h or 8 : 16 h light /dark cycle (LP and SP, respectively). RFRP was expressed widely in the sheep hypothalamus and increased modestly overall with exposure to LP. Interestingly, RFRP expression in the ependymal cells surrounding the base of the third ventricle was highly photoperiodic, with levels being undetectable in animals held on SP but consistently high under LP. These data are inconsistent with a conserved reproductive role for RFRP across mammals. Additionally, we cloned the ovine homologue of the cognate RFRP receptor, rfr-2 (NPFF1) and found localised expression in the suprachiasmatic nuclei and in the pars tuberalis. Taken together, these data strengthen the emerging view that interplay between ependymal cells and the pars tuberalis might be important for the seasonal timing system.

Changes in GnRH I, bradykinin and their receptors and GnIH in the ovary of Calotes versicolor during reproductive cycle

The aim of this study was to investigate changes in the abundance of gonadotrophin releasing hormone I (GnRH I) and GnRH I receptor in the ovary of Calotes versicolor during the reproductive cycle and correlate them with the changes in gonadotrophin inhibitory hormone (GnIH), bradykinin and bradykinin B(2) receptor in order to understand their interaction during ovarian cycle. GnRH I, bradykinin and their receptors and GnIH, were localized immunohistochemically in the ovary. Relative intensity of these peptides was estimated from the contralateral ovary using slot/Western blot followed by densitometry. The immunostaining of GnRH I, bradykinin and their receptors and GnIH were localized in the granulosa cells of previtellogenic follicles and stroma cells, whereas in the peripheral part of the cytoplasm in oocytes of vitellogenic and ovulatory follicles. The GnRH I immunostaining was relatively higher in inactive phase, but was low during active preovulatory phase suggesting inverse correlation with circulating estradiol level. The study showed a positive correlation between the expression pattern of GnRH I and GnIH, but showed a negative correlation between GnIH with GnRH I receptor in the ovary. This study further suggests a possibility for bradykinin regulating GnRH I synthesis in the ovary. An increase in the immunostaining of both GnRH I and GnIH in the oocyte prior to ovulation suggests their involvement in the oocyte maturation. It is thus concluded that the ovary of C. versicolor possesses GnRH I-GnIH-bradykinin system and interaction between these neuropeptides may be involved in the regulation of follicular development and oocyte maturation.

Potent action of RFamide-related peptide-3 on pituitary gonadotropes indicative of a hypophysiotropic role in the negative regulation of gonadotropin secretion

We identified a gene in the ovine hypothalamus encoding for RFamide-related peptide-3 (RFRP-3), and tested the hypothesis that this system produces a hypophysiotropic hormone that inhibits the function of pituitary gonadotropes. The RFRP-3 gene encodes for a peptide that appears identical to human RFRP-3 homolog. Using an antiserum raised against RFRP-3, cells were localized to the dorsomedial hypothalamic nucleus/paraventricular nucleus of the ovine brain and shown to project to the neurosecretory zone of the ovine median eminence, predicating a role for this peptide in the regulation of anterior pituitary gland function. Ovine RFRP-3 peptide was tested for biological activity in vitro and in vivo, and was shown to reduce LH and FSH secretion in a specific manner. RFRP-3 potently inhibited GnRH-stimulated mobilization of intracellular calcium in gonadotropes. These data indicate that RFRP-3 is a specific and potent mammalian gonadotropin-inhibiting hormone, and that it acts upon pituitary gonadotropes to reduce GnRH-stimulated gonadotropin secretion.

Recent advances in reproductive neuroendocrinology: a role for RFamide peptides in seasonal reproduction?

Most temperate-zone species use photoperiod to coordinate breeding and ensure that offspring are born during favourable conditions. Although photoperiodic influences on the reproductive axis have been well characterized, the precise mechanisms by which photoperiodic information and other seasonal cues are integrated to regulate reproductive function remain less well specified. Two recently discovered neuropeptides, kisspeptin and gonadotropin-inhibitory hormone, have pronounced opposing influences on reproductive function. This paper will review recent evidence for a role of these peptides in seasonal reproduction and propose a theoretical framework by which these novel regulatory peptides may serve to regulate seasonal breeding. Understanding the mechanisms regulating appropriate changes in reproductive status will serve to advance a wide range of life science disciplines.

Gonadotrophin-inhibitory hormone receptor expression in the chicken pituitary gland: potential influence of sexual maturation and ovarian steroids

Gonadotrophin-inhibitory hormone (GnIH), a hypothalamic RFamide, has been found to inhibit gonadotrophin secretion from the anterior pituitary gland originally in birds and, subsequently, in mammalian species. The gene encoding a transmembrane receptor for GnIH (GnIHR) was recently identified in the brain, pituitary gland and gonads of song bird, chicken and Japanese quail. The objectives of the present study are to characterise the expression of GnIHR mRNA and protein in the chicken pituitary gland, and to determine whether sexual maturation and gonadal steroids influence pituitary GnIHR mRNA abundance. GnIHR mRNA quantity was found to be significantly higher in diencephalon compared to either anterior pituitary gland or ovaries. GnIHR mRNA quantity was significantly higher in the pituitaries of sexually immature chickens relative to sexually mature chickens. Oestradiol or a combination of oestradiol and progesterone treatment caused a significant decrease in pituitary GnIHR mRNA quantity relative to vehicle controls. GnIHR-immunoreactive (ir) cells were identified in the chicken pituitary gland cephalic and caudal lobes. Furthermore, GnIHR-ir cells were found to be colocalised with luteinising hormone (LH)beta mRNA-, or follicle-stimulating hormone (FSH)beta mRNA-containing cells. GnIH treatment significantly decreased LH release from anterior pituitary gland slices collected from sexually immature, but not from sexually mature chickens. Taken together, GnIHR gene expression is possibly down regulated in response to a surge in circulating oestradiol and progesterone levels as the chicken undergoes sexual maturation to allow gonadotrophin secretion. Furthermore, GnIHR protein expressed in FSHbeta or LHbeta mRNA-containing cells is likely to mediate the inhibitory effect of GnIH on LH and FSH secretion.

The control of reproductive physiology and behavior by gonadotropin-inhibitory hormone

Gonadotropin-releasing hormone (GnRH) controls the reproductive physiology and behavior of vertebrates by stimulating synthesis and release of gonadotropin from the pituitary gland. In 2000, another hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), was discovered in quail and found to be an inhibiting factor for gonadotropin release. GnIH homologs are present in the brains of vertebrates, including birds, mammals, amphibians, and fish. These peptides, categorized as RF amide-related peptides (RFRPs), possess a characteristic LPXRF-amide (X = L or Q) motif at their C-termini. GnIH/RFRP precursor mRNA encodes a polypeptide that is possibly cleaved into three mature peptides in birds and two in mammals. The names of these peptides are GnIH, GnIH-related peptide-1 (GnIH-RP-1) and GnIH-RP-2 in birds, and RFRP-1 and RFRP-3 in mammals. GnIH/RFRP is synthesized in neurons of the paraventricular nucleus of the hypothalamus in birds and the dorsomedial hypothalamic area in mammals. GnIH neurons project to the median eminence, thus providing a functional neuroanatomical infrastructure to regulate anterior pituitary function. In quail, GnIH inhibits gonadal activity by decreasing synthesis and release of gonadotropin. The widespread distribution of GnIH/RFRP immunoreactive fibers in all animals tested suggests various actions within the brain. In accordance, GnIH/RFRP receptor mRNA is also expressed widely in the brain and the pituitary. GnIH/RFRP immunoreactive axon terminals are in probable contact with GnRH neurons in birds and mammals, and we recently demonstrated expression of GnIH receptor mRNA in GnRH-I and GnRH-II neurons in European starlings. Thus, GnIH/RFRP may also inhibit gonadotropin synthesis and release by inhibiting GnRH neurons in addition to having direct actions on the pituitary gland. Intracerebroventricular administration of GnIH/RFRP further inhibits reproductive behaviors in songbirds and rodents, possibly via direct actions on the GnRH system. The expression of GnIH/RFRP is regulated by melatonin which is an internal indicator of day length in vertebrates. Stress stimuli also regulate the expression of GnIH/RFRP in songbirds and rodents. Accordingly, GnIH/RFRP may serve as a transducer of environmental information and social interactions into endogenous physiology and behavior of the animal. Recently, it was shown that GnIH/RFRP and its receptor are also expressed in the gonads of birds, rodents and primates. In sum, the existing data suggest that GnIH/RFRP is an important mediator of reproductive function acting at the level of the brain, pituitary, and the gonad in birds and mammals.

Gonadotropin-inhibitory hormone neurons interact directly with gonadotropin-releasing hormone-I and -II neurons in European starling brain

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic dodecapeptide (SIKPSAYLPLRF-NH(2)) that directly inhibits gonadotropin synthesis and release from quail pituitary. The action of GnIH is mediated by a novel G-protein coupled receptor. This gonadotropin-inhibitory system may be widespread in vertebrates, at least birds and mammals. In these higher vertebrates, histological evidence suggests contact of GnIH immunoreactive axon terminals with GnRH neurons, thus indicating direct regulation of GnRH neuronal activity by GnIH. In this study we investigated the interaction of GnIH and GnRH-I and -II neurons in European starling (Sturnus vulgaris) brain. Cloned starling GnIH precursor cDNA encoded three peptides that possess characteristic LPXRF-amide (X = L or Q) motifs at the C termini. Starling GnIH was further identified as SIKPFANLPLRF-NH(2) by mass spectrometry combined with immunoaffinity purification. GnIH neurons, identified by in situ hybridization and immunocytochemistry (ICC), were clustered in the hypothalamic paraventricular nucleus. GnIH immunoreactive fiber terminals were present in the external layer of the median eminence in addition to the preoptic area and midbrain, where GnRH-I and GnRH-II neuronal cell bodies exist, respectively. GnIH axon terminals on GnRH-I and -II neurons were shown by GnIH and GnRH double-label ICC. Furthermore, the expression of starling GnIH receptor mRNA was identified in both GnRH-I and GnRH-II neurons by in situ hybridization combined with GnRH ICC. The cellular localization of GnIH receptor has not previously been identified in any vertebrate brain. Thus, GnIH may regulate reproduction of vertebrates by directly modulating GnRH-I and GnRH-II neuronal activity, in addition to influencing the pituitary gland.

The general and comparative biology of gonadotropin-inhibitory hormone (GnIH)

The decapeptide gonadotropin-releasing hormone (GnRH) is the primary factor responsible for the hypothalamic control of gonadotropin secretion. Gonadal sex steroids and inhibin inhibit gonadotropin secretion via feedback from the gonads, but a neuropeptide inhibitor of gonadotropin secretion was, until recently, unknown in vertebrates. In 2000, we identified a novel hypothalamic dodecapeptide that inhibits gonadotropin release in cultured quail pituitaries and termed it gonadotropin-inhibitory hormone (GnIH). To elucidate the mode of action of GnIH, we then identified a novel G protein-coupled receptor for GnIH in quail. The GnIH receptor possesses seven transmembrane domains and specifically binds to GnIH. The GnIH receptor is expressed in the pituitary and several brain regions including the hypothalamus. These results indicate that GnIH acts directly on the pituitary via GnIH receptor to inhibit gonadotropin release. GnIH may also act on the hypothalamus to inhibit GnRH release. To demonstrate the functional significance of GnIH and its potential role as a key regulatory neuropeptide in avian reproduction, we investigated GnIH actions on gonadal development and maintenance in quail. Chronic treatment with GnIH inhibited gonadal development and maintenance by decreasing gonadotropin synthesis and release. GnIH was also found in the hypothalamus of other avian species including sparrows and chickens and also inhibited gonadotropin synthesis and release. The pineal hormone melatonin may be a key factor controlling GnIH neural function, since quail GnIH neurons express melatonin receptor and melatonin treatment stimulates the expression of GnIH mRNA and mature GnIH peptide. Thus, GnIH is capable of transducing photoperiodic information via changes in the melatonin signal, thereby influencing the reproductive axis. It is concluded that GnIH, a newly discovered hypothalamic neuropeptide, is a key factor controlling avian reproduction. The discovery of avian GnIH opens a new research field in reproductive neuroendocrinology.

Rat RFamide-related peptide-3 stimulates GH secretion, inhibits LH secretion, and has variable effects on sex behavior in the adult male rat

A recently described avian neuropeptide, gonadotropin inhibitory hormone (GnIH), has been shown to have seasonal regulatory effects on the hypothalamic-pituitary-gonadotropin axis (HPG) in several avian species. In the bird, GnIH expression is increased during the photorefractory period and has inhibitory effects on the HPG. A recently described mammalian neuropeptide, RF-amide-related peptide-3 (RFRP-3), may be genetically related and functionally similar to this avian neuropeptide. The purposes of this study were to first see if rat RFRP-3 is expressed in the male rat brain and second to determine if ICV injections of RFRP-3 will have effects on feeding and sex behaviors, as well as hormone release from the anterior pituitary. Results confirm other studies in that immunoreactive cell bodies and fibers are observable in areas of the male rat brain known to control the HPG and feeding and sex behaviors. RFRP-3 fibers are also observed in close proximity to GnRH immunoreactive cell bodies. Behavioral tests indicate that high but not low ICV RFRP-3 (500 vs. 100 ng, respectively) significantly (p<0.05) suppressed all facets of male sex behavior while not having any observable effects on their ability to ambulate. Sex behavior was later exhibited when those same male rats received the ICV vehicle. While suppressing sex behavior, ICV RFRP-3 significantly (p<0.05) increased food intake compared to controls. ICV RFRP-3 also significantly reduced plasma levels of luteinizing hormone but increased growth hormone regardless of the time of day; however, at no time did RFRP-3 alter plasma levels of FSH, thyroid hormone, or cortisol. These results indicate that although RFRP-3 has similar effects on LH as observed with GnIH in avian species, in the rat RFRP-3 has additional roles in regulating feeding and growth.

Driving reproduction: RFamide peptides behind the wheel

The availability of tools for probing the genome and proteome more efficiently has allowed for the rapid discovery of novel genes and peptides that play important, previously uncharacterized roles in neuroendocrine regulation. In this review, the role of a class of neuropeptides containing the C-terminal Arg-Phe-NH(2) (RFamide) in regulating the reproductive axis will be highlighted. Neuropeptides containing the C-terminal Phe-Met-Arg-Phe-NH(2) (FMRFamide) were first identified as cardioregulatory elements in the bi-valve mollusk Macrocallista nimbosa. During the past two decades, numerous studies have shown the presence of structurally similar peptides sharing the RFamide motif across taxa. In vertebrates, RFamide peptides have pronounced influences on opiatergic regulation and neuroendocrine function. Two key peptides in this family are emerging as important regulators of the reproductive axis, kisspeptin and gonadotropin-inhibitory hormone (GnIH). Kisspeptin acts as the accelerator, directly driving gonadotropin-releasing hormone (GnRH) neurons, whereas GnIH acts as the restraint. Recent evidence suggests that both peptides play a role in mediating the negative feedback effects of sex steroids. This review presents the hypothesis that these peptides share complementary roles by responding to internal and external stimuli with opposing actions to precisely regulate the reproductive axis.

Mode of action and functional significance of avian gonadotropin-inhibitory hormone (GnIH): a review

Neuropeptide control of gonadotropin secretion at the level of the anterior pituitary gland is primarily through the stimulatory action of the hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH). However, a hypothalamic neuropeptide acting at the level of the pituitary to negatively regulate gonadotropin secretion has, until recently, remained unknown in any vertebrate. In 2000, we discovered a novel hypothalamic neuropeptide inhibiting gonadotropin release at the level of the pituitary in quail and termed it gonadotropin-inhibitory hormone (GnIH). A gonadotropin-inhibitory system is an intriguing concept and provides us with an unprecedented opportunity to study the regulation of avian reproduction from an entirely novel standpoint. To elucidate the mode of action of GnIH, we further identified the receptor for GnIH and characterized its expression and binding activity in quail. The identified GnIH receptor possessed seven transmembrane domains and specifically bound to GnIH in a concentration-dependent manner. The expression of GnIH receptor was found in the pituitary and several brain regions including the hypothalamus. These results suggest that GnIH acts directly on the pituitary via GnIH receptor to inhibit gonadotropin release. GnIH may also act on the hypothalamus to inhibit GnRH release. To understand the functional significance of GnIH in avian reproduction, we also investigated the mechanism that regulates GnIH expression. Interestingly, melatonin induced dose-dependently GnIH expression and melatonin receptor (Mel(1c)) was expressed in GnIH neurons. Thus melatonin appears to act directly on GnIH neurons via its receptor to induce GnIH expression. Based on these studies, GnIH is likely an important neuropeptide for the regulation of avian reproduction.