Distribution of 26RFa binding sites and GPR103 mRNA in the central nervous system of the rat

Brain RFamide Neuropeptides in Stress-Related Psychopathologies

The RFamide peptide family is a group of proteins that share a common C-terminal arginine-phenylalanine-amide motif. To date, the family comprises five groups in mammals: neuropeptide FF, LPXRFamides/RFamide-related peptides, prolactin releasing peptide, QRFP, and kisspeptins. Different RFamide peptides have their own cognate receptors and are produced by different cell populations, although they all can also bind to neuropeptide FF receptors with different affinities. RFamide peptides function in the brain as neuropeptides regulating key aspects of homeostasis such as energy balance, reproduction, and cardiovascular function. Furthermore, they are involved in the organization of the stress response including modulation of pain. Considering the interaction between stress and various parameters of homeostasis, the role of RFamide peptides may be critical in the development of stress-related neuropathologies. This review will therefore focus on the role of RFamide peptides as possible key hubs in stress and stress-related psychopathologies. The neurotransmitter coexpression profile of RFamide-producing cells is also discussed, highlighting its potential functional significance. The development of novel pharmaceutical agents for the treatment of stress-related disorders is an ongoing need. Thus, the importance of RFamide research is underlined by the emergence of peptidergic and G-protein coupled receptor-based therapeutic targets in the pharmaceutical industry.

The role of QRFP43 in the secretory activity of the gonadotrophic axis in female sheep

In mammals reproduction is regulated by many factors, among others by the peptides belonging to the RFamide peptide family. However, the knowledge concerning on the impact of recently identified member of this family (QRFP43) on the modulation of the gonadotrophic axis activity is still not fully understood and current research results are ambiguous. In the present study we tested the in vivo effect of QRFP43 on the secretory activity of the gonadotrophic axis at the hypothalamic-pituitary level in Polish Merino sheep. The animals (n = 48) were randomly divided into three experimental groups: controls receiving an icv infusion of Ringer-Locke solution, group receiving icv infusion of QRFP43 at 10 μg per day and 50 μg per day. All sheep received four 50 min icv infusions at 30 min intervals, on each of three consecutive days. Hypothalamic and pituitaries were collected and secured for further immunohistochemical and molecular biological analysis. In addition, during the experiment a blood samples have been collected for subsequent RIA determinations. QRFP43 was found to downregulate Kiss mRNA expression in the MBH and reduce the level of IR material in ME. This resulted in a reduction of GnRH IR material in the ME. QRFP43 increased plasma FSH levels while decreasing LH levels. Our findings indicate that QRFP43 inhibits the activity of the gonadotropic axis in the ovine at the level of the hypothalamus and may represent another neuromodulator of reproductive processes in animals.

The 26RFa (QRFP)/GPR103 Neuropeptidergic System: A Key Regulator of Energy and Glucose Metabolism

BACKGROUND

Obesity and type 2 diabetes are strongly associated pathologies, currently considered as a worldwide epidemic problem. Understanding the mechanisms that drive the development of these diseases would enable to develop new therapeutic strategies for their prevention and treatment. Particularly, the role of the brain in energy and glucose homeostasis has been studied for 2 decades. In specific, the hypothalamus contains well-identified neural networks that regulate appetite and potentially also glucose homeostasis. A new concept has thus emerged, suggesting that obesity and diabetes could be due to a dysfunction of the same, still poorly understood, neural networks.

SUMMARY

The neuropeptide 26RFa (also termed QRFP) belongs to the family of RFamide regulatory peptides and has been identified as the endogenous ligand of the human G protein-coupled receptor GPR103 (QRFPR). The primary structure of 26RFa is strongly conserved during vertebrate evolution, suggesting its crucial roles in the control of vital functions. Indeed, the 26RFa/GPR103 peptidergic system is reported to be involved in the control of various neuroendocrine functions, notably the control of energy metabolism in which it plays an important role, both centrally and peripherally, since 26RFa regulates feeding behavior, thermogenesis and lipogenesis. Moreover, 26RFa is reported to control glucose homeostasis both peripherally, where it acts as an incretin, and centrally, where the 26RFa/GPR103 system relays insulin signaling in the brain to control glucose metabolism.

KEY MESSAGES

This review gives a comprehensive overview of the role of the 26RFa/GPR103 system as a key player in the control of energy and glucose metabolism. In a pathophysiological context, this neuropeptidergic system represents a prime therapeutic target whose mechanisms are highly relevant to decipher.

Interactions between the regulatory peptide 26RFa (QRFP) and insulin in the regulation of glucose homeostasis in two complementary models: The high fat 26RFa-deficient mice and the streptozotocin insulin-deficient mice

The regulatory peptide 26RFa (QRFP) is involved in the control of glucose homeostasis at the periphery by acting as an incretin, and in the brain by mediating the central antihyperglycemic effect of insulin, indicating the occurrence of a close relationship between 26RFa and insulin in the regulation of glucose metabolism. Here, we investigated the physiological interactions between 26RFa and insulin in two complementary models i.e. a model of obese/hyperglycemic mice deficient for 26RFa and a model of diabetic mice deficient for insulin. For this, transgenic 26RFa-deficient mice were made obese and chronically hyperglycemic by a 3-month high fat diet (HFD) and second group of mice was made diabetic by destruction of the β cells of the pancreatic islets using a single injection of streptozotocin. Our data reveal that 26RFa deficiency does not impact significantly the "glycemic" phenotype of the HFD mice. The pancreatic islets, liver, white adipose tissue masses are not altered by the lack of 26RFa production but the brown adipose tissue (BAT) weight is significantly increased in these animals. In diabetic insulin-deficient mice, the injection of 26RFa does not exhibit any beneficial effect on the impaired glucose homeostasis characterizing this model. Finally, we show that streptozotocin diabetic mice display lowered plasma 26RFa levels as compared to untreated mice, whereas the expression of the peptide in the duodenum is not affected. Taken together, the present results indicate that dysregulation of glucose homeostasis in obese/hyperglycemic mice is not aggravated by the absence of 26RFa that may be compensated by the increase of BAT mass. In diabetic insulin-deficient mice, the antihypergycemic effect of 26RFa is totally blunted probably as a result of the impaired insulin production characterizing this model, avoiding therefore the action of the peptide.

Blood Levels of Neuropeptide 26RFa in Relation to Anxiety and Aggressive Behavior in Humans-An Exploratory Study

26RFa, also referred to as QRFP, is a hypothalamic neuropeptide mainly known for its role in the regulation of appetite and glucose metabolism. Its possible relevance to emotional regulation is largely unexplored. To address this, in the present exploratory study, we analyzed the plasma concentrations of 26RFa in humans characterized by different levels of anxiety and aggressive behavior. For this purpose, the study included 13 prison inmates who have committed violent crimes and 19 age-matched healthy men from the general population as controls. Anxiety, depression and aggressive behavior were evaluated in both groups using standard questionnaires. The inmate group was characterized by increased aggression and anxiety compared to the controls. We found that the mean plasma levels of 26RFa did not significantly differ between the inmates and the controls. However, several high outliers were present only in the inmate group. The plasma levels of 26RFa correlated positively with the anxiety scores in all the studied subjects and controls. After removing the high outliers in the inmate group, positive correlations of 26RFa with anxiety and a subscale of hostility in the aggression scale were also recorded in this group. No significant correlations of 26RFa with depression scores or other parameters of aggressive behavior were found. Thus, the present results did not support an involvement of 26RFa in aggressive behavior in humans but pointed to a link between this neuropeptide and anxiety. Nevertheless, considering the exploratory nature of the present study, this conclusion should be verified in a larger cohort, including the clinical degree of anxiety.

The hypothalamic RFamide, QRFP, increases feeding and locomotor activity: The role of Gpr103 and orexin receptors

Here we show that central administration of pyroglutamylated arginine-phenylamine-amide peptide (QRFP/26RFa) increases both food intake and locomotor activity, without any significant effect on energy expenditure, thermogenesis or reward. Germline knock out of either of the mouse QRFP receptor orthologs, Gpr103a and Gpr103b, did not produce a metabolic phenotype. However, both receptors are required for the effect of centrally administered QRFP to increase feeding and locomotor activity. As central injection of QRFP activated orexin/hypocretin neurons in the lateral hypothalamus, we compared the action of QRFP and orexin on behaviour. Both peptides increased arousal and locomotor activity. However, while orexin increased consummatory behaviour, QRFP also affected other appetitive behaviours. Furthermore, the feeding but not the locomotor response to QRFP, was blocked by co-administration of an orexin receptor 1 antagonist. These results suggest that QRFP agonism induces both appetitive and consummatory behaviour, but only the latter is dependent on orexin/hypocretin receptor signalling.

The 26RFa (QRFP)/GPR103 neuropeptidergic system in mice relays insulin signalling into the brain to regulate glucose homeostasis

AIMS/HYPOTHESIS

26RFa (pyroglutamilated RFamide peptide [QRFP]) is a biologically active peptide that regulates glucose homeostasis by acting as an incretin and by increasing insulin sensitivity at the periphery. 26RFa is also produced by a neuronal population localised in the hypothalamus. In this study we investigated whether 26RFa neurons are involved in the hypothalamic regulation of glucose homeostasis.

METHODS

26Rfa^+/+, 26Rfa^-/- and insulin-deficient male C57Bl/6J mice were used in this study. Mice received an acute intracerebroventricular (i.c.v.) injection of 26RFa, insulin or the 26RFa receptor (GPR103) antagonist 25e and were subjected to IPGTTs, insulin tolerance tests, acute glucose-stimulated insulin secretion tests and pyruvate tolerance tests (PTTs). Secretion of 26RFa by hypothalamic explants after incubation with glucose, leptin or insulin was assessed. Expression and quantification of the genes encoding 26RFa, agouti-related protein, the insulin receptor and GPR103 were evaluated by quantitative reverse transcription PCR and RNAscope in situ hybridisation.

RESULTS

Our data indicate that i.c.v.-injected 26RFa induces a robust antihyperglycaemic effect associated with an increase in insulin production by the pancreatic islets. In addition, we found that insulin strongly stimulates 26Rfa expression and secretion by the hypothalamus. RNAscope experiments revealed that neurons expressing 26Rfa are mainly localised in the lateral hypothalamic area, that they co-express the gene encoding the insulin receptor and that insulin induces the expression of 26Rfa in these neurons. Concurrently, the central antihyperglycaemic effect of insulin is abolished in the presence of a GPR103 antagonist and in 26RFa-deficient mice. Finally, our data indicate that the hypothalamic 26RFa neurons are not involved in the central inhibitory effect of insulin on hepatic glucose production, but mediate the central effects of the hormone on its own peripheral production.

CONCLUSION/INTERPRETATION

We have identified a novel mechanism in the hypothalamic regulation of glucose homeostasis, the 26RFa/GPR103 system, and we provide evidence that this neuronal peptidergic system is a key relay for the central regulation of glucose metabolism by insulin.

New Peptides as Potential Players in the Crosstalk Between the Brain and Obesity, Metabolic and Cardiovascular Diseases

According to the World Health Organization report published in 2016, 650 million people worldwide suffer from obesity, almost three times more than in 1975. Obesity is defined as excessive fat accumulation which may impair health with non-communicable diseases such as diabetes, cardiovascular diseases (hypertension, coronary artery disease, stroke), and some cancers. Despite medical advances, cardiovascular complications are still the leading causes of death arising from obesity. Excessive fat accumulation is caused by the imbalance between energy intake and expenditure. The pathogenesis of this process is complex and not fully understood, but current research is focused on the role of the complex crosstalk between the central nervous system (CNS), neuroendocrine and immune system including the autonomic nervous system, adipose tissue, digestive and cardiovascular systems. Additionally, special attention has been paid to newly discovered substances: neuropeptide 26RFa, preptin, and adropin. It was shown that the above peptides are synthesized both in numerous structures of the CNS and in many peripheral organs and tissues, such as the heart, adipose tissue, and the gastrointestinal tract. Recently, particular attention has been paid to the role of the presented peptides in the pathogenesis of obesity, metabolic and cardiovascular system diseases. This review summarizes the role of newly investigated peptides in the crosstalk between brain and peripheral organs in the pathogenesis of obesity, metabolic, and cardiovascular diseases.

Is there a hypothalamic basis for anorexia nervosa?

The hypothalamus has long been known to control food intake and energy metabolism through a complex network of primary and secondary neurons and glial cells. Anorexia nervosa being a complex disorder characterized by abnormal feeding behavior and food aversion, it is thus quite surprising that not much is known concerning potential hypothalamic modifications in this disorder. In this chapter, we review the recent advances in the fields of genetics, epigenetics, structural and functional imaging, and brain connectivity, as well as neuroendocrine findings and emerging animal models, which have begun to unravel the importance of hypothalamic adaptive processes to our understanding of the pathology of eating disorders.

Glucose homeostasis is impaired in mice deficient in the neuropeptide 26RFa (QRFP)

INTRODUCTION

26RFa (pyroglutamyl RFamide peptide (QRFP)) is a biologically active peptide that has been found to control feeding behavior by stimulating food intake, and to regulate glucose homeostasis by acting as an incretin. The aim of the present study was thus to investigate the impact of 26RFa gene knockout on the regulation of energy and glucose metabolism.

RESEARCH DESIGN AND METHODS

26RFa mutant mice were generated by homologous recombination, in which the entire coding region of prepro26RFa was replaced by the iCre sequence. Energy and glucose metabolism was evaluated through measurement of complementary parameters. Morphological and physiological alterations of the pancreatic islets were also investigated.

RESULTS

Our data do not reveal significant alteration of energy metabolism in the 26RFa-deficient mice except the occurrence of an increased basal metabolic rate. By contrast, 26RFa mutant mice exhibited an altered glycemic phenotype with an increased hyperglycemia after a glucose challenge associated with an impaired insulin production, and an elevated hepatic glucose production. Two-dimensional and three-dimensional immunohistochemical experiments indicate that the insulin content of pancreatic β cells is much lower in the 26RFa^-/- mice as compared with the wild-type littermates.

CONCLUSION

Disruption of the 26RFa gene induces substantial alteration in the regulation of glucose homeostasis, with in particular a deficit in insulin production by the pancreatic islets. These findings further support the notion that 26RFa is an important regulator of glucose homeostasis.

QRFP administration into the medial hypothalamic nuclei improves memory in rats

Even though several of RFamide peptides have been shown to modify memory and learning processes in different species, almost nothing is known regarding cognitive effects of recently discovered neuropeptide QRFP. Considering multiple physiological functions of QRFP, localization of QRFP-synthesizing neurons in the hypothalamus and its' widely spread binding sites within the CNS, the present study was designed to investigate the possible role of QRFP in the consolidation of spatial memory. As target area for microinjection, the medial hypothalamic area, including dorsomedial (DMN) and ventromedial (VMN) nuclei, has been chosen. At first, the effects of two doses (200 ng and 400 ng) of QRFP were investigated in Morris water maze. After that receptor antagonist BIBP3226 (equimolar amount to the effective dose of neuropeptide) was applied to elucidate whether it can prevent effects of QRFP. To reveal possible changes in anxiety level, animals were tested in Elevated plus maze. The higher dose of QRFP (400 ng) improved short-term memory consolidation in Morris water maze. Pretreatment with antagonist BIBP3226 abolished cognitive effects of QRFP. The neuropeptide did not affect anxiety level of rats. This study provides unique evidence regarding the role of QRFP in the consolidation of memory and gives the basis for further investigations of neuropeptide's cognitive effects.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

The neuropeptide 26RFa in the human gut and pancreas: potential involvement in glucose homeostasis

OBJECTIVE

Recent studies performed in mice revealed that the neuropeptide 26RFa regulates glucose homeostasis by acting as an incretin and by increasing insulin sensitivity. However, in humans, an association between 26RFa and the regulation of glucose homeostasis is poorly documented. In this study, we have thus investigated in detail the distribution of 26RFa and its receptor, GPR103, in the gut and the pancreas, and determined the response of this peptidergic system to an oral glucose challenge in obese patients.

DESIGN AND METHODS

Distribution of 26RFa and GPR103 was examined by immunohistochemistry using gut and pancreas tissue sections. Circulating 26RFa was determined using a specific radioimmunoassay in plasma samples collected during an oral glucose tolerance test.

RESULTS

26RFa and GPR103 are present all along the gut but are more abundant in the stomach and duodenum. In the stomach, the peptide and its receptor are highly expressed in the gastric glands, whereas in the duodenum, ileum and colon they are present in the enterocytes and the goblet cells. In the pancreatic islets, the 26RFa/GPR103 system is mostly present in the β cells. During an oral glucose tolerance test, plasma 26RFa profile is different between obese patients and healthy volunteers, and we found strong positive correlations between 26RFa blood levels and the BMI, and with various parameters of insulin secretion and insulin resistance.

CONCLUSION

The present data suggest an involvement of the 26RFa/GPR103 peptidergic system in the control of human glucose homeostasis.

Neuropeptide 26RFa (QRFP) is a key regulator of glucose homeostasis and its activity is markedly altered in obese/hyperglycemic mice

Recent studies have shown that the hypothalamic neuropeptide 26RFa regulates glucose homeostasis by acting as an incretin and increasing insulin sensitivity. In this study, we further characterized the role of the 26RFa/GPR103 peptidergic system in the global regulation of glucose homeostasis using a 26RFa receptor antagonist and also assessed whether a dysfunction of the 26RFa/GPR103 system occurs in obese hyperglycemic mice. First, we demonstrate that administration of the GPR103 antagonist reduces the global glucose-induced incretin effect and insulin sensitivity whereas, conversely, administration of exogenous 26RFa attenuates glucose-induced hyperglycemia. Using a mouse model of high-fat diet-induced obesity and hyperglycemia, we found a loss of the antihyperglcemic effect and insulinotropic activity of 26RFa, accompanied with a marked reduction of its insulin-sensitive effect. Interestingly, this resistance to 26RFa is associated with a downregulation of the 26RFa receptor in the pancreatic islets, and insulin target tissues. Finally, we observed that the production and release kinetics of 26RFa after an oral glucose challenge is profoundly altered in the high-fat mice. Altogether, the present findings support the view that 26RFa is a key regulator of glucose homeostasis whose activity is markedly altered under obese/hyperglycemic conditions.

Microinjection of 26RFa, an endogenous ligand for the glutamine RF-amide peptide receptor (QRFP receptor), into the rostral ventromedial medulla (RVM), locus coelureus (LC), and periaqueductal grey (PAG) produces an analgesic effect in rats

26RFa is an endogenous ligand for the QRFP receptor. We previously found that intracerebroventricular injection of 26RFa produces an analgesic effect in a rat formalin test. In the present study, we directly tested the hypothesis that the analgesic effects of 26RFa in the formalin test are mediated in well-recognized regions of the descending inhibitory pain pathways, such as the rostral ventromedial medulla (RVM), locus coeruleus (LC), and periaqueductal grey (PAG) in rats. Injection cannulae were stereotaxically placed in the RVM, LC, or PAG through a burr hole. 26RFa (15 μg) or saline was delivered in a total volume of 0.5 μL. In a formalin test, 50 μL of 5% formalin was injected subcutaneously into the hind paw. In an antagonist study, idazoxan, an α-2 antagonist, or naloxone, an opioid receptor antagonist, was administered. Microinjection of 26RFa into the RVM had no effect compared with that in saline-injected rats. Microinjection of 26RFa into the LC contralateral, but not ipsilateral, to the formalin injection site significantly decreased the number of flinching behaviors compared with that of saline-injected rats. This effect was antagonized by intrathecal injection of idazoxan. Microinjection of 26RFa into the contralateral, but not ipsilateral, PAG produced an analgesic effect, and this effect was partly antagonized by intraperitoneal naloxone. These data suggest that 26RFa microinjected into the contralateral LC induced noradrenaline release in the spinal cord and produced an analgesic effect. In the contralateral PAG, 26RFa activated the opioid system, and some analgesic effects were mediated by opioid system activation.

Design, Synthesis, Molecular Dynamics Simulation, and Functional Evaluation of a Novel Series of 26RFa Peptide Analogues Containing a Mono- or Polyalkyl Guanidino Arginine Derivative

26RFa, the endogenous QRFPR ligand, is implicated in several physiological and pathological conditions such as the regulation of glucose homeostasis and bone mineralization; hence, QRFPR ligands display therapeutic potential. At the molecular level, functional interaction occurs between residues Arg²⁵ of 26RFa and Gln¹²⁵ of QRFPR. We have designed 26RFa_(20-26) analogues incorporating arginine derivatives modified by alkylated substituents. We found that the Arg²⁵ side chain length was necessary to retain the activity of 26RFa_(20-26) and that N-monoalkylation of arginine was accommodated by the QRFPR active site. In particular, [(Me)^ωArg²⁵]26RFa_(20-26) (5b, LV-2186) appeared to be 25-fold more potent than 26RFa_(20-26) and displayed a position in a QRFPR homology model slightly different to that of the unmodified heptapeptide. Other peptides were less potent than 26RFa_(20-26), exhibited partial agonistic activity, or were totally inactive in accordance to different ligand-bound structures. In vivo, [(Me)^ωArg²⁵]26RFa_(20-26) exerted a delayed 26RFa-like hypoglycemic effect. Finally, N-methyl substituted arginine-containing peptides represent lead compounds for further development of QRFPR agonists.

Orphan G protein-coupled receptors: The role in CNS disorders

There are various types of receptors in the central nervous system (CNS). G protein-coupled receptors (GPCRs) have the highest expression with a wide range of physiological functions. A newer sub group of these receptors namely orphan GPCRs have been discovered. GPR3, GPR6, GPR17, GPR26, GPR37, GPR39, GPR40, GPR50, GPR52, GPR54, GPR55, GPR85, GPR88, GPR103, and GPR139 are the selected orphan GPCRs for this article. Their roles in the central nervous system have not been understood well so far. However, recent studies show that they may have very important functions in the CNS. Hence, in the present study, we reviewed most recent findings regarding the physiological roles of the selected orphan GPCRs in the CNS. After a brief presentation of each receptor, considering the results from genetic and pharmacological manipulation of the receptors, their roles in the pathophysiology of different diseases and disorders including anxiety, depression, schizophrenia, epilepsy, Alzheimer's disease, Parkinson's disease, and substance abuse will be discussed. At present, our knowledge regarding the role of GPCRs in the brain is very limited. However, previous limited studies show that orphan GPCRs have an important place in psychopharmacology and these receptors are potential new targets for the treatment of major CNS diseases.

The Arg-Phe-amide peptide 26RFa/glutamine RF-amide peptide and its receptor: IUPHAR Review 24

The RFamide neuropeptide 26RFa was first isolated from the brain of the European green frog on the basis of cross-reactivity with antibodies raised against bovine neuropeptide FF (NPFF). 26RFa and its N-terminally extended form glutamine RF-amide peptide (QRFP) have been identified as cognate ligands of the former orphan receptor GPR103, now renamed glutamine RF-amide peptide receptor (QRFP receptor). The 26RFa/QRFP precursor has been characterized in various mammalian and non-mammalian species. In the brain of mammals, including humans, 26RFa/QRFP mRNA is almost exclusively expressed in hypothalamic nuclei. The 26RFa/QRFP transcript is also present in various organs especially in endocrine glands. While humans express only one QRFP receptor, two isoforms are present in rodents. The QRFP receptor genes are widely expressed in the CNS and in peripheral tissues, notably in bone, heart, kidney, pancreas and testis. Structure-activity relationship studies have led to the identification of low MW peptidergic agonists and antagonists of QRFP receptor. Concurrently, several selective non-peptidic antagonists have been designed from high-throughput screening hit optimization. Consistent with the widespread distribution of QRFP receptor mRNA and 26RFa binding sites, 26RFa/QRFP exerts a large range of biological activities, notably in the control of energy homeostasis, bone formation and nociception that are mediated by QRFP receptor or NPFF2. The present report reviews the current knowledge concerning the 26RFa/QRFP-QRFP receptor system and discusses the potential use of selective QRFP receptor ligands for therapeutic applications.

A potential role for the secretogranin II-derived peptide EM66 in the hypothalamic regulation of feeding behaviour

EM66 is a conserved 66-amino acid peptide derived from secretogranin II (SgII), a member of the granin protein family. EM66 is widely distributed in secretory granules of endocrine and neuroendocrine cells, as well as in hypothalamic neurones. Although EM66 is abundant in the hypothalamus, its physiological function remains to be determined. The present study aimed to investigate a possible involvement of EM66 in the hypothalamic regulation of feeding behaviour. We show that i.c.v. administration of EM66 induces a drastic dose-dependent inhibition of food intake in mice deprived of food for 18 hours, which is associated with an increase of hypothalamic pro-opiomelanocortin (POMC) and melanocortin-3 receptor mRNA levels and c-Fos immunoreactivity in the POMC neurones of the arcuate nucleus. By contrast, i.c.v. injection of EM66 does not alter the hypothalamic expression of neuropeptide Y (NPY), or that of its Y1 and Y5 receptors. A 3-month high-fat diet (HFD) leads to an important decrease of POMC and SgII mRNA levels in the hypothalamus, whereas NPY gene expression is not affected. Finally, we show that a 48 hours of fasting in HFD mice decreases the expression of POMC and SgII mRNA, which is not observed in mice fed a standard chow. Taken together, the present findings support the view that EM66 is a novel anorexigenic neuropeptide regulating hypothalamic feeding behaviour, at least in part, by activating the POMC neurones of the arcuate nucleus.

Role of Melatonin, Galanin, and RFamide Neuropeptides QRFP26 and QRFP43 in the Neuroendocrine Control of Pancreatic β-Cell Function

Glucose homeostasis is finely regulated by a number of hormones and peptides released mainly from the brain, gastrointestinal tract, and muscle, regulating pancreatic secretion through cellular receptors and their signal transduction cascades. The endocrine function of the pancreas is controlled by islets within the exocrine pancreatic tissue that release hormones like insulin, glucagon, somatostatin, pancreatic polypeptide, and ghrelin. Moreover, both exocrine and endocrine pancreatic functions are regulated by a variety of hormonal and neural mechanisms, such as ghrelin, glucagon-like peptide, glucose-dependent insulinotropic polypeptide, or the inhibitory peptide somatostatin. In this review, we describe the role of neurohormones that have been less characterized compared to others, on the regulation of insulin secretion. In particular, we will focus on melatonin, galanin, and RFamide neuropeptides QRFP26 and QRFP43, which display either insulinotropic or insulinostatic effects. In fact, in addition to other hormones, amino acids, cytokines, and a variety of proteins, brain-derived hormones are now considered as key regulators of glucose homeostasis, representing potential therapeutic targets for the treatment of diabetes and obesity.

The Neuropeptide 26RFa (QRFP) and Its Role in the Regulation of Energy Homeostasis: A Mini-Review

This mini-review deals with the neuropeptide 26RFa (or QRFP) which is a member of the RFamide peptide family discovered simultaneously by three groups in 2003. 26RFa (or its N-extended form 43RFa) was subsequently shown to be the endogenous ligand of the human orphan receptor GPR103. In the brain, 26RFa and GPR103mRNA are primarily expressed in hypothalamic nuclei involved in the control of feeding behavior, and at the periphery, the neuropeptide and its receptor are present in abundance in the gut and the pancreatic islets, suggesting that 26RFa is involved in the regulation of energy metabolism. Indeed, 26RFa stimulates food intake when injected centrally, and its orexigenic effect is even more pronounced in obese animals. The expression of 26RFa is up-regulated in the hypothalamus of obese animals, supporting that the 26RFa/GPR103 system may play a role in the development and/or maintenance of the obese status. Recent data indicate that 26RFa is also involved in the regulation of glucose homeostasis. 26RFa reduces glucose-induced hyperglycemia, increases insulin sensitivity and insulinemia. Furthermore, an oral ingestion of glucose strongly stimulates 26RFa release by the gut, indicating that 26RFa is a novel incretin. Finally, 26RFa is able to prevent pancreatic β cell death and apoptosis. This brief overview reveals that 26RFa is a key neuropeptide in the regulation of energy metabolism. Further fields of research are suggested including the pathophysiological implication of the 26RFa/GPR103 system.

Regulation of the expression of corticotropin-releasing factor gene by pyroglutamylated RFamide peptide in rat hypothalamic 4B cells

Pyroglutamylated RFamide peptide (QRFP), an important regulator of metabolism and energy homeostasis, has orexigenic effects. QRFP acts via a specific receptor, Gpr103. Gpr103 mRNA is expressed in the rat hypothalamic paraventricular nucleus (PVN). In the PVN, corticotropin-releasing factor (CRF), which plays a central role in regulating the stress response and is produced in response to stress, stimulates the release of adrenocorticotropic hormone from the anterior pituitary. We hypothesized that QRFP regulates CRF gene expression directly in the hypothalamus, and thus examined the direct effect of QRFP on the promoter activity and mRNA levels of CRF in hypothalamic cells. To examine these pathways, we used hypothalamic 4B cells, a homologous PVN neuronal cell line. Gpr103a and Gpr103b mRNA, and Gpr103 (a and b) proteins were expressed in the hypothalamic cells. The Gpr103 mRNA and protein levels were increased by QRFP. QRFP also stimulated CRF mRNA levels and CRF promoter activity directly in 4B cells following their transfection with the CRF promoter. The protein kinase A (PKA) and protein kinase C (PKC) pathways were involved in the QRFP-induced increases in CRF promoter activity. QRFP stimulated cAMP response element-binding protein (CREB) phosphorylation. CREB phosphorylation was inhibited by a PKC inhibitor. PKC-dependent signaling would be upstream of the CREB phosphorylation. Thus, QRFP-dependent pathways are involved in the regulation of CRF gene expression in the hypothalamus.

Anxiolytic effect of the GPR103 receptor agonist peptide P550 (homolog of neuropeptide 26RFa) in mice. Involvement of neurotransmitters

The GPR103 receptor is a G protein-coupled receptor, which plays a role in several physiological functions. However, the role of the GPR103 receptor in anxiety has not been clarified. The first aim of our study was to elucidate the involvement of the GPR103 receptor in anxious behavior. Mice were treated with peptide P550, which is the mouse homolog of neuropeptide 26RFa and has similar activity for the GPR103 receptor as neuropeptide 26RFa. The anxious behavior was investigated using an elevated plus-maze paradigm. The second aim of our study was to investigate the underlying neurotransmissions. Accordingly, mice were pretreated with a nonselective muscarinic acetylcholine receptor antagonist, atropine, a γ-aminobutyric acid subunit A (GABAA) receptor antagonist, bicuculline, a non-selective 5-HT2 serotonergic receptor antagonist, cyproheptadine, a mixed 5-HT1/5-HT2 serotonergic receptor antagonist, methysergide, a D2, D3, D4 dopamine receptor antagonist, haloperidol, a nonselective α-adrenergic receptor antagonist, phenoxybenzamine and a nonselective β-adrenergic receptor antagonist, propranolol. Our results demonstrated that peptide P550 reduces anxious behavior in elevated plus maze test in mice. Our study shows also that GABAA-ergic, α- and β-adrenergic transmissions are all involved in this action, whereas 5-HT1 and 5-HT2 serotonergic, muscarinic cholinergic and D2, D3, D4 dopaminergic mechanisms may not be implicated.

QRFP and Its Receptors Regulate Locomotor Activity and Sleep in Zebrafish

The hypothalamus plays an important role in regulating sleep, but few hypothalamic sleep-promoting signaling pathways have been identified. Here we demonstrate a role for the neuropeptide QRFP (also known as P518 and 26RFa) and its receptors in regulating sleep in zebrafish, a diurnal vertebrate. We show that QRFP is expressed in ∼10 hypothalamic neurons in zebrafish larvae, which project to the hypothalamus, hindbrain, and spinal cord, including regions that express the two zebrafish QRFP receptor paralogs. We find that the overexpression of QRFP inhibits locomotor activity during the day, whereas mutation of qrfp or its receptors results in increased locomotor activity and decreased sleep during the day. Despite the restriction of these phenotypes to the day, the circadian clock does not regulate qrfp expression, and entrained circadian rhythms are not required for QRFP-induced rest. Instead, we find that QRFP overexpression decreases locomotor activity largely in a light-specific manner. Our results suggest that QRFP signaling plays an important role in promoting sleep and may underlie some aspects of hypothalamic sleep control.

SIGNIFICANCE STATEMENT

The hypothalamus is thought to play a key role in regulating sleep in vertebrate animals, but few sleep-promoting signaling pathways that function in the hypothalamus have been identified. Here we use the zebrafish, a diurnal vertebrate, to functionally and anatomically characterize the neuropeptide QRFP. We show that QRFP is exclusively expressed in a small number of neurons in the larval zebrafish hypothalamus that project widely in the brain. We also show that QRFP overexpression reduces locomotor activity, whereas animals that lack QRFP signaling are more active and sleep less. These results suggest that QRFP signaling participates in the hypothalamic regulation of sleep.

New Insights in Anorexia Nervosa

Anorexia nervosa (AN) is classically defined as a condition in which an abnormally low body weight is associated with an intense fear of gaining weight and distorted cognitions regarding weight, shape, and drive for thinness. This article reviews recent evidences from physiology, genetics, epigenetics, and brain imaging which allow to consider AN as an abnormality of reward pathways or an attempt to preserve mental homeostasis. Special emphasis is put on ghrelino-resistance and the importance of orexigenic peptides of the lateral hypothalamus, the gut microbiota and a dysimmune disorder of neuropeptide signaling. Physiological processes, secondary to underlying, and premorbid vulnerability factors-the "pondero-nutritional-feeding basements"- are also discussed.

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.

[The neuropeptide 26RFa and its role in the regulation of energy metabolism]

The neuropeptide 26RFa, also referred to as QRFP (for pyroglutamilated RFamide peptide), is the latest member of the RFamide peptide family to be discovered. 26RFa and its N-extended form, 43RFa, have been characterized in all vertebrate classes as the endogenous ligands of the human orphan receptor GPR103. In the brain, 26RFa and GPR103mRNA are primarily expressed in hypothalamic nuclei involved in the control of feeding behavior, and in the periphery, the neuropeptide and its receptor are present in abundance in the gut and the pancreatic islets, suggesting that 26RFa is involved in the regulation of energy metabolism. Indeed, 26RFa stimulates food intake when centrally injected, and its orexigenic effect is even more pronounced in obese animals. The expression of 26RFa is up-regulated in the hypothalamus of obese animals, supporting the view that 26RFa may play a role in the development and/or maintenance of the obese status. Recent data indicate that 26RFa is also involved in the regulation of glucose homeostasis. 26RFa reduces glucose-induced hyperglycemia, increases insulin sensitivity and insulinemia. Furthermore, an oral ingestion of glucose strongly stimulates 26RFa release by the gut, indicating that 26RFa is a novel incretin. Finally, 26RFa is able to prevent pancreatic β cell death and apoptosis. In conclusion, this overview of the literature reveals that 26RFa is a key neuropeptide in the regulation of energy metabolism. Further fields of research are suggested including the pathophysiological implication of the 26RFa/GPR103 system.

Effects of direct QRFP-26 administration into the medial hypothalamic area on food intake in rats

The RFamide peptide family comprises a number of biologically active peptides sharing RF motif at their C-terminal end. These peptides are involved in the control of multiple physiological functions including regulation of metabolism and feeding behavior. QRFP-43 as well as its 26-aminoacid residue QRFP-26 are able to cause orexigenic effect when administered to the rodents' cerebral ventricles. QRFPs have been suggested as the endogenous ligands of the previously orphan GPR103 receptors. GPR103 receptors share amino acid identity with other receptors of neuropeptides involved in feeding (NPY, NPFF, galanin). QRFP-26 expressing neurons and binding sites are densely present in the rat medial hypothalamus (MHA), an area directly responsible for the regulation of feeding. QRFP-26 was delivered to the target area by direct intrahypothalamic microinjection, and the consumption of liquid food was measured over a 60 min period. Both doses (100 and 200 ng) significantly increased food intake. Non-specific receptor antagonist BIBP3226 eliminated the orexigenic effect caused by QRFP-26 administration. Effective doses of QRFP-26 did not modify general locomotor activity and behavioral patterns examined in the open-field test. This study is the first reporting feeding modulating effects following direct intrahypothalamic QRFP-26 administration.

Hypothalamic Neuropeptide 26RFa Acts as an Incretin to Regulate Glucose Homeostasis

26RFa is a hypothalamic neuropeptide that promotes food intake. 26RFa is upregulated in obese animal models, and its orexigenic activity is accentuated in rodents fed a high-fat diet, suggesting that this neuropeptide might play a role in the development and maintenance of the obese status. As obesity is frequently associated with type 2 diabetes, we investigated whether 26RFa may be involved in the regulation of glucose homeostasis. In the current study, we show a moderate positive correlation between plasma 26RFa levels and plasma insulin in patients with diabetes. Plasma 26RFa concentration also increases in response to an oral glucose tolerance test. In addition, we found that 26RFa and its receptor GPR103 are present in human pancreatic β-cells as well as in the gut. In mice, 26RFa attenuates the hyperglycemia induced by a glucose load, potentiates insulin sensitivity, and increases plasma insulin concentrations. Consistent with these data, 26RFa stimulates insulin production by MIN6 insulinoma cells. Finally, we show, using in vivo and in vitro approaches, that a glucose load induces a massive secretion of 26RFa by the small intestine. Altogether, the present data indicate that 26RFa acts as an incretin to regulate glucose homeostasis.

Orexin receptors exert a neuroprotective effect in Alzheimer's disease (AD) via heterodimerization with GPR103

Orexins are neuropeptides that regulate the sleep-wake cycle and feeding behaviour. QRFP is a newly discovered neuropeptide which exerts similar orexigenic activity, thus playing an important role in energy homeostasis and regulation of appetite. The exact expression and signalling characteristics and physiological actions of QRFP and its receptor GPR103 are poorly understood. Alzheimer's disease (AD) patients experience increased nocturnal activity, excessive daytime sleepiness, and weight loss. We hypothesised therefore that orexins and QRFP might be implicated in the pathophysiology of AD. We report that the down-regulation of hippocampal orexin receptors (OXRs) and GPR103 particularly in the cornu ammonis (CA) subfield from AD patients suffering from early onset familial AD (EOFAD) and late onset familial AD (LOAD). Using an in vitro model we demonstrate that this downregulation is due to to Aβ-plaque formation and tau hyper-phosphorylation. Transcriptomics revealed a neuroprotective role for both orexins and QRFP. Finally we provide conclusive evidence using BRET and FRET that OXRs and GPR103 form functional hetero-dimers to exert their effects involving activation of ERK1/2. Pharmacological intervention directed at the orexigenic system may prove to be an attractive avenue towards the discovery of novel therapeutics for diseases such as AD and improving neuroprotective signalling pathways.

Molecular basis of agonist docking in a human GPR103 homology model by site-directed mutagenesis and structure-activity relationship studies

BACKGROUND AND PURPOSE

The neuropeptide 26RFa and its cognate receptor GPR103 are involved in the control of food intake and bone mineralization. Here, we have tested, experimentally, the predicted ligand-receptor interactions by site-directed mutagenesis of GPR103 and designed point-substituted 26RFa analogues.

EXPERIMENTAL APPROACH

Using the X-ray structure of the β2 -adrenoceptor, a 3-D molecular model of GPR103 has been built. The bioactive C-terminal octapeptide 26RFa(19-26) , KGGFSFRF-NH2 , was docked in this GPR103 model and the ligand-receptor complex was submitted to energy minimization.

KEY RESULTS

In the most stable complex, the Phe-Arg-Phe-NH2 part was oriented inside the receptor cavity, whereas the N-terminal Lys residue remained outside. A strong intermolecular interaction was predicted between the Arg(25) residue of 26RFa and the Gln(125) residue located in the third transmembrane helix of GPR103. To confirm this interaction experimentally, we tested the ability of 26RFa and Arg-modified 26RFa analogues to activate the wild-type and the Q125A mutant receptors transiently expressed in CHO cells. 26RFa (10(-6)  M) enhanced [Ca(2+) ]i in wild-type GPR103-transfected cells, but failed to increase [Ca(2+) ]i in Q125A mutant receptor-expressing cells. Moreover, asymmetric dimethylation of the side chain of arginine led to a 26RFa analogue, [ADMA(25) ]26RFa(20-26) , that was unable to activate the wild-type GPR103, but antagonized 26RFa-evoked [Ca(2+) ]i increase.

CONCLUSION AND IMPLICATIONS

Altogether, these data provide strong evidence for a functional interaction between the Arg(25) residue of 26RFa and the Gln(125) residue of GPR103 upon ligand-receptor activation, which can be exploited for the rational design of potent GPR103 agonists and antagonists.

RFamide peptides 43RFa and 26RFa both promote survival of pancreatic β-cells and human pancreatic islets but exert opposite effects on insulin secretion

RFamide peptides 43RFa and 26RFa have been shown to promote food intake and to exert different peripheral actions through G-protein-coupled receptor 103 (GPR103) binding. Moreover, 26RFa was found to inhibit pancreatic insulin secretion, whereas the role of 43RFa on β-cell function is unknown, as well as the effects of both peptides on β-cell survival. Herein, we investigated the effects of 43RFa and 26RFa on survival and apoptosis of pancreatic β-cells and human pancreatic islets. In addition, we explored the role of these peptides on insulin secretion and the underlying signaling mechanisms. Our results show that in INS-1E β-cells and human pancreatic islets both 43RFa and 26RFa prevented cell death and apoptosis induced by serum starvation, cytokine synergism, and glucolipotoxicity, through phosphatidylinositol 3-kinase/Akt- and extracellular signal-related kinase 1/2-mediated signaling. Moreover, 43RFa promoted, whereas 26RFa inhibited, glucose- and exendin-4-induced insulin secretion, through Gαs and Gαi/o proteins, respectively. Inhibition of GPR103 expression by small interfering RNA blocked 43RFa insulinotropic effect, but not the insulinostatic action of 26RFa. Finally, 43RFa, but not 26RFa, induced cAMP increase and glucose uptake. In conclusion, because of their survival effects along with the effects on insulin secretion, these findings suggest potential for 43RFa and 26RFa as therapeutic targets in the treatment of diabetes.

Molecular evolution of GPCRs: 26Rfa/GPR103

Neuropeptides possessing the Arg-Phe-NH2 (RFamide) motif at their C-termini (designated as RFamide peptides) have been characterized in a variety of animals. Among these, neuropeptide 26RFa (also termed QRFP) is the latest member of the RFamide peptide family to be discovered in the hypothalamus of vertebrates. The neuropeptide 26RFa/QRFP is a 26-amino acid residue peptide that was originally identified in the frog brain. It has been shown to exert orexigenic activity in mammals and to be a ligand for the previously identified orphan G protein-coupled receptor, GPR103 (QRFPR). The cDNAs encoding 26RFa/QRFP and QRFPR have now been characterized in representative species of mammals, birds, and fish. Functional studies have shown that, in mammals, the 26RFa/QRFP-QRFPR system may regulate various functions, including food intake, energy homeostasis, bone formation, pituitary hormone secretion, steroidogenesis, nociceptive transmission, and blood pressure. Several biological actions have also been reported in birds and fish. This review summarizes the current state of identification, localization, and understanding of the functions of 26RFaQRFP and its cognate receptor, QRFPR, in vertebrates.

New Hits as Antagonists of GPR103 Identified by HTS

Preclinical data indicate that GPR103 receptor and its endogenous neuropeptides QRFP26 and QRFP43 are involved in appetite regulation. A high throughput screening (HTS) for small molecule GPR103 antagonists was performed with the clinical goal to target weight management by modulation of appetite. A high hit rate from the HTS and initial low confirmation with respect to functional versus affinity data challenged us to revise the established screening cascade. To secure high quality data while increasing throughput, the binding assay was optimized on quality to run at single concentration. This strategy enabled evaluation of a larger fraction of chemical clusters and singletons delivering 17 new compound classes for GPR103 antagonism. Representative compounds from three clusters are presented. One of the identified clusters was further investigated, and an initial structure-activity relationship study is reported. The most potent compound identified had a pIC50 of 7.9 with an improved ligand lipophilic efficiency.

Involvement of Mammalian RF-Amide Peptides and Their Receptors in the Modulation of Nociception in Rodents

Mammalian RF-amide peptides, which all share a conserved carboxyl-terminal Arg-Phe-NH2 sequence, constitute a family of five groups of neuropeptides that are encoded by five different genes. They act through five G-protein-coupled receptors and each group of peptide binds to and activates mostly one receptor: RF-amide related peptide group binds to NPFFR1, neuropeptide FF group to NPFFR2, pyroglutamylated RF-amide peptide group to QRFPR, prolactin-releasing peptide group to prolactin-releasing peptide receptor, and kisspeptin group to Kiss1R. These peptides and their receptors have been involved in the modulation of several functions including reproduction, feeding, and cardiovascular regulation. Data from the literature now provide emerging evidence that all RF-amide peptides and their receptors are also involved in the modulation of nociception. This review will present the current knowledge on the involvement in rodents of the different mammalian RF-amide peptides and their receptors in the modulation of nociception in basal and chronic pain conditions as well as their modulatory effects on the analgesic effects of opiates.

RFamide Peptides in Early Vertebrate Development

RFamides (RFa) are neuropeptides involved in many different physiological processes in vertebrates, such as reproductive behavior, pubertal activation of the reproductive endocrine axis, control of feeding behavior, and pain modulation. As research has focused mostly on their role in adult vertebrates, the possible roles of these peptides during development are poorly understood. However, the few studies that exist show that RFa are expressed early in development in different vertebrate classes, perhaps mostly associated with the central nervous system. Interestingly, the related peptide family of FMRFa has been shown to be important for brain development in invertebrates. In a teleost, the Japanese medaka, knockdown of genes in the Kiss system indicates that Kiss ligands and receptors are vital for brain development, but few other functional studies exist. Here, we review the literature of RFa in early vertebrate development, including the possible functional roles these peptides may play.

Hypothalamic QRFP: regulation of food intake and fat selection

QRFP, a member of the RFamide-related peptide family, is a strongly conserved hypothalamic neuropeptide that has been characterized in various species. Prepro-QRFP mRNA expression is localized to select regions of the hypothalamus, which are involved in the regulation of feeding behavior. The localization of the peptide precursor has led to the assessment of QRFP on feeding behaviors and the orexigenic effects of QRFP have been detected in mice, rats, and birds. QRFP acts in a macronutrient specific manner in satiated rats to increase the intake of a high fat diet, but not the intake of a low fat diet, and increases the intake of chow in food-restricted rats. Studies suggest that QRFP's effects on food intake are mediated by the adiposity signal, leptin, and hypothalamic neuropeptides. Additionally, QRFP regulates the expression and release of hypothalamic Neuropeptide Y and proopiomelanocortin/α-Melanocyte-Stimulating Hormone. QRFP binds to receptors throughout the brain, including regions associated with food intake and reward. Taken together, these data suggest that QRFP is a mediator of motivated behaviors, particularly the drive to ingest high fat food. The present review discusses the role of QRFP in the regulation of feeding behavior, with emphasis on the intake of dietary fat.

Identification, localization and function of a novel neuropeptide, 26RFa, and its cognate receptor, GPR103, in the avian hypothalamus

Several neuropeptides possessing the RFamide motif at their C-termini (designated RFamide peptides) have been characterized in the hypothalamus of a variety of vertebrates. Since the discovery of the 26-amino acid RFamide peptide (termed 26RFa) from the frog brain, 26RFa has been shown to exert orexigenic activity in mammals and to be a ligand of the previously identified orphan G protein-coupled receptor GPR103. Recently, we have identified 26RFa in the avian brain by molecular cloning of the cDNA encoding the 26RFa precursor and mass spectrometry analysis of the mature peptide. 26RFa-producing neurons are exclusively located in the hypothalamus whereas GPR103 is widely distributed in the avian brain. Furthermore, avian 26RFa stimulates feeding behavior in broiler chicks. This review summarizes the advances in the identification, localization, and functions of 26RFa and its cognate receptor GPR103 in vertebrates and highlights recent progress made in birds.

Endogenous mammalian RF-amide peptides, including PrRP, kisspeptin and 26RFa, modulate nociception and morphine analgesia via NPFF receptors

Mammalian RF-amide peptides are encoded by five different genes and act through five different G protein-coupled receptors. RF-amide-related peptides-1 and -3, neuropeptides AF and FF, Prolactin releasing peptides, Kisspeptins and RFa peptides are currently considered endogenous peptides for NPFF1, NPFF2, GPR10, GPR54 and GPR103 receptors, respectively. However, several studies suggest that the selectivity of these peptides for their receptors is low and indicate that expression patterns for receptors and their corresponding ligands only partially overlap. In this study, we took advantage of the cloning of the five human RF-amide receptors to systematically examine their affinity for and their activation by all human RF-amide peptides. Binding experiments, performed on membranes from CHO cells expressing GPR10, GPR54 and GPR103 receptors, confirmed their high affinity and remarkable selectivity for their cognate ligands. Conversely, NPFF1 and NPFF2 receptors displayed high affinity for all RF-amide peptides. Moreover, GTPγS and cAMP experiments showed that almost all RF-amide peptides efficiently activate NPFF1 and NPFF2 receptors. As NPFF is known to modulate morphine analgesia, we undertook a systematic analysis in mice of the hyperalgesic and anti morphine-induced analgesic effects of a representative set of endogenous RF-amide peptides. All of them induced hyperalgesia and/or prevented morphine analgesia following intracerebroventricular administration. Importantly, these effects were prevented by administration of RF9, a highly selective NPFF1/NPFF2 antagonist. Altogether, our results show that all endogenous RF-amide peptides display pain-modulating properties and point to NPFF receptors as essential players for these effects.

Role of neuropeptide FF in central cardiovascular and neuroendocrine regulation

Neuropeptide FF (NPFF) is an octapeptide belonging to the RFamide family of peptides that have been implicated in a wide variety of physiological functions in the brain including central cardiovascular and neuroendocrine regulation. The effects of these peptides are mediated via NPFF1 and NPFF2 receptors that are abundantly expressed in the rat and human brain. Herein, we review evidence for the role of NPFF in central regulation of blood pressure particularly within the brainstem and the hypothalamic paraventricular nucleus (PVN). At a cellular level, NPFF demonstrates distinct responses in magnocellular and parvocellular neurons of the PVN, which regulate the secretion of neurohypophyseal hormones and sympathetic outflow, respectively. Finally, the presence of NPFF system in the human brain and its alterations within the hypertensive brain are discussed.

Study of the role of novel RF-amide neuropeptides in affecting growth hormone secretion in a representative non-human primate (Macaca mulatta)

RF amide peptide family with distinctive terminal -Arg-Phe-NH(2) signature is evolutionarily conserved from invertebrates to mammals. These neuropeptides have been shown to affect diverse functions in invertebrates and vertebrates including influencing pituitary hormone secretion. More recently, two members of this family 26-amino acid and 43-amino acid RF amide peptide (26RFa and 43RFa, respectively) originally isolated from frog have been cloned in rats and humans. Actions of these peptides on hormone secretion have not been studied in primates. In the present study, effect of iv administration of three different doses of human 26RFa and 43RFa on GH secretion was studied in a representative higher primate, the rhesus monkey. As control against these two peptides, normal saline and a scrambled sequence of 26RFa was administered. A set of four intact adult male monkeys received the administration in a random order. Peripheral blood samples were obtained from the chairrestrained but fully conscious animals for a period of 30 min before and 240 min after the administration at 15-min intervals. For quantitative measurement of GH concentration, a human GH chemiluminescent immunometric assay was used. Peripheral administration of 38 and 76 nmol doses of 26RFa significantly (P < 0.05) stimulated GH AUC during a 0-120 min period after injection of 26RFa. In contrast to 26RFa, administration of 43RFa appeared to suppress GH levels during the later stages of the sampling i.e. from 120 to 240 min period. Mean AUC during the period was significantly (P < 0.05) reduced by 76 nmol dose of 43RFa, while 38 nmol dose of 43RFa also had similar effect but lacked full statistical significance (P = 0.058). To our knowledge present study reports for the first time-specific stimulatory effect of 26RFa on the GH secretion and a novel inhibitory and delayed effect of 43RFa on the GH secretion in higher primates. In conclusion, present findings extend evidence for endocrine actions of RF amides in primates and suggest differential effect of these peptides on GH secretion in primates.

Rational design of a low molecular weight, stable, potent, and long-lasting GPR103 aza-β3-pseudopeptide agonist

26RFa, a novel RFamide neuropeptide, is the endogenous ligand of the former orphan receptor GPR103. Intracerebroventricular injection of 26RFa and its C-terminal heptapeptide, 26RFa((20-26)), stimulates food intake in rodents. To develop potent, stable ligands of GPR103 with low molecular weight, we have designed a series of aza-β(3)-containing 26RFa((20-26)) analogues for their propensity to establish intramolecular hydrogen bonds, and we have evaluated their ability to increase [Ca(2+)](i) in GPR103-transfected cells. We have identified a compound, [Cmpi(21),aza-β(3)-Hht(23)]26RFa((21-26)), which was 8-fold more potent than 26RFa((20-26)) in mobilizing [Ca(2+)](i). This pseudopeptide was more stable in serum than 26RFa((20-26)) and exerted a longer lasting orexigenic effect in mice. This study constitutes an important step toward the development of 26RFa analogues that could prove useful for the treatment of feeding disorders.

Study of the effect of 26RF- and 43RF-amides on testosterone and prolactin secretion in the adult male rhesus monkey (Macaca mulatta)

RF-amides (RFa), a superfamily of evolutionary-conserved neuropeptides, are expressed in both invertebrates and vertebrates. While some endocrine functions have been attributed to these peptides in lower vertebrates and few mammalian models, not much is known about their actions in primates. Therefore, the present study was designed to examine the effects of peripheral administration of two recently cloned human RFa peptides, 26RFa and 43RFa, on testosterone and prolactin secretion in the adult male adult male rhesus monkey (Macaca mulatta). For control purposes, a scrambled sequence of 26RFa (Sc-26RFa) and normal saline (1ml) were injected. Three different doses of 26RFa and 43RFa (19-nmol, 38-nmol and 76-nmol) and a single dose (38-nmol) of Sc-26RFa were tested. A set of four chair-restraint habituated monkeys was used. Comparison of post-treatment T levels with respective pre levels showed that none of the doses of both 26RFa and 43RFa changed T release. Similarly, Sc-26RFa and saline administration also did not affect T levels. In contrast, all doses of 26RFa and 43RFa significantly (P<0.05) stimulated prolactin secretion. 43RFa dose dependently increased prolactin secretion while dose dependency was not observed for 26RFa. Saline and Sc-26RFa injection had no effect on prolactin concentrations. Thus, present study demonstrated that peripheral administration of 26RFa and 43RFa, in the doses tested, have no effect on T secretion, suggesting possible selective lack of their neuroendocrine role in controlling hypothalamic-pituitary-gonadal axis in the adult male primates. The prominent stimulation of prolactin suggests a neuroendocrine role of RFa peptides in regulation of prolactin release in primates.

Orexigenic neuropeptide 26RFa: new evidence for an adaptive profile of appetite regulation in anorexia nervosa

CONTEXT

Restrictive anorexia nervosa (AN) presents an adaptive appetite regulating profile including mainly high levels of ghrelin. Because this adaptive mechanism is not effective on food intake, other appetite-regulating peptides need to be explored. 26RFa is a hypothalamic neuropeptide that stimulates appetite, gonadotropin release, and bone metabolism.

OBJECTIVE

The objective of the study was to evaluate the circadian levels of 26RFa in AN patients compared with healthy subjects, other eating disorders, and constitutional thinness (CT).

DESIGN AND SETTINGS

This was a cross-sectional study performed in an endocrine unit and an academic laboratory.

INVESTIGATED SUBJECTS

Five groups of age-matched young women were included in the study: 19 restrictive AN, 10 AN with bingeing/purging episodes, 14 with CT, 10 bulimic, and 10 normal-weight controls.

MAIN OUTCOME MEASURES

Twelve-point circadian profiles of plasma 26RFa levels were measured in each subject.

RESULTS

Significant circadian variations of 26 RFA were noticed in controls with higher values in the morning and abrupt decrease at noon. Twenty-four-hour mean 26RFa levels were significantly increased in restrictive AN and AN with bingeing/purging episodes (P < 0.001), predominantly in the afternoon and evening when compared with controls. Preprandial rises of 26 RFA were noticed in AN patients. Mean 26RFa levels trend to be higher in CT than in controls (P = 0.06) and significantly lower than in AN. The bulimic patients presented a circadian profile of 26RFa similar to that of controls.

CONCLUSION

High levels of circulating 26RFa observed in AN patients might reflect an adaptive mechanism of the organism to promote energy intake and to increase fat stores in response to undernutrition.

The RFamide neuropeptide 26RFa and its role in the control of neuroendocrine functions

Identification of novel neuropeptides and their cognate G protein-coupled receptors is essential for a better understanding of neuroendocrine regulations. The RFamide peptides represent a family of regulatory peptides that all possess the Arg-Phe-NH2 motif at their C-terminus. In mammals, seven RFamide peptides encoded by five distinct genes have been characterized. The present review focuses on 26RFa (or QRFP) which is the latest member identified in this family. 26RFa is present in all vertebrate phyla and its C-terminal domain (KGGFXFRF-NH2), which is responsible for its biological activity, has been fully conserved during evolution. 26RFa is the cognate ligand of the orphan G protein-coupled receptor GPR103 that is also present from fish to human. In all vertebrate species studied so far, 26RFa-expressing neurons show a discrete localization in the hypothalamus, suggesting important neuroendocrine activities for this RFamide peptide. Indeed, 26RFa plays a crucial role in the control of feeding behavior in mammals, birds and fish. In addition, 26RFa up-regulates the gonadotropic axis in mammals and fish. Finally, evidence that the 26RFa/GPR103 system regulates steroidogenesis, bone formation, nociceptive transmission and arterial blood pressure has also been reported. Thus, 26RFa appears to act as a key neuropeptide in vertebrates controlling vital neuroendocrine functions. The pathophysiological implication of the 26RFa/GPR103 system in human is totally unknown and some fields of investigation are proposed.

Structure-activity relationships of a series of analogues of the RFamide-related peptide 26RFa

26RFa is a new member of the RFamide peptide family that has been identified as the endogenous ligand of the orphan GPCR GPR103. As the C-terminal heptapeptide (26RFa((20-26))) mimics the action of the native peptide on food intake and gonadotropin secretion in rodents, we have synthesized a series of analogues of 26RFa((20-26)) and measured their potency to induce [Ca(2+)](i) mobilization in Gα(16)-hGPR103-transfected CHO cells. Systematic replacement of each residue by an alanine (Ala scan) and its D-enantiomer (D scan) showed that the last three C-terminal residues were very sensitive to the substitutions while position 23 tolerated rather well both modifications. Most importantly, replacement of Ser(23) by a norvaline led to an analogue, [Nva(23)]26RFa((20-26)), that was 3-fold more potent than the native heptapeptide. These new pharmacological data, by providing the first information regarding the structure-activity relationships of 26RFa analogues, should prove useful for the rational design of potent GPR103 receptor ligands with potential therapeutic application.