Quantitative autoradiographic distribution of NPFF1 neuropeptide FF receptor in the rat brain and comparison with NPFF2 receptor by using [125I]YVP and [(125I]EYF as selective radioligands

NPFFR2-deficient mice fed a high-fat diet develop strong intolerance to glucose

A previous study on neuropeptide FF receptor 2 (NPFFR2)-deficient mice has demonstrated that NPFFR2 is involved in the control of energy balance and thermogenesis. Here, we report on the metabolic impact of NPFFR2 deficiency in male and female mice that were fed either a standard diet (STD) or a high-fat diet (HFD) and each experimental group consisted of ten individuals. Both male and female NPFFR2 knockout (KO) mice exhibited severe glucose intolerance that was exacerbated by a HFD diet. In addition, reduced insulin pathway signaling proteins in NPFFR2 KO mice fed a HFD resulted in the development of hypothalamic insulin resistance. HFD feeding did not cause liver steatosis in NPFFR2 KO mice of either sex, but NPFFR2 KO male mice fed a HFD had lower body weights, white adipose tissues, and liver and lower plasma leptin levels compared with their wild-type (WT) controls. Lower liver weight in NPFFR2 KO male mice compensated for HFD-induced metabolic stress by increased liver PPARα and plasma FGF21 hepatokine, which supported fatty acid β-oxidation in the liver and white adipose tissue. Conversely, NPFFR2 deletion in female mice attenuated the expression of Adra3β and Pparγ, which inhibited lipolysis in adipose tissue.

Lipidated PrRP31 metabolites are long acting dual GPR10 and NPFF2 receptor agonists with potent body weight lowering effect

Prolactin-releasing peptide (PrRP) is an endogenous neuropeptide involved in appetite regulation and energy homeostasis. PrRP binds with high affinity to G-protein coupled receptor 10 (GPR10) and with lesser activity towards the neuropeptide FF receptor type 2 (NPFF2R). The present study aimed to develop long-acting PrRP31 analogues with potent anti-obesity efficacy. A comprehensive series of C18 lipidated PrRP31 analogues was characterized in vitro and analogues with various GPR10 and NPFF2R activity profiles were profiled for bioavailability and metabolic effects following subcutaneous administration in diet-induced obese (DIO) mice. PrRP31 analogues acylated with a C18 lipid chain carrying a terminal acid (C18 diacid) were potent GPR10-selective agonists and weight-neutral in DIO mice. In contrast, acylation with aliphatic C18 lipid chain (C18) resulted in dual GPR10-NPFF2R co-agonists that suppressed food intake and promoted a robust weight loss in DIO mice, which was sustained for at least one week after last dosing. Rapid in vivo degradation of C18 PrRP31 analogues gave rise to circulating lipidated PrRP metabolites maintaining dual GPR10-NPFF2R agonist profile and long-acting anti-obesity efficacy in DIO mice. Combined GPR10 and NPFF2R activation may therefore be a critical mechanism for obtaining robust anti-obesity efficacy of PrRP31 analogues.

Development of Multifunctional and Orally Active Cyclic Peptide Agonists of Opioid/Neuropeptide FF Receptors that Produce Potent, Long-Lasting, and Peripherally Restricted Antinociception with Diminished Side Effects

We previously reported that a multifunctional opioid/neuropeptide FF receptor agonist, DN-9, achieved peripherally restricted analgesia with reduced side effects. To develop stable and orally bioavailable analogues of DN-9, eight lactam-bridged cyclic analogues of DN-9 between positions 2 and 5 were designed, synthesized, and biologically evaluated. In vitro cAMP assays revealed that these analogues, except 7, were multifunctional ligands that activated opioid and neuropeptide FF receptors. Analogue 1 exhibited improved potency for κ-opioid and NPFF₂ receptors. All analogues exhibited potent, long-lasting, and peripherally restricted antinociception in the tail-flick test without tolerance development after subcutaneous administration and produced oral analgesia. Oral administration of the optimized compound analogue 1 exhibited powerful, peripherally restricted antinociceptive effects in mouse models of acute, inflammatory, and neuropathic pain. Remarkably, orally administered analogue 1 had no significant side effects, such as tolerance, dependence, constipation, or respiratory depression, at effective analgesic doses.

Neuropeptide FF and Its Receptors: Therapeutic Applications and Ligand Development

The endogenous neuropeptide FF (NPFF) and its two cognate G protein-coupled receptors, Neuropeptide FF Receptors 1 and 2 (NPFFR1 and NPFFR2), represent a relatively new target system for many therapeutic applications including pain regulation, modulation of opioid side effects, drug reward, anxiety, cardiovascular conditions, and other peripheral effects. Since the cloning of NPFFR1 and NPFFR2 in 2000, significant progress has been made to understand their pharmacological roles and interactions with other receptor systems, notably the opioid receptors. A variety of NPFFR ligands with different mechanisms of action (agonists or antagonists) have been discovered although with limited subtype selectivities. Differential pharmacological effects have been observed for many of these NPFFR ligands, depending on assays/models employed and routes of administration. In this Perspective, we highlight the therapeutic potentials, current knowledge gaps, and latest updates of the development of peptidic and small molecule NPFFR ligands as tool compounds and therapeutic candidates.

Spinal DN-9, a Peptidic Multifunctional Opioid/Neuropeptide FF Agonist Produced Potent Nontolerance Forming Analgesia With Limited Side Effects

The development of multitarget opioid drugs has emerged as an attractive therapeutic strategy to eliminate opioid-related side effects. Our previous study developed a series of opioid and neuropeptide FF pharmacophore-containing chimeric peptides, including DN-9 (Tyr-D.Ala-Gly-NMe.Phe-Gly-Pro-Gln-Arg-Phe-NH₂), which produced potent nontolerance forming analgesia at the supraspinal level. In the present study, the antinociceptive effects of DN-9 in a series of preclinical pain models and the potential side-effects were investigated at the spinal level in mice. In the tail-flick test, intrathecal injection of DN-9 produced potent analgesia with an ED₅₀ value at 1.33 pmol, and the spinal antinociception of DN-9 was mainly mediated by μ- and κ-opioid receptors. In addition, DN-9-induced spinal antinociception was augmented by the neuropeptide FF receptors antagonist. Furthermore, DN-9 could decrease both the frequency and amplitude of sEPSCs in lamina IIo neurons of the spinal cord, which were mediated by opioid receptors. In contrast to morphine, chronic intrathecal treatments with DN-9 did not induce analgesic tolerance, c-Fos expression or microglial activation. Intrathecal injection of DN-9 showed potent analgesia with antinociceptive ED₅₀ values between .66 and 55.04 pmol in different pain models, including the formalin test, acetic acid-induced writhing test, carrageenan-induced inflammatory pain and neuropathic pain. Moreover, DN-9 did not show side effects in locomotor function and coordination, gastrointestinal transit inhibition, the cardiovascular system, and body temperature regulation at antinociceptive doses. Taken together, the present study showed DN-9 produced effective, nontolerance forming analgesia with reduced side effects at the spinal level. DN-9 might be a promising compound for developing multifunctional opioid analgesics with limited adverse effects. PERSPECTIVE: This article presents the potent and nontolerance forming analgesia effects of DN-9 in a series of preclinical pain models with less opioid related adverse effects at the spinal level in mice. This study also demonstrates that DN-9 has translational potential into an intrathecal analgesic.

Aminoguanidine Hydrazone Derivatives as Nonpeptide NPFF1 Receptor Antagonists Reverse Opioid Induced Hyperalgesia

Neuropeptide FF receptors (NPFF1R and NPFF2R) and their endogenous ligand neuropeptide FF have been shown previously to display antiopioid properties and to play a critical role in the adverse effects associated with chronic administrations of opiates including the development of opioid-induced hyperalgesia and analgesic tolerance. In this work, we sought to identify novel NPFF receptors ligands by focusing our interest in a series of heterocycles as rigidified nonpeptide NPFF receptor ligands, starting from already described aminoguanidine hydrazones (AGHs). Binding experiments and functional assays highlighted AGH 1n and its rigidified analogue 2-amino-dihydropyrimidine 22e for in vivo experiments. As shown earlier with the prototypical dipeptide antagonist RF9, both 1n and 22e reduced significantly the long lasting fentanyl-induced hyperalgesia in rodents. Altogether these data indicate that AGH rigidification maintains nanomolar affinities for both NPFF receptors, while improving antagonist character toward NPFF1R.

NPFFR2 Activates the HPA Axis and Induces Anxiogenic Effects in Rodents

Neuropeptide FF (NPFF) belongs to the RFamide family and is known as a morphine-modulating peptide. NPFF regulates various hypothalamic functions through two receptors, NPFFR1 and NPFFR2. The hypothalamic-pituitary-adrenal (HPA) axis participates in physiological stress response by increasing circulating glucocorticoid levels and modulating emotional responses. Other RFamide peptides, including neuropeptide AF, neuropeptide SF and RFamide related peptide also target NPFFR1 or NPFFR2, and have been reported to activate the HPA axis and induce anxiety- or depression-like behaviors. However, little is known about the action of NPFF on HPA axis activity and anxiety-like behaviors, and the role of the individual receptors remains unclear. In this study, NPFFR2 agonists were used to examine the role of NPFFR2 in activating the HPA axis in rodents. Administration of NPFFR2 agonists, dNPA (intracerebroventricular, ICV) and AC-263093 (intraperitoneal, IP), time-dependently (in rats) and dose-dependently (in mice) increased serum corticosteroid levels and the effects were counteracted by the NPFF receptor antagonist, RF9 (ICV), as well as corticotropin-releasing factor (CRF) antagonist, α-helical CRF(9-41) (intravenous, IV). Treatment with NPFFR2 agonist (AC-263093, IP) increased c-Fos protein expression in the hypothalamic paraventricular nucleus and induced an anxiogenic effect, which was evaluated in mice using an elevated plus maze. These findings reveal, for the first time, that the direct action of hypothalamic NPFFR2 stimulates the HPA axis and triggers anxiety-like behaviors.

Activation of NPFF2 receptor stimulates neurite outgrowth in Neuro 2A cells through activation of ERK signaling pathway

Neurite outgrowth is an important process in neural regeneration and plasticity, especially after neural injury, and recent evidence indicates that several G_αi/o protein-coupled receptors play an important role in neurite outgrowth. The neuropeptide (NP)FF system contains two G_αi/o protein-coupled receptors, NPFF₁ and NPFF₂ receptors, which are mainly distributed in the central nervous system. The aim of the present study was to determine whether the NPFF system is involved in neurite outgrowth in Neuro 2A cells. We showed that Neuro 2A cells endogenously expressed NPFF₂ receptor, and the NPFF₂ receptor agonist dNPA inhibited cyclic adenosine monophosphate (cAMP) production stimulated by forskolin in Neuro 2A cells. We also demonstrated that NPFF and dNPA dose-dependently induced neurite outgrowth in Neuro 2A cells, which was completely abolished by the NPFF receptor antagonist RF9. Pretreatment with mitogen-activated protein kinase inhibitors PD98059 and U0126 decreased dNPA-induced neurite outgrowth. In addition, dNPA increased phosphorylation of extracellular signal-regulated kinase (ERK) in Neuro 2A cells, which was completely antagonized by pretreatment with U0126. Our results suggest that activation of NPFF₂ receptor stimulates neurite outgrowth in Neuro 2A cells through activation of the ERK signaling pathway. Moreover, NPFF₂ receptor may be a potential therapeutic target for neural injury and degeneration in the future.

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

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

What's in a Name? Roles of RFamide-Related Peptides Beyond Gonadotrophin Inhibition

RFamide-related peptides (RFRPs) have been heavily implicated in the control of reproductive function subsequent to their discovery more than 16 years ago. However, recent studies using genetic and pharmacological tools have challenged their importance in regulating the hypothalamic-pituitary-gonadal axis. It is generally accepted that RFRPs act as part of a wider RFamide system, which involves two receptors, called the neuropeptide FF receptors (NPFFR1 and R2), and includes the closely-related neuropeptide NPFF. NPFF has been studied ever since the 1980s and many of the functions of NPFF are also shared by RFRPs. The current review questions whether these functions of NPFF are indeed specific to just NPFF alone and presents evidence from both neuroendocrine and pharmacological perspectives. Furthermore, recently emerging new functions of RFRPs are discussed with the overall goal of clarifying the functions of RFRPs beyond the hypothalamic-pituitary-gonadal axis.

Chronic activation of NPFFR2 stimulates the stress-related depressive behaviors through HPA axis modulation

Neuropeptide FF (NPFF) is a morphine-modulating peptide that regulates the analgesic effect of opioids, and also controls food consumption and cardiovascular function through its interaction with two cognate receptors, NPFFR1 and NPFFR2. In the present study, we explore a novel modulatory role for NPFF-NPFFR2 in stress-related depressive behaviors. In a mouse model of chronic mild stress (CMS)-induced depression, the expression of NPFF significantly increased in the hypothalamus, hippocampus, medial prefrontal cortex (mPFC) and amygdala. In addition, transgenic (Tg) mice over-expressing NPFFR2 displayed clear depression and anxiety-like behaviors with hyperactivity in the hypothalamic-pituitary-adrenal (HPA) axis, reduced expression of glucocorticoid receptor (GR) and neurogenesis in the hippocampus. Furthermore, acute treatment of NPFFR2 agonists in wild-type (WT) mice enhanced the activity of the HPA axis, and chronic administration resulted in depressive and anxiety-like behaviors. Chronic stimulation of NPFFR2 also decreased the expression of hippocampal GR and led to persistent activation of the HPA axis. Strikingly, bilateral intra-paraventricular nucleus (PVN) injection of NPFFR2 shRNA predominately inhibits the depressive-like behavior in CMS-exposed mice. Antidepressants, fluoxetine and ketamine, effectively relieved the depressive behaviors of NPFFR2-Tg mice. We speculate that persistent NPFFR2 activation, in particular in the hypothalamus, up-regulates the HPA axis and results in long-lasting increases in circulating corticosterone (CORT), consequently damaging hippocampal function. This novel role of NPFFR2 in regulating the HPA axis and hippocampal function provides a new avenue for combating depression and anxiety-like disorder.

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.

Cholesterol-rich lipid rafts are involved in neuropeptide FF anti-nociceptin/orphanin FQ effect

The participation of a signaling platform to the anti-nociceptin/orphanin FQ (N/OFQ) effect of neuropeptide FF (NPFF) receptors was investigated in both acutely dissociated neurons and SH-SY5Y human neuroblastoma cells. The NPFF anti-N/OFQ, not anti-μ opioid effect, on the Ca²⁺ transient triggered by depolarization was reversed by methyl-β-cyclodextrin which depletes cholesterol from cell membranes. While the inactive α-cyclodextrin had no effect. By using [³⁵ S]GTPγS binding assay, a significant 20% decrease in the activity of nociceptin/orphanin FQ peptide receptors induced by the NPFF analog 1DMe was observed in detergent-resistant membranes, but not in total membranes of SH-SY5Y cells. Moreover, siRNA knock-down of G-protein-coupled receptor kinase 2 indicated that G-protein-coupled receptor kinase 2, but not protein kinase C, acted as the mediator in the NPFF anti-N/OFQ process. These data indicate that cholesterol-rich lipid rafts play an important role in the anti-N/OFQ effect of NPFF receptors. We proposed the participation of a signaling platform to the anti-Nociceptin/Orphanin FQ (N/OFQ) effect of Neuropeptide FF (NPFF) receptors both in mouse neurons and SH-SY5Y cells, with GRK2 protein acting as the mediator in this process. These findings should provide a more precise way to understand the anti-opioid effect of NPFF. NOP, Nociceptin/Orphanin FQ peptide.

Kisspeptin regulation of arcuate neuron excitability in kisspeptin receptor knockout mice

The G protein-coupled receptor 54 (GPR54) is critical for kisspeptin to activate GnRH neurons to modulate fertility. However, the often mismatching distribution of kisspeptin and GPR54 in the brain suggests that kisspeptin may also act on other receptors. The arcuate nucleus (ARN) is one brain region with a very high density of kisspeptin fibers but only limited evidence for the expression of GPR54. Using acute brain slice electrophysiology in combination with Gpr54 knockout (GPR54KO) mouse models, we examined whether actions of kisspeptin in the ARN were dependent upon GPR54. Cell-attached recordings from unidentified ARN neurons in wild-type mice revealed that approximately one third of neurons were either excited or inhibited by kisspeptin in a dose-dependent manner. The responses of ARN neurons to kisspeptin were exactly the same in GPR54KO mice despite effects of kisspeptin on GnRH neurons being abolished. To evaluate whether kisspeptin may be acting through neuropeptide FF receptors, the effects of an agonist RFamide-related peptide 3 (RFRP-3) and antagonists RF9 and BIBP-3226 were evaluated. Both the excitatory and inhibitory effects of kisspeptin were mimicked by the agonist RFRP-3. RF9 itself activated ARN neurons and suppressed only the inhibitory actions of kisspeptin. BIBP-3226 suppressed kisspeptin actions in 50% of neurons. Whole-cell recordings in GPR54KO mice demonstrated that both kisspeptin and RFRP-3 acted directly on the same ARN neurons and activated the same ion channels. Together, these studies demonstrate that kisspeptin can act partly through neuropeptide FF receptors to modulate neuronal activity independent of GPR54 in the mouse brain.

Nonpeptide small molecule agonist and antagonist original leads for neuropeptide FF1 and FF2 receptors

Neuropeptide FF1 and FF2 receptors (NPFF1-R and NPFF2-R), and their endogenous ligand NPFF, are one of only several systems responsible for mediating opioid-induced hyperalgesia, tolerance, and dependence. Currently, no small molecules displaying good affinity or selectivity for either subtype have been reported, to decipher the role of NPFF2-R as it relates to opioid-mediated analgesia, for further exploration of NPFF1-R, or for medication development for either subtype. We report the first nonpeptide small molecule scaffold for NPFF1,2-R, the guanidino-piperidines, and SAR studies resulting in the discovery of a NPFF1 agonist (7b, K(i) = 487 ± 117 nM), a NPFF1 antagonist (46, K(i) = 81 ± 17 nM), and a NPFF2 partial antagonist (53a, K(i) = 30 ± 5 nM), which serve as leads for the development of pharmacological probes and potential therapeutic agents. Testing of 46 alone was without effect in the mouse 48 °C warm-water tail-withdrawal test, but pretreatment with 46 prevented NPFF-induced hyperalgesia.

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.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

From gonadotropin-inhibitory hormone to SIFamides: are echinoderm SALMFamides the "missing link" in a bilaterian family of neuropeptides that regulate reproductive processes?

Gonadotropin-inhibitory hormone (GnIH) belongs to a family of vertebrate neuropeptides with a C-terminal PxRFamide motif, which exert effects by activating the G-protein coupled receptors NPFF1 and/or NPFF2. Comparative analysis of genome sequence data has revealed that orthologs of NPFF1/NPFF2-type receptors occur throughout the Bilateria and the neuropeptide ligand that activates the Drosophila NPFF1/NPFF2-type receptor has been identified as AYRKPPFNGSIFamide ("SIFamide"). Therefore, SIFamide-type neuropeptides, which occur throughout protostomian invertebrates, probably share a common evolutionary origin with vertebrate PxRFamide-type neuropeptides. Based on structural similarities, here SALMFamide neuropeptides are identified as candidate ligand components of this ancient bilaterian peptide-receptor signaling system in a deuterostomian invertebrate phylum, the echinoderms (e.g., starfish, sea urchins). Furthermore, functional studies provide evidence that PxRFamide/SALMFamide/SIFamide-type neuropeptides have evolutionarily conserved roles in regulation (typically inhibitory) of reproductive processes.

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.

Crypteins derived from the mouse neuropeptide FF (NPFF)A precursor display NPFF-like effects in nociceptive tests in mice

NPFF precursor, pro-NPFF(A) contains three known bioactive sequences: NPFF (FLFQPQRF-NH(2)), neuropeptide AF (NPAF; AGEGLSSPFWSLAAPQRF-NH(2)) and neuropeptide SF (NPSF; SLAAPQRF-NH(2)). The key-feature of these fragments is their common PQRF-amidated sequence at their C termini. Here, we evaluated the biological activity of two other sequences derived from the mouse NPFF(A) precursor, that does not have PQRF-amidated C-terminus. One peptide was residing between positions 85 and 99 in the mice pro-NPFF(A). This peptide was referred to as neuropeptide SA (NPSA; SAWGSWSKEQLNPQA), assigned due to its flanking amino acids. Another sequence used in the experiments was N-terminal fragment of NPSA, here referred to as neuropeptide SS (NPSS; SAWGSWS). These two peptides, classified as crypteins, were synthesized and tested in the hot-plate and tail immersion tests in mice for their pharmacological activity in morphine-induced antinociception. The effects of both crypteins were compared to NPFF. Our experiments indicated that both crypteins inhibited morphine antinociception and their effects were reversed by RF9, an antagonist of NPFF receptors. These data show that NPSA and NPSS possess NPFF-like anti-opioid activity in these behavioral tests.

Selective mode of action of guanidine-containing non-peptides at human NPFF receptors

The binding pocket of both NPFF receptors was investigated, focusing on subtype-selective behavior. By use of four nonpeptidic compounds and the peptide mimetics RF9 and BIBP3226, agonistic and antagonistic properties were characterized. A set of Ala receptor mutants was generated. The binding pocket was narrowed down to the upper part of transmembrane helices V, VI, VII and the extracellular loop 2. Positions 5.27 and 6.59 have been shown to have a strong impact on receptor activation and were suggested to form an acidic, negatively charged binding pocket in both NPFF receptor subtypes. Additionally, position 7.35 was identified to play an important role in functional selectivity. According to docking experiments, the aryl group of AC-216 interacts with position 7.35 in the NPFF(1) but not in the NPFF(2) receptor. These results provide distinct insights into the receptor specific binding pockets, which is necessary for the development of drugs to address the NPFF system.

Stimulatory effect of RFRP-3 on the gonadotrophic axis in the male Syrian hamster: the exception proves the rule

In seasonal mammals, a distinct photoneuroendocrine circuit that involves the pineal hormone melatonin tightly synchronizes reproduction with seasons. In the Syrian hamster, a seasonal model in which sexual activity is inhibited by short days, we have previously shown that the potent GnRH stimulator, kisspeptin, is crucial to convey melatonin's message; however, the precise mechanisms through which melatonin affects kisspeptin remain unclear. Interestingly, rfrp gene expression in the neurons of the dorsomedial hypothalamic nucleus, a brain region in which melatonin receptors are present in the Syrian hamster, is strongly down-regulated by melatonin in short days. Because a large body of evidence now indicates that RFamide-related peptide (RFRP)-3, the product of the rfrp gene, is an inhibitor of gonadotropin secretion in various mammalian species, we sought to investigate its effect on the gonadotrophic axis in the Syrian hamster. We show that acute central injection of RFRP-3 induces c-Fos expression in GnRH neurons and increases LH, FSH, and testosterone secretion. Moreover, chronic central administration of RFRP-3 restores testicular activity and Kiss1 levels in the arcuate nucleus of hamsters despite persisting photoinhibitory conditions. By contrast RFRP-3 does not have a hypophysiotrophic effect. Overall, these findings demonstrate that, in the male Syrian hamster, RFRP-3 exerts a stimulatory effect on the reproductive axis, most likely via hypothalamic targets. This places RFRP-3 in a decisive position between the melatonergic message and Kiss1 seasonal regulation. Additionally, our data suggest for the first time that the function of this peptide depends on the species and the physiological status of the animal model.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

RFamide Peptides: Structure, Function, Mechanisms and Pharmaceutical Potential

Different neuropeptides, all containing a common carboxy-terminal RFamide sequence, have been characterized as ligands of the RFamide peptide receptor family. Currently, five subgroups have been characterized with respect to their N-terminal sequence and hence cover a wide pattern of biological functions, like important neuroendocrine, behavioral, sensory and automatic functions. The RFamide peptide receptor family represents a multiligand/multireceptor system, as many ligands are recognized by several GPCR subtypes within one family. Multireceptor systems are often susceptible to cross-reactions, as their numerous ligands are frequently closely related. In this review we focus on recent results in the field of structure-activity studies as well as mutational exploration of crucial positions within this GPCR system. The review summarizes the reported peptide analogs and recently developed small molecule ligands (agonists and antagonists) to highlight the current understanding of the pharmacophoric elements, required for affinity and activity at the receptor family. Furthermore, we address the biological functions of the ligands and give an overview on their involvement in physiological processes. We provide insights in the knowledge for the design of highly selective ligands for single receptor subtypes to minimize cross-talk and to eliminate effects from interactions within the GPCR system. This will support the drug development of members of the RFamide family.

Opioid-modulating properties of the neuropeptide FF system

Opioid receptors are involved in the control of pain perception in the central nervous system together with endogenous neuropeptides, termed opioid-modulating peptides, participating in a homeostatic system. Neuropeptide FF (NPFF) and related peptides possess anti-opioid properties, the cellular mechanisms of which are still unclear. The purpose of this review is to detail the phenomenon of cross-talk taking place between opioid and NPFF systems at the in vivo pharmacological level and to propose cellular and molecular models of functioning. A better knowledge of the mechanisms underlying opioid-modulating properties of NPFF has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

Prolactin-releasing peptide effects in the rat brain are mediated through the Neuropeptide FF receptor

Prolactin-releasing peptide (PrRP), an RF amide peptide present in the brain, generates a wide variety of centrally generated autonomic responses, including increases in arterial blood pressure and heart rate. The identity of the receptor mediating the effects of PrRP is unknown. In addition to GPR10, which is its putative endogenous receptor, PrRP demonstrates a high binding affinity for Neuropeptide FF (NPFF) receptors, specifically the NPFF2 receptor. In the present study, we examined whether the central cardiovascular effects of PrRP in the intact animal and its cellular effects on parvocellular paraventricular nucleus (PVN) neurons are mediated via NPFF receptors. In conscious rats, intracerebroventricular (i.c.v.) PrRP caused an increase in arterial blood pressure and heart rate, which was blocked with RF9, a specific NPFF receptor antagonist. These PrRP-evoked cardiovascular effects were preserved in the Otsuka Long-Evans Tokushima Fatty (OLETF) rat strain, in which the GRP10 receptor gene was mutated. In rat brain slices, whole-cell patch clamp recordings of parvocellular paraventricular nucleus neurons show PrRP caused a decrease in evoked and miniature GABAergic inhibitory postsynaptic currents (IPSCs), effects that were antagonized by RF9, but not neuropeptide Y, a putative GPR10 receptor antagonist. The effects of PrRP on IPSCs in OLETF rats were similar to those in wild-type rats. Both in vivo and in vitro data strongly suggest that certain PrRP effects in the brain are expressed via NPFF receptors, probably NPFF2, rather than the GPR10 receptor. These observations may assume clinical relevance as RF amide peptides such NPFF and PrRP become therapeutic targets for a variety of autonomically related disorders.

Neuropeptide FF2 receptor distribution in the human brain. An immunohistochemical study

Human neuropeptide FF2 (hFF2) receptor has been postulated to mediate central autonomic regulation by virtue of its ability to bind with high affinity to many amidated neuropeptides. In the present immunohistochemical study, we identified hFF2 positive neurons in the forebrain and medulla oblongata of individuals, who died suddenly of mechanical trauma or hypothermia. Morphologically, these neurons demonstrated features identified with both projection neurons and interneurons. In the forebrain, the highest density of hFF2 expressing neurons was observed in the anterior amygdaloid area and dorsomedial hypothalamic nucleus, especially in its caudal part. A lesser density of hFF2 neurons was identified in the ventromedial hypothalamic nucleus, lateral and posterior hypothalamic areas whereas few cells were visualized in the paraventricular hypothalamic nucleus, perifornical nucleus, horizontal limb of the diagonal band, ventral division of the bed nucleus of the stria terminalis, nucleus basalis of Meynert and ventral tegmental area. In the medulla, significant numbers of hFF2 neurons were observed in the dorsal motor nucleus of vagus and to a lesser extent in the area of catecholaminergic cell groups, A1/C1. These data provide first immunohistochemical evidence of hFF2 localization in the human brain, which is consistent with that reported for tissue distribution of FF2 mRNA and FF2 binding sites within the brain of a variety of mammalian species. The distribution of hFF2 may help in identifying the role of amidated neuropeptides in the human brain within the context of central autonomic and neuroendocrine regulation.

Decreased motivational properties of morphine in mouse models of cancerous- or inflammatory-chronic pain: implication of supraspinal neuropeptide FF(2) receptors

Our main purpose was to evaluate the influence of cancer pain on the rewarding properties of morphine. Opioids are very addictive when used by healthy persons, conversely the occurrence of an opioid addiction seems very low when patients suffering from cancer are treated with morphine. We investigated the reinforcing properties of morphine in the place preference paradigm on a new model of mice suffering from a cancer pain induced by syngenic melanoma cells injected in the hind paw. These data were compared with mice suffering either from a short-term- or a chronic-inflammatory pain induced respectively by injection of carrageenan or complete Freund's adjuvant. Remarkably, mice suffering from cancer pain or chronic inflammatory pain did not develop any preference for the environment associated with the injection of morphine. In mice injected with melanoma cells, the specific binding of [(125)I]EYWSLAAPQRF-NH(2), an agonist of neuropeptide FF(2) receptors, was increased in several brain areas involved in the rewarding properties of opiates, including the shell of the nucleus accumbens, the major islands of Calleja, the ventral endopiriform nucleus and the amygdaloid area. Our study is the first to reveal a modification of morphine rewarding properties under cancer pain in rodents. We postulate that anti-opioid neuropeptides might contribute to the suppression of morphine rewarding effects in this murine model of cancer pain.

Central neuropeptide FF reduces feed consumption and affects hypothalamic chemistry in chicks

Information on the physiological functions of neuropeptide FF; NPFF, a morphine modulating octapeptide in avians is lacking. Thus, we designed a study to investigate the effects of central NPFF with particular emphasis on appetite-related processes. Cobb-500 chicks were intracerebroventricularly (ICV) injected with 0, 4.16, 8.32 or 16.6nmol NPFF, and feed and water intake were quantified. Feed intake was linearly decreased as NPFF dose increased, and this effect decayed over time and was not significant by 120min post-injection. Water intake was not affected by ICV NPFF. In a second exp, we observed that naloxone completely reversed the NPFF-induced decrease in feed intake. The amount of time a visible marker took to travel through the total length of the alimentary canal linearly increased as NPFF dose increased. We measured neuronal activation in the lateral hypothalamus (LH), paraventricular nucleus (PVN) dorsomedial nucleus (DMN) and ventromedial hypothalamus (VMN) of the hypothalamus, and nucleus dorsomedialis posterior thalami (DMP) of the thalamus. The DMN, DMP, PVN and VMH were all activated by ICV NPFF while the LH was not affected. Finally, we determined that the anorexigenic effect of ICV NPFF is primarily behavior specific, since behaviors unrelated to ingestion were not increased the same duration of time as was consumatory pecking. We conclude that NPFF causes anorexigenic effects in chicks that are primarily behavior specific.

Neuropeptide FF (NPFF) reduces the expression of morphine- but not of ethanol-induced conditioned place preference in rats

Neuropeptide FF (NPFF) has been described as an anti-opioid peptide. It plays a role in opioid antinociception, dependence and tolerance. Previous study has indicated that 1DMe ([D-Tyr(1), (NMe)Phe(3)]NPFF), a stable analog of NPFF, inhibits acquisition of the rewarding effect of morphine but not of ethanol in mice. The rewarding effects of these drugs were measured in the unbiased paradigm of conditioned place preference (CPP). The present study examines the influence of NPFF on the expression of morphine- and ethanol-induced CPP in the biased procedure in rats. Our experiments showed that NPFF, given intracerebroventricularly (i.c.v.) at the doses of 5, 10 and 20 nmol, inhibited the expression of morphine-induced CPP. NPFF gave itself, neither induced place preference nor aversion, although a tendency to aversive effect was seen at the highest dose of 20 nmol. NPFF did not indicate fear behavior in the elevated plus maze test, and did not disturb locomotor activity of rats. However, NPFF was unable to inhibit the expression of ethanol-induced CPP. Probably this effect is due to the fact that ethanol reward is a more complex process and apart from the role of opioids, there are other neurotransmitters also involved in this mechanism. These results suggest that NPFF is involved in the expression of morphine reward. Moreover, our study supports an anti-opioid character of this peptide.

Neuropeptide FF and neuropeptide VF inhibit GABAergic neurotransmission in parvocellular neurons of the rat hypothalamic paraventricular nucleus

Neuropeptide FF (NPFF) and neuropeptide VF (NPVF) are octapeptides belonging to the RFamide family of peptides that have been implicated in a wide variety of physiological functions in the brain, including central autonomic and neuroendocrine regulation. The effects of these peptides are mediated via NPFF1 and NPFF2 receptors that are abundantly expressed in the rat brain, including the hypothalamic paraventricular nucleus (PVN), an autonomic nucleus critical for the secretion of neurohormones and the regulation of sympathetic outflow. In this study, we examined, using whole cell patch-clamp recordings in the brain slice, the effects of NPFF and NPVF on inhibitory GABAergic synaptic input to parvocellular PVN neurons. Under voltage-clamp conditions, NPFF and NPVF reversibly and in a concentration-dependent manner reduced the evoked bicuculline-sensitive inhibitory postsynaptic currents (IPSCs) in parvocellular PVN neurons by 25 and 31%, respectively. RF9, a potent and selective NPFF receptor antagonist, blocked NPFF-induced reduction of IPSCs. Recordings of miniature IPSCs in these neurons following NPFF and NPVF applications showed a reduction in frequency but not amplitude, indicating a presynaptic locus of action for these peptides. Under current-clamp conditions, NPVF and NPFF caused depolarization (6-9 mV) of neurons that persisted in the presence of TTX but was abolished in the presence of bicuculline. Collectively, these data provide evidence for a disinhibitory role of NPFF and NPVF in the hypothalamic PVN via an attenuation of GABAergic inhibitory input to parvocellular neurons of this nucleus and explain the central autonomic effects of NPFF.

Biochemical anti-opioid action of NPFF2 receptors in rat spinal cord

Neuropeptide FF (NPFF) agonists counteract the cellular opioid actions. We demonstrate for the first time a biochemical anti-opioid effect of NPFF receptors in the rat spinal cord by using the [(35)S]GTPgammaS binding assay in autoradiography. The mu agonist DAMGO as well as the potent and selective NPFF(2) agonist dNPA, stimulated [(35)S]GTPgammaS binding at different optimal GDP concentrations. dNPA decreased the effects induced by DAMGO alone; the maximal of G-protein coupling was decreased but not the potency of opioid agonist. We conclude that NPFF(2) receptors are coupled to G-protein in the rat spinal cord and could exert a molecular anti-opioid effect.

Physical association between neuropeptide FF and micro-opioid receptors as a possible molecular basis for anti-opioid activity

Neuropeptide FF (NPFF) modulates the opioid system by exerting functional anti-opioid activity on neurons, the mechanism of which is unknown. By using a model of SH-SY5Y cells, we recently postulated that anti-opioid activity likely takes place upstream from the signaling cascade, suggesting that NPFF receptors could block opioid receptors by physical interaction. In the present study, fluorescence techniques were used to monitor the physical association and the dynamic of NPFF2 and micro-opioid (MOP) receptors tagged with variants of the green fluorescent protein. Importantly, cyan fluorescent protein-tagged NPFF2 receptors retained their capacity to antagonize opioid receptors. Fluorescence resonance energy transfer (FRET) and coimmunoprecipitation studies indicate that NPFF and MOP receptors are close enough to generate a basal FRET signal. The opioid agonist Tyr-D-Ala-Gly-NMe-Phe-Gly-ol disrupts by 20-30% this FRET signal, mainly because it concomitantly induces 40% internalization of receptors. In contrast, the NPFF analog 1DMe significantly increases by 10-15% the basal FRET signal, suggesting an association between both receptors. In addition, 1DMe reduces, by half, MOP receptor internalization, indicating that, besides a functional blockade of opioid receptors, the NPFF analog also inhibits their internalization. Finally, as a first report showing the modulation of the mobility of a G-protein-coupled receptor by another one, fluorescence recovery after photobleaching analysis reveals that 1DMe modifies the lateral diffusion of MOP receptors in the cell membrane, changing them from a confined to a freely diffusing state. By promoting NPFF-MOP receptor heteromerization, 1DMe could disrupt the domain organization of MOP receptors in the membrane, resulting in a reduction of opioid response.

Functional differences between NPFF1 and NPFF2 receptor coupling: high intrinsic activities of RFamide-related peptides on stimulation of [35S]GTPgammaS binding

By using an optimized [(35)S]GTPgammaS binding assay, the functional activities (potency and efficacy) of peptides belonging to three members of the RFamide family; Neuropeptide FF (NPFF), prolactin-releasing peptide (PrRP) and 26RFamide, were investigated on NPFF(1) and NPFF(2) receptors stably expressed in Chinese Hamster Ovary (CHO) cells. Despite their large differences in affinity and selectivity, all analogues tested behaved as agonists toward NPFF(1) and NPFF(2) receptors. High NaCl concentration in the assay strongly increased the efficacy toward NPFF(2) receptors and augmented differences among agonists. In low sodium conditions, whereas the potencies of agonists correlated with their affinities for NPFF(1) receptors, NPFF(2) receptors exhibited an extraordinary activity since all compounds tested displayed EC(50) values of GTPgammaS binding lower than their K(I) values. Comparisons of functional values between NPFF(1) and NPFF(2) receptors revealed unexpected potent selective NPFF(2) agonists especially for the PLRFamide and the VGRFamide sequences. By using blocker peptides, we also show that Galpha(i3) and Galpha(s) are the main transducers of NPFF(1) receptors while NPFF(2) are probably coupled with Galpha(i2), Galpha(i3), Galpha(o) and Galpha(s) proteins. Our data indicate that NPPF(1) and NPFF(2) receptors are differently coupled to G proteins in CHO cells.

Facilitation of spinal morphine analgesia in normal and morphine tolerant animals by neuropeptide SF and related peptides

Neuropeptide FF and related synthetic amidated peptides have been shown to elicit sustained anti-nociceptive responses and potently augment spinal anti-nociceptive actions of spinal morphine in tests of thermal and mechanical nociception. Recent studies have described the occurrence of another octapeptide, neuropeptide SF (NPSF) in the spinal cord and the cerebrospinal fluid and demonstrated its affinity for the NPFF receptors. This study examined the effects of NPSF and two putative precursor peptides, EFW-NPSF and NPAF, on the spinal actions of morphine in normal and opioid tolerant rats using the tailflick and pawpressure tests. In normal rats, NPSF demonstrated weak intrinsic activity but sub-effective doses of the peptide significantly increased the magnitude and duration of spinal morphine anti-nociception in both tests. A low-dose of NPSF also augmented the spinal actions of a delta receptor agonist, deltorphin. The morphine-potentiating effect of NPSF was shared by EFW-NPSF and the octadecapeptide NPAF. In animal rendered tolerant by continuous intrathecal infusion of morphine for 6 days, low dose NPSF itself elicited a significant anti-nociceptive response and potently increased morphine-induced response in both tests. In animals made tolerant by repeated injections of intrathecal morphine, administration of NPSF, EFW-NPSF, and NPAF with morphine reversed the loss of the anti-nociceptive effect and restored the agonist potency. The results demonstrate that in normal animals NPSF and related peptides exert strong potentiating effect on morphine anti-nociception at the spinal level and in tolerant animals these agents can reverse the loss of morphine potency.

Anti-opioid activities of NPFF1 receptors in a SH-SY5Y model

In order to elucidate the mechanisms of the neuronal anti-opioid activity of Neuropeptide FF, we have transfected the SH-SY5Y neuroblastoma cell line, which expresses mu- and delta-opioid receptors, with the human NPFF1 receptor. The SH1-C7 clone expresses high affinity NPFF1 receptors in the same range order of density as opioid receptors. Similarly to the opioids, acute stimulation with the NPFF1 agonist NPVF inhibits adenylyl cyclase activity and voltage-gated (N-type) Ca2+ currents and enhances the intracellular Ca2+ release triggered by muscarinic receptors activation. In contrast, preincubation of cells with NPVF decreases the response to opioids on both calcium signaling, thus reproducing the cellular anti-opioid activity described in neurons. SH1-C7 cells are therefore a suitable model to investigate the interactions between NPFF and opioid receptors.

Structure-activity relationships of neuropeptide FF and related peptidic and non-peptidic derivatives

Neuropeptide FF, a member of the RFamide family of peptides, has demonstrated an interesting array of pharmacological effects. To date however, little information has been obtained as to the exact pharmacological roles of the individual NPFF1 and NPFF2 receptors. Through peptide analogs of NPFF and related peptides, the essential pharmacophore has emerged somewhat. Yet, the field is lacking small molecule ligands selective for each receptor. This review of the structure-activity relationships of the reported NPFF peptide analogs and some non-selective small molecule ligands highlights the current understanding of the pharmacophoric elements required for affinity and activity at the NPFF receptors. The lack of mutagenesis data on the receptor as well as a crystal structure has also hindered the understanding of ligand recognition at the receptor level. If the targets can be further investigated as to their requirements for ligand recognition, the successful development of highly selective ligands should follow.

Rat NPFF(1) receptor-mediated signaling: functional comparison of neuropeptide FF (NPFF), FMRFamide and PFR(Tic)amide

Neuropeptide FF (NPFF) participates in many physiological functions associated with opioids in the mammalian CNS. We established a pheochromocytoma PC-12 cell line clone stably expressing rat NPFF1 receptors. Both NPFF and FMRFamide activated NPFF1 receptors to couple with Gi/o protein and inhibited adenylyl cyclase activity in PC-12/rNPFF1 cells, but there were no effects on MAPKs (ERK1/2 and p38 MAPK) or PI3K/Akt pathway. FMRFamide also inhibited DARPP-32/Thr34 phosphorylation in the presence of forskolin. Similarly, PFR(Tic)amide, a 'super-agonist' of NPFF receptors, inhibited the production of cAMP and slightly decreased DARPP-32/Thr34 phosphorylation in PC-12/rNPFF1 cells. Intracerebroventricular injections of PFR(Tic)amide blocked behavioral sensitization of locomotor activity to amphetamine, and intrathecal injection of PFR(Tic)amide caused a dose-dependent antinociception in vivo in rats. Thus, "over-activation" of NPFF receptors by PFR(Tic)amide induced different bio-effects from those induced by NPFF in vivo.