Modulation of naloxone-precipitated morphine withdrawal syndromes in rats by neuropeptide FF analogs

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.

A bifunctional-biased mu-opioid agonist-neuropeptide FF receptor antagonist as analgesic with improved acute and chronic side effects

Opioid analgesics, such as morphine, oxycodone, and fentanyl, are the cornerstones for treating moderate to severe pain. However, on chronic administration, their efficiency is limited by prominent side effects such as analgesic tolerance and dependence liability. Neuropeptide FF (NPFF) and its receptors (NPFF1R and NPFF2R) are recognized as an important pronociceptive system involved in opioid-induced hyperalgesia and analgesic tolerance. In this article, we report the design of multitarget peptidomimetic compounds that show high-affinity binding to the mu-opioid receptor (MOPr) and NPFFRs. In vitro characterization of these compounds led to identification of KGFF03 and KGFF09 as G-protein-biased MOPr agonists with full agonist or antagonist activity at NPFFRs, respectively. In agreement with their biased MOPr agonism, KGFF03/09 showed reduced respiratory depression in mice, as compared to the unbiased parent opioid agonist KGOP01. Chronic subcutaneous administration of KGOP01 and KGFF03 in mice rapidly induced hyperalgesia and analgesic tolerance, effects that were not observed on chronic treatment with KGFF09. This favorable profile was further confirmed in a model of persistent inflammatory pain. In addition, we showed that KGFF09 induced less physical dependence compared with KGOP01 and KGFF03. Altogether, our data establish that combining, within a single molecule, the G-protein-biased MOPr agonism and NPFFR antagonism have beneficial effects on both acute and chronic side effects of conventional opioid analgesics. This strategy can lead to the development of novel and potent antinociceptive drugs with limited side effects on acute and chronic administration.

Supraspinal inhibitory effects of chimeric peptide MCRT on gastrointestinal motility in mice

Objectives

Chimeric peptide MCRT, based on morphiceptin and PFRTic-NH2, was a bifunctional ligand of μ- and δ-opioid receptors (MOR-DOR) and produced potent analgesia in tail-withdrawal test. The study focused on the supraspinal effects of morphiceptin, PFRTic-NH2 and MCRT on gastrointestinal motility. Moreover, opioid receptor antagonists, naloxone (non-selective), cyprodime (MOR selective) and naltrindole (DOR selective) were utilized to explore the mechanisms.

Methods

Intracerebroventricular administration was achieved via the implanted cannula. Gastric emptying and intestinal transit were measured to evaluate gastrointestinal motility.

Key findings

(1) At supraspinal level, morphiceptin, PFRTic-NH2 and MCRT significantly decreased gastric emptying and intestinal transit; (2) MCRT at 1 nmol/mouse, far higher than its analgesic dose (ED50 = 29.8 pmol/mouse), failed to regulate the gastrointestinal motility; (3) MCRT-induced gastrointestinal dysfunction could be completely blocked by naloxone and naltrindole, but not affected by cyprodime.

Conclusions

(1) Morphiceptin and PFRTic-NH2 played important roles in the regulation of gastrointestinal motility; (2) MCRT possessed higher bioactivity of pain relief than gastrointestinal regulation, suggesting its promising analgesic property; (3) MCRT-induced motility disorders were sensitive to DOR but not to MOR blockade, indicating the pain-relieving specificity of speculated MOR subtype or splice variant or MOR-DOR heterodimer.

Anti-opioid Effects of RFRP-3 on Magnocellular Neuron Activity in Morphine-naïve and Morphine-treated Female Rats

Neuropeptide FF receptors (NPFFR1 and NPFFR2) have been proposed to possess anti-opioid properties, and be involved in the development of opiate tolerance and dependence. However, there is no evidence to date supporting such opioid effects at the cellular level in vivo. Using in vivo electrophysiological recordings from vasopressin and oxytocin neurons in the supraoptic nucleus, we aimed to determine the effects of NPFFRs on opiate inhibition, tolerance, and dependence at a cellular level. Both vasopressin and oxytocin neurons are acutely inhibited by opioids and develop opiate tolerance. Oxytocin neurons also develop cellular opiate dependence and undergo withdrawal hyperexcitation upon cessation of opiate administration. Here, the classical μ-opioid receptor agonist, morphine robustly inhibited the spontaneous firing rate of vasopressin and oxytocin neurons, and this inhibition was attenuated by pretreatment with the NPFFR1 agonist, RFamide-related peptide-3. In rats infused with morphine for 6 d, vasopressin neurons were unresponsive to morphine, indicating the development of cellular tolerance, but pretreatment with the NPFFR antagonist, GJ14, restored acute morphine inhibition. In morphine-infused rats, RFamide related peptide-3 did not induce withdrawal excitation in oxytocin neurons and GJ14 did not reverse naloxone-precipitated withdrawal excitation. This is the first evidence of anti-opioid effects of the NPFFR system at a cellular level in vivo. Our results suggest that the anti-opioid properties of the NPFFR system reduce morphine sensitivity during tolerance but that it is not involved in dependence.

A possible correlation between oxytocin-induced and angiotensin IV-induced anti-hyperalgesia at the spinal level in rats

In our previous study, we showed that intrathecal (i.t.) administration of angiotensin IV (Ang IV), an insulin-regulated aminopeptidase (IRAP) inhibitor, attenuated inflammatory hyperalgesia in rats. Using the plantar test in rats with carrageenan-induced paw inflammation, we investigated the possible mechanism(s) of this effect. Because i.t. oxytocin was reported to produce a dose-dependent anti-hyperalgesia in rats with inflammation, we speculate that there is a possible correlation between oxytocin-induced and Ang IV-induced anti-hyperalgesia. Using i.t. co-administered atosiban (oxytocin receptor antagonist), the anti-hyperalgesia by Ang IV was completely abolished. This indicated that oxytocin could be the major IRAP substrate responsible for the anti-hyperalgesia by Ang IV. When Ang IV was co-administered with a low dose of oxytocin, there was a significant enhancing effect of Ang IV on oxytocin-induced anti-hyperalgesia. In recent reports, electrical stimulation on the paraventricular hypothalamic nucleus (PVN) was proved to increase oxytocin release at the spinal cord. Our results also showed that Ang IV could prolong the anti-hyperalgesia induced by PVN stimulation. This suggests a possible protective effect of Ang IV on endogenous oxytocin degradation/dysfunctioning. Moreover, we examined the local effect of intraplantarly injected Ang IV in the same model. Our results showed no effect of local Ang IV on hyperalgesia and paw edema, indicating that Ang IV may not regulate the peripheral inflammatory process. Overall, our study suggests that Ang IV may act through the inhibition of the activity of IRAP to reduce the degradation of oxytocin at the spinal cord, thereby leading to anti-hyperalgesia in rats with inflammation.

LVV-hemorphin 7 and angiotensin IV in correlation with antinociception and anti-thermal hyperalgesia in rats

Hemorphins, a family of atypical endogenous opioid peptides, are produced by the cleavage of hemoglobin β-chain. Hemorphins were proved to bind to the μ-opioid receptors (agonist) and angiotensin IV receptors (insulin-regulated aminopeptidase; IRAP) (inhibitor). Among the hemorphins, LVV-hemorphin-7 (LVV-H7) was found to be abundant and with a longer half life in the CNS. Using intrathecal and intracerebroventricular injections, LVV-H7 and angiotensin IV were given to the rats, which were then subjected to the plantar test and the tail-flick test. Our results showed that LVV-H7 attenuated carrageenan-induced hyperalgesia at the spinal level, which could not be reversed by the co-administration of naloxone. At the supraspinal level, LVV-H7 also produced a significant anti-hyperalgesia effect but with a lower extent. Angiotensin IV showed a similar anti-hyperalgesia effect at the spinal level, but had no effect at the supraspinal level. In the tail-flick test and paw edema test, both peptides showed no effect. These results suggest that LVV-H7 mainly exert the anti-hyperalgesia effect at the spinal level, possibly through IRAP but not μ-opioid receptors. In addition, we observed the expression of IRAP in the CNS of animals with/without carrageenan-induced hyperalgesia. Our results showed a significant expression of IRAP in the spinal cord of rats. However, there was no significant quantitative change of IRAP after the development of hyperalgesia. The serum level of LVV-H7 was also found to be with no change caused by hyperalgesia. These results indicated that the endogenous LVV-H7 and IRAP may not regulate the severity of hyperalgesia through a quantitative change.

Involvement of neuropeptide FF receptors in neuroadaptive responses to acute and chronic opiate treatments

BACKGROUND AND PURPOSE Opiates remain the most effective compounds for alleviating severe pain across a wide range of conditions. However, their use is associated with significant side effects. Neuropeptide FF (NPFF) receptors have been implicated in several opiate-induced neuroadaptive changes including the development of tolerance. In this study, we investigated the consequences of NPFF receptor blockade on acute and chronic stimulation of opioid receptors in mice by using RF9, a potent and selective antagonist of NPFF receptors that can be administered systemically. EXPERIMENTAL APPROACH The effects of RF9 were investigated on opioid pharmacological responses including locomotor activity, antinociception, opioid-induced hyperalgesia, rewarding properties and physical dependence. KEY RESULTS RF9 had no effect on morphine-induced horizontal hyperlocomotion and slightly attenuated the decrease induced in vertical activity. Furthermore, RF9 dose-dependently blocked the long-lasting hyperalgesia produced by either acute fentanyl or chronic morphine administration. RF9 also potentiated opiate early analgesic effects and prevented the development of morphine tolerance. Finally, RF9 increased morphine-induced conditioned place preference without producing any rewarding effect by itself and decreased naltrexone-precipitated withdrawal syndrome following chronic morphine treatment. CONCLUSION AND IMPLICATIONS The NPFF system is involved in the development of two major undesirable effects: tolerance and dependence, which are clinically associated with prolonged exposure to opiates. Our findings suggest that NPFF receptors are interesting therapeutic targets to improve the analgesic efficacy of opiates by limiting the development of tolerance, and for the treatment of opioid dependence.

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.

Structure-activity studies of RFamide peptides reveal subtype-selective activation of neuropeptide FF1 and FF2 receptors

Selectivity is a major issue in closely related multiligand/multireceptor systems. In this study we investigated the RFamide systems of hNPFF₁R and hNPFF₂R that bind the endogenous peptide hormones NPFF, NPAF, NPVF, and NPSF. By use of a systematic approach, we characterized the role of the C-terminal dipeptide with respect to agonistic properties using synthesized [Xaa 7]NPFF and [Xaa 8]NPFF analogues. We were able to identify only slight differences in potency upon changing the position of Arg 7, as all modifications resulted in identical behavior at the NPFF₁R and NPFF₂R. However, the C-terminal Phe 8 was able to be replaced by Trp or His with only a minor loss in potency at the NPFF₂R relative to the NPFF₁R. Analogues with shorter side chains, such as α-amino-4-guanidino butyric acid ([Agb 7]NPFF) or phenylglycine ([Phg 8]NPFF), decreased efficacy for the NPFF₁ R to 25-31 % of the maximal response, suggesting that these agonist-receptor complexes are more susceptible to structural modifications. In contrast, mutations to the conserved Asp 6.59 residue in the third extracellular loop of both receptors revealed a higher sensitivity toward the hNPFF₂R receptor than toward hNPFF₁R. These data provide new insight into the subtype-specific agonistic activation of the NPFF₁ and NPFF(2) receptors that are necessary for the development of selective agonists.

Dansyl-PQRamide, a putative antagonist of NPFF receptors, reduces anxiety-like behavior of ethanol withdrawal in a plus-maze test in rats

Much evidence indicates that endogenous opioid peptides are involved in effects caused by ethanol. The aim of the present study was to determine whether dansyl-PQR amide, a putative antagonist of receptors for an anti-opioid peptide-neuropeptide FF (NPFF) could affect anxiety-like behavior measured during withdrawal from acute-, and chronic ethanol administration in the elevated plus maze test in rats. Our study indicated that intracerebroventricular (i.c.v.) administration of dansyl-PQRamide (2.4 and 4.8 nmol) reversed anxiety-like behavior measured as a percent time spent in the open arms, and a percent open arm entries onto the open arms in the elevated plus-maze test in rats. These effects were inhibited by NPFF (10 and/or 20 nmol, i.c.v.) in the experiments performed during withdrawal from acute- and chronic ethanol administration. During withdrawal from acute ethanol, naloxone (1mg/kg, i.p.), a nonselective opioid receptor antagonist, attenuated only an increased percent time spent in the open arms induced by dansyl-PQR amide (4.8 nmol). Dansyl-PQR amide, NPFF and naloxone given alone to naive rats did not have influence on spontaneous locomotor activity of animals. Furthermore, NPFF potentiated anxiety-like behavior during withdrawal from chronic, but not acute, ethanol administration in rats. Our data suggest that NPFF system is involved in regulation of affective symptoms of ethanol withdrawal. It seems that involvement of the NPFF system in ethanol withdrawal anxiety-like behavior is associated with regulation of the opioid system activity.

Pharmacological effects of the dansylated neuropeptide FF analogues on body temperature and morphine analgesia

In our previous work, the two putative agonists (dansyl-GSRFamide and dansyl-PQRFamide) and the two putative antagonists (dansyl-GSRamide and dansyl-PQRamide) on neuropeptide FF (NPFF) receptors were synthesized to evaluate the importance of Phe(8) of NPFF. In the present study, these putative NPFF agonists/antagonists containing different N-terminal sequences were further examined for their pharmacological profiles in thermoregulatory and nociceptive tests. The results indicated that the two dansylated agonists potently possessed similar thermoregulation (rank order of potencies: dansyl-GSRFamide>>NPFF>dansyl-PQRFamide) and different modulation of opioid-induced analgesia; in contrast, both of the two putative antagonists exhibited marked hypothermia (rank order of potencies: dansyl-PQRamide>dansyl-GSRamide) and facilitation of morphine analgesia (rank order of potencies: dansyl-PQRamide > dansyl-GSRamide). These data reveal that the difference of the N-terminal residues of the two putative agonists causes their dissociation of pharmacological pro- and anti-opioid effects. In addition, their N-terminal part is important to determine the potency of the dansylated agonists/antagonists. Our work might be helpful to develop a highly potent and fluorescent NPFF ligand.

In vivo inhibition of neuropeptide FF agonism by BIBP3226, an NPY Y1 receptor antagonist

BIBP3226 {(R)-N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)-methyl]-argininamide} was recently shown to display relatively high affinities for neuropeptide FF (NPFF) receptors and exhibit antagonist activities towards NPFF receptors in vitro. The present study was undertaken to investigate the antagonistic effects of BIBP3226 on several in vivo pharmacologic profiles induced by exogenous NPFF and NPVF. (1) BIBP3226 (5 nmol) injected into the third ventricle completely antagonized the hypothermic effects of NPFF (30 nmol) and NPVF (30 nmol) after cerebral administration in mice; (2) BIBP3226 (5 nmol, i.c.v.) prevented the anti-morphine actions of NPFF (10 nmol, i.c.v.) in the mouse tail-flick assay; (3) in urethane-anaesthetized rats, both NPFF (200 nmol/kg, i.v.) and NPVF (200 nmol/kg, i.v.) increased the mean arterial blood pressure, which were significantly reduced by pretreatment with BIBP3226 (500 nmol/kg, i.v.). Collectively, these data suggest that BIBP3226, a mixed antagonist of NPY Y1 and NPFF receptors, shows in vivo antagonistic effects on NPFF receptors. In addition, it seems to be clear that the in vivo pharmacological profiles of NPFF are mediated directly by NPFF receptors.

Anatomical distribution and biochemical characterization of the novel RFamide peptide 26RFa in the human hypothalamus and spinal cord

26RFa is a novel RFamide peptide originally isolated in the amphibian brain. The 26RFa precursor has been subsequently characterized in various mammalian species but, until now, the anatomical distribution and the molecular forms of 26RFa produced in the CNS of mammals, in particular in human, are unknown. In the present study, we have investigated the localization and the biochemical characteristics of 26RFa-like immunoreactivity (LI) in two regions of the human CNS--the hypothalamus and the spinal cord. Immunohistochemical labeling using specific antibodies against human 26RFa and in situ hybridization histochemistry revealed that in the human hypothalamus 26RFa-expressing neurons are located in the paraventricular and ventromedial nuclei. In the spinal cord, 26RFa-expressing neurons were observed in the dorsal and lateral horns. Characterization of 26RFa-related peptides showed that two distinct molecular forms of 26RFa are present in the human hypothalamus and spinal cord, i.e. 26RFa and an N-terminally elongated form of 43 amino acids designated 43RFa. These data provide the first evidence that 26RFa and 43RFa are actually produced in the human CNS. The distribution of 26RF-LI suggests that 26RFa and/or 43RFa may modulate feeding, sexual behavior and transmission of nociceptive stimuli.

In vitro and in vivo studies of dansylated compounds, the putative agonists and antagonists on neuropeptide FF receptors

To further evaluate the importance of C-terminal modification of neuropeptide FF (NPFF), in the present work, four dansylated NPFF analogues, including two putative agonists (dansyl-PQRFamide and dansyl-GSRFamide) and two putative antagonists (dansyl-PQRamide and dansyl-GSRamide), were synthesized and investigated to address their potencies and efficacies in a series of in vitro and in vivo assays. (1) In the isolated mouse colon bioassay, the four dansylated compounds showed agonistic profiles: both dansyl-GSRFamide (1-10 microM) and dansyl-GSRamide (1-10 microM) dose-dependently caused colonic contractions, which were attenuated by pretreatment with BIBP3226; dansyl-PQRFamide and dansyl-PQRamide evoked modest colonic contractions at a high dose of 50 microM. (2) In urethane-anaesthetized rats, both dansyl-PQRFamide (50-300 nmol/kg, i.v.) and dansyl-GSRFamide (15-50 nmol/kg, i.v.) dose-dependently increased the mean arterial pressure and heart rate in a manner similar to NPFF (50-300 nmol/kg, i.v.); on the contrary, the two putative antagonists (100-800 nmol/kg, i.v.) decreased blood pressure in a dose-dependent manner. All the results suggest that dansyl-PQRFamide and dansyl-GSRFamide are NPFF full agonists; in contrast, dansyl-GSRamide and dansyl-PQRamide behave as agonists in vitro and antagonists in vivo on NPFF receptors. The findings reveal that the C-terminal Phe might be a crucial residue to determine the efficacy. In addition, the novel analogue dansyl-GSRFamide may be developed as a highly potent agonist to investigate the NPFF system.

Cardiovascular effects of neuropeptide FF antagonists

The neuropeptide FF (NPFF) antagonist desaminotyrosyl-Phe-Leu-Phe-Gln-Pro-Gln-Arg-NH2 dose-dependently reversed NPFF-induced elevation of blood pressure in anesthetized rats after intravenous injection without causing a significant change of blood pressure and heart rate by itself. However, another antagonist dansyl-Pro-Gln-Arg-NH2 produced a significant drop of the mean arterial pressure only at a large dose (10 micromol/kg body weight), but reversal of the NPFF-induced hypertension was modest. Consequently and contrary to the conclusions of a previous study, NPFF antagonists cannot be identified simply by measuring the changes in the hemodynamic parameters upon the injection of the compounds alone and without a subsequent NPFF challenge.

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.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.

Functional properties of Pfr(Tic)amide and BIBP3226 at human neuropeptide FF2 receptors

The functional characteristics of two putative neuropeptide FF (NPFF) antagonists, BIBP3226 and PFR(Tic)amide, on the human neuropeptide FF receptor subtype 2 (hNPFF2) were investigated. Surprisingly, PFR(Tic)amide was shown to exhibit agonist properties in the [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding assay. The efficacy of PFR(Tic)amide was significantly greater than that of (1DMe)Y8Fa, a stable analog of NPFF, and PFR(Tic)amide can therefore be classified as a 'super-agonist'. BIBP3226 did act as a reversible competitive antagonist on the hNPFF2 receptor. However, high concentrations of BIBP3226 also non-specifically increased [35S]GTP-gammaS binding. The usefulness of BIBP3226 as an antagonist tool on the NPFF receptor is thus limited.

Neuropeptide FF receptor 2 (NPFF2) is localized to pain-processing regions in the primate spinal cord and the lower level of the medulla oblongata

Studies have suggested that NPFF-like peptides and their receptors play important roles in physiological and pathological conditions. Here, we show, using multiple expression modalities, that the type 2 NPFF receptor (hNPFF2) is expressed in regions of the primate spinal cord and brainstem mediating pain sensation. In situ hybridization using an NPFF2 riboprobe, and immunohistochemistry using a novel NPFF2 antibody, demonstrated strong NPFF2 expression in the superficial layer of the dorsal horn, and in the spinal trigeminal nucleus of the brainstem of the African green monkey (AGM). In addition, autoradiography using a radiolabeled NPFF analog ([125I]1DMe) revealed dense binding signal in the superficial layer of the dorsal horn in the spinal cord. The distribution pattern of hNPFF2 in the AGM spinal cord and the lower level of the brainstem are consistent with a hypothesized potential role for NPFF peptides in modulation of sensory input, opioid analgesia and morphine tolerance through spinal and supraspinal mechanisms.

Endomorphin-1 and -2 induce naloxone-precipitated withdrawal syndromes in rats

In 1997, endomorphin-1 (EM-1) and -2 (EM-2) were identified as the most specific endogenous mu-opioid ligands. These two peptides have shown analgesic effects and many other opioid functions. In the present study, we attempt to investigate the possible ability of endomorphins to induce naloxone-precipitated withdrawal in comparison with that induced by morphine. Using the previously established scoring system in rats, 12 withdrawal signs (chewing, sniffing, grooming, wet-dog shakes, stretching, yawning, rearing, jumping, teeth grinding, ptosis, diarrhea, and penile erection) were observed and scored following naloxone (4 mg/kg, i.p.) challenge. Compared with the sham control, EM-1 and EM-2 (20 microg, i.c.v., b.i.d. for 5 days) both produced significant naloxone-induced withdrawal syndromes with similar severity to that induced by the same dose of morphine. There was no significant difference between EM-1, EM-2, and morphine-treated group for naloxone-induced withdrawal signs, except for grooming. EM-1 and EM-2 induced more grooming than that caused by morphine. Although EM-1 and EM-2 both led to the withdrawal, they displayed different potency for certain signs and suggest their distinct regulations. The present results indicate EM-1 and EM-2 could initiate certain mechanism involved opiate dependence.

Immobilized artificial membrane chromatography coupled with atmospheric pressure ionization mass spectrometry

Liquid chromatographic separations on monolayers of cell membrane phospholipids covalently immobilized to silica particles at high molecular density is used for mimicking solute partitioning into biological membranes that generally correlates with membrane transport. This technique called immobilized artificial membrane chromatography usually employs ultraviolet (UV) detection where a single compound is analyzed in a chromatographic run limiting thereby its throughput for drug discovery applications. For coupling with atmospheric pressure ionization mass spectrometry, the phosphate-buffered saline mobile phase was replaced with one that used ammonium acetate as a volatile buffer. While atmospheric pressure chemical ionization accommodated a purely aqueous effluent, interfacing with electrospray ionization required effluent splitting and the addition of an organic modifier (5%, v/v, acetonitrile). Neuropeptide FF antagonists as early-phase drug candidates were used for the comparative evaluation of the methods. Whereas electrospray ionization produced essentially no fragment ions, several compounds involved in our study yielded low-abundance molecular ions with atmospheric pressure chemical ionization. The use of mass spectrometry yielded data that correlated well with those obtained by the method employing UV detection. Both atmospheric pressure ionization methods permitted the simultaneous determination of the k'(IAM), capacity factors and, therefore, an increased-throughput ranking of potential new leads emerged from the drug discovery process based on affinity to artificial membranes.

Isolation, characterization, and distribution of a novel neuropeptide, Rana RFamide (R-RFa), in the brain of the European green frog Rana esculenta

A novel neuropeptide of the RFamide peptide family was isolated in pure form from a frog (Rana esculenta) brain extract by using reversed-phase high performance liquid chromatography in combination with a radioimmunoassay for mammalian neuropeptide FF (NPFF). The primary structure of the peptide was established as Ser-Leu-Lys- Pro-Ala-Ala-Asn-Leu-Pro-Leu- Arg-Phe-NH(2). The sequence of this neuropeptide, designated Rana RFamide (R-RFa), exhibits substantial similarities with those of avian LPLRFamide, gonadotropin-inhibitory hormone, and human RFRP-1. The distribution of R-RFa was investigated in the frog central nervous system by using an antiserum directed against bovine NPFF. In the brain, immunoreactive cell bodies were primarily located in the hypothalamus, i.e., the anterior preoptic area, the suprachiasmatic nucleus, and the dorsal and ventral hypothalamic nuclei. The most abundant population of R-RFa-containing neurons was found in the periependymal region of the suprachiasmatic nucleus. R-RFa- containing fibers were widely distributed throughout the brain from the olfactory bulb to the brainstem, and were particularly abundant in the external layer of the median eminence. In the spinal cord, scattered immunoreactive neurons were found in the gray matter. R-RFa-positive processes were found in all regions of the spinal cord, but they were more abundant in the dorsal horn. This study provides the first characterization of a member of the RFamide peptide family in amphibians. The occurrence of this novel neuropeptide in the hypothalamus and median eminence and in the dorsal region of the spinal cord suggests that, in frog, R-RFa may exert neuroendocrine activities and/or may be involved in the transmission of nociceptive stimuli.

Dansyl-PQRamide, a possible neuropeptide FF receptor antagonist, induces conditioned place preference

Neuropeptide FF (NPFF) is an endogenous anti-opioid peptide. NPFF could potentiate the naloxone-precipitated morphine withdrawal syndromes in morphine-dependent rats, indicating the possible involvement of the endogenous NPFF system in opioid analgesia and dependence. The present study was performed to examine the effects of dansyl-PQRamide (dns-PQRa), a putative NPFF antagonist, on conditioned place preference (CPP), in addition, its interaction with the opioid system. Two CPP experiments were conducted. First, rats were treated with dns-PQRa (4-13 mg/kg, i.p.) and paired with the non-preferred compartment while the vehicle was paired with the preferred compartment. Second, similar to experiment 1 except naloxone (1 mg/kg, i.p.) was given 10 min prior to each dns-PQRa administration. The post-drug place preference was examined after 4 alternative pairings. Another group of animals after repetitive dns-PQRa treatments were analyzed for levels of neurotransmitters in discrete brain areas. Dns-PQRa (4-13 mg/kg, i.p.) induced a significant dose-dependent CPP. The dns-PQRa-induced CPP was completely blocked by pretreatment with 1 mg/kg i.p. naloxone, while naloxone alone did not induce any place aversion. The chronic dns-PQRa-treated (13 mg/kg, i.p., b.i.d.) rats caused a significant increase in 3,4-dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid in the olfactory tubercle compared to the vehicle-treated controls. There was also an increase in the turnover of serotonin in the olfactory tubercle, nucleus accumbens and medial prefrontal cortex. These results suggest that blockade of the NPFF system produces rewarding, possibly via an inhibition of the anti-opioid action of NPFF. These results also reveal a close relationship between NPFF, drug rewarding and the dopaminergic and serotoninergic neurons in the mesolimbic system.

Effects of peptides on animal and human behavior: a review of studies published in the first twenty years of the journal Peptides

This review catalogs effects of peptides on various aspects of animal and human behavior as published in the journal Peptides in its first twenty years. Topics covered include: activity levels, addiction behavior, ingestive behaviors, learning and memory-based behaviors, nociceptive behaviors, social and sexual behavior, and stereotyped and other behaviors. There are separate tables for these behaviors and a short introduction for each section.

Identification and characterization of two G protein-coupled receptors for neuropeptide FF

The central nervous system octapeptide, neuropeptide FF (NPFF), is believed to play a role in pain modulation and opiate tolerance. Two G protein-coupled receptors, NPFF1 and NPFF2, were isolated from human and rat central nervous system tissues. NPFF specifically bound to NPFF1 (K(d) = 1.13 nm) and NPFF2 (K(d) = 0.37 nm), and both receptors were activated by NPFF in a variety of heterologous expression systems. The localization of mRNA and binding sites of these receptors in the dorsal horn of the spinal cord, the lateral hypothalamus, the spinal trigeminal nuclei, and the thalamic nuclei supports a role for NPFF in pain modulation. Among the receptors with the highest amino acid sequence homology to NPFF1 and NPFF2 are members of the orexin, NPY, and cholecystokinin families, which have been implicated in feeding. These similarities together with the finding that BIBP3226, an anorexigenic Y1 receptor ligand, also binds to NPFF1 suggest a potential role for NPFF1 in feeding. The identification of NPFF1 and NPFF2 will help delineate their roles in these and other physiological functions.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

The cardiovascular effects of PFRFamide and PFR(Tic)amide, a possible agonist and antagonist of neuropeptide FF (NPFF)

Neuropeptide FF (NPFF), an endogenous opioid-related neuromodulater, has been reported to show significant effects on the cardiovascular system, namely elevation of arterial blood pressure (BP) and heart rate (HR) in rats. In the present study, we synthesized two novel NPFF analogs, PFRFamide (putative NPFF agonist) and PFR(Tic)amide (putative NPFF antagonist), and examined their cardiovascular effect on BP and HR in anesthetized rats. The arterial mean BP and HR were measured by way of direct femoral artery catheterization. The data showed that PFRFamide increased BP in a dose-dependent manner, while PFR(Tic)amide decreased BP dose-dependently. These results revealed the possibility of PFRFamide and PFR(Tic)amide to be NPFF agonist and antagonist (or inverse agonist), respectively. These two NPFF analogs may possess potential in new drug design, and the NPFF system could be very important in mammalian cardiovascular function.