Neuropeptide FF: new molecular insights

The kisspeptin derivative kissorphin reduces the acquisition, expression, and reinstatement of ethanol-induced conditioned place preference in rats

Research has shown that opioids are involved in the rewarding effects of ethanol. Neuropeptide FF (NPFF) has been described as an anti-opioid peptide because, in many cases, it inhibits opioid and ethanol effects in rodents. Kissorphin (KSO) is a new peptide derived from kisspeptin-10 with structural similarities to NPFF. This peptide possesses NPFF-like biological activity in vitro. The aim of the current study was to investigate whether KSO (Tyr-Asn-Trp-Asn-Ser-Phe-NH2) influences the acquisition, expression, and reinstatement of ethanol-induced conditioned place preference (ethanol-CPP) in rats. The ethanol-CPP was established (conditioning for 5 days) by intraperitoneal (i.p.) administration of ethanol (1 g/kg, 20%, w/v) using an unbiased procedure. After that, one group of rats was used in final post-conditioning testing (expression of CPP) and the other group received a priming injection of ethanol after 10 days of extinction (reinstatement of CPP). Our experiments showed that KSO, given intravenously (i.v.) at the doses of 1, 3, and 10 nmol before every ethanol administration, inhibited the acquisition and, given acutely before the post-conditioning test or before the priming dose of ethanol, inhibited the expression and reinstatement of ethanol-CPP, respectively, in a dose-dependent manner. KSO given by itself neither induced place preference nor aversion and did not alter locomotor activity and coordination of rats. These results suggest that KSO can alter rewarding/motivational effects of ethanol. These data suggest this peptide possesses an anti-opioid character.

Genes encoding neuropeptide receptors are epigenetic markers in patients with head and neck cancer: a site-specific analysis

Staging and pathological grading systems are useful but imperfect predictors of recurrence in head and neck squamous cell carcinoma (HNSCC). To identify potential prognostic markers, we examined the methylation status of eight neuropeptide receptor gene promoters in 231 head and neck squamous cell carcinomas. The NPFFR1, NPFFR2, HCRTR1, HCRTR2, NPY1R, NPY2R, NPY4R, and NPY5R promoters were methylated in 80.5%, 79.2%, 67.1%, 73.2%, 35.1%, 36.4%, 38.5%, and 35.9% of the samples, respectively. In a multivariate Cox proportional hazards analysis, the odds ratio for recurrence was 2.044 (95% confidence interval [CI], 1.323–3.156; P = 0.001) when the NPY2R promoter was methylated. In patients without lymph node metastasis (n = 100), methylation of NPY2R (compared with methylation of the other seven genes) best correlated with poor disease-free survival (DFS) (odds ratio, 2.492; 95% CI, 1.190–5.215; P = 0.015). In patients with oral cancer (n = 69), methylated NPY1R and NPY2R were independent prognostic factors for poor DFS, both individually and, even more so, in combination (odds ratio, 3.90; 95% CI, 1.523–9.991; P = 0.005). Similar findings were observed for NPY2R and NPY4R in patients with oropharyngeal cancer (n = 162) (odds ratio, 5.663; 95% CI, 1.507–21.28; P = 0.010).

Identification and functional characterization of the phosphorylation sites of the neuropeptide FF2 receptor

The neuropeptide FF2 (NPFF2) receptor belongs to the rhodopsin family of G protein-coupled receptors and mediates the effects of several related RFamide neuropeptides. One of the main pharmacological interests of this system resides in its ability to regulate endogenous opioid systems, making it a potential target to reduce the negative effects of chronic opioid use. Phosphorylation of intracellular residues is the most extensively studied post-translational modification regulating G protein-coupled receptor activity. However, until now, no information concerning NPFF2 receptor phosphorylation is available. In this study, we combined mass spectrometric analysis and site-directed mutagenesis to analyze for the first time the phosphorylation pattern of the NPFF2 receptor and the role of the various phosphorylation sites in receptor signaling, desensitization, and trafficking in a SH-SY5Y model cell line. We identified the major, likely GRK-dependent, phosphorylation cluster responsible for acute desensitization, (412)TNST(415) at the end of the C terminus of the receptor, and additional sites involved in desensitization ((372)TS(373)) and internalization (Ser(395)). We thus demonstrate the key role played by phosphorylation in the regulation of NPFF2 receptor activity and trafficking. Our data also provide additional evidence supporting the concept that desensitization and internalization are partially independent processes relying on distinct phosphorylation patterns.

A high-affinity, radioiodinatable neuropeptide FF analogue incorporating a photolabile p-(4-hydroxybenzoyl)phenylalanine

A new radioiodinated photoaffinity compound, [(125)I]YE(Bpa)WSLAAPQRFNH2, derived from a peptide present in the rat neuropeptide FF (NPFF) precursor was synthesized, and its binding characteristics were investigated on a neuroblastoma clone, SH-SY5Y, stably expressing rat NPFF2 receptors tagged with the T7 epitope. The binding of the probe was saturable and revealed a high-affinity interaction (KD=0.24nM) with a single class of binding sites. It was also able to affinity label NPFF2 receptor in a specific and efficient manner given that 38% of the bound radioligand at saturating concentration formed a wash-resistant binding after ultraviolet (UV) irradiation. Photoaffinity labeling with [(125)I]YE(Bpa)WSLAAPQRFamide showed two molecular forms of NPFF2 receptor with apparent molecular weights of 140 and 95kDa in a 2:1 ratio. The comparison of the results between photoaffinity labeling and Western blot analysis suggests that all receptor forms bind the probe irreversibly with the same efficiency. On membranes of mouse olfactory bulb, only the high molecular weight form of NPFF2 receptor is observed. [(125)I]YE(Bpa)WSLAAPQRFamide is an excellent radioiodinated peptidic ligand for direct and selective labeling of NPFF2 receptors in vitro.

GRK2 protein-mediated transphosphorylation contributes to loss of function of μ-opioid receptors induced by neuropeptide FF (NPFF2) receptors

Neuropeptide FF (NPFF) interacts with specific receptors to modulate opioid functions in the central nervous system. On dissociated neurons and neuroblastoma cells (SH-SY5Y) transfected with NPFF receptors, NPFF acts as a functional antagonist of μ-opioid (MOP) receptors by attenuating the opioid-induced inhibition of calcium conductance. In the SH-SY5Y model, MOP and NPFF(2) receptors have been shown to heteromerize. To understand the molecular mechanism involved in the anti-opioid activity of NPFF, we have investigated the phosphorylation status of the MOP receptor using phospho-specific antibody and mass spectrometry. Similarly to direct opioid receptor stimulation, activation of the NPFF(2) receptor by [D-Tyr-1-(NMe)Phe-3]NPFF (1DMe), an analog of NPFF, induced the phosphorylation of Ser-377 of the human MOP receptor. This heterologous phosphorylation was unaffected by inhibition of second messenger-dependent kinases and, contrarily to homologous phosphorylation, was prevented by inactivation of G(i/o) proteins by pertussis toxin. Using siRNA knockdown we could demonstrate that 1DMe-induced Ser-377 cross-phosphorylation and MOP receptor loss of function were mediated by the G protein receptor kinase GRK2. In addition, mass spectrometric analysis revealed that the phosphorylation pattern of MOP receptors was qualitatively similar after treatment with the MOP agonist Tyr-D-Ala-Gly (NMe)-Phe-Gly-ol (DAMGO) or after treatment with the NPFF agonist 1DMe, but the level of multiple phosphorylation was more intense after DAMGO. Finally, NPFF(2) receptor activation was sufficient to recruit β-arrestin2 to the MOP receptor but not to induce its internalization. These data show that NPFF-induced heterologous desensitization of MOP receptor signaling is mediated by GRK2 and could involve transphosphorylation within the heteromeric receptor complex.

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.

Electrophysiological analysis of the inhibitory effects of FMRFamide-like peptides on the pacemaker activity of gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons in the terminal nerve (TN) show endogenous pacemaker activity, which is suggested to be dependent on the physiological conditions of the animal. The TN-GnRH neurons have been suggested to function as a neuromodulatory neuron that regulates long-lasting changes in the animal behavior. It has been reported that the TN-GnRH neurons are immunoreactive to FMRFamide. Here, we find that the pacemaker activity of TN-GnRH neuron is inhibited by FMRFamide: bath application of FMRFamide decreased the frequency of pacemaker activity of TN-GnRH neurons in a dose-dependent manner. This decrease was suppressed by a blockage of G protein-coupled receptor pathway by GDP-β-S. In addition, FMRFamide induced an increase in the membrane conductance, and the reversal potential for the FMRFamide-induced current changed according to the changes in [K(+)](out) as predicted from the Nernst equation for K(+). We performed cloning and sequence analysis of the PQRFamide (NPFF/NPAF) gene in the dwarf gourami and found evidence to suggest that FMRFamide-like peptide in TN-GnRH neurons of the dwarf gourami is NPFF. NPFF actually inhibited the pacemaker activity of TN-GnRH neurons, and this inhibition was blocked by RF9, a potent and selective antagonist for mammalian NPFF receptors. These results suggest that the activation of K(+) conductance by FMRFamide-like peptide (≈NPFF) released from TN-GnRH neurons themselves causes the hyperpolarization and then inhibition of pacemaker activity in TN-GnRH neurons. Because TN-GnRH neurons make tight cell clusters in the brain, it is possible that FMRFamide-like peptides released from TN-GnRH neurons negatively regulates the activities of their own (autocrine) and/or neighboring neurons (paracrine).

Characterization of the inhibitory roles of RFRP3, the mammalian ortholog of GnIH, in the control of gonadotropin secretion in the rat: in vivo and in vitro studies

RF-amide related peptides (RFRP), as putative mammalian orthologs of the avian gonadotropin-inhibitory hormone (GnIH), have been proposed as key regulators of gonadotropin secretion in higher vertebrates. Yet considerable debate has arisen recently on their physiological relevance and potential mechanisms and sites of action. Present studies were undertaken to further characterize the effects of RFRP on LH and FSH secretion by a combination of in vivo and in vitro approaches in male and female rats. Initial screening via intracerebroventricular (icv) administration of different analogs of RFRP1 (RFRP1-12 and RFRP1-20) and RFRP3 (RFRP3-8 and RFRP3-17), as well as the related neuropeptide FF (NPFF8), to gonadectomized (GNX) female rats evidenced significant, albeit modest, inhibitory effects on LH secretion only for RFRP3-8 and RFRP3-17, which were detectable at the high dose rage (1 nmol for RFRP3-8, 5 nmol for RFRP3-17). This moderate inhibitory action was also documented after icv administration of RFRP3-8 to intact and GNX male rats. In addition, systemic (intravenous) administration of RFRP3-8 decreased the circulating levels of both gonadotropins in GNX male rats. Likewise, RFRP3-8 inhibited basal and GnRH-stimulated LH secretion by pituitaries from GNX males in vitro. This inhibitory effect was blocked by the antagonist of RFRP receptors, RF9. In summary, our results support a putative inhibitory role of RFRP3 as ortholog of GnIH in the regulation of gonadotropin secretion in mammals, which appears to involve direct pituitary actions as well as potential central (hypothalamic) effects.

Characterization of the potent gonadotropin-releasing activity of RF9, a selective antagonist of RF-amide-related peptides and neuropeptide FF receptors: physiological and pharmacological implications

Identification of RF-amide-related peptides (RFRP), as putative mammalian orthologs of the avian gonadotropin-inhibitory hormone, has drawn considerable interest on its potential effects and mechanisms of action in the control of gonadotropin secretion in higher vertebrates. Yet, these analyses have so far relied mostly on indirect approaches, while direct assessment of their physiological roles has been hampered by the lack of suitable antagonists. RF9 was recently reported as a selective and potent antagonist of the receptors for RFRP (RFRPR) and the related neuropeptides, neuropeptide FF (NPFF) and neuropeptide AF (NPFF receptor). We show here that RF9 possesses very strong gonadotropin-releasing activities in vivo. Central administration of RF9 evoked a dose-dependent increase of LH and FSH levels in adult male and female rats. Similarly, male and female mice responded to intracerebroventricular injection of RF9 with robust LH secretory bursts. In rats, administration of RF9 further augmented the gonadotropin-releasing effects of kisspeptin, and its stimulatory effects were detected despite the prevailing suppression of gonadotropin secretion by testosterone or estradiol. In fact, blockade of estrogen receptor-alpha partially attenuated gonadotropin responses to RF9. Finally, systemic administration of RF9 modestly stimulated LH secretion in vivo, although no direct effects in terms of gonadotropin secretion were detected at the pituitary in vitro. Altogether, these data are the first to disclose the potent gonadotropin-releasing activity of RF9, a selective antagonist of RFRP (and NPFF) receptors. Our findings support a putative role of the RFRP/gonadotropin-inhibitory hormone system in the central control of gonadotropin secretion in mammals and have interesting implications concerning the potential therapeutic indications and pharmacological effects of RF9.

Modulation by neuropeptide FF of the interaction of mu-opioid (MOP) receptor with G-proteins

The Neuropeptide FF (NPFF) system is known to modulate the effects of opioids in vivo and in vitro. In the present study, we have investigated the effect of NPFF agonists on the coupling of the Mu-opioid (MOP) receptor to G-proteins in a model of SH-SY5Y cells transfected with NPFF(2) receptor, in which the neuronal anti-opioid activity of NPFF was previously reproduced. Activation of G-proteins was monitored by [(35)S]GTPgammaS binding assay and analysis of G-protein subunits associated with MOP receptors was performed by Western blotting after immunoprecipitation of the receptor. The results demonstrate that concentrations of NPFF agonists that produce a cellular anti-opioid effect, did not affect the ability of the opioid agonist DAMGO to activate G-proteins. However, at saturating concentration of agonist or when expression of receptor was high, opioid and NPFF agonists did not stimulate [(35)S]GTPgammaS binding in an additive manner, indicating that both receptors share a common fraction of a G-protein pool. In addition, stimulation of NPFF receptors in living cells modified the G-protein environment of MOP receptor by favoring its interaction with alpha(s), alpha(i2) and beta subunits. This change in G-protein coupling to MOP receptor might participate in the mechanism by which NPFF agonists reduce the inhibitory activity of opioids.

Central locomotor and cognitive effects of a NPFF receptor agonist in mouse

NPFF receptors are expressed in several brain regions directly or indirectly involved in cognition and behavior. However, the cognitive effects of the NPFF system have been poorly studied. Therefore, the aim of our study was to analyze the effects of i.c.v. injections of 1 DMe, a stable agonist of NPFF receptors, on behavioral and cognitive performances in C57BL/6J mice. We measured locomotor activity, and an open field with objects was used to estimate the ability of mice to react to spatial changes and to measure short-term retention of information. The Morris navigation task was used to evaluate the acquisition, as well as long-term retention of a hippocampo-dependent spatial memory with a distributed training procedure. Finally, 1 DMe was tested in a contextual fear conditioning paradigm to study its effect on long-term memory of contextual information acquired in a single training session. Altogether, our results demonstrate a small but complex influence of the NPFF system on mouse behavior. 1 DMe injected i.c.v. induces a delayed hyperlocomotion and mildly impairs both short-term and long-term spatial memory processing without affecting contextual fear memory.

Pharmacological characterization of the mouse NPFF2 receptor

This study presents the binding and functional properties of the mouse NPFF(2) (mNPFF(2)) receptor, in comparison with its human counterpart (hNPFF(2)). Binding experiments were performed by using the NPFF(2) selective radioligand [(3)H]-EYF in membranes from CHO cells transfected with mouse and human NPFF(2) receptors and compared to membranes from mouse olfactory bulb, the brain region expressing the highest density of NPFF(2) receptors in mouse. mNPFF(2) receptors exhibited a high affinity (Kd=0.2-0.4 nM) for [(3)H]-EYF, comparable to that of hNPFF(2) receptors. Also, the binding selectivity profile of mNPFF(2) receptors was comparable to that of hNPFF(2) receptors, except for three ligands (NPSF, NPVF, RF9) that were about tenfold more potent and active on mouse receptors than on human receptors. In particular, compared to hNPFF(2) receptors, mNPFF(2) receptors were less discriminative towards the proNPFF(B)-derived peptide. This suggests some species-related differences in the binding properties of NPFF(2) receptors that could have repercussion when evaluating the pharmacological properties of drugs in vivo.

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.

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.

Neuropeptide FF (NPFF) reduces the expression of cocaine-induced conditioned place preference and cocaine-induced sensitization in animals

The endogenous brain opioid system is believed to play an important role in mediating reward mechanisms. Opioid innervation is high in many limbic regions and reinforcing actions of many drugs of abuse, including cocaine, are thought to be mediated via endogenous opioid system. The aim of the present study was to indicate whether the anti-opioid peptide, neuropeptide FF (NPFF; FLFQPQRF-NH2) was able to modify the rewarding effect of cocaine (5 mg/kg) measured in the expression of conditioned place preference (CPP) test in rats and the expression of sensitization to hyperlocomotor effect of cocaine (10 mg/kg) in mice. Our results indicate that NPFF (5, 10, and 20 nmol) given intracerebroventricularly (i.c.v.) inhibited the expression of cocaine-induced CPP at the dose of 10 nmol (P<0.01) and 20 nmol (P<0.001). Moreover, NPFF inhibited the expression of cocaine-induced sensitization to its hyperlocomotor effect at the dose of 20 nmol (P<0.05) and acute hyperlocomotor effect of cocaine at doses of 5 nmol (P<0.01), 10 nmol (P<0.01), and 20 nmol (P<0.05). Our study suggests that NPFF may participate in a rewarding effect of cocaine measured in the CPP paradigm. On the other hand, our experiments indicate that NPFF is involved in the mechanism of expression of sensitization to cocaine hyperlocomotion but this effect seems to be non-specific because NPFF also inhibited the acute hyperlocomotor effect of cocaine.

Staurosporine differentiation of NPFF2 receptor-transfected SH-SY5Y neuroblastoma cells induces selectivity of NPFF activity towards opioid receptors

Activation of the NPFF(2) receptor reduces the inhibitory effect of opioids on the N-type Ca(2+) channel. Although this anti-opioid effect is specific for opioid receptors in neurons and tissues, it also affects NPY Y2 and alpha(2)-adrenoreceptors in undifferentiated SH-SY5Y cells stably expressing the NPFF(2) receptor. To test whether this difference could be due to the immaturity of these cells, they were differentiated to a noradrenergic neuronal phenotype with staurosporine. The differentiated cells ceased to divide and grew long, thin neurites. The inhibition of the depolarization-triggered Ca(2+) transient by activation of G(i)-coupled receptors was either unaffected (micro-opioid), increased (NPY), reduced (NPFF(2)) or lost (alpha(2)-adrenoreceptors). Following a 20 min incubation with 1DMe, the effect of DAMGO was reduced, as in undifferentiated cells, but the effect of NPY was no longer affected. Staurosporine differentiation did not modify the coupling of the micro-opioid and NPFF(2) receptors to the G(i/o) proteins. We suggest that the specificity of the effect of NPFF may not reside in the molecular mechanism of its anti-opioid activity itself but in the organization of receptors within the membrane.

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.

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.

A neuropeptide FF agonist blocks the acquisition of conditioned place preference to morphine in C57Bl/6J mice

Neuropeptide FF behaves as an opioid-modulating peptide that seems to be involved in morphine tolerance and physical dependence. Nevertheless, the effects of neuropeptide FF agonists on the rewarding properties of morphine remain unknown. C57BL6 mice were conditioned in an unbiased balanced paradigm of conditioned place preference to study the effect of i.c.v. injections of 1DMe (D-Tyr1(NMe)Phe3]NPFF), a stable agonist of the neuropeptide FF system, on the acquisition of place conditioning by morphine or alcohol (ethanol). Morphine (10 mg/kg, i.p.) or ethanol (2 g/kg, i.p.) induced a significant place preference. Injection of 1DMe (1-20 nmol), given 10 min before the i.p. injection of the reinforcing drug during conditioning, inhibited the rewarding effect of morphine but had no effect on the rewarding effect of ethanol. However, a single injection of 1DMe given just before place preference testing was unable to inhibit the rewarding effects of morphine. By itself, 1DMe was inactive but an aversive effect of this agonist could be evidenced if the experimental procedure was biased. These results suggest that neuropeptide FF, injected during conditioning, should influence the development of rewarding effects of morphine and reinforce the hypothesis of strong inhibitory interactions between neuropeptide FF and opioids.

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.

Neuropeptide FF (NPFF) control of magnocellular neurosecretory cells of the rat hypothalamic paraventricular nucleus (PVN)

Neuropeptide FF (NPFF) is an octapeptide belonging to an extended family of RF amide peptides that have been implicated in a wide variety of physiological functions in the brain. NPFF and its receptors are abundantly expressed in the rat brain and spinal cord 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 sought to examine the effects of NPFF on GABAergic inhibitory synaptic input to magnocellular neurosecretory cells (MNCs) of the PVN, which secrete the neurohormones, vasopressin and oxytocin from their terminals in the neurohypophysis. Whole cell patch clamp recordings under voltage clamp conditions were performed from PVN MNCs in the brain slice. Bicuculline-sensitive inhibitory postsynaptic currents (IPSCs) were isolated in the presence of glutamate receptor blockers. In tetrodotoxin, NPFF (5 microM) caused an increase in frequency, but not amplitude of miniature inhibitory postsynaptic currents (mIPSCs) in MNCs indicating a presynaptic locus of action for this peptide. Intracerebroventricular application of NPFF resulted in an activation of GABAergic neurons located adjacent to the PVN as revealed by immunohistochemistry for Fos protein and in situ hybridization for glutamic acid decarboxylase (GAD67) mRNA. Based on these observations we conclude that NPFF facilitates inhibitory input to MNCs of the PVN via GABAergic interneurons located in immediate vicinity of the nucleus. These findings provide a cellular and anatomic basis for the NPFF-induced inhibition of vasopressin release has been reported consequent to hypovolemia and hyperosmolar stimulation.

RF9, a potent and selective neuropeptide FF receptor antagonist, prevents opioid-induced tolerance associated with hyperalgesia

Neuropeptide FF (NPFF) has been proposed to play a role in pain modulation, opioid tolerance, and several other physiological processes. However, pharmacological agents that would help define physiological roles for this peptide are still missing. Here we report the discovery of a potent and selective NPFF receptor antagonist, RF9, that can be administered systemically. This compound does not show any effects by itself but can block efficiently the increase in blood pressure and heart rate evoked by NPFF. When chronically coinjected with heroin, RF9 completely blocks the delayed and long-lasting paradoxical opioid-induced hyperalgesia and prevents the development of associated tolerance. Our data indicate that NPFF receptors are part of a bona fide antiopioid system and that selective antagonists of these receptors could represent useful therapeutic agents for improving the efficacy of opioids in chronic pain treatment.

Anti-analgesia of a selective NPFF2 agonist depends on opioid activity

NPFF agonists designed to be selective NPFF(2) receptor probes were synthesized. D.Asn-Pro-(N-Me)Ala-Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH(2) (dNPA) displays a very high affinity (0.027nM) for NPFF(2) receptors transfected in CHO cells, and a very high selectivity with a discrimination ratio greater than 100 versus NPFF(1) receptors. dNPA acts as a potent and selective agonist in [(35)S]GTPgammaS binding experiments and inhibits intracellular cAMP production with the same efficacy as NPA-NPFF. In SH-SY5Y cells expressing NPFF(2) receptors dNPA, in the presence of carbachol, stimulates Ca(2+) release from the intracellular stores. In vivo, after intracerebroventricular injection dNPA increases body temperature in mice and reverses the morphine-induced analgesia. Also, dNPA displays anti-opioid activity after systemic administration. So far, dNPA exhibits the highest affinity and selectivity for NPFF(2) receptors and reveals that its behavioral anti-opioid activity depends on the degree of opioid-induced analgesia.

Rodent strain differences in the NPFF1 and NPFF2 receptor distribution and density in the central nervous system

The present study evaluates the putative differences between NPFF1 and NPFF2 receptor distribution and density throughout the central nervous system between rat and mouse strains by using in vitro quantitative autoradiography. The binding of [125I]YVP ([125I]YVPNLPQRF-NH2) and [125I]EYF ([125I]EYWSLAAPQRF-NH2), used to label NPFF1 and NPFF2 receptors, respectively, was compared between Sprague-Dawley and Wistar rats and between Swiss and C57BL/6-SV129 mice. In contrast to Wistar, Sprague-Dawley brains contained NPFF1 binding sites in the cortical and spinal cord areas, the accumbens nucleus, the anterodorsal thalamic nucleus, the parafascicular thalamic nucleus, the inferior colliculus and the nucleus of the solitary tract. The distribution of NPFF2 binding sites was also different between the two strains of rats. As compared to Swiss, C57BL/6-SV129 mice showed higher basal NPFF2 receptor levels in cortical areas, telencephalon and some other regions. In contrast, they showed lower amounts in thalamic structures, except the reuniens nucleus, and in mesencephalic and rhombencephalic regions. In the cervical spinal cord the levels of NPFF2 receptors were similar. The NPFF1 binding levels were nearly the same in telencephalic structures while distinct in the forebrain. Differences in amount of NPFF receptor subtypes among these strains of rats or mice could lead to differences in NPFF control of opioid nociception.

Opposite alterations of NPFF1 and NPFF2 neuropeptide FF receptor density in the triple MOR/DOR/KOR-opioid receptor knockout mouse brains

Mice lacking the mu-delta-kappa-opioid receptor (MOR/DOR/KOR) genes and their corresponding wild-type littermates have been used to quantify NPFF(1) and NPFF(2) (neuropeptide FF) receptors by in vitro autoradiography in the central nervous tissues. Adjacent coronal sections were labelled with [125I]YVP ([125I]YVPNLPQRF-NH(2)) and [125I]EYF ([125I]EYWSLAAPQRF-NH(2)) as specific radioligands for NPFF(1) and NPFF(2) receptors, respectively. NPFF(2) receptors are predominantly expressed in both genotypes, but their density increases significantly in non cortical regions of mutant mice: 64% in the amygdaloid area, 89, 308, 1214 and 49% in the nucleus of the vertical limb of the diagonal band, substantia nigra, the vestibular nucleus and the spinal cord, respectively. In contrast, the density of the NPFF(1) subtype is lower than NPFF(2) in both genotypes and significantly decreased in some brain areas of mutant mice: -99, -90 and -90% in the nucleus of the vertical limb of the diagonal band, substantia nigra and the spinal cord, respectively. This study shows that mice lacking opioid receptors have brain region-dependent increases (NPFF(2)) and decreases (NPFF(1)) in NPFF receptors densities and suggests a different functional participation of each NPFF receptor subtype in the actions of opioids.

Detailed distribution of neuropeptide FF receptors (NPFF1 and NPFF2) in the rat, mouse, octodon, rabbit, guinea pig, and marmoset monkey brains: a comparative autoradiographic study

The distribution of neuropeptide FF receptors (NPFF(1) and NPFF(2)) was analyzed throughout the central nervous system of rodents (rat, mouse, Octodon degus, and guinea pig), rabbit, and marmoset monkey brains, representing three orders of mammals. Quantitative in vitro receptor autoradiography with [(125)I]EYF ([(125)I]EYWSLAAPQRF-NH(2)) and [(125)I]YVP ([(125)I]YVPNLPQRF-NH(2)) as specific radioligands for NPFF(2) and NPFF(1) receptors, respectively, was used. The NPFF(2) receptor is predominantly expressed in all species, except in the central nervous system of Octodon degus, in which it is undetectable. The density of the NPFF(1) subtype is low in rat and mice, moderate in octodon, rabbit, and monkey, and relatively high in the guinea pig. The present study reveals prominent species differences in the NPFF receptors expression in the brain. The distribution pattern of NPFF(2) receptors in the diencephalon and the superficial layers of the spinal cord is consistent with a hypothesized potential role for NPFF in the modulation of sensory input and opioid analgesia. In contrast, the constant presence of NPFF(1) receptors in the septum, the nucleus of the tractus solitarius, and the hypothalamus suggest its participation in neuroendocrine functions.

Identification of proNeuropeptide FFA peptides processed in neuronal and non-neuronal cells and in nervous tissue

Peptides which should be generated from the neuropeptide FF (NPFF) precursor were identified in a neuronal (human neuroblastoma SH-SY5Y) cell line and in COS-7 cells after transient transfection of the human proNPFFA cDNA and were compared with those detected in the mouse spinal cord. After reverse-phase high performance liquid chromatography of soluble material, NPFF-related peptides were immunodetected with antisera raised against NPFF and identified by using on-line capillary liquid chromatography/nanospray ion trap tandem mass spectrometry. Neuronal and non-neuronal cells generated different peptides from the same precursor. In addition to NPFF, SQA-NPFF (Ser-Gln-Ala-Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-amide) and NPAF were identified in the human neuroblastoma while only NPFF was clearly identified in COS-7 cells. In mouse, in addition to previously detected NPFF and NPSF, SPA-NPFF (Ser-Pro-Ala-Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-amide), the homologous peptide of SQA-NPFF, were characterized. These data on intracellular processing of proNeuropeptide FFA are discussed in regard to the known enzymatic processing mechanisms.

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

The selectivity of two new radioligands, [(125)I]YVP ([(125)I]YVPNLPQRF-NH(2)) and [(125)I]EYF ([(125)I]EYWSLAAPQRF-NH(2)), for neuropeptide FF (NPFF) receptor subtypes was determined using HEK293 cells expressing hNPFF(1) and CHO cells expressing hNPFF(2) receptors. Saturation binding and displacement experiments showed that [(125)I]YVP and [(125)I]EYF bound selectively with a very high affinity, K(D)=0.18 nM and 0.06 nM, to NPFF(1) and NPFF(2) receptors respectively. By using in vitro autoradiography with these radioligands and frog pancreatic polypeptide (PP) as selective unlabelled competitor of NPFF(2) binding sites, NPFF(1) and NPFF(2) receptor distribution was analyzed throughout the rat CNS. The highest densities of [(125)I]EYF binding sites were seen in the most external layers of the dorsal horn of the spinal cord, the parafascicular thalamic nucleus, laterodorsal thalamic nucleus and presubiculum of hippocampus. All specific binding of this radioligand was inhibited by 200 nM frog PP. The density of 0.1 nM [(125)I]YVP binding was much smaller in all brain areas and frog PP-insensitive binding sites (NPFF(1) receptor subtype) were detected in septal, thalamic and hypothalamic areas but were absent in the spinal cord. The restricted distribution of NPFF(1) receptors in the CNS supports its specific role in a limited number of neuronal functions. In contrast to the rat spinal cord where the NPFF(1) system is absent, there is no strict separation between NPFF(1) and NPFF(2) system at the supraspinal level.

Identification of neuropeptide FF-related peptides in human cerebrospinal fluid by mass spectrometry

Several neuropeptide FF (NPFF)-related peptides, known as modulators of the opioid system, have been previously characterized in bovine and rodent brain. Reverse-phase high pressure liquid chromatography (HPLC) fractions of a human with normal pressure hydrocephalus cerebrospinal fluid (CSF), co-migrating with NPFF-related synthetic peptides, were characterized by capillary HPLC coupled on-line to nanospray ion trap tandem mass spectrometry. Two peptides present in the pro-NPFF(A) precursor, NPAF (AGEGLNSQFWSLAAPQRF-NH2) and NPSF (SLAAPQRF-NH2), were identified. The monitoring of NPFF-related peptides in human CSF can be helpful to understand their roles in pain sensitivity.

Pharmacological characterization of human NPFF1 and NPFF2 receptors expressed in CHO cells by using NPY Y1 receptor antagonists

Neuropeptide FF (NPFF) belongs to an opioid-modulatory system including two precursors (pro-NPFF(A) and pro-NPFF(B)) and two G-protein coupled receptors (NPFF(1) and NPFF(2)). The pharmacological and functional profiles of human NPFF(1) and NPFF(2) receptors expressed in Chinese hamster ovary (CHO) cells were compared by determining the affinity of several peptides derived from both NPFF precursors and by measuring their abilities to inhibit forskolin-induced cAMP accumulation. Each NPFF receptor recognizes peptides from both precursors with nanomolar affinities, however, with a slight preference of pro-NPFF(A) peptides for NPFF(2) receptors and of pro-NPFF(B) peptides for NPFF(1) receptors. BIBP3226 ((R)-N(2)-(diphenylacetyl)-N-[(4-hydroxyphenyl)-methyl]-argininamide) and BIBO3304 ((R)-N(2)-(diphenylacetyl)-N-[4-(aminocarbonylaminomethyl)-benzyl]-argininamide trifluoroacetate), two selective neuropeptide Y (NPY) Y(1) receptor antagonists, display relative high affinities for NPFF receptors and exhibit antagonist properties towards hNPFF(1) receptors. The structural determinants responsible for binding of these molecules to NPFF receptors were investigated and led to the synthesis of hNPFF(1) receptor antagonists with affinities from 40 to 80 nM. Our results demonstrate differences in pharmacological characteristics between NPFF(1) and NPFF(2) receptors and the feasibility of subtype-selective antagonists.

Peptide nucleic acids targeted to the mouse proNPFF(A) reveal an endogenous opioid tonus

Pharmacological studies have implicated the anti-opioid neuropeptide FF (NPFF) in the modulation of pain transmission. Since its physiological role has not yet been fully elucidated, the present study examined whether antisense peptide nucleic acid (PNA) complementary to the NPFF precursor (proNPFF(A)) modified pain sensitivity. Mice received three intraperitoneal (i.p.) injections (10mg/kg) of antisense PNA (As-proNPFF(A)) over a period of 24h. As-proNPFF(A) treatment significantly increased the basal tail withdrawal latency in the tail-flick test. This analgesia persisted during 2 days and was completely reversed by naloxone. Thus, antisense PNAs, by decreasing anti-opioid effects, revealed a basal endogenous opioid activity. Our results evidence a physiological interplay between NPFF and opioid systems and further support the use of PNA as effective antisense agents, for studying gene function in vivo.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.