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

An Inhibitory Circuit From Brainstem to GnRH Neurons in Male Mice: A New Role for the RFRP Receptor

RFamide-related peptides (RFRPs, mammalian orthologs of gonadotropin-inhibitory hormone) convey circadian, seasonal, and social cues to the reproductive system. They regulate gonadotropin secretion by modulating gonadotropin-releasing hormone (GnRH) neurons via the RFRP receptor. Mice lacking this receptor are fertile but exhibit abnormal gonadotropin responses during metabolic challenges, such as acute fasting, when the normal drop in gonadotropin levels is delayed. Although it is known that these food intake signals to the reproductive circuit originate in the nucleus tractus solitarius (NTS) in the brainstem, the phenotype of the neurons conveying the signal remains unknown. Given that neuropeptide FF (NPFF), another RFamide peptide, resides in the NTS and can bind to the RFRP receptor, we hypothesized that NPFF may regulate GnRH neurons. To address this question, we used a combination of techniques: cell-attached electrophysiology on GnRH-driven green fluorescent protein-tagged neurons in acute brain slices; calcium imaging on cultured GnRH neurons; and immunostaining on adult brain tissue. We found (1) NPFF inhibits GnRH neuron excitability via the RFRP receptor and its canonical signaling pathway (Gi/o protein and G protein-coupled inwardly rectifying potassium channels), (2) NPFF-like fibers in the vicinity of GnRH neurons coexpress neuropeptide Y, (3) the majority of NPFF-like cell bodies in the NTS also coexpress neuropeptide Y, and (4) acute fasting increased NPFF-like immunoreactivity in the NTS. Together these data indicate that NPFF neurons within the NTS inhibit GnRH neurons, and thus reproduction, during fasting but prior to the energy deficit.

Analgesic activities of the mixed opioid and NPFF receptors agonist DN-9 in a mouse model of formalin-induced orofacial inflammatory pain

Orofacial pain is one of the most common pain conditions and compromises the quality of life of the sufferer. Several studies have shown that opioid agonists produced significant analgesia in the orofacial pain, and combination of opioids with drugs belonging to other classes induced synergism in the orofacial pain. However, combination therapy of different analgesic drugs improves the risk of drug-drug interactions. Against this background, we sought to investigate the analgesic effects of the multi-functional opioid peptide DN-9, a mixed opioid and NPFF receptors agonist that produced robust analgesia in acute and inflammatory pain models, on formalin-induced orofacial pain. Our results showed that formalin injection caused significant spontaneous pain behaviors and increased the expressions of the mu-opioid receptor, c-Fos and phosphorylated extracellular signal-regulated kinase (p-ERK1/2) in the ipsilateral trigeminal ganglion (TG). In mice pretreated with DN-9, there was a significant reduction in nociceptive behaviors, which was selectively mediated by the mu- and kappa-opioid receptors, independently of the NPFF system. Four hours after formalin injection, the level of c-Fos immunoreactivity in the ipsilateral TG neurons was much lower in mice pretreated with DN-9 or morphine. In addition, DN-9 exhibited a significant inhibition in the expression of p-ERK1/2, which was reversed by the selective antagonists of the mu- and kappa-opioid receptors. In conclusion, our present results demonstrate that central administration of DN-9 produces potential antinociceptive effects via the mu- and kappa-opioid receptors, independently of the NPFF system, and this antinociceptive action is tightly linked with the intracellular ERK activation in TG neurons.

The role of orphan G protein-coupled receptors in the modulation of pain: A review

G protein-coupled receptors (GPCRs) comprise a large number of receptors. Orphan GPCRs are divided into six families. These groups contain orphan receptors for which the endogenous ligands are unclear. They have various physiological effects in the body and have the potential to be used in the treatment of different diseases. Considering their important role in the central and peripheral nervous system, their role in the treatment of pain has been the subject of some recent studies. At present, there are effective therapeutics for the treatment of pain including opioid medications and non-steroidal anti-inflammatory drugs. However, the side effects of these drugs and the risks of tolerance and dependence remain a major problem. In addition, neuropathic pain is a condition that does not respond to currently available analgesic medications well. In the present review article, we aimed to review the most recent findings regarding the role of orphan GPCRs in the treatment of pain. Accordingly, based on the preclinical findings, the role of GPR3, GPR7, GPR8, GPR18, GPR30, GPR35, GPR40, GPR55, GPR74, and GPR147 in the treatment of pain was discussed. The present study highlights the role of orphan GPCRs in the modulation of pain and implies that these receptors are potential new targets for finding better and more efficient therapeutics for the management of pain particularly neuropathic pain.

The new kisspeptin derivative - kissorphin (KSO) - attenuates acute hyperlocomotion and sensitization induced by ethanol and morphine in mice

Kissorphin (KSO) is a new peptide derived from kisspeptin-10. This peptide possesses neuropeptide FF (NPFF)-like biological activity in vitro; NPFF, in many cases, inhibits opioid and ethanol effects in rodents. Therefore, the current study explored the influence of KSO on acute ethanol- and morphine-induced hyperactivity, and on the development and expression of locomotor sensitization induced by these drugs. In the present study, sensitization to locomotor effects was induced by repeated exposure to ethanol (2.4 g/kg, intraperitoneally [i.p.], 1 × 4 days) or morphine (10 mg/kg, subcutaneously [s.c.], 1 × 7 days). We found that KSO (1-10 nmol/300 μL, intravenously [i.v.]) did not have an impact on locomotor activity of naïve mice. However, it reduced both acute ethanol- (10 nmol/300 μL) and morphine-induced hyperactivity (3 and 10 nmol/300 μL). Pretreatment of animals with KSO (10 nmol/300 μL), before every ethanol or morphine injection during development of sensitization or before the ethanol or morphine challenge, attenuated the development, as well as the expression of locomotor sensitization to both substances. Moreover, prior administration of the NPFF receptor antagonist RF9 (10 nmol/300 μL, i.v.) inhibited the ability of KSO (10 nmol/300 μL) to reduce the expression of ethanol and morphine sensitization. KSO given alone, at all used doses, did not influence the motor coordination measured via the rotarod test. The results from this study show that KSO effectively attenuated acute and repeated effects of ethanol and morphine. Thus, KSO possesses NPFF-like anti-opioid activity in these behavioral studies.

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.

Effects of neuropeptide FF and related peptides on the antinociceptive activities of VD-hemopressin(α) in naive and cannabinoid-tolerant mice

Neuropeptide FF (NPFF) system has recently been reported to modulate cannabinoid-induced antinociception. The aim of the present study was to further investigate the roles of NPFF system in the antinociceptive effects induced by intracerebroventricular (i.c.v.) administration of mouse VD-hemopressin(α), a novel endogenous agonist of cannabinoid CB1 receptor, in naive and VD-hemopressin(α)-tolerant mice. The effects of NPFF system on the antinociception induced by VD-hemopressin(α) were investigated in the radiant heat tail-flick test in naive mice and VD-hemopressin(α)-tolerant mice. The cannabinoid-tolerant mice were produced by given daily injections of VD-hemopressin(α) (20 nmol, i.c.v.) for 5 days and the antinociception was measured on day 6. In naive mice, intracerebroventricular injection of NPFF dose-dependently attenuated central analgesia of VD-hemopressin(α). In contrast, neuropeptide VF (NPVF) and D.NP(N-Me)AFLFQPQRF-NH2 (dNPA), two highly selective agonists for Neuropeptide FF1 and Neuropeptide FF2 receptors, enhanced VD-hemopressin(α)-induced antinociception in a dose-dependent manner. In addition, the VD-hemopressin(α)-modulating activities of NPFF and related peptides were antagonized by the Neuropeptide FF receptors selective antagonist 1-adamantanecarbonyl-RF-NH2 (RF9). In VD-hemopressin(α)-tolerant mice, NPFF failed to modify VD-hemopressin(α)-induced antinociception. However, both neuropeptide VF and dNPA dose-dependently potentiated the antinociception of VD-hemopressin(α) and these cannabinoid-potentiating effects were reduced by RF9. The present works support the cannabinoid-modulating character of NPFF system in naive and cannabinoid-tolerant mice. In addition, the data suggest that a chronic cannabinoid treatment modifies the pharmacological profiles of NPFF, but not the cannabinoid-potentiating effects of neuropeptide VF and dNPA.

Role of opioid receptor heterodimerization in pain modulation and tolerance development

Protein to protein interactions leading to homo/heteromerization of receptor is well documented in literature. These interactions leading to dimeric/oligomers formation of receptors are known to modulate their function, particularly in case of G-protein coupled receptors. The opioid receptor heteromers having changed pharmacological properties than the constituent protomers provides preferences for novel drug targets that could lead to potential analgesic activity devoid of tolerance and physical dependence. Heterodimerization of opioid receptors appears to generate novel binding properties with improved specificity and lack of side effects. Further the molecules which can interact simultaneously to both the protomers of the heteromer, or to both the binding sites (orthosteric and allosteric) of a receptor protein could be potential therapeutic molecules. This review highlights the recent advancements in exploring the plausible role of heteromerization of opioid receptors in induction of tolerance free antinociception.

Opposite effects of neuropeptide FF on central antinociception induced by endomorphin-1 and endomorphin-2 in mice

Neuropeptide FF (NPFF) is known to be an endogenous opioid-modulating peptide. Nevertheless, very few researches focused on the interaction between NPFF and endogenous opioid peptides. In the present study, we have investigated the effects of NPFF system on the supraspinal antinociceptive effects induced by the endogenous µ-opioid receptor agonists, endomorphin-1 (EM-1) and endomorphin-2 (EM-2). In the mouse tail-flick assay, intracerebroventricular injection of EM-1 induced antinociception via µ-opioid receptor while the antinociception of intracerebroventricular injected EM-2 was mediated by both µ- and κ-opioid receptors. In addition, central administration of NPFF significantly reduced EM-1-induced central antinociception, but enhanced EM-2-induced central antinociception. The results using the selective NPFF1 and NPFF2 receptor agonists indicated that the EM-1-modulating action of NPFF was mainly mediated by NPFF2 receptor, while NPFF potentiated EM-2-induecd antinociception via both NPFF1 and NPFF2 receptors. To further investigate the roles of µ- and κ-opioid systems in the opposite effects of NPFF on central antinociception of endomprphins, the µ- and κ-opioid receptors selective agonists DAMGO and U69593, respectively, were used. Our results showed that NPFF could reduce the central antinociception of DAMGO via NPFF2 receptor and enhance the central antinociception of U69593 via both NPFF1 and NPFF2 receptors. Taken together, our data demonstrate that NPFF exerts opposite effects on central antinociception of endomorphins and provide the first evidence that NPFF potentiate antinociception of EM-2, which might result from the interaction between NPFF and κ-opioid systems.

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.

Study of the N-terminal part of peptidic selective NPFF₂ agonists

Neuropeptide FF (NPFF) has been shown to act as an endogenous anti-analgesic peptide. In this paper, several peptide analogs of the selective ligand dNP(NMe)AFLFQPQRF-NH(2) modified in the putative address segment, were designed to be selective NPFF(2) receptor probes, synthesized and assayed. One peptide dA(NMe)AAFLFQPQRF-NH(2) displays a very high affinity for NPFF(2) receptors transfected in CHO cells, and a high selectivity versus NPFF(1) receptors. The exact residues carried in the N-terminal part of the ligands are not decisive to obtain a high affinity only the length of the peptide in itself seems important to create selectivity.

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.

Nonpeptide ligands of neuropeptide FF: current status and structural insights

Neuropeptide FF (NPFF) was first isolated from the bovine brain in 1985 and is linked with a variety of biological activities. NPFF, which belongs to the RF-amide family of peptides, interacts with two distinct G-protein-coupled receptors, NPFF(1) and NPFF(2). These receptors are distributed throughout the body. The NPFF system was initially described as an anti-opioid system and, while the NPFF system does affect the opioid system, it also has been implicated in pain modulation, changes in arterial blood pressure, feeding behavior and regulation of core body temperature and of monoamine systems. Most of this pharmacology has been realized from the peptide NPFF itself or through peptide analogs. The quest for nonpeptide tools for this receptor system has been limited by low selectivity and poor pharmacokinetic properties. Herein, we summarize the current knowledge from the scientific and patent literature that demonstrates a clear need for future medicinal chemistry efforts.

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.

Effects of neuropeptide FF system on CB₁ and CB₂ receptors mediated antinociception in mice

It has been demonstrated that opioid and cannabinoid receptor systems can produce similar signal transduction and behavioural effects. Neuropeptide FF (NPFF) belongs to an opioid-modulating peptide family. NPFF has been reported to play important roles in control of pain and analgesia through interactions with the opioid system. We were interested in whether the central and peripheral antinociception of cannabinoids could be influenced by supraspinal NPFF system. The present study examined the effects of NPFF and related peptides on the antinociceptive activities induced by the non-selective cannabinoid receptors agonist WIN55,212-2, given by supraspinal and intraplantar routes. In mice, the central and peripheral antinociception of WIN55,212-2 are mediated by cannabinoid CB(1) and CB(2) receptors, respectively. Interestingly, central administration of NPFF significantly reduced central and peripheral analgesia of cannabinoids in dose-dependent manners. In contrast, dNPA and NPVF (i.c.v.), two highly selective agonists for NPFF(2) and NPFF(1) receptors, dose-dependently augmented the antinociception caused by intracerebroventricular and intraplantar injection of WIN55,212-2. Additionally, pretreatment with the NPFF receptors selective antagonist RF9 (i.c.v.) markedly reduced the cannabinoid-modulating activities of NPFF and related peptides in nociceptive assays. These data provide the first evidence for a functional interaction between NPFF and cannabinoid systems, indicating that activation of central NPFF receptors interferes with cannabinoid-mediated central and peripheral antinociception. Intriguingly, the present work may pave the way for a new strategy of using combination treatment of cannabinoid and NPFF agonists for pain management. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

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.

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.

Opposite control of body temperature by NPFF1 and NPFF2 receptors in mice

Neuropeptide FF (NPFF) is a neurotransmitter known to modulate opioid functions. This study investigates the effects of RF9, a new antagonist of NPFF receptors, on the roles of NPFF1 and NPFF2 receptors in thermoregulation in mice. RF9 (10 nmol) injected into the third ventricle did not modify the body temperature as compared to saline, but it completely antagonized the hypothermic effects of 10 nmol NPVF, a NPFF1 selective agonist, as well as the hyperthermic actions of dNPA (5 nmol), a NPFF2 selective agonist. The use of a specific antagonist demonstrates here that central NPFF1 and NPFF2 receptors control in an opposite manner the body temperature in mice.

Metabolic and stress-related roles of prolactin-releasing peptide

In the modern world, improvements in human health can be offset by unhealthy lifestyle factors, including the deleterious consequences of stress and obesity. For energy homeostasis, humoral factors and neural afferents from the gastrointestinal tract, in combination with long-term nutritional signals, communicate information to the brain to regulate energy intake and expenditure. Energy homeostasis and stress interact with each other, and stress affects both food intake and energy expenditure. Prolactin-releasing peptide, synthesized in discrete neuronal populations in the hypothalamus and brainstem, plays an important role in integrating these responses. This review describes how prolactin-releasing peptide neurons receive information concerning both internal metabolic states and environmental conditions, and play a key role in energy homeostasis and stress responses.

Pressor and tachycardic responses to intrathecal administration of neuropeptide FF in anesthetized rats

Neuropeptide FF (NPFF) belongs to a neuropeptide family including two precursors (pro-NPFF(A) and pro-NPFF(B)) and two receptors (NPFF(1) and NPFF(2)). NPFF and NPFF receptor mRNAs have been reported to be highly expressed and localized in the rat and human spinal cord. In the present study, the i.t. action of NPFF system on blood pressure and heart rate were examined using NPFF and two related agonists, NPVF and dNPA, which exhibit highest selectivities for NPFF(1) and NPFF(2) receptors, respectively. In urethane-anesthetized rats, NPFF and related peptides (5-40 nmol, i.t.) produced significant pressor and tachycardic responses at the spinal cord level. These effects were dose-dependent and similar with respect to time-course for the three peptides. Furthermore, i.t. injection of RF9 (20 nmol), a selective NPFF antagonist, significantly antagonized the cardiovascular responses to 20 nmol NPFF and related peptides (i.t.). Moreover, pretreatment of the rats with alpha-adrenoceptor antagonist phentolamine (1mg/kg, i.v.) significantly reduced the pressor effects of NPFF. Nevertheless, pretreatment with muscarinic receptor and adrenoceptor antagonists (i.v.) could block the tachycardic effects induced by NPFF. Collectively, our results suggested that i.t. administration of NPFF and related peptides increased MAP and HR which were possibly mediated by the activation of both NPFF(1) and NPFF(2) receptors in the rat spinal cord. In addition, our results showed that the muscarinic receptor and adrenoceptor participated in the tachycardic response to i.t. NPFF, while alpha-adrenoceptor played an important role in the regulation of pressor effect of NPFF.

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.

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.

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.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Morphine withdrawal modifies antinociceptive effects of acute morphine in rats

Repeated opioid use is known to cause tolerance of antinociceptive effects. Whether opioid abstinence modifies antinociceptive effects is unknown. Here we reported that morphine withdrawal for 18 h and 4 days after repeated morphine treatment largely reduced tail-flick latencies compared with control, while the rats showed severe withdrawal syndromes. However, the latencies and withdrawal syndromes were restored to control level at 20 days withdrawal. Similarly, antinociceptive effects of acute morphine were decreased at 18 h and further decreased at 4 days but restored to control level at 20 days withdrawal. Behavioral stress that was given to the rats at 18 h withdrawal further reduced tail-flick latencies and antinociceptive effects. Conversely, the glucocorticoid receptor antagonist RU38486 increased tail-flick latencies and antinociceptive effects at 4 days withdrawal. These results suggest that morphine withdrawal could evoke behavioral stress to modify antinociceptive effects, implicating a significant influence of opioid abstinence on chronic pain treatment.