Bidirectional modulatory effect of orphanin FQ on morphine-induced analgesia: antagonism in brain and potentiation in spinal cord of the rat

Substance P N-terminal fragment SP(1-7) attenuates chronic morphine tolerance and affects dynorphin B and nociceptin in rats

The N-terminal substance P fragment SP(1-7) is known to modulate hyperalgesia and opioid withdrawal in animal models. This study examined the effects of intraperitoneal (i.p.) injections of SP(1-7) on chronic morphine tolerance and on the levels of dynorphin B (DYN B) and nociceptin/orphanin FQ (N/OFQ) in various brain areas of male Sprague-Dawley rats. Morphine tolerance was induced by subcutaneous injections of the opioid (10mg/kg) twice daily for 7 days. SP(1-7) injected i.p. (185 nmol/kg) 30 min prior to morphine reduced the development of morphine tolerance. Immunoreactive (ir) DYN B and N/OFQ peptide levels were measured in several areas of the central nervous system. Levels of ir DYN B in rats treated with SP(1-7) and morphine were decreased in the nucleus accumbens, substantia nigra and ventral tegmental area and increased in the frontal cortex. The ir N/OFQ levels were increased in the periaqueductal gray and decreased in the nucleus accumbens. Since the concentration profiles of the two peptides were altered by SP(1-7) in the areas that are implicated in the modulation of opioid tolerance and analgesia, it is suggested that DYN B and N/OFQ systems may be involved in the effects of SP(1-7) on opioid tolerance.

The role of nociceptin and dynorphin in chronic pain: implications of neuro-glial interaction

Nociceptin-opioid peptide (NOP) receptor, also known as opioid receptor like-1 (ORL1), was identified following the cloning of the kappa-opioid peptide (KOP) receptor, and the characterization of these receptors revealed high homology. The endogenous ligand of NOP, nociceptin (NOC), which shares high homology to dynorphin (DYN), was discovered shortly thereafter, and since then, it has been the subject of several investigations. Despite the many advances in our understanding of the involvement of NOC and DYN systems in pain, tolerance and withdrawal, the precise function of these systems has not been fully characterized. Here, we review the recent literature concerning the distribution of the NOC and DYN systems in the central nervous system and the involvement of these systems in nociceptive transmission, especially under chronic pain conditions. We discuss the use of endogenous and exogenous ligands of NOP and KOP receptors in pain perception, as well as the potential utility of NOP ligands in clinical practice for pain management. We also discuss the modulation of opioid effects by NOC and DYN. We emphasize the important role of neuro-glial interactions in the effects of NOC and DYN, focusing on their presence in neuronal and non-neuronal cells and the changes associated with chronic pain conditions. We also present the dynamics of immune and glial regulation of neuronal functions and the importance of this regulation in the roles of NOC and DYN under conditions of neuropathic pain and in the use of drugs that alter these systems for better control of neuropathic pain.

Antinociceptive effects of spinally administered nociceptin/orphanin FQ and its N-terminal fragments on capsaicin-induced nociception

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand for the N/OFQ peptide (NOP) receptors, has been shown to be metabolized into some fragments. We examined to determine whether intrathecal (i.t.) N/OFQ (1-13), (1-11) and (1-7) have antinociceptive activity in the pain-related behavior after intraplantar injection of capsaicin. The i.t. administration of N/OFQ (0.3-1.2 nmol) produced an appreciable and dose-dependent inhibition of capsaicin-induced paw-licking/biting response. The N-terminal fragments of N/OFQ, (1-13) and (1-11), were antinociceptive with a potency lower than N/OFQ. Calculated ID₅₀ values (nmol, i.t.) were 0.83 for N/OFQ, 2.5 for N/OFQ (1-13) and 4.75 for N/OFQ (1-11), respectively. The time-course effect revealed that the antinociceptive effects of these N-terminal fragments lasted longer than those of N/OFQ. Removal of amino acids down to N/OFQ (1-7) led to be less potent than N/OFQ and its fragments, (1-13) and (1-11). Antinociception induced by N/OFQ or N/OFQ (1-13) was reversed significantly by i.t. co-injection of [Nphe¹]N/OFQ (1-13)NH₂, a peptidergic antagonist for NOP receptors, whereas i.t. injection of the antagonist did not interfere with the action of N/OFQ (1-11) and (1-7). Pretreatment with the opioid receptor antagonist naloxone hydrochloride did not affect the antinociception induced by N/OFQ and its N-terminal fragments. These results suggest that N-terminal fragments of N/OFQ are active metabolites and may modulate the antinociceptive effect of N/OFQ in the spinal cord. The results also indicate that N/OFQ (1-13) still possess antinociceptive activity through NOP receptors.

Buprenorphine: a unique drug with complex pharmacology

Buprenorphine, an opioid with mixed agonist-antagonist activity at classical opioid receptors, has been approved recently for the treatment of opioid dependency. Buprenorphine is also used as an analgesic. The buprenorphine dose-response curve is sometimes submaximal, or even bell-shaped, in nociceptive assays, depending upon the nature and intensity of the noxious stimulus. Moreover, buprenorphine, when administered with full agonists, such as morphine, antagonizes the action of these drugs. Partial agonism at the mu opioid receptor and, in some cases, antagonism at the kappa or delta opioid receptor have been considered as possible underlying mechanisms for the ceiling effect and bell-shaped dose-response curve of buprenorphine. While ceiling effects can be explained by partial agonist activity of buprenorphine, the bell-shaped dose-response curve cannot be a consequence of this property of the drug. Recently, buprenorphine has been shown to activate the opioid receptor-like (ORL-1; also known as NOP) receptor. Supraspinal activation of the ORL-1 receptor counteracts the antinociceptive and rewarding actions of morphine, raising the possibility that these actions of buprenorphine can also be altered by its ability to concomitantly activate the ORL-1 receptor. The use of molecular biological techniques has advanced our knowledge regarding the role of opioid receptors in modulation of pain and reward. In particular, generation of opioid receptor knockout mice has proven useful in this regard. Indeed, using knockout mice, we have recently shown that the antinociceptive effect of buprenorphine mediated primarily by the mu opioid receptor is attenuated by the ability of the drug to activate the ORL-1 receptor. Thus, the goal of this review is to provide evidence demonstrating that the ORL-1 receptor plays a functional role not only in the antinociceptive effect of buprenorphine but also in other actions of the drug as well.

Nociceptin/Orphanin FQ in PAG modulates the release of amino acids, serotonin and norepinephrine in the rostral ventromedial medulla and spinal cord in rats

High density Nociceptin/Orphanin FQ (N/OFQ) and its receptor (NOPr) have been found in the ventrolateral periaqueductal gray (vlPAG), a main output pathway involved in the descending pain-control system. Our previous study demonstrated that the microinjection of N/OFQ into the vlPAG markedly facilitated nociceptive responses of spinal dorsal horn neurons. The aim of the present work was to further provide evidence for the supraspinal mechanisms of action for N/OFQ-mediated nociceptive facilitation by examining the effect of N/OFQ in the vlPAG on neurotransmitter release in the descending pain-control system, including the nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis (NGC) and dorsal horn of the spinal cord. The results showed that the microinjection of N/OFQ into the vlPAG produced robust decreases in 5-hydroxytryptamine (5-HT, serotonin), norepinephrine (NE), and gamma-aminobutyric acid (GABA), and increase in glutamate (Glu) release in the spinal dorsal horn. Spinal application of 5-HT, 2-Me-5-HT (5-HT(3) receptor agonist), muscimol (GABA(A) receptor agonist), and baclofen (GABA(B) receptor agonist) significantly blocked intra-vlPAG-induced facilitation on nociceptive responses. However, the extracellular concentrations of these neurotransmitters in the NRM and NGC exhibited diversity following intra-vlPAG of N/OFQ. In the NRM, intra-vlPAG injection of N/OFQ significantly decreased 5-HT, NE, and Glu, but increased GABA release. Differently, in the NGC, both NE and GABA releases were attenuated by intra-vlPAG of N/OFQ, whereas the concentration of 5-HT and Glu exhibited a trend to increase. These findings provide direct support for the hypothesis that intra-PAG of N/OFQ-induced facilitation of nociceptive responses is associated with the release of 5-HT, NE, and amino acids.

Receptor mediation and nociceptin inhibition of bradykinin-induced plasma extravasation in the knee joint of the rat

OBJECTIVE AND DESIGN

The aim was to investigate the signaling mechanisms and regulation of bradykinin (BK)-induced inflammation in rat knee joint.

MATERIALS AND METHODS

Knee joints of anesthetized rats were perfused with BK (0.1-1.0 microM), and synovial plasma extravasation (PE) was evaluated by spectrophotometrical measurement of Evans Blue leakage. To examine the signaling pathway, B1 antagonist [des-Arg10]-HOE140 (0.1-1.0 microM) and B2 antagonist HOE140 (0.05-1.0 microM), calcitonin gene-related peptide (CGRP) antagonist CGRP8-37 (0.5-1.0 microM), prostaglandin E2 antagonist AH-6809 (0.1-1.0 microM), and histamine H1 antagonist mepyramine (0.1-1.0 microM) were used. Nociceptin (0.0001-1.0 microM) and antagonist J-113397 were tested for modulation of BK-induced PE. The analyses were compared side-by-side with 5-hydroxytryptamine-induced PE.

RESULTS

BK perfusion dose-dependently induced PE, which was blocked by HOE140, CGRP8-37, AH-6809, and mepyramine. It was also inhibited by nociceptin, which could be reversed by antagonist J-113397. In contrast, 5-hydroxytryptamine-induced PE was biphasically regulated by nociceptin and was not antagonized by CGRP8-37.

CONCLUSIONS

BK-induced PE is mediated by B2 receptors and may involve CGRP, prostaglandin, and histamine pathways. BK-induced PE is inhibited by nociceptin through the activation of ORL1 receptors. There are differences between BK- and 5-hydroxytryptamine-induced inflammation in signaling and modulation.

Nociceptin/orphanin FQ receptor activation attenuates antinociception induced by mixed nociceptin/orphanin FQ/mu-opioid receptor agonists

Activation of brain nociceptin/orphanin FQ (NOP) receptors leads to attenuation of mu-opioid receptor (MOP receptor)-mediated antinociception. Buprenorphine, a high-affinity partial MOP receptor agonist also binds to NOP receptors with 80 nM affinity. The buprenorphine-induced inverted U-shaped dose-response curve for antinociception may be due to NOP receptor activation, given that, in the presence of the NOP receptor antagonist, 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (J113397), or in NOP receptor knockout mice, buprenorphine has a steeper dose-response curve and acts as a full agonist. To further explore the involvement of the direct activation of NOP receptors by buprenorphine and other compounds that activate both NOP and MOP receptors, the antinociceptive effects of 1-(1-(2,3,3alpha,4,5,6-hexahydro-1H-phenalen-1-yl)piperidin-4-yl)-indolin-2-one. (SR16435), 3-ethyl-1-(1-(4-isopropylcyclohexyl)piperidin-4-yl)-indolin-2-one (SR16507), buprenorphine, pentazocine, and morphine, compounds with varying levels of MOP and NOP receptor affinity and efficacy, were assessed in mice using the tail-flick assay. The ability of the selective NOP receptor antagonist (-)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111) to potentiate antinociception induced by the above compounds was examined to investigate whether activation of NOP receptors leads to attenuation of MOP receptor-mediated antinociception. SB-612111 potentiated antinociception induced by buprenorphine and the other mixed NOP/MOP receptor agonists SR16435 and SR16507. However, SB-612111 had no effect on pentazocine or morphine antinociception, two compounds with no NOP receptor-binding affinity. These results further support the hypothesis that activation of NOP receptors can lead to attenuation of MOP receptor-mediated antinociception elicited by mixed NOP/MOP receptor compounds such as buprenorphine, SR16435, and SR16507 and that, although buprenorphine has low efficacy in vitro, it has significant NOP receptor agonist activity in vivo.

Activity of new NOP receptor ligands in a rat peripheral mononeuropathy model: potentiation of morphine anti-allodynic activity by NOP receptor antagonists

The effect of new NOP receptor agonists and antagonists in the rat chronic constriction injury model was investigated. Intraperitoneally administered NOP receptor agonist SR14150 and antagonists SR16430 and SR14148, had no effect on mechanical allodynia when given alone. The nonselective NOP/mu-opioid receptor agonist SR16435, however, produced an anti-allodynic response, similar to morphine and reversible by naloxone. Notably, co-administration of the NOP receptor antagonists potentiated the anti-allodynic activity of both morphine and SR16435. Increased levels of the NOP receptor are implicated in the reduced efficacy of morphine in neuropathic pain. Our results suggest the utility of NOP receptor antagonists for potentiating opioid efficacy in chronic pain.

Neuropeptides in depression: role of VGF

The monoamine hypothesis of depression is increasingly called into question by newer theories that revolve around changes in neuronal plasticity, primarily in the hippocampus, at both the structural and the functional levels. Chronic stress negatively regulates hippocampal function while antidepressants ameliorate the effects of stress on neuronal morphology and activity. Both stress and antidepressants have been shown to affect levels of brain-derived neurotrophic factor (BDNF) whose transcription is dependent on cAMP response element binding protein (CREB). BDNF itself has antidepressant-like actions and can induce transcription of a number of molecules. One class of genes regulated by both BDNF and serotonin (5-HT) are neuropeptides including VGF (non-acryonimic) which has a novel role in depression. Neuropeptides are important modulators of neuronal function but their role in affective disorders is just emerging. Recent studies demonstrate that VGF, which is also a CREB-dependent gene, is upregulated by antidepressant drugs and voluntary exercise and is reduced in animal models of depression. VGF enhances hippocampal synaptic plasticity as well as neurogenesis in the dentate gyrus but the mechanisms of antidepressant-like actions of VGF in behavioral paradigms are not known. We summarize experimental data describing the roles of BDNF, VGF and other neuropeptides in depression and how they may be acting through the generation of new neurons and altered synaptic activity. Understanding the molecular and cellular changes that underlie the actions of neuropeptides and how these adaptations result in antidepressant-like effects will aid in developing drugs that target novel pathways for major depressive disorders.

Hypocretin/orexin and nociceptin/orphanin FQ coordinately regulate analgesia in a mouse model of stress-induced analgesia

Stress-induced analgesia (SIA) is a key component of the defensive behavioral "fight-or-flight" response. Although the neural substrates of SIA are incompletely understood, previous studies have implicated the hypocretin/orexin (Hcrt) and nociceptin/orphanin FQ (N/OFQ) peptidergic systems in the regulation of SIA. Using immunohistochemistry in brain tissue from wild-type mice, we identified N/OFQ-containing fibers forming synaptic contacts with Hcrt neurons at both the light and electron microscopic levels. Patch clamp recordings in GFP-tagged mouse Hcrt neurons revealed that N/OFQ hyperpolarized, decreased input resistance, and blocked the firing of action potentials in Hcrt neurons. N/OFQ postsynaptic effects were consistent with opening of a G protein-regulated inwardly rectifying K+ (GIRK) channel. N/OFQ also modulated presynaptic release of GABA and glutamate onto Hcrt neurons in mouse hypothalamic slices. Orexin/ataxin-3 mice, in which the Hcrt neurons degenerate, did not exhibit SIA, although analgesia was induced by i.c.v. administration of Hcrt-1. N/OFQ blocked SIA in wild-type mice, while coadministration of Hcrt-1 overcame N/OFQ inhibition of SIA. These results establish what is, to our knowledge, a novel interaction between the N/OFQ and Hcrt systems in which the corticotropin-releasing factor and N/OFQ systems coordinately modulate the Hcrt neurons to regulate SIA.

Neural mechanism underlying acupuncture analgesia

Acupuncture has been accepted to effectively treat chronic pain by inserting needles into the specific "acupuncture points" (acupoints) on the patient's body. During the last decades, our understanding of how the brain processes acupuncture analgesia has undergone considerable development. Acupuncture analgesia is manifested only when the intricate feeling (soreness, numbness, heaviness and distension) of acupuncture in patients occurs following acupuncture manipulation. Manual acupuncture (MA) is the insertion of an acupuncture needle into acupoint followed by the twisting of the needle up and down by hand. In MA, all types of afferent fibers (Abeta, Adelta and C) are activated. In electrical acupuncture (EA), a stimulating current via the inserted needle is delivered to acupoints. Electrical current intense enough to excite Abeta- and part of Adelta-fibers can induce an analgesic effect. Acupuncture signals ascend mainly through the spinal ventrolateral funiculus to the brain. Many brain nuclei composing a complicated network are involved in processing acupuncture analgesia, including the nucleus raphe magnus (NRM), periaqueductal grey (PAG), locus coeruleus, arcuate nucleus (Arc), preoptic area, nucleus submedius, habenular nucleus, accumbens nucleus, caudate nucleus, septal area, amygdale, etc. Acupuncture analgesia is essentially a manifestation of integrative processes at different levels in the CNS between afferent impulses from pain regions and impulses from acupoints. In the last decade, profound studies on neural mechanisms underlying acupuncture analgesia predominately focus on cellular and molecular substrate and functional brain imaging and have developed rapidly. Diverse signal molecules contribute to mediating acupuncture analgesia, such as opioid peptides (mu-, delta- and kappa-receptors), glutamate (NMDA and AMPA/KA receptors), 5-hydroxytryptamine, and cholecystokinin octapeptide. Among these, the opioid peptides and their receptors in Arc-PAG-NRM-spinal dorsal horn pathway play a pivotal role in mediating acupuncture analgesia. The release of opioid peptides evoked by electroacupuncture is frequency-dependent. EA at 2 and 100Hz produces release of enkephalin and dynorphin in the spinal cord, respectively. CCK-8 antagonizes acupuncture analgesia. The individual differences of acupuncture analgesia are associated with inherited genetic factors and the density of CCK receptors. The brain regions associated with acupuncture analgesia identified in animal experiments were confirmed and further explored in the human brain by means of functional imaging. EA analgesia is likely associated with its counter-regulation to spinal glial activation. PTX-sesntive Gi/o protein- and MAP kinase-mediated signal pathways as well as the downstream events NF-kappaB, c-fos and c-jun play important roles in EA analgesia.

Acute myocardial ischemia up-regulates nociceptin/orphanin FQ in dorsal root ganglion and spinal cord of rats

Nociceptin/orphanin FQ (N/OFQ) possesses modulatory effects on somatic noxious signals in spinal cord, while the potential role in visceral nociception remains elusive. We designed this study to investigate the hypothesis that cardiac nociceptive signals from acute ischemic myocardium to the spinal cord are transmitted or modulated by mechanisms including N/OFQ. We examined the changes of N/OFQ and its mRNA in the dorsal root ganglia and spinal cord of upper thoracic segments innervating the heart of rats. Thoracic epidural anesthesia was performed to confirm neural mechanism underlying the changes. We observed that selective coronary artery occlusion significantly up-regulated N/OFQ and ppN/OFQ mRNA in the dorsal root ganglia and spinal cord. Thoracic epidural anesthesia abolished the changes in the expression of N/OFQ and its mRNA. The observations indicate that cardiac noxious neural afferent drive is responsible for the up-regulation of N/OFQ in the primary afferent neurons and intrinsic spinal neurons.

Activities of mixed NOP and mu-opioid receptor ligands

BACKGROUND AND PURPOSE

Compounds that activate both NOP and mu-opioid receptors might be useful as analgesics and drug abuse medications. Studies were carried out to better understand the biological activity of such compounds.

EXPERIMENTAL APPROACH

Binding affinities were determined on membranes from cells transfected with NOP and opioid receptors. Functional activity was determined by [(35)S]GTPgammaS binding on cell membranes and using the mouse vas deferens preparation in vitro and the tail flick antinociception assay in vivo.

KEY RESULTS

Compounds ranged in affinity from SR14150, 20-fold selective for NOP receptors, to buprenorphine, 50-fold selective for mu-opioid receptors. In the [(35)S]GTPgammaS assay, SR compounds ranged from full agonist to antagonist at NOP receptors and most were partial agonists at mu-opioid receptors. Buprenorphine was a low efficacy partial agonist at mu-opioid receptors, but did not stimulate [(35)S]GTPgammaS binding through NOP. In the mouse vas deferens, each compound, except for SR16430, inhibited electrically induced contractions. In each case, except for N/OFQ itself, the inhibition was due to mu-opioid receptor activation, as determined by equivalent results in NOP receptor knockout tissues. SR14150 showed antinociceptive activity in the tail flick test, which was reversed by the opioid antagonist naloxone.

CONCLUSIONS AND IMPLICATIONS

Compounds that bind to both mu-opioid and NOP receptors have antinociceptive activity but the relative contribution of each receptor is unclear. These experiments help characterize compounds that bind to both receptors, to better understand the mechanism behind their biological activities, and identify new pharmacological tools to characterize NOP and opioid receptors.

Receptor binding properties and antinociceptive effects of chimeric peptides consisting of a micro-opioid receptor agonist and an ORL1 receptor antagonist

Receptor binding properties and antinociceptive activities of chimeric peptides linked by spacers were investigated. The peptides consisted of the micro-opioid receptor ligand dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH(2)) or its analog YRFB (Tyr-D-Arg-Phe-betaAla-NH(2)) linked to the ORL1 receptor ligand Ac-Arg-Tyr-Tyr-Arg-Ile-Lys-NH(2) (Ac-RYYRIK-NH(2)). All chimeric peptides were found to possess high receptor binding affinities for both micro-opioid and ORL1 receptors in mouse brain membranes although their binding affinities for both receptors in spinal membranes were significantly lower. Among them, chimeric peptide 2, which consists of dermorphin and Ac-RYYRIK-NH(2) connected by a long spacer, had the highest binding affinity towards both receptors. In the tail-flick test following intrathecal (i.t.) administration to mice, all chimeric peptides showed potent and dose-dependent antinociceptive activities with an ED(50) of 1.34-4.51 (pmol/mouse), nearly comparable to dermorphin alone (ED(50); 1.08 pmol/mouse). In contrast to their micro-opioid receptor binding profiles, intracerebroventricular (i.c.v.) administration of the chimeric peptides resulted in much less potent antinociceptive activity (ED(50) 5.55-100< pmol/mouse) than when administered i.t. (ED(50): 1.34-4.51 pmol/mouse). These results suggest the involvement of nociceptin-like agonistic effects of the Ac-RYYRIK pharmacophore in the peptides, and the regulation of mu-opioid receptor-mediated antinociception in brain. The present chimeric peptides may be useful as pharmacological tools for studies on micro-opioid receptor/ORL1 receptor heterodimers.

Nociceptin (1-13)NH2 inhibits stimulated calcitonin-gene-related-peptide release from primary cultures of rat trigeminal ganglia neurones

In this work we have developed and characterized primary cultures of neonatal rat trigeminal ganglia neurones; calcitonin-gene-related-peptide (CGRP) released from cells was taken as a marker of neuronal function. A significant and consistent increase in CGRP secretion was elicited by non-specific (56 mm KCl or veratridine) or specific (capsaicin) depolarizing stimuli. This paradigm was subsequently used to investigate the effects of nociceptin, an opioid-like peptide involved in central and peripheral control of nociception. We found that the nociceptin analogue nociceptin (1-13)NH2 (NOC) did not affect baseline CGRP release, but it reduced in a concentration-dependent manner CGRP release induced by all tested stimuli. NOC-induced reduction was statistically significant from 0.01 nm onward and achieved maximal effects at 10 nm. Such effects of NOC were seemingly mediated by the activation of specific ORL1 receptors, as a well-known nociceptin antagonist, N(Phe1)nociceptin (1-13)NH2, was able to completely revert NOC inhibition of capsaicin-stimulated CGRP release.

Involvement of local orphanin FQ in the tolerance induced by repeated microinjections of morphine into ventrolateral periaqueductal gray in rats

The present study evaluated the role of ventrolateral periaqueductal gray (vlPAG)-located orphanin-FQ (OFQ) in the opioid tolerance induced by repeated microinjections of morphine (MOR) into vlPAG. Microinjection of MOR (5 microg/0.5 microl) into vlPAG caused antinociception as quantified with the tail flick and the hot plate tests. When MOR microinjection was repeated twice daily, the antinociceptive effect disappeared within 2 days (tolerance). However, if MOR microinjection was preceded by the OFQ receptor antagonist nocistatin (NST; 1 ng/0.5 microl), the microinjections of MOR did not induce tolerance. If NST microinjections were suspended, subsequent MOR microinjections induced tolerance. In MOR-tolerant rats, a single NST microinjection into vlPAG was enough to restore the antinociceptive effect of MOR. Furthermore, if OFQ (1 ng/0.5 microl) was microinjected into vlPAG, then a MOR microinjection administered 15 min later into vlPAG did not elicit antinociception. Finally, opioid tolerance induced by repeated systemic MOR injections (5 mg/kg, i.p.) was reversed by a single microinjection of NST into vlPAG. This emphasizes the central importance of vlPAG-located OFQ in the MOR tolerance.

Anti-nociceptive and anti-allodynic effects of a high affinity NOP hexapeptide [Ac-RY(3-Cl)YRWR-NH2] (Syn 1020) in rodents

There has been a flurry of activity to develop agonists and antagonists for the member of the opioid receptor family, NOP receptor (also known as ORL1), in part to understand its role in pain. Modifications of a hexapeptide originally identified from a combinatorial library have led to the discovery of a high affinity hexapeptide agonist Ac-RY(3-Cl)YRWR-NH2 (Syn 1020). In the following experiments we characterized the anti-nociceptive effects of Syn 1020 in the tail-flick model of acute pain and the diabetic neuropathy model of chronic pain in mice and rats, respectively. Acute antinociception was assessed using the tail-flick assay in mice in which animals received intracerebroventricular (i.c.v.) or subcutaneous (s.c.) injections of Syn 1020 alone or with morphine and were tested for tail-flick latencies. In the chronic pain model, diabetic neuropathy was induced by injections of streptozotocin in rats. Tactile allodynia was measured, with von Frey hair filaments, following intraperitoneal (i.p.) injections of Syn 1020 or gabapentin (positive control). In mice, i.c.v. injections of Syn 1020 did not have any pro- or anti-nociceptive effects, however, Syn 1020 reversed morphine antinociception with a similar potency as N/OFQ (the natural ligand to NOP). S.c. injections of Syn 1020 in mice also produced analgesic effects. In rats, i.p, injections of Syn 1020 produced anti-allodynic effects. Thus, Syn 1020, a NOP receptor directed peptide, administered systemically has anti-nociceptive activity in both acute and chronic pain models in mice and rats respectively.

SR 16435 [1-(1-(bicyclo[3.3.1]nonan-9-yl)piperidin-4-yl)indolin-2-one], a novel mixed nociceptin/orphanin FQ/mu-opioid receptor partial agonist: analgesic and rewarding properties in mice

We identified a novel nociceptin/orphanin FQ (NOP)/mu-opioid receptor agonist, SR 16435 [1-(1-(bicyclo[3.3.1]nonan-9-yl)piperidin-4-yl)indolin-2-one], with high binding affinity and partial agonist activity at both receptors. It was hypothesized that SR 16435 would produce antinociception and yet, unlike morphine, would have diminished rewarding properties and tolerance development. Antinociception was assessed in mice using the tail-flick assay, whereas behavioral and rewarding effects were assessed using the place conditioning (PC) paradigm. PC was established by pairing drug injections with a distinct compartment. Behavioral effects were measured after acute and repeated drug administration, and the test for PC was carried out 24 h after four drug- and vehicle-pairing sessions. SR 16435 produced an increase in tail-flick latency, but SR 16435-induced antinociception was lower than that observed with morphine. Given that naloxone blocked SR 16435-induced antinociception, it is highly likely that this effect was mediated by mu-opioid receptors. Compared with morphine, chronic SR 16435 treatment resulted in reduced development of tolerance to its antinociceptive effects. SR 16435-induced conditioned place preference (CPP) was evident, an effect that was probably mediated via mu-opioid receptors, as it was reversed by coadministration of naloxone. NOP agonist activity was also present, given that SR 16435 decreased global activity, and this effect was partially reversed with the selective NOP antagonist, SR 16430 [1-(cyclooctylmethyl)-4-(3-(trifluoromethyl)phenyl)piperidin-4-ol]. Naloxone, however, also reversed the SR 16435-induced decrease in activity, indicating that both opioid and NOP receptors mediate this behavior. In summary, the mixed NOP/mu-opioid partial agonist SR 16435 exhibited both NOP and mu-opioid receptor-mediated behaviors.

Synthesis and receptor binding properties of chimeric peptides containing a mu-opioid receptor ligand and nociceptin/orphanin FQ receptor ligand Ac-RYYRIK-amide

Four chimera peptides composed of ORL1 receptor ligand Ac-RYYRIK-NH2 and a mu-opioid receptor agonist dermorphin YAFGYPS-NH2 or YRFB-NH2, with a spacer linking the two pharmacophores, were synthesized and tested for their receptor binding properties. Chimera peptides with long spacers (a Lys and five or eight Gly residues) showed synergistically improved affinity for both the mu-opioid receptor and ORL1 receptor, while the chimera peptides with short spacers (Lys residue only) showed decreased or similar affinity compared to the monomeric receptor ligands. Chimera peptides containing long spacers may prove to be useful tools for studying ORL1 receptor/mu-opioid receptor heterodimers.

Influence of nociceptin(1-17) fragments and its tyrosine-substituted derivative on morphine-withdrawal signs in rats

Our previous studies demonstrated that endogenous ligand of nociceptin (NOP) receptor, nociceptin(1-17) (also known as orphanin FQ), inhibits morphine-withdrawal syndrome measured as wet dog shakes in rats [Life Sci. 66 (2000) PL119]. This peptide is metabolized in the spinal cord, both in vitro and in vivo, to shorter fragments, including nociceptin(1-11) and nociceptin(1-6). These fragments, formed after cleavage by endogenous peptidase, are behaviorally active and modulate nociception in a bi-phasic process [Peptides 20 (1999) 239]. As these peptides induced transient naloxone-reversible analgesia in behavioral tests [Peptides 20 (1999) 239], in the present study we tested the influence of nociceptin(1-11) (10 and 20 microg) and nociceptin(1-6) (10, 20 and 40 microg) on the morphine-withdrawal syndrome in rats. Furthermore, the modified fragment of nociceptin(1-6) with an opioid-message domain achieved by replacement of Phe1 with Tyr was tested. Morphine-withdrawal syndrome was precipitated by the i.p. injection of naloxone hydrochloride (2 mg/kg), 72 h after implantation of morphine pellets. The wet-dog shakes were chosen for statistical analyses of the abstinence signs. The results show that nociceptin(1-11) and (1-6) attenuate this morphine-withdrawal symptom. The replacement of Phe1 with Tyr in nociceptin(1-6) fragment did not potentiate the influence of nociceptin(1-6) on wet dog shakes precipitated by naloxone in morphine-dependent rats.

Cerebrospinal fluid biomarkers in experimental spinal nerve root injury

STUDY DESIGN

Cerebrospinal fluid biomarkers were evaluated in a setup using established pig models to mimic clinical disc herniation.

OBJECTIVES

To investigate biomarkers for nerve tissue injury, inflammation, and pain in cerebrospinal fluid after mechanical compression and/or nucleus pulposus application to spinal nerve roots.

SUMMARY OF BACKGROUND DATA

The association between mechanical compression, biochemical effects of nucleus pulposus, and nerve root injury in degenerative disc disorders is incompletely investigated.

METHODS

The unilateral S1 nerve root was exposed in 20 pigs. The animals were divided into four groups (n = 5 each): 1) slow-onset mechanical compression with an ameroid constrictor; 2) autologous nucleus pulposus application; 3) mechanical compression plus nucleus pulposus; and 4) sham operation. After 1 week, 6 mL of cerebrospinal fluid was collected, and four structural nerve proteins, neurofilaments, S-100, glial fibrillary acidic protein, neuron-specific enolase, the proinflammatory cytokine interleukin-8, the neurotransmitter nociceptin, and substance P endopeptidase activity were analyzed using immunoassays.

RESULTS

The concentration of neurofilament was increased in the mechanical compression group (17.0 microg/L +/- 5.0) and in the mechanical compression plus nucleus pulposus group (19.8 +/- 12.1 microg/L) compared with the sham group (0.9 +/- 0.9 microg/L) and the nucleus pulposus group (0.4 +/- 0.1 microg/L) (P < 0.01 for both). The concentration of nociceptin was increased significantly in the mechanical compression group (24.0 +/- 8.6 fm/mL) and in the mechanical compression plus nucleus pulposus group (31.2 +/- 6.6 fm/mL) compared with the sham group (7.0 +/- 1.3 fm/mL) (P < 0.05 and P < 0.01, respectively). A correlation was found between concentrations of neurofilament and nociceptin (r = 0.50, P < 0.05). There were no intergroup differences regarding glial fibrillary acidic protein, neuron-specific enolase, S-100, interleukin-8, or substance P endopeptidase activity.

CONCLUSIONS

The present study demonstrates increased concentrations of neurofilament and nociceptin in cerebrospinal fluid after nerve root compression. A simultaneous application of nucleus pulposus did not increase the response.

Structure-activity studies on high affinity NOP-active hexapeptides

Nociceptin/orphanin FQ (N/OFQ) is a 17 amino acid peptide that is the endogenous ligand for the G-protein coupled receptor ORL1 (NOP), a member of the opioid receptor family. Although it is clear that this receptor system is involved in a variety of physiologic functions, including analgesia, the precise actions of N/OFQ remain largely uncharacterized. One reason for this has been limited number of high-affinity ligands to NOP, and particularly the lack of availability of useful specific antagonists. Herein, we describe the pharmacologic activity of a series of modified amino acid containing modifications of the hexapeptide Ac-RYYRWR-NH2, with high affinity for NOP. These compounds were tested for binding affinity using [3H]N/OFQ binding to human NOP in CHO cells, and functional activity by measuring stimulation of [35S]GTPgammaS-binding in CHO cell membranes. These studies suggest that each Arg of the hexapeptide is required to maintain high-binding affinity. The peptide maintains high affinity if the Tyr2 or Tyr3 are modified, but at least one of these residues must maintain its hydroxyl group or there is a large decrease in intrinsic activity of the peptide.

Evidence for an exclusive antinociceptive effect of nociceptin/orphanin FQ, an endogenous ligand for the ORL1 receptor, in two animal models of neuropathic pain

Nociceptin/orphanin FQ (noci/OFQ), the endogenous ligand for the orphan ORL1 (opioid receptor-like1), has been shown to be anti- or pronociceptive and modify morphine analgesia in rats after central administration. We comparatively examined the effect of noci/OFQ on hyperalgesia and morphine analgesia in two experimental models of neuropathic pain: diabetic (D) and mononeuropathic (MN) rats. Noci/OFQ, when intrathecally (i.t.) injected (0.1, 0.3, or 1, to 10 microg/rat) was ineffective in normal rats, but reduced and suppressed mechanical hyperalgesia (paw-pressure test) in D and MN rats, respectively. This spinal inhibitory effect was suppressed by naloxone (10 microg/rat, i.t.) in both models. Combinations of systemic morphine with spinal noci/OFQ resulted in a strong potentiation of analgesia in D rats. In MN rats, an isobolographic analysis showed that the morphine+noci/OFQ association (i.t.) suppressed mechanical hyperalgesia in a superadditive manner. In summary, the present findings reveal that spinal noci/OFQ produces a differential antinociception in diabetic and traumatic neuropathic pain according to the etiology of neuropathy, an effect possibly mediated by opioid receptors. Moreover, noci/OFQ combined with morphine produces antinociceptive synergy in experimental neuropathy, opening new opportunities in the treatment of neuropathic pain.

Minireview: Characterization of influence of central nociceptin/orphanin FQ on consummatory behavior

Nociceptin/orphanin FQ (N/OFQ), a peptide closely related to dynorphin A, is the endogenous agonist of the NOP receptor that moderately increases food intake under various conditions. Its orexigenic properties are mediated by the brain circuitry. In the present review, we focus on discussing the nature of hyperphagic effects of N/OFQ with special emphasis on its function within feeding-related neural networks. Although some of N/OFQ's orexigenic effects resemble those induced by opioids, reward-dependent feeding appears to be affected in a different manner by agonists of the NOP and classical opioid receptors. Also, data suggest that N/OFQ may not only promote feeding initiation, but rather its role may be to inhibit signaling responsible for inhibition of consummatory behavior. Central systems involved in termination of feeding that seem to be influenced by N/OFQ encompass oxytocin, alpha-MSH, and CRH.

Biostable aptamers with antagonistic properties to the neuropeptide nociceptin/orphanin FQ

The neuropeptide nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid receptor-like 1 (ORL1) receptor, has been shown to play a prominent role in the regulation of several biological functions such as pain and stress. Here we describe the isolation and characterization of N/OFQ binding biostable RNA aptamers (Spiegelmers) using a mirror-image in vitro selection approach. Spiegelmers are L-enantiomeric oligonucleotide ligands that display high affinity and specificity to their targets and high resistance to enzymatic degradation compared to D-oligonucleotides. A representative Spiegelmer from the selections performed was size-minimized to two distinct sequences capable of high affinity binding to N/OFQ. The Spiegelmers were shown to antagonize binding of N/OFQ to the ORL1 receptor in a binding-competition assay. The calculated IC(50) values for the Spiegelmers NOX 2149 and NOX 2137a/b were 110 nM and 330 nM, respectively. The competitive antagonistic properties of these Spiegelmers were further demonstrated by their effective and specific inhibition of G-protein activation in two additional models. The Spiegelmers antagonized the N/OFQ-induced GTPgammaS incorporation into cell membranes of a CHO-K1 cell line expressing the human ORL1 receptor. In oocytes from Xenopus laevis, NOX 2149 showed an antagonistic effect to the N/OFQ-ORL 1 receptor system that was functionally coupled with G-protein-regulated inwardly rectifying K(+) channels.

Distribution of nociceptin/orphanin FQ in adult human brain

Nociceptin/orphanin FQ (N/OFQ), the endogenous agonist for the opioid receptor-like receptor 1 (ORL1), shows significant similarities to dynorphin A in structure and distribution in rat central nervous system. The distribution of N/OFQ in human brain has not been studied. We measured the concentrations of N/OFQ in 47 microdissected areas of the central nervous system of adult human brain using radioimmunoassay (RIA). Significant heterogeneity was found in the levels of N/OFQ concentration in the various analyzed regions. The highest concentrations were measured in the dorsal central gray matter (periaqueductal gray), the locus coeruleus, the ventromedial nucleus of hypothalamus, the septum and the dorsal horn of the spinal cord. High concentrations were also detected in other hypothamamic nuclei, the inferior colliculus, the ventral central gray matter, the pontine tegmentum, the amygdala, the reticular formation and the spinal trigeminal nucleus. Considerable similarity with the distribution of N/OFQ in rat CNS was observed. The widespread distribution in CNS predicts multifaceted functions for N/OFQ.

Cholecystokinin fails to block the spinal inhibitory effects of nociceptin in sham operated and neuropathic rats

Cholecystokinin (CCK) has a number of roles in the central nervous system and can reduce the analgesic effect of activation of mu (micro), delta (delta) and kappa (kappa) opioid receptors. CCK has been proposed to be a major reason for reduced effects of morphine after nerve injury. This study examines if CCK modulates the effect of the Opioid Receptor Like-1 (ORL1) agonist, nociceptin on dorsal horn neurone activity in vivo in the spinal nerve ligation model of neuropathic pain compared with sham-operated and naive rats. In naive and neuropathic rats nociceptin alone inhibited the C-fibre evoked response, post-discharge, wind-up and input, while in sham operated rats nociceptin did not cause any inhibition but by contrast caused a facilitation of post-discharge and wind-up. CCK alone had no significant effect, although did cause slight facilitation in the three groups. In the presence of CCK the inhibitory effect of nocieceptin was blocked in naive animals, but in contrast the inhibitory effect of nociceptin was enhanced by CCK in sham and neuropathic rats. These results emphasize the differences between ORL1 and other opioid receptors. This loss of the inhibitory effect of CCK on nociceptin after nerve injury may be of clinical interest in the treatment of neuropathic pain.

Nociceptin/orphanin FQ increases anxiety-related behavior and circulating levels of corticosterone during neophobic tests of anxiety

Intracranial administration of nociceptin/orphanin FQ (N/OFQ) increases circulating concentrations of adrenocorticotrophic hormone and corticosterone in unstressed rats, and elevates the responsiveness of these hormones during mild stress. Furthermore, N/OFQ and its cognate receptor are both abundant in a variety of limbic nuclei, and stress exposure decreases neuronal N/OFQ content in forebrain neurons. In light of these and other findings, we examined the potential involvement of N/OFQ in regulation of anxiety-related behaviors in rats. In the open field, elevated plus maze, and dark-light neophobic tests, intracerebroventricular N/OFQ (1.0 pmole-1.0 nmole) increased the expression of anxiety-related behaviors. Specifically, N/OFQ increased the latency to enter, decreased the number of entries into, and decreased the time spent in the exposed or brightly lit environments of all three tests. N/OFQ also enhanced thigmotactic responses in the open field test. The effects of diazepam and of the benzodiazepine inverse agonist FG 7142 were also assessed in independent groups of rats. In all three tests, the behavioral effects of N/OFQ resembled the anxiogenic actions of FG 7142, and contrasted with the anxiolytic actions of diazepam. N/OFQ administration also increased circulating concentrations of corticosterone during anxiety testing, in comparison with the concentrations in vehicle-treated controls. We conclude that N/OFQ administration is anxiogenic, and elevates responsiveness of the hypothalamic pituitary-adrenal axis during neophobic tests of anxiety. This supports the possibility that N/OFQ neurotransmission participates in processing of emotionally-salient and stressful stimuli, and suggests that normal functioning of the N/OFQ system may be important in physiological and psychological well-being.

N-terminal modifications leading to peptide ORL1 partial agonists and antagonists

Nociceptin/Orphanin FQ (N/OFQ) is a 17 amino acid peptide that is the endogenous ligand for the G protein-coupled receptor (opioid receptor like 1, ORL1), a member of the opioid receptor family. Although it is clear that this receptor system is involved in a variety of physiological functions, including analgesia, the precise actions of N/OFQ remain largely uncharacterized. One reason for this has been limited high affinity ligands to ORL1, and particularly the lack of availability of useful specific antagonists. Herein we describe the pharmacological activity of a series of N-terminally modified hexapeptides with high affinity for ORL1. These compounds were tested for binding affinity using [3H]N/OFQ binding to human ORL1 in CHO cells, and functional activity by measuring stimulation of [35S]GTPgammaS binding in CHO cell membranes. The N-terminal modifications have produced compounds that maintained very high receptor affinity, but led to significant changes in intrinsic activity. One compound, pentanoyl-RYYRWR-NH2, with barely measurable agonist activity was tested in vivo. It was found to possess modest analgesic activity, but it was unable to block the morphine modulatory activity of N/OFQ.

Modification of Nociception and Morphine Tolerance by the Selective Opiate Receptor-Like Orphan Receptor Antagonist (–)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111)

(-)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111) is a novel human opiate receptor-like orphan receptor (ORL-1) antagonist that has high affinity for the clonal human ORL-1 receptor (hORL-1 K(i) = 0.33 nM), selectivity versus mu-(174-fold), delta-(6391-fold), and kappa (486-fold)-opioid receptors and is able to inhibit nociceptin signaling via hORL-1 in a whole cell gene reporter assay. SB-612111 has no measurable antinociceptive effects in vivo in the mouse hot-plate test after intravenous administration but is able to antagonize the antimorphine action of nociceptin [ED(50) = 0.69 mg/kg, 95% confidence limit (CL) = 0.34-1.21]. SB-62111 administration can also reverse tolerance to morphine in this model, established via repeated morphine administration. In addition, intravenous SB-612111 can antagonize nociceptin-induced thermal hyperalgesia in a dose-dependent manner (ED(50) = 0.62 mg/kg i.v., 95% CL = 0.22-1.89) and is effective per se at reversing thermal hyperalgesia in the rat carrageenan inflammatory pain model. These data show that an ORL-1 receptor antagonist may be a useful adjunct to chronic pain therapy with opioids and can be used to treat conditions in which thermal hyperalgesia is a significant component of the pain response.

The rostral ventromedial medulla mediates the facilitatory effect of microinjected orphanin FQ in the periaqueductal gray on spinal nociceptive transmission in rats

Single unit extracellular recordings from spinal dorsal horn neurons were obtained with glass micropipettes in pentobarbital-anesthetized rats. A total of 50 wide dynamic range (WDR) neurons were studied in 25 rats. Microinjection of orphanin FQ (OFQ, 0.1 microg/0.1 microl) (a potent endogenous ligand of the opioid receptor-like receptor (ORL-1)) into the ipsilateral ventrolateral parts of periaqueductal gray (vlPAG) significantly increased C-response and post-discharge activity in most of the WDR neurons. Pre-microinjection of lidocaine (4%) into the nucleus raphe magnus (NRM) (0.5 microl), ipsilateral nucleus reticularis gigantocellularis (NGC) (0.6 microl), or nucleus gigantocellularis pars alpha (NGCalpha) and nucleus reticularis paragigantocellularis lateralis (NPGL) (0.5 microl) markedly reduced intra-vlPAG microinjection of OFQ-induced facilitatory effects on nociceptive responses of WDR neurons. Furthermore, if the NRM and ipsilateral NGC were simultaneously pre-microinjected with lidocaine, the intra-vlPAG microinjection of OFQ-induced facilitation on nociceptive responses of WDR neurons was eliminated. Also, a similar effect was observed when all the NRM, ipsilateral NGC, NGCalpha and NPGL were blocked with lidocaine. No significant effect on nociceptive responses of WDR neurons per se was found after blocking the NRM, ipsilateral NGC, NGCalpha/NPGL, or all the NRM, ipsilateral NGC, and NGCalpha/NPGL with lidocaine. These results indicate that (1) the facilitatory effect evoked by microinjection of OFQ into the vlPAG on nociceptive responses of WDR neurons in the spinal dorsal horn is primarily mediated by the NRM and ipsilateral NGC; (2) the NRM, ipsilateral NGC, and NGCalpha/NPGL do not mediate tonic descending inhibition of the spinal dorsal horn neurons.

Sex-related differences in the distribution of opioid receptor-like 1 receptor mRNA and colocalization with estrogen receptor mRNA in neurons of the spinal trigeminal nucleus caudalis in the rat

We recently reported that exogenously applied orphanin FQ, the endogenous ligand for opioid receptor-like 1 (ORL(1)) receptor, produces sex-specific modulation of trigeminal nociception, and that estrogen contributes to these sex-related differences. Estrogen could produce these sex-related differences by altering the expression of the ORL(1)-receptor gene in the trigeminal nucleus caudalis. Utilizing in situ hybridization, we compared levels of ORL(1) receptor mRNA and investigated its colocalization with estrogen receptor mRNA in trigeminal neurons. Our results showed that in male rats, ORL(1) receptor mRNA is abundantly expressed in the rostral part of the trigeminal nucleus caudalis, and at the junction of caudalis and interpolaris (Vc/Vi). In comparison with males, levels of ORL(1) receptor mRNA were not significantly different in proestrus females, but were significantly higher in the rostral trigeminal nucleus caudalis and at the junction of Vc/Vi of diestrus females. In addition, ovariectomy raised the levels in the rostral trigeminal nucleus caudalis, and at the junction of Vc/Vi. Levels were reduced to proestrus levels in these regions following estradiol replacement. Our results also showed that ORL(1) receptor mRNA is present in majority of estrogen receptor (alpha and/or beta) mRNA-containing neurons. We conclude that there are sex-related differences in the ORL(1)-receptor gene expression in the trigeminal nucleus caudalis, which appear to be determined in part by estrogen levels.

Distribution of ORL-1 receptor binding and receptor-activated G-proteins in rat forebrain and their experimental localization in anterior cingulate cortex

Opioid receptor-like (ORL-1) receptors and ORL-1-activated G-proteins are found in high levels in the forebrain, particularly cingulate cortex, an area involved in processing of nociceptive stimuli. [(3)H]nociceptin/orphanin FQ (N/OFQ) and N/OFQ-stimulated [(35)S]GTPgammaS autoradiography in rat brain were used to localize ORL-1 receptors and activated G-proteins, respectively. N/OFQ binding and activated G-proteins were highest in anterior cingulate, agranular insula, piriform, perirhinal and entorhinal cortices; midline and intralaminar thalamic nuclei; and subnuclei of the amygdala and hippocampus. In anterior cingulate area 24, [(3)H]N/OFQ and N/OFQ-stimulated [(35)S]GTPgammaS binding were highest in layers V and VI. The cellular localization of ORL-1 receptors and activated G-proteins in area 24 was examined using two strategies: ibotenic acid injection into the cortex or undercut lesions to remove afferent axons, followed by autoradiography. Ibotenic acid lesions that destroyed neurons in the anterior cingulate cortex decreased [(3)H]N/OFQ binding by 75-80% and reduced N/OFQ-stimulated [(35)S]GTPgammaS binding to basal levels seen in the absence of agonist. Deafferentation lesions increased [(3)H]N/OFQ binding by 40-50%, with no significant change in N/OFQ-stimulated [(35)S]GTPgammaS binding. These data demonstrate that ORL-1 receptors in layer V of anterior cingulate cortex are located on somatodendritic elements and that deafferentation increases ORL-1 receptor binding.

Binding and GTPgammaS autoradiographic analysis of preproorphanin precursor peptide products at the ORL1 and opioid receptors

Utilizing agonist-stimulated GTPgammaS autoradiography, we analyzed the ability of preproorphanin FQ (ppOFQ) peptides to stimulate [35S]-GTPgammaS binding in adult rat brain. Orphanin FQ (OFQ) stimulated [35S]-GTPgammaS binding in a pattern similar to that described for [125I]-OFQ at the endogenous opioid receptor-like (ORL1) receptor. The ppOFQ peptides nocistatin and orphanin FQ2 (OFQ II(1-17)) had no effect, suggesting that they do not mediate their reported analgesic effects via a G(i/o)-coupled receptor (i.e. opioid or ORL1). Unlike OFQ II(1-17), high concentrations of its C-terminal extension, OFQ II(1-28), stimulated [35S]-GTPgammaS binding in a mu (mu) opioid receptor-like distribution and the effect was blocked by naloxone. To explore these observations, we evaluated the receptor binding profile of OFQ II(1-28) at the cloned ORL1 and mu opioid receptors. OFQ II(1-28) had no specific binding at either ORL1 or mu opioid receptors at concentrations up to 50 microM. This lack of affinity was not consistent with a mu-mediated effect, as suggested by preliminary observation using functional autoradiography in rat brain sections. Although behavioral studies suggest that OFQ II(1-28) possesses analgesic activity, this effect does not appear to be mediated via direct binding at the mu opioid receptor. Taken together, these findings support the view that (1) OFQ is the only ppOFQ peptide that binds to and activates the ORL1 receptor and (2) OFQ II(1-28) does not bind or stimulate [35S]-GTPgammaS binding in cells expressing the mu opioid receptor.

Normal sensitivity to acute pain, but increased inflammatory hyperalgesia in mice lacking the nociceptin precursor polypeptide or the nociceptin receptor

Nociceptin/orphanin FQ (N/OFQ) is the endogenous agonist of the N/OFQ peptide receptor (NOP receptor). It is released from a larger precursor polypeptide, called prepro-nociceptin (ppN/OFQ) from which, in addition to N/OFQ, other biologically active neuropeptides may be derived. Increasing evidence indicates that exogenous application of N/OFQ to the central nervous system of mice and rats induces pro- and antinociceptive effects depending on the dose and site of administration. Much less is known about a potential contribution of endogenous N/OFQ to pain control. Here, we have used a genetic approach to address this topic. Mice deficient in either the NOP receptor (NOP-R-/- mice) or the N/OFQ precursor polypeptide (ppN/OFQ-/- mice) or both (double knockout mice) were compared with wild-type littermates in animal models of acute and tonic pain. Nociceptive responses to acute noxious heat of all three types of mutant mice were indistinguishable from those of wild-type mice. Accordingly, nociceptive behaviour was very similar in the early phase of the formalin test. However, NOP-R-/-, ppN/OFQ-/- and double knockout mice showed markedly stronger nociceptive responses during prolonged nociceptive stimulation in the second phase of the formalin test and significantly lower thermal pain thresholds in inflamed tissue after zymosan A injection. These results indicate that N/OFQ contributes significantly to endogenous pain control during prolonged nociceptive stimulation but does not affect acute pain sensitivity. Among the three types of mutant mice nociceptive behaviour was nearly identical, indicating that the lack of other potential ppN/OFQ products in the ppN/OFQ-/- mice was apparently without effect on the nociceptive phenotype.

Mechanical stimulation activates small fiber mediated nociceptive responses in the nucleus gigantocellularis

We characterized nociceptive discharges induced by mechanical stimulation and the modulating effects of orphanin FQ on noxious responses in the rat brain stem gigantocellular reticular nucleus (Gi). A pressure pulse of constant force and rising rate was delivered by a mechanical stimulator with feedback control, allowing responses to be analyzed statistically. A pressure pulse of 300 g, which evoked C-fiber mediated nerve responses, was delivered to the tail. Two excitatory (45/58) and one inhibitory (13/58) types of extracellular unit discharges were recorded in Gi. One of the excitatory types was a phasic discharge (13/45) elicited at the onset and/or the end of stimulation. Latencies of the phasic discharges (0.104+/-0.1 s) were shorter than those of other type (tonic) discharges (0.43+/-0.2 s). The tonic discharges (32/45), which frequently persisted past the end of stimulation without adaptation, were classified into two groups. The first group of tonic type units (23/45) was high threshold, like nociceptive specific neurons in the primary sensory cortex, while the second group of neurons (9/45) responded to a wide range of stimulus intensities. The mean frequency, response duration and spike numbers gradually increased with stimulus intensity change in all nine neurons. The neurons encode mechanical stimulus intensity with discharge frequency, response duration and evoked spike numbers. Local injection of orphanin FQ (200 ng/2 microl) changed high threshold tonic type spike numbers in a biphasic manner, i.e., there was an early phase suppression (5-30 min, p=0.016) and a late phase enhancement (30-60 min, p=0.027). In contrast, phasic type discharges did not show an altered discharge pattern in response to orphanin FQ. Thus, orphanin FQ affects small fiber-mediated nociceptive responses and may behave as a complex modulator of pain systems in the brain stem.

Effects of nociceptin/orphanin FQ receptor ligands on blood pressure, heart rate, and plasma catecholamine concentrations in guinea pigs

Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ peptide receptor (NOP) and has been shown previously to produce bradycardia and hypotension in rodents. In this study we have measured the effects of intravenous N/OFQ, and the NOP antagonists [Nphe(1)]N/OFQ(1-13)-NH(2) ([Nphe(1)]) and [Nphe(1),Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101) on cardiovascular parameters and plasma catecholamine concentrations. Female Hartley guinea pigs were anesthetized with pentobarbital and ventilated artificially. MAP and HR were measured via a femoral arterial catheter and ECG, respectively. Plasma catecholamine concentrations were measured by HPLC. Animals received saline, N/OFQ (0.25, 1.25, 6.25 and 25 nmol cumulatively at 10-min intervals), [Nphe(1)] (600 nmol) and UFP-101 (60 nmol) i.v. in various combinations. After establishing a stable baseline, MAP and HR measurements and blood sampling were performed at the beginning and 3 min following each drug administration. N/OFQ significantly decreased MAP, HR and the plasma noradrenaline concentrations in a dose dependent manner (maximally by 29.1+/-1.8%, 13.8+/-0.8% and 46.6+/-7.8%, respectively) To the contrary, N/OFQ tended to increase plasma adrenaline concentration but did not affect plasma dopamine concentrations. There was a significant correlation between percent change in MAP (0.69, P<0.01) or HR (0.84, P<0.01) and that in plasma noradrenaline. [Nphe(1)], but not UFP-101, alone significantly decreased MAP. [Nphe(1)] partially antagonized N/OFQ-induced hypotension, bradycardia and the decrease in plasma concentration of noradrenaline. UFP-101 fully prevented the effects of N/OFQ in this model. In conclusion, the present study shows that intravenous N/OFQ, via NOP receptors, elicits hypotension and bradycardia also in the anaesthetized guinea pig and that the decrease in MAP and HR are positively correlated with the decrease in the plasma noradrenaline level.

Non-peptidergic OP4 receptor agonist inhibits morphine antinociception but does not influence morphine dependence

The non-peptidergic opioid receptor-like 1 (ORL1, OP4) receptor ligand, Ro 64-6198 [(1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4,5]decan-4-one], is a full agonist of the OP4 receptor. The aim of this study was to evaluate whether this compound influences morphine antinociception and dependence in mice. Ro 64-6198 inhibits the acute analgesic effect of morphine in the tail-immersion test, however, when given chronically during the acquisition of morphine dependence, development of this dependence is not prevented. The acute injection of Ro 64-6198 suppresses withdrawal escape jumps in morphine dependent mice, though this effect may be a result of the loss of locomotor activity induced by this compound and/or its myorelaxant action. The study provides evidence that stimulation of the OP4 receptor suppresses acute morphine antinociception, but is not sufficient to inhibit the development of morphine dependence in mice.

Utilizing functional genomics to identify new pain treatments : the example of nociceptin

Nociceptin/orphanin FQ (noc/oFQ) is the first novel bioactive substance to have been discovered by the implementation of a functional genomics/reverse pharmacology approach. The neuropeptide was indeed identified in brain extracts as the natural ligand of a previously cloned orphan G protein-coupled receptor, the opioid receptor-like 1 (ORL1) receptor. Since its discovery in 1995, noc/oFQ has been the subject of intensive study to establish its role in normal brain function and its possible involvement in neurophysiopathology. Although the neuropeptide, an inhibitor of neuronal activity, has been found to have a wide spectrum of pharmacological effects in vivo, none has been as intensively investigated as its action on nociception and nociceptive processing. There is now substantial evidence that noc/oFQ has a modulatory role in nociception. However, dependent on the dose and site of injection, and possibly the animal's genetic background and even psychological status, the peptide has been variously reported to cause allodynia, hyperalgesia, analgesia, and even pain, in rodents. Overall, noc/oFQ tends to facilitate pain when administered supraspinally, and to inhibit it when administered spinally. These opposing effects beg the obvious, yet still unanswered, question as to what would be the net effect on nociception of an ORL1 receptor ligand, agonist or antagonist, able to target supraspinal and spinal sites simultaneously. Owing to the research effort of several drug companies, such ligands, i.e. nonpeptidic, brain-penetrating agonists and antagonists, have recently been produced whose systematic screening in animal models of acute and inflammatory pain may help validate the ORL1 receptor as the target for novel, non-opioid analgesics.

Dimerization of morphine and orphanin FQ/nociceptin receptors: generation of a novel opioid receptor subtype

Although orphanin FQ/nociceptin (OFQ/N) receptors are a member of the opioid receptor family of receptors, they bind traditional opioids with very poor affinity. We now demonstrate that mu opioid receptors can physically associate with OFQ/N receptors, resulting in a complex with a unique binding selectivity profile. Immunoprecipitation of epitope-tagged OFQ/N receptors co-precipitates mu receptors. When the two receptors were co-expressed in CHO cells, [3H]OFQ/N retained its high binding affinity for its receptor. However, co-expression of the two receptors increased by up to 250-fold the affinity of a series of opioids in [3H]OFQ/N binding assays. This enhanced affinity was limited to agonists with high affinity for mu receptors. Selective kappa(1) and delta opioids did not lower binding. Despite the dramatic increase in affinity for the opioid agonists in co-expressing cells, the opioid antagonists naloxone and diprenorphine failed to compete [3H]OFQ/N binding.

Degradation of nociceptin (orphanin FQ) by mouse spinal cord synaptic membranes is triggered by endopeptidase-24.11: an in vitro and in vivo study

We analyzed spinal metabolic pathway of nociceptin/orphanin FQ related to pain-transmission or modulation in the both in vitro and in vivo experiments. Nociceptin was degraded by spinal synaptic membranes. Major metabolites of nociceptin were free phenylalanine, nociceptin (1-13) and nociceptin (14-17). Both the degradation of nociceptin and the accumulation of the major cleavage metabolites, nociceptin (1-13) and nociceptin (14-17), were strongly inhibited by a metal chelator and also by specific inhibitors of endopeptidase-24.11, thiorphan and phosphoramidon. Furthermore, purified endopeptidase-24.11 hydrolyzed nociceptin at the cleavage site (Lys(13)-Leu(14) bond) identical to that by spinal synaptic membranes. Recently, we have found that nociceptin, injected intrathecally at small doses (fmol order) elicits a behavioral response consisting of scratching, biting and licking in mice. In the present study, we have examined the effect of peptidase inhibitors on the behavioral response elicited by intrathecal injection of nociceptin in mice. Phosphoramidon simultaneously injected with nociceptin additively enhanced nociceptin-induced behavioral response, whereas the nociceptin-induced behavioral response was unaffected by either bestatin, an aminopeptidase inhibitor or captopril, an angiotensin-converting enzyme inhibitor. However, the nociceptin effect was potentiated by combined injection of phosphoramidon and bestatin, indicating that inhibition of aminopeptidase may also contribute to inducing the behavioral response to nociceptin. These data suggest that endopeptidase-24.11 plays a major role in initial stage of nociceptin metabolism at the spinal cord level in mice.

Immunological challenge modulates brain orphanin FQ/nociceptin and nociceptive behavior

Orphanin FQ/Nociceptin (OFQ/N), an endogenous peptide found throughout the central nervous system, has been attributed with a wide range of functions, including modulation of motivational and emotional behavior, but most prominently, facilitation of hyperalgesia. It has also been shown that brain OFQ/N is stimulated during locally-induced peripheral inflammation, a condition well known to increase pain sensitivity. However, few studies have addressed whether specific immunological challenge using T-cell dependent and independent stimuli alters OFQ/N gene activation in the brain. Consequently, male C57BL/6J mice were challenged with 5 microg of lipopolysaccharide (LPS) or a T-cell-activating bacterial superantigen, Staphyloccocal enterotoxin A (SEA), and levels of brain OFQ/N precursor, pNOC, mRNA were analyzed by semi-quantitative RT-PCR. In addition, nociceptive thresholds were examined in immunologically challenged mice using the hotplate test. Initial results on a combined region of the brain containing various limbic components, revealed increased levels of pNOC mRNA in response to SEA challenge, but not to LPS. Further analysis of more discrete brain regions revealed increased pNOC mRNA in the hypothalamus and amygdala in response to SEA. Interestingly, challenge with SEA, but not LPS, significantly reduced hindpaw-lick latency in the hot plate test, although this effect was observed only if the hotplate environment was unfamiliar, suggesting an interaction between immunological stimulation and novelty-induced stress. Since SEA induces various cytokines, including TNF-alpha, these results are consistent with a growing literature documenting the effects of cytokines on nociceptive functions, and a possible involvement of the OFQ/nociceptin system.

Does nociceptin play a role in pain disorders in man?

Nociceptin-immunoreactive cellbodies were detected in the human trigeminal ganglion, while no such fibers were identified in the temporal artery or in dermal tissue from the neck region. In four healthy subjects receiving nociceptin into the temporal muscle in an open labeled design no pain was detected. In 10 healthy subjects who received 200pmol of nociceptin into tender non-dominant trapezius muscles in a placebo-controlled, randomized, balanced, and double-blinded design local tenderness increased (P=0.025) while no pain was noted. Thus, the action of nociceptin should be searched for in the trigeminal ganglion and/or in the central nervous system (CNS).