Molecular cloning and tissue distribution of a putative member of the rat opioid receptor gene family that is not a mu, delta or kappa opioid receptor type

Orphanin FQ is the major OFQ1-17-containing peptide produced in the rodent and monkey hypothalamus

In order to investigate the processing of OFQ containing peptides in the hypothalamus we have developed a sensitive and quantitative radioimmunoassay for OFQ. We fractionated rodent and monkey hypothalamic extracts by reversed-phase high performance liquid chromatography and found that the extracts contained multiple peaks of OFQ immunoreactivity with the major peak co-eluting with synthetic OFQ1-17. Mouse hypothalamic extracts were also fractionated by SDS-PAGE to determine the apparent molecular weights of molecules containing the OFQ peptide. Multiple peaks of OFQ immunoreactivity, ranging in size from approximately 1 to 30 kilodaltons, were detected by this method. These results suggest that OFQ1-17 is processed to smaller peptides in mouse and monkey hypothalamic neurons.

Naltrexone, but not atropine or yohimbine, antagonizes suppression of formalin-induced spinal sensitization by intrathecal nociceptin

We investigated the effects of spinal nociceptin on formalin-induced spinal sensitization and examined the role of the opioidergic, alpha 2-adrenergic and muscarinic cholinergic receptors in the nociceptin-produced suppression of spinal sensitization. The results demonstrated that spinal nociceptin suppressed the formalin-induced spinal sensitization in a dose-dependent manner (1, 5 and 10 nmol). The inhibitory effect of 10 nmol of nociceptin on spinal sensitization, was readily antagonized by naltrexone, but not by atropine or yohimbine. Each of the antagonists, naltrexone, atropine or yohimbine, alone had no effect on the formalin-induced spinal sensitization. Our results show that spinal nociceptin elicits dose-dependent, naltrexone-reversible suppression of spinal sensitization evoked by injection of formalin.

Distribution of nociceptin/orphanin FQ receptor transcript in human central nervous system and immune cells

We have examined the distribution of the opioid receptor-like-1 (ORL-1) transcript in the human CNS as well as human immune cells by RT-PCR and RNAse protection. The hORL-1 mRNA was distributed throughout the brain and particularly abundant in cortical areas, striatum, thalamus and hypothalamus. In the immune system, gene transcription was observed in normal circulating lymphocytes and monocytes as well as in T, B and monocytic cell lines. A splice variant, lacking 15 nucleotides at the junction between exon 1 and exon 2, showed a distribution similar to the already known ORL-1 transcript. Altogether these results show comparable expression levels of the hORL-1 gene in both nervous and immune systems, suggesting that the ORL-1-encoded receptor may participate to neuronal and non-neuronal physiological functions in humans.

Nitric oxide release mediates vasodilator responses to endomorphin 1 but not nociceptin/OFQ in the hindquarters vascular bed of the rat

We have recently shown that endomorphin 1, an endogenous ligand for the mu-opioid receptor, and nociceptin (Orphanin FQ; OFQ), an endogenous ligand for the ORL1 receptor, have substantial vasodilator activity in the hindquarters vascular bed of the rat. In the present study, the role of nitric oxide, vasodilator prostaglandins, and the opening of K+ ATP channels in mediating vasodilator responses to endomorphin 1, PL017, and DAMGO was investigated in the regional vascular bed in the rat. Under constant-flow conditions, injections of the mu-selective agonists endomorphin 1, PL017 ([N-MePhe3,D-Pro4]-morphiceptin), and DAMGO, and the ORL1 receptor agonist nociceptin/ OFQ produced dose-dependent decreases in hindquarters perfusion pressure. Vasodilator responses to endomorphin 1, PL017, and DAMGO, and the endothelium-dependent vasodilators acetylcholine and adrenomedullin were attenuated by the nitric oxide synthase inhibitor L-NAME (50 mg/kg IV) at a time when vasodilator responses to nociceptin/OFQ were not altered. Vasodilator responses to isoproterenol and prostaglandin E1, agents known to increase cAMP levels, and the nitric oxide donor DEA/NO were not altered by the nitric oxide synthase inhibitor. Responses to endomorphin 1, PL017, DAMGO, and nociceptin/OFQ were not altered by sodium meclofenamate at a time when vasodilator responses to arachidonic acid were reduced significantly or after administration of U-37883A at a time when vasodilator responses to levcromakalim were reduced significantly. The results of these studies indicate that responses to endomorphin 1, PL017, and DAMGO are mediated in large part by the release of nitric oxide, while responses to nociceptin/OFQ are mediated by an L-NAME-insensitive mechanism. Moreover, these results demonstrate that responses to these peptides are not mediated by the release of vasodilator prostaglandins or the opening of K+ATP channels the hindquarters vascular bed.

Orphanin FQ acts as an anxiolytic to attenuate behavioral responses to stress

Orphanin FQ (OFQ, Nociceptin) is a recently discovered 17-amino acid neuropeptide that is structurally related to the opioid peptides but does not bind opioid receptors. OFQ has been proposed to act as an anti-opioid peptide, but its widespread sites of action in the brain suggest that it may have more general functions. Here we show that OFQ plays an important role in higher brain functions because it can act as an anxiolytic to attenuate the behavioral inhibition of animals acutely exposed to stressful/anxiogenic environmental conditions. OFQ anxiolytic-like effects were consistent across several behavioral paradigms generating different types of anxiety states in animals (light-dark preference, elevated plus-maze, exploratory behavior of an unfamiliar environment, pharmacological anxiogenesis, operant conflict) and were observed at low nonsedating doses (0.1-3 nmol, intracerebroventricular). Like conventional anxiolytics, OFQ interfered with regular sensorimotor function at high doses (>3 nmol). Our results show that an important role of OFQ is to act as an endogenous regulator of acute anxiety responses. OFQ, probably in concert with other major neuropeptides, exerts a modulatory role on the central integration of stressful stimuli and, thereby, may modulate anxiety states generated by acute stress.

Nociceptin/orphanin FQ and the opioid receptor-like ORL1 receptor

Homology cloning and, more recently, the sequencing of whole genomes, have identified many open reading frames encoding proteins of unknown function, in particular putative G protein-coupled membrane receptors. Identification of orphan receptors in this way has marked the advent of 'reverse pharmacology' to identify the corresponding physiological ligands. This approach has led to the discovery of the ORL1 (Opioid Receptor-Like 1) receptor, and of its natural ligand, nociceptin/orphanin FQ (noc/oFQ), the basic components of a new peptide-based signalling pathway in the nervous system. Based on genetic criteria, the ORL1 and opioid receptors belong to the same family, as do noc/oFQ and opioid peptides. The marked structural analogy between the ORLI and opioid receptors, especially the kappa-opioid receptor, and the noc/oFQ and opioid peptides, particularly dynorphin A, is not reflected anatomically since noc/oFQ and opioid peptides appear to be located in separate neuronal circuits. Noc/oFQ triggers the same G protein-mediated signalling pathways as do opioids, however, to produce pharmacological effects that sometimes differ from, and even oppose, those of opioids. Noc/oFQ stimulates an outward K+ current and/or inhibits voltage-gated Ca2+ channels, thereby reducing synaptic efficacy, i.e. neuronal activity. In the rat, noc/oFQ is endowed with supraspinal pronociceptive/anti-opioid properties (it suppresses opioid-mediated analgesia), while convergent electrophysiological and behavioural data indicate that the peptide is a spinal analgesic. Noc/oFQ has not yet been found to precipitate withdrawal in morphine-tolerant rats. Nor does it elicit motivational effects, suggesting it lacks abuse liability. Also, by acting supraspinally, noc/oFQ impairs motor performance, suppresses spatial learning, induces feeding, and regulates basal and stress-induced release of pituitary hormones. Noc/oFQ is also active when administered intravenously, exhibiting potent smooth muscle relaxant, diuretic, and antinatriuretic properties. Last but not least, noc/oFQ appears to regulate stimulated immune function, and to be involved in neuronal differentiation. The discovery of noc/oFQ, a neuropeptide with multiple functions, will certainly improve our knowledge of brain physiology, and may find therapeutic applications, for example in the management of pain or hyponatremic and water-retaining diseases. However, given the wide distribution of noc/oFQ and its receptor, the pharmacological profile of noc/oFQ is likely to be incomplete, and other as yet unknown functions of the peptide remain to be discovered. Most helpful in this respect will be the identification of new ligands of the ORL1 receptor, particularly antagonists. If research on noc/oFQ carries on unabated at the present pace, potentially clinically interesting new compounds could become available in the not too distant future.

Functional expression of opioid receptor-like receptor and its endogenous specific agonist nociceptin/orphanin FQ during mouse embryogenesis

Expression of opioid receptor-like receptor (ORL1) and its endogenous peptide agonist nociceptin/orphanin FQ (N/OFQ) during mouse embryogenesis have been investigated. Transcripts of ORL1 and N/OFQ were detected by RT-PCR in mouse brain of day 8 embryo (E8) and the expression continued afterwards. Northern blot analysis revealed abundant expression of ORL1 at postnatal day 1 (P1) and N/OFQ at E17 and P1 in the brain but none was detected in other embryonic tissues. The presence of functional ORL1 in mouse embryonic brain was also confirmed by specific binding of [3H] N/OFQ (kd = 1.3 +/- 0.5 nM and Bmax = 72 +/- 9 fmol/mg protein) as well as by N/OFQ-stimulated G protein activation.

Circuitry underlying antiopioid actions of orphanin FQ in the rostral ventromedial medulla

Several laboratories recently identified a 17 amino-acid peptide, termed "nociceptin" or "orphanin FQ (OFQ)", as the endogenous ligand for the LC132 (or "opioid receptor-like1") receptor. Taken together with the fact that the cellular effects of OFQ are to a large extent opioid-like, the close relationship between the LC132 receptor and known opioid receptors raised expectations that the behavioral effects of this peptide would resemble those of opioids. However studies of the role of OFQ in nociception have not provided a unified view. The aim of the present study was to use a combination of electrophysiological and pharmacological techniques to characterize the actions of OFQ in a brain region in which the circuitry mediating the analgesic actions of opioids has been relatively well characterized, the rostral ventromedial medulla (RVM). Single-cell recording was combined with opioid administration and local infusion of OFQ in the RVM of rats lightlyanesthetized with barbiturates. The tail flick reflex was used as a behavioral index of nociceptive responsiveness. Two classes of physiologically identifiable RVM neurons with distinct responses to opioids have been characterized. -cells are activated, although indirectly, by opioids, and there is strong evidence that this activation is crucial to opioid antinociception. -cells, thought to enable nociception, are directly inhibited by opioids. Cells of a third class, cells, do not respond to opioids and whether or not they have any role in nociceptive modulation remains an open question. OFQ infused within the RVM profoundly suppressed the firing of all classes of RVM neurons, blocking opioid-induced activation of -cells. The antinociceptive effects of a micro-opioid agonist infused at the same site were significantly attenuated in these animals. Those of systemically administered morphine, which can produce its antinociceptive effects by acting at a number of CNS sites, were not blocked by RVM OFQ. Inasmuch as activation of -cells can account for the antinociceptive action of opioids within the RVM, these results demonstrate that, at least within the medulla, OFQ can exert a functional "antiopioid" effect by suppressing firing of this cell class. However to the extent that antinociceptive and pronociceptive outflows from various brain regions involved in both transmission and modulation of nociception are active under different conditions, focal application of OFQ in different regions could potentially produce either hypalgesia or hyperalgesia.

Partial loss of tolerance liability to morphine analgesia in mice lacking the nociceptin receptor gene

In mice lacking the nociceptin (or orphanin FQ) receptor gene, when 10 mg/kg of morphine was subcutaneously given, a potent analgesia in the tail pinch test was observed. The analgesic effect of morphine was equivalent among wild-type, heterozygous and homozygous mutant mice. When morphine was given to such mice in a dose of 10 mg/kg once per day for 5 days, wild-type and heterozygous mice showed marked tolerance or reduction in the morphine analgesia on the 5th day, while homozygous mice showed only 50% reduction in the peripheral analgesia of morphine. These findings suggest that nociceptin or its receptor plays important roles in the in vivo mechanism for the development of morphine tolerance.

Characterization of nociceptin-stimulated in situ [35S]GTPgammaS binding in comparison with opioid agonist-stimulated ones in brain regions of the mice

We studied the characterization of receptor-mediated G protein activity by nociceptin throughout brain regions, using in situ GTPgammaS binding autoradiography. Nociceptin-stimulated GTPgammaS binding was markedly observed in amygdala, hippocampal pyramidal cell layers, temporal and entorhinal cortex, infralimbic organ, anterior olfactory nucleus, and rostral part of thalamus. These nociceptin-stimulated activities were not affected by naloxone, naltrindol nor norbinaltorphimine which completely blocked mu-, delta- or kappa-opioid agonist-stimulated GTPgammaS binding, respectively. In addition, the distribution of nociceptin-stimulated activities throughout brain regions was found to be different from such opioid receptor-mediated ones.

Nociceptin/orphanin FQ activates protein kinase C, and this effect is mediated through phospholipase C/Ca2+ pathway

The effect of nociceptin/orphanin FQ (N/OFQ) on protein kinase C (PKC) was investigated in Chinese hamster ovary cells stably expressing opioid receptor-like (ORL1) receptor (CHO-ORL1 cells). N/OFQ significantly activated PKC in CHO-ORL1 cells with EC50 of 0.2 nM. This response was blocked by PKC inhibitors chelerythrine and Gö 6976, and by pretreatment of cells with pertussis toxin (PTX). The inhibition of PKC activation by N/OFQ was also achieved by use of Ca(2+)-chelators and phospholipase C (PLC) inhibitor U-73122. These results indicate that N/OFQ can effectively activate PKC via ORL1 receptor, and suggest the activation involve the PLC/Ca2+ system.

Comparison of the structure-activity relationships of nociceptin and dynorphin A using chimeric peptides

The aim of the present study was to delineate the functional domains of nociceptin (noc), a neuropeptide which is structurally related to dynorphin A (dyn). The binding and biological potencies towards the nociceptin (ORL1) and dynorphin A (kappa-opioid) receptors of twenty dyn/noc and noc/dyn hybrid peptides were compared with those of the parent heptadecapeptides. Replacement of as many as eleven residues in the C-terminus of dynorphin by the corresponding nociceptin sequence has no significant effect on binding and biological activity towards the kappa-opioid receptor. In marked contrast, replacement of as few as six residues (RKLANQ) in the C-terminus of nociceptin by the corresponding dynorphin sequence (LKWDNQ) dramatically impairs both affinity and activity towards the ORL1 receptor. This clearly indicates that the two neuropeptides have different functional architectures, despite the dual structural homology of both ligands and receptors. Moreover, the recombinant peptide approach led us to identify hybrids whose sequences differ only at positions 5 and 6 and displaying opposite or no receptor selectivity. One contains the dynorphin Leu5-Arg6 sequence and prefers the kappa-opioid receptor, whereas the other comprises the nociceptin Thr5-Gly6 sequence and prefers the ORL1 receptor. A third, containing the mixed dynorphin/nociceptin Leu5-Gly6 sequence, does not discriminate between the two types of receptor.

Presence of nociceptin (orphanin FQ) receptors in rat retina: comparison with receptors in striatum

Nociceptin (orphanin FQ), a heptadecapeptide with some sequence homology to dynorphin A, has been proposed as an endogenous ligand for a previously cloned orphan receptor with significant homology to opioid receptors. Utilizing [(125)I][Tyr14]nociceptin as ligand, saturable and high affinity nociceptin binding sites were detected and characterized in rat retina and striatum. For retina, Bmax = 44.0 +/- 4.5 fmol/mg and Kd = 32.4 +/- 2.7 pM; for striatum, Bmax = 51.6 +/- 7.7 fmol/mg and Kd = 98.6 +/- 11.3 pM. In competition studies, nociceptin bound with picomolar affinity, dynorphin A with nanomolar affinity, naloxone and dynorphan A-(1-8) with micromolar affinity, while [des-Tyr1]dynorphin (dynorphin A-(2-17)), several other opioids, morphine and benzomorphans failed to compete for binding at 1-10 microM. Gpp(NH)p plus NaCl markedly decreased binding, consistent with involvement of a G protein-linked receptor. It is concluded that rat retina contains nociceptin receptors similar in concentration to those present in striatum. Properties of both the retinal and the striatal receptors are similar to those previously found for rat hypothalamus.

Orphanin FQ has an inhibitory effect on the guinea pig ileum and the mouse vas deferens

The activity of the recently isolated endogenous opioid-like peptide orphanin FQ (OFQ) was measured in two classical bioassays of opioid action. OFQ potently and concentration-dependently suppressed the electrically stimulated contractions of the guinea pig ileum (GPI) and the mouse vas deferens (MVD) with EC50 values of 1.82 +/- 0.16 and 2.97 +/- 0.01 nM, and Emax values of 56 +/- 3% and 96 +/- 4%, respectively. This effect of OFQ, in both the GPI and MVD, was insensitive to the opioid receptor antagonist naloxone. OFQ competed with [3H]diprenorphine binding to mu-, delta- or kappa-opioid receptors stably expressed in Chinese hamster ovary cell lines with IC50 values of 2.1 +/- 0.4, 2.2 +/- 0.3, 0.75 +/- 0.3 microM, respectively. Low affinity for the classical opioid receptors together with the inability of naloxone to antagonize its effect suggest that the inhibitory action of OFQ is mediated via a distinct OFQ receptor in the GPI and MVD. Consequently, the MVD could serve as a valuable bioassay of potential OFQ receptor antagonists.

Nociceptin activation of the human ORL1 receptor expressed in Chinese hamster ovary cells: functional homology with opioid receptors

Opioid receptor-like 1 (ORL1) receptor, a member of the superfamily of G-protein-coupled receptors has significant primary sequence homology to the mu-, delta- and kappa-opioid receptors. The ORL1 receptor is selectively activated by the recently discovered peptide nociceptin. To probe the functional homology amongst these receptors, a Chinese hamster ovary (CHO) cell line expressing the human ORL1 receptor has been characterized. Nociceptin inhibited forskolin-stimulated increases in intracellular cAMP with an IC50 of 70 pM. Stimulation by nociceptin caused a 2-fold increase in the rate of [35S]GTPgammaS binding to membranes derived from CHO cells expressing the ORL1 receptor. Following incubation with nociceptin mitogen-activated protein kinase activity was increased by 2-fold in cells expressing the ORL1 receptor. In non-transfected CHO cells, nociceptin had no effect on cAMP accumulation, the rate of [35S]GTPgammaS binding or mitogen-activated protein kinase activity. Human ORL1 receptors expressed in CHO cells selectively bound [125I][Tyr14]nociceptin with a Kd of 2.1 pM and a Bmax of 2.6 pmol/mg protein. Similar to opioid receptors, nociceptin binding to the ORL1 receptor was altered by Na+, GTPgammaS and dithiothreitol. Na+ increased the Kd of nociceptin binding to the ORL1 receptor. GTPgammaS decreased the apparent Bmax of [125I][Tyr14]nociceptin binding but had no effect on the Kd of the remaining sites. Pretreatment with dithiothreitol inhibited nociceptin binding to the ORL1 receptor. Nociceptin binding was insensitive to low nanomolar concentrations of opioid receptor-selective agonists and antagonists. However, high micromolar levels of opioid receptor-selective agents inhibited the binding. Morphine, naloxone, naltrindole, nor-Binaltorphimine and CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) inhibited nociceptin binding to ORL1 receptor with Ki values of 36, 24, 0.4, 8 and 28 microM, respectively. These results imply that ORL1 is a G-protein-coupled receptor with functional as well as structural homology to opioid receptors. In addition, opioid receptor ligands may serve as starting templates for the development of ORL1 specific ligands.

Anatomical distribution of mu, delta, and kappa opioid- and nociceptin/orphanin FQ-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate binding in guinea pig brain

An in vitro autoradiographic technique has recently been developed to visualize receptor-activated G-proteins by using agonist-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding in the presence of excess guanosine 5'-diphosphate. This technique was used to localize opioid-activated G-proteins in guinea pig brain, a species that contains the three major types of opioid receptors. This study used selective mu, delta, and kappa opioid agonists as well as nociceptin or orphanin FQ (N/OFQ) peptide, an endogenous ligand for an orphan opioid receptor-like (ORL1) receptor, to stimulate [35S]GTPgammaS binding in guinea pig brain sections. Opioid receptor specificity was confirmed by blocking agonist-stimulated [35S] GTPgammaS binding with the appropriate antagonists. In general, the distribution of agonist-stimulated [35S]GTPgammaS binding correlated with previous reports of receptor binding autoradiography, although quantitative differences suggest regional variations in receptor coupling efficiency. Mu, delta, and kappa opioid-stimulated [35S]GTPgammaS binding was found in the caudate-putamen, nucleus accumbens, amygdala, and hypothalamus. Mu-stimulated [35S]GTPgammaS binding predominated in the hypothalamus, amygdala, and brainstem, whereas kappa-stimulated [35S]GTPgammaS binding was particularly high in the substantia nigra and cortex and was moderate in the cerebellum. N/OFQ-stimulated [35S] GTPgammaS binding was highest in the cortex, hippocampus, and hypothalamus and exhibited a unique anatomical distribution compared with opioid-stimulated [35S]GTPgammaS binding. The present study extends previous reports on opioid and ORL1 receptor localization by anatomically demonstrating functional activity produced by mu, delta, and kappa opioid and ORL1 receptor activation of G-proteins.

Nociceptin-like immunoreactivity in autonomic nuclei of the rat spinal cord

Immunohistochemical techniques were used to localize nociceptin-like immunoreactivity (NOCI-LI) in the rat spinal cords. NOCI-LI nerve fibers were distributed in three fairly well-define regions: superficial layers of the dorsal horn, central canal area, and intermediolateral cell column (ILp) of lower cervical, thoracic, upper lumbar, and sacral segments of the spinal cord. A few NOCI-LI somata of small diameter were noted in the dorsal horn; NOCI-LI cell bodies were infrequently observed in the ILp or ventral horn. Concentration of NOCI-LI in nerve fibers of the superficial layers and in fibers projecting into the spinal sympathetic and parasympathetic nuclei suggests that the peptide may participate in sensory as well as autonomic functions.

Relationship between binding affinity and functional activity of nociceptin/orphanin FQ

The relationship between binding affinity and functional activity of nociceptin/orphanin FQ binding was studied in brain membranes, membranes of Chinese hamster ovary cells transfected with ORL1, and intact CHO-ORL1 cells. Binding affinities were compared with potency for the stimulation of [35S]GTP gamma S binding in cell membranes, and inhibition of forskolin-stimulated cAMP accumulation in intact cells. Binding was conducted with [3H]14-Tyr-nociceptin, and in brain or cell membranes the affinity was found to be 50-100 pM. The binding of [3H]14-Tyr-nociceptin was found to be regulated by Na+ and GTP, as expected for an opioid-like receptor. In intact cells, saturation produced a curvilinear Scatchard Plot. Non-linear analysis indicated two states of the receptor, with the vast majority of binding being to a low affinity state of approximately 8 nM. This low affinity component is consistent with the lower potency derived from the inhibition of cAMP accumulation, stimulation of [35S]GTP gamma S binding, and other functional assays.

Nociceptin and its receptor in guinea-pig sympathetic ganglia

The occurrence and distribution of nociceptin, a 17-amino acid peptide with structural similarity to dynorphin A, and its receptor, opioid receptor-like-1 (ORL1) receptor, were investigated in the guinea-pig sympathetic nervous system by means of immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR), respectively. Immunofluorescence revealed varicose nociceptin-immunoreactive axons and some paraganglionic cells in prevertebral (coeliaco-superior mesenteric, inferior mesenteric), but not in paravertebral (superior cervical, stellate, lumbar chain) sympathetic ganglia. Messenger RNA for the ORL1 receptor, however, was detected by RT-PCR in both para- and prevertebral ganglia. The findings suggest participation of the nociceptin/ORL1 receptor signalling pathway in processing of information within prevertebral ganglia, and a general responsiveness of sympathetic neurons to nociceptin.

Molecular approaches to receptors as targets for drug discovery

The cloning of a great number of receptors and channels has revealed that many of these targets for drug discovery can be grouped into superfamilies based on sequence and structural similarities. This review presents an overview of how molecular biological approaches have revealed a plethora of receptor subtypes, led to new definitions of subtypes and isoforms, and played a role in the development of high selective drugs. Moreover, the diversity of subtypes has molded current views of the structure and function of receptor families. Practical difficulties and limitations inherent in the characterization of the ligand binding and signaling properties of expressed recombinant receptors are discussed. The importance of evaluating drug-receptor interactions that differ with temporally transient and distinct receptor conformational states is emphasized. Structural motifs and signal transduction features are presented for the following major receptor superfamilies: ligand-gated ion channel, voltage-dependent ion channel, G-protein coupled, receptor tyrosine-kinase, receptor protein tyrosine-phosphatase, cytokine and nuclear hormone. In addition, a prototypic receptor is analyzed to illustrate functional properties of a given family. The review concludes with a discussion of future directions in receptor research that will impact drug discovery, with a specific focus on orphan receptors as targets for drug discovery. Methods for classifying orphan receptors based upon homologies with members of existing superfamilies are presented together with molecular approaches to the greater challenge of defining their physiological roles. Besides revealing new orphan receptors, the human genome sequencing project will result in the identification of an abundance of novel receptors that will be molecular targets for the development of highly selective drugs. These findings will spur the discovery and development of an exciting new generation of receptor-subtype specific drugs with enhanced therapeutic specificity.

Activation of mitogen-activated protein kinase by the nociceptin receptor expressed in Chinese hamster ovary cells

Activation of the nociceptin receptor stably expressed in Chinese hamster ovary cells induced a transient mitogen-activated protein kinase (MAPK) activation, via pertussis toxin-sensitive G-proteins. The nociceptin receptor-mediated MAPK activation was partially blocked by down-regulation or inhibition of protein kinase C, and suppressed by pretreatment with a phosphatidylcholine-specific phospholipase C inhibitor, D609. Furthermore, a tyrosine protein kinase inhibitor, genistein, and phosphatidylinositol 3-kinase inhibitors, wortmannin and LY294002, affected the nociceptin-induced MAPK activity. The nociceptin-induced MAPK activation may lead to activation of phospholipase A2 and induce changes in gene expression.

Nociceptin/orphanin FQ stimulates extracellular acidification and desensitization of the response involves protein kinase C

A Chinese hamster ovary (CHO) cell line, CHO-ORL1, stably expressing human opioid receptor-like receptor 1 (ORL1) has been used to determine ORL1-mediated signaling events using microphysiometry. Nociceptin/orphanin FQ (N/OFQ), a specific endogenous agonist of ORL1, induced an increase in extracellular acidification rate (ECAR) in CHO-ORL1 cells. The ECAR response stimulated by N/OFQ was concentration-dependent and pertussis toxin-sensitive. Repeated exposures of the cells to N/OFQ caused desensitization of ORL1. The ECAR response was recovered at the half-life of approximately 12 min after the initial challenge. Pretreatment with inhibitor of cAMP-dependent kinase did not affect desensitization of ORL1. However, specific inhibitors for protein kinase C almost abolished N/OFQ-induced desensitization of extracellular acidification responsiveness, indicating the involvement of protein kinase C in the process.

Autoradiographic localization of [3H]nociceptin binding sites from telencephalic to mesencephalic regions of the mouse brain

The binding sites of [3H]nociceptin (also named Orphanin FQ), the endogenous ligand of the ORL1 (opiate receptor like 1) receptor, were localized in the central nervous system of the mouse using an autoradiographic procedure. A high density of binding sites was seen in the cerebral cortex, paraventricular nucleus of the thalamus, amygdaloid complex, suprachiasmatic nucleus, medial thalamus and medial geniculate nucleus. Moderate binding was observed in the nucleus accumbens, lateral septum, lateral thalamus, hippocampus, periaqueductal grey matter and pons. Finally, low levels of binding were seen in the striatum, olfactory tubercle, hypothalamus and substantia nigra. Thus, it appears that the ORL1 receptor is particularly abundant in the cerebral cortex and limbic system of the mouse brain.

Presence and characterization of nociceptin (orphanin FQ) receptor binding in adult rat and human fetal hypothalamus

High affinity and saturable nociceptin (orphanin FQ) receptors were detected and characterized in adult rat and human fetal hypothalamic membranes, utilizing [125I]Tyr12-nociceptin as ligand. Nociceptin bound with picomolar affinity, dynorphin A with nanomolar affinity, naloxone and dynorphan A(1-8) with micromolar while des-Tyr1-dynorphin (dynorphin A(2-17)), several other opioids, morphine and benzomorphans failed to compete for binding at 1-10 microM. Gpp(NH)p together with sodium ion markedly decreased binding, consistent with involvement of a G protein-linked receptor.

Opiate and cocaine addictions: challenge for pharmacotherapies

Neurobiological and behavioral studies, as well as basic and applied clinical research studies, may all contribute to the development of a pharmacotherapy for a specific addictive disease. This paper reviews recent findings from research work, primarily from one laboratory along with collaborative laboratories, that could have some relevance for the development of pharmacotherapy for cocaine dependency. The much earlier experiences of this laboratory in the development of a pharmacotherapy for opiate addiction will be addressed in the context of providing both some specific suggestions for addictive disease pharmacotherapy development and some warnings about the complexities of the introduction and implementation of a pharmacotherapy once developed. Finally, based on both the earlier perspectives and the more recent research findings, some very specific, though speculative, suggestions will be made about the development of novel pharmacotherapies for early opiate addiction, especially for cocaine abuse or addiction and prevention of relapse to cocaine use. The complex and diverse nature of the challenge for pharmacotherapy for the addictive diseases is presented, including specifically a mandate for broadening educational efforts concerning the basis of addictive diseases and the need for treatment, in parallel with the scientific efforts to develop increasingly sophisticated and targeted pharmacotherapies.

Nociceptin, a novel endogenous ligand for the ORL1 receptor, has potent erectile activity in the cat

The heptadecapeptide nociceptin, also known as orphanin FQ, is a newly discovered endogenous ligand for the opioid-like G protein-coupled receptor ORL1. The present study was undertaken to investigate the effects of intracavernosal injections of nociceptin on penile erection in anesthetized cats. Responses to nociception were compared with erectile responses elicited by intracavernosal injection of vasoactive intestinal polypeptide (VIP), adrenomedullin (ADM), the novel nitric oxide donor diethylaminenitric oxide complex sodium (DEA/NO), and the control triple-drug combination (papaverine, phentolamine, and prostaglandin E1). The order of potency was VIP > ADM > nociceptin > DEA/NO. Intracavernosal injections of nociceptin in doses of 0.3-30 nmol elicited dose-related increases in cavernosal pressure and penile length that were comparable to those induced by the triple-drug combination, which is used in the treatment of erectile dysfunction. The response to nociceptin was rapid in onset, and the duration of the peak pressure increase and total response was significantly shorter than the response to the control triple-drug combination but longer in duration than responses to VIP and ADM. Intracavernosal injection of the triple-drug combination resulted in a greater decrease in mean systemic arterial blood pressure than did nociceptin. These data demonstrate that intracavernosal injection of this novel endogenous ligand for the ORL1 receptor induces a potent and relatively long-lasting erectile response in the cat.

Modulation of excitatory synaptic transmission by nociceptin in superficial dorsal horn neurones of the neonatal rat spinal cord

1. The modulatory actions of nociceptin/orphanin FQ on excitatory synaptic transmission were studied in superficial dorsal horn neurones in transverse slices from 7 to 14 day old rats. 2. Glutamatergic excitatory postsynaptic currents (e.p.s.cs) were recorded from the somata of the neurones in the whole-cell patch-clamp configuration. E.p.s.cs were evoked by extracellular electrical stimulation (100 microns, 3-10 V) of the ipsilateral dorsal root entry zone by use of a glass electrode. E.p.s.cs with constant short latency (< 2.3 ms) and with no failures upon stimulation were assumed to be monosynaptic. These e.p.s.cs occurred with an average latency of 1.72 +/- 0.098 ms and exhibited a fast decay with a time constant, tau, of 4.8 +/- 0.53 ms (n = 30). 3. Nociceptin reversibly reduced the amplitudes of e.p.s.cs in a concentration-dependent manner in 25 out of 27 cells tested. Average maximum inhibition was 51.6 +/- 5.7% (mean +/- s.e.mean; n = 9), at concentrations > 3 microM. EC30 was 485 +/- 47 nM and the Hill coefficient was 1.29 +/- 0.09. 4. Inhibition of synaptic transmission by nociceptin (10 microM) was insensitive to the non-specific opioid receptor antagonist naloxone (10 microM) indicating that nociceptin did not act via classical opioid receptors. 5. In order to determine the site of action of nociceptin spontaneous miniature e.p.s.cs (m-e.p.s.cs) were recorded. Nociceptin reduced the frequency of m-e.p.s.cs in 6 out of 7 cells but had no effect on their amplitude distribution or on their time course. These findings suggest a pre- rather than a postsynaptic modulatory site of action. This is in line with the finding that current responses elicited by extracellular application of L-glutamate (10 microM) were not affected by nociceptin (10 microM; n = 7). 6. No positive correlation was found between the degree of inhibition by nociceptin (10 microM) and by the mixed delta- and mu-receptor agonist methionine-enkephalin (10 microM). This suggests that both neuropeptides acted on different but perhaps overlapping populations of synaptic connections. 7. Our results indicate that nociceptin inhibits excitatory synaptic transmission in the superficial layers of the rat dorsal horn by acting on presynaptic, presumably ORL1 receptors. This may be an important mechanism for spinal sensory information processing including nociception.

Overexpression of a neuropeptide nociceptin/orphanin FQ precursor gene, N23K/N27K, induces neurite outgrowth in mouse NS20Y cells

We recently discovered N23K and its splicing variant N27K as transcripts upregulated in mouse NS20Y cells after differentiation induced by dibutylyl cyclic AMP (dbcAMP) treatment. N23K and N27K encode precursor proteins for an opioid neuropeptide, nociceptin/orphanin FQ, but the transient expression of N23K and N27K suggests that it may be involved in neuronal differentiation. In the present study, we report that NS20Y cells transfected with N23K and/or N27K but not with vector alone formed neurites, with the expressed protein distributed in the perinuclear region and distal parts of the neurites. The granular staining of the N23K and N27K proteins was also colocalized with secretogranin I, indicating incorporation into large dense core vesicles. This cellular targeting of the N23K and/or N27K protein is similar to that of dbcAMP-induced processes in nontransfected NS20Y cells. In addition, the neurites of transfectants that expressed both N23K and N27K were longer than those of the transfectants that expressed N23K or N27K alone. Our results demonstrate that N23K and N27K participate in the regulation of neurite outgrowth.

Molecular characterization and functional expression of opioid receptor-like1 receptor

The opioid receptor-like1 receptor (ORL1), an orphan receptor whose human and murine complementary DNAs, has been characterized recently. ORL1 transcripts are particularly abundant in the central nervous system. We demonstrated that ORL1 expressed in human neuroblastoma SK-N-SH and SH-SY5Y cell lines by radioligand binding assay, reverse transcription polymerase chain reaction (RT-PCR) and Northern analysis in the present study. Stimulation with ORL1 specific agonist, nociceptin/orphanin FQ, increased [35S]GTP gamma S binding to SK-N-SH cell membranes (EC50 = 14 +/- 0.45 nM), and attenuated forskolin-stimulated accumulation of cellular cAMP (EC50 = 0.80 +/- 0.45 nM), indicative that activation of ORL1 activates G proteins and inhibits adenylyl cyclase. Activation of ORL1 receptor was also accessed using CHO:hORL1 cell line by microphysiometer. Treatment of nociceptin/orphanin FQ increased extracellular acidification rate significantly.

Characterization of nociceptin hyperalgesia and allodynia in conscious mice

1. Intrathecal (i.t.) administration of nociceptin and high doses of morphine induced allodynia in response to innocuous tactile stimuli, and i.t. nociceptin evoked hyperalgesia in response to noxious thermal stimuli in conscious mice. Here we have characterized the nociceptin-induced allodynia and compared it with the morphine-induced allodynia and the nociceptin-evoked hyperalgesia. 2. Nociceptin-induced allodynia was evoked by the first stimulus 5 min after i.t. injection, reached a maximum at 10 min, and continued for a 50 min experimental period. Dose-dependency of the allodynia showed a bell-shaped pattern from 50 pg to 5 ng kg-1, and the maximum effect was observed at 2.5 ng kg-1. 3. Morphine-induced allodynia reached the maximum effect at 15 min and declined progressively until cessation by 40-50 min. The dose-response curve showed a bell-shaped pattern, similar to that induced by nociceptin, with a maximum effect at 0.5 mg kg-1, five orders of magnitude higher than that of nociceptin. 4. The allodynia evoked by nociceptin and morphine were dose-dependently blocked by glycine, D(-)-2-amino-5-phosphonovaleric acid (D-AP5, an N-methyl-D-aspartate (NMDA) receptor antagonist), gamma-D-glutamylaminomethyl sulphonic acid (GAMS, a non-NMDA receptor antagonist) and methylene blue (a soluble guanylate cyclase inhibitor), but were not affected by muscimol (a gamma-aminobutyric acidA (GABAA) receptor agonist) and baclofen (a GABAB receptor agonist). 5. Morphine did not inhibit forskolin-stimulated cyclicAMP formation in cultured cells expressing the nociceptin receptor. 6. Nociceptin-induced hyperalgesia was evoked 10-15 min after i.t. injection. Nociceptin produced a monophasic hyperalgesic action over a wide range of doses from 5 fg to 50 ng kg-1. The nociceptin-induced hyperalgesia was blocked by glycine only among the agents examined. 7. None of the pain responses evoked by nociceptin and morphine were blocked by naloxone. 8. These results demonstrate that, whereas the mechanisms of the nociceptin-induced allodynia and hyperalgesia are evidently distinct, they involve a common neurochemical event beginning with the disinhibition of the inhibitory glycinergic response. Morphine may induce allodynia through a pathway common to nociceptin, but the nociceptin receptor does not mediate the action of high doses of morphine.

Interaction of [3H]orphanin FQ and 125I-Tyr14-orphanin FQ with the orphanin FQ receptor: kinetics and modulation by cations and guanine nucleotides

The heptadecapeptide orphanin FQ (OFQ) has been identified as the endogenous ligand for a G protein-coupled receptor (OFQ-R), which, despite its high degree of sequence similarity to opioid receptors, fails to bind opioid ligands. We developed two radioligands for the OFQ-R: a tritiated native OFQ peptide ([3H]OFQ) and a radioiodinated form in which Leu14 was substituted by tyrosine (125I-Tyr14-OFQ). Their binding properties were examined in human embryonic kidney (HEK) 293 and Chinese hamster ovary (CHO) cells heterologously expressing the OFQ-R at different levels (HEK 293 expressed 40-fold more OFQ-R than did CHO). Both ligands exhibited rapid, monophasic association kinetics in each cell line. Dissociation of both ligands from OFQ-R expressed in HEK 293 cells was biphasic, whereas dissociation of 125I-Tyr14-OFQ from OFQ-R expressed in CHO cells was monophasic and slow. Saturation binding analysis revealed two affinity states in HEK 293 cells with binding parameters in accord with those determined kinetically. In CHO cells, 125I-Tyr14-OFQ detected a single affinity state with an intermediate Kd value of 54 pM. Optimal binding of the radioligands required 1-5 mM MgCl2, whereas millimolar concentrations of ZnCl2, CaCl2, MnCl2, and NaCl reduced specific binding of both ligands. A nonhydrolyzable GTP analog [guanosine-5'-(beta,gamma-imido)triphosphate] reduced the affinity of both OFQ ligands to their receptor without significant changes in the total binding capacity, indicating functional interactions between the OFQ-R and G proteins. In rat brain membranes, specific, saturable binding of 125I-Tyr14-OFQ was demonstrated to be pharmacologically identical to the heterologously expressed OFQ-R. Taken together, these results indicate that 125I-Tyr14-OFQ and [3H]OFQ exhibit virtually identical characteristics and are suitable for the pharmacological analysis of the OFQ-R.

Unrestrained nociceptive response and disregulation of hearing ability in mice lacking the nociceptin/orphaninFQ receptor

In the G-protein-coupled receptor superfamily, the opioid receptor subfamily is constituted of the three distinct opioid receptors (namely delta-, mu- and kappa-subtypes) and the receptor for nociceptin (also designated orphaninFQ). The members of the opioid receptor subfamily were known to mediate a variety of cellular inhibitory effects. The three opioid receptors are known to play central roles in mediating analgesia and many other physiological activities; however, the nociceptin receptor was identified recently and less is known about its physiological roles. Here we report the generation and characterization of mice lacking the nociceptin receptor. The knockout mice showed no significant differences in nociceptive threshold and locomotor activity compared with control mice, but they lost nociceptin-induced behavioral responses. These results indicate that the nociceptin system is not essential for regulation of nociception or locomotor activity. On the other hand, we found insufficient recovery of hearing ability from the adaptation to sound exposure in the mutant mice. Thus, the nociceptin system appears to participate in the regulation of the auditory system.

Antisense mapping DOR-1 in mice: further support for delta receptor subtypes

In contrast to the pharmacological studies implicating delta-opioid receptor subtypes, cloning studies have identified only a single cDNA encoding a delta receptor, DOR-1. Antisense studies have established the importance of DOR-1 in delta analgesia in mice. Antisense mapping extends this approach to include oligodeoxynucleotides which are targeted against each of the exons of the gene. Five different antisense oligodeoxynucleotides based upon the three DOR-1 exons all block both spinal and supraspinal analgesic actions of the delta2 ligand [D-Ala2,Glu4]deltorphin, consistent with the suggestion that DOR-1 encodes the delta2 receptor. At the spinal level, [D-Pen2,D-Pen5]enkephalin (DPDPE) acts also acts through delta2 receptors and all the antisense probes block spinal DPDPE analgesia. However, supraspinally only the two antisense probes targeting exon 3 block DPDPE analgesia. The remaining three antisense probes based upon exons 1 and 2 are inactive. Thus, the delta receptors responsible for spinal and supraspinal DPDPE analgesia can be discriminated at the molecular level by antisense mapping.

Functional coupling of the nociceptin/orphanin FQ receptor with the G-protein-activated K+ (GIRK) channel

Nociceptin/orphanin FQ is a heptadecapeptide which was recently isolated from brains. It induces hyperalgesia, in contrast to the analgesic effects of opioid ligands, although it and its receptor structurally resemble opioid peptides and opioid receptors, respectively. To investigate the molecular mechanism underlying nociceptin/orphanin FQ actions, we performed Xenopus oocyte expression assays, in situ hybridization histochemistry and electrophysiological analyses of neurons. We found that the nociceptin/orphanin FQ receptor is functionally coupled with the G-protein-activated K+ (GIRK) channel in Xenopus oocytes, and that the receptor mRNA and GIRK1 mRNA co-exist in various neurons, including hippocampal pyramidal cells. Furthermore, we found that nociceptin/orphanin FQ induces hyperpolarizing currents via inward-rectifier K+ channels in hippocampal pyramidal cells, suggesting that the nociceptin/orphanin FQ receptor couples with the GIRK channel in this region. We conclude that the nociceptin/orphanin FQ receptor couples with the GIRK channel in various neurons, including hippocampal pyramidal cells, thereby modulating neuronal excitability.

Naloxone-insensitive inhibition of acetylcholine release from parasympathetic nerves innervating guinea-pig trachea by the novel opioid, nociceptin

The novel peptide, nociceptin and the mu-opioid agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) produced a concentration-dependent inhibition of electrical field stimulation (EFS)-evoked release of acetylcholine (ACh) from cholinergic nerves innervating guinea-pig trachea. The non-selective opioid receptor antagonist, naloxone, did not antagonize the inhibitory action of nociceptin under conditions where the inhibition of ACh release evoked by DAMGO was completely reversed. It is suggested that DAMGO and nociceptin can inhibit cholinergic, parasympathetic neurotransmission to the airways via the activation of classical (naloxone-sensitive) and novel (naloxone-insensitive) opioid receptors, respectively.

Spinal analgesic activity of orphanin FQ/nociceptin and its fragments

Previous work reveals that orphanin FQ/nociceptin (OFQ/N) administered supraspinally produces an initial hyperalgesic response followed by analgesia. Spinally, OFQ/N elicits a rapidly appearing, naltrexone-reversible, dose-dependent analgesia in the tailflick assay without any indication of hyperalgesia. Two OFQ/N fragments, OFQ/N (1-7) and OFQ/N (1-11), are active, but far weaker. Blockade of sigma receptors with haloperidol enhances the analgesic potency of spinal OFQ/N, OFQ/N (1-7) and OFQ/N (1-11), but not as dramatically as supraspinal OFQ. Antisense probes targeting the second and third coding exons, but not the first exon, of the cloned mouse OFQ/N receptor (KOR-3) partially block OFQ/N analgesia.

Recognition and activation of the opioid receptor-like ORL 1 receptor by nociceptin, nociceptin analogs and opioids

Nociceptin, also known as orphanin FQ, was recently identified as the naturally occurring agonist of orphan opioid receptor-like ORL1 receptor (Meunier et al., 1995, Nature 377, 532; Reinscheid et al., 1995, Science 270, 792). Nociceptin is a heptadecapeptide which, although it resembles dynorphin A, the endogenous agonist of the kappa-opioid receptor, displays very low potency in competing with binding of [3H]diprenorphine to or inhibiting adenylate cyclase via mu-, delta- and kappa-opioid receptors. Tritium-labeled nociceptin ([3H]nociceptin) was used here to establish a pharmacological profile in vitro of the ORL 1 receptor. In membranes from recombinant Chinese hamster ovary (CHO) cells expressing the ORL 1 receptor, equilibrium binding of [3H]nociceptin is highly specific, saturable (Bmax in the range 1.3-1.8 pmol/mg protein) and of high affinity (Kd approximately equal to 0.1 nM). It is selectively decreased in the presence of Na+ ions and/or of the GTP analog 5'-guanylylimido-diphosphate, an allosteric regulation that is analogous to that of opiate binding to opioid receptors. A few opiates, namely lofentanil, a 4-anilinopiperidine derivative and etorphine, a 6,14-endo-ethenotetrahydrothebaine derivative, were found to be quite potent not only in competing with binding of [3H]nociceptin at the ORL 1 receptor but also in inhibiting forskolin-induced accumulation of cyclic AMP in intact recombinant CHO cells. In a preliminary attempt to delineate active parts of the neuropeptide, nociceptin analogs were also tested, including N- and C-terminal truncation products. Our results suggest that the highly basic, internal core of nociceptin might be essential in conferring on the peptide both affinity for and activity at the ORL 1 receptor. In this respect, the message and address division of dynorphin A, nociceptin's closest structural analog, do not seem to apply to nociceptin.

Lack of cross-tolerance between the antinociceptive effect of intrathecal orphanin FQ and morphine in the rat

We assessed whether tolerance develops to the antinociceptive effect of intrathecal (i.t.) orphanin FQ, the endogenous ligand of the orphan opioid-like receptor, and whether there is a cross-tolerance between the antinociceptive effect of i.t. orphanin FQ and the mu opioid receptor agonist morphine. Orphanin FQ administered i.t. at 10 micrograms induced strong thermal antinociception as assessed with the tail flick test without inducing motor impairment or sedation. Tolerance developed in 4 days to the antinociceptive effect of orphanin FQ upon twice daily injections. However, rats rendered tolerant to orphanin FQ did not exhibit cross-tolerance to morphine. Moreover, in rats made tolerant to morphine, orphanin FQ induced similar antinociceptive effect as in morphine naive rats. The present results are the first to show that there is no cross-tolerance between orphanin FQ and morphine in eliciting spinal antinociception, supporting the notion that orphanin FQ produced spinal antinociception through a site which is different from classical opioid receptors.

Functional expression, activation and desensitization of opioid receptor-like receptor ORL1 in neuroblastoma x glioma NG108-15 hybrid cells

Neuroblastoma x glioma NG108-15 hybrid cells have been examined for the expression of opioid receptor-like receptor (ORL1). [3H]Nociceptin/orphanin FQ (OFQ) bound to the cell membrane specifically (Kd = 3.6 +/- 0.6 nM) and inhibited forskolin-stimulated cAMP accumulation (EC50 = 0.72 +/- 0.3 nM). The responsiveness of NG108-15 cells to nociceptin/OFQ was blocked by pertussis toxin but not by naltrindole. The inhibitory activity of nociceptin/OFQ was significantly reduced after a prechallenge with the same peptide but was not influenced by DPDPE pretreatment, indicating acute and homologous desensitization of ORL1 receptors. Naltrindole caused the overshoot of cAMP in DPDPE-pretreated cells but not in nociceptin/OFQ-pretreated cells. The results indicate that ORL1 is functionally expressed and does not cross-interact with specific ligands of the delta opioid receptor in NG108-15 cells.

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

1. The present study was designed to investigate further the effects of the newly discovered orphanin FQ (OFQ)-the endogenous ligand for the orphan opioid receptor (called, e.g., ORL, and LC132)-on pain modulation in the rat. We used the tail-flick assay as a nociceptive index. 2. When injected into a cerebral ventricle, OFQ (4 fmol-10 nmol) has no effect on basal tail-flick latency by itself at any dose, but dose-dependently antagonizes systemic morphine analgesia (400 fmol 50 nmol). 3. Injected intrathecally, OFQ (3 and 10 nmol) displayed an analgesic effect without producing motor dysfunction, and potentiated morphine analgesia (1 and 10 nmol). 4. The anti-opioid effect of OFQ in rat brain and the high level of expression of LC132/ORL, receptor in the locus coeruleus indicated a possible role of OFQ in the precipitation of opiate withdrawal symptoms. However, no such precipitation was observed by OFQ in morphine-dependent rats.

Association of aminopeptidase N and endopeptidase 24.15 inhibitors potentiate behavioral effects mediated by nociceptin/orphanin FQ in mice

The behavioral effects induced by central administration in mice of the endogenous ORL1 (opioid receptor-like1) ligand, nociceptin/orphanin FQ, were investigated in the absence or presence of inhibitors of aminopeptidase N (bestatin) and endopeptidase 24.15 (Z-(L,D)Phe psi(PO2CH2)(L,D)Ala-Arg-Phe) recently shown to be involved in the metabolism of the heptadecapeptide in vitro. A severe reduction in motor activity induced by nociceptin/orphanin FQ was measured in two tests (spontaneous motor activity and open field). This pharmacological effect was shown to be potentiated by the association of bestatin and Z-(L,D)Phe psi(PO2CH2)(L,D)Ala-Arg-Phe, confirming in vivo the involvement of these peptidases in nociceptin/orphanin FQ inactivation. In our conditions, these inhibitors were devoid of intrinsic effects, suggesting a low tonic regulation by the heptadecapeptide of the measured behaviour.

Biochemical evidence for orphanin FQ/nociceptin receptor heterogeneity in mouse brain

A recently identified novel peptide, orphanin FQ/nociceptin (OFQ/N), is an endogenous ligand for a unique member of the cloned opioid receptor family. Saturation studies in mouse brain membranes reveal curvilinear Scatchard plots with both a higher (KD 3.8 pM, Bmax 31.6 fmol/mg protein) and a lower (KD 896 pM, Bmax 233 fmol/mg protein) affinity site in brain tissue, compared to only a single site in transfected CHO cells (KD 36 pM). Competition studies confirm the high affinity of OFQ/N for this site, but shallow Hill slopes suggest heterogeneity. Traditional opioids have poor affinity for this receptor and OFQ/N and its fragments do not label traditional opioid receptors. In brain homogenates, both OFQ/N and OFQ/N(1-11) inhibit forskolin-stimulated camp accumulation with IC50 values of 1 nM or less, an action which is readily reversed by opioid antagonists. OFQ/N(1-7) shows little activity. Together, these studies suggest the presence of heterogeneous, functionally active OFQ/N receptors in mouse brain.

Nociceptin, an endogenous ligand for the ORL1 receptor, decreases cardiac output and total peripheral resistance in the rat

The heptadecapeptide nociceptin, also known as Orphanin FQ, is a newly discovered endogenous ligand for the opioid-like G-protein coupled receptor, ORL1. In the present study, responses to intravenous injections of nociceptin were investigated in the systemic vascular bed of the rat. Nociceptin induced dose-related decreases in systemic arterial pressure and total peripheral resistance when injected in doses of 1-30 nmol/kg i.v.. Nociceptin decreased heart rate and in doses of 10 and 30 nmol/kg i.v., significantly decreased cardiac output. In terms of relative vasodilator activity, nociceptin was approximately 10-fold less potent than the beta-adrenergic receptor agonist isoproterenol. These data show that nociceptin has novel vasodilator activity in the systemic vascular bed of the rat.

Nociceptin, an endogenous ligand for the ORL1 receptor, has novel hypotensive activity in the rat

The heptadecapeptide nociceptin, also known as Orphanin FQ, is a newly discovered endogenous ligand for the opioid-like G-protein coupled receptor, ORL1. In the present study responses to intravenous administration of nociceptin were investigated in the systemic vascular bed of the rat. Nociceptin induced dose-related decreases in systemic arterial pressure when injected in doses of 1-30 nmol/kg i.v. In terms of relative vasodepressor activity, nociceptin was approximately 10-fold more potent than the nitric oxide donor, DEA/NO, and 10-fold less potent than adrenomedullin. The duration of the vasodepressor response to nociceptin was shorter than adrenomedullin but longer than DEA/NO. These results show that nociceptin has significant vasodepressor activity in the rat.

Decreases in systemic arterial and hindquarters perfusion pressure in response to nociceptin are not inhibited by naloxone in the rat

Nociceptin, the endogenous ligand for the ORL1 receptor, has been shown to decrease systemic arterial and hindquarters perfusion pressures in the rat. The present study was undertaken to determine if decreases in systemic arterial and hindquarters perfusion pressures, in response to nociceptin, are mediated by a naloxone-sensitive mechanism. Injections of nociceptin decreased systemic arterial and hindquarters perfusion pressures in a dose-related manner. The decreases in systemic arterial and hindquarters perfusion pressure in response to nociceptin were not altered by the administration of naloxone in a dose of 2 mg/kg i.v. Met-enkephalin decreased systemic arterial and hindquarters perfusion pressures and responses to the opioid receptor agonist were significantly reduced by naloxone, whereas decreases in systemic arterial pressure in response to the nitric oxide donor, DEA/NO, were not altered. The results of the present study show that decreases in systemic arterial and hindquarters perfusion pressure in response to nociceptin are not mediated by a naloxone-sensitive mechanism in the rat.

Nociceptin, an endogenous ligand for the ORL1 receptor, has vasodilator activity in the hindquarters vascular bed of the rat

The heptadecapeptide nociceptin, also known as Orphanin FQ, is a newly discovered endogenous ligand for the G-protein coupled, opioid-like receptor ORL1. In the present study, responses to intra-arterial injections of nociceptin were investigated in the hindquarters vascular bed of the rat. Under constant-flow conditions nociceptin induced dose-dependent decreases in hindquarters perfusion pressure when injected in doses of 1-30 nmol into the hindquarters perfusion circuit. The decreases in hindquarters perfusion pressure were rapid in onset and pressure returned to control values over a 3-6 min period. In terms of relative vasodilator activity, nociceptin was approximately equipotent to the nitric oxide donor, DEA/NO, and 30-fold less potent than adrenomedullin. These data demonstrate that nociceptin has significant vasodilator activity in the hindquarters vascular bed of the rat.