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

Opioid activity profiles indicate similarities between the nociceptin/orphanin FQ and opioid receptors

Nociceptin (orphanin FQ) is the recently discovered peptide agonist for the orphan receptor opioid receptor-like 1 (ORL1). Despite the high sequence homology between ORL1 and the opioid receptors, most opioids lack affinity for the nociceptin receptor. The affinity and functional profile of opioids possessing activity at the nociceptin receptor was determined using [3H]nociceptin and nociceptin-stimulated [35S]GTPgammaS binding. The mu-opioid receptor-selective agonist lofentanil potently and competitively displaced [3H]nociceptin at rat brain receptors (IC(50) 62 nM). Lofentanil exhibited full agonism for enhancement of [35S]GTPgammaS binding to human recombinant ORL1 receptors (EC(50) 50 nM). The related piperidines ohmefentanyl and sufentanil and the nonselective opioid receptor agonist etorphine were less potent nociceptin receptor agonists. The kappa(1)+kappa(3)-opioid receptor agonist/mu-opioid receptor antagonist naloxone benzoylhydrazone was a pure antagonist at both rat brain and human ORL1 receptors. The nonselective opioid receptor partial agonist buprenorphine and the nonselective opioid receptor antagonist (-)-quadazocine exhibited pure antagonism at rat brain receptors, but displayed partial agonism at human ORL1 receptors. Thus, opioids displaying full agonism at the nociceptin receptor are also opioid receptor agonists, whereas opioids that are antagonists or partial agonists at the nociceptin receptor show antagonism or partial agonism at opioid receptors. In addition, the stereospecificity required at opioid receptors appears to be retained at the nociceptin receptor, since (+)-quadazocine is inactive at both receptors. These findings illustrate the structural and functional homology of the opioid recognition site on these two receptor classes and suggest that opioids may provide leads for the design of nonpeptide nociceptin receptor agonists and antagonists lacking affinity for the classical opioid receptors.

Fos expression in feeding-related brain areas following intracerebroventricular administration of orphanin FQ in rats

While the influence of orphanin FQ (OFQ) on the regulation of food intake has been substantiated, little is known about feeding-related brain regions that mediate OFQ-induced feeding. To further investigate this, we injected OFQ intracerebroventricularly and evaluated c-Fos immunoreactivity in brain areas thought to be involved in the regulation of food intake. Altered c-Fos expression as a consequence of OFQ injection was observed in the nucleus of the solitary tract, paraventricular nucleus of the hypothalamus, supraoptic nucleus, central nucleus of amygdala, lateral septal nucleus and lateral habenular nucleus. Presumably, OFQ modulates food ingestion through its action on these brain regions, most probably by activating feeding signals as well as suppressing satiety mechanisms.

Orphanin FQ/nociceptin inhibits morphine withdrawal

The influence of orphanin FQ/nociceptin (OFQ/N) on the morphine-withdrawal symptom was investigated. Withdrawal syndrome was induced in the morphine-dependent rats by an intraperitoneal (i.p.) injection of 2 mg/kg naloxone hydrochloride--an opioid receptors antagonist. Wet-dog shakes were used as a measure of the abstinence syndrome. Intraventricular injections of OFQ/N (5-20 microg/animal) caused significant inhibition of the withdrawal signs at doses between 15-20 microg, in the morphine-dependent rats. OFQ/N alone did not change behavior of the morphine-dependent animals. The obtained results indicate that OFQ/N can inhibit the morphine withdrawal symptoms induced by naloxone.

Supraspinal and spinal effects of [Phe1psi(CH2-NH)Gly2]-nociceptin(1-13)-NH2 on nociception in the rat

A new derivative of the neuropeptide nociceptin (NC) has recently been developed. This molecule, the pseudopeptide [Phe1psi(CH2-NH)Gly2]-nociceptin(1-13)-NH2 was found to antagonize NC inhibitory effects in peripheral smooth muscle preparations in vitro. However, contrasting results have appeared as regards its pharmacodynamic profile in the CNS. Here, we investigated the pseudopeptide effects, in vivo, on nociceptive responses in the rat. [Phe1psi(CH2-NH)Gly2]-nociceptin(1-13)-NH2 was administered intracerebroventricularly (i.c.v.) or intrathecally (i.t.) (alone or in combination with NC), and tail-flick latencies (TFL) to radiant heat were assessed. I.c.v. [Phe1psi(CH2-NH)Gly2]-nociceptin(1-13)-NH2 (1-10 nmol/rat) caused a short-lasting decrease (5 min) of TFL and did not antagonize the threshold lowering effect of i.c.v. NC (1 nmol/rat). At the spinal level, the i.t. administration (0.2-10 nmol/rat) of [Phe1psi(CH2-NH)Gly2]-nociceptin(1-13)-NH2 produced a dose-dependent and long-lasting antinociceptive effect that was not modified by the administration of a high dose (30 nmol/rat i.t.) of the opioid antagonist naloxone. The i.t. co-administration of the pseudopeptide (10 nmol/rat) did not block the antinociceptive effect of i.t. NC (10 nmol/rat). These data indicate that the pseudopeptide behaves as an NC agonist at supraspinal and spinal levels in the rat tail-flick test of nociception. These different profiles in the periphery and the CNS could suggest differences between central and peripheral NC receptor/s and provide a basis for further development of antagonist molecules suitable for their characterization.

Opiate modulating properties of nociceptin/orphanin FQ

The recently discovered peptide nociceptin/orphanin FQ (N/OFQ) and its receptor NOR share many structural similarities with the opioid peptides and their receptors. The anatomical distributions of N/OFQ and NOR are similar to those of opioid peptides and receptors. In addition, NOR and opiate receptors couple via the same G-proteins to similar effectors, such as Ca(2+) channels, K(+) channels, adenylyl cyclase, and several protein kinases. Thus, the behavioral effects of N/OFQ have been investigated in the context of known opiate effects, and a possible connection has been sought between the effects of these two homologous signaling systems. Originally characterized as a nociception-producing peptide, N/OFQ has now been shown to have diverse effects on nociception, as well as effects on many other behaviors. With regard to nociception, the peptide has been reported to produce hyperalgesia, reversal of opioid-mediated analgesia, analgesia, and allodynia. N/OFQ also has effects on other behaviors, such as locomotion, feeding, anxiety, spatial attention, reproductive behaviors, and opiate tolerance. The relationship between opiates and N/OFQ is strengthened by the fact that opiates also affect these behaviors. However, the exact nature of the relationship of N/OFQ with opiates-opiate-like versus antiopiate-remains controversial. This review will detail the diverse effects of N/OFQ and suggest that this peptide, like other putative antiopiate peptides, can be described as 'opiate modulating. '

Characterization of nociceptin/orphanin FQ-induced pain responses in conscious mice: neonatal capsaicin treatment and N-methyl-D-aspartate receptor GluRepsilon subunit knockout mice

Activation of primary afferent C fibers gives rise to spinal release of substance P and glutamate, and these mediators facilitate the cascade of nociceptive processing. We recently showed that intrathecal administration of nociceptin or orphanin FQ (hereafter called nociceptin) induced hyperalgesia to noxious thermal stimuli and allodynia to innocuous tactile stimuli applied to conscious mice. In the present study, we designed experiments to elucidate the pathways and mediators of nociceptin-evoked pain responses. Neonatal capsaicin treatment eliminated the induction of hyperalgesia and allodynia by nociceptin. Whereas this treatment markedly reduced the content of substance P in the spinal cord, it did not affect the nociceptin content or the expression of nociceptin receptors and GluRvarepsilon and GluRzeta subunits of N-methyl-D-aspartate receptors in it. The substance P antagonists CP96,345 and CP99,994 blocked the nociceptin-induced hyperalgesia, but not the allodynia. In contrast, the nociceptin-evoked allodynia, but not hyperalgesia, disappeared in N-methyl-D-aspartate receptor GluRvarepsilon1 subunit knockout mice. Both nociceptin-evoked hyperalgesia and allodynia were attenuated by morphine in a dose-dependent manner. Taken together, these results demonstrate that capsaicin-sensitive primary afferent fibers are involved not only in thermal hyperalgesia but also in tactile allodynia induced by nociceptin, but in different pathways; the former is mediated by substance P and the latter is mediated by glutamate through the N-methyl-D-aspartate receptor comprising the GluRvarepsilon1 subunit.

Identification of a high-affinity orphanin FQ/nociceptin(1-11) binding site in mouse brain

The presence of pairs of basic amino acids within the orphanin FQ/Nociceptin (OFQ/N) sequence has raised the possibility that truncated versions of the peptide might be physiologically important. OFQ/N(1-11) is pharmacologically active in mice, despite its poor affinity in binding assays (K(i) > 250 nM) for the OFQ/N receptor. Using an analog of OFQ/N(1-11), [(125)I][Tyr(10)]OFQ/N(1-11), we identified a high-affinity binding site (K(D) 234 pM; B(max) 43 fmol/mg protein) with a selectivity profile distinct from the OFQ/N receptor and all the traditional opioid receptors. This site had very high affinity for OFQ/N and its related peptides. The most striking differences between the new site and the OFQ/N receptor previously observed in brain were seen with traditional opioids. Dynorphin A analogs and alpha-neoendorphin competed with [(125)I][Tyr(10)]OFQ/N(1-11) binding in mouse brain with K(i) values below 10 nM, while naloxone benzoylhydrazone (K(i) 3.9 nM) labeled the [(125)I][Tyr(10)]OFQ/N(1-11) binding site as potently as many traditional opioid receptors. Several other opioids, including fentanyl, (-)cyclazocine, levallorphan, naltrindole, and diprenorphine, also displayed moderate affinities for this site. Finally, the [(125)I][Tyr(10)]OFQ/N(1-11) site had a unique regional distribution consistent with a distinct receptor. Thus, [(125)I][Tyr(10)]OFQ/N(1-11) labels a novel site in brain with a selectivity profile intermediate between that of either opioid or OFQ/N receptors.

[(125)I]orphanin FQ/nociceptin binding in Raji cells

Western blots using an antibody which recognizes the orphanin FQ/nociceptin (OFQ/N) receptor reveals a band at approximately 69 kD in several cell lines, including the Raji human B cell lymphoma cell line. RT-PCR confirms the presence of this receptor in the Raji cells. Binding studies revealed a high affinity [(125)I][Tyr(14)]OFQ/N site in the Raji cells. The affinity of [(125)I][Tyr(14)]OFQ/N in the Raji cells (K(D) 68.4 pM) was similar to that in the transfected receptor (K(D) 36.7 pM). Its selectivity profile also was quite similar. OFQ/N competed binding quite potently (K(i) 65 pM), as did [Tyr(14)]OFQ/N (K(i) 33 pM). Traditional opioids displayed no appreciable affinity for the binding at any concentration examined, with the exception of naloxone benzoylhydrazone, which had only a very modest affinity. The receptors in the Raji cells were functionally active. OFQ/N inhibited forskolin-stimulated cyclase by 72% with an IC(50) value of approximately 1 nM.

Nociceptin (1 - 7) antagonizes nociceptin-induced hyperalgesia in mice

Nociceptin and its N-terminal fragment, nociceptin (1 7), were administered intrathecally (i.t.) into conscious mice. Nociceptin (3.0 fmol) produced a significant reduction in the nociceptive thermal threshold (hyperalgesia) measured as the tail-flick and paw-withdrawal responses. Nociceptin (1-7), injected i.t., at 150-1200 fmol had no significant effect. However, when nociceptin (1-7) (150 1200 fmol) was injected simultaneously with nociceptin (3.0 fmol), nociceptin-induced hyperalgesia was significantly reduced. Analgesia induced by a high dose (1200 pmol) of nociceptin was not antagonized by co-administration of nociceptin (1-7) (1200 fmol). These results suggest that N-terminal fragments of nociceptin formed endogenously could modulate the hyperalgesic action of nociceptin in the spinal cord.

Involvement of tachykinin NK1 receptors in nociceptin-induced hyperalgesia in mice

Intrathecal (i.t.) injection of nociceptin at small doses (3.0 and 30.0 fmol) produced a significant hyperalgesic response as assayed by the tail-flick test. This hyperalgesic effect peaked at 15 min following i.t. administration of nociceptin (3.0 fmol) and returned to control level within 30 min. Hyperalgesia elicited by nociceptin was inhibited dose-dependently by i.t. co-administration of tachykinin NK1 receptor antagonists, CP-99,994 and sendide. A significant antagonistic effect of [D-Phe7, D-His9] substance P (6-11), a selective antagonist for substance P, was observed against the nociceptin-induced hyperalgesia. Pretreatment with i.t. substance P antiserum and i.t. capsaicin resulted in a complete block of the reduced threshold produced by nociceptin. The NK2 receptor antagonist, MEN-10,376 and pretreatment with neurokinin A antiserum did not alter the behavioural effect of nociceptin. The N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine (MK-801) and D(-)-2-amino-5-phosphonovaleric acid (D-APV), and L-NG-nitro arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, failed to inhibit nociceptin-induced hyperalgesia. The results obtained suggest that the hyperalgesic effect of nociceptin may be mediated through tachykinin NK1 receptors in the spinal cord.

Effects of orphanin FQ on endomorphin-1 induced analgesia

Orphanin FQ (also known as nociceptin) is a 17-amino-acid peptide which acts as a potent endogenous agonist of the orphan opioid receptor-like (ORL1) receptor. Endomorphin-1, a 4-amino-acid peptide discovered recently, is a potent and selective endogenous agonist for the mu-opiate receptor. In the present study, the effect of OFQ or/and endomorphin-1 on the response to noxious thermal stimuli was observed using the tail-flick test in rats. Intracerebroventricular (i.c.v.) administration of OFQ (1, 5 microg) could shorten tail-flick latency; In contrast, intrathecal (i.t.) administration of OFQ (1, 2 or 10 microg) could increase the latency; i.c.v. (1, 2, 5 microg) or i.t. (0.2, 2, 5 microg) administration of endomorphin-1 dose-dependently increased the latency, indicating an analgesic effect. Furthermore, OFQ (0.1-5 microg) when intraventricularly injected together with endomorphin-1 (5 microg), could dose-dependently reverse the analgesia induced by the latter. On the contrary, OFQ (1 microg) intrathecally injected together with endomorphin-1 (0.2 microg) could further increase the tail-flick latency. The results showed that OFQ at the supraspinal level produces hyperalgesia and is antagonistic to endomorphin-1, while at the spinal level it produces analgesia and is synergic with endomorphin-1. Different interaction mechanism between OFQ and endomorphin-1 in the brain and the spinal cord is thus suggested.

Supraspinal hyperalgesia and spinal analgesia by [Phe1psi(CH2-NH)Gly2]nociceptin-(1-13)-NH2 in rat

[Phe1psi(CH2-NH)Gly2]nociceptin-(1-13)-NH2, a pseudopeptide analog of nociceptin, was originally seen as an antagonist of nociceptin receptors. In the present study, it was observed that intracerebroventricular (i.c.v.) injection of this pseudopeptide (1, 5, 10 microg) significantly decreased the tail-flick latency of rats, indicating a hyperalgesic effect, while intrathecal (i.t.) injection of it (1, 2.5, 10 microg) dramatically increased the tail-flick latency, indicating an analgesic effect. This strengthened the in vivo evidence that [Phe1psi(CH2NH)Gly2]nociceptin-(1-13)-NH2 might be an agonist of nociceptin receptors.

Distribution of the nociceptin and nocistatin precursor transcript in the mouse central nervous system

The distribution of prepronociceptin messenger RNA, the recently identified endogenous ligand of the ORL1 receptor (opioid receptor-like-1), has been studied in the adult mouse central nervous system using in situ hybridization. Prepronociceptin is a new peptide precursor that generates, upon maturation, at least three bioactive peptides: nociceptin, noc2 and the recently described nocistatin. Considering both the density of labeled neurons per region and their intensity of labeling, the distribution of prepronociceptin messenger RNA-containing neurons can be summarized as follows: the highest level of prepronociceptin messenger RNA expression was detected in the septohippocampal nucleus, bed nucleus of the stria terminalis, central amygdaloid nucleus, and in selective thalamic nuclei such as the parafascicular, reticular, ventral lateral geniculate and zona incerta. High to moderate levels of prepronociceptin messenger RNA expression were detected in the lateral, ventral and medial septum, and were evident in brainstem structures implicated in descending antinociceptive pathways (e.g., the gigantocellular nucleus, raphe magnus nucleus, periaqueductal gray matter), and also observed in association with auditory relay nuclei such as the inferior colliculi, lateral lemniscus nucleus, medioventral preolivary nucleus and lateral superior nucleus. A moderate level of prepronociceptin messenger RNA expression was observed in the medial preoptic nucleus, ventromedial preoptic nucleus, periventricular nucleus, pedonculopontine tegmental nucleus, solitary tract nucleus and spinal trigeminal nucleus. A weak level of prepronociceptin messenger RNA expression was present in some areas, such as the cerebral cortex, endopiriform cortex, hippocampal formation, medial amygdaloid nucleus, anterior hypothalamic area, medial mammillary hypothalamic nuclei, retrorubral field and substantia nigra pars compacta. No labeled cells could be found in the caudate-putamen, nucleus accumbens and ventral tegmental area. The present data confirm that nociceptin is expressed in a broad array of regions of the central nervous system. In good correlation with the presently known physiological actions of nociceptin, they include, amongst others, brain areas conveying/integrating pain and auditory sensory afferences.

Bv8, a small protein from frog skin and its homologue from snake venom induce hyperalgesia in rats

From skin secretions of Bombina variegata and Bombina bombina, we isolated a small protein termed Bv8. The sequence of its 77 amino acids was established by peptide analysis and by cDNA cloning of the Bv8 precursor. Bv8 stimulates the contraction of the guinea-pig ileum at nanomolar concentrations. The contraction is not inhibited by a variety of antagonists. Injection of a few micrograms of Bv8 into the brain of rats elicits, as assessed by the tail-flick test and paw pressure threshold, a marked hyperalgesia which lasts for about 1 h. Bv8 is related to protein A, a component of the venom of the black mamba. After i.c.v. injection, protein A is even more active than Bv8 in inducing hyperalgesia.

Accelerated release and production of orphanin FQ in brain of chronic morphine tolerant rats

Orphanin FQ has been shown to possess anti-opioid activity at supraspinal level. Our previous work revealed that chronic morphine tolerance could be reversed by intracerebroventricular (i.c.v.) injection of OFQ IgG to rats. In this study, we used radioimmunoassay (RIA) to assess the changes of Orphanin FQ immunoreactivity (OFQ-ir) in cerebroventricular perfusate, periaqueductal gray (PAG) and amygdala of rats made tolerance to morphine (10-60 mg/kg, s.c., t.i.d., for 5 days). The results indicated that: (1) In rats administrated with morphine for 3 and 5 days, the content of OFQ-ir in cerebroventricular perfusate increased by 25% and 52% over the NS control group. (2) The content of OFQ-ir in PAG of rats receiving 1d, 3d and 5d injections of morphine showed an increase of 17%, 48% and 81% respectively over NS group. (3) The content of OFQ-ir in amygdala of rats given 3d and 5d of morphine showed a 36% and 55% increase compared with corresponding control group. It is suggested that continuous use of high doses of morphine accelerated the release and biosynthesis of OFQ in rat brain to antagonize the effect of opioids, which may play a role in the development of morphine tolerance, and that brain OFQ may serve as a delayed negative feedback control on opioid analgesia.

Nociceptin/orphanin FQ: role in nociceptive information processing

Recently, opioid receptor like1 (ORL1) receptor was identified. The ORL1 receptor is a G protein coupled receptor and the sequence of the ORL1 receptor is closely related to that of the opioid receptors. Nociceptin/orphanin FQ has been identified as a potent endogenous agonist of the ORL1 receptor and the sequence of nociceptin/orphanin FQ is closely related to that of dynorphin A. Nociceptin/orphanin FQis not active at the classical opioid receptors, such as mu, kappa and delta receptors. The distribution of prepronociceptin mRNA is distinct from that of the opioid peptide precursor. Mice lacking the ORL1 receptor showed no significant differences in nociceptive threshold compared with wild mice. The role of nociceptin/orphanin FQ on nociceptive transmission is unclear. Intracerebroventricular (i.c.v.) injection of nociceptin/orphanin FQ produced hyperalgesia and allodynia and antagonized morphine analgesia. On the other hand, intrathecal injection of low dose nociceptin/orphanin FQ produces allodynia, but high dose of nociceptin/orphanin FQ produces an analgesic effect. Although we do not fully understand the mechanisms that produce the difference between the effect of i.c.v. injection of nociceptin/orphanin FQ and that of intrathecal injection of nociceptin/orphanin FQ, we believe that spinal ORL1 receptor may be the next receptor which should be targeted by drugs designed for the treatment of pain.

In vivo molecular signal transduction of peripheral mechanisms of pain

Although we have obtained a number of pharmacological tools and mutant mice lacking specific genes related to the pain, the distinct molecular basis of the pain-producing mechanism has remained to be fully clarified since we have been using conventional paradigms of the nociception test that may drive multiple endogenous molecules affecting nociception at the same time. Here, I will introduce a new paradigm of the nociception test. In this test, we focused on polymodal C-fibers by measuring nociceptive flexor responses induced by the peripheral application of a single species of nociceptive molecule. In addition, we identified the site of drug actions on nociceptor endings by the fact that the nociception was abolished by the intrathecal pretreatment with antisense oligodeoxynucleotide for receptors. Throughout experiments using this paradigm of the nociception test, it was firstly revealed that substance P, a major neurotransmitter of polymodal C-fibers, directly stimulates nociceptor endings through activation of Gq/11 and phospholipase C, followed by Ca2+ influx through plasma membrane-bound inositol trisphosphate receptors, and that bradykinin and histamine, both endogenous representative pain-producing substances, share this mechanism. Another unique mechanism is through Gi-coupled receptors such as receptors for nociceptin (orphanin FQ) or kyotorphin (tyrosine-arginine). The latter mechanism was found to be mediated through a substance P release from nociceptor endings. Future studies including some modifications of this paradigm should be also clinically useful for neuropathic pain research as well as understanding of pain physiology.

Strain-dependent effects of supraspinal orphanin FQ/nociceptin on thermal nociceptive sensitivity in mice

Despite much study since its discovery in 1995, the effects of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the 'orphan' opioid receptor, on nociceptive sensitivity remain unclear. Different laboratories have variously reported hyperalgesic, analgesic, anti-analgesic or no effect of the peptide on thermal assays following supraspinal injection in rodents. We and others have argued previously that methodological inconsistencies and experimental parameters may explain some of the contradictions in the literature, especially in mice where intracerebroventricular (i.c.v.) injections proceed directly through the skull. We report presently that both the magnitude of stress-induced analgesia (SIA) produced by such i.c.v. injections, and the ability of OFQ/N to antagonize this opioid-mediated SIA, are strain-dependent. Specifically, significant injection-related SIA was observed in four of six strains studied (outbred: CD-1, SW; inbred: AKR/J, BALB/cJ, C3H/HeJ, CBA/J) on the 47.5 degrees C tail-withdrawal assay, and OFQ/N blocked this SIA in two strains. These data suggest that genetic variability among subject populations may underlie the inconsistent findings among researchers, and may in addition provide a promising avenue for future study of this novel neuromodulator.

Orphanin FQ (nociceptin) modulates responses of trigeminal neurons evoked by excitatory amino acids and somatosensory stimuli, and blocks the substance P-induced facilitation of N-methyl-D-aspartate-evoked responses

The present investigation details the modulation of medullary dorsal horn neuron responses to excitatory amino acids and peripheral cutaneous stimuli by orphanin FQ (nociceptin), an endogenous ligand for the opioid receptor-like, receptor. Effects of orphanin FQ, administered microiontophoretically or given intracerebroventricularly, were tested on the responses of nociceptive-specific, wide dynamic range and low threshold neurons recorded in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in anesthetized (urethane or pentobarbital) male rats. Microiontophoretic application of orphanin FQ reduced the N-methyl-D-aspartate-evoked responses in 86% (71/82) of neurons, and the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-evoked responses in 86% (30/35) of neurons. However, orphanin FQ produced a longer lasting inhibitory effect on the N-methyl-D-aspartate-evoked responses relative to the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-evoked responses. The inhibitory effect of orphanin FQ was not modality-specific, responses evoked by noxious as well as non-noxious stimuli were reduced in 22/23 neurons. However, the inhibitory effect was more pronounced on noxious stimulus-evoked responses. Naloxone applied at currents that antagonized the inhibitory effects of selective agonists at mu and kappa opioid receptors failed to inhibit the effects of orphanin FQ. Microiontophoretic co-application of substance P with N-methyl-D-aspartate facilitated the N-methyl-D-aspartate-evoked responses in 52% (26/50) of nociceptive neurons. Orphanin FQ blocked or reduced the substance P-induced facilitation by 86+/-24.4% (n = 14). In order to compare electrophysiological data with previous behavioral observations, effects of orphanin FQ administered intracerebroventricularly were tested on the excitatory amino acid-evoked responses. Orphanin FQ reduced the N-methyl-D-aspartate-evoked responses in 85% (11/13) of neurons whereas the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-evoked responses were facilitated in 69% (9/13) of neurons. We suggest that orphanin FQ produces a predominantly inhibitory effect on, (i) noxious stimuli evoked responses, (ii) excitatory amino acid receptor-mediated transmission and, (iii) the substance P-induced facilitation of the N-methyl-D-aspartate-evoked responses. We conclude that orphanin FQ primarily produced an antinociceptive action at the level of the dorsal horn of the medulla.

Nociceptin/orphanin FQ increases blood pressure and heart rate via sympathetic activation in sheep

This study was undertaken to examine the cardiovascular effects of nociceptin/Orphanin FQ (OFQ). Nociceptin/OFQ (10-300 nmol/kg, IV) stimulates an increase in mean blood pressure (MBP) and heart rate (HR) in chronically catheterized sheep. Pretreatment with phenoxybenzamine (5 mg/kg) attenuated the pressor response, consistent with sympathetically mediated vasoconstriction. Furthermore, the lack of a reflex bradycardia suggests either blunting of the baroreflex by nociceptin/OFQ or direct beta-adrenergic activation. The bradycardic response to norepinephrine (0.6 microg/kg, IV) remained intact after nociceptin/OFQ administration, demonstrating that nociceptin/OFQ does not blunt the baroreflex. Additionally, the increase in HR was completely reversed by pretreatment with propranolol. These data suggest that nociceptin/OFQ plays a role in cardiovascular regulation via sympathetic activation.

Peripheral orphanin FQ/nociceptin analgesia in the mouse

Orphanin FQ/Nociceptin (OFQ/N) administered peripherally was an effective analgesic in the tailflick test in mice (ED50 16.3 microg). It had a peak effect at 5 min and lasted up to 30 min. The kappa3 analgesic naloxone benzoylhydrazone was also active peripherally (ED50 3.8 microg). The analgesic actions of both agents were blocked by naloxone. Neither OFQ/N(1-11) nor OFQ/N(1-7) had appreciable peripheral activity. Antisense mapping both compounds against the murine orphan opioid receptor (KOR-3) confirmed the importance of this clone in their actions. Antisense probes targeting the second and third coding exons significantly lowered the analgesic effects of both compounds. However, the antisense targeting the first coding exon blocked only the actions of OFQ/N and not kappa3 analgesia.

Orphan receptors and the concept of reverse physiology: discovery of the novel neuropeptide orphanin FQ/nociceptin

The cloning of numerous orphan members from the supergene family of G protein-coupled receptors implies the existence of many as yet undiscovered neurotransmitters and neuropeptides. Recently, new technologies were developed to isolate natural ligands for orphan receptors, using the receptor as a biological sensor during the purification process. This manuscript will present the concept and technology of an approach which starts from a cloned receptor to ultimately describe the physiological functions of the transmitter system. This strategy inverts the classical order of biomedical research and was thus termed "reverse physiology". The first natural ligand isolated by this strategy is a peptide with significant similarity to the opioid peptides and has been named orphanin FQ or nociceptin (OFQ/NOC). Evidence for characterizing OFQ/NOC as a genuine neuropeptide will be reviewed. OFQ/NOC is biosynthetically derived from a larger precursor protein which may encode additional bioactive peptides. Since its discovery, a large number of studies have described numerous physiological functions of OFQ/NOC. Because of its relation to the opioid system, much attention has been focused on the involvement of OFQ/NOC in nociception, sometimes with controversial results. However, the pharmacological profile of the OFQ/NOC system suggests a clear separation from the opioids. The discovery of OFQ/NOC and the subsequent analyses of its physiological functions is an example which has already been followed by the identification of two other novel neuropeptides. The orphan receptor strategy holds a lot of promises for the postgenomic era, helping to fill the vast amount of sequence data with life.

Orphanin FQ stimulates prolactin and growth hormone release in male and female rats

Intracerebroventricular administration of Orphanin FQ (5.5, 55 or 550 pmol) caused a dose-related increase in prolactin secretion in both male and female rats and stimulated GH secretion in males. The magnitude of the prolactin secretory response was greater in females than in males. These effects of OFQ on prolactin and growth hormone release are the same as the stimulatory effects of the endogenous opioid peptides.

ORL-1 and mu opioid receptor antisera label different fibers in areas involved in pain processing

Mu opioid receptors (MOR) mediate the analgesic effects of opioid drugs such as morphine. The opioid receptor-like (ORL-1) receptor is structurally related to opioid receptors and the ORL-1 receptor agonist, orphanin FQ/nociceptin, induces analgesia at the spinal level, but appears to recruit different circuitry than that used by mu opioids. When administered intracerebroventricularly, orphanin FQ/nociceptin produces hyperalgesia and/or reverses opioid analgesia. The functionally distinct actions elicited by MOR and ORL-1 receptors, which activate similar intracellular signaling systems and show similar regional distributions, could be explained by their differential cellular localization. By using double label immunohistochemistry and confocal microscopy, the present study investigates the distribution of MOR and ORL-1 receptors in regions of the rat nervous system that are involved with nociceptive processing. In general co-localization of MOR and ORL-1 receptor immunoreactivity was not observed in either perikarya or neuropil in the dorsal root ganglia, nor in the Lissauer's tract and superficial laminae of the spinal cord. Likewise, there was no evidence for co-localization of these receptors within the periaqueductal gray, the nucleus raphe magnus, the gigantocellular reticular nucleus, and the nucleus of the solitary tract. These observations indicate that MOR and ORL-1 receptors are expressed predominantly on different fiber systems in these regions. This differential distribution is consistent with the distinct pharmacology of ORL-1 and MOR receptor agonists and suggests that the antisera to MOR and ORL-1 receptors may provide useful markers for further investigations of analgesic and counteranalgesic pathways modulating pain perception.

Identification and differential regional expression of KOR-3/ORL-1 gene splice variants in mouse brain

KOR-3, also known as ORL-1, is a member of the opioid receptor family, encoding the murine receptor for orphanin FQ/nociceptin. In the current studies we have identified five different splice variants of KOR-3 in mouse brain, three of which have not been previously reported. In addition to variants with a 15 bp deletion at the 3'-end of the first coding exon (KOR-3d) and an 81 bp insertion between the second and third coding exons (KOR-3e), three new variants with insertions of 34 (KOR-3a), 98 (KOR-3b), and 139 bp (KOR-3c) between the first and second coding exons have been obtained. The expression of the three variants in mouse brain varies markedly among brain regions with a distribution which is quite distinct from KOR-3 itself. Of greatest interest was the presence of high levels of KOR-3a in the striatum, a region with no demonstrable KOR-3, and in the cortex. KOR-3c was seen in the periaqueductal gray and hypothalamus, regions where KOR-3 predominated. The brainstem had similar levels of KOR-3, KOR-3a, and KOR-3d. In contrast, KOR-3d was most prominent in the cerebellum. KOR-3b levels were very low throughout.

Pharmacological characterization of the nociceptin receptor mediating hyperalgesia in the mouse tail withdrawal assay

1. The newly discovered neuropeptide nociceptin (NC) has recently been reported to be the endogenous ligand of the opioid-like orphan receptor. Despite its structural similarity to opioids, when injected intracerebroventricularly (i.c.v.) in the mouse, NC exerts a direct hyperalgesic effect and reverses opioid-induced analgesia. In the present investigation, these two effects of NC were evaluated under the same experimental conditions; in addition, a pharmacological characterization of the receptor mediating these central effects of NC was attempted. 2. NC caused a dose dependent (0.1-10 nmol/mouse), naloxone-insensitive reduction of tail withdrawal latency with a maximal effect of about 50% of the reaction time observed in saline injected mice. In the same range of doses, NC inhibited morphine (1 nmol/mouse) induced analgesia. 3. The effects of the natural peptide were mimicked by NCNH2 and NC(1-13)NH2 (all tested at 1 nmol/mouse) while 1 nmol NC(1-9)NH2 was found to be inactive either in reducing tail withdrawal latency or in preventing morphine analgesia. 4. [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 ([F/G]NC(1-13)NH2), which has been shown to antagonize NC effects in the mouse vas deferens, acted as an agonist, mimicking NC effects in both the experimental paradigms. In addition, when NC and [F/G]NC(1-13)NH2 were given together, their effects were additive. 5. These results demonstrate that both the direct hyperalgesic action and the anti-morphine effect of NC can be studied under the same experimental conditions in the mouse tail withdrawal assay. Moreover, the pharmacological characterization of the NC functional site responsible for these actions compared with the peripherally active site, indicates the existence of important differences between peripheral and central NC receptors.

Nociceptin/orphanin FQ-induced nociceptive responses through substance P release from peripheral nerve endings in mice

We have studied the in vivo signaling mechanisms involved in nociceptin/orphanin FQ (Noci)-induced pain responses by using a flexor-reflex paradigm. Noci was 10,000 times more potent than substance P (SP) in eliciting flexor responses after intraplantar injection into the hind limb of mice, but the action of Noci seems to be mediated by SP. Mice pretreated with an NK1 tachykinin receptor antagonist or capsaicin, or mice with a targeted disruption of the tachykinin 1 gene no longer respond to Noci. The action of Noci appears to be mediated by the Noci receptor, a pertussis toxin-sensitive G protein-coupled receptor that stimulates inositol trisphosphate receptor and Ca2+ influx. These findings suggest that Noci indirectly stimulates nerve endings of nociceptive primary afferent neurons through a local SP release.

Anti-nociceptive responses produced by human putative counterpart of nocistatin

b-nocistatin is a heptadecapeptide produced from bovine prepronociceptin and blocks the induction of hyperalgesia and touch-evoked pain (allodynia) by intrathecal administration of nociceptin or prostaglandin E2 (PGE2). Human prepronociceptin may generate a 30-amino acid peptide different in length from b-nocistatin. Here, we examine whether the human putative counterpart of nocistatin (h-nocistatin) possessed the same biological activities as b-nocistatin. Simultaneous intrathecal injection of h-nocistatin in mice blocked the induction of allodynia by nociceptin and PGE2 in a dose-dependent manner with ID50 values of 329 pg kg(-1) and 16.6 ng kg(-1), respectively. h-nocistatin was about 10 times less potent than b-nocistatin. h-nocistatin also attenuated the nociceptin- and PGE2-induced hyperalgesia. These results demonstrate that h-nocistatin is biologically active and may be involved in the processing of pain at the spinal level in humans.

Nociceptin receptor-mediated Ca2+ channel inhibition and its desensitization in NG108-15 cells

It has been shown that the membrane of hybrid NG108-15 neuroblastoma x glioma cells contains a high-affinity binding site for nociceptin. In the present study, we first demonstrated the expression of nociceptin receptor mRNA in NG108-15 cells. Application of nociceptin to NG108-15 cells produced a concentration-dependent (EC50 = 29 nM) inhibition of Ca2+ channel currents in a pertussis toxin-sensitive fashion. This nociceptin-induced inhibition of Ca2+ channel currents was prevented in the presence of omega-conotoxin GVIA, a blocker of the N-type Ca2+ channel, and had both voltage-dependent and -independent components. Prolonged application of nociceptin elicited homologous desensitization of the inhibition with a time constant of 5.3 min. These results indicate that the nociceptin receptor is coupled to the N-type Ca2+ channel via pertussis toxin-sensitive G proteins in NG108-15 cells and that this coupling is associated with rapid and homologous desensitization.

[Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2, a proposed antagonist of the nociceptin receptor, is a potent and stable agonist in the rat spinal cord

[Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 is an nociceptin analogue which has been shown to be a selective antagonist of the nociceptin receptor in peripheral tissues. We now report that intrathecal [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 produced a dose-dependent depression of the nociceptive flexor reflex in rats, an effect that is similar to nociceptin. The duration of depression produced by [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 was significantly more prolonged than by nociceptin. The reflex depressive effect of nociceptin was not blocked by [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2. The results indicated that the proposed nociceptin receptor antagonist [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 is a potent agonist in rat spinal cord and more resistant to enzymatic degradation compared to nociceptin.

Inhibition of rat oxytocin and vasopressin supraoptic nucleus neurons by nociceptin in vitro

The effects of nociceptin (orphanin FQ) on the excitability of electrophysiologically-identified oxytocin and vasopressin neurons were investigated in rat hypothalamic supraoptic nucleus slices in vitro, using whole-cell patch-clamp recording techniques. Nociceptin inhibited the spontaneous discharge of 9/20 (45%) of supraoptic nucleus neurons tested, while in the remaining 11/20 neurons it inhibited firing rate and induced repetitive burst-firing. There were no differences between the effects of nociceptin on oxytocin and vasopressin neurons. When recordings were made using EGTA-containing patch pipettes, nociceptin caused inhibition in all 30 supraoptic nucleus neurons tested, and burst-firing was not seen. The inhibitory effects of nociceptin persisted in low Ca, Co medium, and were not antagonized by naloxone at concentrations sufficient to antagonize the inhibitory actions of morphine and U50488. The actions of nociceptin on supraoptic nucleus neurons are therefore likely to be mediated by postsynaptic opioid receptor-like (ORL1) receptors that are distinct from known opioid receptors. The inhibitory responses to nociceptin were also insensitive to naloxone benzoylhydrazone, which itself had no effect on the spontaneous discharge of the supraoptic nucleus neurons. Our findings demonstrate that endogenous nociceptin may have a functional role in regulating oxytocin and vasopressin secretion through its actions on hypothalamic supraoptic nucleus neurons.

Orphanin FQ/nociceptin: a role in pain and analgesia, but so much more

The publication of the delta opioid receptor sequence led to the cloning of three homologous receptors: the mu and kappa opioid receptors, and a novel opioid-like orphan receptor. The orphan receptor's endogenous ligand, a 17-amino-acid peptide that resembles dynorphin, was named 'orphanin FQ' and 'nociceptin' (OFQ/N1-17). The OFQ/N1-17 receptor is expressed widely in the nervous system, and it is becoming clear that the peptide is likely to participate in a broad range of physiological and behavioral functions. At the cellular level, OFQ/N1-17 has much in common with the classical opioids; however, functional studies are now revealing distinct actions of this peptide. Identified only two years ago, OFQ/N1-17 has already attracted a great deal of attention. The number and diversity of papers focused on OFQ/N1-17 at the recent meeting of the Society for Neuroscience augur an exciting future for this new peptide.

Catecholaminergic CATH.a cells express predominantly delta-opioid receptors

CATH.a cells are a catecholaminergic cell line of neuronal origin. The opioid receptor complement expressed by CATH.a cells was defined pharmacologically and by reverse transcription-polymerase chain reaction (RT-PCR). CATH.a cells were found to express mRNA encoding all three of the major subtypes of opioid receptors. The relative abundance of CATH.a cell opioid receptor transcripts was delta > kappa> mu. Pharmacological and functional data were in agreement with the results of RT-PCR inasmuch as delta-opioid receptor was identified as the most abundant opioid receptor subtype expressed by CATH.a cells. In addition, at least one of the opioid signalling pathways, inhibition of adenylyl cyclase activity, was found to be operant in this cell line. CATH.a cells should be of general utility for the study of opioid receptor signalling mechanisms in the context of catecholaminergic neurons.

Endogenous orphanin FQ: evidence for a role in the modulation of electroacupuncture analgesia and the development of tolerance to analgesia produced by morphine and electroacupuncture

1. Our previous work has demonstrated that exogenously administered orphanin FQ (OFQ) antagonizes morphine analgesia and electroacupuncture analgesia (EAA) in the brain and potentiates morphine analgesia and EAA in the spinal cord of the rat. In the present study we evaluated the role of endogenously released OFQ in the development of tolerance to morphine and electroacupuncture (EA) and the analgesia produced by electroacupuncture, by use of the IgG fraction of an anti-OFQ antibody (OFQ-Ab) microinjected into the rat central nervous system (CNS). 2. EAA was produced by stimulating rats at a frequency of 100 Hz. Rats were classified as either high responders (HR) or low responders (LR) based on the analgesic effects of EA. LRs could be converted into HRs by the intracerebroventricular (i.c.v.) microinjection of OFQ-Ab at both 1:1 and 1:10 dilutions but not 1:100. HRs could be changed into LRs by the intrathecal (i.t.) injection of OFQ-Ab at both 1:1 and 1:10 dilutions, but not 1:100. 3. Acute morphine tolerance was induced in rats by repeated subcutaneous (s.c.) injections of morphine (5 mg kg, every 2 h) for 16 h. When injected i.c.v. the OFQ-Ab (1:1 dilution) had no effect on the development of acute morphine tolerance. 4. Chronic morphine tolerance was produced in rats by repeated injection of morphine (5-60 mg kg, s.c., 3 x a day) for 6 days. I.c.v. injection of OFQ-Ab (1:1 dilution) reversed this type of morphine tolerance in rats by 50% (P < 0.01). 5. Acute tolerance to the analgesia produced by EA developed after 6 h of continuous (100 Hz, 3 mA) stimulation. This tolerance was almost completely reversed by the i.c.v. injection of OFQ-Ab (1:1 dilution) (P < 0.05). 6. Chronic tolerance to the analgesic effect of EA was produced by repeatedly administering increasing current (1, 2 and 3 mA, each lasting for 10 min, for a total of 30 min) at a frequency of 100 Hz once a day for 6 days. I.c.v. injection of OFQ-Ab (1:1 dilution) reversed this kind of tolerance by 50% (P < 0.01). 7. Together these results suggest that 100 Hz EA may enhance the release of endogenous OFQ in the CNS of the rat, which in turn may act to antagonize EA-produced analgesia in the brain but potentiate EA produced analgesia in the spinal cord. Therefore, OFQ appears to play an important role in the development of tolerance to the analgesic effects produced by EA. 8. The mechanisms underlying the development of acute morphine tolerance and chronic morphine tolerance appear to be different. Central OFQ may play an important role in the development of tolerance after chronic morphine administration.

Antinociceptive analogs of orphanin FQ/nociceptin(1-11)

The presence of pairs of basic amino acids within the sequence of orphanin FQ/nociceptin (OFQ/N) peptide, the endogenous ligand for the ORL1/KOR-3 receptor, has raised the possibility that processing might generate pharmacologically important truncated peptides, including OFQ/N(1-11). OFQ/N(1-11) is pharmacologically active in vivo with a potency comparable to OFQ/N. Several tyrosine-containing analogs of OFQ/N(1-11) have been synthesized and examined for antinociceptive activity. Like OFQ/N(1-11), [Tyr1]OFQ/N(1-11), [Tyr10]OFQ/N(1-11) and [IodoTyr10]OFQ/N(1-11) given supraspinally in mice were antinociceptive in the tailflick assay in mice. The tyrosine analogs showed similar potencies as OFQ/N(1-11) but longer durations of action. This response was readily reversed by the opioid antagonist naloxone despite poor affinities for these analogs at opioid receptors. Another compound, [Tyr11]OFQ/N(1-11) was highly epileptogenic, inducing naloxone-sensitive seizures in greater than 50% of the mice tested at doses comparable to those examined with the other analogs. These results indicate that it is possible to make analgesic OFQ/N(1-11) analogs. The activity of [IodoTyr10]OFQ/N(1-11) suggests that it may prove useful as a radioligand in exploring potential OFQ/N(1-11) binding sites.

Opiate tolerance and dependence: receptors, G-proteins, and antiopiates

Despite the existence of a large body of information on the subject, the mechanisms of opiate tolerance and dependence are not yet fully understood. Although the traditional mechanisms of receptor down-regulation and desensitization seem to play a role, they cannot entirely explain the phenomena of tolerance and dependence. Therefore, other mechanisms, such as the presence of antiopiate systems and the coupling of opiate receptors to alternative G-proteins, should be considered. A further complication of studies of opiate tolerance and dependence is the multiplicity of endogenous opiate receptors and peptides. This review will focus on the endogenous opioid system--peptides, receptors, and coupling of receptors to intracellular signaling via G-proteins--in the context of their roles in tolerance and dependence. Opioid peptides include the recently discovered endomorphins and those encoded by three known genes--pro-opiomelanocortin, pro-enkephalin, and pro-dynorphin. They bind to three types of receptors--mu, delta, and kappa. Each of the receptor types is further divided into multiple subtypes. These receptors are widely known to be coupled to G-proteins of the Gi and Go subtypes, but an increasing body of results suggests coupling to other G-proteins, such as Gs. The coupling of opiate receptors to Gs, in particular, has implications for tolerance and dependence. Alterations at the receptor and transduction level have been the focus of many studies of opiate tolerance and dependence. In these studies, both receptor down-regulation and desensitization have been demonstrated in vivo and in vitro. Receptor down-regulation has been more easily observed in vitro, especially in response to morphine, a phenomenon which suggests that some factor which is missing in vitro prevents receptors from down-regulating in vivo and may play a critical role in tolerance and dependence. We suggest that antiopiate peptides may operate in vivo in this capacity, and we outline the evidence for the antiopiate properties of three peptides: neuropeptide FF, orphanin FQ/nociceptin, and Tyr-W-MIF-1. In addition, we provide new results suggesting that Tyr-W-MIF-1 may act as an antiopiate at the cellular level by inhibiting basal G-protein activation, in contrast to the activation of G-proteins by opiate agonists.

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.

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.

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.

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.