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

Characteristics of electroacupuncture-induced analgesia in mice: variation with strain, frequency, intensity and opioid involvement

The present study was conducted to evaluate the characteristics of electroacupuncture (EA)-induced analgesia in mice. Three inbred strains of mice (DBA/2, C57BL/6J, BALB/c) and three outbred strains (ICR, LACA, NIH) were used in the experiment. Two pairs of metallic needles were inserted into acupoints ST 36 and SP 6 connected to an electric pulse generator. EA parameters were set as constant current output with alteration of a positive and negative square wave, 0.6 ms in pulse width for 2 Hz and 0.3 ms for 100 Hz. Tail-flick latencies evoked by radiant heat were measured before, during and after EA stimulation. We found that (1) DBA/2 mice showed a significantly more potent analgesic effect than the other five strains in response to both 100 and 2 Hz EA. In this case, the intensities were 1.0-2.0-2.0 mA, 10 min for each intensity totally 30 min. (2) EA analgesia increased as the intensity of stimulation increased from 0.5 to 2.0 mA, but it remained at this plateau when the intensity further increased from 2.0 to 3.0 mA. (3) 10.0 mg x kg(-1) naloxone was needed to block the analgesic effect induced by 2 Hz EA of 2.0 mA, but to block that by 100 Hz, 25.0 mg x kg(-1) was necessary. (4) A positive correlation was observed between analgesia induced by morphine at the dose of 5.0 mg x kg(-1) and by 100 Hz EA in two tested strains DBA/2 and C57BL/6J. In conclusion, EA induces reliable, strain-dependent analgesia in mice. The naloxone-reversibility of EA, a measure of whether it is opioid or non-opioid mediated, is dependent upon intensity and frequency.

Endocrine effects of centrally injected nociceptin in the rat

In the present study we investigated the mechanisms involved in the endocrine effect of nociceptin/orphanin FQ (OFQ) in the rat and the possible interaction between OFQ and morphine in the control of growth hormone (GH) secretion. The intracerebroventricular administration of OFQ (2.3 or 23 microg/rat, i.c.v.) in freely moving male rats caused an increase in the secretion of both GH and prolactin (PRL). The possible involvement of the catecholaminergic (CA) system was studied by administering OFQ to CA-depleted rats (rats given 200 mg/kg of alpha-methyl-p-tyrosine subcutaneously 2 h before the i.c.v. dose of OFQ). In these CA-depleted rats, administration of OFQ (23 microg/rat, i.c.v.) did not stimulate GH secretion, whereas it significantly enhanced PRL secretion. In rats anesthetized with ketamine, which induces a significant increase of GH, PRL and corticosterone secretion by activating the sympathetic tone, OFQ (23 microg/rat, i.c.v.) did not modify GH and corticosterone levels, whereas again it significantly potentiated PRL secretion. Overall these results indicate that CA system is involved in the stimulatory action of OFQ on GH but not on PRL secretion. In fact the stimulation of PRL, but not that of GH, was still evident after impairment of the CA system. Pretreatment with OFQ (23 microg/rat, i.c.v.) attenuated the GH secretion induced by morphine (1 mg/kg, given by intra-arterial injection), thus showing a negative interaction between OFQ and morphine in the control of GH secretion.

Nociceptin, OP4 receptor ligand in different models of experimental epilepsy

The anticonvulsive activity of nociceptin, endogenous OP4 receptors agonist was investigated in pentylenetetrazole (PTZ), N-methyl D-aspartic acid (NMDA), bicucculine (BCC) and electrically evoked seizure models of experimental epilepsy. Nociceptin, at the dose of 10 nmol, suppressed the clonic seizures induced by PTZ, NMDA and BCC. [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1-13)-NH2 which has been proposed to be selective antagonist OP4 receptors, did not prevent the action of nociceptin. The effect of [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1-13)-NH2 on seizures induced by PTZ, NMDA and BCC was very similar to that of nociceptin. These data support the hypothesis that it possesses agonistic properties. Naloxone did not reverse the anticonvulsive action of nociceptin as well as [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1-13)-NH2 which excludes the participation of opioid receptor in this action. On the other hand in the electroconvulsive model of generalized seizures, nociceptin as well as [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1-13)-NH2 influenced neither the electroconvulsive threshold nor the maximal electroshock test. The data suggest that nociceptin and [Phe1(psi)(CH2-NH)Gly2]nociceptin-(1-13)-NH2 can exert anticonvulsive action. These properties depend on OP4 but not opioid receptors activation.

Pharmacological properties of nociceptin/orphanin FQ-induced stimulation and inhibition of cyclic AMP formation in distinct layers of rat olfactory bulb

1. We recently reported that nociceptin/orphanin FQ (N/OFQ) inhibited forskolin-stimulated adenylyl cyclase activity and increased basal enzyme activity in membranes of the external plexiform layer (EPL) and granule cell layer (GRL), respectively, of the rat main olfactory bulb. In the present study we have characterized the pharmacological profile of the inhibitory and stimulatory responses by examining the effects of various N/OFQ receptor agonists and antagonists. 2. N/OFQ(1 - 13)NH(2) fully mimicked the inhibitory and stimulatory effects of N/OFQ with EC(50) values of 0.9 and 6.5 nM, respectively. N/OFQ(1 - 7) was inactive at concentrations up to 1 microM, whereas Ac-RYYRIK-NH(2) and [Phe(1)Psi(CH(2)NH)Gly(2)]N/OFQ(1 - 13)-NH(2) behaved as partial agonists in eliciting both responses. 3. The nonpeptidyl N/OFQ receptor antagonist J-113397 competitively counteracted the inhibitory and stimulatory effects of N/OFQ with pA(2) values of 8.63 and 8.70, respectively. Similarly, the peptidyl antagonist [Nphe(1)]N/OFQ(1 - 13)NH(2) potently antagonized the two effects with pA(2) values of 8.03 and 8.45, respectively. None of the antagonists per se affected adenylyl cyclase activity. 4. These data show that in distinct layers of rat olfactory bulb both the inhibitory and stimulatory effects of N/OFQ on cyclic AMP formation display pharmacological properties consistent with the involvement of N/OFQ receptors.

Characterization of rat prepro-orphanin FQ/nociceptin((154-181)): nociceptive processing in supraspinal sites

Orphanin FQ/nociceptin (OFQ/N), the endogenous ligand for the orphan receptor-like/kappa(3)-like opioid receptor clone, produces a variety of behavioral responses, including those associated with pronociception and antinociception. The OFQ/N precursor rattus-proOFQ (rppOFQ/N) contains several paired basic amino acids, which raises the possibility that post-translational processing can be responsible for the production of a number of additional biologically active peptide fragments. One of these putative peptides, rppOFQ/N (rppOFQ/N(154-181)), was examined for antinociceptive and pronociceptive processes in four brain sites involved in pain inhibition: the ventrolateral periaqueductal gray (vlPAG), the amygdala, the locus coeruleus (LC), and the rostroventromedial medulla (RVM). Endogenous rppOFQ/N(154-181) was identified in each region. rppOFQ/N(154-181) produced a dose-dependent antinociception in all four sites using the tailflick assay. Injections into misplaced cannula sites failed to exert effects. Antinociception in the four sites differed in their response to the opioid antagonist naloxone. Naloxone pretreatment completely blocked rppOFQ/N(154-181)-induced antinociception in the vlPAG and the amygdala, but not in the LC or RVM. In contrast rppOFQ/N(154-181) was hyperalgesic in the LC and RVM, but not in the vlPAG or amygdala. rppOFQ/N(154-181) also was compared with either its N-terminal 17-amino acid peptide (rppOFQ/N(154-170), also known as OFQ2) or its 8-amino acid C-terminal fragment (rppOFQ/N(174-181)). Although both rppOFQ/N(154-181) and rppOFQ/N(154-170) produced antinociception, the latter was less effective because the C-terminal fragment was inactive. Thus, rppOFQ/N(154-181) has complex antinociceptive and pronociceptive actions within the brain, and the pharmacological specificity of its actions differs among supraspinal sites.

Nociceptin/orphanin FQ into the rat periaqueductal gray decreases the withdrawal latency to heat and loading, an effect reversed by (Nphe(1))nociceptin(1-13)NH(2)

The present study investigated the effect of intraperiaqueductal grey injection of nociceptin/orphanin FQ (N/OFQ) and an antagonist (Nphe(1))nociceptin(1-13)NH(2) on the hindpaw withdrawal response to thermal and mechanical stimulation in rats. N/OFQ (5 nmol) significantly decreased the nociceptive thresholds in both tests and 1, 5 and 10 nmol of (Nphe(1))nociceptin(1-13)NH(2) significantly reversed this effect in a dose dependent way. Our results demonstrate, that N/OFQ has a nociceptive action, possibly through inhibition of PAG neurons. This effect is blocked by the antagonist (Nphe(1))nociceptin(1-13)NH(2) probably via ORL1 receptors in the periaqueductal grey.

Expression of orphanin FQ and the opioid receptor-like (ORL1) receptor in the developing human and rat brain

The orphanin peptide system, although structurally similar to the endogenous opioid family of peptides and receptors, has been established as a distinct neurochemical entity. The distribution of the opioid receptor-like (ORL1) receptor and its endogenous ligand orphanin FQ (OFQ) in the central nervous system of the adult rat has been recently reported, and although diffusely disseminated throughout the brain, this neuropeptide system is particularly expressed within stress and pain circuitry. Little is known concerning the normal expression of the orphanin system during gestation, nor how opiate or stress exposure may influence its development. Using in situ hybridization techniques, the present study was undertaken to determine the normal pattern of expression of ORL1 mRNA in the human and rat brain at various developmental stages. Rat embryos, postnatal rat brains and postmortem human brains were collected, frozen and cut into 15 microm coronal sections. In situ hybridization was performed using riboprobes generated from cDNA containing representative human and rat ORL1 and OFQ sequences. Both ORL1 and OFQ mRNA is detected as early as E12 in the cortical plate, basal forebrain, brainstem and spinal cord. Expression for both ORL1 and OFQ is strongest during the early postnatal period, remaining strong in the spinal cord, brainstem, ventral forebrain, and neocortex into the adult. Human ORL1 and OFQ expression is observed at 16 weeks gestation, remaining relatively unchanged up to 36 weeks. The influence of early orphanin expression on maturation of stress and pain circuitry in the developing brain remains unknown.

Potentiation of opioid analgesia in dopamine2 receptor knock-out mice: evidence for a tonically active anti-opioid system

Dopamine systems are intimately involved with opioid actions. Pharmacological studies suggest an important modulatory effect of dopamine and its receptors on opioid analgesia. We have now examined these interactions in a knock-out model in which the dopamine(2) (D(2)) receptor has been disrupted. Loss of D(2) receptors enhances, in a dose-dependent manner, the analgesic actions of the mu analgesic morphine, the kappa(1) agonist U50,488H and the kappa(3) analgesic naloxone benzoylhydrazone. The responses to the delta opioid analgesic [d-Pen(2),d-Pen(5)]enkephalin were unaffected in the knock-out animals. Loss of D(2) receptors also potentiated spinal orphanin FQ/nociceptin analgesia. Antisense studies using a probe targeting the D(2) receptor revealed results similar to those observed in the knock-out model. The modulatory actions of D(2) receptors were independent of final sigma receptor systems because the final sigma agonist (+)-pentazocine lowered opioid analgesia in all mice, including the D(2) knock-out group. Thus, dopamine D(2) receptors represent an additional, significant modulatory system that inhibits analgesic responses to mu and kappa opioids.

Endogenous nociceptin signaling and stress-induced analgesia

Nociceptin/orphanin FQ (NC) and its receptor (OP4) have been implicated in pain transmission. The aim of the present study was to investigate the role of the NC/OP4 system in stress-induced analgesia (SIA). The tail-withdrawal assay was performed in mice stressed by forced swimming in water at 15 degrees C (high severity swims) or 32 degrees C (low severity swims). High severity swims produced a naloxone-insensitive antinociceptive effect which was blocked by supraspinal NC (1 nmol). The selective OP4 receptor antagonist, [Nphe1]NC(-13)NH2 (30 nmol), was inactive by itself, but prevented the effect of NC. Low severity swims produced a milder analgesic effect that was partially antagonized by naloxone, completely blocked by NC and potentiated by [Nphe1]NC(-13)NH2. These findings confirm the anti-analgesic role of supraspinal NC and suggest that endogenous NC signaling counteracts the opioid component of SIA.

Orphanin FQ/nociceptin attenuates the development of morphine tolerance in rats

1. Recent evidence from studies in mice lacking the opioid receptor-like (ORL-1) receptor and from experiments using antibodies raised against orphanin FQ/nociceptin (OFQ/N) suggest that this peptide may be involved in morphine tolerance. In the present study we sought to investigate if administration of exogenous OFQ/N would modulate the development of tolerance to the antinociceptive effect of morphine. 2. Rats were treated for 3 days with either saline or morphine (10 mg kg(-1), s.c.) followed, 15 and 75 min later, by two intracerebroventricular injections of either artificial cerebrospinal fluid (aCSF) or OFQ/N. The dose of OFQ/N was doubled each day (7.5, 15, 30 nmol). On day 4, rats were tested on a hot plate apparatus before and 30, 60 and 90 min after morphine administration. 3. Repeated OFQ/N treatment did not affect basal nociceptive responses or morphine-induced antinociception. However, the same treatment significantly attenuated the development of morphine tolerance. 4. Since learning and memory could contribute to the development of morphine tolerance, in subsequent studies, we examined the effect of OFQ/N administered in the CA3 region of the hippocampus, where OFQ/N has been shown to block LTP and impair spatial memory. A greater attenuation of morphine tolerance with no alteration of baseline hot plate latency or morphine-induced antinociception was observed when OFQ/N was administered in this area of the rat brain. 5. Taken together, our results demonstrate that OFQ/N may act in the hippocampus to attenuate morphine tolerance.

Potentiation of opioid analgesia in dopamine2 receptor knock-out mice: evidence for a tonically active anti-opioid system

Dopamine systems are intimately involved with opioid actions. Pharmacological studies suggest an important modulatory effect of dopamine and its receptors on opioid analgesia. We have now examined these interactions in a knock-out model in which the dopamine(2) (D(2)) receptor has been disrupted. Loss of D(2) receptors enhances, in a dose-dependent manner, the analgesic actions of the mu analgesic morphine, the kappa(1) agonist U50,488H and the kappa(3) analgesic naloxone benzoylhydrazone. The responses to the delta opioid analgesic [d-Pen(2),d-Pen(5)]enkephalin were unaffected in the knock-out animals. Loss of D(2) receptors also potentiated spinal orphanin FQ/nociceptin analgesia. Antisense studies using a probe targeting the D(2) receptor revealed results similar to those observed in the knock-out model. The modulatory actions of D(2) receptors were independent of final sigma receptor systems because the final sigma agonist (+)-pentazocine lowered opioid analgesia in all mice, including the D(2) knock-out group. Thus, dopamine D(2) receptors represent an additional, significant modulatory system that inhibits analgesic responses to mu and kappa opioids.

Changes in brain content of nociceptin/orphanin FQ and endomorphin 2 in a rat model of neuropathic pain

Orphanin FQ (OFQ) and endomorphins (EM) are newly characterized members of opioid peptide family. OFQ has been shown to antagonize morphine analgesia at supraspinal level, whereas endomorphins are highly selective endogenous ligands for mu receptor, showing analgesic effect at both spinal and supraspinal level. OFQ and EM-2 (EM2) immunoreactivity (ir) was measured by radioimmunoassay in nociception-related brain areas of rats subjected to L5/L6 spinal nerve ligation, using sham operated rats as control. It was found that: (1) the content of EM2-ir of spinal nerve ligated rats showed a significant increase (778%) in periaqueductal gray (PAG), and a significant decrease (43%) in striatum, compared with the control group. (2) a significant increase of the content of OFQ-ir was found in amygdala (+841%) and PAG (+459%), respectively in spinal nerve ligated rats. High pressure liquid chromatography showed that the EM2-ir and OFQ-ir were both heterogeneous with the major part eluting at the position of EM2 and OFQ standard, respectively. These results suggest that spinal nerve ligation induces significant changes in the content of EM2-ir and OFQ-ir in some discrete brain areas, which may play a role in nociceptive modulation.

Analgesia elicited by OFQ/nociceptin and its fragments from the amygdala in rats

The heptadecapeptide, orphanin FQ/nociceptin (OFQ/N), binds with high affinity to the ORL-1/KOR-3 opioid receptor clone, yet binds poorly with traditional opioid receptors. OFQ/N has a complex functional profile with relation to nociceptive processing, displaying pro-nociceptive properties in some studies, acting as an inhibitor of stress-induced analgesia in others, yet producing both spinal and supraspinal antinociceptive actions in other studies. Among the intracerebral sites at which OFQ/N might produce one or more of these actions is the amygdala which has been intimately implicated in both antinociceptive and stress-related responses. Therefore, the present study assessed whether microinjections into the amygdala of equimolar doses of OFQ/N(1-17) or its shorter-chained active fragments, OFQ/N(1-11) or OFQ/N(1-7), would produce analgesia as measured by either reactivity to high-intensity radiant heat or reactivity to electric shock, and produce hyperalgesia as measured by reactivity to lower-intensity radiant heat. OFQ/N(1-17) in the amygdala produced a dose-dependent and time-dependent increase in high-intensity tail-flick latencies with maximal effects observed at a dose range of 0.75-3 nmol, and lesser effects at lower (0.015-0.15 nmol) and higher (5.5-30 nmol) doses. Both OFQ/N(1-11) and OFQ/N(1-7) in the amygdala displayed lower magnitudes of analgesia than OFQ/N(1-17) on this measure, with OFQ/N(1-11) displaying maximal effects at higher (15-30 nmol) doses and OFQ/N(1-7) displaying maximal effects at lower (0.15-1.5 nmol) doses. In contrast to traditional mu and kappa opioids and beta-endorphin, none of the OFQ/N fragments in the amygdala exhibited any analgesic responses on the jump test. Finally, using a low-intensity radiant heat assay capable of detecting hyperalgesic responses, each of the OFQ/N fragments in the amygdala increased tail-flick latencies on this measure. Therefore, OFQ/N fragments appear to exert only analgesic responses in the amygdala with quantitative and qualitative differences relative to traditional opioid agonists.

Induction by carrageenan inflammation of prepronociceptin mRNA in VR1-immunoreactive neurons in rat dorsal root ganglia

Nociceptin (orphanin FQ) may act on primary afferents and be involved in the regulation of nociceptive processing. We have shown, using reverse transcription-polymerase chain reaction (RT-PCR), that carrageenan-produced peripheral inflammation induces the expression of prepronociceptin (PPN) mRNA in the dorsal root ganglia (DRG). The present experiments were conducted to determine the localization of PPN mRNA in primary sensory neurons after peripheral inflammation, using in situ hybridization. An intraplantar injection of carrageenan induced the expression of PPN mRNA in small and medium sized neurons in the DRG; the effect peaked 0.5 h after carrageenan and subsided by 6 h. All neurons positive for PPN mRNA were positive for vanilloid receptor subtype 1 (VR1)-like immunoreactivity and some VR1-immunoreactive neurons were negative for PPN mRNA. The results suggest that peripheral inflammation induces the production of nociceptin in a sub-population of VR1-positive primary sensory neurons and support the idea that nociceptin produced there is involved in the regulation of nociceptive processing.

Nocistatin, a peptide reversing acute and chronic morphine tolerance

It has been reported that intracerebroventricular (i.c.v.) injection of nociception/orphanin FQ (OFQ) can antagonize morphine analgesia, whereas i.c.v. OFQ antibody can reverse morphine tolerance. Nocistatin (NST) is a recently characterized neuropeptide possessing an antagonizing effect on OFQ. Here we examine whether i.c.v. NST would result in a reversal of morphine tolerance. The results showed that: (1) i.c.v. NST at doses of 0.005, 0.05, 0.5, 5 or 50 ng per rat produced a bell-shaped dose-dependent reversal of chronic morphine tolerance, with maximum response at 0.5 ng. (2) Acute morphine tolerance could also be reversed, albeit partially, by i.c.v. NST at 0.5 ng. (3) The reversal of acute and chronic morphine tolerance by NST was completely abolished when NST (0.5 ng) was co-injected with (8 microg) OFQ. Since OFQ and NST are derived from the same preprohormone, the profile of its splicing in the CNS may play an important role in determining the effectiveness of morphine analgesia.

Enhanced spinal nociceptin receptor expression develops morphine tolerance and dependence

The tolerance and dependence after chronic medication with morphine are thought to be representative models for studying the plasticity, including the remodeling of neuronal networks. To test the hypothesis that changes in neuronal plasticity observed in opioid tolerance or dependence are derived from increased activity of the anti-opioid nociceptin system, the effects of chronic treatments with morphine were examined using nociceptin receptor knock-out (NOR(-/-)) mice and a novel nonpeptidic NOR antagonist, J-113397, which shows a specific and potent NOR antagonist activity in in vitro [(35)S]GTPgammaS binding assay and in vivo peripheral nociception test. The NOR(-/-) mice showed marked resistance to morphine analgesic tolerance without affecting morphine analgesic potency in tail-pinch and tail-flick tests. The NOR(-/-) mice also showed marked attenuation of morphine-induced physical dependence, manifested as naloxone-precipitated withdrawal symptoms after repeated morphine treatments. Similar marked attenuation of morphine tolerance was also observed by single subcutaneous (10 mg/kg) or intrathecal (1 nmol) injection of J-113397, which had been given 60 min before the test in morphine-treated ddY mice. However, the intracerebroventricular injection (up to 3 nmol) did not affect the tolerance. On the other hand, morphine dependence was markedly attenuated by J-113397 that had been subcutaneously given 60 min before naloxone challenge. There was also observed a parallel enhancement of NOR gene expression only in the spinal cord during chronic morphine treatments. Together, these findings suggest that the spinal NOR system develops anti-opioid plasticity observed on morphine tolerance and dependence.

Antagonism of ORLI receptor produces an algesic effect in the rat formalin test

The authors investigated the role of endogenously released nociceptin (also known as orphanin FQ) spinal and supraspinal nociceptive transmission during the rat formalin test by examining the effect of intrathecal and intracerebroventricular injection of J-113397, a non-peptidyl ORL1 receptor selective antagonist. When J-113397 was injected intrathecally or intracerebroventricularly 10 min before the formalin injection, it enhanced the agitation behavior induced by paw formalin injection. This suggested that paw formalin injection induced nociceptin release in the spinal cord and the supraspinal brain sites, that this endogenously released nociceptin produced an analgesic effect and that J-113397 antagonized this analgesic effect of nociceptin and produced an algesic effect in the rat formalin test.

Disparate spinal and supraspinal opioid antinociceptive responses in beta-endorphin-deficient mutant mice

The role of endogenous opioid systems in the analgesic response to exogenous opiates remains controversial. We previously reported that mice lacking the peptide neurotransmitter beta-endorphin, although unable to produce opioid-mediated stress-induced antinociception, nevertheless displayed intact antinociception after systemic administration of the exogenous opiate morphine. Morphine administered by a peripheral route can activate opioid receptors in both the spinal cord and brain. However, beta-endorphin neuronal projections are confined predominantly to supraspinal nociceptive nuclei. Therefore, we questioned whether the absence of beta-endorphin would differentially affect antinociceptive responses depending on the route of opiate administration. Time- and dose-response curves were obtained in beta-endorphin-deficient and matched wild-type C57BL/6 congenic control mice using the tail-immersion/withdrawal assay. Null mutant mice were found to be more sensitive to supraspinal (i.c.v.) injection of the micro-opioid receptor-selective agonists, morphine and D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin. In contrast, the mutant mice were less sensitive to spinal (i.t.) injection of these same drugs. Quantitative receptor autoradiography revealed no differences between genotypes in the density of mu, delta, or kappa opioid receptor binding sites in either the spinal cord or pain-relevant supraspinal areas. Thus we report that the absence of a putative endogenous ligand for the mu-opioid receptor results in opposite changes in morphine sensitivity between discrete areas of the nervous system, which are not simply caused by changes in opioid receptor expression.

Postsynaptic K+ current induced by nociceptin in medullary dorsal horn neurons

The actions of the endogenous ORL1 receptor (opioid receptor-like1) ligand nociceptin on the membrane properties of rat trigeminal nucleus caudalis neurons were examined by use of whole cell and perforated patch clamp recording in brain slices. Nociceptin produced an outward current in all neurons tested (EC50 112 nM). The outward current produced by nociceptin was completely reversed with the addition of the non-peptide ORL1 antagonist J-113397. Outward currents reversed polarity at -99+/-2 mV, close to the potential for K+ of -102 mV, suggesting that they were mediated by an increased K+ conductance. These results suggest that the analgesic action of nociceptin might be mediated by direct postsynaptic inhibition within the dorsal horn.

Morphine tolerance and dependence in nociceptin/orphanin FQ transgenic knock-out mice

It has been hypothesized that morphine tolerance and dependence in mice following chronic exposure may reflect increased compensatory activity of antiopioid systems. The endogenous peptide nociceptin/orphanin FQ has been shown to have anti-opioid effects, for example antagonizing morphine analgesia. Moreover, chronic morphine administration increases synthesis of the peptide, and morphine tolerance and dependence can be attenuated or reversed by antagonists and agonists of the nociceptin/orphanin FQ receptor, respectively. The present study seeks to confirm a role for nociceptin/orphanin FQ in opioid tolerance and dependence by comparing morphine ED(50) values and naloxone-precipitated withdrawal jumping in mice homozygous (knock-out) and heterozygous for a null mutation of the Npnc1 gene encoding the nociceptin/orphanin FQ propeptide, and their wild type littermates, following chronic morphine exposure. Relative to morphine-naive control mice, significant rightward shifts in the morphine dose-response curve, resulting in increased morphine ED(50) values (approximately two to three-fold), was observed for all genotypes following three days of repeated systemic morphine injections. However, no differences between genotypes in the magnitude of tolerance were observed. In contrast, knock-out mice displayed significantly increased naloxone-precipitated withdrawal jumping relative to heterozygous and wild-type mice following implantation with a morphine pellet (25mg) for 72h. Use of nociception/orphaninFQ transgenic knock-out mice thus demonstrate the differential involvement of nociceptin/orphanin FQ in morphine tolerance and dependence.

Bidirectional actions of nociceptin/orphanin FQ on A delta-fibre-evoked responses in rat superficial spinal dorsal horn in vitro

The present study investigated the modulatory actions of nociceptin/orphanin FQ on excitatory glutamatergic transmission in spinal dorsal horn. In transverse spinal cord slices with an attached dorsal root, mono- and polysynaptic A delta-fibre-evoked extracellular field potentials were recorded from superficial dorsal horn. Nociceptin/orphanin FQ showed bidirectional effects on monosynaptic transmission with a potentiation at lower concentrations (100-300 nM) and a dose-dependent depression at higher concentrations (1-3 microM). The polysynaptic field potential was dose-dependently depressed by nociceptin/orphanin FQ (100 nM-3 microM). None of the actions of nociceptin/orphanin FQ was reversed by the non-specific opioid receptor antagonist naloxone, the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid or the peptide nocistatin. The bidirectional actions of nociceptin/orphanin FQ on the monosynaptic field potential may provide an in vitro model for the bidirectional actions of nociceptin/orphanin FQ in behavioural studies showing hyperalgesia at low doses of intrathecal nociceptin/orphanin FQ and analgesia at higher doses.

Bi-directional modulation of 5-hydroxytryptamine-induced plasma extravasation in the rat knee joint by nociceptin

The role of nociceptin, the endogenous ligand for the opioid receptor-like (ORL1) receptor, in nociceptive processing is controversial. Most studies demonstrate hyperalgesia following supraspinal administration, analgesia following intrathecal and peripheral administration at higher doses, and hyperalgesia following intrathecal and peripheral application at lower doses. The present study investigates the effect of nociceptin on synovial plasma extravasation and its ability to modulate 5-hydroxytryptamine-induced synovial plasma extravasation using the rat knee joint model of inflammation. Nociceptin alone does not alter synovial plasma extravasation from baseline. Nociceptin at concentrations up to 1 nM enhances 5-hydroxytryptamine-induced synovial plasma extravasation (up to 50%) and nociceptin at concentrations above 100 nM inhibits 5-hydroxytryptamine-induced synovial plasma extravasation (down to 45%). The novel, selective ORL1 receptor antagonist J-113397 potently inhibits the pro-inflammatory effect of nociceptin, but only partly inhibits, at higher concentrations, the anti-inflammatory effects of nociceptin.These findings demonstrate a dose-dependent bi-directional effect of nociceptin on inflammatory processes and may indicate a target for novel therapeutics.

Orphanin FQ produces gender-specific modulation of trigeminal nociception: behavioral and electrophysiological observations

The present study aimed to determine if orphanin FQ, an endogenous ligand for the opioid receptor like-1 receptor, produces gender-specific effects in the modulation of N-methyl-D-aspartate (NMDA)-evoked responses of trigeminal nociceptive neurons, and in the NMDA-induced nociceptive behavior. Single-unit extracellular recordings were made from nociceptive-specific and wide dynamic range neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in anesthetized (1.5 g/kg urethane) rats. In the proestrous female, orphanin FQ applied microiontophoretically produced facilitation of the NMDA-evoked responses in 50% (16/32) of nociceptive neurons, inhibition in 31% (10/32), and biphasic effects in 19% (6/32). In contrast, in the male, it inhibited the responses in 86% (18/21), and facilitated the responses in 14% (4/21). In ovariectomized animals, orphanin FQ inhibited the responses in 75% (9/12) of nociceptive neurons, facilitated the responses in 17% (2/12) and produced biphasic effects in 8% (1/12). In contrast, in estradiol-treated ovariectomized rats, it facilitated the responses in 46% (5/11), inhibited the responses in 36% (4/11) and produced biphasic effects in 18% (2/11). For behavioral studies, NMDA-induced scratching behavior was used to assess the effects of orphanin FQ. Twenty-eight male, ovariectomized and estradiol-treated ovariectomized rats were microinjected with NMDA (2 nmol in 10 microl) alone through a cannula implanted in the medullary region, while another 27 rats were microinjected with orphanin FQ (10 nmol in 10 microl) 10 min prior to giving NMDA. Orphanin FQ reduced the NMDA-induced nociceptive scratching behavior by 92% in the male, and by 96% in ovariectomized rats. In contrast, in estradiol-treated ovariectomized animals, orphanin FQ facilitated the NMDA-induced scratching behavior by 210%. We conclude from these studies that orphanin FQ is primarily pronociceptive in the female and primarily antinociceptive in the male. Furthermore, we suggest that estrogen is involved in generating the gender-specific effects of orphanin FQ.

Blockade of mu-opioid receptors reveals the hyperalgesic effect of orphanin FQ/nociceptin in the rat hot plate test

Orphanin FQ (OFQ, also known as nociceptin) has been proposed to oppose the antinociceptive effect of endogenous opioid peptides in the brain. We sought to determine whether, conversely, the endogenous opioid peptides counteract a pronociceptive action of OFQ. In testing this hypothesis, naloxone, a non-selective opioid receptor antagonist, was used to block the action of endogenous opioid peptides. We then examined whether OFQ would produce hyperalgesia in the absence of such an endogenous opioidergic tone. Neither naloxone (1 mg kg(-1); s.c.) nor OFQ (up to 30 nmol; i.c.v.) alone induced any significant change in mean hot plate latency. However, OFQ dose-dependently produced hyperalgesia in rats pretreated with naloxone, implying that OFQ can indeed produce hyperalgesia once an endogenous opioidergic tone is inhibited. In subsequent studies, we used subtype selective opioid receptor antagonists to determine which class of opioid receptor is involved in this response. The effect of naloxone was reproduced using the selective mu-opioid receptor antagonist CTOP (D-Phe-Cyc-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2), but not by administration of the delta-opioid receptor antagonist, naltrindole (NTI) or the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI). These results suggest that endogenous opioid peptides acting at the mu-, but not kappa- or delta-opioid receptor may be counteracting the hyperalgesic effect of OFQ in rats.

Intradermal application of nociceptin increases vascular permeability in rats: the possible involvement of histamine release from mast cells

Intradermal application of nociceptin was used to investigate its in vivo effect on the inflammatory response in rats. Intradermal nociceptin (5 pmol/site-5 nmol/site) increased vascular permeability in a dose-dependent manner. The increased vascular permeability by nociceptin (5 nmol/site) was dose-dependently inhibited by the histamine H1 receptor antagonist pyrilamine (50 pmol/site-5 nmol/site). In rat peritoneal mast-cell preparation, nociceptin (10(-8)-10(-4) M) dose-dependently stimulated histamine release. The effect of nociceptin (10(-5) M) occurred rapidly (within 30 s) and was inhibited by pertussis toxin, Ca2+, but was not sensitive to naloxone, a classical opioid receptor antagonist. These characteristics are in agreement with features of the opioid-receptor-like 1 (ORL1) receptor, a non-classical opioid receptor linked to a pertussis toxin-sensitive G protein. Taken together, these data suggest that nociceptin, likely acting via the ORL1 receptor at the site of inflammation, might be critical for the enhancement of the inflammatory response by stimulating histamine release from mast cells.

The pharmacological basis of contemporary pain management

Pain management has become an increasingly well researched area in medicine over recent years, and there have been advances in a number of areas. While opioids remain an integral part of pain-management strategies, there is now an emphasis on the use of adjuvant drugs, such as paracetamol and anti-inflammatory agents, which through physiological or pharmacological synergism, both enhance pain control and reduce opioid use. The management of neuropathic pain continues to be a challenge. Anti-epileptics and antidepressants, together with clonidine and ketamine, provide the foundations for treatment. Another area of interest has been the widespread use of patient-controlled analgesia and the administration of some drugs, especially opioids, by means other than traditional oral and parenteral routes. The number of new drugs that have reached the stage of clinical trials has been small, yet they offer exciting possibilities. The epibatidine analogue ABT-594 and zinconitide both offer novel approaches to the management of neuropathic pain states, while selective cyclo-oxygenase-2 inhibitors and nitroaspirins may see advances in the management of nociceptive pain states.

Enhanced spinal nociceptin receptor expression develops morphine tolerance and dependence

The tolerance and dependence after chronic medication with morphine are thought to be representative models for studying the plasticity, including the remodeling of neuronal networks. To test the hypothesis that changes in neuronal plasticity observed in opioid tolerance or dependence are derived from increased activity of the anti-opioid nociceptin system, the effects of chronic treatments with morphine were examined using nociceptin receptor knock-out (NOR(-/-)) mice and a novel nonpeptidic NOR antagonist, J-113397, which shows a specific and potent NOR antagonist activity in in vitro [(35)S]GTPgammaS binding assay and in vivo peripheral nociception test. The NOR(-/-) mice showed marked resistance to morphine analgesic tolerance without affecting morphine analgesic potency in tail-pinch and tail-flick tests. The NOR(-/-) mice also showed marked attenuation of morphine-induced physical dependence, manifested as naloxone-precipitated withdrawal symptoms after repeated morphine treatments. Similar marked attenuation of morphine tolerance was also observed by single subcutaneous (10 mg/kg) or intrathecal (1 nmol) injection of J-113397, which had been given 60 min before the test in morphine-treated ddY mice. However, the intracerebroventricular injection (up to 3 nmol) did not affect the tolerance. On the other hand, morphine dependence was markedly attenuated by J-113397 that had been subcutaneously given 60 min before naloxone challenge. There was also observed a parallel enhancement of NOR gene expression only in the spinal cord during chronic morphine treatments. Together, these findings suggest that the spinal NOR system develops anti-opioid plasticity observed on morphine tolerance and dependence.

Autoradiographic localization of [3H]nociceptin binding sites in the rat brain

The binding sites of nociceptin (also named orphanin FQ), the endogenous ligand of ORL1 (opiate receptor like 1), were localized in rat brain, using an autoradiographic procedure. High levels of binding were observed in the cingulate, retrosplenial, perirhinal, insular and occipital cortex, anterior and posteromedial cortical amygdaloid nuclei, basolateral amygdaloid nucleus, amygdaloid complex, posterior hippocampus, dorsal endopiriform, central medial thalamic, paraventricular, rhomboid thalamic, suprachiasmatic, ventromedial hypothalamic nuclei, mammillary complex, superficial gray layer of the superior colliculus, locus coeruleus, dorsal raphe nucleus. More moderate labelling was observed in the prefrontal, fronto-parietal, temporal, piriform cortex, dentate gyrus, anterior olfactory nucleus, olfactory tubercle, shell of nucleus accumbens, claustrum, lateral septum, laterodorsal thalamic, medial habenular, subthalamic, reuniens thalamic nuclei, subiculum, periaqueductal grey matter and pons. A lower binding site density was observed in the anterior and medial hippocampus, olfactory bulb, caudate putamen, the core of the nucleus accumbens, medial septum, ventrolateral, ventroposterolateral and mediodorsal thalamic nuclei, lateral and medial geniculate nuclei, hypothalamic area, substantia nigra, ventral tegmentum area and interpedoncular nucleus. A moderate and similar labelling was found in the dorsal and ventral horn of the spinal cord. No labelling was apparent in the corpus callosum. Thus, it appears that the ORL1 receptor is particularly abundant in the cerebral cortex, limbic system of the rat brain and some areas involved in pain perception.

Nociceptin attenuates opioid and gamma-aminobutyric acid(B) receptor-mediated analgesia in the mouse tail-flick assay

Nociceptin (NC) and the opioid receptor like-1 receptors are widely distributed in areas of the neuraxis that are part of the descending modulatory pain system. We used the tail-flick assay in mice to assess the interaction between NC and other analgesic compounds acting on different areas of the descending pathway. Given by intracerebroventricular injection, NC induced hyperalgesia at 10 nmol (39% of reduction vs. control group). The same dose of NC reversed analgesia induced by distinct classes of analgesia-producing compounds such as morphine, dynorphin A or baclofen. NC caused a reduction of their antinociceptive effects: 61, 41 and 27%, respectively. Thus, NC at the supraspinal level appears to interact with both opioid and gamma-aminobutyric acid(B) systems producing anti-analgesic effects probably through the descending pathway for pain control.

The effect of [Phe(1)psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2) on feeding and c-Fos immunoreactivity in selected brain sites

Nociceptin/orphanin FQ (N/OFQ) is an endogenous ligand of the ORL1 receptor. N/OFQ, when administered centrally, stimulates feeding in a fashion similar to other opioids. Intracerebroventricular administration of N/OFQ induces changes in c-Fos immunoreactivity in several feeding-related brain sites. A synthetic pseudopeptide, [Phe(1)iota(CH(2)-NH)Gly(2)]-nociceptin(1-13)-NH(2) (hereafter: [FG]N/OFQ(1-13)NH(2)), has been labeled both as an ORL1 agonist and antagonist. The present study was designed to examine the influence of [FG]N/OFQ(1-13)NH(2) on food intake in rats. We also evaluated c-Fos immunoreactivity in those areas of the brain which have been shown to exhibit altered c-Fos expression upon N/OFQ administration. We found that [FG]N/OFQ(1-13)NH(2) increases food consumption in satiated rats. This effect is short-lasting and can be reversed by the opioid antagonist naloxone. Co-administration of [FG]N/OFQ(1-13)NH(2) does not affect orexigenic response to N/OFQ. Intracerebroventricularly-injected [FG]N/OFQ(1-13)NH(2) induces c-Fos expression in the nucleus of the solitary tract, hypothalamic paraventricular and supraoptic nuclei, central nucleus of amygdala, lateral septal and lateral habenular nuclei-brain areas that have been shown to be activated by N/OFQ. These results support the hypothesis that [FG]N/OFQ(1-13)NH(2) acts as an agonist of ORL1 receptor in vivo.

Evidence of the synthesis of opioid receptor like 1 receptor in nociceptinergic neurons in rat brain suggests the existence of autoreceptor: a confocal double staining study

Opioid receptor like 1 (ORL1) receptor is a novel member of the opioid receptor family, which was not bound by any of the typical opioid receptor ligands but bound by the recently discovered nociceptin (also termed orphanin FQ) with high affinity. By using double staining of fluorescent in situ hybridization and immunohistochemistry, we observed the expression of ORL1 receptor mRNA in nociceptin-like immunoreactive neurons in multiple areas in rat brain including the hippocampus, arcuate nucleus in the hypothalamus, ventralateral periaquiductal gray (PAG) and raphe nuclei in brain stem. The expression of ORL1 mRNA in nociceptinergic neurons suggests that these receptors mediate, at least in part, the presynaptic autoreceptor functions. Further anatomical and functional significance of the autoreceptor of nociceptinergic neurons remain to be elucidated.

Nocistatin inhibits food intake in rats

Nocistatin, a product of the same precursor as nociceptin/orphanin FQ (N/OFQ), has been shown to antagonize effects of N/OFQ. N/OFQ stimulates feeding, most probably by inhibiting activation of neurons containing oxytocin (OT) and vasopressin (VP), peptides considered as satiety factors, and implicated in the development of conditioned taste aversion (CTA). The present study was designed to investigate whether intracerebroventricularly (ICV) injected nocistatin (a) affects deprivation- and N/OFQ-induced feeding, (b) causes CTA, and (c) induces activation of hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei, as well as OT and VP neurons present in these regions. C-Fos immunohistochemistry was used as a marker of cellular activation. Nocistatin (1-3 nmol) significantly reduced food intake in deprived rats during the first and second hour post-injection. Doses of 1-3 nmol suppressed N/OFQ-induced feeding. Nocistatin at the highest (3 nmol) dose did not cause CTA. It also did not affect activation of the PVN or SON. In nocistatin-treated animals, the percentage of Fos-positive OT and VP neurons was similar to controls. We conclude that nocistatin antagonizes the influence of N/OFQ on feeding and suppresses deprivation-induced food consumption through mechanisms other than aversion. Nocistatin does not, however, activate the PVN or SON. It does not exert its effects via VP or OT neurons.

Selective suppression of inhibitory synaptic transmission by nocistatin in the rat spinal cord dorsal horn

Nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) are two recently identified neuropeptides with opposing effects on several CNS functions, including spinal nociception. The cellular mechanisms that underlie this antagonism are not known. Here, we have investigated the effects of both peptides on synaptic transmission mediated by the three fast neurotransmitters l-glutamate, glycine, and GABA in the superficial layers of the rat spinal cord horn, which constitute the first important site of integration of nociceptive information in the pain pathway. NST selectively reduced transmitter release from inhibitory interneurons via a presynaptic Bordetella pertussis toxin-sensitive mechanism but left excitatory glutamatergic transmission unaffected. In contrast, N/OFQ only inhibited excitatory transmission. In the rat formalin test, an animal model of tonic pain in which N/OFQ exerts antinociceptive activity, NST induced profound hyperalgesia after intrathecal application. Similar to glycine and GABA(A) receptor antagonists, NST had no significant effects in the rat tail-flick test, a model of acute thermal pain. Our results provide a cellular basis for the antagonism of N/OFQ and NST and suggest the existence of a so far unidentified membrane receptor for NST. In addition, they support a role of NST as an endogenous inhibitor of glycinergic and GABAergic neurotransmission in the sensory part of the spinal cord and as a mediator of spinal hyperalgesia.

Effects of an antisense oligonucleotide to pronociceptin and long-term prevention of morphine actions by nociceptin

To further characterize the anti-opioid action of the neuropeptide nociceptin, we examined the effects of the repeated intracerebroventricular (i.c.v.) treatment (once daily for 4 days) with an antisense oligodeoxynucleotide complementary to pronociceptin mRNA, in the rat. We also investigated possible changes of the antinociceptive and hyperthermic effects induced by the i.c.v. administration of morphine, in rats i.c.v. pretreated with nociceptin 3 h before. The pretreatment with the antisense oligodeoxynucleotide, but not with a mismatched sequence (used as a control), caused an increase in spontaneous locomotor activity and produced a potentiation of the antinociceptive effect of a submaximal dose of i.c.v. morphine (1 microgram/rat). The i.c.v. pretreatment with nociceptin (2 nmol/rat, 3 h before) prevented both the antinociceptive and the hyperthermic effects of morphine (10 microgram/rat i.c.v.). These results strengthen the hypothesis of an anti-opioid action of nociceptin at supraspinal level and suggest that the neuropeptide may exert long-term modulatory effects.

Cellular actions of nociceptin: transduction mechanisms

The recent identification of the nociceptin receptor-nociceptin system and the description of its role in nociceptive processing has produced numerous investigative studies. A fundamental part of this research is to understand the cellular signaling events (i.e. the building blocks) upon which the pharmacology of this intriguing system is based. As anticipated, nociceptin receptor activation inhibits the formation of cAMP formation via a pertussis toxin-sensitive G-protein. This indicates that nociceptin receptor couples to the G(i)/G(o) class of G-protein(s). However, there is now growing evidence for nociceptin activation of additional signaling pathways, including MAP kinase and phospholipase C/[Ca(2+)](i). These signaling events are discussed in this review.

Functional studies using antibodies against orphanin FQ/nociceptin

Orphanin FQ/nociceptin (OFQ) is a recently discovered endogenous ligand for the novel opioid receptor-like receptor (ORL-1). There are numerous reports in the literature demonstrating paradoxical effects of exogenous OFQ on pain modulation. For example, OFQ produces a pronociceptive effect in the brain and an analgesic effect in the spinal cord. In order to better understand the physiological actions of OFQ, the present study focused on the pain-modulatory effect of endogenously released OFQ measured using antibody microinjection techniques. We found that electroacupuncture analgesia (EA) was increased by intracerebroventricular (i.c.v.) injection of an OFQ-antibody and decreased following intrathecal injection. Furthermore, i.c.v. OFQ-antibody partially reversed tolerance to both chronic morphine and chronic EA. These data suggest that endogenously released OFQ plays an important role in pain modulation, where pain sensitivity in the brain and spinal cord is increased and decreased, respectively.

Nociceptin/orphanin FQ in spinal nociceptive mechanisms under normal and pathological conditions

Nociceptin and its receptor are present in dorsal spinal cord, indicating a possible role for this peptide in pain transmission. The majority of functional studies using behavioral and electrophysiological studies have shown that nociceptin applied at spinal level produces antinociception through pre- and post-synaptic mechanisms. The spinal inhibitory effect of nociceptin is not sensitive to antagonists of opioid receptors such as naloxone. Thus, nociceptin-induced antinociception is mediated by a novel mechanism independent of activation of classic opioid receptors. This has raised the possibility that agonists of the nociceptin receptor may represent a novel class of analgesics. Supporting this hypothesis, several groups have shown that intrathecal nociceptin alleviated hyperalgesic and allodynic responses in rats after inflammation or partial peripheral nerve injury. Electrophysiological studies have also indicated that the antinociceptive potency of spinal nociceptin is maintained or enhanced after nerve injury. It is concluded that the predominant action of nociceptin in the spinal cord appears to be inhibitory. The physiological role of nociceptin in spinal nociceptive mechanisms remains to be defined. Moreover, further evaluation of nociceptin as a new analgesic calls the development of non-peptide brain penetrating agents.

Nocistatin: a novel neuropeptide encoded by the gene for the nociceptin/orphanin FQ precursor

We identified a novel neuropeptide and named it "nocistatin." Its presence was expected by analysis of the precursor for the neuropeptide nociceptin or orphanin FQ (Noc/OFQ), previously identified as an endogenous ligand for the orphan opioid receptor-like receptor. The precursor prepronociceptin/orphanin FQ (ppNoc/OFQ) comprises at least two bioactive peptides, nocistatin and Noc/OFQ. Noc/OFQ is involved in a broad range of pharmacological actions in various tissues from the central nervous system to the periphery. In pain transmission, Noc/OFQ is reported to have different effects including nociception, no effect, and analgesia, depending on the animal species tested, doses, route of administration, and so on. We found that intrathecal administration of Noc/OFQ induced pain responses including allodynia and hyperalgesia. Simultaneous administration of nocistatin blocked the allodynia and hyperalgesia induced by Noc/OFQ, whereas anti-nocistatin antibody decreased the threshold for the Noc/OFQ-induced allodynia. The endogenous heptadecapeptide nocistatin was isolated from bovine brains and recently identified in mouse, rat, and human brain and in human cerebrospinal fluid. Although human, rat and mouse ppNoc/OFQ produced larger respective counterparts with 30, 35, and 41 amino acid residues, all peptides showed the antinociceptive activity. This activity was ascribed to the carboxyl-terminal hexapeptide of nocistatin, Glu-Gln-Lys-Gln-Leu-Gln, which is conserved beyond species. Nocistatin also attenuated the allodynia and hyperalgesia evoked by prostaglandin E(2) and the inflammatory hyperalgesia induced by formalin or carrageenan/kaolin, and reversed the Noc/OFQ-induced inhibition of morphine analgesia at picogram doses. Furthermore, nocistatin counteracted the impairment of learning and memory induced by Noc/OFQ or scopolamine. Nocistatin is widely present in the spinal cord and brain. Although nocistatin did not bind to the Noc/OFQ receptor, it bound to the membrane of mouse brain and spinal cord with a high affinity. Nocistatin is a novel bioactive peptide produced from the same precursor as Noc/OFQ, and it plays important roles in the regulation of pain transmission and learning and memory processes in the central nervous system.

Selective suppression of inhibitory synaptic transmission by nocistatin in the rat spinal cord dorsal horn

Nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) are two recently identified neuropeptides with opposing effects on several CNS functions, including spinal nociception. The cellular mechanisms that underlie this antagonism are not known. Here, we have investigated the effects of both peptides on synaptic transmission mediated by the three fast neurotransmitters l-glutamate, glycine, and GABA in the superficial layers of the rat spinal cord horn, which constitute the first important site of integration of nociceptive information in the pain pathway. NST selectively reduced transmitter release from inhibitory interneurons via a presynaptic Bordetella pertussis toxin-sensitive mechanism but left excitatory glutamatergic transmission unaffected. In contrast, N/OFQ only inhibited excitatory transmission. In the rat formalin test, an animal model of tonic pain in which N/OFQ exerts antinociceptive activity, NST induced profound hyperalgesia after intrathecal application. Similar to glycine and GABA(A) receptor antagonists, NST had no significant effects in the rat tail-flick test, a model of acute thermal pain. Our results provide a cellular basis for the antagonism of N/OFQ and NST and suggest the existence of a so far unidentified membrane receptor for NST. In addition, they support a role of NST as an endogenous inhibitor of glycinergic and GABAergic neurotransmission in the sensory part of the spinal cord and as a mediator of spinal hyperalgesia.

Species differences in the efficacy of compounds at the nociceptin receptor (ORL1)

Recent studies have identified compounds with reduced efficacy relative to nociceptin/orphanin FQ at the opioid-like receptor ORL1. Utilizing stimulation of [(35)S]GTPgammaS binding as in vitro assays, it was determined that both [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)NH(2) and the hexapeptide Ac-RYYRIK-NH(2) act as partial agonists in CHO cells transfected with either human or mouse ORL1. Maximal activity for both [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)NH(2) and Ac-RYYRIK-NH(2) was significantly greater in cells transfected with the human receptor (90% and 73% in a high expressing clone, 76% and 68% in low expressing clone) rather than the mouse receptor (37.5 and 33%), regardless of receptor number in individual clones. In vitro studies in cells transfected with exaggerated receptor numbers can lead to unreliable estimates of agonist and antagonist activity, however, these studies suggest that animal experiments on the activity of novel compounds may not always be better predictors of the ultimate activity in humans.

Effects of supraspinal orphanin FQ/nociceptin

The first reported behavioral action of the endogenous ligand for the "orphan" opioid receptor was a seemingly paradoxical increased sensitivity to nociception (i.e. hyperalgesia) after supraspinal injection into the cerebral ventricles of mice. In the continuing absence of an appropriate in vivo receptor antagonist, studies attempting to define the role of orphanin FQ/nociceptin (OFQ/N) in pain modulation and other behaviors have also featured central injection of peptide. This article reviews the findings of such studies. There appears to be concordance around the observation of anti-opioid actions of supraspinally injected OFQ/N, whereas the observations of hyperalgesia and/or analgesia are much less clear. A portion of the discrepant data may be explained in terms of methodological issues, stress-induced analgesia accompanying experimental protocols, and genotypic variation among subjects. Clarification of OFQ/N's role in nociception, as with other putative biologic functions, will probably depend upon the availability of a selective receptor antagonist.

Tissue distribution of the opioid receptor-like (ORL1) receptor

The ORL1 receptor is a G protein-coupled receptor structurally related to the opioid receptors, whose endogenous ligand is the heptadecapeptide nociceptin/orphanin FQ. In this review, data which have contributed to the mapping of the anatomic distribution of the ORL1 receptor have been collated with an emphasis on their relation to physiological functions. The ORL1 receptor is widely expressed in the central nervous system, in particular in the forebrain (cortical areas, olfactory regions, limbic structures, thalamus), throughout the brainstem (central periaqueductal gray, substantia nigra, several sensory and motor nuclei), and in both the dorsal and ventral horns of the spinal cord. Regions almost devoid of ORL1 receptors are the caudate-putamen and the cerebellum. ORL1 mRNA and binding sites exhibit approximately the same distribution pattern, indicating that the ORL1 receptor is located on local neuronal circuits. The ORL1 receptor is also expressed at the periphery in smooth muscles, peripheral ganglia, and the immune system. The anatomic distribution of ORL1 receptor suggests a broad spectrum of action for the nociceptin/orphanin FQ system (sensory perception, memory process, emotional behavior, etc.).

Pharmacological characterization of the nociceptin receptor, ORL1. Insight from the inward rectifier activation in the periaqueductal gray

A novel opioid receptor-like orphan receptor (ORL1) was cloned and identified to be homologous to classical opioid receptors but insensitive to traditional opioids. A heptadecapeptide, termed orphanin FQ or nociceptin (OFQ/N), was identified as its endogenous ligand. OFQ/N shares overlapping distribution sites in pain-processing areas and common cellular mechanisms with opioids but exerts diverse effects on nociceptive responses. Of the two reported ORL1 antagonists, [Phe(1)psi(CH(2)-NH)- Gly(2)] nociceptin-(1-13)-NH(2) (Phepsi) and naloxone benzoylhydrazone (NBZ), antagonisms were validated in the activation of inward rectifying K channels induced by OFQ/N, using the patch clamp technique in ventrolateral periaqueductal gray slices. Results showed that Phepsi acted as a partial agonist and NBZ was a weak nonselective antagonist of ORL1. It is comparable with most but not all of the findings from other tissues. Comparing all the reports supports the above inference for these two antagonists. The possible causes for the discrepancy were discussed. A brief review on the putative ORL1 antagonists, acetyl-RYYRIK-NH2, some sigma-ligands and the functional antagonist, nocistatin, is also included. It indicates that a potent and selective ORL1 antagonist is expecting to elucidate the physiological role of OFQ/N.

A cellular mechanism for the bidirectional pain-modulating actions of orphanin FQ/nociceptin

Orphanin FQ/nociceptin (OFQ/N) and its receptor share substantial structural features and cellular actions with classic opioid peptides and receptors, but have distinct pharmacological profiles and behavioral effects. Currently there is an active debate about whether OFQ/N produces hyperalgesia or analgesia. Using a well-defined brainstem pain-modulating circuit, we show that OFQ/N can cause either an apparent hyperalgesia by antagonizing mu opioid-induced analgesia or a net analgesic effect by reducing the hyperalgesia during opioid abstinence. It presumably produces these two opposite actions by inhibiting two distinct groups of neurons whose activation mediates the two effects of opioid administration. OFQ/N antagonism of the hyperalgesia may have significance for the treatment of opioid withdrawal and sensitized pain.

Unaltered peripheral excitatory actions of nociceptin contrast with enhanced spinal inhibitory effects after carrageenan inflammation: an electrophysiological study in the rat

Nociceptin (orphanin FQ) is the endogenous agonist of the opioid receptor-like (ORL-1) receptor. The actions of this peptide have been studied extensively at a number of sites with diverse actions being reported. Here, in a rat model of peripheral inflammation, we examine the effects of nociceptin on the responses of dorsal horn neurones when applied directly to the spinal cord and, in separate studies, into the peripheral receptive fields in the hindpaw of the halothane anaesthetized rat. As changes in the receptor density and expression of the message for nociceptin have been reported after inflammation we have compared these actions to previously reported effects in normal animals. The dose-dependent inhibitory actions of nociceptin on C-fibre evoked responses and input (measures of presumed pre-synaptic excitability) are increased 3-4 h after inflammation whereas its inhibitory effects on post-synaptic mechanisms (wind-up) remain unchanged. These inhibitory effects were partly reversible by high doses of naloxone. This increased potency of nociceptin after inflammation is consistent with an increased receptor density in the superficial spinal cord. In contrast, the peripheral administration of nociceptin produced dose-dependent excitations of dorsal horn neurones and a degree of sensitization to mechanical stimuli. This peripheral action was unchanged after inflammation. These diverse site-dependent actions of nociceptin further emphasize the complexities of this novel opioid system.

Pharmacology of nociceptin and its receptor: a novel therapeutic target

Nociceptin (NC), alias Orphanin FQ, has been recently identified as the endogenous ligand of the opioid receptor-like 1 receptor (OP(4)). This new NC/OP(4) receptor system belongs to the opioid family and has been characterized pharmacologically with functional and binding assays on native (mouse, rat, guinea-pig) and recombinant (human) receptors, by using specific and selective agonists (NC, NC(1 - 13)NH(2)) and a pure and competitive antagonist, [Nphe(1)]NC(1 - 13)NH(2). The similar order of potency of agonists and affinity values of the antagonist indicate that the same receptor is present in the four species. OP(4) is expressed in neurons, where it reduces activation of adenylyl cyclase and Ca(2+) channels while activating K(+) channels in a manner similar to opioids. In this way, OP(4) mediates inhibitory effects in the autonomic nervous system, but its activities in the central nervous system can be either similar or opposite to those of opioids. In vivo experiments have demonstrated that NC modulates a variety of biological functions ranging from nociception to food intake, from memory processes to cardiovascular and renal functions, from spontaneous locomotor activity to gastrointestinal motility, from anxiety to the control of neurotransmitter release at peripheral and central sites. These actions have been demonstrated using NC and various pharmacological tools, as antisense oligonucleotides targeting OP(4) or the peptide precursor genes, antibodies against NC, an OP(4) receptor selective antagonist and with data obtained from animals in which the receptor or the peptide precursor genes were knocked out. These new advances have contributed to better understanding of the pathophysiological role of the NC/OP(4) system, and ultimately will help to identify the therapeutic potential of new OP(4) receptor ligands.

The opioid-receptor-like 1 (ORL-1) as a potential target for new analgesics

Anew sequence, which encoded a novel G protein-coupled receptor, was disclosed by two different groups, using the nucleic acid probes based on the delta opioid receptor, first cloned in 1992. The new receptor, which Meunier called opioid-receptor-like 1 (ORL-1), was shown to share high homology with the opioid receptors and therefore thought to be a potential target for new analgesics. In this respect, the present review reports on the literature referring to ORL-1, to its natural ligand (nociceptin or orphanin FQ) and to several synthetic analogues recently described, both as agonists or antagonists at the receptor.

Central changes in nociceptin dynorphin B and Met-enkephalin-Arg-Phe in different models of nociception

The newly identified neuropeptide nociceptin/orphanin FQ (NOC) was measured in different rat brain areas related to the descending anti-nociceptive pathways and compared to two opioid peptides, dynorphin B (DYN B) and Met-enkephalinArgPhe (MEAP). Two experimental models of chronic nociception, one neurogenic and one inflammatory, used in this study, reveal how different pathological conditions may influence these endogenous systems. Nerve injury is induced by ligation of the sciatic nerve and inflammation by a carrageenan injection in the gluteal muscle, 2 weeks prior to decapitation. Selected brain areas were dissected out and frozen. NOC-, DYN B- and MEAP-like immunoreactivity (LI) is determined by radioimmunoassay. Nerve injury increased the NOC-LI levels in the cortex cinguli, DYN B-LI levels in the dorsal and the ventral part of the spinal cord, whereas a decrease in the MEAP-LI levels is seen in the dorsal part of the periaqueductal grey (PAG). After inflammation, the NOC-LI levels increased in cortex cinguli, hypothalamus and in the dorsal spinal cord, whereas DYN B-LI levels increased in the dorsal part of the PAG. A general increase in MEAP-LI levels is found after inflammation in all analyzed brain areas except in hippocampus. In conclusion, increased levels of NOC-LI were found in cortex cinguli in both treatment groups and in hypothalamus and spinal cord following carrageenan treatment. The changes in the NOC-LI concentrations were not parallelled by changes in DYN B-LI and MEAP-LI, suggesting that NOC and opioid peptides elicit different reactions in the systems of nociception/antinociception.