Involvement of endogenous orphanin FQ in electroacupuncture-induced analgesia

The nociceptin/orphanin FQ receptor system as a target to alleviate cancer-induced bone pain in rats: Model validation and pharmacological evaluation

BACKGROUND AND PURPOSE

Cancer-induced bone pain remains inadequately controlled, and current standard of care analgesics is accompanied by several side effects. Nociceptin/orphanin FQ peptide (NOP) receptor agonists have demonstrated broad analgesic properties in rodent neuropathic and inflammatory pain models. Here, we investigate the analgesic potential of NOP receptor activation in a rodent cancer-induced bone pain model.

EXPERIMENTAL APPROACH

Model validation by intratibial inoculation in male Sprague Dawley rats was performed with varying MRMT-1/Luc2 cell quantities (0.5-1.5 × 106 ·ml-1 ) and a behavioural battery (>14 days post-surgery) including evoked and non-evoked readouts: paw pressure test, cold plate, von Frey, open field, and weight distribution. Anti-allodynic potential of the endogenous NOP receptor ligand nociceptin (i.t.) and NOP receptor agonist Ro65-6570 ( i.p.) was tested using von Frey filaments, followed by a combination experiment with Ro65-6570 and the NOP receptor antagonist J-113397 (i.p.). Plasma cytokine levels and NOP receptor gene expression in dorsal root ganglion (DRG, L4-L6) and bone marrow were examined.

KEY RESULTS

Inoculation with 1.5 × 106 ·ml-1 of MRMT-1/Luc2 cells resulted in a robust and progressive pain-related phenotype. Nociceptin and Ro65-6570 treatment inhibited cancer-induced mechanical allodynia. J-113397 selectively antagonized the effect of Ro65-6570. MRMT-1/Luc2-bearing animals demonstrated elevated plasma cytokine levels of IL-4, IL-5, IL-6 and IL-10 plus unaltered NOP-r gene expression in DRG and reduced expression in bone marrow.

CONCLUSION AND IMPLICATIONS

Nociceptin and Ro65-6570 selectively and dose-dependently reversed cancer-induced bone pain-like behaviour. The NOP receptor system may be a potential target for cancer-induced bone pain treatment.

LINKED ARTICLES

This article is part of a themed issue on The molecular pharmacology of bone and cancer-elated bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

Electroacupuncture anesthesia for laparotomy in goats

Background

The objective of the study to evaluate the effect of electroacupuncture during laparotomy in goats.

Aim

To study the abdominal anesthesia in goats by electroacupuncture using the physiological variables, vital parameters, hematological, biochemical, cortisol hormone, pain threshold, and wound healing for laparotomy in goats.

Methods

Fifteen healthy adult bucks were used to receive electroacupuncture in 10 newly selected acupoints. The data (M ± SD) were assessed at intervals (0 minutes) before induction (control group), (5 minutes), (10 minutes), (15 minutes), and (20 minutes) during induction, (30 minutes), (45 minutes), and (60 minutes) throughout surgery and (24 hours) after surgery, cortisol levels in serum at (0), (24 hours), and (72 hours) throughout laparotomy.

Results

The goats of the study showed improvement in the rates of eyelid closure, head, and neck relaxation, rumen motility, and tympany which were graded into mild (+), moderate (++), and severe (+++) degrees. The respiratory rates, body temperatures, and capillary fill times were not significantly different. The total mean of hematocrit was (19.9 ± 2.68), the total mean of hemoglobin was (9.9 ± 0.94), the total mean of red blood cells was (7.9 ± 0.8), the total mean of platelets was (244,861.3 ± 138,444.8) and the total mean of SPO2 was (70.5 ± 4.6). ALT and AST showed no significance. The significant mean cortisol level was (2.6 ± 2.01) and the significant mean pain threshold level was (0.02 ± 0.03). The results proved that electroacupuncture had a lot of significant parameters. The wound healing was improved by early epithelization and immature granulation tissue (at 7 days). Thick keratinized epithelization and collagen deposition in the dermal tissue with enhanced angiogenesis (at 14 days). Mild restoration of skin and the dermal tissue was well-organized (at 21 days). Besides, well-formed scar tissue covering a highly cellular organized dermal tissue (at 28 days).

Conclusions

Electroacupuncture had been considered a powerful anesthetic for abdominal surgery in goats. Moreover, wound healing proved excellent and better healing.

Let-7b-5p promotes electroacupuncture tolerance by downregulating Penk1 gene in CFA-induced inflammatory nociception rats

BACKGROUND

Studies showed that increased let-7b-5p microRNA during repeated electroacupuncture (EA) treatment was associated the formation of EA tolerance, which manifested as gradually decreased nociceptive threshold. Proenkephalin (PENK) is the precursor of enkephalin which is a pivot neuropeptide responsible for the decreased nociceptive threshold in EA. The aim of this study was to evaluate the relationship between let-7b-5p and PENK in EA tolerance.

METHODS

The target gene of let-7b-5p microRNA was determined through the dual-luciferase reporter assay in cortical neurons. Seventy-two Sprague Dawley rats received a combination of EA and intracerebroventricular injection of microRNA (let-7b-5p agomir, antagomir or their controls). The nociceptive thresholds were assessed with radiant heat tail-flick method. PENK and let-7b-5p were measured with Western Blot and qPCR, respectively, after administration of let-7b-5p agomir, antagomir, and their controls at day 1, 4 and 7.

RESULTS

Let-7b-5p targeted the 3' untranslated region of Penk1. The nociceptive thresholds in Let-7b-5p agomir + EA group were decreased (p < 0.05) compared with those in Let-7b-5p antagomir + EA group at day 1 to 7. Compared with Let-7b-5p agomir + EA group, the expression level of PENK in Let-7b-5p antagomir + EA group was increased at days 1, 4, and 7 (p < 0.05) CONCLUSION: Let-7b-5p may be a new potential target for decreasing the EA tolerance effect and facilitating the application of EA in treating chronic nociception of patients.

The Involvement of Descending Pain Inhibitory System in Electroacupuncture-Induced Analgesia

Chronic pain is a major health problem, which can impair quality of life and reduce productivity. Electroacupuncture (EA), a modality of medicine based on the theories of Traditional Chinese Medicine (TCM), presents great therapeutic effects on chronic pain. Its clinical application has gained increasing popularity, and in parallel, more research has been performed on the mechanisms of EA-induced analgesia. The past decades have seen enormous advances both in neuronal circuitry of needle-insertion and in its molecular mechanism. EA may block pain by activating the descending pain inhibitory system, which originates in the brainstem and terminates at the spinal cord. This review article synthesizes corresponding studies to elucidate how EA alleviate pain via the mediation of this descending system. Much emphasis has been put on the implication of descending serotonergic and noradrenergic pathways in the process of pain modulation. Also, other important transmitters and supraspinal regions related to analgesic effects of EA have been demonstrated. Finally, it should be noticed that there exist some shortcomings involved in the animal experimental designed for EA, which account for conflicting results obtained by different studies.

■ REVIEW : ORL-1: An Awkward Child of the Opioid Receptor Family

The cloning of the δ-opioid receptor allowed for the rapid cloning of the two other classically defined opioid receptors, the μ- and κ-opioid receptors. However, several groups cloned a fourth receptor (ORL-1, for opioid receptor-like) that had high homology to the opioid receptors but did not bind any known endogenous opioid peptides (i.e., endorphins) or exogenous opiates. Recently, two independent groups isolated a 17- amino-acid peptide that is an endogenous ligand for ORL-1; one group named it orphanin FQ (OFQ), the other named it nociceptin (N). It was reported that intracerebroventricular administration of this heptadeca peptide (OFQ/N) in mice induced an increased responsiveness to painful stimuli, an effect in striking contrast to the analgesia that is a hallmark of classical opiate drugs. Further research has revealed that OFQ/N has complex effects on pain perception: OFQ/N has been touted as having analgesic, hyperalgesic, and anti opioid properties. In addition to discussing these disparate findings, this review highlights the structural and pharmacological parallels between ORL-1 and opioid receptors as well as their respective endogenous ligands. NEUROSCIENTIST 4:172-184, 1998

MiRNAs are involved in chronic electroacupuncture tolerance in the rat hypothalamus

Acupuncture tolerance is the gradual decrease in analgesic effect due to its prolonged application. However, its mechanism in terms of miRNA is still unknown. To explore the role of miRNAs in electroacupuncture (EA) tolerance of rats using deep sequencing, rats with more than a 50 % increase in tail flick latency (TFL) in response to EA were selected for this experiment. EA tolerance was induced by EA once daily for eight consecutive days. The hypothalami were harvested for deep sequencing. As a result, 49 differentially expressed miRNAs were identified and validated by real-time PCR. Of them, let-7b-5p, miR-148a-3p, miR-124-3p, miR-107-3p, and miR-370-3p were further confirmed to be related to EA tolerance by an intracerebroventricular injection of agomirs or antagomirs of these miRNAs. Potential targets of the 49 miRNAs were enriched in 9 pathways and 282 gene ontology (GO) terms. Five miRNAs were confirmed to participate in EA tolerance probably through the functional categories related to nerve impulse transmission, receptor signal pathways, and gene expression regulation, as well as pathways related to MAPK, neurotrophin, fatty acid metabolism, lysosome, and the degradation of valine, leucine, and isoleucine. Our findings reveal a characterized panel of the differentially expressed miRNAs in the hypothalamus in response to EA and thus provide a solid experimental framework for future analysis of the mechanisms underlying EA-induced tolerance.

Functional plasticity of the N/OFQ-NOP receptor system determines analgesic properties of NOP receptor agonists

Despite high sequence similarity between NOP (nociceptin/orphanin FQ opioid peptide) and opioid receptors, marked differences in endogenous ligand selectivity, signal transduction, phosphorylation, desensitization, internalization and trafficking have been identified; underscoring the evolutionary difference between NOP and opioid receptors. Activation of NOP receptors affects nociceptive transmission in a site-specific manner, with antinociceptive effects prevailing after peripheral and spinal activation, and pronociceptive effects after supraspinal activation in rodents. The net effect of systemically administered NOP receptor agonists on nociception is proposed to depend on the relative contribution of peripheral, spinal and supraspinal activation, and this may depend on experimental conditions. Functional expression and regulation of NOP receptors at peripheral and central sites of the nociceptive pathway exhibits a high degree of plasticity under conditions of neuropathic and inflammatory pain. In rodents, systemically administered NOP receptor agonists exerted antihypersensitive effects in models of neuropathic and inflammatory pain. However, they were largely ineffective in acute pain while concomitantly evoking severe motor side effects. In contrast, systemic administration of NOP receptor agonists to non-human primates (NHPs) exerted potent and efficacious antinociception in the absence of motor and sedative side effects. The reason for this species difference with respect to antinociceptive efficacy and tolerability is not clear. Moreover, co-activation of NOP and μ-opioid peptide (MOP) receptors synergistically produced antinociception in NHPs. Hence, both selective NOP receptor as well as NOP/MOP receptor agonists may hold potential for clinical use as analgesics effective in conditions of acute and chronic pain.

Minocycline enhances the effectiveness of nociceptin/orphanin FQ during neuropathic pain

Nociceptin/orphanin FQ (N/OFQ) antinociception, which is mediated selectively by the N/OFQ peptide receptor (NOP), was demonstrated in pain models. In this study, we determine the role of activated microglia on the analgesic effects of N/OFQ in a rat model of neuropathic pain induced by chronic constriction injury (CCI) to the sciatic nerve. Repeated 7-day administration of minocycline (30 mg/kg i.p.), a drug that affects microglial activation, significantly reduced pain in CCI-exposed rats and it potentiates the analgesic effects of administered N/OFQ (2.5-5 μg i.t.). Minocycline also downregulates the nerve injury-induced upregulation of NOP protein in the dorsal lumbar spinal cord. Our in vitro study showed that minocycline reduced NOP mRNA, but not protein, level in rat primary microglial cell cultures. In [(35)S]GTPγS binding assays we have shown that minocycline increases the spinal N/OFQ-stimulated NOP signaling. We suggest that the modulation of the N/OFQ system by minocycline is due to the potentiation of its neuronal antinociceptive activity and weakening of the microglial cell activation. This effect is beneficial for pain relief, and these results suggest new targets for the development of drugs that are effective against neuropathic pain.

Orphanin-FQ/nociceptin: Lack of antinociceptive, hyperalgesic or allodynic effects in acute thermal or mechanical tests following intracerebroventricular or intrathecal administration to mice or rats

A recent review calls attention to the discrepant results resulting from studies that have examined the nociceptive or antinociceptive properties of orphanin-FQ/nociceptin (Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-ArgLys-Leu-Ala-Asn-Gln; OFQ/N), the heptadecapeptide isolated from rat (nociceptin) and pig (orphanin FQ) brain that binds with high affinity to the opioid 'orphan' receptor (a seven transmembrane protein with sequence homology to opioid receptors), but exhibits only low affinity binding with conventional opioid ligands. Some of the discrepancy might result from differences in species, test, route of administration or time-course. We undertook a comprehensive examination of the effects of spinal (i.t.) or supraspinal (i.c.v.) administration of OFQ/N in mice and rats. Mice treated with OFQ/N either i.t. or i.c.v. demonstrated no significant nociceptive effect in the hot plate, warm-water or radiant heat tail-flick tests (except for the highest and most sedative dose of 10 nmol i.c.v. in the mouse warm-water tail-flick test). Pretreatment with the opioid antagonist naloxone or with peptidase inhibitors did not enhance the nociceptive effects of OFQ/N peptide in the warm-water tail-flick test. The motor activity in mice administered OFQ/N i.c.v. decreased significantly compared to controls. Rats administered i.c.v. or i.t. OFQ/N displayed no significant difference from vehicle-treated animals in similar noxious stimulus tests and OFQ/N-treated rats did not exhibit allodynia in a paw-withdrawal test. Overall, OFQ/N was ineffective in significantly altering response to noxious stimuli, regardless of whether the peptide was given at supraspinal or spinal sites in mice or in rats. In addition, i.c.v. or i.t. application of antisense or mismatch ODN to the orphan receptor did not modify tail-flick latency in either mice or rats, arguing against a tonic nociceptive tone mediated via the OFQ/N receptor.

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

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

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

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

Cellular uptake and biological activity of peptide nucleic acids conjugated with peptides with and without cell-penetrating ability

A 12-mer peptide nucleic acid (PNA) directed against the nociceptin/orphanin FQ receptor mRNA was disulfide bridged with various peptides without and with cell-penetrating features. The cellular uptake and the antisense activity of these conjugates were assessed in parallel. Quantitation of the internalized PNA was performed by using an approach based on capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). This approach enabled a selective assessment of the PNA moiety liberated from the conjugate in the reducing intracellular environment, thus avoiding bias of the results by surface adsorption. The biological activity of the conjugates was studied by an assay based on the downregulation of the nociceptin/orphanin FQ receptor in neonatal rat cardiomyocytes (CM). Comparable cellular uptake was found for all conjugates and for the naked PNA, irrespective of the cell-penetrating properties of the peptide components. All conjugates exhibited a comparable biological activity in the 100 nM range. The naked PNA also exhibited extensive antisense activity, which, however, proved about five times lower than that of the conjugates. The found results suggest cellular uptake and the bioactivity of PNA-peptide conjugates to be not primarily related to the cell-penetrating ability of their peptide components. Likewise from these results it can be inferred that the superior bioactivity of the PNA-peptide conjugates in comparison with that of naked PNA rely on as yet unknown factors rather than on higher membrane permeability. Several hints point to the resistance against cellular export and the aggregation propensity combined with the endocytosis rate to be candidates for such factors.

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

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

Nocistatin and nociceptin given centrally induce opioid-mediated gastric mucosal protection

Nociceptin (N/OFQ) and nocistatin (NST) are two endogenous neuropeptides derived from the same precursor protein, preproN/OFQ. The aim of the present work was to study the effect of NST on the ethanol-induced mucosal damage compared with that of N/OFQ following intracerebroventricular (i.c.v.) administration in the rat and to analyze the mechanism of the gastroprotective action. It was found that both NST and N/OFQ reduced the mucosal lesions in the same dose range (0.2-1 nmol i.c.v.), but in higher doses (2-5 nmol i.c.v.) the gastroprotective effect of both peptides was highly diminished. The gastroprotective effect of N/OFQ (1 nmol), but not that of NST (1 nmol), was reduced by the selective nociceptin receptor antagonist J-113397 (69 nmol i.c.v.). Similarly, decrease of the gastroprotective effect was observed after the combination of NST (1 nmol) with N/OFQ (0.6 or 1 nmol). However, addition of the gastroprotective effects was observed, when lower dose (0.2 nmol) of NST was given prior to N/OFQ (0.6 nmol). The gastroprotective effect of both N/OFQ and NST was antagonized by naloxone (27 nmol), beta-funaltrexamine (20 nmol), naltrindole (5 nmol) and norbinaltorphimine (14 nmol), the mu-, delta- and kappa-opioid receptor antagonists, respectively, given i.c.v. The mucosal protection was significantly decreased after bilateral cervical vagotomy. The present findings suggest that NST similar to N/OFQ, may also induce gastric mucosal protective action initiated centrally in a vagal-dependent mechanism. Opioid component is likely to be involved in the gastroprotective effect of both NST and N/OFQ.

Neural mechanism underlying acupuncture analgesia

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

Blockade of the nociceptin/orphanin FQ/NOP receptor system in the rat ventrolateral periaqueductal gray potentiates DAMGO analgesia

Nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP) are involved in various biological functions including pain. High density of NOP receptor has been found in the ventrolateral periaqueductal gray (vlPAG), the main output pathway involved in descending pain-control system. The aim of our work was to evaluate the involvement of the N/OFQ/NOP system in the modulation of MOP analgesia in the rat vlPAG using UFP-101, a selective NOP antagonist. N/OFQ significantly blocked DAMGO (a selective MOP agonist) analgesia, while UFP-101 enhanced the effect of the opioid given at a subanalgesic dose. These results confirm our hypothesis of an antiopioid role for N/OFQ in the vlPAG.

Involvement of nociceptin/orphanin FQ and its receptor in electroacupuncture-produced anti-hyperalgesia in rats with peripheral inflammation

The neuropeptide nociceptin/orphanin FQ (N/OFQ), the endogenous agonist of the N/OFQ peptide receptor (NOP receptor), has been demonstrated to be involved in many physiological and pathological functions including pain regulation. In the present study, the involvement of N/OFQ-NOP receptor system in electroacupuncture (EA)-produced anti-hyperalgesia was investigated in rats with peripheral inflammation. Intrathecal (i.t.) administration of N/OFQ (15 nmol) or EA at acupoints GB30 and GB34 could significantly attenuate hyperalgesia which was induced by subcutaneously injecting complete Freund's adjuvant (CFA) into one hindpaw of rats, manifesting as decreased paw withdrawal latency (PWL) to the noxious thermal stimulus. The anti-nociceptive effect of N/OFQ or EA was significantly blocked by intrathecal injection of [Nphe(1)]nociceptin(1-13)NH(2) (20 nmol), a selective antagonist of the NOP receptor, indicating the NOP-receptor-mediated mechanism. Additionally, the combination of N/OFQ injection with EA treatment could enhance anti-hyperalgesia compared to that produced by each component alone. These findings suggested that the spinal N/OFQ-NOP system might be involved in EA analgesia, which may be one of the mechanisms underlying the anti-nociceptive effect of EA in rat's peripheral inflammatory pain.

Effect of supraspinal Nocistatin on Nociceptin/Orphanin FQ antagonism of selective opioid analgesia

Nocistatin and Nociceptin/Orphanin FQ are two neuropeptides derived from the same precursor protein, pre-pro-Nociceptin. Nocistatin does not bind to Nociceptin/Orphanin FQ peptide (NOP) receptor but it antagonizes the allodynic and hyperalgesic effect of intrathecal (i.t.) Nociceptin. In this study, we examined the effect of Nocistatin on nociception and opioid analgesia by itself and the nociceptive effect of Nociceptin and antagonistic effect of nociceptin on opioid receptors in tail flick test when given the i.c.v. route. More precisely, supraspinal Nocistatin by itself had no significative effect on nociception and opioid analgesia in the tail flick test but, at the dose of 0.5ng/rat, it reversed the nociceptive effect of Nociceptin and also the antagonistic effect of Nociceptin against analgesia caused by the selective opioid agonists: DAMGO, DPDPE, Deltorphin II and U50 488H. These data suggest that Nocistatin antagonizes the effect of Nociceptin on opioid analgesia and could play an important role in the regulation of nociceptive transmission.

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

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

Effects of electroacupuncture on orphanin FQ immunoreactivity and preproorphanin FQ mRNA in nucleus of raphe magnus in the neuropathic pain rats

Orphanin FQ (OFQ) is an endogenous ligand for opioid receptor-like-1 (ORL1) receptor. Previous studies have shown that both OFQ immunoreactivity and preproorphanin FQ (ppOFQ) mRNA expression could be observed in the brain regions involved in pain modulation, e.g., nucleus of raphe magnus (NRM), dorsal raphe nucleus (DRN), and ventrolateral periaqueductal gray (vlPAG). It was reported that electroacupuncture (EA) has analgesic effect on neuropathic pain, and the analgesic effect was mediated by the endogenous opioid peptides. In the present study, we investigated the effects of EA on the changes of OFQ in the neuropathic pain rats. In the sciatic nerve chronic constriction injury (CCI) model, we investigated the changes of ppOFQ mRNA and OFQ immunoreactivity in NRM after EA by in situ hybridization (ISH) and immunohistochemistry methods, respectively. Then, the ppOFQ mRNA-positive and OFQ immunoreactive cells were counted under a computerized image analysis system. The results showed that expression of ppOFQ mRNA decreased and OFQ immunoreactivity increased after EA treatment in the neuropathic pain rats. These results indicated that EA modulated OFQ synthesis and OFQ peptide level in NRM of the neuropathic pain rats.

Enhancement of intracellular concentration and biological activity of PNA after conjugation with a cell-penetrating synthetic model peptide

In order to evaluate the ability of the cell-penetrating alpha-helical amphipathic model peptide KLALKLALKALKAALKLA-NH(2) (MAP) to deliver peptide nucleic acids (PNAs) into mammalian cells, MAP was covalently linked to the 12-mer PNA 5'-GGAGCAGGAAAG-3' directed against the mRNA of the nociceptin/orphanin FQ receptor. The cellular uptake of both the naked PNA and its MAP-conjugate was studied by means of capillary electrophoresis combined with laser-induced fluorescence detection, confocal laser scanning microscopy and fluorescence-activated cell sorting. Incubation with the fluorescein-labelled PNA-peptide conjugate led to three- and eightfold higher intracellular concentrations in neonatal rat cardiomyocytes and CHO cells, respectively, than found after exposure of the cells to the naked PNA. Correspondingly, pretreatment of spontaneously-beating neonatal rat cardiomyocytes with the PNA-peptide conjugate and the naked PNA slowed down the positive chronotropic effect elicited by the neuropeptide nociceptin by 10- and twofold, respectively. The main reasons for the higher bioavailability of the PNA-peptide conjugate were found to be a more rapid cellular uptake in combination with a lowered re-export and resistance against influences of serum.

Nocistatin potentiates electroacupuncture antinociceptive effects and reverses chronic tolerance to electroacupuncture in mice

Nocistatin (NST) and nociception/orphanin FQ (OFQ) are peptides derived from the same precursor that play opposing roles in pain modulation. OFQ antagonizes morphine analgesia and electroacupuncture (EA)-induced antinociceptive effect. The present study investigates whether NST potentiates EA-induced antinociceptive effect and reverses chronic tolerance to EA in mice. Injection of NST (0.5, 5.0 and 50.0 ng) intracerebroventricularly had no effect on basal thermal latency, but produced a dose-dependent potentiation of EA-induced antinociceptive effect in mice with the maximum response at 5.0 ng. NST (5.0 ng) partly reversed chronic tolerance to EA. These results suggest that NST in the brain might play roles in EA-induced antinociceptive effect and the development of chronic tolerance to EA in mice.

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

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

Orphanin FQ/nociceptin: from neural circuitry to behavior

Orphanin FQ/nociceptin (OFQ/N), the endogenous ligand for the "orphan" opioid receptor ORL-1 (NOP(1)) was first identified in 1995. In the years since its discovery, a large body of evidence has accumulated showing that OFQ/N and its receptor are widely distributed in the nervous system, and showing that OFQ/N has potent and indiscriminate inhibitory actions on neurons in many regions. However, numerous studies investigating the functional role of OFQ/N in physiology or behavior have failed to provide a coherent view. Pain and analgesia have been the best studied, and administration of OFQ/N is reported to have no effect, to produce hyperalgesia, analgesia or anti-hyperalgesia. Effects of OFQ/N receptor antagonists have proved similarly contentious. In an attempt to resolve this controversy, we investigated the actions of OFQ/N on the activity of physiologically characterized neurons in the rostral ventromedial medulla, a region with a well-documented role in pain modulation(Heinricher et al., 1997). The results of those experiments demonstrate that this peptide is neither "anti-opioid" or "anti-hyperalgesic". It is simply inhibitory. For this reason, the effects seen in functional studies will only be fully understood when examined in the context of identified neural circuits.

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

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

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

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

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.

Blockade effects of (Nphe1)Nociceptin(1-13)-NH(2) on anti-nociception induced by intrathecal administration of nociceptin in rats

The present study investigated the roles of the opioid-receptor-like (ORL1) receptor and its endogenous ligand nociceptin on nociception in the spinal cord of rats. Intrathecal administration of 10 nmol of nociceptin produced significant increases in hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation. There were no significant changes of average maximum angles in inclined plane tests after intrathecal injection of 10 nmol of nociceptin in rats. The intrathecal nociceptin-induced increases in HWL were antagonized by intrathecal administration of (Nphe1)Nociceptin(1-13)-NH(2), a selective antagonist of ORL1 receptor, in a dose-dependent manner. The results demonstrated that ORL1 receptor is involved in the nociceptin-induced anti-nociceptive effect in the spinal cord of rats.

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.

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.

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.

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.

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.

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.

Characterization of [Nphe(1)]nociceptin(1-13)NH(2), a new selective nociceptin receptor antagonist

1.. Nociceptin (orphanin FQ) is a novel neuropeptide capable of inducing a variety of biological actions via activation of a specific G-protein coupled receptor. However, the lack of a selective nociceptin receptor antagonist has hampered our understanding of nociceptin actions and the role of this peptide in pathophysiological states. As part of a broader programme of research, geared to the identification and characterization of nociceptin receptor ligands, we report that the novel peptide [Nphe(1)]nociceptin(1-13)NH(2) acts as the first truly selective and competitive nociceptin receptor antagonist and is devoid of any residual agonist activity. 2. [Nphe(1)]nociceptin(1-13)NH(2) binds selectively to recombinant nociceptin receptors expressed in Chinese hamster ovary (CHO) cells (pK(i) 8.4) and competitively antagonizes the inhibitory effects of nociceptin (i) on cyclic AMP accumulation in CHO cells (pA(2) 6.0) and (ii) on electrically evoked contractions in isolated tissues of the mouse, rat and guinea-pig with pA(2) values ranging from 6.0 to 6.4. 3. [Nphe(1)]nociceptin(1-13)NH(2) is also active in vivo, where it prevents the pronociceptive and antimorphine actions of intracerebroventricularly applied nociceptin, measured in the mouse tail withdrawal assay. Moreover, [Nphe(1)]nociceptin(1-13)NH(2) produces per se a dose dependent, naloxone resistant antinociceptive action and, at relatively low doses, potentiates morphine-induced analgesia. 4. Collectively our data indicate that [Nphe(1)]nociceptin(1-13)NH(2), acting as a nociceptin receptor antagonist, may be the prototype of a new class of analgesics.

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.

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. '

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.

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.

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.

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.