Antinociceptive effects of the ORL1 receptor agonist nociceptin/orphanin FQ in diabetic mice.

The antinociceptive potency of nociceptin/orphanin FQ, an opioid-like orphan receptor agonist, was examined using the tail-flick test and the formalin-induced nociception test in diabetic mice. Nociceptin/orphanin FQ, at doses of 0.1 to 10 nmol, intrathecal (i.t.), produced a marked and dose-dependent inhibition of the tail-flick response in both non-diabetic and diabetic mice. The antinociceptive effect of nociceptin/orphanin FQ in the tail-flick test in diabetic mice was greater than that in non-diabetic mice. The antinociceptive effect of nociceptin/orphanin FQ was not antagonized by pretreatment with either beta-funaltrexamine, a selective mu-opioid receptor antagonist, naltrindole, a selective delta-opioid receptor antagonist, or nor-binaltorphimine, a selective kappa-opioid receptor antagonist. The antinociceptive effects of nociceptin/orphanin FQ in diabetic, but not in non-diabetic mice, were abolished when mice were pretreated with capsaicin i.t. 24 h before testing. In the formalin test, nociceptin/orphanin FQ also produced a marked and dose-dependent antinociceptive effect on the first-phase response, but not the second phase-response, in both diabetic and non-diabetic mice. Furthermore, nociceptin/orphanin FQ significantly and dose-dependently reduced the flinching responses to i.t.-administered substance P in diabetic mice, but not in non-diabetic mice. The results of the present experiments clearly indicate that the antinociceptive potency of nociceptin/orphanin FQ is significantly greater in diabetic mice than in non-diabetic mice. Furthermore, the results of this study suggest that the reduction of substance P-mediated nociceptive transmission in the spinal cord may be responsible for the antinociceptive effect of nociceptin/orphanin FQ.     

A potent and highly selective nonpeptidyl nociceptin/orphanin FQ receptor (ORL1) antagonist: J-113397

We discovered a potent nociceptin/orphanin FQ receptor (ORL1) receptor antagonist, J-113397 (1-[(3R, 4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1, 3-dihydro-2H-benzimidazol-2-one). J-113397 inhibited [125I][Tyr(14)]nociceptin binding to Chinese hamster ovary (CHO) cells expressing ORL1 receptor in a dose-dependent manner (IC(50); 2. 3 nM), but showed 600-fold or less affinity for mu-, delta- and kappa-opioid receptors. Nociceptin/orphanin FQ-induced suppression of cyclic AMP accumulation elicited by forskolin was completely inhibited by J-113397 with an IC(50) value of 26 nM. These results indicate that J-113397 is a potent and selective nonpeptidyl antagonist of the ORL1 receptor.     

In vitro characterization of J-113397, a non-peptide nociceptin/orphanin FQ receptor antagonist

The lack of availability of a selective, highly potent, competitive antagonist for the nociceptin receptor (OP4) devoid of residual agonistic activity has hampered studies in this area. We report here the in vitro pharmacological properties of the novel non-peptide OP4 antagonist, J-113397, which was recently discovered by Banyu Pharmaceutical investigators. The compound was synthesized as a racemic mixture in our laboratories. J-113397 was shown to antagonize (pA2 7.52) the nociceptin-induced inhibition of cAMP formation in cells expressing the recombinant human OP4 receptor (CHOhOP4) and to displace [125I]Tyr14nociceptin from CHOhOP4 membranes with a pKi of 8.56. It also competitively antagonized the contractile actions of nociceptin in the mouse colon (pA2 8.07) and the inhibitory effect of nociceptin in electrically stimulated preparations such as the mouse vas deferens (pA2 7.85), the guinea pig ileum (7.75), and the rat vas deferens (7.77). At high concentrations (10 microM), the compound was devoid of agonist activity in the mouse vas deferens and CHOhOP4, while it contracted the mouse colon and increased the twitch response of the rat vas deferens, and produced a naloxone-sensitive inhibition of the electrically evoked twitches in the guinea pig ileum. pA2 values for the new antagonist against deltorphin I in the mouse vas deferens (OP1 receptors), or against dermorphin in the guinea pig ileum (OP3 receptors), etorphine in the rat vas deferens (OP receptors), U69593 in the rabbit vas deferens (OP2 receptors) and endomorphin 1 in the mouse colon (OP3 receptors) were lower than 6. Taken together, these data indicate that J-113397 is a high-affinity, selective and competitive antagonist of the OP4 receptor; this novel pharmacological tool will be of great value in studies directed at evaluating the physiological roles of the nociceptin/OP4 system.     

The role of the ORL1 receptor in the modulation of spinal neurotransmission by nociceptin/orphanin FQ and nocistatin

Nociceptin/orphanin FQ and nocistatin are two neuropeptides with opposing effects on spinal neurotransmission and nociception. Nociceptin/orphanin FQ selectively suppresses excitatory glutamatergic neurotransmission, while nocistatin selectively interferes with glycinergic and gamma-aminobutyric acid (GABA)-ergic transmission. Here, we performed whole-cell patch-clamp recordings from superficial rat spinal cord dorsal horn neurons to investigate the role of the opioid receptor-like (ORL)1 receptor for modulatory actions of these peptides. The partial ORL1 receptor antagonist [phe1psi(CH(2)-NH)Gly(2)]nociceptin-(1-13)NH(2) competitively reversed the effects of nociceptin/orphanin FQ on excitatory neurotransmission (estimated pA(2) 6.43), but left the suppression of inhibitory synaptic transmission by nocistatin unaffected. These results indicate that the inhibitory action of nociceptin/orphanin FQ on glutamatergic transmission is mediated via ORL1 receptors, while nocistatin acts via a different so far unidentified receptor.     

1,4-Dioxolane-triazaspirodecanone derivatives as nociceptin/orphanin FQ receptor ligands

A series of N-substituted analogs based upon the spiropiperidine core of the lead compound Spiroxatrine was synthesized. In particular, the new compounds were obtained by replacing the benzodioxane moiety of the Spiroxatrine with several 2-substituted 1,3-dioxolanes. Thus the designed derivatives were synthesized and evaluated as possible NOP receptor ligands. As a conclusion of these studies, the new triazaspirodecanone derivatives showed unique and significant SAR as NOP receptor agonists. In particular, the present study demonstrated that 1-phenyl-1,3,8-triazaspiro[4.5]decan-4-one portion together with appropriate 1,3-dioxolane substituents could lead to a new promising class of NOP receptor ligands.     

UFP-101, a peptide antagonist selective for the nociceptin/orphanin FQ receptor

Nociceptin/orphanin FQ modulates various biological functions at central and peripheral levels by selectively activating a G-protein coupled receptor named N/OFQ peptide (NOP) receptor. For extending our knowledge on the biological roles of the N/OFQ-NOP receptor system the identification of selective NOP ligands, especially antagonists, is mandatory. [Nphe1, Arg14, Lys15] N/OFQ-NH2 (UFP-101) is a novel NOP ligand that was designed by combining, in the same molecule, the [Nphe1] chemical modification which eliminates efficacy and the [Arg14, Lys15] substitution which increases ligand potency and duration of action in vivo. In the present article, we summarize the pharmacological features of UFP-101 as determined in a series of in vitro and in vivo assays. Moreover, some biological actions and possible therapeutic indications of NOP ligands are discussed on the basis of results obtained with UFP-101. Data obtained with this compound were compared with those generated using other NOP antagonists, especially J-113397 and [Nphe1]N/OFQ(1-13)-NH2, receptor or peptide knockout mice and other pharmacological tools useful for blocking N/OFQ - NOP receptor signaling. The analysis of the available data demonstrates that UFP-101 is a useful pharmacological tool for the investigation of the central and peripheral biological functions regulated by the N/OFQ-NOP receptor system and for defining the therapeutic potential of NOP receptor ligands.     

The nociceptin/orphanin FQ receptor (NOP) as a target for drug abuse medications

Several studies show that the nociceptin receptor NOP plays a role in the regulation of reward and motivation pathways related to substance abuse. Administration of the NOP's natural peptide ligand, Nociceptin/Orphanin FQ (N/OFQ) or synthetic agonist Ro 64-6198 has been shown to block rewarding effects of cocaine, morphine, amphetamines and alcohol, in various behavioral models of drug reward and reinforcement, such as conditioned place preference and drug self-administration. Administration of N/OFQ has been shown to reduce drug-stimulated levels of dopamine in mesolimbic pathways. The NOP-N/OFQ system has been particularly well examined in the development of alcohol abuse in animal models. Furthermore, the efficacy of the mixed-action opioid buprenorphine, in attenuating alcohol consumption in human addicts and in alcohol-preferring animal models, at higher doses, has been attributed to its partial agonist activity at the NOP receptor. These studies suggest that NOP receptor agonists may have potential as drug abuse medications. However, the pathophysiology of addiction is complex and drug addiction pharmacotherapy needs to address the various phases of substance addiction (craving, withdrawal, relapse). Further studies are needed to clearly establish how NOP agonists may attenuate the drug addiction process and provide therapeutic benefit. Addiction to multiple abused drugs (polydrug addiction) is now commonplace and presents a treatment challenge, given the limited pharmacotherapies currently approved. Polydrug addiction may not be adequately treated by a single agent with a single mechanism of action. As with the case of buprenorphine, a mixed-action profile of NOP/opioid activity may provide a more effective drug to treat addiction to various abused substances and/or polydrug addiction.     

Characterisation and comparison of novel ligands for the nociceptin/orphanin FQ receptor

Studies of nociceptin/orphanin FQ (NC) have been hampered by the paucity of available ligands with activity at the nociceptin receptor (NCR). In this study we have compared the agonist profile of NC and a novel NCR agonist, Ro65-6570, in a series of radioligand binding studies and effects on forskolin-stimulated cAMP formation in Chinese hamster ovary (CHO) cells expressing the recombinant human NCR (CHOhNCR). In addition, we report the effects of three antagonists, [Nphe1]NC(1-13)NH2, J-113397 and III-BTD, on these responses. In radioligand binding studies Ro65-6570, [Nphe1]NC(1-13)NH2, J-113397 and III-BTD displaced [3H]NC with similar pKi values (8.4-8.8). This compares with a pK(D) of 10.2 for NC in a direct saturation experiment. [Nphe1]NC(1-13)NH2 and J-113397 showed at least 100-fold selectivity over classical opioid receptors. Both NC and Ro65-6570 produced a concentration-dependent inhibition of cAMP formation with pEC50 values of 9.56+/-0.06 and 8.68+/-0.04, respectively. Maximum inhibition achieved was 100%. [Nphe1]NC(1-13)NH2, J-113397 and III-BTD produced a parallel rightward shift in the concentration-response curves to both NC and Ro65-6570 with pK(B) values of approximately 6.5, approximately 7.5 and approximately 7.7, respectively. Importantly, all three antagonists were devoid of residual agonist activity. Collectively, these data indicate the value of Ro65-6570, [Nphe1]NC(1-13)NH2, J-113397 and III-BTD in studies of the physiological role played by NC. However, due to the relatively poor selectivity of Ro65-6570 and III-BTD caution should be exercised when using tissues that co-express micro-opioid receptors.     

Structure-activity studies of the Phe(4) residue of nociceptin(1-13)-NH(2): identification of highly potent agonists of the nociceptin/orphanin FQ receptor.

A total of 32 compounds was prepared to investigate the functional role of Phe(4) in NC(1-13)-NH(2), the minimal sequence maintaining the same activity as the natural peptide nociceptin. These compounds could be divided into three series in which Phe(4) was replaced with residues that would (i) alter aromaticity or side chain length, (ii) introduce steric constraint, and (iii) modify the phenyl ring. Compounds were tested for biological activity as (a) inhibitors of the electrically stimulated contraction of the mouse vas deferens; (b) competitors of the binding of [(3)H]-NC-NH(2) to mouse brain membranes; and (c) inhibitors of forskolin-stimulated cAMP accumulation in CHO cells expressing the recombinant human OP(4) receptor. Results indicate that all compounds of the first and second series were inactive or very weak with the exception of [N(CH(3))Phe(4)]NC(1-13)-NH(2), which was only 3-fold less potent than NC(1-13)-NH(2). Compounds of the third series showed higher, equal, or lower potencies than NC(1-13)-NH(2). In particular, [(pF)Phe(4)]NC(1-13)-NH(2) (pF) and [(pNO(2))Phe(4)]NC(1-13)-NH(2) (pNO(2)) were more active than NC(1-13)-NH(2) by a factor of 5. In the mVD, these compounds showed the following order of potency: (pF) = (pNO(2)) > or = (pCN) > (pCl) > (pBr) > (pI) = (pCF(3)) = (pOCH(3)) > (pCH(3)) > (pNH(2)) = (pOH). (oF) and especially (mF) maintained high potencies but were less active than (pF). Similar orders of potency were observed in binding competition and cAMP accumulation studies. There was a strong (r(2) > or = 0.66) correlation between data observed in these assays. Biological activity data of compounds of the third series were plotted against some Hansch parameters that are currently used to quantify physicochemical features of the substituents. In the three biological assays agonist potency/affinity positively correlates with the electron withdrawal properties of the groups in the p-position of Phe(4) and inversely with their size.     

Antidepressant- and anxiolytic-like effects of nociceptin/orphanin FQ receptor ligands

Many studies point toward the nociceptin/orphanin FQ (N/OFQ) and the N/OFQ peptide receptor (NOP) as targets for the development of innovative drugs for treating affective disorders. It has been reported that the activation of NOP receptors produces anxiolytic-like effects in rodents in a large series of behavioral assays, i.e., elevated plus maze, light-dark aversion, operant conflict, fear-potentiated startle, pup ultrasonic vocalizations, and hole board tests. In contrast, the blockade of N/OFQ signaling obtained with NOP-selective antagonists promotes antidepressant-like effects in the forced swimming and tail suspension tests. In these assays, N/OFQ is inactive per se, but reverses the antidepressant-like effects of NOP antagonists. NOP receptor knockout mice show an antidepressant-like phenotype, and NOP antagonists are inactive in these animals. Thus, the activation of the NOP receptor seems to evoke anxiolytic-like effects while its blockade antidepressant-like effects. This appears to be a rather unique behavioral profile since the activation or the blockade of a given neuropeptide receptor produces, in most of the cases, both antidepressant- and anxiolytic-like effects. This particular behavioral profile, the possible mechanisms of action, and the therapeutic potential of NOP receptor ligands for the treatment of depression and anxiety disorders are discussed in this review article.     

Enhanced nicotine sensitivity in nociceptin/orphanin FQ receptor knockout mice

The opioid peptide nociceptin (orphanin FQ) has been implicated in reward, reinforcement and addiction. The current study sought evidence of a role of endogenous nociceptin in nicotine responses by studying nociceptin receptor (NOP) knockout mice. The results were: (1) NOP receptor knockout mice showed enhanced anxiety-like behavior on an elevated plus maze. Whereas nicotine (0.05-0.5 mg/kg) tended to be anxiogenic in wild-type mice, NOP receptor KO mice were resistant to this effect, though interpretation was confounded by their stronger anxiety-like behavior. (2) When presented increasing nicotine concentrations (3-50 μg/ml) in a bottle choice drinking paradigm, there were no genotype-dependent differences in nicotine preference. However, NOP receptor knockout mice consumed more 3 μg/ml nicotine solution when considered in absolute terms. (3) NOP receptor knockout mice showed stronger hypothermic responses to nicotine (1 or 2 mg/kg) administration. (4) There was modest evidence that NOP receptor KO mice showed attenuated behavioral sensitization to a low dose of nicotine (0.05 mg/kg) during repeated daily treatment. (5) NOP receptor knockout mice more rapidly tolerated the sedative effect of nicotine (1 mg/kg), due partially to slightly lower locomotion on first treatment. (6) NOP receptor knockout mice, unlike wild-type mice, showed a significant mecamylamine (2.5 mg/kg) induced conditioned place aversion to nicotine (24 mg/kg/day) withdrawal. These results show that mice lacking the influence of endogenous N/OFQ mice are hypersensitive to nicotine in most measures, showing a role of endogenous nociceptin in modulating or mediating the acute effects of nicotine, and possibly nicotine addiction.     

Modulation of synaptic transmission by nociceptin/orphanin FQ and nocistatin in the spinal cord dorsal horn of mutant mice lacking the nociceptin/orphanin FQ receptor

Nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) are two neuropeptides derived from the same precursor protein that exhibit opposing effects on spinal neurotransmission and nociception. Here, we have used whole-cell, patch-clamp recordings from visually identified neurons in spinal cord dorsal horn slices of genetically modified mice to investigate the role of the N/OFQ receptor (N/OFQ-R) in the modulatory action of both peptides on excitatory glutamatergic and inhibitory glycinergic and gamma-aminobutyric acid (GABA)-ergic synaptic transmission. In wild-type mice, N/OFQ selectively suppressed excitatory transmission in a concentration-dependent manner but left inhibitory synaptic transmission unaffected. In contrast, NST reduced only inhibitory but not alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated excitatory synaptic transmission. N/OFQ-mediated inhibition of excitatory transmission was completely absent in N/OFQ-R receptor-deficient (N/OFQ-R(-/-)) mice and significantly reduced in heterozygous (N/OFQ-R(+/-)) mice, whereas the action of NST on inhibitory neurotransmission was completely retained. To test for the relevance of these results for spinal nociception, we investigated the effects of intrathecally injected N/OFQ in the mouse formalin test, an animal model of tonic pain. N/OFQ (3 nmol/mouse) induced significant antinociception in wild-type mice, but had no antinociceptive effects in N/OFQ-R(-/-) mice. These results indicate that the inhibitory action of N/OFQ on excitatory glutamatergic synaptic transmission and its spinal antinociceptive action are mediated via the N/OFQ receptor, whereas the action of NST is independent of this receptor.     

High affinity conformationally constrained nociceptin/orphanin FQ(1-13) amide analogues

A series of cyclic analogues with a lactam linkage were prepared by solid phase peptide synthesis to explore possible biologically active conformation(s) of nociceptin/orphanin FQ (N/OFQ). cyclo[D-Asp(7),Lys(10)]- and cyclo[Asp (6),Lys(10)]N/OFQ(1-13)NH2 exhibit high affinity (Ki = 0.27 and 0.34 nM, respectively) and high potency in the GTPgammaS assay (EC 50 = 1.6 and 4.1 nM, respectively) at human nociceptin/orphanin FQ peptide (NOP) receptors. These analogues exhibit 2- to 3-fold higher affinity and 2- to 5-fold higher potency than the parent peptide.     

Effects of chronic nociceptin/orphanin FQ exposure on cAMP accumulation and receptor density in Chinese hamster ovary cells expressing human nociceptin/orphanin FQ receptors

Since the therapeutic efficacy of Li+ in the treatment of mood disorder is observed only after chronic administration, we examined whether long-term Li+ treatment with a therapeutic concentration affected the elevation of intracellular-free Ca2+ concentration ([Ca2+]i) induced by carbachol, a muscarinic receptor agonist, in 1321N1 human astrocytoma cells. Carbachol caused [Ca2+]i elevation through phosphoinositide hydrolysis in a concentration-dependent manner. Treatment of the cells with phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, for 2 min resulted in a reduction of the carbachol-induced [Ca2+]i elevation. However, PMA did not reduce the carbachol-induced [Ca2+]i elevation in cells treated with PMA for 48 h, reflecting the down-regulation of protein kinase C. Although Li+ at a therapeutic concentration (1 mM) did not affect the carbachol-induced [Ca2+]i elevation in normal cells, it potently inhibited the [Ca2+]i elevation in protein kinase C down-regulated cells. This inhibitory action of Li+ was observed in a concentration- and time-dependent manner. When protein kinase C activity was directly determined, Li+ treatment did not restore protein kinase C activity in protein kinase C down-regulated cells. [3H]Quinuclidinyl benzylate, a muscarinic receptor ligand, bound to membranes derived from normal and protein kinase C down-regulated cells with a similar Kd and Bmax, and Li+ did not affect these parameters of [3H]quinuclidinyl benzylate binding. These results indicated that Li+ at a therapeutic concentration reduced the muscarinic receptor-mediated increased in [Ca2+]i under the protein kinase C-deficient condition without affecting muscarinic receptor or protein kinase C activity.     

Pharmacological profile of nociceptin/orphanin FQ receptors

1. Nociceptin/orphanin FQ (NC) and its receptor (OP4) represent a novel peptide/receptor system pharmacologically distinct from classical opioid systems. 2. Via OP4 receptor activation, NC regulates several biological functions, both at peripheral and central levels; therefore, the OP4 receptor may be viewed as a novel target for drug development. However, the pharmacology of this receptor is still in its infancy, with few molecules interacting selectively with this receptor. 3. In the present article, we review the findings of studies that have investigated the pharmacological profile of ligands selective for the OP4 receptor, these being two antagonists, the peptide [Nphe1]NC(1-13)NH2 and the non-peptide J-113397, and two agonists, the peptide [Arg14,Lys15]NC, and the non- peptide Ro 64-6198. 4. The results of these studies indicate that agents that selectively activate or block the OP4 receptor may represent new potential drugs for the treatment of human diseases.     

The ORL-1 (NOP1) receptor ligand nociceptin/orphanin FQ (N/OFQ) inhibits neurogenic dural vasodilatation in the rat

The effects of the ORL-1 (NOP(1)) receptor ligand nociceptin (N/OFQ) and the nociceptin antagonists [Nphe(1)]N/OFQ-(1-13)-NH(2) (Nphe) and nocistatin (NST) on neurogenic dural vasodilatation (NDV) in the rat dura mater evoked by electrical stimulation of a closed cranial window were studied. The middle meningeal artery was visualised using intravital microscopy, and the vessel diameter analysed using a video dimension analyser. N/OFQ (1, 10, 100 nmol kg(-1); i.v., n=10) significantly and dose-dependently suppressed NDV maximally by 65% (P<0.01). Neither Nphe (100 nmol kg(-1); n=5) nor NST (100 nmol kg(-1); n=4) alone had an effect on NDV (P>0.05). Baseline vessel diameter was not significantly affected by application of N/OFQ, NST or Nphe. Application of the selective N/OFQ antagonist Nphe (10, 100 nmol kg(-1) i.v., n=8) dose-dependently and significantly (P<0.01) reversed the inhibition of NDV induced by application of N/OFQ (10 nmol kg(-1)). NST (10, 100 nmol kg(-1); n=7) failed to reverse the effects elicited by N/OFQ. Application of N/OFQ elicited a dose-dependent transient decrease in arterial blood pressure (P<0.01). Nphe dose-dependently reversed the cardiovascular effects induced by application of N/OFQ (10 nmol kg(-1)) (P<0.01),while NST did not alter the blood pressure reaction elicited by N/OFQ. The results show that N/OFQ inhibits NDV, an effect which is antagonised by Nphe, but not by NST. ORL-1 (NOP(1)) receptors located on trigeminal sensory fibres may be involved in the regulation of dural vessel diameter and hence may play a role in migraine pathophysiology.     

Development of LC-MS/MS-based receptor occupancy tracers and positron emission tomography radioligands for the nociceptin/orphanin FQ (NOP) receptor

Currently, a lack of sufficient tools has limited the understanding of the relationship between neuropsychiatric disorders and the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor. Herein, we describe the discovery and development of an antagonist NOP receptor occupancy (RO) tracer and a novel positron emission tomography (PET) radioligand suitable to probe the NOP receptor in human clinical studies. A thorough structure-activity relationship (SAR) around the high-affinity 3-(2'-fluoro-4',5'-dihydrospiro[piperidine-4,7'-thieno[2,3-c]pyran]-1-yl)-2-(2-halobenzyl)-N-alkylpropanamide scaffold identified a series of subnanomolar, highly selective NOP antagonists. Subsequently, these unlabeled NOP ligands were evaluated in vivo by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in rat to determine brain uptake, kinetics and specific binding. (S)-27 was identified as a suitable unlabeled preclinical RO tracer to accurately quantify NOP receptor engagement in rat brain. Three compounds were selected for evaluation in nonhuman primates as PET tracers: (-)-26, (-)-30, and (-)-33. Carbon-11 labeling of (+)-31 yielded [(11)C]-(S)-30, which exhibited minimal generation of central nervous system (CNS) penetrant radiometabolites, improved brain uptake, and was an excellent PET radioligand in both rat and monkey. Currently [(11)C]-(S)-30 is being evaluated as a PET radiotracer for the NOP receptor in human subjects.     

Involvement of the GABA/benzodiazepine receptor in the axiolytic-like effect of nociceptin/orphanin FQ

We investigated the mechanism underlying the anxiolytic actions of the neuropeptide nociceptin/orphanin FQ (N/OFQ) with an elevated plus-maze test. In mice, intracerebroventricular (i.c.v.) infusions of N/OFQ (0.1 and 0.32 nmol) led to an increase in time spent in the open arms (anxiolytic-like effects). A non-peptidyl N/OFQ receptor (NOP) antagonist, J-113397(1-{(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl}-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one), (1.0 and 3.2 mg/kg, s.c.) blocked the increase induced by N/OFQ. On the other hand, a benzodiazepine receptor antagonist, flumazenil, (10 mg/kg, i.p.) and a GABAA receptor antagonist, (+)-bicuculline, (5.6 mg/kg, i.p.) also inhibited the increase induced by N/OFQ. In rats, microinfusions of N/OFQ (10 and 32 pmol) into the amygdala led to an increase in time spent in the open arms. However, intracranial infusions of N/OFQ (10-100 pmol) into the dorsal hippocampus did not affect the time spent in the open arms. These findings suggest that the anxiolytic-like effects of N/OFQ may be related to the GABA/benzodiazepine system in the amygdala.