Selective N/OFQ Peptide Receptor Research Articles

Discovery of small-molecule nonpeptide antagonists of nociceptin/orphanin FQ receptor: The studies of design, synthesis, and structure–activity relationships for (4-arylpiperidine substituted-methyl)-[bicyclic (hetero)cycloalkanobenzene] derivatives

Nociceptin/orphanin FQ (N/OFQ) and N/OFQ peptide (NOP) receptor are expressed and distributed in various regions such as central nervous system (CNS), peripheral nervous system, immune system, and peripheral tissues. N/OFQ and NOP receptor have important roles on a variety of physiological, pathophysiological, regulatory, and dysregulatory mechanisms in the living body. Both activation and blockade of NOP receptor function have displayed clinical potential of NOP receptor agonists and antagonists for the treatment of various diseases or pathophysiological conditions, respectively. Potent and selective NOP receptor agonists/antagonists are also useful tools to investigate the various mechanisms mediated by NOP receptor–N/OFQ system. As the present study, a series of (4-arylpiperidine substituted-methyl)-[bicyclic (hetero)cycloalkanobenzene] analogs was designed, synthesized, and biologically evaluated in vitro to seek and identify potent and selective, small-molecules of nonpeptide NOP receptor antagonists, which resulted in the discovery of novel potent small-molecule 15 with high human NOP receptor selectivity over human μ receptor. The structure–activity relationship (SAR) of the potency and selectivity, structure–metabolic stability relationship (SMR), and SAR of hERG (human ether-a-go-go related gene) potassium ion channel binding affinity for the analogs in the present studies in vitro provided or suggested significant and/or useful structural determinants and insights for the respective purposes. The superior profiles of compound 15 are discussed with a viewpoint of multisite interactions between ligand and NOP receptor, together with the results of previous NOP receptor agonist/antagonist studies.

Efficient design, synthesis and structure–activity relationship studies of 1-(3′-substituted propyl)-4-arylpiperidines as non-peptide antagonists of nociceptin/orphanin FQ receptor: biological activities, metabolic stabilities and hERG channel bindings

Nociceptin/orphanin FQ (N/OFQ) is an endogenous heptadecapeptide that is a metabolite of precursor polypeptide (prepro-N/OFQ), and N/OFQ peptide (NOP) receptor (or opioid-receptor-like-1 receptor) is a G-protein-coupled receptor (GPCR) that is distinct from classical opioid peptide receptors, whereas the receptors share high sequence-similarities. As well, [35S]-guanosine 5′-(γ-thiotriphosphate) binding that was stimulated by N/OFQ–NOP receptor binding was not affected by various GPCR antagonists including opioid receptor antagonists. N/OFQ and NOP receptor are located in the spinal cord, the peripheral nervous systems and other peripheral tissues that are related to pain-inhibitory signal transmissions, and in the corticolimbic regions that are involved in the integration of the emotional components. Indeed, potent and selective new-class NOP receptor agonists as systemically potent analgesic against neuropathic pain and as orally potent anxiolytic with respective unique safe-profiles have been discovered in our studies. Besides, the blockade of N/OFQ actions via N/OFQ–NOP receptor system has been displayed as anti-depression effect, anti-hyperphasia effect and anti-hypotension effect in animal model studies, which might show potential utilities of NOP receptor antagonists to modulate/attenuate N/OFQ activity for regulation of human physiologies in pharmacological and clinical viewpoints. Hence, the design, synthesis, structure–activity relationship in vitro and structure–metabolic stability relationship in vitro of various 1-(3′-substituted propyl)-4-arylpiperidine derivatives were investigated to seek and identify potent and selective new-class NOP receptor antagonists with metabolic stabilities and little hERG ion channel binding affinities by multi-viewpoint and integrated drug-design strategies, with clarifying structural features and physicochemical properties as key factors for the purposes. The unique and efficient studies, and their exclusive results and findings for the analogues are described herein. Efficient and novel methodology of multi-analogue synthesis for 1-[3′-(N,N-disubstituted amino)propyl]-4-arylpiperidine derivatives 3.

Acute and chronic antiparkinsonian effects of the novel nociceptin/orphanin FQ receptor antagonist NiK‐21273 in comparison with SB‐612111

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonists have been proposed as a novel therapeutic approach to Parkinson's disease. Main limitations of previous studies were the use of structurally similar compounds and the evaluation of their acute effects only. We report here on the acute and long-term antiparkinsonian effects of the novel compound 2-[3-[4-(2-chloro-6-fluoro-phenyl)-piperidin-1-ylmethyl]-2-(morpholine-4-carbonyl)-indol-1-yl]-acetamide (NiK-21273) in comparison with the potent and selective NOP receptor antagonist SB-612111. Basic pharmacological properties of NiK-21273 were studied in cell lines and isolated tissues (mouse and rat vas deferens). Antiparkinsonian effects were studied in reserpinized mice and 6-hydroxydopamine hemilesioned rats under both acute and chronic administration protocols. In vitro, NiK-21273 behaved as a potent (pA(2) 7.7) and selective NOP receptor antagonist. In vivo, it reduced hypokinesia in reserpinized mice at 0.1 and 1 but not 10 mg·kg(-1), whereas SB-612111 (0.01-1 mg·kg(-1)) provided a dose-dependent antiparkinsonian effect. NiK-21273 ameliorated motor performance in 6-hydroxydopamine hemilesioned rats at 0.5 and 5 but not 15 mg·kg(-1). SB-612111 replicated these effects in the 0.01-1 mg·kg(-1) range without loss of efficacy. Both antagonists synergized with L-DOPA at subthreshold doses. Chronic administration of NiK-21273 provided delayed improvement in baseline activity at 0.5 and 1.5 mg·kg(-1), although tolerance to the higher dose was observed. Conversely, SB-612111 (1 mg·kg(-1)) maintained its effects over time without modifying baseline activity. NOP receptor antagonists provide motor benefit in parkinsonism models although the 'therapeutic' window and long-term effects may vary between compounds.

Discovery of 1-(β-amino substituted-β-alanyl)-N,N-dimethylindoline-2-carboxamides as novel nonpeptide antagonists of nociceptin/orphanin FQ receptor: Efficient design, synthesis, and structure–activity relationship studies

Since the discovery of endogenous nociceptin/orphanin FQ (N/OFQ) peptide and N/OFQ peptide (NOP) receptor [or opioid-receptor-like-1 (ORL1) receptor], the structures, distribution, and pharmacology have been reported in detail. N/OFQ and NOP receptor are located in the corticolimbic regions that are involved in the integration of the emotional activity, and located in the spinal cord, the peripheral nervous systems or other peripheral tissues that are related to pain as well as urinary signal transmissions, with a pattern distinct from that of classical opioid peptides and their receptors in rodents or primates. Furthermore, N/OFQ–NOP receptor system plays an important role in the regulation of various human physiologies such as depression effect, hyperphasia effect, and blood pressure effect. In this study, the structure–activity relationship of novel NOP receptor antagonist for various 1-(β-amino substituted-β-alanyl)-N,N-dimethylindoline-2-carboxamides was investigated in vitro to elucidate structural requisites to identify and develop potent and selective NOP receptor antagonists, which resulted in the discovery of 1-{3-[4-(substituted phenyl)piperidin-1-yl]propanoyl}-N,N-dimethylindoline-2-carboxamide analogues that display potent and selective human NOP (hNOP) receptor binding affinity and potent hNOP receptor antagonist activity. The efficient design, synthesis, and structure–activity relationship studies for potent and selective novel NOP receptor antagonists and significant findings in vitro, that include insights for binding and functional mechanisms via receptor–ligand interactions, are reported herein.

Nociceptin/orphanin FQ receptor knockout rats: In vitro and in vivo studies

Nociceptin/orphanin FQ (N/OFQ) regulates several biological functions via selective activation of the N/OFQ peptide (NOP) receptor. Recently knockout rats for the NOP receptor gene (NOP(−/−)) have been generated; these animals were used in the present study to investigate their emotional (open field, elevated plus maze, and forced swimming test), locomotor (drag and rotarod test), and nociceptive (plantar and formalin test) phenotypes in comparison with their NOP(+/+) littermates. In addition, N/OFQ sensitivity has been assessed in electrically stimulated vas deferens tissues taken from NOP(+/+) and NOP(−/−) rats. In the elevated plus maze and forced swimming tests NOP(−/−) rats showed anxiety- and anti-depressant-like phenotype, respectively. No differences were found in the open field test. NOP(−/−) rats outperformed their NOP(+/+) littermates in two motor behaviour assays. Genetic ablation of the NOP receptor gene produced a statistically significant increase in nociceptive behaviour of the mutant rats in the formalin test. Finally, in the electrically stimulated rat vas deferens taken from NOP(+/+) tissues, N/OFQ inhibited in a concentration-dependent manner the electrically induced twitches while the peptide was inactive in tissues taken from NOP(−/−) animals. These results, in line with previous findings obtained with selective NOP receptor antagonists in mice and rats and with mouse knockout studies, clearly indicate that endogenous N/OFQ–NOP receptor signalling plays an important role in controlling anxiety- and mood-related behaviours, exercise-driven locomotor activity and nociception. These observations are relevant for defining the therapeutic indications (and contraindications) of NOP receptor antagonists.

Discovery of {1-[4-(2-{hexahydropyrrolo[3,4- c]pyrrol-2(1 H)-yl}-1 H-benzimidazol-1-yl)piperidin-1-yl]cyclooctyl}methanol, systemically potent novel non-peptide agonist of nociceptin/orphanin FQ receptor as analgesic for the treatment of neuropathic pain: Design, synthesis, and structure–activity relationships

Neuropathic pain is a serious chronic disorder caused by lesion or dysfunction in the nervous systems. Endogenous nociceptin/orphanin FQ (N/OFQ) peptide and N/OFQ peptide (NOP) receptor [or opioid-receptor-like-1 (ORL1) receptor] are located in the central and peripheral nervous systems, the immune systems, and peripheral organs, and have a crucial role in the pain sensory system. Indeed, peripheral or intrathecal N/OFQ has displayed antinociceptive activities in neuropathic pain models, and inhibitory effects on pain-related neurotransmitter releases and on synaptic transmissions of C- and Aδ-fibers. In this study, design, synthesis, and structure–activity relationships of peripheral/spinal cord-targeting non-peptide NOP receptor agonist were investigated for the treatment of neuropathic pain, which resulted in the discovery of highly selective and potent novel NOP receptor full agonist {1-[4-(2-{hexahydropyrrolo[3,4- c]pyrrol-2(1 H)-yl}-1 H-benzimidazol-1-yl)piperidin-1-yl]cyclooctyl}methanol 1 (HPCOM) as systemically (subcutaneously) potent new-class analgesic. Thus, 1 demonstrates dose-dependent inhibitory effect against mechanical allodynia in chronic constriction injury-induced neuropathic pain model rats, robust metabolic stability and little hERG potassium ion channel binding affinity, with its unique and potentially safe profiles and mechanisms, which were distinctive from those of N/OFQ in terms of site-differential effects.

Comparative biochemical and pharmacological characterization of a novel, NOP receptor selective hexapeptide, Ac-RYYRIR-ol

Nociceptin/orphanin FQ (N/OFQ) is an endogenous neuropeptide, which is widely distributed in central and peripheral nervous system. Some N/OFQ sequence unrelated hexapeptides can effectively bind to the N/OFQ peptide (NOP) receptor and they were used as template for structure-activity studies that lead to discovery of the new NOP selective ligands. In the present study, the pharmacological profile of the novel hexapeptide Ac-RYYRIR-ol was investigated using various in vitro assays including receptor binding and G-protein activation in rat brain membranes, mouse and rat vas deferens, guinea pig ileum, mouse colon and Ca 2+ mobilization assay in chinese hamster ovary (CHO) cells co-expressing the human recombinant NOP receptor and the C-terminally modified Gα qi5 protein. In rat brain membranes Ac-RYYRIR-ol displaced both [ 3H]nociceptin/OFQ and [ 3H]Ac-RYYRIK-ol with high affinity (p K i 9.35 and 8.81, respectively) and stimulated [ 35S]GTPγS binding showing however lower maximal effects than N/OFQ ( α = 0.28). The stimulatory effect of Ac-RYYRIR-ol was antagonized by the selective NOP receptor antagonist UFP-101. In the electrically stimulated mouse vas deferens Ac-RYYRIR-ol displayed negligible agonist activity while antagonizing in a competitive manner (pA 2 7.99) the inhibitory effects of N/OFQ. Similar results were obtained in the rat vas deferens. In the mouse colon Ac-RYYRIR-ol produced concentration dependent contractile effects with similar potency and maximal effects as N/OFQ. Finally, in the Ca 2+ mobilization assay performed with CHO-hNOP-Gα qi5 cells Ac-RYYRIR-ol displayed lower potency and maximal effects ( α = 0.87) compared with N/OFQ. In conclusion, the novel NOP receptor selective hexapeptide Ac-RYYRIR-ol has been shown to have fine selectivity, high potency, furthermore agonist and antagonist effects toward the NOP receptors were measured in various assays; this is likely due to its partial agonist pharmacological activity.

Nociceptin Modulates Bronchoconstriction Induced by Sensory Nerve Activation in Mouse Lung

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand for the N/OFQ peptide receptor (NOP), inhibits tachykinin release in the airway of several animal models. The aim of this study was to investigate the role of the N/OFQ-NOP receptor system in bronchoconstriction induced by sensory nerve activation in the isolated mouse lung. We used C57BL/6J NOP(+/+), NOP(-/-), and Balb/C mice sensitized (or not) to ovalbumin. Bronchopulmonary function coupled with measurements of endogenous N/OFQ levels before and after capsaicin-induced bronchoconstriction in the presence or absence of NOP-selective agonists/antagonists are presented. N/OFQ significantly inhibited capsaicin-induced bronchoconstriction in both naive and sensitized mice, these latter animals displaying airway hyperresponsiveness to capsaicin. The inhibitory effect of N/OFQ were not observed in NOP(-/-) mice, and were mimicked/abolished by the selective NOP agonist/antagonist University of Ferrara Peptide (UFP)-112/UFP-101 in NOP(+/+) mice. UFP-101 alone potentiated the effect of capsaicin in naive mice, but not in sensitized mice. Endogenous N/OFQ levels significantly decreased in sensitized mice relative to naive mice. We have demonstrated that a reduction in endogenous N/OFQ, or the lack of its receptor, causes an increase in capsaicin-induced bronchoconstriction, implying a role for the N/OFQ-NOP receptor system in the modulation of capsaicin effects. Moreover, for the first time, we document differential airway responsiveness to capsaicin between naive and sensitized mice due, at least in part, to decreased endogenous N/OFQ levels in sensitized mice.

Pharmacological Characterization of the Nociceptin/Orphanin FQ Receptor Antagonist SB-612111 [(–)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol]: In Vivo Studies

The excellent pharmacological profile displayed by the selective nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonist SB-612111 [(-)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol] in vitro prompted us to investigate the actions of this compound in vivo. In the mouse tail withdrawal assay, SB-612111 given i.p. up to 3 mg/kg did not modify per se tail withdrawal latencies but was able to prevent the pronociceptive and the antinociceptive action of 1 nmol of N/OFQ given i.c.v. and i.t., respectively. In food intake studies performed in sated mice, SB-612111 (1 mg/kg i.p.) had no effect on food consumption but fully prevented the orexigenic effect of 1 nmol of N/OFQ i.c.v. In 17-h food-deprived mice, the opioid receptor antagonist naltrexone (1 mg/kg s.c.), but not SB-612111 (1 and 10 mg/kg i.p.), produced a statistically significant reduction of food intake. In the mouse forced swimming and tail suspension tests, SB-612111 (1-10 mg/kg) reduced immobility time. The antidepressant-like effect elicited by SB-612111 in the forced swimming test was reversed by the i.c.v. injection of 1 nmol of N/OFQ and no longer evident in mice knockout for the NOP receptor gene. In conclusion, the present findings demonstrate that SB-612111 behaves in vivo as a potent and selective NOP antagonist and suggest that the N/OFQ-NOP receptor endogenous system plays an important role in regulating mood-related behaviors. The use of SB-612111 in future pathophysiological studies will certainly contribute to define the therapeutic potential of selective NOP receptor antagonists in different disease areas.

Effects of Intrathecally Administered Nociceptin/Orphanin FQ in Monkeys: Behavioral and Mass Spectrometric Studies

Nociceptin/orphanin FQ (N/OFQ) is a heptadecapeptide that is an endogenous ligand for the N/OFQ peptide (NOP) receptor. The aim of this study was to investigate the behavioral responses of N/OFQ and its major fragment N/OFQ(2-17) in monkeys following i.t. administration. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was used to quantify the amounts of N/OFQ and N/OFQ(2-17) in the cerebrospinal fluid at specific time points when effects of i.t. N/OFQ were sustained and disappeared. Intrathecal administration of N/OFQ dose dependently (10-100 nmol) produced long-lasting antinociception against a noxious stimulus, 50 degrees C water, and did not elicit itch/scratching responses in monkeys. Subcutaneous pretreatment with a selective NOP receptor antagonist, (+)J-113397 [(1-[3R,4R)-1-cyclooctymethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3,-dihydro-2H-benzimidazol-2-one] (0.1 mg/kg), completely blocked i.t. N/OFQ (100 nmol)-induced antinociception. In contrast, a classic opioid receptor antagonist, naltrexone (0.01 and 1 mg/kg), failed to reverse i.t. N/OFQ-induced antinociception. MALDI-TOF-MS showed that the amount of N/OFQ(2-17) was 4-fold higher than that of N/OFQ at 1.5 h after i.t. administration of 100 nmol N/OFQ. Intrathecal N/OFQ-induced antinociception disappeared at 4.5 h, which corresponded to nearly undetectable cerebrospinal fluid levels of N/OFQ. No other metabolite of N/OFQ was detected at appreciable levels at either the 1.5- or 4.5-h time points. Although significant amounts of N/OFQ(2-17) were detected at the 1.5- and 4.5-h time points, 100 nmol N/OFQ(2-17) i.t. was inactive in changing the monkeys' nociceptive threshold. These results provide the first functional evidence of spinal N/OFQ-induced antinociception in primates and indicate that activation of spinal NOP receptors may be a potential target for spinal analgesics.

Pharmacodynamic Characterization of ZP120 (Ac-RYYRWKKKKKKK-NH2), a Novel, Functionally Selective Nociceptin/Orphanin FQ Peptide Receptor Partial Agonist with Sodium-Potassium-Sparing Aquaretic Activity

In conscious rats, intravenous (i.v.) administration of the hexapeptide Ac-RYYRWK-NH(2), a partial agonist of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor, produces a selective water diuresis without marked cardiovascular or behavioral effects. The present study examined the in vitro and in vivo pharmacodynamic profile of the novel and potentially metabolically stable NOP receptor ligand ZP120 (Ac-RYYRWKKKKKKK-NH(2)), which was created by conjugation of a structure-inducing probe (SIP) (i.e., K(6) sequence) to Ac-RYYRWK-NH(2). In cells transfected with human NOP receptors, both Ac-RYYRWK-NH(2) and ZP120 displaced [(3)H]N/OFQ (both peptides, pK(i) = 9.6), and similar to N/OFQ inhibited forskolin-induced cAMP formation (Ac-RYYRWK-NH(2), pEC(50) = 9.2; ZP120, 9.3; N/OFQ, 9.7). In the mouse vas deferens assay (MVD), Ac-RYYRWK-NH(2) and ZP120 behaved as partial agonists, inhibiting electrically induced contractions with similar pEC(50) values (9.0 and 8.6, respectively) but with submaximal efficacy compared with N/OFQ. In MVD, both peptides blocked the responses to N/OFQ, with ZP120 being approximately 50-fold more potent than Ac-RYYRWK-NH(2). In vivo, dose-response studies in rats showed that at doses (i.v. bolus or i.v. infusion) that produced a sodium-potassium-sparing aquaresis, ZP120 and Ac-RYYRWK-NH(2) elicited a mild vasodilatory response without reflex tachycardia. However, the renal responses to ZP120 were of greater magnitude and duration. Finally, each peptide blocked the bradycardia and hypotension to N/OFQ in conscious rats, but the effect of ZP120 was of much greater duration. Together, these findings demonstrate that ZP120 is a novel, functionally selective SIP-modified NOP receptor partial agonist with increased biological activity and sodium-potassium-sparing aquaretic activity, the actions of which may be useful in the management of hyponatremia/hypokalemia in water-retaining states.

The effects of [Arg 14, Lys 15] nociceptin/orphanin FQ, a highly potent agonist of the NOP receptor, on in vitro and in vivo gastrointestinal functions

Nociceptin/orphanin FQ (N/OFQ) administered into the lateral left cerebral ventricle of rats has been reported to inhibit in vivo gut motor and secretory functions. Recently, a novel N/OFQ analog, [Arg 14, Lys 15] N/OFQ, was synthesized and demonstrated to behave as a highly potent agonist at the human recombinant N/OFQ peptide (NOP) receptors and to produce long-lasting effects in vivo in mice compared with the natural ligand N/OFQ. In the present study, the pharmacological profile of [Arg 14, Lys 15] N/OFQ was further evaluated and compared with that of N/OFQ in vitro on guinea pig exocrine pancreas and in vivo on gastric emptying, colonic propulsion and gastric acid secretion in rats. [Arg 14, Lys 15] N/OFQ and N/OFQ significantly decreased the KCl-evoked amylase secretion from isolated pancreatic lobules of the guinea pig. In in vivo experiments, [Arg 14, Lys 15] N/OFQ mimicked the effects of N/OFQ, inducing, after intracerebroventricular injection, a delay (up to 70%) in the gastric emptying of a phenol red meal, an increase (about 40 times) of the mean bead colonic expulsion time and a decrease (up to 90%) of gastric acid secretion in water loaded rats after 90 min pylorus ligature. In all these assays, [Arg 14, Lys 15] N/OFQ was more effective than N/OFQ, and its effective doses were at least 10-fold lower than N/OFQ effective doses. The highly selective NOP receptor antagonist, UFP-101, decreased the efficacy of [Arg 14, Lys 15] N/OFQ in in vitro and in vivo assays above reported. These findings: (a) show that pancreatic NOP receptors mediate an in vitro inhibitory effect on stimulated guinea pig amylase secretion; (b) confirm that the stimulation of central NOP receptors exerts an inhibitory control on gastric emptying, colonic motility and gastric secretion in rats and (c) put in evidence that [Arg 14, Lys 15] N/OFQ, being more potent and effective than the natural ligand N/OFQ, represents a new pharmacological tool for the study of the physiological and pharmacological roles mediated by the N/OFQ–NOP receptor system.

RAPID COMMUNICATION: Blockade of nociceptin/orphanin FQ transmission in rat substantia nigra reverses haloperidol‐induced akinesia and normalizes nigral glutamate release

AbstractWe recently showed that pharmacological blockade of nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors located in the substantia nigra stimulates the nigrostriatal dopaminergic pathway and motor behavior (Marti et al. J. Neurosci. 2004, 24, 6659–6666). To investigate whether such motor‐stimulating action was dependent on functional dopaminergic transmission, the selective NOP receptor peptide antagonist [Nphe1,Arg14,Lys15]N/OFQ‐NH2 (UFP‐101) was microinjected into the substantia nigra reticulata of rats made cataleptic by systemic haloperidol administration. UFP‐101 reduced haloperidol‐induced akinesia as measured by immobility time in the bar test. UFP‐101 also induced contralateral turning in cataleptic rats. To investigate the mechanisms involved in the anti‐akinetic action of UFP‐101, nigral glutamate release was monitored by microdialysis technique. The anti‐akinetic action of UFP‐101 correlated with normalization of nigral glutamate release, previously elevated by haloperidol injection. We conclude that endogenous N/OFQ in the substantia nigra sustains akinesia generated by impaired DA transmission and subthalamic nucleus overactivation. NOP receptor antagonists may be beneficial in the symptomatic therapy of parkinsonism, via normalization of subthalamonigral glutamatergic transmission.

Blockade of nociceptin/orphanin FQ receptor signaling in rat substantia nigra pars reticulata stimulates nigrostriatal dopaminergic transmission and motor behavior.

A multidisciplinary approach was followed to investigate whether the opioid-like peptide nociceptin/orphanin FQ (N/OFQ) regulates the nigrostriatal dopaminergic pathway and motor behavior. Nigrostriatal dopaminergic cells, which express N/OFQ peptide (NOP) receptors, are located in the substantia nigra pars compacta and extend their dendrites in the substantia nigra pars reticulata, thereby modulating the basal ganglia output neurons. In vitro electrophysiological recordings demonstrated that N/OFQ hyperpolarized the dopaminergic cells of the substantia nigra pars compacta and inhibited their firing activity. In vivo dual-probe microdialysis showed that N/OFQ perfused in the substantia nigra pars reticulata reduced dopamine release in the ipsilateral striatum, whereas UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ(1-13)-NH2) (a selective NOP receptor peptide antagonist) stimulated it. N/OFQ microinjected in the substantia nigra pars reticulata impaired rat performance on a rotarod apparatus, whereas UFP-101 enhanced it. Electromyography revealed that N/OFQ and UFP-101 oppositely affected muscle tone, inducing relaxation and contraction of triceps, respectively. The selective NOP receptor nonpeptide antagonist J-113397 (1-[3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one), either injected intranigrally or given systemically, also elevated striatal dopamine release and facilitated motor activity, confirming that these effects were caused by blockade of endogenous N/OFQ signaling. The inhibitory role played by endogenous N/OFQ on motor activity was additionally strengthened by the finding that mice lacking the NOP receptor gene outperformed wild-type mice on the rotarod. We conclude that NOP receptors in the substantia nigra pars reticulata, activated by endogenous N/OFQ, drive a physiologically inhibitory control on motor behavior, possibly via modulation of the nigrostriatal dopaminergic pathway.