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

Regulator of G-Protein Signalling 4 (RGS4) negatively modulates nociceptin/orphanin FQ opioid receptor signalling: Implication for l-Dopa-induced dyskinesia

BACKGROUND AND PURPOSE

Regulator of G-protein signalling 4 (RGS4) is a signal transduction protein that accelerates intrinsic GTPase activity of Gαi/o and Gαq subunits, suppressing GPCR signalling. Here, we investigate whether RGS4 modulates nociceptin/orphanin FQ (N/OFQ) opioid (NOP) receptor signalling and if this modulation has relevance for l-Dopa-induced dyskinesia.

EXPERIMENTAL APPROACH

HEK293T cells transfected with NOP, NOP/RGS4 or NOP/RGS19 were challenged with N/OFQ and the small-molecule NOP agonist AT-403, using D1-stimulated cAMP levels as a readout. Primary rat striatal neurons and adult mouse striatal slices were challenged with either N/OFQ or AT-403 in the presence of the experimental RGS4 chemical probe, CCG-203920, and D1-stimulated cAMP or phosphorylated extracellular signal regulated kinase 1/2 (pERK) responses were monitored. In vivo, CCG-203920 was co-administered with AT-403 and l-Dopa to 6-hydroxydopamine hemilesioned rats, and dyskinetic movements, striatal biochemical correlates of dyskinesia (pERK and pGluR1 levels) and striatal RGS4 levels were measured.

KEY RESULTS

RGS4 expression reduced NOFQ and AT-403 potency and efficacy in HEK293T cells. CCG-203920 increased N/OFQ potency in primary rat striatal neurons and potentiated AT-403 response in mouse striatal slices. CCG-203920 enhanced AT-403-mediated inhibition of dyskinesia and its biochemical correlates, without compromising its motor-improving effects. Unilateral dopamine depletion caused bilateral reduction of RGS4 levels, which was reversed by l-Dopa. l-Dopa acutely up-regulated RGS4 in the lesioned striatum.

CONCLUSIONS AND IMPLICATIONS

RGS4 physiologically inhibits NOP receptor signalling. CCG-203920 enhanced NOP responses and improved the antidyskinetic potential of NOP receptor agonists, mitigating the effects of striatal RGS4 up-regulation occurring during dyskinesia expression.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Haloperidol-induced catalepsy as an animal model for parkinsonism: A systematic review of experimental studies

Several useful animal models for parkinsonism have been developed so far. Haloperidol-induced catalepsy is often used as a rodent model for the study of motor impairments observed in Parkinson's disease and related disorders and for the screening of potential antiparkinsonian compounds. The objective of this systematic review is to identify publications that used the haloperidol-induced catalepsy model for parkinsonism and to explore the methodological characteristics and the main questions addressed in these studies. A careful systematic search of the literature was carried out by accessing articles in three different databases: Web of Science, PubMed and SCOPUS. The selection and inclusion of studies were performed based on the abstract and, subsequently, on full-text analysis. Data extraction included the objective of the study, study design and outcome of interest. Two hundred and fifty-five articles were included in the review. Publication years ranged from 1981 to 2020. Most studies used the model to explore the effects of potential treatments for parkinsonism. Although the methodological characteristics used are quite varied, most studies used Wistar rats as experimental subjects. The most frequent dose of haloperidol used was 1.0 mg/kg, and the horizontal bar test was the most used to assess catalepsy. The data presented here provide a framework for an evidence-based approach to the design of preclinical research on parkinsonism using the haloperidol-induced catalepsy model. This model has been used routinely and successfully and is likely to continue to play a critical role in the ongoing search for the next generation of therapeutic interventions for parkinsonism.

CCRD based development of bromocriptine and glutathione nanoemulsion tailored ultrasonically for the combined anti-parkinson effect

Bromocriptine Mesylate (BRM) acts as a dopamine receptor agonist along with antioxidant effect and is utilized in the treatment of Parkinson's disease (PD). Glutathione (GSH) is a thiol- reducing agent having antioxidant properties in the brain. Replenishment of GSH inside the brain can play a major role in the management of PD. Both BRM and GSH suffer from low oral bioavailability and poor absorption. The objective of the present study was to develop BRM and GSH loaded nanoemulsion for the combined and synergistic effect delivered through the intranasal route for the better and effective management of PD. After extensive screening experiments, Capmul PG-8 NF was selected as oil, polyethylene glycol (PEG) 400 as a surfactant and propylene glycol as co-surfactant. Ultrasonication technique was employed for the fabrication of nanoemulsion. Central composite rotatable design (CCRD) was used to obtain the best formulation by optimization. Oil (%), S_mix (%), and sonication time (second) were chosen as independent variables for the optimization. Particle size, PDI, zeta potential, % transmittance, pH, refractive index, viscosity and conductivity of the optimized nanoemulsion were found to be 80.71 ± 2.75 nm, 0.217 ± 0.009, -12.60 ± 0.10 mV, 96.00 ± 3.05 %, 6.48 ± 0.28, 1.36 ± 0.03, 30.12 ± 0.10 mPas and 214.28 ± 2.79 μS/cm respectively. Surface morphology demonstrated that nanoemulsion possessed spherical and globular nature of the particle which showed 3.4 times and 1.5 times enhancement in drug permeation in the case of BRM and GSH respectively as compared to suspension. MTT assay done on neuro-2a cell lines revealed that nanoemulsion was safe for intranasal delivery. Behavioural studies were carried out to prove the efficacy of optimized nanoemulsion in PD using forced swimming test, locomotor activity test, catalepsy test, rota-rod test, and akinesia test in Wistar rats. The outcomes of the behavioural studies revealed that BRM and GSH loaded nanoemulsion treatment showed significant improvement in behavioural activities of PD (haloperidol-induced) rats after intranasal administration. This study concluded that BRM and GSH loaded nanoemulsion could be promising for the combined and synergistic anti-parkinson effect for the effective management of PD.

Effects of non-peptide nociceptin/orphanin FQ receptor ligands on methylphenidate-induced hyperactivity in mice: Implications for bipolar disorders

Bipolar disorder is a psychiatric pathology characterized by biphasic mood episodes of mania or hypomania and depression. The pharmacotherapy of bipolar disorder has significant adverse effects impairing treatment adherence and patient quality of life. The N/OFQ-NOP receptor system has been widely implicated with mood disorders. Clinical and preclinical findings suggest antidepressants actions for NOP antagonists. More recently, the administration of NOP agonists has shown to promote depressant states. The present study aimed to investigate the effects of non-peptide NOP ligands in methylphenidate-induced manic-like behavior in mice. The NOP agonist Ro 65-6570 (0.01-1 mg/kg, ip), at the higher dose, did not affect spontaneous locomotion per se, but prevented the methylphenidate (10 mg/kg, sc)-induced hyperlocomotion. The NOP partial agonist AT-090 (0.001-0.03 mg/kg, ip) and the NOP antagonist SB-612111 (1-10 mg/kg, ip) did not significantly affect the psychostimulant-induced hyperactivity. Experiments performed with mice lacking the NOP receptor (NOP(-/-)) demonstrated that the treatment with methylphenidate induced similar hyperlocomotion in NOP(-/-) and NOP(+/+) mice. In conclusion, these findings suggest a potential role for NOP agonists in the prevention of manic states, especially by counteracting the hyperactivity symptom of bipolar patients. However, more studies are necessary in order to evaluate these compounds in other features of bipolar disorder.

Managing Parkinson's disease: moving ON with NOP

The opioid-like neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP receptor) contribute to Parkinson's disease (PD) and motor complications associated with levodopa therapy. The N/OFQ-NOP receptor system is expressed in cortical and subcortical motor areas and, notably, in dopaminergic neurons of the substantia nigra compacta. Dopamine depletion, as in rodent models of PD results in up-regulation of N/OFQ transmission in the substantia nigra and down-regulation of N/OFQ transmission in the striatum. Consistent with this, NOP receptor antagonists relieve motor deficits in PD models by reinstating the physiological balance between excitatory and inhibitory inputs impinging on nigro-thalamic GABAergic neurons. NOP receptor antagonists also counteract the degeneration of nigrostriatal dopaminergic neurons, possibly by attenuating the excitotoxicity or modulating the immune response. Conversely, NOP receptor agonists attenuate levodopa-induced dyskinesia by attenuating the hyperactivation of striatal D1 receptor signalling in neurons of the direct striatonigral pathway. The N/OFQ-NOP receptor system might represent a novel target in the therapy of PD.

NOP Receptor Ligands and Parkinson's Disease

Nociceptin/Orphanin FQ (N/OFQ) and its NOP receptor are highly expressed in motor areas of the rodent, nonhuman, and human primate brain, such as primary motor cortex, thalamus, globus pallidus, striatum, and substantia nigra. Endogenous N/OFQ negatively regulates motor behavior and dopamine transmission through NOP receptors expressed by dopaminergic neurons of the substantia nigra compacta. Consistent with the existence of an N/OFQ tone over dopaminergic transmission, blockade of NOP receptor antagonists increases striatal dopamine release. In this chapter, we will review the evidence linking the N/OFQ-NOP receptor system to Parkinson's disease (PD). We will first discuss data showing that the central N/OFQ-NOP receptor system undergoes plastic changes in different basal ganglia nuclei following dopamine depletion. Then we will show that NOP receptor antagonists relieve motor deficits in different rodent and nonhuman primate models of PD. Mechanistically, NOP receptor blockade in substantia nigra reticulata results in rebalancing of the inhibitory GABAergic and excitatory glutamatergic inputs impinging on nigro-thalamic GABAergic neurons, leading to thalamic disinhibition. We will also present data showing that, in addition to motor symptoms, N/OFQ also plays a role in the parkinsonian neurodegeneration. In fact, NOP receptor antagonists possess neuroprotective/neurorescue properties in in vitro and in vivo models of PD.

A systematic review of the role of the nociceptin receptor system in stress, cognition, and reward: relevance to schizophrenia

Schizophrenia is a debilitating neuropsychiatric illness that is characterized by positive, negative, and cognitive symptoms. Research over the past two decades suggests that the nociceptin receptor system may be involved in domains affected in schizophrenia, based on evidence aligning it with hallmark features of the disorder. First, aberrant glutamatergic and striatal dopaminergic function are associated with psychotic symptoms, and the nociceptin receptor system has been shown to regulate dopamine and glutamate transmission. Second, stress is a critical risk factor for first break and relapse in schizophrenia, and evidence suggests that the nociceptin receptor system is also directly involved in stress modulation. Third, cognitive deficits are prevalent in schizophrenia, and the nociceptin receptor system has significant impact on learning and working memory. Last, reward processing is disrupted in schizophrenia, and nociceptin signaling has been shown to regulate reward cue salience. These findings provide the foundation for the involvement of the nociceptin receptor system in the pathophysiology of schizophrenia and outline the need for future research into this system.

Anti-Parkinsonian and anti-dyskinetic profiles of two novel potent and selective nociceptin/orphanin FQ receptor agonists

BACKGROUND AND PURPOSE

We previously showed that nociceptin/orphanin FQ opioid peptide (NOP) receptor agonists attenuate the expression of levodopa-induced dyskinesia in animal models of Parkinson's disease. We now investigate the efficacy of two novel, potent and chemically distinct NOP receptor agonists, AT-390 and AT-403, to improve Parkinsonian disabilities and attenuate dyskinesia development and expression.

EXPERIMENTAL APPROACH

Binding affinity and functional efficacy of AT-390 and AT-403 at the opioid receptors were determined in radioligand displacement assays and in GTPγS binding assays respectively, conducted in CHO cells. Their anti-Parkinsonian activity was evaluated in 6-hydroxydopamine hemi-lesioned rats whereas the anti-dyskinetic properties were assessed in 6-hydroxydopamine hemi-lesioned rats chronically treated with levodopa. The ability of AT-403 to inhibit the D₁ receptor-induced phosphorylation of striatal ERK was investigated.

KEY RESULTS

AT-390 and AT-403 selectively improved akinesia at low doses and disrupted global motor activity at higher doses. AT-403 palliated dyskinesia expression without causing sedation in a narrow therapeutic window, whereas AT-390 delayed the appearance of abnormal involuntary movements and increased their duration at doses causing sedation. AT-403 did not prevent the priming to levodopa, although it significantly inhibited dyskinesia on the first day of administration. AT-403 reduced the ERK phosphorylation induced by SKF38393 in vitro and by levodopa in vivo.

CONCLUSIONS AND IMPLICATIONS

NOP receptor stimulation can provide significant albeit mild anti-dyskinetic effect at doses not causing sedation. The therapeutic window, however, varies across compounds. AT-403 could be a potent and selective tool to investigate the role of NOP receptors in vivo.

Pharmaco-toxicological effects of the novel third-generation fluorinate synthetic cannabinoids, 5F-ADBINACA, AB-FUBINACA, and STS-135 in mice. In vitro and in vivo studies

INTRODUCTION

5F-ADBINACA, AB-FUBINACA, and STS-135 are 3 novel third-generation fluorinate synthetic cannabinoids that are illegally marketed as incense, herbal preparations, or research chemicals for their psychoactive cannabis-like effects.

METHODS

The present study aims at investigating the in vitro and in vivo pharmacological activity of 5F-ADBINACA, AB-FUBINACA, and STS-135 in male CD-1 mice, comparing their in vivo effects with those caused by the administration of Δ⁹ -THC and JWH-018. In vitro competition binding experiments revealed a nanomolar affinity and potency of the 5F-ADBINACA, AB-FUBINACA, and STS-135 on mouse and human CB₁ and CB₂ receptors. Moreover, these synthetic cannabinoids induced neurotoxicity in murine neuro-2a cells.

RESULTS

In vivo studies showed that 5F-ADBINACA, AB-FUBINACA, and STS-135 induced hypothermia; increased pain threshold to both noxious mechanical and thermal stimuli; caused catalepsy; reduced motor activity; impaired sensorimotor responses (visual, acoustic, and tactile); caused seizures, myoclonia, and hyperreflexia; and promoted aggressiveness in mice. Behavioral and neurological effects were fully prevented by the selective CB₁ receptor antagonist/inverse agonist AM 251. Differently, the visual sensory response induced by STS-135 was only partly prevented by the AM 251, suggesting a CB₁ -independent mechanism.

CONCLUSIONS

For the first time, the present study demonstrates the pharmaco-toxicological effects induced by the administration of 5F-ADBINACA, AB-FUBINACA, and STS-135 in mice and suggests their possible detrimental effects on human health.

Effect of the novel synthetic cannabinoids AKB48 and 5F-AKB48 on "tetrad", sensorimotor, neurological and neurochemical responses in mice. In vitro and in vivo pharmacological studies

RATIONALE

AKB48 and its fluorinate derivate 5F-AKB48 are two novel synthetic cannabinoids belonging to a structural class with an indazole core structure. They are marketed as incense, herbal preparations or chemical supply for their psychoactive Cannabis-like effects.

OBJECTIVES

The present study was aimed at investigating the in vitro and in vivo pharmacological activity of AKB48 and 5F-AKB48 in male CD-1 mice and comparing their in vivo effects with those caused by the administration of Δ⁹-THC and JWH-018.

RESULTS

In vitro competition binding experiments performed on mouse and human CB₁ and CB₂ receptors revealed a nanomolar affinity and potency of the AKB48 and 5F-AKB48. In vivo studies showed that AKB48 and 5F-AKB48, induced hypothermia, increased pain threshold to both noxious mechanical and thermal stimuli, caused catalepsy, reduced motor activity, impaired sensorimotor responses (visual, acoustic and tactile), caused seizures, myoclonia, hyperreflexia and promoted aggressiveness in mice. Moreover, microdialysis study in freely moving mice showed that systemic administration of AKB48 and 5F-AKB48 stimulated dopamine release in the nucleus accumbens. Behavioural, neurological and neurochemical effects were fully prevented by the selective CB₁ receptor antagonist/inverse agonist AM 251.

CONCLUSIONS

For the first time, the present study demonstrates the overall pharmacological effects induced by the administration of AKB48 and 5F-AKB48 in mice and suggests that the fluorination can increase the power and/or effectiveness of SCBs. Furthermore, this study outlines the potential detrimental effects of SCBs on human health.

Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems

The NOP receptor (nociceptin/orphanin FQ opioid peptide receptor) is the most recently discovered member of the opioid receptor family and, together with its endogenous ligand, N/OFQ, make up the fourth members of the opioid receptor and opioid peptide family. Because of its more recent discovery, an understanding of the cellular and behavioral actions induced by NOP receptor activation are less well developed than for the other members of the opioid receptor family. All of these factors are important because NOP receptor activation has a clear modulatory role on mu opioid receptor-mediated actions and thereby affects opioid analgesia, tolerance development, and reward. In addition to opioid modulatory actions, NOP receptor activation has important effects on motor function and other physiologic processes. This review discusses how NOP pharmacology intersects, contrasts, and interacts with the mu opioid receptor in terms of tertiary structure and mechanism of receptor activation; location of receptors in the central nervous system; mechanisms of desensitization and downregulation; cellular actions; intracellular signal transduction pathways; and behavioral actions with respect to analgesia, tolerance, dependence, and reward. This is followed by a discussion of the agonists and antagonists that have most contributed to our current knowledge. Because NOP receptors are highly expressed in brain and spinal cord and NOP receptor activation sometimes synergizes with mu receptor-mediated actions and sometimes opposes them, an understanding of NOP receptor pharmacology in the context of these interactions with the opioid receptors will be crucial to the development of novel therapeutics that engage the NOP receptor.

Genetic and pharmacological evidence that endogenous nociceptin/orphanin FQ contributes to dopamine cell loss in Parkinson's disease

To investigate whether the endogenous neuropeptide nociceptin/orphanin FQ (N/OFQ) contributes to the death of dopamine neurons in Parkinson's disease, we undertook a genetic and a pharmacological approach using NOP receptor knockout (NOP(-/-)) mice, and the selective and potent small molecule NOP receptor antagonist (-)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111). Stereological unbiased methods were used to estimate the total number of dopamine neurons in the substantia nigra of i) NOP(-/-) mice acutely treated with the parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP), ii) naïve mice subacutely treated with MPTP, alone or in combination with SB-612111, iii) rats injected with a recombinant adeno-associated viral (AAV) vector overexpressing human mutant p.A53T α-synuclein, treated with vehicle or SB-612111. NOP(-/-) mice showed a 50% greater amount of nigral dopamine neurons spared in response to acute MPTP compared to controls, which was associated with a milder motor impairment. SB-612111, given 4 days after MPTP treatment to mimic the clinical condition, prevented the loss of nigral dopamine neurons and striatal dopaminergic terminals caused by subacute MPTP. SB-612111, administered a week after the AAV injections in a clinically-driven protocol, also increased by 50% both the number of spared nigral dopamine neurons and striatal dopamine terminals, and prevented accompanying motor deficits induced by α-synuclein. We conclude that endogenous N/OFQ contributes to dopamine neuron loss in pathogenic and etiologic models of Parkinson's disease through NOP receptor-mediated mechanisms. NOP receptor antagonists might prove effective as disease-modifying agents in Parkinson's disease, through the rescue of degenerating nigral dopamine neurons and/or the protection of the healthy ones.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

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

BACKGROUND AND PURPOSE

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.

EXPERIMENTAL APPROACH

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.

KEY RESULTS

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.

CONCLUSIONS AND IMPLICATIONS

NOP receptor antagonists provide motor benefit in parkinsonism models although the 'therapeutic' window and long-term effects may vary between compounds.

Dopamine-nociceptin/orphanin FQ interactions in the substantia nigra reticulata of hemiparkinsonian rats: involvement of D2/D3 receptors and impact on nigro-thalamic neurons and motor activity

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonists proved to be effective in alleviating experimental parkinsonism. Nonetheless, loss of effectiveness or even worsening of parkinsonian symptoms have been observed at high doses. With the aim of clarifying the circuitry underlying the dual action of NOP receptor antagonists and the role of endogenous dopamine, the NOP receptor antagonist 1-benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl]pyrrolidine-2-carboxamide (Compound 24) and the D(2)/D(3) receptor antagonist raclopride were used in 6-hydroxydopamine hemilesioned rats. Systemically administered Compound 24 improved motor activity in the 0.1-10mg/kg dose range being ineffective at 30 mg/kg. To confirm NOP selectivity, Compound 24 improved motor performance in wild-type mice at 1 and 10mg/kg and inhibited it at 60 mg/kg, being ineffective in NOP receptor knockout mice. To prove that the bell-shaped profile was mediated by nigral NOP receptors, reverse dialysis of Compound 24 (0.03 μM) in substantia nigra reticulata ameliorated akinesia whereas Compound 24 (3 μM) was ineffective. To demonstrate that motor responses were mediated by tuning inhibitory and excitatory inputs to nigro-thalamic neurons, the low concentration elevated GABA and reduced glutamate in substantia nigra, simultaneously reducing GABA levels in ventro-medial thalamus. Conversely, the higher concentration reduced nigral and elevated thalamic GABA, without affecting nigral glutamate levels. Co-perfusion with raclopride (1 μM) abolished the antiakinetic action of Compound 24 (0.03 μM) and turned the ineffectiveness of Compound 24 (3 μM) into an antiakinetic effect. The low concentration reduced nigral but did not affect thalamic GABA whereas the higher concentration elevated nigral and reduced thalamic GABA. Neither concentration affected nigral glutamate. We conclude that dual motor effects of Compound 24 in hemiparkinsonian rats are accomplished through blockade of nigral NOP receptors resulting in opposite modulation of nigro-thalamic neurons. Endogenous dopamine contributes to these responses affecting the level of GABAergic inhibition of the nigral output via D(2)/D(3) receptors.

Further evidence for an involvement of nociceptin/orphanin FQ in the pathophysiology of Parkinson's disease: a behavioral and neurochemical study in reserpinized mice

The contribution of nociceptin/orphanin FQ (N/OFQ) to reserpine-induced Parkinsonism was evaluated in mice. A battery of motor tests revealed that reserpine caused dose-dependent and long-lasting motor impairment. Endogenous N/OFQ sustained this response because N/OFQ peptide (NOP) receptor knockout (NOP(-/-) ) mice were less susceptible to the hypokinetic action of reserpine than wild-type (NOP(+/+) ) animals. Microdialysis revealed that reserpine elevated glutamate and reduced GABA levels in substantia nigra reticulata, and that resistance to reserpine in NOP(-/-) mice was accompanied by a milder increase in glutamate and lack of inhibition of GABA levels. To substantiate this genetic evidence, the NOP receptor antagonist 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one (J-113397) simultaneously reduced akinesia and nigral glutamate levels in reserpinized NOP(+/+) mice, being ineffective in NOP(-/-) mice. Moreover, repeated J-113397 administration in reserpinized mice resulted in faster recovery of baseline motor performance which was, however, accompanied by a loss of acute antiakinetic response. The short-term beneficial effect of J-113397 was paralleled by normalization of nigral glutamate levels, whereas loss of acute response was paralleled by loss of the ability of J-113397 to inhibit glutamate levels. We conclude that endogenous N/OFQ contributes to reserpine-induced Parkinsonism, and that sustained NOP receptor blockade produces short-term motor improvement accompanied by normalization of nigral glutamate release.

Brain interstitial nociceptin/orphanin FQ levels are elevated in Parkinson's disease

Expression and release of nociceptin/orphanin FQ (N/OFQ) are elevated in the substantia nigra reticulata of 6-hydroxydopamine-hemilesioned rats, suggesting a pathogenic role for N/OFQ in Parkinson's disease. In this study, we investigated whether elevation of N/OFQ expression in 6-hydroxydopamine-hemilesioned rats selectively occurs in substantia nigra and whether hypomotility following acute haloperidol administration is accompanied by a rise in nigral N/OFQ levels. Moreover, to prove a link between N/OFQ and idiopathic Parkinson's disease in humans, we measured N/OFQ levels in the cerebrospinal fluid of parkinsonian patients undergoing surgery for deep brain stimulation. In situ hybridization demonstrated that dopamine depletion was associated with increase of N/OFQ expression in substantia nigra (compacta +160%, reticulata +105%) and subthalamic nucleus (+45%), as well as reduction in caudate putamen (-20%). No change was observed in globus pallidus, nucleus accumbens, thalamus, and motor cortex. Microdialysis coupled to the bar test allowed to demonstrate that acute administration of haloperidol (0.8 and 3 mg/kg) increased nigral N/OFQ levels (maximally of +47% and +53%, respectively) in parallel with akinesia. A correlation with preclinical studies was found by analyzing N/OFQ levels in humans. Indeed, N/OFQ levels were found to be approximately 3.5-fold elevated in the cerebrospinal fluid of parkinsonian patients (148 fmol/ml) compared with nonparkinsonian neurologic controls (41 fmol/ml). These data represent the first clinical evidence linking N/OFQ to idiopathic Parkinson's disease in humans. They strengthen the pathogenic role of N/OFQ in the modulation of parkinsonism across species and provide a rationale for developing N/OFQ receptor antagonists as antiparkinsonian drugs.

MPTP treatment increases expression of pre-pro-nociceptin/orphanin FQ mRNA in a subset of substantia nigra reticulata neurons

Antagonists selectively inhibiting activation of the nociceptin/orphanin FQ (N/OFQ) receptor reduce motor symptoms in experimental models of Parkinson's disease, and genetic deletion of the ppN/OFQ gene offers partial protection of mid-brain dopamine neurons against the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP increased ppN/OFQ mRNA expression in the substantia nigra (SN). We have evaluated the temporal relationship of dopamine cell loss to increased ppN/OFQ mRNA expression in the substantia nigra after MPTP treatment, and characterized the cellular locations in which increased ppN/OFQ mRNA expression was observed after MPTP treatment. MPTP increased by about 5-fold the number of neurons expressing ppN/OFQ mRNA in the pars reticulata of SN (SNr) by 24 h after treatment and the elevation remained significant for at least 7 days. This period coincided with the timing of the loss of dopamine neurons from the pars compacta of substantia nigra (SNc) after MPTP. The increased expression of ppN/OFQ mRNA co-localized with a neuronal marker in the SNr. MPTP treatment resulted in a small increase in the numbers of neurons expressing ppN/OFQ in the SNc in mice from one mouse colony but the increase did not reach statistical significance in mice from another colony. No changes in ppN/OFQ-mRNA expression were observed in the ventral tegmental area (VTA), the caudate-putamen, the subthalamic nucleus, or in two other brains areas. These results demonstrate that increased N/OFQ expression in the SNr is closely associated with the MPTP-induced loss of dopamine neurons in the SNc in a widely used animal model of Parkinson's disease.

Pharmacological profile and antiparkinsonian properties of the novel nociceptin/orphanin FQ receptor antagonist 1-[1-cyclooctylmethyl-5-(1-hydroxy-1-methyl-ethyl)-1,2,3,6-tetrahydro-pyridin-4-yl]-3-ethyl-1,3-dihydro-benzoimidazol-2-one (GF-4)

In this study we provided a pharmacological characterization of the recently synthesized nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP) antagonist 1-[1-Cyclooctylmethyl-5-(1-hydroxy-1-methyl-ethyl)-1,2,3,6-tetrahydro-pyridin-4-yl]-3-ethyl-1,3-dihydro-benzoimidazol-2-one (GF-4) and investigated its antiparkinsonian properties. GF-4 inhibited N/OFQ binding to CHO(hNOP) cell membranes (pK(i) 7.46), and antagonized N/OFQ effects in a calcium mobilization assay and electrically stimulated isolated tissues (pK(B) 7.27-7.82), showing a approximately 5-fold selectivity over classical opioid receptors. In vivo, GF-4 dually modulated stepping activity in wild-type mice, causing facilitation in the 0.01-10mg/kg dose range and inhibition at 30mg/kg. These effects were mediated by NOP receptors since GF-4 was ineffective in NOP receptor knock-out mice. Antiparkinsonian properties of GF-4 were investigated in 6-hydroxydopamine hemilesioned rats. GF-4 ameliorated akinesia, bradykinesia and overall gait ability in the 0.1-10mg/kg dose range, but inhibited motor activity at 30mg/kg. To investigate the circuitry underlying motor facilitating and inhibitory effects of GF-4, microdialysis coupled to behavioral testing (akinesia test) was performed. An anti-akinetic dose of GF-4 (1mg/kg) reduced glutamate (GLU) and enhanced GABA release in SNr, while the pro-akinetic dose of GF-4 (30mg/kg) evoked opposite effects. Moreover, the anti-akinetic dose of GF-4 reduced GABA and increased GLU release in ventro-medial thalamus, the pro-akinetic dose decreasing GABA without affecting GLU release in this area. We conclude that GF-4 is an effective NOP receptor antagonist able to attenuate parkinsonian-like symptoms in vivo via inhibition of the nigro-thalamic pathway.

UFP-112 a potent and long-lasting agonist selective for the Nociceptin/Orphanin FQ receptor

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions via selective activation of the N/OFQ peptide receptor (NOP). [(pF)Phe(4) Aib(7) Arg(14) Lys(15) ]N/OFQ-NH(2) (UFP-112) is an NOP receptor ligand designed using a combination of several chemical modifications in the same peptide sequence that increase NOP receptor affinity/potency and/or reduce susceptibility to enzymatic degradation. In the present review article, we summarize data from the literature and present original findings on the in vitro and in vivo pharmacological features of UFP-112. Moreover, important biological actions and possible therapeutic indications of NOP receptor agonists are discussed based on the results obtained with UFP-112 and compared with other peptide and nonpeptide NOP receptor ligands.

Endogenous nociceptin/orphanin FQ (N/OFQ) contributes to haloperidol-induced changes of nigral amino acid transmission and parkinsonism: a combined microdialysis and behavioral study in naïve and nociceptin/orphanin FQ receptor knockout mice

The contribution of endogenous nociceptin/orphanin FQ (N/OFQ) to neuroleptic-induced parkinsonism has been evaluated in haloperidol-treated mice. Pharmacological blockade of N/OFQ receptors (NOP) via systemic administration of 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one (J-113397, 0.01-10 mg/kg i.p.) or central injection of [Nphe(1),Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101, 10 nmol i.c.v.) attenuated (0.8 mg/kg) haloperidol-induced motor deficits as evaluated by a battery of behavioral tests providing complementary information on motor parameters: the bar, drag and rotarod tests. A combined neurochemical and behavioral approach was then used to investigate whether the substantia nigra reticulata could be involved in antiakinetic actions of J-113397. Microdialysis combined to the bar test revealed that haloperidol (0.3 and 0.8 mg/kg i.p.) caused a dose-dependent and prolonged elevation of immobility time (i.e. akinesia) which was associated with an increase in nigral glutamate and a reduction in GABA release. Conversely, J-113397 (1 mg/kg) alone reduced glutamate and elevated nigral GABA release, and when challenged against haloperidol, counteracted its behavioral and neurochemical effects. Microdialysis coupled to behavioral testing also demonstrated that NOP receptor knockout mice were resistant to haloperidol (0.3 mg/kg) compared to wild-type mice, lack of response being associated with a reversal of glutamate release facilitation into inhibition and no change in nigral GABA release. This study provides pharmacological and genetic evidence that endogenous N/OFQ contributes to haloperidol-induced akinesia and changes of amino acid transmission in mice. Moreover, it confirms the view that NOP receptor antagonists are capable of reversing akinesia across species and genotypes and may prove effective in relieving neuroleptic-induced parkinsonism.

Pharmacological characterization of the nociceptin/orphanin FQ receptor non peptide antagonist Compound 24

Compound 24, 1-benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl] pyrrolidine-2-carboxamide was recently identified as a nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP) ligand. In this study, the in vitro and in vivo pharmacological profiles of Compound 24 were investigated. In vitro studies were performed measuring receptor and [(35)S]GTPgammaS binding and calcium mobilization in cells expressing the recombinant NOP receptor as well as using N/OFQ sensitive tissues. In vivo studies were conducted using the tail withdrawal assay in mice. Compound 24 produced a concentration-dependent displacement of [(3)H]N/OFQ binding to CHO(hNOP) cell membranes showing high affinity (pK(i) 9.62) and selectivity (1000 fold) over classical opioid receptors. Compound 24 antagonized with high potency the following in vitro effects of N/OFQ: stimulation of [(35)S]GTPgammaS binding in CHO(hNOP) cell membranes (pA(2) 9.98), calcium mobilization in CHO(hNOP) cells expressing the Galpha(qi5) chimeric protein (pK(B) 8.73), inhibition of electrically evoked twitches in the mouse (pA(2) 8.44) and rat (pK(B) 8.28) vas deferens, and in the guinea pig ileum (pK(B) 9.12). In electrically stimulated tissues, Compound 24 up to 1 microM did not modify the effects of classical opioid receptor agonists. Finally in vivo, in the mouse tail withdrawal assay, Compound 24 at 10 mg/kg antagonized the pronociceptive and antinociceptive effects of 1 nmol N/OFQ given supraspinally and spinally, respectively. Under the same experimental conditions Compound 24 did not affect the antinociceptive action of 3 nmol endomorphin-1 injected intrathecally. The present study demonstrated that Compound 24 is a pure, competitive, and highly potent non-peptide NOP receptor selective antagonist.

Alterations of N/OFQ and NOP receptor gene expression in the substantia nigra and caudate putamen of MPP+ and 6-OHDA lesioned rats

It has been suggested that the opioid-like neuropeptide nociceptin/orphanin FQ(N/OFQ) and its receptor (NOPr) may contribute to Parkinson's disease. Based on this idea, the aim of our study was to investigate the involvement of the N/OFQ-NOPr system in an animal model of Parkinson's disease and to evaluate if this neuropeptidergic system is acting through mechanisms involving glutamate and/or GABA. We injected the neurotoxins MPP+ or 6-OHDA into the cerebral ventricles and 10 days later measured N/OFQ and NOPr gene expression in caudate putamen (CP) and substantia nigra (SN), by RT-PCR. A large reduction in N/OFQ and NOPr mRNAs was observed in the CP of rat treated with either MPP+ or 6-OHDA, MPP+ being more effective than 6-OHDA. Both the neurotoxins induced an increase in N/OFQ gene expression in the SN, but only MPP+ evoked a significant down-regulation of NOPr in this area, showing a slight trend of reduction in 6-OHDA treated rats. Moreover, a reduction in the levels of glutamic acid decarboxylase (GAD65/67), an enzyme that converts the excitatory neurotransmitter glutamate to the inhibitory neurotransmitter y-aminobutyric acid (GABA), was also observed in the SN following 6-OHDA. These data suggest that DA modulates N/OFQ-NOPr system gene expression in SN and CP, strengthening the hypothesis that this neuropeptidergic system could be implicated in the mechanisms underlying Parkinson's disease. Our data might also suggest that the GABAergic system plays a role in the regulation of nigral function, although further studies are necessary to confirm this hypothesis. In agreement with previous studies, we also support the hypothesis of a potential value for NOP receptor antagonists to attenuate symptoms related to the degeneration of nigrostriatal dopaminergic pathway.

Nociceptin/orphanin FQ modulates motor behavior and primary motor cortex output through receptors located in substantia nigra reticulata

This study was set to investigate whether motor effects of nociceptin/orphanin FQ (N/OFQ) can be related to changes in primary motor cortex output. N/OFQ injected i.c.v. biphasically modulated motor performance, low doses being facilitating and higher ones inhibitory. These effects were counteracted by the N/OFQ receptor antagonist [Nphe(1) Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101) confirming the specificity of N/OFQ action. However, UFP-101 alone facilitated motor performance, suggesting that endogenous N/OFQ inhibits motor function. N/OFQ and UFP-101 injected into the substantia nigra reticulata but not motor cortex replicated these effects, suggesting motor responses were mediated by subcortical circuits involving the basal ganglia. Intracortical microstimulation technique showed that i.c.v. N/OFQ also biphasically modulated motor cortex excitability and movement representation. Low N/OFQ doses caused a leftward shift of threshold distribution curve in the forelimb area without affecting the number of effective sites. Conversely, high N/OFQ doses increased unresponsive and reduced excitable (movement) sites in vibrissa but not forelimb area. However, increased threshold currents and rightward shift of threshold distribution curve were observed in both areas, suggesting an overall inhibitory effect on cortical motor output. UFP-101 alone evoked effects similar to low N/OFQ doses, suggesting tonic inhibitory control over forelimb movement by endogenous N/OFQ. As shown in behavioral experiments, these effects were replicated by intranigral, but not intracortical, N/OFQ or UFP-101 injections. We conclude that N/OFQ receptors located in the substantia nigra reticulata mediate N/OFQ biphasic control over motor behavior, possibly through changes of primary motor cortex output.

In vivo microdialysis in Parkinson's research

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is primarily characterized by the degeneration of dopamine (DA) neurons in the nigrostriatal system, which in turn produces profound neurochemical changes within the basal ganglia, representing the neural substrate for parkinsonian motor symptoms. The pathogenesis of the disease is still not completely understood, but environmental and genetic factors are thought to play important roles. Research into the pathogenesis and the development of new therapeutic intervention strategies that will slow or stop the progression of the disease in human has rapidly advanced by the use of neurotoxins that specifically target DA neurons. Over the years, a broad variety of experimental models of the disease has been developed and applied in diverse animal species. The two most common toxin models used employ 6-hydroxydopamine (6-OHDA) and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/1-methyl-4-phenilpyridinium ion (MPTP/MPP+), either given systemically or locally applied into the nigrostriatal pathway, to resemble PD features in animals. Both neurotoxins selectively and rapidly destroy catecolaminergic neurons, although with different mechanisms. Since in vivo microdialysis coupled to high-performance liquid chromatography is an established technique for studying physiological, pharmacological, and pathological changes of a wide range of low molecular weight substances in the brain extracellular fluid, here we review the most prominent animal and human data obtained by the use of this technique in PD research.

Nociceptin/orphanin FQ receptor blockade attenuates MPTP-induced parkinsonism

Endogenous nociceptin/orphanin FQ (N/OFQ) inhibits the activity of dopamine neurons in the substantia nigra and affects motor behavior. In this study we investigated whether a N/OFQ receptor (NOP) antagonist, J-113397, can modify movement in naive mice and nonhuman primates and attenuate motor deficits in MPTP-treated parkinsonian animals. J-113397 facilitated motor activity in naïve mice at low doses (0.1-1 mg/kg) and inhibited it at higher ones (10 mg/kg). Likewise, in MPTP-treated mice, J-113397 reversed motor deficit at 0.01 mg/kg but worsened hypokinesia at higher doses (1 mg/kg). In naïve nonhuman primates, J-113397, ineffective up to 1 mg/kg, produced inconsistent motor improvements at 3 mg/kg. Conversely, in parkinsonian primates J-113397 (0.01 mg/kg) reversed parkinsonism, being most effective against hypokinesia. We conclude that endogenous N/OFQ modulates motor activity in mice and nonhuman primates and contributes to parkinsonian symptoms in MPTP-treated animals. NOP receptor antagonists may represent a novel approach to Parkinson's disease.

The nociceptin/orphanin FQ receptor antagonist J-113397 and L-DOPA additively attenuate experimental parkinsonism through overinhibition of the nigrothalamic pathway

By using a battery of behavioral tests, we showed that nociceptin/orphanin FQ receptor (NOP receptor) antagonists attenuated parkinsonian-like symptoms in 6-hydroxydopamine hemilesioned rats (Marti et al., 2005). We now present evidence that coadministration of the NOP receptor antagonist 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one (J-113397) and L-DOPA to 6-hydroxydopamine hemilesioned rats produced an additive attenuation of parkinsonism. To investigate the neurobiological substrates underlying this interaction, in vivo microdialysis was used in combination with behavioral measurements (bar test). J-113397 and L-DOPA alone reduced the time on bars (i.e., attenuated akinesia) and elevated GABA release selectively in the lesioned substantia nigra reticulata. J-113397 also reduced nigral glutamate levels, whereas L-DOPA was ineffective. J-113397 and L-DOPA coadministration produced additive antiakinetic effect, which was associated with additive increase in nigral GABA release but no additional reductions in glutamate levels. To investigate whether the increase in nigral GABA release could translate to changes in nigrothalamic transmission, GABA release was monitored in the ventromedial thalamus (one of the main target areas of the nigrothalamic projections). J-113397 and L-DOPA decreased thalamic GABA release and attenuated akinesia, their combination resulting in a more profound effect. These actions were prevented by perfusing the voltage-dependent Na+ channel blocker tetrodotoxin or the GABA(A) receptor antagonist bicuculline in the substantia nigra reticulata. These data demonstrate that J-113397 and L-DOPA exert their antiparkinsonian action through overinhibition of nigrothalamic transmission and suggest that NOP receptor antagonists may be useful as an adjunct to L-DOPA therapy for Parkinson's disease.

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.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Blockade of nociceptin/orphanin FQ transmission attenuates symptoms and neurodegeneration associated with Parkinson's disease

The opioid-like neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP) are expressed in the substantia nigra (SN), a brain area containing dopamine neurons that degenerate in Parkinson's disease. Endogenous N/OFQ facilitates nigral glutamate release and inhibits nigrostriatal dopamine transmission and motor behavior. Here, we present evidence suggesting that endogenous N/OFQ may contribute to Parkinson's disease. Pharmacological blockade of the SN N/OFQ-NOP receptor system attenuated parkinsonian-like akinesia/hypokinesia in 6-hydroxydopamine hemilesioned or haloperidol-treated rats, whereas deletion of the NOP receptor gene conferred mice partial protection from haloperidol-induced motor depression. The antiparkinsonian action of NOP receptor antagonists was associated with reduction of glutamate release in the SN. In 6-hydroxydopamine hemilesioned rats, enhancement of N/OFQ expression and release was detected in the lesioned compared with the unlesioned SN, indicating that parkinsonism may be associated with overactivation of the N/OFQ-NOP receptor system in the SN. Finally, deletion of the N/OFQ gene conferred mice partial protection against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced loss of SN dopamine neurons. Based on these data, we propose that NOP receptor antagonists may represent a novel approach for combined (symptomatic and neuroprotective) therapy of Parkinson's disease.