Nociceptin/orphanin FQ receptor agonists attenuate L-DOPA-induced dyskinesias

Differential effects of opioid receptor antagonism on the anti-dyskinetic and anti-parkinsonian effects of sub-anesthetic ketamine treatment in a preclinical model

Sub-anesthetic ketamine treatment has been shown to be an effective therapy for treatment-resistant depression and chronic pain. Our group has previously shown that sub-anesthetic ketamine produces acute anti-parkinsonian, and acute anti-dyskinetic effects in preclinical models of Parkinson's disease (PD). Ketamine is a multifunctional drug and exerts effects through blockade of N-methyl-d-aspartate receptors but also through interaction with the opioid system. In this report, we provide detailed pharmacokinetic rodent data on ketamine and its main metabolites following an intraperitoneal injection, and second, we explore the pharmacodynamic properties of ketamine in a rodent PD model with respect to the opioid system, using naloxone, a pan-opioid receptor antagonist, in unilateral 6-hydroxydopamine-lesioned male rats, treated with 6 mg/kg levodopa (l-DOPA) to establish a model of l-DOPA-induced dyskinesia (LID). As previously reported, we showed that ketamine (20 mg/kg) is highly efficacious in reducing LID and now report that the magnitude of this effect is resistant to naloxone (3 and 5 mg/kg). The higher naloxone dose of 5 mg/kg, however, led to an extension of the time-course of the LID, indicating that opioid receptor activation, while not a prerequisite for the anti-dyskinetic effects of ketamine, still exerts an acute modulatory effect. In contrast to the mild modulatory effect on LID, we found that naloxone added to the anti-parkinsonian activity of ketamine, further reducing the akinetic phenotype. In conclusion, our data show opioid receptor blockade differentially modulates the acute anti-parkinsonian and anti-dyskinetic actions of ketamine, providing novel mechanistic information to support repurposing ketamine for individuals with LID.

Antagonism of kappa opioid receptors accelerates the development of L-DOPA-induced dyskinesia in a preclinical model of moderate dopamine depletion

Levels of the opioid peptide dynorphin, an endogenous ligand selective for kappa-opioid receptors (KORs), its mRNA and pro-peptide precursors are differentially dysregulated in Parkinson's disease (PD) and following the development of l-DOPA-induced dyskinesia (LID). It remains unclear whether these alterations contribute to the pathophysiological mechanisms underlying PD motor impairment and the subsequent development of LID, or whether they are part of compensatory mechanisms. We sought to investigate nor-BNI, a KOR antagonist, 1) in the dopamine (DA)-depleted PD state, 2) during the development phase of LID, and 3) via measuring of tonic levels of striatal DA. While nor-BNI (3 mg/kg; s.c.) did not lead to functional restoration in the DA-depleted state, it affected the dose-dependent development of abnormal voluntary movements (AIMs) in response to escalating doses of l-DOPA in a rat PD model with a moderate striatal 6-hydroxdopamine (6-OHDA) lesion. We tested five escalating doses of l-DOPA (6, 12, 24, 48, 72 mg/kg; i.p.), and nor-BNI significantly increased the development of AIMs at the 12 and 24 mg/kg l-DOPA doses. However, after reaching the 72 mg/kg l-DOPA, AIMs were not significantly different between control and nor-BNI groups. In summary, while blocking KORs significantly increased the rate of development of LID induced by chronic, escalating doses of l-DOPA in a moderate-lesioned rat PD model, it did not contribute further once the overall severity of LID was established. While we observed an increase of tonic DA levels in the moderately lesioned dorsolateral striatum, there was no tonic DA change following administration of nor-BNI.

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.

Regulation of N-type calcium channels by nociceptin receptors and its possible role in neurological disorders

Activation of nociceptin opioid peptide receptors (NOP, a.k.a. opioid-like receptor-1, ORL-1) by the ligand nociceptin/orphanin FQ, leads to G protein-dependent regulation of Cav2.2 (N-type) voltage-gated calcium channels (VGCCs). This typically causes a reduction in calcium currents, triggering changes in presynaptic calcium levels and thus neurotransmission. Because of the widespread expression patterns of NOP and VGCCs across multiple brain regions, the dorsal horn of the spinal cord, and the dorsal root ganglia, this results in the alteration of numerous neurophysiological features. Here we review the regulation of N-type calcium channels by the NOP-nociceptin system in the context of neurological conditions such as anxiety, addiction, and pain.

Striatal and nigral muscarinic type 1 and type 4 receptors modulate levodopa-induced dyskinesia and striato-nigral pathway activation in 6-hydroxydopamine hemilesioned rats

Acetylcholine muscarinic receptors (mAChRs) contribute to both the facilitation and inhibition of levodopa-induced dyskinesia operated by striatal cholinergic interneurons, although the receptor subtypes involved remain elusive. Cholinergic afferents from the midbrain also innervate the substantia nigra reticulata, although the role of nigral mAChRs in levodopa-induced dyskinesia is unknown. Here, we investigate whether striatal and nigral M1 and/or M4 mAChRs modulate dyskinesia and the underlying striato-nigral GABAergic pathway activation in 6-hydroxydopamine hemilesioned rats. Reverse microdialysis allowed to deliver the mAChR antagonists telenzepine (M1 subtype preferring), PD-102807 and tropicamide (M4 subtype preferring), as well as the selective M4 mAChR positive allosteric modulator VU0152100 in striatum or substantia nigra, while levodopa was administered systemically. Dyskinetic movements were monitored along with nigral GABA (and glutamate) and striatal glutamate dialysate levels, taken as neurochemical correlates of striato-nigral pathway and cortico-basal ganglia-thalamo-cortical loop activation. We observed that intrastriatal telenzepine, PD-102807 and tropicamide alleviated dyskinesia and inhibited nigral GABA and striatal glutamate release. This was partially replicated by intrastriatal VU0152100. The M2 subtype preferring antagonist AFDX-116, used to elevate striatal acetylcholine levels, blocked the behavioral and neurochemical effects of PD-102807. Intranigral VU0152100 prevented levodopa-induced dyskinesia and its neurochemical correlates whereas PD-102807 was ineffective. These results suggest that striatal, likely postsynaptic, M1 mAChRs facilitate dyskinesia and striato-nigral pathway activation in vivo. Conversely, striatal M4 mAChRs can both facilitate and inhibit dyskinesia, possibly depending on their localization. Potentiation of striatal and nigral M4 mAChR transmission leads to powerful multilevel inhibition of striato-nigral pathway and attenuation of dyskinesia.

µ Opioid Receptor Agonism for L-DOPA-Induced Dyskinesia in Parkinson's Disease

Parkinson's disease (PD) is characterized by severe locomotor deficits and is commonly treated with the dopamine precursor L-DOPA, but its prolonged usage causes dyskinesias referred to as L-DOPA-induced dyskinesia (LID). Several studies in animal models of PD have suggested that dyskinesias are associated with a heightened opioid cotransmitter tone, observations that have led to the notion of a LID-related hyperactive opioid transmission that should be corrected by µ opioid receptor antagonists. Reports that both antagonists and agonists of the µ opioid receptor may alleviate LID severity in primate models of PD and LID, together with the failure of nonspecific antagonist to improve LID in pilot clinical trials in patients, raises doubt about the reliability of the available data on the opioid system in PD and LID. After in vitro characterization of the functional activity at the µ opioid receptor, we selected prototypical agonists, antagonists, and partial agonists at the µ opioid receptor. We then showed that both oral and discrete intracerebral administration of a µ receptor agonist, but not of an antagonist as long thought, ameliorated LIDs in the gold-standard bilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned female macaque model of PD and LID. The results call for a reappraisal of opioid pharmacology in the basal ganglia as well as for the development of brain nucleus-targeted µ opioid receptor agonists.SIGNIFICANCE STATEMENT µ opioid receptors have long been considered as a viable target for alleviating the severity of L-DOPA-induced hyperkinetic side effects, induced by the chronic treatment of Parkinson's disease motor symptoms with L-DOPA. Conflicting results between experimental parkinsonism and Parkinson's disease patients, however, dampened the enthusiasm for the target. Here we reappraise the pharmacology and then demonstrate that both oral and discrete intracerebral administration of a µ receptor agonist, but not of an antagonist as long thought, ameliorates LIDs in the gold-standard bilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned macaque model of Parkinson's disease, calling for a reappraisal of the opioid pharmacology as well as for the development of brain nucleus-targeted µ receptor agonists.

Discovery and Structure-Activity Relationships of Nociceptin Receptor Partial Agonists That Afford Symptom Ablation in Parkinson's Disease Models

A novel series of C(3)-substituted piperdinylindoles were developed as nociceptin opioid receptor (NOP) partial agonists to explore a pharmacological hypothesis that NOP partial agonists would afford a dual pharmacological action of attenuating Parkinson's disease (PD) motor symptoms and development of levodopa-induced dyskinesias. SAR around the C-3 substituents investigated effects on NOP binding, intrinsic activity, and selectivity and showed that while the C(3)-substituted indoles are selective, high affinity NOP ligands, the steric, polar, and cationic nature of the C-3 substituents affected intrinsic activity to afford partial agonists with a range of efficacies. Compounds 4, 5, and 9 with agonist efficacies between 25% and 35% significantly attenuated motor deficits in the 6-OHDA-hemilesioned rat model of PD. Further, unlike NOP antagonists, which appear to worsen dyskinesia expression, these NOP partial agonists did not attenuate or worsen dyskinesia expression. The NOP partial agonists and their SAR reported here may be useful to develop nondopaminergic treatments for PD.

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.

The Rodent Models of Dyskinesia and Their Behavioral Assessment

Dyskinesia, a major motor complication resulting from dopamine replacement treatment, manifests as involuntary hyperkinetic or dystonic movements. This condition poses a challenge to the treatment of Parkinson's disease. So far, several behavioral models based on rodent with dyskinesia have been established. These models have provided an important platform for evaluating the curative effect of drugs at the preclinical research level over the past two decades. However, there are differences in the modeling and behavioral testing procedures among various laboratories that adversely affect the rat and mouse models as credible experimental tools in this field. This article systematically reviews the history, the pros and cons, and the controversies surrounding rodent models of dyskinesia as well as their behavioral assessment protocols. A summary of factors that influence the behavioral assessment in the rodent dyskinesia models is also presented, including the degree of dopamine denervation, stereotaxic lesion sites, drug regimen, monitoring styles, priming effect, and individual and strain differences. Besides, recent breakthroughs like the genetic mouse models and the bilateral intoxication models for dyskinesia are also discussed.

NOP-Targeted Nonpeptide Ligands

The development of nonpeptide systemically active small-molecule NOP-targeted ligands has contributed tremendously to validating the NOP receptor as a promising target for therapeutics. Although a NOP-targeted compound is not yet approved for clinical use, a few NOP ligands are in clinical trials for various indications. Both successful and failed human clinical trials with NOP ligands provide opportunities for rational development of new and improved NOP-targeted compounds. A few years after the discovery of the NOP receptor in 1994, and its de-orphanization upon discovery of the endogenous peptide nociceptin/orphanin FQ (N/OFQ) in 1995, there was a significant effort in the pharmaceutical industry to discover nonpeptide NOP ligands from hits obtained from high-throughput screening campaigns of compound libraries. Depending on the therapeutic indication to be pursued, NOP agonists and antagonists were discovered, and some were optimized as clinical candidates. Advances such as G protein-coupled receptor (GPCR) structure elucidation, functional selectivity in ligand-driven GPCR activation, and multi-targeted ligands provide new scope for the rational design of novel NOP ligands fine-tuned for successful clinical translation. This article reviews the field of nonpeptide NOP ligand drug design in the context of these exciting developments and highlights new optimized nonpeptide NOP ligands possessing interesting functional profiles, which are particularly attractive for several unmet clinical applications involving NOP receptor pharmacomodulation.

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.

Acute Low Alcohol Disrupts Hippocampus-Striatum Neural Correlate of Learning Strategy by Inhibition of PKA/CREB Pathway in Rats

The hippocampus and striatum guide place-strategy and response-strategy learning, respectively, and they have dissociable roles in memory systems, which could compensate in case of temporary or permanent damage. Although acute alcohol (AA) treatment had been shown to have adverse effects on hippocampal function, whether it causes the functional compensation and the underlying mechanisms is unknown. In this study, rats treated with a low dose of AA avoided a hippocampus-dependent spatial strategy, instead preferring a striatum-dependent response strategy. Consistently, the learning-induced increase in hippocampal, but not striatal, pCREB was rendered less pronounced due to diminished activity of pPKA, but not pERK or pCaMKII. As rats approached the turn-decision area, Sp-cAMP, a PKA activator, was found to mitigate the inhibitory effect of AA on intra- and cross-structure synchronized neuronal oscillations, and rescue response-strategy bias and spatial learning deficits. Our study provides strong evidence of the critical link between neural couplings and strategy selection. Moreover, the PKA/CREB-signaling pathway is involved in the suppressive effect of AA on neural correlates of place-learning strategy. The novel important evidence provided here shows the functional couplings between the hippocampus and striatum in spatial learning processing and suggests possible avenues for therapeutic intervention.

Safinamide Modulates Striatal Glutamatergic Signaling in a Rat Model of Levodopa-Induced Dyskinesia

Safinamide (Xadago) is a novel dual-mechanism drug that has been approved in the European Union and United States as add-on treatment to levodopa in Parkinson's disease therapy. In addition to its selective and reversible monoamine oxidase B inhibition, safinamide through use-dependent sodium channel blockade reduces overactive glutamatergic transmission in basal ganglia, which is believed to contribute to motor symptoms and complications including levodopa-induced dyskinesia (LID). The present study investigated the effects of safinamide on the development of LID in 6-hydroxydopamine (6-OHDA)-lesioned rats, evaluating behavioral, molecular, and neurochemical parameters associated with LID appearance. 6-OHDA-lesioned rats were treated with saline, levodopa (6 mg/kg), or levodopa plus safinamide (15 mg/kg) for 21 days. Abnormal involuntary movements, motor performance, molecular composition of the striatal glutamatergic synapse, glutamate, and GABA release were analyzed. In the striatum, safinamide prevented the rearrangement of the subunit composition of N-methyl-d-aspartate receptors and the levodopa-induced increase of glutamate release associated with dyskinesia without affecting the levodopa-stimulated motor performance and dyskinesia. Overall, these findings suggest that the striatal glutamate-modulating component of safinamide's activity may contribute to its clinical effects, where its long-term use as levodopa add-on therapy significantly improves motor function and "on" time without troublesome dyskinesia.

Predictive Value of Parkinsonian Primates in Pharmacologic Studies: A Comparison between the Macaque, Marmoset, and Squirrel Monkey

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate is the gold-standard animal model of Parkinson disease (PD) and has been used to assess the effectiveness of experimental drugs on dyskinesia, parkinsonism, and psychosis. Three species have been used in most studies-the macaque, marmoset, and squirrel monkey-the last much less so than the first two species; however, the predictive value of each species at forecasting clinical efficacy, or lack thereof, is poorly documented. Here, we have reviewed all the published literature detailing pharmacologic studies that assessed the effects of experimental drugs on dyskinesia, parkinsonism, and psychosis in each of these species and have calculated their predictive value of success and failure at the clinical level. We found that, for dyskinesia, the macaque has a positive predictive value of 87.5% and a false-positive rate of 38.1%, whereas the marmoset has a positive predictive value of 76.9% and a false-positive rate of 15.6%. For parkinsonism, the macaque has a positive predictive value of 68.2% and a false-positive rate of 44.4%, whereas the marmoset has a positive predictive value of 86.9% and a false-positive rate of 41.7%. No drug that alleviates psychosis in the clinic has shown efficacy at doing so in the macaque, whereas the marmoset has 100% positive predictive value. The small number of studies conducted in the squirrel monkey precluded us from calculating its predictive efficacy. We hope our results will help in the design of pharmacologic experiments and will facilitate the drug discovery and development process in 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.

Maladaptive Synaptic Plasticity in L-DOPA-Induced Dyskinesia

The emergence of L-DOPA-induced dyskinesia (LID) in patients with Parkinson disease (PD) could be due to maladaptive plasticity of corticostriatal synapses in response to L-DOPA treatment. A series of recent studies has revealed that LID is associated with marked morphological plasticity of striatal dendritic spines, particularly cell type-specific structural plasticity of medium spiny neurons (MSNs) in the striatum. In addition, evidence demonstrating the occurrence of plastic adaptations, including aberrant morphological and functional features, in multiple components of cortico-basal ganglionic circuitry, such as primary motor cortex (M1) and basal ganglia (BG) output nuclei. These adaptations have been implicated in the pathophysiology of LID. Here, we briefly review recent studies that have addressed maladaptive plastic changes within the cortico-BG loop in dyskinetic animal models of PD and patients with PD.

Impairment of cocaine-mediated behaviours in mice by clinically relevant Ras-ERK inhibitors

Ras-ERK signalling in the brain plays a central role in drug addiction. However, to date, no clinically relevant inhibitor of this cascade has been tested in experimental models of addiction, a necessary step toward clinical trials. We designed two new cell-penetrating peptides - RB1 and RB3 - that penetrate the brain and, in the micromolar range, inhibit phosphorylation of ERK, histone H3 and S6 ribosomal protein in striatal slices. Furthermore, a screening of small therapeutics currently in clinical trials for cancer therapy revealed PD325901 as a brain-penetrating drug that blocks ERK signalling in the nanomolar range. All three compounds have an inhibitory effect on cocaine-induced ERK activation and reward in mice. In particular, PD325901 persistently blocks cocaine-induced place preference and accelerates extinction following cocaine self-administration. Thus, clinically relevant, systemically administered drugs that attenuate Ras-ERK signalling in the brain may be valuable tools for the treatment of cocaine addiction.

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 deletion of Rhes or pharmacological blockade of mTORC1 prevent striato-nigral neurons activation in levodopa-induced dyskinesia

Ras homolog enriched in striatum (Rhes) is a small GTP-binding protein that modulates signal transduction at dopamine receptors, and also activates mammalian target of rapamycin complex 1 (mTORC1). Rhes binding to mTORC1 is hypothesized to play a role in motor disorders such as levodopa-induced dyskinesia. Here, we investigate the behavioral and in vivo neurocircuitry changes associated with genetic deletion of Rhes or inhibition of mTORC1 signaling in the mouse model of levodopa-induced dyskinesia. 6-Hydroxydopamine-hemilesioned Rhes knockout mice and wild-type littermates were chronically treated with levodopa. In parallel, 6-hydroxydopamine-hemilesioned naïve mice were chronically treated with levodopa or levodopa plus rapamycin, to block mTORC1 pathway activation. Dyskinetic movements were monitored during levodopa treatment along with motor activity on the rotarod. Finally, dyskinetic mice underwent microdialysis probe implantation in the dopamine-depleted striatum and ipsilateral substantia nigra reticulata, and GABA and glutamate levels were monitored upon acute challenge with levodopa. Both Rhes knockouts and rapamycin-treated mice developed less dyskinesia than controls, although only rapamycin-treated mice fully preserved rotarod performance on levodopa. Levodopa elevated nigral GABA and glutamate in controls but not in Rhes knockouts or rapamycin-treated mice. Levodopa also stimulated striatal glutamate in controls and Rhes knockouts but not in rapamycin-treated mice. We conclude that both genetic deletion of Rhes and pharmacological blockade of mTORC1 significantly attenuate dyskinesia development by reducing the sensitization of striato-nigral medium-sized spiny neurons to levodopa. However, mTORC1 blockade seems to provide a more favorable behavioral outcome and a wider effect on neurochemical correlates of dyskinesia.

Eltoprazine prevents levodopa-induced dyskinesias by reducing striatal glutamate and direct pathway activity

BACKGROUND

Preclinical and clinical evidence that the serotonergic system plays a major role in levodopa-induced dyskinesias has been provided. Selective serotonin (5-hydroxytryptamine; 5-HT) 5-HT1A or 5-HT1B receptor agonists, and, very recently, the mixed 5-HT1A /5-HT1B receptor agonist, eltoprazine, proved effective in inhibiting L-dopa-induced dyskinesias in experimental animals and parkinsonian patients. Here, we investigate the mechanisms underlying this effect.

METHODS

Microdialysis was employed in 6-hydroxydopamine-hemilesioned rats chronically treated with L-dopa alone or in combination with eltoprazine. Gamma-aminobutyric acid (GABA) and glutamate levels were monitored on L-dopa in the dopamine-depleted striatum and ipsilateral SNr. Motor activity on the rotarod was assessed, both off and on L-dopa. Western blot was used to quantify ex vivo striatal levels of phosphorylated extracellular signal-regulated kinase 1 and 2. Striatal and nigral amino acid levels, as well as striatal dopamine levels, were also monitored in L-dopa-primed dyskinetic rats acutely challenged with L-dopa and eltoprazine.

RESULTS

Eltoprazine attenuated the development and expression of dyskinesias, preserving motor coordination on the rotarod. Eltoprazine prevented the rise of nigral amino acids and striatal glutamate levels, as well as the increase in striatal phosphorylated extracellular signal-regulated kinase 1 and 2, associated with dyskinesias. However, eltoprazine did not affect the L-dopa-induced increase in striatal dopamine.

CONCLUSIONS

Eltoprazine inhibits the sensitization of striatonigral medium-sized GABA spiny neurons (the direct pathway) to L-dopa and their overactivation associated with dyskinesias appearance. Activation of 5-HT1A and 5-HT1B receptors regulating striatal glutamate transmission, but not striatal ectopic dopamine release, might underlie the symptomatic effect of eltoprazine.

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.

Intranigral administration of substance P receptor antagonist attenuated levodopa-induced dyskinesia in a rat model of Parkinson's disease

Levodopa (L-dopa) remains the most effective drug in the treatment of Parkinson's disease (PD). However, L-dopa-induced dyskinesia (LID) has hindered its use for PD patients. The mechanisms of LID are not fully understood. Substance P (SP) receptor antagonist has been shown to reduce parkinsonism in animal models of PD, and ameliorate LID in PD rats. But the concrete mechanism is not fully understood. To address this issue, we produced a rat model of PD using 6-hydroxydompamine (6-OHDA) injections, and valid PD rats were intranigrally administrated with different doses of SP receptor antagonist LY303870 (5 nmol/day, 10 nmol/day and 20 nmol/day) following L-dopa (6 mg/kg/day, i.p.) plus benserazide (12 mg/kg/day, i.p.) for 23 days. We found that nigral SP levels were increased on days 3, 7 and 14 and decreased on day 21 after 6-hydroxydompamine lesions. But nigral SP levels kept increasing after repeated L-dopa administration in PD rats. Intranigral administration of low and moderate LY303870 reduced abnormal involuntary movements (AIMs) while improving motor deficits in PD rats treated with L-dopa plus benserazide. Microdialysis revealed that LY303870 (10 nmol/day) treatment attenuated the increase of striatal dopamine and the reduction of γ-aminobutyric acid in ventromedial thalamus of PD rats primed with L-dopa. Additionally, LY303870 (10 nmol/day) treatment prior to L-dopa administration reduced the phosphorylated levels of dopamine- and cyclic adenosine monophosphate-regulated phosphoprotein of 32 kDa at Thr 34 and extracellular signal-regulated kinases 1/2 as well as the levels of activity-regulated cytoskeleton-associated protein and Penk in L-dopa-primed PD rats. Taken together, these data showed that low and moderate SP receptor antagonists LY303870 could ameliorate LID via neurokinin 1 receptor without affecting therapeutic effect of L-dopa.

Stimulation of δ opioid receptor and blockade of nociceptin/orphanin FQ receptor synergistically attenuate parkinsonism

δ opioid peptide (DOP) receptors are considered a therapeutic target in Parkinson's disease, although the use of DOP agonists may be limited by side effects, including convulsions. To circumvent this issue, we evaluated whether blockade of nociceptin/orphanin FQ (N/OFQ) tone potentiated the antiparkinsonian effects of DOP agonists, thus allowing for reduction of their dosage. Systemic administration of the N/OFQ receptor (NOP) antagonist J-113397 [(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one] and the DOP receptor agonist SNC-80 [(+)-4-[(αR)-α-(2S,5R)-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N-N-diethylbenzamide] revealed synergistic attenuation of motor deficits in 6-hydroxydopamine hemilesioned rats and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice. In this model, repeated administration of the combination produced reproducible antiparkinsonian effects and was not associated with rescued striatal dopamine terminals. Microdialysis studies revealed that either systemic administration or local intranigral perfusion of J-113397 and SNC-80 led to the enhancement of nigral GABA, reduction of nigral Glu, and reduction of thalamic GABA levels, consistent with the view that NOP receptor blockade and DOP receptor stimulation caused synergistic overinhibition of nigro-thalamic GABA neurons. Whole-cell recording of GABA neurons in nigral slices confirmed that NOP receptor blockade enhanced the DOP receptor-induced effect on IPSCs via presynaptic mechanisms. Finally, SNC-80 more potently stimulated stepping activity in mice lacking the NOP receptor than wild-type controls, confirming the in vivo occurrence of an NOP-DOP receptor interaction. We conclude that endogenous N/OFQ functionally opposes DOP transmission in substantia nigra reticulata and that NOP receptor antagonists might be used in combination with DOP receptor agonists to reduce their dosage while maintaining their full therapeutic efficacy.

L-dopa-induced dyskinesia: beyond an excessive dopamine tone in the striatum

L-dopa remains the mainstay treatment for Parkinson's disease (PD), although in later stages, treatment is complicated by L-dopa-induced dyskinesias (LID). Current evidence links LID to excessive striatal L-dopa-derived dopamine (DA) release, while the possibility of a direct involvement of L-dopa itself in LID has been largely ignored. Here we show that L-dopa can alter basal ganglia activity and produce LID without enhancing striatal DA release in parkinsonian non-human primates. These data may have therapeutic implications for the management of advanced PD since they suggest that LID could result from diverse mechanisms of action of L-dopa.

Combined treatment with acupuncture reduces effective dose and alleviates adverse effect of L-dopa by normalizing Parkinson's disease-induced neurochemical imbalance

This study first showed the behavioural benefits of novel combination therapy of L-dopa with acupuncture on Parkinson's disease, and its underlying mechanisms within basal ganglia. The previous study reported that acupuncture may improve the motor function of a Parkinson's disease (PD) mouse model by increasing the dopamine efflux and turnover ratio of dopamine. Hence, we hypothesised that combining L-dopa with acupuncture would have a behavioural benefit for those with PD. We performed unilateral injections of 6-OHDA into the striatum of C57Bl/6 mice to model hemi-Parkinsonian attributes. To test motor function and dyskinetic anomalies, we examined cylinder behaviour and abnormal involuntary movement (AIM), respectively. We found that (1) a 50% reduced dose of L-dopa (7.5 mg/kg) combined with acupuncture showed an improvement in motor function that was comparable to mice given the standard dose of L-dopa treatment (15 mg/kg) only, and that (2) the combination treatment (L-dopa +acupuncture) was significantly superior in reducing AIM scores when equivalent doses of L-dopa were used. The combination treatment also significantly reduces the abnormal increase of GABA contents in the substantia nigra compared to the standard L-dopa treatment. Furthermore, abnormal expression of FosB, the immediate early gene of L-dopa induced dyskinesia (LID), was mitigated in the striatum by the combination treatment. All of these results indicate that acupuncture enhances the benefits of L-dopa on motor function with reduced dose of L-dopa and alleviating LID by normalising neurochemical imbalance within the basal ganglia.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

D1 receptor agonist improves sleep-wake parameters in experimental parkinsonism

Both excessive daytime sleepiness (EDS) and rapid eye movement (REM) sleep deregulation are part of Parkinson's disease (PD) non-motor symptoms and may complicate dopamine replacement therapy. We report here that dopamine agonists act differentially on sleep architecture in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine macaque monkey. Continuous sleep and wake electroencephalographic monitoring revealed no effect of the selective dopamine D2 receptor agonist quinpirole on EDS, whereas the selective dopamine D1 receptor agonist SKF38393 efficiently alleviated EDS and restored REM sleep to baseline values. The present results question the relevance of abandoning D1 receptor agonist treatment in PD as it might actually improve sleep-related disorders.

Pharmacological and genetic evidence for pre- and postsynaptic D2 receptor involvement in motor responses to nociceptin/orphanin FQ receptor ligands

A combined pharmacological and genetic approach was undertaken to investigate the contribution of endogenous dopamine to the motor actions of nociceptin/orphanin FQ (N/OFQ) receptor (NOP receptor) ligands. Motor activity was evaluated by a battery of behavioural tests in mice. The involvement of the various DA receptor subtypes in the motor effects of N/OFQ and NOP receptor antagonists was evaluated pharmacologically, using D1/D5 (SCH23390), D2/D3 (raclopride, amisulpride) and D3 (S33084) receptor antagonists, and by using D2 receptor knockout mice. Low doses of N/OFQ and NOP receptor antagonists promoted movement whereas higher doses inhibited it. Motor facilitation was selectively prevented by raclopride while motor inhibition was prevented by amisulpride. Amisulpride also attenuated the hypolocomotion induced by the D2/D3 receptor agonist pramipexole and dopamine precursor l-3,4-dihydroxyphenylalanine, whereas raclopride (and S33084) worsened it. To dissect out the contribution of pre- and postsynaptic D2 receptors, mice lacking the D2 receptor (D2R(-/-)) or its long isoform (D2L(-/-)) were used. Motor facilitation induced by N/OFQ and NOP receptor antagonists was lost in D2R(-/-) and D2L(-/-) mice whereas motor inhibition induced by NOP receptor antagonists (and pramipexole) was lost in D2R(-/-) but preserved in D2L(-/-) mice. N/OFQ-induced hypolocomotion was observed in both genotypes. We demonstrate that motor actions of NOP receptor ligands rely on the modulation of endogenous dopamine. Motor facilitation induced by NOP receptor antagonists as well as low dose N/OFQ is mediated through D2L postsynaptic receptors whereas motor inhibition observed with higher doses of N/OFQ occurs by direct inhibition of mesencephalic DA neurons. Motor inhibition seen with high doses of NOP receptor antagonists appears to be mediated through the D2 presynaptic autoreceptors. These data confirm that endogenous N/OFQ is a powerful modulator of dopamine transmission in vivo and that the effects of NOP receptor antagonists on motor function reflect the blockade of this endogenous N/OFQ tone.

GluN2A and GluN2B NMDA receptor subunits differentially modulate striatal output pathways and contribute to levodopa-induced abnormal involuntary movements in dyskinetic rats

Dual probe microdialysis was used to investigate whether GluN2A and GluN2B NMDA receptor subunits regulate striatal output pathways under dyskinetic conditions. The preferential GluN2A antagonist NVP-AAM077 perfused in the dopamine-depleted striatum of 6-hydroxydopamine hemilesioned dyskinetic rats reduced GABA and glutamate levels in globus pallidus whereas the selective GluN2B antagonist Ro 25-6981 elevated glutamate without affecting pallidal GABA. Moreover, intrastriatal NVP-AAM077 did not affect GABA but elevated glutamate levels in substantia nigra reticulata whereas Ro 25-6981 elevated GABA and reduced nigral glutamate. To investigate whether GluN2A and GluN2B NMDA receptor subunits are involved in motor pathways underlying dyskinesia expression, systemic NVP-AAM077 and Ro 25-6981 were tested for their ability to attenuate levodopa-induced abnormal involuntary movements. NVP-AAM077 failed to prevent dyskinesia while Ro 25-6981 mildly attenuated it. We conclude that in the dyskinetic striatum, striatal GluN2A subunits tonically stimulate the striato-pallidal pathway whereas striatal GluN2B subunits tonically inhibit striato-nigral projections. Moreover, GluN2A subunits are not involved in dyskinesia expression whereas GluN2B subunits minimally contribute to it.