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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 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]