RGS4 negatively modulates Nociceptin/Orphanin FQ opioid receptor signaling: implication for L-Dopa induced dyskinesia

Background and purpose\ud \ud Regulator of G-protein signal 4 (RGS4) is a signal transduction protein that accelerates intrinsic GTPase activity of Gαi/o and Gαq subunits, suppressing GPCR signaling. Here we investigate whether RGS4 modulates nociceptin/orphanin FQ (N/OFQ) opioid (NOP) receptor signaling and this modulation has relevance for L-Dopa-induced dyskinesia.\ud Experimental approach\ud \ud 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 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.\ud Key results\ud \ud 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 upregulated RGS4 in the lesioned striatum.\ud Conclusions and Implications\ud \ud RGS4 physiologically inhibits NOP receptor signaling. CCG-203920 enhanced NOP responses and improved the antidyskinetic potential of NOP receptor agonists, mitigating the effects of striatal RGS4 upregulation occurring during dyskinesia expression.