Background and purpose: 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 opioid (NOP) receptor signaling and whether 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 N/OFQ or AT-403 in the presence of the RGS4 inhibitor, CCG-203920, and D1-stimulated cAMP or pERK responses were monitored. In vivo, CCG-203920 was co-administered with AT-403 and levodopa 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 levodopa. Levodopa acutely upregulated RGS4 in the lesioned striatum. Conclusions and Implications: RGS4 physiologically inhibits NOP receptor signaling and an RGS4 inhibitor enhances NOP responses. Furthermore, an RGS4 inhibitor improved the antidyskinetic potential of NOP receptor agonists, mitigating the effects of upregulation of striatal RGS4 levels occurring during dyskinesia expression.

Background and purpose: Muscarinic receptors contribute to both the facilitation and inhibition of levodopa-induced dyskinesia operated by striatal cholinergic interneurons, although the receptor subtypes and the mechanisms involved have not been clearly identified. Also muscarinic receptors in substantia nigra reticulata, activated by cholinergic midbrain afferents, regulate striatal functions although their role in levodopa-induced dyskinesia remains to be proven. Here, we investigate whether striatal and nigral muscarinic M1 and/or M4 receptors modulate dyskinesia expression and the underlying striato-nigral GABAergic pathway activation in 6-hydroxydopamine hemilesioned rats. Experimental approach: Reverse microdialysis allowed to deliver the M1 and M4 preferential antagonists telenzepine, PD-102807, tropicamide and the selective M4 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 levels, taken as neurochemical markers of striato-nigral pathway activation. Key results: Intrastriatal telenzepine, PD-102807 and tropicamide alleviated dyskinesia and inhibited nigral GABA and striatal glutamate release. This was partially replicated by intrastriatal VU0152100 that, however, failed to inhibit striatal glutamate. The M2 preferential 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. Conclusions and Implications: Striatal M1 and M4, likely postsynaptic, receptors facilitate dyskinesia and striato-nigral pathway activation. Striatal M4 receptors, possibly located presynaptically, also inhibit dyskinesia. Potentiation of striatal and nigral M4 transmission leads to powerful multilevel inhibition of striato-nigral pathway providing a new strategy to tackle dyskinesia.