Selective blockade of D3 dopamine receptors enhances the antiparkinsonian properties of ropinirole and levodopa in the MPTP-lesioned primate

To date, the lack of highly selective antagonists at the dopamine D-3 receptor has hampered clarification of their involvement in the actions of currently used therapies in Parkinson's disease. However, the novel benzopyranopyrrole, S33084, displays greater than 100-fold selectivity as an antagonist for D-3 versus D-2 receptors and all other sites tested. S33084 was administered to I -methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmosets previously primed with levodopa to elicit dyskinesia. Administered alone, S33084 exerted a modest, but significant, anti-parkinsonian effect without provoking dyskinesia. At low D-3-selective doses (0.16 and 0.64 mg/kg), S33084 potentiated, though to different extents and in qualitatively different ways, the anti-parkinsonian actions of both ropinirole and levodopa. At these doses, S33084 did not significantly modify levodopa-induced or ropinirole-induced dyskinesia. These data suggest that ropinirole and levodopa do not exert their anti-parkinsonian or pro-dyskinetic actions via D-3 receptor stimulation. Indeed, stimulation of D-3 receptors may be detrimental to the anti-parkinsonian properties of D-2/D-3 agonists. Selectivity for stimulation of D-2, over D-3, receptors may therefore be a beneficial property of dopamine receptor agonists in management of motor symptoms of Parkinson's disease patients with established dyskinesia. (C) 2004 Elsevier Inc. All rights reserved.