Potent activation of dopamine D3 /D2 heterodimers by the antiparkinsonian agents, S32504, pramipexole and ropinirole.

Recombinant, human dopamine D₃ and D₂ receptors form functional heterodimers upon co-expression in COS-7 cells. Herein, actions of the antiparkinsonian agents, S32504, ropinirole and pramipexole, at D₃ /D_2L heterodimers were compared to their effects at the respective monomers and at split, chimeric D_3trunk /D_2tail and D_2trunk /D_3tail receptors: the trunk incorporated transmembrane domains (TDs) I–V and the tail TDs VI and VII. In binding assays with the antagonist [³ H]nemonapride, all agonists were potent ligands of D₃ receptors showing, respectively, 100-, 18- and 56-fold lower affinity at D_2L receptors, mimicking the selective D₃ receptor antagonist, S33084 (100-fold). At D_3trunk /D_2tail receptors, except for ropinirole, all drugs showed lower affinities than at D₃ sites, whereas for D_2trunk /D_3tail receptors, affinities of all drugs were higher than at D_2L sites. The proportion of high affinity binding sites recognized by S32504, pramipexole and ropinirole in membranes derived from cells co-expressing D₃ and D_2L sites was higher than in an equivalent mixture of membranes from cells expressing D₃ or D_2L sites, consistent with the promotion of heterodimer formation. In contrast, the percentage of high and low affinity sites (biphasic isotherms) recognized by S33084 was identical. Functional actions were determined by co-transfection of a chimeric adenylyl cyclase (AC)-V/VI insensitive to D₃ receptors. Accordingly, D₃ receptor-transfected cells were irresponsive whereas, in D_2L receptor-transfected cells, agonists suppressed forskolin-stimulated cAMP production with modest potencies. In cells co-transfected with D₃ and D_2L receptors, S32504, ropinirole and pramipexole potently suppressed AC-V/VI with EC₅₀ s 33-, 19- and 11-fold lower than at D_2L receptors, respectively. S32504 also suppressed AC-V/VI activity at split D_3trunk /D_2tail and D_2trunk /D_3tail chimeras transfected into COS-7 cells. In conclusion, antiparkinson agents behave as potent agonists at D₃ /D₂ ‘heterodimers’, though any role in their actions in vivo remains to be demonstrated. [ABSTRACT FROM AUTHOR]