L-3,4-dihydroxyphenylalanine (L-DOPA) by itself has been believed to be an inert amino acid that exerts action and effectiveness in Parkinson's disease through its conversion to dopamine by aromatic L-amino acid decarboxylase. We propose that L-DOPA is a neurotransmitter. We recently identified a G-protein coupled receptor 143 (GPR143), as a receptor for L-DOPA. GPR143 is distributed in some specific brain regions including striatum and substantia nigra in which dopamine receptors are highly expressed. To investigate the role of GPR143 in dopaminergic transmission, we examined the effect of quinpirole, a dopamine D2 receptor (D2R) agonist, on mouse behavior. Quinpirole (0.03 mg/kg, i.p.) decreased locomotor activity in wild-type (WT) mice. We found that this behavioral response to quinpirole was attenuated in Gpr143 gene-deficient (Gpr143-/y) mice when compared to WT mice. In contrast, SKF81297 (2.5 mg/kg, i.p.), a D1R agonist, increased locomotor activity in Gpr143-/y mice to a similar extent to that in WT mice. To validate GPR143 function after its depletion, we performed rescue experiment of GPR143 using adeno-associated virus encoding GPR143-P2A-enhanced green fluorescent protein (EGFP) or EGFP in Gpr143-/y mice. The decreased responsiveness to quinpirole in Gpr143-/y mice was rescued by re-expression of GPR143 in the dorsal striatum. These results indicate that GPR143 positively regulates D2R function in the dorsal striatum.