We propose that L-3,4-dihydroxyphenylalanine(L-DOPA) by itself is a neurotransmitter. Recently, a G-protein coupled receptor GPR143, a gene product of ocular albinism 1, was identified as a receptor for L-DOPA. However, L-DOPA is an unstable compound, and GPR143 ligands other than L-DOPA have not been identified yet. In this study, we found compound X, a novel ligand for GPR143, and examined its pharmacological actions. Using zero-maze test, we found that Gpr143 gene-deficient (Gpr143^-/y) mice exhibited more anxiety-like behavior when compared to wild-type (WT) mice. Intraventricular administration of compound X caused anxiety-like behavior in WT mice. The effect of compound X was not observed in Gpr143^-/y mice. These results indicate that compound X showed an antagonistic activity against GPR143. Compound X may be useful to clarify the role of L-DOPA/GPR143.