One or more oxidized fatty acids selected from among 9-hydroxyoctadecadienic acid, 9-hydroperoxyoctadecadienic acid, 13-hydroxyoctadecadienic acid, hydroxyeicosatetraenic acid and ricinolic acid are used as G2A agonists. By using these oxidized fatty acids as controls, a novel G2A agonist, an inverse agonist thereto or an antagonist thereto is screened.