The present invention provides a ligand to GPR8, in particular polypeptides capable of binding to GPR8 or its amides or esters, or salts thereof, as well as DNA, encoding for such polypeptides. The ligand to GPR8 of the present invention is useful in developing a receptor-binding assay system with the use of a GPR8 expression system, for screening candidate compounds for drugs such as preventive/therapeutic agents for obesity, appetite stimulants, and prolactin production inhibitors.