A full-length cDNA encoding a human-derived G protein-coupled receptor protein is isolated by screening a human hippocampus library. Also, a rat-derived cDNA corresponding to the human-derived cDNA is isolated. Proteins encoded by these cDNAs have an activity of lowering intracellular cAMP concentration under stimulation with histamine. These proteins are usable as tools in screening ligands thereof or in screening candidate compounds for drugs capable of regulating signal transduction from the above proteins.