Natural variations of wild Caenorhabditis elegans isolates having either Phe-215 or Val-215 in NPR-1, a putative orphan neuropeptide Y-like G protein-coupled receptor, result in either "social" or "solitary" feeding behaviors (de Bono, M., and Bargmann, C. I. (1998) Cell 94, 679-689). We identified a nematode peptide, GLGPRPLRF-NH2 (AF9), as a ligand activating the cloned NPR-1 receptor heterologously expressed in mammalian cells. Shifting cell culture temperatures from 37 to 28 degrees C, implemented 24 h after transfections, was essential for detectable functional expression of NPR-1. AF9 treatments linked both cloned receptor variants to activation of Gi/Go proteins and cAMP inhibition, thus allowing for classification of NPR-1 as an inhibitory G protein-coupled receptor. The Val-215 receptor isoform displayed higher binding and functional activity than its Phe-215 counterpart. This finding parallels the in vivo observation of a more potent repression of social feeding by the npr-1 gene encoding the Val-215 form of the receptor, resulting in dispersing (solitary) animals. Since neuropeptide Y shows no sequence homology to AF9 and was functionally inactive at the cloned NPR-1, we propose to rename NPR-1 and refer to it as an AF9 receptor, AF9-R1.