We report here the isolation and functional expression of a neuropeptide Y (NPY) receptor from the river lamprey, Lampetra fluviatilis. The receptor displays < 50% aminoacid sequence identity to all previously cloned Y1-subfamily receptors including Y1, Y4, and y6 and the teleost subtypes Ya, Yb and Yc. Phylogenetic analyses point to a closer relationship with Y4 and Ya/b/c suggesting that the lamprey receptor could possibly represent a pro-orthologue of some or all of those gnathostome receptors. Our results support the notion that the Y1 subfamily increased in number by genome or large-scale chromosome duplications, one of which may have taken place prior to the divergence of lampreys and gnathostomes whereas the second duplication probably occurred in the gnathostome lineage after this split. Functional expression of the lamprey receptor in a cell line facilitated specific binding of the three endogenous lamprey peptides NPY, peptide YY and peptide MY with picomolar affinities. Binding studies with a large panel of NPY analogues revealed indiscriminate binding properties similar to those of another nonselective Y1-subfamily receptor, zebrafish Ya. RT-PCR detected receptor mRNA in the central nervous system as well as in several peripheral organs suggesting diverse functions. This lamprey receptor is evolutionarily the most distant NPY receptor that clearly belongs to the Y1 subfamily as defined in mammals, which shows that subtypes Y2 and Y5 arose even earlier in evolution.Keywords: NPY; PYY; evolution; gene duplication; G-protein coupled receptor; Lamprey.Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) are closely related 36-amino-acid neuroendocrine peptides found in all tetrapods. NPY and PYY have also been found in nontetrapod gnathostomes as well as agnathans [1]. These peptides have a large number of physiological effects in the nervous system, the circulatory system and the gastrointestinal tract. NPY is most abundant in the nervous system and stimulates food intake, regulates blood pressure and influences release of pituitary peptides [2,3]. PYY and PP are found in the gastrointestinal tract and are released upon food intake. In nontetrapod vertebrates PYY is also present in the central nervous system (CNS) [4,5]. PY, a PYY-like peptide is only found in certain teleost fishes, and appears to have arisen from a fairly recent gene duplication [1,6]. In the river lamprey, Lampetra fluviatilis, the ancestral PYY gene appears to have undergone a separate gene duplication, resulting in peptide YY and peptide MY [4,7].The effects of the NPY family of peptides are relayed by a family of G-protein coupled receptors [8 -10]. Five receptors have been cloned in mammals; Y1, Y2, Y4, Y5 and y6, most of which have high affinities for NPY and PYY. Y4 is the only receptor for which PP has higher affinity than NPY or PYY. The y6 gene still lacks a physiological correlate, hence it is designated with a lower case 'y'. In humans and other primates, the y6 gene is a pseudogene. Sequence comparisons ...