Neuropeptide Y (NPY) is a 36-amino acid peptide discovered in the early 1980s that belongs to a family of peptides that includes pancreatic polypeptides (PP) and peptide YY (PYY) (Tatemoto and Mutt, 1980). NPY is widely distributed in the central and peripheral nervous systems (for reviews, see Blomqvist and Herzog, 1997;Gehlert, 1999;Wieland et al., 2000;Kask et al., 2002;Malmstrom, 2002;Thorsell and Heilig, 2002). In the brain, NPY is the most abundant neuropeptide yet identified and its localization suggests an involvement in a variety of physiological processes, including anxiety, food intake, water consumption, circadian rhythms, hormone release, learning, and memory.The use of various cloning techniques has resulted in the identification of five receptors to date (Y 1 , Y 2 , Y 4 , Y 5 , and y 6 ) (Herzog et al., 1992;Larhammar et al., 1992;Bard et al., 1995;Gerald et al., 1995Gerald et al., , 1996Gregor et al., 1996;Hu et al., 1996;Matsumoto et al., 1996;Weinberg et al., 1996), all of them belong to the superfamily of G protein-coupled receptors. All NPY receptor subtypes are expressed in several species, including human, except the y 6 , which is absent in rat and not functional in the human and primates (Blomqvist and Herzog, 1997