Depicted are the main locations of production for each peptide, although many of these molecules are detectable in smaller quantities at other sites in the GI system. In addition, most of them are also synthesized within the brain, including CCK, APO AIV, GLP1, oxyntomodulin, PYY, enterostatin, ghrelin, gastrin-releasing peptide (GRP), neuromedin B (NMB), and possibly PP. GI peptides that regulate appetite and do not seem to be produced within the brain include leptin, insulin, glucagon, and amylin.