Cholecystokinins are a series of peptides of heterogeneous length (5 to 58 amino acids) that are derived from preprocholecystokinin and are found in gastrointestinal tissues and the central nervous system. Gastrin is a related peptide with 5 C-terminal amino acids identical to those of cholecystokinin. Two GPCRs, CCK1 (CCKA) and CCK2 (CCKB), bind to CCK and/or gastrin to mediate the biological effects of the peptides. CCK2 binds to CCK and gastrin with similar affinity, whereas CCK1 selectively binds sulfated CCK. Binding of ligands to CCK2 stimulates mobilization of intracellular calcium by activation of Gq/11. In the periphery, CCK2 is present in the stomach, where it mediates gastrin-stimulated gastric acid secretion. In the CNS, CCK2 has been implicated in anxiety, depression, schizophrenia, depression and opioid analgesia.