Cholecystokinin (
CCK or
CCK-PZ; from
Greek chole, "bile";
cysto, "sac";
kinin, "move"; hence,
move the bile-sac (gallbladder)) is a
peptide hormone of the
gastrointestinal system responsible for stimulating the
digestion of
fat and
protein. Cholecystokinin, previously called
pancreozymin, is synthesized and secreted by
enteroendocrine cells in the duodenum, the first segment of the
small intestine, and causes the release of
digestive enzymes and
bile from the
pancreas and
gallbladder, respectively. It also acts as a
hunger suppressant. Recent evidence has suggested that it also plays a major role in inducing drug
tolerance to
opioids like
morphine and heroin, and is partly implicated in experiences of pain hypersensitivity during opioid
withdrawal.