Cholecystokinin

Cholecystokinin (CCK; 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 secreted by 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 suppresant. 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.

Structure
CCK is composed of varying numbers of amino acids (e.g., CCK58, CCK33, CCK8) depending on post-translational modification of the CCK gene product, preprocholecystokinin. CCK is very similar in structure to gastrin, another of the gastrointestinal hormones, so much so that the last five C-terminal amino acids are same as those of gastrin. CCK58 is comprised of a helix-turn-helix configuration.

Function
CCK mediates a number of physiological processes, including digestion and satiety.

Digestion
CCK is secreted by the duodenal mucosa when fat- or protein-rich chyme leaves the stomach and enters the duodenum. The hormone acts on the pancreas to stimulate the secretion of a juice rich in digestive enzymes, including trypsinogen, chymotrypsinogen (which are converted to trypsin and chymotrypsin in the duodenum), amylase, lipase, and maltase. Together these pancreatic digestive enzymes catalyze the digestion of fat, protein, and carbohydrate.

CCK also causes the increased production of hepatic bile, and stimulates the contraction of the gallbladder and the relaxtion of the Sphincter of Oddi (Glisson's sphincter), resulting in the delivery of bile into the duodenal part of the small intestine. Bile salts form amphipathic micelles that emulsify fats, aiding in their digestion and absorption.

Neurobiology
As a neuropeptide, CCK mediates satiety by acting on the CCK receptors distributed widely throughout the central nervous system. In humans, CCK administration causes nausea and anxiety, and weakly decreases the desire to eat.

The effects of CCK vary between individuals. For example, in rats, CCK administration significantly reduces hunger in young males, but is less effective in older subjects, and even less effective in females. The hunger-suppressive effects of CCK also diminish in obese rats.