cholecystokinin (CCK) is a peptide hormone that causes contraction of the gall bladder (the meaning of its name) and also release of digestive enzymes from the pancreas. Originally it was thought that these were the actions of two different hormones, cholecystokinin and pancreozymin, but they were later found to be identical. The hormone originates in endocrine cells in the gut — mainly in the duodenum, iejunum, and ileum.

This same peptide is found in neurons of the peripheral nervous system, including those in the gut, and in the brain, but little in the spinal cord. The highest concentrations are found in the cerebral cortex, hippocampus, thalamus, amygdala, and hypothalamus. The release of cholecystokinin from the endocrine cells of the gut can be promoted by the presence of digested egg yolk, cream, or olive oil in the duodenum, which also cause the gall bladder to contract. The release of cholecystokinin from neurones can be achieved in experimental studies by depolarization, like most neurotransmitters, but little is known about its release from nerves under normal conditions.

The functions of cholecystokinin in the gut are clearly related to the need for bile salts and pancreatic enzymes to digest meals rich in fat and protein, with the partially-digested material acting as the stimulus for release of bile and enzymes into the alkaline gut, once the acid phase of digestion has been completed in the stomach. Cholecystokinin is sometimes injected into patients as a test of gall bladder function.

The neurotransmitter functions in the brain are as yet unclear, although it has been suggested that cholecystokinin is involved in the regulation of hormone secretion, e.g. by promoting release of growth hormone and inhibiting release of thyroid stimulating hormone.

Thus a substance first identified as a blood-borne hormone is apparently widely used in the body to carry ‘chemical messages’ between cells.

Alan W. Cuthbert

See also alimentary system; hormones; neurotransmitters.