Cholecystokinin

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Cholecystokinin

Cholecystokinin (CCK; from Greek

Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
chole, "bile"; cysto, "sac"; kinin, "move"; hence, move the bile-sac (gallbladder
Gallbladder

The gallbladder is a small non-vital Organ which aids in the digestive process and concentrates bile produced in the liver....
)) is a peptide hormone

Peptide hormone

Peptide hormones are a class of peptide that are secreted into the blood stream and have endocrine functions in living animals. Peptide hormones are increasingly being identified in plants with important roles in cell-to-cell communication and plant defence....
of the gastrointestinal system responsible for stimulating the digestion

Digestion

Digestion is the mechanical and chemical breaking down of food into smaller components, to a form that can be Absorption, for instance, by a blood stream....
of fat

Fat

Fats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. Chemistry, fats are generally ester of glycerol and fatty acids....
and protein

Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
.

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Cholecystokinin (CCK; from Greek

Greek language

Peptide hormone

Digestion

Digestion is the mechanical and chemical breaking down of food into smaller components, to a form that can be Absorption, for instance, by a blood stream....
of fat

Fat

Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
. Cholecystokinin, previously called pancreozymin, is synthesised by I-cells in the mucosal epithelium of the small intestine and secreted in the duodenum

Duodenum

The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum....
, the first segment of the small intestine

Small intestine

In vertebrates, including mammals, reptiles, birds, and bony fish, the small intestine is the part of the gastrointestinal tract following the stomach, and is where the vast majority of digestion takes place....
, and causes the release of digestive enzyme

Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
s and bile

Bile

Bile or gall is a bitter yellow or green fluid secreted by hepatocytes from the liver of most vertebrates. In many species, bile is stored in the gallbladder between meals and upon eating is discharged into the duodenum where the bile aids the process of digestion of lipids....
from the pancreas

Pancreas

Gallbladder

The gallbladder is a small non-vital Organ which aids in the digestive process and concentrates bile produced in the liver....
, 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

Morphine

Morphine is a highly potent opiate analgesic Medication, is the principal active agent in opium, and is considered to be the prototypical opioid....
and heroin

Heroin

Heroin is a opioid synthesized from morphine, a derivative of the opium poppy. It is the 3,6-acetate ester of morphine . The white crystalline form is commonly the hydrochloride salt diacetylmorphine hydrochloride, however heroin Freebase may also appear as a white powder....
, and is partly implicated in experiences of pain hypersensitivity during opioid withdrawal

Withdrawal

Withdrawal, also known as withdrawal/abstinence syndrome, refers to the characteristic signs and symptoms that appear when a drug that causes physical dependence is regularly used for a long time and then suddenly discontinued or decreased in dosage....
.

Structure

CCK is composed of varying numbers of amino acid

Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
s depending on post-translational modification of the CCK gene product, preprocholecystokinin. Thus CCK is actually a family of hormones identified by number of amino acids, e.g., CCK58, CCK33, and CCK8. CCK58 assumes a helix-turn-helix

Helix-turn-helix

In proteins, the helix-turn-helix is a major structural motif capable of binding DNA. It is composed of two alpha helix joined by a short strand of amino acids and is found in many proteins that regulate gene expression....
configuration. Its existence was first suggested in 1905 by the British physiologist Joy Cohen. CCK is very similar in structure to gastrin

Gastrin

In humans, gastrin is a hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the stomach, duodenum, and the pancreas....
, another of the gastrointestinal hormone

Gastrointestinal hormone

The gastrointestinal hormones constitute a group of hormones secreted by enteroendocrine cells in the stomach, pancreas, and small intestine that control various functions of the digestive organs....
s. CCK and gastrin share the same five amino acids at their C-termini.

Functions

CCK mediates a number of physiological

Physiology

Physiology is the study of the mechanical, physical, and biochemical functions of living organisms. Physiology has traditionally been divided between plant physiology and animal and all living things physiology but the principles of physiology are universal, no matter what particular organism is being studied....
processes, including digestion

Digestion

Digestion is the mechanical and chemical breaking down of food into smaller components, to a form that can be Absorption, for instance, by a blood stream....
and satiety.

Digestion

Secretion of CCK by the duodenal and intestinal mucosa is stimulated by fat- or protein-rich chyme

Chyme

Chyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum. In other words, chyme is half digested food. Also known as Chymus, it is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum....
entering the duodenum

Duodenum

Gastric acid

Gastric acid is one of the main secretions of the stomach, together with several enzymes and intrinsic factor. Chemically it is an acid solution with a pH of 1 to 2 in the stomach lumen , consisting mainly of hydrochloric acid , and large quantities of potassium chloride and sodium chloride ....
secretion and mediates digestion in the duodenum. It stimulates acinar cell of pancreas

Pancreas

The pancreas is a gland Organ in the digestive system and endocrine system of vertebrates. It is both an endocrine gland , as well as an exocrine gland, secreting pancreatic juice containing Digestion enzymes that pass to the small intestine....
to release water and ions and stimulates the secretion of a juice rich in pancreatic digestive enzymes. Together these enzymes catalyze the digestion of fat, protein, and carbohydrates. Thus the levels of the substances which stimulated the release of CCK drop and the concentration of the hormone drops as well. The release of CCK is also inhibited by somatostatin

Somatostatin

Somatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones....
.

CCK also causes the increased production of hepatic bile, and stimulates the contraction of the gall bladder and the relaxation of the Sphincter of Oddi

Sphincter of Oddi

The Sphincter of Oddi is a muscular valve that controls the flow of digestive juices through the ampulla of Vater into the second part of the duodenum ....
(Glisson's sphincter), resulting in the delivery of bile

Bile

Micelle

A micelle is an aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic tail regions in the micelle centre....
s that emulsify fats, aiding in their digestion and absorption.

Neurobiology

As a neuropeptide

Neuropeptide

A neuropeptide is any of the variety of peptides found in neural tissue; e.g. endorphins, enkephalins. At present about 100 different peptides are known to be released by different populations of neurons in the mammalian brain....
, CCK mediates satiety by acting on the CCK receptors distributed widely throughout the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms....
. In humans, it has been suggested that CCK administration causes nausea

Nausea

Nausea is the sensation of unease and discomfort in the stomach with an urge to vomit....
and anxiety

Anxiety

Anxiety is a psychological and physiological state characterized by cognitive, somatic, emotional, and behavioral components. These components combine to create an unpleasant feeling that is typically associated with uneasiness, fear, or worry....
, and weakly decreases the desire to eat is the reason for CCK administration to induce a satiating effect. Some studies have given a strong correlation for the satiating effect, but have not proven or disproven that CCK administration causes nausea or anxiety. The mechanism for this hunger suppression is thought to be a decrease in the rate of gastric emptying.

CCK also has stimulatory effects on the vagus nerve

Vagus nerve

The vagus nerve is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem and extends, through the jugular foramen, down below the head , to the neck, chest and abdomen, where it contributes to the innervation of the viscera....
, effects that can be inhibited by capsaicin

Capsaicin

Capsaicin is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an Irritation for mammals, including humans, and produces a sensation of burning in any Biological tissue with which it comes into contact....
. The stimulatory effects of CCK oppose those of ghrelin

Ghrelin

Ghrelin is a hormone produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates appetite....
, which has been shown to inhibit the vagus nerve. The CCK tetrapeptide fragment CCK-4

CCK-4

Cholecystokinin tetrapeptide is a peptide fragment derived from the larger peptide hormone cholecystokinin. Unlike cholecystokin which has a variety of roles in the gastrointenstinal system as well as central nervous system effects, CCK-4 acts primarily in the brain as an anxiogenic, although it does retain some GI effects, but not as much a...
(Trp

Tryptophan

Tryptophan is one of the 20 List of standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG....
-Met

Methionine

Methionine is an a-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This Essential amino acid is classified as nonpolar....
-Asp-Phe

Phenylalanine

Phenylalanine is an a-amino acid with the chemical formula HO2CCHCH2C6H5, which is found naturally in the breast milk of mammals and manufactured for food and drink products and are also sold as nutritional supplements for their reputed analgesic and antidepressant effects....
-NH₂) reliably causes anxiety when administered to humans, and is commonly used in scientific research to induce panic attack

Panic attack

Panic attacks are very sudden, discrete periods of intense anxiety, mounting physiological arousal, fear, stomach problems and discomfort that are associated with a variety of somatic and cognitive symptoms....
s for the purpose of testing new anxiolytic

Anxiolytic

An anxiolytic is a Medication prescribed for the treatment of symptoms of anxiety. Some anxiolytics have been shown to be useful in the treatment of anxiety disorders as have antidepressants such as the class of selective serotonin reuptake inhibitors ....
drugs.

The effects of CCK vary between individuals. For example, in rat

Rat

Rats are various medium sized, long-tailed rodents of the Family Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus....
s, CCK administration significantly reduces hunger in young males, but is slightly less effective in older subjects, and even slightly less effective in females. The hunger-suppressive effects of CCK also are reduced in obese rats.

Cholecystokinin

Structure

Functions

Digestion

Neurobiology

See also

External links