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Definition

Endorphins are small neuropeptides that are produced by the body and act to reduce pain hence, the name endorphin (a shortened version of endogenous morphine). The term "enkephalin" (meaning literally "in the head") is also applied to endorphins, but usually refers to smaller molecules that have pain-relieving properties 1.

Related Peptides

There are 3 types of Endorphins:

Enkephalins: Met- and Leu-
Endorphins
Dynorphins
Endorphins are neuropeptides that can range from 2 to 39 amino acids in length. Neuropeptides are peptide molecules produced and released in the nervous system that act like transmitters 2. There are three different neuropeptide sequences including enkephalins, endorphins, and dynorphins 3

Discovery

In 1975, John Hughes and Hans W. Kosterlitz of the University of Aberdeen isolated two naturally occurring peptides in the brain that bound tightly to the opiate receptors and named them enkephalins. The endorphin molecule was subsequently isolated from the pituitary gland 4.

Structural Characteristics

Four distinct groups of endorphins have been identified to date. They have been termed: a-endorphin, a polypeptide with 16 residues; ß-endorphin, a polypeptide with 31 residues; ?-endorphin, a polypeptide with 17 residues; and S-endorphin, a polypeptide with 27 residues. These different types of endorphins, like all known polypeptide hormones, are synthesized in a "prepro" form that is one gigantic polypeptide with a signal sequence and additional sequences that are cleaved out during posttranslational maturation of the polypeptide. The most interesting example of this is the pituitary multihormone precursor termed pro-opiomelanocortin that contains the sequences for ß-lipotropin, melanocyte-stimulating hormone (MSH), endorphins, enkephalins, and adrenocorticotropic hormone (ACTH). After synthesis, this peptide is cleaved in the pituitary to generate ACTH and ß-lipotropin, while processing in the central nervous system produces endorphins and enkephalins, along with some other products 5.

Mode of Action

Receptors enable endorphins to perform their specific function. Opioid receptors are large protein molecules embedded in the semi-fluid matrix of the cell membrane of the receiving neuron. The surface of the receptor protein contains a region that is the precise size and shape to match the structure of the endorphin molecule. The endorphin molecule precisely fits into the specific receptor site. The binding of the neuropeptide with its specific receptor (opioid receptor) alters the three-dimensional shape of the receptor protein, thereby causing a neuron to be excited or inhibited6. As in the case of endorphins, inhibition of the neuron will reduce the release of substance P. In other words, the opioid receptor translates the precise messages encoded by the molecular structure of the endorphin molecule into a specific physiological response. Thus, receptors act as a control mechanism thereby regulating the function of endorphins 7.

Functions

Endorphins are not considered to be neurotransmitter molecules, but are instead classified as neuromodulatory, that is, they modify the action of neurotransmitters through a number of effects associated with pain or pleasure. Endorphins exhibit a number of neurological effects associated with the relief of pain. The administration of exogenous endorphins (those prepared outside the body) stimulates the release of many other hormones including prolactin, ACTH, and antidiuretic hormone. The analgesic effects of morphine are commonly believed to be caused by binding to receptor sites for endorphins, but few beneficial effects of treatment with exogenous endorphins have been reported. Early speculations concerning the function of endorphins suggested that they were natural painkillers that the body produced to alleviate pain in circumstances requiring an individual to continue functioning in spite of injury or stress. Examples of such situations might include childbirth, exercise, and combat. Several procedures that treat chronic pain (acupuncture, direct electrical stimulation of the brain and even hypnosis) may act by inducing the release of enkephalins or endorphins in the brain and spinal cord. This hypothesis is based on the finding that the effectiveness of treating pain implemented by these procedures is blocked by administration of naloxone, a drug that specifically blocks the binding of morphine to the opiate receptor 1.

References

Book: Textbook of Biochemistry: With Clinical Correlations by Devlin TM.
Book: Animal Physiology by Eckert R.
Book: Neurobiology by Shepherd GM.
Book: The Brain by Iverson L.
Book: Molecular Expressions: Exploring the World of Optics and Microscopy Michael WD.
Book: Neural and Integrative Animal Physiology by Prosser CL.
Book:. Neuroscience by Barker RA