endorphin

[en-dawr-fin]
noun
any of a group of peptides occurring in the brain and other tissues of vertebrates, and resembling opiates, that react with the brain's opiate receptors to raise the pain threshold.

Origin:
1970–75; end(ogenous) (m)orphine, with -ine respelled as -in2

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World English Dictionary
endorphin (ɛnˈdɔːfɪn)
 
n
any of a class of polypeptides, including enkephalin, occurring naturally in the brain, that bind to pain receptors and so block pain sensation
 
[C20: from endo- + morphine]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

endorphin
1975, from Fr. endorphine, from endogène "endogenous, growing within" (from endo- + gene) + (mo)rphine.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

endorphin en·dor·phin (ěn-dôr'fĭn)
n.
Any of a group of peptide hormones that bind to opiate receptors and are found mainly in the brain. Endorphins reduce the sensation of pain and affect emotions.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
endorphin   (ěn-dôr'fĭn)  Pronunciation Key 
Any of a group of peptide substances secreted by the anterior portion of the pituitary gland that inhibit the perception of painful stimuli. Endorphins act as neurotransmitters in the pain pathways of the brain and spinal cord. Narcotic drugs may stimulate the secretion of endorphins.

Our Living Language  : Endorphins are long chains of amino acids, or polypeptides, that are able to bind to the neuroreceptors in the brain and are capable of relieving pain in a manner similar to that of morphine. There are three major types of endorphins: beta-endorphins are found almost entirely in the pituitary gland, while enkephalins and dynorphins are both distributed throughout the nervous system. Scientists had suspected that analgesic opiates, such as morphine and heroin, worked effectively against pain because the body had receptors that were activated by such drugs. They reasoned that these receptors probably existed because the body itself had natural painkilling compounds that also bonded to those receptors. When scientists in the 1970s isolated a biochemical from a pituitary gland hormone that showed analgesic properties, Choh Li, a chemist from Berkeley, California, named it endorphin, meaning "the morphine within." Besides behaving as a pain reducer, endorphins are also thought to be connected to euphoric feelings, appetite modulation, and the release of sex hormones. Prolonged, continuous exercise contributes to an increased production of endorphins and, in some people, the subsequent "runner's high."
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary
endorphins [(en-dawr-finz)]

Substances produced by the brain that have painkilling and tranquillizing effects on the body. Endorphins are thought to be similar to morphine and are usually released by the brain during times of extreme body stress. The release of endorphins may explain why trauma victims sometimes cannot feel the pain associated with their injuries.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
But the endorphins from running keep her mind clear and make her spirits soar,
  she said.
The reason is almost certainly the effects of endogenous opioids, better known
  as endorphins.
When a heroin user shoots up, the opiates in the drug plug into the nerve
  receptors normally occupied by endorphins.
She attributes this effect to the release of endorphins in the brain, which she
  says occurs during reflexology.
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