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Around the same time, in a calf brain, Rabi Simantov and Solomon H. Snyder of the United States found what Eric Simon (who independently discovered opioid receptors in vertebral brains) later termed "endorphin" by an abbreviation of "endogenous morphine", meaning "morphine produced naturally in the body". Importantly, recent studies have demonstrated that human and diverse animal tissues are in fact capable of producing morphine itself, which is not a peptide.
Scientists sometimes debate whether specific activities release measurable levels of endorphins. Much of the current data comes from animals which may not be relevant to humans. The studies that do involve humans often measure endorphin plasma levels, which do not necessarily correlate with levels in the central nervous system. Other studies use a blanket opioidantagonist (usually naloxone) to indirectly measure the release of endorphins by observing the changes that occur when any endorphin activity that might be present is blocked.[medical citation needed]
Endorphins are known to play a role in depersonalization disorder. The opioid antagonistsnaloxone and naltrexone have both been proven to be successful in treating depersonalization. To quote a 2001 naloxone study, "In three of 14 patients, depersonalization symptoms disappeared entirely and seven patients showed a marked improvement. The therapeutic effect of naloxone provides evidence for the role of the endogenous opioid system in the pathogenesis of depersonalization."[non-primary source needed]
From the words ἔνδον / Greek: éndon meaning "within" (endogenous, ἐνδογενής / Greek: endogenes, "proceeding from within") and morphine, from Morpheus (Μορφεύς / Ancient Greek: Morpheús, the god of sleep in the Greek mythology, thus 'endo(genous) (mo)rphine’.
^Hughes J, Smith T, Kosterlitz H, Fothergill L, Morgan B, Morris H (1975). "Identification of two related pentapeptides from the brain with potent opiate agonist activity". Nature258 (5536): 577–80. doi:10.1038/258577a0. PMID1207728.