Endorphins (contracted from "endogenous morphine"[note 1]) are endogenous opioid neuropeptides and peptide hormones in humans and other animals. They are produced by the central nervous system and the pituitary gland. The term "endorphins" implies a pharmacological activity (analogous to the activity of the corticosteroid category of biochemicals) as opposed to a specific chemical formulation. It consists of two parts: endo- and -orphin; these are short forms of the words endogenous and morphine, intended to mean "a morphine-like substance originating from within the body". The class of endorphins includes three compounds – α-endorphin, β-endorphin, and γ-endorphin – which preferentially bind to μ-opioid receptors. The principal function of endorphins is to inhibit the transmission of pain signals; they may also produce a feeling of euphoria very similar to that produced by other opioids.
Opioid neuropeptides were first discovered in 1974 by two independent groups of investigators:
- John Hughes and Hans Kosterlitz of Scotland isolated – from the brain of a pig – what some called "enkephalins" (from the Greek εγκέφαλος, cerebrum).
- 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". Studies have demonstrated that human and diverse animal tissues are capable of producing morphine, which is not a peptide.
Endorphins are naturally produced in response to pain, but their production can also be triggered by various human activities. Vigorous aerobic exercise can stimulate the release of β-endorphin, a potent μ-opioid receptor agonist, in the human brain, which contributes to a phenomenon known as a "runner's high". Laughter may also stimulate endorphin production; a 2011 study showed that attendees at a comedy club showed increased resistance to pain.
Endorphins are suspected to play a role in depersonalization disorder. The opioid antagonists naloxone and naltrexone have both been proven to be successful in treating depersonalization. Quoting a 2001 study involving the drug naloxone, "in  of 14 patients, depersonalization symptoms disappeared entirely, and  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 (Ancient Greek: Μορφεύς, translit. Morpheús, the god of dreams in the Greek mythology, thus 'endo(genous) (mo)rphine’.
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Positive evolutionary pressure has apparently preserved the ability to synthesize chemically authentic morphine, albeit in homeopathic concentrations, throughout animal phyla. ... The apparently serendipitous finding of an opiate alkaloid-sensitive, opioid peptide-insensitive, µ3 opiate receptor subtype expressed by invertebrate immunocytes, human blood monocytes, macrophage cell lines, and human blood granulocytes provided compelling validating evidence for an autonomous role of endogenous morphine as a biologically important cellular signalling molecule (Stefano et al., 1993; Cruciani et al., 1994; Stefano and Scharrer, 1994; Makman et al., 1995). ... Human white blood cells have the ability to make and release morphine
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Different patterns of tissue-specific cleavage is another step by which a diversity of signaling peptides can be generated. Posttranslational processing can be illustrated by the cleavage and modification of proopiomelanocortin (POMC), the precursor of several peptides with distinct biological actions, including adrenocorticotropic hormone (ACTH), α-melanocytestimulating hormone (α-MSH; also called melanocortin), and β-endorphin.
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