Endomorphin
Endogenous agonist for m-opioid receptor

[The discovery and isolation of a potent and selective agonist for the m- opiate receptor from brain]

More than three opioid-receptor subtypes, termed m, d, k have been reported.
The endogenous ligand for d -receptor is enkephalin, and that for k -receptor is dynorphin.
Although opiate alkaloids morphine is well-known for the m-selective and potent ligand, endogenous ligand "endogenous morphine" has never reported.

Recently, endogenous agonists for the m-opiate receptor, were first isolated from bovine brain[Nature,386, 499 (1997)]. These are endomorphin-1; Tyr-Pro-Trp-Phe-NH2, and endomorphin-2; Tyr-Pro-Phe-Phe-NH2. The isolation of relatively large amounts of endomorphin-1 and endomorphin-2 from human brain cortex has also reported[Peptides,18, 1635 (1997)].
The immunoreactivity for endomorphin was also detected in rat brain near the m-opiate receptor containing neurons, but the molecular form of the rat peptide has not yet been elucidated[Peptides,18, 1641 (1997)]. These observations may be considered to indicate that endomorphin-1 and 2 are common m-opiate agonists in mammals. In addition to the originally discovered m-opiate agonistic function, these peptides have been reported to show hypotensive activity and Ca²⁺ channel current inhibitory activity[Biochem. Biophys. Res. Commun.,235, 567 (1997)]. Endomorphins would be useful for the study of pain perception together with some other biological functions in the body.