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Endomorphin – Endogenous agonist for μ-opioid receptor  

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

More than three opioid-receptor subtypes, termed μ, δ, κ have been reported. The endogenous ligand for δ -receptor is enkephalin, and that for κ -receptor is dynorphin. Although opiate alkaloids morphine is well-known for the μ-selective and potent ligand, endogenous ligand “endogenous morphine” has never reported.

Recently, endogenous agonists for the μ-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 μ-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 μ-opiate agonists in mammals. In addition to the originally discovered μ-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.

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