Researchers discovered endomorphin-1 and endomorphin-2, the first known endogenous peptides with high selectivity for the mu-opioid receptor.
First mu-selective endogenous opioidsEndomorphin-1 and -2 filled a 20+ year gap as the missing natural ligands for the mu-opioid receptor
What the researchers found
Two novel tetrapeptides, endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), were identified as potent and selective endogenous agonists of the mu-opioid receptor.
Why it matters
This discovery filled a major gap in opioid biology — the missing endogenous ligand for the mu receptor. Understanding the body's own mu-opioid system could lead to pain treatments that mimic natural mechanisms with fewer side effects than morphine.
How the study worked
Peptide isolation from bovine brain, receptor binding assays across opioid receptor subtypes (mu, delta, kappa), and functional activity testing to establish selectivity and potency.
What this study cannot tell us
Initial discovery paper — full physiological roles not yet characterized. Brain distribution and biosynthetic pathway were not fully mapped. The precursor protein for endomorphins remained elusive.
How to read the evidence
Published in Nature, one of the highest-impact scientific journals. Landmark discovery with rigorous receptor binding and selectivity characterization.
When this study was published
Published in 1997, this seminal discovery has been foundational to subsequent opioid peptide research and remains widely cited.
The bigger picture
The discovery of endomorphins completed the picture of endogenous opioid peptide families. Each major opioid receptor now has known natural ligands: enkephalins for delta, dynorphins for kappa, and endomorphins for mu. This has profound implications for pain research, addiction science, and understanding mood regulation.
Questions still open
- What is the precursor protein from which endomorphins are derived?
- Could synthetic endomorphin analogs provide pain relief with fewer side effects than morphine?
- How do endomorphin levels change in chronic pain conditions?
Common questions
What are endomorphins?
Why was this discovery important?
Read the original research
A potent and selective endogenous agonist for the mu-opiate receptor.
Nature, 386(6624), 499-502
Citation
Zadina, J E; Hackler, L; Ge, L J; Kastin, A J. (1997). A potent and selective endogenous agonist for the mu-opiate receptor.. Nature, 386(6624), 499-502.