ABSTRACT
Enkephalins were discovered during studies in the early 1970s that were focused on the mechanism of opium addiction. Several groups discovered that brain plasma membranes contain high-affinity binding sites for purified opiates, such as the alkaloid morphine (a narcotic, analgesic drug). Since such receptors exist in the brains of all vertebrates, from sharks to primates, the question was raised why vertebrates should have highly specific receptors for alkaloids produced by opium poppy plants, and why these receptors should have survived evolution. Since none of the neurotransmitters and peptides then known were structurally similar to morphine, nor could they serve as agonists for these receptors, a search was begun for natural compounds that could. This led to the discovery of two pentapeptides, Met-enkephalin and Leu-enkephalin, both of which bind to “opiate” receptors in the brain, and have the same effect as morphine when injected into the ventricles (cavities) of brains in experimental animals. Enkephalins, dynorphins, and endorphins appear to act by inhibiting neurons that transmit or perceive pain impulses: presumably these neurons contain abundant “opiate peptide” receptors.
