When a new naturally occurring substance is discovered, medical researchers are justifiably eager to determine whether abnormalities in its disposition are relevant to disease states. Such investigations are more readily conducted with the opioid peptides than with most other substances. These opioids are generically termed endorphins and include the enkephalins and β-endorphin. There are several different endorphins. Met-enkephalin and leu-enkephalin are pentapeptides that differ only in their carboxyl-terminal amino acid.1 The two enkephalins are stored in separate populations of neurons in specific brain areas that have high concentrations of opiate receptors — establishing the enkephalins as probable neurotransmitters at opiate.
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