- either of two pentapeptides that bind to morphine receptors in the central nervous system and have opioid properties of relatively short duration; one pentapeptide (Met enkephalin ) has the amino acid sequence Tyr-Gly-Gly-Phe-Met and the other (Leu enkephalin ) has the sequence Tyr-Gly-Gly-Phe-Leu.
- a chemical occurring in the brain, having effects similar to those of morphine
