Any of a class of endogenous opioids consisting of five amino acids and found principally in the central nervous system. They bind to opioid receptors, chiefly in the brain, and their release controls levels of pain and other sensations, for example by blocking the release of substance P, a neurotransmitter involved in nociception. Enkephalins themselves can act as neurotransmitters in the brain. See also dynorphin; endorphin.