enkephalin /enkef″əlin/ [Gk, enkepalos, brain, in, within] , one of two pain-relieving pentapeptides produced in the body, located in the pituitary gland, brain, and GI tract. Axon terminals that release enkephalins are concentrated in the posterior horn of the gray matter of the spinal cord, in the central part of the thalamus, and in the amygdala of the limbic system of the cerebrum. Enkephalins function as neurotransmitters or neuromodulators and inhibit neurotransmitters in the pathway for pain perception, thereby reducing the emotional as well as the physical impact of pain. Methionine-enkephalin and isoleucine-enkephalin are each composed of five amino acids, four of which are identical in both compounds. These two neuropeptides can depress neurons throughout the central nervous system. Although it is not known exactly how these neuropeptides function, the enkephalins are natural pain killers and may be involved, with other neuropeptides, in the development of psychopathological behavior in some cases. Compare endorphin.
