An enkephalin is a pentapeptide involved in regulating nociception in the body. The enkephlins are termed endogenous ligands, or specifically endorphins, as they are internally derived and bind to the body"s opiod receptors. They function as neurotransmitters or neuromodulators and inhibit neurotransmitters in the pathway for pain perception. There are two forms of enkephalin, one containing leucine, and the other containing methionine. 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, wth other neuropeptides, in the development of psychopathologic behavior in some cases.