APA Dictionary of Psychology

a substance produced in the body that has the analgesic and euphoric effects of morphine. Three families of endogenous opioids are well known: the enkephalins, endorphins, and dynorphins. All are neuropeptides that bind to opioid receptors in the central nervous system; they are mostly inhibitory, acting like opiates to block pain. They bind relatively nonselectively to opioid receptors, although enkephalins bind preferentially to the delta opioid receptors and dynorphins to kappa opioid receptors. Three other endogenous opioid peptides have been identified: nociceptin/orphanin FQ (N/OFQ) and endomorphins 1 and 2. Also called opioid neurotransmitter.