Opioids exert their psychoactive effects through opioid receptors in the brain. Of the three opioid receptor genes, OPRM1, OPRD1, and OPRK1, which encode the mu, delta, and kappa opioid receptors, OPRM1 is the most studied with respect to opioid addiction, as it is the main site of action of commonly used opioids such as heroin, morphine, and methadone [117]. It should also be noted that behavioral reinforcement via endogenous opioids plays a role in the formation and maintenance of many other types of substance use disorders, such as alcoholism [93].
