In the interim, animal models of addiction continue to provide insights into potential candidate genes that would benefit from more directed study in humans. A number of these studies have targeted the known neurobiological systems regulating the dopamine reward system and the endogenous opioid system. Dopamine plays a key role in reward behavior, yet the association with alcoholism and other drugs remains controversial. There are equally prominent association studies of DRD2 with alcoholism and other drug addictions and failures to replicate (Gelernter et al., 1991; Parsian et al., 1991, Le Foll et al, 2009). Similarly, the endogeneous opioid system clearly plays a role in addiction, and an amino acid change in the μ opioid receptor (OPRM1) displays functional changes with up to threefold variation in the affinity of the receptor to bind beta-endorphin, the endogenous opioid (Bond et al., 1998). However, a large scale meta-analysis does not demonstrate that this variant alters the risk of developing addiction (Arias et al., 2006).
