Alcohol abuse is a major public health burden in the United States, yet few pharmacotherapies are approved to treat this disease. Naltrexone, a non-selective opioid receptor antagonist, has been effective in clinical trials, as it appears to block the ethanol-evoked activation of brain reward pathways by endogenous opioids. Recent evidence suggests that the A118G polymorphism in exon 1 of the mu-opioid receptor gene (OPRM1) is a clinically relevant risk factor associated with alcohol abuse and dependence. Carrying the G- allele appears to increase the risk of developing alcohol use disorders and predicts treatment response to opioid antagonism in clinical trials, although the etiology of this effect is unclear. This proposal outlines behavioral pharmacology experiments that will employ a humanized mouse model of the A118G polymorphism to determine if sensitivity to the rewarding effects of alcohol is different in mice homozygous for the 118G-allele (h/m118GG) compared to those homozygous for the 118A-allele (h/m118AA). Experiments are also proposed that will determine if the sensitivity of alcohol reward to antagonism by naltrexone differs between h/m118GG and h/m118AA mice. Pharmacological effects of opioids in vitro will be compared between h/m118AA and h/m118GG mice in a site that is critical to alcohol reward to determine if differences in the synaptic effects of opioids may explain behavioral differences in vivo. Intracranial self-stimulation (ICSS), an operant behavioral method, will be used to determine the rewarding effects of alcohol, morphine, and cocaine and to measure the reward-devaluing effects of naltrexone and a kappa-opioid receptor agonist, U69,593. Opioid signaling in the ventral tegmental area, a major target of opioid activity in the brain reward system, will be characterized with whole-cell patch clamp electrophysiology. If funded, these studies will address important unanswered questions about how the A118G polymorphism alters the rewarding effects of alcohol and increase understanding of the mechanism through which this genetic variant increases risk for alcohol abuse. The proposed training plan will provide a solid educational and professional foundation on which to pursue a career as a physician-scientist in the field of addiction medicine.