The rewarding properties of nicotine are mediated in part by μ-opioid receptors(MOR) encoded by the OPRM1 locus.3, 18 The reinforcing effects of nicotine areattenuated in mice lacking MOR19, 20 and MOR antagonists suppress nicotineself-administration in rats.21, 22 In humans, cigarette smoking increases therelease of the endogenous MOR ligand β-endorphin,23 while naloxone, a MOR antagonist, decreases nicotinereward.24 Also, MOR availability inreward-related brain regions correlates with nicotine dependence andreward.25, 26. This evidence concerns the gene OPRM1 and nicotine dependence.