Interestingly, the pattern of OPRM1 Asn40Asp and DRD4 VNTR findings are consistent with Robinson and Berridge’s (1993) incentive sensitization model of drug motivation. From this standpoint, the ascending corticomesolimbic dopamine circuit is largely responsible for attributions of incentive salience, or “wanting,” whereas opioidergic and other neurotransmitter systems variously subserve the hedonic impact, or “liking,” of both natural and drug rewards (Kelley & Berridge, 2002; Robinson & Berridge, 1993). Similarly, in the current study and other recent studies, functional genetic variation in the dopamine system has been associated with more pronounced craving (“wanting”) responses (e.g., Hutchison et al., 2002; McGeary et al., 2006; cf. van den Wildenberg et al., 2007b), whereas functional genetic variation in the endogenous opioid system has been associated with variation in the psychoactive effects of alcohol (Ray & Hutchison, 2004, 2007). This is also consistent with the post-hoc findings from this study suggesting that when controlling for urge to drink, carriers of the Asp40 allele drank 4.73 more drinks per episode than Asn40 homozygous and the OPRM1 × urge to drink interaction suggesting that urge to drink
