Further evidence for the relevance of the OPRM1 A118G polymorphism comes from cognitive neuroscience studies. A functional neuroimaging study revealed that G-allele carriers had greater hemodynamic response in mesocorticolimbic areas both before and after a priming dose of alcohol (Filbey et al., 2008). A recent study combining alcohol administration with positron emission tomography (PET) (Ramchandani et al., 2011)showed that G-allele carriers displayed a more potent striatal dopamine response to alcohol, compared to A-allele homozygotes. In conjunction, these studies support the biological plausibility of this polymorphism as a determinant of alcohol-induced reward, both in terms of hemodynamic response (Filbey et al., 2008) and dopamine (DA) release in the striatum following alcohol exposure(Ramchandani et al., 2011). This is consistent with the putative role of endogenous opioids in mediating the reinforcing effects of alcohol and suggest that ideal phenotypes to test candidate gene should probe the effects of alcohol on the reward circuitry(Ray et al., 2012).
