reduced gray matter in the limbic system increases risk for alcohol addiction due to reward deficiencies and exaggerated processing of incentive values of substance-related stimuli (Blum et al., 2000). In addition, those with the AA genotype of SLC6A2 rs36021 had higher rates of marijuana use. Few studies have tested the role of SLC6A2 on marijuana use, though one study demonstrated a significant difference across genotypes on the SLC6A2 SNP rs785143, whereby those homozygous for the C allele reported more lifetime marijuana use (Dlugos, Hamidovic, Palmer, & de Wit, 2009). As noted, prior work demonstrates that elevated noradrenergic signaling is related to increased susceptibility to psychological stress (Fitzgerald, 2013). In turn, stress relief is reported as a reason for initiating marijuana use (Copeland, Swift, & Rees, 2001). Thus, those with SLC6A2 variants may use marijuana for coping motives. Findings supporting both direct and indirect effects of genetic variants are in line with conclusions reached by other researchers that an individual’s liability for addiction is partly due to a general externalizing factor and partly due to genetic factors that are disorder- and mechanism-specific (Dick & Agrawal, 2008).
