The cocaine-derived score identified within the current training sample predicted only cocaine dependence severity in the testing sample; it did not account for genetic variance common across substances. This could be due to the specific relationship between dopamine and cocaine. There is extensive genetic, pharmacologic, and functional evidence, in both human and non-human animal models, explicating the direct relationship between cocaine and the dopaminergic system (e.g., Dackis & Gold, 1985; Haile et al., 2007; Kuhar et al., 1991). While a substantial research literature supports an association between dopamine and other substances (e.g., alcohol, tobacco, and marijuana; Blum et al., 2000; Derauf et al., 2009), the effects are generally not as strong and the posited mechanism(s) of action not as direct as between dopamine and cocaine.
