Opioid drugs are highly addictive and their abuse has a strong genetic load. Dopamine-glutamate interactions are hypothesized to be important for regulating neural systems central for addiction vulnerability. Balanced dopamine-glutamate interaction is mediated through several functional associations, including a physical link between discs, large homolog 4 (Drosophila) (DLG4, PSD-95) and dopamine receptor 1 (DRD1) within the postsynaptic density to regulate DRD1 trafficking. To address whether genetic associations with heroin abuse exist in relation to dopamine and glutamate and their potential interactions, we evaluated single-nucleotide polymorphisms of key genes within these systems in three populations of opiate abusers and controls, totaling 489 individuals from Europe and the United States. Despite significant differences in racial makeup of the separate samples, polymorphisms of DRD1 and DLG4 were found to be associated with opiate abuse. In addition, a strong gene-gene interaction between homer 1 homolog (Drosophila) (HOMER1) and DRD1 was predicted to occur in Caucasian subjects. This interaction was further analyzed by evaluating DRD1 genotype in relation to HOMER1b/c protein expression in postmortem tissue from a subset of Caucasian subjects. DRD1 rs265973 genotype correlated with HOMER1b/c levels in the striatum, but not cortex or amygdala; the correlation was inversed in opiate abusers as compared with controls. Cumulatively, these results support the hypothesis that there may be significant, genetically influenced interactions between glutamatergic and dopaminergic pathways in opiate abusers.