An important consideration in human studies is genetics which confer an increased vulnerability to heroin addiction (13, 14) and gene variants encoding opioid receptors have been shown to be associated with this disorder.(15–18) Numerous single nucleotide polymorphisms (SNPs) have been identified in the human MOR gene (OPRM1) and while the functional relevance of some variants to MOR signaling(19–21) and expression(22) has been described, the downstream transcriptional events have not been well characterized.
