The importance of individual factors to striatal ELK1 expression was also evident in our study, both in relation to drug use history as supported by our animal model and by mutation of the OPRM1 gene. The rs2075572 OPRM1 variant has not been studied in opioid abuse and the functional consequences of this variant on MOR activity are not known. However, a similar genotype-dose effect pattern was evident for the well-known A118G SNP in heroin abusers, suggesting that these mutations relate to general functioning of MOR and its downstream signaling. Although mutations of OPRM1 contributed to variability in the individual ELK1 expression levels, it is important to emphasize that heroin use still enhanced ELK1 at each respective genotype and thus, history of drug use plays a significant role in ELK1 regulation.
