Epigenetics plays a key role in regulating gene expression. Studies have shown that drug exposure causes changes in DNA methylation that lead to alterations in transcription. Acute cocaine treatment was reported to increase the expression of DNA methyltransferase genes, Dnmt3a and Dnmt 3b, in mouse NAc, resulting in DNA hypermethylation and the increased binding of methyl CpG binding protein 2 (MeCP2) at the promoter of protein phosphatase-1 catalytic subunit gene (Pp1c). As a result, Pp1c expression was decreased (Anier, Malinovskaja, Aonurm-Helm, Zharkovsky, & Kalda, 2010). In contrast, chronic cocaine administration was found to decrease expression of Dnmt3a. The attenuation of DNA methylation led to potentiated cocaine reward (LaPlant et al., 2010). Acute and repeated cocaine administration was also shown to cause hypomethylation at the FosB promoter, leading to upregulation of FosB expression (Anier et al., 2010). It has also been shown that in heroin addicts, there was elevated methylation at several CpG sites in the promoter of a μ-opioid receptor gene, OPRM1, in lymphocytes, which might result in reduced expression of that gene (Nielsen et al., 2009). Differential DNA methylation
