Substance abuse and addiction to drugs and other addictive agents lead to cellular and molecular changes and are also caused in part by adaptations in epigenetic regulation and gene expression that can be measured in cells. Addictive behaviors are the outcome of allostatic maladaptation of neural circuitries (Goldman, Oroszi, & Ducci, 2005; Koob & Le Moal, 2001). Although great efforts have been made to understand the molecular basis of addiction, the mechanisms are elusive, in part because they are likely to be multiple. However, the study of gene expression in the addicted brain has already yielded valuable insights to the molecular mechanisms of maladaption. In model organisms and cellular models, several important pathway-related changes induced by acute and chronic drug exposure have been discovered. Human studies enabled by the availability of postmortem brain tissues from addicted individuals (Albertson et al., 2004; Albertson, Schmidt, Kapatos, & Bannon, 2006; Bannon, Kapatos, & Albertson, 2005; Kristiansen, Bannon, & Meador-Woodruff, 2009; Lehrmann et al., 2003; Lewohl et al., 2011; Liu, Chen, Lerner, Brackett, & Matsumoto, 2005; Mash et al., 2007; Ponomarev, Wang, Zhang, Harris,
