It is clear that several differences in type of exposure alter the pattern of altered gene expression. One such factor is course of the exposure. Specific changes in early response genes and signal transduction pathways are more visible in the early stages of drug-induced neural adaptive processes (Celentano et al., 2009; Marie-Claire et al., 2007; Zhou et al., 2011), whereas prolonged exposure leads to widespread transcriptional changes of genes involved in diverse cellular functions such as ion transport, chromosome remodeling, stress and immune response, cell adhesion, cell cycle, apoptosis, protein and lipid metabolism, and mitochondrial functions (Albertson et al., 2004; Bannon et al., 2005; Mash et al., 2007; Renthal et al., 2007; Zhou et al., 2011). The impact of drug exposure on transcription is also brain region specific. In two components of the mesolimbic system, the dorsal striatum and nucleus accumbens (NAc), the expression of genes involved in dopaminergic, glutamatergic, and GABAergic transmission (Ghasemzadeh, Mueller, & Vasudevan, 2009; Hyman & Malenka, 2001; McClung et al., 2005; Schumann & Yaka, 2009) and that play key roles in drug-reward and drug-seeking behavior
