The discovery of microRNAs (miRNAs) and their mechanisms of action is revolutionizing our understanding of gene regulation (Ambros, 2001; Filipowicz et al., 2008). These short (∼17–24 nucleotides long) non-coding RNAs act as post-transcriptional modulators of gene expression by binding to miRNA-recognition elements (MREs) in their target genes. This direct targeting generally results in either suppression of translation or degradation of the targeted mRNA transcript, or both (Filipowicz et al., 2008; Breving and Esquela-Kerscher, 2010). There are instances, however, when miRNAs can increase the expression of a target gene by enhancing mRNA translation (Vasudevan et al., 2007). miRNAs are highly abundant in the brain and play important roles in multiple biological processes such as neuronal differentiation (Cheng et al., 2009), brain development (Fiore et al., 2008), synapse formation and plasticity (Schratt et al., 2006), and neurodegeneration (Schaefer et al., 2007; Bushati and Cohen, 2008). miRNAs also appear to mediate the cellular adaptations induced by exposure to a number of drugs of abuse, including nicotine (Huang and Li, 2008), cocaine (Chandrasekar and Dreyer, 2009), opioids (He et al., 2010), and alcohol (Sathyan
