appear to bypass dopamine inputs from the VTA (Hyman et al., 2006). Studies examining the effect of selective agonist and antagonist drugs have indicated that multiple neurotransmitters, including dopamine, serotonin, acetylcholine, glutamate, GABA, and various peptides, are involved in activation of reward circuits and the production of multiple forms of neuronal plasticity that can convert drug-induced signals into long-term alterations in behavior (Bardo, 1998; Hyman et al., 2006). At the molecular level, complex gene expression mechanisms coordinate long-lasting alterations in dendrite and synapse structure and function. Among these mechanisms, the local control of mRNA translation in neuronal dendrites can account for the tight spatial regulation of plasticity at the level of individual dendrites or spines (Sutton and Schuman, 2006; Schratt, 2009). Recent evidence discussed below indicates that miRNAs play extensive roles in the regulation of these local processes and the development of addictive animal behaviors. We summarize this evidence in a hypothetical model presented in Figure 2.
