The mesocorticolimbic system comprises several interconnected brain regions, including the ventral tegmental area (VTA) and substantia nigra, dorsal striatum, ventral striatum (nucleus accumbens, NAcb) and the amygdala, as well as the frontal cortical regions that correspond to rat prefrontal cortex or human anterior cingulate (Goldstein and Volkow, 2002; Ongur and Price, 2000). The VTA, NAcb, and frontal cortex comprise an integral part of the motivational circuit (Figure 1) (Mogenson et al., 1993). The major source of dopamine (DA) to forebrain structures, such as the prefrontal cortex and NAcb, arises from cell bodies in the VTA of the midbrain (Fields et al., 2007). The important and complex role of DA in motivated behavior and learning has been previously reviewed (Berke and Hyman, 2000; El-Ghundi et al., 2007; Nicola et al., 2000) and previous work supports the hypothesis that the NAcb, a primary target of the VTA, serves as a limbic-motor interface that processes reward valence and modulates motivational drives in order to execute both novel and more habitual responding (Kelley, 2004; Koob and Le Moal, 2001; Mogenson et al., 1993; Nestler,
