The cortical components of the limbic circuit consist of mesocortical and prefrontal areas, as well as the allocortical regions, such as the hippocampal formation and basolateral amygdala (BLA) (Gerfen 1992, Yin and Knowlton 2006). These cortical regions project predominantly to the nucleus accumbens (NAc) (although the BLA does give rise to an appreciable projection to the DMS). The NAc can be further subdivided into the “core” and “shell” subregions, and these have been implicated in different aspects of learning and responses to substances of abuse as will be discussed. Thalamic projections to the NAc from antero-medial subregions are also part of the limbic circuitry. The dopaminergic input to the NAc comes predominantly from the ventral tegmental area (VTA). The NAc shell subregion also receives a more robust noradrenergic input in comparison to DS and the NAc core, and this arises from the nucleus of the solitary tract (Berridge, Stratford et al. 1997, Delfs, Zhu et al. 1998). Some MSNs within the NAc send their projections to the ventral mesencephalon (VM), while others innervate the ventral pallidum (VP). While the VM
