islands.60,61 Secondly, a subterritory of the VS, the shell region, divides the VS into two parts, a medial/ventral shell region and a central core region.62 Experiments aimed at delineating the functional significance of these two regions have been instrumental in understanding the circuitry underlying goal-directed behaviors, behavioral sensitization, and changes in affective states. Studies in rodents have been particularly important in demonstrating the organization of the shell and core and their relationship to distinct ventral striatal afferent and efferent projections.63,64 Whereas several transmitter and receptor distribution patterns distinguish the shell/core subterritories, lack of calbindin-positive staining is the most consistent marker for the shell across species.65 Thirdly, while the basic cortical basal ganglia loop is similar in all basal ganglia circuits, the VS alone receives an additional subcortical input from the amygdala and from the hippocampus, for which there is no comparable input to the other basal ganglia territories.61,66 Finally, the DA transporter is relatively low throughout the VS, including the core. This pattern is consistent with the fact that the dorsal-tier DA neurons that project to the VS express relatively low levels of messenger RNA for the DA transporter, compared with the ventral tier.67
