Basal amygdaloid complex afferents to the rat nucleus accumbens are compartmentally organized

The basal amygdaloid complex (BAC) topographically projects to the nucleus accumbens (Acb) in patchy, inhomogeneous patterns. These termination patterns may be related to the histological features of the Acb that define the shell, core, and adjacent ventral caudate-putamen (CPv), and the ventral striatal compartments providing output to different autonomic, motor, and endocrine targets. Knowledge of the relationships of BAC afferents with these compartments is essential for understanding the activities of amygdalostriatal circuits. Therefore, anterograde tracing experiments were performed, combined with calbindin-D28K (CaB) immunohistochemistry or Nissl staining. The results demonstrated that the caudal parvicellular basal amygdala (Bpc) projected primarily to cell clusters in the dorsal shell of the medial Acb, and to patches in the core/CPv. Fibers from the caudal accessory basal nucleus (AB) selectively reached CaB-immunoreactive cell clusters in the ventral shell, avoiding the core/CPv. The rostral AB projected to the same ventral shell compartments as the caudal AB; in addition, dense terminations were found in the matrix of the core/CPv, avoiding the patches. Caudal magnocellular basal amygdala (Bmg) fibers reached ventral parts of the shell, including the CaB-immunoreactive cell clusters. The caudal Bmg projected strongly to the patches of the core/CPv, evading the matrix. Finally, the rostral Bmg densely innervated the moderately CaB-immunoreactive lateral shell and the patches of the core/CPv, largely avoiding the matrix. These results indicate the specific compartmental relationships of the patchy BAC terminations and suggest that BAC subregions differentially influence particular ventral striatal outputs.