Convergence and segregation of ventral striatal inputs and outputs.

The ventral striatum, which prominently includes the nucleus accumbens (Acb), is a heterogeneous area. Within the Acb of rats, a peripherally located shell and a centrally situated core can be recognized that have different connectional, neurochemical, and functional identities. Although the Acb core resembles in many respects the dorsally adjacent caudate-putamen complex in its striatal character, the Acb shell has, in addition to striatal features, a more diverse array of neurochemical characteristics, and afferent and efferent connections. Inputs and outputs of the Acb, in particular of the shell, are inhomogeneously distributed, resulting in a mosaical arrangement of concentrations of afferent fibers and terminals and clusters of output neurons. To determine the precise relationships between the distributional patterns of various afferents (e.g., from the prefrontal cortex, the basal amygdaloid complex, the hippocampal formation, and the midline/intralaminar thalamic nuclei) and efferents to the ventral pallidum and mesencephalon, neuroanatomical anterograde and retrograde tracing experiments were carried out. The results of the double anterograde, double retrograde, and anterograde/retrograde tracing experiments indicate that various parts of the shell (dorsomedial, ventromedial, ventral, and lateral) and the core (medial and lateral) have different input-output characteristics. Furthermore, within these Acb regions, various populations of neurons can be identified, arranged in a cluster-like fashion, onto which specific sets of afferents converge and that project to particular output stations, distinct from the input-output relationships of neighboring, cluster-like neuronal populations. These results support the idea that the nucleus accumbens may consist of a collection of neuronal ensembles with different input-output relationships and, presumably, different functional characteristics.

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.