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.

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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.

The nucleus accumbens, in view of its afferent and efferent fiber connections, appears to hold a key position for "limbic" (e.g., hippocampal and amygdaloid) influences to reach somatomotor and autonomic brain structures, and it has therefore been considered as a limbic-motor interface. The nucleus accumbens can be subdivided into a shell and a core region, which both contain further inhomogeneities. The present account first summarizes the detailed topographical anatomical relationships of inputs from different dorso-ventral parts of the hippocampus and different rostrocaudal parts of the basal amygdaloid complex at the level of the accumbens. Subsequently, the electrophysiological characteristics of hippocampal and amygdaloid inputs in the accumbens are described. Interactions between hippocampal and amygdaloid inputs appear to exist primarily in the medial parts of both the shell and the core of the nucleus accumbens. In the short term, stimulating amygdaloid inputs appear to facilitate hippocampal throughput (heterosynaptic paired pulse facilitation), whereas stimulation of hippocampal inputs depresses amygdaloid throughput in a paired pulse paradigm. Tetanic stimulation of hippocampal inputs to the accumbens leads to a decrementallong-term potentiation (LTP) of this fiber pathway (homosynaptic LTP) but, along a similar time range, to a depression of amygdaloid inputs (heterosynaptic long-term depression). The involvement of dopaminergic, GABAergic, and glutamatergic mechanisms in these interactions is discussed. Finally, it is suggested that the interactions between hippocampal and amygdaloid inputs at the level of the nucleus accumbens playa role in different aspects of associative learning.