Although many workers have appreciated the striking cytologic and neurochemical similarities
of neostriatum, accumbens and olfactory tubercle, a compelling case for regarding
these areas as territories in a striatal complex awaited the arguments made by Heimer
and his colleagues based on their investigations of connections. A number of recent
papers support this viewpoint and extend it with the characterization of three accumbal
subterritories: core, shell and rostral pole. The case for separate classifications
of systems traversing the accumbens has become more compelling with each study that
demonstrates connectional, cytoarchitectural and neurochemical specificity conforming
to the boundaries separating the core and its downstream targets from the shell and
its projection fields. Furthermore, its apparent composite of core-like and shell-like
characteristics distinguishes the rostral pole as yet another unique subterritory.
Differences in compartmental organization distinguish the accumbens and neostriatum.
The available data are consistent with the periventricular and rostrolateral enkephalin-rich
zones being ventralmost parts of the neostriatal patch and matrix compartments, respectively.
The accumbal cell cluster compartment, on the other hand, appears to be a separate
entity, with connectional and neurochemical features that are dissimilar to both patch
and matrix of neostriatum. Boundaries between the accumbens and caudate-putamen remain
elusive, and the point of view that such boundaries do not exist but, rather, are
represented by "transition zones" must to a large degree reflect the reality. Likewise,
it is important to acknowledge that the boundaries between accumbal subterritories
are not necessarily distinct or observed faithfully by all of the afferent systems.
"Transition zones" appear to be particularly significant organizational features in
rostral and lateral parts of the accumbens. Interestingly, histochemically distinct
cell clusters tend to be numerous in boundary regions between adjacent territories
and subterritories. The predominant organizational pattern appears to be one in which
the core, shell and rostral pole engage different forebrain systems that possibly
subserve entirely different functions mediated by distantly related mechanisms. In
this regard, it is of paramount interest that the processing of information conveyed
to the accumbens by diverse cortical and subcortical inputs occurs within distinct
and perhaps very different dopaminergic environments in the core, shell and rostral
pole (e.g., see Refs 24, 34, 90, 110).