The striatum, which is the major component of the basal ganglia, displays a complex mosaic organization of neurochemical systems that are related to its neuroanatomical connections. This mosaic organization reflects multiple levels of functional compartments. The first level is determined by the segregation of two major striatal output systems, one to the globus pallidus (external segment) and the other to the entopeduncular nucleus and substantia nigra. The second level segregates the cortical outputs of sublaminae of layer V between the patch and matrix compartments of the striatum, which project to the dopaminergic and GABAergic neurons in the substantia nigra, respectively. The third level is related to the topography of cortical inputs by which regions of the striatum may be functionally defined on the basis of the cortical areas with which they are connected. Neurochemical markers display complex mosaic patterns in the striatum that, when examined in the context of the multi-level compartmental organization of the striatum, reveal the highly organized manner by which the striatum processes cortical information.

When classical neuroanatomical techniques such as cytoarchitectural staining, which distinguishes layers in the cortex, or Golgi impregnation, which characterizes neuronal cell types, are applied to the striatum, they reveal a cytoarchitecturally homogeneous structure composed of principally one neuronal type, the medium spiny neuron. Against this background, neurochemical and axonal tracing techniques have revealed an intriguing mosaic organization. Mosaically organized neuroanatomical markers within the striatum include neurotransmitter receptors, neuropeptide immunoreacfivity, second messenger systems, and patterns of afferent, efferent and intrinsic connections. Relating these mosaic patterns to function has proved difficult because different mosaic patterns are often not consistent with one another, some of the neurochemical markers have not been adequately localized to specific morphological structures, and heterogeneous patterns of distribution sometimes represent an artifact of the histochemical process rather than an underlying organization. In this article a conceptual framework of the mosaic compartmental organization of the striatum that may be related to its functions is presented, and is based on the following principles. Compartmental organization in the striatum is defined on the basis of morphological structure, ie the neurons of the striatum, their axonal projections and their afferent inputs. Neurochemical markers are used to characterize subsets of neuronal populations. The static nature of morphological structure is used as a starting point to characterize striatal compartmentalization, whereas neurochemical markers serve a dual purpose in aiding the delineation of subsets of striatal neurons and, as the levels of neurochemical markers may be altered by functional manipulations, they aid in discerning functional organization of the stfiatum.