The neostriatal mosaic. I. compartmental organization of projections from the striatum to the substantia nigra in the rat

Abstract

Combined neuroanatomical techniques were used to examine the organization of the striatal projection to the substantia nigra in the rat. Both double anterograde axonal tracing methods (Phaseolus vulgaris leuco‐agglutinin (PHA‐L) and 3 H‐amino acid tract tracing) and double fluorescent retrograde axonal transport tracing methods were used to examine the relationship among striatal neurons projecting to separate areas of the substantia nigra. Additionally, the distributions of retrogradely labeled striatonigral projection neurons were charted relative to the neurochemically distinct striatal “patch” compartment, identified by substance P‐or leuenkephalin‐like immunoreactivity, and the complementary “matrix” compartment, identified by somatostatin‐like immunoreactive fibers. These studies show two distinct types of organization in the striatonigral projections. One type is topographic in that the mediolateral relationships among these striatal efferent neurons are roughly maintained by their termination patterns in the substantia nigra, while the dorsoventral relationships are inverted. Projections from any part of the striatum, however, are distributed throughout the rostrocaudal axis of the substantia nigra. Despite their general topographic organization, the variable and dispersed nature of such projections from individual striatal loci results in partial overlap of afferent fields from separate striatal areas. The second type of organization is nontopographic and provides a different system for convergence of inputs from separated striatal areas that is superimposed on the rough topographic system. In this other projection system the mediolateral and dorsoventral relationships typical of the topographically ordered system are not maintained and are sometimes reversed. For example, PHA‐L injected into the dorsal striatum labels a topographic (inverted relationship) projection to the ventral substantia nigra pars reticulata but also a smaller and separate projection to the dorsal pars reticulata and adjacent pars compacta. Retrograde tracer deposits in the pars compacta label neurons in the ventral striatum (the inverted relationship) but also clusters of neurons in the dorsal striatum. These clusters are in the neurochemically defined patch compartment whereas neurons in the matrix are labeled by injections into the pars reticulata. The dendrites of both retrogradely filled patch and matrix neurons are confined to the compartment containing their cell bodies, suggesting a restriction that would functionally segregate extrinsic striatal afferents shown in other studies to be confined to either patches or matrix. Finally, somatostatin‐immunoreactive neurons are found in both patch and matrix compartments, but somatostatin fibers are found primarily in the matrix. These neurons are suggested to be interneurons that link the patches to the matrix. These results suggest that the striatal patch and matrix compartments provide segregated parallel input‐output pathways that are linked locally via intrinsic striatal neurons. This modular organization is reiterated throughout the striatum for integrative processing of cortical inputs by the basal ganglia.