Compartmental origins of the striatopallidal projection in the primate

The organization of striatopallidal projection neurons in the primate was studied by injecting horseradish peroxidase conjugated with wheat germ agglutinin and fluorescent markers (latex microspheres, Fluorogold, Diamidino Yellow or Nuclear Yellow) into the globus pallidus of 20 adult squirrel monkeys (Saimiri sciureus). Single injections of horseradish peroxidase conjugated with wheat germ agglutinin were placed so as to involve predominantly either one or both pallidal segments. In the double-tracer experiments, fluorescent tracer injections were centered in the external pallidum and deposits of horseradish peroxidase conjugated with wheat germ agglutinin were made in the internal pallidum. In control cases, injections were made in nearby parts of the internal capsule or striatum. Distributions of retrogradely labeled neurons in the striatum were analysed in relation to its striosomal architecture as demonstrated by histochemistry and immunohistochemistry. Three principal findings emerged. (1) Both the external and the internal segments of the primate pallidum receive input from both the caudate nucleus and the putamen, but different sets of striatal cells within these nuclei project to the two segments. (2) The striatopallidal projection in the primate originates mainly in the extrastriosomal matrix, although striosomes in the fields of labeling almost always contain some labeled neurons. (3) Heterogeneous groupings of striatopallidal projection neurons exist in the matrix and appear to be parts of three-dimensional projection-neuron arrays. We conclude that in the primate, separate lines of conduction lead from the striatum to the external and the internal pallidal segments, and raise the possibility that the cells of origin of these pathways form a mosaic in the extrastriosomal matrix.