Striatal axons to the globus pallidus, entopeduncular nucleus and substantia nigra come mainly from separate cell populations in cat

The extent to which individual striatal neurons send collaterals to the globus pallidus, entopeduncular nucleus and substantia nigra in the cat brain was determined by double-retrograde tracing with rhodamine fluorescent latex microspheres in combination with either horseradish peroxidase or the fluorescent nuclear dye Diamidino Yellow. In each case, two of the three target nuclei were injected, each with a different tracer, until all three possible combinations of two had been obtained several times. In all cases in which the tracer encroaches upon a striatal target, there are cells labeled in the striatum of a size and shape that is consistent with the observation that they mainly belong to the category of medium striatal cells. Since the striatal projections to the globus pallidus, entopeduncular nucleus and substantia nigra are each topographically organized, the zones of cell-labeling within the striatum vary depending upon the portion of the target nucleus involved by the deposit. Thus, in many cases the fields of striatal cells containing one label overlap only slightly with those in which cells containing the other label occur. In other cases, however, there is extensive overlap of the striatal zones containing cells marked with either tracer. In all cases, very few double-labeled cells can be found, even where hundreds of cells labeled with either tracer are freshly intermingled. Doubly labeled cells occur somewhat more frequently in those cases where the tracers are placed in the entopeduncular nucleus and substantia nigra than in those with the other two combinations, suggesting that striatal axons branch more often to the entopeduncular nucleus and substantia nigra than to the globus pallidus and nigra or globus pallidus and entopeduncular nucleus. These findings confirm, that, in the cat as in the primate, the striatal axons to the substantia nigra arise from cells that are largely separate from the striatopallidal population, and further show that the axons to the globus pallidus and entopeduncular nucleus also emanate mainly from different cells.