Organization of the striatum: collateralization of its efferent axons.

The anatomical structure of the basal ganglia indicates that the input from the cerebral cortex is funnelled through the striatum to the globus pallidus and substantia nigra. This structure implies integration of the information as it is transferred through the basal ganglia. In order to investigate this integration, we studied the collateralization of striatal efferents to the globus pallidus and the substantia nigra-ventral tegmental area. Retrogradely transported fluorescent tracers were injected into the target areas of striatal efferents. Nuclear yellow or propidium iodide was injected into the substantia nigra-ventral tegmental area (SN-VTA) and 4-acetamido, 4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) into the globus pallidus (GP) of adult albino rats. SITS was chosen for the pallidal injections because it is not taken up by fibers-of-passage. The pressure injections resulted in large injection sites which covered the majority of each efferent target area, and as a result retrogradely labeled cell bodies were found throughout the entire extent of the striatum. Cell bodies double-labeled with both dyes were found intermingled with single-labeled cell bodies. In rats injected with propidium iodide in the SN-VTA and SITS in the GP, 70% of all neurons (as revealed by Nissl staining) were labeled. Of these labeled cells, 40% were double labeled, 20% contained only SITS and 40% contained only propidium iodide. Thus a substantial number of the striatal neurons that project to the SN-VTA also possess collateral axons to the GP. Some striatal neurons appear to project to only the SN-VTA or only to the GP. The cells projecting to only one of these striatal target regions tend to cluster together in patches. The organizational pattern of these patches does not seem to coincide in any simple way with the mosaic pattern of striatal opiate receptors, nor with the previously described mosaic pattern of striatal afferents and various neurotransmitter substances.