Organization of efferent projections from the internal segment of globus pallidus in primate as revealed by fluorescence retrograde labeling method.

The exact cellular origin and the degree of collateralization of the major efferent projections from the internal segment of globus pallidus (GPi) in squirrel monkey (Saimiri sciureus) were studied using Evans blue (EB) and a mixture of DAPI-primuline (DP) as retrograde fluorescent tracers. After the concomitant injection of EB in VA/VL thalamic nuclei and of DP in habenula on the same side, numerous EB-labeled cells were found in the central portion of GPi compared to a much smaller number of DP-labeled neurons mostly encountered at the periphery of GPi. Only very few double-labeled cells were visualized in these experiments indicating that the pallidohabenular and pallidothalamic pathways arise largely from two different cell populations, each having a preferential distribution in GPi. On the other hand, a multitude of both EB- and DP-labeled cells occurred in the central portion of GPi after the concomitant injection of EB in VA/VL nuclei and of DP in nucleus tegmenti pedunculopontinus of the midbrain tegmentum. Although the EB-labeled cells tend to be more abundant in the dorsolateral half, and the DP-labeled cells more numerous in the ventromedial half of GPi, about 70-75% of the cells in the core of GPi were double-labeled in such a case. This indicates that the pallidothalamic and pallidotegmental fibers arise largely from the same neurons in the core of GPi. A number of DP-labeled cells was also found in the contralateral GPi revealing that the pallidotegmental pathway is partly (15-20%) crossed. In addition, numerous DP-labeled cells (projecting to brain stem) occurred in the medial two-thirds of the substantia nigra pars reticulata (SNr), whereas EB-labeled cells (projecting to the thalamus) abounded in the lateral third of SNr. A small number of double-labeled SNr cells were also encountered after thalamus-midbrain injection. These findings suggest that in regard to its output elements, the primate GPi is organized according to a complex pattern consisting of: (1) a central 'motor' zone where most neurons send axonal branches to both thalamus and midbrain; and (2) a peripheral 'limbic' zone which encroaches largely upon the lateral hypothalamus and whose cells project only to habenula. These two pallidal zones are furthermore embedded in a peripallidal neuronal network composed of large acetylcholinesterase-containing cells related to nucleus basalis and projecting diffusely to neocortex.