The organization of the descending ventral pallidal projections in the monkey.

This study describes the organization and topography of the descending efferent projections from the monkey ventral pallidum. The main efferent projections from the globus pallidus are to the subthalamus, to the thalamus, and to the substantia nigra. Although these projections have been well established for the dorsal pallidum, the projections of the ventral pallidum have not been explored in primates. The results of this study add an important link in how information from the limbic lobe is channeled through the basal ganglia in monkeys. Anterograde tracers, Phaseolus vulgaris-leucoagglutinin, and tritiated amino acids were injected into various regions of the ventral pallidum. The descending efferent projection from the ventral pallidum in monkeys terminates primarily in the subthalamic nucleus and adjacent lateral hypothalamus, in the substantia nigra, and in the lateral habenular nucleus. Although terminals are also found in the thalamus, these are relatively sparse. The projections to the subthalamic nucleus and the lateral hypothalamus are topographically arranged, while those to the substantia nigra are not. These results suggest that pathways from distinct pallidal regions that receive specific striatal input terminate in distinct regions of the subthalamic/hypothalamic regions, thus maintaining a topographic arrangement. Projections to the substantia nigra, however, overlap extensively, suggesting convergence of terminals from different ventral pallidal regions. The relatively small projection to the thalamus raises the question that without a prominent thalamic projection, is this system parallel to that described for the dorsal globus pallidus?