Evidence for interconnections between the two segments of the globus pallidus in primates: a PHA-L anterograde tracing study.

Small injections of the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L) in the external segment of the pallidum (GPe) in the squirrel monkey (Saimiri sciureus) and in the rhesus monkey (Macaca mulatta) led to anterograde labeling of fibers in the internal segment of the pallidum (GPi). These fibers formed numerous large varicosities reminiscent of terminal boutons, which closely surrounded GPi cell bodies and primary dendrites. Conversely, PHA-L injections in the GPi of squirrel monkeys produced anterograde fiber labeling in the GPe. However, in contrast to fibers in GPi, those in GPe did not make prominent pericellular contacts. Instead, they displayed a rather linear course, had long intervaricose segments, and appeared to contact several GPe neurons along their course by close appositions on cell bodies and primary dendrites. These results suggest the existence of a reciprocal connection between the two pallidal segments, which may play a crucial role in the functional organization of the basal ganglia in primates.