Course of motor and associative pallidothalamic projections in monkeys

As a result of the frequent performance of lesioning and electrical stimula tion procedures targeting the globus pallidus internus (GPi) to treat medic ally intractable hypokinetic and hyperkinetic movement disorders, the cours e of the pallidothalamic projections originating, in particular, from the m otor territory of GPi has important clinical relevancy. To assess the organ ization of pallidothalamic projections originating from motor and associati ve portions of GPi, small quantities of the anterograde/retrograde tracer, biotinylated dextran amine (BDA) were injected into localized regions of th e caudal GPi in squirrel monkeys. The localization to motor and associative territories in GPi was confirmed by examining the corresponding regions of retrograde labeling in the striatum and subthalamic nucleus (STN). The lab eled pallidothalamic fibers projected principally medially across the infer ior edge of the internal capsule. The fiber bundle ventral to the caudal GP i was mainly devoid of labeling. Fibers labeled along the medial and inferi or borders of GPi at centrorostral levels were traceable to the medial edge of the injections. The densest fiber labeling at rostral levels was produc ed by those injections with the greatest extent of rostral labeling of neur ons. In opposition to generally accepted schemes, the findings from this st udy suggest that the pallidothalamic fibers originating from the caudal por tions of GPi, including the motor territory, do not course ventromedially t o form the ansa lenticularis, but rather, travel predominately medially thr ough the lenticular fasciculus en route to the thalamus. Thus, proposed sur gical schemes to target fibers ventral to the caudal GPi or at the rostral pole of GPi appear to be misguided. J. Comp. Neurol. 429:490-501, 2001. (C) 2001 Wiley-Liss, Inc.