EFFERENT CONNECTIONS OF THE INTERNAL GLOBUS-PALLIDUS IN THE SQUIRREL-MONKEY .1. TOPOGRAPHY AND SYNAPTIC ORGANIZATION OF THE PALLIDOTHALAMICPROJECTION

The objectives of this study were, on one hand, to better understand h ow the segregated functional pathways from the cerebral cortex through the striatopallidal complex emerged in the projections to the thalamu s and, on the other hand, to compare the ultrastructure and synaptic o rganization of the pallidal efferents to the ventrolateral (VL) and ce ntromedian (CM) thalamic nuclei in primates. These aims were achieved by injections of the retrograde-anterograde tracer, biotinylated dextr an amine (BDA), in different functional regions of the internal pallid um (GPi) in squirrel monkeys. The location of retrogradely labelled ce lls in the striatum was determined to ascertain the functional specifi city of the injection sites. Injections in the ventrolateral two-third s of the GPi (group 1) led to retrograde labelling in the postcommissu ral region of the putamen (''sensorimotor striatum'') and plexuses of labelled fibers in the rostral one-third of the principal ventrolatera l nucleus (VLp) and the central part of the CM. On the other hand, inj ections in the dorsal one-third (group 3) and the rostromedial pole (g roup 4) of the GPi led to retrogradely labelled cells in the body of t he caudate nucleus (''associative striatum'') and the ventral striatum (''limbic striatum''), respectively. After those injections, dense pl exuses of anterogradely labelled varicosities were found in common tha lamic nuclei, including the parvocellular ventral anterior nucleus (VA pc), the dorsal VL (VLd), and the rostrodorsal part of the parafascicu lar nucleus (PF). In the caudal two-thirds of the CM/PF, the labelled fibers formed a band that lay along the dorsal border of the complex i n a region called the dorsolateral PF (PFdl) in this study. The ventro medial nucleus (VM) was densely labelled only after injections in the rostromedial GPi, whereas the dorsal part of the zona incerta was labe lled in both groups. At the electron microscopic level, the BDA-positi ve terminals in the VLp were larger and more elongated than those in t he CM but, overall, displayed the same pattern of synaptic organizatio n. Our findings indicate 1) that some associative and limbic cortical information, which is largely processed in segregated corticostriatopa llidal channels, converges to common thalamic nuclei and 2) that the P F is a major target of associative and limbic GPi efferents in monkeys . J. Comp. Neurol. 382:323-347, 1997. (C) 1997 Wiley-Liss, Inc.