Pallidal and cerebellar inputs to thalamocortical neurons projecting to the supplementary motor area in Macaca fuscata: a triple-labeling light microcopic study

We investigated the interrelationship between the supplementary motor area (SMA) thalamocortical projection neurons and the pallidothalamic and cerebe llothalamic territories in the monkey (Macaca fuscata) using a combination of three tracers in a triple labeling paradigm. Thalamic labeling was analy zed following injections of the anterograde tracers, biotinylated dextran a mine (BDA) into the internal segment of the globus pallidus (GPI) and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the co ntralateral cerebellar interpositus and dentate nuclei. In addition, the re trograde tracer cholera toxin subunit B (CTB) was injected into the physiol ogically identified hand/arm representation of SMA. The tissue was processe d sequentially using different chromogens in order to visualize all three t racers in a single section. We found that the SMA thalamocortical neurons o ccupied a wide band extending from the ventral anterior nucleus pars princi palis (VApc) through the ventral lateral nucleus pars oralis (VLo) and the ventral lateral nucleus pars medialis (VLm) and into to the ventral lateral nucleus pars caudalis (VLc) including a portion of ventral posterior later al nucleus pars oralis (VPLo) and nucleus X. The heaviest CTB labeling was found in VLo with dense plexuses of BDA labeled pallidothalamic fibers and swellings often observed superimposed upon retrogradely labeled CTB cells. In addition, dense foci of cerebellothalamic WGA-HRP anterograde label were observed coinciding with the occasional retrogradely CTB labeled neurons i n VLc and transitional zones between VApc, VLo and VPLo. Our light microsco pic results suggest that the SMA receives thalamic inputs with afferents la rgely derived from GPi and minor inputs originating from the cerebellum.