Pallidothalamic projections in the rat

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Abstract
The purpose of this investigation was to examine pallidofugal projections to the thalamus of the rat using a selective silver degeneration method and autoradiography. Degenerating pallidothalamic projections emerging from the anterior part of the pallidum formed two diffuse groups of fibers; one coursed ventromedially directly through the internal capsule without forming a lenticular fasciculus and terminated uniformly in the reticular, VA, VD, VM and the medial part of the VE. The other group coursed ventrolaterally in the internal capsule and, without forming an ansa lenticularis, entered the zona incerta, swept medially through and ventral to the medial lemniscus, and terminated in the medial part of the VE and throughout the VD. Degenerating fibers from both of these groups also terminated in the rhomboid, gelatinous, and medial dorsal nuclei of the thalamus. Fibers from the intermediate and posterior parts of the pallidum crossed the internal capsule in diffuse bundles and terminated in the medial and posterior zones of the reticular nucleus respectively. Most of the fibers passed through the reticular nucleus and terminated uniformly in the VA, VD, VM and the ventral portion of the VE. Some terminated in the medial and midline thalamic nuclear groups. The character of terminal degeneration differed markedly. In the ventral aspect of the VE it had a fine dust-like appearance whereas in the VA, VD, and VM it was coarsely granular. Autoradiography confirmed the course and terminations described above except that reduced silver grains were not observed in the rhomboid, gelatinous and medial dorsal nuclei of the thalamus. Also, autoradiography confirmed that the dust-like terminal degeneration in the ventral VE was of the anterograde type since the leucine label was concentrated in precisely the same area.