NIGROTHALAMIC PROJECTIONS IN MONKEY DEMONSTRATED BY AUTORADIOGRAPHIC TECHNIQUES

Despite repeated demonstrations by degeneration techniques, nigrothalamic fibers are regarded with some skepticism. Attempts were made to race nigral efferent projections in the [rhesus] monkey by autoradiographic techniques. Tritiated amino acids (L-leucine, L-lysine and L-proline), injected into portions of the substantia nigra (SN), labeled cells in 4 regions, designated as rostralateral, caudolateral, rostromedial, and central. Rostrolateral nigral neurons transported radioactive label preferentially and abundantly to thalamic nuclei; localized isotope was found in parts of 3 thalamic nuclei, the medial part of the ventral lateral nucleus (VLm), the magnocellular part of the ventral anterior nucleus (VAmc) and the paralaminar part of the dorsomedial nucleus (DMpl). Lateral neurons in the caudal half of the SN transmitted radioactive label to the same thalamic nuclei as rostrolateral nigral neurons. Isotope transported to portions of the striatum was modest and localized. Radioactive label taken up by large cells in the caudal third of the SN was transported to portions of the striatum, but not to thalamic nuclei. Labeled nigral neurons in the medial 2/3 of the rostral half of the SN, and in the middle third of the central part of the SN, transported the label mainly to parts of the caudate nucleus and putamen. In these animals modest radioactive label was seen in VLm and VAmc, but not in other thalamic nuclei. There was no evidence that nigral neurons project to the subthalamic nucleus. No radioactive transport from nigral neurons was detected in the superior colliculus, the midbrain tegmentum or the red nucleus, and none was transported to more caudal brain stem nuclei. Nigrothalamic fibers arose particularly from cells in rostral and lateral parts of the substantia nigra. While some cells in other parts of the nigra projected to thalamic nuclei, these appeared scattered and less numerous. Large cells in caudal parts of the SN did not project to thalamic nuclei. These observations confirm nigrothalamic projections to VLm and VAmc, and identify a new nigral projection to part of the dorsomedial nucleus of the thalamus DMpl. No nigral efferent fibers projected to any of the intralaminar thalamic nuclei.