Nigrothalamic pathway in the cat demonstrated by autoradiography and electron microscopy

Light microscopic autoradiography and electron microscopy were used to trace the nigrothalamic projections and to study the sites of termination of this pathway in the cat. Injections of tritiated amino acids or electrolytic lesions were placed in the substantia nigra pars reticularis (SNr). An accumulation of radioactivity was found in the ventral medial nucleus and in the ventromedial part of the ventral anterior nucleus. At the ultrastructural level degenerating medium size synaptic boutons and medium size myelinated fibers were observed in these nuclei. The boutons contained clear pleomorphic veiscles and formed symmetrical type synaptic contacts with regular type dendrites and vesicel-containing dendrites. The present findings indicate that the ventral medial nucleus is the principal site of termination of nigrothalamic projections in the cat.