Fine structure of the magnocellular subdivision of the ventral anterior thalamic nucleus (VAmc) of Macaca mulatta: II. Organization of nigrothalamic afferents as revealed with EM autoradiography.

An EM-autoradiographic technique was used to identify the ultrastructural features and synaptic sites of nigral afferents to the ventral anterior nucleus pars magnocellularis (VAmc) of the rhesus monkey thalamus. The findings demonstrate that the nigral boutons are of medium-sized to large, with the majority being of the en passant type. These boutons form symmetric synaptic contacts, and contain pleomorphic or entirely flat vesicles and numerous mitochondria. The nigral input is heavily biased towards thalamocortical projection neurons (PN), whose somata and dendrites represent about 82% of the postsynaptic sites of labeled boutons. The distal dendrites of local circuit neurons (LCN) comprise 13% of the postsynaptic sites. Nigral terminals appear to represent a single extrinsic afferent input to the somata and primary dendrites of thalamocortical projection neurons. A nigral input to LCN somata was not demonstrated but the possibility could not be excluded. Although the basic ultrastructural features of nigral boutons in the monkey are similar to those described earlier in the cat (Kultas-Ilinsky et al.: J. Comp. Neurol. 216:390-405, '83), essential species differences exist in the intensity of the nigral input and its distribution on thalamic neurons.