MODE OF TERMINATION OF PALLIDAL AFFERENTS TO THE THALAMUS - A LIGHT AND ELECTRON-MICROSCOPIC STUDY WITH ANTEROGRADE TRACERS AND IMMUNOCYTOCHEMISTRY IN MACACA-MULATTA

The mode of termination of individual pallidothalamic fibers in the de nsicellular subdivision of the ventral anterior thalamic nucleus (VAdc ) of Macaca mulatta was analyzed with light and electron microscopy af ter injections of anterograde tracers in the medial globus pallidus. T hree tracers were utilized: tritiated leucine, biotinylated dextran am ine, and wheat germ agglutinin conjugated to horseradish peroxidase in combination with postembedding immunocytochemistry for gamma-aminobut yric acid (GABA). Pallidothalamic fibers, upon entering the VAdc, gave off several collaterals that formed plexuses of varicose terminal bra nches within different cell clusters. The varicosities were aligned al ong somata and proximal dendrites of projection neurons providing dens e input to each individual cell. At the electron microscopic level, la beled boutons displayed a predominantly flat and elongated shape. They contained a moderate number of pleomorphic synaptic vesicles and very large amounts of mitochondria, displayed symmetric synaptic contacts, and were immunoreactive for GABA. In the total sample of 128 autoradi ographically labeled terminals, 64% were in synaptic contact with soma ta and primary dendrites of projection neurons, 14% formed synapses on proximal dendrites of undefined order, and only 7% established synapt ic contacts on distal dendrites. Fifteen percent of the labeled bouton s established synapses on distal dendrites of GABAergic local circuit neurons (LCN). Pallidal boutons were also found in complex synaptic ar rangements: triads with three GABAergic synapses, and serial synapses with LCN dendrites that in turn established synaptic contacts on proje ction neuron somata or dendrites. These anatomical results suggest a d ual effect of pallidal afferents to projection neurons: direct inhibit ion and disinhibition mediated by LCN. The findings indicate that the fine structure of pallidothalamic terminals in the monkey is similar t o that described earlier in the cat. There are, however, interspecies differences in the distribution of pallidal input on postsynaptic targ ets and its participation in complex synaptic arrangements. (C) 1997 W iley-Liss, Inc.