PALLIDAL AFFERENT TERRITORY OF THE MACACA-MULATTA THALAMUS - NEURONALAND SYNAPTIC ORGANIZATION OF THE VADC

Ventral anterior thalamic nucleus pars densicellularis (VAdc) as delin eated earlier (Ilinsky and Kultas-Ilinsky [1987] J. Comp. Neurol. 262: 331-364) was analyzed by using qualitative and quantitative neuroanato mical techniques. Projection neurons (PN), retrogradely labeled with w heat germ agglutinin conjugated horseradish peroxidase from the cortex , were small to medium in size (mean area, 312 mu m(2)) with numerous primary dendrites displaying a tufted branching pattern. Local circuit neurons (LCN), immunoreactive for gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase, were small (mean area, 110 mu m(2)), and gave off few dendrites. Two subpopulations of GABA positive boutons (F 1 type) were distinguished: large (mean area, 2.6 mu m(2)) terminals w ith symmetric synapses containing few pleomorphic vesicles and numerou s mitochondria densely covered proximal PN sites; smaller F1 boutons w ith a slightly different morphology contacted mostly distal PN dendrit es. Two subpopulations of terminals containing round vesicles and form ing asymmetric synapses were distinguished by bouton size (mean areas, 0.4 mu m(2) and 1.6 mu m(2), respectively). These targeted mainly dis tal PN dendrites, but some synapsed proximally next to large F1 bouton s. On distal dendrites, representatives of both types were labeled fro m the cortex. The density of boutons with symmetric and asymmetric syn apses (the number of boutons per 100 mu m of PN membrane length) was 3 .3:0.2 on primary, 2.5:1.2 on secondary, and 0.8:12 on distal dendrite s. The numerical density of synapses formed by presynaptic LCN dendrit es on all PN levels was 20 to 40 times less than that of axon terminal s at the same sites. Afferent input to LCN from boutons of all types, including that from 50% of labeled cortical boutons, mainly targeted d istal dendrites. Overall, the findings suggest that PN in VAdc receive massive inhibitory input proximally intermingled with some presumably excitatory input, and that LCN contribution to PN inhibition is modes t. (C) 1997 Wiley-Liss, Inc.