Pallidal afferent territory of the Macaca mulatta thalamus: neuronal and synaptic organization of the VAdc.

The Journal of comparative neurology

PubMedID: 9378853

Kultas-Ilinsky K, Reising L, Yi H, Ilinsky IA. Pallidal afferent territory of the Macaca mulatta thalamus: neuronal and synaptic organization of the VAdc. J Comp Neurol. 1997;386(4):573-600.
Ventral anterior thalamic nucleus pars densicellularis (VAdc) as delineated earlier (Ilinsky and Kultas-Ilinsky [1987] J. Comp. Neurol. 262:331-364) was analyzed by using qualitative and quantitative neuroanatomical techniques. Projection neurons (PN), retrogradely labeled with wheat germ agglutinin conjugated horseradish peroxidase from the cortex, were small to medium in size (mean area, 312 microm2) 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 microm2), and gave off few dendrites. Two subpopulations of GABA positive boutons (F1 type) were distinguished: large (mean area, 2.6 microm2) terminals with symmetric synapses containing few pleomorphic vesicles and numerous mitochondria densely covered proximal PN sites; smaller F1 boutons with a slightly different morphology contacted mostly distal PN dendrites. Two subpopulations of terminals containing round vesicles and forming asymmetric synapses were distinguished by bouton size (mean areas, 0.4 microm2 and 1.6 microm2, respectively). These targeted mainly distal PN dendrites, but some synapsed proximally next to large F1 boutons. On distal dendrites, representatives of both types were labeled from the cortex. The density of boutons with symmetric and asymmetric synapses (the number of boutons per 100 microm of PN membrane length) was 3.3:0.2 on primary, 2.5:1.2 on secondary, and 0.8:12 on distal dendrites. The numerical density of synapses formed by presynaptic LCN dendrites on all PN levels was 20 to 40 times less than that of axon terminals at the same sites. Afferent input to LCN from boutons of all types, including that from 50% of labeled cortical boutons, mainly targeted distal 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 modest.