NUCLEUS-RETICULARIS THALAMI CONNECTIONS WITH THE MEDIODORSAL THALAMICNUCLEUS - A LIGHT AND ELECTRON-MICROSCOPIC STUDY IN THE MONKEY

Wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and biotinylated dextran amine (BDA) were used as tracers to study nucleus reticularis (NRT) connections with the mediodorsal nucleus (MD). Inje ctions of WGA-HRP in the MD resulted in retrograde labeling of cells i n the anteromedial segment of the NRT, the so-called rostral NRT pole. Injections of WGA-HRP and BDA in this NRT region resulted in dense an terograde labeling in the MD. Labeled NRT fibers gave off several coll aterals to different MD regions ending with terminal plexuses of thin varicose fibers. In the neuropil, the varicosities were distributed at random, and no tendency to form pericellular baskets was noted. Poste mbeddng immunocytochemistry for GABA was performed on the tissue conta ining anterograde WGA-HRP label for identification of NRT boutons unde r electron microscope. The double-labeled boutons were of small to med ium size, contained a large number of pleomorphic vesicles, few mitoch ondria, and formed multiple symmetric synaptic contacts. The number of contacts established by one bouton ranged from 1 to 4 with an average of 1.8 per bouton. About 60% of these boutons made synapses on distal dendrites of GABAergic local circuit neurons; 33% of synaptic contact s were on distal dendrites of thalamocortical neurons, and the rest on their proximal dendrites and soma. NRT boutons were also found in ser ial synapses and triads. The results demonstrate that the NRT input to the MD is organized so that a single fiber innervates different MD re gions and its terminals form numerous synaptic contacts mostly on the distal dendrites of a large number of local circuit neurons and projec tion neurons.