Organization of projections from the anterior pole of the nucleus reticularis thalami (NRT) to subdivisions of the motor thalamus: Light and electronmicroscopic studies in the rhesus monkey

Projections to the motor-related thalamic nuclei from the anterior pole of the reticular thalamic nucleus (NRT) were studied after injections of bioti nylated dextran amine and wheat germ agglutinin conjugated horseradish pero xidase at light and electron microscopic levels, respectively. Each injecti on resulted in anterograde labeling in the three subdivisions of the ventra l anterior nucleus (pars parvicellularis, VApc; pars densicellularis, VAdc; and pars magnocellularis, VAmc) and in the ventral lateral nucleus (VL). N RT fibers had beaded shapes and coursed in a posterior direction giving ris e to relatively diffuse terminal plexuses. The average size of the beads (0 .7 mu m(2)) and their density per 100 mu m of fiber length (23.7-25.7) were similar between the nuclei studied. At the electron microscopic level, ant erogradely labeled boutons displayed positive immunoreactivity for gamma-am inobutyric acid (GABA), contained pleomorphic synaptic vesicles, and formed relatively long (similar to 0.4 mu m) symmetric synaptic contacts. Usually , a single terminal formed synapses on more than one postsynaptic structure . Synaptic contacts were on projection and local circuit neurons and target ed mainly their distal dendrites. In the VAmc, synapses on local circuit ne urons composed 48% of the total sample, in the VAdc/VApc and in the VL the proportion was higher, 65% and 62%, respectively. The results suggest that the input from the anterior pole of the monkey reticular nucleus to the mot or-related thalamic nuclei is organized differently from what is known on t he organization of connections of NRT with sensory thalamic nuclei in other species in that the terminal fields of individual fibers are diffuse rathe r than focal and that at least 50% of synapses are established on GABAergic local circuit neurons. (C) 1999 Wiley-Liss, Inc.