Synaptology of trigemino- and spinothalamic lamina I terminations in the posterior ventral medial nucleus of the macaque

• We used the electron microscope to examine lamina I trigemino- and spinothalamic (TSTT) terminations in the posterior part of the ventral medial nucleus (VMpo) of the macaque thalamus. Lamina I terminations were identified by anterograde labeling with biotinylated dextran, and 109 boutons on 38 terminal fibers were closely studied in series of ultrathin sections. Five unlabeled terminal boutons of similar appearance were also examined in detail. Three-dimensional, volume-rendered computer models were reconstructed from complete series of serial sections for 29 boutons on 10 labeled terminal fibers and one unlabeled terminal fiber. In addition, postembedding immunogold staining for GABA was obtained in alternate sections through 23 boutons. Lamina I TSTT terminations in VMpo generally have several large boutons (mean length = 2.16 ╡m, mean width = 1.29 ╡m) that are densely packed with vesicles and make asymmetric synaptic contacts on low-order dendrites of VMpo neurons (mean diameter 1.45 ╡m). They are closely associated with GABAergic presynaptic dendrites (PSDs), and nearly all form classic triadic arrangements (28 of 29 reconstructed boutons). Consecutive boutons on individual terminal fibers make multiple contacts with a single postsynaptic dendrite and can show evidence of progressive complexity. Dendritic appendages that enwrap and invaginate the terminal bouton constitute additional anatomic evidence for secure, high-fidelity synaptic transfer. These observations provide direct ultrastructural evidence supporting the hypothesis that VMpo is a lamina I TSTT thalamocortical relay nucleus in primates that subserves pain, temperature, itch, and other sensations related to the physiological condition of the body.

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