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The synaptic organization of the mediodorsal thalamic nucleus (MD) in the cat have been studied with the electron microscope, and correlated with the termination of the medial prefrontal corticothalamic afferents using the method of anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The neuropil of MD was divided into glomerular and extraglomerular regions. A synaptic glomerulus was composed of a central dendrite and some different presynaptic profiles with astroglial ensheathment. The prevalent presynaptic elements in glomeruli were presynaptic dendrites (PSDs) that contained pleomorphic vesicles, and formed symmetric synaptic contacts and puncta adhaerentia junctions with central dendrites. One or two large terminals with round vesicles and asymmetric specializations (LR) also participated in glomerular formations. They were invariably presynaptic to central dendrites and PSDs. As terminals less frequently found within glomeruli, there were small-sized terminals with round vesicles and asymmetric synaptic junctions with PSDs. In the extraglomerular neurophils, small to medium-sized terminals with pleomorphic vesicles (SMP) were found besides the presynaptic profiles identified in glomerular regions. These SMP terminals formed axodendritic and axosomatic symmetric synapses. WGA-HRP injections into the medial prefrontal cortex resulted in anterograde labelings in not only SR but also LR presynaptic terminals. SR boutons made up the majority of labeled terminals, and they were found only in the extraglomerular neuropil. While labeled LR terminals were detected in the extraglomerular neuropil and synaptic glomeruli with less encounter. The results of the present study show that the synaptic organization in MD of the cat is similar to that in other thalamic relay nuclei and in MD of the monkey. Further, MD receiving two subsets of synaptic terminations from the prefrontal cortex might play a different functional role in regulating the neural circuit between MD and the prefrontal cortex in comparison with that in the sensory and motor thalamic nuclei that receive only SR terminals from the sensorimotor cortex.
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