Thalamocortical synapses between axons from the mediodorsal thalamic nucleus and pyramidal cells in the prelimbic cortex of the rat.

A combined anterograde axonal degeneration and Golgi electron microscopic (Golgi-EM) study was undertaken to identify thalamocortical synaptic connections between axon terminals from the mediodorsal thalamic nucleus (MD) and pyramidal cells in layers III and V of the agranular prelimbic cortex in the rat. The morphological characteristics of thalamocortical synapses from MD were also examined by labeling axon terminals with anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). WGA-HRP labeled axon terminals from MD to the prelimbic cortex were small in size (0.5-1 microns in diameter), contained round synaptic vesicles, and formed axospinous synapses with asymmetrical membrane thickenings. With Golgi-EM methods, gold-toned apical dendrites in layer III were analyzed by reconstruction of serial ultrathin sections. Following lesions in the thalamus, degenerating thalamocortical axon terminals formed asymmetrical contacts exclusively on dendritic spines of the identified apical dendrites. More thalamocortical synapses were found on apical dendrites of layer V pyramidal cells than on apical dendrites of layer III pyramidal cells. In addition to thalamocortical synapses, a very few unlabeled symmetrical synapses were found on apical dendrites and somata of pyramidal cells, but they were not quantified and their sources are unknown.