Ultrastructure and Distribution of Corticothalamic Fiber Terminals From the Posterior Cingulate Cortex and the Presubiculum to the Anteroventral Thalamic Nucleus of the Rat
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Abstract
The ultrastructure of axon terminals in the anteroventral thalamic nucleus arising
in the cingulate cortex and in the presubiculum was examined using the anterograde
transport of wheat germ agglutinin conjugated to horseradish peroxidase in rats. Anterogradely
labeled axonal arborizations arising from the posterior cingulate cortex were concentrated
bilaterally in the ventral part of the anteroventral nucleus. In electron micrographs
these thalamic terminals arising from the posterior cingulate cortex were consistently
small, contained round vesicles, and established asymmetric contacts on distal dendritic
processes. In contrast, the axonal arborizations arising from the presubiculum were
concentrated ipsilaterally in the dorsal part of the anteroventral nucleus and comprised
two identifiable populations of terminals. The smaller terminals, which contained
densely packed round vesicles, established asymmetric synaptic contacts on distal
dendritic processes and resembled the posterior cingulate cortex terminals described
above. The other population of the presubiculum terminals consisted of medium-sized
terminals. These contained loosely packed round vesicles and established asymmetric
synaptic contacts on proximal dendritic processes. These results indicate that the
posterior cingulate cortex and the presubiculum project differentially upon the anteroventral
thalamic nucleus. They also indicate that although the posterior cingulate cortex
gives rise to only one type of corticothalamic terminal, the presubiculum gives rise
to two types of corticothalamic terminals. When taken together, these data suggest
that these different limbic cortical areas might subserve distinct roles in the anteroventral
thalamic nucleus function.