DIRECT SYNAPTIC CONNECTIONS BETWEEN THALAMOCORTICAL AXON TERMINALS FROM THE MEDIODORSAL THALAMIC NUCLEUS (MD) AND CORTICOTHALAMIC NEURONS TO MD IN THE PREFRONTAL CORTEX

A combined anterograde axonal degeneration with ibotenic acid and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) retrograde tracing study revealed that some degenerating thalamocortical axon terminals from the mediodorsal thalamic nucleus (MD) directly formed asymmetrical synaptic contacts predominantly with dendritic spines of apical dendrites of WGA-HRP-labeled corticothalamic projection neurons to MD in the prelimbic cortex of the rat. This result suggests that there is a monosynaptic feedback loop from and to MD via deeper layer neurons in the prelimbic cortex.