Role of Inhibitory Cortical Interneurons in the Neural Circuit between the Prefrontal Cortex and the Mediodorsal Nucleus of the Thalamus

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The neural circuit between the prefrontal cortex and the mediodorsal nucleus of the thalamus (MD) is well investigated and known to be reciprocally interconnected in a topographical manner. However, it has not been investigated in detail how inhibitory cortical interneurons are involved in the neural circuit. To address this issue, we conducted three combined neuroanatomical studies on the rat brain. First, it has been determined that the frequency and the spatial distribution of synapses made by reconstructed dendrites of nonpyramidal neurons identified by impregnation with the Golgi method and thalamocortical terminals identified by degeneration. Degenerating terminals from MD were found to make direct synaptic contacts with either small-sized dendritic shafts or spines of Golgi-impregnated nonpyramidal cells with very sparse dendritic spines. Secondly, a combined study of the anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and postembedding GABA immunocytochemist … More ry resulted I that PHA-L-labeled terminals from MD made synaptic junctions with dendritic shafts and spines labeled with immunogold particles indicative of GABA immunoreactivity. Interestingly, non-labeled dendritic spines were found to receive both axonal inputs from MD with PHA-L labelings and from GABAergic cells with immunogold particles. In addition, there found synapses between dendritic shafts and axon terminals, both of which were labeled with immunogold particles. Thirdly, synaptic connections between corticothalamic neurons projecting to MD and GABAergic terminals were investigated using wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and postembedding GABA immunocytochemistry. GABAergic terminals in the prelimbic cortex made synaptic contacts with retrogradely labeled corticothalamic neurons to MD. All of the synapses were found on cell somata and thick dendritic trunks.These results provide the first demonstration of synaptic contacts in the prelimbic cortex not only between thalammocortical terminals from MD and GABAergic interneurons but also between GABAergic terminals and corticothalamic neurons to MD. The anatomical findings suggest that GABAergic interneurons have a modulatory influence on the excitatory reverberation between MD and the prefrontal cortex. Less