Importance of the ventral midline thalamus in driving hippocampal functions.

The ventral midline of the thalamus encompasses the reuniens and rhomboid (ReRh) nuclei. These nuclei are bidirectionally connected with the hippocampus and the medial prefrontal cortex (mPFC). About 8% of the neurons of the Re have collaterals in both structures. The ReRh nuclei provide the major thalamic input to the hippocampus. Their stimulation induces long-term potentiation in region CA1, suggesting a role in hippocampal plasticity. Experimental manipulations of the ReRh nuclei such as lesions, reversible inactivations, or optogenetic stimulations produce alterations of cognitive functions, especially in tasks known for their sensitivity to lesions of the hippocampus, but also of the mPFC. Behavioral approaches suggest that the ReRh nuclei might relay incoming signals from the mPFC both to the hippocampus and back to the mPFC. Thus, the Re and Rh nuclei have a role in orchestrating the information flow between the hippocampus and the mPFC, and this orchestration has both "online" and "off-line" implications in cognitive functions.