Reciprocal connections between the entorhinal cortex and hippocampal fields CA1 and the subiculum are in register with the projections from CA1 to the subiculum

The topology of the connections between the entorhinal cortex (EC), area CA1, and the subiculum is characterized by selective and restricted origin and termination along the transverse or proximodistal axis of CA1 and the subiculum. In the present study, we analyzed whether neurons in CA1 and the subiculum that receive EC projections are interconnected and give rise to return projections to EC, such that they terminate deep in the area of origin of the EC-to-CA1/subiculum projections. Both for the lateral and medial subdivision of EC, the projections to CA1/subiculum, as well as the projections from CA1 to the subiculum and back to EC, are rather divergent. Interestingly, we only rarely observed evidence for the presence of "reentry loops," i.e., cells in layer III of EC giving rise to projections to interconnected neurons in CA1 and the subiculum, while the targeted CA1 neurons also projected back to the deep layers of the area of origin of the pathway in EC. We conclude that although fibers originating from a restricted part of EC distribute extensively in a divergent way along the longitudinal axis of CA1 and the subiculum, only restricted portions of the latter two areas, receiving inputs from the same entorhinal area, are interconnected. Moreover, only a small percentage of the CA1 neurons that project to the correspondingly innervated subicular neurons give rise to projections that return to the deep layers of the originating part of EC. The present findings are taken to indicate that the EC-hippocampal circuitry functionally comprises many parallel-organized specific "reentry loops."