The organization of the reciprocal connections between the subiculum and the entorhinal cortex in the cat: I. A neuroanatomical tracing study.

The connections between the subiculum (SUB) and the entorhinal cortex (EC) were studied in the cat with retrograde and anterograde tracing techniques. Injections of the retrogradely transported tracer WGA-HRP at different levels along the septotemporal axis of the subiculum result in labeled neurons predominantly in the medial entorhinal cortex (MEA) in the superficial layers II and III. In the deep layers labeled cells are found more widespread over the EC. The superficially located labeled EC neurons are topographically distributed in a lateromedial gradient, which corresponds to a septotemporal gradient along the longitudinal axis of the subiculum. This organization of the EC-SUB projection system could be substantiated by the use of injections anterogradely transported radioactively labeled amino acids in EC. The SUB to EC projections were investigated with the anterograde transport of WGA-HRP and with radioactively labeled amino acids that were injected at different levels along the septotemporal axis of the subiculum. This results in a patch of anterogradely labeled fibers and terminals in MEA, predominantly in layers II and III, with a wider band of label in the deep layers. Again, a topographical distribution along the lateromedial axis of the EC corresponding to the septotemporal axis of the SUB was observed. Contralateral reciprocal connections between EC and SUB are also present, and exhibit a similar topographical organization.