Laminar origin and septotemporal distribution of entorhinal and perirhinal projections to the hippocampus in the cat.

The projections of the entorhinal and perirhinal cortices to the hippocampus in the cat have been studied with retrograde and anterograde tracing techniques. Retrogradely transported tracers, which were injected at different levels along the septotemporal longitudinal hippocampal axis, result in labeled neurons in superficial entorhinal cortical layers II and III. Occasionally, labeled cells were also observed in the deepest entorhinal layer as well as in the superficial layers of the perirhinal area 35. It could further be shown that labeled neurons located superficially in the entorhinal cortex are topographically distributed in a lateromedial gradient, which corresponds to a septotemporal gradient along the longitudinal axis of the hippocampus. This topographical organization of the entorhinal-hippocampal projection system could be substantiated by the use of anterograde tracing of radioactively labeled amino acids. Injections in the entorhinal cortex produce labeled fibers in the hippocampus. Injections in the perirhinal area 35 result also in labeling over the hippocampus, whereas area 36 does not seem to distribute fibers to the hippocampus. As anticipated from the results of the retrograde tracing experiments, injections located laterally, in or close to the posterior rhinal sulcus, produce prominent labeling over the septal pole of the hippocampus, whereas progressively more medially located injections result in progressively more temporally located labeling. This topographical distribution of perforant path fibers along the septotemporal axis of the hippocampus, which is related to a lateromedial axis in the entorhinal cortex, has been observed following injections in the lateral entorhinal area (LEA) as well as in the medial entorhinal area (MEA). The present observations are discussed in regard of other connectional and putative functional differences between the septal and temporal hippocampus.