Topographic activation of the medial entorhinal cortex by presubicular commissural projections.

Previous investigations have shown that presubicular commissural fibers traveling in the caudal part of the dorsal hippocampal commissure (PSD) selectively activated the dorsalmost portion of the entorhinal cortex (EC), where they discharged perforant path neurons to the dorsal dentate gyrus. The dentate activation was followed by that of the dorsal hippocampus. The aim of the present study was to ascertain whether presubiculum commissural projections traveling in the PSD can also activate ventral levels of the EC and, if so, whether this activation is followed by that of the dentate gyrus-hippocampal. system in the ventral hippocampus. The experiments were carried out in adult, anesthetized guinea pigs by field potential analysis. The results showed that presubicular fibers traveling at different PSD loci selectively activated specific EC portions, with caudal fibers activating only the dorsal EC and more rostral fibers activating ventral EC points. The region activated by PSD projections corresponded to the medial EC. Current source-density (CSD) analysis revealed that at both dorsal and ventral EC levels excitatory synaptic potentials followed by neuron discharge were generated in layer II, site of origin of the perforant path to the dentate gyrus. Activation of either dorsal or ventral levels of the EC was followed by activation of the dentate gyrus-hippocampal system in corresponding hippocampal segments. The results provide physiological evidence that the commissural presubicular projections activate the EC in a topographic manner. The massive activation of perforant path neurons at all EC levels suggests that presubicular signals may strongly influence the functions played by the EC-dentate-hippocampal system.