The topography of activity transmission between lateral entorhinal cortex and subfield CA1 of the hippocampus

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It is important to establish how information is transmitted through the hippocampal formation because of the structure's critical role in memory and spatial processing. Here we provide evidence that challenges the hypothesis that information is processed in parallel closed entorhinal-CA1 loops. We tested the hypothesis by mapping, throughout hippocampal subfield CA1, field potentials evoked by stimulation of different sites in lateral entorhinal cortex in awake rats, thereby establishing the topography of electrophysiological transmission between the entorhinal cortex and CA1. The results established that antidromic and orthodromic responses evoked from the same entorhinal site occurred in spatially separated CA1 areas, with antidromic responses being located more septally than orthodromic responses. Thus, an entorhinal site receives information from a CA1 area located closer to the septal pole of the hippocampus and transmits it to the next CA1 area located closer to the temporal pole. Accordingly, processing in the hippocampal formation is by open rather than closed loops. Activation occurred first in CA1 close to its septal pole and spread towards its temporal pole. Four successively activated CA1 areas, oriented at an angle to the longitudinal axis of the hippocampus, were distinguished. Overall, the findings indicate that information can potentially be transmitted from the septal to the temporal end of the hippocampal formation via an ordered succession of hippocampal and entorhinal areas that form a three-dimensional spiral pathway.

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