Hippocampal efferents to the ipsilateral entorhinal area: An experimental study in the rat

Abstract A hitherto undescribed pathway from the hippocampus to the entorhinal area has been found in the albino rat in experimental studies by the silver‐impregnation technique of Fink and Heimer ('67) and a modification there of by the present author ('70). Animals in which the hippocampal formation had been deprived of commissural and other afferents by a preliminary operation at the age of eight days were used to faciliate the search for the site of origin of the pathway. Lesions in various parts of the hippocampus showed the origin to be restricted to subfield CA3 in approximately the temporal third of the hippocampus. The axons run caudally in the stratum radiatum and stratum lacunosummoleculare, and terminate in the medial part of the entorhinal area in a distinct zone corresponding to layer IV of Lorente de Nó ('33, '34). By the aid of alternating series of sections impregnated with silver and incubated for acetyl‐cholinesterase, the zone of termination was shown to be identical with the acetyl‐cholinesterase positive band at the level of layer IV. In the dorso‐ventral sense the fibers exhibit a topical organization, terminal degeneration at different levels of the entorhinal area depending on the levels of CA3 damaged. The mean course of the fibers from each level of CA3 seems to be roughly horizontal. A hippocampoentorhinal neuron circuit is proposed.