Projections from the hippocampal and parahippocampal regions to the entorhinal cortex. An anterograde and retrograde tract-tracing study in the cat

Projections from the hippocampal and parahippocampal regions to the entorhi nal cortex IEC) were examined in the cat by anterograde and retrograde trac t-tracing with Phaseolus vulgaris leucoagglutinin and cholera toxin B subun it. CAI fibers to EC were distributed more densely in the medial EC than in the lateral EC: these were seen in all EC layers, but most densely in laye rs II and III. The septotemporal axis of the area of origin of CA1-EC fiber s corresponded to a caudal-lo-rostral axis of the area of their termination in the EC. CA2 and CA3 also sent a small number of fibers to the EC. The s ubiculum sent fibers mainly to the lateral EC; more densely to layers IV-VI than to layers I-III. The septotemporal axis of the area of origin of subi culum-EC fibers corresponded to a caudolateral-to-rostromedial axis of thei r termination in the EC. Distribution pattern of fibers from the prosubicul um regions close to CA1 or from prosubiculum regions close to the subiculum was similar to that of CA1 fibers or subiculum fibers, respectively. The p resubiculum sent fibers mainly to the medial EC; most densely to layers I a nd III. The parasubiculum sent fibers mainly to the medial EC; most densely to layer II. Fibers to the contralateral EC were detected only from the pr esubiculum; they originated from the superficial layers and terminated in l ayer III of the medial entorhinal area. (C) 2001 Elsevier Science Ireland L td and the Japan Neuroscience Society. All rights reserved.