Organization of connectivity of the rat presubiculum: I. Efferent projections to the medial entorhinal cortex.

The organization of the laminar and topographical projections from the presubiculum to the entorhinal area was studied in the rat by anterograde labeling with Phaseolus vulgaris leucoagglutinin and retrograde labeling with horseradish peroxidase conjugated to wheat germ agglutinin. We found that the pattern of presubiculo-entorhinal projections differs between the superficial and deep layers of the presubiculum. The superficial layers (layers II and III) of the presubiculum gave rise to bilateral projections to layers I-VI of the medial entorhinal area (MEA). Many terminals were distributed in layer III, fewer in layer II and the deep portion of layer I, and many fewer terminals in the deep layers (layers V and VI) of MEA. In contrast, the deep layers (layers V and VI) of the presubiculum gave rise to ipsilateral projections to the entorhinal area. Many axon terminals were distributed in layers V and VI of MEA and the most superficial portion of layer I of MEA, but very few in layers II and III. In addition, the ramifications in layer I extended to the lateral entorhinal area (LEA). Using two-dimensional unfolded maps of parahippocampal cortices, we elucidated the distinct topographical relationship in the presubiculo-entorhinal projection: 1) The septotemporal or longitudinal axis of the presubiculum corresponded to the axis on the MEA/LEA boundary, where the septal presubiculum projected toward the rhinal fissure and the temporal presubiculum projected away from the fissure. 2) The proximodistal axis of the presubiculum corresponded to the axis from the MEA/LEA boundary to the MEA/parasubiculum boundary that was virtually perpendicular to the MEA/LEA boundary, where the proximal portion of the presubiculum (close to the subiculum) projected to the region near the MEA/LEA boundary.