Intrinsic connections of the retrohippocampal region in the rat brain: III. The lateral entorhinal area

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Abstract
This paper describes the retrohippocampal projections of individual layers of the lateral entorhinal area as studied by the method of anterograde transport of the lectin Phaseolus vulgaris leucoagglutinin (PHA‐L) in the rat. As in the medial entorhinal area (EA), (Köhler, '86a) PHA‐L injections restricted to individual layers of the lateral EA resulted in labeling of sparse projections to the subicular complex (e.g., subiculum, pre‐ and parasubiculum), whereas projections to the perirhinal area and piriform cortex were prominent. All PHA‐L injections resulted in the labeling of axons projecting longitudinally within the entorhinal area, in both dorsal and ventral directions, albeit the ventral projections were the most prominent ones.PHA‐L injections into layers 2a and 2b resulted in labeling of axons that could be followed into layers 2a, 2b, and layer 1 on both sides of the injection site. Whereas numerous axons appeared to terminate in layer 2, most fibers ascended into layer 1, where they ran in a medial direction, passing the medial EA, around the parasubiculum to the presubiculum. Numerous axons were found to take a lateral route running past the lateral aspect of the lateral EA to the piriform cortex. The axons running medial in layer 2 did not enter the medial EA.After PHA‐L injections into layer 3, a large number of axons left the labeled cells on both sides of the injection site, in addition to massive projections that ascended into layers 2b, 2a and 1, just above the injection. Few axons entered layers 2‐6 of the medial EA, but numerous axons innervated layer 1, where they were found to run in the outer half of this layer. The axons running in a medial direction reached layer 1 of the presubiculum, whereas the laterally oriented ones innervated the molecular layer of the piriform cortex. PHA‐L injections into layer 4 resulted in massive labeling of projections to all superficially located layers. Layers 1, and 2b through 5 were innervated lateral to, and layer 4 medial to, the injection site.After a PHA‐L injection into layer 5, ascending projections were found innervating layers 1 through 4. The terminal fields were found to be particularly dense in the deep parts of layer 3 and in layer 1. This projection expanded laterally, but few projections reached into the medial sector of the lateral EA or into the medial EA.PHA‐L injections into layer 6 resulted in massive projections to layers 1 through 6 of the lateral EA. Layer 4 was among the most densely innervated layers of the lateral EA after this injection, and the innervation of layers 4 through 6 expanded into the medial EA after layer 6 injections.Taken together, these findings have shown that: (1) most of the projections from individual layers of the lateral EA are confined within this cortical area or run to extrahippocampal areas by way of the piriform cortex, and (2) with the exception of layers 4 and 6, the lateral EA sends few projections to layers 2 through 6 of the media EA or to the deep layers of the subicular complex.

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