Intrinsic projections of the retrohippocampal region in the rat brain. I. The subicular complex

The intrahippocampal projections of the subicular complex were studied in the rat with the aid of the anterogradely transported lectin Phaseolus vulgaris leucoagglutinin (PHA‐L). After iontophoretic injections of the lectin into the subiculum proper, presubiculum, or the parasubiculum, axons and terminal processes immunoreactive for PHA‐L were traced to their respective terminal fields within the hippocampal region. After subicular injections PHA‐L‐stained axons could be followed both in a caudal and a rostral direction. The caudally directed fibers course around or within the angular bundle to enter layers VI and V of the medial entorhinal area (MEA). Many fibers penetrate through these layers to terminate in layer IV of the medial and the lateral entorhinal area, which contains a major terminal field of this projection. At more ventral levels, all layers of the entorhinal area are innervated by cells located in the subiculum. Other retrohippocampal projections of the subiculum proper include the deep and the outer two layers of the presubiculum and the medial sector of the parasubiculum, in addition to a massive projection which terminates in the retrosplenial cortex. The rostrally directed projections from the subiculum form a dense innervation of strata lacunosum, radiatum, oriens, and of individual pyramidal cells in the regio superior of the Ammon's horn. All these projections of the subiculum are exclusively ipsilateral.