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

Abstract

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. After injections of PHA‐L into layers n and III of the presubiculum, both ipsi‐ and contralateral projections were traced to the outer three layers of the medial entorhinal area; the lateral entorhinal area apparently receives no innervation from the presubiculum. The innervation of layer III is very dense while in layer II and deep layer I, restricted zones of innervation are found. The fibers reach these layers via the deep layers of the MEA and through the molecular layer after first coursing around the parasubiculum. In addition, a minor projection from the presubiculum to the pyramidal cell layer of the subiculum and to the molecular layer of the hippocampal formation was found. PHA‐L injections into the parasubiculum labeled fibers that form a dense innervation of layer II in the MEA and the medial part of the lateral EA, and of the most medial sector of layer III in the MEA. Layer I and the superficial part of layer II of the contralateral MEA also contain a dense terminal network after PHA‐L injections into the parasubiculum. The commissural fibers reach the contralateral side via the dorsal hippocampal commissural system, the angular bundle, and finally through all layers of the contralateral EA. With the exception of a small projection to the molecular layer of the hippocampal formation, the parasubiculum appears not to innervate any of the other hippocampal subfields. Taken together, these studies have shown that each field of the subicular complex has projections restricted to separate layers of the entorhinal area, and thus, that each field participates in its own unique way in the control of entorhinal function.