Afferents to the hippocampus of the rat studied with the method of retrograde transport of horseradish peroxidase

Horseradish peroxidase (HRP) injected into rat hippocampus was transported to the perikarya of neurons which project to the hippocampus. HRP-labeled cells were present in both medial and lateral entorhinal cortex; cells of the medial entorhinal cortex appeared to be topographically organized. The mediaal septal nucleus contained stained cells; its mediaal aspect was labeled after dorsal hippocampal injections, while ventral hippocampal injections resulted in the labeling of more laterally located cells. Stained cells were also observed in the ipsilateral nucleus locus coeruleus, dorsal and median raphe nuclei and areas CA3–4 of the contralateral hippocampus. In additions, cells in the supramammillary region, an area not previously recognized to project to the hippocampus, were labeled. Finally, the mossy fiber terminal zone and the CA3–4 terminal zone in the dentate molecular layer of the ipsilateral hippocampus demonstrated HRP activity, presumably the result of orthograde axonal transport from the injection site.