Connectivity of the hilar region of the hippocampal formation in the rat.

Injections of horseradish peroxidase (HRP) into the hilar region (fascia dentata/CA4/CA 3c) of the dorsal hippocampal formation in the rat resulted in labeling of cell bodies in the ipsilateral fields CA 4 and CA 3, and in the contralateral field CA 4. Injection into CA 3 labeled many granular cells in the ipsilateral fascia dentata, and a few neurons in CA 4. A small number of CA 4-cells were labeled after HRP-injections into the retrosplenial cortex. The results support the notions, suggested by some anatomical reports as well as electrophysiological data, (1) that there are projections from CA 3 to CA 4 and/or the fascia dentata, (2) that a pathway exists from CA 4 to CA 3, and (3) that axons from CA 4-neurons project beyond the hilus.