Ipsilateral projection from the subiculum to the retrosplenial cortex in the guinea pig

A projection from the subiculum to the retrosplenial cortex (area 29a and b) was studied by autoradiographic tracing of anterogradely transported proteins following injections of radioactive amino acids into ventral and dorsal parts of the subiculum. Only the dorsal parts of the subiculum projected to the retrosplenial cortex. The projection was exclusively ipsilateral, and topographically arranged along both the longitudinal septotemporal axis and the transverse plane of the subiculum. The termination in the retrosplenial cortex was confined to the relatively cell-free superficial layer (lamina 1) and the underlying cell-rich layers (laminae 3-4). Between these two terminations a narrow zone (lamina 2) was only weakly labeled. The projection constitutes an output route for hippocampal activity in addition to the fimbria-fornix system and the subicular projections to the parahippocampal areas.