Development and topographical organization of projections from the hippocampus and parahippocampus to the retrosplenial cortex

The rat hippocampal formation (HF), parahippocampal region (PHR), and retrosplenial cortex (RSC) play critical roles in spatial processing. These regions are interconnected, and functionally dependent. The neuronal networks mediating this reciprocal dependency are largely unknown. Establishing the developmental timing of network formation will help to understand the emergence of this dependency. We questioned whether the long-range outputs from HF-PHR to RSC in Long Evans rats develop during the same time periods as previously reported for the intrinsic HF-PHR connectivity and the projections from RSC to HF-PHR. The results of a series of retrograde and anterograde tracing experiments in rats of different postnatal ages show that the postnatal projections from HF-PHR to RSC display low densities around birth, but develop during the first postnatal week, reaching adult-like densities around the time of eye-opening. Developing projections display a topographical organization similar to adult projections. We conclude that the long-range projections from HF-PHR to RSC develop in parallel with the intrinsic circuitry of HF-PHR and the projections of RSC to HF-PHR.