Closing the loop: Development and topography in subicular-to-parahippocampal projections
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The hippocampal formation (HF) and the parahippocampal region (PHR) are two stronglyand reciprocally connected components of the hippocampal memory system. While theentorhinal cortex provides the main input from PHR to HF, the subiculum is a major outputstructure of the HF, projecting back to the PHR. In adults, the connectivity between the lattertwo regions shows a striking topology along the transverse axis. This topology may havefunctional consequences for the hippocampal memory system. Before a system can befunctional, a rudimentary structural connectivity of the system must be in place. In this study, development and topography of subicular-to-PHR projections are described. The anterograde tracers biotinylated dextran amine (BDA), Alexa Fluor 488 dextran amine(Alexa 488) and phaseolus vulgaris leucoagglutinin (PHA-L) were injected into the subiculumin rats aged between postnatal day 7 (P7) and P17 (n=57). Injections were made along theentire proximal-distal axis at intermediate dorsoventral levels. The results indicate thatsubicular-to-PHR projections are present by P8 and adult-like by P15. Furthermore, theresults confirm and extend previously reported subicular-to-PHR topology, revealing apreviously unreported topography in parahippocampal target regions; the transverse positionof cells in the subiculum determines the proximodistal location of terminal plexuses in preandparasubiculum. Furthermore, the results suggest that the laminar position in distalsubiculum may determine the laminar termination pattern in the medial entorhinal cortex.