Abstract
The rat parahippocampal region (PHR) and retrosplenial cortex (RSC) are cortical areas important for spatial cognition. In PHR, head-direction cells are present before eye-opening, earliest detected in postnatal day (P)11 animals. Border cells have been recorded around eye-opening (P16), while grid cells do not obtain adult-like features until the fourth postnatal week. In view of these developmental time-lines, we aimed to explore when afferents originating in RSC arrive in PHR. To this end, we injected rats aged P0-P28 with anterograde tracers into RSC. First, we characterized the organization of RSC-PHR projections in postnatal rats and compared these results with data obtained in the adult. Second, we described the morphological development of axonal plexus in PHR. We conclude that the first arriving RSC-axons in PHR, present from P1 onwards, already show a topographical organization similar to that seen in adults, although the labeled plexus does not obtain adult-like densities until P12.
Highlights
The parahippocampal region (PHR) is important for learning and memory
For a summary and direct comparison of the different nomenclatures, we refer to a recent review on the retrosplenial cortex (RSC)-hippocampal formation (HF)-PHR connectivity (Sugar et al, 2011)
After fast blue injections in PHR of pups we identified labeled neurons in RSC in superficial layer V at P5 (Figure 9A) and P11 (Figure 9B), which suggested that the RSC-PHR projections are adult-like with respect to the layer in which the neurons are located already during the first postnatal week of development
Summary
The parahippocampal region (PHR) is important for learning and memory. It consists of two functionally different networks, one of which, involved in spatial functions, comprises the presubiculum (PrS), parasubiculum (PaS), medial entorhinal cortex (MEC) and postrhinal cortex (POR). Adult-like grid cells in layer II of MEC first appear during the fourth postnatal week (Langston et al, 2010; Wills et al, 2010). The early presence of head-direction cells is apparently paralleled by a early developed shared connectivity (Bjerknes et al, 2015), while the late development of grid cells is paralleled by a corresponding late development of the relevant intrinsic connectivity in MEC (Langston et al, 2010; Couey et al, 2013).
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