Postnatal development of retrosplenial projections to the parahippocampal region of the rat

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.

DOI: http://dx.doi.org/10.7554/eLife.13925.001

eLife digest

Our ability to navigate critically depends on part of the brain called the parahippocampal region. Within this region, there are several different types of brain cells (or neurons) whose activity “codes” different aspects of navigation, such as position, direction and speed.

To understand how parahippocampal neurons are able to form these activity patterns, we need to understand how they develop connections with neurons from other brain regions that are important for navigation, such as the retrosplenial cortex. If inputs from retrosplenial neurons are important for generating the activity patterns observed in the parahippocampal region, the connections between the two groups of neurons should be fully mature before the activity patterns emerge. In rats, this should occur around 11–16 days after birth.

Sugar and Witter have now assessed how the retrosplenial inputs are organized in the parahippocampal region of rats. This revealed that, when the rats are born, there are very few retrosplenial inputs present in the parahippocampal region. However, the few inputs that are present are organized similarly to how they eventually will be organized in adults. After birth, the number of inputs gradually increases until the rats are approximately 12 days old, at which point the pattern of connections is indistinguishable from what we observe in adults. Thus it appears that retrosplenial inputs are fully mature before activity patterns emerge in the parahippocampal region.

In the future, Sugar and Witter would like to investigate how inputs to the parahippocampal region are able to organize themselves during early development. The importance of retrosplenial inputs could also be investigated by manipulating them during development and adulthood.

DOI: http://dx.doi.org/10.7554/eLife.13925.002