Expansion and retraction of hippocampal mossy fibers during postweaning development: Strain-specific effects of NMDA receptor blockade

We have recently discovered differences in the distribution of the mossy fiber terminal field (MFTF) between adult Long‐Evans rats (LER) and Wistar rats(WR): the suprapyramidal MFTF extends into distal stratum oriens (dSO) in LER, but is nearly absent in WR (Holahan et al.,2006, Hippocampus 16:560–570). To our knowledge, there is no developmental evidence that sheds light on how this strain‐dependent MFTF innervation in the adult is achieved. Accordingly, the present study examined the time course of MFTF development from postnatal days 0 to 40 and the effect of NMDA‐receptor antagonist 3‐(2‐carboxypiperazin‐4‐yl) propyl‐1‐phosphonic acid (CPP) on this developmental organization. In both LER and WR, a MFTF projection to dSO was observed between P18 and P21. By P24, the dSO projection in WR was no longer detectable whereas in LER, the dSO projection seen on P21 remained. We suggest that in WR a retraction of the MFTF projection from dSO to stratum lucidum between P21 and P24 leads to its adult pattern. In WR, CPP administration enhanced the dSO projection, possibly by blocking the retraction process. In LER, CPP administration reduced the dSO projection. Thus, in each strain, NMDA receptor blockade effectively reversed the developmental course of MFTF pattern of innervation. The present results lend strong support to the view that NMDA receptor regulation of input‐dependent processes during development is of critical importance in promoting the motility and target selection of presynaptic MF axons. This regulation extends later into development than had previously been thought. © 2006 Wiley‐Liss, Inc.