Rapid extension of axons into the CA3 region by adult-generated granule cells.

The dentate gyrus continues to produce granule neurons throughout adulthood. The present study examined the extension of axons by adult-generated granule neurons into hippocampal area CA3. We injected the fluorescent retrograde tracers Fast blue (FB) and FluoroRuby (FR) into area CA3 of adult male rats at various times after the administration of 5'-bromo-2'-deoxyuridine (BrdU), a marker of proliferating cells and their progeny. We report that immature granule cells extend axons into CA3 as rapidly as 4-10 days after mitosis. A significant increase in the percentage of BrdU-labeled cells that were labeled with FB or FR was observed by 2 weeks after BrdU administration. This proportion remained roughly constant up to 3 weeks after BrdU-labeling, a time at which markers of a mature neuronal phenotype are expressed. BrdU-labeled cells that contained either FB or FR often were located far from the tracer injection site, indicating that these cells had extended relatively long axons. Collectively these results suggest that adult-generated granule neurons may influence normal hippocampal function, even at a very early stage after their production.