Ongoing neurogenesis in the adult hippocampus is proposed to contribute to memory formation and retrieval. However, although adult-born granule cells have been shown to integrate into the dentate gyrus and form functional connections, evidence for a role in memory processes is lacking. A study by Kee et al. now implicates these cells in spatial memory processing.
Previous studies that investigated the role of adult neurogenesis in memory processing by suppressing cell division gave mixed results, depending on the conditions used. Kee et al. took a different approach, choosing to visualize the activity of the new-born cells in a well-known test of spatial memory, the water maze.
The authors labelled adult-born neurons in the hippocampus of mice using 5-bromo-2'-deoxyuridine (BrdU). At various intervals after BrdU injection the mice were trained in the water maze, and 10 weeks after the injection a retrieval trial took place. 90 minutes after testing, the authors examined the expression levels of two markers of neuronal activity, the immediate early gene c-Fos and the activity-regulated cytoskeletal-associated protein Arc, to identify cells that were active during memory retrieval. They identified cells labelled for both BrdU and either c-Fos or Arc, indicating that adult-born granule cells were recruited into networks that were active during spatial memory processing.
The recruitment of the BrdU-labelled cells was dependent on the interval between injection and training. In mice that began training 2 weeks after injection, the proportion of BrdU-labelled cells recruited was low, whereas after a 6–10 week interval, BrdU-labelled cells were twice as likely to co-express c-Fos as existing granule cells, suggesting the preferential recruitment of more mature adult-born granule cells.
This study does not show that these cells are essential for memory formation, and defining their role in spatial memory processes will require further work. However, the preferential recruitment of these cells suggests that their contribution is likely to be important.