The relevance of adult neurogenesis in hippocampal function is well documented, as is the potential
impact stress has on the adult neurogenic niche. Adult born neurons are generated from neural precursors
in the dentate gyrus (DG), although the pointin postnatal development that these cell precursors originate
is not known. This is particularly relevant if we consider the effects stress may have on the development
of neural precursors, and whether such effects on adult neurogenesis and behavior may persist in the
long-term. We have analyzed the proportion of neural precursors in the adult murine hippocampus born
on specific days during postnatal development using a dual birth-dating analysis, and we assessed their
sensitivity to dexamethasone (DEX) on the peak day of cell generation. We also studied the consequences
of postnatal DEX administration on adult hippocampal-dependent behavior. Postnatal day 6 (P6) is a
preferred period for proliferating neural stem cells (NSCs) to become the precursors that remain in a
proliferative state throughout adulthood. This window is independent of gender, the cell’s location in
the DG granule cell layer or their rostro-caudal position. DEX administration at P6 reduces the size of
the adult NSC pool in the DG, which is correlated with poor learning/memory capacity and increased
anxiety-like behavior. These results indicate that aNSCs are generated non-uniformly during postnatal
development, with peak generation on day P6, and that stress receptor activation during the key period of
postnatal NSC generation has a profound impact on both adult hippocampal neurogenesis and behavior.