Loss of Rac1 in postmitotic neurons of the hippocampus affects NPC cycle regulation. A, Comparison of relative mRNA expression levels of cell cycle regulators in the DG. The experimental setup was the same as in Figure 7A, except that learner and runner-learner groups were killed on day 8 (after 1 week of learning). Microdissected DG tissue from naive, learners, and runner-learners was subjected to qRT-PCR analysis. Values were normalized to those of β-actin for each sample and then normalized to the control naive group. No changes in mRNA expression for Cdk 2, 4, and 6 and cyclin D1 and D2 were observed when comparing Rac1 control groups with Rac1 mutant groups. However, Rac1 mutant learners had significantly higher levels of Cdk inhibitor 1B (p27 Kip1) compared with all the other groups (for all groups, n = 5, ANOVA with post hoc test, *p < 0.05). B, Immunostaining of p27 Kip1 in the DG. p27 Kip1 protein expression (red) was predominantly found in the subgranular cell layer (SGCL) and to a lesser extent in the granular cell layer (GCL) of the DG. Expression colocalized with Dcx⁺ cells (blue) in the SGCL and with NeuN⁺ cells in the GCL. Scale bars, 50 μm. C, Increased cell cycle exit in Rac1 mutant mice during learning. Double immunofluorescence for Ki67 (red) and IdU (green) after 24 h survival to a single IdU pulse at training day 7. Cells labeled only with IdU were more abundant in Rac1 mutant learners. Scale bar, 40 μm. Quantification of the fraction of cells leaving the mitotic cycle (quit fraction: percentage of the number of IdU⁺, Ki67⁻ cells among the total of IdU⁺ cells). In Rac1 mutant learners, the number of cells that leave the cell cycle was significantly increased compared with control learners (n = 3–4, ANOVA, F_(1,5) = 6.7, *p < 0.05). D, Relative abundance of TGF-β1 mRNA in the DG. Rac1 mutant mice had significantly higher levels of TGF-β1 expression as measured by qRT-PCR compared with their control groups (control naive vs mutant naive and control learners vs mutant learners, n = 3–5, ANOVA with post hoc test, **p < 0.01).