Conditional ablation of Rac1 in postmitotic neurons of the DG. A, Schematic representation: conditional loss of Rac1 was achieved via CaMKIIα-driven Cre-mediated ablation of exon 3 of rac1 (Chrostek et al., 2006). At the same time recombined cells were visualized by Cre-mediated induction of a conditional reporter such as β-gal. B, β-gal (Rosa26 reporter mouse, top; Soriano, 1999) and EGFP (Z/EG, bottom; Novak et al., 2000) expression in the granular cell layer (GCL) of the DG. Reporter-positive green cells colabeled with the postmitotic neuronal marker NeuN (red, arrowheads). Scale bars, 20 μm. C, Percentage of doublecortin (Dcx), Dcx/NeuN, and NeuN-positive cells that express the reporters (GFP or β-gal). Less than 1% of Dcx⁺ precursor cells colabeled with either reporter. Reporter expression was detected in postmitotic Dcx/NeuN⁺ intermediate neurons and mature NeuN⁺ neurons. D, Rac1 protein expression in the GCL of Rac1 control mice (left) and Rac1 mutant mice (right). Rac1 (red) was abundantly expressed in NeuN-positive GCL neurons and in Dcx⁺ NPCs (blue, arrows). Rac1 expression in Rac1 mutant mice was downregulated in the mature neurons but still present in Dcx⁺ neurons, as seen in higher magnification on the right. E, Relative Rac1 and Rac3 mRNA levels in cell fractions enriched for mature neurons of the hippocampus. Significantly lower Rac1 expression is seen in Rac1 mutant mature neurons of naive animals (controls, n = 8; mutants, n = 6; ANOVA, F_(1,12) = 6.03, *p < 0.05). Rac3 expression was also evaluated as an internal control and no change in expression was detected. Rac1 expression was significantly upregulated during 7 d of learning in Rac1 control mice but remained low in Rac1 mutant learners (n > 5 for all groups of mice; ANOVA with post hoc test, **p < 0.01, ***p < 0001; overall genotype effect, two-way ANOVA, F_(9,44) = 5.24; p < 0.001). F, Significant upregulation of Rac1 mRNA levels is also detected in cell fractions enriched for NPCs of the hippocampus during 7 d of learning in Rac1 control mice (control naives, n = 10; control learners, n = 5; ANOVA with post hoc test, p < 0001). No significant differences were detected in Rac1 expression between Rac1 control and mutant mice. *p < 0.05, ***p < 0.001, ns, not significant.