Fig. 4. A chronic increase in neurogenesis preserves hippocampal learning throughout life.

a Experimental layout of mice injected with GFP or 4D viruses at 4 months of age (green arrow) and tested for over 1 year (as indicated). b–d Freezing response during a contextual discrimination test performed by altering the presentation order of a context A and B each day for 7 consecutive days (as depicted in b and in Supplementary Fig. 4A), with ratios for days with A-B (orange) and B-A (grey) order during the discrimination phase (b) and box–whisker plot quantifications of relative freezing response (%) during a probe trial (c) and second training phase (d) performed different months later (as indicated) for GFP and 4D chronically treated mice. Note the positive discrimination ratio of 4D-treated mice independent from the presentation order during both testing (b) and probe trial (c) and the more efficient reacquisition of memory after a second training (d). e, f Assessment of the relative contributions of navigational strategies of 14 months old mice (i.e. 2 months after tests shown in d) (e) and box–whisker plot quantification of quadrant preference during probe trials performed 2 and 6 weeks later (f). Note the improvements of 4D-treated mice during flexible re-learning after reversal (e; bottom) and the preservation of a space memory absent in controls (f; right). g Fluorescence pictures of the DG and striatum (left) of mice tested for remote memory (f) stained for c-Fos and DAPI (as indicated). Insets (bottom) indicate the dorso-lateral or medial (DL or DM, respectively) striatal area within which quantifications were performed (right). Data represent activation ratios extracted from total c-Fos+ cells per area (mm²) (Supplementary Fig. 4E) in the respective brain structures, as indicated. Scale bar = 100 μm (DG) and 500 μm (striatum). N = 9 (b–d) or 9 and 8 (e and f); n = 4 and 3 (b, representing days with A-B or B-A presentation order, respectively); N = 3 and 4 (g). Error bars = SD (b and g). *p < 0.05; **p < 0.01; ***p < 0.001 assessed as paired or unpaired Student’s t test (for intra or inter-group comparisons, respectively; b–d and f, g) and Wald-test (e).