Figure 1. Experiment 1 indicated that the number of doublecortin‐positive new neurons was highest in rats genetically predisposed to a high response to training and in which aerobic fitness consequently increased significantly in response to voluntary wheel running .

Abbreviations: C, control; gcl, granule cell layer; HIT, high‐intensity interval training; HRT, high‐response trainer; IHC, immunohistochemistry; LRT, low‐response trainer; mo, months old; RW, running wheel; and Sed, sedentary. A, outline of the experiment. B, running in rats allowed access to running wheels throughout the first 7 days. The HRT rats increased running more compared with LRT rats. C, throughout the whole 7 week training period, the running distance was comparable in both rat lines and changed in a quadratic manner. D, running speed increased in both LRT and HRT rats in response to either HIT or RW. The biggest improvement was seen in HRT rats voluntarily training on a running wheel. E, there was no baseline difference in adult hippocampal neurogenesis in the two rat lines. F, however, when allowed to run freely on a wheel in the home cage for 7 weeks, the HRT rats showed a higher number of new neurons in the hippocampus compared with rats of the same rat line either not trained at all (Sed) or subjected to HIT three times a week. G, representative examples of doublecortin‐positive cells (brown) in the dentate gyrus in each group of rats are illustrated. The background stain (Cresyl Violet) shows cell bodies of mature neurons in light blue. The hilus and the granule cell layer are indicated in the top left panel. The scale bar length is 50 μm, and the scale is the same in all panels (original magnification, ×400). Insets depict cell groups positive for doublecortin at ×1000 magnification, and arrows point to these cells in the ×400 photos. In B–F, vertical lines depict the SEM. Asterisks refer to statistically significant effects/differences, as follows: *P < 0.05, **P < 0.01 and ***P < 0.001.