Running throughout middle-age improves memory function, hippocampal neurogenesis, and BDNF levels in female C57BL/6J mice

Abstract

Age-related memory loss is considered to commence at middle-age and coincides with reduced adult hippocampal neurogenesis and neurotrophin levels. Consistent physical activity at midlife may preserve brain-derived neurotrophic factor (BDNF) levels, new cell genesis, and learning. In the present study, 9-month-old female C57Bl/6J mice were housed with or without a running wheel and injected with bromodeoxyuridine (BrdU) to label newborn cells. Morris water maze learning, open field activity and rotarod behavior were tested 1 and 6 months after exercise onset. Here we show that long-term running improved retention of spatial memory and modestly enhanced rotarod performance at 15 months of age. Both hippocampal neurogenesis and mature BDNF peptide levels were elevated after long-term running. Thus, regular exercise from the onset and during middle-age may maintain brain function.

Figure 1

Timeline of short and long-term running effects on behavior and neurogenesis. Animals were housed with a running wheel from 9 months of age. In order to analyze the survival of newborn cells, bromodeoxyuridine (BrdU) (50 mg/kg) was injected i.p. for the first 10 days of individual housing. After 1 month (B1) and 6 months (B2) an identical battery of behavioral testing was performed. Once the second testing session was complete, the animals were euthanized at 17 months of age. The table below shows the housing conditions, mouse strain used, sex and the number of mice per group at the beginning of the experiment.

Figure 2

Effects of short and long-term running paradigms on cognitive, locomotor and anxiety-related behaviors. Morris water maze training (A) and a 24-hr probe trial (C), to test for retrieval of spatial memory, after 1 month of running showed no difference between the groups (N=9-10 per group). At 6 months after housing with a running wheel, the runners, unlike the controls, showed a clear preference for the target quadrants on the 4-hr (B) and the 24-hr (D) probe trials (N=9 per group). At the 1 month timepoint, runners displayed significant improvements on performance in the open field in terms of distance travelled (E) and a greater percentage of distance travelled in the center versus the peripheral area of the arena (F) compared to controls (N=9-10 per group) . These effects were no longer apparent after 6 months of running (E,F) (N=9 per group). Though not significantly different, the rotarod performance of the runners indicated strong trends towards an increase in the latency to fall (G) at 1 month (N=9-10 per group) and a decrease in the number of falls (H) at 6 months (N=9 per group) compared to controls. Data represents the mean ± SEM. (*p<0.05; ^ represents a strong trend). Dashed lines in B, C and D represent 15 seconds or one quarter of the time allotted for completion of the probe trial.

Figure 3

Consistent running results in increased hippocampal neurogenesis. The number of BrdU⁺ cells (A) as well as the total number of new neurons (B) was significantly increased in runners as compared to controls (N=5-9 per group) . The number of type D cells in the suprapyramidal layer of the dentate gyrus (C) in the runners was greater than controls but it did not reach significance (N=9 per group). There was no difference in type C cells in the suprapyramidal layer of the dentate gyrus between the groups (N=9 per group) (D). Examples of photomicrographs of BrdU⁺/NeuN⁺ labeling of newborn neurons in controls (E) and runners (G). Photomicrographs displaying doublecortin (DCX) expressing immature neurons in the dentate gyrus of control (F) and running (H) mice. Arrows and labels indicate the type D and type C cells analyzed in this study. Scale bars = 50μm. Data represents the mean ± SEM. (*p<0.05; ^ represents a strong trend).

Figure 4

Long-term running leads to an increase in hippocampal levels of mature BDNF peptide. Runners have increased levels of mature BDNF peptide compared to controls after 8 months of running (A). Data were normalized to β-tubulin loading controls. (B) Image of immunoblot showing increased mature BDNF peptide levels in the runners. BDNF band is a positive control. Data represents the mean ± SEM. (*p<0.05).