Running is the neurogenic and neurotrophic stimulus in environmental enrichment

Abstract

Environmental enrichment (EE) increases dentate gyrus (DG) neurogenesis and brain-derived neurotrophic factor (BDNF) levels. However, running is considered an element of EE. To dissociate effects of physical activity and enrichment on hippocampal neurogenesis and BDNF levels, young female C57Bl/6 mice were housed under control, running, enrichment, or enrichment plus running conditions, and injected with bromodeoxyuridine. Cell genesis was assessed after 12 d and differentiation was analyzed 1 mo later. In addition, locomotor activity in the open field and hippocampal mature BDNF peptide levels were measured. Open-field adaptation was improved in all groups, compared to controls, but more so with running. New cell proliferation, survival, neuron number, and neurotrophin levels were enhanced only when running was accessible. We conclude that exercise is the critical factor mediating increased BDNF levels and adult hippocampal neurogenesis.

Figure 1.

Housing of the experimental animals. Female C57Bl/6 mice (n = 10 per group) were housed in large cages (30″ × 33″ × 8″) as shown. (A) Control (CON). (B) Running (RUN); cage with 10 running wheels for voluntary physical activity. (C) Enriched environment only (EEO). (D) Enrichment and running (EER); the cage contained enrichment objects similar to C, as well as 10 running wheels. (E) Overview of the experimental cages.

Figure 2.

Dentate gyrus cell proliferation and neurogenesis, and open-field locomotion. Photomicrographs (A–L) and quantification (M,N) of BrdU-positive cells 1 d (A–D,M) and 4 wk (E–H,N) after the last BrdU injection in CON (A,E,I), EEO (B,F,J), RUN (C,G,K), and EER (D,H,L) mice. (I–L) Confocal images of BrdU-positive cells in CON (I), EEO (J), RUN (K), and EER (L) 4 wk after the last injection. Sections were immunofluorescent double-labeled for BrdU (green) and NeuN (red) indicating neuronal phenotype. (O) Open-field behavior was evaluated on day 30. RUN and EER groups habituated more rapidly to the open field than CON and EEO (P < 0.04). EEO differed significantly from CON (P < 0.01).

Figure 3.

Hippocampal BDNF levels. (A) Mature BDNF peptide levels were significantly increased in the hippocampi of mice in the running (RUN, n = 8) and enriched plus running (EER, n = 9) groups compared to the enriched only (EEO, n = 10) and control (CON, n = 8) groups. Specific comparisons showed that RUN, EER, and EEO groups differed (P < 0.04), as well as RUN, EER and CON groups (P < 0.01). (B) One replicate of the Western blot is shown for each condition, as well as recombinant mature human BDNF peptide as a positive control and β-tubulin as an endogenous control.