Voluntary Wheel Running in Old C57BL/6 Mice Reduces Age-Related Inflammation in the Colon but Not in the Brain

Abstract

Inflammation is considered a possible cause of cognitive decline during aging. This study investigates the influence of physical activity and social isolation in old mice on their cognitive functions and inflammation. The Barnes maze task was performed to assess spatial learning and memory in 3, 9, 15, 24, and 28 months old male C57BL/6 mice as well as following voluntary wheel running (VWR) and social isolation (SI) in 20 months old mice. Inflammatory gene expression was analyzed in hippocampal and colonic samples by qPCR. Cognitive decline occurs in mice between 15 and 24 months of age. VWR improved cognitive functions while SI had negative effects. Expression of inflammatory markers changed during aging in the hippocampus (Il1a/Il6/S100b/Iba1/Adgre1/Cd68/Itgam) and colon (Tnf/Il6/Il1ra/P2rx7). VWR attenuates inflammaging specifically in the colon (Ifng/Il10/Ccl2/S100b/Iba1), while SI regulates intestinal Il1b and Gfap. Inflammatory markers in the hippocampus were not altered following VWR and SI. The main finding of our study is that both the hippocampus and colon exhibit an increase in inflammatory markers during aging, and that voluntary wheel running in old age exclusively attenuates intestinal inflammation. Based on the existence of the gut-brain axis, our results extend therapeutic approaches preserving cognitive functions in the elderly to the colon.

Keywords: Barnes maze; aging; gut-brain axis; hippocampus; inflammaging; intestine; physical activity; social isolation; spatial learning.

Figure 1

Effect of aging on cognitive function in male mice in the Barnes maze test. Aging significantly increased (A) primary latency and decreased (B) cognitive score during the training period. (C) Search strategies used during the training period differed between age groups. (D) The estimated training time (ETT) to achieve optimal test results differs between age groups and shows a significant association to age in linear regression. Significant effects of aging were also evident in the (E) probe trial and (F) retention test while the (G) reversal test showed no differences. ETT is shown as median, all other parameters are shown as mean ± SEM, the success rate in finding the target hole is presented as %. Specific significance levels for (A) primary latency and (B) cognitive scores are given in the tables above the graph. The training period was tested with a mixed ANOVA (A-B) and with a Kruskal-Wallis test (D). The probe trial, retention test, and reversal test were analyzed with a univariate ANOVA, and the success rate was tested with Fisher’s exact test, * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Figure 2

Effect of running and isolation on cognitive functions in male mice in the Barnes maze test. Isolated runners and group-housed mice were each compared to the same group of isolated controls. Running decreased (A) primary latency and increased (B) the cognitive score during the training period. (C) Isolation increased primary latency and decreased (D) the cognitive score. (E) Search strategies differed between isolated runners, isolated controls, and group-housed mice. (F) The probe trial, (G) the retention test, and (H) the reversal test were significantly different with respect to the primary latency after VWR. Cognitive score and primary latency are shown as mean per day ± SEM, mixed ANOVA. All other parameters are shown as mean ± SEM, t-test. The success rate in finding the target hole is presented as % and was tested with Fisher’s exact test. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Figure 3

Transcript expression in the (A) HC and (B) colon after aging, running, and social isolation. For aging, 20 versus 3 months old animals were compared. For running, 20 months old isolated runners were compared with isolated control mice; for isolation, 20 months old isolated mice were compared with group-housed mice. The mRNA transcription ratios are displayed as geometric mean ± SEM. Statistically significant ratios are in bold, Mann-Whitney-U test, * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001, n = 8–10. Specific p-values for each significant mRNA transcription ratio are given in the text. Protein-protein association network (STRING v11) of significantly differentially expressed genes in the (C) HC and (D) colon. The network is based on experimental interaction, pathway databases, or co-expression evidence. Confidence in associations is displayed by line thickness.