Little exercise, big effects: reversing aging and infection-induced memory deficits, and underlying processes

Abstract

We have previously found that healthy aged rats are more likely to suffer profound memory impairments following a severe bacterial infection than are younger adult rats. Such a peripheral challenge is capable of producing a neuroinflammatory response, and in the aged brain this response is exaggerated and prolonged. Normal aging primes, or sensitizes, microglia, and this appears to be the source of this amplified inflammatory response. Among the outcomes of this exaggerated neuroinflammatory response are impairments in synaptic plasticity and reductions of brain-derived neurotrophic factor (BDNF), both of which have been associated with cognitive impairments. Since it has been shown that physical exercise increases BDNF mRNA in the hippocampus, the present study examined voluntary exercise in 24-month-old F344×BN rats as a neuroprotective therapeutic in our bacterial infection model. Although aged rats ran only an average of 0.7 km per week, this small amount of exercise was sufficient to completely reverse infection-induced impairments in hippocampus-dependent long-term memory compared with sedentary animals. Strikingly, exercise prevented the infection-induced exaggerated neuroinflammatory response and the blunted BDNF mRNA induction seen in the hippocampus of sedentary rats. Moreover, voluntary exercise abrogated age-related microglial sensitization, suggesting a possible mechanism for exercise-induced neuroprotection in aging.

Figure 1.

a, b, Distance run (a), and body weight changes (b) over a 6 week period. Means ± SEM are plotted. a, Rats (n = 16) ran an average of 1028 ± 120 m the first week of the study, and showed a steady decrease over the course of the study with distances stabilizing ∼470 ± 67 m in weeks 5 and 6. b, Inset (starting body weights), A one-way ANOVA showed that there were no differences in body weight at the start of the study among the 3 groups: F_(2,31) = 0.87, p > 0.05; n = 8–16. Percentage body weight changes over 6 weeks, A repeated-measures ANOVA showed there was a significant main effect of physical activity (sedentary, locked, runners), F_(2,29) = 42.12, p < 0.0001; a significant main effect of time, F_(5,10) = 8.02, p < 0.0001; and a significant physical activity × time interaction, F_(10,145) = 7.90, p < 0.0001. Fisher's post hoc tests revealed that runners (n = 16) had lost significantly more weight than did both sedentary rats (n = 8) and those with a locked wheel (n = 8) beginning with the first week of the study and continuing on through the last (p < 0.0001, each week). Rats housed with a locked wheel also showed a weight reduction compared with sedentary rats, but these were only significant beginning on week 4 and continuing through week 6 (p < 0.01, each week). Sedentary rats gained an average of 0.55%, while rats housed with locked wheels lost an average of 1.73%, and runners lost an average of 5.52% of their starting weight.

Figure 2.

a, b, Freezing to the conditioned fear context (a) and an alternate context with conditioning having occurred 4 d after immune challenge (b) in 24-month-old rats, following 6 weeks of physical activity. Means ± SEM are plotted. a, There was a significant main effect of physical activity (sedentary, locked, runners), F_(2,26) = 4.12, p < 0.03, and a significant main effect of immune challenge (vehicle, E. coli), F_(1,26) = 11.91, p < 0.002. There was also a significant physical activity × immune challenge interaction (F_(2,26) = 4.05, p < 0.03). Fisher's post hoc tests revealed that sedentary E. coli-treated rats (n = 4) froze significantly less than vehicle-treated controls (p < 0.05; n = 4) and E. coli-treated runners (p < 0.01; n = 8). Locked-wheel E. coli-treated rats (n = 4) also froze significantly less than vehicle controls (p < 0.01; n = 4) and E. coli-treated runners (p < 0.01). E, coli-treated runners showed no difference from their vehicle controls (p > 0.05; n = 8). b, In the alternate context there was not a main effect, either of physical activity, F_(2,26) = 1.58, p > 0.05, or of immune challenge F_(1,26) = 0.011, p > 0.05.

Figure 3.

a, BDNF mRNA expression levels in CA1 region of the hippocampus 2 h after conditioning in rats that received either vehicle or E. coli 4 d before conditioning. Percentages of vehicle HCC ± SEM are plotted. There was a significant main effect of physical activity (HCC, sedentary, locked, runners), F_(3,43) = 50.144, p < 0.0001, and a significant main effect of immune challenge (vehicle, E. coli) F_(1,43) = 8.698, p < 0.005. There was also a significant physical activity × immune challenge interaction (F_(3,43) = 3.14, p < 0.04). Fisher's post hoc tests showed that all conditioned groups had significantly greater BDNF expression than did vehicle-treated HCC rats. Vehicle-treated HCC (n = 5) BDNF levels did not differ significantly from those of E. coli-treated HCC rats (p > 0.05; n = 6). Vehicle-treated runners (n = 9) exhibited greater BDNF expression compared with vehicle-treated locked wheel rats (p < 0.05; n = 6), but not compared with vehicle-treated sedentary rats (n = 6). Sedentary E. coli-treated rats (n = 6) exhibited significantly blunted BDNF expression compared with their vehicle controls (p < 0.05) and E. coli-treated runners (p < 0.0005; n = 8). BDNF mRNA levels from locked-wheeled E. coli-treated rats (n = 5) were also significantly lower compared with their vehicle-treated controls (p < 0.01) and E. coli-treated runners (p < 0.0001). BDNF mRNA levels in E. coli-treated runners did not differ from their vehicle-treated controls (p = 0.80). b, c, Representative photomicrographs of vehicle-treated HCC (non-conditioned) (b) and conditioned hippocampal (c) slices.

Figure 4.

a, b, IL-1β protein levels in hippocampus (a) and liver (b) 4 d after immune challenge in animals that were housed with a running wheel, a locked wheel, or no wheel for 6 weeks. Means ± SEM are plotted. a, Hippocampus. There was a significant main effect of physical activity (sedentary, locked, runners), F_(2,35) = 6.11, p > 0.005, a significant main effect of immune challenge (vehicle, E. coli), F_(1,35) = 15.21, p < 0.0005, and a significant physical activity × immune challenge interaction, F_(2,35) = 3.57, p < 0.05. Fisher's post hoc tests showed that all vehicle-treated rats exhibited comparable IL-1β protein levels, and did not differ across conditions. Sedentary E. coli-treated IL-1β levels (n = 5) were significantly higher compared with their vehicle controls (p < 0.01; n = 6), E. coli-treated locked-wheel rats (p < 0.05; n = 5), and E. coli-treated runners (p < 0.0005; n = 8). Hippocampal IL-1β levels of locked-wheeled E. coli-treated rats were also significantly higher compared with their vehicle-treated controls (p < 0.005; n = 6) and E. coli-treated runners (p = 0.05). IL-1β levels in E. coli-treated runners were not different from their vehicle-treated controls (p = 0.73; n = 11). b, Liver. There was a significant main effect of physical activity, F_(2,36) = 5.18, p < 0.05, a significant main effect of immune challenge, F_(1,36) = 41.98, p < 0.0001, and a significant physical activity × immune challenge interaction, F_(2,36) = 6.05, p < 0.005. Again, all vehicle-treated rats exhibited comparable IL-1β protein levels, and did not differ across activity conditions. However, IL-1β levels in sedentary E. coli-treated rats (n = 7) were significantly higher compared with their vehicle controls (p < 0.0005; n = 7), E. coli-treated locked wheel rats (p < 0.05; n = 4), and E. coli-treated runners (p < 0.05; n = 8). Liver IL-1β levels of locked wheel E. coli-treated rats were also significantly higher compared with their vehicle-treated controls (p < 0.0001; n = 5), but not different from E. coli-treated runners (p > 0.05). Liver IL-1β levels in E. coli-treated runners were also significantly higher compared with their vehicle-treated controls (p < 0.001; n = 11).

Figure 5.

a–c, TNFα (a), IL-1β (b), and IL-6 (c) gene expression in isolated microglia 2 h following stimulation with increasing doses of LPS. Means ± SEM are plotted; n = 4 in each group. a, TNFα. There was a significant main effect of physical activity (locked, runners), F_(1,24) = 37.03, p < 0.001, a significant main effect of LPS dose (0, 0.1, 1, 10, 100 ng), F_(4,24) = 206.29, p < 0.0001, and a significant physical activity × LPS dose interaction, F_(4,24) = 22.45, p < 0.0001. At all doses of LPS, even the 0 ng dose, TNF gene expression was significantly higher in microglia of locked wheel rats than that of runners (p < 0.05). b, IL-1β. There was a significant main effect of physical activity, F_(1,24) = 17.23, p < 0.01, a significant main effect of LPS dose, F_(4,24) = 224.78, p < 0.0001, and a significant physical activity × LPS dose interaction, F_(4,24) = 4.07, p < 0.01. IL-1β gene expression was significantly higher in microglia of locked wheel rats than that of runners at the 0 ng (p < 0.002), 0.1 ng (p = 0.05), 1 ng (p < 0.01), and 10 ng (p < 0.05) doses, but not the 100 ng dose (p > 0.05). c, IL-6. There was no significant main effect of physical activity, F_(1,24) = 4.35, p > 0.05, a significant main effect of LPS dose, F_(4,24) = 117.19, p < 0.0001, and a significant physical activity × LPS dose interaction, F_(4,24) = 7.35, p < 0.0005. IL-6 gene expression was significantly higher in microglia of locked wheel rats than that of runners only at the 100 ng dose (p < 0.005).