Modulatory Effect of Aerobic Physical Activity on Synaptic Ultrastructure in the Old Mouse Hippocampus

Abstract

Aerobic physical exercise (APE) leads to improved brain functions. To better understand the beneficial effect of APE on the aging brain, a morphometric study was carried out of changes in hippocampal synapses of old (>27 months) Balb/c mice undergoing treadmill training (OTT) for 4 weeks in comparison with old sedentary (OS), middle-aged sedentary (MAS) and middle-aged treadmill training (MATT) mice. The inner molecular layer of the hippocampal dentate gyrus (IMLDG) and the molecular stratum of Ammon's horn1 neurons (SMCA1) were investigated. The number of synapses per cubic micron of tissue (numeric density, Nv), overall synaptic area per cubic micron of tissue (surface density, Sv), average area of synaptic contact zones (S), and frequency (%) of perforated synapses (PS) were measured in electron micrographs of ethanol-phosphotungstic acid (E-PTA) stained tissue. Data were analyzed with analysis of variance (ANOVA). In IMLDG, an effect of age was found for Nv and Sv, but not S and %PS. Similar results were found for exercise and the interaction of age and exercise. In post hoc analysis Nv was higher (60.6% to 75.1%; p < 0.001) in MATT vs. MAS, OS and OTT. Sv was higher (32.3% to 54.6%; p < 0.001) in MATT vs. MAS, OS and OTT. In SMCA1, age affected Nv, Sv and %PS, but not S. The effect of exercise was significant for Sv only. The interaction of age and exercise was significant for Nv, Sv and %PS. In post hoc analysis Nv was lower in OS vs. MAS, MATT and OTT (-26.1% to -32.1%; p < 0.038). MAS and OTT were similar. Sv was lower in OS vs. MAS, MATT and OTT (-23.4 to -30.3%, p < 0.004). MAS and OTT were similar. PS frequency was higher in OS vs. MAS, MATT and OTT (48.3% to +96.6%, p < 0.023). APE positively modulated synaptic structural dynamics in the aging hippocampus, possibly in a region-specific way. The APE-associated reduction in PS frequency in SMCA1 of old mice suggests that an increasing complement of PS is a compensatory phenomenon to maintain synaptic efficacy. In conclusion, the modulation of synaptic plasticity by APE gives quantitative support to the concept that APE protects from neurodegeneration and improves learning and memory in aging.

Keywords: aging; exercise; hippocampus; physical activity; synaptic morphology.

Figure 1

Representative transmission electron micrographs of synapses in the mouse hippocampus. (A) The ethanol phosphotungstic acid (E-PTA) technique evidences the synaptic contact zones against an unstained background and enables reliable measurements of synaptic ultrastructural features. (B) Arrows indicate two perforated synaptic contacts: pre-synaptic apposition appears as a range of peaks, while the post-synaptic junction appears as a dark uniform line. Bars: 500 nm.

Figure 2

(A–D) Mean value ± standard error of the mean (SEM) of synaptic ultrastructural variables measured in the inner molecular layer of dentate gyrus (IMLDG) in middle-aged sedentary (MAS), middle-aged running (MATT), old sedentary (OS) and old treadmill-exposed (OTT) mice. Nv, numeric density of synapses; Sv, surface density of synapses; S, average size of synaptic contacts; PS, percent of perforated synapses. Asterisk indicates between-group statistically significant differences (p ≤ 0.05; Bonferroni’s post hoc test).

Figure 3

(A–D) Mean values ± SEM of synaptic ultrastructural variables measured in stratum moleculare of hippocampal Ammon’s horn1 pyramidal cells (SMCA1) in MAS, middle-aged running (MATT), OS and old treadmill-exposed (OTT) mice. Nv, numeric density of synapses; Sv, surface density of synapses; S, average size of synaptic contacts; PS, percent of perforated synapses. Asterisk indicates between-group statistically significant differences (p ≤ 0.05; Bonferroni’s post hoc test).