Effect of cardiorespiratory exercise intervention on the volume of dentate gyrus and CA3 subfields of the hippocampus
Jo, Yongho Christopher
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Alzheimer’s disease (AD) is widely accepted as being linked with abnormal atrophy of the hippocampus. In the nonhuman-focused literature, the hippocampus has been identified as one of the prominent regions of interest with mechanisms of adult neurogenesis from aerobic exercise. Several human studies over the past decade have shown the effect of exercise that improves cardiorespiratory fitness on the size and function of the hippocampus in participants. However, the size of hippocampal subfields, especially the dentate gyrus (DG), has not been examined in humans even though various animal studies have identified the DG subfield as the primary region of adult neurogenesis induced by aerobic exercise. The point of this investigation, therefore, was to investigate the effect of an exercise intervention on the size of the DG subfield and the related subfield of cornu ammonis (CA) 3. The hypothesis was that an endurance training intervention, designed to improve cardiorespiratory fitness, would increase the volume of the DG and CA3 subfields of the hippocampus more than a resistance training intervention, designed to increase strength, flexibility, and balance, and that improvement in cardiorespiratory fitness would positively correlate with the change in volumes of these subfields. For this investigation, 32 participants (young adults from age 20 to 33 with sedentary lifestyles) were selected from a data set collected for an ongoing study by the Brain Plasticity and Neuroimaging (BPN) Laboratory at Boston University School of Medicine (Boston, MA, USA). The fitness data and T1-weighted and T2-weighted structural magnetic resonance imaging (MRI) data were used in the analysis. FreeSurfer v6.0 software was used to extract volumetric data of the hippocampal subfields using a hippocampal subfield segmentation algorithm. Analysis of variance (ANOVA) with repeated measures and linear regression were used to analyze the statistical significance of the results. The change in volumes for the whole hippocampus, DG, and CA3 did not show any statistically significant differences after endurance training compared with after resistance training. The effect of exercise on the volume of the CA3 subfield appeared to be asymmetrical from left to right, with heavier impact on the left CA3 than on the right CA3. There was no statistically significant correlation between the change in cardiorespiratory fitness and the change in volume of any of the regions analyzed. However, the left whole hippocampus showed a slight trend (p = 0.078; R = 0.317) of weak positive correlation between its volume change and the cardiorespiratory fitness change of the participants. This result was consistent with the previous human literature. Although statistically not significant, most data showed that the endurance training group saw more preservation or increase in volume. This result is encouraging and should be explored further to validate the efficacy of cardiorespiratory exercise as a possible prevention mechanism against AD for young adults later in life.