Effects Of Short-term Running Exercise On The Myelinated Nerve Fibers And Capillaries In The Brain Of Middle Aged Rats

Aging is an inevitable physiological process. The brain function, especially the cognitive function, is declined with aging. Exercise is a facile and convenient intervention to reduce the age-related cognitive decline and delay the onset of various neurodegenerative disorders. However, the underlying mechanism of the exercise-induced improvement of brain function is still unknown. Hippocampal formation, which is one of the most vulnerable regions to the process of aging, is structurally and functionally changed after exercise. Exercise increases neurogenesis, synaptic plasticity and dendritic complexity in hippocampal formation. The myelinated nerve fibers are responsible for the fast nerve conduction in the hippocampal formation. The exercise-induced changes of the myelinated nerve fibers in the hippocampal formation may contribute to the exercise-induced changes in cognitive function. Until now, however, there are no quantitative studies investigating the effects of exercise on the myelinated fibers in the hippocampal formation. Capillaries provide oxygen and nutrients for the nervous tissue and play an important role in normal neural activities. The changes of the capillaries in brain must influence brain function. Until now, in the researches on the effects of exercise on the capillaries of cerebral cortex, we could not find any study that used stereological methods to make three-dimensional quantification of capillaries. The obtained results in previous studies were the density values of capillaries. However, the density values cannot precisely represent the total quantities of capillaries. In the present study, therefore, we used 14-month Sprague-Dawley rats as a middle-aged animal model to study the effects of 4-month treadmill running on the myelinated nerve fibers in the hippocampal formation and the capillaries in the cerebral cortex with new stereological methods.PARTⅠEFFECTS OF SHORT-TERM RUNNING EXERCISE ON THE HIPPOCAMPAL FORMATION AND THE MYELINATED FIBERS THEREIN OF MIDDLE-AGED RATS1. Materials and methods1.1 Ten 14-month female Sprague-Dawley rats (280±20 g) were randomly divided into two groups: 5 rats in sedentary group and 5 rats in exercise group.1.2 Exercise rats were forced to run on a motorized treadmill for 20 min/day in 5 consecutive days per week for 4 months. Rats ran at a speed of 10 m/min in the first week, 15 m/min in the second week and 20 m/min from the third week to the end of the forth month. The sedentary rats were without any exercise.1.3 Spatial learning capacity was examined with a Morris water maze task for consecutive five days with hidden platform trials in first four days and visible platform trials on the fifth day.1.4 The rats were anaesthetized and perfusion-fixed. One hemisphere was sampled from each rat brain. From the sampled hemisphere, 5 hippocampal formation blocks (1 mm3) were sampled in a systematic random fashion. One 60-nm section was cut from each sampled block. From each cut section, 15 fields of view were randomly photographed under TEM with the magnification of 9000.1.5 The volume of the hippocampal formation, the length density and the total length of the myelinated fibers, the volume density and the total volume of the myelinated fibers and the mean diameter of the myelinated fibers in the hippocampal formation were investigated with the stereological methods.2. Results2.1 In the hidden platform trials, the time latency of the exercise group was significantly decreased when compared to the sedentary group (p<0.05). In the visible platform trials, there was no significant difference in the time latency between the exercise group and sedentary group.2.2 The volume of the hippocampal formation was 82.7±6.5 mm3 in the exercise group and 74.4±4.6 mm3 in the sedentary group. When compared to the sedentary group, the hippocampal formation volume in the exercise group was significantly increased by 11.2% (p<0.05).2.3 The total length of the myelinated fibers in the hippocampal formation was 15.1±1.7 km in the exercise group and 8.3±1.1 km in the sedentary group. The total length of the myelinated fibers in the hippocampal formation in the exercise group was significantly increased by 81.9% (p<0.01) when compared to the sedentary group.2.4 The total volume of the myelinated fibers in the hippocampal formation was 7.1±1.6 mm3 in the exercise group and 5.7±1.4 mm3 in the sedentary group. There was no significant difference in the total volume of the myelinated fibers in the hippocampal formation between the exercise group and the sedentary group (p>0.05).2.5 The mean diameter of the myelinated fibers in the hippocampal formation was 0.66±0.04μm in the exercise group and 0.80±0.09μm in the sedentary group. When compared to the sedentary group, the mean diameter of the myelinated fibers in the hippocampal formation was significantly decreased by 17.5% (p<0.05).PARTⅡEFFECTS OF SHORT-TERM RUNNING EXERCISE ON THE CORTEX AND THE CAPPILARIES IN THE CORTEX OF MIDDLE-AGED RATS 1. Materials and methods1.1 Ten female 14-month and ten male 14-month Sprague-Dawley rats (280±20 g) were randomly divided into two groups: sedentary group (5 females and 5 males) and exercise group (5 females and 5 males).1.2 Exercise rats were forced to run on a motorized treadmill for 20 min/day in 5 consecutive days per week for 4 months. Rats ran at a speed of 10 m/min in the first week, 15 m/min in the second week and 20 m/min from the third week to the end of the forth month. The sedentary rats were without any exercise.1.3 The rats were anaesthetized and perfusion-fixed. One hemisphere was randomly sampled from each rat brain and then coronally cut into 2-mm consecutive slabs. One 4-μm isotropic, uniform and random section was cut from each slab. The sections were stained with rabbit polyclonal anti-collagen IV antibody. From each section, 4~6 fields of view in the cortex were randomly photographed under an oil objective lens with the magnification of 100.1.4 The volume of the cerebral cortex, the length density and the total length of the capillaries in the cortex, the volume density and the total volume of the capillaries in the cortex, the surface area density and the total surface area of the capillaries in the cortex and the mean diameter of the capillaries in the cortex were investigated with the new stereological methods. 2. Results2.1 In female groups, the volume of the cortex was 582.5±37.1 mm3 in the exercise group and 474.2±75.3 mm3 in the sedentary group. The volume of the cortex was significantly increased in the exercise group by 22.8% (p<0.05) when compared to the sedentary group. In male groups, the volume of the cortex was 605.0±23.1 mm3 in the exercise group and 598.8±51.4 mm3 in the sedentary group. There was no significant difference in the volume of the cortex between the exercise group and sedentary group (p>0.05).2.2 In the female groups, the total length of the capillaries in the cortex was 299.5±42.6 m in the exercise group and 235.1±27.1 m in the sedentary group. When compared to the sedentary group, the total length of the capillaries in the cortex of the exercise group was significantly increased by 27.4% (p<0.05). In male groups, the total length of the capillaries in the cortex was 425.1±34.9 m in the exercise group and 353.2±81.9 m in the sedentary group. There was no significant difference in the total length of the capillaries in the cortex between the exercise group and sedentary group (p>0.05).2.3 In the female groups, the total volume of the capillaries in the cortex was 9.1±1.0 mm3 in the exercise group and 8.0±1.2 mm3 in the sedentary group, with no significant difference between the two groups (p>0.05). In the male groups, the total volume of the capillaries in the cortex was 9.7± 2.5 mm3 in the exercise group and 9.4±1.9 mm3 in the sedentary group, with no significant difference between the two groups (p>0.05).2.4 In the female groups, the total surface area of the capillaries in the cortex in the exercise group was 51.4±6.3 cm2 and 41.8±5.4 cm2 in the sedentary group. When compared to the sedentary group, the total surface area of the capillaries in the cortex of the exercise group was significantly increased by 23.0% (p<0.05). In male groups, the total surface area of the capillaries in the cortex was 65.1±11.6 cm2 in the exercise group and 62.0±9.6 cm2 in the sedentary group, with no significant difference between the two groups (p>0.05).2.5 In female groups, the mean diameter of the capillaries in the cortex was 5.5±0.4μm in the exercise group and 5.6±0.2μm in the sedentary group, with no significant difference between the two groups (p>0.05). In the male groups, the mean diameter of the capillaries in the cortex was 4.9±0.5μm in the exercise group and 5.7±1.0μm in the sedentary group, with no significant difference between the two groups (p>0.05).GENERAL CONCLUSIONS1. The present study for the first time quantitatively study the effects of short-term running exercise on the myelinated nerve fibers in the hippocampal formation as well as on the capillaries in the cortex of middle-aged rats by the use of new stereological techniques.2. The present study confirmed that short-term running exercise remarkably improved the spatial learning capacity of the middle-aged female rats.3. The present study found that there were sex differences in the effects of short-term running exercise on the capillaries in the cortex of middle-aged rats.4. The present study for the first time found that the total length of the myelinated fibers in the hippocampal formation as well as the total length of the capillaries and the total surface area of the capillaries in the cerebral cortex of the middle-aged female rats was significantly increased after running exercise, when compared to the sedentary rats. The running exercise-induced increases of the myelinated fibers in the hippocampal formation and the capillaries in the cortex might be the structural basis for the exercise-induced improvement in the spatial learning capacity of the middle-aged female rats. These results provide an important scientific basis for the future studies that search for the behavioral strategies to delay the progress of brain aging.