Age-Related Changes Of The Myelinated Nerve Fibers In The Hippocampal Formation And The Effects Of Running Exercise And Enriched Environment On The Spatial Learning Capacity Of Rats

With life's prolonging, the impacts of senescence have been an important problem that we have to face, especially the senescence of the brain. Hippocampal formation is especially vulnerable to the process of aging in the brain. Due to the fact that hippocampus plays important role in the learn and memory functions and it is especially vulnerable to aging process, the age-related changes of hippocampus have been the focus of the neuroscience researches. For example, there have been a lot of studies on the age-related changes of hippocampus volume and on the age-related changes of the total number of the neurons and synapses in hippocampus. However, with the help of modern design-based stereological methods, the acceptance of neuronal death and synapse loss as substrates of the age-dependent senility was challenged. What leads to the decline of the brain function in normal aging needs to be investigated. Nerve fibers, especially the myelinated nerve fibers, are essential for the nerve impulse conduction in the hippocampal formation. Therefore, the changes of the myelinated fibers during normal aging are likely to influence the cognition. However, there have been no reports of the quantitative study on the myelinated fibers in the hippocampal formation. Tang and Nyengaard for the first time used the stereological methods to quantify the myelinated fibers in the white matter of aged brain so that effectual methods for the quantitation of the nerve fibers in three dimensions were given. Therefore, the present study was designed to investigate the age-related changes of the myelinated fibers in the hippocampal formation of Long-Evans rat brain by the use of stereological techniques and transmission electronic microscope technique. The aging brains have more plasticity than we think. At present, more and more investigators found that exercise and beneficial social activities are helpful to inhibit the aging process. Therefore, we used SD rats to study if the running exercise and enriched environment can delay the process of the age-related decline of brain functions. The structural basis for the effects of the running exercise and enriched environment on the spatial learning capacity are being investigated.PARTⅠ1. Materials and methods1.1 Five young (6-month-old), five middle-aged (18-month-old) and six aged (28-month-old) female Long-Evans rats were used. Body weight is 280±20 g.1.2 One hemisphere was sampled from each rat brain. From the sampled hemisphere, 5 or 6 hippocampal formation blocks were sampled in a systematic random fashion. One ultra-thin section was cut from each sampled block. From each cut section, 15 fields of view were randomly photographed under TEM with the magnification of 7000.1.3 The volume of the hippocampal formation, the length density and total length of the myelinated fibers in the hippocampal formation, the volume density and total volume of the myelinated fibers in the hippocampal formation and the mean diameter of the myelinated fibers in the hippocampal formation were investigated with the new stereological methods.2. Results2.1 The volume of the hippocampal formation was 85.9±14.0 mm3 in young rats, 95.6±22.5 mm3 in middle-aged rats and 79.5±10.1 mm3 in aged rats. The differences among three groups were non-significant (p>0.05).2.2 The total length of the myelinated fibers in hippocampal formation was 9.1±3.8 km in young rats, 14.9±5.3 km in middle-aged rats and 7.8±2.4 km in aged rats. When compared to young rats, the total length of the myelinated fibers in middle-aged rats was significantly increased by 63.6% (p < 0.05). The total length of the myelinated fibers in aged rats was significantly decreased by 47.5% when compared to middle-aged rats (p < 0.05). The total length of the myelinated fibers in hippocampal formation was not significantly difference between young and aged rats (p>0.05).2.3 The total volume of the myelinated fibers in hippocampal formation was 4.3±1.2 mm3 in young rats, 5.1±1.0 mm3 in middle-aged rats and 4.4±1.4 mm3 in aged rats, with no significant differences among three groups (p>0.05).2.4 The mean diameter of the myelinated fibers in hippocampal formation was 0.74±0.07μm in young rats, 0.70±0.10μm in middle-aged rats and 0.75±0.04μm in aged rats. The differences were non-significant (p>0.05).PARTⅡ1. Materials and methods1.1 Running exercise1.1.1 Twenty 14 month-old SD rats (10 females and 10 males) were randomly divided into exercise group (5 females and 5 males) and controlled group (5 females and 5 males). So were twenty 24 month-old SD rats (10 females and 10 males).1.1.2 Rats of the exercise groups were made to run on treadmills for 20 min once a day for 5 consecutive days in a week, whereas those of the controlled groups were not subject to any exercises. The exercise load consisted of running at a speed of 10 m/min in the first week, at 15 m/min in the second week, and at 20 m/min from the third week to the end of the forth month.1.1.3 Spatial learning capacity was examined in 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. Time latency was recorded.1.2 Enriched environment1.2.1 Forty 14 month-old SD rats (20 females and 20 males) were randomly divided into enriched environment (EE) group (10 females and 10 males) and standard environment (SE) group (10 females and 10 males). So were twenty 24 month-old SD rats (20 females and 20 males).1.2.2 Female and male rats of EE groups were housed in large wooden cages of 120×60×50 cm3, respectively. Food and water were available. These cages were equipped with various objects during enrichment session of four months, including pipes and toys. The objects were changed, recombined and placed in different locations once a week at the time of cage cleaning. In SE groups, three or four rats were housed in a small wired cage of 40×30×30 cm3 without any other objects than food and water.1.2.3 Spatial learning capacity was examined in 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. Time latency was recorded.2. Results2.1 Running exercise2.1.1 The time latency in hidden platform trials of 14 month-old female exercised rats was significantly shorter than that of controlled rats (p<0.05), while the time latency in visible platform trials between those groups was non-significantly different. The time latencies in hidden platform trials and in visible platform trials were both non-significantly different between 14 month-old male exercised rats and controlled rats (p>0.05).2.1.2 The time latencies in hidden platform trials and in visible platform trials were both non-significantly different between 24 month-old female exercised rats and controlled rats. Neither were the time latencies between24 month-old male exercised rats and controlled rats (p>0.05).2.2 Enriched environment2.2.1 The time latency in hidden platform trials of 14 month-old female enriched rats was significantly shorter than that of controlled rats in SE group (p<0.05), while the time latency in visible platform trials between those groups was non-significantly different. The time latencies in hidden platform trials and in visible platform trials were both non-significantly different between 14 month-old male enriched rats and controlled rats in SE group.2.2.2 The time latencies in hidden platform trials and in visible platform trials were both non-significantly different between 24 month-old female enriched rats and controlled rats. Neither were the time latencies between24 month-old male enriched rats and controlled rats (p>0.05).General Conclusions1. The present study for the first time in the world combined the new stereological techniques and transmission electronic microscope technique to quantitatively study the age-related changes of the myelinated fibers in the hippocampal formation of female rat brain.2. The present study for the first time found that the total length of the myelinated in the hippocampal formation was significantly decreased during normal aging.3. Although the reason why the total length of the myelinated fibers in the hippocampal formation of middle-aged rats was significantly longer than that of young rats needs further investigations, the present results together with our previous findings in white matter and cortex further suggested that there are age-related changes of the myelinated fibers in normal aged brain.4. The present study provided the normal data of the hippocampal formation volume, the total length, total volume and mean diameter of the myelinated fibers in the hippocampal formation of female Long-Evans rats. These data will provide the important baseline value to compare with future studies of the effects of the neurodegenerative diseases in CNS on the hippocampal formation and the myelinated fibers in hippocampal formation of this animal model.5. Running exercise and enriched condition remarkably improved the spatial learning capacity of middle-aged female rats. This result provided an important theoretical basis for the application of similar strategy to delay the progress of brain function decline with aging.