The Age-Related Changes Of Myelin Sheath Of Myelinated Nerve Fibers In Rat Hippocampus And The Changes Of The Behavior And Hippocampus In Early Period Of Transgenic AD Mice

The hippocampal formation plays important role in learning ability and memory. Initially, the researchers found that hippocampal volume was decreased with age, and generally agreed that this change in hippocampal volume was mainly due to hippocampal neuron loss. Using new stereological methods, however, some researchers found that the number of hippocampal neurons did not have significant age-related changes. If there was no significant neuronal death in the age hippocampus, then, what is the neurobiological basis of the hippocampus-related functional decline in old age? With the increasing of the aging population, more and more attentions are paid to the age-related diseases such as Alzheimer's disease (AD). AD is a neurodegenerative disease, characterised by progressive cognitive and memory dysfunction. It is generally believed that senile plaques and neurofibrillary tangles are the characteristics of Alzheimer's disease pathology, but the large number of studies found that the pathological process seems not accompanied with clinical manifestations. What is the change of the learning-memory ability, the concentration of Aβprotein and the hippocampal structure in the early period of AD? What is the relationship among these changes? Therefore, the present study used the new stereological methods and transmission electron microscopic technique to investigate the age-related changes of the myelin sheaths of the myelinated nerve fibers in rat hippocampus and the relationship among the behavier, Aβprotein and volume of hippocampus in transgenic AD mice at different times. The present results would provide a scientific basis for understanding the pathogenesis of AD, particularly the early pathogenesis of AD. PARTâ… 1. Materials and methods1.1 16 female Long-Evans rats were divided into three groups: young group (6-month old, five rats), middle-aged group (18-month old, five rats) and aged group (27-month old, six rats).1.2 One hemisphere was sampled from each rat brain. From the sampled hemisphere, 5 6 tissue blocks were sampled from hippocampus in a systematic random fashion. From each cut section, 15 fields of view were randomly photographed under TEM with a magnification of 9000.1.3 The mean diameter of the myelinated fibers and the mean diameter of the axons, the inner perimeter, outer perimeter and mean thickness of the myelin sheaths in the myelinated fibers of hippocampus were investigated with the new stereological methods and transmission electron microscope technique.2. Results2.1 When compared to middle-aged rats, the mean diameter of the myelinated fibers in the hippocampus of aged rats was significantly increased by 10.8%(p<0.05). The mean diameter of the myelinated fibers in the hippocampus was not significantly different between young rats and aged rats(p>0.05).2.2 The mean thickness of the myelin sheaths of the myelinated fibers with the diameter of less than 0.3μm in the middle-aged rat was significantly increased by 45.8%(p<0.05)when compared to young rats, and that in the aged rat hippocampus was significantly decreased by 24.4%(p<0.05)when compared to middle-aged rats. The mean thickness of the myelin sheaths of the myelinated fibers with the diameter of 0.4⁰.5μm in the middle-aged rat was significantly increased by 26.2%(p<0.05)when compared to young rats, and that in the aged rat hippocampus was significantly increased by 21.4%(p<0.05)when compared to young rats. The mean thickness of the myelin sheaths of the myelinated fibers with the diameter of 0.5⁰.6μm in the middle-aged rat was significantly increased by 16%(p<0.05)when compared to young rats, and that in the aged rat hippocampus was significantly increased by 16%(p<0.05)when compared to young rats. The mean thickness of the myelin sheaths of the myelinated fibers with the diameter of 0.7⁰.8μm in the aged rat hippocampus was significantly increased by 14.8%(p<0.05)when compared to young rats.2.3 The difference between inner diameter and outer diameter of the myelin sheaths with the diameter of less than 0.3μm in the middle-aged rat was significantly increased by 29.6%(p<0.05)when compared to young rats, and that in the aged rat hippocampus was significantly decreased by 21.6%(p<0.05)when compared to middle-aged rats.2.4 The outer perimeter of the myelin sheaths with the diameter of less than 0.3μm in the middle-aged rat was significantly increased by 33.4%(p<0.05)when compared to young rats, and that in the aged rat hippocampus was significantly decreased by 17% ( p < 0.05 ) when compared to middle-aged rats. The outer perimeter of the myelin sheaths with the diameter of 0.8⁰.9μm in the middle-aged rat was significantly increased by 19.5%(p<0.05)when compared to young rats, and that in the aged rat hippocampus was significantly increased by 14.6%( p < 0.05 ) when compared to young rats.2.5 The inner perimeter of the myelin sheaths with the diameter of less than 0.3μm in the middle-aged rat was significantly increased by 32.5%(p<0.05)when compared to young rats, and that in the aged rat hippocampus was significantly decreased by 24.9%(p<0.05)when compared to middle-aged rats. The inner perimeter of the myelin sheaths with the diameter of 0.8⁰.9μm in the aged rat was significantly increased by 13.3%(p<0.05)when compared to young rats.2.6 The difference between inner perimeter and outer perimeter of the myelin sheaths with the diameter of more than 1.2μm in the middle-aged rat was significantly increased by 43.9%(p<0.05)when compared to young rats.2.7 The proportion of inner perimeter and outer perimeter of myelin sheaths with the diameter of 0.4⁰.5μm in the middle-aged rat was significantly increased by 13.2%(p<0.05)when compared to young rats, and that in the aged rat hippocampus was significantly decreased by 13.1%(p<0.05)when compared to middle-aged rats. PARTâ…¡1. Materials and methods1.1 Transgenic 2567 mice were used: 4 month-old group (14 mice), 6 month-old group (14 mice), 8 month-old group (20 mice) and 10 month-old group (12 mice) with age-matched control groups (non-transgenic mice).1.2 Spatial learning and memory capacity was examined with a Morris water maze for consecutive seven days with spatial learning and memory trials in first six days and spatial exploratory trials on the seventh day. Both the percentage of time that animals appear in the platform quadrant and the number of thoughing platform were recorded.1.3 6 mice were sampled from each group in a systematic random fashion after the Morris water maze trials. The concentration of Aβin hippocampus of each mice was calculated with Elisa method.1.4 The rest mice of each group were investigated with the new stereological methods. One hemisphere was sampled from each mouse brain, and hippocampus tissue blocks were sampled from the sampled hemisphere. The volume of hippocampus was calculated with the new stereological methods.2 Results2.1 In the 4 month-old, 6 month-old and 8 month-old mice, there were no significant differences of the searching platform latency (SPL) in the spatial learning and memory trials between the TG group and the control group, and so were in the spatial exploratory trials. In the 10 month-old mice, there were significant differences of the SPL in the spatial learning and memory trials between the TG group and the control group (p<0.05), while the SPL in the spatial exploratory trials between the two groups was non-significantly different.2.2 There was significant increase in the concentration of Aβ40 in hippocampus between the TG group and the control group in all month-old mice (p <0.05). In the 6 month-old mice, the concentration of Aβ₄₂ in hippocampus of the TG group was significantly increased compared to the control group (p <0.05). In the 4 month-old, 8 month-old and 10 month-old mice, the concentrations of Aβ₄₂ in hippocampus of the TG group were no-significantly increased when compared to the control group (p >0.05). The concentrations of Aβ40 and Aβ₄₂ protein in model mice were non-significantly different among the 4 month-old, 6 month-old, 8 month-old and 10 month-old mice.2.3 The volume of hippocampus was non-significantly different between the TG group and the control group.General Conclusions1. The present study for the first time combined the new stereological techniques and transmission electron microscope technique to quantitatively study the age-related changes of the myelin sheathes of the myelinaed nerve fibers in the hippocampus of normal rat brain.2. The present study provided the normal data of the mean diameter of myelinated fibers and the mean diameter of axons, the inner perimeter, outer perimeter and mean thickness of the myelin sheaths in the myelinated fibers of hippocampus in rat brain. These values will provide important reference for future studies.3. The present results indicated that there was the demyelination of the myelinated fibers with small diameter, as well as the continued myelination of the myelinated fibers with medium-size diameter in the hippocampus of normal aged brains.4. The present study provided the normal data of the behavier, Aβ protein and volume of hippocampus in transgenic AD mice at different times. These values will provide important reference for future studies.5. The present results indicated that there were no significant changes in the learning and memory of the 4 month-old, 6 month-old and 8 month-old transgenic AD mice, but there were significant decreases in the learning (no memory) of the 10 month-old transgenic AD mice. There was significant increase in the concentration of Aβ40 in hippocampus between the TG group and the control group in all month-old mice. In the 6 month-old mice, the concentration of Aβ₄₂ in hippocampus of the TG group was significantly increased compared to the control group, but in the 4 month-old, 8 month-old and 10 month-old mice, the concentrations of Aβ₄₂ in hippocampus of the TG group were no-significantly increased when compared to the control group. The volume of hippocampus was non-significantly different between the TG group and the control group.