| Background and objective: Exposure to chronic stress is known to precipitate or exacerbate many neuropsychiatric disorders such as depression and PTSD. Although the responsible brain sites in the pathogenesis of these disorders have been obscure, the hippocampus is thought to be involved. Indeed, chronic stress in rats induces atrophy of apical dendrites of the pyramidal neurons in the hippocampal CA3 subfield. In addition, these stressed rats show depression- or PTSD-like behavior. Since the hippocampal formation is the important structure in brain involving memory, emotion and cognitive function, and these functions are impaired in patients with depression or PTSD, the stress-induced hippocampal atrophy may be implicated in the pathogenesis of these disorders. Several morphological studies suggest that this atrophy is caused by the damage of cytoskeleton such as microtubule, microfilament and intermediate filament. Among them, microtubule is the most important segment of cytoskeleton in dendrites of neurons. Therefore, the damage of it may be involved in atrophy of apical dendrites of the pyramidal neurons. The purpose of this study is to clarifythe mechanism of the chronic stress-induced atrophy of the hippocampal neurons. To this point, we investigated the effects of chronic stress on the morphology of microtubules, the level of phosphorylated MAP2 in dendrites and the concentration of [Ca2+]i in the hippocampal CA3 pyramidal neurons.Materials and methods: Male SD rats were divided into two groups: control and stress. Rats in the stress group were subjected to forced-swimming for a 20-minute a day for four weeks, the diameter and length of dendrite shaft of the rat hippocampal CA3 pyramidal neurons were measured by using Golgi staining. The ultrastructure of dendrite, especially, microtubules of rat hippocampal CA3 pyramidal neurons were observed by using an electron-microscope. The level of phosphorylated MAP2 of rat hippocampal CA3 pyramidal neurons was measured quantitatively by using the immunohistochemistry method and the computerized image technique. LSCM was used to measure the concentration of [Ca2+]i in the CA3 pyramidal neurons.Result: The diameter of dendritic shaft of the hippocampal CA3 pyramidal neurons was significantly thicker in the stress group (6.40um ±0.94um) than that in the control group (5.65um±0.85um) (p<0.05); the length of them (125.35um±18.69um) was shorter than that in the control group (98.51um ± 17.48um) (p<0.01). The following ultrastructural changes of dendrites were observed in the hippocampalCA3 pyramidal neurons of the stress group: breaking into fragmentation of microtubule, widening of spacing between parallel microtubules in longitudinal-section, irregularity of annular and maldistribution of monome in cross-section, crista clouding or degeneration as vacuoles of mitochondrion.The average gray degree of phosphorylated MAP2 in hippocampal CA3 pyramidal neurons was significantly lower in the stress group (145 ±4.40) than that in the control group (149±1.81) (p<0.05). As the average gray degree is correlated negatively with expression of intensity, the decrease of phosphorylated MAP2 average gray degree in stress group indicates that the chronic stress enhances the phosphorylation of MAP2. Moreover, the number of phosphorylated MAP2 positive pyramidal neurons was significantly decreased in the stress group (40.36 ±1.35) compared to the control group (42.73 ± 1.56) (p<0.05).The fluorescent intensity of [Ca2+]i in hippocampal CA3 pyramidal neurons was significantly stronger in the stress group (1548.83 ± 118.53 ) than that in the control group (598.71 ±48.02) (p<0.01), which indicatedthat chronic stress increase the concentration of [Ca2+ ]i in the pyramidal neurons.Conclusions: Chronic stress could damage the microtubules of dendrites leading to dendritic shaft retraction in rat hippocampal CA3 pyramidal neurons. In addition, chronic stress enhances phosphorylationof MAP2 and increases the concentration of [Ca2+]i in the CA... |
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