X线照射大脑对幼鼠学习记忆能力的影响

目的：探讨多次临床剂量头颅X线检查对生长发育期大鼠学习记忆能力的影响。方法：取出生后21 d的SD大鼠54只，随机分为照射组（3 d照射组及7 d照射组）和对照组，每组18只。照射组大鼠于35 d龄时每天接受1次临床头颅CT检查剂量［以临床中婴幼儿（0～6岁）所采用剂量为基准，1次扫描剂量为285 mGy］的X线照射，其中3 d照射组连续照射3 d，7 d照射组连续照射7 d；对照组不作X线照射。待大鼠60 d龄时进行Morris水迷宫试验检测其学习记忆能力；之后处死大鼠，免疫荧光染色观察大鼠海马CA1区N-甲基-D-天冬氨酸2B亚基（NR2B）的表达，Fluoro-Jade B（FJB）染色观察神经元退变情况，蛋白质印迹法检测海马总NR2B、突触后膜致密物-95 （PSD-95）蛋白的表达，并通过电镜观察海马CA1区的超微结构。结果：与对照组比较，7 d照射组第1～4天逃避潜伏期均延长（P<0.01），原站台象限停留时间均缩短、穿越原站台次数均减少（P＜0.01）。7 d照射组海马CA1区退变神经元显著多于对照组（P＜0.01）。7 d照射组海马CA1区NR2B表达增强，海马总NR2B、PSD 95表达明显高于对照组（P＜0.05）。7 d照射组部分突触结构受损。3 d照射组与对照组比较，在Morris水迷宫试验各指标及海马CA1区退变神经元、NR2B表达和海马总NR2B、PSD-95表达方面差异均无统计学意义（均P>0.05）。结论：头颅X线检查对学习记忆能力损伤属于确定性效应，该损伤与海马区的NR2B、PSD-95过表达导致兴奋性氨基酸毒性作用有关。

Effect of cerebral X-ray irradiation on learning and memory function in young SD rats

Objective: To investigate the effect of cerebral X-ray irradiation on learning and memory function in young rats. Methods: Fifty-four SD rats aged 35 d were randomly divided into 3 groups with 18 in each group: rats in 3-d group and 7-d group received X-ray irradiation with a dose of 28.5 mGy/d for 3 d and 7 d, respectively; rats in control group received sham X-ray irradiation. Morris water maze （MWM）was tested when animals at age of 60 d; then the animals were sacrificed and brain samples were taken. The neurodegeneration was observed by Fluro-Jade B staining; the expression of N-methyl-aspartate （NMDA）receptors subunit 2B （NR2B）and postsynaptic density protein-95 （PSD-95）in the hippocampus were analyzed by immunofluorescence and Western blot methods, respectively, and ultrastructure of CA1 region was observed with electron microscopy. Results: No significant difference in 1-4 d escape latency as shown in MWM test was noted between 3-d group and control group （P＞0.05）; while the escape latency in 7-d group was significantly longer than that in control group （P <0.01）. No significant differences in lingering in the quadrant and the frequency of passing through the original platform between 3-d group and control group （P＞0.05）, while those in 7-d group were significantly lower than those in control group （P＜0.01）. Compared to control group, the number of FJB positive cells in 7-d group was increased （P＜0.01）; the expressions of NR2B and PSD-95 in hippocampus CA1 region were also increased （P＜0.05）. The ultrastructure observation in 7-d group showed that the synapse structure of some neurons was impaired. Conclusion: X-ray irradiation may affect learning and memory function of young rats, which is associated with overexpression of NR2B and PSD-95 in hippocampal regions.