幼期相对低剂量X线辐射对小鼠成年后学习记忆及海马神经发生的影响

目的研究幼期相对低剂量电离辐射对小鼠成年后学习记忆及海马神经发生的影响,并探讨相对低剂量电离辐射、学习记忆和神经发生之间的关系。方法出生21 d的昆明小鼠20只,随机对半分为对照组(假辐射)、辐射组(单次5 Gy全身辐射)。辐射前、2月龄、4月龄分别记录每只小鼠体重。4月龄分别对两组小鼠行神经行为学实验;实验结束后对每只小鼠灌注固定取脑后行冰冻切片、免疫组化染色实验,检测海马齿状回标记增殖神经元的Ki67蛋白、标记迁移分化神经元的DCX蛋白、标记成熟神经元的CB、PV蛋白的阳性神经元的数量。结果(1)2月龄、4月龄辐射组小鼠体重增加百分比低于对照组(t=3.02,P<0.05;t=4.22,P<0.01)。(2)旷场实验中5 min内自由探索的总路程、新物体识别实验中对新旧物体的识别记忆能力、条件性恐惧实验中环境关联性僵直时间和线索关联性僵直时间、Morris水迷宫定位航行实验中逃避潜伏期和空间探查实验中穿越平台次数,两组小鼠均无统计学差异(t=-1.672,P>0.05;t=0.753,P>0.05;t=-0.885,P>0.05;t=0.15,P>0.05;F=2.30,P>0.05;t=-1.07,P>0.05)。(3)免疫组化检测,辐射组小鼠海马齿状回区Ki67标记的增殖神经元、DCX标记的分化神经元、CB和PV标记的中间成熟神经元数量均低于对照组(t=2.80,P<0.05;t=6.63,P<0.001;t=3.75,P<0.01;t=5.39,P<0.001)。结论幼期相对低剂量(5 Gy)电离辐射会导致小鼠体重增加缓慢;小鼠幼期相对低剂量电离辐射并未造成其成年后学习记忆障碍,但海马神经发生能力减弱;电离辐射引起神经发生的减弱包括破坏神经元的增殖、分裂、分化、迁移、成熟等一系列复杂渐近性过程。

Effects of relatively low dose of X-rays irradiation on adult cognition and hippocampal neurogenesis in juvenile mice

Objective To investigate the effects of relatively low dose of ionizing radiation on adult cognition and hippocampal neurogenesis in juvenile mice,and to illuminate the relationship between relatively low dose of irradiation,cognitive function and neurogenesis.Methods 20 Kunming mice were randomly divided into two groups at postnatal 21 days:a radiation group and a control group(P21)(n=10 each group).The radiation group was exposured with an acute whole body irradiation with a dose of 5 Gy at P21,the normal control group was sham irradiated.The two groups of mice were weighed before being exposured,at 2 months old and 4 months old,respectively.At 4-month-old,each mouse was experimented with open field test(OPT),new object recognition experiment(NOR),fear conditioning test(FC)and Morris Water Maze test(MWM).Brain sections were then immunostaining for different markers including Ki67(mark proliferation neuron),doublecortin(DCX,mark differentiation and migration neuron),calbindin(CB,mark interneuron),and parvalbumin(PV,mark interneuron)in hippocampal dentate gyrus(DG).Results(1)In the body weight test,both at 2 and 4 months of age,the percentage of mice’body weight gain in the radiated group was significantly lower than that of the control group(t=3.02,P<0.05;t=4.22,P<0.01).(2)There were no significant differences in the distance traveled within 5 min in OPT(t=-1.672,P>0.05),the DI in NOR(t=0.753,P>0.05),the contextual memory related freezing time(t=-0.885,P>0.05)and the clued memory related freezing time(t=0.15,P>0.05)in FC,the average escape latency(F=2.30,P>0.05)in MWM and the times of crossing platform(t=-1.07,P>0.05)between the two groups.(3)However,Ki67,DCX,CB and PV labeled neurons number in DG in irradiation group were significantly lower than that of the control group(Ki67,t=2.80,P<0.05;DCX,t=6.63,P<0.001;CB,t=3.75,P<0.01;PV,t=5.39,P<0.001).Conclusions(1)Relatively low doses(5 Gy)of ionizing radiation in juvenile stage cause slow body weight gain in mice.(2)Relatively low doses(5 Gy)of irradiation exposure to juvenile mice do not cause learning and memory disorder when mice were tested in adult,but it resulted in declining of neurogenesis in SGZ.(3)The decreasing of neurogenesis induced by irradiation includes injury of a series of processes such as proliferation,division,differentiation,migration and maturation of neurons.