| Objective:Chronic cerebral hypoperfusion(CCH),which is a common event in elderly people,has been identified as a notable risk factor of dementia in patients with cerebrovascular disease.However,its underlying mechanism is poorly understood and no effective treatment is available.The present study is designed to explore learning and memory impairment related the change of silent synapses and dendritic spine density in hippocampal CA1 by direct visualization in a rat model of CCH.Material and method:Adult male Sprague–Dawley rats were randomly divided into control group and cerebral ischemia group,and then divided into 4 subgroups,that is 1 week,4 weeks,12 weeks and 24 weeks group.The rats were subjected to permanent bilateral common carotid artery occlusion(BCCAO)to reproduce CCH.The learning and memory effects of CCH were tested in Morris water maze and object recognition task(ORT)at 1,4,12 and 24 weeks after BCCAO,followed by immunofluorescent confocal microscopy to detect glutamate receptor 2(GluR2)and N-methyl-D-aspartate receptor 1(NMDAR1)subunits colocalized with synaptophysin,Golgi-Cox staining to observe dendritic spine density.Results:1.BCCAO rats exhibited recognition memory deficit from 4 weeks persisted to 24 weeks after surgery,spatial learning and memory,working memory impairment from 1 week persisted to 24 weeks after surgery;2.There was about 20% silent synapse(puncta containing only NMDAR1/synaptophysin without GluR2 colocalization)in adult hippocampus of rats.The results showed approximately 20% NMDAR1-only puncta(19.46±1.63%,20.19±1.31%,19.33±1.66%,and 19.95±1.11% at time points of 1,4,12,and 24 weeks,respectively)in the control subgroups;3.The percentage of puncta containing only NMDAR1/synaptophysin(without GluR2 colocalization,in contact with synaptophysin puncta)increased by 29.81–55.08%(25.26±1.08% vs.19.46±1.63%,29.43±1.52% vs.20.19±1.31%,30.37±1.21% vs.19.33±1.66%,29.48±1.34% vs.19.95±1.11% at time points of 1,4,12,and 24 weeks,respectively)compared with controls(P<0.001),and the number of NMDAR1/synaptophysin puncta also increased at 4,12,and 24 weeks compared with controls(P<0.01).Accordingly,the total number of puncta triple-labeled for GluR2-NMDAR1-synaptophysin was significantly decreased from 4 weeks after surgery(3.57±0.82 vs.4.13±1.25,P>0.05;3.50±0.74 vs.4.13±1.01,P=0.005;3.22±0.72 vs.3.97±0.81,P<0.001;3.37±0.93 vs.4.50±1.17,P<0.001 at time points of 1,4,12,and 24 weeks,respectively).These results suggested that BCCAO increased the number of silent synapse,but decreased the expression of active ones;4.We used Golgi staining to examine whether the increase in the number silent synapse reflected a decrease in dendritic spine density.Compared to control groups,dendritic spine density in hippocampal CA1 of BCCAO groups significantly decreased(11.90±1.69 vs.13.46±2.42,9.83±1.93 vs.13.63±2.71,8.65±2.22 vs.13.25±2.82,8.85±2.00 vs.13.85±2.95 at time points of 1,4,12,and 24 weeks,respectively,P<0.001).Dendritic spine density progressively decreased from 1 week to 12 weeks after surgery(4 weeks compared with 1 week,P<0.001;12 weeks compared with 4 weeks,P<0.01)and stabilized from 12 weeks to 24 weeks after surgery in the model group(24 weeks compared with 12 weeks,P=0.924).This observation suggested that dendritic spine density was significantly disrupted under BCCAO.Conclusion:This study suggests CCH can induce long-lasting cognitive deficit,which accompanied by persistent increase in silent synapse,as well as the decrease in dendritic spine density is correlated with the decrease in the number of functional synapse.The results suggest a potential mechanism by which CCH can induce learning and memory dysfunction. |
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