Experimental Research On Establishment Of Mild Cognitive Impairment Models In Elderly Rats And Effect Of Folic Acid On Its Learning And Memory

Backgroud Mild cognitive impairment refers to the clinical condition between normalaging and AD in which persons experience memory loss to a greater extent than onewould expect for age, yet they do not meet currently accepted criteria for clinicallyprobable AD. About10%^-15%of patients with MCI develop to AD each year, so MCIwill become high-risk group for dementia. Due to the irreversibility progess and badtreatment effect, we have focused our studies on the early prevention and positivetreatment of MCI. The purpose of the subject is not only to establish MCI models inaging rats and study the pathophysiological features of them, but also to investigate themechanism of folic acid on the improvement of its learning and memory abilities.This study was divided into the following two parts:PartⅠ Establishment and evaluation of mild cognitive dysfunction models inelderly ratsMethods Totally40SD rats（14to18-month-old） were randomly divided into2groups: the model group（n=20） and the sham operation group（n=20）. Bilateralcarotidartery stenosis was operated in the model group while bilateral carotidartery wasseperated with no bilateral narrowing in the shamoperation group.30days after theoperation, Morris water maze test was performed, pathomorphological and electron microscopic observations of the cerebral tissue were examined and the expression ofprotein-coupled receptor kinase2in hippocampus tissue were detected by reversetranscription polymerase chain reaction（RT-PCR） and Western blotting.Results The mortality in model grouprats was only10%. Pathological morphologyand ultrastructure showed that hippocampal tissue structure was almost normal in shamoperated group, but in model group, hippocampal CA1pyramidal cells were ischemicdemyelination, arranged loose,and part of the cells appeared nucleus pyknosis, deeplystained; there was no obvious infarct in white matter, part of the white matter fiberbecame thinner and disorder,nucleolus became smaller and steped aside, cytoplasmicelectron density increased, appeared lipofuscin occasionally; Rough endoplasmicreticulum and Golgi expanded, cytosolic free ribosomes increased, part of mitochondriabecame swelled, vacuolated. Morris water maze test result showed that the averageescape latency in model group was longer than sham operated group（P<0.05）. In spatialprobe test,the average time of crossing the first original platform in model rats wassignificantly longer than the sham operated group[（36.80±7.68）s vs （20.87±6.16） s,（P<0.05）]. The average number of crossing the original platform in60seconds in modelgroup was significantly less than the sham operated group[（1.43±0.51） vs （3.10±1.45）,（P<0.05）]. The expression of GRK2mRNA and protein in the hippocampus wassignificantly increased in model group rats than the sham group（P<0.05）.Conclusions （1） The model of severe CCA stenosis in elderly rats can be applied forMCI animal models with good stability and repeatability. Compared with shamgroup,the cells morphology and ultrastructure in model group appeared more obviouspathological changes and mild impairments in cognitive function.（2） GRK2may playan important role in the development of MCI. Part ⅡThe mechanism of folic acid for improving the learning and memoryabilities of aging rats with mild cognitive impairment.Methods Forty aging male SD rats were randomly divided into four groups consistingof a sham-operated contorl（SC） group, a model control（MC） group, a low dose（LD）group and a high dose（HD） group（n=10in each group）. A aging rat model of MCI wasestablished by means of incompletely bilateral carotid occlusion.30days later, Morriswater maze was taken to eliminate out the rats which do not conform to the standards ofMCI. LD group(low-dose folic acid4mg·kg^-1·d^-1) and HD group(high-dose folic acid12mg·kg^-1·d^-1) were given intragastric administration with folic acid tablets dissovled insaline daily for30days. The rats in SC group and MC group were given intragastricadministration with the same dose of saline daily also for30days. Morris water mazewas taken again to messure its learning and memory abilities on the30th day ofintragastric administration. Fresh hippocampal tissues were taken out after Morris watermaze, so was that serum folic acid was measured by chemical immune assay andplasma homocysteine measured by high performance liquid chromatography. GRK2mRNA and protein expression changes in hippocampal tissues were observed by usingreverse transcription polymerase chain reaction and western blot.Results In place navigation test, escape incubation period to find hidden platform ofHD and LD groups were all significantly shorter than MC group. The same indexes inHD group were shorter than LD group. In spatial probe test, times across the hiddenplatform and the percentage of swimming time in the platform of HD and LD groupswere all significantly higher than MC group. The same indexes in HD group werehigher than LD group. Levers of serum folic acid concentration were significantlyhigher in HD and LD groups than those in MC group. The same indexe in HD groupwere higher than LD group. Plasma homocysteine concentrations were obviously lower in HD and LD groups than those in MC group. The same indexe in HD group werelower than LD group. GRK2mRNA and protein expression were obviously higher inMC group than those in HD and LD groups. The same indexe in HD group were lowerthan LD group.Conclusions （1）Supplementation of folic acid can increase sertum folic acidconcentration in aging MCI rats.（2） Supplementation of folic acid improve the abilityof learning and memory. The mechanism may be related with reducing plasmahomocysteine concentration and decreasing GRK2gene and protein expression inhippocampal tissues.