Involvement Of Postoperative Cognitive Dysfunction In A Rat Model Of Single Intracerebroventricular Injection Of STZ

Background and objectiveAlzheimer’s disease(AD)is one of the most common neurodegenerative diseases with dementia as its clinical manifestation.With the acceleration of the aging process in China,the incidence rate of AD is increasing year by year,which brings serious harm to the society and families.The onset of the disease is insidious,the pathogenesis is still unclear,there is no effective treatment.It is an important topic in the field of Neurology to reveal the molecular mechanism of cognitive impairment in AD and subsequently to find effective prevention and treatment for this disease.Intracerebroventricular(icv)injection of streptozotocin(STZ)impairs spatial learning and memory in rats or mice,which can mimic some characteristics of most sporadic AD.Therefore,icv injection of STZ in rats or mice is a commonly used animal model for AD studies.Most studies revealed that STZ induced spatial learning and memory impairment occurred 4 weeks after injection,but a small number of studies suggested that STZ induced spatial learning and memory impairment occurred2 and 3 weeks after injection.Postoperative cognitive dysfunction(POCD)is a common complication characterized by postoperative neurocognitive dysfunction,which may occur between24 and 72 hours after surgery,and the peak incidence is 1 week after surgery.It is characterized by impaired memory,attention,language comprehension and social skills,but the specific pathogenesis is still unclear.It has not been reported whether the spatial learning and memory impairment in the second or third week after icv injection of STZ is POCD,and what is the possible mechanism involved in the impairment.Recent studies have shown that inflammasomes play a key role in the pathogenesis of both AD and POCD.The increased expression of inflammasomes can promote Tau phosphorylation and related inflammatory responses in the brain,which in turn leads to cognitive dysfunction.In this study,the AD rat model was established by single icv injection of STZ into the lateral ventricle.Morris water maze was used to detect the changes of spatial learning and memory ability of rats after injection of STZ.q RT-PCR was used to detect the m RNA expression level of some inflammasomes in hippocampus.The protein expression of inflammasomes,related inflammatory factors,and the phosphorylation level of Tau protein in hippocampus were detected by Western blot.This study will provide the possible molecular mechanism of the involvement of POCD in the early postoperative period of the AD model by icv injection of STZ.Methods1.To explore the cognitive function of rats injected with lateral ventricle STZThe rat model of AD was established by single icv injection of STZ(3 mg/kg).The rats were used as control group by icv injection of ACSF.At 2 and 3 weeks after injection,the spatial learning and memory ability of rats was tested by Morris water maze behavior test.2.To explore the possible mechanisms of cognitive dysfunction after icv injection of STZRats were sacrificed at different periods after STZ injection,and the hippocampal tissues were extracted.The m RNA levels of inflammasomes including NLRP1,NLRP3 and NLRC4 were detected by q RT-PCR.Western blot was used to detect the protein expressions of NLRP1,NLRP3,NLRC4,ASC,Pro casp-1,Casp1p12,Casp1 p10,IL-1β,IL-18 and the phosphorylation level of Tau-p S396.Results1.The expression of NLRP3,related proteins and Tau-p S396 in the hippocampus was increased in STZ group but not in ACSF group 72 hours after single icv injection.There were no significant changes in inflammasomes and other related indexes in the hippocampus of rats(see method for details)24 h after single icv injection of ACSF or STZ.72 hours after injection of STZ,the m RNA and protein expressions of NLRP3 in the hippocampus were increased.The protein expressions of ASC,Pro casp-1,Casp1 p12,Casp1 p10,IL-1β,IL-18 and Tau-p S396 were significantly up-regulated,while the m RNA and protein expressions of NLRP1 and NLRP4 were not significantly changed in STZ group 72 hours after icv injection.2.One week after single icv injection,Tau-p S396 in the hippocampus of both ACSF group and STZ group was significantly increased.There was no statistical difference between the groups about the protein expressions of NLRP3 and NLRP1 one week after single icv injection of ACSF or STZ.The m RNA and protein expression levels of NLRC4 in STZ group were significantly increased in STZ group.There was no significant difference in the protein expression levels of ASC,Pro Casp-1,Casp1 p12,Casp1 p10,IL-1β,IL-18 and total Tau among all groups.The expression of Tau-p S396 in the hippocampus of both ACSF group and STZ group was significantly increased.3.Two weeks after single icv injection,the level of Tau-p S396 in the hippocampus was significantly increased in STZ group,while the level of Tau-p S396 returned to normal level in ACSF group.Two weeks after single icv injection of ACSF or STZ,m RNA expressions of NLRP1,NLRP3 and NLRC4 were detected in the hippocampus of rats in each group,and there was no statistical difference between the groups.The protein expression of Tau-p S396 in the hippocampus was increased in STZ group,but there was no significant difference between ACSF group and non-operation group.The first Morris water maze test showed that the the times of crossing the platform and the percentage of time spent in the target quadrant were significantly lower in the STZ compared with ACSF group.However,no significant differences were found among the three groups in the second Morris water maze test of another batch of rats.4.Three weeks after single icv injection,the level of Tau-p S396 in the hippocampus of STZ group was significantly higher than that of ACSF group.There was no significant difference in the m RNA expression of NLRP1,NLRP3 and NLRC4 in the hippocampus between ACSF group and STZ group 3 weeks after single icv injection of ACSF or STZ.The level of Tau-p S396 in STZ group was significantly higher than that in ACSF group.Morris water maze test showed that there was no significant difference in spatial learning and memory ability between STZ and ACSF groups.Conclusion:In the animals with a single icv injection of ACSF,the level of Tau-p S396 increased significantly in the first week after injection,and returned to normal level in the second week after operation.In the animals with a single icv injection of STZ,the level of Tau-p S396 began to significantly increase 72 h after injection,and continued to be expressed at a high level for a long time(more than 3 weeks).A single icv injection can cause transient increase in the level of Tau-p S396,which may mediate the cognitive dysfunction after the icv injection.However,further study is needed to determine whether Tau phosphorylation is triggered by inflammasome.Our study indicates that postoperative cognitive dysfunction affects results of the spatial behavioral test in the early stage in icv-injected STZ animal model.Therefore,the behavioral test of this model should be performed at or above 4 weeks after the icv injection to avoid the influence of postoperative cognitive dysfunction.