Swimming Alleviating Cognitive Impairment By Reducing Insulin Resistance In Obese Mice

Objective:With the development of the social economy and the improvement of people’s living standards,cognitive dysfunction caused by obesity has become a significant concern in the academic world.Cognitive dysfunction is an intermediate stage between normal and dementia and is considered to be reversible.Regular exercise or physical activity can enhance the cognitive function of the brain and improve age-related brain age-related degeneration and degenerative neurological diseases.Moreover,exercise to improve insulin sensitivity in tissues and organs has been widely studied,so exercise to improve insulin sensitivity in the hippocampal region of the brain may be one of the deeper mechanisms of exercise to improve cognitive dysfunction,which may provide a valid theoretical basis for enhancing the concept of safe and comfortable exercise.Methods:Thirty-five 4-week-old SPF healthy C57BL/6 male mice were randomly divided into 25 in the high-fat diet（High fat diet,HFD）group and 10 in the maintenance diet（NC）group after 1 week of acclimatization feeding.After 12 weeks of feeding,the successfully modeled mice were randomly divided into 2 groups:the obese control group（OC）,and the obesity exercise group（OE）for the 9 weeks non-weight-bearing swimming intervention,and the rest of the groups served as controls.Immediately after the last exercise intervention,a new object recognition experiment was performed and the Novel recognition index（NOI）was calculated as an evaluation index of cognitive function.At the end of the behavioral experiments,a portion of fresh hippocampal tissue was selected for freezing and the rest for tissue sectioning.The tissue section specimens were observed using HE,Nissler,Neu N and DCX staining.Protein immunoblotting（Western blot,WB）was used to detect the altered expression of insulin resistance,neurogenesis and apoptosis-related proteins in mouse hippocampal tissues.Results:1.High-fat diet increased the Lee’s index of mice:During the modeling period,compared with NC group,the Lee’s index of HFD group increased significantly（p<0.001）.2.Swimming exercise reduced body weight in obese mice:During the exercise intervention,compared with OC group,the weight of OE group decreased significantly after 8 weeks of swimming intervention（p<0.05）3.Swimming exercise improved cognitive dysfunction in obese mice:Novel recognition index（NOI）showed that the time to explore two identical objects did not change significantly in each group of mice and the difference was not statistically significant（p>0.05）.The time to explore new objects and the new object recognition index in the OC group of mice were significantly reduced compared to the NC group（p<0.01）,while the time to explore new objects and the new object recognition index in the OE group of mice increased compared to the OC group（p<0.05,p<0.01）.The time of exploring new objects and new object recognition index increased in the OE group compared with the OC group（p<0.05,p<0.01）.The results of new object recognition experiment showed that swimming exercise could improve the cognitive dysfunction of obese mice.4.Swimming exercise inhibits hippocampal neuronal deformation in obese mice4.1 Swimming activates insulin signaling pathway in hippocampus of obese mice.:Compared with the NC group,neuronal damage,disorganized and sparse neuronal arrangement and a reduced number of mature neurons were seen in the OC group mice.Moreover,the OE group showed convergence to normal brain tissue in hippocampal tissue.It is suggested that the exercise intervention reduced these obesity-induced neuronal damage in the hippocampus.4.2 Swimming increases the number of Nissl bodies in hippocampus of obese mice.:The number of Nissler vesicles in hippocampal tissue was reduced in the OC group compared with the NC group,with significant differences（p<0.01）;the number of Nissler vesicles in hippocampal tissue was significantly increased and statistically significant in the OE group compared with the OC group（p<0.05）;a large number of Nissler vesicles were seen in hippocampal tissue in both the NC and OE groups,with no statistically significant differences（p>0.05）.4.3 Swimming promotes the maturation of nerve cells in hippocampus of mice:Compared with the NC group,the percentage of Neu N positive cell expression in the OC group was low,with significant differences（p<0.05）;compared with the OC group,the percentage of Neu N positive cells in the OE group was significantly higher and statistically significant（p<0.05）;a large number of Neu N positive cells were seen in both the NC and OE groups,with no statistically significant differences（p>0.05）.4.4 Swimming improves neurogenesis in hippocampus of obese mice:Compared with the NC group,the percentage of DCX-positive cells in the OC group was low,with significant differences（p<0.01）;compared with the OC group,the percentage of DCX-positive cells in the OE group was significantly higher and statistically significant（p<0.05）;a large number of DCX-positive cells were expressed in the NC group and the OE group,with no statistically significant differences（p>0.05）.5.WB results5.1 Swimming exercise activated insulin signaling pathway in hippocampal tissue of obese mice:Compared with NC group,the ratio of p-IRS^ser307/IRS-1 in hippocampus of OC group increased（p<0.05）;in contrast,p-PI3K p85/PI3K and p-Akt^ser473/Akt ratios were significantly lower（p<0.05）,indicating that high-fat diet can cause insulin resistance.The decrease in p-IRS^ser307/IRS-1 ratio（p<0.05）and the decrease in p-PI3K p85/PI3K and p-Akt^ser473/Akt ratio（p<0.05,p<0.01）in the hippocampal tissue of mice in the OE group compared to the OC group,suggesting that swimming exercise stimulates the activation of the insulin signaling pathway.5.2 Swimming exercise upregulated the expression of neurotrophic factors and synapse-related proteins in the hippocampal tissue of obese mice:The expression of PGC-1α,BDNF and PSD95 in the hippocampal tissue of mice in the OC group was significantly lower compared with that in the NC group（p<0.05,p<0.01）.The expression of PGC-1α,BDNF and PSD95 was elevated in the hippocampal tissues of mice in the OE group compared with the OC group（p<0.05,p<0.01）.Therefore,the swimming exercise intervention was beneficial to reverse the down-regulated PGC-1α,BDNF,and PSD95 in the hippocampal tissue of mice induced by long-term high-fat diet,thus enhancing neurotrophic factor and synaptic plasticity.5.3 Swimming exercise inhibited neuronal apoptosis in obese mice:Compared with the NC group,Bcl-2 was significantly lower and Bax was significantly higher in hippocampal tissues of the OC group mice（p<0.05）;compared with the OC group,Bcl-2 was higher and Bax was lower in hippocampal tissues of the OE group mice.Although the differences of Bax between the OC and the OE groups were not statistically significant（p>0.05）,the Bax expression in the OE group showed a decreasing trend compared with the OC group.However,in the Bcl-2/Bax ratio,the NC group was significantly higher than the OC group（p<0.05）and the OC group was significantly lower than the OE group（p<0.05）.These results suggest that a long-term high-fat diet can trigger apoptosis of hippocampal neurons,and swimming exercise can rescue this phenomenon to some extent.Conclusion:The study concludes that swimming intervention reverses weight gain and impaired learning memory caused by a chronic high-fat diet,and rescues impaired learning memory in obese mice by reducing obesity,activating insulin signaling pathways,promoting the production and secretion of neurotrophic factors,inhibiting hippocampal neuronal apoptosis and promoting neuronal regeneration in adolescent mice.