An Experimental Study On The AVP(4-8) Improving Cognitive Behavior And Hippocampal Synaptic Plasticity In APP/PS1 Mice Of Alzheimer's Disease

Objective:Memory deficits with aging are related to neurophysiological and neurochemical dysfunctions in human and animals.Such changes include a reduction in arginine vasopressin?AVP?in the brain of Alzheimer's disease?AD?.AVP?4-8?,a metabolic fragment of vasopressin,preserves neuroprotective effects while without pressor and/or antidiuretic activity of AVP.The present study investigated for the first time the effects of AVP?4-8?on the cognitive behaviors and in vivo hippocampal synaptic plasticity in APP/PS1-AD mice.Methods:8-month-old APPswe/PS1dE9?APP/PS1?-AD mice and wild-type?WT?mice were randomly divided into four groups:WT+Saline,WT+AVP?4-8?,APP/PS1+Saline and APP/PS1+AVP?4-8??n=10-13/group?.AVP?4-8??2?g/kg?or equivalent saline?0.9%NaCl?was administered intranasally three times a day.After 4 weeks of drug application,Y-maze and Morris water maze?MWM?tests were performed.Afterwards,in vivo hippocampal LTP and paired pulse facilitation?PPF?were recorded,and then followedWestern blotting and ELISA experiments.Y-maze test:Each animal?n=9/group?was placed in the central triangle region and was allowed to move freely in the Y-maze for 8 min.A correct spontaneous alternation was identified as a set of three different arm entries.A higher percentage of correct spontaneous alteration indicated better working memory.Morris water maze?MWM?test:During the acquisition phase(1^st-5^thh day),mice were trained 4 times in each day for the underwater platform searching.The escape latency?the time climbing on the underwater platform?and swimming trails were recorded.In the probe trials?6^thh day?,mice were allowed to swim freely for 1 min in the water without the platform.The time spent on the target quadrant and the swimming speed were recorded.After that,a visible platform test was performed to assess the visual and motor ability of mice by recording the time when mice arrived at the escape platform.In vivo hippocampal LTP recording:Mice were anesthetized with chloral hydrate and placed in a stereotaxic apparatus.Then,a bound stimulating/recording electrode was inserted into the Schaffer collateral/CA1 region.Test stimuli were delivered to the Schaffer-collateral/commissural pathway with frequency of 0.033 Hz for evoking field exitatory postsynaptic potentials?fEPSPs?,which was recorded for 30 min as a stable baseline control.PPF was observed by paired stimuli with an interval of 50ms,and the percentage of fEPSP2/fEPSP1 was calculated.Then,LTP was induced by HFS?three times of 20 pulses at 200 Hz with an interval of 30 s?.After that,the fEPSPs were evoked and recorded again for at least 60 min with the same intensity of test stimuli.Molecular biology experiment:Mice were sacrificed to obtain the whole brains,and the hippocampus was dissected quickly on ice to subject to Western blot and ELISA experiments for detecting the levels of SYP,PSD95 and NGF.Results:1.In Y-maze test,total arm entries did not show any significant difference among the 4 groups?P>0.05?.In contrast,a significant decrease in spontaneous alternation?%?was seen in the mice of APP/PS1+Saline group when compared with WT+Saline group?P<0.001?;And AVP?4-8?treatment caused a significant increase in spontaneous alternation?%?in WT+AVP?4-8??P<0.05?and APP/PS1+AVP?4-8?group?P<0.001?compared with their respective saline control groups.These results suggested a working memory enhancement in normal mice by AVP?4-8?,and an impairment in APP/PS1 mice and its reversal by AVP?4-8?.2.In MWM test,APP/PS1+Saline group spent more time on finding the hidden platform than WT+Saline group on day 3-5?day 3:P<0.001;day 4:P<0.001;and day 5:P<0.001?and APP/PS1+AVP?4-8?group on day 4 and 5?day 4:P<0.05;day 5:P<0.05?,respectively.It suggests that the spatial learning impairment of APP/PS1 could be partly rescued by AVP?4-8?treatment.In the probe test,reference memory deficit also occurred in APP/PS1+Saline group,with fewer swimming time percent in the target quadrantthan its WT control?P<0.001?;AVP?4-8?treatment in APP/PS1+AVP?4-8?group showed a significant increase?P<0.05?compared with the APP/PS1+Saline group.The results above suggest the spatial learning and memory impairment of APP/PS1 could be partly rescued by AVP?4-8?treatment.Moreover,similar swimming speed in the probe test and swimming time to the escape platform in the visible platform test suggested the differences in spatial learning and memory did not result from the alteration of motor ability and visual acuity.3.In hippocampal LTP experiment,at 30 min and 60 min after HFS,the fEPSP slopes in APP/PS1+Saline group significantly decreased?n=6;30 min:P<0.01;60 min:P<0.001?compared to that in the WT+Saline group.After treatment with AVP?4-8?,the fEPSP slopes in APP/PS1+AVP?4-8?group significantly increased compared to that in APP/PS1+Saline group?30 min:P<0.05;60 min:P<0.001?Besides,there was no significant difference in the PPF ratio between the four groups.These results demonstrated that AVP?4-8?partly rescued the LTP maintenance in APP/PS1 mice,which might be involved in a postsynaptic,not presynaptic,mechanism.4.In molecular biology experiment,the expression levels of SYP?P<0.01?and PSD95?P<0.001?in the APP/PS1+Saline group were significantly decreased when compared with that in the WT+Saline group,while AVP?4-8?application significantly up-regulated the levels of SYP?P<0.05?and PSD95?P<0.001?in the APP/PS1+AVP?4-8?group.Similarly,a distinct increase in the NGF level was found in the WT+AVP?4-8??P<0.05?and APP/PS1+AVP?4-8??P<0.05?groups compared to their corresponding saline control group,but without any significant difference between WT+Saline and APP/PS1+Saline groups.The result of ELISA for measuring NGF was in accordance with that of the Western blot.Conclusion:These results revealed for the first time the beneficial effects of AVP?4-8?on the cognitive behaviors and the in vivo hippocampal LTP in APP/PS1-AD mice,indicating that the exogenous administration of AVP?4-8?could alleviate the cognitive deficits in APP/PS1-AD mice,while the enhancement of hippocampal synaptic plasticity and the elevation of SYP,PSD95 and NGF levels in the hippocampus might be involved in the neuroprotective mechanisms of AVP?4-8?.