Effects And Mechanisms Of Aquaporin-4Knockout On Hippocampal-dependent Learning Memory And Long-term Depression Of Mice

Part I Effects of aquaporin-4knockout on hippocampal-dependent learning memory and synaptic plasticityAims:In the central neuronal system, astrocytes have been generally believed to play important roles in regulating the activities and functions of neurons. A lot of evidence suggests that astrocytes are important in signal transmission, synaptic plasticity and learning memories. Aquaporin-4(AQP-4), as one of the AQP family of water channels, is primarily expressed in astrocytes. A growing number of evidence shows that AQP-4plays a protential role in the regulation of astrocytic function. However, there is little known about the function of AQP-4for synaptic plasticity in the hippocampus. Therefore, we evaluated the changes in synaptic plasticity in the hippocampus by AQP-4knockout.Methods:Both field potentials recording and whole-cell recording methods were used to record the field excitatory potentials (fEPSPs) and excited postsynaptic currents (EPSCs) in the CA3-CA1pathway in hippocampus from AQP-4wild type and knockout type mice. Novel-object recognition behavioral test and fear conditioning test were used to evaluate hippocampal-dependent learning and memories in AQP-4WT and KO mice. One-way repeated-measure-ANVOA followed by Student’s t test was used to statistically evaluate the data. Two-way repeated-measure-ANOVA followed by Bonferroni post hoc test were used to statistically evaluate the data.Results:AQP-4knockout impaired novel object recognition. Compared with WT mice, KO mice showed decreased time that spent on exploring novel objects during examine phase, and the discrimination index of KO mice is much lower than WT mice. AQP-4knockout facilitated long-term depression (LTD) without alter the basal synaptic transmission in the CA3-CA1pathway in the hippocampus. AQP-4deficiency facilitated. LTD was NMDA receptor-dependent LTD, but not mGluR-dependent LTD. Administration of NMDA receptor antagonist50μM APV decreased AQP-4knockout-induced LTD. Furthermore, AQP-4knockout significantly and selectively increased NMDA receptor-mediated EPSCs, but had no effects on AMPA receptor-mediated EPSCs.Conclusion:These results suggest a role for AQP-4in synaptic plasticity and learning memories in the hippocampus by regulating the function of NMDA receptors. Part Ⅱ Effects and mechanisms of synaptic and extrasynaptic NMDA receptors in AQP-4knockout-induced changes in hippocampal synaptic plasticity and learning memory of miceAims:It is generally believed that synaptic plasticity is the prime candidate for mediating learning and memories. LTP and LTD are two forms of synaptic plasticity, and they share some but not all the properties and mechanisms. Evidence shows that LTP is related to the requiring and formation of memories, while LTD is related to the extinction of memories. Hippocampus as a part of limbic system plays an important role in regulating of learning and memories. Synaptic and extrasynaptic NMDA receptors play different roles in regulating of the hippocampal synaptic plasticity. Therefore, this study investigated the effects and mechanisms of synaptic and extrasynaptic NMDA receptors in AQP-knockout-induced hippocampal learning and memories in mice.Methods:Field potentials recording and whole-cell recording methods were used to evaluate the effects of syanptic and extrasynaptic NMDA receptors on long-term depression in the hippocampus of AQP-4WT and KO mice. Tow-way repeated-measure-ANOVA followed by Bonferroni post hoc test were used to statistically evaluate the data. Novel-object recognition behavioral test and fear conditioning test were used to evaluate the effects and mechanism of synaptic and extrasynaptic NMDA receptors in hippocampal dependent learning and memories in AQP-4WT and KO mice. One-way repeated-measure-ANVOA followed by Student’s t test was used to statistically evaluate the data.Results:Application of NR2B selective antagonist3μM ifenprodil and1μM Ro256981decreased AQP-4knockout-induced LTD, while NR2A selective antagonist0.4μM PEAQX had no effects on the induction of LTD. Furthermore, AQP-4knockout significantly and selectively increased NR2B-NMDA receptor mediated EPSCs, but had no effects on NR2A-NMDA receptor mediated EPSCs.The activation of NR2B-NMDAR but not NR2A-NMDAR contributed to the impairment of novel object recognition by AQP-4knockout. In the fear conditioning test, AQP-4knockout facilitated the extinction of fear memories at24h,72h and120h after the extinction training. Application of NR2B receptor antagonist3mg/kg ifenprodil and2.5mg/kg HA96630min before extinction training significantly attenuated AQP-4knockout-induced fear extinction, while NR2A receptor antagonist5mg/kg PEAQX had no effects on fear extinction.Conclusion:AQP-4knockout facilitates NMDAR-dependent LTD, the extinction of fear memories and impaired novel object recognition by enhancement of extrasynaptic NR2B-NMDA receptor activities.