Distinct Trafficking And Expression Mechamisms Underlie LTP And LTD Of NMDAR Receptor-mediated Synaptic Responses

Although extensive studies have examined long-term potentiation(LTP) and long-term depression(LTD)of AMPA receptor-mediated postsynaptic currents,the prevailing view is that NMDARs function as the trigger of LTP and LTD and that the primary expression mechanism of synaptic plasticity involves alterations in the function,number,and/or subunit composition of synaptic AMPARs,but much less is known about the mechanisms responsible for LTP and LTD of NMDAR receptor (NMDAR)-mediated EPSCs.Thus,we are interested that whether NMDARs have involved in and mediated the LTP/LTD,and what are their mechanisms? In the present studies,using whole cell recording techniques we investigated the trafficking and expression mechanisms underlying the LTP versus LTD of NMDAR-mediated synaptic responses in Schaffer-CA1 synapses of hippocampal brain slice.The main findings are as follows:1.NMDARs have involved in and mediated the LTP/LTD in hippocampal Schaffer-CA1 synapses In hippocampal brain slice,we found that LTP-producing protocol induced a slowly developing enhancement of NMDAR EPSC that reached a stable magnitude around 20 min later,while the peak of magnitude of AMPAR-mediated EPSCs was obtained as soon as the same LTP protocol was applied to,and AMDAR EPSCs reached a stable magnitude later. Respectively perfusion of NMDARs and mGluRs with their antagonist AP5(100μM)and MCPG(0.25μM)inhibited the potentiation of NMDAR-mediated EPSCs,and loading CA1 cells(drugs were added to the patch solution)with the calcium chelator BAPTA(15 mM)also blocked LTP of NMDAR EPSCs(LTP_NMDA).Then,we found LTD of NMDAR EPSCs(LTD_NMDA)was induced immediately,later reached stable amplitude as AMPAR-mediated LTD.AP5,MCPG and BAPTA blocked LTD of NMDAR EPSCs as their effect on NMDAR-mediated LTP.These findings suggest that there are NMDAR-mediated LTP and LTD in Schaffer-CA1 synapses of hippocampal brain slice,calcium and both activation of NMDARs and mGluRs were required for LTP/LTD of NMDAR EPSCs.2.The trafficking of NMDARs have involved in LTP_NMDA/LTD_NMDAIn order to further understand the trafficking and expression of NMDAR-mediated LTP/LTD,we loaded CA1 cells(drugs were added to the patch solution)with Tetanus toxin(0.1μM)which should impair SNARE-dependent exocytosis,this manipulation has been found to block LTP of NMDAR EPSCs.Then we loaded cells with GDPβs(600μM), which should impair all GTP-dependent activity,including that of dynamin-dependent endocytosis.And this manipulation has been found to obviously facilitate LTP of NMDAR EPSCs.Loading cells with D15(2 mM)which specially inhibited the dynamin-dependent endocytosis had the same effect on LTP of NMDAR EPSCs as GDPβs.Our results indicated that LTP-producing protocol made more NMDARs insert into the postsynaptic membrane which potentiated the NMDAR-mediated EPSCs. In contrast,inhibition of endocytosis and exocytosis had little impact on LTD of NMDAR-mediated EPSCs,so differences in trafficking and expression mechanisms underlying the LTP versus LTD of NMDAR-mediated synaptic responses in hippocampal Schaffer-CA1 synapses.3.calcium-dependent regulation of actin depolymerization contribute to LTP_NMDAand LTD_NMDAWe have referred to differences in trafficking and expression mechanisms underlying the LTP versus LTD of NMDAR-mediated synaptic responses,in order to understand the differences in details,we therefore respectively examined whether loading cells with Latruculin B (200μM),which inhibits actin polymerization and promotes depolymerization,and phalloidin(100 mM),which inhibits actin depolymerization and stabilizes actin,affect LTP_NMDAand LTD_NNDA.These findings suggest phalloidin facilitated LTP_NMDA,but Latruculin B blocked LTP_NMDA.In contract,Latruculin B had no effect on LTD_NMDAbut blocked LTP_NMDA.These results indicate calcium-dependent regulation of actin depolymerization contribute to LTP_NMDAand LTD_NMDA.4.Differential switches on NR2 subunit composition were found to accompany with LTP_NMDAand LTD_NMDAWe found there were differential switches on NR2 subunit compositions accompanying with LTP_NMDAand LTD_NMDA.The ratio of NR2A/NR2B increased during LTP_NMDA,but the decay time(τ)of NMDAR EPSC decreased.Whereas ratio of NR2A/NR2B decreased during LTD_NMDA,but the decay time(τ)of NMDAR EPSC increased.These studies suggest that more NR2A-containing NMDAR insert into postsynaptic during LTP_NNDA, while there are relative more NR2B-containing NMDAR in postsynaptic during LTD_NMDA.In conclusion,our results suggest that both subunit-specific regulation of NMDAR receptor trafficking and calcium-dependent regulation of actin depolymerization contribute to LTP_NMDAand LTD_NMDA.