MK-801对清醒大鼠习得性LTP中海马DG区谷氨酸浓度的影响

Effect of MK-801 on extracellular levels of glutamate in the hippocampal dentate gyrus during learning-dependent LTP in free-moving conscious rats

目的观察条件反射的建立和消退过程中,N-甲基-D-门冬氨酸(N-methy1-D-asparate,NMDA)受体的非竞争性抑制剂MK-801对自由活动大鼠海马齿状回(dentate gyrus,DG)细胞外液中谷氨酸(Glutamate,Glu)浓度的影响,并进一步探讨它们与习得性突触效应长时程增强(learning dependenatlong-term potentiation,LD-LTP)和行为学习之间的关系。方法该实验选用Wistar系雄性大鼠,随机分4组(每组8只):对照1组和实验1组在用脑部微量注射法往海马DG区分别注射Ringer液和MK-801的条件下,利用脑部微量透析法和高效液相色谱法测定海马DG区细胞外液中Glu浓度,同时利用慢性埋植电极技术测定海马DG区群体峰电位(population spike,PS。LTP形成的指标)幅值;对照2组和实验2组进行条件反射训练,每天训练前往DG区分别注射Ringer液和MK-801,测定DG区Glu浓度和PS幅值,并观察大鼠行为正确反应率。将未进行处理前的Glu浓度和PS幅值分别定为100%。结果对照1组:海马DG区微量注射Ringer液后,DG区内Glu浓度与注射前相比无明显变化(P>0.05)。实验1组:海马DG区微量注射60μmol/L的MK-801 1μL后,Glu水平减少到注射前的(30.67±17.76)%,与注射前相比差异有显著性(P<0.01)。对照2组:随着条件反射的建立,DG区Glu出现明显增加,增加到训练开始前的(275.19±11.17)%,与训练前相比差异有显著性(P<0.001),随着条件反射的消退,Glu水平又逐渐回降,这与DG区PS幅值以及大鼠行为正确反应率的变化呈平行关系。实验2组:DG区微量注射MK-801,显著推迟条件反射的形成,而且条件反射形成时的PS幅值及Glu浓度的变化明显小于对照2组(P<0.01)。结论NMDA受体参与谷氨酸(Glu)介导的海马DG区习得性LTP的形成过程,而且用MK-801阻断NMDA受体的作用可明显抑制突触前膜Glu的释放。

[Objective] To investigate the neurochemical mechanisms responsible for learning dependent longterm potentiation （LTP）, the effect of MK-801 on extraceUular levels of Glutamate （Glu） was measured in hippocampal dentate gyms （DG） region during the period of establishment and extinction of the conditioned reflex in freemoving conscious rats. [Methods] The concentration of glutamate （Glu） was measured by mierodialysis and highperformance liquid chromatography （HPLC） methods and MK-801 was administrated by local-microinjection. The population spike （PS）, a parameter of LTP, and conditioned reflex were examined by implanted electrodes and combining electrophysiological-behavioral methods. [Results] The extracellular levels of Glu in the DG region were significantly reduced by local-microinjection of MK-801 （60 nmol/L） without training （P 〈0.05）. In the control group 2 and experiment group 2, the extracellular levels of Glu （P 〈0.001） in the DG region were significantly increased during the establishment of conditioned reflex and gradually returned following the extinction of conditioned reflex. The changes of Glu were correlated to the change of PS and both changes roughly follow the correct rate of behavior. Nevertheless, the changes of Glu and PS were significantly attenuated by local-microinjection of MK-801, as well as the establishment of conditional response was significantly inhibited by MK-801. [ Conclusion] NMDA receptor participates in the Gin-associated learning-dependent LTP in hippocampus DG region, it may be partly dependent on modulation of presynaptic release of Glu.