Tovar K R, Westbrook G L
Vollum Institute, Oregon Health Sciences University, Portland, Oregon 97201-3098, USA.
J Neurosci. 1999 May 15;19(10):4180-8. doi: 10.1523/JNEUROSCI.19-10-04180.1999.
Activity-dependent synaptic rearrangements during CNS development require NMDA receptor activation. The control of NMDA receptor function by developmentally regulated subunit expression has been proposed as one mechanism for this receptor dependence. We examined the phenotype of synaptic and extrasynaptic NMDA receptors during the development of synaptic load using the NMDA receptor 2B (NR2B)-selective antagonist ifenprodil. In cultured rat hippocampal neurons when relatively few synapses had formed, the ifenprodil block of EPSCs was less than whole-cell currents, the latter of which included both synaptic and extrasynaptic receptors. At the same developmental stage, we found that extrasynaptic receptors outnumbered synaptic receptors by 3:1; thus whole-cell currents were dominated by the extrasynaptic population. We used the macroscopic kinetics of ifenprodil block to distinguish between the receptor populations. The ifenprodil kinetics of whole-cell currents from neurons before and during the development of synaptic load was comparable with that of whole-cell currents in HEK293 cells transfected with NR1 and NR2B cDNA, indicating that extrasynaptic receptors are largely NR1/NR2B heteromers. In contrast, synaptic receptors included both a highly ifenprodil-sensitive (NR1/NR2B) component as well as a second population with lower ifenprodil sensitivity; the reduced ifenprodil block of EPSCs was attributable to synaptic receptors with lower ifenprodil sensitivity rather than to the appearance of ifenprodil-insensitive (NR1/NR2A) receptors. Our data indicate that the synaptic NMDA receptor complement changes quickly after synapse formation. We suggest that synapses containing predominately NR1/NR2B heteromers represent "immature" sites, whereas mature sites express NMDA receptors with a distinct, presumably triheteromeric, subunit composition.
中枢神经系统发育过程中依赖活动的突触重排需要NMDA受体激活。通过发育调控的亚基表达来控制NMDA受体功能被认为是这种受体依赖性的一种机制。我们使用NMDA受体2B(NR2B)选择性拮抗剂艾芬地尔,研究了突触负载发育过程中突触和突触外NMDA受体的表型。在培养的大鼠海马神经元中,当形成的突触相对较少时,艾芬地尔对兴奋性突触后电流(EPSCs)的阻断作用小于全细胞电流,后者包括突触和突触外受体。在相同的发育阶段,我们发现突触外受体的数量比突触受体多3倍;因此全细胞电流主要由突触外群体主导。我们利用艾芬地尔阻断的宏观动力学来区分受体群体。突触负载发育之前和期间神经元全细胞电流的艾芬地尔动力学与转染了NR1和NR2B cDNA的HEK293细胞全细胞电流的动力学相当,这表明突触外受体主要是NR1/NR2B异聚体。相比之下,突触受体包括一个对艾芬地尔高度敏感的(NR1/NR2B)成分以及另一个对艾芬地尔敏感性较低的群体;EPSCs的艾芬地尔阻断作用降低归因于对艾芬地尔敏感性较低的突触受体,而不是出现了对艾芬地尔不敏感的(NR1/NR2A)受体。我们的数据表明,突触形成后突触NMDA受体的组成变化很快。我们认为,主要含有NR1/NR2B异聚体的突触代表“不成熟”位点,而成熟位点表达具有独特的、可能是三聚体亚基组成的NMDA受体。