Shi J, Aamodt S M, Townsend M, Constantine-Paton M
Departments of Biology and Brain and Cognitive Science and The McGovern Brain Research Institute, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Neurosci. 2001 Aug 15;21(16):6233-44. doi: 10.1523/JNEUROSCI.21-16-06233.2001.
Slabs of slow-release plastic (Elvax) containing NMDA or solvent were implanted over the rat colliculus beginning on postnatal day 8 (P8). Whole-cell patch clamping in the superficial superior collicular layers (sSCs) from P10 to P21 demonstrated a severe decrease in spontaneous EPSC frequency after chronic NMDA treatment. The decrease was not attributable to an increase in GABA(A) receptor-mediated inhibition and was present only when NMDA receptor (NMDAR) current was blocked by Mg(2+). Analysis of miniature EPSCs indicated that many active sites on NMDA-treated neurons lacked functional AMPA and kainate receptor (AMPA/KAR) currents, and AMPA/KAR:NMDAR current ratios of evoked EPSCs were also significantly reduced. In addition, the normal downregulation of NMDAR decay time in sSC neurons at P11 was absent after NMDA treatment. Nevertheless, neither AMPA nor NMDA receptor subunit expression was altered by NMDA treatment, and experiments with the NMDAR antagonist ifenprodil suggested that incorporation of NR2A-containing NMDARs at the sSC synapses was unperturbed. Thus, disrupting but not blocking NMDARs suppresses the development of AMPA/KAR currents. The absence of the P11 NMDAR current downregulation is likely a secondary effect resulting from the reduction of AMPA/KAR function. Chronic agonist application reduces but does not eliminate NMDAR conductances. Therefore these data support an active role for NMDAR currents in synaptic development. Prolonged NMDA treatment in vivo, which couples reduced postsynaptic Ca(2+) responses with normally developing afferent activity, produces a long-lasting synaptic depression and stalls glutamatergic synaptogenesis, suggesting that the correlation between robust NMDAR activation and afferent activity is an essential component during normal development.
从出生后第8天（P8）开始，将含有N-甲基-D-天冬氨酸（NMDA）或溶剂的缓释塑料板（聚乙烯乙酸乙烯酯）植入大鼠的上丘。在出生后第10天（P10）至第21天，对上丘浅层（sSCs）进行全细胞膜片钳记录，结果显示，慢性NMDA处理后，自发性兴奋性突触后电流（EPSC）频率显著降低。这种降低并非由于γ-氨基丁酸A型（GABA(A)）受体介导的抑制作用增强所致，且仅在NMDA受体（NMDAR）电流被镁离子（Mg(2+)）阻断时才出现。对微小兴奋性突触后电流（mEPSC）的分析表明，NMDA处理的神经元上许多活性位点缺乏功能性的α-氨基-3-羟基-5-甲基-4-异恶唑丙酸（AMPA）和海人藻酸受体（AMPA/KAR）电流，且诱发性EPSC的AMPA/KAR:NMDAR电流比值也显著降低。此外，NMDA处理后，出生后第11天（P11）时sSC神经元中NMDAR衰减时间的正常下调消失。然而，NMDA处理并未改变AMPA或NMDAR亚基的表达，使用NMDAR拮抗剂ifenprodil进行的实验表明，sSC突触处含NR2A的NMDAR的掺入未受干扰。因此，破坏而非阻断NMDAR会抑制AMPA/KAR电流的发育。P11时NMDAR电流下调的缺失可能是AMPA/KAR功能降低的继发效应。慢性应用激动剂会降低但不会消除NMDAR电导。因此，这些数据支持NMDAR电流在突触发育中发挥积极作用。体内长期进行NMDA处理，使突触后钙离子（Ca(2+)）反应降低与传入活动正常发育相结合，会导致长期的突触抑制并使谷氨酸能突触发生停滞，这表明在正常发育过程中，强大的NMDAR激活与传入活动之间的相关性是一个重要组成部分。
