Joshi Indu, Yang Yi-Mei, Wang Lu-Yang
The Program for Neurosciences and Mental Health and Division of Neurology, The Hospital for Sick Children, and Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5G 1X8.
J Neurosci. 2007 Sep 12;27(37):9989-99. doi: 10.1523/JNEUROSCI.2506-07.2007.
NMDA receptors (NMDARs) are usually downregulated in developing central synapses, but underlying mechanisms and functional consequences are not well established. Using developing calyx of Held synapses in the mouse auditory brainstem, we show here that pairing presynaptic stimulation with postsynaptic depolarization results in a persistent downregulation in the summated amplitude of NMDAR-mediated EPSCs (NMDAR-EPSCs) during a train of stimuli (100/200 Hz, 100 ms) at both 22 degrees C and 35 degrees C. In contrast, the amplitude of single NMDAR-EPSCs or AMPA receptor-mediated EPSCs in the same synapses is not significantly altered, implying a preferential downregulation of perisynaptic/extrasynaptic NMDARs. Induction of this downregulation is blocked by antagonists for NMDARs or group I metabotropic glutamate receptors (mGluRs), suggesting that coincident activation of these two receptors is required. When the postsynaptic neuron is loaded with the fast Ca2+ buffer BAPTA or depolarized to +60 mV to reduce the driving force for Ca2+ influx, downregulation of the summated NMDAR-EPSCs is abolished, indicating Ca2+ plays a critical role in the induction. The expression of this downregulation depends on ongoing synaptic activity, and is attenuated by a dynamin peptide (D15) that blocks clathrin-dependent internalization. We further demonstrated that the same induction paradigm specifically reduces NMDAR-dependent plateau potential and aberrant spike firings during repetitive activity. Together, our results suggest that coincident activation of mGluRs and NMDARs during intense synaptic activity may lead to selective endocytosis of NMDARs in the perisynaptic/extrasynaptic domain, and implicate that mGluRs are potentially important for gating development of high-fidelity neurotransmission at this synapse.
N-甲基-D-天冬氨酸受体（NMDARs）在发育中的中枢突触中通常会下调，但其潜在机制和功能后果尚未完全明确。利用小鼠听觉脑干中发育中的Held壶腹突触，我们在此表明，在22摄氏度和35摄氏度下，将突触前刺激与突触后去极化配对，会导致在一系列刺激（100/200赫兹，100毫秒）期间，NMDAR介导的兴奋性突触后电流（NMDAR-EPSCs）的总和幅度持续下调。相比之下，同一突触中单个NMDAR-EPSCs或AMPA受体介导的EPSCs的幅度没有显著变化，这意味着突触周围/突触外NMDARs存在优先下调。这种下调的诱导被NMDARs或I组代谢型谷氨酸受体（mGluRs）的拮抗剂阻断，表明这两种受体的同时激活是必需的。当突触后神经元加载快速Ca2+缓冲剂BAPTA或去极化到+60毫伏以降低Ca2+内流的驱动力时，总和NMDAR-EPSCs的下调被消除，表明Ca2+在诱导过程中起关键作用。这种下调的表达取决于持续的突触活动，并被阻断网格蛋白依赖性内化的发动蛋白肽（D15）减弱。我们进一步证明，相同的诱导模式在重复活动期间特异性地降低了NMDAR依赖性平台电位和异常的动作电位发放。总之，我们的结果表明，在强烈的突触活动期间，mGluRs和NMDARs的同时激活可能导致突触周围/突触外区域NMDARs的选择性内吞，并暗示mGluRs对于在这个突触处控制高保真神经传递的发育可能具有重要意义。
