Joshi Indu, Shokralla Shahira, Titis Paul, Wang Lu-Yang
The Program for Brain and Behavioral Research and Division of Neurology, The Hospital for Sick Children and Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5G, 1X8.
J Neurosci. 2004 Jan 7;24(1):183-96. doi: 10.1523/JNEUROSCI.1074-03.2004.
During early postnatal development of auditory synapses, the decay time course of AMPA receptor (AMPAR) EPSCs accelerates markedly, but the mechanisms underlying this process remain uncertain. Using the developing calyx of Held synapse in the mouse auditory brainstem, we have examined presynaptic and postsynaptic elements that may regulate decay kinetics of AMPAR EPSCs. We found that the decay time kinetics was voltage dependent in both immature and mature synapses, being slower at positive potentials than negative potentials. By recording evoked miniature events in extracellular Ca2+ or Sr2+, we revealed a significant decrease in decay time constants of EPSCs as maturation progresses. On the basis of internal and external polyamine block of AMPAR EPSCs and immunohistochemistry assays with subunit-specific antibodies, we demonstrated that the glutamate receptor (GluR) 2 subunit is virtually absent at all developmental ages. Antibody staining patterns suggest a gradual shift in subunit composition from GluR1- to GluR3/4-dominant phenotypes. Kinetic analyses of deactivation, desensitization, and recovery from desensitization in outside-out patches in response to ultrafast application of glutamate lend supportive evidence that such a shift in the gating phenotype likely accounts for the accelerated time course throughout development. Finally, by pharmacologically manipulating AMPAR gating and using simulated EPSCs to evoke action potentials, we demonstrated that rapid decay kinetics of AMPAR EPSCs is essential for this synapse to accommodate high-frequency firing without compromising spike amplitude. Hence, developmental alterations in the subunit composition likely dictate changes in the time course of AMPAR EPSCs and play an indispensable role in the refinement of high-fidelity neurotransmission at the calyx of Held synapse.
在听觉突触的出生后早期发育过程中，AMPA受体（AMPAR）介导的兴奋性突触后电流（EPSCs）的衰减时间进程显著加快，但其背后的机制仍不明确。利用小鼠听觉脑干中发育中的Held壶腹突触，我们研究了可能调节AMPAR EPSCs衰减动力学的突触前和突触后元件。我们发现，在未成熟和成熟突触中，衰减时间动力学均依赖于电压，在正电位时比负电位时更慢。通过记录细胞外Ca2+或Sr2+中诱发的微小事件，我们发现随着成熟进程，EPSCs的衰减时间常数显著降低。基于对AMPAR EPSCs的内部和外部多胺阻断以及用亚基特异性抗体进行的免疫组织化学分析，我们证明谷氨酸受体（GluR）2亚基在所有发育阶段实际上都不存在。抗体染色模式表明亚基组成从以GluR1为主逐渐转变为以GluR3/4为主的表型。对响应超快施加谷氨酸的外向膜片的失活、脱敏和脱敏恢复的动力学分析提供了支持性证据，表明门控表型的这种转变可能是整个发育过程中时间进程加快的原因。最后，通过药理学操纵AMPAR门控并使用模拟EPSCs诱发动作电位，我们证明AMPAR EPSCs的快速衰减动力学对于该突触在不影响峰电位幅度的情况下适应高频放电至关重要。因此，亚基组成的发育变化可能决定了AMPAR EPSCs时间进程的变化，并在Held壶腹突触的高保真神经传递优化中发挥不可或缺的作用。
