Department of Physiology and Biomedical Sciences, Seoul National University College of Medicine, Seoul, South Korea 03080, and Neuroscience Research Institute, Medical Research Center, Seoul National University College of Medicine, Seoul, South Korea, 03080.
Department of Physiology and Biomedical Sciences, Seoul National University College of Medicine, Seoul, South Korea 03080, and Neuroscience Research Institute, Medical Research Center, Seoul National University College of Medicine, Seoul, South Korea, 03080
J Neurosci. 2020 Oct 28;40(44):8426-8437. doi: 10.1523/JNEUROSCI.3029-19.2020. Epub 2020 Sep 28.
Synaptic strength and reliability are determined by the number of vesicles released per action potential and the availability of release-competent vesicles in the readily releasable pool (RRP). Compared with release of a single vesicle (univesicular release), multivesicular release (MVR) would speed up RRP depletion, yet whether the RRP is refilled differently during the two different release modes has not been investigated. Here, we address this question by quantitative optical imaging with an axon-targeting glutamate sensor, iGluSnFRpre. We found that hippocampal synapses preferentially release multiple vesicles per action potential at high extracellular calcium or by paired-pulse stimulation. When MVR prevails, the RRP is recovered very rapidly with a time constant of 430 ms. This rapid recovery is mediated by dynamin-dependent endocytosis followed by direct reuse of retrieved vesicles. Furthermore, our simulation proved that the portion of retrieved vesicles that directly refill the RRP increases dramatically (>70%) in MVR compared with that in univesicular release (<10%). These results suggest that the contribution of rapid and direct recruitment of retrieved vesicle to the RRP changes dynamically with release mode at the level of individual synapses, which suggests a form of presynaptic homeostatic plasticity for reliable synaptic transmission during various synaptic activity. The number of vesicles released in response to an action potential and the number of release competent vesicles in the readily releasable pool (RRP) are the fundamental determinants of synaptic efficacy. Despite its functional advantages, releasing multiple vesicles, especially at small synapses, can deplete the RRP after a couple of action potentials. To prevent failure of synaptic transmission, the RRP should be refilled rapidly, yet whether the RRP replenishment process is regulated by the release mode has not been investigated. Here, using quantitative optical glutamate imaging and simulation, we demonstrate that the contribution of the fast refilling mechanism changes with release mode at the level of individual synapses, suggesting a rapid form of presynaptic homeostatic plasticity during various synaptic activity.
突触强度和可靠性取决于每个动作电位释放的囊泡数量和易释放池（RRP）中具有释放能力的囊泡的可用性。与单个囊泡的释放（单囊泡释放）相比，多囊泡释放（MVR）会加速 RRP 的消耗，但在两种不同的释放模式下，RRP 是否以不同的方式重新填充尚不清楚。在这里，我们通过使用靶向轴突的谷氨酸传感器 iGluSnFRpre 进行定量光学成像来解决这个问题。我们发现，海马突触在高细胞外钙或成对脉冲刺激下优先每个动作电位释放多个囊泡。当 MVR 占主导地位时，RRP 会以 430ms 的时间常数快速恢复。这种快速恢复是由 dynamin 依赖性内吞作用介导的，随后是回收囊泡的直接再利用。此外，我们的模拟证明，与单囊泡释放（<10%）相比，MVR 中直接再填充 RRP 的回收囊泡的比例显著增加（>70%）。这些结果表明，在单个突触水平上，随着释放模式的变化，快速和直接招募回收囊泡对 RRP 的贡献会动态变化，这表明在各种突触活动中，突触传递的可靠性存在一种形式的突触前稳态可塑性。对动作电位的反应中释放的囊泡数量和易释放池中具有释放能力的囊泡数量是突触效能的基本决定因素。尽管具有功能优势，但在小突触中释放多个囊泡，尤其是在几个动作电位后，会耗尽 RRP。为了防止突触传递失败，RRP 应该快速补充，但 RRP 的补充过程是否受到释放模式的调节尚不清楚。在这里，我们使用定量光学谷氨酸成像和模拟表明，在单个突触水平上，快速补充机制的贡献会随着释放模式的变化而变化，这表明在各种突触活动中存在一种快速的突触前稳态可塑性形式。
