Department of Cell and Molecular Biology, Tulane University, New Orleans, Louisiana 70118.
Department of Cell and Molecular Biology, Tulane University, New Orleans, Louisiana 70118
J Neurosci. 2020 Aug 5;40(32):6112-6120. doi: 10.1523/JNEUROSCI.0119-20.2020. Epub 2020 Jun 30.
Retrieval of synaptic vesicles via endocytosis is essential for maintaining sustained synaptic transmission, especially for neurons that fire action potentials at high frequencies. However, how neuronal activity regulates synaptic vesicle recycling is largely unknown. Here we report that Na substantially accumulated in the mouse calyx of Held terminals of either sex during repetitive high-frequency spiking. Elevated presynaptic Na accelerated both slow and rapid forms of endocytosis and facilitated endocytosis overshoot, but did not affect the readily releasable pool size, Ca influx, or exocytosis. To examine whether this facilitation of endocytosis is related to the Na-dependent vesicular content change, we dialyzed glutamate into the presynaptic cytosol or blocked the vesicular glutamate uptake with bafilomycin and found that the rate of endocytosis was not affected by regulating the vesicular glutamate content. Endocytosis is critically dependent on intracellular Ca, and the activity of Na/Ca exchanger (NCX) may be altered when the Na gradient is changed. However, neither NCX inhibitor nor change of extracellular Na concentration affected the endocytosis rate. Moreover, two-photon Ca imaging showed that presynaptic Na did not affect the action potential-evoked intracellular Ca transient and decay. Therefore, we revealed a novel mechanism of cytosolic Na in accelerating vesicle endocytosis. During high-frequency synaptic transmission, when large numbers of synaptic vesicles were fused, the rapid buildup of presynaptic cytosolic Na promoted vesicle recycling and sustained synaptic transmission. High-frequency firing neurons are widely distributed in the CNS. A large number of synaptic vesicles are released during high-frequency synaptic transmission; accordingly, synaptic vesicles need to be recycled rapidly to replenish the vesicle pool. Synaptic vesicle exocytosis and endocytosis are tightly coupled, and their coupling is essential for synaptic function and structural stability. We showed here that intracellular Na concentration at the calyx of Held terminal increased rapidly during spike activity and the increased Na accelerated endocytosis. Thus, when large numbers of synaptic vesicles are released during high-frequency synaptic transmission, Na accumulated in terminals and facilitated vesicle recycling. These findings represent a novel cellular mechanism that supports reliable synaptic transmission at high frequency in the CNS.
通过内吞作用回收突触囊泡对于维持持续的突触传递至关重要，特别是对于高频放电的神经元。然而，神经元活动如何调节突触囊泡的再循环在很大程度上仍是未知的。在这里，我们报告在雄性和雌性小鼠的外侧膝状体终末中，重复高频放电会导致钠离子大量积聚。升高的突触前钠离子既加速了慢速和快速的内吞作用，又促进了内吞作用的过冲，但不影响易释放池的大小、钙离子流入或胞吐作用。为了研究这种内吞作用的促进是否与钠离子依赖性囊泡内容物变化有关，我们将谷氨酸灌流到突触前胞质中，或用巴弗洛霉素阻断囊泡谷氨酸摄取，发现通过调节囊泡谷氨酸含量，内吞作用的速率不受影响。内吞作用对细胞内钙离子有严格的依赖性，当钠离子梯度改变时，钠离子/钙交换器（NCX）的活性可能会改变。然而，NCX 抑制剂或细胞外钠离子浓度的变化都没有影响内吞作用的速率。此外，双光子钙离子成像显示，突触前钠离子并不影响动作电位诱发的细胞内钙离子瞬变和衰减。因此，我们揭示了一种加速囊泡内吞作用的细胞质钠离子的新机制。在高频突触传递过程中，当大量的突触囊泡融合时，突触前细胞质中钠离子的快速积累促进了囊泡的回收和持续的突触传递。高频放电神经元广泛分布于中枢神经系统中。在高频突触传递过程中，会释放大量的突触囊泡；因此，需要快速回收突触囊泡以补充囊泡库。突触囊泡的胞吐作用和内吞作用紧密偶联，它们的偶联对于突触功能和结构稳定性是至关重要的。我们在这里显示，在尖峰活动期间，外侧膝状体终末内的细胞内钠离子浓度迅速增加，而增加的钠离子加速了内吞作用。因此，在高频突触传递过程中大量的突触囊泡释放时，在终末中积累的钠离子促进了囊泡的回收。这些发现代表了一种新的细胞机制，支持中枢神经系统中高频的可靠突触传递。
