Shahrezaei Vahid, Cao Alex, Delaney Kerry R
Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6.
J Neurosci. 2006 Dec 20;26(51):13240-9. doi: 10.1523/JNEUROSCI.1418-06.2006.
Neurotransmitter release is triggered by the cooperative action of approximately five Ca2+ ions entering the presynaptic terminal through Ca2+ channels. Depending on the organization of the active zone (AZ), influx through one or many channels may be needed to cause fusion of a vesicle. Using a combination of experiments and modeling, we examined the number of channels that contribute Ca2+ for fusion of a single vesicle in a frog neuromuscular AZ. We compared Ca2+ influx to neurotransmitter release by measuring presynaptic action potential-evoked (AP-evoked) Ca2+ transients simultaneously with postsynaptic potentials. Ca2+ influx was manipulated by changing extracellular [Ca2+] (Ca(ext)) to alter the flux per channel or by reducing the number of open Ca2+ channels with omega-conotoxin GVIA (omega-CTX). When Ca(ext) was reduced, the exponent of the power relationship relating release to Ca2+ influx was 4.16 +/- 0.62 (SD; n = 4), consistent with a biochemical cooperativity of approximately 5. In contrast, reducing influx with omega-CTX yielded a power relationship of 1.7 +/- 0.44 (n = 5) for Ca(ext) of 1.8 mM and 2.12 +/- 0.44 for Ca(ext) of 0.45 mM (n = 5). Using geometrically realistic Monte Carlo simulations, we tracked Ca2+ ions as they entered through each channel and diffused in the terminal. Experimental and modeling data were consistent with two to six channel openings per AZ per AP; the Ca2+ that causes fusion of a single vesicle originates from one or two channels. Channel cooperativity depends mainly on the physical relationship between channels and vesicles and is insensitive to changes in the non-geometrical parameters of our model.
神经递质的释放是由大约五个钙离子通过钙离子通道进入突触前终末的协同作用触发的。根据活性区（AZ）的组织结构，可能需要一个或多个通道的内流来引发囊泡融合。我们结合实验和建模方法，研究了青蛙神经肌肉活性区中为单个囊泡融合提供钙离子的通道数量。我们通过同时测量突触前动作电位诱发（AP诱发）的钙离子瞬变和突触后电位，比较了钙离子内流与神经递质释放的关系。通过改变细胞外[Ca2+]（Ca(ext)）以改变每个通道的通量，或用ω-芋螺毒素GVIA（ω-CTX）减少开放钙离子通道的数量来操纵钙离子内流。当Ca(ext)降低时，将释放与钙离子内流相关的幂关系指数为4.16±0.62（标准差；n = 4），这与大约5的生化协同性一致。相比之下，用ω-CTX减少内流时，对于1.8 mM的Ca(ext)，幂关系为1.7±0.44（n = 5），对于0.45 mM的Ca(ext)，幂关系为2.12±0.44（n = 5）。使用几何逼真的蒙特卡罗模拟，我们追踪了钙离子通过每个通道进入并在终末中扩散的过程。实验和建模数据与每个动作电位每个活性区有两到六个通道开放一致；导致单个囊泡融合的钙离子来自一到两个通道。通道协同性主要取决于通道与囊泡之间的物理关系，并且对我们模型的非几何参数变化不敏感。
