Presynaptic calcium channels and the depletion of synaptic cleft calcium ions
Presynaptic calcium channels and the depletion of synaptic cleft calcium ions
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DOI：
10.1152/jn.2000.83.1.477
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发表时间：
2000-01-01
影响因子：
2.5
通讯作者：
Stanley, EF
中科院分区：
文献类型：
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作者：
Stanley, EF
钙离子（Ca ~（2+））通过电压门控钙通道进入突触前神经末梢是神经递质释放的重要步骤。由于钙通道聚集在释放位点，Ca 2+流入末端不可避免地将离子从相邻的细胞外空间（突触间隙）中移除。我们使用鸡睫状神经节的大萼型突触来测试突触间隙Ca 2+耗竭。在-80 mV的保持电位（V-H）下对端子进行电压钳位，并将去极化脉冲施加到一定范围的电位（-60至+60 mV）。电压脉冲激活了持续的内向钙电流，随后，通过内向钙尾电流将膜电位恢复到V-H。尾电流的幅度反映了电压脉冲结束时开放钙通道的数量和Ca 2+电化学梯度。外部钡取代钙作为电荷携带离子，因为最初的实验表明突触前钙通道的钙依赖性失活。尾电流招聘进行了比较，在萼神经末梢，仍然附着在突触后神经元，因此保留了突触间隙，终端已被完全隔离。在孤立的终端，尾电流的募集曲线，可以拟合的玻尔兹曼分布的平均V-1/2为0.4 mV和斜率因子为5.4。然而，在附加的肾盏尾电流招聘偏向去极化电位与平均V-1/2为11.9 mV和斜率因子为12.0。附着肾盏募集曲线的偏斜程度与阶跃去极化诱发的内向电流的幅度相关。这些发现的最简单的解释是，在去极化脉冲期间，Ba 2+从突触间隙中去除的速度比补充的速度快，从而通过降低离子进入的驱动力来降低尾电流。突触前钙通道激活过程中的Ca 2+耗竭可能是快速突触化学传递的一般性质，其对持续分泌的持续时间设置了功能限制。突触可能已经进化到最小化间隙消耗，通过开发钙高效机制来门控递质释放，仅需要同时打开几个低电导钙通道。
The entry of calcium ions (Ca2+) through voltage-gated calcium channels is an essential step in the release of neurotransmitter at the presynaptic nerve terminal. Because the calcium channels are clustered at the release sites, the flux of Ca2+ into the terminal inevitably removes the ion from the adjacent extracellular space, the synaptic cleft. We have used the large calyx-type synapse of the chick ciliary ganglion to test for synaptic cleft Ca2+ depletion. The terminal was voltage clamped at a holding potential (V-H) of -80 mV and a depolarizing pulse was applied to a range of potentials (-60 to +60 mV). The voltage pulse activated a sustained inward calcium current and was followed, on return of the membrane potential to V-H by an inward calcium tail current. The amplitude of the tail current reflects both the number of open calcium channels at the end of the voltage pulse and the Ca2+ electrochemical gradient. External barium was substituted for calcium as the charge-carrying ion because initial experiments demonstrated calcium-dependent inactivation of the presynaptic calcium channels. Tail current recruitment was compared in calyx nerve terminals that remained attached to the postsynaptic neuron and therefore retained a synaptic cleft, with terminals that had been fully isolated. in isolated terminals, the tail currents exhibited recruitment curves that could be fit by a Boltzmann distribution with a mean V-1/2 of 0.4 mV and a slope factor of 5.4. However, in attached calyces tail current recruitment was skewed to depolarized potentials with a mean V-1/2 of 11.9 mV and a slope factor of 12.0. The degree of skew of the recruitment curve in the attached calyces correlated with the amplitude of the inward current evoked by the step depolarization. The simplest interpretation of these findings is that during the depolarizing pulse Ba2+ is removed from the synaptic cleft faster than it is replenished, thus reducing the tail current by reducing the driving force for ion entry. Ca2+ depletion during presynaptic calcium channel activation is likely to be a general property of chemical transmission at fast synapses that sets a functional limit to the duration of sustained secretion. The synapse may have evolved to minimized cleft depletion by developing a calcium-efficient mechanism to gate transmitter release that requires the concurrent opening of only a few low conductance calcium channels.