Tank D W, Regehr W G, Delaney K R
Biological Computation Research Department, AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA.
J Neurosci. 1995 Dec;15(12):7940-52. doi: 10.1523/JNEUROSCI.15-12-07940.1995.
Augmentation and posttetanic potentiation--two forms of short-term synaptic enhancement produced by repetitive presynaptic action potentials--are dependent on the buildup and decay of nerve terminal residual calcium that occurs on the seconds to minutes time scale. With the goal of providing a quantitative understanding of these kinetics, we measured the buildup and decay of calcium ions in nerve terminals at the crayfish neuromuscular junction under a variety of intracellular buffer conditions and stimulation paradigms. The calcium extrusion process in the terminals was characterized by analysis of calcium levels reached during long stimulus trains as a function of action potential frequency. The extrusion was linearly dependent on the free calcium ion concentration. Using this result, we developed a mathematical model and computer simulation of the residual calcium kinetics. The model demonstrates the experimentally observed dependence of decay rate on exogenous calcium buffer concentration, and can be explicitly solved to provide an expression for the limiting exponential time course of calcium decay following trains in terms of calcium buffer and extrusion characteristics. Methods to determine the calcium influx per action potential, characteristics of endogenous buffer, and the rate of calcium extrusion are suggested by our analysis and demonstrated experimentally.
增强作用和强直后增强——由重复的突触前动作电位产生的两种短期突触增强形式——取决于在数秒到数分钟时间尺度上发生的神经末梢残余钙的积累和衰减。为了定量理解这些动力学，我们在各种细胞内缓冲条件和刺激模式下，测量了小龙虾神经肌肉接头处神经末梢中钙离子的积累和衰减。通过分析长时间刺激序列中达到的钙水平作为动作电位频率的函数，对末梢中的钙外排过程进行了表征。外排与游离钙离子浓度呈线性相关。利用这一结果，我们建立了残余钙动力学的数学模型和计算机模拟。该模型证明了实验观察到的衰减速率对外源钙缓冲液浓度的依赖性，并且可以明确求解，以根据钙缓冲液和外排特性提供一系列刺激后钙衰减的极限指数时间进程的表达式。我们的分析提出了确定每个动作电位的钙内流、内源性缓冲液的特性以及钙外排速率的方法，并通过实验进行了验证。
