Regehr W G, Atluri P P
Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA.
Biophys J. 1995 May;68(5):2156-70. doi: 10.1016/S0006-3495(95)80398-X.
Calcium ions act presynaptically to modulate synaptic strength and to trigger neurotransmitter release. Here we detect stimulus-evoked changes in residual free calcium ([Ca2+]i) in rat cerebellar granule cell presynaptic terminals. Granule cell axons, known as parallel fibers, and their associated boutons, were labeled with several calcium indicators. When parallel fibers were extracellularly activated with stimulus trains, calcium accumulated in the terminals, producing changes in the fluorescence of the indicators. During the stimulus train, the fluorescence change per pulse became progressively smaller with the high affinity indicators Fura-2 and calcium green-2 but remained constant with the low affinity dyes BTC and furaptra. In addition, fluorescence transients of high affinity dyes were slower than those of low affinity indicators, which appear to accurately report the time course of calcium transients. Simulations show that differences in the observed transients can be explained by the different affinities and off rates of the fluorophores. The return of [Ca2+]i to resting levels can be approximated by an exponential decay with a time constant of 150 ms. On the basis of the degree of saturation in the response of high affinity dyes observed during trains, we estimate that each action potential increases [Ca2+]i in the terminal by several hundred nanomolar. These findings indicate that in these terminals [Ca2+]i transients are much larger and faster than those observed in larger boutons, such as those at the neuromuscular junction. Such rapid [Ca2+]i dynamics may be found in many of the terminals in the mammalian brain that are similar in size to parallel fiber boutons.
钙离子在突触前发挥作用，调节突触强度并触发神经递质释放。在此，我们检测了大鼠小脑颗粒细胞突触前终末中残余游离钙（[Ca2+]i）的刺激诱发变化。颗粒细胞轴突，即平行纤维，及其相关的突触小体，用几种钙指示剂进行了标记。当用刺激序列对平行纤维进行细胞外激活时，钙在终末中积累，导致指示剂荧光发生变化。在刺激序列期间，高亲和力指示剂Fura-2和钙绿-2的每个脉冲荧光变化逐渐变小，但低亲和力染料BTC和氟罗帕特拉的荧光变化保持恒定。此外，高亲和力染料的荧光瞬变比低亲和力指示剂的荧光瞬变更慢，而低亲和力指示剂似乎能准确报告钙瞬变的时间进程。模拟结果表明，观察到的瞬变差异可以通过荧光团的不同亲和力和解离速率来解释。[Ca2+]i恢复到静息水平可以用时间常数为150毫秒的指数衰减来近似。根据在刺激序列期间观察到的高亲和力染料反应的饱和程度，我们估计每个动作电位使终末中的[Ca2+]i增加数百纳摩尔。这些发现表明，在这些终末中，[Ca2+]i瞬变比在较大的突触小体（如神经肌肉接头处的突触小体）中观察到的瞬变更大、更快。在哺乳动物大脑中，许多与平行纤维突触小体大小相似的终末可能都存在这种快速的[Ca2+]i动态变化情况。
