Connor E A, Dunaevsky A, Griffiths D J, Hardwick J C, Parsons R L
Department of Biology, University of Massachusetts, Amherst 01003, USA.
J Neurophysiol. 1997 Feb;77(2):749-60. doi: 10.1152/jn.1997.77.2.749.
Twitch and tonic muscle fibers of snake skeletal muscle differ in their synpatic as well as mechanical properties. These experiments were aimed at detemining the basis of the difference in vesicular release properties of nerve terminals at twitch and tonic endplates. Miniature endplate currents (MEPCs) were recorded from voltage-clamped garter snake muscle fibers depolarized by high K+ in either a control Ca2+ or high-Ca2+ solution. MEPC frequency increased at twitch and tonic endplates and remained elevated for 8 h during depolarization in control Ca2+. At twitch endplates depolarized in the presence of high Ca2+, an increase in MEPC frequency was followed by a progressive decline. In contrast, MEPC frequency remained elevated in high Ca2+ at tonic endplates. The observed decrease in MEPC frequency at depolarized twitch endplates in high Ca2+ was not a function of the level of depolarization or initial MEPC frequency, nor was it due to a reduction in MEPC amplitude and loss of MEPCs in baseline noise. An optical assay of presynaptic function in which the activity-dependent dye FM1-43 was used confirmed that quantal releases differs at twitch and tonic endplates. Most twitch nerve terminals were labeled by FM1-43 during prolonged depolarization with control Ca2+ or after brief depolarization with high Ca2+. In contrast, the number of twitch nerve terminals and the degree to which they were stained was greatly reduced after prolonged exposure to high K+ and high Ca2+, whereas depolarized tonic endplates were well stained by FM1-43 during brief and prolonged exposure to high Ca2+. FM1-43 staining also revealed variable levels of quantal release between individual boutons at twitch endplates after prolonged depolarization in high-Ca2+ solution. The observed reduction in presynaptic function at twitch nerve terminals after prolonged depolarization in high-Ca2+ solution was reversible and therefore not due to irreversible damage to terminal boutons. MEPC frequency increased at both twitch and tonic endplates when either Sr2+ or Ba2+ was substituted for high Ca2+ during K(+)-induced depolarization. Over time, in Sr2+ or Ba2+ solutions, MEPC frequency remained elevated at tonic endplates but declined at twitch endplates with a time course similar to that observed in high Ca2+. MEPC amplitudes at both endplates remained constant. We conclude that the regulation of quantal release differs in nerve terminals innervating twitch and tonic endplates and postulate that differential intraterminal accumulation of Ca2+ may underlie the observed difference in presynaptic function.
蛇骨骼肌的抽搐型和紧张型肌纤维在突触及机械特性方面存在差异。这些实验旨在确定抽搐型和紧张型终板处神经末梢囊泡释放特性差异的基础。在对照Ca²⁺或高Ca²⁺溶液中，通过高K⁺使膜电位钳制的束带蛇肌纤维去极化，记录微小终板电流（MEPCs）。在对照Ca²⁺中去极化时，抽搐型和紧张型终板处的MEPC频率增加，并在8小时内保持升高。在高Ca²⁺存在下去极化的抽搐型终板处，MEPC频率增加后逐渐下降。相比之下，在高Ca²⁺时紧张型终板处的MEPC频率保持升高。在高Ca²⁺下去极化的抽搐型终板处观察到的MEPC频率下降，不是去极化水平或初始MEPC频率的函数，也不是由于MEPC幅度减小和基线噪声中MEPCs丢失所致。使用活性依赖染料FM1 - 43的突触前功能光学检测证实，抽搐型和紧张型终板处的量子释放不同。在对照Ca²⁺长时间去极化期间或高Ca²⁺短暂去极化后，大多数抽搐型神经末梢被FM1 - 43标记。相比之下，长时间暴露于高K⁺和高Ca²⁺后，抽搐型神经末梢的数量及其染色程度大大降低；而在短暂和长时间暴露于高Ca²⁺期间，去极化的紧张型终板被FM1 - 43良好染色。FM1 - 43染色还显示，在高Ca²⁺溶液中长时间去极化后，抽搐型终板的各个突触小体之间量子释放水平存在差异。在高Ca²⁺溶液中长时间去极化后，抽搐型神经末梢突触前功能的观察到的降低是可逆的，因此不是由于终末小体的不可逆损伤。在K⁺诱导的去极化过程中，当用Sr²⁺或Ba²⁺替代高Ca²⁺时，抽搐型和紧张型终板处的MEPC频率均增加。随着时间推移，在Sr²⁺或Ba²⁺溶液中，紧张型终板处的MEPC频率保持升高，而抽搐型终板处的MEPC频率下降，其时间进程与在高Ca²⁺中观察到的相似。两个终板处的MEPC幅度保持恒定。我们得出结论，支配抽搐型和紧张型终板的神经末梢中量子释放的调节不同，并推测终末内Ca²⁺的差异积累可能是观察到的突触前功能差异的基础。
