Max Planck Institute for Biophysical Chemistry, D-37077 Göttingen, Germany.
J Neurosci. 2012 Jan 25;32(4):1261-72. doi: 10.1523/JNEUROSCI.3838-11.2012.
Voltage-gated Ca2+ channels (VGCCs) of the P/Q-type, which are expressed at a majority of mammalian nerve terminals, show two types of Ca2+-dependent feedback regulation-inactivation (CDI) and facilitation (CDF). Because of the nonlinear relationship between Ca2+ influx and transmitter release, CDI and CDF are powerful regulators of synaptic strength. To what extent VGCCs inactivate or facilitate during spike trains depends on the dynamics of free Ca2+ ([Ca2+]i) and the Ca2+ sensitivity of CDI and CDF, which has not been determined in nerve terminals. In this report, we took advantage of the large size of a rat auditory glutamatergic synapse--the calyx of Held--and combined voltage-clamp recordings of presynaptic Ca2+ currents (ICa(V)) with UV-light flash-induced Ca2+ uncaging and presynaptic Ca2+ imaging to study the Ca2+ requirements for CDI and CDF. We find that nearly half of the presynaptic VGCCs inactivate during 100 ms voltage steps and require several seconds to recover. This inactivation is caused neither by depletion of Ca2+ ions from the synaptic cleft nor by metabotropic feedback inhibition, because it is resistant to blockade of metabotropic and ionotropic glutamate receptors. Facilitation of ICa(V) induced by repetitive depolarizations or preconditioning voltage steps decays within tens of milliseconds. Since Ca2+ buffers only weakly affect CDI and CDF, we conclude that the Ca2+ sensors are closely associated with the channel. CDI and CDF can be induced by intracellular photo release of Ca2+ resulting in [Ca2+]i elevations in the low micromolar range, implying a surprisingly high affinity of the Ca2+ sensors.
电压门控钙通道（VGCCs）的 P/Q 型，在大多数哺乳动物神经末梢表达，表现出两种类型的钙依赖性反馈调节失活（CDI）和易化（CDF）。由于钙内流和递质释放之间的非线性关系，CDI 和 CDF 是突触强度的有力调节因子。在尖峰列车期间，VGCC 失活或易化的程度取决于游离钙（[Ca2+]i）的动力学和 CDI 和 CDF 的钙敏感性，这在神经末梢尚未确定。在本报告中，我们利用大鼠听觉谷氨酸能突触——海氏复合体的大尺寸，并结合电压钳记录的突触前 Ca2+电流（ICa（V））与紫外光闪光诱导的 Ca2+去笼和突触前 Ca2+成像，研究 CDI 和 CDF 的 Ca2+需求。我们发现，在 100ms 电压步长期间，近一半的突触前 VGCCs 失活，需要数秒才能恢复。这种失活既不是由于突触小泡中 Ca2+离子的耗竭引起的，也不是由于代谢型反馈抑制引起的，因为它不受代谢型和离子型谷氨酸受体阻断的影响。重复去极化或预处理电压步长诱导的 ICa（V）易化在数十毫秒内衰减。由于 Ca2+缓冲剂对 CDI 和 CDF 的影响较弱，我们得出结论，Ca2+传感器与通道密切相关。CDI 和 CDF 可通过细胞内光释放 Ca2+诱导，导致[Ca2+]i 升高至低微摩尔范围，这意味着 Ca2+传感器具有惊人的高亲和力。
