脑功能依赖于化学突触处神经递质的囊泡释放。释放概率取决于动作电位诱发的突触前Ca2进入,而且在静息的Ca2+水平上。突触前钙稳态的这些基本方面是否沿轴突路径显示出任何一致的趋势,以及它们如何被本地网络活动控制,仍然知之甚少。这里,我们利用最近先进的基于FLIM的方法以纳摩尔灵敏度监测突触前Ca2+。我们发现,在皮质锥体神经元中,动作电位诱发的钙进入(范围10-300nM),但不是静息的Ca2+水平(范围10-100nM),倾向于随着轴突分支的高阶而增加。阻断星形胶质细胞谷氨酸的摄取会减少诱发的Ca2进入,但对静息的Ca2几乎没有影响,而两者似乎都受到1/2组代谢型谷氨酸受体的组成性激活的推动。我们发现瞬时体细胞去极化或超极化对突触前Ca2进入或其基础水平没有一致的影响。结果揭示了皮质回路中突触前机制的一些关键方面,揭示大脑突触连接的基本原理。
Brain function relies on vesicular release of neurotransmitters at chemical synapses. The release probability depends on action potential-evoked presynaptic Ca2+ entry, but also on the resting Ca2+ level. Whether these basic aspects of presynaptic calcium homeostasis show any consistent trend along the axonal path, and how they are controlled by local network activity, remains poorly understood. Here, we take advantage of the recently advanced FLIM-based method to monitor presynaptic Ca2+ with nanomolar sensitivity. We find that, in cortical pyramidal neurons, action potential-evoked calcium entry (range 10-300 nM), but not the resting Ca2+ level (range 10-100 nM), tends to increase with higher order of axonal branches. Blocking astroglial glutamate uptake reduces evoked Ca2+ entry but has little effect on resting Ca2+ whereas both appear boosted by the constitutive activation of group 1/2 metabotropic glutamate receptors. We find no consistent effect of transient somatic depolarization or hyperpolarization on presynaptic Ca2+ entry or its basal level. The results unveil some key aspects of presynaptic machinery in cortical circuits, shedding light on basic principles of synaptic connectivity in the brain.