Wheeler D B, Randall A, Tsien R W
Neurosciences Program, Stanford University School of Medicine, California 94305, USA.
J Neurosci. 1996 Apr 1;16(7):2226-37. doi: 10.1523/JNEUROSCI.16-07-02226.1996.
It has been established that multiple types of Ca2+ channels participate in triggering neurotransmitter release at central synapses, but there is uncertainty about the nature of their combined actions. We investigated synaptic transmission at CA3-CA1 synapses of rat hippocampal slices and asked whether the dependence on omega-CTx-GVIA-sensitive N-type channels and omega-Aga-IVA-sensitive P/Q-type Ca2+ channels can be altered by physiological mechanisms. The reliance on multiple types of Ca2+ channels was not absolute but depended strongly on the amount of Ca2+ influx through individual channels, which was manipulated by prolonging the presynaptic action potential with the K+ channel blocker 4-aminopyridine (4-AP) and by varying the extracellular Ca2+ concentration ([Ca2+]o). We quantified the influence of spike broadening on Ca2+ influx through various Ca2+ channels by imposing mock action potentials on voltage-clamped cerebellar granule neurons. In field recordings of the EPSP in hippocampal slices, action potential prolongation increased the EPSP slope by 2-fold and decreased its reliance on either N-type or P/Q-type Ca2+ channels. The inhibition of synaptic transmission by N-type channel blockade was virtually eliminated in the presence of 4-AP, but it could be restored by lowering [Ca2+]o. These results rule out a scenario in which a significant fraction of presynaptic terminals rely solely on N-type channels to trigger transmission. The change in sensitivity to the neurotoxins with 4-AP could be explained in terms of a nonlinear relationship between Ca2+ entry and synaptic strength, which rises steeply at low [Ca2+]o, but approaches saturation at high [Ca2+]o. This relationship was evaluated experimentally by varying [CA2+]o in the absence and presence of 4-AP. One consequence of this relationship is that down-modulation of presynaptic Ca2+ channels by various modulators would increase the relative impact of spike broadening greatly.
已经确定多种类型的Ca2+通道参与触发中枢突触处的神经递质释放，但它们联合作用的性质尚不确定。我们研究了大鼠海马切片CA3-CA1突触处的突触传递，并询问对ω-芋螺毒素GVIA敏感的N型通道和对ω-银环蛇毒素IVA敏感的P/Q型Ca2+通道的依赖性是否可通过生理机制改变。对多种类型Ca2+通道的依赖并非绝对，而是强烈依赖于通过单个通道的Ca2+内流的量，这可通过用K+通道阻滞剂4-氨基吡啶（4-AP）延长突触前动作电位以及改变细胞外Ca2+浓度（[Ca2+]o）来操纵。我们通过在电压钳制的小脑颗粒神经元上施加模拟动作电位，量化了动作电位展宽对通过各种Ca2+通道的Ca2+内流的影响。在海马切片的场兴奋性突触后电位（EPSP）记录中，动作电位延长使EPSP斜率增加了2倍，并降低了其对N型或P/Q型Ca2+通道的依赖性。在存在4-AP的情况下，N型通道阻断对突触传递的抑制作用几乎消除，但通过降低[Ca2+]o可恢复。这些结果排除了很大一部分突触前终末仅依赖N型通道来触发传递的情况。4-AP导致的对神经毒素敏感性的变化可以根据Ca2+内流与突触强度之间的非线性关系来解释，这种关系在低[Ca2+]o时急剧上升，但在高[Ca2+]o时接近饱和。通过在不存在和存在4-AP的情况下改变[Ca2+]o对这种关系进行了实验评估。这种关系的一个结果是，各种调节剂对突触前Ca2+通道的下调将大大增加动作电位展宽的相对影响。
