Delaney K R, Zucker R S, Tank D W
Department of Molecular and Cell Biology, University of California, Berkeley 94720.
J Neurosci. 1989 Oct;9(10):3558-67. doi: 10.1523/JNEUROSCI.09-10-03558.1989.
We have used fura-2 fluorescence to study the effects of repetitive stimulation producing posttetanic potentiation (PTP) at crayfish neuromuscular junctions on presynaptic calcium concentration. Fura-2 was injected into the preterminal axon of the excitor motor neuron to the claw opener muscle of a walking leg. Pictures of presynaptic terminals on the muscle surface were obtained with a charge-coupled device camera, ratioed, and converted to spatial images of intracellular calcium concentration. Stimulation of the motor nerve for 7-10 min at 20-33 Hz produced potentiation during the tetanus and PTP following the tetanus. Presynaptic calcium levels in terminal boutons and varicosities rose to about 2 microM during the tetanus and decayed at first rapidly and then slowly back to levels near the initial concentration of about 200 nM. The decay rate of potentiated synaptic transmission was the same as the decay rate of the elevated calcium concentration during the posttetanic period dominated by PTP, when facilitation and augmentation had dissipated. A 13-fold potentiation corresponded to a 500 nM elevation of calcium to about 700 nM. The linear dependence we observed is not consistent with the power law formulation of a residual calcium hypothesis for PTP. During the tetanus, the enhancement of synaptic transmission due to facilitation, augmentation, and potentiation exceeded that expected from the correspondence between PTP and posttetanic calcium levels. This may occur because during the tetanus there is insufficient time for calcium to equilibrate spatially between action potentials, and the submembrane calcium will be higher than the volume-average calcium levels that we detect. Following low-frequency trains (typically 8 Hz for about 35 sec), enhanced synaptic transmission and elevated presynaptic calcium decayed rapidly, within a few seconds. Short high-frequency trains (50-100 Hz for 1-2 min) elicited an additional hours-long elevation of presynaptic calcium, corresponding to, and perhaps responsible for, part of the long-term potentiation of transmission that such stimulation produces at this synapse.
我们利用fura - 2荧光来研究重复性刺激在小龙虾神经肌肉接头处产生强直后增强（PTP）时对突触前钙浓度的影响。将fura - 2注入到行走腿爪 opener肌肉的兴奋性运动神经元的终末前轴突中。用电荷耦合器件相机获取肌肉表面突触前终末的图像，进行比率测定，并转换为细胞内钙浓度的空间图像。以20 - 33Hz的频率刺激运动神经7 - 10分钟，在强直期间产生增强，并在强直后产生PTP。在强直期间，终末小体和曲张体中的突触前钙水平上升至约2微摩尔，最初迅速衰减，然后缓慢回到接近初始浓度约200纳摩尔的水平。在由PTP主导的强直后时期，当易化和增强作用消失时，增强的突触传递的衰减速率与升高的钙浓度的衰减速率相同。13倍的增强对应于钙浓度从约200纳摩尔升高500纳摩尔至约700纳摩尔。我们观察到的线性相关性与PTP的残余钙假说的幂律公式不一致。在强直期间，由于易化、增强和增强作用导致的突触传递增强超过了PTP与强直后钙水平之间对应关系所预期的增强。这可能是因为在强直期间，钙在动作电位之间没有足够的时间进行空间平衡，并且膜下钙将高于我们检测到的体积平均钙水平。在低频串刺激（通常为8Hz，持续约35秒）后，增强的突触传递和升高的突触前钙在几秒钟内迅速衰减。短时间的高频串刺激（50 - 100Hz，持续1 - 2分钟）引发突触前钙的额外长达数小时的升高，这与这种刺激在该突触产生的部分长期传递增强相对应，并且可能是其原因。
