Vyshedskiy A, Lin J W
Department of Biology, Boston University, Massachusetts 02215, USA.
J Neurophysiol. 1997 May;77(5):2300-15. doi: 10.1152/jn.1997.77.5.2300.
Facilitation at the crayfish neuromuscular inhibitor synapse was investigated with the use of a presynaptic voltage control method in which 5-ms presynaptic pulses were used to activate and monitor facilitation. A single 5-ms pulse was able to activate facilitation with a decay time constant similar to that of the F2 component of facilitation activated by action potentials. The quality of the control of presynaptic potential during F2 facilitation was evaluated by measuring the amplitude of presynaptic pulses and by analyzing the shape of the depolarization-release coupling plot during facilitation. Both approaches suggested that neither the amplitude of presynaptic depolarizations nor the space clamp of the presynaptic axon was changed during F2 facilitation. The activation of facilitation was examined by changing the amplitude of conditioning pulses systematically and using a test pulse of a constant amplitude to monitor facilitation. We found that a significant amount of facilitation could be activated by conditioning pulses that were subthreshold to the activation of transmitter release. Facilitation plateaued before the inhibitory postsynaptic potentials (IPSPs) activated by conditioning pulses reached their maximum. A double logarithm plot of facilitation magnitude against the conditioning IPSP amplitude yielded a slope of 0.34, which implies that the calcium ion cooperativity of activating facilitation is about one third of the secretion process. These findings enabled us to activate near maximal facilitation, by a burst of subthreshold conditioning pulses, without any conditioning transmitter release, and, therefore, to avoid complications associated with previous transmitter release. The detection of facilitation was examined by changing test pulse amplitude systematically to evaluate the ability of the test pulse to detect a constant level of facilitation. The magnitude of normalized facilitation decreased with increasing test pulse amplitude. The magnitude of absolute facilitation (the amplitude of the facilitated minus the control IPSP) increased with increasing test pulse amplitude. A double logarithm plot between facilitated and control IPSPs gave rise to a slope of 0.77, which suggests that the calcium cooperativity of transmitter release was decreased during facilitation.
利用突触前电压控制方法研究了小龙虾神经肌肉抑制性突触的易化作用，其中使用5毫秒的突触前脉冲来激活并监测易化作用。单个5毫秒的脉冲能够激活易化作用，其衰减时间常数与动作电位激活的易化作用的F2成分相似。在F2易化作用期间，通过测量突触前脉冲的幅度以及分析易化作用期间去极化-释放偶联图的形状，来评估突触前电位的控制质量。两种方法均表明，在F2易化作用期间，突触前去极化的幅度和突触前轴突的空间钳制均未改变。通过系统地改变条件刺激脉冲的幅度，并使用恒定幅度的测试脉冲来监测易化作用，对易化作用的激活进行了研究。我们发现，低于递质释放激活阈值的条件刺激脉冲能够激活大量的易化作用。在条件刺激脉冲激活的抑制性突触后电位（IPSP）达到最大值之前，易化作用达到平稳状态。易化幅度相对于条件IPSP幅度的双对数图的斜率为0.34，这意味着激活易化作用的钙离子协同性约为分泌过程的三分之一。这些发现使我们能够通过一串低于阈值的条件刺激脉冲激活接近最大程度的易化作用，而无需任何条件刺激递质释放，从而避免与先前递质释放相关的并发症。通过系统地改变测试脉冲幅度来评估测试脉冲检测恒定水平易化作用的能力，对易化作用的检测进行了研究。标准化易化作用的幅度随测试脉冲幅度的增加而降低。绝对易化作用的幅度（易化后的IPSP幅度减去对照IPSP幅度）随测试脉冲幅度的增加而增加。易化后的IPSP与对照IPSP之间的双对数图的斜率为0.77，这表明在易化作用期间递质释放的钙协同性降低。
