Reiser G, Miledi R
Department of Biophysics, University College London, U.K.
Brain Res. 1989 Feb 6;479(1):83-97. doi: 10.1016/0006-8993(89)91338-3.
Acetylcholine (ACh)-activated channels in end-plates of frog sartorius muscle were studied at various times after denervation. Mean open times of the synaptic membrane channels were derived from the time constant of decay of miniature end-plate currents (tau MEPC) evoked by ACh quanta released from Schwann cells, which replace the motor nerve terminals after these degenerate. Membrane current noise, elicited by iontophoretic application of ACh to voltage-clamped end-plates, was also used to determine mean open time (tau noise) and conductance of the ion channels. About 1 week after denervation, soon after Schwann cell MEPCs appeared, they had a tau similar to that of the neural MEPC in innervated end-plates. However, 5-6 weeks after denervation tau MEPC was increased by a factor of about 5. Circa 4 weeks after denervation, cholinesterase activity of the denervated muscle decreased to 76% of that in the contralateral, innervated muscle, and even 4 months after the operation it was still 64%. Thus, it is unlikely that a change in acetylcholinesterase activity is the main factor responsible for the increase in tau of Schwann cell MEPC. About 1 week after denervation tau noise was close to that in innervated end-plates (about 2 ms). Twelve to 24 days after denervation the average channel open time was 4.5 +/- 1.0 ms, with some end-plates still showing normal 'fast' channels. However, in muscles denervated for 47-113 days the open time was 12.9 +/- 1.9 ms. In the early and intermediate periods, ACh-induced noise spectra with two components were obtained from many end-plates, indicating the simultaneous activation of two different types of channels. At some end-plates during the early and intermediate periods after denervation, but not after about 5 weeks, neostigmine caused the appearance of a component, which was as fast as that of normal end-plate channels. In other experiments small doses of alpha-bungarotoxin were applied in order to predominantly block extra-junctional receptors. In the early period of denervation, when two components were present in the noise spectra, alpha-bungarotoxin eliminated the slow component leaving channels as fast as in innervated end-plates. After prolonged denervation, a component with tau of about 5.5 ms was occasionally disclosed by application of alpha-bungarotoxin. tau noise and tau MEPC from the same end-plate closely agreed. Our results indicate that at frog end-plates the open time of the majority of the synaptic channels opened by ACh becomes longer with increasing time after denervation.
在去神经后的不同时间，对青蛙缝匠肌终板中乙酰胆碱（ACh）激活的通道进行了研究。突触膜通道的平均开放时间由施万细胞释放的ACh量子诱发的微小终板电流（tauMEPC）的衰减时间常数得出，施万细胞在运动神经末梢退化后取代了它们。通过向电压钳制的终板离子电泳施加ACh引发的膜电流噪声，也用于确定离子通道的平均开放时间（tau噪声）和电导。去神经后约1周，施万细胞MEPCs出现后不久，它们的tau与受神经支配终板中的神经MEPC相似。然而，去神经后5 - 6周，tauMEPC增加了约5倍。去神经后约4周，去神经肌肉的胆碱酯酶活性降至对侧受神经支配肌肉的76%，甚至术后4个月仍为64%。因此，乙酰胆碱酯酶活性的变化不太可能是施万细胞MEPC的tau增加负责的主要因素。去神经后约1周，tau噪声接近受神经支配终板中的噪声（约2毫秒）。去神经后12至24天，平均通道开放时间为4.5 +/- 1.0毫秒，一些终板仍显示正常的“快速”通道。然而，在去神经47 - 113天的肌肉中，开放时间为12.9 +/- 1.9毫秒。在早期和中期，从许多终板获得了具有两个成分的ACh诱导噪声谱，表明两种不同类型的通道同时被激活。在去神经后的早期和中期，在一些终板处，但约5周后没有，新斯的明导致出现一个成分，其与正常终板通道的成分一样快。在其他实验中，应用小剂量的α - 银环蛇毒素以主要阻断接头外受体。在去神经的早期，当噪声谱中有两个成分时，α - 银环蛇毒素消除了慢成分，使通道与受神经支配终板中的通道一样快。长时间去神经后，应用α - 银环蛇毒素偶尔会揭示一个tau约为5.5毫秒的成分。来自同一终板的tau噪声和tauMEPC非常一致。我们的结果表明，在青蛙终板处，由ACh打开的大多数突触通道的开放时间随着去神经时间的增加而变长。
