Slutsky I, Wess J, Gomeza J, Dudel J, Parnas I, Parnas H
The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem 91904, Israel.
J Neurophysiol. 2003 Apr;89(4):1954-67. doi: 10.1152/jn.00668.2002.
We have previously suggested that presynaptic M(2)-muscarinic receptors (M(2)R) are involved in the control of the time course of evoked acetylcholine release in the frog neuromuscular junction. The availability of knockout mice lacking functional M(2)R (M(2)-KO) enabled us to address this issue in a more direct way. Using the phrenic diaphragm preparation, we show that in wild-type (WT) mice experimental manipulations known to affect Ca(2+) entry and removal, greatly affected the amount of acetylcholine released (quantal content). However, the time course of release remained unaltered under all these experimental treatments. On the other hand, in the M(2)-KO mice, similar experimental treatments affected both the quantal content and the time course of release. In general, a larger quantal content was accompanied by a longer duration of release. Similarly, the rise time of the postsynaptic current produced by axon stimulation was sensitive to changes in Ca(2+) or Mg(2+) in M(2)-KO mice but not in WT mice. Measurements of Ca(2+) currents revealed that the shorter rise time of the postsynaptic current seen in high Mg(2+) in M(2)-KO mice was not produced by a shorter wave of the presynaptic Ca(2+) current. These results support our earlier findings and provide direct evidence for the major role that presynaptic M(2)-muscarinic receptors play in the control of the time course of evoked acetylcholine release under physiological conditions.
我们之前曾提出，突触前M2型毒蕈碱受体（M2R）参与调控青蛙神经肌肉接头处诱发的乙酰胆碱释放的时间进程。缺乏功能性M2R的基因敲除小鼠（M2-KO）的出现，使我们能够更直接地解决这个问题。利用膈神经膈肌标本，我们发现，在野生型（WT）小鼠中，已知会影响钙离子内流和外流的实验操作，会极大地影响乙酰胆碱的释放量（量子含量）。然而，在所有这些实验处理下，释放的时间进程保持不变。另一方面，在M2-KO小鼠中，类似的实验处理既影响量子含量，也影响释放的时间进程。一般来说，较大的量子含量伴随着较长的释放持续时间。同样，轴突刺激产生的突触后电流的上升时间，在M2-KO小鼠中对细胞外钙离子浓度（[Ca2+]o）或细胞外镁离子浓度（[Mg2+]o）的变化敏感，而在WT小鼠中则不然。钙离子电流测量结果显示，M2-KO小鼠在高细胞外镁离子浓度下突触后电流较短的上升时间，并非由突触前钙离子电流较短的波峰产生。这些结果支持了我们早期的发现，并为突触前M2型毒蕈碱受体在生理条件下控制诱发的乙酰胆碱释放的时间进程中所起的主要作用提供了直接证据。
