Lo Y J, Poo M M
Department of Biological Sciences, Columbia University, New York, New York 10027.
J Neurosci. 1994 Aug;14(8):4684-93. doi: 10.1523/JNEUROSCI.14-08-04684.1994.
Activity-dependent competition between two spinal neurons coinnervating an embryonic myocyte was studied in Xenopus cell culture. We have characterized in detail the phenomenon of heterosynaptic suppression by which tetanic stimulation of one neuron results in functional suppression of the synapse made by the untetanized neuron (Lo and Poo, 1991). Fluorescence labeling of the neurons using two different fluorophores revealed that the coinnervating nerve terminals on the spherical myocyte were in close proximity. Heterosynaptic suppression could be induced when the postsynaptic cell was held under either current-clamp or voltage-clamp conditions during the tetanic stimulation. This finding, together with the observation that repetitive postsynaptic depolarization of the myocyte by direct current injection was much less effective in inducing synaptic depression, suggests that postsynaptic ACh receptor activation plays a dominant role in the induction of heterosynaptic suppression. The heterosynaptic suppression appears to be mediated by a rise of Ca2+ levels in the postsynaptic cell, since it was not observed when the cytosolic Ca2+ concentration of the myocyte was buffered at a low level with intracellular loading of a Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA). The dependence of heterosynaptic suppression on the pattern of tetanic stimulation was also studied. At a stimulation frequency of 2 Hz, detectable heterosynaptic suppression could be induced after 20 repetitive stimuli were applied to one of the presynaptic neurons and the suppression was more effective with increasing number of stimuli. Over the range of 0.5-5 Hz, the extent of suppression was independent of the frequency of tetanic stimulation and, in some cells, detectable suppression could be induced at a frequency as low as 0.05 Hz. Except for a few cases, heterosynaptic suppression was found to last for as long as the recording was made after tetanus (up to 1 hr). The fact that the mean amplitude of spontaneous synaptic currents remained the same before and after the suppression while the evoked synaptic currents exhibited higher fluctuation after suppression suggests that the observed synaptic suppression involves a reduction of evoked ACh release from the nerve terminal, although postsynaptic changes have not been excluded. Finally we found that spontaneous synaptic activity may also contribute in part to the synaptic competition between coinnervating nerve terminals. Taken together, these findings provide a quantitative basis for further understanding of activity-dependent competition between developing neuromuscular synapses.
在非洲爪蟾细胞培养中研究了共同支配一个胚胎肌细胞的两个脊髓神经元之间的活动依赖性竞争。我们详细描述了异突触抑制现象，即对一个神经元进行强直刺激会导致未受强直刺激的神经元所形成的突触发生功能性抑制（Lo和Poo，1991年）。使用两种不同的荧光团对神经元进行荧光标记显示，球形肌细胞上共同支配的神经末梢彼此紧邻。在强直刺激期间，当突触后细胞处于电流钳或电压钳条件下时，可诱导异突触抑制。这一发现，连同通过直流电注入对肌细胞进行重复性突触后去极化在诱导突触抑制方面效果差得多的观察结果，表明突触后乙酰胆碱受体激活在异突触抑制的诱导中起主导作用。异突触抑制似乎是由突触后细胞中钙离子水平升高介导的，因为当用钙离子螯合剂1,2-双（2-氨基苯氧基）乙烷-N,N,N',N'-四乙酸（BAPTA）进行细胞内加载将肌细胞的胞质钙离子浓度缓冲在低水平时未观察到这种抑制。还研究了异突触抑制对强直刺激模式的依赖性。在2 Hz的刺激频率下，对其中一个突触前神经元施加20次重复刺激后可诱导出可检测到的异突触抑制，并且随着刺激次数增加抑制更有效。在0.5 - 5 Hz范围内，抑制程度与强直刺激频率无关，并且在一些细胞中，在低至0.05 Hz的频率下也可诱导出可检测到的抑制。除了少数情况外，发现异突触抑制在破伤风后进行记录的整个期间（长达1小时）都持续存在。抑制前后自发突触电流的平均幅度保持不变而诱发突触电流在抑制后表现出更高的波动这一事实表明，观察到的突触抑制涉及神经末梢诱发的乙酰胆碱释放减少，尽管尚未排除突触后变化。最后我们发现自发突触活动也可能部分促成共同支配神经末梢之间的突触竞争。综上所述，这些发现为进一步理解发育中的神经肌肉突触之间的活动依赖性竞争提供了定量基础。
