Taschenberger H, von Gersdorff H
The Vollum Institute, Oregon Health Sciences University, Portland, Oregon 97201, USA.
J Neurosci. 2000 Dec 15;20(24):9162-73. doi: 10.1523/JNEUROSCI.20-24-09162.2000.
Fast, precise, and sustained synaptic transmission at high frequency is thought to be crucial for the task of sound localization in the auditory brainstem. However, recordings from the calyx of Held synapse have revealed severe frequency-dependent synaptic depression, which tends to degrade the exact timing of postsynaptic spikes. Here we investigate the functional changes occurring throughout the critical period of synapse refinement from immature calyx terminal [postnatal day 5 (P5)] to after the onset of hearing (P12-P14). Surprisingly, for recordings near physiological temperature (35 degrees C), we find that P14 synapses are already able to follow extremely high input rates of up to 800 Hz. This ability stems in part from a remarkable shortening of presynaptic action potentials, which may lead to a lowering of release probability and decrease in synaptic delays during development. In addition, AMPA receptor-mediated EPSCs as well as quantal synaptic currents acquired progressively faster kinetics, although their mean amplitudes did not change significantly. NMDA receptor-mediated EPSCs, however, diminished with age, as indicated by a 50% reduction in mean amplitude and faster decay kinetics. Finally, the degree of synaptic depression was greatly attenuated with age, presumably because of a 2.5-fold or larger increase in the releasable pool of vesicles, which together with a decreasing release probability produces a fairly constant EPSC amplitude. This finely tuned orchestra of developmental changes thus simultaneously promotes speed while preventing premature vesicle pool depletion during prolonged bouts of firing. A few critical days in postnatal development can thus have a large impact on synaptic function.
高频下快速、精确且持续的突触传递被认为对听觉脑干中的声音定位任务至关重要。然而，对Held壶腹突触的记录显示出严重的频率依赖性突触抑制，这往往会使突触后峰电位的确切时间退化。在这里，我们研究了从未成熟的壶腹终末（出生后第5天，P5）到听力开始后（P12 - P14）整个突触精细化关键期内发生的功能变化。令人惊讶的是，对于接近生理温度（35摄氏度）的记录，我们发现P14突触已经能够跟随高达800赫兹的极高输入率。这种能力部分源于突触前动作电位的显著缩短，这可能导致发育过程中释放概率降低和突触延迟减少。此外，AMPA受体介导的兴奋性突触后电流（EPSCs）以及量子突触电流的动力学逐渐加快，尽管它们的平均幅度没有显著变化。然而，NMDA受体介导的EPSCs随着年龄增长而减少，平均幅度降低50%以及更快的衰减动力学表明了这一点。最后，突触抑制程度随着年龄增长而大大减弱，推测这是由于可释放囊泡池增加了2.5倍或更多，这与降低的释放概率一起产生了相当恒定的EPSC幅度。因此，这种精细调节的发育变化协同作用，在长时间放电期间既促进了速度，又防止了囊泡池过早耗尽。因此，出生后发育中的几个关键天数会对突触功能产生很大影响。
