Nair Anu G, Bollmohr Nasrin, Schökle Levin, Keim Jennifer, Melero José María Mateos, Müller Martin
Department of Molecular Life Sciences, University of Zurich, Zurich, Switzerland.
Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden.
J Physiol. 2024 Aug 26. doi: 10.1113/JP286176.
Repetitive synaptic stimulation can induce different forms of synaptic plasticity but may also limit the robustness of synaptic transmission by exhausting key resources. Little is known about how synaptic transmission is stabilized after high-frequency stimulation. In the present study, we observed that tetanic stimulation of the Drosophila neuromuscular junction (NMJ) decreases quantal content, release-ready vesicle pool size and synaptic vesicle density for minutes after stimulation. This was accompanied by a pronounced increase in quantal size. Interestingly, action potential-evoked synaptic transmission remained largely unchanged. EPSC amplitude fluctuation analysis confirmed the post-tetanic increase in quantal size and the decrease in quantal content, suggesting that the quantal size increase counteracts release depression to maintain evoked transmission. The magnitude of the post-tetanic quantal size increase and release depression correlated with stimulation frequency and duration, indicating activity-dependent stabilization of synaptic transmission. The post-tetanic quantal size increase persisted after genetic ablation of the glutamate receptor subunits GluRIIA or GluRIIB, and glutamate receptor calcium permeability, as well as blockade of postsynaptic calcium channels. By contrast, it was strongly attenuated by pharmacological or presynaptic genetic perturbation of the GTPase dynamin. Similar observations were made after inhibition of the H-ATPase, suggesting that the quantal size increase is presynaptically driven. Additionally, dynamin and H-ATPase perturbation resulted in a post-tetanic decrease in evoked amplitudes. Finally, we observed an increase in synaptic vesicle diameter after tetanic stimulation. Thus, a presynaptically-driven quantal size increase, likely mediated by larger synaptic vesicles, counterbalances post-tetanic release depression, thereby conferring robustness to synaptic transmission on the minute time scale. KEY POINTS: Many synapses transmit robustly after sustained activity despite the limitation of key resources, such as release-ready synaptic vesicles. We report robust synaptic transmission after sustained high-frequency stimulation of the Drosophila neuromuscular junction despite a reduction in release-ready vesicle number. An increased postsynaptic response to individual vesicles, likely driven by an increase in vesicle size due to endocytosis defects, stabilizes synaptic efficacy for minutes after sustained activity. Our study provides novel insights into the mechanisms governing synaptic stability after sustained neural activity.
重复性突触刺激可诱导不同形式的突触可塑性，但也可能因耗尽关键资源而限制突触传递的稳健性。关于高频刺激后突触传递如何稳定，人们知之甚少。在本研究中，我们观察到，对果蝇神经肌肉接头（NMJ）进行强直刺激后，刺激后数分钟内量子含量、易释放囊泡池大小和突触囊泡密度均降低。与此同时，量子大小显著增加。有趣的是，动作电位诱发的突触传递基本保持不变。兴奋性突触后电流（EPSC）幅度波动分析证实了强直刺激后量子大小增加和量子含量减少，这表明量子大小增加可抵消释放抑制，从而维持诱发传递。强直刺激后量子大小增加的幅度和释放抑制与刺激频率和持续时间相关，表明突触传递的活动依赖性稳定。在谷氨酸受体亚基GluRIIA或GluRIIB基因敲除、谷氨酸受体钙通透性以及突触后钙通道阻断后，强直刺激后量子大小增加仍持续存在。相比之下，通过对GTP酶发动蛋白进行药理学或突触前基因干扰，这种增加会被强烈减弱。在抑制H-ATP酶后也有类似观察结果，表明量子大小增加是由突触前驱动的。此外，发动蛋白和H-ATP酶干扰导致强直刺激后诱发幅度降低。最后，我们观察到强直刺激后突触囊泡直径增加。因此，由较大突触囊泡介导的突触前驱动的量子大小增加可抵消强直刺激后释放抑制，从而在分钟时间尺度上赋予突触传递稳健性。要点：尽管关键资源（如易释放突触囊泡）有限，但许多突触在持续活动后仍能稳健传递。我们报告了对果蝇神经肌肉接头进行持续高频刺激后，尽管易释放囊泡数量减少，但突触传递仍很稳健。对单个囊泡的突触后反应增加，可能是由于内吞缺陷导致囊泡大小增加所驱动，在持续活动后数分钟内稳定了突触效能。我们的研究为持续神经活动后控制突触稳定性的机制提供了新见解。
