State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangdong Provincial Key Laboratory of Ophthalmology and Visual Science, Guangzhou 510060, China
eNeuro. 2022 May 18;9(3). doi: 10.1523/ENEURO.0074-22.2022. Print 2022 May-Jun.
Excitatory amino acid transporters (EAATs) control visual signal transmission in the retina by rapidly removing glutamate released from photoreceptors and bipolar cells (BCs). Although it has been reported that EAAT2 and EAAT5 are expressed at presynaptic terminals of photoreceptors and some BCs in mammals, the distinct functions of these two glutamate transporters in retinal synaptic transmission, especially at a single synapse, remain elusive. In this study, we found that EAAT2 was expressed in all BC types while coexisting with EAAT5 in rod bipolar (RB) cells and several types of cone BCs from mice of either sex. Our immunohistochemical study, together with a recently published literature (Gehlen et al., 2021), showed that EAAT2 and EAAT5 were both located in RB axon terminals near release sites. Optogenetic, electrophysiological and pharmacological analyses, however, demonstrated that EAAT2 and EAAT5 regulated neurotransmission at RB→AII amacrine cell synapses in significantly different ways: EAAT5 dramatically affected both the peak amplitude and kinetics of postsynaptic responses in AIIs, whereas EAAT2 had either relatively small or opposite effects. By contrast, blockade of EAAT1/GLAST, which was exclusively expressed in Müller cells, showed no obvious effect on AII responses, indicating that glutamate uptake by Müller cells did not influence synaptic transmission from RB terminals. Furthermore, we found that temporal resolution at RB→AII synapses was reduced substantially by blockade of EAAT5 but not EAAT2. Taken together, our work reveals the distinct functions of EAAT2 and EAAT5 in signal transmission at RB ribbon synapses.
兴奋性氨基酸转运体（EAATs）通过快速清除从光感受器和双极细胞（BCs）释放的谷氨酸来控制视网膜中的视觉信号传递。尽管已经报道 EAAT2 和 EAAT5 在哺乳动物光感受器和某些 BCs 的突触前末端表达，但这两种谷氨酸转运体在视网膜突触传递中的独特功能，尤其是在单个突触上，仍然难以捉摸。在这项研究中，我们发现 EAAT2 在所有 BC 类型中表达，而在杆状双极（RB）细胞和来自雌雄小鼠的几种类型的 cone BCs 中与 EAAT5 共存。我们的免疫组织化学研究，以及最近发表的文献（Gehlen 等人，2021）表明，EAAT2 和 EAAT5 都位于 RB 轴突末梢附近的释放位点。然而，光遗传学、电生理学和药理学分析表明，EAAT2 和 EAAT5 以明显不同的方式调节 RB→AII 无长突细胞突触的神经传递：EAAT5 显著影响 AII 中的突触后反应的峰值幅度和动力学，而 EAAT2 的影响相对较小或相反。相比之下，仅在 Müller 细胞中表达的 EAAT1/GLAST 的阻断对 AII 反应没有明显影响，表明 Müller 细胞摄取的谷氨酸不会影响 RB 末梢的突触传递。此外，我们发现 EAAT5 的阻断显著降低了 RB→AII 突触的时间分辨率，但 EAAT2 的阻断则没有。总之，我们的工作揭示了 EAAT2 和 EAAT5 在 RB 带状突触信号传递中的不同功能。
