Truhlsen Eye Institute, Departments of Ophthalmology & Visual Sciences and Pharmacology & Experimental Neuroscience, University of Nebraska Medical Center, Omaha, NE, 68198, USA.
Pflugers Arch. 2021 Sep;473(9):1469-1491. doi: 10.1007/s00424-021-02548-9. Epub 2021 Mar 29.
Light-evoked voltage responses of rod and cone photoreceptor cells in the vertebrate retina must be converted to a train of synaptic vesicle release events for transmission to downstream neurons. This review discusses the processes, proteins, and structures that shape this critical early step in vision, focusing on studies from salamander retina with comparisons to other experimental animals. Many mechanisms are conserved across species. In cones, glutamate release is confined to ribbon release sites although rods are also capable of release at non-ribbon sites. The role of non-ribbon release in rods remains unclear. Release from synaptic ribbons in rods and cones involves at least three vesicle pools: a readily releasable pool (RRP) matching the number of membrane-associated vesicles along the ribbon base, a ribbon reserve pool matching the number of additional vesicles on the ribbon, and an enormous cytoplasmic reserve. Vesicle release increases in parallel with Ca channel activity. While the opening of only a few Ca channels beneath each ribbon can trigger fusion of a single vesicle, sustained release rates in darkness are governed by the rate at which the RRP can be replenished. The number of vacant release sites, their functional status, and the rate of vesicle delivery in turn govern replenishment. Along with an overview of the mechanisms of exocytosis and endocytosis, we consider specific properties of ribbon-associated proteins and pose a number of remaining questions about this first synapse in the visual system.
光激发的脊椎动物视网膜中的视杆和视锥光感受器细胞的电压响应必须转换为一连串的突触囊泡释放事件，以便向下游神经元传递。这篇综述讨论了塑造视觉这一关键早期步骤的过程、蛋白质和结构，重点是来自蝾螈视网膜的研究，并与其他实验动物进行了比较。许多机制在物种间是保守的。在锥体中，谷氨酸的释放仅限于带状释放部位，尽管杆状细胞也能够在非带状部位释放。在杆状细胞中，非带状释放的作用仍不清楚。突触带中的释放涉及至少三个囊泡池：与带状基底上的膜相关囊泡数量相匹配的易释放池 (RRP)、与带状上的额外囊泡数量相匹配的带状储备池以及巨大的细胞质储备池。囊泡释放与 Ca 通道活性平行增加。虽然每个带状物下方只有几个 Ca 通道的开放可以触发单个囊泡的融合，但在黑暗中的持续释放速率受 RRP 补充的速率控制。空释放位点的数量、它们的功能状态以及囊泡的传递速率反过来又控制着补充。除了对胞吐作用和胞吞作用机制的概述外，我们还考虑了与带状相关的蛋白质的特定性质，并提出了关于视觉系统中这个第一个突触的许多悬而未决的问题。
