Transmitter Release From Mammalian Horizontal Cells.

描述（由申请人提供）：光感受器突触三联体，由光感受器末端、双极细胞树突和水平细胞末端组成，是一种非常重要的特殊突触复合体。从生理学上讲，这是光感受器传递视觉信息的初始位置，信息传递的保真度对视觉处理至关重要。水平细胞介导外视网膜双极细胞树突和光感受器末端的抑制性反馈；然而，哺乳动物水平细胞释放递质的机制尚不清楚。例如，突触三联体中的水平细胞末端具有相对较少的小的、核心清晰的囊泡，并且它们缺乏传统定义的突触前膜特化。相反，这些细胞表达已建立的突触囊泡蛋白，包括囊泡GABA转运蛋白（VGAT）和SV2A，一组突触蛋白，包括SNAP-25和络合蛋白，与胞吐和l型Ca2+通道相关，表明GABA是通过囊泡机制释放的。这种机制不同于先前建立的非哺乳动物水平细胞的GABA质乳转运机制。

DESCRIPTION (provided by applicant): The photoreceptor synaptic triad, consisting of a photoreceptor terminal, bipolar cell dendrites and horizontal cell endings, is a specialized synaptic complex of great importance. Physiologically, this is the site of initial transfer of visual information from photoreceptors and the fidelity of information transfer is critically important for visual processing. Horizontal cells mediate inhibitory feedback in the outer retina at bipolar cell dendrites and photoreceptor terminals; however, the mechanisms that underlie transmitter release from mammalian horizontal cells are poorly understood. For instance, horizontal cell endings in the synaptic triad have relatively few small, clear-core vesicles and they lack conventionally defined presynaptic membrane specializations. In contrast, these cells express established synaptic vesicle proteins, including the vesicular GABA transporter (VGAT) and SV2A, a complement of synaptic proteins, including SNAP-25 and complexin, that are associated with exocytosis, and L-type Ca2+ channels, suggesting GABA is released by a vesicular mechanism. This mechanism differs from a previously established GABA plasmalemmal transporter mechanism described for non-mammalian horizontal cells. The long-term objective of this project is to understand the functional role of mammalian horizontal cells in visual information processing. This application will address this objective by examining the hypothesis that the cellular structure and biochemical machinery that mediate vesicular transmitter release are present in mammalian horizontal cells. This hypothesis will be examined as follows: Specific aim 1: Characterize vesicles in horizontal cell endings in the synaptic triad. Vesicle type, number and distribution in horizontal cell dendrites that innervate cone pedicles and axonal terminals that innervate rod spherules will be examined with conventional electron microscopy. Specific aim 2: Examine the distribution of the principal vesicular proteins which are involved in regulated exocytosis in horizontal cell endings by determining the expression of a) VGAT and b) VAMP isoforms, SV2A, synaptotagmin 1 and 2, Rab3 isoforms, and the vacuolar proton pump with RT-PCR and immunohistochemistry. Specific aim 3: Examine the distribution of key synaptic proteins, which participate in vesicle docking, priming and fusion in regulated transmitter release, in horizontal cell endings by determining the expression of syntaxin, SNAP-25 and complexin 1-4, Munc 13-1 and 18, and RIM1 with RT-PCR and immunohistochemistry. These studies will further elucidate the structural and biochemical basis of transmitter release from horizontal cells, thus providing a better understanding of the functional role of horizontal cells in the outer retina. These findings will aid in the development of therapeutic strategies for the treatment of pathological changes in the outer retina related to retinal disease, such as macular degeneration.