FUNCTION OF BASAL SYNAPSES AT PHOTORECEPTORS

该项目的目标是对该机制进行定量描述 视锥感光细胞与非双极细胞之间的信号传递 在哺乳动物的视网膜中。锥体形成两种类型的突触：带状突触和突触 基本的。带状突触在锥体和后脑之间的传递 水平突触和双极细胞。这些带状突触有 对接的突触小泡，因此类似于传统的突触 由大脑中的其他神经元。基底突触介导信息传递 在锥体和突触后脱离双极细胞之间。在基底突触， 突触前和突触后细胞膜接近 对位，但没有明显的停靠的小泡，也没有活动 区域。基底突触在结构上不同于常规突触 突触，对它们的功能知之甚少。重要的是要知道 基础突触是如何发挥作用的，因为这些突触在 在明亮的光照下，发出信号的途径减少。有在 关于基础突触如何运作的至少三个假设：1)递质是 缺钙时，基底节的钙/2+依赖性胞吐释放 形态特化(局部囊泡释放)；2)胞吐作用 仅发生在带状突触和基底突触通过 溢出(远程囊泡释放)；以及，3)发生变送器释放 在基底突触通过钙/2+非依赖性转运蛋白介导 机制。基本突触的功能将在一个 哺乳动物视网膜切片制剂中膜电压的变化 锥体细胞(突触前细胞)和超极化双极细胞 (突触后细胞)与Patch同时控制- 移液管电压钳。已经设计了专门的实验来 区分了三种机制。实验将首先 通过以下方法确定基底突触的传递是否为囊泡状 表征其钙离子和电压依赖关系。到那时，实验将 通过表征时间来区分本地释放和远程释放- 和电压依赖的锥体内囊泡融合率，形状 量子反应，神经递质在突触间隙的停留时间， 以及突触后谷氨酸受体的特性。

The goal of this project is a quantitative description of the mechanism of signal transmission between cone photoreceptors and Off bipolar cells in the mammalian retina. Cones make two types of synapse: ribbon and basal. Ribbon synapses mediate transmission between cones and post- synaptic horizontal and On bipolar cells. The ribbon synapses have docked synaptic vesicles, and thus resemble conventional synapses made by other neurons in the brain. Basal synapses mediate transmission between cones and post-synaptic Off bipolar cells. At basal synapses, the membranes of pre- and post-synaptic cells come into close apposition, but there are no obvious docked vesicles and no active zones. Basal synapses are structurally different from conventional synapses, and little is known of their function. It is important to know how basal synapses function, since these synapses form a crucial link in the pathway that signals decreases in bright illumination. There are at least three hypotheses for how basal synapses operate: 1) Transmitter is released by Ca/2+-dependent exocytosis at basal junctions in the absence of morphological specialization (local vesicular release); 2) Exocytosis occurs only at ribbon synapses and basal synapses receive transmitter by overflow (remote vesicular release); and, 3) Transmitter release occurs at the basal synapse by a Ca/2+-independent, transporter-mediated mechanism. The function of basal synapses will be characterized in a mammalian retinal slice preparation in which the membrane voltage of both a cone (the pre-synaptic cell) and a hyperpolarizing bipolar cell (the post-synaptic cell) are controlled simultaneously with patch- pipette voltage clamps. Specific experiments have been designed to differentiated between three mechanisms. Experiments will first determine whether transmission at the basal synapse is vesicular by characterizing its Ca/2+ and voltage dependence. Experiments will then distinguish between local and remote release by characterizing the time- and voltage-dependent rate of vesicle fusion in a cone, the shape of the quantal response, the dwell-time of transmitter in the synaptic cleft, and the properties of the post-synaptic glutamate receptors.