FUNCTION OF BASAL SYNAPSES AT PHOTORECEPTORS

本项目的目标是对该机制进行定量描述 视锥细胞和Off双极细胞之间的信号传输 在哺乳动物的视网膜上。视锥细胞形成两种突触：带状突触和 基底的带状突触介导视锥细胞和突触后细胞之间的信息传递。 突触水平和双极细胞。带状突触 对接的突触囊泡，因此类似于传统的突触， 大脑中的其他神经元。基底突触介导传递 视锥细胞和突触后关闭双极细胞之间。在基底突触， 突触前和突触后细胞的膜紧密结合 并置，但没有明显的对接囊泡，也没有活性 区界限基底突触在结构上不同于传统的 突触，而很少有人知道他们的功能。重要的是要知道 基础突触是如何运作的，因为这些突触形成了 信号通路在明亮的照明下减少。存在 关于基底突触如何运作，至少有三个假设：1）递质是 在缺乏Ca/2+的情况下，通过基底连接处的Ca/2+依赖性胞吐释放 形态特化（局部囊泡释放）; 2）胞吐作用 只发生在带状突触和基底突触接收发射器， 溢出（远程囊泡释放）;以及，3）发生发射器释放 在基底突触的Ca/2+非依赖性，转运蛋白介导的 机制基底突触的功能将在一个 哺乳动物视网膜切片制备，其中， 锥体（突触前细胞）和超极化双极细胞 (the突触后细胞）与贴片同时控制， 移液管电压钳。已经设计了具体的实验， 区别于三种机制。实验将首先 通过以下方式确定基底突触处的传输是否是囊泡性的 表征其Ca/2+和电压依赖性。实验将 通过描述时间来区分本地和远程释放- 和电压依赖性速率的囊泡融合在一个圆锥体，形状的 量子反应，递质在突触间隙中的停留时间， 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.