Dynamic Modulation of Retinal Ribbon-Type Synapses

视网膜能够快速、可靠地检测和传输大量的视觉信息。带状突触是脊椎动物视网膜回路的关键组成部分，构成了通向大脑的信号通路中的第一和第二突触前元件。功能区的特殊形态和功能可能赋予了它们一种独特的能力，可以大量而快速地释放神经递质，这被认为是视觉信息有效处理和编码所必需的。然而，在非常不同的环境光条件下和在白天/夜间周期中，调节和维持带状突触的递质输出的潜在细胞机制尚不清楚。由于它们的体积很大，我们能够膜片钳单个金鱼双极细胞终末。这使我们能够测量突触前钙电流和膜电容的诱发变化，从而实时检测突触小泡的胞吐和内吞作用。我们发现，突触终末在夜间的释放效率大大降低，因此大的钙电流引起少量的胞吐。相反，相对较小的钙电流在白天会引起更多的胞吐作用。此外，我们还发现，众所周知的中间代谢产物NAD+和NADH可能通过与一种新的主要带状成分蛋白Ribeye相互作用来调节带状突触的输出。因此，要检验的第一个假设是，胞吐的效率在昼夜节律周期中在突触的带状突触上发生变化(这与突触的带状形态和代谢状态的变化相平行)，并且带状功能受到反映细胞能量水平的代谢物的调节。我们还发现，体内氯离子水平的升高抑制了带状突触的内吞速率。因此，需要检验的第二个假设是，氯-通过末端强大的GABA能输入直接调节囊泡回收的速率，抑制内吞作用，这是回收过程的第一步。这一发现表明，氯离子作为第二信使调节囊泡循环具有新的作用。最后，我们有初步证据表明，去磷酸化大大抑制了双极细胞终末内突触小泡的流动性。关于带状突触如何调节囊泡的移动和聚集，我们知之甚少。需要检验的第三个假说是，磷酸化调节突触带状突触的囊泡循环和移动。因此，这些研究将增加我们对带状突触作为动态结构的理解，这些结构适应不同的条件，从而有效地传递广泛的刺激。

The retina detects and transmits large amounts of visual information quickly and reliably. Ribbon synapses are key components of the vertebrate retinal circuitry, forming the first and second presynaptic elements in the signaling pathway to the brain. The specialized morphology and function of the ribbons presumably endows them with a unique capacity for copious and fast neurotransmitter release, which is thought to be essential for the efficient processing and encoding of visual information. Nevertheless, the underlying cellular mechanisms that modulate and maintain transmitter output from ribbon synapses under vastly different ambient light conditions and during the daytime/nighttime cycle are poorly understood. Due to their large size, we are able to patch-clamp single goldfish bipolar cell terminals. This allows us to measure both presynaptic Ca currents and evoked changes in membrane capacitance that assay synaptic vesicle exocytosis and endocytosis in real time. We have found that synaptic terminals have a greatly reduced efficiency of release at night, so that large Ca currents evoke small amounts of exocytosis. Conversely, relatively smaller Ca currents evoke much larger amounts of exocytosis during daytime. In addition, we have found that the well-known intermediate metabolites NAD+ and NADH modulate ribbon synapse output, perhaps via their interaction with a novel and major ribbon constituent protein RIBEYE. Thus, the first hypothesis to be tested is that the efficiency of exocytosis changes at a ribbon synapse in a circadian cycle (that parallels changes in ribbon morphology and metabolic state of the synapse), and that ribbon function is modulated by metabolites that reflect cellular energy levels. We have also found that elevated levels of internal Cl-ions inhibit the rate of endocytosis at ribbon synapses. Therefore, the second hypothesis to be tested is that Cl-influx via the strong GABAergic input at the terminal directly modulates the rate of vesicle recycling by inhibiting endocytosis, the first step in the recycling process. This finding suggests a novel role for Cl- ions as second messengers that modulate vesicle cycling. Finally, we have preliminary evidence that dephosphorylation drastically inhibits the mobility of synaptic vesicles within bipolar cell terminals. Very little is known about how ribbon synapses regulate vesicle mobility and clustering. The third hypothesis to be tested is that phosphorylation regulates vesicle recycling and mobility at ribbon synapses. These studies will thus increase our understanding of ribbon synapses as dynamic structures that adapt to diverse conditions so as to efficiently transmit a wide array of stimuli.