Van Hook Matthew J, Parmelee Caitlyn M, Chen Minghui, Cork Karlene M, Curto Carina, Thoreson Wallace B
Department of Ophthalmology and Visual Sciences and Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, NE 68198
Department of Mathematics, University of Nebraska-Lincoln, Lincoln, NE 68588.
J Gen Physiol. 2014 Nov;144(5):357-78. doi: 10.1085/jgp.201411229. Epub 2014 Oct 13.
At the first synapse in the vertebrate visual pathway, light-evoked changes in photoreceptor membrane potential alter the rate of glutamate release onto second-order retinal neurons. This process depends on the synaptic ribbon, a specialized structure found at various sensory synapses, to provide a supply of primed vesicles for release. Calcium (Ca(2+)) accelerates the replenishment of vesicles at cone ribbon synapses, but the mechanisms underlying this acceleration and its functional implications for vision are unknown. We studied vesicle replenishment using paired whole-cell recordings of cones and postsynaptic neurons in tiger salamander retinas and found that it involves two kinetic mechanisms, the faster of which was diminished by calmodulin (CaM) inhibitors. We developed an analytical model that can be applied to both conventional and ribbon synapses and showed that vesicle resupply is limited by a simple time constant, τ = 1/(Dρδs), where D is the vesicle diffusion coefficient, δ is the vesicle diameter, ρ is the vesicle density, and s is the probability of vesicle attachment. The combination of electrophysiological measurements, modeling, and total internal reflection fluorescence microscopy of single synaptic vesicles suggested that CaM speeds replenishment by enhancing vesicle attachment to the ribbon. Using electroretinogram and whole-cell recordings of light responses, we found that enhanced replenishment improves the ability of cone synapses to signal darkness after brief flashes of light and enhances the amplitude of responses to higher-frequency stimuli. By accelerating the resupply of vesicles to the ribbon, CaM extends the temporal range of synaptic transmission, allowing cones to transmit higher-frequency visual information to downstream neurons. Thus, the ability of the visual system to encode time-varying stimuli is shaped by the dynamics of vesicle replenishment at photoreceptor synaptic ribbons.
在脊椎动物视觉通路的第一个突触处，光诱发的光感受器膜电位变化会改变谷氨酸释放到二级视网膜神经元上的速率。这一过程依赖于突触带，一种在各种感觉突触中发现的特殊结构，以提供用于释放的预充囊泡供应。钙（Ca(2+)）加速了视锥细胞带突触处囊泡的补充，但这种加速的机制及其对视觉的功能影响尚不清楚。我们使用虎螈视网膜中视锥细胞和突触后神经元的配对全细胞记录来研究囊泡补充，发现它涉及两种动力学机制，其中较快的一种被钙调蛋白（CaM）抑制剂减弱。我们开发了一个可应用于传统突触和带突触的分析模型，并表明囊泡再供应受一个简单的时间常数限制，即τ = 1/(Dρδs)，其中D是囊泡扩散系数，δ是囊泡直径，ρ是囊泡密度，s是囊泡附着的概率。电生理测量、建模以及单个突触囊泡的全内反射荧光显微镜观察相结合表明，CaM通过增强囊泡与突触带的附着来加速补充。使用视网膜电图和光反应的全细胞记录，我们发现增强的补充提高了视锥细胞突触在短暂闪光后信号黑暗的能力，并增强了对高频刺激的反应幅度。通过加速囊泡向突触带的再供应，CaM扩展了突触传递的时间范围，使视锥细胞能够将更高频率的视觉信息传递给下游神经元。因此，视觉系统编码随时间变化的刺激的能力是由光感受器突触带处囊泡补充的动力学塑造的。
