Understanding synaptic complexity: Parallel feedback mechanisms in the mouse retina

视觉系统的第一突触在所有后续处理步骤中起着至关重要的作用：它必须在广泛的光照条件下，从视锥光感受器向视网膜的二级神经元提供可靠、高保真的信号流。在这里，视锥光感受器与水平细胞和双极细胞的树突形成复杂的突触。不同类型的双极细胞将锥体输出传递到具有不同功能特性的信号通路。水平细胞形成一个单独的电偶联的横向中间神经元网络，通过反馈突触调节锥体输出，从而调节锥体输出增益，作为网络感知到的背景光照的函数。从水平细胞到视锥细胞的连接已经被深入研究了几十年，但到目前为止，我们仍然缺乏一个全面和连贯的机制，即hc调节视锥细胞输出的机制及其在视网膜光适应中的作用。已经描述了三种不同的突触机制来介导反馈：gaba介导的反馈，半通道介导的触觉反馈和质子（pH-）介导的反馈。不同的适应状态和/或不同的实验条件可能导致单一物种内多种反馈机制的矛盾结果。因此，目前尚不清楚不同的反馈机制是并行的，还是像最近的研究提出的那样，它们是多方面反馈网络的一部分。此外，尚不清楚这些通路是否/如何在功能上连接。通过结合三种遗传方法- (i)转基因小鼠系，其中锥体选择性表达遗传编码的钙生物传感器，（ii）水平细胞转染编码钙和氯化物生物传感器的病毒，以及（iii）基因沉默技术敲除水平细胞中的半通道-双光子钙成像，我们希望解开小鼠外视网膜中光适应的突触机制。特别是，我们的目标是探索在不同背景光水平下HC反馈机制的存在，首先直接在锥体轴突末端（作为锥体输出的影响），然后在HC树突（作为产生反馈的突触前信号）。为了深入了解不同反馈机制的时空方面，我们将分析不同光谱（绿色和蓝色）锥体类型的色度输入如何在解剖和功能水平上被整合到hc中，然后研究这种输入如何转化为（局部和/或全局网络）反馈。我们认为，一个全面的外视网膜反馈模型系统可能为研究大脑其他神经元回路中的平行突触机制提供新的动力。

The first synapse of the visual system plays a fundamental role for all subsequent processing steps: It must provide a reliable, high-fidelity signal flow from the cone photoreceptors to second order neurons of the retina under a wide range of illumination conditions. Here, cone photoreceptors form complex synapses with dendrites of both horizontal cells and bipolar cells. Different types of bipolar cells relay the cone output to signal pathways with different functional properties. Horizontal cells form a single electrically-coupled lateral interneuron network and modulate cone output via feedback synapses, and thus, adjust the cone output gain as a function of background illumination as sensed by the network. The connections from horizontal cells to cones have been intensively studied for several decades but to date we still lack a comprehensive and coherent picture of the mechanisms underlying the modulation of cone output by HCs and their role in retinal light adaptation. Three different synaptic mechanisms have been described to mediate feedback: GABA-mediated feedback, hemichannel-mediated ephaptic feedback, and proton- (pH-) mediated feedback. Contradictory findings with respect to multiple feedback mechanisms within a single species may result from different adaptation states and/or the different experimental conditions. Thus, it is still unclear if the different feedback mechanisms act in parallel or if they part of a multifaceted feedback network as proposed recent studies. Furthermore, it is not clear whether/how the pathways might be functionally connected. By combining three genetic approaches - (i) a transgenic mouse line, in which cones selectively express a genetically encoded calcium biosensor, (ii) horizontal cells transfected with viruses encoding calcium and chloride biosensors, and (iii) gene-silencing techniques to knockdown hemichannels in horizontal cells - with two-photon calcium imaging we want to unravel the synaptic mechanisms that underlie light adaptation in the mouse outer retina. In particular, we aim at probing the HC feedback mechanisms present at different background light levels first directly in the cone axon terminals (as an effect on cone output), and then in HC dendrites (as the presynaptic signal that generates feedback). To gain insight into the spatio-temporal aspects of different feedback mechanisms, we will analyze how chromatic input from spectrally different (green and blue) cone types is integrated in HCs both at the anatomical and the functional level, and then study how this input is translated into (local and/or global network) feedback. We think that a comprehensive outer retina feedback model system may provide new impulses for studies of parallel synaptic mechanisms in other neuronal circuits of the brain.