Synaptic mechanisms and circuitry of retinal interneurons

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Abstract The goal of this proposal is to understand how visual signals are transmitted from the outer to the inner retina through cone bipolar cells (CBC), with a focus on detailed synaptic mechanisms and functional circuits between morphologically and physiologically identified cone bipolar cell types and specific ganglion and amacrine cell types in the whole-mount mouse retina. This investigation is motivated by an important observation that, although recent transcriptomic, connectomic, and imaging studies have significantly advanced our understanding of bipolar cell classification and anatomical structure, our knowledge of the detailed physiology of the synapses and circuits formed by these classified CBC types remains very limited. To address this fundamental gap in our understanding of retinal processing, we developed a new experimental approach, using dual pre- and post-synaptic patch-clamp recording from pairs of morphologically identified CBC types and ganglion cell types in the whole-mount mouse retina in conjunction with two-photon optical recording and targeted expression of genetically encoded glutamate and Ca sensors. This approach allowed us to correlate the anatomical structure of each morphological CBC type with its intrinsic and receptive-field physiology and, more importantly, to directly measure, at a millisecond resolution and under voltage-clamp condition, synaptic transmission from identified CBC types to their postsynaptic targets in a structurally intact retina. Our preliminary results revealed novel kinetic and circuit properties of CBC that suggested a new mechanism of synaptic integration using both chemical and electrical synaptic transmission. Based on these preliminary results, we propose a dual-mode synaptic mechanism by which CBCs transmit both a direct synaptic signal and a gap junction-coupled network signal to their postsynaptic targets. This hypothesized mechanism will be tested through the three Specific Aims. Aim 1, dual patch-clamp characterization of synaptic transmission and functional connectivity between morphologically identified CBC types and a diffused ganglion cell type (W3) in the whole-mount retina. Aim 2, understand the chemical and electrical synaptic interactions underlying signal transmission from identified CBC types to W3 cells in the whole-mount retina. Aim 3, determine the functional impact of different modes of CBC synaptic transmission on diffused and narrowly stratified postsynaptic target cells. Results from this study are expected to provide novel insights into the synaptic mechanisms and functional circuitry of cone bipolar cell types in the mammalian retina and shed light on chemical and electrical synaptic integration in the CNS in general.