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Katrin Franke, Philipp Berens, Timm Schubert, Thomas Euler, Tom Baden; A glutamate map of the inner retina. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2616.
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© ARVO (1962-2015); The Authors (2016-present)
The retina decomposes spatio-temporal photoreceptor activation patterns into specific parallel channels. In mice, ≥13 types of bipolar cell (BC) systematically transform the photoreceptor input in different parallel pathways and provide the excitatory drive for downstream retinal circuits (Euler et al., 2014). Here, we present a detailed functional map of BC output at the level of glutamate release across the inner plexiform layer (IPL).
To selectively record BC output, we used an AAV transduction strategy to express the fluorescent glutamate sensor iGluSnFR (Marvin et al., 2013) cell type-specifically in transgenic Cre lines (ChATCre, PVCre) or pan-neuronally. Light stimulus-evoked glutamate release was recorded using two photon imaging in whole-mounted retina at the level of individual processes in the IPL. A correlation-based algorithm was applied to place regions of interest (ROIs), which were restricted to the equivalent size of individual BC terminals. We implemented a probabilistic clustering framework for separating response profiles of more than 10,000 ROIs (17 mice) into functional clusters, which were then grouped according to stratification level and functional similarity.
We obtained ~14 functional BC types (~7 ON, ~6 OFF, and 1 ON-OFF) with strikingly diverse properties. In line with previous studies (Baden et al., 2013; Borghuis et al., 2013), sustained BCs mainly stratified at the IPL borders, whereas more transient BCs stratified closer to the IPL centre. Furthermore, we obtained a detailed description of each functional BC type including stratification profile, frequency and contrast preference, response delay, receptive field size and surround properties. These response properties varied strongly as a function of IPL-depth forming multiple overlapping organisational maps. In addition, we demonstrated that functional BC types with similar stratification depths can show key differences in specific response characteristics, such as response delay and surround properties.
Here, we functionally classified the mouse BCs based on their glutamate release in response to a wide range of light stimuli. This unbiased, terminal- instead of cell-centric approach allows us to systematically chart the main excitatory drive in the IPL with unprecedented detail.
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