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Seunghoon Lee, Minggang Chen, Yuelin Shi, Xiaochun Yang, Z Jimmy Zhou; Glycinergic suppression of a M1-ipRGC subpopulation by vGluT3 amacrine cells in the mouse retina. Invest. Ophthalmol. Vis. Sci. 2019;60(9):1375.
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To understand the circuitry and role of amacrine cell inputs in M1-ipRGC function.
We recorded the responses of M1-ipRGCs to optogenetic (ChR2) stimulation of vGluT3 amacrine cells (GACs) in flat-mount retinas from OPN4-GFP/vGluT3/ChR2-YFP mice under pharmacological blockade of photoreceptor-driven light responses in bipolar cells. Dendritic morphology, synaptic circuitry, and receptive field properties of M1-ipRGCs were characterized with electrophysiology and two-photon imaging.
Optogenetic stimulation of ChR2-expressing GACs evoked direct postsynaptic glycinergic responses (~150 pA at 0 mV on average) from a subset of M1-ipRGCs, which constituted ~50% of GFP-labeled M1-ipRGCs recorded. When illuminated by a small center spot of light (100 μm in dia.), the subpopulation of M1-ipRGCs (M1-1) that received a large glycinergic input from GACs displayed a much larger light-evoked inhibitory input than did the other sub-population (M1-2) that received little GAC input. This light-evoked inhibitory input to M1-1 ipRGCs was largely mediated by glycine, with a spatiotemporal property closely matching the receptive field property of GACs. Under on-cell recording, M1-1 ipRGCs often responded to a small center light spot (100 μm in dia.) with a brief suppression of spikes, but showed little suppression of spikes in response to a large light spot. This suppressive feature of the M1-1 ipRGC receptive field is reminiscent of the suppressed-by-contrast trigger feature of uniformity detectors, which also received a strong glycinergic inhibition from GACs. In contrast, the M1-2 ipRGCs showed little detectable transient spike suppression in response to a small light spot. In addition, the two subpopulations of M1 ipRGCs showed difference in spike frequency in response to full-field light stimulation and current injection. Finally, though both M1 subpopulations share the characteristic M1 morphology, the detailed branching pattern and density of dendrites appeared slightly, but significantly different between the two subpopulations.
The study demonstrates that GACs provide a glycinergic inhibition to a subset of M1-ipRGCs, revealing a previously unknown suppressed-by-contrast trigger feature in M1-1 ipRGCs. The suppressive response to small bright light may help M1-1 cells encode ambient background light more reliably by making the cells less responsive to local contrast signals.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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