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Xiwu Zhao, Kwoon Y Wong; Gap-junction networks of intrinsically photosensitive retinal ganglion cells (ipRGCs). Invest. Ophthalmol. Vis. Sci. 2017;58(8):4124.
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© ARVO (1962-2015); The Authors (2016-present)
We previously found that in the ganglion cell layer (GCL), all amacrine cells with spiking, tonic ON photoresponses receive gap-junction input from ipRGCs (Reifler et al. 2015 Curr Biol). Here we test whether ipRGCs are coupled to additional kinds of GCL cells.
Dark-adapted mouse retinas containing GFP-labeled ipRGCs (Ecker et al. 2010 Neuron) were flattened and superfused by Ames medium, and GFP+ somas visualized with a two-photon laser. The gap-junction-permeable dye PoPro1 (Hoshi et al. 2006 J Histochem Cytochem) was injected using sharp electrodes into GFP+ somas to label ipRGC-coupled cells, which were whole-cell-recorded and Alexa-filled to study their photoresponses and morphologies. While recording from an ipRGC-coupled cell, individual nearby GFP+ somas were illuminated with a localized two-photon stimulus to excite one ipRGC at a time, in order to reveal how many ipRGCs signaled to that cell. PoPro1 was injected into some ipRGC-coupled cells to identify cells that were coupled to them.
Each ipRGC was coupled to 2 – 20 GCL cells, ~70% of which were morphologically and physiologically similar to those reported in Reifler et al. 2015: S5-monostratifying and S1/S5-bistratifying amacrine cells with medium and wide fields, all displaying spiking, tonic ON responses. The remaining ipRGC-coupled cells included S1-monostratifying medium-field amacrine cells, starburst amacrines, and OFF RGCs including OFF alpha cells. Though some of these new types of ipRGC-coupled cells had spiking, tonic ON photoresponses, many showed novel light responses, e.g. OFF responses to dim light but ON responses to bright light; ON responses to dim light but OFF responses to bright light; and non-spiking ON responses. These cells could generate melanopsin-driven light responses during pharmacological block of rod/cone signaling, confirming they received ipRGC input. Each ipRGC-coupled amacrine cell was coupled to other amacrines, and could be excited by two-photon activation of up to four nearby ipRGC somas.
ipRGCs are coupled with and signal to a far greater variety of GCL cells than previously appreciated, including conventional RGCs – this is the first report of coupling between different types of mouse RGCs. Each ipRGC is coupled to many amacrines and each amacrine cell may get input from multiple ipRGCs. Coupling with OFF RGCs enables ipRGCs (which are ON cells) to influence the OFF channel.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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