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Xiwu Zhao, DiJon D. Hill, Ben K. Stafford, Kwoon Y. Wong; Synaptic Inputs and Receptive Field Organization of Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs). Invest. Ophthalmol. Vis. Sci. 2012;53(14):2725.
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© ARVO (1962-2015); The Authors (2016-present)
Five types of ipRGCs (M1 - M5) have been identified in the mouse but a proper analysis of their light responses remains lacking, because in all published whole-cell recordings, these cells were identified using intense epifluorescence and thus severely light-adapted. We wanted to examine ipRGC photoresponses under more dark-adapted conditions and address three questions: 1) Previous studies detected bipolar- and amacrine-driven photoresponses only at light onset - are there OFF inputs as well? 2) Rod/cone-driven ipRGC light responses are unusually tonic - what are the underlying mechanisms? 3) Do ipRGC receptive fields exhibit surround inhibition?
Retinas were isolated from dark-adapted mice whose ipRGCs were selectively labeled with EGFP (Ecker et al. 2010 Neuron). EGFP-labeled cells were visualized on a multiphoton microscope using 950nm laser and targeted for whole-cell recording. To selectively study bipolar or amacrine cell input, ipRGCs were voltage-clamped at ECl or 0 mV respectively. All stimuli were white light generated by an OLED monitor. In the receptive field mapping experiment, the stimuli were light spots of various diameters centered within each cell’s receptive field. In all other experiments, the stimuli were full-field step increases in light intensity.
So far we have analyzed M1 - M4 ipRGCs. All cells generated sustained ON bipolar-mediated light responses, and a few M1 and M2 cells also showed L-AP4-resistant OFF bipolar-evoked responses. All cells generated amacrine-driven photoresponses; these responses were transient and for most cells occurred only at light on, although some M1 and M4 cells showed L-AP4-resistant off responses as well. In the mapping experiment, both the bipolar- and amacrine-driven photoresponses of M3 cells progressively increased in amplitude as stimulus diameter increased from 20 µm to 500 µm, but became smaller as the diameter exceeded 500 µm. For M1, M2 and M4 cells, such a suppressive effect of large-diameter spots was absent.
Some ipRGCs receive inputs from OFF bipolar and OFF amacrine cells in addition to ON inputs. Curiously, these include some M2 and M4 cells, whose dendrites are confined within the "on" sublamina of the inner plexiform layer. The photoresponses evoked by amacrine input are far more transient than those induced by bipolar input; thus, during a light step, excitation outlasts inhibition, resulting in sustained depolarization. M1, M2 and M4 cells lack inhibitory surrounds, just like primate ipRGCs (Dacey et al. 2005 Nature). By contrast, all M3 cells tested showed pronounced surround suppression, suggesting that this cell type may mediate spatial analysis and hence image-forming vision.
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