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Maureen E Estevez, Lauren E Quattrochi, Inkyu Kim, David M Berson; Circuit properties of an S-ON/M-OFF chromatic signal in the M5 melanopsin ganglion cell. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1232.
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© ARVO (1962-2015); The Authors (2016-present)
Intrinsically photosensitive retinal ganglion cells (ipRGCs) express melanopsin, detect light directly, and are further influenced by rods and cones. There are five subtypes of ipRGCs in mice (M1-M5), with different forms, responses, and projections. The M1 subtype is critical for non-image-forming functions, such as circadian photoentrainment. Other subtypes (e.g., M4 & M5) project to brain targets important for pattern vision, but their roles are unclear. We recently discovered that the M5 type carries an S-ON/M-OFF synaptically-driven chromatic signal. However, the circuitry and spatial properties of this signal are unknown. Because M5 cells stratify only in the ON IPL, we speculated that the long-wavelength OFF signal would be conferred by sign-inverting amacrine cells.
We performed whole-cell voltage clamp recordings of M5 cells from adult mice of the Opn4cre/+;Z/EG line, in which all known ipRGC subtypes express GFP. We targeted M5 cells by their small, round somas. We delivered light stimuli (1s steps; 360 or 520 nm) using small spots that fit within the cells’ receptive field center and larger ones that extended well beyond it. We recorded light-evoked currents while holding at 0, -64, and -69 mV, then applied L-AP4 to block the ON channel.
M5 ipRGCs, distinguishable from other ipRGC subtypes by their morphology, also prove to have unique synaptically-driven physiology. In response to center-only stimuli, M5 cells exhibited inward currents in response to both 360 and 520 nm light. However, when both center and surround were stimulated, M5 cells exhibited outward currents in response to 520 nm light, but inward currents at 360 nm light. The outward current triggered by long-wavelengths was abolished when holding at the reversal potential for chloride. All synaptically-driven currents were abolished by L-AP4. Thus, the OFF-channel makes no obvious contribution to the response of these cells, and the long-wavelength inhibition is presumably driven by ON-type GABAergic or glycinergic amacrine cells fed at least partly by M cones. We did not observe chromatic opponency in other ipRGC subtypes.
The M5 type of ipRGCs constructs an S-ON center and M-OFF surround using circuitry reminiscent of that reported for blue OFF-center ganglion cells in the ground squirrel (but with inverted chromatic preference). This suggests a role for the M5 ipRGC in spatial and chromatic vision.
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