Selective glycinergic input from vGluT3 amacrine cells confers a suppressed-by-contrast trigger feature in a subtype of M1 ipRGCs in the mouse retina.

Abstract

KEY POINTS\nM1 ipRGCs is known to encode absolute light intensity (irradiance) for non-image-forming visual functions (subconscious vision), such as circadian photoentrainment and pupillary light reflex. It has been unclear how M1 cells respond to relative light intensity (contrast) and patterned visual signals. This study discovered a special form of contrast sensitivity (suppressed-by-contrast) in M1 cells, suggesting a role of patterned visual signals in regulating non-image-forming vision and a potential role of M1 ipRGCs in encoding image-forming visual cues. It also uncovered a synaptic mechanism and a retinal circuit mediated by vGluT3 amacrine cells that underlie the suppressed-by-contrast response of M1 cells. The study further revealed two M1 ipRGC subtypes (M1a and M1b), which are distinguishable based on synaptic connectivity with vGluT3 amacrine cells, receptive field properties, intrinsic photo sensitivity and membrane excitability, and morphological features, suggesting a division of visual tasks among discrete M1 subpopulations.\n\n\nABSTRACT\nThe M1 type ipRGC (intrinsically photo-sensitive retinal ganglion cell) is known to encode ambient light signals for non-image-forming visual functions such as circadian photo-entrainment and pupillary light reflex. Here, we report that a subpopulation of M1 cells (M1a) in the mouse retina possess the suppressed-by-contrast (sbc) trigger feature which is a receptive field property previously found only in ganglion cells mediating image-forming vision. Using optogenetics and dual patch clamp, we discovered that vGluT3 amacrine cells make glycinergic, but not glutamatergic, synapses specifically onto M1a cells. The spatiotemporal and pharmacological properties of visually evoked responses of M1a cells closely matched the receptive field characteristics of vGluT3 cells, suggesting a major role of the vGluT3 amacrine cell input in shaping the sbc trigger feature of M1a cells. We found that the other subpopulation of M1 cells (M1b), which did not receive a direct vGluT3 cell input, lacked the sbc trigger feature and were distinctively different from M1a cells in intrinsic photo responses, membrane excitability, receptive-field characteristics, and morphological features. Together, the results uncovered a retinal circuit that uses the sbc trigger feature to regulate irradiance coding and potentially send image-forming cues to non-image-forming visual centers in the brain. This article is protected by copyright. All rights reserved.