Abstract
Purpose :
The pupillary light reflex (PLR) constricts the pupil in response to light. Intrinsically photosensitive retinal ganglion cells (ipRGCs) transmit signals to the brain, inducing the PLR. ipRGCs receive rod and cone inputs; however, the distinct roles of these three photo-sensitive cells in the PLR is still unclear. Previous studies of the PLR have used mutant mouse models that may induce developmental compensations. We used N-methyl-N-nitrosourea (NMU) to chemically ablate rod/cone photoreceptors in several days, which would avoid compensatory changes to uncover the cellular mechanisms of the PLR.
Methods :
PLR was evoked by a 10s green light flash at mesopic, photopic, and high-photopic levels in C57 and rod/cone double knockout (Cgna3-/-, Gnat1-/-) mice (dKO), which was video recorded. Five days after NMU intraperitoneal injection, PLR was recorded with the same stimuli, and retinas were removed to examine the photoreceptor layer and ipRGCs by melanopsin staining. Videos were analyzed by measuring pupil diameter over time.
Results :
Both the level of pupil constriction and speed of the response onset increased as stimulus light intensity did, in control mice. Five days after NMU injection, the outer segment and outer nuclear layers (rod/cone photoreceptor layers) were ablated, while other layers were unaffected. Melanopsin staining revealed ipRGCs remained at the same density displayed in saline-injected mice. These mice had no PLR at the mesopic level, indicating exclusive rod/cone contributions. In response to photopic light, PLR was absent in some mice, while others exhibited a weak, slow onset, and long-lasting PLR, suggesting both rods/cones and ipRGC contributions to PLR, but rods/cones shape the fast response. Surprisingly, in response to high photopic light, the PLR was identical to control recordings with fast onset and recovery, indicating that ipRGCs exclusively evoked the PLR. dKO mice showed similar responses to the NMU-injected mice at all three light levels.
Conclusions :
Both rods/cones have been shown to contribute to the PLR at all light levels. However, our findings show that ipRGCs exclusively evoke the PLR at high photopic light stimulus, indicating an unseen cellular mechanism for the PLR by ipRGCs.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.