Slowing Photoreceptor Inactivation Kinetics Slows Photopic Vision in Mice

Abstract

Purpose: : Study the relationship between temporal visual sensitivity and photoresponse inactivation kinetics.

Methods: : We tested the visual sensitivity of wildtype (C57BL/6J), R9AP-/- and R9AP overexpressing mice by measuring their threshold for optomotor responses to rotating sinusoidal gratings (Optomotry^©) using a double blind protocol. R9AP is a part of the RGS9-Gβ5-R9AP GTPase activating complex that controls transducin inactivation. The expression level of this complex rate-limits the inactivation of the phototrasnduction cascade in rods. Overexpression of R9AP results in overexpression of this complex and acceleration of the photoresponse recovery, while R9AP knockout slows this process. We measured the temporal contrast sensitivity for a range of spatial frequencies under both scotopic (-4.5 log cd/m²) and photopic (1.8 log cd/m²) conditions.

Results: : Visual sensitivity of WT mice is tuned to stimulus speed for all spatial frequencies (peak: ~12 deg/sec) under photopic conditions and to the temporal frequency of rotating pattern (peak: ~0.75 Hz) under scotopic conditions. In contrast, R9AP -/- mice lacking the GTPase activating complex in both rods and cones, are tuned to stimulus speed only at low spatial frequencies (0.128 cyc/deg) as is the case for WT mice under scotopic conditions. Under scotopic conditions WT and R9AP -/- mice exhibit similar temporal contrast sensitivities tuned to the temporal frequency of the rotating grating (~0.75 Hz). R9AP -/- mice show "rod-like" temporal visual sensitivity under photopic conditions. Overexpressing RGS9 in rods of the R9AP95 mice does not impact their vision under any light level.

Keywords: contrast sensitivity • gene modifiers