Over-expression of the GAP complex in rods increases temporal contrast sensitivity of mice determined with an operant behavior assay

Abstract

Purpose: Past work has assigned the photoreceptor frequency response as one mechanism that limits Temporal Contrast Sensitivity (TCS); however, this concept has not been demonstrated empirically. Our overall hypothesis is that under mesopic conditions, when rods integrate multiple photon events, rod photoresponse recovery kinetics limits rod-driven temporal contrast sensitivity. Here we apply an operant behavioral assay to measure TCS in mouse while removing the confounding factors inherent to the optomotor assay (see our ARVO abstract, 2013).

Methods: We applied a novel operant behavioral assay to R9AP95 transgenic mice, which display accelerated rod photo-response recovery kinetics due to the over-expression of transducin GTPase activating protein (GAP) complex selectively in rods (Krispel et al, 2006). We selectively isolated rod-mediated vision in mesopic conditions by determining TCS functions for R9AP95 mice in animals (GNAT2^cpfl3) that have no cone responses due to a missense mutation in the cone-specific transducin α-subunit gene (Gnat2). We measured TCS as a function of temporal frequency (1.5-48 Hz) in response to full-field flicker. Flicker ERGs were determined to measure retinal sensitivity.

Results: Comparison of operant responses in R9AP95(+)::GNAT2^cpfl3 vs R9AP95(-)::GNAT2^cpfl3 mice reveals that the effect of GAP overexpression on TCS depends on the level of background illumination. GAP overexpression resulted in: 1) increased TCS in response to intermediate (3 - 15 Hz) but not to high temporal frequencies (20 - 42 Hz) at low mesopic lights (20 R*/rod/s), when rods integrate photons; 2) no change in TCS at scotopic lights (0.2 R*/rod/s), when rods count photons. The flicker ERG sensitivities were in line with visual sensitivities but their dynamic range was limited to temporal frequencies <24 Hz.

Conclusions: Our data indicate that photoreceptor response kinetics constrain operation of rod pathways under specific illumination conditions and temporal frequencies. Candidate mechanisms include shorter integration times in R9AP95 rods (Krispel et al; 2006), lower sensitivity to background lights and extended response range in ‘fast’ rods (Chen et al; 2010), and/ or transmission of enhanced OFF signals along selective (eg, tertiary) rod pathways (Li et al; 2010, Pang et al; 2010). Circuit constraints may limit responses to high temporal frequencies.