R9AP-overexpression in rods increases rod-driven optomotor sensitivity to high temporal frequencies at mesopic illumination levels

Abstract

Purpose: Our recent work (e.g. Umino et al, 2012) has demonstrated that when rods function as single photon counters they do not limit visual temporal contrast sensitivity. This implies that postsynaptic elements limit the speed of visual responses under scotopic conditions. However, in mesopic conditions when rods act as integrators, they operate largely via secondary (and tertiary) rod circuits characterized by lower luminance sensitivity and faster kinetics. We propose that under mesopic conditions, rod photoresponse recovery kinetics limits rod-driven temporal contrast sensitivity.

Methods: We tested this hypothesis using an existing transgenic mouse line with accelerated rod photoresponse recovery kinetics (R9AP95: transgenic overexpression of the Gβ5:RGS9:R9AP complex in rods, see Krispel et al, 2006). To selectively analyze rod-mediated vision in mesopic conditions in the absence of cone function, we isolated rod- from cone-mediated vision by breeding R9AP95 lines onto the GNAT2^cpfl3 background. GNAT2^cpfl3 mice carry a spontaneous point mutation in the GNAT2 gene that reduces cone phototransduction efficacy (Chang et al, 2006). Using the optomotor response to sinusoidal gratings we determined contrast sensitivity as a function of light intensity and temporal frequency. Spatial frequency of the stimulus was optimal (0.128 cyc/deg).

Results: In lights eliciting > 5 R*/rod/s contrast sensitivities were higher in R9AP95 GNAT2^cpfl3 double mutant mice than in GNAT2^cpfl3 littermate controls. The increase in sensitivity depended both on luminance and temporal frequency of stimulus grating. With moderate lights (1700 R*/rod/s) contrast sensitivities were higher at temporal frequencies exceeding 1.0 Hz, showing an overall 2.0-fold increase. In lights producing < 5 R*/rod/s the optomotor responses were not altered by the overexpression of R9AP in rods. Control studies with GNAT1^-/- GNAT2^cpfl3 double mutant mice (GNAT1^-/- mice exhibit loss of rod function; see Calvert et al, 2000) confirmed that the responses of R9AP95 GNAT2^cpfl3 mice were driven by rods and not by cones.

Conclusions: These results suggest that photoresponse inactivation is a critical factor that limits rod-driven optomotor responses to high temporal frequencies at moderate (mesopic) illumination levels when rods integrate multiple photon events.

Keywords: 648 photoreceptors • 730 temporal vision • 689 retina: distal (photoreceptors, horizontal cells, bipolar cells)