Behavioral Analysis of the Contributions of Rod and Cone Transducins to Vision

Abstract

Abstract: : Purpose: To confirm the physiological roles of rod and cone transducin and to evaluate the ranges of illumination over which rods and cones contribute to vision. Methods: The visual sensitivities of mice with mutations that specifically disable the rod transducin gene (GNAT1) or the cone transducin gene (GNAT2) were evaluated using a vision–dependent behavior. Wild–type, GNAT1–/– and GNAT2–/– mice were trained to find a black wall in a white–walled water maze using a method described previously by Hayes and Balkema (Behav. Genet. 23:395 (1993)). Visual sensitivity was evaluated by measuring the time required for mice to find the black wall under a wide range of illumination conditions. Results: Wild–type mice can use visual cues when the white wall reflects as little as 0.01 photons/µm²/s and more than 10⁷ photons/µm²/s onto the mouse cornea. The lower limit corresponds to ∼1 photoisomerisation per 400 rods per second. In the absence of rod transducin (GNAT1–/–) mice are unable to effectively use visual cues until illumination exceeds ∼200 photons/µm²/s at the cornea. GNAT2–/– mice can use visual cues at low levels of illumination similar to wild–type mice. However, they are unable to use visual cues effectively at levels of illumination above ∼20,000 photons/µm²/s at the cornea. Conclusions: Transducins are essential for rods and cones to contribute to vision. Rods in GNAT2–/– mice can contribute to vision–dependent behavior when the flux of photons onto the cornea ranges from 0.01 to 20,000 photons/µm²/s. Cones in GNAT1–/– mice contribute only at fluxes greater than ∼200 photons/µm²/s. These findings suggest that there are about 1.5 to 2 log units of illumination where rod and cone function in the mouse visual system overlap. The behavior exhibited by the GNAT1–/– and GNAT2–/– strains is consistent with the current concept of cone and rod functions. Further development of the types of analyses described here will provide a method for analyzing vision in mouse mutants and for studying rod and cone contributions to seeing specific types of stimuli.

Keywords: photoreceptors • vision and action • perception