Rod and Cone Visual Pigments and Phototransduction through Pharmacological, Genetic, and Physiological Approaches

Vladimir J. Kefalov

doi:10.1074/jbc.r111.303008

Abstract

Activation of the visual pigment by light in rod and cone photoreceptors initiates our visual perception. As a result, the signaling properties of visual pigments, consisting of a protein, opsin, and a chromophore, 11-cis-retinal, play a key role in shaping the light responses of photoreceptors. The combination of pharmacological, physiological, and genetic tools has been a powerful approach advancing our understanding of the interactions between opsin and chromophore and how they affect the function of visual pigments. The signaling properties of the visual pigments modulate many aspects of the function of rods and cones, producing their unique physiological properties.

Highlights

The human retina has one type of rod for dim light vision and three types of cone cells that allow color discrimination
Rods are so sensitive that they can detect a single photon of light [9], which makes them perfectly suited for dim light vision
Considering that the activation of multiple G proteins by a single photoactivated pigment molecule represents the first amplification step in the phototransduction cascade, it is likely that changes in the activity or lifetime of the photoactivated pigment will have substantial effects on the light responses of photoreceptors

Summary

Rod Versus Cone Pigment Signaling

One way of investigating how visual pigments determine the function of rod and cone photoreceptors is to compare their signaling properties directly. Such studies have been extended to transgenic mouse rod photoreceptors This approach has the great advantage that it enables the use of rhodopsin knock-out animals to generate and functionally characterize mice with rod photoreceptors expressing exclusively cone opsins (26 –29). Such a pigment substitution produces a dramatic shift in the spectral sensitivity of these transgenic rods, rendering them most sensitive to ultraviolet light [29], consistent with the peak of the absorption spectrum of mouse S-opsin, or to red light [28], consistent with the peak of the absorption spectrum of human L-opsin. These results demonstrate that rod and cone pigments are equivalent with respect to signaling downstream in phototransduction: first, the active life-

Spontaneous Thermal Activation of Visual Pigment

Spontaneous Cone Pigment Dissociation

Lifetime of Photoactivated Pigment

Pigment Decay and Regeneration

Dark Adaptation

Chromophore Traffic

Findings

Chromophore Recycling