Rod and cone visual pigments and phototransduction through pharmacological, genetic, and physiological approaches

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.

FIGURE 1.

Difference in kinetics and sensitivity between rod and cone light responses. A, suction electrode recording from a salamander cone. The inner segment of a dissociated cone is drawn in the electrode to collect and measure the membrane current flowing through the cell. The reduction in the current produced by the light stimulation represents the light response of the cell. B, family of flash responses from a salamander rod and red cone to flashes of increasing intensity. Note the difference in rod and cone response kinetics. C, intensity-response functions of salamander rod and red cone based on the responses in B. Note the right shift of the cone intensity-response curve, demonstrating the lower sensitivity of cones compared with rods. This figure was reprinted from Ref. with permission. PIGM, pigment.

FIGURE 2.

Measurements of Meta II decay time constant from rod and cone pigments. The deletion of arrestin in rods renders the decay of Meta II rate-limiting for the response shutoff. As a result, the rate of rod pigment Meta II decay can be measured from the decay rate of the dim flash light response in mouse rods lacking arrestin (A). Using the same approach, the Meta II decay rate of mouse S- and L-cone pigments can be estimated from transgenic mouse rods expressing S-opsin (B) or L-opsin (C) and lacking arrestin (arr^−/−) and the endogenous rod pigment (rho^−/−). This figure was reprinted from Refs. and with permission.

FIGURE 3.

Schematics of two visual cycles in vertebrate eye. The canonical RPE visual cycle (left) recycles all-trans-retinol (at ROL) released from rods and cones following a bleach to 11-cis-retinal (11c ROL), which can be used by both rods and cones for pigment regeneration. The retina visual cycle (right) relies on the Müller cells to recycle all-trans-retinol released from cones to 11-cis-retinol, which only cones can move to their outer segments and oxidize to 11-cis-retinal for regeneration of the pigment. IPM, interphotoreceptor matrix. hν, photon of light.