The photosensitive pigment in the rods is called rhodopsin (visual purple). Rhodopsin is composed of retinal, an aldehyde of vitamin A, and a protein called opsin. Because of the importance of vitamin A in the synthesis of retinal, it is not surprising that a deficiency in this vitamin produces visual abnormalities (Clinical Box 1).
Clinical Box 1
Opsin has a molecular weight of 41 kDa. It is found in the membranes of the rod disks and makes up 90% of the total protein in these membranes. Opsin is part of the large family of G-protein-coupled receptors (GPCR). Retinal is parallel to the surface of the membrane (Figure 1) and is attached to a lysine residue at position 296 in the seventh transmembrane domain.
Fig1. Diagrammatic representation of the structure of rhodopsin, showing the position of retinal in the rod disk membrane. Retinal (R) is located in parallel to the surface of the membrane and is attached to a lysine residue at position 296 in the 7th transmembrane domain.
The sequence of events in photoreceptors by which incident light leads to production of a signal in the next succeeding neural unit in the retina is summarized in Figure2. In the dark, the retinal in rhodopsin is in the 11-cis configuration. The only action of light is to change the shape of the retinal, converting it to the all-trans isomer. This, in turn, alters the configuration of the opsin, and the opsin change activates its associated heterotrimeric G-protein, which in this case is called transducin, which has several subunits Tα, Gβ1, and Gγ1. After 11-cis retinal is converted to the all-trans configuration, it separates from the opsin in a process called bleaching. This changes the color from the rosy red of rhodopsin to the pale yellow of opsin.
Fig2. Sequence of events involved in phototransduction in rods and cones.
Some of the all-trans retinal is converted back to the 11-cis retinal by retinal isomerase, and then again associates with scotopsin, replenishing the rhodopsin supply. Some 11-cis retinal is also synthesized from vitamin A. All of these reactions, except the formation of the all-trans isomer of retinal, are independent of the light intensity, proceeding equally well in light or darkness. The amount of rhodopsin in the receptors therefore varies inversely with the incident light level.
The G-protein transducin exchanges GDP for GTP, and the α subunit separates. This subunit remains active until its intrinsic GTPase activity hydrolyzes the GTP. Termination of the activity of transducin is also accelerated by its binding of β-arrestin. The α subunit activates cGMP phosphodiesterase, which converts cGMP to 5′-GMP.
In darkness, when phosphodiesterase activity is low, cGMP-gated Na+ channels are maintained in an open state so both Na+ and Ca2+ enter the photoreceptor so the cell is depolarized and glutamate is released. The light-induced decline in the cytoplasmic cGMP concentration causes some cGMP gated Na+ channels to close, reducing the entry of Na+ and Ca2+. This produces the hyperpolarizing potential. This cascade of reactions occurs very rapidly and amplifies the light signal. The amplification helps explain the remarkable sensitivity of rod photoreceptors; these receptors are capable of producing a detectable response to as little as one photon of light.
Calcium ions exert a negative feedback effect on the phototransduction process. In darkness, the relatively high intra cellular Ca2+ concentration inhibits guanylyl cyclase activity, decreases the activity of the cGMP-gated Na+ channels, and increases the activity of rhodopsin. The light-induced decrease in Ca2+ concentration thus influences these components of the phototransduction cascade.
