Definition
Phototransduction is the process through which photons, elementary particles of light, are converted into electrical signals. Visual phototransduction occurs in the retina through photoreceptors, cells that are sensitive to light.
Photoreceptor Cells
Two types of photoreceptors: rods and cones
There are two types of photoreceptors distributed unevenly across the retina: rods and
cones. Rods are very sensitive cells specialized for night vision. In bright light conditions the response of the rods is saturated and cones, faster but less sensitive photoreceptors, mediate day vision. There are three types of cones, each one of them responding best to different wavelengths (short, middle, and long). Their combined responses generate color vision.
Opsin, the key molecule for phototransduction
Both rods and cones contain opsin, a G protein-coupled receptor. Opsin is bound to a light-absorbing chromophore, 11-cis-retinal (an aldehyde of vitamin A). Different types of opsins are involved in transducing light of different intensities and wavelengths. Rhodopsin is present in rods and transduces dim light while photopsins are present in cones cells and generate color vision.
Phototransduction step by step
In the absence of light, the photoreceptors are depolarized to a membrane resting potential of -40mV. Light will hyperpolarize the photoreceptors to -70mV (Figure 1). This is in contrast to most other neuronal types, which depolarize following excitation.
Figure 1: An intracellular recording from a single cone stimulated with different amounts of light. Each trace represents the response to a brief flash that was varied in intensity. At the highest light levels, the response amplitude saturates. (After Schnapf and Baylor, 1987.)
A key second messenger molecule responsible for maintaining a depolarized rest state in photoreceptors is the nucleotide cyclic guanosine 3’-5’ monophospate (cGMP). High cGMP levels keep cGMP-gated ion channels in the open state and allow them to pass an inward Na+ current.
Phototransduction involves three main biochemical events, outlined below for rhodopsin.
Light entering the eye activates the opsin molecules in the photoreceptors
Upon photon absorption, 11-cis-retinal undergoes an isomerization to the all-trans form, causing a conformational change in the rhodopsin. The activated rhodopsin is called metarhodopsin II.
The precursor for 11-cis-retinal is all-trans-retinol (vitamin A). A diet rich in vitamin A is crucial for vision, since vitamin A cannot be synthesized by humans.
Activated rhodopsin causes a reduction in the cGMP intracellular concentration
The cytoplasmic cGMP levels are controlled by cGMP phosphodiesterase, an enzyme that breaks down cGMP. In the dark, the activity of this enzyme is relatively week. When the photoreceptor is exposed to light, metarhodopsin II stimulates the activity of cGMP phosphodiesterase via transducin, a G protein. GDP-bound inactive transducin will exchange GDP for GTP following interaction with activated rhodopsin. GTP-bound active transducing will increase the activity of cGMP phosphodiesterase. The result is decreased levels of cGMP in the cytoplasm.
The photoreceptor is hyperpolarized following exposure to light
Decreased levels of cGMP cause the closing of cGMP-gated ion channels which will lead to membrane hyperpolarization.
Termination of the transduction cascade
The light response is terminated by two mechanisms. Transducin has GTPase activity and therefore it will inactivate itself by hydrolyzing bound GTP to GDP. The other shutoff mechanism involves phosphorylation of the activated rhodopsin by the opsin kinase. Phosphorylated rhodopsin will be inactivated by binding to arrestin.
Amplification in the visual cascade
The activation of a single rhodopsin by a single photon is sufficient to cause a significant change in the membrane conductance. This is possible due to amplification steps present in the transduction cascade.
A single photoactivated rhodopsin catalyses the activation of 500 transducin molecules. Each transducing can stimulate one cGMP phosphodiesterase molecule and each cGMP phosphodiesterase molecule can break down 10^3 molecules of cGMP per second. Therefore, a single activated rhodopsin can cause the hydrolysis of more than 10^5 molecules of cGMP per second.
References
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