BIO254:Phototransduction

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.

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.

Figure 2: Cyclic GMP-gated channels in the outer segment membrane are responsible for the light-induced changes in the electrical activity of photoreceptors.

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.

Figure 3 summarizes the phototransduction cascade.

Figure 3: Details of phototransduction in rod photoreceptors. (A) The molecular structure of rhodopsin, the pigment in rods. (B) The second messenger cascade of phototransduction. Light stimulation of rhodopsin in the receptor disks leads to the activation of a G-protein (transducin), which in turn activates a phosphodiesterase (PDE). The phosphodiesterase hydrolyzes cGMP, reducing its concentration in the outer segment and leading to the closure of sodium channels in the outer segment membrane.

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.

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