Rhodpsin Studies

Rhodopsin Studies

Roll of Rhodopsin in the eye

Rhodopsin is sensitive enough to respond to a single photon of light, this response takes place in the form of isomeriztion, figure1. Therefore, when a photon of light enters the eye, it is absorbed by the retinal and causes a change in its configuration from 11-cis retinal to all-trans retinal. This isomeriztion induces conformational changes in Rhodopsin that activates the G-protein and triggers a second messenger cascade ending with impulses being sent along the optical nerve to the brain.

Figure1. Isomerization of the retinal from 11-cis to all-trans upon absorbing a photon.

In the bright light, Rhodopsin breaks down into its component; the opsin and the retinal, which are both colorless and that is to protect the rod cells from over stimulation.

In the dim light, the process is reserved. Opsin and retinal combine again to form the purple-reddish Rhodopsin. This process is called “dark adaptation”. Therefore adaptation to light is, in fact, a matter of generation more 11-cis retinal from vitamin A. [See structure of Rhodopsin].

The spectra of Rhodopsin before and after illumination are shown in figure2.

Figure2. FTIR difference spectra of rhodopsin before and after illumination. (Red) difference spectrum of rhodopsin regenerated with the mixture of locked retinals; (green) difference spectrum of the 11-cis-retinal regenerated sample (control measurement).

A plot showing absorbance vs. wave number of Rhodopsin before and after bleaching is shown in figure3 below. Before bleaching the absorbance was 498nm, so it appears purple.

Figure3. Rhodopsin before and after bleaching.

Step by step with phototransduction

Phototransduction is the process through which photons are converted into electrical signals.

The visual pigment Rhodopsin is bound to the plasma membrane of the rod cell. Figure4 illustrates Rhodopsin forming transmembrane complex within the membrane.

Figure4. Opsin (the G-protein coupled receptor) is bound to the light-absorbing chromophore, 11-cis-retinal (the aldehyde of vitamin A) forming the visual pigment Rhodopsin which is embedded within the plasma membrane.

Also within the disk membrane is a cGMP (nucleotide cyclic guanosine 3-5 monophosphate). The level of this cGMP is controlled by cGMP phosphodiesterase which is basically an enzyme that breaks down the cGMP.

When light enters the eye, figure5, a photon is absorbed by Rhodopsin and the 11-cis-retinal undergoes isomerizaion to all-trans form, this activated Rhodopsin is called metarhodopsin II.

Figure5. Light is absorbed by the Rhodopsin and the retinal component changes its conformation and dissociates from the opsin.

Metarhodopsin II stimulates the activity of the enzyme which in turn will decrease the level of cGMP in the cytoplasm. Decreasing the level of cGMP will cause the closing of sodium channels which will lead to decrease the sodium level causing hyper polarization which closes the Ca+2 channels. With less Ca2+ entering the cell, the release of transmitter decreases so the graded potential passing on to the optic nerve is smaller.

In the dark, the activity of this enzyme is weak. High level of GMP causes the opening of the sodium channels in the plasma membrane therefore sodium ions are moving into the cell and the photoreceptor is depolarized. In response to depolarization Calcium channels open for calcium ions to enter the cell causing the release of transmitter to increase which causes graded potentials passing on the optic nerve to be greater.

Figure6a&b illustrates how sodium channels opens in the dark and close in the light and the decrease in transmitter upon illumination respectively.

Figure6a. Cyclic GMP-gated channels in the outer segment membrane are responsible for the light-induced changes in the electrical activity of photoreceptors. (Neuroscience, Purves et al., 2001).

Figure6b. Change in the level of transmitters upon illumination.

Colorado Physiology

In conclusion, in the light, less transmitter is released which means less signals are passed on to the next cells in the visual pathway. Therefore the rod cells function in detecting light only the under dim.

References

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