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Researchers discover mechanism that helps humans see in bright and low light
Date:10/13/2009

cells and travel to the nearby pigment epithelium near the retina. There the chromophore is restored and returned to the photoreceptor cells.

Earlier this year, the research team removed the pigment epithelium layer in salamander retinas, so that pigment molecules could not be recycled that way. Then they exposed retinal cells both to bright light and to darkness. The rods no longer worked, but the cones continued to function properly, even without the eye's pigment epithelium.

Exposure to bright light destroyed visual pigments in rods, and those cells could not recycle chromophores, says principal investigator Vladimir J. Kefalov, Ph.D., assistant professor of ophthalmology and visual sciences. Pigments in cones, by contrast, quickly regenerated and continued to detect light even without the pigment epithelium, so it was clear a second pathway was involved.

In the new study, Kefalov did the same experiments in cells from mice, primates and humans with the same result.

To learn how cones were able to recycle pigments without pigment epithelium, Kefalov's team has focused on a particular type of cell in the retina. Called Mller cells, these cells support and interact with rods and cones. The researchers treated mouse retinas with a chemical that destroyed the Mller cells, then exposed the retina to bright light, followed by darkness.

When we blocked the function of Mller cells, the retinal visual pathway could not function because cones ran out of photopigment and could not adapt to dark, Kefalov says.

The new paper, published in the journal Current Biology, suggests Mller cells are key to this pathway in mammals, including humans.

When those cells function properly, cones in the mouse, primate and human retinas are able to function in bright light and adapt to darkness, independently of the pigment epithelium, Kefalov says.

He says this discovery means it may one day be possible to manipu
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Contact: Jim Dryden
jdryden@wustl.edu
314-286-0110
Washington University School of Medicine
Source:Eurekalert

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