New Insights Into Vision

(Ivanhoe Newswire) -- Melanopsin, a light sensor that sets the circadian rhythm- the body's biological clock- also plays an important role in vision, according to this study.
Melanopsin's messengers, called melanopsin-expressing retinal ganglion cells (mRGCs), forward information about the brightness of incoming light directly to visual centers in the brain.
The results show a new role for mRGCs during image-forming vision, and suggest that these cells could make a significant contribution to assessing the intensity of light and supporting vision even in people with advanced retinal degeneration.

"Millions of people worldwide suffer varying degrees of blindness because of rod and cone degeneration or dysfunction, but many of them can still perceive differences in brightness," senior author, Satchidananda (Satchin) Panda, Ph.D., an assistant professor in the Regulatory Biology Laboratory at the Salk Institute for Biological Studies was quoted as saying. "Melanopsin-expressing RGCs typically survive even complete rod and cone loss and could explain the light responses under these conditions."

It was previously thought that the ability to convert light into electrical signals in the retina was restricted to only 2 types of photoreceptors: rods and cones. His changed completely when Dr. Panda discovered the existence of a third type of photoreceptor. Melanopsin is only present in a few thousand cells deep in the retina.

Melanopsin, is completely different from the classical rod and cone opsins, which help us see. It is much less sensitive to light and has far less spatial resolution-characteristics that fit perfectly with this light sensor's primary function of signaling changes in ambient light levels to the brain throughout the day. It sends signals directly to the circadian clock, which sits just about the point where the optic nerves cross. It synchronizes the body's daily rhythms with day and night, telling the body when to sleep, when to be hungry, and when to wake up. These tiny photopigment also controls pupil size, allowing the visual field to be manipulated.

Until now, however, it was unknown whether mRGCs also contribute to conventional image-forming vision, especially the as-yet poorly understood mechanism of "brightness" and "lightness" perception.

To find out, Dr. Panda and his colleagues at the University of Manchester traced individual mRGCs' axons-long, slender projections that connect with other neurons-from the retina through the circadian clock and onward. They discovered that the axons reached all the way to the lateral geniculate nucleus, LGN, the primary processing center for visual information received from the retina.

"We found widespread light responses in the LGN and visual cortex, even in mice lacking functional rods and cones, which are often used as a model of advanced retinal degeneration," says Panda, who hopes that one day it might be possible to impart vision to blind individuals by gene therapy with a re-engineered melanopsin.

"The density of mRGCs in the retina is too low for any meaningful resolution," Dr. Panda says. "But if we could express melanopsin in a greater number of cells, we might be able to increase resolution to a point that allows blind people to safely navigate their environment."

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SOURCE: PLoS Biology, published online December 7, 2010

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