Melanopsin looks on the bright side of life

Dec 07, 2010
Melanopsin-expressing retinal ganglion cells extend their axons-long, slender projections that connect with other neurons-all the way to the primary processing center for visual information received from the brain. Credit: Courtesy of Dr. Satchin Panda, Salk Institute for Biological Studies and Dr. James Fitzpatrick, Waitt Advanced Biophotonics Center

Better known as the light sensor that sets the body's biological clock, melanopsin also plays an important role in vision: Via its messengers-so-called melanopsin-expressing retinal ganglion cells, or mRGCs-it forwards information about the brightness of incoming light directly to conventional visual centers in the brain, reports an international collaboration of scientists in this week's issue of PLoS Biology.

The findings reveal 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, the researchers say.

"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," says senior author Satchidananda (Satchin) Panda, Ph.D., an assistant professor in the Regulatory Biology Laboratory at the Salk Institute for Biological Studies. "Melanopsin-expressing RGCs typically survive even complete rod and cone loss and could explain the light responses under these conditions," he adds.

For the greater part of 100 years, it was thought that the ability to convert light into in the mammalian retina was restricted to only two types of photoreceptors: rods and cones. This view changed dramatically when Panda discovered the existence of a third type of mammalian photoreceptor, which is only present in a few thousand cells embedded in the deeper layers of the retina.

Melanopsin, a photopigment that measures the intensity of incoming light, is fundamentally different from the classical rod and cone opsins, which help us see. For one, 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 its signals directly to the human circadian clock, which sits just above the point where the optic nerves cross. Although only half the size of a pencil eraser, it synchronizes the body's daily rhythms with the rising and setting of the sun, telling the body when it is time to go to sleep, when to be hungry and when to wake up. But it does more than that. Just like a meter in a camera that allows the aperture to be adjusted, mRGCs also control pupil size.

Melanopsin, a light receptor that measures the intensity of incoming light is found in rare, specialized cells embedded in the retina. The different colors corresponds to different depths of the imaged tissue sample. Credit: Courtesy of Dr. Satchin Panda, Salk Institute for Biological Studies and Dr. James Fitzpatrick, Waitt Advanced Biophotonics Center

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, Panda and his collaborators 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 LGN, short for lateral geniculate nucleus, 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 ," 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," he 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."

Explore further: New research characterizes novel aspects of maize reproduction

Related Stories

Bright lights, not-so-big pupils

Dec 31, 2008

A team of Johns Hopkins neuroscientists has worked out how some newly discovered light sensors in the eye detect light and communicate with the brain. The report appears online this week in Nature.

An 'eye catching' vision discovery

Jul 26, 2009

Nearly all species have some ability to detect light. At least three types of cells in the retina allow us to see images or distinguish between night and day. Now, researchers at the Johns Hopkins School of ...

Scientists Uncover Inner Workings of Rare Eye Cells

Jan 27, 2005

Three years ago, Brown University researchers discovered new eye cells – indeed a parallel visual system. Now, in a report in Nature, they explain how these exotic cells harness light energy to do their chief job: setting ...

More than meets the eye to staying awake, alert

May 13, 2010

Think twice before falling asleep alongside the glare of your computer and TV screens: exposure to dim light from ordinary room lights, computer screens and other electronic devices late at night may be interfering with our ...

Recommended for you

The origins of polarized nervous systems

13 hours ago

(Phys.org)—There is no mistaking the first action potential you ever fired. It was the one that blocked all the other sperm from stealing your egg. After that, your spikes only got more interesting. Waves ...

New fat cells created quickly, but they don't disappear

17 hours ago

Once fat cells form, they might shrink during weight loss, but they do not disappear, a fact that has derailed many a diet. Yale researchers in the March 2 issue of the journal Nature Cell Biology descri ...

A single target for microRNA regulation

18 hours ago

It has generally been believed that microRNAs control biological processes by simultaneously, though modestly, repressing a large number of genes. But in a study published in Developmental Cell, a group ...

Sizing up cells: Study finds possible regulator of growth

Mar 02, 2015

Modern biology has attained deep knowledge of how cells work, but the mechanisms by which cellular structures assemble and grow to the right size largely remain a mystery. Now, Princeton University researchers ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.