Bright lights, not-so-big pupils

December 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.

These light sensors are a small number of nerve cells in the retina that contain melanopsin molecules. Unlike conventional light-sensing cells in the retina--rods and cones--melanopsin-containing cells are not used for seeing images; instead, they monitor light levels to adjust the body's clock and control constriction of the pupils in the eye, among other functions.

"These melanopsin-containing cells are the only other known photoreceptor besides rods and cones in mammals, and the question is, 'How do they work?'" says Michael Do, Ph.D., a postdoctoral fellow in neuroscience at Hopkins. "We want to understand some fundamental information, like their sensitivity to light and their communication to the brain."

Using mice, the team first tested the light sensitivity of these cells by flashing light at the cells and recording the electrical current generated by one cell. They found that these cells are very insensitive to light, in contrast to rods, which are very sensitive and therefore enable us to see in dim light at night, for example. According to Do, the melanopsin-containing cells are less sensitive than cones, which are responsible for our vision in daylight.

"The next question was, What makes them so insensitive to light? Perhaps each photon they capture elicits a tiny electrical signal. Then there would have to be bright light--giving lots of captured photons--for a signal large enough to influence the brain. Another possibility is that these cells capture photons poorly," says Do.

To figure this out, the team flashed dim light at the cells. The light was so dim that, on average, only a single melanopsin molecule in each cell was activated by capturing a photon. They found that each activated melanopsin molecule triggered a large electrical signal. Moreover, to their surprise, the cell transmits this single-photon signal all the way to the brain.

Yet the large signal generated by these cells seemed incongruous with their need for such bright light. "We thought maybe they need so much light because each cell might also contain very few melanopsin molecules, decreasing their ability to capture photons," says King-Wai Yau, Ph.D., a professor of neuroscience at Hopkins. When they did the calculations, the research team found that melanopsin molecules are 5,000 times sparser than other light-capturing molecules used for image-forming vision.

"It appears that these cells capture very little light. However, once captured, the light is very effective in producing a signal large enough to go straight to the brain," says Yau. "The signal is also very slow, so it is not intended for detecting very brief changes in ambient light, but slow changes over time instead."

Curious about how these cells bear on behavior, the researchers examined pupil constriction in mice that had been genetically altered to be free of rod and cone function in order to focus on activity resulting from only melanopsin-containing cells. Flashing light at mice sitting in the dark, the team measured the degree of pupil constriction. They found that, on average, roughly 500 light-activated melanopsin molecules are enough to trigger a pupil response. "But it takes a lot of light to activate 500 molecules of melanopsin," says Yau. "Thus, the pupils close maximally only in bright light."

"In terms of controlling the pupils and the body clock, it makes sense to have a sensor that responds slowly and only to large light changes," says Yau. "You wouldn't want your body to think every cloud passing through the sky is nightfall."

"These melanopsin-containing cells signal light to many different parts of the brain to drive different behaviors, from setting the circadian clock to affecting mood and movement," says Do. "I want to know how these signals are processed and whether they are abnormal in disorders like seasonal affective disorder and jetlag--this is what we hope to tackle next."
-end-
This study was funded by a National Research Service Award, the Visual Neuroscience Training Program at Johns Hopkins supported by the National Eye Institute, and grants from the National Institutes of Health.

Authors on the paper are Michael Tri H. Do, Shin H. Kang, Tian Xue, Haining Zhong, Hsi-Wen Liao, Dwight E. Bergles, and King-Wai Yau, all of Johns Hopkins.

On the Web:
http://neuroscience.jhu.edu/KingWaiYau.php
http://neuroscience.jhu.edu/
http://www.nature.com/nature/index.html

Johns Hopkins Medicine

Related Brain Articles from Brightsurf:

Glioblastoma nanomedicine crosses into brain in mice, eradicates recurring brain cancer
A new synthetic protein nanoparticle capable of slipping past the nearly impermeable blood-brain barrier in mice could deliver cancer-killing drugs directly to malignant brain tumors, new research from the University of Michigan shows.

Children with asymptomatic brain bleeds as newborns show normal brain development at age 2
A study by UNC researchers finds that neurodevelopmental scores and gray matter volumes at age two years did not differ between children who had MRI-confirmed asymptomatic subdural hemorrhages when they were neonates, compared to children with no history of subdural hemorrhage.

New model of human brain 'conversations' could inform research on brain disease, cognition
A team of Indiana University neuroscientists has built a new model of human brain networks that sheds light on how the brain functions.

Human brain size gene triggers bigger brain in monkeys
Dresden and Japanese researchers show that a human-specific gene causes a larger neocortex in the common marmoset, a non-human primate.

Unique insight into development of the human brain: Model of the early embryonic brain
Stem cell researchers from the University of Copenhagen have designed a model of an early embryonic brain.

An optical brain-to-brain interface supports information exchange for locomotion control
Chinese researchers established an optical BtBI that supports rapid information transmission for precise locomotion control, thus providing a proof-of-principle demonstration of fast BtBI for real-time behavioral control.

Transplanting human nerve cells into a mouse brain reveals how they wire into brain circuits
A team of researchers led by Pierre Vanderhaeghen and Vincent Bonin (VIB-KU Leuven, Université libre de Bruxelles and NERF) showed how human nerve cells can develop at their own pace, and form highly precise connections with the surrounding mouse brain cells.

Brain scans reveal how the human brain compensates when one hemisphere is removed
Researchers studying six adults who had one of their brain hemispheres removed during childhood to reduce epileptic seizures found that the remaining half of the brain formed unusually strong connections between different functional brain networks, which potentially help the body to function as if the brain were intact.

Alcohol byproduct contributes to brain chemistry changes in specific brain regions
Study of mouse models provides clear implications for new targets to treat alcohol use disorder and fetal alcohol syndrome.

Scientists predict the areas of the brain to stimulate transitions between different brain states
Using a computer model of the brain, Gustavo Deco, director of the Center for Brain and Cognition, and Josephine Cruzat, a member of his team, together with a group of international collaborators, have developed an innovative method published in Proceedings of the National Academy of Sciences on Sept.

Read More: Brain News and Brain Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.