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(Media-Newswire.com) - COLUMBUS, Ohio – Scientists have long believed that the eye uses the retina’s rod cells for vision at night and cone cells to see during the day. New research at The Ohio State University Medical Center, however, shows that rods and cones team up to boost night vision.
The study, published in a recent issue of the journal Neuron, shows that at night, rod cells send electrical signals to neighboring cone cells through tiny channels, or gap junctions, between the cells. These channels close each morning and open each evening.
The study also shows that a 24-hour molecular clock present in the retina, and not the level of ambient light, controls this cyclic change. This diurnal pattern continues even when the retina is kept in constant darkness for several days and nights.
The findings could lead to a better understanding of vision in very dim light and of progressive, blinding eye diseases such as retinitis pigmentosa, and to new ways to treat and prevent blindness.
“We found that cones actually can detect very dim light at night, not on their own, but because they are getting signals from rods through these channels,” says principal investigator Stuart C. Mangel, professor of neuroscience at Ohio State’s College of Medicine.
The channels were first observed by scientists more than 30 years ago, but they were thought to be always closed, or to open only slightly under certain circumstances, he says.
“Our findings clearly show that they open dramatically at night, which could enable a variety of molecules such as survival factors or, in the case of disease, toxic factors to diffuse from one cell to another.”
For this study, Mangel and his colleagues, Christophe P. Ribelayga, assistant professor of neuroscience, and Yu Cao, a post-doctoral researcher in Mangel’s laboratory, used cone cells in intact retinas from goldfish kept in starlight levels of darkness for an hour before surgery. The surgery itself was done in darkness while wearing night-vision infrared goggles.
In continual darkness, the scientists used microelectrodes to monitor the small electric signals generated by individual cones and the input they received from neighboring rods.
By injecting a tracer molecule into individual cone cells, they discovered that the tracer remained within a cone cell during daytime hours but diffused into more than a thousand rod cells and a hundred cone cells at night.
“We were surprised by how dramatic the difference was between day and night,” Mangel says.
The researchers then exposed the cells to moonlight-level illumination at night to see if the increased light caused the channels to close. “It made absolutely no difference,” Mangel says. “The channels opened during nighttime hours and closed during the day as before, indicating that the retina’s molecular clock regulates the coupling between the rods and cones, not the level of ambient light.”
Furthermore, the study linked the closing of the channels to a daytime rise in the level of the neurotransmitter dopamine in the retina. “The action of the clock,” Mangel says, “increases the dopamine level and activates a particular type of dopamine receptor on the rods and cones. This closes the channels and uncouples the rods and cones.”
Last, the researchers showed that the same cyclic coupling and uncoupling of cones and rods also occurs in mice. This, too, involved the retinal clock and release of dopamine.
“Our study shows that the gap junctions between rods and cones open dramatically at night, and we believe that this improves the eye’s ability to see large objects in dim light.”
Funding from the National Eye Institute supported this research.
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