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Not Blinded by the Light

Our eyes work well in a remarkably wide range of lighting conditions--from high noon in the desert to a candlelit restaurant. Now researchers have discovered a trick the eye uses to adapt to changing light. In experiments with rats, they found that photoreceptor cells shuttle a key signaling protein away from the outer, most exposed edge of the retina to the cell interior where it can no longer play its part in light detection.

Photoreceptor cells detect light through a special complex of molecules located in the outer segments of the cells--where their exposure to light is maximized. Once activated, these molecules pass on the signal to transducin, a protein that acts as an amplifier and ultimately triggers an electrical message to the brain. But bright light can overload the signaling mechanism. Scientists suspected that photoreceptors may compensate, in part, by moving transducin away from the light-capturing equipment, but the evidence was inconclusive.

Neuroscientist Vadim Arshavsky of Harvard Medical School in Boston and his team sought to settle the issue. They exposed rat retinas to light of varying brightness, then used a new technique to track transducin. Within an hour of exposure to bright light, most transducin moved away from the light-capturing complexes to the photoreceptors' inner segments, the team reports in the 28 March issue of Neuron. At the same time, the electrical output of photoreceptors decreased.

Arshavsky interprets this as evidence that the movement is important in adapting the eyes to changes in brightness. Although much faster mechanisms--involving changes in the biochemical signaling pathways--are probably more important in quick adaptation to, say, stepping outside on a sunny day, Arshavsky says that shuttling transducin could help maintain the comfortable operation of the visual system as light levels change throughout the course of the day. He likens the situation to putting on a pair of sunglasses during the day and taking them off at night.

Uwe Wolfrum, a biologist at the University of Mainz in Germany, says the study "finally demonstrates that protein movement in response to light changes occurs." He agrees that the transducin shuffle is likely to help the eyes adapt to light changes and adds that the protein-tracking technique invented by Arshavsky's team could be used to explore the role of protein movements in other tissues.

Related links
Arshavsky's site
Primer on human vision
More detailed information on vision