Long before sunglasses came on the scene, animals had evolved ways, such as shutter-like irises, to deal with the large differences in lighting that their eyes encounter. After all, many animals can see on both a starry night and a sunny day.
Two research teams have now uncovered a novel molecular mechanism that assists animals’ vision. Proteins that are central to the complex light-sensing systems of the eye migrate from one part of a retinal cell to another to adjust the cell’s sensitivity.
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“This is a new theme” in light adaptation, says Vadim Arshavsky of Harvard Medical School in Boston, an author of one of the reports that appear in the March 28 Neuron.
Mammalian eyes depend on rods and cones, retinal cells known as photoreceptors. These cells sport protein complexes that respond to light by creating an electrical signal that travels to the brain. When photons hit a light-sensing protein called rhodopsin, another protein–transducin–amplifies the signal. A photon hitting a single rhodopsin molecule might activate hundreds of transducin molecules.
More than a decade ago, several research groups reported that transducin moves away from the outer, photon-sensing segments of rods when rodent eyes were exposed to light. After other scientists challenged the data as an experimental artifact, the work was largely forgotten.
Using more-accurate techniques, Arshvasky’s group has now confirmed the earlier findings. When rodents were exposed to moderate daylight, up to 90 percent of the transducin moved from the outer segments to another part of the rod cells, the researchers report.
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By separating much of the transducin from the rhodopsin, this movement seems to reduce the strength of the signaling cascade. In other words, for a given amount of light, rods would generate a smaller electrical signal. Arshvasky’s team demonstrated this adaptation by measuring the electrical signals of rat retinas. The group used an electrode affixed to a tiny contact lens on an animal’s eye. The adaptation extends the range of illumination in which rods can work by a factor of 10.
In a similar vein, Armin Huber of the University of Karlsruhe in Germany and his colleagues studied fruit fly photoreceptor cells, focusing on a protein channel that regulates the flow of ions through cells. In flies reared in the dark, the protein is abundant in light-sensing complexes. After flies are exposed to light, however, it disappears from the complexes. Within an hour of such exposure, 70 percent of the protein has moved to a storage compartment within the cell, Huber’s team reports in Neuron.
Huber points out that related ion channels play roles in many nonvision processes, such as sensing pain and temperature. “The most important impact of our paper,” says Huber, “may lie in the possibility that other members of this family undergo translocations as well.”
“These studies represent significant advances to our understanding of photo-receptor adaptation,” says Roger Hardie of Cambridge University in the United Kingdom in a Neuron commentary.