That has changed as the result of a technical advance developed by Daniel C. Tu and Donald Zhang, both Medical Scientist Training Program students in Van Gelder's lab, and co-first authors of this study. Tu and Zhang used a multi-electrode array technique in which tiny, individual electrodes are placed about 200 microns apart. Each electrode is a mere 30 microns in size -- there are 25,400 microns per inch --and 60 electrodes are contained on a grid.
"This spacing turns out to be perfect for a retina," Van Gelder says. "You can remove the retina and place it, ganglion cell-side down, on this array. Then the electrodes pick up the impulses of the ganglion cells when those cells react to light."
Whereas the original brain injection technique allowed researchers to study only one or two ipRGCs per day, the multi-electrode array allows Van Gelder's team to study 30 times that many. Those studies have revealed a cell population that reacts quickly and consistently to light.
"If you give the cells a series of identical pulses of light and look at how fast they fire, the reaction is identical every time," Van Gelder says. "The ganglion cells detect brightness, and they're extremely good at it. You could make a good light meter for a camera out of these cells because they are consistent in their response to brightness over the equivalent of almost 10 f-stops on a camera. That's completely different from the rods and cones in the retina. Those visual cells can't detect brightness very well. They detect contrast, sensitivity and motion."
Studying these populations of ipRGCs, Van Gelder also found the cells require a protein called melanopsin to sense and react to pulses of light. When the group examined retinas of mice that were genetically engineered to lack melanopsin, they found that the ganglion cells lost all sensitivity to light.
The ability to study many of these cells a
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Contact: Jim Dryden
jdryden@wustl.edu
314-286-0110
Washington University School of Medicine
21-Dec-2005