Scientists at Stanford Bio-X, the institution's department for breakthrough discoveries about the human body and disease, have modified mice with gene therapy so that their sensitivity to pain can be altered by shining light on their paws.

This application of the neuromodulation technique called optogenetics starts with the insertion of light-sensitive proteins called opsins into the nerves of the mice. The researchers then showed that that exposure to one color of light can increase pain sensitivity in the mice whilst another reduces it.

The research is helping to develop a better understanding of what pain is and why it occurs. It is also hoped that it will provide some clues as to why some people feel pain more or less than others and the extent to which light might ultimately be able to treat pain in humans, in particularly those living with chronic and debilitating pain conditions.

The mice were modified by injecting a virus directly into their nerves that had been engineered to contain opsin-producing DNA. After a few weeks it was found that only the nerves that control pain had taken on the opsin proteins. As a result, the nerves were either more or less likely to fire depending on the color of light to which they were exposed.

"This powerful approach shows great potential for helping the millions who suffer pain from nerve damage," said Linda Porter, the pain policy adviser at the National Institute of Neurological Disorders and Stroke and a leader of the NIH's Pain Consortium. "Now, with a flick of a switch, scientists may be able to rapidly test new pain relieving medications and, one day, doctors may be able to use light to relieve pain."

Different opsins are now being produced that will react differently to different colors of light and the use of viruses to deliver the opsins means that they can be tested very quickly.

"Because we used a viral approach we could, in the future, quickly turn around and use newer opsins," says study leader Kate Montgomery in a press release.

Optogenetics was developed as a means of activating precise regions of the brain to better understand its functions. During a previous piece of research in which optogenetics was being used to control nerves that excite muscles, it was found that the opsins would on occasion be accidentally placed into the nerves that signal pain. It was this occurrence that triggered the research into the control of pain sensitivity.

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