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We have some hopeful news now for people who like to crank their music up to 11. Researchers say they've discovered a drug that can partially reverse hearing loss caused by exposure to extremely loud sounds. So far, the drug has only been tried in mice. But NPR's Jon Hamilton reports that it's an important step toward a treatment that could help millions of people.

JON HAMILTON, BYLINE: The trouble with really loud sounds is that they can injure or kill cells in the inner ear known as hair cells. Albert Edge is a hearing researcher at Harvard. He says hair cells turn sounds into electrical signals that are sent to the brain.

DR. ALBERT EDGE: The unusual thing about these hair cells is that what you're born with is what you have throughout life, and there aren't very many of them. And once they're gone, that's why deafness tends to be permanent. It doesn't just go away.

HAMILTON: Hair cells can be damaged by diseases, certain medications or as a part of normal aging. And Edge says hearing specialists are seeing a new group of relatively young people who have lost hair cells.

EDGE: What's important in our society right now are a lot of the soldiers coming back from overseas who have been exposed to even a single loud noise which can seriously damage hearing.

HAMILTON: Tens of thousands of veterans have hearing loss because they were in a fire fight or near a bomb blast that produced a sound many times louder than even the loudest rock concert. Because permanent hearing loss is so common in people, scientists have been studying species in which deafness is only temporary. Fish, for example, can grow new hair cells to replace damaged ones. And Edge says in the 1980s, researchers showed that birds, warm-blooded creatures that are more like us, had the same ability.

EDGE: You can, in fact, deafen a chick, for example, and over the course of a couple of weeks they completely recover their hearing and the hair cells grow back.

HAMILTON: That finding got scientists looking for a way to accomplish the same feat in mammals, which don't naturally re-grow hair cells. And in the current issue of the journal Neuron, Edge and an international team report that they've partially succeeded. The team took a bunch of mice and exposed one ear to a very loud noise. Then they looked to see whether the ear was still producing electrical signals in response to a sound.

EDGE: And so these mice, when we start the experiments, no matter how loud we make the sound, there's no electrical signal. Their ear is essentially dead.

HAMILTON: Next, the team administered a drug to the inner ear. The drug, called a gamma secretase inhibitor, was developed to treat Alzheimer's disease. It never worked out for that purpose. But the drug turned out to have an interesting side effect in mice. Edge says it caused so-called support cells in the inner ear to transform into hair cells.

EDGE: And to our delight these hair cells were functioning hair cells that improved the hearing of the animal.

HAMILTON: Edge says this shows it is possible to grow new hair cells in a mammal. And because mice and humans have very similar hearing systems, he says, the approach is likely to work in people too. It's not a complete cure though. The mice got only about 20 percent of their hearing back, and they still couldn't hear certain sound frequencies. Even so, Ed Rubel at the University of Washington says the result is significant.

DR. ED RUBEL: In terms of the recovery of hearing, it's fairly modest. But it's real and it's a step in the right direction.

HAMILTON: Rubel is one of the scientists who discovered back in the 1980s that birds could create new hair cells. Ever since, he's been involved in the effort to restore hearing. And Rubel says the drug used in this experiment is just one approach. Other researchers are trying different methods including gene therapy. Rubel says he's pretty sure something is going to work.

RUBEL: Oh, I'm hugely optimistic. There's no question that sometime in the future, we will restore hearing in humans through regeneration.

HAMILTON: Rubel says the first treatments for people, though, may be a couple of decades off. Jon Hamilton, NPR News.

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