Researchers here uncover a novel mechanism that contributes to age-related hearing loss, involving changes in nerve cell communication in older animals. At the present time, much of the work on potential ways to treat loss of hearing with age is focused on hair cells and their regeneration. Some progress has been made on this front, with mouse studies demonstrating partial recovery of hearing through restoration of hair cell populations. It will be unfortunate if it turns out that this approach isn't sufficient on its own to form an effective therapy, but the presence of another important mechanism would explain the partial results seen to date:
Conventional wisdom has long blamed age-related hearing loss almost entirely on the death of sensory hair cells in the inner ear, but new information about the workings of nerve cells suggests otherwise. Researchers have verified an increased number of connections between certain sensory cells and nerve cells in the inner ear of aging mice. Because these connections normally tamp down hearing when an animal is exposed to loud sound, the scientists think these new connections could also be contributing to age-related hearing loss in the mice, and possibly in humans. "The nerve cells that connect to the sensory cells of the inner ear are known to inhibit hearing, and although it's not yet clear whether that's their function in older mice, it's quite likely. If confirmed, our findings give us new ideas for how physicians may someday treat or prevent age-related hearing loss."
The new research builds on the knowledge that inside the ear lies a coiled row of sensory cells responsible for converting sound waves into electrical signals sent through nerve cells to the brain, which processes and tells animals what they "hear." Each of those nerve cells is like a one-way street, taking signals either from the ear to the brain or vice versa. The nerve cells that take signals to the ear are known to turn down the amplification provided by outer hair cells when an animal is, for example, exposed to a noisy environment for an extended period of time. But studies over the last decade have suggested that changes over time also occur in the connections between hair cells and the nerve cells to which they are attached.
The researchers painstakingly recorded electrical signals from within the inner hair cells of young and old mice. They found that the incoming nerve cells were indeed active and that their activity levels correlated with the animals' hearing abilities: The harder of hearing an animal was, the higher the activity of its incoming nerve cells. "These nerve cell connections seem to be reverting back to the way they worked during early development before the animals' sense of hearing was operating. We don't know why the new connections form, but it might be as simple as a lack of competition for space once the outgoing nerve cells have retracted."