New clues point to possible origin of tinnitus

Brain scientists at Johns Hopkins University in the USA have discovered how cells in the developing ear make their own noise, long before the ear is able to detect sound around them.

The finding, reported in the journal Nature, helps to explain how the developing auditory system generates brain activity in the absence of sound. It may also explain why people sometimes experience tinnitus and hear sounds that seem to come from nowhere.

The research team made their discovery while studying the properties of non-nerve cells in the ears of young rats. These so-called support cells were thought to be silent bystanders not directly involved in nerve communication. However, to the researchers' surprise, these cells showed robust electrical activity, similar to nerve cells. Furthermore, this activity occurred spontaneously, without sound or any external stimulus. The team discovered that the cells were using ATP (adenosine triphosphate), a chemical most often used as a cell's energy currency, as a signal to communicate with other cells.

ATP was being released from the supporting cells near the hair cells, the cells that are responsible for transferring sound information to auditory nerves. The team found that hair cells then also showed spontaneous electrical activity, which occurred at the same time as the responses in neighbouring support cells. In a domino-like effect, ATP then signals the hair cells to release another chemical, glutamate, which activates the nerve cells that project into the brain.

"It is as if ATP substitutes for sound when the ear is still immature and physically incapable of detecting sound," said Professor Dwight Bergles, the lead researcher on the study, adding that "the cells we have been studying seem to be warming up the machinery that will later be used to transmit sound signals to the brain."

While this activity is likely to be essential for the auditory system's proper development, it could be bad in the adult, mature nervous system as it would trigger electrical signals in the absence of sound. Although there is no ATP floating around by the time the ear matures and sounds can be heard, the hair cells continue to respond to it, and injury to the cochlea can trigger the release of ATP. Bergles suspects that "if ATP were released by the remaining support cells, it may cause the sensation of sound when there is none", the condition known as tinnitus.