Ringing in the ears? The cells of the developing ear may provide an explanation for tinnitus

Scientists from Johns Hopkins University have discovered how certain cells in the stages of development of the auditory system send signals even before the ear is able to receive noises from the environment

ScientificDaily (5/11/07)

Scientists from Johns Hopkins University have discovered how certain cells in the stages of development of the auditory system send signals even before the ear is able to receive noises from the environment. These findings, which were published in the journal Nature, help explain how the brain area responsible for hearing is shaped and developed without external sounds. In addition, this discovery may explain why we sometimes hear "ringing" (tinton) in our ears and sounds coming from nowhere.

The research, done in young rats, focused on supporting cells in the ear, which are not nerve cells. Until now, these cells were considered "silent observers" that are not directly involved in neural communication. To the researchers' surprise, they noticed that these cells display a lot of electrical activity, similar to nerve cells. This activity occurred spontaneously and without any sound or external stimulation.

According to Dr. Dwight Bergles, a senior lecturer in the Department of Neuroscience at Hopkins University, it is commonly thought that during the development of the auditory system, nerve cells that connect the organ of hearing to the brain need to experience sounds or other design activity in order to find their way to the part of the brain responsible for voice processing. Therefore, when the researchers noticed that the supporting cells can generate electrical activity independently, they suspected that they might be involved in initiating the activity required to direct the nerve cells.

In order to find out how these cells create electrical pulses, Dr. Bergles and his group tested the effect of various inhibitors and drugs on the cells during the development of the cochlea (the cochlea, a small, hollow area in the inner ear, filled with fluid that plays a role in converting sound waves into electrical signals). They found that the drugs that disrupt the electrical signals are the ones that interfere with the activity of ATP, which is a molecule used as a signal for communication between cells.

According to Dr. Bergles, the breakthrough occurred when it was discovered that ATP causes the supporting cells to change their shape. The researchers discovered that ATP is released from the supporting cells near the hair cells, which are the cells responsible for transmitting sound information to the auditory nerves. Since there are receptors for ATP on the surface of the hair cells, the researchers hypothesized that they are also affected by this molecule. Indeed, the researchers found that the hair cells display spontaneous electrical activity that corresponds to the reactions in the nearby supporting cells, and is affected by the presence of inhibitors that block the ATP receptors.

The presence of ATP in the environment of the hair cells causes these cells to release another chemical, glutamate, which activates the nerve cells that reach the brain. In this way, a situation is created in which the presence of ATP mimics a situation of external sounds in the developing ear even before it is able to receive sounds from the environment. This is a kind of preparatory stage for the system that will transmit auditory information to the brain.

According to the researchers, only a few of the supporting cells release ATP, and only a part of the hair cells next to them are activated at a time. In this way, they allow the nerve cells to recognize their neighbors and their location, and in the process, create appropriate connections between them and shape the auditory area of ​​the brain.

A question that arises from this research is, why is it necessary to "hear" during the development of the ear, before birth? The hypothesis is that the ability to recognize small sound differences, as sometimes reflected in speech, requires many very fine adjustments, which are based on the connections between the nerve cells in the brain. It is possible that the electrical activity described, which activates a limited number of nerve cells at a time, enables this subtle auditory ability.

While this activity of the supporting cells is essential for the proper development of the auditory system, it may be harmful in an adult if it evokes neural-electrical activity without the stimulation of external sounds. However, it was found that in the first two weeks of the rat's life, most of the ATP-releasing cells disappear. At this point the rat can hear, and the spontaneous electrical activity stops.

Although the adult does not have ATP in the hair cell environment, they are still able to respond to it. The exposure to loud sounds from the environment can cause the release of ATP in the ear. In this condition, known as "ringing in the ears", we hear sounds even though they don't exist. In addition, in these cases, there may be changes in the connections between the nerve cells in the brain, similar to the processes that occur during the development of the auditory system. These changes may damage nerve cell activity and hearing.

For the announcement of Johns Hopkins University