Activity in non-sensory cells of the developing ear may help form neural pathways in hearing,
transient group of non-sensory cells in the developing cochlea, the hearing chamber of the inner ear, generates auditory activity in the absence of sound, according to a study published this week in Nature. This activity, present before the onset of hearing, may help form the auditory map in the brain, and may provide insights into hearing conditions such as tinnitus, the researchers say.
"Understanding the cochlea in development has been left out in the progress of neuroscience," said Timothy Jones of East Carolina University in North Carolina, who was not involved in the research. "This study will be kindling. This will trigger a great interest in pre-hearing."
During development, the ear gradually develops its ability to respond to auditory stimuli. Before the ear is capable of hearing, researchers have observed spontaneous activity in the auditory nerve that mimics the ear's response to sound but occurs without an external stimulus. But how this activity is initiated and what purpose it serves has remained unclear.
Using intracellular and extracellular imaging, the researchers traced this activity to a group of non-sensory supporting cells located in a part of an epithelial ridge known as Kölliker's organ, which disappears after the onset of hearing. The Kölliker cells surround the inner hair cells, the ear's sensory receptors.
"These [supporting cells] are cells that, until now, we thought weren't doing a hell of a lot," said Jonathan Gale, a coauthor of the study. "A lot of research was on the inner hair cells, the sensory cells themselves."
Using a combination of in vitro electrophysiology and imaging techniques, such as intrinsic optical imaging as well as calcium-imaging dyes, the researchers found that the Kölliker cells release ATP in spontaneous bursts, causing the inner hair cells to release glutamate, thus triggering action potentials in primary auditory neurons.
"What we were struck with was that ATP was doing the job that sound would eventually do in the developed cochlea," said Dwight Bergles of Johns Hopkins School of Medicine, corresponding author of the study. "Before the ear is mature enough to detect sound, hair cells respond to ATP."
Bergles explained that as the ear develops and the Kölliker's organ disappears, the synchronous bursts of activity decrease in frequency, which is desirable, since they would interfere with normal hearing.
The authors of the study make the analogy to the visual system: there is electrical activity in the eye before the animal opens its eyes and can see. This activity helps develop the visual topography of the brain. They suggest that the spontaneous activity in the inner ear may serve a similar developmental function, but further research must investigate this claim.
While Bergles noted that the researchers' results were preliminary, he said they may provide insights into the mechanisms of tinnitus (chronic ringing after auditory trauma) and deafness. Gale explained that studies have already shown how noise trauma causes the release of ATP. Thus, injury to the cochlea could induce the release of ATP and cause the ringing in one's ears.
Research on tinnitus and deafness has largely focused on the central nervous system and the brain, since, for example, soldiers who return from war still experience chronic ringing even after their auditory nerve is severed, Bergles explained. "It's pretty clear that after it becomes full-blown, it must be in the brain," he said, but peripheral cells such as those the study identified may play a greater role than previously supposed.
The study is "essentially opening up a new area, and actually underscoring the importance of cells we normally ignore," said Jones.