When we hear a sound, sensory cells in our inner ear trigger the release of a chemical – called a neurotransmitter – to neighboring nerve cells, which, in turn, relay the auditory message to our brain. When our ears are exposed to very loud sounds, such as the blast of a firecracker, too much of the neurotransmitter is released, damaging these auditory nerve cells and causing hearing loss.
Researchers at the Massachusetts Eye and Ear Infirmary, Harvard Medical School, have found that auditory nerve cells temporarily reduce the expression of a key neurotransmitter receptor on their surfaces when exposed to loud noise, and they wanted to know why.
Supported by the National Institute on Deafness and Other Communication Disorders, one of the National Institutes of Health, the scientists are presenting their findings at the 2009 Midwinter Meeting of the Association for Research in Otolaryngology in Baltimore.
In a new study on mice, the researchers used a drug to block the ability of the auditory nerve cells to remove the receptor and then exposed the mice to a moderately loud sound that, under normal conditions, would not damage the nerve cells. They found that the mice given the blocking drug experienced hearing loss for at least six hours following exposure to the normally harmless sound. Also, the blocker accelerated the death of auditory nerve cells that had been incubated in the lab with neurotransmitter chemicals that are normally released during sound stimulation.
The researchers suggest that the auditory nerve regulates the expression of these surface receptors as a way to protect itself against the chemical overload caused by loud noise. Although the scientists believe that auditory nerve cells can rid their surfaces of the receptor by as much as 50 percent, this may not be enough protection against all loud sounds.