"Our data are the first to directly show that chloride ions are crucial for our exquisite sense of hearing," said Joseph Santos-Sacchi, professor in the Departments of Surgery and Neurobiology and first author of the study in the Journal of Neuroscience. "These data also indicate that the hearing in marine and other mammals could be affected by environmental toxins, such as TBT (tributyl tin), because they appear to alter the balance of chloride ions in the outer hair cell."
Sensitive hearing in mammals relies on cochlear amplification resulting from the motor activity of outer hair cells. They are the only group of animals that have outer hair cells. Additionally, TBT is known to damage the immune and hormonal systems of marine mammals.
In this study on guinea pigs, Santos-Sacchi tested whether TBT or salicylate, which is a chemical that occurs naturally in plants and is a component of aspirin, interfered with the guinea pigs' ability to hear. He found that TBT, salicylate, or otherwise altering the extracellular chloride levels in the cochleas, interfered with the balance of chloride in the outer hair cells and caused profound changes in sound amplification in the inner ear.
In his previous study using TBT on isolated cells only, Santos-Sacchi had proposed that the ear's ability to perceive sound would be compromised. He also speculated that whales and other marine mammals exposed to TBT would have altered sound localization abilities. The present findings confirm that their hearing is altered in mammals.
"Since many marine mammals use echolocation or sonar to get around, this could be contributing to whales and dolphins beaching and hitting ships," Santos-Sacchi said.
Co-authors include Lei Song, M.D., Jiefu Zheng, M.D., and Alfred Nuttall. The study was funded in part by the National Institutes of Health and the National Institute on Deafness and Other Communication Disorders.
Journal of Neuroscience: (published online April 12, 2006 DOI 10.1523/JNEUROSCI.4548-05.2006)