More than 50 million Americans struggle with tinnitus, hearing a sound in the ears in the absence of an external source. A symptom rather than a disease itself, tinnitus in patients ranges from annoying to debilitating and has no cure. Some treatments help some people, but no one treatment seems to work for everyone. A major risk factor for tinnitus is hearing loss.
Shaowen Bao, Ph.D., an associate professor of physiology at the University of Arizona College of Medicine–Tucson, and colleagues are closing in on potential treatments for tinnitus by connecting brain inflammation to the condition. They found inflammation in a sound-processing region of the brain triggers evidence of tinnitus in mice that have noise-induced hearing loss, according to a study published in the journal PLOS Biology in June 2019. Coauthors include 2019 Emerging Research Grants scientist Hao Luo, M.D., Ph.D.
Credit: National Institute on Aging, NIH via Flickr.
Recent studies indicate that noise-induced hearing loss causes inflammation—the immune system’s response to injury or infection—in the brain auditory pathway. How it contributes to hearing loss–related conditions, such as tinnitus, however, is not well understood. Bao and colleagues examined neuroinflammation (inflammation in the nervous system) in the auditory cortex of the brain following noise-induced hearing loss, and its role in tinnitus.
Their research showed mice with noise-induced hearing loss (under anesthesia) had elevated levels of molecules called proinflammatory cytokines and the activation of non-neuronal cells called microglia, two defining features of neuroinflammatory responses, in the primary auditory cortex in the brain.
The research also showed that the cytokine TNF-alpha (tumor necrosis factor alpha), a cell signaling protein (cytokine) involved in systemic inflammation, is necessary for noise-induced neuroinflammation, tinnitus, and synaptic imbalance (an altered pattern of signaling between neurons). When the researchers used a pharmacological drug to block the TNF-alpha, the mice no longer showed signs of tinnitus.
Bao has been examining the role of TNF-alpha in tinnitus since 2011. “People have found clues for the cause of tinnitus but because many parallel components are involved, we would block one component, then we would have to block another, then another still,” Bao says.
“Neuroinflammation seems to be involved in many of these components. We hope blocking neuroinflammation will give us a better chance to block them all, thereby stopping the tinnitus.”
The findings suggest that neuroinflammation may be a therapeutic target for treating tinnitus and other hearing loss-related disorders. However, Bao says, “We have more work to do to confirm the mechanism that is causally linked to tinnitus and determine if the results translate to humans. While promising, we still have a long way to go from research to patient care.”
This originally appeared as a University of Arizona press release. A research associate at Wayne State University in Detroit, Hao Luo, M.D., Ph.D., is a 2019 Emerging Research Grants scientist generously funded by the General Grand Chapter Royal Arch Masons International.
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