If you've ever been kept awake by that ringing in your ears when everything else is quiet, you'd be familiar with how frustrating tinnitus can be. Now, research led by the University of Arizona may have uncovered a potential new treatment target – not in the ears but within the brain itself. The study suggests that neuroinflammation is to blame, and could be a new way to fix the problem.
Tinnitus itself is a symptom of hearing loss, induced by loud noises. Stand too close to the speakers at too many gigs, or spend a lifetime working in a saw mill with no hearing protection, and you'll permanently damage your hearing. But rather than just reducing the range of sounds you can hear, that damage can trigger tinnitus as a ringing or a hiss in the ears.
Recent studies may have found the connection. Inflammation is the body's response to damage or infection, so noise-induced hearing loss (NIHL) could cause inflammation in the auditory pathway. That in turn could be what triggers tinnitus, but exactly how is still unknown.
To investigate, the researchers on the new study focused on mice with NIHL, looking at neuroinflammation in the auditory cortex. The team found that in that part of their brains, those mice had higher levels of molecules called proinflamatory cytokines, and cells called microglia appeared to be activated in higher amounts. Both of these are known to be involved in neuroinflammatory responses.
The team also singled out a molecule called tumor necrosis factor alpha (TNF-α) as being involved in neuroinflammation and tinnitus. When the researchers knocked out the gene responsible for TNF-α, they found that neuroinflammation was prevented and the mice no longer showed signs of tinnitus. The same result was shown when TNF-α was blocked using drugs in another group of mice.
To check the reverse of the connection between tinnitus and TNF-α, the researchers also infused more of the molecule into the primary auditory cortex of other groups of mice. Sure enough, the extra molecules were found to trigger tinnitus behaviors both in mice with normal hearing and those engineered to lack TNF-α.
If the findings translate over to humans, drugs that block TNF-α could become a brand new treatment for tinnitus. Other potential treatments in the works include a headset developed by University of Michigan researchers that delivers small electrical pulses into the brain.
The research was published in the journal PLOS Biology.
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