For some patients, the very medications that elevate their mood can also hinder their ability to find quiet. Scientists have uncovered how serotonin—the key brain chemical targeted by many antidepressants—can amplify signals in the brain’s auditory system, worsening tinnitus symptoms. The findings of this study were published in the journal Proceedings of the National Academy of Sciences.
These results provide a clearer biological explanation for why tinnitus sometimes intensifies during treatment, transforming patient reports into something researchers can now test and measure.
Within the auditory circuits of mice, the signal arose in areas where mood-related brain cells directly connect into sound-processing pathways.
At Oregon Health & Science University (OHSU), Lawrence Trussell linked this activity to serotonin and demonstrated that stronger signaling correlates with behavior consistent with phantom ringing in the ears.
The effect was not limited to a vague shift in the brain’s state; it traveled along a specific pathway into cells involved in processing sound.
This does not imply that every drug that boosts serotonin levels will worsen symptoms, but it raises a more critical question about why this ringing in the ears can become so persistent.
For patients, tinnitus—hearing sounds without an external source—can persist as ringing, buzzing, roaring, or hissing throughout the day. Damaged auditory pathways may transmit altered nerve signals after injury or with age, and the brain may fill the silence with unwanted noise.
According to a global study, 14.4% of adults have experienced tinnitus, with about 2% reporting severe symptoms that can over time impair sleep and concentration.
Many patients encounter this issue when taking selective serotonin reuptake inhibitors (SSRIs), antidepressants that maintain higher serotonin activity between brain cells.
These medications can alleviate depression and anxiety; however, some individuals report increased ringing in the ears after starting treatment or adjusting the dosage.
“This study underscores the importance of doctors recognizing and validating patient reports of medication-related increases in tinnitus,” said Trussell.
Careful monitoring is essential because untreated symptoms of mood disorders can cause harm if medication is abruptly discontinued.
A more detailed investigation pointed to the dorsal cochlear nucleus, an early brain region that helps sort sounds before they reach consciousness. The influx of serotonin into this brain region excited certain cells responsible for shaping sound, causing them to activate more rapidly before higher brain areas interpreted the signals.
Noise exposure also elevated serotonin levels in this auditory region during tinnitus-like behavior in mice, helping to pinpoint exactly where the chemical signal acts within a specific signaling pathway.
To test whether this mechanism truly caused a similar effect, the team turned to optogenetics—a method that uses light to control individual brain cells.
By shining light through an optical fiber onto serotonin-producing neurons, the researchers were able to trigger activity with high precision. After stimulation, the mice in startle response tests reacted as if some hidden sound had broken the silence at other moments.
Although animal behavior cannot fully reflect human sensations, it can reveal brain activity associated with perceiving sound as a target. The researchers also tested whether reducing activity in the same pathway could alleviate tinnitus-like symptoms.
Using chemogenetics—a technique that employs artificially created receptors and specially designed drugs to control specific cells—they suppressed the serotonin pathway feeding the auditory region.
Blocking the serotonin receptor yielded similar results, pointing to a more precise drug target and suggesting that future treatments could aim at these receptors to reduce symptoms.
For patients already struggling with constant noise, these findings offer a more straightforward way to build conversations with doctors. A treating physician can align mood relief with changes in tinnitus, adjusting dose or timing, rather than ignoring either issue.
“People suffering from tinnitus should collaborate with their healthcare provider to design a treatment plan that balances relief from psychiatric symptoms like depression and anxiety while minimizing the perception of tinnitus,” said Trussell.
No one should stop antidepressants on their own, as sudden changes can worsen symptoms or trigger withdrawal syndrome, and must always be supervised by a doctor.
Findings from mouse studies allow scientists to control conditions in ways typically unattainable in human research, especially regarding the brain’s tiniest neural networks, while avoiding invasive investigative methods.
This control enabled the identification of a cause-and-effect relationship, as researchers could increase or decrease one variable at will in the same animals.
Nevertheless, studying tinnitus in humans takes into account hearing loss history, stress, sleep, medication dosage, ear damage, and the brain’s attentional systems. Thus, the model developed in mice serves as a guide for human research rather than a simple rule for every patient or prescription.
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