Serotonin May Play a Role in Tinnitus Severity
New research suggests that serotonin, a neurotransmitter typically associated with mood regulation, could also impact the severity of tinnitus. In a study involving mice conducted by scientists from the US and China, it was found that heightened serotonin signaling in a particular brain circuit led to behaviors linked to this neurological disorder.
Given that serotonin is often used to mitigate symptoms of depression and anxiety, these insights might help steer the development of treatments that alleviate those conditions without worsening tinnitus.
“We’ve had some suspicion that serotonin is involved in tinnitus, but the details were unclear. Using mice, we’ve pinpointed a specific brain circuit that connects serotonin to the auditory system, and we’ve observed that it can induce tinnitus-like effects,” explains Zheng-Quan Tang, a neuroscientist at Anhui University in China.
“When we deactivated that circuit, we saw a significant reduction in tinnitus symptoms. This provides a clearer understanding of what’s happening in the brain and opens up new treatment avenues.”
Tinnitus is generally described as a ‘phantom’ noise, such as ringing or buzzing, that only the patient can hear. While certain causes—like hearing loss or earwax—are understood, many cases appear to stem from neurological issues in the brain’s auditory system.
Describing the relentless anguish of this auditory disturbance is challenging, but many individuals with tinnitus report feelings of depression, anxiety, and even suicidal thoughts. Treatments for mood disorders often incorporate selective serotonin reuptake inhibitors (SSRIs), which elevate serotonin levels by blocking its reabsorption in neurons.
Though numerous investigations over the years have pointed to serotonin’s potential involvement in tinnitus, concrete evidence linking the two has been elusive. To explore this connection, the researchers designed experiments centered on mice.
Initially, they traced the pathway from the dorsal raphe nucleus, a serotonin-producing area in the brainstem, to the dorsal cochlear nucleus, which plays a role in auditory processing.
Then, they genetically modified mice to activate serotonin-releasing neurons in the dorsal raphe nucleus using light or drugs.
Both the modified mice and control groups were subjected to various tests to determine if they exhibited behaviors suggestive of experiencing a phantom sound.
A key indicator was the mice’s inability to notice silent gaps in sounds—this is a common measure for assessing tinnitus in animal studies.
“When these serotonergic neurons are stimulated, we observe activity in the auditory region of the brain,” comments Laurence Trussell, a neuroscientist at Oregon Health & Science University. “The animals displayed behaviors that indicated they were perceiving tinnitus, suggesting symptoms mirroring those found in humans.”
When the researchers inhibited the serotonergic-auditory circuit, they noted a reduction in the tinnitus-like behavior. Additionally, when loud sounds were used to induce tinnitus, the mice behaved similarly to when serotonin levels were increased.
The results imply a tangible connection between serotonin and tinnitus, indicating that this brain circuit might directly contribute to the phantom sounds that patients experience, prompting further investigation in humans.
This research also highlights the necessity of careful management when treating depression or anxiety in individuals with tinnitus.
“Our study indicates a delicate balance,” Trussell notes.
“There might be potential in developing specific drugs targeting particular brain regions to enhance serotonin in some areas while avoiding effects in others. This approach could separate the beneficial aspects of antidepressants from any adverse impacts on hearing.”
The findings have been published in the Proceedings of the National Academy of Sciences.
