Nerve stimulation may thwart tinnitus: Study

LONDON - A new study found that researchers were able to eliminate tinnitus in a group of rats by stimulating a nerve in the neck while simultaneously playing a variety of sound tones over an extended period of time.

This new therapy was found to help retrain the part of the brain that interprets sound so that errant neurons reverted back to their original state and the ringing disappeared. The research was conducted by scientists from the University of Texas at Dallas and MicroTransponder Inc., in Dallas.

Tinnitus is a symptom some people experience as a result of hearing loss. When sensory cells in the inner ear are damaged, such as from loud noise, the resulting hearing loss changes some of the signals sent from the ear to the brain. For reasons that are not fully understood, some people will develop tinnitus as a result.

Michael Kilgard, associate professor of behavior and brain sciences at UT-Dallas, and a co-principal investigator on the study, along with co-principal investigator Navzer Engineer, of MicroTransponder, Inc., and others on the research team first sought to induce changes in the auditory cortex of a group of rats by pairing stimulation of the vagus nerve, a large nerve that runs from the head and neck to the abdomen, with the playing of a single tone. When the vagus nerve is stimulated, it releases acetylcholine, norepinephrine, and other chemicals that help encourage changes in the brain. They wanted to find out if they could induce more brain cells to become responsive to that tone over a period of time.

For 20 days, 300 times a day, researchers played a high-pitched tone, at 9 kilohertz (kHz), to eight rats. At the same time that the tone was played, an electrode delivered a very small electrical pulse to the vagus nerve. The researchers found that the number of neurons tuned to the 9 kHz frequency had jumped by 79 percent in comparison to the control rats.

In a second group of rats, they randomly played two different tones-one at 4 kHz and the other at 19 kHz-but stimulated the vagus nerve only for the higher tone. Neurons tuned to the higher frequency increased by 70 percent while neurons tuned to the 4 kHz tone actually decreased in number, indicating that the tone alone was not enough to initiate the change. It had to be accompanied by vagus nerve stimulation (VNS).

Next, the researchers tested whether tinnitus could be reversed in noise-exposed rats by increasing the numbers of neurons tuned to frequencies other than the tinnitus frequency. A group of the noise-exposed rats with tinnitus received VNS that was paired with different tones surrounding the tinnitus frequency 300 times a day for about three weeks. Rats in the control group received VNS with no tones, tones with no VNS, or no therapy. For both groups, measurements were taken four weeks after noise exposure, then 10 days after therapy began, and one day, one week, and three weeks after therapy ended.

Rats that received the VNS paired with tones showed promising results for each time point after therapy began, including midway through therapy, indicating that the ringing had stopped for the treated rats.

Conversely, the data from control rats indicated their tinnitus had continued throughout the testing period. What’s more, the researchers followed two treated and two control rats for an additional two months and found that the treated rats maintained this benefit for 3.5 months after noise exposure, while the controls continued to be impaired.

The researchers also evaluated neural responses in the auditory cortex in these same rats and found that neurons in the treated rats had returned to their normal levels, where they remained. This indicated that the tinnitus had disappeared. However, the control group levels continued to be distorted, indicating that the tinnitus persisted. Overall, the researchers found that the VNS treatment paired with tones had not only reorganized the neurons to respond to their original frequencies, but it also made the brain responses sharper, decreased excitability, and decreased synchronization of auditory cortex neurons.

“The key is that, unlike previous treatments, we’re not masking the tinnitus, we’re not hiding the tinnitus. We are retuning the brain from a state where it generates tinnitus to a state that does not generate tinnitus. We are eliminating the source of the tinnitus,” said Kilgard.

The study has been published in the advance online publication of the journal Nature. (ANI)