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January 24, 2011
Tinnitus Cure May Lie in the Brain
Scientists were able to eliminate tinnitus — a persistent ringing in the ears — in rats by stimulating a nerve in the neck while playing a variety of sound tones over an extended period of time. The finding gives hope for a future tinnitus cure in humans.
Tinnitus usually comes in the form of a high-pitched tone in one or both ears, but can also sound like a clicking, roaring or whooshing sound. While tinnitus isn't fully understood, it is known to be a sign that something is wrong in the auditory system: the ear, the auditory nerve that connects the inner ear to the brain, or the parts of the brain that process sound. Something as simple as a piece of earwax blocking the ear canal can cause tinnitus, but it can also arise from a number of health conditions. For example, when sensory cells in the inner ear are damaged from loud noise, the resulting hearing loss changes some of the signals in the brain to cause tinnitus.
There's no known cure for tinnitus. Current treatments generally involve masking the sound or learning to ignore it. A research team led by Dr. Michael Kilgard at the University of Texas at Dallas and Dr. Navzer Engineer at MicroTransponder, Inc. set out to see if they could develop a way to reverse tinnitus by essentially resetting the brain's auditory system. Their work was funded in part by NIH’s National Institute on Deafness and Other Communication Disorders (NIDCD).
The researchers paired electrical stimulation of the vagus nerve — a large nerve that runs from the head to the abdomen — with the playing of a tone. Vagus nerve stimulation (VNS) is known to release chemicals that encourage changes in the brain. This technique, the scientists reasoned, might induce brain cells (neurons) to tune to frequencies other than the tinnitus one. For 20 days, 300 times a day, they played a high-pitched tone to 8 rats during VNS.
In the advance online edition of Nature on January 12, 2011, the researchers reported that the number of neurons tuned to the high frequency had jumped by 79% compared to control rats. The scientist then tested 2 different tones in a second group of rats but stimulated the vagus nerve only for the higher one. The neurons tuned to the higher tone increased by 70%, while those tuned to the lower one decreased in number. This showed that the tone alone wasn’t enough to initiate the change; it had to be accompanied by VNS.
The researchers next tested whether tinnitus could be reversed in noise-exposed rats. The animals received VNS paired with various tones other than the tinnitus frequency 300 times a day for about 3 weeks. Rats that received the treatment showed behavioral changes indicating that the ringing had stopped. Neural responses in the brain's auditory cortex returned to their normal levels as well, indicating that the tinnitus had disappeared.
"The key is that, unlike previous treatments, we're not masking the tinnitus, we're not hiding the tinnitus," Kilgard says. "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."
VNS is already used to treat people with epilepsy or depression. The scientists are now planning to conduct clinical studies of VNS paired with tones in tinnitus patients.