Exposure to loud noises induces plasticity in hippocampus

Experiments find functioning changed by noise in previously undetected ways

Neuroscientists at The University of Texas at Dallas are examining whether multiple areas of the brain are culpable in causing tinnitus, research that could enable new medical interventions against the disabling effects of severe "ringing in the ears."

Dr. Tres Thompson, associate professor in UT Dallas' School of Behavioral and Brain Sciences, found that exposure to loud noises induces plasticity in the hippocampus, a section of the brain not primarily associated with hearing but known for learning-related plasticity. This neuroplasticity - changes in the function of the brain in reaction to experiences - could open the door to long-term tinnitus, he said.

A three-year, $135,000 grant from the American Tinnitus Association supports this work in Thompson's lab. The next stage of research could focus on drug treatments aimed at reducing or reversing plasticity. Thompson wants to test whether certain drugs targeting plasticity mechanisms might inhibit or change plasticity, protecting against tinnitus.

The hippocampus neurons of laboratory rats exhibited plasticity within 15 minutes of exposure to intense noise that can induce tinnitus in humans and rats.

"The animals' whole map of their world changed," Thompson said. "The activity of many neurons changed completely in reaction to the noise exposure."

Instead of mapping the "normal" world, the neurons conveyed an altered picture of the world around, one that rapidly changed.

The electrical signaling in the brain continued to change for another 12 hours after noise exposure. Electrical monitoring of the rats' brain cells indicated plasticity started in the hippocampus long before outward signs of tinnitus, such as ringing in the ears, showed up.

Tinnitus is defined as the perception of sound in the ears or head where no external source exists. It affects about 50 million people in the United States and 250 million worldwide. It is especially common among combat veterans and is a major disability faced by the current generation of American soldiers.

Major remapping of neurons could help explain why people exposed to extremely loud or continual noise experience lingering hearing disruption, Thompson said. Researchers previously concentrated only on the auditory portions of the brain involved in tinnitus.

Thompson, whose research focuses primarily on aging and memory, began looking at the role of the hippocampus in tinnitus while investigating how brain cells change as they learn.

"It turns out that tinnitus is, in effect, a learning event," he said.