| | | Soft sounds numb pain. Researchers may now know why
Experiments in mice show how sound tamps down on pain processing in the brain
7 JUL 20222:00 PM
BY TESS JOOSSE
Science.org
In 1960, a group of dentists published a curious study: when they played music for their patients during operations, the people experienced less pain. Some didn’t even need nitrous oxide or local anesthesia to get through unpleasant procedures.
Now a new paper untangles why this works—at least in mice. It’s an “elegant” study, says Eduardo Garza-Villarreal, a neurobiologist at the National Autonomous University of Mexico, Juriquilla, who wasn’t involved with the research. The findings could give scientists new ways to treat pain in humans, he says.
In the decades since the 1960 study, researchers and medical providers have tested the numbing effect of sound with everything from Mozart to Michael Bolton. Both seem to work: in one study, patients with fibromyalgia had fewer aches when listening to their favorite music, Mozart and Bolton included.
To get a better sense of why music helps with pain, Yuanyuan Liu, a neurobiologist at the U.S. National Institute of Dental and Craniofacial Research, and his colleagues turned to mice. For 20 minutes a day, they played pleasant-sounding (at least to human ears) symphonic music— Bach’s Réjouissance—to the rodents at 50 or 60 decibels in a room where the background noise clocked in at 45 decibels.
During these sessions, the scientists injected the mice’s paws with a painful solution. Then, they prodded the paws with thin filaments at differing pressure levels to see how the rodents responded. If they flinched, licked, or pulled their paws back, the researchers took that as a clue the mice were feeling pain.
Only noise at the lower volume, 50 decibels, seemed to numb the animals—a real surprise, Liu says. When the researchers poked their inflamed paws, the mice didn’t flinch. With louder noise, the animals were much more sensitive to the stimulus. It took only one-third as much pressure on the their paws to make them respond, the same as with no music. “It turns out that this intensity is the key,” Liu says.
The team also tested dissonant music (Réjouissance pitch-shifted to sound unpleasant) and white noise. All numbed pain, as long as they were played at levels only slightly above background noise, the researchers report today in Science.
The scientists repeated the experiments while tracking a red fluorescent dye injected in the mice’s auditory cortex, the brain region that processes sounds. They found lots of fluorescence in certain dense regions of the thalamus, the hub of sensory processing, suggesting connections between this region and the auditory cortex are involved in the pain suppression. Tiny electrodes implanted into the animals’ brains further revealed that relatively soft sounds decreased activity coming out of the auditory cortex. And when the team artificially blocked the connection between the auditory cortex and the thalamus by targeting pulses of light at these specific neurons, the mice appeared to feel less pain.
Overall, low sounds seem to blunt neurological signals between the auditory cortex and thalamus, tamping down on pain processing in the thalamus, the team concluded. The analgesic effects lasted up to 2 days after the mice stopped hearing the sound. The researchers next want to figure out why a low sound over background noise is the “sweet spot,” says author Zhi Zhang, a neurobiologist at the University of Science and Technology of China.
The ultimate goal, however, is to manage pain in humans—and there are many differences between mice and people, notes Clifford Woolf, a neurobiologist at Harvard University who was not involved with the study. Though scientists can’t probe the auditory cortex-thalamus connections in the human brain with invasive methods, they can play similar low sounds for people and monitor their thalamus activity with MRI scans. “That now needs to be tested in humans,” Woolf says. “Many would have anticipated you need to listen to Mozart to get pain relief,” he says. “But maybe all we need to do is give patients a tiny level of buzzing noise.”
Beyond making dentist visits more bearable, the findings could provide researchers with a cheap and easy way to shield rodents from pain during experiments without confounding the results, Zhang says. “Pain relief is part of the basic welfare of animals” in research, he says. Playing these sounds could have “a striking effect.”
Soft sounds numb pain. Researchers may now know why | Science | AAAS |
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