In mice given acupuncture, a molecule that's known to reduce pain is released, and when combined with drugs, offers even more relief from pain.
The natural pain-relieving chemical might help explain how acupuncture works, at least in mice.
Understanding how acupuncture affects humans is trickier because of how complicated pain is.
Uncovering the biological secrets of acupuncture could help legitimize it and convince more insurers to pay for it.
When they get acupuncture, mice release a natural pain-relieving molecule that scientists have never linked with the treatment before.
While it's not clear yet whether the finding will apply to humans, unraveling the biological secrets of acupuncture could help the therapy become a mainstream way to tackle pain.
"I think it's important that the Western world take acupuncture seriously," said Maiken Nedergaard, a neuroscientist at the University of Rochester in New York. "Many patients have unnecessary pain. I hope this can improve pain treatment."
Acupuncture has been around for thousands of years, and the World Health Organization has endorsed it for more than 20 conditions, but Western medicine continues to be skeptical about the practice.
For managing pain, sessions last about 30 minutes and involve inserting small needles into specific acupuncture points, then periodically rotating the needles, using electrical stimulation or adding heat.
Studies have shown a clear link between acupuncture and pain relief in animals, but research on people has offered more confusing results, said Richard Harris, who studies chronic pain and acupuncture at the University of Michigan, Ann Arbor. Many large trials have found that pretend acupuncture works just as well as -- and sometimes even better than -- real acupuncture.
"The acupuncture community feels that acupuncture is doing something, but differentiating that from the effects of sham acupuncture is more difficult to do," Harris said, partly because pain is so complicated in people, and the placebo effect is powerful.
"When a patient complains of chronic pain, there's not just the intensity of painful stimulation," Harris said. "There are also issues about whether the person can still work and provide for their family, whether there is a disability or they are under litigation to receive money to compensate them for their illness. A lot of factors go into pain."
In his own studies, Harris and his group have found that sham acupuncture works by increasing levels of endorphins and other feel-good chemicals in the brain, while true acupuncture works by targeting the receptors for those chemicals, increasing the number of receptors that are activated or helping them bind more tightly.
Still other studies have shown that certain parts of the brain get turned off during acupuncture therapy, leading to analgesic effects.
In the new study, just published in the journal Nature Neuroscience, Nedergaard and colleagues looked instead at adenosine, a molecule that influences inflammation, sleep and the heart, and also acts as a natural painkiller. Using mice with injured paws, the scientists first demonstrated that adenosine was released during acupuncture.
Next, they found that acupuncture reduced the amount of pain that the mice felt by two-thirds. The researchers measured pain levels by how quickly the animals moved their legs away when touched. In mice that didn't have adenosine receptors and so couldn't benefit from any of the molecule's effects, there was no relief from pain.
Finally, the researchers gave the mice an approved leukemia drug that lengthens the amount of time that adenosine sticks around. With the drug, the animals experienced relief for three times as long: three hours instead of an hour.
If the same strategy worked in people, it might be possible to treat chronic pain more effectively with, say, a combination of acupuncture and drugs that enhance acupuncture's benefits. One hope is that acupuncture could cut into the amount of medicine people need to take, Nedergaard said. That could reduce side effects and lower the chances of addiction to narcotics.
That's a big if, Harris warned. These kinds of pain studies in rodents often don't translate to humans, and the new study didn't compare needle pricks at specific acupuncture points with pricks in other areas.
Still, he said, by pinpointing the science of how acupuncture works, studies like this give legitimacy to acupuncture and help move the field forward.
"There are definitely people out there that are suffering from chronic pain that could get better if they had access to treatment and if their insurance companies covered it," Harris said. "I really do think that acupuncture offers hope for some people."