A molecule which may control how acupuncture relieves pain has been pinpointed by US researchers.
Experiments in mice showed that levels of adenosine - a natural painkiller - increased in tissues near acupuncture sites.
The Nature Neuroscience study also found that in mice resistant to the effects of adenosine, acupuncture had no effect.
Pain experts said the findings may partly explain how the treatment works.
Adenosine is known to have many roles in the body including regulating sleep and reducing inflammation, the researchers said.
Other research has shown that it becomes active in the skin after an injury to act as a local painkiller.
In the latest study, the researchers were looking at the effects of the molecule in the deeper tissues which acupuncturists target with fine needles.
The team performed a 30-minute acupuncture session at a pressure point in the knee of mice that had discomfort in one paw.
They found that in mice with normal functioning levels of adenosine, acupuncture reduced soreness by two-thirds, as assessed by nerve sensitivity measurements.
In mice specially engineered to lack the receptor for adenosine, acupuncture had no effect.
And during and immediately after an acupuncture treatment, the level of adenosine in the tissues near the needles was 24 times greater than before the treatment, the researchers said.
Then using a drug which extends the effects of adenosine, they found that the benefits of acupuncture lasted three times as long.
Study leader Dr Maiken Nedergaard, a neuroscientist at the University of Rochester Medical Center, said: "Acupuncture has been a mainstay of medical treatment in certain parts of the world for 4,000 years, but because it has not been understood completely, many people have remained skeptical.
"In this work, we provide information about one physical mechanism through which acupuncture reduces pain in the body," she added.
Acupuncture is used for a wide range of treatments but on the NHS its use is limited to lower back pain.
Experts pointed out that acupuncture may mediate its effects in a number of different ways.
A spokesman from the British Pain Society said: "We have known for a long time that acupuncture alters the response to pain by modulation of some of the pain pathways in the spinal cord, and also by the release of endorphins.
"It is very interesting that scientists have found an alteration in the tissue levels of adenosine, which helps to explain some of the modulatory effects of acupuncture on pain perception."
Professor Edzard Ernst, professor of complementary medicine at Peninsula Medical School agreed the study might go some way towards explaining how acupuncture reduces pain.
"We need, I would argue, independent replications with more rigorous controls before we can fully accept its findings.
"The curious thing with acupuncture is that we seem to understand better and better how it might work and, at the same time, we have more and more reason to doubt that it works."