From Scientific American.
"Scientists tried the technique on mice that had a pain in the paw, inserting and rotating the needles in the mouse version of one of the most effective acupoints in Chinese medicine. And they found that the tissues around the treated acupoint get flooded with adenosine, a chemical that provides relief by preventing pain signals from reaching the brain.
This biochemical blockade reduced the animals’ discomfort, as did treating them with drugs that boost the amount of adenosine in the tissue. The scientists say the pain relief stems from the body’s natural response to minor tissue injury. So acupuncture’s analgesic effect may have finally been pinned down."