Acupuncture works by inducing body's own painkiller

By Yun Xie, Ars Technica

The art of sticking and manipulating fine needles in specific body parts to relieve pain and fix other ailments has been around for thousands of years. More recently, acupuncture has spread out of China and has been gaining popularity worldwide. While many practitioners swear by acupuncture’s therapeutic powers, there are few scientific studies of how it works, and one of those suggested that any needle stick would do. This has led many people to suspect that the whole process induces little more than a placebo effect.

An article in a recent issue of Nature Neuroscience indicates that at least one of acupuncture’s reported benefits may finally have concrete support and a proposed mechanism of action thanks to laboratory experiments. Researchers from the University of Rochester Medical Center, Boston University School of Medicine, and the National Institute of Health report that a neuromodulator (a chemical agent secreted by neurons) called adenosine is the key to why acupuncture lessens pain associated with inflammation and chronic neuropathic problems.

Adenosine is a natural pain killer that the body produces, so the first thing the researchers wanted to do was figure out if acupuncture increased the concentration of adenosine in the tissues surrounding the needle. They inserted needles into mice right below the knee and rotated the needles every five minutes over half an hour. Mice receiving acupuncture had about 24 times more adenosine in their tissue fluids compared to their baseline concentrations, and the levels remained high even an hour after treatment.

The next question was whether the increased amount of adenosine affected a mouse's response to pain. The researchers performed acupuncture on two groups of mice: one suffering from inflammatory pain and one experiencing neuropathic pain. They found that a half hour acupuncture session alleviated symptoms in both populations. Treated mice with inflammatory pain showed a higher thermal pain threshold (10.6 seconds before withdrawal from a heat source compared to 3.9 seconds without acupuncture) and over a 3-fold improvement to touch sensitivity. Similarly, animals with neuropathic pain became less sensitive to thermal pain (11.4 seconds compared to 3.1 seconds without treatment) and saw over a twofold recovery in touch sensitivity.

Notably, the acupuncture needles must be periodically rotated during the treatment for the positive effects to take place. If the needles were just left in the mice for 30 minutes, there would be no pain alleviation. The effects are directly related to adenosine’s binding to a receptor (the adenosine A1 receptor) on the cell surface. Mice lacking the receptors didn’t benefit from acupuncture.

Adenosine’s interaction with its receptor changes how the nervous system processes painful stimulation; it can interfere with pain signals that go to the anterior cingulate cortex (a part of your brain that regulates a myriad of functions, like emotion and heart rate). The researchers found that acupuncture reduced the field excitatory postsynaptic potentials in the anterior cingulate cortex by almost 50 percent after one hour of treatment. This is slow relative to the action of a synthetic, adenosine-A1-receptor-binding drug that is directly injected in the source of pain. When the scientists injected a strong dose of CCPA (2-chloro-N(6)-cyclopentyladenosine) into the knee of mice, the drug significantly impacted the activity in the anterior cingulate cortex in as little as six minutes.

The comparatively slow action of acupuncture on the brain implies that adenosine gradually accumulates in the tissue surrounding the needles. The researchers also suspect that there could be additional factors that contribute to the pain-relieving properties, such as inducing the release of peptides that have effects similar to opiates.

Since adenosine accumulation during acupuncture is a major factor in pain reduction, the researchers proposed that it might be possible to extend the relief if they inhibited the natural enzymes that the body employs to degrade the substance. They found that deoxycoformycin, a drug approved by the FDA for leukemia treatment, inhibited the enzymes and subsequently increased the concentration of adenosine by nearly threefold. On average, mice that received 30 minutes of acupuncture experienced reduced pain for 1.0-1.5 hours. Deoxycoformycin extended the period of pain relief to 3.0-3.5 hours, leading the scientists to suggest that enzyme suppression by drugs can “be used as an adjuvant to acupuncture.”

While it is certainly good to know that acupuncture’s positive effects can be extended, it may be counterproductive to do so with medication—many people choose acupuncture as an alternative to pharmaceuticals. In any case, the authors seem to have cleared up what's going on after the needle stick of acupuncture occurs and have tied it to a known neural signaling pathway.