Jun 02 2010

Potential New Mechanism of Pain Relief Discovered

The development of drugs and other treatments for specific symptoms or conditions relies heavily on either serendipity (the chance finding of a beneficial effect) or on an understanding of underlying mechanisms. In pain, for example, there are limited ways in which we can block pain signals – such as activating opiate receptors, or inhibiting prostaglandins. There are only so many ways in which you can interact with these systems. The discovery of a novel mechanism of modulating pain is therefore most welcome, and has the potential of leading to entirely new treatments that may have a better side effect profile than existing treatments and also have an additive clinical effect.

A recent study by Nana Goldman et. al., published in Nature Neuroscience, adds to our understanding of pain relief by identifying the role of adenosine in reducing pain activity in the peripheral nervous system. The researchers, in a nice series of experiments, demonstrated that producing a local painful stimulus in mice causes the local release of ATP (adenosine triphosphate) that peaks at about 30 minutes. This correlates with a decreased pain response in the mice. Further, if drugs are given that prolong the effect of adenosine, the analgesic effect itself is prolonged.

Also, if drugs are given that activate the adenosine A1 receptor, the observed analgesic effect is replicated. When these experiments are replicated in knockout mice that do not have the gene for the adenosine A1 receptor, there is no observed analgesic effect.

Together these experiments are fairly solid evidence that local pain results in the local release of adenosine that in turn binds to the adenosine A1 receptor inhibiting the pain response. This is potentially very exiting – it should lead to further investigation of the adenosine A1 receptor and the effects of activating and inhibiting it. This may lead to the development of drugs or other interventions that activate these receptors and may ultimately be a very useful addition to our ability to treat acute and chronic pain.

Now Comes the Spin

In a rational science-based world the above would be the end of this blog entry. But that is not the world we currently live in. In this study the chosen method of provoking pain was the insertion of an acupuncture needle into the “Zusanli point” of the lower extremity and rotating it. This allowed the authors of this study to spin the results as if they were validation for acupuncture itself – and the mainstream press dutifully followed suit.

The introduction to this study is an apology for acupuncture itself, and sets the tone for the rest of the paper as well as the press release and comments to follow:

Acupuncture is a procedure in which fine needles are inserted into an individual at discrete points and then manipulated, with the intent of relieving pain. Since its development in China around 2,000 B.C., acupuncture has become worldwide in its practice. Although Western medicine has treated acupuncture with considerable skepticism, a broader worldwide population has granted it acceptance. For instance, the World Health Organization endorses acupuncture for at least two dozen conditions and the US National Institutes of Health issued a consensus statement proposing acupuncture as a therapeutic intervention for complementary medicine. Perhaps most tellingly, the U.S. Internal Revenue Service approved acupuncture as a deductible medical expense in 1973.

That this paragraphs appears in a high impact peer-reviewed journal is very curious (to put it mildly). It is full of misinformation and logical fallacies. Acupuncture is used for more than pain relief, and so making statements about the efficacy of “acupuncture” go way beyond the scope of this study, which only involves pain. The authors should have specifically pointed out that this study cannot be used to explain any indication for acupuncture that does not involve local pain relief. Acupuncture as it was practiced in China 4000 years ago bears little resemblance to what is practiced today, and may have been more of a form a bloodletting.

The authors then make a false dichotomy between “Western medicine” and, presumably, “Eastern medicine” – which is an anti-scientific and culturally bigoted point of view. This is followed by an argument from popularity, and a particularly bizarre argument from authority – noting that the IRS accepts acupuncture as a medical expense. I was unaware that the IRS is a scientific medical organization.

The authors also fail to put this study into its proper context by reviewing existing evidence – the more traditional use of the introduction to peer-reviewed research. The clinical research on acupuncture, which is quite extensive, finds that any measured symptomatic effect is almost certainly due mostly or entirely to placebo effects. Further, it does not matter where you stick the acupuncture needle, nor even if you do stick the need through the skin. Twisting toothpicks against the skin in random locations has the same effect.

My interpretation of the research is that acupuncture (placing needles at specific acupuncture points to manipulate chi) does not work. There may, however, be non-specific physiological responses to the mechanical stimulation of sticking needles at random locations, or just poking toothpicks. This study, if anything, supports this interpretation – it, if fact, has identified a local mechanism of analgesia that can help explain a non-specific response to acupuncture, sham acupuncture, or placebo acupuncture and therefore is consistent with the clinical evidence showing no difference among these interventions.

This is also not the first study to identify potential local mechanisms of pain relief from mechanical stimulation. A 2002 study correlated insertional activity (depolarization of muscle fibers in response to needle insertion) or electrical stimulation with pain relief. This would explain why, in this study, constant twisting of the needle was needed to provoke pain relief – perhaps insertional muscle activity is needed to release adenosine, or increases its release.

By focusing on what is really going on here we can best understand how to develop methods to optimally capitalize on these local mechanisms for pain relief. It must be noted, however, that needle insertion gives very unimpressive results in clinical trials. It may be that the effect is too temporary to be worthwhile, at least as mechanically provoked – pharmacologically activating the adenosine A1 receptor may be a better strategy. Also, it is very difficult to extrapolate from mouse data as they are much smaller than humans, and therefore their nerves and motor end-plates (the location where the nerves innervate the muscles) are much closer together and superficial – closer to the skin.

Mechanisms are interesting, but net clinical outcomes in humans are the only kind of scientific data that really tells us if a modality works or not.


Finally, it has to be emphasized that this study says nothing about acupuncture itself, except for providing a possible mechanism for a non-specific local response. The term “acupuncture”, in fact, is becoming increasingly problematic and is confusing the scientific literature, not to mention the public. What is acupuncture? If we use the term broadly enough to mean any use of needles, with or without electrical stimulation, at any points, with or without skin penetration, etc. then the term is too broad to be useful. If we use the term narrowly – to mean sticking needles to a certain depth in specific acupuncture points that work through a novel mechanism specific to those locations, then we can say, based upon extensive research, that “acupuncture” does not work and its proposed underlying mechanisms are nothing more than pre-scientific superstition.

This study is an excellent example of the mischief caused by confusing the non-specific use of the term “acupuncture” with its more traditional use. Research involving acupuncture in its vaguest sense is used to promote “acupuncture” in the traditional sense. This is highly deceptive and scientifically sloppy.

The researchers of this current study could have used other controls to see if the effect they discovered is in any way specific to any acupuncture variables. For example – they could have used a non-acupuncture point as a control, or other forms of mechanical pain production that do not involve needles. I suspect any local pain production or mechanical trauma beyond a certain threshold would result in the same adenosine response – which certainly seems like a non-specific mechanism to modulate pain.

Because they did not do this they did not actually research “acupuncture”. The description of this research in the published paper and in the press should have been as I discussed in the opening of this post. Instead genuinely interesting research that may lead to novel pain treatments is being diverted as propaganda for an ancient superstition.


Note: This article is cross-posted over at Science-Based Medicine

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