What a silly question! At least this is what most sceptics would say: if we are not sure that it works, we do not need to spend any thoughts on a potential mechanism!
However, in the realm of acupuncture, the potential mode of action remains a hotly debated and fundamentally relevant issue.
The TCM folks, of course, ‘knew’ all along how acupuncture works: it re-balances the life-forces yin and yang. This is a nice theory – it has but one disadvantage: it has no bearing whatsoever on reality. Vitalistic ideas such as this one have long been proven to be nothing but fantasy.
Meanwhile, several more plausible hypotheses have been developed, and hundreds of papers have been published on the subject. One recent article, for instance, suggests a range of mechanisms including microinjury, increased local blood flow, facilitated healing, and analgesia. Acupuncture may trigger a somatic autonomic reflex, thereby affecting the gastric and cardiovascular functions. Acupuncture may also change the levels of neurotransmitters such as serotonin and dopamine, thereby affecting the emotional state and craving… By affecting other pain-modulating neurotransmitters such as met-enkephalin and substance P along the nociceptive pathway, acupuncture may relieve headache. Acupuncture may affect the hypothalamus pituitary axis and reduce the release of the luteinizing hormone…
Another article states that the Western explanation for acupuncture effectiveness is based upon more than half a century of basic and clinical research, which identified the activation of sensory system and the subsequent activity-dependent regulation of neurotransmitters, neurohormones, and several classes of neuromodulators as plausible mechanism for the acupuncture‘s therapeutic properties. The regulation of neurotrophins’ expression and activity is one of the possible neurophysiological mechanisms underlying acupuncture‘s effects on neuropathic pain, nerve injury, neurodegeneration, and even in the regulation of gonadal functions…
Recently Burnstock proposed that mechanical deformation of the skin by needles and application of heat or electrical current leads to release of large amounts of ATP from keratinocytes, fibroblasts and other cells in skin; the ATP then occupies specific receptor subtypes expressed on sensory nerve endings in the skin and tongue; the sensory nerves send impulses through ganglia to the spinal cord, the brain stem, hypothalamus and higher centres; the brain stem and hypothalamus contain neurons that control autonomic functions, including cardiovascular, gastrointestinal, respiratory, urinogenital and musculo-skeletal activity. Impulses generated in sensory fibres in the skin connect with interneurons to modulate (either inhibition or facilitation) the activities of the motoneurons in the brain stem and hypothalamus to change autonomic functions; specifically activated sensory nerves, via interneurons, also inhibit the neural pathways to the pain centres in the cortex.
A brand-new article in the journal SCIENTIFIC AMERICAN puts the hypothesis in perspective:
…scientists have been studying a roster of potential biological pathways by which needling might relieve pain. The most successful of these efforts has centered on adenosine, a chemical believed to ease pain by reducing inflammation. A 2010 mouse study found that acupuncture needles triggered a release of adenosine from the surrounding cells into the extracellular fluid that diminished the amount of pain the rodents experienced. The mice had been injected with a chemical that made them especially sensitive to heat and touch. The researchers reported a 24-fold increase in adenosine concentration in the blood of the animals after acupuncture, which corresponded to a two-thirds reduction in discomfort, as revealed by how quickly they recoiled from heat and touch. Injecting the mice with compounds similar to adenosine had the same effect as acupuncture needling. And injecting compounds that slowed the removal of adenosine from the body boosted the effects of acupuncture by making more adenosine available to the surrounding tissue for longer periods. Two years later a different group of researchers went on to show that an injection of PAP, an enzyme that breaks other compounds in the body down into adenosine, could relieve pain for an extended chunk of time by increasing the amount of adenosine in the surrounding tissue. They dubbed that experimental procedure “PAPupuncture.”
Both sets of findings have excited researchers—and for good reason. The current options for treating pain are limited and rely mostly on manipulating the body’s natural pain-management system, known as the opioid system. Opioid-based painkillers are problematic for several reasons. Not only does their efficacy tend to wane over time, but they have been linked to an epidemic of addiction and overdose deaths across the U.S.—so much so that the Centers for Disease Control and Prevention has recently advised doctors to seriously restrict their use. The available nonopioid pain treatments are few; many of them require multiple injections or catheterization to work; and they often come with side effects, such as impaired movement. Adenosine offers an entirely new mechanism to exploit for potential treatments—one that may come with fewer side effects and less potential for addiction. What is more, adenosine can be made to circulate in the body for prolonged stretches. Pharmaceutical companies are actively investigating adenosine-related compounds as potential drugs.
But however promising adenosine may be as a treatment, the findings from this research do not prove that acupuncture itself “works.” For one thing, the researchers did not show that the release of adenosine was specific to acupuncture. Acupuncture needles might cause adenosine to flood the surrounding tissue, but so might a hard pinch, or applied pressure, or any number of other physical insults. In fact, both of the studies found that when adenosine was turned on in mouse tissue by other mechanisms, the pain response was equal to or better than the response generated by acupuncture. For another thing, the study results offered no support for the use of acupuncture to treat any of the other conditions for which the procedure is often advertised. A localized adenosine response may mitigate localized pain. That does not mean it can also cure insomnia or infertility.
It may well be that the reams of research scientists have done on acupuncture have lit the path toward improved understanding of—and eventually better treatments for—intractable pain. But it may also be time to take whatever bread crumbs have been laid out by that work and move on.
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As we see, there is no shortage of potential explanations as to HOW acupuncture works. The most plausible theory still is that it works largely or even exclusively via a placebo effect.
Due to this type of mechanistic research, acupuncture has gained much credibility. The question is, does it deserve it? In my view, it would be much more fruitful to first make sure THAT acupuncture works (beyond a placebo response) and, if so, for what conditions. The question HOW it works is unquestionably interesting but in the final analysis it probably is secondary.