Although acupuncture has been practiced for over 4,000 years, it has been difficult to establish its biological basis. Our findings indicate that adenosine is central to the mechanistic actions of acupuncture. We found that insertion and manual rotation of acupuncture needles triggered a general increase in the extracellular concentration of purines, including the transmitter adenosine (), which is consistent with the observation that tissue damage is associated with an increase in extracellular nucleotides and adenosine36
. Because the anti-nociceptive effects of peripheral, spinal and supraspinal adenosine A1 receptors are well established37,38
, we asked whether peripheral injection of an A1 receptor agonist suppressed hyper-algesia25,37
(). We found that the A1 receptor agonist CCPA sharply reduced inflammatory and neurogenic pain and that suppression of pain mediated by acupuncture required adenosine A1 receptor expression (). These findings suggest that A1 receptor activation is both necessary and sufficient for the clinical benefits of acupunctures. To the best of our knowledge, adenosine A1 receptors have not previously been implicated in the anti-nociceptive actions of acupuncture.
One may speculate that other non-allopathic treatments of chronic pain, such as chiropractic manipulations and massage, modalities that involve the mechanical manipulation of joints and muscles, might also be associated with an efflux of cytosolic ATP that is sufficient to elevate extracellular adenosine. As in acupuncture, adenosine may accumulate during these treatments and dampen pain in part by the activation of A1 receptors on sensory afferents of ascending nerve tracks. Notably, needle penetration has been reported to not confer an analgesic advantage over nonpenetrating (placebo) needle application39
, as opposed to our observations (Supplementary Figs. 2 and 3
) and those of others40,41
. However, it is possible that ATP release from keratinocytes in response to mechanical stimulation of the skin results in an accumulation of adenosine that transiently reduces pain, as A1 receptors are probably expressed by nociceptive axon terminal in epidermis37
. In fact, vibratory stimulation applied to the skin depressed the activity of nociceptive neurons in the lower lumbar segments of cats by release of adenosine42
. However, this effect differs from the anti-nociceptive effect of acupuncture, which does not depend on the afferent innervation of the skin4
. Acupuncture is typically applied to deep tissue, including muscle and connective tissue, and acupoints may better overlap with their proximity to ascending nerve tracks than to the density of cutaneous afferents.
Most patients have reported that acupuncture in itself is not a painful procedure, except for a pinching sensation in tissue below the acupuncture needle. Because ATP is released during acupuncture (), the pinching sensation may be mediated by nociceptive P2X3 receptors, which are expressed by small-diameter, primary afferent neurons, some of which are sensitive to capsaicin10
. The most likely explanation for the lack of direct pain during acupuncture is that extracellular ATP does not reach high enough concentrations to activate P2X3 and other nociceptive P2X receptors because of its rapid degradation (). However, activation of P2X receptors may nonetheless contribute to the anti-analgesic effects of acupuncture, as was recently suggested27
, perhaps by subthreshold-activating P2X receptors or by more complex mechanisms involving dimerization of P2X and A1 receptors43
. In addition to the use of acupuncture for treatment of chronic pain, acupuncture is also frequently employed in diseases with a local inflammatory component, such as arthritis and tendinitis44
. Adenosine has anti-inflammatory properties and we found that acupuncture increased extracellular adenosine36
Quantification of extracellular purines in microdialysis samples collected nearby the acupuncture point revealed that the extracellular adenosine concentration rose following the release of ATP, which was dephosphorylated to ADP, AMP and adenosine by potent ectonucleotidases, and that AMP dephosphorylation represents the rate-limiting step in this reaction (Supplementary Fig. 5
). As with most other transmitters, adenosine has a short lifespan in the extracellular space as a result of facilitated uptake by nucleoside transporters and concurrent degradation to inosine33
. After reuptake, adenosine is quickly converted to AMP by cytosolic adenosine kinase (Km
~20 nM), thereby facilitating the rapid clearance of adenosine in the extracellular space36
and shortening the anti-nociceptive effects of acupuncture. Moreover, our analysis confirmed the prior observation that AMP deaminase activity is high in muscle/subcutis33
and that only a fraction of AMP is dephosphorylated to adenosine. AMP deaminases constitute the primary enzymatic pathway for elimination of extracellular AMP and this pathway bypasses adenosine. Thus, acupuncture combined with pharmacological suppression of AMP deaminase activity should theoretically increase the availability of adenosine and thereby enhance the clinical benefits of acupuncture. As a proof of principle, we found that mice treated with a Food and Drug Administration approved deaminase inhibitor, deoxycoformycin, exhibited more potent increases in adenosine and benefitted from a longer-lasting suppression of chronic pain following acupuncture. In summary, we found that the anti-nociceptive action of acupuncture is mediated by activation of A1 receptors located on ascending nerves. Thus, medications that interfere with A1 receptors or adenosine metabolism may improve the clinical benefit of acupuncture.