Mechanisms of Action of Acupuncture for Chronic Pain Relief – Polymodal Receptors Are the Key Candidates

Therapeutic benefits of acupuncture for chronic pain patients have been clearly identified in recent clinical trials. Underlying mechanisms of acupuncture action mediated by endogenous opioids have been well demonstrated. The existence of pain inhibitory systems in the central nervous system has also been clarified and acupuncture seems to be a potent stimulus for activating the analgesic systems, although the pain mechanisms in acute and chronic states are essentially different. On the other hand, the exact nature of the acupuncture point still remains unclear. Here, we propose a key role of polymodal receptors (PMR) in acupuncture and moxibustion and offer a rational explanation of the acupuncture point as a sensitised PMR. Moxibustion (burning of moxa) therapy has been shown by medical historians to predate the use of acupuncture, and the meridian theory developed in association with moxibustion treatment. A variety of sensory receptors are activated by acupuncture and/or moxibustion, but there are very few that can be excited by both stimuli. PMRs are one of the most promising candidates. The functional characteristics of PMRs correspond with those of acupuncture action in the periphery; and tender or trigger points, one of the primitive features of acupuncture points, are assumed to be the sites of sensitised PMRs. Diffuse noxious inhibitory control (DNIC) is proposed as a possible mechanism of immediate action of acupuncture, and inputs for the development of DNIC seem to be the PMRs. In our experimental model, repeated eccentric contractions of muscle produced local tenderness at the palpable band and induced a typical referred pain pattern on application of pressure. Repeated indomethacin injections inhibited the production of the experimental trigger point. These lines of evidence suggest that the acupuncture points are the sites where the PMRs are sensitised and that such conditions might be repeatedly produced by various biomechanical stressors, insufficient blood supply and metabolic products.