Acupuncture at distant myofascial trigger spots enhances endogenous opioids in rabbits: a possible mechanism for managing myofascial pain

Background and aim Acupuncture applied at myofascial trigger points (MTrPs) of distant anatomical regions, to reduce pain in a patient's area of primary complaint, is one strategy that is available to manage myofascial pain. However, the endogenous opioid-mediated analgesic mechanism of distant acupuncture associated with pain control is still unclear. This aims of this study were to evaluate the changes in enkephalin and beta-endorphin in serum, spinal cord, dorsal root ganglion (DRG) and muscle induced by acupuncture at distant myofascial trigger spots (MTrSs, similar to human MTrPs) in rabbits, to explore its underlying remote analgesic mechanism. Methods Acupuncture at MTrSs of a distant muscle (gastrocnemius) was performed either for one session or five daily sessions in rabbits. The levels of enkephalin and beta-endorphin in proximal muscle (biceps femoris), serum, DRGs and spinal cords (L5-S2) were then determined by immunoassay immediately and 5 days after treatment. Results Immediately after treatment, acupuncture comprising both one dose and five doses significantly enhanced spinal enkephalin expression and serum beta-endorphin levels (p<0.05). However, only five-dose acupuncture significantly enhanced the beta-endorphin levels in the biceps femoris and DRGs (p<0.05), while 1-dose acupuncture did not (p>0.05). Furthermore, 5 days after treatment, significantly increased levels of spinal enkephalin and serum beta-endorphin persisted in animals that received 5-dose acupuncture (p<0.05). Conclusions This study demonstrates that interactions within the endogenous opioid system may be involved in the remote effects of acupuncture treatment and could be a potential analgesic mechanism underlying MTrP pain management.