Mast Cell-Associated Serotonin in Acupoint Contributes to Acupuncture Analgesia in Arthritis Rats by Mediating ATP Release

Background: The activation of subcutaneous mast cells (MCs) helps to trigger the analgesic effect induced by acupuncture (AP), a traditional oriental therapy, that has been gradually accepted worldwide. This work aimed to reveal whether the serotonin (5-hydroxytryptamine, 5-HT) released from MCs plays an important role in this process, which has a controversial effect in the mechanism of pain. Methods: In vivo tests, a 20-min session of AP was applied at Zusanli acupuncture point (acupoint) of acute ankle arthritis rats. Pain thresholds of the injured hindpaw were assessed to reflect the pain state, and the targeting substances in the interstitial space of the treated acupoint were sampled by microdialysis. In vitro experiments, exogenous 5-HT (exo-5-HT) was introduced to mediate adenosine triphosphate (ATP) release from cultured MCs. Results: Needling promoted 5-HT accumulation at the Zusanli acupoint, which was prevented by sodium cromolyn. AP's analgesic effect was suppressed by the inhibition of 5-HT receptors at the acupoint, especially 5-HT1A subtype. In vitro tests, mechanical perturbation mimicking needling stimulation induced MCs to release 5-HT. 1 mu M and 10 mu M of exo-5-HT facilitated ATP release, which was restrained by blocking of 5-HT1 receptors rather than 5-HT3 receptors. As 5-HT, ATP and adenosine were also transiently accumulated in the treated acupoint during needling. Promoting ATP hydrolysis or activation adenosine A1 receptors duplicated AP analgesic effect. Finally, the inhibition of ATP receptors by suramin or pyridoxal phosphate-6-azo tetrasodium salt hydrate (PPADS) prevented AP analgesic effect. Conclusions: Our results suggest that MC-associated 5-HT release at acupoints contributes to AP analgesia, and the mediation of ATP secretion through 5-HT1A receptors might be the underlying mechanism at play. ATP could facilitate adenosine production or the propagation of needling signals.