The Synergistic Effect of Heat Therapy and Electroacupuncture Treatment in Inflammatory Pain Mouse Models

Background: Heat therapy (HT) and electroacupuncture (EA) are widely utilized pain relief methods, but the analgesic mechanisms of their combined application remain unclear. Methods: In acetic acid (AA)-induced writhing test and complete Freund's adjuvant (CFA)-induced inflammatory pain tests, mice received one of three treatments: EA at bilateral ST36, HT via a 45 °C heating pad, or the combination (EA + HT). To probe underlying pathways, separate groups were pretreated with caffeine, DPCPX (a selective adenosine A₁ receptor antagonist), or naloxone (an opioid receptor antagonist). Spinal expression of glial fibrillary acidic protein (GFAP) and phosphorylated p38 (p-p38) was examined by Western blot and immunofluorescence. Results: Both EA and HT individually reduced AA-induced writhing, with the combination (EA + HT) exhibiting the greatest analgesic effect. EA's analgesic effect was reversed by caffeine and DPCPX and partially by naloxone, while HT's effect was reversed by caffeine and DPCPX but was unaffected by naloxone. AA injection elevated spinal p-p38 and GFAP expression, which were attenuated by either EA or HT, with the most substantial suppression observed in the EA + HT group. In the CFA model, both treatments alleviated mechanical allodynia, while the combined treatment resulted in significantly greater analgesia compared to either treatment alone. Conclusions: EA combined with HT synergistically enhances analgesia in both AA and CFA pain models, accompanied by reduced spinal inflammation and astrocyte activation. EA's analgesic effects appear to involve adenosine A₁ receptor pathways and, to a lesser extent, opioid receptor mechanisms, whereas HT's effects involve adenosine A₁ receptor pathways.