Mechanical Acupuncture at ST36 Attenuates Inflammatory Pain Involving TRPV1 Signaling in Mice

We recently developed a mechanical acupuncture instrument (MAI) that applies mechanical stimulation to acupuncture points in effectively treating hypertension and addiction in animal models. However, its analgesic effect on inflammatory pain remains unclear. Here, we aimed to determine the optimal duration of MAI treatment at any given acupuncture point to improve analgesic effects. Adult male ICR mice (20-25 g, 6 weeks old, n = 6 per group) were used to evaluate whether MAI administration or TRPV1 (transient receptor potential vanilloid 1) inhibition had analgesic effects. Then, we investigated whether it affected TRPV1 expression and glial cells in the spinal cord of mice. The capsaicin test was used to identify the most effective acupoints and optimal treatment times for MAI. Additionally, we induced inflammatory pain in mice by administering a 2% carrageenan via intraplantar injection. To assess the analgesic effects of MAI treatment and TRPV1 inhibition, we evaluated pain-related behavior using von Frey filaments and a thermal stimulator applied to the hind paw. MAI treatment significantly suppressed pain-related behaviors. In particular, paw-licking duration was markedly reduced in the group treated with MAI for 60 s at ST36 compared to the capsaicin-treated group (p < 0.05), suggesting a robust analgesic effect. Additionally, MAI and capsazepine administration significantly attenuated carrageenan-induced mechanical allodynia and thermal hyperalgesia compared to the carrageenan-only group (p < 0.05 to p < 0.001). Additionally, MAI treatment and capsazepine administration effectively suppressed the carrageenan-induced upregulation of TRPV1 and glial cells in the spinal cord. In conclusion, our findings show that MAI administration at ST36 significantly alleviated inflammatory pain and was associated with downregulation of TRPV1 expression and microglial activation in the spinal cord. The present findings suggest that TRPV1 signaling is involved in the analgesic effects of mechanical acupuncture; however, a direct causal relationship has yet to be established.