Effect of electroacupuncture at "Neiguan" (PC6) on pain and brain orexin 1 receptor in mice with inflammatory pain.

Objectives: To observe the effect of electroacupuncture (EA) at "Neiguan" (PC6) on pain response in mice injected with complete Freund's adjuvant (CFA) in the hind paw, so as to investigate the mechanism of orexin 1 receptor (OX1R) -endogenous cannabinoid 1 receptor (CB1R) pathway in acupuncture analgesia.

Methods: A total of 48 male C57BL/6 mice were used in the present study. In the first part of this study, 18 mice were randomized into control, model and EA groups, with 6 mice in each group. In the second part of this study, 30 mice were randomized into control, model, EA, EA+Naloxone, EA+OX1R antagonist (SB33486) groups, with 6 mice in each group. Inflammatory pain model was established by subcutaneous injection of 20 μL CFA solution in the left hind paw. EA (2 Hz, 2 mA ) was applied to bilateral PC6 for 20 min, once a day for 5 consecutive days. The mice in the EA+Naloxone and EA+SB33486 groups were intraperitoneally injected with naloxone (10 mg/kg) or SB33486 (15 mg/kg) 15 min before EA intervention on day 5, respectively. Tail-flick method and Von Frey method were used to detect the thermal pain threshold and mechanical pain threshold of mice. Quantitative real-time PCR was used to detect the expression level of β-endorphin mRNA in periaqueductal gray (PAG) of mice. The expression of OX1R positive cells in the lateral hypothalamic area (LH) and CB1R positive cells in the ventrolateral periaqueductal gray (vlPAG) were detected by immunofluorescence.

Results: Compared with the control group, the thermal pain threshold and mechanical pain threshold of the model group were decreased (P<0.001), the expression level of β-endorphin mRNA in PAG was decreased (P<0.001), and the numbers of OX1R positive cells in LH and CB1R positive cells in vlPAG were decreased (P<0.05, P<0.001). Compared with the model group, the thermal pain threshold and mechanical pain threshold of the EA group were significantly increased (P<0.001), and the numbers of OX1R positive cells in LH and CB1R positive cells in vlPAG were increased (P<0.01, P<0.001). Compared with the EA group, the mechanical pain threshold in the EA+SB33486 group was significantly decreased (P<0.01), but there was no significant difference in the mechanical pain threshold between the EA+Naloxone group and EA group, and the numbers of OX1R positive neurons in LH and CB1R positive neurons in vlPAG were decreased in the EA+SB33486 group (P<0.001).

Conclusions: EA at PC6 can achieve analgesic effect on CFA mice by activating the OX1R-CB1R pathway in the brain, and this effect is opioid-independent.