To observe the law of changes in mechanical paw-withdrawal threshold (MPWT) and thermal paw-withdrawal latency (TPWL) on the modeled side of rats with pain transition (hyperalgesic priming [HP]), and the expression of prostaglandin E receptor 1 (EP1) and transient receptor potential vanilloid type 1 (TRPV1) in the dorsal root ganglion (DRG) of the affected side of rats. To observe the effects of electroacupuncture (EA) on the TPWL of on the modeled side of rats with pain transition HP and regulation of EP1 and TRPV1 expression in DRG.

Part 1: Healthy male SD rats were selected and randomly divided into control, sham HP, and HP groups with randome nuber table method. The modeling comprised two injections. The rats in the HP group were subcutaneously injected 1% carrageenan 100 μL into the left hind paw in the first injection (those in the control and sham HP groups were injected with saline).The second injection was administered 8 days later by injecting prostaglandin E2 (PGE2) 100 ng/25 μL into the dorsum of the paw (the rats in the control group were administered saline and those in the sham HP group PGE2), thereby developing a pain transition model. TPWL and MPWT were measured before rat modeling, 4 h, and 1, 2, 3, and 7 days after the first injection, and 1, 4, 24, and 48 h after the second injection (8 days after the first injection). The expression rates of EP1- and TRPV1-positive cells in the affected DRG were measured by immunofluorescence. Part 2: Healthy male SD rats were selected and randomly divided into sham HP, HP, and EP1 antagonist groups with randome nuber table method for the detection of TPWL. Rats in the EP1 antagonist group were injected with EP1 antagonist 5 min before PGE2 injection. The expression rate of TRPV1-positive cells in the affected DRG 48 h after PGE2 injection was detected using the immunofluorescence(IF) method.Part 3: Healthy male SD rats were selected and randomly divided into a sham HP, HP, sham EA, and EA groups for the detection of TPWL with randome nuber table method. Following the injection of carrageenan, rats in the EA group were intervened at “Zúsānlĭ (足三里 ST36)” and “Kūnlún(昆仑 BL60)” acupoints bilaterally, once a day. The expression rate of EP1- and TRPV1-positive cells in the affected DRG was detected by the immunofluorescence(IF) method.

Part 1: 1. Compared with the control and sham HP groups, the MPWT of the affected paw of the rats in the HP group was significantly reduced at 4, 24, 48, and 72 h after carrageenan injection, and at 4, 24, and 48 h after PGE2 injection (all P < 0.01). 2. Compared with the control group, the TPWL of the affected paw was significantly decreased in the HP group at 4, 24, 48, and 72 h after carrageenan injection, and at 4, 24, and 48 h after PGE2 injection (all P < 0.01); compared with the MPWT of the HP rats, the TPWL of the HP rats increased and recovered 4 h after PGE2 injection, while there was no such trend in the MPWT. 3. The expression of EP1- and TRPV1-positive cells in the affected DRG of rats in the HP group was increased compared with that of the control and sham HP rats (all P < 0.01). Part 2: 1. TPWL was significantly increased in the EP1 antagonist group compared with the HP group at 1, 4, 24, and 48 h after PGE2 injection (1 h P < 0.01, 4 h P < 0.05, 24 h P < 0.05, and 48 h P < 0.05). 2. Compared with the HP group, the expression rate of TRPV1-positive cells in the affected DRG of the rats in the EP1 antagonist group was significantly lower 48 h after PGE2 injection (P < 0.01), while there was no statistical difference compared with the sham HP group.Part 3: 1. EA significantly elevated the TPWL on the affected side at 24, 48, and 72 h after the first injection, and at 4, 24, and 48 h after the second injection, compared with the HP group (all P < 0.01), whereas there was no such change in the sham EA group. 2. The expression rates of EP1- and TRPV1-positive cells in the affected DRG were increased in the HP group compared with the sham HP group (both P < 0.01). Compared with the HP group, the expression rates of EP1- and TRPV1-positive cells in the affected DRG were decreased in the EA group (both P < 0.01); however, there was no significant change in the sham EA group.

1. HP induces decreased MPWT and TPWL and prolonged hyperalgesia in rats, resulting in the transition from acute to chronic pain; however, the changing pattern of TPWL differs from that of MPWT; 2. EP1-TRPV1 signaling pathway in DRG is involved in HP and promotes the transition from acute to chronic pain; 3. The analgesic effect of EA on HP thermal pain in rats may relate to the inhibition of the expression of the EP1-TRPV1 signaling pathway; thus, inhibiting the transition from acute to chronic pain.