Effects of Repeated Electroacupuncture with Different Intervals on Hypothalamic Opioid Peptide Gene Expression in Inflammatory-pain Rats

Objective: To observe the analgesic effect of repeated electroacupuncture (EA) with different intervals in a day in adjuvant arthritis (AA) rats so as to find the best time interval of EA for analgesia. Methods: A total of 96 SD rats were evenly randomized into normal, model and EA groups which were further divided into 3 h , 6 h , 12 h and 24 h subgroups, with 8 cases in each subgroup. AA model was established by subcutaneous injection of Freund’s Complete Adjuvant ( 0.1 mL ) into the right hind paw. EA (15/ 100 Hz , dense-sparse waves, 2- 4 V ) was applied to the right “Kunlun” (BL 60)-“Xuanzhong” (GB 39) for 20 min every time, with intervals of 3 h , 6 h , 12 h and 24 h everyday and continuously for 6 days. The pain grade (according to Xu’s method~ 1 ) and pain threshold (PT, the latency of radiant heat irradiation induced leg withdrawal reaction) of AA rats were determined after EA treatment. The rat’s hypothalamus tissue was sampled to cut into sections (5- 6 μm ) to be stained with in-situ hybridization histochemistry for detecting proopiomelanocortin (POMC) and proenkephalin (PENK) mRNA expression. Results: Contrasted with 3 h , 6 h , 12 h and 24 h of control group, the pain grades of both model and EA groups were apparently higher than those of normal control group, while those of EA group were lower than those of model group. Compared with control group, the PT values of model group ( 3 h , 6 h , 12 h and 24 h subgroups) and EA group ( 3 h and 6 h subgroups) were significant lower (P0.01, 0.05), while comparison between model group and EA group showed that the PT values of EA group were higher ( 3 h , 6 h and 12 h subgroups) or significantly higher ( 24 h subgroup) than those of model group (P0.01). In EA group, the PT value of 24 h subgroup was significantly higher than those of 3 h , 6 h and 12 h subgroups (P0.05, 0.01). The POMC mRNA and PENK mRNA expression-positive cells of the 4 subgroups of both model and EA groups were significantly more than those of control group (P0.01); and those of the 4 subgroups of EA group were significantly more than those of model group (P 0.01). In EA group, the POMC mRNA and PENK mRNA expression-positive cells of 24 h subgroup were significantly more than those of 3 h , 6 h and 12 h subgroups (P0.01, 0.05). Conclusion: Repeated EA has a definite analgesic effect and can obviously promote POMC mRNA and PENK mRNA expression in AA rats, and the effects of 24 h subgroup is the best.