[Effects of electroacupuncture on pain reactions, expression of spinal kappa-opioid receptor and contents of enkephalin and beta-endorphin in periaqueductal gray of midbrain in dysmenorrhea model rats].

Objective: To observe effects of electroacupuncture (EA) on the expression of kappa-opioid receptor in the dorsal horn of the spinal cord and contents of enkephalin(ENK) and beta-endorphin (beta-EP) in the periaqueductal gray (PAG) of midbrain in dysmenorrheal rats, so as to reveal its underlying mechanism in relieving dysmenorrhea.

Methods: A total of 80 female SD rats were randomized into saline control (control), model, Sanyinjiao (SP 6), Xuanzhong (GB 39), non-acupoint groups (16 rats/group). Dysmenorrhea model was established by subcutaneous injection of estradiol benzoate (0.5 mg/rat on the 1st day and 10th day, 0.2 mg/rat from the 2nd day to the 9th day). One hour after the last injection, oxytocin (2 U/rat) was given intraperitoneally, for rats of the control group, the same dose of saline was given (i. p.). On the 10th day, EA (2 Hz/100 Hz, 0.1-0.3 mA) was applied to "Sanyinjiao" (SP 6), "Xuanzhong" (GB 39) and non-acupoint (the mid-point between the Stomach Meridian and Gallbladder Meridian, and in parallel with GB 39) for 20 min, respectively. Latency and number of writhing response, and writhing score (according to Schmauss's and Yaksh's method) were recorded. The expression of kappa-opioid receptor (kappa-OR) in T13, L1 , L2, L6 and S1 segments of spinal cord was determined by immunohistochemistry, and the contents of ENK and beta-EP in the midbrain PAG were assayed by ELISA.

Results: (1) Compared with the saline control group, the writhing latency of the model group was significantly shortened (P < 0.01), while the writhing times and writhing score of the model group were increased significantly (P < 0.01). Compared with the model group, the writhing latency of SP 6 group was significantly prolonged (P < 0.05), while the writhing scores and writhing times of the SP 6, GB 39 and the non-acupoint groups decreased significantly (P < 0.01). (2) In comparison with the control group, kappa-OR expression in the dorsal horn of L2 segment of spinal cord was upregulated significantly in the model group (P < 0.05). Compared to the model group, kappa-OR expression levels in the dorsal horns (DHs) of spinal T13, L1, L2, L6 and S1 segments in the SP 6 group were upregulated significantly (P < 0.01). ENK and beta-EP contents of PAG in the SP 6 and GB 39 groups were increased considerably (P < 0.01, P < 0.05). The effects of SP 6 were significantly superior to those of GB 39 in upregulating kappa-OR expression of spinal L1, L2 and L6 DHs and in upregulating beta-EP content of PAG; and superior to non-acupoint in upregulating kappa-OR expression of spinal T13, L1, L2, L6 and S1 DHs and in increasing both ENK and beta-EP contents of PAG (P < 0.01, PF < 0.05). No significant differences were found between the non-acupoint group and the model group in writhing latency, kappa-OR expression levels of spinal T13, L1, L2 and S1 DHs, and in ENK and beta-EP contents of PAG (P > 0.05).

Conclusion: EA of SP 6 can significantly alleviate pain reactions in dysmenorrhea rats, which is closely associated with its functions in upregulating spinal kappa-OR expression and ENK and beta-EP contents in PAG. EA of SP 6, GB 39 and non-acupoint has some different degrees of efficacies in relieving dysmenorrhea and in upregulating spinal K-OR expression.