Objective To observe effects of electroacupuncture(EA) on the expression of κ-opioid receptor in the dorsal horn of the spinal cord and contents of enkephalin(ENK) and beta-endorphin(β-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 intrape-ritoneally

for rats of the control group

the same dose of saline was given(i.p.).On the10th 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 κ-opioid receptor(κ-OR) in T13

L1

L2

L6 and S1 segments of spinal cord was determined by immunohistochemistry

and the contents of ENK and β-EP in the midbrain PAG were assayed by ELISA.Results ①Compared 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

κ-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

κ-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 β-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 κ-OR expression of spinal L1

L2 and L6 DHs and in upregulating β-EP content of PAG;and superior to non-acupoint in upregulating κ-OR expression of spinal T13

L1

L2

L6 and S1 DHs and in increasing both ENK and β-EP contents of PAG(P<0.01

P<0.05).No significant differences were found between the non-acupoint group and the model group in writhing latency

κ-OR expression levels of spinal T13

L1

L2 and S1 DHs

and in ENK and β-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 κ-OR expression and ENK and β-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 κ-OR expression.