Objective To observe the effect of different frequencies of electroacupuncture(EA)stimulation on pain threshold(PT)and expression of vascular endothelial growth factor(VEGF)in dorsal horns(DHs)of the lumbar spinal cord in resiniferatoxin(RTX)-induced post-herpetic neuralgia(PHN)rats

so as to reveal its mechanism in alleviating PHN.Methods Male SD rats were randomized into control

model

2 Hz-EA

15 Hz-EA

100 Hz-EA and sham EA groups(n=16 in each).The PHN model was induced by a single intraperitoneal injection of RTX(250!g/kg)

and rats of the control group received intraperitoneal injection of the same dose of vehicle(10% Tween 80

10% alcohol and 0.9% NaCl).Rats of EA treatment groups received EA stimulation(2 Hz

15 Hz or 100 Hz

1 mA)at the left"Huantiao"(GB 30)and"Yanglingquan"(GB 34)for 30 min

once every other day for35 days

starting from 1 week after RTX injection.For sham control

acupuncture needles were inserted ipsilaterally into GB 30 and GB 34 for 30 min without electrical stimulation or manual needle manipulation.The mechanical allodynia was quantified with Von Frey filaments.The expression of mRNA and protein of VEGF in the DHs of lumbar spinal cord 4-6 segments(sampled under light microscope)was detected by quantitative polymerase chain reaction(qPCR)and Western blot

respectively.Results A single RTX injection gradually induced tactile allodynia(significant reduction of the mechanical PT)within 3 weeks relevant to the control group(P<0.01).EA applied to GB 30 and GB 34 at 2 Hz and 15 Hz

but not 100 Hz

significantly decreased the tactile allodynia after the treatment(2 Hz from 2 weeks on and 15 Hz from 3 weeks on)in RTX-treated rats(P<0.05).RTX administration increased the mRNA and protein expression of VEGF in the lumbar spinal cord compared with the control group(P<0.05).Moreover

2 Hz

but not 15 Hz and 100 Hz EA significantly reduced VEGF mRNA and protein expression(P<0.05).The expression of both VEGF mRNA and protein was negatively correlated with mechanical PT in RTX-induced PHN rats.Conclusion EA at 2 Hz can significantly reduce VEGF expression in the lumbar spinal cord DHs of PHN rats

which is possibly in part related to its effect in alleviating the mechanical allodynia.Our study suggests that 2 Hz EA is the best stimulation frequency for relieving PHN.