ABSTRACT
Objective To investigate the effect of electro-acupuncture on the spinal nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signal pathway in a rat model of neuropathic pain (NP). Methods Forty-eight pathogen-free male SD rats weighing 190-210 g were randomly divided into 3 groups (n = 16 each):group Ⅰ sham operation (group S); group Ⅱ group NP and group Ⅲ electro-acupuncture + NP (group E). NP was induced by chronic constrictive injury (CCI). Right sciatic nerve was exposed and 4 loose ligatures were placed on the sciatic nerve at 1 mm intervals with 4-0 chromic catgut. In group E Huantiao and Weizhong points on the operated side were stimulated with electric stimulator (frequency 2 Hz, wave length 0.6 ms, starting at a voltage of 1mA and increasing by 1 mA every 10 min) for 30 min once a day at 11-17 d after CCI. Pain threshold to mechanical and thermal nociceptive stimuli was measured before (T0 , baseline) and at 10 and 16 d after CCI (T1, T2). The animals were sacrificed at 17 d after CCI and the lumbar segment (L4-6) was removed for determination of the activities of total NO synthase (tNOS), induced and neural NOS (iNOS, nNOS) (by spectrophotometry), NO content (by nitrate reductase method) and cGMP content (by immuno-histochemistry) in the spinal cord in 8 animals and the expression of iNOS and nNOS in the dorsal horn of the spinal cord (by immuno-histochemistry) in another 8 animals in each group. Results CCI significantly decreased the mechanical and thermal pain threshold at T1 and T2 as compared with the baseline at T0 in group NP and E. Hyperalgesia induced by CCI was significantly attenuated by electro-acupuncture at T2 in group E as compared with group NP.CCI significantly increased tNOS and nNOS activities, NO and cGMP content in the spinal cord and up-regulated nNOS expression in the spinal dorsal horn. Electro-acupuncture significantly attenuated the CCI-induced increases in tNOS and nNOS activities, NO and cGMP content in the spinal cord and nNOS expression in the spinal dorsal horn. There was no significant difference in the iNOS activity among the 3 groups. Conclusion NO-cGMP signal pathway in the spinal cord is involved in the acupuncture analgesia.