BACKGROUND: Previous studies have shown that extracellular signal-regulated kinase 1/2 (ERK1/2) and nitric oxide activation play a pivotal role in central sensitization and long-term neuronal plasticity induced by noxious stimulation. However, their effects on compressive neuralgia formation and maintenance remain poorly understood.OBJECTIVE: To investigate effects of the specific inhibitor of ERK1/2 signal pathway U0126 on neuronal nitric oxide synthase (nNOS) expression in the dorsal h of the spinal cord in a compressive neuralgia rat model.DESIGN, TIME AND SETTING: A randomized, controlled experiment was performed at the Institute of Otolaryngology, Head and Neck Surgery, First Hospital of Jilin University from July 2008 to March 2009.MATERIALS: U0126 (Bio-Mol, USA) was used in this study.METHODS: A total of 84 rats were randomly assigned to two groups. In the first part of the experiment, 24 rats were used for behavioral testing, and they were randomly assigned to three sub-groups (n =8): U0126, dimethyl sulfoxide (DMSO) and model control. In the second part of the experiment, 60 rats were used for immunofluorescence and West blot analysis, and they were randomly assigned to six sub-groups (n = 10): sham surgery, model control, U0126 post-injection at 0.5, 2, 12 and 24 hours. Neuropathic pain was produced by chronic compression to the dorsal root ganglion in rats from each sub-group. Rats in the U0126 group were administered a 5-ug U0126 intrathecal injection, and rats in the DMSO group were administered a 10-μL 5% DMSO intrathecal injection.MAIN OUTCOME MEASURES: Changes in mechanical and thermal hyperalgesia were observed using von Frey filaments and thermalqia stimular. Thermal and mechanical hyperalgesia were stimulated at different time points following intrathecal injection of U0126. nNOS activation and expression in the spinal cord dorsal h were determined by immunofluorescence and West blot analysis.RESULTS: Intrathecal injection of U0126 significantly attenuated chronic compression of dorsal root ganglion-induced mechanical and thermal hyperalgesia. Immunofluorescence staining results demonstrated that, compared to the sham surgery group, the number of nNOS-positive neurons was significantly increased in the injured spinal dorsal h in the model control group (P＜0.01). However, compared to the model control group, there were significantly decreasing numbers of nNOS-positive neurons in the U0126 post-injection groups at 0.5-hour, 2-hour, and 12-hour (P＜0.05). West blot analysis revealed similar results. CONCLUSION: Decreased activity in the ERK signal pathway resulted in down regulated nNOS expression in the dorsal h of the spinal cord. These results suggested that ERK is involved in nitric oxide reaction to neuropathic pain.