Three experiments were conducted in order to investigate the possible involvement of the reactive oxygen species in the nociception within the subnucleus caudalis of the spinal trigeminal nucleus (Vc). In the first experiment the extracellular level of hydrogen peroxide was evaluated by microdialysis in the Vc of two groups of six rats before and after a formalin (group 1) or saline solution (group 2) injection into the upper lip. In the second experiment the formalin test was conducted in three groups of 6 rats after a microinjection of 2-methoxyestradiol (2-ME, a superoxide-dismutase inhibitor; group 1) or N-acetylcysteine (NAC, an oxygen intermediate scavenger; group 2) or saline solution (group 3) into the Vc. In the third experiment an histochemical assay for superoxide dismutase activity was performed on two groups of 4 rats each 2 h after a formalin (group 1) or saline solution (group 2) injection into the upper lip. The results showed that (1) the level of hydrogen peroxide increases into the Vc during facial pain (134\% of baseline); (2) the inhibition of superoxide dismutase or the removal of oxygen intermediate within the Vc decreases the sensibility to facial pain stimuli; and (3) persistent facial pain stimuli decrease the superoxide activity into the Vc (90\% of counter-lateral). These data indicate that reactive oxygen species are produced in the Vc during persistent facial pain and are necessary for the transmission of pain.
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