Peroxynitrite-activated divalent metal transporter 1 without iron-responsive element [DMT1(-)IRE] overexpression leads to abnormal iron accumulation in remifentanil-induced postoperative hyperalgesia. Iron accumulation in spinal cord was shown as blue deposits in Perl stain micrographs. Hardly any iron accumulation was seen in the vehicle (A), there were mild iron accumulation in rats receiving incision (B) and remifentanil (C) individually, and pronounced iron accumulation in incision–remifentanil-treated rats (D). Intraperitoneal delivery of hydrogen-rich saline (10 ml/kg) protected against abnormal iron accumulation, respectively (F, G, H), but not its vehicle (E). n = 4 rats per group, scale bar = 50 μm.