Papers of the Week

Reactive oxygen species such as superoxide anion have varied roles in inflammation and pain, which can be mimicked by potassium superoxide (KO), the superoxide anion donor. Interleukin (IL)-33 has pleiotropic functions by activating its receptor suppression of tumorigenicity 2 (ST2). However, the role of IL-33/ST2 signaling in inflammatory pain initiated by reactive oxygen species (ROS) such as superoxide anion has not been investigated, which was the aim of the present study. IL-33 levels were assessed by enzyme-linked immunosorbent assay (ELISA). Mechanical and thermal hyperalgesia and overt pain were evaluated by electronic von Frey, hot plate, and abdominal writhing/paw flinching/licking, respectively. Edema and leukocyte recruitment (myeloperoxidase assay and total/differential cell count), antioxidant capacity, superoxide anion production and lipid peroxidation were assessed. Paw skin and spinal cord messenger ribonucleic acid (mRNA) expression of pro-inflammatory mediators and glial markers in the spinal cord were evaluated. Immunofluorescence was used to detect spinal glial and neuronal c-Fos activation. KO injection triggered IL-33 production in the paw skin and spinal cord of mice, induced hyperalgesia, edema, neutrophil recruitment to the paw tissue, overt pain-like behavior, and leukocyte recruitment to the peritoneum that were reduced in ST2 deficient mice. In the paw skin and spinal cord, KO triggered IL-33/ST2-dependent oxidative stress, and mRNA expression of inflammatory molecules, which were reduced by ST2 deficiency. KO induced spinal cord glial (at mRNA/protein levels) and neuronal activation in IL-33/ST2-dependent manner. IL-33/ST2 signaling mediates, at least in part, superoxide anion-induced inflammatory pain by modulating local and spinal inflammatory events.