Background/Purpose: More thorough understanding of age-related molecular interactions that drive inflammation and inflammatory pain is required to help guide evidenced-based, age appropriate treatment strategies that improve arthritis care. Our earlier studies indicate that age is an important determinant of collagen-induced arthritis (CIA) progression and resolution. This study characterized age-related differences in an inflammatory pain pathway involving pro-inflammatory high mobility box 1 protein (HMGB1) and pro-resolution resolvin among different age groups of rats with and without arthritis.

Methods:  Juvenile (5 wks old) and young adult (13 wks old) male Wistar rats were immunized with an emulsion of bovine type II collagen/incomplete Freund’s adjuvant. Naïve juvenile and adult rats served as controls. For each animal, the Maximum Daily Arthritis Score (MDAS) was recorded. Fourteen days after arthritis onset, animals were euthanized and blood and tissues collected. Dorsal root ganglia immunohistochemistry assessed pro-inflammatory high mobility group box-1 (HMGB1), receptor for advanced glycation end products (RAGE) and the resolvin D1 receptors G-protein coupled receptor 32 (GPR32) and formyl peptide receptor 2 (FPR2).

Results: The MDAS for juvenile rats with CIA were significantly lower than the adult rats with CIA (3.96 + 0.17 and 5.25 + 0.16 respectively; p<0.0001). During the first week after onset of CIA, adult rats had a slightly higher MDAS than juvenile rats (p=0.04). During the second week of CIA, the MDAS for juvenile rats decreased (p=0.03) while the MDAS for adult rats increased significantly (p=0.002). At the end of the second week of CIA, adult rats had a significantly higher MDAS than juveniles (p<0.0001).

Adult naïve rats had significantly higher percentages of neurons expressing HMGB1 (p<0.0001) and RAGE (p=0.02) than juvenile naïve rats. Two weeks after CIA onset, percentage of neurons positive for HMGB1 (p<0.0001) and RAGE (p=0.0002) were significantly higher in the adult CIA group compared to the juvenile naïve group. In contrast, Juvenile naïve rats have significantly higher percentages of GPR32 (p=0.02) and FPR2 (p=0.04) expressing neurons than adult naïve rats. After two weeks, Juvenile CIA rats also had a higher percentage of neurons expressing GPR32 than the Adult CIA rats (p=0.02). No differences in the percentage of neurons expressing FPR2 were observed between Juvenile CIA and Adult CIA rats.

Conclusion:  Results indicate age-related differences in arthritis severity are related to the balance between HMGB1 and resolvins and their respective receptors with the balance favoring promotion of inflammation in adults associated with higher levels of HMGB1 and lower levels of resolvin receptors. Juveniles have more available resolvin receptors that may impede transition from acute to chronic arthritis in this model. We postulate that anti-inflammatory effects of endongenous resolvins are mediated by impeding production of HMGB1 and/or HMGB1 binding to its receptors.