Background: Accumulating evidence suggests that extracellular signal-regulated protein kinase (ERK), p38, and c-Jun N-terminal kinase (JNK) might be involved in hypersensitivity of various pain models. However, there is a lack of direct evidence for actual involvement of peripheral ERK, p38, and JNK in induction and maintenance of arthritic pain and the development of arthritis.
Methods: We evaluated the effects of preemptive and therapeutic intra-articular administration of selective inhibitors of p38 (SB203580) and JNK (SP600125), and indirect inhibition of ERK with a blocker (PD98059) of the kinase that activates ERK (i.e., MEK, the mitogen-activated protein kinase [MAPK]/ERK kinase), on arthritic pain-related behavior such as reduction of weight load and the inflammatory responses such as neutrophil infiltration into the synovium and knee joint diameter in rats. In addition, arthritis-induced phosphorylation of ERK, p38, and JNK in synovium of knee joint was examined.
Results: Pretreatments with PD98059, SB203580, and SP600125 prevented the reduction of weight load induced by the carrageenan injected into the knee joint cavity, but their effects showed different time course patterns. Therapeutic administration of PD98059 and SB203580 partially reversed carrageen-induced reduction of weight load, and their effects showed a similar time course pattern. However, therapeutic administration of SP600125 had no effect on the reduction of weight load. Hematoxylin and eosin staining revealed that carrageenan-induced neutrophil infiltration into the synovium was inhibited by pretreatment with SB203580 or SP600125, but not PD98059. Western blot measurements showed distinct expression of phosphorylated ERK, p38, and JNK in the synovium at different time points after carrageenan injection.
Conclusion: These results suggest that ERK, p38, and JNK signaling pathways at the peripheral level may play different roles in arthritic pain and arthritis of the knee joint.