Analysis of the relationship between knee osteoarthritis weight–bearing pain and transient receptor potential vanilloid 1 using the CatWalk system

Osteoarthritis of the knee (knee OA) is a common disease in the elderly, and the chief complaint of these patients is knee pain. Knee OA pain reduces the activities of daily living and the quality of life in these patients. The degree of pain does not correlate with radiographic grades, and the intense wear of the cartilage does not neces sarily mean intense pain. However, the cellular mechanism of chronic pain in this disease has been unclear. This study aimed to investigate the effects of the transient receptor potential vanilloid 1 (TRPV 1 ) on weight–bearing pain–related behaviors in knee OA model rats using the CatWalk system. To generate the knee OA model, monosodium iodoacetate (MIA) was injected in the right knee joint of male adult Sprague–Dawley rats. Four weeks after the MIA injection, we used the knee OA rats for the behavioral study. Using the CatWalk system, we investigated the effects of the intra–articular injection of the TRPV 1 selective agonist capsaicin, TRPV 1 selective antagonist capsazepine, and saline on the nociceptive behaviors of the rats with knee OA at 1 , 2 , 7 , and 10 days after the intra–articular injection. The rats treated with intra–articular capsazepine injection showed significantly greater improvement in swing speed, a pain–related behavioral parameter of the CatWalk system, than those treated with capsaicin on day 2 after injection. The maximum contact area on day 10 showed significantly greater improvement in the capsazepine group than in the capsaicin group. These results suggest that TRPV 1 is an important contributor to weight–bearing pain–related behavior of patients with knee OA. However, in this study, these effects were observed at later time points. Thus, TRPV 1 could be related to not only the peripheral nociceptive pain mechanism, but also to the secondary pain mechanism.