Femur bone densities and tibialis anterior muscle properties of type II collagen-induced arthritic mice were determined. Furthermore, voluntary running activities of arthritic mice were compared with those of controls. Arthritis was induced by an intradermal injection of type II collagen in the adjuvant. Body and muscle weights were lower in arthritic mice than in controls. Cortical and trabecular bone densities and muscle fiber crosssectional areas were decreased by arthritis. After classifying the arthritic severity into slight, intermediate, and severe levels based on the degree of knuckle swelling, cortical and trabecular bone densities, fiber cross-sectional areas, and fiber succinate dehydrogenase activities were lowest when arthritis was most severe. Furthermore, arthritic mice, especially those with intermediate and severe levels, showed lower voluntary running activities. These findings indicate that lower bone density and muscle atrophy of type II collagen-induced arthritic mice are related to arthritic severity and decreased motor activity.Key words : bone density, cross-sectional area, motor activity, mouse, muscle fiber, succinate dehydrogenase activity, type II collagen-induced arthritis.Degeneration in the musculoskeletal system, including muscle atrophy and bone osteoporosis, is widely accepted as being induced by aging [1][2][3][4][5], hind-limb suspension [6][7][8], or exposure to microgravity [9][10][11][12]. These changes are considered to result from decreased motor activity.Previous studies observed that animal models [13-16] and patients [17] with arthritis showed decreased spontaneous motor activities because of severe chronic pain in the joints. Type II collagen-induced arthritis is a model of polyarthritis induced in susceptible rodents by immunization with type II collagen [18,19]. Muscle atrophy and decreased bone density occur in type II collageninduced arthritic rodents because these animals may have lower motor activities as a result of their distal joint pain. Therefore this study investigated cortical and trabecular bone densities of the femur and fiber cross-sectional areas and succinate dehydrogenase (SDH) activities of the tibialis anterior muscle in type II collagen-induced arthritic mice. Furthermore, voluntary running activities of arthritic mice were compared with those of controls.
METHODSAll experimental procedures and animal care were conducted in accordance with the Guide for the Care and Use of Laboratory Animals prepared by the Physiological Society of Japan. This study was also approved by the Institutional Animal Care and Use Committee of Kyoto University.Antigens and immunizations. Sixty-day-old DBA/1J male mice were randomly divided into control ( n = 5) and arthritic ( n = 13) groups. All mice were individually housed in cages of the same size. The mice in the arthritic group were immunized from 60 to 81 days after birth. Type II collagen was solubilized from fetal bovine articular cartilage by limited proteolysis with pepsin [20]. Type II collagen was...