To analyze the effects of capsaicin administered to the arthritic rat model, immunohistochemical stains for c-fos protein in the spinal dorsal horn and for substance P and CGRP in the dorsal root ganglia (DRG) were done. Kaolin and carageenan were administered to the knee joint cavity of adult rats to induce arthritis, and capsaicin was administered immediately after kaolin-carageenan injection. A count was conducted of the c-fos immunoreactive dorsal horn neurons and substance P and CGRP immunoreactive cells in L5 and L6 DRG to elucidate the effect of capsaicin. The number of c-fos immunoreactive neurons in the superficial dorsal horn was increased markedly 2 hours after the kaolin and carageenan injection to the knee joint, and decreased gradually to the control level 1 week after injection. The number of c-fos immunoreactive neurons in the deep dorsal horn was increased later than those in the superficial dorsal horn and reached peak level 16 hours after the kaolin and carageenan injection, and decreased gradually thereafter. After capsaicin treatment, the number of c-fos immunoreactive neurons in the superficial and deep dorsal horn of capsaicin-treated rats was less than in those rats not treated with capsaicin. The number of substance P and CGRP immunoreactive DRG neurons increased 24 hours after the kaolin and carageenan injection to the knee joint, and also apparently increased 1 week after injection. The number of substance P and CGRP immunoreactive DRG neurons of capsaicin-treated rats was less than in those rats not treated with capsaicin administered rats. Capsaicin reduces the number of c-fos immunoreactive neurons in the spinal dorsal horn, and also reduces the number of substance P and CGRP immunoreactive neurons in the DRG of the arthritic rat model, which may be closely related to the analgesic effects of capsaicin.

Keywords: capsaicin, c-fos, substance P, calcitonin gene-related peptide (CGRP), spinal cord dorsal horn, dorsal root ganglia (DRG), arthritis, pain