Effects of Capsaicin on the c-fos in the Spinal Dorsal Horn and Substance P- and CGRP-like Immunoreactivities in the Dorsal Root Ganglia of the Experimental Arthritic Rat Model

Abstract

To analyze the effects of capsaicin adm inistered to the arthritic rat m odel, im m unohistochem ical stains for c-fos protein in the spinal dorsal horn and for substance P and C G RP in the dorsal root ganglia (DR G ) were done. Kaolin and carageenan were adm inistered to the knee joint cavity of adult rats to induce arthritis, and capsaicin was adm inistered im m ediately after kaolin-carageenan injection. A count was conducted of the c-fos im m unoreactive dorsal horn neurons and substance P and CG RP im m unoreactive cells in L5 and L6 DRG to elucidate the effect of capsaicin. The num ber of c-fos im m unoreactive neurons in the superficial dorsal horn was increased m arkedly 2 hours after the kaolin and carageenan injection to the knee joint, and decreased gradually to the control level 1 week after injection. The num ber of c-fos im m unoreactive neurons in the deep dorsal horn w as 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 treatm ent, the num ber of c-fos im m unoreactive neurons in the superficial and deep dorsal horn of capsaicin-treated rats was less than in those rats not treated with capsaicin. The num ber of substance P and CG R P im-m unoreactive D RG neurons increased 24 hours after the kaolin and carageenan injection to the knee joint, and also apparently increased 1 w eek after injection. The number of substance P and C G RP im m unoreactive DR G neurons of capsaicin-treated rats w as less than in those rats not treated with capsaicin adm inistered rats. Capsaicin reduces the num ber of c-fos im m unoreactive neurons in the spinaldorsal horn, and also reduces the num ber of substance P and CG RP im m unoreactive neurons in the D RG of the arthritic rat m odel, w hich m ay be closely related to the analgesic effects of capsaicin.