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The anti-nociceptive effect of thalidomide on zymosan-induced articular knee joint incapacitation in rats was investigated. Thalidomide (5–45mg/kg), given 30min before but not 2h after the intra-articular injection of zymosan, inhibited the nociceptive response in a dose-dependent manner. Furthermore, thalidomide pretreatment significantly reduced the concentration of tumor necrosis factor-alpha (TNF-α, −68.4%) in the exudate of zymosan-injected joints, but not those of interleukin-1β, interleukin-6, CINC-1 or interleukin-10. The expression of TNF-α, determined by immunohistochemical staining, in synovial tissues obtained from articular joints injected with zymosan was also inhibited by thalidomide pretreatment. The anti-nociceptive effect of thalidomide was not reversed by the co-administration of an opioid receptor antagonist, naloxone, suggesting that endogenous opioids do not mediate the anti-nociceptive effect of thalidomide in this model. In conclusion, the anti-nociceptive activity of thalidomide in zymosan-induced articular incapacitation is associated with the inhibition of TNF-α by resident synovial cells.