Participation of cyclooxygenases in cutaneous thermal nociception under non-inflamed and inflamed conditions

Objective and Design: Effects of cyclooxygenase inhibitors on noxious thermal stimuli were investigated in non-inflamed and inflamed rats. Materials: Male Sprague-Dawley rats were used in this study. Treatment: Cyclooxygenase inhibitors, indomethacin, mofezolac, NS-398, and JTE-522 were administered orally at a dose of 10 mg/kg 1 h prior to and 4 h after the intravenous injection of lipopolysaccharide (1 mg/kg). Methods: The nociceptive response was evaluated from the escape latency of foot withdrawal to the thermal stimuli with a beam of light. Expression of cyclooxygenase was examined by reverse transcription-polymerase chain reaction. Results: In normal rat, administration of indomethacin, relatively cyclooxygenase-1-selective inhibitor, mofezolac, or cyclooxygenase-2-selective inhibitors, NS-398 and JTE-522 had no effects on the escape latency against thermal stimuli. Injection of lipopolysaccharide into rat induced the expression of mRNA for cyclooxygenase-2 in the subcutaneous tissue of foot pad. The escape latency at 8 h was significantly shortened by the injection. This hyperalgesia could be reversed by pretreatment of rat with NS-398 or JTE-522, but not with mofezolac. Conclusions: Cyclooxygenases may have little participation in peripheral skin thermal nociception in non-inflamed condition, although cyclooxygenase-2 could be responsible for the hyperalgesia during inflammation induced by lipopolysaccharide.