Differential effect of selective cyclooxygenase-2 (COX-2) inhibitor NS 398 and diclofenac on formalin-induced nociception in the rat.

Prostaglandins (PGs) are known to be involved in inflammatory and nociceptive processing. Since the discovery of at least two isozymes of cyclooxygenase (COX), inhibition of COX-2 has been suggested to be responsible for the therapeutic effects of nonsteroidal anti-inflammatory drugs (NSAIDs). In the present study, the effects of a rather selective COX-2 inhibitor, NS-398 (0.3-27 mg/kg i.p.), were studied using the rat formalin test as a model of acute nociception. Diclofenac (non-selective COX inhibitor; 0.3-27 mg/kg i.p.) was used as a control. NS-398 revealed antinociceptive activity only at a dose (27 mg/kg) which results in plasma concentrations which most likely do not selectively inhibit COX-2. By contrast, diclofenac inhibited formalin-induced flinching behaviour over the whole dose range tested. Our results suggest that PGs mediating nociception in the formalin test of the rat are most likely produced via the COX-1 as well as COX-2 pathways. Thus, in an acute model of nociception a non-selective COX inhibitor may offer advantages as compared to a selective COX-2 inhibitor.