Background: Contradictory reports have been published relating the role of NO in signal processing of inflammation and pain. The present study was designed to evaluate the level of peripheral NO production following administration of different concentrations of formalin (as an inflammatory pain inducer) and celecoxib (an analgesic and anti-inflammatory drug).

Materials and Methods: Acute and chronic inflammatory pain was induced in adult rats through formalin injections (1%, 2.5% and 5%). spectrophotometric analysis of the serum nitrite on acute (1.5h and 24h postinjection) and following 24h after the last 4 day chronic formalin injection was performed. The effect of acute and chronic injections of celecoxib, (10, 20 and 40 mg/kg/ip) either as a pretreatment or alone, on NO production was also evaluated at the same time intervals. Two-way ANOVA was used for data analysis.

Results: Administration of formalin resulted in the enhancement of serum nitrite dose- and time-dependently. Except to the ineffectiveness of celecoxib (40mg/kg) on nitrite level 1.5 h following 5% formation injction, in other cases it prevened nitrite production. Meanwhile, 1.5h following the administration of celecoxib (40mg/kg) alone, a significant increase of serum nitrite was observed.

Conclusions: It seems that NO affects formalin-dependent signal transduction pathways. Celecoxib, depending on the administration conditions, exerts different effects on the peripheral NO production.