EVALUATION OF THE FORMALIN TEST TO ASSESS THE ANALGESIC ACTIVITY OF DIFLUNISAL IN THE RAT

The formalin test was evaluated to assess the analgesic activity of di flunisal in the rat. Fifty microliters of a 5% formalin solution was i njected into the hindpaw of rats and two distinct nociceptive behavior s, i.e. flinching/shaking and licking/biting of the injected paw, were recorded over 120 min. The effect of factors such as age of the anima l, time of injection (morning vs, afternoon), site of injection (right vs, left hind paw) were evaluated. Both nociceptive behaviors exhibit ed a biphasic time course. Rats weighing 210-220 grams showed a more i ntense response compared to older rats weighing 240-250 or 270-280 gra ms. The nociceptive behavior response was affected by the time of form alin injection and was more pronounced in the morning. Diflunisal (100 mg/kg, i.v. infusion over 3 min) caused a significant delay in the fl inching/shaking response vs. time curve, whereas the licking/biting re sponse was significantly inhibited. When carried out under carefully c ontrolled conditions, the formalin test may be useful to study the ana lgesic effect of diflunisal in the rat. It seems to be less sensitive, however, than other commonly used nociceptive tests. (C) 1998 Elsevie r Science B.V. All rights reserved.