A new model to assess analgesic activity: Pain-induced functional impairment in the rat (PIFIR)

A new experimental model to assess analgesic activity of both analgesic and non-steroidal antiinflammatory drugs in described. It uses the unilateral intra-articular knee injection of an uric acid suspension in mineral oil to produce acute inflammation, pain, and functional motor impairment. The model, named “pain-induced functional impairment in the rat” (PIFIR) assesses analgesic activity by measuring the capacity to walk with the injured extremity. The procedure determines both the potencies of analgesic drugs and the time course of the effect. Analgesia of selected reference agents was followed for 4 h and the effect versus time curves were constructed. The area under the curve (effect versus time), an expression of the overall activity during the observation period, increased in a dose-dependent manner. The area under the curve, Emax, TEmax, and ED50 of reference agents tested are reported. The PIFIR procedure was sensitive to opiate and nonopiate analgetics (nonsteroidal antiinflammatory drugs) and possibly steroidal antiinflammatory drugs. These characteristics make it suitable for screening purposes. © 1993 Wiley-Liss, Inc.