Analgesic effectiveness of ketorolac compared to meperidine in the rat formalin test

The rat formalin test is an analgesic behavioral observation assessment method that demonstrates two phases of nociceptive behavior. The test consists of injecting the right hind paw with a 5% formalin solution and then observing the animal for specific nociceptive behavior. The phases represent two different types of pain. Phase 1 is pain produced by direct nerve stimulation and phase 2 is an inflammation-induced pain. The nociceptive behavior measured in this experiment was licking and biting the injected paw. A comparison of nociceptive behavior was made when ketorolac and meperidine were injected (i.p.) 10 min prior to formalin injection. As expected, a biphasic pattern of licking and biting the injected paw ensued. It was found that ketorolac had no significant reduction in licking and biting, while meperidine dramatically reduced the nociceptive response in phase 1. In phase 2, both ketorolac and meperidine caused a reduction in licking and biting; however, meperidine reduced the nociceptive response to a greater extent. This experiment demonstrates that ketorolac, when compared to meperidine, is less effective in treating pain from inflammatory origin and is not effective in treating pain from direct nerve stimulation.