A modified formalin test for measuring analgesia in mice.

A simple method for assessing pain and analgesia in mice was developed by modification of the formalin test. Formalin was injected into the forepaw of a mouse and the durations (in sec) spent in licking and biting responses were measured. These behavioral responses being very distinct, pain assessment was easily performed without the experimenter's subjective assessment. The use of the duration of pain during each 5 min block of 2 peak points enables determination of the analgesic effect quantitatively and shortening of the experimental time. As a low concentration of formalin (0.5%) was used in this method, the analgesia of weak nonnarcotic analgesics was detectable as well as that of narcotic ones and tissue damage was not elicited. Furthermore, this method enables to distinguish roughly the action site of analgesics between the central and peripheral one.