Formalin Test in Mice

The formalin model used in this study was as previously described (Tjølsen et al., 1992). Mice were allowed to habituate for at least 2 h in testing room prior to the experiment and the behaviors were conducted between 8:00 am and 1:00 pm. Mice (n = 10/group) were orally administered with vehicle, pregabalin (60 mg kg⁻¹) or LPM580098 (16 mg kg⁻¹, 32 mg kg⁻¹, 64 mg kg⁻¹) 60 min prior to intraplantar injection of diluted formalin solution, and acclimatized individually in custom-made Plexiglass boxes 10 min before the experiment. 20 μL of 2.5% formalin (37% formaldehyde diluted in saline) was injected into the plantar surface of the right hind paw of the mice. Observation was conducted immediately after formalin injection. The time (in seconds) spent flinching and licking the right hind paw from 0 to 5 min after injection was considered phase I, whereas phase II was the total time spent performing these responses from 15 to 40 min, and nociception was quantified based on the flinching/licking time lasted in both phases.