2.6.1 Hot plate test
Heat thresholds were measured in the hot plate test. Animals were placed in their home cages and allowed to acclimate for at least 30 min before testing. After habituation, an unrestrained animal was placed on the metal surface of the hot plate, which was maintained at a constant temperature of 55℃. The response latency (i.e., the time before nocifensive behavior was exhibited) was recorded. Nocifensive behaviors included forepaw withdrawal or licking, hind paw withdrawal or licking, stamping, leaning posture, and jumping. If no behavioral response was elicited after 40 s (cutoff time), the animal was removed from the hot plate to prevent tissue damage. This procedure was repeated three times and the average across trials was used for data analysis.