Thermal hyperalgesia

Intellective hot plate equipment (cat. no. YLS-6 B; Shanghai Zhenghua Medical Equipment Co., Ltd) was used to measure the paw withdrawal latency (PWL) by two blinded experimenters. One experimenter numbered all the animals in the different groups, and the other, who was blinded to the animal grouping, measured the PWL. Animals were allowed to acclimate to the environment for 1 h before testing by placing a hot plate (50°C). The PWL was defined as the time (sec) when the rat showed a positive response (a clear paw withdrawal). The test was repeated three times at 10-min intervals. The mean of the three trials was regarded as the PWL. A cut-off time of 30 s was used to avoid tissue damage.