Pain threshold testing
Pain threshold was measured by the Von Frey testing (NC12775-99, North Coast, USA). The mechanical withdrawal threshold (MWT) was assessed by independent
observer blind to different groups rats. Briefly, we placed the rats in a chamber with a wire mesh floor (22 × 12 × 12 cm3 ) and acclimated to the environment for 20 min. The plantar surface of the right hind paw was stimulated by Von Frey filaments ranging from 2.0 g to 15.0 g in sequence. The MWT value was recorded while withdrawing the paw. The results were presented as the following formula: 50% threshold (g) = (10[Xf+ Kδ]) /10000. Where Xf referred to as the value of the final von Frey filaments, k as coefficient value for the pattern of positive/negative responses, and δ was equal to 0.224 [26].