2.5 Response to mechanical stimuli
We evaluated mechanical hypersensitivity by measuring the hind paw withdrawal response to von Frey filament stimulation. Mice were placed individually in Plexiglas cages positioned on a wire‐mesh stand. Mice were allowed to acclimate to the environment for approximately 30 min before testing. Low force (0.07 g) and high force (0.45 g) monofilaments (Stoelting Co., Wood Dale, IL) were applied to a fixed point on the midplantar surface of each hind paw (Guan, Liu, Tang, Raja, Anderson & Dong, 2010). Each von Frey filament was applied 10 times successively at approximately l–2-s intervals. Vertical elevation of the paw and licking associated with the mechanical application were considered withdrawal responses. Paw withdrawal frequencies (PWF) were calculated for both hind paws.