von Frey testing

Rats were placed in individual Plexiglas chambers with a wire mesh bottom and allowed to acclimate for 10 min. Each paw was tested only when the weight of the rat was equally distributed on all four paws, or as much as possible post-SCI. Plastic monofilaments of varying bending forces (von Frey monofilaments; Stoelting Co., Wood Dale, IL) were applied to the plantar surface of the forepaw in the up-down method with an interval of 2–4 min between applications. A total of 10 von Frey monofilament stimulus applications were collected for each paw for each day of testing, beginning with the 5.18g force von Frey hair. Immediate forepaw withdrawal with evidence of supraspinal awareness (licking or looking at the paw, moving away from the stimulus, vocalization, etc.) was considered as a positive response, followed by application of von Frey hair of next lower force value. Absence of forepaw withdrawal was noted as a negative response, and the next higher force von Frey hair was applied. Behavior indicating supraspinal awareness of tactile stimuli (glancing at paw, licking, and vocalization) was recorded. Paw withdrawal threshold (PWT) was determined as the lowest force (g) that produced a forepaw withdrawal and supraspinal behaviors in at least 50% of the applications at that force. Order of paw testing was randomized to minimize fatigue or an order effect. SCI rats that demonstrated a >50% reduction in PWT at 28 dpi compared to baseline were assigned to pain subgroups.^40,46,47