Mechanical allodynia (von Frey hair test)

The cutaneous nociceptive threshold of the operated hind paws was assessed according to a previous method reported by Chaplan³². Briefly, rats were placed in test cages with a metal-mesh floor and were allowed to habituate for at least 30 min. Von Frey filaments (Touch-Test^® Sensory Evaluators, NC12775-99, North Coast Medical Inc, San Jose, CA, USA) of 0.6, 1.0, 1.4, 2.0, 4.0, 6.0, 8.0, 10.0, 15.0 and 26.0 g were applied to the plantar surface of the left hind paw. A positive or negative response was defined as a paw withdrawal response from the pressure of a filament or the lack of a response within 6 s, respectively. Initially, a 2.0-g force filament was used for the nerve-injured rats, and a 6.0-g force filament was used for the sham rats. If a positive response to a given filament occurred, the next-smaller filament was then used. If a negative response occurred, the next-larger filament was used. The test continued until four responses were collected after the first change in response. The tactile stimulus producing a 50% likelihood of paw withdrawal threshold (PWT) was used to calculate the cutaneous nociceptive threshold using an adaptation of the Dixon up-down paradigm.