Mechanical withdrawal thresholds - Mechanical withdrawal testing was performed as described previously [14]. Briefly, mechanical nociceptive thresholds were assayed using nylon von Frey filaments according to the “up-down” algorithm described by Chaplan et al. [49]. In these experiments mice were placed on wire mesh platforms in plastic cylinders. After acclimation, fibers of sequentially increasing stiffness were applied to the plantar surface of a hind paw according to the up-down paradigm. Testing proceeded in this manner until 4 fibers had been applied after the first one causing a withdrawal response allowing the estimation of the mechanical withdrawal threshold using curve fitting of the response data [50]. Our index of mechanical OIH was calculated as the percentage decrease in baseline mechanical nociceptive threshold resulting from chronic morphine administration.
