Thermal withdrawal latency (Hargreaves’ testing) - Response latencies to noxious thermal stimulation of the plantar surface of the hindpaw were measured as described previously [13]. Briefly, mice were placed on a temperature controlled glass platform (29°C) in plastic cylinders. After acclimation, focused light was focused on the plantar surface of the hind paw between the foot pads. The time to withdrawal of the foot was measured to 0.1 sec, and a 20-second cutoff was used to prevent tissue damage. The light beam intensity was adjusted to provide an approximate 10 second baseline latency for the C57BL/6 index strain prior to morphine treatment and the same light intensity was used for all subsequent experiments for all strains. Two measurements were made per animal per test session. Latencies were measured at baseline, and again 18 hours after the final dose of morphine when thermal and mechanical OIH were maximal. Our index of thermal OIH was calculated as the percentage decrease in baseline thermal withdrawal latency resulting from chronic morphine administration.
