Hot Plate Analgesia Assays.

Analgesia was measured using a 52 °C hot plate apparatus (IITC Life Sciences) as previously described (29). Animals were habituated in the room for 1 h, and the baseline latencies to jump or lick the hind paw were measured. Morphine was administered s.c., and, 30 min later, mice were placed on the hot plate and the latencies to jump or lick the hind paw were monitored. A cutoff time of 40 s was used to avoid tissue damage and inflammation (31). For tolerance studies, this procedure was repeated for 4 to 8 consecutive days using a high dose of morphine (15 mg/kg, s.c.). Data are expressed as percentages of maximal possible effect [MPE = (latency − baseline)/(cutoff − baseline)].