2.7.1 |. Hot plate test

The Hot Plate test was conducted using the Hot Plate Analgesia Meter (Columbus Instruments, OH). Mice were placed on the plate surface, which was heated to 55°C, and the time when animals first lift their paws and lick them (paw-lick response) was recorded as the basal response time. The maximal time of the test was 30 s (cut off time). The procedure was repeated 30 min after injection of morphine (5 mg/kg, s.c.), and the antinociceptive morphine response was calculated as a percentage of maximal possible effect using the formula %MPE = 100% × (morphine response time − basal response time)/(cutoff time − basal response time). The MPEs were compared by Mann–Whitney test.