Cumulative morphine dose–response curves were constructed as described previously [11, 12]. Briefly, the thermal withdrawal latencies of gently restrained mice were measured using a tail-flick apparatus (Columbus Instruments, Columbus, OH) with 0.1 second precision. Two measurements were made per mouse. The lamp intensity was the same for all strains. For the assessment of morphine analgesic tolerance, these dose–response experiments were made on morphine naïve mice and again 18 hours after the final dose of morphine given as part of the chronic morphine administration protocol. The cumulative doses of morphine used were 0,1,2,4,8,16 and 32 mg/kg s.c.. Tail flick latency was determined 25 minutes after morphine administration as demonstrated previously to be the time of peak morphine effect. The parameter %MPE (percent maximal possible effect) was determined according to the following formula:
