Tail flick test in rats

A tail flick assay in rats was performed as described in our previous study (21), for which an automated tail flick test device (SW-200 Analgesia Meter; Chengdu Taimeng Science And Technology Co., Ltd.) was used. Rat tails (distal 1/3rd) were painted with black ink to enhance absorption of heat radiation. An adjustable heat source was directly placed under the tail. The assay is based on the animal flicking its tail away to avoid pain induced by the source of heat. Tail flick latencies required for the rat to remove its tail were determined in sec as an index of nociceptive threshold (22). Pain threshold elongation indicated the analgesic effect of the drug. The rats with latency of 3–20 sec by duplicated detection were selected to be used in the formal experiment. They were randomized into 9 groups and were respectively administered (i.p.) 0.9% N.S. (n=9), nefopam (2.5 and 5 mg/kg; n=9 per group), acetaminophen (42, 84 and 168 mg/kg; n=8 per group), or combinations of N and A (1.25 N + 21 A, 2.5 N + 42 A, 5.0 N +84 A mg/kg; n=9,9 and 8) prior to their subjection to the tail flick test. The latency of tail flicking was determined at 15, 30, 45, 60 and 75 min after drug administration. The maximal latency was limited to 30 sec to protect tail tissues from damage in instances of thermal pain endurance.