The magnitude and duration of the analgesic effect of morphine, butorphanol, and buprenorphine in rats and mice

This study was designed to determine the magnitude and duration of the anal gesic effect of three commonly used opioids: buprenorphine (0.5 mg/kg for r ats; 2.0 mg/kg for mice), butorphanol (2.0 mg/kg for rats; 5.0 mg/kg for mi ce), and morphine (10 mg/kg for rats and mice). We used two standard tests, the hot plate and tail flick assays, to measure opioid analgesia in 62 mal e, 200 to 300 g Sprague-Dawley rats and 61 male, 25 to 35 g ICR mice. We ob tained five baseline measurements then administered the drugs subcutaneousl y. Morphine gave the highest analgesic effect and was intermediate in durat ion (2 to 3 h in rats and mice) of analgesia. Butorphanol provided the lowe st level of and shortest (1 to 2 h in rats and mice) analgesia. Buprenorphi ne had an intermediate analgesic effect and the longest duration (6 to 8 h in rats and 3 to 5 h in mice). In light of our results, we recommend the us e of morphine (with frequent redosing) for severe pain, butorphanol for mil d pain of short duration, and buprenorphine for mild to moderate pain of in creased duration. The dosing intervals suggested by our study are 2 to 3 h for morphine in both rats and mice, I to 2 h for butorphanol in both rats a nd mice; and 6 to 8 h in rats and 3 to 5 h in mice for buprenorphine.