Spinal and supraspinal sites for morphine and nefopam analgesia in the mouse

Using the tail-flick and hot-plate assays, morphine and nefopam were tested for analgesic activity following intraperitoneal (i.p.), intracranial (i.c.) and intraspinal (i.s.) injections in mice. By the i.p. route, morphine was equipotent on both analgesic tests. Nefopam was one-third as potent as morphine on the hot-plate test, but did not affect the tail-flick. Intracranial morphine was more effective on the hot-plate than on the tail-flick, but i.s. morphine was most potent on the tail-flick. Naloxone, 0.5 mg/kg i.p., totally reversed morphine's effects on the tail-flick, but only partially reversed these actions on the hot-plate, suggesting the possibility that morphine's effects on the mouse hot-plate test may be mediated via multiple receptor types. Nefopam was more potent by the i.c. route than by the i.p. route, but it was inactive spinally. Nefopam analgesia was unaffected by naloxone treatment. It is concluded that nefopam is a novel, centrally acting, non-narcotic analgesic.