Naloxone blocks the analgesic action of levorphanol but not of dextrorphan in the leopard frog

Intraspinal injection of levorphanol (3 μg) at the lumbar area of the leopard frog, Rana pipiens, induced analgesia which was completely blocked by co-injection of naloxone (3 μg), whereas dextrorphan (3 μg) induced analgesia which was unaffected by naloxone. Subcutaneous levorphanol (20 or 80 mg/kg) induced a dose-dependent analgesia which was blocked by concurrent naloxone (2 mg/kg), while only the higher dose of dextrorphan (80 mg/kg) induced analgesia which was unaffected by concurrent naloxone at 8 or 80 mg/kg. These data are the first to indicate naloxone-insensitive, dextrorphan-induced analgesia.