Subcutaneous injection of 0.02 mL of 4
aminopyridine (4 AP, 2.6 mmol·L 1 ) into the dorsal surface of the neck was
used as a nociceptive stimulus which caused the mouse to scratch the injected site by the hindpaws. Effects of various analgesics and other related drugs on this behavior were determined by counting the number of mice with scratching response within 5 min after injection of 4 AP. The narcotic analgesics morphine (10 mg·kg 1 , sc), pethidine (25 mg·kg 1 , sc) and fentanyl (0.14 mg·kg 1 ) inhibited the scratching response markedly. The mild analgesics rotundine, analgine, sodium salicylate and the nonanalgesics were inactive. Local
anesthetics procaine (0.25%), lidocaine (0.1%) or tetracaine (0.01%) coinjected sc with 4 AP also abolished the scratching behavior. When procaine and 4 AP were injected sc into two separated sites, no effect on scratching response was observed. The results suggest that the inhibition of 4 AP induced scratch may provide a simple method for assessing analgesic activity and local anesthetic activity of drugs.