A novel method for detecting the analgesic effect of drugs by quantifying body movement after noxious stimulation in...

We developed a method for quantifying nociceptive struggling in neonatal rats induced by bolus, subcutaneous injection of capsaicin (3-3000 ng). The response was quantified by using an audio speaker and electrical instruments such as an amplifier, a rectifier, and a monostable multivibrator. Using this method, we were able to quantify the nociceptive response which appeared immediately after injection of capsaicin. The response peaked at 0-1 min, and then decayed during next 1-3 min. Furthermore, this method also detected that the magnitude of the response increased dose-dependently up to the maximum dose of 3000 ng. In an experiment testing the suitability of this method for screening analgesics, it was shown that morphine (0.03-0.3 mg/kg, s.c.) inhibited the response dose-dependently with an ID50 value of 0.089 mg/kg and that the analgesic effect of morphine (1 mg/kg, s.c.) was reversed by naloxone (0.03-0.1 mg/kg, s.c.). Thus it can be concluded that this method is useful to quantify the capsaicin-induced nociceptive response and is suitable for screening analgesics.