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

Abstract

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 ID 50 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.