Abstract
A novel analgesic test using neonatal rats was developed. In neonatal rats, body movement was induced by bolus, subcutaneous (s.c.) injection of algesic substances. The body movement was quantified by using a device composed of an audio speaker as a detector. Capsaicin, injected s.c., induced the body movement including scratching and struggling responses. The response peaked at 0-1 min and then decayed during the next 1-3 min. Furthermore, this method also detected that the magnitude of the response increased dosedependently up to the maximum dose of 3000 ng. Endogenous algesic substances such as bradykinin, serotonin and histamine induced responses similar to those induced by capsaicin. Exogenous algesic substances such as formalin and acetic acid evoked more prolonged responses than those induced by capsaicin and endogenous substances. The effects of drugs on the capsaicin-induced response were examined in the following experiments. Opioid analgesics, morphine, buprenorphine and pentazocine, inhibited the response. Non-steroidal anti-inflammatory drugs, indomethacin, ibuprofen and acetaminophen, did not exert any effect on the response. Non-steroidal ani-inflammatory drugs, indomethacin, ibuprofen and acetaminophen, did not exert any effect on the response. Neither the sedative diazepam nor the sedative chlorpromazine inhibited the response. It is concluded that this method is useful to quantify the body movement induced by noxious stimulation and is suitable for screening centrally acting analgesics with μ-opioid receptor agonist activity.
