Kazufumi Kubota, Mikie Kubota-Watanabe, Kenji Fujibayashi and Koji Saito
Biological Research Laboratories, Sankyo Co., Ltd., 2-58, Hiromachi 1-chome, Shinagawa-ku, Tokyo 140-8710, Japan
Abstract: In neonatal rats, nociceptive responses induced by capsaicin were characterized pharmacologically. Capsaicin, injected subcutaneously (s.c.), induced body movement including scratching and struggling responses, and the responses were quantified by using a device composed of an audio speaker as a detector. The capsaicin-induced body movement was inhibited by a tachykinin NK1-receptor antagonist RP-67580 with an ID50 value of 3.5 mg/kg, s.c. Opioid analgesics, morphine, buprenorphine and pentazocine, also inhibited the body movement with ID50 values of 0.085, 0.0079 and 0.92 (mg/kg, s.c.), respectively. Non-steroidal anti-inflammatory drugs, indomethacin, ibuprofen and acetaminophen, did not exert any effect on the capsaicin-induced body movement. Neither the sedative diazepam nor the sedative chlorpromazine inhibited the body movement. It is concluded that the capsaicin-induced body movement in neonatal rats, which is considered to be nociceptive responses mediated by substance P, is sensitive to centrally acting analgesics with mu-opioid receptor agonist activity.
Keywords: Capsaicin, Analgesia, Opioid, Quantification of nociceptive response, Neonatal rat