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