Influence of Y-20003, an analgesic agent, on the endogenous opioid peptide system in rats

Changes of endogenous opioid peptide system by pain stimuli were examined using adjuvant-induced arthritic rats. Following the 5th day after adjuvant inoculation, met-enkephalin (ME)-like peptide content in plasma decreased, while after the 12th day, the peptide content increased in adrenal medulla. A significant correlationship was found between pain threshold and plasma ME-like peptide content. Y-20003, a new nonsteroidal agent without inhibition of prostaglandin biosynthesis, showed an analgesic effect which was significantly antagonized by naloxone or Win 44,441-3, suggesting that endogenous opioid peptide system may be involved in the analgesic mechanism of Y-20003.