Abstract
Changes of endogenous opioid peptide system by pain stimuli were examined using adjuvant-induced arthritic rats. Following the 5th day after adjuvant inoculation, the met-enkephalin-like peptide content in plasma was significantly decreased while after the 12th day, the peptide content was markedly increased in adrenal medulla. Significant correlationships were found between pain threshold and met-enkephalin-like peptide contents in adrenal medulla and plasma. It was suggested that a long term of lowering of plasma opioid peptide level was necessary to lower the pain threshold, and was followed by compensatory promotion of the opioid peptide processing in adrenal medulla. Y-20003, a new nonsteroidal antiinflammatory agent without inhibition of prostaglandin biosynthesis, showed an analgesic effect which was significantly antagonized by naloxone or Win 44, 441-3, suggesting that the endogenous opioid peptide system may be involved in the analgesic mechanism of Y-20003.
