The role of prostaglandins in the nociceptive response induced by intraperitoneal injection of zymosan in mice

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Abstract
1 Intraperitoneal injection of zymosan (1 mg in 0.5 ml saline) in mice induces a transient writhing response accompanied by the synthesis of small amounts of prostaglandin E2 (PGE2, < 2 ng) and larger amounts of PGI2 (200 ng per mouse), measured as its non-enzymatic breakdown product, 6-keto-PGF1α. 2 Although both centrally-acting analgesics (morphine, clonidine) and prostaglandin biosynthesis inhibitors (aspirin, indomethacin, ibuprofen) blocked the writhing response to intraperitoneal injection of zymosan, only the latter reduced prostaglandin levels in the peritoneal cavity. 3 The writhing response correlated equally well with PGE2 levels and 6-keto-PGF1α levels when data from mice treated with centrally-acting analgesics were excluded. However, intraperitoneal injection of PGI2, but not PGE2, reversed the analgesia induced by indomethacin in zymosan-injected mice. 4 Centrally-acting agents, but not ibuprofen, blocked the ability of PGI2 to reverse the analgesic activity of indomethacin. 5 PGI2 (2 μg per mouse), injected intraperitoneally in otherwise untreated mice, induced writhing. 6 These data indicate that PGI2 is the prostaglandin involved in mediation of the writhing response to zymosan and that prostaglandin biosynthesis inhibitors, but not centrally-acting analgesics, exert their analgesic activity by reducing the peritoneal level of PGI2. It is possible that PGI2 may have the ability to stimulate pain receptors directly in the mouse peritoneal cavity, in addition to its previously recognized ability to sensitize pain receptors to other pain-producing stimuli.