Role of central and peripheral mGluR5 receptors in post-operative pain in rats

Metabotropic glutamate receptors (mGluRs) have previously been shown to play a role in pain transmission during inflammatory or neuropathic pain states. However, the role of mGluR5 in post-operative pain remains to be fully investigated. The present study was conducted to characterize analgesic activity of 2-methyl-6-(phenylethynyl)-pyridine (MPEP) in the skin-incision-induced post-operative pain model in rats. MPEP is a potent and selective mGluR5 antagonist with high affinity (K-i = 6.3 +/- 0.9 nM) in rat cortex using [H-3]-MPEP as a radioligand, while not competing with the mGluR1-selective radioligand [H-3]-R214127 (K-i > 10,000 nM) in rat cerebellum. Post-operative pain was examined 2 h following surgery using weight-bearing (WB) difference between injured and uninjured paws as a measure of nonevoked pain. In this model, MPEP, as morphine, showed dose-dependent effects and full efficacy after systemic administration (ED50 = 15 mg/kg, i.p. for MPEP, ED50 = 1.3 mg/kg, s.c. for morphine). In addition, intrathecal (i.t.) and intracerebroventricular (i.c.v.) MPEP reduced WB difference (ED50 = 65 mu g/rat i.t. and ED50 = 200 mu g/rat i.c.v.). Interestingly, intraplantar (i.pl.) injection of MPEP either before or after surgery induced a similar reduction in WB difference (ED50 = 90 mu g/rat, i.pl.) while contralateral i.pl. MPEP injection did not produce any effect. These results demonstrate that both peripheral and central mGluR5 receptors play a role in nociceptive transmission observed during post-operative pain. In addition, the data suggest that mGluR5 antagonists could offer a new therapeutic approach to the treatment of post-operative pain. (c) 2004 International Association for the Study of Pain, Published by Elsevier B.V. All rights reserved.