A Study About Mechanism Of Morphine-induced Antinociceptive Tolerance Related With Excitatory Glutamate Receptors

Objective: Morphine is one of the most powerful and extensive drugs used in pain management.Chronic morphine-induced tolerance and hyperalgesia reduce their analgesic efficacy,and also limit their long-term use especially in the patients who suffer from chronic pain,such as neuropathic pain.The aim of this study is to investigate the crosstalk between mGluR5 and NMDAR during tolerance process,and how analgesic effect of chronic morphine presents under disruption of the link between mGluR5 and NMDAR.Methods: In the first part,repeated morphine treatment through intrathecal injection(i.t.10 μg/10μl twice daily for 5 days)was used to induce anti-nociceptive tolerance and thermal hyperalgesia,which was determined by tail-flick latency and calculated as percentage of maximum possible effects(%MPE).Expression of NMDAR subunits(NR1,NR2 A and NR2B)and phosphorylation of NR2 B in cortex were evaluated by Western Blot.Colocalization of mGluR5,PSD-95,Shank3 and NR2 B were tested by immunofluorescence.In part two,mGluR5 knockout(KO),Shank3 ?C mutant mice and their littermate controls were used to study the relationship between postsynaptic proteins and morphine tolerance.Behavior test and Western Blot were repeated in different mouse lines.Results: In part one,the mouse model of morphine tolerance was successfully established.Chronic morphine treatment increased the total expression of mGluR5 and NR2 A,but not NR1 and NR2 B in the cortex,and an enhanced phosphorylation of NR2 B was also observed in wildtype mice after tolerance developed.In part two,mGluR5 knockout could attenuate morphine-induced tolerance and thermal hyperalgesia,and also inhibit chronic morphine-induced phosphorylation of NR2 B,but not total NR2 A expression.Shank3 ?C mice were used to imitate loss of physiological connection between mGluR5 and NMDAR.The mice showed a similar analgesic effect of morphine as their littermate control,which didn’t affect morphine-induced NMDAR subunits alterations as compared with control in the development of morphine-induced tolerance.Conclusions: mGluR5 may regulate NMDAR through increasing the activation of NMDAR especially the phosphorylation of NR2 B instead of through the physiological link to induce the anti-nociceptive effect induced by chronic morphine treatment in mice cortex.