The Mechanism Of Glucocorticoid Receptor In The Development Of Chronic Morphine Tolerance

Objective: Opioids are commonly used in treatment of acute and chronic pain, but long-term application can lead to chronic opioid tolerance. Clarifying the mechanism of opioid tolerance is important to its prevention and treatment. There have been many articles that illustrate the mechanism of morphine tolerance. Recent reports indicate that glucocorticoid receptor in spinal cord is involved in the development of chronic opioid tolerance. But the mechanisms in cellular level are still unknown.Whether the activation of ERK is regulated by spinal glucocorticoid receptor after chronic morphine treatment has not been reported. In this dissertation, we study the effect of glucocorticoid receptor in morphine tolerance, anticipating providing important information for future treatment and studies associated with morphine tolerance.Methods:Experiment one, sixty Sprague-Dawley rats weighed 300～350g were randomly divided into 4 groups (n= 15):control group (C group),chronic morphine tolerance group (M group), morphine+glucocorticoid receptor antagonist group (MR group) and morphine+glucocorticoid receptor agonist group (MD group). They were intrathecally (i.t.) administrated saline 10μl, morphine 10μg, morphine 10μg+ RU38486 2μg, morphine 10Lg+dexamethasone 4μg respectively twice daily for 6 days. Tail flick test and hot plat test are done on 8:30 on day 1,3,5,7. At the end, the rats are euthanasiaed.The segment of spinal L3～L5 were taken away for Western blotting (each group 5 rats) and immunofluorescence staining (each group 5 rats) to examine the expression of glucocorticoid receptor, phosphorylated ERK1/2 and mu opioid receptor in the spinal cord dorsal horn. TUNEL was used to test the spinal apoptotic cells (each group 5 rats).Experiment two, twenty rats were randomly divided into 4 groups (n= 5): Vehicle group, chronic morphine tolerance group (M group), PD98059+morphine+ dexamethasone (PDM group), PD98059+morphine (PM group). PD98059 is an inhibitor of an upstream kinase in ERK pathway, and morphine or dexamethasone was injected 30min after the injection of PD98059. All drugs are given intrathecally twice daily for 7 days. Tail flick test was taken 30min after administration every morning to evaluate their thermal hyperalgesia.Results:After the continuous injection of morphine, the latency of both tail flick and hot plat decline gradually. The morphine tolerance was inhibited by RU38486 and Dex facilitated the development of morphine tolerance. Compared with C group, the expression of glucocorticoid receptor and p-ERK1/2 were up-regulated and the mu opioid receptor was down-regulated in M group; compared with M group, the expression of glucocorticoid receptor was up-regulated and mu opioid receptor and p-ERK1/2 were down-regulated in MD group. There is co-existence of GR and p-ERK 1/2 in spinal cord dorsal horn by immunofluorescence double staining. TUNEL staining showed that compared with the C group, apoptotic rate of neurons in M and MD group increased (P<0.05); compared with the M group, apoptotic rate of neurons decreased in MR, but increased in MD Group (P<0.05). The result of experiment 2 shows that pretreatment of PD98059 (10μg) delayed the development of chronic morphine tolerance.Conclusion:Spinal glucocorticoid receptor may play a significant role in the development of morphine tolerance through the ERK signal pathway. Moreover, glucocorticoid receptors contribute to morphine tolerance by inducing neuronal apoptosis in spinal cord dorsal horn.In summary, the model of chronic morphine tolerance is established by intrathecal administration of morphine. In our studies, we find that glucocorticoid receptor plays an important role in the development of morphine tolerance through the ERK signal pathway, indicating that we can change the activation of glucocorticoid receptor to prevent and treat morphine tolerance.