The Study Of Intracellular Mechanism Of Glucocorticoid Receptor Involved In Chronic Morphine Tolerance

Opioids, such as morphine, are the most effective analgesics clinically used in treatment of acute and chronic pain, but the application is hindered by analgesic tolerance and hyperalgesia. Since opioids are the main medicine for the treatment of pain, clarifying the mechanism of opioid tolerance and recovering their efficiency are very significant to clinical use. 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.The phosphatidylinositol-3-kinase/protein kinase B (PI3K/Akt) signaling pathway is widely spread in different cells and is one of the important signaling pathways intracellularly. It plays a very pivotal role in inhibiting apoptosis and promoting proliferation of cells though affecting the activation state of the downstream effector molecules.It is reported that there is neuronal apoptosis in the cytotoxic mechanism of morphine tolerance. Also, there are studies pointing out that the glucocorticoid receptor agonists might affect the activity of PI3K/Akt. It is still unclear that how glucocorticoid receptor is involved in intracellular mechanism of chronic morphine tolerance, furthermore, whether the PI3K/Akt signaling pathway is regulated by spinal glucocorticoid receptor following 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.Objective:To study the effect of glucocorticoid receptor in the development of morphine tolerance and its intracellular signaling pathway.Methods:Eighty male Sprague-Dawley rats weighed280～320g were randomly divided into4groups (n=20):control group (group C), chronic morphine tolerance group (group M), morphine+glucocorticoid receptor agonist group (group MD) and morphine+glucocorticoid receptor antagonist group (group MR). They were intrathecally (IT) administrated saline10μ1, morphine10μg, morphine10μg+dexamethasone4μg, morphine10μg+RU384862μg respectively twice daily for6days. Tail flick test was done on8:30on day1,3,5,7. At the end, the rats were euthanasiaed. The segment of spinal L3-5were taken away for Western blot (each group6rats) and immunofluorescence staining (each group6rats) to examine the expression of PI3K, phosphorylated Akt and Caspase-3in the spinal cord dorsal horn.Results:After the continuous injection of morphine, the latency of tail flick declined gradually and this indicated the chronic morphine tolerance models were successful. The morphine tolerance was delayed by RU38486and Dex facilitated the development of morphine tolerance. Compared with group C, the expression of PI3K and pAkt were down-regulated and Caspase-3was up-regulated in group M; Compared with group M, the expression of PI3K and pAkt were up-regulated and Caspase-3was down-regulated in group MR. Conversely, the expression of PI3K and pAkt were down-regulated and Caspase-3was up-regulated in group MD. There is co-existence of GR and PI3K in spinal cord dorsal horn by immunofluorescence double staining.Conclusion:Spinal glucocorticoid receptor may play a significant role in the development of morphine tolerance through the PI3K/Akt signaling pathway. Moreover, this effect may be closely associated with 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 PI3K/Akt signaling pathway, indicating that we can change the activation of glucocorticoid receptor to prevent and treat morphine tolerance.