Effects Of Morphine On Acid-sensing Ion Channels And The Molecular Mechanisms

Background: Morphine is one of the major alkaloids extracted from the fruit of Papaver somniferum (content of about 10-15%). It has been widely used as an analgesic, sedative and antidiarrheal agent. It aiso could treat cardiac asthma. Recently, several studies have demonstrated effects of morphine upon activating K+ channels and inhibiting voltage-gated Ca2 + channels. However, no detailed studies have been performed concerning the effects of morphine on the acid-sensing ion channels (ASICs). The aim of our study is to characterize the role of morphine on current and protein expression of acid-sensing ion channels in rat trigeminal ganglions and to elucidate the molecular mechanisms of these changes.Methods: Whole-cell patch clamp technique was used to examine the effects of morphine on the current amplitude of ASIC1 and ASIC3 in isolated rat trigeminal ganglion neurons. In animal experiments, rats in two control groups were administrated with 0.9% normal saline, and rats in two experimental groups were administrated with 4% formalin solution.One control group and one model group were injected intraperitoneally with morphine.Western blot technique was employed to analyze the protein expression of ASIC1 and ASIC3 in rat trigeminal ganglions of all groups.Results: Morphine can decrease the current amplitude of ASIC1 and ASIC3 in isolated rat trigeminal ganglion neurons in a concentration-dependent manner. Morphine can decrease the protein expression of ASIC1 and ASIC3 in both control group and formalin-induced inflammation group.Conclusion: The findings demonstrate that morphine relieved the pain partly through its modulation on the current amplitude and protein expression of acid-sensing ion channels. Background: Opioid receptors are G protein-coupled receptors and their ligands are opioid peptides. Opioid receptor family consists of a lot of subtypes, among which the most classical types areμreceptor,δreceptor andκreceptor. Opioid receptors distribute widely in the body and have complex biological effects. Morphine could open some ion channels on the neuron membranes by acting on opioid receptors, then cause the transmembrane transport of certain ions to exert analgesic effect. The aim of this part is to characterize the role of opioid receptors during the process of morphine regulating the current amplitude and protein expression of acid-sensing ion channels and to elucidate further the analgesic mechanisms of morphine.Methods: Whole-cell patch clamp technique was used to examine the effects of morphine (0.3μM/L, 1μM/L, 3μM/L,10μM/L) on current amplitude of ASIC1a in Chinese Hamster Ovary cells. In animal experiments, rats in two control groups were administrated with 0.9% normal saline, and one of them was injected intraperitoneally with naloxone additionally. Rats in three model groups were administrated with 4% formalin solution. One model group was injected intraperitoneally with morphine only, another was carried out the same thing after injected intraperitoneally with naloxone for 10 minutes and another was done nothing. Western blot technique was employed to analyze the changes of ASIC1 and ASIC3 protein levels in rat trigeminal ganglions of all groups.Results: Morphine can decrease the current amplitude of ASIC1a in CHO cells in a concentration-dependent manner.The IC50 of the dose-response curve is 1.35±0.57μmol/L and the Hill coefficient is 1.08±0.37. Morphine can decrease protein expression of ASIC1 and ASIC3 in formalin model groups whether naloxone presence or not.Conclusion: These findings demonstrate that the analgesic effect of morphine is partially through its inhibition effect on the current amplitude and protein expression of ASICs. Meanwhile, this modulation effect is independent of opioid receptors.