The Role And The Mechanism Of Metformin Inhibting Morphineconditioned Place Preference Through Glucagon-like Peptide-1 Receptor

Opioid drugs are widely used as analgesics in clinic,and its long-term use can lead to addiction.Opioid addiction has always been a difficult problem on clinical medicine.The specific mechanism of opioid addiction,such as morphine is still not very clear.Morphine addiction is a brain disorder,which is closely related to various brain areas,including the ventral tegmental area(VTA),nucleus accumbens(NAc),prefrontal cortex(PFC)and hippocampus(Hip).Metformin(Met)is the most commonly used prescription drug for treatment of type 2 diabetes(T2DM),through reducing hepatic gluconeogenesis and promoting the use of peripheral glucose to decrease glucose in blood.Studies have shown that metformin plays the roles in treating polycystic ovarian syndrome(PCOS),nonalcoholic fatty liver disease(NAFLD),and small cell lung cancer(SCLC)and pancreatic cancer in addition to T2DM.Glucagon-like peptide-1(GLP-1)is a brain-intestinal peptide substance secreted by ileal endocrine cells in the body.Proglucagon gene,(GCG)is formed by encoding and promotes the synthesis of insulin in the body.GLP-1 functions mainly through the GLP-1R.Studies have shown that GLP-1R is the target of metformin in vivo,and GLP-1R is also the target of T2DM drugs.The previous results showed that the expression of GCG was changed after the formation of conditioned place preference(CPP)induced by morphine in mice,which suggests that GLP-1R plays a key role in the formation of CPP.Exendin-(9-39)is a specific competitive antagonist of GLP-1R,which inhibits the insulinotropic effect of GLP-1.GLP-1 can affect the survival of neurons.Thioredoxin-1(Trx-1)is a redox regulatory protein,which can catalyze various redox reactions through its active center.The previous study of our research group found that Trx-1 overexpressed transgenic mice resist the formation of CPP caused by morphine and methamphetamine.The purpose of this study is to explore the role of metformin in suppressing formation of morphine-CPP and clarify the mechanism on metformin inhibiting the espressions of morphine addiction-related molecules through GLP-1R.We used the morphine intraperitoneal injection to construct morphine-CPP.Metformin was intragastrically administrated before morphine-CPP construction.Before inducing the formation of morphine-CPP,the metformin was administrated for one week,and then the morphine-CPP model of mice was constructed.The differences in CPP scores of the control group,morphine group,morphine+metformin group and metformin group were compared.Secondly,GLP-1R inhibitor,Exendin-(9-39)was used to study the role of GLP-1R in metformin suppressing morphine-CPP.The differences in CPP scores of control group,morphine group,morphine+metformin group,morphine+metformin+inhibitor group and morphine+inhibitor group were compared.The expressions of addiction-related molecules in the VTA were detected by western blot(WB).Our present study found that metformin reduces the CPP score induced by morphine in mice.The expressions of GLP-1R andγ-aminobutyric receptor B,(GABA_BR)in the VTA were decreased by morphine,which were restored by metformin.The level of p-AMP-activated protein kinase(p-AMPK),the expressions D1 and Trx-1 were increased by morphine in the VTA,which were inhibited by metformin.After Exendin-(9-39)administration,the CPP scores of mice in the morphine+metformin+inhibitor group were higher than morphine+metformin group,which suggests that Exendin-(9-39)inhibits the effects of metformin on morphine-CPP.Moreover,Exendin-(9-39)inhibits the effects of metformin on the above addiction-related molecules in the VTA.In conclusion:this study shows that morphine-CPP was suppressed by metformin.Metformin suppressed morphine-CPP through increasing GLP-1R in the VTA.This study provides new idea and target for metformin treating morphine addcition.