The Role Of Inflammatory Response In Gut Microbiota Imbalance And Neurotransmitter Metabolism Disorder In Depressed Mice

Objective : The neurotransmitter hypothesis is currently accepted to cause depression.It shows that the reduction of monoamine neurotransmitters is an important biological basis for depression.Recently,theeffects of brain-gut axis in depression development attracted more attention,especially the crosstalk between inflammation and neurotransmittors.However,the interaction mechanism of inflammation response,gut microbiota and neurotransmitter in the occurrence and development of depression is still poorly understood.In the current study,a mouse model of depression-like behaviorestablished by chronic restraint stress(CRS)was intervened with the anti-inflammatory drug dexamethasone to examine the effects of complications in depressive behavior on the gut microbiota and neurotransmitters,and explore the interaction among the gut microbiota,neurotransmitters and depressive behaviors.Therefore,it will be beneficial to decipher the potential mechanisms of depression,and provide new targets for clinical prevention and treatment of depression.Methods:Forty male C57BL/6J mice were divided into 4 groups randomly: control group,dexamethasone group,CRS group,CRS+dexamethasone group.The mice in the CRS group were given chronic restraint stress stimulation for 5 weeks to establish a depression mouse model.The mice in dexamethasone group were given1 mg/kg dexamethasone solution daily,the control group was given the same volume of saline.Depressive-like behavior tests,including the sucrose preference test,open field test,elevated plus maze and forced swimming test were conducted.The protein expression of pro-inflammatory factors IL-1?,IL-2,IL-6,TNF-?;brain-derived neurotrophic factor(BDNF)and the NF-?B signaling pathway in hippocampus,prefrontal cortex,and striatum were tested by western bolt.The immunohistochemical assay was performed to further examine the expression of IL-1?,IL-2,IL-6,TNF? and BDNF in hippocampal CA1,prelimbic cortex and striatum.The levels of monoamine neurotransmitters and the corresponding metabolites in mouse plasma were detected by ultra-high-performance liquid chromatography-mass spectrometry(UHPLC-MS/ MS),and the distribution of bacterial flora in murine feces was detected by 16 S r RNA gene sequencing.Results:1.The behavioral tests showed that CRS decreased sucrose preference and shorted the time in the open field central square.Meanwhile,it increased time of closed armsin the elevated plus maze and immobility time in forced swimming.Western bolt results showed that CRS increased the expression of inflammatory mediators in hippocampus,prefrontal cortex and striatum,and decreased the expression level of BDNF.The intervention of dexamethasone partly reversed the inflammatory level in those three tissues,and increased the expression level of BDNF.2.The neurotransmitter metabolomics results showed the levels of plasma 5-HT,DA,NE and the corresponding metabolitesdecreased in CRS mice.The gut microbiota fecal metabolites with the 16 S r RNA gene sequencing indicated that CRS caused marked microbiota dysbiosis in mice,with a significant increase in Helicobacter,Lactobacillusand Oscillibacterwhile a decrease in Parabacteroides,Ruminococcus,and Prevotella.Notably,CRS-induced depressive behaviours,disturbance of neurotransmitter metabolism and microbiota dysbiosis can be substantially restored by DXMS administration.Furthermore,a Pearson heatmap focusing on correlations among the microbiota,behaviours and neurotransmitters showed that Helicobacter,Lactobacillus,and Oscillibacter were positively correlated withdepressive behaviours but were negatively correlated with neurotransmitter metabolism.Parabacteroides and Ruminococcus were negatively correlated withdepressive behaviours but were positively correlated with neurotransmitter metabolism.Conclusions:1.CRS induced depression-like behavior in mice and inflammatory response,and reduced BDNF levels,so as to significantly reverse the inhibition of the inflammatory response.This phenomenon indicated that CRS may promote inflammatory response,resulting in the reduction of BDNF levels,and thereby damaging central nervous system,which contributed to the occurrenceof depression-like behavior.2.CRS induced significant flora imbalance and significant reduction of monoaminergic neurotransmitters and related metabolites in mice.The above changes have been partially improved after dexamethasone anti-inflammatory treatment,suggesting that anti-inflammatory treatments can not only directly inhibit inflammation,but possibleindirectly improve the level of monoaminergic neurotransmitters in depressed mice through the intestinal flora-inflammation axis,so as to improve depression-like behaviors in mice.