The Clinical Association And Underlying Mechanisms Researches Of Neurotransmitters In Major Depression

Background:Major depressive disorder(MDD)is a debilitating brain-psychiatric illness.However,there is currently no objective laboratory-based diagnostic tests for this disorder.Although,some hypotheses have been implicated in MDD,the perturbation of neurotransmitter is a prominent feature of the disease.Currently,a comprehensive global evaluation of the relationship between neurotransmitter and diagnosis and severity of depression has not yet been performed.Our previous studies found that disruption of gut microbiota was a potential new cause of depression.via the “microbiota-gut-brain” axis.The gut microbiota may regulate the neurotransmitter system via the “microbiota-gut-brain” axis.Objectives:1.We sought to characterize metabolite changes in the early stage of MDD to identify potential diagnostic biomarkers for depression and evaluate its clinical diagnostic performance for MDD.2.We used Germ-free mice(GF)to evaluate how the gut bacteria shape hippocampal neurotransmitters system expression,and monitored behavior changes due to absence of gut microbiota.Methods:1.Using a GC-MS coupled with LC-MS/MS-based targeted metabolomics approach,we simultaneously quantified the levels of 24 plasma metabolites involved in GABAergic,catecholaminergic,and serotonergic neurotransmitter systems in 50 first-onset drug-na?ve MDD subjects and 50 healthy controls to identify potential metabolite biomarkers for MDD(training set).2.An independent sample cohort comprising 49 MDD patients,30 bipolar disorder(BD)patients and 40 healthy controls(testing set)was used to validate diagnostic generalizability and specificity of the candidate biomarkers.3.The expression of neurotransmitter genes in the hippocampus of germ-free(GF)mice and specific-pathogen-free(SPF)mice were detected by real-time PCR microarrays to explore the effect of gut microbiota on the expression of hippocampal neurotransmitter related gene.The serum and hippocampal neurotransmitters and their metabolites of GF and SPF mice were detected by targeted metabolomics.4.Western Blotting and behavioral tests,including open field test(OFT)and novelty suppressed feeding test were proceed to validate the differential genes.5.A set of GF mice recolonized with commensal microbiota in adolescence were used to determine whether the altered genes,proteins,metabolism,and behavioral phenotypes could be reversed by microbiota colonization.Results:1.The neurotransmitter metabolites were significantly changed in MDD subjects.These metabolites were mainly involved in GABAergic,catecholaminergic and serotonergic systems.The GABAergic and catecholaminergic had better diagnostic value than serotonergic pathway.2.A panel of four candidate plasma metabolite biomarkers(GABA,dopamine,tyramine,kynurenine)could distinguish MDD subjects from health controls with an AUC of 0.968 and 0.953 in the training and testing set,respectively.Furthermore,this panel distinguished MDD subjects from BD subjects with high accuracy.3.27 differential expressed genes were identified between GF and SPF mice.Bioinformatic analysis shows these gut microbiota related genes are highly enriched for gene sets related to 12 neuropsychiatric disorders,such as depression,anxiety,schizophrenia,and Huntingdon's disease.KEGG Mapper analysis show that dopaminergic synapse and serotonergic synapse were significantly altered in GF mice.Consistent with the results of PCR array,western blotting showed that the expression of dopa decarboxylase,monoamine oxidase B and dopamine receptor D3 in the hippocampus of GF mice were up-regulated.The absence of gut microbiota can significantly affect metabolism of peripheral and central neurotransmitters.The levels of dopamine,tyramine,3-hydroxyanthranilic acid,tryptamine in serum of GF mice were increased compared with SPF mice,while the levels of SA,?-ketoglutaric acid,glutamine,glutamic acid,3,4-dihydroxyphenylacetic acid,L-tyrosine,and N-acetyl-serotonin were decreased.The levels of glutamine and 3-hydroxyanthranilic acid in hippocampus of GF mice were up-regulated4.In the open field test,GF mice significant increased in total move distance and center distance compared to SPF mice.In novelty suppressed feeding test,GF mice significant decreased in latency to feed relative to SPF mice.5.15 altered genes were reversed by microbiota colonization,while the altered proteins,metabolism,and behavioral phenotypes could not be reversed.Conclusions:1.This study is the first to globally evaluate multiple neurotransmitters in MDD plasma.The altered plasma neurotransmitter metabolite profile has potential differential diagnostic value for MDD.2.The absence of commensal microbiota during early life markedly affects peripheral and hippocampal neurotransmitters,which may be a new potential factor for depression.