MDD is a complex, heterogeneous psychiatric disorder, partly attribute to secondary effects of illness chronicity and/or antipsychotic medication. Therefore, we chose first-episode, antidepressant drug-naïve to reduce the samples heterogeneity29. Presently, the diagnosis of MDD remains primarily subjective. Therefore, people who are depressed are often not correctly diagnosed, and others who do not have this disorder are too often misdiagnosed and prescribed antidepressants30,31. A major barrier to effective care is inaccurate assessment32. The aim of this study was to examine the feasibility of an empirical laboratory-based method to diagnose MDD. Here, by targeted assessment of plasma metabolites from multiple neurotransmitter systems, we identified a plasma neurometabolite signature able to distinguish first-episode, antidepressant drug-naı̈ve depressed patients from healthy controls. Moreover, this biomarker panel was able to accurately diagnose blinded samples with both high sensitivity and high specificity.

Numerous recent studies have identified hundreds of potential biomarkers for depression; however, their roles in depressive illness are unclear and they have been unable to enhance diagnosis, treatment or prognosis33. This lack of progress is partially due to the heterogeneity of depression, in conjunction with methodological heterogeneity within the published papers34. The most prominent molecular endophenotypes and biomarkers of depression are neurotransmitters, including dopamine and GABA, and components of the serotonin pathway35,36,37. This is the first report to globally evaluate multiple neurotransmitters in the plasma of MDD patients, although changes in neurotransmitter levels have been implicated in many neuropsychiatric diseases. Our previous studies also found disturbance of some neurotransmitters in MDD animal models and patients26,38,39,40,41,42. Therefore, in the present study, metabolites involved in GABA, catecholamine and tryptophan metabolism in the plasma of depressed subjects and healthy controls were assessed by targeted metabolomics to identify those that are significantly differentially expressed in MDD subjects. Furthermore, the combination of GC-MS and LC-MS/MS used here can enhance detection and overcome their individual disadvantages.

In clinical practice, BD cases are often misdiagnosed as MDD because of the similarity in clinical symptoms43. Recently, researchers have investigated the psychopathological characteristics of bipolar and unipolar depression44 and found different pathophysiologic processes underlying the depressive episodes in MDD and BD, especially in the neural circuitry regulating emotion, reward and attention45. Our group previously identified candidate biomarkers for diagnosing MDD11,12,46,47,48,49 and BD50,51,52,53, respectively. These biomarkers are capable of accurately distinguishing MDD and BD patients from healthy controls. However, it remained unknown whether these biomarkers can be used to differentiate MDD from BD. To address this issue, 30 BD subjects were also recruited in the current study to validate the diagnostic specificity of the biomarker panel. We found that this diagnostic panel could effectively discriminate the 49 MDD subjects from 30 BD subjects, with an AUC of 0.901 (95% confidence interval: 0.813–957).

Here, we found that plasma GABA levels in MDD subjects were increased in cohorts 1 and 2. GABA is an inhibitory transmitter that has long been associated with mental illnesses, including anxiety, depression, and schizophrenia54,55,56. Studies of patients and animal models increasingly suggest a key role for functional imbalances between the major excitatory and inhibitory neurotransmitters, including GABA and its receptors57. Dysregulated GABA neurotransmission in MDD has been reported in the plasma, CSF and cortex of depressed subjects58,59,60. Consistent with our results, environmental factors, including stress and excessive alcohol use, may increase GABA, causing symptoms of depression or mania61. Indeed, the panel of biomarkers in the GABA pathway (SA, GABA, α-KG, and Gln) effectively discriminated MDD subjects from healthy controls, with an AUC of 0.904, suggesting that perturbations in GABAergic neurotransmission may be causal for depressive disorders.

Many diseases such as depression, BD, Parkinson’s disease and attention deficit hyperactivity disorder are associated with abnormal catecholamine neurotransmitter levels. Kunugi et al.62 proposed a subtype of depressed patients: a dopamine-related subset of patients who present with anhedonia and respond well to aripiprazole. Indeed, the presence of such subgroups might underlie the discrepancies between previous studies, and furthermore, they highlight the need for stratified treatment. Zhao et al.63 reported that dopamine dysfunction in depressive patients might be a sign of diathetic depression or a depressive subtype, with medication unable to alter dopamine levels. In the present study, we systematically evaluated the plasma concentrations of dopamine and its metabolites, and found that plasma dopamine concentration was upregulated in depressed patients. This is in keeping with a previous report64, showing a marked increase in plasma and urinary norepinephrine in patients with major affective disorders, such as depression, bipolar depression, and unipolar depression. Furthermore, abnormal catecholaminergic neurotransmitters levels were detected in the prefrontal lobe of a depressed mouse model in our previous study26. Collectively, these findings suggest that plasma catecholamine neurotransmitters are comparatively reliable biological markers for MDD.

We also found altered tryptophan metabolism in MDD subjects. Numerous studies suggest that brain serotonin plays a critical role in patients with depression, and the relationship between tryptophan metabolism intermediates and depression has recently been highlighted65,66. Several authors have described non-targeting metabolomics methods to determine the concentration of metabolites in the 5-HT and kynurenine pathways67,68. Here, we used a reliable targeted metabolomic method using GC-MS combined with LC-MS/MS to quantitate Trp, Kyn, 5-HT, 5-HIAA, 3-HA, NAS, and Tra. We found that the levels of Trp and Kyn were decreased in the plasma of first-episode, antidepressant drug-naïve depressed subjects compared with healthy controls. In line with this speculation, a recent study showed that plasma metabolites related to the kynurenine pathway are downregulated during high suicidal ideation58. However, in this study, the diagnostic efficacy of serotonergic pathway was not good as GABAergic and catecholaminergic pathway, suggesting that single peripheral serotonergic system could not discriminate MDD from healthy controls well. Serotonergic system combined with other neurotransmitters may performed better as a biomarker in diagnosis of MDD. The biomarker panel in this study, involving three pathways, can discriminate depressed patients from healthy controls and BD subjects with high accuracy.

Limitation

There are some limitations that should be noted in this study. First, the altered neurotransmitters identified in this study should be validated by metabolomic analysis of cerebrospinal fluid or brain tissues obtained from depressed patients. Second, as the MDD is a heterogeneous psychiatric disorder, we could not cover all the subtype. The subjects may not be very well described in clinical dimensions, although all the subjects were recruited with relatively strict criteria. Further studies using a larger sample size with more detailed clinical characteristics are required to validate the diagnostic performance. Lastly, all subjects were recruited from the same site; thus, site-specific biases cannot be ruled out. Further studies recruiting heterogeneous subjects from different clinical sites are required.