Altered fecal microbiota composition in the Flinders sensitive line rat model of depression

The gut microbiome from middle-aged women with depression modulates depressive-like behaviors and plasma fatty acid metabolism in female middle-aged mice

BACKGROUND

Intestinal dysbacteriosis has frequently been involved in the context of depression. Nonetheless, only scant information is available about the features and functional changes of gut microbiota in female middle-aged depression (MAD).

OBJECTIVE

This study aims to explore whether there are characteristic changes in the gut microbes of female MAD and whether these changes are associated with depressive-like behaviors. Meanwhile, this study observed alterations in the lipid metabolism function of gut microbes and further examined changes in plasma medium- and long-chain fatty acids (MLCFAs) in mice that underwent fecal microbiota transplantation (FMT).

METHODS

Stool samples obtained from 31 MAD, along with 24 healthy individuals (HC) were analyzed by 16 S rRNA gene sequencing. Meanwhile, 14-month-old female C57BL/6J mice received antibiotic cocktails and then oral gavage of the microbiota suspension of MAD or HC for 3 weeks to reconstruct gut microbiota. The subsequent depressive-like behaviors, the composition of gut microbiota, as well as MLCFAs in the plasma were evaluated.

RESULTS

A noteworthy disruption in gut microbial composition in MAD individuals compared to HC was observed. Several distinct bacterial taxa, including Dorea, Butyricicoccus, and Blautia, demonstrated associations with the demographic variables. A particular microbial panel encompassing 49 genera effectively differentiated MAD patients from HC (AUC = 0.82). Fecal microbiome transplantation from MAD subjects led to depressive-like behaviors and dysfunction of plasma MLCFAs in mice.

CONCLUSIONS

These findings suggest that microbial dysbiosis is linked to the pathogenesis of MAD, and its role may be associated with the regulation of MLCFAs metabolism.

BDNF/CREB signaling pathway contribution in depression pathogenesis: A survey on the non-pharmacological therapeutic opportunities for gut microbiota dysbiosis

Emerging evidence supports the gut microbiota and the brain communication in general health. This axis may affect behavior through modulating neurotransmission, and thereby involve in the pathogenesis and/or progression of different neuropsychiatric disorders such as depression. Brain-derived neurotrophic factor and cAMP response element-binding protein known as CREB/BDNF pathway plays have critical functions in the pathogenesis of depression as the same of mechanisms related to antidepressants. However, the putative causal significance of the CREB/BDNF signaling cascade in the gut-brain axis in depression remains unknown. Also interventions such as probiotics supplementation and exercise can influence microbiome also improve bidirectional communication of gut and brain. In this review we aim to explain the BDNF/CREB signaling pathway and gut microbiota dysfunction and then evaluate the potential role of probiotics, prebiotics, and exercise as a therapeutic target in the gut microbiota dysfunction induced depression. The current narrative review will specifically focus on the impact of exercise and diet on the intestinal microbiota component, as well as the effect that these therapies may have on the microbiota to alleviate depressive symptoms. Finally, we look at how BDNF/CREB signaling pathway may exert distinct effects on depression and gut microbiota dysfunction.

Microbiota-Induced Epigenetic Alterations in Depressive Disorders Are Targets for Nutritional and Probiotic Therapies

Major depressive disorder (MDD) is a complex disorder and a leading cause of disability in 280 million people worldwide. Many environmental factors, such as microbes, drugs, and diet, are involved in the pathogenesis of depressive disorders. However, the underlying mechanisms of depression are complex and include the interaction of genetics with epigenetics and the host immune system. Modifications of the gut microbiome and its metabolites influence stress-related responses and social behavior in patients with depressive disorders by modulating the maturation of immune cells and neurogenesis in the brain mediated by epigenetic modifications. Here, we discuss the potential roles of a leaky gut in the development of depressive disorders via changes in gut microbiota-derived metabolites with epigenetic effects. Next, we will deliberate how altering the gut microbiome composition contributes to the development of depressive disorders via epigenetic alterations. In particular, we focus on how microbiota-derived metabolites such as butyrate as an epigenetic modifier, probiotics, maternal diet, polyphenols, drugs (e.g., antipsychotics, antidepressants, and antibiotics), and fecal microbiota transplantation could positively alleviate depressive-like behaviors by modulating the epigenetic landscape. Finally, we will discuss challenges associated with recent therapeutic approaches for depressive disorders via microbiome-related epigenetic shifts, as well as opportunities to tackle such problems.

Dose-dependent action of cordycepin on the microbiome-gut-brain-adipose axis in mice exposed to stress

The high risk for anxiety and depression among individuals with stress has become a growing concern globally. Stress-related mental disorders are often accompanied by symptoms of metabolic dysfunction. Cordycepin is a Chinese herbal medicine commonly used for its metabolism-enhancing effects. We aimed to investigate the dose-dependent effects of cordycepin on psycho-metabolic disorders induced by stress. Our behavioral tests revealed that 12.5 mg/kg cordycepin by oral gavage significantly attenuated the anxiety- and depression-like behaviors induced by stress in mice. At 25 mg/kg, cordycepin restored the reduced weight and cell size of adipose tissues caused by stress. Besides ameliorating the metabolic dysbiosis of gut microbiota due to stress, cordycepin significantly reduced the elevated contents of 5-hydroxyindoleacetic acid in the serum and prefrontal cortex at 12.5 mg/kg and reversed the decrease in adipose induced by stress at 25 mg/kg. Correlation analyses further revealed that 12.5 mg/kg cordycepin reversed stress-induced changes in the intestinal microbiome of NK4A214_group and decreased serum Myristic acid and PC(15:0/18:1(11Z)) and cytokines, such as IFN-γ and IL-1β. 25 mg/kg cordycepin reversed stress-induced changes in the abundances of Prevoteaceae_UCG-001 and Desulfovibrio, increased serum L-alanine level, and decreased serum Inosine-5'-monophosphate level. Cordycepin thereby ameliorated the anxiety- and depression-like behaviors as well as disturbances in the adipose metabolism of mice exposed to stress. Overall, these findings offer evidence indicating that the prominent effects of cordycepin in the brain and adipose tissues are dose dependent, thus highlight the importance of evaluating the precise therapeutic effects of different cordycepin doses on psycho-metabolic diseases.

Antibiotic-induced socio-sexual behavioral deficits are reversed via cecal microbiota transplantation but not androgen treatment

Recent evidence has demonstrated a sex-specific role of the gut microbiome on social behavior such as anxiety, possibly driven by a reciprocal relationship between the gut microbiome and gonadal hormones. For instance, gonadal hormones drive sex differences in gut microbiota composition, and certain gut bacteria can produce androgens from glucocorticoids. We thus asked whether the gut microbiome can influence androgen-dependent socio-sexual behaviors. We first treated C57BL/6 mice with broad-spectrum antibiotics (ABX) in drinking water to deplete the gut microbiota either transiently during early development (embryonic day 16-postnatal day [PND] 21) or in adulthood (PND 60-85). We hypothesized that if ABX interferes with androgens, then early ABX would interfere with critical periods for sexual differentiation of brain and thus lead to long-term decreases in males' socio-sexual behavior, while adult ABX would interfere with androgens' activational effects on behavior. We found that in males but not females, early and adult ABX treatment decreased territorial aggression, and adult ABX also decreased sexual odor preference. We then assessed whether testosterone and/or cecal microbiota transplantation (CMT) via oral gavage could prevent ABX-induced socio-sexual behavioral deficits in adult ABX-treated males. Mice were treated with same- or other-sex control cecum contents or with testosterone for two weeks. While testosterone was not effective in rescuing any behavior, we found that male CMT restored both olfactory preference and aggression in adult ABX male mice, while female CMT restored olfactory preference but not aggression. These results suggest sex-specific effects of the gut microbiome on socio-sexual behaviors, independent of androgens.

Hypoglycemic effects of different molecular weight konjac glucomannans via intestinal microbiota and SCFAs mediated mechanism

The hypoglycemic effects of konjac glucomannans (KGMs) are well recognized, and our previous study showed KGMs with different molecular weight have different hypoglycemic effects on diabetes rats, but the detailed mechanisms still remain unclear. In this study, KGMs with medium molecular weight (KGM-M, 757.1 kDa) and low molecular weight (KGM-L, 87.3 kDa) were utilized to investigate the possible mechanism on hypoglycemic effects of type 2 diabetic (T2DM) rats. The results revealed that KGM-M had better effects than KGM-L on decreasing fasting blood glucose, mitigating insulin resistance and improving inflammation. Further mechanism analysis showed that KGM-M better enriched gut flora diversity and the abundance of Ruminococcus and Lachnoclostridium, which was accompanied by increased short chain fatty acids (SCFAs) production and expression of G protein-coupled receptors (GPCRs), and improved regulation on bile acid synthesis. Antibiotics treatment eliminated the beneficial effects of KGMs on gut flora, SCFAs, GPCRs and bile acid synthesis. By contrast, fecal microbiota transplantation (FMT) treatment restored the structure of intestinal microbiota. And after FMT treatment, KGM-M displayed higher hypoglycemic activity than KGM-L, probably due to the better effects on intestinal microbiota, SCFAs production, GPCRs expression and bile acid synthesis inhibition.

Gut Microbiota in Anxiety and Depression: Unveiling the Relationships and Management Options

The gut microbiota is critical for maintaining human health and the immunological system. Several neuroscientific studies have shown the significance of microbiota in developing brain systems. The gut microbiota and the brain are interconnected in a bidirectional relationship, as research on the microbiome-gut-brain axis shows. Significant evidence links anxiety and depression disorders to the community of microbes that live in the gastrointestinal system. Modified diet, fish and omega-3 fatty acid intake, macro- and micro-nutrient intake, prebiotics, probiotics, synbiotics, postbiotics, fecal microbiota transplantation, and 5-HTP regulation may all be utilized to alter the gut microbiota as a treatment approach. There are few preclinical and clinical research studies on the effectiveness and reliability of various therapeutic approaches for depression and anxiety. This article highlights relevant research on the association of gut microbiota with depression and anxiety and the different therapeutic possibilities of gut microbiota modification.

Gut microbiota-induced microRNA-206-3p increases anxiety-like behaviors by inhibiting expression of Cited2 and STK39

BACKGROUND

Anxiety disorder is highly prevalent worldwide and represents a chronic and functionally disabling condition, with high levels of psychological stress characterized by cognitive and physiological symptoms. The purpose of this study is to evaluate the clinical significance of gut microbiota regulating microRNA (miR)-206-3p as a biomarker in the anxiety-like behaviors.

METHODS

Initially, bioinformatics analysis was performed to predict the related factors for gut microbiota affecting anxiety-like behaviors. Next, the anxiety-like behaviors in mice were measured by multiple experiments. Western blot analysis, immunohistochemistry, and enzyme-linked immunosorbent assay (ELISA) were utilized to measure the levels of 5-hydroxytryptamine (5-HT), brain derived neurotrophic factor (BDNF), and neutrophil expressed (NE) in brain tissues and serum and cAMP responsive element binding protein 1 (CREB) phosphorylation in brain tissues of germ-free (GF) mice. Dual-luciferase reporter gene assay was employed to verify the relationship between miR-206-3p and Cbp/p300 interacting transactivator with Glu/Asp rich carboxy-terminal domain 2 (Cited2)/serine/threonine kinase 39 (STK39). Ectopic expression and depletion experiments of miR-206-3p were conducted to determine the expression of miR-206-3p and mRNA and protein levels of Cited2, and STK39 in HT22 cells and brain tissues. Finally, transmission electron microscope (TEM) was used to observe the effects of miR-206-3p on hippocampal mitochondria and synapses.

RESULTS

Gut microbiota could elevate miR-206-3p expression in brain tissues to increase the anxiety-like behaviors. GF mice displayed the increased levels of 5-HT, BDNF, and NE in brain tissues and serum and CREB phosphorylation in brain tissues. Cited2/STK39 was identified as the target genes of miR-206-3p. Upregulated miR-206-3p increased anxiety-like behaviors by promoting degeneration of mitochondria and synapses in hippocampus via downregulation of Cited2 and STK39.

CONCLUSIONS

In conclusion, the key findings of the current study demonstrate that gut microbiota aggravated anxiety-like behaviors via the miR-206-3p/Cited2/STK39 axis.

Antidepressive Effect of Natural Products and Their Derivatives Targeting BDNF-TrkB in Gut-Brain Axis

Modern neurological approaches enable detailed studies on the pathophysiology and treatment of depression. An imbalance in the microbiota-gut-brain axis contributes to the pathogenesis of depression. This extensive review aimed to elucidate the antidepressive effects of brain-derived neurotrophic factor (BDNF)-targeting therapeutic natural products and their derivatives on the gut-brain axis. This information could facilitate the development of novel antidepressant drugs. BDNF is crucial for neuronal genesis, growth, differentiation, survival, plasticity, and synaptic transmission. Signaling via BDNF and its receptor tropomyosin receptor kinase B (TrkB) plays a vital role in the etiopathogenesis of depression and the therapeutic mechanism of antidepressants. This comprehensive review provides information to researchers and scientists for the identification of novel therapeutic approaches for neuropsychiatric disorders, especially depression and stress. Future research should aim to determine the possible causative role of BDNF-TrkB in the gut-brain axis in depression, which will require further animal and clinical research as well as the development of analytical approaches.

SCFAs Ameliorate Chronic Postsurgical Pain-Related Cognition Dysfunction via the ACSS2-HDAC2 Axis in Rats

Patients with chronic postsurgical pain (CPSP) frequently exhibit comorbid cognitive deficits. Recent observations have emphasized the critical effects of gut microbial metabolites, like short-chain fatty acids (SCFAs), in regulating cognitive function. However, the underlying mechanisms and effective interventions remain unclear. According to hierarchical clustering and 16S rRNA analysis, over two-thirds of the CPSP rats had cognitive impairment, and the CPSP rats with cognitive impairment had an aberrant composition of gut SCFA-producing bacteria. Then, using feces microbiota transplantation, researchers identified a causal relationship between cognitive-behavioral and microbic changes. Similarly, the number of genera that generated SCFAs was decreased in the feces from recipients of cognitive impairment microbiota. Moreover, treatment with the SCFAs alleviated the cognitive-behavioral deficits in the cognitively compromised pain rats. Finally, we observed that SCFA supplementation improved histone acetylation and abnormal synaptic transmission in the medial prefrontal cortex (mPFC), hippocampal CA1, and central amygdala (CeA) area via the ACSS2 (acetyl-CoA synthetase2)-HDAC2 (histone deacetylase 2) axis. These findings link pain-related cognition dysfunction, gut microbiota, and short-chain fatty acids, shedding fresh insight into the pathogenesis and therapy of pain-associated cognition dysfunction.

Antidepressant Shugan Jieyu Capsule Alters Gut Microbiota and Intestinal Microbiome Function in Rats With Chronic Unpredictable Mild Stress -Induced Depression

Shugan Jieyu Capsule (SG) has been widely used in China to treat mild to moderate depression. Hypericum perforatum L. (St John’s Wort, SJW) is the main ingredient of SG and has been used as herbal medicine to treat depression in western countries. However, it is known that SJW has low bioavailability and does not easily get through the blood-brain barrier. Therefore, how SG plays an antidepressant effect in the central nervous system (CNS) remains an urgent problem to be solved. Mounting research has described the relationship between antidepressants and intestinal microbiota to illuminate antidepressive mechanisms in the CNS. We aimed to investigate the effects of therapy with SG on the function of gut microbiota and intestinal microbiota in rats with chronic unpredictable mild stress (CUMS)-induced depression. The psychophysiological state and the hypothalamic-pituitary-adrenal axis function of rats are evaluated through behavioral experiments, corticosterone levels, serotonin levels, and adrenal index measurements. 16S rDNA amplicon sequencing is used to test the changes in gut microbiota and make functional predictions of genes. With treatment of SG, the depression-like behaviors of CUMS-induced rats were reversed; the corticosterone levels and the adrenal index decreased significantly; the level of serotonin increased significantly; and the alpha and beta diversity analysis of microbiota showed an increase in the richness and uniformity of the flora were increased. SG regulated the relative abundance of Actinobacteria, Erysipelotrichaceae, Bifidobacteriaceae, Atopobiaceae, Dubosiella, and Bifidobacterium; Linear discriminant analysis effect size analysis demonstrated that Lactobacillaceae (family level), Lactobacillus (genus level), Lactobacillales (order level), Bacilli (class level), and Lactobacillus-reuteri (species level) were biomarkers in the SG group samples, and also likely to modulate metabolic pathways, such as those involved in carbohydrate metabolism, amino acid metabolism, and signal transduction. These data clearly illustrated the effect of SG on gut microbiome, thus laying the foundation for uncovering more insights on the therapeutic function of the traditional Chinese antidepressants. The potential of SG on mechanisms of antidepression to alter gut microbiota and intestinal microbiome function exposed to CUMS can be explored.

Effects of 'Healthy' Fecal Microbiota Transplantation against the Deterioration of Depression in Fawn-Hooded Rats

Depression is a recurrent, heterogeneous mood disorder occurring in more than 260 million people worldwide. Gut microbiome dysbiosis is associated with the development of depressive-like behaviors by modulating neuro-biochemical metabolism through the microbiome-gut-brain (MGB) axis. Fecal microbiota transplantation (FMT) has been proposed as a potential therapeutic solution for depression, but the therapeutic efficiency and mechanism are unknown. Here, we performed an FMT from Sprague-Dawley (SD) rats (‘healthy’ controls) to Fawn-hooded (FH) rats (depression model). Pre-FMT, the FH rats exhibited significantly elevated depressive-like behaviors and distinct neurotransmitter and cytokine levels compared with SD rats. Post-FMT, FH recipients receiving FH fecal microbiota (FH-FH rats) showed aggravated depressive-like behaviors, while the ones receiving SD microbiota (FH-SD rats) had significantly alleviated depressive symptoms, a significant increase in hippocampal neurotransmitters, and a significant decrease of some hippocampal cytokines than FH-FH rats. SD-FMT resulted in the FH-SD rats’ gut microbiome resembling the SD donors, and a significant shift in the serum metabolome but not the hippocampal metabolome. Co-occurrence analysis suggests that SD-FMT prevented recipients’ depression development via the significant decrease of gut microbial species such as Dialister sp., which led to the recipients’ metabolic modulation in serum and hippocampus through the enteric nervous system, the intestinal barrier, and the blood-brain barrier. Our results provided new data pointing to multiple mechanisms of interaction for the impact of gut microbiome modulation on depression therapy.

IMPORTANCE Depression is a chronic, recurrent mental disease, which could make the patients commit suicide in severe cases. Considering that gut microbiome dysbiosis could cause depressive symptoms in animals through the MGB axis, the modification of gut microbiota is expected to be a potential therapy for depression, but the daily administration of probiotics is invalid or transient. In this study, we demonstrated that the gut microbiome transferred from a healthy rat model to a depressive rat model could regulate the recipient’s neurobiology and behavior via the systematic alternation of the depressive gut microbiota followed by the serum and hippocampal metabolism. These results underline the significance of understanding the impact of gut microbiota on mental disorders and suggest that ‘healthy’ microbiota transplantation with the function to solve the host’s cerebral inflammation may serve as a novel therapeutic strategy for depression.

One Giant Leap from Mouse to Man: The Microbiota-Gut-Brain Axis in Mood Disorders and Translational Challenges Moving towards Human Clinical Trials

The microbiota–gut–brain axis is a bidirectional communication pathway that enables the gut microbiota to communicate with the brain through direct and indirect signaling pathways to influence brain physiology, function, and even behavior. Research has shown that probiotics can improve several aspects of health by changing the environment within the gut, and several lines of evidence now indicate a beneficial effect of probiotics on mental and brain health. Such evidence has prompted the arrival of a new term to the world of biotics research: psychobiotics, defined as any exogenous influence whose effect on mental health is bacterially mediated. Several taxonomic changes in the gut microbiota have been reported in neurodevelopmental disorders, mood disorders such as anxiety and depression, and neurodegenerative disorders such as Alzheimer’s disease. While clinical evidence supporting the role of the gut microbiota in mental and brain health, and indeed demonstrating the beneficial effects of probiotics is rapidly accumulating, most of the evidence to date has emerged from preclinical studies employing different animal models. The purpose of this review is to focus on the role of probiotics and the microbiota–gut–brain axis in relation to mood disorders and to review the current translational challenges from preclinical to clinical research.

Microbial Ecosystem Therapeutic-2 Intervention in People With Major Depressive Disorder and Generalized Anxiety Disorder: Phase 1, Open-Label Study

Background

Recent studies have investigated the potential of treatments that modify the gut microbiome, such as fecal microbiota transplantation and probiotics, in individuals with psychiatric illnesses.

Objective

The aim of this study was to investigate the safety, tolerability, and efficacy of a novel gut microbiome therapeutic, Microbial Ecosystem Therapuetic-2 (MET-2), in people with depression and anxiety.

Methods

In this phase 1, open-label trial, 12 adults diagnosed with major depressive disorder, generalized anxiety disorder, or both were recruited. Over 8 weeks, participants consumed three capsules per day, orally, of an encapsulated microbial therapeutic (MET-2), which contained 40 strains of bacteria that were purified and lab-grown from the stool of a single healthy donor. Participants were assessed biweekly using clinical scales and questionnaires in order to evaluate the safety, efficacy, and tolerability of the therapeutic.

Results

The therapeutic was found to be generally safe and tolerable, with limited adverse events and side effects and no serious adverse events. Of the 12 individuals included in this study, 9 (75%) responded to treatment (50% improvement in Montgomery-Asberg Depression Rating Scale [MADRS] scores, 7-item Generalized Anxiety Disorder scale [GAD-7] scores, or both, from baseline to the week-8 visit). Over the course of 10 weeks, MET-2 significantly decreased mean MADRS and GAD-7 scores (MADRS: F2.731, 30.05=8.784, P<.001; GAD-7: F2.778, 30.55= 9.638, P<.001). Multiple comparisons with Bonferroni adjustments showed a significant reduction in MADRS scores from baseline (mean 19.00, SD 4.843) to week 6 (mean 11.25, SD 8.001; P=.009), week 8 (mean 8.667, SD 8.732; P=.002), and week 10 (mean 8.250, SD 9.304; P=.006). Multiple comparisons showed a significant reduction in GAD-7 scores from baseline (mean 13.58, SD 4.010) to week 4 (mean 9.167, SD 5.096; P=.03), week 6 (mean 7.667, SD 4.539; P=.004), week 8 (mean 7.333, SD 6.583; P=.03), and week 10 (mean 7.500, SD 6.448; P=.03).

Conclusions

The findings from this study are the first to provide evidence for the role of microbial ecosystem therapy in treating depression and anxiety. However, a double-blind, randomized controlled trial with a larger sample size is needed for more conclusive results.

Trial Registration

ClinicalTrials.gov NCT04052451; https://www.clinicaltrials.gov/ct2/show/NCT04052451

International Registered Report Identifier (IRRID)

RR2-10.2196/17223

Association Between Non-Suicidal Self-Injury and Gut Microbial Characteristics in Chinese Adolescent

PURPOSE

By exploring the gut-related microbiota differences of adolescents with non-suicidal self-injury (NSSI) and depression (without NSSI) and healthy volunteers, we provide a theoretical basis for the prevention and control of NSSI in adolescents through intestinal microecological regulation.

PATIENTS AND METHODS

A total of 99 subjects were recruited in Guangdong Province, China, including 51 adolescents with NSSI (KD), 24 healthy adolescents (NOR1), and 24 depression adolescents without NSSI (NOR2). General clinical data and fecal samples were collected from all subjects, who were assessed using the NSSI Behavioral Questionnaire and the 24-item Hamilton Depression Scale. The taxonomic composition of the gut microbiota was determined using the 16S rDNA gene sequencing method.

RESULTS

There were significant differences in diversity between the KD and NOR1, and the species uniformity index of the KD according to the Shannon and Simpson indices was significantly reduced compared with that of the NOR1 (4.81 vs 5.21, p<0.01; 0.02 vs 0.01, p<0.05). The relative abundances were different among the KD, NOR1 and NOR2, as reflected at the taxonomic levels of class, order, family, genus, and species. Bacteroides were the dominant flora of the KD and NOR2, while Mitsuokella was the dominant flora that distinguished the KD from the NOR2.

CONCLUSION

We found that gut microbiota diversity was decreased in adolescents with NSSI, and the relative abundance was altered at different taxonomic levels. These results enrich the understanding of the relationship between NSSI and depression and the gut microbiota, Supporting that NSSI and depression are not homologous disorders. What is more, it establishes the basis for exploring the mechanisms of flora action in NSSI, providing a possible direction for NSSI to achieve a better prognosis and prevent relapse.

Proteomics analysis of the gut-brain axis in a gut microbiota-dysbiosis model of depression

Major depressive disorder (MDD) is a serious mental illness. Increasing evidence from both animal and human studies suggested that the gut microbiota might be involved in the onset of depression via the gut-brain axis. However, the mechanism in depression remains unclear. To explore the protein changes of the gut-brain axis modulated by gut microbiota, germ-free mice were transplanted with gut microbiota from MDD patients to induce depression-like behaviors. Behavioral tests were performed following fecal microbiota transplantation. A quantitative proteomics approach was used to examine changes in protein expression in the prefrontal cortex (PFC), liver, cecum, and serum. Then differential protein analysis and weighted gene coexpression network analysis were used to identify microbiota-related protein modules. Our results suggested that gut microbiota induced the alteration of protein expression levels in multiple tissues of the gut-brain axis in mice with depression-like phenotype, and these changes of the PFC and liver were model specific compared to chronic stress models. Gene ontology enrichment analysis revealed that the protein changes of the gut-brain axis were involved in a variety of biological functions, including metabolic process and inflammatory response, in which energy metabolism is the core change of the protein network. Our data provide clues for future studies in the gut-brain axis on protein level and deepen the understanding of how gut microbiota cause depression-like behaviors.

Faecal microbiota transplantation from patients with depression or healthy individuals into rats modulates mood-related behaviour

Differences in gut microbiota composition have been observed in patients with major depressive disorder (MDD) compared to healthy individuals. Here, we investigated if faecal microbiota transplantation (FMT) from patients with MDD into rats could induce a depressive-like phenotype. We performed FMT from patients with MDD (FMT-MDD) and healthy individuals (FMT-Healthy) into male Flinders Sensitive Line (FSL) and Flinders Resistant Line (FRL) rats and assessed depressive-like behaviour. No behavioural differences were observed in the FSL rats. In FRL rats, the FMT-Healthy group displayed significantly less depressive-like behaviour than the FMT-MDD group. However, there was no difference in behaviour between FMT-MDD FRL rats and negative controls, indicating that FMT-Healthy FRL rats received beneficial bacteria. We additionally found different taxa between the FMT-MDD and the FMT-Healthy FRL rats, which could be traced to the donors. Four taxa, three belonging to the family Ruminococcaceae and the genus Lachnospira, were significantly elevated in relative abundance in FMT-MDD rats, while the genus Coprococcus was depleted. In this study, the FMT-MDD group was different from the FMT-Healthy group based on behaviour and intestinal taxa.

The role of microbiota-gut-brain axis in neuropsychiatric and neurological disorders

Emerging evidence indicates that the gut microbiota play a crucial role in the bidirectional communication between the gut and the brain suggesting that the gut microbes may shape neural development, modulate neurotransmission and affect behavior, and thereby contribute to the pathogenesis and/or progression of many neurodevelopmental, neuropsychiatric, and neurological conditions. This review summarizes recent data on the role of microbiota-gut-brain axis in the pathophysiology of neuropsychiatric and neurological disorders including depression, anxiety, schizophrenia, autism spectrum disorders, Parkinson's disease, migraine, and epilepsy. Also, the involvement of microbiota in gut disorders co-existing with neuropsychiatric conditions is highlighted. We discuss data from both in vivo preclinical experiments and clinical reports including: (1) studies in germ-free animals, (2) studies exploring the gut microbiota composition in animal models of diseases or in humans, (3) studies evaluating the effects of probiotic, prebiotic or antibiotic treatment as well as (4) the effects of fecal microbiota transplantation.

Altered Fecal Metabolites and Colonic Glycerophospholipids Were Associated With Abnormal Composition of Gut Microbiota in a Depression Model of Mice

The microbiota–gut–brain axis has been considered to play an important role in the development of depression, but the underlying mechanism remains unclear. The gastrointestinal tract is home to trillions of microbiota and the colon is considered an important site for the interaction between microbiota and host, but few studies have been conducted to evaluate the alterations in the colon. Accordingly, in this study, we established a chronic social defeated stress (CSDS) mice model of depression. We applied 16S rRNA gene sequencing to assess the gut microbial composition and gas and liquid chromatography–mass spectroscopy to identify fecal metabolites and colonic lipids, respectively. Meanwhile, we used Spearman’s correlation analysis method to evaluate the associations between the gut microbiota, fecal metabolites, colonic lipids, and behavioral index. In total, there were 20 bacterial taxa and 18 bacterial taxa significantly increased and decreased, respectively, in the CSDS mice. Further, microbial functional prediction demonstrated a disturbance of lipid, carbohydrate, and amino acid metabolism in the CSDS mice. We also found 20 differential fecal metabolites and 36 differential colonic lipids (in the category of glycerolipids, glycerophospholipids, and sphingolipids) in the CSDS mice. Moreover, correlation analysis showed that fecal metabolomic signature was associated with the alterations in the gut microbiota composition and colonic lipidomic profile. Of note, three lipids [PC(16:0/20:4), PG(22:6/22:6), and PI(18:0/20:3), all in the category of glycerophospholipids] were significantly associated with anxiety- and depression-like phenotypes in mice. Taken together, our results indicated that the gut microbiota might be involved in the pathogenesis of depression via influencing fecal metabolites and colonic glycerophospholipid metabolism.

Gut microbiota alteration and modulation in psychiatric disorders: Current evidence on fecal microbiota transplantation

The micro-organisms residing within the gastrointestinal tract, namely gut microbiota, form a dynamic population proper of each individual, mostly composed by bacteria which co-evolved symbiotically with human species. The advances of culture-independent techniques allowed the understanding of the multiple functions of the gut microbiota in human physiology and disease, the latter often recognising a predisposing condition in an imbalanced intestinal microbial ecosystem (dysbiosis). A complex mutual interconnection between the central nervous system (CNS), the intestine and the gut microbiota, known as "microbiota-gut-brain axis", has been hypothesized to play a pivotal role in maintaining central and peripheral functions, as well as mental health. Thus, dysbiosis with specific microbiota imbalances seems to be strongly associated with the onset psychiatric disorders by altering neurodevelopment, enhancing neurodegeneration, affecting behaviour and mood. Fecal microbiota transplantation (FMT) consists of transferring the fecal matter from a donor into the gastrointestinal tract of a recipient, and it is used to quickly modulate the gut microbiota. This review focuses on the uses of FMT in psychiatric disorders. FMT has been used to induce dysbiosis and to study the disease development, or to heal dysbiosis-related mental disorders. Overall, FMT of impaired microbiota resulted effective in enhancing psychiatric-like disturbances (mainly depression and anxiety) in recipient animals, plausibly by impairing immune system, inflammatory and metabolic pathways, neurochemical processes and neuro-transmission. On the other side, preclinical and clinical data suggest that reversing or mitigating dysbiosis seems a promising strategy to restore behavioural impairments or to obtain psychiatric symptom relief. However, current evidence is limited by the lack of procedural standardization, the paucity of human studies in the vastity of psychiatric conditions and the need of a microbiota-targeted donor-recipient matching.

The Gut Microbiome in Schizophrenia and the Potential Benefits of Prebiotic and Probiotic Treatment

The gut microbiome (GMB) plays an important role in developmental processes and has been implicated in the etiology of psychiatric disorders. However, the relationship between GMB and schizophrenia remains unclear. In this article, we review the existing evidence surrounding the gut microbiome in schizophrenia and the potential for antipsychotics to cause adverse metabolic events by altering the gut microbiome. We also evaluate the current evidence for the clinical use of probiotic and prebiotic treatment in schizophrenia. The current data on microbiome alteration in schizophrenia remain conflicting. Longitudinal and larger studies will help elucidate the confounding effect on the microbiome. Current studies help lay the groundwork for further investigations into the role of the GMB in the development, presentation, progression and potential treatment of schizophrenia.

A diet-induced gut microbiota component and related plasma metabolites are associated with depressive-like behaviour in rats

It is well-established in preclinical studies that various probiotics may improve behaviours related to psychiatric disease. We have previously shown that probiotics protected against high-fat diet (HFD)-induced depressive-like behaviour in Flinders Sensitive Line (FSL) rats, whereas FSL rats on control (CON) diet were unaffected. Therefore, we hypothesised that a dysmetabolic component of depression may exist that involves the gut microbiota and that such component may be reflected in the plasma metabolome. The aims of the present study post hoc analyses were 1) to study the effect of probiotics on gut microbiota composition and its association with depressive-like behaviour in FSL rats, and 2) to identify plasma metabolites associated with gut microbiota and depressive-like behaviour. Forty-six FSL rats were fed CON or HFD and treated with multi-species probiotics (nine Bifidobacterium, Lactococcus and Lactobacillus species) for 12 weeks. Faecal samples were collected for 16S rRNA (VR4) gene amplicon sequencing (Illumina MiSeq), and an untargeted plasma metabolomics was performed. We found that probiotics increased the relative faecal abundance of the Bifidobacterium, Lactococcus and Lactobacillus genera in HFD-fed rats only. Also, a HFD-induced microbiota component associated with depressive-like behaviour was identified, and probiotics improved the component score. Finally, the plasma levels of 44 metabolites correlated with the depression-related microbiota component, and three such metabolites had good predictive ability for depressive-like behaviour. Potentially, our findings imply that a subtype of depression characterised by a diet-induced, pro-depressant gut microbiota may exist and that analysis of related plasma metabolites may reveal aberrant microbiota functioning related to depression.

Investigating causality with fecal microbiota transplantation in rodents: applications, recommendations and pitfalls

In recent years, studies investigating the role of the gut microbiota in health and diseases have increased enormously - making it essential to deepen and question the research methodology employed. Fecal microbiota transplantation (FMT) in rodent studies (either from human or animal donors) allows us to better understand the causal role of the intestinal microbiota across multiple fields. However, this technique lacks standardization and requires careful experimental design in order to obtain optimal results. By comparing several studies in which rodents are the final recipients of FMT, we summarize the common practices employed. In this review, we document the limitations of this method and highlight different parameters to be considered while designing FMT Studies. Standardizing this method is challenging, as it differs according to the research topic, but avoiding common pitfalls is feasible. Several methodological questions remain unanswered to this day and we offer a discussion on issues to be explored in future studies.

Whole body vibration training improves depression-like behaviors in a rat chronic restraint stress model

Major depressive disorder (MDD) is a prevalent psychiatric disorder that brings great harm and burden to both patients and society. This study aimed to examine the effects of whole-body vibration (WBV) training on a chronic restraint stress (CRS) induced depression rat model and provide an initial understanding of related molecular mechanisms. Adult Sprague-Dawley male rats were randomly divided into the following three groups: a) control group, b) depressive disorder group, and c) depression with WBV training treatment group. Daily food intake, body weight, sucrose preference test, open field test, elevated plus maze, forced swimming test, and Barnes maze task tests were performed. Immunofluorescence staining and ELISA analysis were used to assess neuronal damage, synaptic proteins, glial cells, and trophic factors. The data of behavioral tests and related biochemical indicators were statistically analyzed and compared between groups. Rats undergoing CRS showed increased anxiety-like behavior and memory impairment, along with synaptic atrophy and neuronal degeneration. WBV could reverse behavioral dysfunction, inhibit the degeneration of neurons, alleviate the damage of neurons and the pathological changes of glial cells, enhance trophic factor expression, and ameliorate the downregulation of dendritic and synaptic proteins after CRS. The effect of WBV in rats may be mediated via the reduction of hippocampal neuronal degeneration and by improving expression of synaptic proteins. WBV training exerts multifactorial benefits on MDD that supports its use as a promising new therapeutic option for improving depression-like behaviors in the depressive and/or potentially depressive.

Gut-Brain Axis: Role of Gut Microbiota on Neurological Disorders and How Probiotics/Prebiotics Beneficially Modulate Microbial and Immune Pathways to Improve Brain Functions

The gut microbiome acts as an integral part of the gastrointestinal tract (GIT) that has the largest and vulnerable surface with desirable features to observe foods, nutrients, and environmental factors, as well as to differentiate commensals, invading pathogens, and others. It is well-known that the gut has a strong connection with the central nervous system (CNS) in the context of health and disease. A healthy gut with diverse microbes is vital for normal brain functions and emotional behaviors. In addition, the CNS controls most aspects of the GI physiology. The molecular interaction between the gut/microbiome and CNS is complex and bidirectional, ensuring the maintenance of gut homeostasis and proper digestion. Besides this, several mechanisms have been proposed, including endocrine, neuronal, toll-like receptor, and metabolites-dependent pathways. Changes in the bidirectional relationship between the GIT and CNS are linked with the pathogenesis of gastrointestinal and neurological disorders; therefore, the microbiota/gut-and-brain axis is an emerging and widely accepted concept. In this review, we summarize the recent findings supporting the role of the gut microbiota and immune system on the maintenance of brain functions and the development of neurological disorders. In addition, we highlight the recent advances in improving of neurological diseases by probiotics/prebiotics/synbiotics and fecal microbiota transplantation via the concept of the gut-brain axis.

Effect of fecal microbiota transplant on symptoms of psychiatric disorders: a systematic review

BACKGROUND

The Gut-Brain-Axis is a bidirectional signaling pathway between the gastrointestinal (GI) tract and the brain. The hundreds of trillions of microorganisms populating the gastrointestinal tract are thought to modulate this connection, and have far reaching effects on the immune system, central and autonomic nervous systems, and GI functioning. These interactions Diagnostic and statistical manual of mental disorders have also been linked to various psychiatric illnesses such as depression, anxiety, substance abuse, autism spectrum disorder, and eating disorders. It is hypothesized that techniques aimed at strengthening and repopulating the gut microbiome, such as Fecal Microbiota Transplant (FMT), may be useful in the prevention and treatment of psychiatric illnesses.

METHODS

A systematic search of five databases was conducted using key terms related to FMT and psychiatric illnesses. All results were then evaluated based on specific eligibility criteria.

RESULTS

Twenty-one studies met the eligibility criteria and were analysed for reported changes in mood and behavioural measures indicative of psychiatric wellbeing. The studies included were either entirely clinical (n = 8), preclinical with human donors (n = 9), or entirely preclinical (n = 11). All studies found a decrease in depressive and anxiety-like symptoms and behaviours resulting from the transplantation of healthy microbiota. The inverse was also found, with the transmission of depressive and anxiety-like symptoms and behaviours resulting from the transplantation of microbiota from psychiatrically ill donors to healthy recipients.

CONCLUSION

There appears to be strong evidence for the treatment and transmission of psychiatric illnesses through FMT. Further research with larger sample sizes and stronger scientific design is warranted in order to fully determine the efficacy and safety of this potential treatment. Registered on PROSPERO, IRD: CRD42019126795.

Fecal Microbiota Transplantation and Hydrocortisone Ameliorate Intestinal Barrier Dysfunction and Improve Survival in a Rat Model of Cecal Ligation and Puncture-Induced Sepsis

INTRODUCTION

Sepsis is a life-threatening syndrome which can progress to multiple organ dysfunction with high mortality. Intestinal barrier failure exerts a central role in the pathophysiological sequence of events that lead from sepsis to multiple organ dysfunction. The present study investigated the role of hydrocortisone (HC) administration and fecal microbiota transplantation (FMT) in several parameters of the gut barrier integrity, immune activation, and survival, in a model of polymicrobial sepsis in rats.

METHODS

Forty adults male Wistar rats were randomly divided into four groups: sham (group I), cecal ligation and puncture (CLP) (group II), CLP + HC (2.8 mg/kg, intraperitoneally single dose at 6 h) (group III), and CLP + FMT at 6 h (group IV). At 24 h post-CLP, ileal tissues were harvested for histological and immunohistochemical analyses while endotoxin, IL-6, and IL-10 levels in systemic circulation were determined. In a second experiment the same groups were observed for 7 days for mortality, with daily administration of hydrocortisone (group III) and FMT (group IV) in surviving rats.

RESULTS

HC administration and FMT significantly reduced mortality of septic rats by 50%. These interventions totally reversed intestinal mucosal atrophy by increasing villous density and mucosal thickness (μm, mean ± SD: Group I: 620 ± 35, Group II: 411 ± 52, Group III: 622 ± 19, Group IV: 617 ± 44). HC and FMT reduced the apoptotic body count in intestinal crypts whereas these increased the mitotic/apoptotic index. Activated caspase-3 expression in intestinal crypts was significantly reduced by HC or FMT (activated caspase-3 (+) enterocytes/10 crypts, mean ± SD: Group I: 1.6 ± 0.5, Group II: 5.8 ± 2.4, Group III: 3.6 ± 0.9, Group IV: 2.3 ± 0.6). Both treatments increased Paneth cell count and decreased intraepithelial CD3(+) T lymphocytes and inflammatory infiltration of lamina propria to control levels. In the sham group almost the total of intestinal epithelial cells expressed occludin (92 ± 8%) and claudin-1 (98 ± 4%) and CLP reduced this expression to 34 ± 12% for occludin and 35 ± 7% for claudin-1. Administration of HC significantly increased occludin (51 ± 17%) and claudin-1 (77 ± 9%) expression. FMT exerted also a significant restoring effect in tight junction by increasing occludin (56 ± 15%) and claudin-1 (84 ± 7%) expression. The beneficial effects of these treatments on gut barrier function led to significant reduction of systemic endotoxemia (EU/mL, mean ± SD: Group I: 0.93 ± 0.36, Group II: 2.14 ± 1.74, Group III: 1.48 ± 0.53, Group IV: 1.61 ± 0.58), while FMT additionally decreased IL-6 and IL-10 levels.

CONCLUSION

Fecal microbiota transplantation and stress dose hydrocortisone administration in septic rats induce a multifactorial improvement of the gut mechanical and immunological barriers, preventing endotoxemia and leading to improved survival.

Animal Models for Functional Gastrointestinal Disorders

Functional gastrointestinal disorders (FGID), such as functional dyspepsia (FD) and irritable bowel syndrome (IBS) are characterized by chronic abdominal symptoms in the absence of an organic, metabolic or systemic cause that readily explains these complaints. Their pathophysiology is still not fully elucidated and animal models have been of great value to improve the understanding of the complex biological mechanisms. Over the last decades, many animal models have been developed to further unravel FGID pathophysiology and test drug efficacy. In the first part of this review, we focus on stress-related models, starting with the different perinatal stress models, including the stress of the dam, followed by a discussion on neonatal stress such as the maternal separation model. We also describe the most commonly used stress models in adult animals which brought valuable insights on the brain-gut axis in stress-related disorders. In the second part, we focus more on models studying peripheral, i.e., gastrointestinal, mechanisms, either induced by an infection or another inflammatory trigger. In this section, we also introduce more recent models developed around food-related metabolic disorders or food hypersensitivity and allergy. Finally, we introduce models mimicking FGID as a secondary effect of medical interventions and spontaneous models sharing characteristics of GI and anxiety-related disorders. The latter are powerful models for brain-gut axis dysfunction and bring new insights about FGID and their comorbidities such as anxiety and depression.

Implications of gut microbiota dysbiosis and metabolic changes in prion disease

Evidence of the gut microbiota influencing neurodegenerative diseases has been reported for several neural diseases. However, there is little insight regarding the relationship between the gut microbiota and prion disease. Here, using fecal samples of 12 prion-infected mice and 25 healthy controls, we analyzed the structure of the gut microbiota and metabolic changes by 16S rRNA sequencing and LC-MS-based metabolomics respectively as multi-omic analyses. Additionally, SCFAs and common amino acids were detected by GC-MS and UPLC respectively. Enteric changes induced by prion disease affected both structure and abundances of the gut microbiota. The gut microbiota of infected mice displayed greater numbers of Proteobacteria and less Saccharibacteria at the phylum level and more Lactobacillaceae and Helicobacteraceae and less Prevotellaceae and Ruminococcaceae at the family level. A total of 145 fecal metabolites were found to be significantly different in prion infection, and most (114) of these were lipid metabolites. Using KEGG pathway enrichment analysis, we found that 3 phosphatidylcholine (PC) compounds significantly decreased and 4 hydrophobic bile acids significantly increased. Decreases of 8 types of short-chain acids (SCFAs) and increases of Cys and Tyr and decreases of His, Trp, and Arg were observed in prion infection. Correlation analysis indicated that the gut microbiota changes observed in our study may have been the shared outcome of prion disease. These findings suggest that prion disease can cause significant shifts in the gut microbiota. Certain bacterial taxa can then respond to the resulting change to the enteric environment by causing dramatic shifts in metabolite levels. Our data highlight the health impact of the gut microbiota and related metabolites in prion disease.

The Role of Bacteria and Its Derived Metabolites in Chronic Pain and Depression: Recent Findings and Research Progress

BACKGROUND

Chronic pain is frequently comorbid with depression in clinical practice. Recently, alterations in gut microbiota and metabolites derived therefrom have been found to potentially contribute to abnormal behaviors and cognitive dysfunction via the "microbiota-gut-brain" axis.

METHODS

PubMed was searched and we selected relevant studies before October 1, 2019. The search keyword string included "pain OR chronic pain" AND "gut microbiota OR metabolites"; "depression OR depressive disorder" AND "gut microbiota OR metabolites". We also searched the reference lists of key articles manually.

RESULTS

This review systematically summarized the recent evidence of gut microbiota and metabolites in chronic pain and depression in animal and human studies. The results showed the pathogenesis and therapeutics of chronic pain and depression might be partially due to gut microbiota dysbiosis. Importantly, bacteria-derived metabolites, including short-chain fatty acids, tryptophan-derived metabolites, and secondary bile acids, offer new insights into the potential linkage between key triggers in gut microbiota and potential mechanisms of depression.

CONCLUSION

Studying gut microbiota and its metabolites has contributed to the understanding of comorbidity of chronic pain and depression. Consequently, modulating dietary structures or supplementation of specific bacteria may be an available strategy for treating chronic pain and depression.

Modulation of gut microbiota by chlorogenic acid pretreatment on rats with adrenocorticotropic hormone induced depression-like behavior

Gut microbiota dysbiosis has been implicated as a vital element in the development or exacerbation of mental disorders, such as major depressive disorder (MDD). Based on the current interest in the gut-brain axis, we investigate the effects of chlorogenic acid (CGA) on gut microbiota in a rat model of MDD. Depression was induced by the adrenocorticotropic hormone (ACTH, 100 μg per rat) in male Wistar rats, which were intervened with using saline or CGA (500 mg kg-1). Behavioral changes and serum parameters were assessed and fecal samples were analyzed by 16S rRNA gene sequencing. Our studies demonstrated that CGA pretreatment ameliorated depression-like behavior (SPT, FST, TST, and OFT) and serum biochemical levels (5-HT, DA, IL-6, and TNF-α) in ACTH-induced depression rats. In addition, CGA ameliorated the decrease in fecal microbiota diversity in ACTH-treated rats. In particular, at the genus level, the changes in the relative abundance of some key bacteria such as Desulfovibrionales, Desulfovibrio, Klebsiella, Burkholderiales, and Bifidobacterium were modulated by CGA pretreatment. These results indicated that CGA could modify the gut microbial community structure, which may contribute to its antidepressant effects.

The human gut bacteria Christensenellaceae are widespread, heritable, and associated with health

The Christensenellaceae, a recently described family in the phylum Firmicutes, is emerging as an important player in human health. The relative abundance of Christensenellaceae in the human gut is inversely related to host body mass index (BMI) in different populations and multiple studies, making its relationship with BMI the most robust and reproducible link between the microbial ecology of the human gut and metabolic disease reported to date. The family is also related to a healthy status in a number of other different disease contexts, including obesity and inflammatory bowel disease. In addition, Christensenellaceae is highly heritable across multiple populations, although specific human genes underlying its heritability have so far been elusive. Further research into the microbial ecology and metabolism of these bacteria should reveal mechanistic underpinnings of their host-health associations and enable their development as therapeutics.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Gut microbiome: An intermediary to neurotoxicity

There is growing recognition that the gut microbiome is an important regulator for neurological functions. This review provides a summary on the role of gut microbiota in various neurological disorders including neurotoxicity induced by environmental stressors such as drugs, environmental contaminants, and dietary factors. We propose that the gut microbiome remotely senses and regulates CNS signaling through the following mechanisms: 1) intestinal bacteria-mediated biotransformation of neurotoxicants that alters the neuro-reactivity of the parent compounds; 2) altered production of neuro-reactive microbial metabolites following exposure to certain environmental stressors; 3) bi-directional communication within the gut-brain axis to alter the intestinal barrier integrity; and 4) regulation of mucosal immune function. Distinct microbial metabolites may enter systemic circulation and epigenetically reprogram the expression of host genes in the CNS, regulating neuroinflammation, cell survival, or cell death. We will also review the current tools for the study of the gut-brain axis and provide some suggestions to move this field forward in the future.

Animal models of major depressive disorder and the implications for drug discovery and development

INTRODUCTION

Depression is a highly debilitating psychiatric disorder that affects the global population and causes severe disabilities and suicide. Depression pathogenesis remains poorly understood, and the disorder is often treatment-resistant and recurrent, necessitating the development of novel therapies, models and concepts in this field. Areas covered: Animal models are indispensable for translational biological psychiatry, and markedly advance the study of depression. Novel approaches continuously emerge that may help untangle the disorder heterogeneity and unclear categories of disease classification systems. Some of these approaches include widening the spectrum of model species used for translational research, using a broader range of test paradigms, exploring new pathogenic pathways and biomarkers, and focusing more closely on processes beyond neural cells (e.g. glial, inflammatory and metabolic deficits). Expert opinion: Dividing the core symptoms into easily translatable, evolutionarily conserved phenotypes is an effective way to reevaluate current depression modeling. Conceptually novel approaches based on the endophenotype paradigm, cross-species trait genetics and 'domain interplay concept', as well as using a wider spectrum of model organisms and target systems will enhance experimental modeling of depression and antidepressant drug discovery.