Stress and the Role of the Gut-Brain Axis in the Pathogenesis of Schizophrenia: A Literature Review

Perturbations in gut microbiota composition in schizophrenia

Schizophrenia is a severe, complex and long-term psychiatric disorder with unclear etiology. Gut microbes influence the central nervous system via the gut-brain axis. Consequently, investigations of the relationship between gut microbes and schizophrenia are warranted. This study involved 29 patients with schizophrenia and 30 age-matched normal controls. After 16S rRNA gene sequencing and whole-genome shotgun metagenomic sequencing, we analyzed microbial diversity, composition, and function. According to 16S rRNA and metagenomic gene sequencing results, patients with schizophrenia had higher abundances of Clostridium and Megasphaera. Functional analysis showed that sphingolipid, phosphonates and phosphinates, as well as glutamine metabolism were associated with the occurrence and development of schizophrenia. Our data suggest that the gut microbiota exerts an effect on patients with schizophrenia, providing valuable insights into the potential regulation of in the context of this disorder.

Pilot study indicates that a gluten-free diet lowers oxidative stress for gluten-sensitive persons with schizophrenia

One-third of people with schizophrenia have elevated levels of anti-gliadin antibodies (AGA IgG). A 5-week randomized double-blind pilot study was performed in 2014-2017 in an inpatient setting to test the effect of a gluten-free diet (GFD) on participants with schizophrenia or schizoaffective disorder who also had elevated AGA IgG (≥ 20 U) but were negative for celiac disease. This earlier pilot study reported that the GFD-group showed improved gastrointestinal and psychiatric symptoms, and also improvements in TNF-α and the inflammatory cytokine IL-23. Here, we performed measurements of these banked plasma samples to detect levels of oxidative stress (OxSt) using a recently developed iridium (Ir)-reducing capacity assay. Triplicate measurements of these samples showed an Intraclass Correlation Coefficient of 0.84 which indicates good reproducibility. Further, a comparison of the OxSt measurements at the baseline and 5-week end-point for this small sample size shows that the GFD-group (N = 7) had lowered OxSt levels compared to the gluten-containing diet group (GCD; N = 9; p = 0.05). Finally, we showed that improvements in OxSt over these 5 weeks were correlated to improvements in gastrointestinal (r = +0.64, p = 0.0073) and psychiatric (r = +0.52, p = 0.039) symptoms. Also, we showed a possible association between the decrease in OxSt and the lowered levels of IL-23 (r = +0.44, p = 0.087), although without statistical significance. Thus, the Ir-reducing capacity assay provides a simple, objective measure of OxSt with the results providing further evidence that inflammation, redox dysregulation and OxSt may mediate interactions between the gut and brain.

A soy protein enzymatic digest mitigates Nrf2-related oxidative stress and attenuates depression-like behavior in a mouse model of sub-chronic restraint stress

Continuous oxidative stress conditions have been identified as a major cause of various neuropsychiatric disorders, including depression. The present study investigated the potential antidepressant-like effects of a soy protein enzymatic digest (SPD) containing soy-deprestatin, which is a soy-derived peptide with reported antidepressant-like effects, as well as its ability to mitigate oxidative stress in the brain caused by sub-chronic restraint stress. Mice were divided into two groups: a control group and restraint stress group. The restraint stress group was further divided into two groups administered water or SPD. After repeated short-time restraints over five days, we evaluated immobility times in the tail suspension test, and antioxidant enzyme activities, glutathione levels, oxidative stress maker levels, and the gene expression levels of Nrf2 and antioxidant enzymes in the brain. The results obtained showed that the oral administration of SPD reduced immobility times in mice exposed to restraint stress. In comparisons with the water-treated restraint group, the administration of SPD restored superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase activities and glutathione levels and prevented restraint stress-induced increases in malondialdehyde, carbonyl protein, and 8-OHdG levels in the restraint stress group. In addition, high expression levels of Nrf2, HO-1, NQO-1 and GCLC were observed in the SPD-treated restraint group. These results suggest that SPD attenuated repeated restraint stress-induced depression-like behaviors by mitigating oxidative stress through the activation of the Nrf2 signaling pathway.

The correlation between gut microbiota and both neurotransmitters and mental disorders: A narrative review

The gastrointestinal tract is embedded with microorganisms of numerous genera, referred to as gut microbiota. Gut microbiota has multiple effects on many body organs, including the brain. There is a bidirectional connection between the gut and brain called the gut-brain-axis, and these connections are formed through immunological, neuronal, and neuroendocrine pathways. In addition, gut microbiota modulates the synthesis and functioning of neurotransmitters. Therefore, the disruption of the gut microbiota in the composition or function, which is known as dysbiosis, is associated with the pathogenesis of many mental disorders, such as schizophrenia, depression, and other psychiatric disorders. This review aims to summarize the modulation role of the gut microbiota in 4 prominent neurotransmitters (tryptophan and serotonergic system, dopamine, gamma-aminobutyric acid, and glutamate), as well as its association with 4 psychiatric disorders (schizophrenia, depression, anxiety disorders, and autism spectrum disorder). More future research is required to develop efficient gut-microbiota-based therapies for these illnesses.

Intricate role of intestinal microbe and metabolite in schizophrenia

BACKGROUND

The brain-gut axis has gained increasing attention due to its contribution to the etiology of various central nervous system disorders. This study aims to elucidate the hypothesis that schizophrenia is associated with disturbances in intestinal microflora and imbalance in intestinal metabolites. By exploring the intricate relationship between the gut and the brain, with the goal of offering fresh perspectives and valuable insights into the potential contribution of intestinal microbial and metabolites dysbiosis to the etiology of schizophrenia.

METHODS

In this study, we used a 16S ribosomal RNA (16S rRNA) gene sequence-based approach and an untargeted liquid chromatography-mass spectrometry-based metabolic profiling approach to measure the gut microbiome and microbial metabolites from 44 healthy controls, 41 acute patients, and 39 remission patients, to evaluate whether microbial dysbiosis and microbial metabolite biomarkers were linked with the severity of schizophrenic symptoms.

RESULTS

Here, we identified 20 dominant disturbances in the gut microbial composition of patients compared with healthy controls, with 3 orders, 4 families, 9 genera, and 4 species. Several unique bacterial taxa associated with schizophrenia severity. Compared with healthy controls, 145 unusual microflora metabolites were detected in the acute and remission groups, which were mainly involved in environmental information processing, metabolism, organismal systems, and human diseases in the Kyoto encyclopedia of genes and genomes pathway. The Sankey diagram showed that 4 abnormal intestinal and 4 anomalous intestinal microbial metabolites were associated with psychiatric clinical symptoms.

CONCLUSIONS

These findings suggest a possible interactive influence of the gut microbiota and their metabolites on the pathophysiology of schizophrenia.

Dissecting the Relationship Between Neuropsychiatric and Neurodegenerative Disorders

Neurodegenerative diseases (NDDs) and neuropsychiatric disorders (NPDs) are two common causes of death in elderly people, which includes progressive neuronal cell death and behavioral changes. NDDs include Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis, and motor neuron disease, characterized by cognitive defects and memory impairment, whereas NPDs include depression, seizures, migraine headaches, eating disorders, addictions, palsies, major depressive disorders, anxiety, and schizophrenia, characterized by behavioral changes. Mounting evidence demonstrated that NDDs and NPDs share an overlapping mechanism, which includes post-translational modifications, the microbiota-gut-brain axis, and signaling events. Mounting evidence demonstrated that various drug molecules, namely, natural compounds, repurposed drugs, multitarget directed ligands, and RNAs, have been potentially implemented as therapeutic agents against NDDs and NPDs. Herein, we highlighted the overlapping mechanism, the role of anxiety/stress-releasing factors, cytosol-to-nucleus signaling, and the microbiota-gut-brain axis in the pathophysiology of NDDs and NPDs. We summarize the therapeutic application of natural compounds, repurposed drugs, and multitarget-directed ligands as therapeutic agents. Lastly, we briefly described the application of RNA interferences as therapeutic agents in the pathogenesis of NDDs and NPDs. Neurodegenerative diseases and neuropsychiatric diseases both share a common signaling molecule and molecular phenomenon, namely, pro-inflammatory cytokines, γCaMKII and MAPK/ERK, chemokine receptors, BBB permeability, and the gut-microbiota-brain axis. Studies have demonstrated that any alterations in the signaling mentioned above molecules and molecular phenomena lead to the pathophysiology of neurodegenerative diseases, namely, Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis, and neuropsychiatric disorders, such as bipolar disorder, schizophrenia, depression, anxiety, autism spectrum disorder, and post-traumatic stress disorder.

Stress induced exacerbation of Alzheimer's disease brain pathology is thwarted by co-administration of nanowired cerebrolysin and monoclonal amyloid beta peptide antibodies with serotonin 5-HT6 receptor antagonist SB-399885

Alzheimer's disease is one of the devastating neurodegenerative diseases affecting mankind worldwide with advancing age mainly above 65 years and above causing great misery of life. About more than 7 millions are affected with Alzheimer's disease in America in 2023 resulting in huge burden on health care system and care givers and support for the family. However, no suitable therapeutic measures are available at the moment to enhance quality of life to these patients. Development of Alzheimer's disease may reflect the stress burden of whole life inculcating the disease processes of these neurodegenerative disorders of the central nervous system. Thus, new strategies using nanodelivery of suitable drug therapy including antibodies are needed in exploring neuroprotection in Alzheimer's disease brain pathology. In this chapter role of stress in exacerbating Alzheimer's disease brain pathology is explored and treatment strategies are examined using nanotechnology based on our own investigation. Our observations clearly show that restraint stress significantly exacerbate Alzheimer's disease brain pathology and nanodelivery of a multimodal drug cerebrolysin together with monoclonal antibodies (mAb) to amyloid beta peptide (AβP) together with a serotonin 5-HT6 receptor antagonist SB399885 significantly thwarted Alzheimer's disease brain pathology exacerbated by restraint stress, not reported earlier. The possible mechanisms and future clinical significance is discussed.

Neuropsychiatric and Neurological Diseases in Relation to the Microbiota-Gut-Brain Axis: From Research to Clinical Care

Neurological disease is on the upswing, the second leading cause of mortality and a significant cause of disability. The term gut-brain axis emphasizes a dynamic two-way communication between the central nervous system and the gastrointestinal system. The microbiome is being linked to more and more clinical and preclinical studies as a major risk factor for neurological diseases. Overall, 288 studies were identified initially. After screening, data extraction, and applying the inclusion and exclusion criteria, 37 articles were included in the study. Changes in the gut microbial population composition have been correlated to many neurological and neurodevelopmental diseases.

The Role of Pharmacogenetics in Personalizing the Antidepressant and Anxiolytic Therapy

Pharmacotherapy for neuropsychiatric disorders, such as anxiety and depression, has been characterized by significant inter-individual variability in drug response and the development of side effects. Pharmacogenetics, as a key part of personalized medicine, aims to optimize therapy according to a patient's individual genetic signature by targeting genetic variations involved in pharmacokinetic or pharmacodynamic processes. Pharmacokinetic variability refers to variations in a drug's absorption, distribution, metabolism, and elimination, whereas pharmacodynamic variability results from variable interactions of an active drug with its target molecules. Pharmacogenetic research on depression and anxiety has focused on genetic polymorphisms affecting metabolizing cytochrome P450 (CYP) and uridine 5'-diphospho-glucuronosyltransferase (UGT) enzymes, P-glycoprotein ATP-binding cassette (ABC) transporters, and monoamine and γ-aminobutyric acid (GABA) metabolic enzymes, transporters, and receptors. Recent pharmacogenetic studies have revealed that more efficient and safer treatments with antidepressants and anxiolytics could be achieved through genotype-guided decisions. However, because pharmacogenetics cannot explain all observed heritable variations in drug response, an emerging field of pharmacoepigenetics investigates how epigenetic mechanisms, which modify gene expression without altering the genetic code, might influence individual responses to drugs. By understanding the epi(genetic) variability of a patient's response to pharmacotherapy, clinicians could select more effective drugs while minimizing the likelihood of adverse reactions and therefore improve the quality of treatment.

The Role of Gut Microbiota in Various Neurological and Psychiatric Disorders-An Evidence Mapping Based on Quantified Evidence

Methods

We searched PubMed, Cochrane Library, and Epistemonikos to identify systematic reviews and meta-analysis (SRs). We searched for neurological diseases and psychiatric disorders, including Alzheimer's disease (AD), attention deficit hyperactivity disorder (ADHD), amyotrophic lateral sclerosis (ALS), autism spectrum disorder (ASD), anorexia nervosa (AN), bipolar disorder (BD), eating disorder (ED), generalized anxiety disorder (GAD), major depressive disorder (MDD), multiple sclerosis (MS), obsessive compulsive disorder (OCD), Parkinson's disease (PD), posttraumatic stress disorder (PTSD), spinal cord injury (SCI), schizophrenia, and stroke. We used A Measurement Tool to Assess Systematic Reviews (AMSTAR-2) to evaluate the quality of included SRs. We also created an evidence map showing the role of gut microbiota in neurological diseases and the certainty of the evidence.

Results

In total, 42 studies were included in this evidence mapping. Most findings were obtained from observational studies. According to the AMSTAR-2 assessment, 21 SRs scored "critically low" in terms of methodological quality, 16 SR scored "low," and 5 SR scored "moderate." A total of 15 diseases have been investigated for the potential association between gut microbiome alpha diversity and disease, with the Shannon index and Simpson index being the most widely studied. A total of 12 diseases were investigated for potential link between beta diversity and disease. At the phylum level, Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, and Verrucomicrobia were more researched. At the genus level, Prevotella, Coprococcus, Parabacteroides, Phascolarctobacterium, Escherichia Shigella, Alistipes, Sutteralla, Veillonella, Odoribacter, Faecalibacterium, Bacteroides, Bifidobacterium, Dialister, and Blautia were more researched. Some diseases have been found to have specific flora changes, and some diseases have been found to have common intestinal microbiological changes.

Conclusion

We found varied levels of evidence for the associations between gut microbiota and neurological diseases; some gut microbiota increased the risk of neurological diseases, whereas others showed evidence of benefit that gut microbiota might be promising therapeutic targets for such diseases.

Low income and schizophrenia risk: a narrative review

Despite decades of research, the precise etiology of schizophrenia is not fully understood. Ample evidence indicates that the disorder derives from a complex interplay of genetic and environmental factors during vulnerable stages of brain maturation. Among the plethora of risk factors investigated, stress, pre- and perinatal insults, and cannabis use have been repeatedly highlighted as crucial environmental risk factors for schizophrenia. Compelling findings from population-based longitudinal studies suggest low income as an additional risk factor for future schizophrenia diagnosis, but underlying mechanisms remain unclear. In this narrative review, we 1) summarize the literature in support of a relationship between low (parental) income and schizophrenia risk, and 2) explore the mediating role of chronic stress, pre- and perinatal factors, and cannabis use as established risk factors for schizophrenia. Our review describes how low income facilitates the occurrence and severity of these established risk factors and thus contributes to schizophrenia liability. The broadest influence of low income was identified for stress, as low income was found to be associated with exposure to a multitude of severe psychological and physiological stressors. This narrative review adds to the growing literature reporting a close relationship between income and mental health.