Gut microbiota's effect on mental health: The gut-brain axis

Ketamine interactions with gut-microbiota in rats: relevance to its antidepressant and anti-inflammatory properties

BACKGROUND

Appreciable evidence suggest that dysbiosis in microbiota, reflected in gut microbial imbalance plays a key role in the pathogenesis of neuropsychiatric disorders including depression and inflammatory diseases. Recently, the antidepressant properties of ketamine have gained prominence due to its fast and long lasting effects. Additional uses for ketamine in inflammatory disorders such as irritable bowel syndrome have been suggested. However, ketamine's exact mechanism of action and potential effects on microbiome is not known. Here, we examined the effects of low dose ketamine, known to induce antidepressant effects, on stool microbiome profile in adult male Wistar rats. Animals (5/group) were injected intraperitoneally with ketamine (2.5 mg/kg) or saline, daily for 7 days and sacrificed on day 8 when intestinal stools were collected and stored at - 80 °C. DNA was extracted from the samples and the 16 S rRNA gene-based microbiota analysis was performed using 16S Metagenomics application.

RESULTS

At genus-level, ketamine strikingly amplified Lactobacillus, Turicibacter and Sarcina by 3.3, 26 and 42 fold, respectively. Conversely, opportunistic pathogens Mucispirillum and Ruminococcus were reduced by approximately 2.6 and 26 fold, respectively, in ketamine group. Low levels of Lactobacillus and Turicibacter are associated with various disorders including depression and administration of certain species of Lactobacillus ameliorates depressive-like behavior in animal models. Hence, some of the antidepressant effects of ketamine might be mediated through its interaction with these gut bacteria. Additionally, high level of Ruminococcus is positively associated with the severity of irritable bowel syndrome (IBS), and some species of Mucispirillum have been associated with intestinal inflammation. Indirect evidence of anti-inflammatory role of Sarcina has been documented. Hence, some of the anti-inflammatory effects of ketamine and its usefulness in specific inflammatory diseases including IBS may be mediated through its interaction with these latter bacteria.

CONCLUSION

Our data suggest that at least some of the antidepressant and anti-inflammatory effects of daily ketamine treatment for 7 days may be mediated via its interaction with specific gut bacteria. These findings further validate the usefulness of microbiome as a target for therapeutic intervention and call for more detailed investigation of microbiome interaction with central mediators of mood and/or inflammatory disorders.

Differences in Anxiety Levels of Various Murine Models in Relation to the Gut Microbiota Composition

Psychobiotics are probiotic strains that confer mental health benefits to the host through the modulation of the gut microbial population. Mounting evidence shows that the gut microbiota play an important role in communication within the gut–brain axis. However, the relationship between the host genetics and the gut microbiota and their influence on anxiety are still not fully understood. Hence, in our research, we attempted to draw a connection between host genetics, microbiota composition, and anxiety by performing an elevated plus maze (EPM) test on four genetically different mice. Four different breeds of 5-week-old mice were used in this experiment: Balb/c, Orient C57BL/6N, Taconic C57BL/6N, and Taconic C57BL/6J. After 1 week of adaptation, their initial anxiety level was monitored using the EPM test via an EthoVision XT, a standardized software used for behavorial testing. Significant differences in the initial anxiety level and microbial composition were detected. Subsequently, the microbiota of each group was modulated by the administration of either a probiotic, fecal microbiota transplantation, or antibiotics. Changes were observed in host anxiety levels in correlation to the shift of the gut microbiota. Our results suggest that the microbiota, host genetics, and psychological symptoms are strongly related, yet the deeper mechanistic links need further exploration.

Gut thinking: the gut microbiome and mental health beyond the head

Background: In recent decades, dominant models of mental illness have become increasingly focused on the head, with mental disorders being figured as brain disorders. However, research into the active role that the microbiome-gut-brain axis plays in affecting mood and behaviour may lead to the conclusion that mental health is more than an internalised problem of individual brains. Objective: This article explores the implications of shifting understandings about mental health that have come about through research into links between the gut microbiome and mental health problems such as depression and anxiety. It aims to analyse the different ways that the lines between mind and body and mental and physical health are re-shaped by this research, which is starting to inform clinical and public understanding. Design: As mental health has become a pressing issue of political and public concern it has become increasingly constructed in socio-cultural and personal terms beyond clinical spaces, requiring a conceptual response that exceeds biomedical inquiry. This article argues that an interdisciplinary critical medical humanities approach is well positioned to analyse the impact of microbiome-gut-brain research on conceptions of mind. Results: The entanglement of mind and matter evinced by microbiome-gut-brain axis research potentially provides a different way to conceptualise the physical and social concomitants of mental distress. Conclusion: Mental health is not narrowly located in the head but is assimilated by the physical body and intermingled with the natural world, requiring different methods of research to unfold the meanings and implications of gut thinking for conceptions of human selfhood.

Autointoxication and historical precursors of the microbiome-gut-brain axis

This article focuses on autointoxication, a discredited medical theory from the late nineteenth century that provides important points of reflection for today's research on the role of microbes in the human gut for mental health. It considers how the theory of autointoxication, which came into great prominence amongst physicians and the general public worldwide, fell from grace by the middle of the twentieth century, and briefly asks why studies of the human microbiome are now back in vogue. It departs from earlier articles on the topic firstly by arguing that autointoxication theory was especially prevalent in France, and secondly by focusing on the application of this theory to mental health. Bringing to light medical treatises and theses from this period which have so far remained unexamined, it shows that examining the development and reception of medical theories form the past can help us today in understanding both the pitfalls and promise of research in this area.

Psychopharmacology: From serendipitous discoveries to rationale design, but what next?

Psychopharmacology really developed as a discipline from the mid-20th century with the discovery of a number of new classes of psychoactive drugs which could modify behaviour. These drugs were discovered as a consequence of clinical observations of patients, often being treated for other conditions. These serendipitous discoveries were the start of an era of drug development which has led to the antidepressants, antipsychotics, anxiolytics and mood stabilisers used today. Subsequent research focused on understanding why these drugs were effective, and used this information to develop a second generation of drugs that were more selective for their therapeutic targets, and therefore had reduced side effects and improved safety and tolerability. After a period of decline in new discoveries and withdrawal of the majority of the major pharmaceutical companies from active development programmes in psychiatry, new avenues are emerging fuelling renewed interest in this area.

Prospective association between combined healthy lifestyles and risk of depressive symptoms in the French NutriNet-Santé cohort

BACKGROUND

Several modifiable lifestyle indicators, including diet, smoking, alcohol consumption, weight and physical activity have been associated with depression; however, their combined effect has been less studied. The aim of this study was to calculate a Healthy Lifestyle Index (HLI) composed of the 5 above-mentioned indicators and investigate its association with incident depressive symptoms.

METHODS

The study sample consisted of 25,837 participants from the NutriNet-Santé study, initially free of depressive symptoms. The HLI was computed by assigning 1 point to each lifestyle indicator namely healthy diet, healthy weight, moderate or high physical activity, never smoking and low alcohol consumption. Depressive symptoms were measured using the Center for Epidemiologic Studies Depression Scale (CES-D). Hazard Ratios were estimated using Cox proportional hazards models and population attributable risks (PAR) were calculated.

RESULTS

A total of 2112 incident cases of depressive symptoms were identified over a mean follow-up of 5 years. After accounting for a wide range of potential confounders, a 1-point increase in the HLI was associated with a 10% (95% CI 6%; 13%) reduction in the risk of depressive symptoms. The estimated PAR representing the proportion of cases that are attributable to non-adherence to specific healthy lifestyle indicators were 8% for healthy diet, 5% for healthy weight, 5% for non-smoking and 14% for the non-adherence to a combination of healthy diet, healthy weight and non-smoking.

LIMITATIONS

Some unmeasured factors related to both depression and lifestyle indicators, such as family history of depressive disorder, stressful life events, and sleep disorders might have led to potential residual confounding.

CONCLUSIONS

Modifying unhealthy lifestyles, especially diet, weight and smoking, is a potential target of major interest in the prevention of depressive symptoms in adults.

Gut Microbiota Profiling and Gut-Brain Crosstalk in Children Affected by Pediatric Acute-Onset Neuropsychiatric Syndrome and Pediatric Autoimmune Neuropsychiatric Disorders Associated With Streptococcal Infections

Pediatric acute-onset neuropsychiatric syndrome (PANS) and pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections syndrome (PANDAS) are conditions that impair brain normal neurologic function, resulting in the sudden onset of tics, obsessive-compulsive disorder, and other behavioral symptoms. Recent studies have emphasized the crosstalk between gut and brain, highlighting how gut composition can influence behavior and brain functions. Thus, the present study investigates the relationship between PANS/PANDAS and gut microbiota ecology. The gut composition of a cohort of 30 patients with PANS/PANDAS was analyzed and compared to control subjects using 16S rRNA-based metagenomics. Data were analyzed for their α- and β-diversity; differences in bacterial distribution were detected by Wilcoxon and LEfSe tests, while metabolic profile was predicted via PICRUSt software. These analyses demonstrate the presence of an altered bacterial community structure in PANS/PANDAS patients with respect to controls. In particular, ecological analysis revealed the presence of two main clusters of subjects based on age range. Thus, to avoid age bias, data from patients and controls were split into two groups: 4-8 years old and >9 years old. The younger PANS/PANDAS group was characterized by a strong increase in Bacteroidetes; in particular, Bacteroides, Odoribacter, and Oscillospira were identified as potential microbial biomarkers of this composition type. Moreover, this group exhibited an increase of several pathways concerning the modulation of the antibody response to inflammation within the gut as well as a decrease in pathways involved in brain function (i.e., SCFA, D-alanine and tyrosine metabolism, and the dopamine pathway). The older group of patients displayed a less uniform bacterial profile, thus impairing the identification of distinct biomarkers. Finally, Pearson's analysis between bacteria and anti-streptolysin O titer reveled a negative correlation between genera belonging to Firmicutes phylum and anti-streptolysin O while a positive correlation was observed with Odoribacter. In conclusion, this study suggests that streptococcal infections alter gut bacterial communities leading to a pro-inflammatory status through the selection of specific bacterial strains associated with gut inflammation and immune response activation. These findings highlight the possibility of studying bacterial biomarkers associated with this disorder and might led to novel potential therapeutic strategies.

The Brain-Intestinal Mucosa-Appendix- Microbiome-Brain Loop

The brain and the gut are connected from early fetal life. The mother's exposure to microbial molecules is thought to exert in utero developmental effects on the fetus. These effects could importantly underpin the groundwork for subsequent pathophysiological mechanisms for achieving immunological tolerance and metabolic equilibrium post birth, events that continue through to 3-4 years of age. Furthermore, it is understood that the microbiome promotes cues that instruct the neonate's mucosal tissues and skin in the language of molecular and cellular biology. Post birth mucosal lymphoid tissue formation and maturation (most probably including the vermiform appendix) is microbiota-encouraged co-establishing the intestinal microbiome with a developing immune system. Intestinal mucosal tissue maturation loops the brain-gut-brain and is postulated to influence mood dispositions via shifts in the intestinal microbiome phyla. A plausible appreciation is that dysregulated pro-inflammatory signals from intestinal resident macrophages could breach the loop by providing adverse mood signals via vagus nerve afferents to the brain. In this commentary, we further suggest that the intestinal resident macrophages act as an upstream traffic controller of translocated microbes and metabolites in order to maintain local neuro-endocrine-immunological equilibrium. When macrophages are overwhelmed through intestinal microbiome and intestinal epithelial cell dysbiosis, pro-inflammatory signals are sustained, which may then lead to mood disorders. The administration of probiotics as an adjunctive medicine co-administered with antidepressant medications in improving depressed mood may have biological and clinical standing.

The choroid plexus epithelium as a novel player in the stomach-brain axis during Helicobacter infection

Several studies suggest a link between shifts in gut microbiota and neurological disorders. Recently, we reported a high prevalence of Helicobacter suis (H. suis) in patients with Parkinson's disease. Here, we evaluated the effect of gastric H. suis infection on the brain in mice. One month of infection with H. suis resulted in increased brain inflammation, reflected in activation of microglia and cognitive decline. Additionally, we detected choroid plexus inflammation and disruption of the epithelial blood-cerebrospinal fluid (CSF) barrier upon H. suis infection, while the endothelial blood-brain barrier (BBB) remained functional. These changes were accompanied by leakage of the gastrointestinal barrier and low-grade systemic inflammation, suggesting that H. suis-evoked gastrointestinal permeability and subsequent peripheral inflammation induces changes in brain homeostasis via changes in blood-CSF barrier integrity. In conclusion, this study shows for the first time that H. suis infection induces inflammation in the brain associated with cognitive decline and that the choroid plexus is a novel player in the stomach-brain axis.