Absence of the gut microbiota enhances anxiety-like behavior and neuroendocrine response to acute stress in rats

Brain-gut-microbiota axis: challenges for translation in psychiatry

PURPOSE

The accruing data linking the gut microbiome to the development and function of the central nervous system has been proposed as a paradigm shift in neuroscience. The gut microbiota can communicate with the brain via neuroimmune, neuroendocrine, and neural pathways comprising the brain-gut-microbiota axis. Dysfunctional neuroimmune pathways are implicated in stress-related psychiatric disorders.

METHODS

Using depression as our primary example, we review both the preclinical and clinical evidence supporting the possible role played by the gut microbiota in stress-related psychiatric disorders. We consider how this can inform future treatment strategies and outline the challenges and necessary studies for moving the field forward.

RESULTS

The role played by the gut microbiota has not been fully elucidated in psychiatric populations. Although tempting to speculate that psychiatric patients may benefit from therapeutic modulation of the brain-gut-microbiota axis, the translational applications of the results obtained in rodent studies have yet to be demonstrated.

CONCLUSIONS

Evidence of altered gut microbiota composition and function in psychiatric patients is limited and cannot be regarded as proven. Moreover the efficacy of targeting the gut microbiota has not yet been established, and needs further investigation.

The microbiome: A key regulator of stress and neuroinflammation

There is a growing emphasis on the relationship between the complexity and diversity of the microorganisms that inhabit our gut (human gastrointestinal microbiota) and health/disease, including brain health and disorders of the central nervous system. The microbiota-gut-brain axis is a dynamic matrix of tissues and organs including the brain, glands, gut, immune cells and gastrointestinal microbiota that communicate in a complex multidirectional manner to maintain homeostasis. Changes in this environment can lead to a broad spectrum of physiological and behavioural effects including hypothalamic-pituitary-adrenal (HPA) axis activation, and altered activity of neurotransmitter systems and immune function. While an appropriate, co-ordinated physiological response, such as an immune or stress response are necessary for survival, a dysfunctional response can be detrimental to the host contributing to the development of a number of CNS disorders. In this review, the involvement of the gastrointestinal microbiota in stress-mediated and immune-mediated modulation of neuroendocrine, immune and neurotransmitter systems and the consequential behaviour is considered. We also focus on the mechanisms by which commensal gut microbiota can regulate neuroinflammation and further aim to exploit our understanding of their role in stress-related disorders as a consequence of neuroinflammatory processes.

The Second Brain: Is the Gut Microbiota a Link Between Obesity and Central Nervous System Disorders?

The gut-brain axis is a bi-directional integrated system composed by immune, endocrine, and neuronal components by which the gap between the gut microbiota and the brain is significantly impacted. An increasing number of different gut microbial species are now postulated to regulate brain function in health and disease. The westernized diet is hypothesized to be the cause of the current obesity levels in many countries, a major socio-economical health problem. Experimental and epidemiological evidence suggest that the gut microbiota is responsible for significant immunologic, neuronal, and endocrine changes that lead to obesity. We hypothesize that the gut microbiota, and changes associated with diet, affect the gut-brain axis and may possibly contribute to the development of mental illness. In this review, we discuss the links between diet, gut dysbiosis, obesity, and immunologic and neurologic diseases that impact brain function and behavior.

A gut (microbiome) feeling about the brain

PURPOSE OF REVIEW

There is an increasing realization that the microorganisms which reside within our gut form part of a complex multidirectional communication network with the brain known as the microbiome-gut-brain axis. In this review, we focus on recent findings which support a role for this axis in modulating neurodevelopment and behavior.

RECENT FINDINGS

A growing body of research is uncovering that under homeostatic conditions and in response to internal and external stressors, the bacterial commensals of our gut can signal to the brain through a variety of mechanisms to influence processes such neurotransmission, neurogenesis, microglia activation, and modulate behavior. Moreover, the mechanisms underlying the ability of stress to modulate the microbiota and also for microbiota to change the set point for stress sensitivity are being unraveled. Dysregulation of the gut microbiota composition has been identified in a number of psychiatric disorders, including depression. This has led to the concept of bacteria that have a beneficial effect upon behavior and mood (psychobiotics) being proposed for potential therapeutic interventions.

SUMMARY

Understanding the mechanisms by which the bacterial commensals of our gut are involved in brain function may lead to the development of novel microbiome-based therapies for these mood and behavioral disorders.

"WHAT'S BUGGING THE GUT IN OCD?" A REVIEW OF THE GUT MICROBIOME IN OBSESSIVE-COMPULSIVE DISORDER

The gut microbiome has become a topic of major interest as of late, with a new focus specifically on psychiatric disorders. Recent studies have revealed that variations in the composition of the gut microbiota may influence anxiety and mood and vice versa. Keeping the concept of this bidirectional "microbiota-gut-brain" axis in mind, this review aims to shed light on how these findings may also be implicated in obsessive-compulsive disorder (OCD); potentially outlining a novel etiological pathway of interest for future research in the field.

Naproxen, a Nonsteroidal Anti-Inflammatory Drug, Can Affect Daily Hypobaric Hypoxia-Induced Alterations of Monoamine Levels in Different Areas of the Brain in Male Rats

Goswami, Ananda Raj, Goutam Dutta, and Tusharkanti Ghosh. Naproxen, a nonsteroidal anti-inflammatory drug can affect daily hypobaric hypoxia-induced alterations of monoamine levels in different areas of the brain in male rats. High Alt Med Biol. 17:133-140, 2016.-The oxidative stress (OS)-induced prostaglandin (PG) release, in hypobaric hypoxic (HHc) condition, may be linked with the changes of brain monoamines. The present study intends to explore the changes of monoamines in hypothalamus (H), cerebral cortex (CC), and cerebellum (CB) along with the motor activity in rats after exposing them to simulated hypobaric condition and the role of PGs on the daily hypobaric hypoxia (DHH)-induced alteration of brain monoamines by administering, an inhibitor of PG synthesis, naproxen. The rats were exposed to a decompression chamber at 18,000 ft for 8 hours per day for 6 days after administration of vehicle or naproxen (18 mg/kg body wt.). The monoamine levels (epinephrine, E; norepinephrine, NE; dopamine, DA; and 5-hydroxytryptamine, 5-HT) in CC, CB, and H were assayed by high-performance liquid chromatography (HPLC) with electrochemical detection, and the locomotor behavior was measured by open field test. The NE and DA levels were decreased in CC, CB, and H of the rat brain in HHc condition. The E and 5-HT levels were decreased in CC, but in H and CB, they remained unaltered in HHc condition. These DHH-induced changes of monoamines in brain areas were prevented after administration of naproxen in HHc condition. The locomotor behavior remained unaltered in HHc condition and after administration of naproxen in HHc condition. The DHH-induced changes of monoamines in the brain in HHc condition are probably linked with PGs that may be induced by OS.

Gut microbiota regulates key modulators of social behavior

Social behavior plays a pivotal role in the mental well-being of an individual. Continuous efforts in the past have led to advancements in the area of how the brain regulates emotion and cognition, while the understanding of human social behavior still remains eluded. A major breakthrough in understanding the etiology of neurological disorders is the recent insight on the role of the gut microbiota (GM). Human GM also referred to as the "forgotten organ" is home to 10(13-14) microorganisms, which is 10 times the number of cells present in the human body. In addition, the gut microbiome (total genome of GM) is 150 times greater as compared to the human genome. An emerging concept gaining worldwide focus and acceptance is that, this much big genome can potentially control human behavior and other biological functions. Herein we hypothesize on the basis of GM's ability to modify brain and behavior and that it can directly or indirectly control social behavior. This review focuses on the association of GM with various domains of social behavior like stress, cognition and anxiety.

Host microbiota modulates development of social preference in mice

BACKGROUND

Mounting evidence indicates that the indigenous gut microbiota exerts long-lasting programming effects on brain function and behaviour.

OBJECTIVE

In this study, we used the germ-free (GF) mouse model, devoid of any microbiota throughout development, to assess the influence of the indigenous microbiota on social preference and repetitive behaviours (e.g. self-grooming).

METHODS AND RESULTS

Using the three-chambered social approach task, we demonstrate that when adult GF mice were given a choice to spend time with a novel mouse or object, they spent significantly more time sniffing and interacting with the stimulus mouse compared to conventionally raised mice (specific pathogen-free, SPF). Time spent in repetitive self-grooming behaviour, however, did not differ between GF and SPF mice. Real-time PCR-based gene expression analysis of the amygdala, a key region that is part of the social brain network, revealed a significant reduction in the mRNA levels of total brain-derived neurotrophic factor (BDNF), BDNF exon I-, IV-, VI-, IX-containing transcripts, and NGFI-A (a signalling molecule downstream of BDNF) in GF mice compared to SPF mice.

CONCLUSION

These results suggest that differential regulation of BDNF exon transcripts in the amygdala by the indigenous microbes may contribute to the altered social development of GF mice.

Microbes & neurodevelopment--Absence of microbiota during early life increases activity-related transcriptional pathways in the amygdala

The mammalian amygdala is a key emotional brain region for eliciting social behaviour, critically involved in anxiety and fear-related behaviours, and hence a focus of research on neurodevelopmental and stress-related disorders such as autism and anxiety. Recently, increasing evidence implicates host-microbe interactions in the aetiology of these conditions. Germ-free (GF) mice, devoid of any microbiota throughout organismal maturation, are a well-established tool to study the effects of absence of the microbiota on host physiology. A growing body of independently replicated findings confirm that GF animals demonstrate altered anxiety-related behaviour and impaired social behaviour. However, the underlying mechanisms of this interaction and the nature of the pathways involved are only insufficiently understood. To further elucidate the molecular underpinnings of microbe-brain interaction, we therefore exploited unbiased genome-wide transcriptional profiling to determine gene expression in the amygdala of GF and GF mice that have been colonised after weaning. Using RNA-sequencing and a comprehensive downstream analysis pipeline we studied the amygdala transcriptome and found significant differences at the levels of differential gene expression, exon usage and RNA-editing. Most surprisingly, we noticed upregulation of several immediate early response genes such as Fos, Fosb, Egr2 or Nr4a1 in association with increased CREB signalling in GF mice. In addition, we found differential expression and recoding of several genes implicated in brain physiology processes such as neurotransmission, neuronal plasticity, metabolism and morphology. In conclusion, our data suggest altered baseline neuronal activity in the amygdala of germ-free animals, which is established during early life and may have implications for understanding development and treatment of neurodevelopmental disorders.

Administration of Lactobacillus helveticus NS8 improves behavioral, cognitive, and biochemical aberrations caused by chronic restraint stress

Increasing numbers of studies have suggested that the gut microbiota is involved in the pathophysiology of stress-related disorders. Chronic stress can cause behavioral, cognitive, biochemical, and gut microbiota aberrations. Gut bacteria can communicate with the host through the microbiota-gut-brain axis (which mainly includes the immune, neuroendocrine, and neural pathways) to influence brain and behavior. It is hypothesized that administration of probiotics can improve chronic-stress-induced depression. In order to examine this hypothesis, the chronic restraint stress depression model was established in this study. Adult specific pathogen free (SPF) Sprague-Dawley rats were subjected to 21 days of restraint stress followed by behavioral testing (including the sucrose preference test (SPT), elevated-plus maze test, open-field test (OFT), object recognition test (ORT), and object placement test (OPT)) and biochemical analysis. Supplemental Lactobacillus helveticus NS8 was provided every day during stress until the end of experiment, and selective serotonin reuptake inhibitor (SSRI) citalopram (CIT) served as a positive control. Results showed that L. helveticus NS8 improved chronic restraint stress-induced behavioral (anxiety and depression) and cognitive dysfunction, showing an effect similar to and better than that of CIT. L. helveticus NS8 also resulted in lower plasma corticosterone (CORT) and adrenocorticotropic hormone (ACTH) levels, higher plasma interleukin-10 (IL-10) levels, restored hippocampal serotonin (5-HT) and norepinephrine (NE) levels, and more hippocampal brain-derived neurotrophic factor (BDNF) mRNA expression than in chronic stress rats. Taken together, these results indicate an anti-depressant effect of L. helveticus NS8 in rats subjected to chronic restraint stress depression and that this effect could be due to the microbiota-gut-brain axis. They also suggest the therapeutic potential of L. helveticus NS8 in stress-related and possibly other kinds of depression.

The gut microbiome and the brain

The human gut microbiome impacts human brain health in numerous ways: (1) Structural bacterial components such as lipopolysaccharides provide low-grade tonic stimulation of the innate immune system. Excessive stimulation due to bacterial dysbiosis, small intestinal bacterial overgrowth, or increased intestinal permeability may produce systemic and/or central nervous system inflammation. (2) Bacterial proteins may cross-react with human antigens to stimulate dysfunctional responses of the adaptive immune system. (3) Bacterial enzymes may produce neurotoxic metabolites such as D-lactic acid and ammonia. Even beneficial metabolites such as short-chain fatty acids may exert neurotoxicity. (4) Gut microbes can produce hormones and neurotransmitters that are identical to those produced by humans. Bacterial receptors for these hormones influence microbial growth and virulence. (5) Gut bacteria directly stimulate afferent neurons of the enteric nervous system to send signals to the brain via the vagus nerve. Through these varied mechanisms, gut microbes shape the architecture of sleep and stress reactivity of the hypothalamic-pituitary-adrenal axis. They influence memory, mood, and cognition and are clinically and therapeutically relevant to a range of disorders, including alcoholism, chronic fatigue syndrome, fibromyalgia, and restless legs syndrome. Their role in multiple sclerosis and the neurologic manifestations of celiac disease is being studied. Nutritional tools for altering the gut microbiome therapeutically include changes in diet, probiotics, and prebiotics.

Prebiotic intake reduces the waking cortisol response and alters emotional bias in healthy volunteers

RATIONALE

There is now compelling evidence for a link between enteric microbiota and brain function. The ingestion of probiotics modulates the processing of information that is strongly linked to anxiety and depression, and influences the neuroendocrine stress response. We have recently demonstrated that prebiotics (soluble fibres that augment the growth of indigenous microbiota) have significant neurobiological effects in rats, but their action in humans has not been reported.

OBJECTIVES

The present study explored the effects of two prebiotics on the secretion of the stress hormone, cortisol and emotional processing in healthy volunteers.

METHODS

Forty-five healthy volunteers received one of two prebiotics (fructooligosaccharides, FOS, or Bimuno®-galactooligosaccharides, B-GOS) or a placebo (maltodextrin) daily for 3 weeks. The salivary cortisol awakening response was sampled before and after prebiotic/placebo administration. On the final day of treatment, participants completed a computerised task battery assessing the processing of emotionally salient information.

RESULTS

The salivary cortisol awakening response was significantly lower after B-GOS intake compared with placebo. Participants also showed decreased attentional vigilance to negative versus positive information in a dot-probe task after B-GOS compared to placebo intake. No effects were found after the administration of FOS.

CONCLUSION

The suppression of the neuroendocrine stress response and the increase in the processing of positive versus negative attentional vigilance in subjects supplemented with B-GOS are consistent with previous findings of endocrine and anxiolytic effects of microbiota proliferation. Further studies are therefore needed to test the utility of B-GOS supplementation in the treatment of stress-related disorders.

Microbial endocrinology: host-bacteria communication within the gut microbiome

The human body is home to trillions of micro-organisms, which are increasingly being shown to have significant effects on a variety of disease states. Evidence exists that a bidirectional communication is taking place between us and our microbiome co-habitants, and that this dialogue is capable of influencing our health in a variety of ways. This review considers how host hormonal signals shape the microbiome, and what in return the microbiome residents may be signalling to their hosts.

Microbiome Disturbances and Autism Spectrum Disorders

Autism spectrum disorders (ASDs) are considered a heterogenous set of neurobehavioral diseases, with the rates of diagnosis dramatically increasing in the past few decades. As genetics alone does not explain the underlying cause in many cases, attention has turned to environmental factors as potential etiological agents. Gastrointestinal disorders are a common comorbidity in ASD patients. It was thus hypothesized that a gut-brain link may account for some autistic cases. With the characterization of the human microbiome, this concept has been expanded to include the microbiota-gut-brain axis. There are mounting reports in animal models and human epidemiologic studies linking disruptive alterations in the gut microbiota or dysbiosis and ASD symptomology. In this review, we will explore the current evidence that gut dysbiosis in animal models and ASD patients correlates with disease risk and severity. The studies to date have surveyed how gut microbiome changes may affect these neurobehavioral disorders. However, we harbor other microbiomes in the body that might impact brain function. We will consider microbial colonies residing in the oral cavity, vagina, and the most recently discovered one in the placenta. Based on the premise that gut microbiota alterations may be causative agents in ASD, several therapeutic options have been tested, such as diet modulations, prebiotics, probiotics, synbiotics, postbiotics, antibiotics, fecal transplantation, and activated charcoal. The potential benefits of these therapies will be considered. Finally, the possible mechanisms by which changes in the gut bacterial communities may result in ASD and related neurobehavioral disorders will be examined.

Systemic effects of gut microbiota and its relationship with disease and modulation

The gut microbiota makes up the majority of the human bacterial population, and although the gut microbiota resides in the intestines, it is able to exert systemic effects. Therefore, many diseases and conditions could be impacted by the gut microbiota when its composition is imbalanced, otherwise known as dysbiosis. However, apart from understanding the illnesses, we must also try to understand the intestinal flora itself to move forward and develop potential treatments.

Gut/brain axis and the microbiota

Tremendous progress has been made in characterizing the bidirectional interactions between the central nervous system, the enteric nervous system, and the gastrointestinal tract. A series of provocative preclinical studies have suggested a prominent role for the gut microbiota in these gut-brain interactions. Based on studies using rodents raised in a germ-free environment, the gut microbiota appears to influence the development of emotional behavior, stress- and pain-modulation systems, and brain neurotransmitter systems. Additionally, microbiota perturbations by probiotics and antibiotics exert modulatory effects on some of these measures in adult animals. Current evidence suggests that multiple mechanisms, including endocrine and neurocrine pathways, may be involved in gut microbiota-to-brain signaling and that the brain can in turn alter microbial composition and behavior via the autonomic nervous system. Limited information is available on how these findings may translate to healthy humans or to disease states involving the brain or the gut/brain axis. Future research needs to focus on confirming that the rodent findings are translatable to human physiology and to diseases such as irritable bowel syndrome, autism, anxiety, depression, and Parkinson's disease.

Gut microbes and the brain: paradigm shift in neuroscience

The discovery of the size and complexity of the human microbiome has resulted in an ongoing reevaluation of many concepts of health and disease, including diseases affecting the CNS. A growing body of preclinical literature has demonstrated bidirectional signaling between the brain and the gut microbiome, involving multiple neurocrine and endocrine signaling mechanisms. While psychological and physical stressors can affect the composition and metabolic activity of the gut microbiota, experimental changes to the gut microbiome can affect emotional behavior and related brain systems. These findings have resulted in speculation that alterations in the gut microbiome may play a pathophysiological role in human brain diseases, including autism spectrum disorder, anxiety, depression, and chronic pain. Ongoing large-scale population-based studies of the gut microbiome and brain imaging studies looking at the effect of gut microbiome modulation on brain responses to emotion-related stimuli are seeking to validate these speculations. This article is a summary of emerging topics covered in a symposium and is not meant to be a comprehensive review of the subject.

Gut microbes and the brain: paradigm shift in neuroscience

The discovery of the size and complexity of the human microbiome has resulted in an ongoing reevaluation of many concepts of health and disease, including diseases affecting the CNS. A growing body of preclinical literature has demonstrated bidirectional signaling between the brain and the gut microbiome, involving multiple neurocrine and endocrine signaling mechanisms. While psychological and physical stressors can affect the composition and metabolic activity of the gut microbiota, experimental changes to the gut microbiome can affect emotional behavior and related brain systems. These findings have resulted in speculation that alterations in the gut microbiome may play a pathophysiological role in human brain diseases, including autism spectrum disorder, anxiety, depression, and chronic pain. Ongoing large-scale population-based studies of the gut microbiome and brain imaging studies looking at the effect of gut microbiome modulation on brain responses to emotion-related stimuli are seeking to validate these speculations. This article is a summary of emerging topics covered in a symposium and is not meant to be a comprehensive review of the subject.

The gut microbiome and diet in psychiatry: focus on depression

PURPOSE OF REVIEW

With depressive disorders the leading source of disability globally, the identification of new targets for prevention and management is imperative. A rapidly emerging field of research suggests that the microbiome-gut-brain axis is of substantial relevance to mood and behaviour. Similarly, unhealthy diet has recently emerged as a significant correlate of and risk factor for depression. This review provides evidence for the gut microbiota as a key factor mediating the link between diet and depressive illness.

RECENT FINDINGS

The development of new technologies is affording a better understanding of how diet influences gut microbiota composition and activity and how this may, in turn, influence depressive illness. New interventions are also suggesting the possible utility of pre and probiotic formulations and fermented food in influencing mental health.

SUMMARY

Although in its early stages, the emerging field of research focused on the human microbiome suggests an important role for the gut microbiota in influencing brain development, behaviour and mood in humans. The recognition that the gut microbiota interacts bidirectionally with other environmental risk factors, such as diet and stress, suggests promise in the development of interventions targeting the gut microbiota for the prevention and treatment of common mental health disorders.

Stress-induced visceral pain: toward animal models of irritable-bowel syndrome and associated comorbidities

Visceral pain is a global term used to describe pain originating from the internal organs, which is distinct from somatic pain. It is a hallmark of functional gastrointestinal disorders such as irritable-bowel syndrome (IBS). Currently, the treatment strategies targeting visceral pain are unsatisfactory, with development of novel therapeutics hindered by a lack of detailed knowledge of the underlying mechanisms. Stress has long been implicated in the pathophysiology of visceral pain in both preclinical and clinical studies. Here, we discuss the complex etiology of visceral pain reviewing our current understanding in the context of the role of stress, gender, gut microbiota alterations, and immune functioning. Furthermore, we review the role of glutamate, GABA, and epigenetic mechanisms as possible therapeutic strategies for the treatment of visceral pain for which there is an unmet medical need. Moreover, we discuss the most widely described rodent models used to model visceral pain in the preclinical setting. The theory behind, and application of, animal models is key for both the understanding of underlying mechanisms and design of future therapeutic interventions. Taken together, it is apparent that stress-induced visceral pain and its psychiatric comorbidities, as typified by IBS, has a multifaceted etiology. Moreover, treatment strategies still lag far behind when compared to other pain modalities. The development of novel, effective, and specific therapeutics for the treatment of visceral pain has never been more pertinent.

Microbiota regulation of the Mammalian gut-brain axis

The realization that the microbiota-gut-brain axis plays a critical role in health and disease has emerged over the past decade. The brain-gut axis is a bidirectional communication system between the central nervous system (CNS) and the gastrointestinal tract. Regulation of the microbiota-brain-gut axis is essential for maintaining homeostasis, including that of the CNS. The routes of this communication are not fully elucidated but include neural, humoral, immune, and metabolic pathways. A number of approaches have been used to interrogate this axis including the use of germ-free animals, probiotic agents, antibiotics, or animals exposed to pathogenic bacterial infections. Together, it is clear that the gut microbiota can be a key regulator of mood, cognition, pain, and obesity. Understanding microbiota-brain interactions is an exciting area of research which may contribute new insights into individual variations in cognition, personality, mood, sleep, and eating behavior, and how they contribute to a range of neuropsychiatric diseases ranging from affective disorders to autism and schizophrenia. Finally, the concept of psychobiotics, bacterial-based interventions with mental health benefit, is also emerging.

The structures of the colonic mucosa-associated and luminal microbial communities are distinct and differentially affected by a prolonged murine stressor

The commensal microbiota of the human gastrointestinal tract live in a largely stable community structure, assisting in host physiological and immunological functions. Changes to this structure can be injurious to the health of the host, a concept termed dysbiosis. Psychological stress is a factor that has been implicated in causing dysbiosis, and studies performed by our lab have shown that restraint stress can indeed shift the cecal microbiota structure as well as increase the severity of a colonic infection caused by Citrobacter rodentium. However, this study, like many others, have focused on fecal contents when examining the effect of dysbiosis-causing stimuli (e.g. psychological stress) upon the microbiota. Since the mucosa-associated microbiota have unique properties and functions that can act upon the host, it is important to understand how stressor exposure might affect this niche of bacteria. To begin to understand whether chronic restraint stress changes the mucosa-associated and/or luminal microbiota mice underwent 7 16-hour cycles of restraint stress, and the microbiota of both colonic tissue and fecal contents were analyzed by sequencing using next-gen bacterial tag-encoded FLX amplicon technology (bTEFAP) pyrosequencing. Both control and stress groups had significantly different mucosa-associated and luminal microbiota communities, highlighting the importance of focusing gastrointestinal community structure analysis by microbial niche. Furthermore, restraint stress was able to disrupt both the mucosa-associated and luminally-associated colonic microbiota by shifting the relative abundances of multiple groups of bacteria. Among these changes, there was a significant reduction in the immunomodulatory commensal genus Lactobacillus associated with colonic mucosa. The relative abundance of Lactobacillus spp. was not affected in the lumen. These results indicate that stressor-exposure can have distinct effects upon the colonic microbiota situated at the mucosal epithelium in comparison to the luminal-associated microbiota.

Gut brain axis: diet microbiota interactions and implications for modulation of anxiety and depression

The human gut microbiome is composed of an enormous number of microorganisms, generally regarded as commensal bacteria. Without this inherent microbial community, we would be unable to digest plant polysaccharides and would have trouble extracting lipids from our diet. Resident gut bacteria are an important contributor to healthy metabolism and there is significant evidence linking gut microbiota and metabolic disorders such as obesity and diabetes. In the past few years, neuroscience research has demonstrated the importance of microbiota in the development of brain systems that are vital to both stress reactivity and stress-related behaviours. Here we review recent literature that examines the impact of diet-induced changes in the microbiota on stress-related behaviours including anxiety and depression.

The utility of Apc-mutant rats in modeling human colon cancer

Prior to the advent of genetic engineering in the mouse, the rat was the model of choice for investigating the etiology of cancer. Now, recent advances in the manipulation of the rat genome, combined with a growing recognition of the physiological differences between mice and rats, have reignited interest in the rat as a model of human cancer. Two recently developed rat models, the polyposis in the rat colon (Pirc) and Kyoto Apc Delta (KAD) strains, each carry mutations in the intestinal-cancer-associated adenomatous polyposis coli (Apc) gene. In contrast to mouse models carrying Apc mutations, in which cancers develop mainly in the small intestine rather than in the colon and there is no gender bias, these rat models exhibit colonic predisposition and gender-specific susceptibility, as seen in human colon cancer. The rat also provides other experimental resources as a model organism that are not provided by the mouse: the structure of its chromosomes facilitates the analysis of genomic events, the size of its colon permits longitudinal analysis of tumor growth, and the size of biological samples from the animal facilitates multiplexed molecular analyses of the tumor and its host. Thus, the underlying biology and experimental resources of these rat models provide important avenues for investigation. We anticipate that advances in disease modeling in the rat will synergize with resources that are being developed in the mouse to provide a deeper understanding of human colon cancer.

Friends with social benefits: host-microbe interactions as a driver of brain evolution and development?

The tight association of the human body with trillions of colonizing microbes that we observe today is the result of a long evolutionary history. Only very recently have we started to understand how this symbiosis also affects brain function and behavior. In this hypothesis and theory article, we propose how host-microbe associations potentially influenced mammalian brain evolution and development. In particular, we explore the integration of human brain development with evolution, symbiosis, and RNA biology, which together represent a “social triangle” that drives human social behavior and cognition. We argue that, in order to understand how inter-kingdom communication can affect brain adaptation and plasticity, it is inevitable to consider epigenetic mechanisms as important mediators of genome-microbiome interactions on an individual as well as a transgenerational time scale. Finally, we unite these interpretations with the hologenome theory of evolution. Taken together, we propose a tighter integration of neuroscience fields with host-associated microbiology by taking an evolutionary perspective.

Serotonin, tryptophan metabolism and the brain-gut-microbiome axis

The brain-gut axis is a bidirectional communication system between the central nervous system and the gastrointestinal tract. Serotonin functions as a key neurotransmitter at both terminals of this network. Accumulating evidence points to a critical role for the gut microbiome in regulating normal functioning of this axis. In particular, it is becoming clear that the microbial influence on tryptophan metabolism and the serotonergic system may be an important node in such regulation. There is also substantial overlap between behaviours influenced by the gut microbiota and those which rely on intact serotonergic neurotransmission. The developing serotonergic system may be vulnerable to differential microbial colonisation patterns prior to the emergence of a stable adult-like gut microbiota. At the other extreme of life, the decreased diversity and stability of the gut microbiota may dictate serotonin-related health problems in the elderly. The mechanisms underpinning this crosstalk require further elaboration but may be related to the ability of the gut microbiota to control host tryptophan metabolism along the kynurenine pathway, thereby simultaneously reducing the fraction available for serotonin synthesis and increasing the production of neuroactive metabolites. The enzymes of this pathway are immune and stress-responsive, both systems which buttress the brain-gut axis. In addition, there are neural processes in the gastrointestinal tract which can be influenced by local alterations in serotonin concentrations with subsequent relay of signals along the scaffolding of the brain-gut axis to influence CNS neurotransmission. Therapeutic targeting of the gut microbiota might be a viable treatment strategy for serotonin-related brain-gut axis disorders.

Minireview: Gut microbiota: the neglected endocrine organ

The concept that the gut microbiota serves as a virtual endocrine organ arises from a number of important observations. Evidence for a direct role arises from its metabolic capacity to produce and regulate multiple compounds that reach the circulation and act to influence the function of distal organs and systems. For example, metabolism of carbohydrates results in the production of short-chain fatty acids, such as butyrate and propionate, which provide an important source of nutrients as well as regulatory control of the host digestive system. This influence over host metabolism is also seen in the ability of the prebiotic inulin to influence production of relevant hormones such as glucagon-like peptide-1, peptide YY, ghrelin, and leptin. Moreover, the probiotic Lactobacillus rhamnosus PL60, which produces conjugated linoleic acid, has been shown to reduce body-weight gain and white adipose tissue without effects on food intake. Manipulating the microbial composition of the gastrointestinal tract modulates plasma concentrations of tryptophan, an essential amino acid and precursor to serotonin, a key neurotransmitter within both the enteric and central nervous systems. Indirectly and through as yet unknown mechanisms, the gut microbiota exerts control over the hypothalamic-pituitary-adrenal axis. This is clear from studies on animals raised in a germ-free environment, who show exaggerated responses to psychological stress, which normalizes after monocolonization by certain bacterial species including Bifidobacterium infantis. It is tempting to speculate that therapeutic targeting of the gut microbiota may be useful in treating stress-related disorders and metabolic diseases.