The anxiolytic effect of Bifidobacterium longum NCC3001 involves vagal pathways for gut-brain communication

Prokaryotes Versus Eukaryotes: Who is Hosting Whom?

Microorganisms represent the largest component of biodiversity in our world. For millions of years, prokaryotic microorganisms have functioned as a major selective force shaping eukaryotic evolution. Microbes that live inside and on animals outnumber the animals' actual somatic and germ cells by an estimated 10-fold. Collectively, the intestinal microbiome represents a "forgotten organ," functioning as an organ inside another that can execute many physiological responsibilities. The nature of primitive eukaryotes was drastically changed due to the association with symbiotic prokaryotes facilitating mutual coevolution of host and microbe. Phytophagous insects have long been used to test theories of evolutionary diversification; moreover, the diversification of a number of phytophagous insect lineages has been linked to mutualisms with microbes. From termites and honey bees to ruminants and mammals, depending on novel biochemistries provided by the prokaryotic microbiome, the association helps to metabolize several nutrients that the host cannot digest and converting these into useful end products (such as short-chain fatty acids), a process, which has huge impact on the biology and homeostasis of metazoans. More importantly, in a direct and/or indirect way, the intestinal microbiota influences the assembly of gut-associated lymphoid tissue, helps to educate immune system, affects the integrity of the intestinal mucosal barrier, modulates proliferation and differentiation of its epithelial lineages, regulates angiogenesis, and modifies the activity of enteric as well as the central nervous system. Despite these important effects, the mechanisms by which the gut microbial community influences the host's biology remain almost entirely unknown. Our aim here is to encourage empirical inquiry into the relationship between mutualism and evolutionary diversification between prokaryotes and eukaryotes, which encourage us to postulate: who is hosting whom?

The intestinal microbiome in early life: health and disease

Human microbial colonization begins at birth and continues to develop and modulate in species abundance for about 3 years, until the microbiota becomes adult-like. During the same time period, children experience significant developmental changes that influence their health status as well as their immune system. An ever-expanding number of articles associate several diseases with early-life imbalances of the gut microbiota, also referred to as gut microbial dysbiosis. Whether early-life dysbiosis precedes and plays a role in disease pathogenesis, or simply originates from the disease process itself is a question that is beginning to be answered in a few diseases, including IBD, obesity, and asthma. This review describes the gut microbiome structure and function during the formative first years of life, as well as the environmental factors that determine its composition. It also aims to discuss the recent advances in understanding the role of the early-life gut microbiota in the development of immune-mediated, metabolic, and neurological diseases. A greater understanding of how the early-life gut microbiota impacts our immune development could potentially lead to novel microbial-derived therapies that target disease prevention at an early age.

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.

Intestinal microbiota in pathophysiology and management of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a functional bowel disorder without any structural or metabolic abnormalities that sufficiently explain the symptoms, which include abdominal pain and discomfort, and bowel habit changes such as diarrhea and constipation. Its pathogenesis is multifactorial: visceral hypersensitivity, dysmotility, psychosocial factors, genetic or environmental factors, dysregulation of the brain-gut axis, and altered intestinal microbiota have all been proposed as possible causes. The human intestinal microbiota are composed of more than 1000 different bacterial species and 10¹⁴ cells, and are essential for the development, function, and homeostasis of the intestine, and for individual health. The putative mechanisms that explain the role of microbiota in the development of IBS include altered composition or metabolic activity of the microbiota, mucosal immune activation and inflammation, increased intestinal permeability and impaired mucosal barrier function, sensory-motor disturbances provoked by the microbiota, and a disturbed gut-microbiota-brain axis. Therefore, modulation of the intestinal microbiota through dietary changes, and use of antibiotics, probiotics, and anti-inflammatory agents has been suggested as strategies for managing IBS symptoms. This review summarizes and discusses the accumulating evidence that intestinal microbiota play a role in the pathophysiology and management of IBS.

The microbiota-gut-brain axis in gastrointestinal disorders: stressed bugs, stressed brain or both?

The gut-brain axis is the bidirectional communication between the gut and the brain, which occurs through multiple pathways that include hormonal, neural and immune mediators. The signals along this axis can originate in the gut, the brain or both, with the objective of maintaining normal gut function and appropriate behaviour. In recent years, the study of gut microbiota has become one of the most important areas in biomedical research. Attention has focused on the role of gut microbiota in determining normal gut physiology and immunity and, more recently, on its role as modulator of host behaviour ('microbiota-gut-brain axis'). We therefore review the literature on the role of gut microbiota in gut homeostasis and link it with mechanisms that could influence behaviour. We discuss the association of dysbiosis with disease, with particular focus on functional bowel disorders and their relationship to psychological stress. This is of particular interest because exposure to stressors has long been known to increase susceptibility to and severity of gastrointestinal diseases.

Converging effects of a Bifidobacterium and Lactobacillus probiotic strain on mouse intestinal physiology

Evidence has grown to support the efficacy of probiotics in the management of gastrointestinal disorders, many of which are associated with dysregulated fluid and electrolyte transport. A growing body of evidence now suggests that the host microbiota and probiotics can influence intestinal ion transport and that these effects often occur in a strain-dependent manner. In this study, we sought to investigate the effects of two therapeutically relevant organisms, Bifidobacterium infantis 35,624 and Lactobacillus salivarius UCC118, on small intestinal transit, fecal output and water content, transepithelial resistance (TER), and colonic secretomotor function. Mice fed either strain displayed significantly reduced small intestinal transit in vivo, though neither strain influenced fecal pellet output or water content. Colon from mice fed both organisms displayed increased colonic TER, without a concomitant change in the gene expression of the tight junction proteins claudin 1 and occludin. However, L. salivarius UCC118 selectively inhibited neurally evoked ion secretion in tissues from animals fed this particular probiotic. Consistent with this finding, the neurotoxin tetrodotoxin (TTx) significantly inhibited the short-circuit current response induced by L. salivarius UCC118 following addition to colonic preparations in Ussing chambers. Responses to B. infantis 35,624 also displayed sensitivity to TTx, although to a significantly lesser degree than L. salivarius UCC118. Both strains similarly inhibited cholinergic-induced ion transport after addition to Ussing chambers. Taken together, these data suggest that B. infantis 35,624 and L. salivarius UCC118 may be indicated in disorders associated with increased small intestinal transit, and, in particular for L. salivarius UCC118, neurally mediated diarrhea.

Update on irritable bowel syndrome program of research

PURPOSE

This article provides an update and overview of a nursing research program focused on understanding the pathophysiology and management of irritable bowel syndrome (IBS).

METHODS

This review includes English language papers from the United States, Europe, and Asia (e.g., South Korea) from 1999 to 2013. We addressed IBS as a health problem, emerging etiologies, diagnostic and treatment approaches and the importance of a biopsychosocial model.

RESULTS

IBS is a chronic, functional gastrointestinal disorder characterized by recurrent episodes of abdominal pain and alterations in bowel habit (diarrhea, constipation, mixed). It is a condition for which adults, particularly women ages 20-45, seek health care services in both the United States and South Korea. Clinically, nurses play key roles in symptom prevention and management including designing and implementing approaches to enhance the patients' self-management strategies. Multiple mechanisms are believed to participate in the development and maintenance of IBS symptoms including autonomic nervous system dysregulation, intestinal inflammation, intestinal dysbiosis, dietary intolerances, alterations in emotion regulation, heightened visceral pain sensitivity, hypothalamic-pituitary-adrenal dysregulation, and dysmotility. Because IBS tends to occur in families, genetic factors may also contribute to the pathophysiology. Patients with IBS often report a number of co-morbid disorders and/or symptoms including poor sleep.

CONCLUSION

The key to planning effective management strategies is to understand the heterogeneity of this disorder. Interventions for IBS include non-pharmacological strategies such as cognitive behavior therapy, relaxation strategies, and exclusion diets.

Effect of probiotic supplementation on schizophrenia symptoms and association with gastrointestinal functioning: a randomized, placebo-controlled trial

OBJECTIVE

A range of immune system abnormalities have been associated with schizophrenia. Probiotic compounds modulate the immune response and offer a potential treatment strategy for schizophrenia. Probiotic compounds have also been observed to improve gastrointestinal dysfunction, which is a common problem in individuals with schizophrenia. We performed a randomized, double-blind, placebo-controlled trial to examine whether probiotic supplementation can reduce symptom severity in patients with schizophrenia receiving antipsychotic treatment and also whether probiotics are associated with bowel functioning.

METHODS

Outpatients with schizophrenia (N = 65) meeting DSM-IV criteria and with at least moderately severe psychotic symptoms were enrolled in the study from December 2010-August 2012. Following a 2-week placebo run-in period, patients were randomly assigned to 14 weeks of double-blind adjunctive probiotic (combined Lactobacillus rhamnosus strain GG and Bifidobacterium animalis subsp. lactis strain Bb12) or placebo therapy. Psychiatric symptoms were assessed biweekly with the Positive and Negative Syndrome Scale (PANSS), and patients were queried weekly about their gastrointestinal functioning.

RESULTS

Repeated-measures analysis of variance showed no significant differences in the PANSS total score between probiotic and placebo supplementation (F = 1.28, P = .25). However, patients in the probiotic group were less likely to develop severe bowel difficulty over the course of the trial (hazard ratio = 0.23; 95% CI, 0.09-0.61, P = .003).

CONCLUSIONS

Probiotic supplementation may help prevent a common somatic symptom associated with schizophrenia.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT01242371.

Hot topics in gut microbiota

The study of gut microbiota is a rapidly moving field of research, and the impact of gut microbial communities on human health is widely perceived as one of the most exciting advancements in biomedicine in recent years. The gut microbiota plays a key role in digestion, metabolism and immune function, and has widespread impact beyond the gastrointestinal tract. Changes in the biodiversity of the gut microbiota are associated with far reaching consequences on host health and development. Further understanding of the importance of developing and maintaining gut microbiota diversity may lead to targeted interventions for health promotion, disease prevention and management. Diet, functional foods and gut microbiota transplantation are areas that have yielded some therapeutic success in modulating the gut microbiota, and warrant further investigation of their effects on various disease states.

Gut vagal afferents differentially modulate innate anxiety and learned fear

Vagal afferents are an important neuronal component of the gut-brain axis allowing bottom-up information flow from the viscera to the CNS. In addition to its role in ingestive behavior, vagal afferent signaling has been implicated modulating mood and affect, including distinct forms of anxiety and fear. Here, we used a rat model of subdiaphragmatic vagal deafferentation (SDA), the most complete and selective vagal deafferentation method existing to date, to study the consequences of complete disconnection of abdominal vagal afferents on innate anxiety, conditioned fear, and neurochemical parameters in the limbic system. We found that compared with Sham controls, SDA rats consistently displayed reduced innate anxiety-like behavior in three procedures commonly used in preclinical rodent models of anxiety, namely the elevated plus maze test, open field test, and food neophobia test. On the other hand, SDA rats exhibited increased expression of auditory-cued fear conditioning, which specifically emerged as attenuated extinction of conditioned fear during the tone re-exposure test. The behavioral manifestations in SDA rats were associated with region-dependent changes in noradrenaline and GABA levels in key areas of the limbic system, but not with functional alterations in the hypothalamus-pituitary-adrenal grand stress. Our study demonstrates that innate anxiety and learned fear are both subjected to visceral modulation through abdominal vagal afferents, possibly via changing limbic neurotransmitter systems. These data add further weight to theories emphasizing an important role of afferent visceral signals in the regulation of emotional behavior.

The role of microbiome in central nervous system disorders

Mammals live in a co-evolutionary association with the plethora of microorganisms that reside at a variety of tissue microenvironments. The microbiome represents the collective genomes of these co-existing microorganisms, which is shaped by host factors such as genetics and nutrients but in turn is able to influence host biology in health and disease. Niche-specific microbiome, prominently the gut microbiome, has the capacity to effect both local and distal sites within the host. The gut microbiome has played a crucial role in the bidirectional gut-brain axis that integrates the gut and central nervous system (CNS) activities, and thus the concept of microbiome-gut-brain axis is emerging. Studies are revealing how diverse forms of neuro-immune and neuro-psychiatric disorders are correlated with or modulated by variations of microbiome, microbiota-derived products and exogenous antibiotics and probiotics. The microbiome poises the peripheral immune homeostasis and predisposes host susceptibility to CNS autoimmune diseases such as multiple sclerosis. Neural, endocrine and metabolic mechanisms are also critical mediators of the microbiome-CNS signaling, which are more involved in neuro-psychiatric disorders such as autism, depression, anxiety, stress. Research on the role of microbiome in CNS disorders deepens our academic knowledge about host-microbiome commensalism in central regulation and in practicality, holds conceivable promise for developing novel prognostic and therapeutic avenues for CNS disorders.

The microbiota-gut-brain axis in functional gastrointestinal disorders

Functional gastrointestinal disorders (FGIDs) are highly prevalent and pose a significant burden on health care and society, and impact patients' quality of life. FGIDs comprise a heterogeneous group of disorders, with unclear underlying pathophysiology. They are considered to result from the interaction of altered gut physiology and psychological factors via the gut-brain axis, where brain and gut symptoms are reciprocally influencing each other's expression. Intestinal microbiota, as a part of the gut-brain axis, plays a central role in FGIDs. Patients with Irritable Bowel Syndrome, a prototype of FGIDs, display altered composition of the gut microbiota compared with healthy controls and benefit, at the gastrointestinal and psychological levels, from the use of probiotics and antibiotics. This review aims to recapitulate the available literature on FGIDs and microbiota-gut-brain axis.

The effects of gut microbiota on CNS function in humans

The role of the gastrointestinal microbiota in human brain development and function is an area of increasing interest and research. Preclinical models suggest a role for the microbiota in broad aspects of human health, including mood, cognition, and chronic pain. Early human studies suggest that altering the microbiota with beneficial bacteria, or probiotics, can lead to changes in brain function, as well as subjective reports of mood. As the mechanisms of bidirectional communication between the brain and microbiota are better understood, it is expected that these pathways will be harnessed to provide novel methods to enhance health and treat disease.

The gut-brain axis rewired: adding a functional vagal nicotinic "sensory synapse"

It is generally accepted that intestinal sensory vagal fibers are primary afferent, responding nonsynaptically to luminal stimuli. The gut also contains intrinsic primary afferent neurons (IPANs) that respond to luminal stimuli. A psychoactive Lactobacillus rhamnosus (JB-1) that affects brain function excites both vagal fibers and IPANs. We wondered whether, contrary to its primary afferent designation, the sensory vagus response to JB-1 might depend on IPAN to vagal fiber synaptic transmission. We recorded ex vivo single- and multiunit afferent action potentials from mesenteric nerves supplying mouse jejunal segments. Intramural synaptic blockade with Ca(2+) channel blockers reduced constitutive or JB-1-evoked vagal sensory discharge. Firing of 60% of spontaneously active units was reduced by synaptic blockade. Synaptic or nicotinic receptor blockade reduced firing in 60% of vagal sensory units that were stimulated by luminal JB-1. In control experiments, increasing or decreasing IPAN excitability, respectively increased or decreased nerve firing that was abolished by synaptic blockade or vagotomy. We conclude that >50% of vagal afferents function as interneurons for stimulation by JB-1, receiving input from an intramural functional "sensory synapse." This was supported by myenteric plexus nicotinic receptor immunohistochemistry. These data offer a novel therapeutic target to modify pathological gut-brain axis activity.-Perez-Burgos, A., Mao, Y.-K., Bienenstock, J., Kunze, W. A. The gut-brain axis rewired: adding a functional vagal nicotinic "sensory synapse."

Probiotic gut effect prevents the chronic psychological stress-induced brain activity abnormality in mice

BACKGROUND

A probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175 combination, Probio'Stick(®) ) displays anxiolytic-like activity and reduces apoptosis in the lymbic system in animal models of depression. Based on the hypothesis that modulation of gut microbiota by this probiotic formulation has beneficial effects on brain activity in stress conditions, we report a set of probiotic-evoked physiological, cellular, and molecular events in the brain of Probio'Stick(®) pretreated mice submitted to chronic psychological stress.

METHODS

Water avoidance stress (WAS) was applied or not (sham). Hypothalamic-pituitary-adrenal (HPA) axis and autonomic nervous system (ANS) responses to the chronic stress were assessed through plasma corticosterone and catecholamine measurements. Specific markers for neuronal activity, neurogenesis, and synaptic plasticity were used to assess brain activity. In addition, gut permeability and tight junction (TJ) proteins levels were also determinated.

KEY RESULTS

We observed that a pretreatment with the probiotic formulation attenuated HPA axis and ANS activities in response to WAS, and reduced cFos expression in different brain areas but Lactobacillus salivarius (a negative control) treatment was ineffective on these parameters. Moreover, probiotic pretreatment prevented the WAS-induced decrease hippocampal neurogenesis and expression changes in hypothalamic genes involved in synaptic plasticity. These central effects were associated with restoration of TJ barrier integrity in stressed mice.

CONCLUSIONS & INFERENCES

These data suggest that chronic stress-induced abnormal brain plasticity and reduction in neurogenesis can be prevented by a pretreatment with the Probio'Stick(®) formulation, suggesting that probiotics modulate neuroregulatory factors and various signaling pathways in the central nervous system involved in stress response.

Physiological underpinnings of irritable bowel syndrome: neurohormonal mechanisms

The gastrointestinal tract is a vast neuroendocrine organ with extensive extrinsic and intrinsic neural circuits that interact to control its function. Circulating and paracrine hormones (amine and peptide) provide further control of secretory, absorptive, barrier, motor and sensory mechanisms that are essential to the digestion and assimilation of nutrients, and the transport and excretion of waste products. Specialized elements of the mucosa (including enteroendocrine cells, enterocytes and immune cells) and the microbiome interact with other intraluminal contents derived from the diet, and with endogenous chemicals that alter the gut's functions. The totality of these control mechanisms is often summarized as the brain-gut axis. In irritable bowel syndrome (IBS), which is the most common gastrointestinal disorder, there may be disturbances at one or more of these diverse control mechanisms. Patients present with abdominal pain in association with altered bowel function. This review documents advances in understanding the pathophysiological mechanisms in the brain-gut axis in patients with IBS. It is anticipated that identification of one or more disordered functions in clinical practice will usher in a renaissance in the management of IBS, leading to effective therapy tailored to the needs of the individual patient.

Chronic functional bowel syndrome enhances gut-brain axis dysfunction, neuroinflammation, cognitive impairment, and vulnerability to dementia

The irritable bowel syndrome (IBS) is a common chronic functional gastrointestinal disorder world wide that lasts for decades. The human gut harbors a diverse population of microbial organisms which is symbiotic and important for well being. However, studies on conventional, germ-free, and obese animals have shown that alteration in normal commensal gut microbiota and an increase in pathogenic microbiota-termed "dysbiosis", impact gut function, homeostasis, and health. Diarrhea, constipation, visceral hypersensitivity, and abdominal pain arise in IBS from the gut-induced dysfunctional metabolic, immune, and neuro-immune communication. Dysbiosis in IBS is associated with gut inflammation. Gut-related inflammation is pivotal in promoting endotoxemia, systemic inflammation, and neuroinflammation. A significant proportion of IBS patients chronically consume alcohol, non-steroidal anti-inflammatories, and fatty diet; they may also suffer from co-morbid respiratory, neuromuscular, psychological, sleep, and neurological disorders. The above pathophysiological substrate is underpinned by dysbiosis, and dysfunctional bidirectional "Gut-Brain Axis" pathways. Pathogenic gut microbiota-related systemic inflammation (due to increased lipopolysaccharide and pro-inflammatory cytokines, and barrier dysfunction), may trigger neuroinflammation enhancing dysfunctional brain regions including hippocampus and cerebellum. These as well as dysfunctional vago-vagal gut-brain axis may promote cognitive impairment. Indeed, inflammation is characteristic of a broad spectrum of neurodegenerative diseases that manifest demntia. It is argued that an awareness of pathophysiological impact of IBS and implementation of appropriate therapeutic measures may prevent cognitive impairment and minimize vulnerability to dementia.

Neuropeptides and the microbiota-gut-brain axis

Neuropeptides are important mediators both within the nervous system and between neurons and other cell types. Neuropeptides such as substance P, calcitonin gene-related peptide and neuropeptide Y (NPY), vasoactive intestinal polypeptide, somatostatin and corticotropin-releasing factor are also likely to play a role in the bidirectional gut-brain communication. In this capacity they may influence the activity of the gastrointestinal microbiota and its interaction with the gut-brain axis. Current efforts in elucidating the implication of neuropeptides in the microbiota-gut-brain axis address four information carriers from the gut to the brain (vagal and spinal afferent neurons; immune mediators such as cytokines; gut hormones; gut microbiota-derived signalling molecules) and four information carriers from the central nervous system to the gut (sympathetic efferent neurons; parasympathetic efferent neurons; neuroendocrine factors involving the adrenal medulla; neuroendocrine factors involving the adrenal cortex). Apart from operating as neurotransmitters, many biologically active peptides also function as gut hormones. Given that neuropeptides and gut hormones target the same cell membrane receptors (typically G protein-coupled receptors), the two messenger roles often converge in the same or similar biological implications. This is exemplified by NPY and peptide YY (PYY), two members of the PP-fold peptide family. While PYY is almost exclusively expressed by enteroendocrine cells, NPY is found at all levels of the gut-brain and brain-gut axis. The function of PYY-releasing enteroendocrine cells is directly influenced by short chain fatty acids generated by the intestinal microbiota from indigestible fibre, while NPY may control the impact of the gut microbiota on inflammatory processes, pain, brain function and behaviour. Although the impact of neuropeptides on the interaction between the gut microbiota and brain awaits to be analysed, biologically active peptides are likely to emerge as neural and endocrine messengers in orchestrating the microbiota-gut-brain axis in health and disease.

Irritable eye syndrome: neuroimmune mechanisms and benefits of selected nutrients

Previous studies showed comorbidity of some ocular, enteral, and affective symptoms comprising irritable eye syndrome. Aims of the present study were to learn more about the pathogenic mechanisms of this syndrome and to evaluate benefits of food supplements on these disorders. In in vitro assay, Lactobacillus acidophilus lysate inhibited interleukin (IL)-1β and tumor necrosis factor (TNF)-α generation of lipopolysaccharide (LPS)-stimulated macrophages in dose- and size-dependent manner. For a prospective, open-label phase I/II controlled clinical trial, 40 subjects affected by ocular dysesthesia and hyperesthesia and comorbid enteral and anxiety-depression symptoms were randomly assigned either into the treated group, which received a composition containing probiotic lysate, vitamins A, B, and D and omega 3 fatty acids, or into the control group, which received vitamins and omega 3 fatty acids. For reference, 20 age- and sex-matched healthy subjects were also selected. White blood count (WBC) and lymphocyte and monocyte counts, as well as IL-6 and TNF-α levels, were significantly above the reference levels in both treated and control groups. After 8 weeks, WBC and lymphocyte and monocyte counts, and cytokine levels significantly decreased, and ocular, enteral, and anxiety-depression symptoms significantly improved in the treated group as compared to the control group. This proof-of-concept study suggested that subclinical inflammation may be a common mechanism connecting ocular, enteral, and anxiety/depression symptoms, and supplements affecting dysbiosis may be a new approach to treating this syndrome.

Microbial genes, brain & behaviour - epigenetic regulation of the gut-brain axis

To date, there is rapidly increasing evidence for host-microbe interaction at virtually all levels of complexity, ranging from direct cell-to-cell communication to extensive systemic signalling, and involving various organs and organ systems, including the central nervous system. As such, the discovery that differential microbial composition is associated with alterations in behaviour and cognition has significantly contributed to establishing the microbiota-gut-brain axis as an extension of the well-accepted gut-brain axis concept. Many efforts have been focused on delineating a role for this axis in health and disease, ranging from stress-related disorders such as depression, anxiety and irritable bowel syndrome to neurodevelopmental disorders such as autism. There is also a growing appreciation of the role of epigenetic mechanisms in shaping brain and behaviour. However, the role of epigenetics in informing host-microbe interactions has received little attention to date. This is despite the fact that there are many plausible routes of interaction between epigenetic mechanisms and the host-microbiota dialogue. From this new perspective we put forward novel, yet testable, hypotheses. Firstly, we suggest that gut-microbial products can affect chromatin plasticity within their host's brain that in turn leads to changes in neuronal transcription and eventually alters host behaviour. Secondly, we argue that the microbiota is an important mediator of gene-environment interactions. Finally, we reason that the microbiota itself may be viewed as an epigenetic entity. In conclusion, the fields of (neuro)epigenetics and microbiology are converging at many levels and more interdisciplinary studies are necessary to unravel the full range of this interaction.

Microbiota modulate behavioral and physiological abnormalities associated with neurodevelopmental disorders

Neurodevelopmental disorders, including autism spectrum disorder (ASD), are defined by core behavioral impairments; however, subsets of individuals display a spectrum of gastrointestinal (GI) abnormalities. We demonstrate GI barrier defects and microbiota alterations in the maternal immune activation (MIA) mouse model that is known to display features of ASD. Oral treatment of MIA offspring with the human commensal Bacteroides fragilis corrects gut permeability, alters microbial composition, and ameliorates defects in communicative, stereotypic, anxiety-like and sensorimotor behaviors. MIA offspring display an altered serum metabolomic profile, and B. fragilis modulates levels of several metabolites. Treating naive mice with a metabolite that is increased by MIA and restored by B. fragilis causes certain behavioral abnormalities, suggesting that gut bacterial effects on the host metabolome impact behavior. Taken together, these findings support a gut-microbiome-brain connection in a mouse model of ASD and identify a potential probiotic therapy for GI and particular behavioral symptoms in human neurodevelopmental disorders.

Bifidobacteria: their impact on gut microbiota composition and their applications as probiotics in infants

This review is aimed at describing the most recent advances in the gut microbiota composition of newborns and infants with a particular emphasis on bifidobacteria. The newborn gut microbiota is quite unstable, whereas after weaning, it becomes more stable and gets closer to the typical adult microbiota. The newborn and infant gut microbiota composition is impaired in several enteric and non-enteric pathologies. The core of this review is the description of the most recent documented applications of bifidobacteria to newborns and infants for their prevention and treatment. Acute diarrhea is the most studied disease for which bifidobacteria are applied with great success, Bifidobacterium longum and Bifidobacterium breve being the most applied species. Moreover, the most recent updates in the use of bifidobacteria for the prevention and treatment of pathologies typical of newborns, such as necrotizing enterocolitis, colics, and streptococcal infections, are presented. In addition, a number of not strictly enteric pathologies have in recent years evidenced a strict correlation with an aberrant gut microbiota in infants, in particular showing a reduced level of bifidobacteria. These diseases represent new potential opportunities for probiotic applications. Among them, allergic diseases, celiac disease, obesity, and neurologic diseases are described in this review. The preliminary use of bifidobacteria in in vitro systems and animal models is summarized as well as preliminary in vivo studies. Only after validation of the results via human clinical trials will the potentiality of bifidobacteria in the prevention and cure of these pathologies be definitely assessed.

The microbiome: stress, health and disease

Bacterial colonisation of the gut plays a major role in postnatal development and maturation of key systems that have the capacity to influence central nervous system (CNS) programming and signaling, including the immune and endocrine systems. Individually, these systems have been implicated in the neuropathology of many CNS disorders and collectively they form an important bidirectional pathway of communication between the microbiota and the brain in health and disease. Regulation of the microbiome-brain-gut axis is essential for maintaining homeostasis, including that of the CNS. Moreover, there is now expanding evidence for the view that commensal organisms within the gut play a role in early programming and later responsivity of the stress system. Research has focused on how the microbiota communicates with the CNS and thereby influences brain function. The routes of this communication are not fully elucidated but include neural, humoral, immune and metabolic pathways. This view is underpinned by studies in germ-free animals and in animals exposed to pathogenic bacterial infections, probiotic agents or antibiotics which indicate a role for the gut microbiota in the regulation of mood, cognition, pain and obesity. Thus, the concept of a microbiome-brain-gut axis is emerging which suggests that modulation of the gut microflora may be a tractable strategy for developing novel therapeutics for complex stress-related CNS disorders where there is a huge unmet medical need.

The primate vaginal microbiome: comparative context and implications for human health and disease

The primate body hosts trillions of microbes. Interactions between primate hosts and these microbes profoundly affect primate physiology, reproduction, health, survival, and ultimately, evolution. It is increasingly clear that primate health cannot be understood fully without knowledge of host-microbial interactions. Our goals here are to review what is known about microbiomes of the female reproductive tract and to explore several factors that influence variation within individuals, as well as within and between primate species. Much of our knowledge of microbial variation derives from studies of humans, and from microbes located in nonreproductive regions (e.g., the gut). We review work suggesting that the vaginal microbiota affects female health, fecundity, and pregnancy outcomes, demonstrating the selective potential for these agents. We explore the factors that correlate with microbial variation within species. Initial colonization by microbes depends on the manner of birth; most microbial variation is structured by estrogen levels that change with age (i.e., at puberty and menopause) and through the menstrual cycle. Microbial communities vary by location within the vagina and can depend on the sampling methods used (e.g., swab, lavage, or pap smear). Interindividual differences also exist, and while this variation is not completely understood, evidence points more to differences in estrogen levels, rather than differences in external physical environment. When comparing across species, reproductive-age humans show distinct microbial communities, generally dominated by Lactobacillus, unlike other primates. We develop evolutionary hypotheses to explain the marked differences in microbial communities. While much remains to be done to test these hypotheses, we argue that the ample variation in primate mating and reproductive behavior offers excellent opportunities to evaluate host-microbe coevolution and adaptation.

Prebiotic feeding elevates central brain derived neurotrophic factor, N-methyl-D-aspartate receptor subunits and D-serine

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Prebiotic feeding elevated BDNF and NR1subunit mRNAs, in the rat hippocampus.

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The GOS prebiotic increased cortical NR1, d-serine, and hippocampal NR2A subunits.

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GOS feeding elevated plasma levels of the gut peptide PYY.

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GOS plasma increased BDNF release from human SH-SY5Y neuroblastoma cells.

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BDNF secretion from cells by GOS plasma was blocked by PYY antisera.

The influence of the gut microbiota on brain chemistry has been convincingly demonstrated in rodents. In the absence of gut bacteria, the central expression of brain derived neurotropic factor, (BDNF), and N-methyl-d-aspartate receptor (NMDAR) subunits are reduced, whereas, oral probiotics increase brain BDNF, and impart significant anxiolytic effects. We tested whether prebiotic compounds, which increase intrinsic enteric microbiota, also affected brain BDNF and NMDARs. In addition, we examined whether plasma from prebiotic treated rats released BDNF from human SH-SY5Y neuroblastoma cells, to provide an initial indication of mechanism of action.

Rats were gavaged with fructo-oligosaccharides (FOS), galacto-oligosaccharides (GOS) or water for five weeks, prior to measurements of brain BDNF, NMDAR subunits and amino acids associated with glutamate neurotransmission (glutamate, glutamine, and serine and alanine enantiomers). Prebiotics increased hippocampal BDNF and NR1 subunit expression relative to controls. The intake of GOS also increased hippocampal NR2A subunits, and frontal cortex NR1 and d-serine. Prebiotics did not alter glutamate, glutamine, l-serine, l-alanine or d-alanine concentrations in the brain, though GOSfeeding raised plasma d-alanine. Elevated levels of plasma peptide YY (PYY) after GOS intake was observed. Plasma from GOS rats increased the release of BDNF from SH-SY5Y cells, but not in the presence of PYY antisera. The addition of synthetic PYY to SH-SY5Y cell cultures, also elevated BDNF secretion.

We conclude that prebiotic-mediated proliferation of gut microbiota in rats, like probiotics, increases brain BDNF expression, possibly through the involvement of gut hormones. The effect of GOS on components of central NMDAR signalling was greater than FOS, and may reflect the proliferative potency of GOS on microbiota. Our data therefore, provide a sound basis to further investigate the utility of prebiotics in the maintenance of brain health and adjunctive treatment of neuropsychiatric disorders.

The microbiota-gut-brain axis: neurobehavioral correlates, health and sociality

Recent data suggest that the human body is not such a neatly self-sufficient island after all. It is more like a super-complex ecosystem containing trillions of bacteria and other microorganisms that inhabit all our surfaces; skin, mouth, sexual organs, and specially intestines. It has recently become evident that such microbiota, specifically within the gut, can greatly influence many physiological parameters, including cognitive functions, such as learning, memory and decision making processes. Human microbiota is a diverse and dynamic ecosystem, which has evolved in a mutualistic relationship with its host. Ontogenetically, it is vertically inoculated from the mother during birth, established during the first year of life and during lifespan, horizontally transferred among relatives, mates or close community members. This micro-ecosystem serves the host by protecting it against pathogens, metabolizing complex lipids and polysaccharides that otherwise would be inaccessible nutrients, neutralizing drugs and carcinogens, modulating intestinal motility, and making visceral perception possible. It is now evident that the bidirectional signaling between the gastrointestinal tract and the brain, mainly through the vagus nerve, the so called “microbiota–gut–vagus–brain axis,” is vital for maintaining homeostasis and it may be also involved in the etiology of several metabolic and mental dysfunctions/disorders. Here we review evidence on the ability of the gut microbiota to communicate with the brain and thus modulate behavior, and also elaborate on the ethological and cultural strategies of human and non-human primates to select, transfer and eliminate microorganisms for selecting the commensal profile.

Melancholic microbes: a link between gut microbiota and depression?

There is a growing awareness of the potential for microbiota to influence gut-brain communication in health and disease. A variety of strategies have been used to study the impact of the microbiota on brain function and these include antibiotic use, probiotic treatments, fecal microbiota transplantation, gastrointestinal infection studies, and germ-free studies. All of these approaches provide evidence to support the view that the microbiota can influence brain chemistry and consequently behavior. Efforts are now turning to investigate the role of microbiota in animal models of psychopathology. Animal models of depression are thus essential in studying the complex interplay between the microbiota and brain. Recent studies published in this Journal and elsewhere demonstrate that there is a distinct perturbation of the composition of gut microbiota in animal models of depression and chronic stress. This has direct implications for the development of psychobiotic-based therapeutic strategies for psychiatric disorders. Moreover, given that affective co-morbidities, such as major depression and anxiety states, are common in patients presenting with irritable bowel syndrome (IBS), it may have implications for functional bowel disorders also. Further studies require appropriately phenotyped patients with depression and/or IBS using a judicious use of techniques including functional imaging and in depth microbial pyrosequencing.

Microbiota in health and irritable bowel syndrome: current knowledge, perspectives and therapeutic options

The gastrointestinal tract is a natural reservoir of microbiota. The gut is germ-free at birth, but rapidly becomes host to various bacteria establishing a progressively mutual relationship. The composition of gut microbiota is individual-specific and depends on the genotype of the host and environmental factors. Novel techniques have been used to characterize gastrointestinal microbiota, including genomic approaches. The bacterial profile shows that dominant and minor phyla are present in the gastrointestinal tract. From the proximal to the distal segments of the gut the bacterial density gradually increases, reaching an estimated 10(11) to 10(12) bacteria per gram of colonic content. Dynamic interactions between gut and microbiota play a physiological role in metabolic, protective and structural functions, while dysbiosis contributes to several diseases. Microbiota appear to play a role in IBS, where qualitative and quantitative changes of bacteriaoccur in IBS subtypes. Initial therapeutic approaches in IBS have focused on microbiota. The relationship between perturbations of the microbiota, mucosal inflammation and IBS remains to be further investigated.

Neural networks in intestinal immunoregulation

Key physiological functions of the intestine are governed by nerves and neurotransmitters. This complex control relies on two neuronal systems: an extrinsic innervation supplied by the two branches of the autonomic nervous system and an intrinsic innervation provided by the enteric nervous system. As a result of constant exposure to commensal and pathogenic microflora, the intestine developed a tightly regulated immune system. In this review, we cover the current knowledge on the interactions between the gut innervation and the intestinal immune system. The relations between extrinsic and intrinsic neuronal inputs are highlighted with regards to the intestinal immune response. Moreover, we discuss the latest findings on mechanisms underlying inflammatory neural reflexes and examine their relevance in the context of the intestinal inflammation. Finally, we discuss some of the recent data on the identification of the gut microbiota as an emerging player influencing the brain function.

Probiotics from research to market: the possibilities, risks and challenges

Probiotic foods can affect large parts of the population, while therapeutic applications have a less wide scope. While commercialization routes and regulatory requirements differ for both applications, both will need good scientific support. Today, probiotics are mainly used for gastrointestinal applications, their use can easily be extended to skin, oral and vaginal health. While most probiotics currently belong to food-grade species, the future may offer new functional microorganisms in food and pharma. This review discusses the crosstalk between probiotic producers, regulatory people, medical care and healthcare workers, and the scientific community.

Bifidobacterium longum NCC3001 inhibits AH neuron excitability

BACKGROUND

Bifidobacterium longum (B. longum) NCC3001 can affect behavior and brain biochemistry, but identification of the cellular targets needs further investigation. Our hypothesis was that the communication with the brain might start with action on enteric sensory neurons.

METHODS

Ileal segments from adult mice were used to create a longitudinal muscle-myenteric-plexus preparation to expose sensory after-hyperpolarizing (AH) neurons in the myenteric plexus to allow access with microelectrodes. The intrinsic excitability of AH neurons was tested in response to the perfusion of conditioned media (B. longum culture supernatant) or unconditioned media (growth medium, MRS).

KEY RESULTS

B. longum conditioned medium significantly reduced the excitability of AH neurons compared to perfusion with the unconditioned medium. Specifically, a reduction was seen in the number of action potentials fired per depolarizing test pulse, the instantaneous and time-dependent input resistances and the magnitude of the hyperpolarization-activated cationic current (Ih ).

CONCLUSIONS & INFERENCES

The probiotic B. longum reduces excitability of AH sensory neurons likely via opening of potassium channels and closing of hyperpolarization-activated cation channels.

Gut feelings: bacteria and the brain

The gut-brain axis—an imaginary line between the brain and the gut—is one of the new frontiers of neuroscience. Microbiota in our gut, sometimes referred to as the “second genome” or the “second brain,” may influence our mood in ways that scientists are just now beginning to understand. Unlike with inherited genes, it may be possible to reshape, or even to cultivate, this second genome. As research evolves from mice to people, further understanding of microbiota’s relationship to the human brain could have significant mental health implications.

Consumption of fermented milk product with probiotic modulates brain activity

BACKGROUND & AIMS

Changes in gut microbiota have been reported to alter signaling mechanisms, emotional behavior, and visceral nociceptive reflexes in rodents. However, alteration of the intestinal microbiota with antibiotics or probiotics has not been shown to produce these changes in humans. We investigated whether consumption of a fermented milk product with probiotic (FMPP) for 4 weeks by healthy women altered brain intrinsic connectivity or responses to emotional attention tasks.

METHODS

Healthy women with no gastrointestinal or psychiatric symptoms were randomly assigned to groups given FMPP (n = 12), a nonfermented milk product (n = 11, controls), or no intervention (n = 13) twice daily for 4 weeks. The FMPP contained Bifidobacterium animalis subsp Lactis, Streptococcus thermophiles, Lactobacillus bulgaricus, and Lactococcus lactis subsp Lactis. Participants underwent functional magnetic resonance imaging before and after the intervention to measure brain response to an emotional faces attention task and resting brain activity. Multivariate and region of interest analyses were performed.

RESULTS

FMPP intake was associated with reduced task-related response of a distributed functional network (49% cross-block covariance; P = .004) containing affective, viscerosensory, and somatosensory cortices. Alterations in intrinsic activity of resting brain indicated that ingestion of FMPP was associated with changes in midbrain connectivity, which could explain the observed differences in activity during the task.

CONCLUSIONS

Four-week intake of an FMPP by healthy women affected activity of brain regions that control central processing of emotion and sensation.

Beyond phylotyping: understanding the impact of gut microbiota on host biology

BACKGROUND

Microbial constituents of the gut microbiome interact with each other and the host to alter the luminal environment and impact development, motility, and homeostasis of the gut. Powerful methods are becoming available to investigate connections between the gut microbiome and human health. While high-throughput sequencing of 16S rRNA genes can be used to identify and enumerate microbes in the gut, advances in several techniques (e.g., metagenomics, metatranscriptomics, metabolomics, and metaproteomics) are providing a clearer view as to the specific activities of the microbiota in the context of functional host-microbe interactions. Testing emergent hypotheses regarding microbial effects on host biology, which arise from analyses of 'Big Data' generated from massive parallel high-throughput sequencing technology and spectroscopic techniques, to guide translational research is an important goal for the future. Insights regarding the fundamental operating principles of the gut microbiota should lay the foundation for rational manipulation of the microbiota to promote human health.

PURPOSE

In this review, we provide an overview of current research on the gut microbiome emphasizing current state-of-the-art technologies, approaches, and directions for improvement of our understanding of the impact of the gut microbiota with specific focus on gastrointestinal motility disorders.

Intestinal microbiota, probiotics and mental health: from Metchnikoff to modern advances: part III - convergence toward clinical trials

Rapid scientific and technological advances have allowed for a more detailed understanding of the relevance of intestinal microbiota, and the entire body-wide microbiome, to human health and well-being. Rodent studies have provided suggestive evidence that probiotics (e.g. lactobacillus and bifidobacteria) can influence behavior. More importantly, emerging clinical studies indicate that the administration of beneficial microbes, via supplementation and/or fecal microbial transplant (FMT), can influence end-points related to mood state (glycemic control, oxidative status, uremic toxins), brain function (functional magnetic resonance imaging fMRI), and mental outlook (depression, anxiety). However, despite the advances in the area of gastro-biological psychiatry, it becomes clear that there remains an urgent need to explore the value of beneficial microbes in controlled clinical investigations. With the history explored in this series, it is fair to ask if we are now on the cusp of major clinical breakthroughs, or are we merely in the quicksand of Autointoxication II?

Gut-brain axis: how the microbiome influences anxiety and depression

Within the first few days of life, humans are colonized by commensal intestinal microbiota. Here, we review recent findings showing that microbiota are important in normal healthy brain function. We also discuss the relation between stress and microbiota, and how alterations in microbiota influence stress-related behaviors. New studies show that bacteria, including commensal, probiotic, and pathogenic bacteria, in the gastrointestinal (GI) tract can activate neural pathways and central nervous system (CNS) signaling systems. Ongoing and future animal and clinical studies aimed at understanding the microbiota-gut-brain axis may provide novel approaches for prevention and treatment of mental illness, including anxiety and depression.

American Journal of Gastroenterology Lecture: Intestinal microbiota and the role of fecal microbiota transplant (FMT) in treatment of C. difficile infection

The vital roles that intestinal flora, now called microbiota, have in maintaining our health are being increasingly appreciated. Starting with birth, exposure to the outside world begins the life-long intimate association our microbiota will have with our diet and environment, and initiates determination of the post-natal structural and functional maturation of the gut. Moreover, vital interactions of the microbiota with our metabolic activities, as well as with the immunological apparatus that constitutes our major defense system against foreign antigens continues throughout life. A perturbed intestinal microbiome has been associated with an increasing number of gastrointestinal and non-gastrointestinal diseases including Clostridium difficile infection (CDI). It has become recognized that fecal microbiota transplantation (FMT) can correct the dysbiosis that characterizes chronic CDI, and effect a seemingly safe, relatively inexpensive, and rapidly effective cure in the vast majority of patients so treated. In addition, FMT has been used to treat an array of other gastrointestinal and non-gastrointestinal disorders, although experience in these other non-CDI diseases is in its infancy. More work needs to be done with FMT to ensure its safety and optimal route of administration. There is a conceptual sea change that is developing in our view of bacteria from their role only as pathogens to that of being critical to health maintenance in a changing world. Future studies are certain to narrow the spectrum of organisms that need to be given to patients to cure disease. FMT is but the first step in this journey.

Psychoactive bacteria Lactobacillus rhamnosus (JB-1) elicits rapid frequency facilitation in vagal afferents

Mounting evidence supports the influence of the gut microbiome on the local enteric nervous system and its effects on brain chemistry and relevant behavior. Vagal afferents are involved in some of these effects. We previously showed that ingestion of the probiotic bacterium Lactobacillus rhamnosus (JB-1) caused extensive neurochemical changes in the brain and behavior that were abrogated by prior vagotomy. Because information can be transmitted to the brain via primary afferents encoded as neuronal spike trains, our goal was to record those induced by JB-1 in vagal afferents in the mesenteric nerve bundle and thus determine the nature of the signals sent to the brain. Male Swiss Webster mice jejunal segments were cannulated ex vivo, and serosal and luminal compartments were perfused separately. Bacteria were added intraluminally. We found no evidence for translocation of labeled bacteria across the epithelium during the experiment. We recorded extracellular multi- and single-unit neuronal activity with glass suction pipettes. Within minutes of application, JB-1 increased the constitutive single- and multiunit firing rate of the mesenteric nerve bundle, but Lactobacillus salivarius (a negative control) or media alone were ineffective. JB-1 significantly augmented multiunit discharge responses to an intraluminal distension pressure of 31 hPa. Prior subdiaphragmatic vagotomy abolished all of the JB-1-evoked effects. This detailed exploration of the neuronal spike firing that encodes behavioral signaling to the brain may be useful to identify effective psychoactive bacteria and thereby offer an alternative new perspective in the field of psychiatry and comorbid conditions.

Voices from within: gut microbes and the CNS

Recent advances in research have greatly increased our understanding of the importance of the gut microbiota. Bacterial colonization of the intestine is critical to the normal development of many aspects of physiology such as the immune and endocrine systems. It is emerging that the influence of the gut microbiota also extends to modulation of host neural development. Furthermore, the overall balance in composition of the microbiota, together with the influence of pivotal species that induce specific responses, can modulate adult neural function, peripherally and centrally. Effects of commensal gut bacteria in adult animals include protection from the central effects of infection and inflammation as well as modulation of normal behavioral responses. There is now robust evidence that gut bacteria influence the enteric nervous system, an effect that may contribute to afferent signaling to the brain. The vagus nerve has also emerged as an important means of communicating signals from gut bacteria to the CNS. Further understanding of the mechanisms underlying microbiome-gut-brain communication will provide us with new insight into the symbiotic relationship between gut microbiota and their mammalian hosts and help us identify the potential for microbial-based therapeutic strategies to aid in the treatment of mood disorders.

The intestinal microbiome, probiotics and prebiotics in neurogastroenterology

The brain-gut axis allows bidirectional communication between the central nervous system (CNS) and the enteric nervous system (ENS), linking emotional and cognitive centers of the brain with peripheral intestinal functions. Recent experimental work suggests that the gut microbiota have an impact on the brain-gut axis. A group of experts convened by the International Scientific Association for Probiotics and Prebiotics (ISAPP) discussed the role of gut bacteria on brain functions and the implications for probiotic and prebiotic science. The experts reviewed and discussed current available data on the role of gut microbiota on epithelial cell function, gastrointestinal motility, visceral sensitivity, perception and behavior. Data, mostly gathered from animal studies, suggest interactions of gut microbiota not only with the enteric nervous system but also with the central nervous system via neural, neuroendocrine, neuroimmune and humoral links. Microbial colonization impacts mammalian brain development in early life and subsequent adult behavior. These findings provide novel insights for improved understanding of the potential role of gut microbial communities on psychological disorders, most particularly in the field of psychological comorbidities associated with functional bowel disorders like irritable bowel syndrome (IBS) and should present new opportunity for interventions with pro- and prebiotics.

Probiotics in the management of functional bowel disorders: promise fulfilled?

Irritable bowel syndrome (IBS) and chronic constipation (CC) are common problems worldwide and are associated with significant impact on activities of daily living and quality of life. Recent interest, in IBS in particular, has focused on the potential roles of the microbiota and its interaction with the host's immune system. Recently, high-quality clinical trials have been performed on prebiotics and probiotics in IBS or CC. Although strategies that seek to modify the microbiota, such as the use of probiotics, offer much promise in IBS and CC, more high-quality trials and, studies of longer duration are required.

Neuropeptide Y, peptide YY and pancreatic polypeptide in the gut-brain axis

The gut-brain axis refers to the bidirectional communication between the gut and the brain. Four information carriers (vagal and spinal afferent neurons, immune mediators such as cytokines, gut hormones and gut microbiota-derived signalling molecules) transmit information from the gut to the brain, while autonomic neurons and neuroendocrine factors carry outputs from the brain to the gut. The members of the neuropeptide Y (NPY) family of biologically active peptides, NPY, peptide YY (PYY) and pancreatic polypeptide (PP), are expressed by cell systems at distinct levels of the gut-brain axis. PYY and PP are exclusively expressed by endocrine cells of the digestive system, whereas NPY is found at all levels of the gut-brain and brain-gut axis. The major systems expressing NPY comprise enteric neurons, primary afferent neurons, several neuronal pathways throughout the brain and sympathetic neurons. In the digestive tract, NPY and PYY inhibit gastrointestinal motility and electrolyte secretion and in this way modify the input to the brain. PYY is also influenced by the intestinal microbiota, and NPY exerts, via stimulation of Y1 receptors, a proinflammatory action. Furthermore, the NPY system protects against distinct behavioural disturbances caused by peripheral immune challenge, ameliorating the acute sickness response and preventing long-term depression. At the level of the afferent system, NPY inhibits nociceptive input from the periphery to the spinal cord and brainstem. In the brain, NPY and its receptors (Y1, Y2, Y4, Y5) play important roles in regulating food intake, energy homeostasis, anxiety, mood and stress resilience. In addition, PP and PYY signal to the brain to attenuate food intake, anxiety and depression-related behaviour. These findings underscore the important role of the NPY-Y receptor system at several levels of the gut-brain axis in which NPY, PYY and PP operate both as neural and endocrine messengers.

On communication between gut microbes and the brain

PURPOSE OF REVIEW

Interest in the microbiota-gut-brain axis is increasing apace and what was, not so long ago, a hypothetical relationship is emerging as a potentially critical factor in the regulation of intestinal and mental health. Studies are now addressing the neural circuitry and mechanisms underlying the influence of gut bacteria on the central nervous system and behavior.

RECENT FINDINGS

Gut bacteria influence development of the central nervous systems (CNS) and stress responses. In adult animals, the overall composition of the microbiota or exposure to specific bacterial strains can modulate neural function, peripherally and centrally. Gut bacteria can provide protection from the central effects of infection and inflammation as well as modulate normal behavioral responses. Behavioral effects described to date are largely related to stress and anxiety and an altered hypothalamus-pituitary-adrenal axis response is a common observation in many model systems. The vagus nerve has also emerged as an important means of communicating signals from gut microbes to the CNS.

SUMMARY

Studies of microbiota-gut-brain communication are providing us with a deeper understanding of the relationship between the gut bacteria and their hosts while also suggesting the potential for microbial-based therapeutic strategies that may aid in the treatment of mood disorders.

Communication between gastrointestinal bacteria and the nervous system

In the past few years, intestinal microbiota has emerged as a novel target for the treatment of gut-brain axis alterations. These include functional gastrointestinal disorders, such as irritable bowel syndrome (IBS), which can be comorbid with stress-related psychiatric conditions. Thus, modulation of the microbiota (e.g. with the use of probiotics) could be proposed as a novel strategy not only for the treatment of IBS but also as an adjuvant for psychiatric treatment of anxiety and depression.

The interplay between the intestinal microbiota and the brain

The intestinal microbiota consists of a vast bacterial community that resides primarily in the lower gut and lives in a symbiotic relationship with the host. A bidirectional neurohumoral communication system, known as the gut-brain axis, integrates the host gut and brain activities. Here, we describe the recent advances in our understanding of how the intestinal microbiota communicates with the brain via this axis to influence brain development and behaviour. We also review how this extended communication system might influence a broad spectrum of diseases, including irritable bowel syndrome, psychiatric disorders and demyelinating conditions such as multiple sclerosis.