Gut microbiome is an integral part of the Gut-Brain axis. It is becoming increasingly recognized that the presence of a healthy and diverse gut microbiota is important to normal cognitive and emotional processing. It was known that altered emotional state and chronic stress can change the composition of gut microbiome, but it is becoming more evident that interaction between gut microbiome and central nervous system is bidirectional. Alteration in the composition of the gut microbiome can potentially lead to increased intestinal permeability and impair the function of the intestinal barrier. Subsequently, neuro-active compounds and metabolites can gain access to the areas within the central nervous system that regulate cognition and emotional responses. Deregulated inflammatory response, promoted by harmful microbiota, can activate the vagal system and impact neuropsychological functions. Some bacteria can produce peptides or short chain fatty acids that can affect gene expression and inflammation within the central nervous system. In this review, we summarize the evidence supporting the role of gut microbiota in modulating neuropsychological functions of the central nervous system and exploring the potential underlying mechanisms.
Over the past decade, experimental data has suggested a complex and bidirectional interaction between the gastrointestinal (GI) tract and the central nervous system (CNS), the so-called “Gut-Brain axis.”1 Derangements of this axis (typically in the brain-to-gut direction) have been implicated in the pathogenesis of symptoms of many functional bowel disorders such as the irritable bowel syndrome (IBS).2,3 In recent years, however, emerging knowledge about gut microbiota has compelled us to re-examine the directionality of this process.4–11 The presence of a healthy and diverse gut microbiota appears to be imperative not only for normal gastrointestinal function, but may also influence a variety of systemic and mental processes. Our understanding of the interaction between gut microbiota and the CNS is incomplete and only at its starting point. In this article, we will review the current evidence in the literature that points towards a role for gut microbiota in various developmental and psychiatric disorders such as anxiety, depression, schizophrenia and autism. We will also review the possible mechanisms through which gut microbiota might be involved in the pathogenesis of these disorders.
The gut microbiota at infancy is usually diverse and highly variable, trending towards its final composition between 6–12 months of age,12 reflecting a combination of genetic factors, maternal health, method of delivery, subsequent nutrition, and maternal and postnatal exposure to antibiotics.13–16 Germ-free mice show developmental abnormality in the GI tract that can be reversed by reconstructing the gut microbiota, suggesting a role for gut microbiota in postnatal development of the enteric nervous system (ENS).17,18 This period is also critical for the development of the CNS leading to the suggestion, based on experimental models, that gut microbiota may be an important factor participating in the development of cognitive, emotional, and behavioral processes shortly after birth.19,20 For example, germ free mice show significant alteration in the concentration of the key neurotransmitters such as serotonin in the hypothalamus.21 Alterations in serotonin concentration can in turn affect several aspects of the development of central nervous system, including synapse formation and connectivity between various regions in the central nervous system and their plasticity.22 The picture becomes more complicated because serotonin is also a key factor in the development of the ENS, and alteration of its concentration in the blood may modulate ENS structure and function23; in turn this can affect the composition of gut microbiota, thus potentially providing a closed loop system for mutual regulation of the 2 nervous systems.
A central issue in any discussion on this topic relates to the question of how microbes that live in the colon can influence a remote organ such as the brain. We are just beginning to scratch the surface of this problem, but theoretically there are multiple, possible overlapping mechanisms, that amplify each other in short as well as long loops (Figure). With the exception of the microbe-epithelial interface, all these mechanisms imply some degree of access of either the microorganism itself or its products to the deeper layers of the gut, in turn activating a myriad of factors. Thus, as is being increasingly recognized, gut permeability is perhaps the most important factor in initiating microbial interactions with the rest of the body. These factors will now be briefly described.
The normal intestinal barrier consists of multiple layers that includes gut flora and external mucus layer, epithelial layer, and lamina propria, to name them from outside to inside.24 Mucus is secreted by goblet cells and acts as a mechanical protective layer that also contains digestive and antibacterial enzymes and antibodies, and will hydrate the epithelial layer and helps it regenerate.25 The epithelial layer, in addition to playing an important part in absorption of the nutrients, also serves as a physical barrier due to the tight junctions between the epithelial cells. Furthermore, enteroendocrine cells are distributed through the epithelial layer.26 This layer along with lamina propria is also the host of the largest repository of immune cells in the body which is known as mucosa-associated immune cells. The population of immune cells in the epithelial layer is mostly CD8+ lymphocytes, while the immune cells in the lamina propria are more diverse and consisted of macrophages, plasma cells, antigen presenting cells, and mast cells in addition to lymphocytes.27
Normal gut microbiota is essential in preventing colonization of the harmful bacteria by competing with them for vital resources such as food and growth factors. If the population of normal gut microbiota is reduced, for example due to antibiotic therapy, pathogenic organisms find the opportunity to colonize the gut epithelium. Toxins produced by pathogenic microorganisms and the focal inflammation created by immune responses to them can increase gut permeability.28 For example,
Impaired intestinal barrier function and consequent increased gut permeability can lead to increased translocation of gut bacteria across the intestinal wall and into the mesenteric lymphoid tissue.34 Increased exposure of the ENS or mucosal immune cells to bacteria can provoke an immune response that can lead to release of inflammatory cytokines and activation of the vagus nerve and spinal afferent neurons. Inflammatory cytokines and the vagal system in turn can modulate the activity of the CNS and ENS.38,39 Furthermore, increased permeability of the gut can also increase the translocation of metabolic products such as lipopolysaccharide (LPS) or neuro-active peptides created by the bacteria that can alter the activity of the ENS and CNS.40 For example, LPS can activate Toll-Like receptors that are present on epithelial cells, enteric neurons, sensory afferent neurons in the spine, and various cells in the brain, modulating their activity and affecting the function of both ENS and CNS.41–44
As mentioned above, the interaction between the gut and brain is bidirectional- the CNS can affect gut permeability and increased gut permeability in turn can alter CNS function. In both animal models of stress and human subjects who were exposed to stress, the intestinal barrier is impaired. It has been shown that both acute and chronic stress can reduce water secretion and increase ion secretion in the intestine, and therefore impair the physical protection of the epithelial layer and lamina propria against adhesion of harmful bacteria and nociceptive chemicals.45–47 Activation of the hypotha-lamic-pituitary-adrenal (HPA) axis and increased production of corticotropin-releasing factor (CRF), altered activation of the vagal system, mast cell activation, and release of certain cytokines such as IFN-γ, TNF-α, and IL-4 are suggested culprits in this interaction.48–54 Additionally, stress can change the function of mucosal-associated immune cells and cause increased antigenic and bacterial uptake.55,56 Multiple studies have been published that have shown that the composition of gut microbiota is changed in the face of acute or chronic stress, and this in turn can subsequently change the function of intestinal barrier as explained above.57–60 There is limited data regarding the changes in intestinal barrier or GI physiology and the underlying mechanisms of it in neuropsychiatric disorders. It has been reported that the frequency of GI symptoms is increased in children with autism but the mechanism is not known.61 In patients with schizophrenia, there are increased intestinal permeability and change in intestinal function.62 Emotional stress and depression have been shown to increase prevalence of disorders of the digestive system.63
Theoretically, bacterial products like other luminal contents, can be absorbed into the blood stream and affect remote sites in the brain. Alternatively, or in addition, bacteria can interact with local elements in the gut such as nerves or endocrine cells that then in turn signal to the brain. Experimental data suggest that a variety of biologically active products derived from gut microbiota can directly or indirectly influence the brain. These include well known, although non-specific, factors such as LPS, which can influence the CNS directly by activating Toll-like receptor 4 on microglial cells causing release of inflammatory cytokines by them within the CNS, or indirectly by inducing release of inflammatory cytokines from the GI tract.64,65 LPS can cause behavioral changes during an acute illness or cause a delayed change in mood after sickness.66,67 IgA and IgM against LPS of gut bacteria are found in the blood of patients with depression or chronic fatigue syndrome, suggesting a potential role for LPS in the pathogenesis of these diseases.66 Other bacterial products reflect the role of colonic microbiota in the fermentation of undigested carbohydrates to short chain fatty acids (SCFA).68 SC-FAs can act as signaling molecules by binding to G protein-coupled receptors, Gpr41, and Gpr43.69–71 It has been shown that Gpr41 and Gpr43 receptors are abundantly present on the surface of gut epithelial and immune cells and are activated by SCFAs. This activation can provoke an inflammatory and immune response that can be helpful in the setting of an acute infection, but dysregulation can produce an exaggerated response leading to increased gut permeability and increased absorption of neuro-active metabolites.72,73 SCFAs can also directly activate the sympathetic nervous system through Gpr41 receptors that are found on sympathetic ganglionic neurons.74 Furthermore, it has been shown that SCFAc can pass through the blood-brain barrier and influence behavior, neural signaling, the production of neurotransmitters and, ultimately, behavior.75–77
Studies have reported on CNS neurotransmitter changes in response to more specific biological factors that may be restricted to certain types of bacteria, thus providing a mechanistic link to changes in microbial metabolism. Germ free mice have elevated levels of dopamine and tryptophan in striatum, but not serotonin or gama-amino butyric acid (GABA).78 Another study has reported increased levels of serotonin in the hippocampus of germ free mice.79 It has been recently shown that indigenous bacteria from gut of mice and humans can induce serotonin production in entrochromaffin cells and increase the level of serotonin in blood.80 Histaminergic pathways are found in areas of the limbic system and also areas in the brain heavily involved in cognitive functions.81
One potential unifying mechanism through which these various processes can influence the activity of CNS is via vagal nerve activity. In animal models, administration of
It has been shown that stress can alter gut permeability as well as the composition of gut microbiota.46,57 In a mice model of stress due to social disruption, Bacteroids are reduced while Clostridia are increased, resulting in a pro-inflammatory change in the profile of cytokines produced by gut microbiota.58 More recently, the interaction between stress and gut microbiome has been shown to be bidirectional, and that gut microbes can modulate the stress response and the activity of the corticosterone pathway orchestrated by the HPA, a key stress regulatory system in the CNS. Germ-free mice show an exaggerated HPA response to stress and the amount of CRF released in response to stress.96 Introduction of
Exposing rats in the early postnatal period to stress by maternal separation also leads to a change in composition of gut microbiota, which is linked to a long-term increase in anxiety-like behavior.100–102 Introducing probiotics containing
Further, it is shown that the behavioral phenotype of anxietyprone strains of mice is also dependent on their existing microbiota. For example, BALB/c mice exhibit a highly anxious phenotype that does not show much exploratory locomotion in a new environment, while NIH Swiss mice show less anxiety and more exploratory motions in the same environment. Transferring gut microbiota from one of the species to another can change their behavior to the one typical of the donor.106 In another study, described above, infecting mice with
It has also been suggested that the modulatory effects of gut microbiota on the level of anxiety are exerted through alterations in serotonin signaling.84 This idea is in part based on the finding that reduced anxiety-like behavior in germ free mice is associated with increased expression of serotonin receptor 1A in the hippocampus.78 However, experimental data from mice showed that while reintroduction of gut microbiota to germ free mice can normalize the anxious behavior, it fails to reverse the changes in serotonin levels in the hypothalamic-pituitary pathway.79 Another mechanism that has been described is increased release of the adreno-corticotropin hormone from the HPA axis in response to stress.96 A link between hypersensitivity of the HPA axis and reduced BDNF expression in the prefrontal cortex and hippocampus, and subsequently reduced N-methyl-d-aspartate receptor expression in germ-free mice is observed and was thought to play a role in regulation of HPA activity.78 Alteration of BDNF expression in the hippocampus was seen in mice that were treated with non-absorbable antibiotics such as neomycin, but not in mice treated with systemic intraperitoneal injection of antibiotics, suggesting that this effect is a result of elimination of gut microbiota, not the antibiotic treatment itself.106
In animal models of depression, it has been reported that the composition of gut microbiota has been changed.107,108 These data however, have not been validated in patients with depression. In one study on human subjects with depression, no significant difference in the composition of gut microbiota was found between depressed patients and a control group.109 However, another recent study examined the composition of fecal microbiota in 46 patients with depression and 30 healthy controls, and reported significant differences with increased population of
In mice, elimination of gut microbiota can alter performance in tasks that require intact spatial memory, hippocampal function or working memory.116 Similarly, altering the composition of gut microbiota in mice by infection or dietary modifications also can change the performance of the animal in memory tasks.117 For example, adding lean beef to the mice diet will alter composition of gut microbiota and will improve their performance in cognitive tasks. In this experiment, a temporal relationship with dietary induced changes in gut microbiota and working memory performance was reported.118 Mice infected with
Given the explosion of interest in the microbiota and the gut-brain axis, it is not surprising that investigators are moving beyond more traditional phenotypes such as anxiety/depression to other neuropsychological syndromes including schizophrenia and autism. Increased gut permeability and translocation of gut bacteria has been shown in schizophrenic patients.122 The fundamental cause of this is unknown and could include both the controversial association with gluten sensitivity and celiac disease123 as well as primary changes in gut microbiota.62,124 These theories may not be mutually exclusive as it is possible that certain compositions of gut microbiota can lead to changed metabolism of certain food products such as gluten, and subsequent production of neuroactive peptides, increased absorption of these products due to local inflammation, and alteration of dopaminergic and serotonergic pathways in individuals who are genetically susceptible to schizophrenia.62 Germ-free mice tend to show a schizoid type behavior, not spending more time in a chamber with another mice in it when put in a 3-chamber sociability test.125 In a mice model that shows behavioral changes that resemble schizophrenia, treatment with
Another area in which information is rapidly evolving is that of autism spectrum disorders (ASD). In a study in rodent model, it was found that the composition of gut microbiota in animals with ASD-like behavior is significantly changed compared with control animals. These changes were similar to those found in human patients with most changes observed in
Some of these findings have parallels in humans. Children with ASD also show altered composition of gut microbiota with a reduced population of
Another bacterial genus that has been linked to autism is
A few small clinical trials have shown beneficial effects for gluten free and casein free diets on symptoms of children with ASD145,146 that could potentially be attributed to the change in gut microbiota.4,8,19,61,128,147 Furthermore, in children with autism, the frequency of GI symptoms is increased148,149 and has been attributed to a low-grade chronic inflammation in the GI tract caused by altered gut microbiota. In a clinical study, oral vancomycin was used as a minimally absorbed antibiotic to treat the GI problems, based on this theory. Interestingly, in addition to improvement in GI symptoms, autistic behavior was also improved in these children.150
The influence of gut microbiota on several aspects of CNS function is increasingly supported by a growing body of experimental data. The mechanism of this influence is complex and involves multiple direct and indirect pathways. Increased gut permeability appears to be the cornerstone of the microbiome-gut-brain interaction. This provides a pathway for gut bacteria and their metabolic products to access the immune system, ENS, the blood stream, and centripetal neural pathways. Much of this evidence comes from rodent studies, and considerable work has to be done to validate these findings in humans before we can understand how best, if at all, to modulate the gut microbiota for clinical benefit.