Communication of gut microbiota and brain via immune and neuroendocrine signaling

1The gastrointestinal tract (GI) is the external environment for our body. Therefore, the GI epithelium has to act as the protective barrier for the organism. Tight junctions connecting the epithelial cells and mucus produced by goblet cells are the physical barriers. Moreover, enterocytes produce antimicrobial peptides (AMPs) which act directly on the bacterial cell membrane, causing its disruption and cell lysis. Furthermore, the gut-associated lymphoid tissue (GALT) defends the organism against pathogens and, more importantly, maintains homeostasis in the GI tract. The molecules which allow our immune system to recognize potentially harmful microorganisms are the microorganism-associated molecular patterns (MAMPs), or in general for pathogens—the pathogen-associated molecular patterns (PAMPs). Such patterns are recognized by pattern recognition receptors (PRRs) present on the host cells of the innate immune system. All these factors contribute to limiting the direct contact of the gastrointestinal epithelium with the microbiota. Therefore, the symbiotic gut microbiota which colonizes our intestines influences the GI epithelium, and interacts with the immune system, but in a limited scope.

2The term gut microbiota describes the microorganisms that exist in the GI, consisting mainly of bacteria, but also of archaea, fungi, and viruses. The term microbiome, which is a broader concept that refers to microorganisms, their genomes, and the habitat they reside in, is often used instead of the term gut microbiota.

3The colonization of the GI tract mostly takes place during birth, but there is evidence of trans-placental gut colonization. It was shown that the gut microbiota profile in humans changes with the age of the host and is most stable in adulthood. In the healthy adult human, there are about 5 × 1013 bacteria (of about 1000 species) in the gut, mostly represented by the two phyla, Firmicutes and Bacteroidetes, as well as less numerous Actinobacteria, Proteobacteria, Fusobacteria, Verrucomicrobia, and Archaea. The available data suggest that gender may have a significant influence on the composition and activity of gut microbiota. Clinical studies have shown that men when compared with women have an increased number of Bacteroides and Prevotella, as well as Clostridia, Bacteroidetes, and Proteobacteria phylum. However, the mechanisms explaining the above observation remain unclear.

4The condition in which the balance between the commensal microflora and pathogenic microorganisms is maintained is called eubiosis. Each deviation from the normal composition of gut microbiota (instability or a decrease in the number of Firmicutes and Bacteroidetes) and the accompanying violation of the intestinal epithelial barrier is referred to as dysbiosis, i.e., a deviation leading to an inflammatory state.

5The host organism creates favorable conditions for the development of gut microbiota, which in turn exert many positive effects on the host. Bacteria, through fermentation processes, produce metabolites such as lactates and short-chain fatty acids (SCFAs), which include acetate (C2), propionate (C3), and butyrate (C4). These substances, synthesized in the gastrointestinal tract, can enter the circulation through the portal vein, and then via the blood to the peripheral tissues and organs, and also to the brain. For example, butyrate is the most abundant of the SCFAs. It is synthesized in the large intestine by the following types of bacteria: Clostridium, Eubacterium, and Butyrivibrio (;;; Table 1). It has been shown that butyrate has anti-inflammatory properties and has a positive effect on the intestinal epithelium, which may have a positive effect on the gut microbiota, increasing the number of the ENS cholinergic neurons, as well as modulating appetite through vagal and hypothalamic stimulation. Additionally, gut microbiota plays an important role in the synthesis of vitamins (K, riboflavin, biotin, nicotinic acid, pantothenic acid, pyridoxine, thiamine, and folic acid), and the metabolism of bile acids. However, gut microbiota can also have adverse effects on the host organism, including the synthesis and release into the blood of lipopolysaccharide (LPS), also called endotoxin, which is a component of the cell wall of gram-negative bacteria and plays a key role in the initiation and progression of inflammation.

6The gut microbiota has been shown to influence the host organism in three major pathways: (a) via immune systems, (b) via neurotransmitters, and (c) via microbial metabolites.

7As previously mentioned, the mucus layer reduces to a large extent the direct contact of the gut microbiota with the GI epithelium. Bacteria that reach deeper into the mucosal surface are sampled by the protrusions of the dendritic cells (DCs), which are the specialized antigen-presenting cells (APCs). Next, the DCs go to the mesenteric lymph node, where the sampled antigens are presented to T and B lymphocytes.

8In the state of eubiosis, physical barriers are intact and molecules such as IgA and AMPs, present in the gut lumen, provide the control mechanisms inhibiting the pathogen spreading. Moreover, symbiotic bacteria itself produces the AMPs against pathogenic strains. Immune cells composing GALT present the tolerogenic phenotype as the result of the influence of transforming growth factor β (TGFβ) produced by the epithelial cells. TGFβ causes the differentiation of immune cells toward the anti-inflammatory tolerogenic phenotype. In addition, CD103 + DCs residing in the lamina propria produce anti-inflammatory interleukin 10 (IL-10), which further influences T and B cell differentiation. B cells in the mesenteric lymph nodes produce the IgA antibodies against the bacterial antigens, and next go to the lamina propria and secrete the IgA into the intestinal lumen. Here, the IgA binds to the bacterial antigens and prevents bacterial translocation through the gut epithelium. In case of bacterial penetration under the epithelium, the resident macrophages and DCs in the lamina propria phagocyte the microorganisms. In turn, naive T cells under the influence of the commensal bacteria antigens are differentiated into regulatory T cells (Tregs). Such a mechanism underlies the mechanism of oral tolerance, which prevents the evoking of an immune response against dietary antigens and against symbiotic microflora. Furthermore, induced Tregs secrete the anti-inflammatory cytokines, IL-10, and TGF-β. When the pathogenic bacteria encounter GALT, naive T cells differentiate into Th1/Th17 subtypes, producing proinflammatory cytokines. Moreover, CD103+ DCs also contribute to the production of retinoic acid (RA) from vitamin A, which is induced by the SCFAs obtained from dietary fibers. RA is also the factor inducing Tregs differentiation. In such a homeostatic state, tolerance to dietary antigens and antigens from commensal microflora is maintained.

9On the other hand, during dysbiosis, pathogens and their harmful metabolites are recognized by the PRRs on the surface of the immune cells of the host, and an innate immune response is evoked. Enterotoxins released by pathogens, such as LPS, cause damage to the intestinal epithelium, which results in gut permeability and the entrance of pathogens into the circulation. The damage to the gut epithelium may also contribute to intestinal inflammation. Environmental factors, such as a western diet, antibiotics, stress, or injury may cause damage to the gut epithelium, allowing bacterial gut penetration, which induces an immune reaction. Mediators released by the immune cells during an inflammatory reaction enter the circulation to attract more immune cells. PAMPs released from damaged pathogens may also enter the circulation.

10Two examples of PRRs are toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors. The interaction of PRRs and PAMPs results in the activation of intracellular signaling pathways, leading to nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) translocation to the nucleus, and results in the activation of gene transcription for proinflammatory cytokines (e.g., interleukin-1, IL-1; tumor necrosis factor α, TNFα; interferon gamma, IFNγ). Interestingly, the TLRs of the innate immune systems can distinguish between the MAMPs of the symbiotic bacteria from those present on the surface of the pathogens.

11The above-mentioned cells and molecules, such as immune cells differentiated under the influence of symbiotic or pathogenic microflora, cytokines, inflammatory mediators, pathogen toxins, and PAMPs after translocation into the systemic circulation, may further infiltrate the central nervous system (CNS) and influence its functioning.

12The main neurotransmitters that may play a role in the gut-brain axis communication are serotonin, dopamine, noradrenaline, and gamma-aminobutyric acid (GABA). These substances are synthesized not only in the CNS but also in enteroendocrine (EEC) cells that have the ability to synthesize neurotransmitters under the influence of intestinal peptides, as well as the gut microbiota itself.

13It is worth emphasizing that one percent of the intestinal epithelial cells are EEC cells, the role of which is to synthesize and release substances into the intestinal lumen in the presence of ingested carbohydrates, triglycerides, and proteins, and to regulate intestinal motility, secretion, and food intake.

14It has been shown that EEC cells synthesize approximately 90 percent of the total serotonin produced in the human body. Moreover, an important role in the synthesis of serotonin is played by the liver expressing the enzyme tryptophan-2,3-dioxygenase (TDO, tryptophan pyrolase), degrading tryptophan to N-formylkynurenine, which is then deformylated by formidases to kynurenine. This reduces the concentration of tryptophan which may be converted to the serotonin in the brain. In the digestive system, serotonin is involved in the activation of innate intestinal reflexes, mediating intestinal-brain communication, regulating the immune system, and has a protective/regenerative effect on neuronal cells and the interstitial cells of Cajal, but on the other hand it may cause enteritis. Studies indicate that the level and activity of serotonin, synthesized both in the CNS and peripherally, is strongly influenced by the gut microbiota. In particular, a few representatives of the gut microbiota such as Candida, Streptococcus, Escherichia, and Enterococcus are capable of producing serotonin directly. Nevertheless, it was noticed that serotonin has a significant effect on the composition and activity of the gut microbiota. observed that elevated intestinal serotonin levels increased the relative abundance of spore-forming bacteria.

15Serotonin as well as other neurotransmitters, including dopamine and norepinephrine, are involved in gut-brain communication. It has been found that both dopamine and norepinephrine can be synthesized directly by the gut microbiota. Dopamine can be synthesized by the following microorganisms: Bacillus cereus, Bacillus mycoides, Bacillus subtilis; Escherichia coli; Hafnia alvei; Klebsiella pneumoniae; Morganella morganii; Proteus vulgaris; and Staphylococcus aureus. In turn, norepinephrine can be synthesized by: Bacillus mycoides, Bacillus subtilis; Escherichia coli; and Proteus vulgaris. Moreover, gut microbiota is capable of synthesizing oxidases such as laccase, which has oxidizing properties, and catabolize catecholamines to reactive oxygen species (ROS) and dopamine quinone (DAQ), which is associated with mitochondrial dysfunction and dementia in patients with Parkinson’s disease. Gut microbiota may also play a role in the transport of catecholamines. In addition, gut microbiota may affect the availability of tryptophan, a precursor to catecholamines. These data appear to have important implications because the nucleus tractussolitarius (NTS), the main site for catecholamine synthesis, receives signals from the phrenic and vagus nerves. In turn, the NTS, via noradrenergic neurons and catecholamines, can activate the hypothalamic-pituitary-adrenal (HPA) axis, which has a significant impact on the composition and activity of the gut microbiota.

16It has also been shown that GABA is directly synthesized by some types of intestinal bacteria, mainly: Bifidobacterium spp. and Lactobacillus spp.. In addition, it was found that lactate, a substrate for the SCFAs produced by gut microbiota, through the pathway dependent on the brain G protein-coupled receptor 81 (GPR81), induces anti-GABA-transmitting effects. On the other hand, the administration of Lactobacillus rhamnosus contributed to an increase in the expression of GABA receptors in the cingulate cortex and a decrease in the expression of GABA receptors in the hippocampus, amygdala, and the locus coeruleus, leading to a reduction in anxiety and depressive-like behavior in adult male BALB/c mice.

17It appears that SCFAs can influence the brain indirectly by activating the immune system and the autonomic nervous system. First, SCFAs are able to stimulate the activity of microglia and change the selectivity of the blood-brain barrier (BBB) permeability. It is likely that the abundant exposure of H+ dependent monocarboxylate transporters (MCTs) in endothelial cells may facilitate the penetration of SCFAs by BBB. Clinical studies have shown the presence of SCFAs in human cerebrospinal fluid (CSF) in the following range: acetate 0–171 μM, propionate 0–6 μM, and butyrate 0–2.8 μM. In addition, the mean level of SCFAs in the human brain is 17.0 pmol/mg of tissue for butyrate, and 18.8 pmol/mg of tissue for propionate. The SCFAs also appear to play a significant role in maintaining the integrity of the BBB. Studies on germ-free mice showed decreased expression of tight junction proteins such as claudin and occludin in the endothelium, leading to increased BBB permeability. In turn, colonization of adult germ-free mice with complex microflora or monocolonization with SCFA-producing bacterial strains restored BBB integrity.

18It has also been shown that SCFAs can act indirectly on the CNS by binding SCFAs to their receptors on EEC cells, stimulating the secretion of GLP-1 and PYY, which can act on the CNS via the vagus nerve. Another mechanism by which SCFAs respond to systemic functions is the inhibition of histone deacetylase (HDAC) activity, thereby promoting the acetylation of a lysine residue present in histones and nucleosomes in various cell populations, including the intestine, the autonomic nervous system (ANS), and the CNS.

19It was also found that SCFAs can affect the synthesis of neurotransmitters in the CNS, butyrate and propionate are able to stimulate the synthesis of dopamine and norepinephrine, and propionic acid can modulate serotonergic neurotransmission and affect the levels of GABA, dopamine, and serotonin. The available data suggest that the SCFAs synthesized by the gut microbiota after entering the host cells (passive diffusion and/or active transport) inhibit the histone deacetylase inhibitor (HDAC) or, by interacting with membrane receptors, activate various intracellular signaling pathways that modify the expression of a given gene. It has been shown that butyric and propionic acids can regulate tyrosine hydroxylase (TH) mRNA levels through various transcription factors, including the activation of the cAMP-response element binding protein (CREB) and, consequently, may lead to increased production of catecholamines.

20Short-chain fatty acids may also modulate the expression of signaling molecules important for learning and memory, such as brain-derived neurotrophic factor (BDNF), N-methyl-D-aspartate receptor subtype 2B (NR2B) subunit, the serotonin transporter, and the neuropeptide Y system. It has been shown that sodium butyrate is able to stimulate BDNF expression, neurogenesis, and neuronal proliferation in rodents, and facilitate long-term consolidation. Moreover, SCFAs influence several nervous functions, such as the regulation of the circadian rhythm and the control of appetite. It appears that SCFAs may affect neuronal function through a pathway dependent on GPR41 and GPR43 receptors or HDAC inhibitory activity.

21The observed concordance of the phylogenetic trees of the gut microbiota and primates indicates co-evolution of the host organism, including humans, with the resident microorganisms. Through co-evolution, the gut microbiota influenced the formation of the host’s immune system, which developed complex mechanisms for identifying and destroying microbes. Moreover, available data suggest that gut microbiota may influence host brain activities such as behavior, appetite regulation, and serotonin metabolism. Changes in gut microbiota composition have been linked to many neurological diseases such as neurodegenerative disorders. The above effects on the host brain are likely to be exerted by gut microbiota via the gut-brain axis (GBA). Moreover, the same pathway enables the CNS to influence the composition and activity of the gut microbiota. The gut-brain axis is a bidirectional signaling pathway between the gut and the CNS, and this action is possible through three GBA communication pathways: (1) immunological; (2) neuroanatomical; and (3) neuroendocrine.

22As described above, the immune mechanisms are shifted to anti-inflammatory responses in the state of eubiosis in the gut. Tregs are generated when antigens are presented by DCs to lymphocytes in the GALT during the eubiotic state. Tregs produce anti-inflammatory cytokines, such as IL-10 and TGFβ, which are responsible for the inhibition of proinflammatory cytokine production, switching the immune response from Th1/Th17-dependent to Th2-dependent, and therefore quenching the immune reaction and promoting the repair process in the damaged tissue. Induced Tregs may also translocate into circulation and inhibit the inflammatory responses in the organism. The depletion of Tregs was shown to increase CNS damage in mice after stroke. It was also presented that Tregs may influence the CNS from the periphery. In rodents with ischemic stroke evoked, injection of Tregs diminished immune cell infiltration, inflammatory response, and ischemic damage of the brain. This was due to the inhibition of matrix metallopeptidase-9 (MMP-9) production by neutrophiles, which suppressed the remodeling of the BBB and decreased its permeability (; Figure 1).

23When dysbiosis occurs, a local inflammatory response in the gut is observed first, and then peripheral inflammation develops when inflammatory mediators, bacteria, metabolites, PAMPs, etc., enter the systemic circulation. Both innate and adaptive immune mechanisms are involved. PAMPs via PRR receptors on the host cells activate innate immunity mechanisms. In addition, the immune cells in GALT are involved during dysbiosis, and lymphocytes under the influence of inflammatory mediators and PAMPs differentiate into proinflammatory subtype (Th1 and Th17), and further produce proinflammatory cytokines, such as interleukin-2 (IL-2), interleukin-12 (IL-12), TNFα, IFNγ, and Th-17. Peripheral inflammation affects the BBB integrity, allowing the infiltration of immune cells and inflammatory mediators into the CNS. Besides, bacterial toxins present in the circulation may also infiltrate into the CNS. Microglia, resident immune cells in the CNS may be further activated by infiltrating proinflammatory cytokines from the periphery, and sterile immune reaction may be evoked, eventually causing CNS injury (;; Figure 1).

24The gut-brain immune communication acts in both ways. After injury of the CNS caused by mechanical injury, stroke, infection, etc., the damaged tissue releases damage-associated molecular patterns (DAMPs) activating resident microglia. Activated microglia of the proinflammatory (M1) phenotype release inflammatory mediators acting as chemoattractants and are responsible for recruiting peripheral immune cells to the inflammation site. Those cells, neutrophils, monocytes, and CD4+ T cells, also produce proinflammatory cytokines, which together with DAMPs may enter the peripheral circulation and affect the peripheral tissues. If the intestinal barrier is reached, gut inflammation may be evoked, causing gut permeability, epithelial injury, and the entrance of pathogenic bacteria into circulation. This may eventually lead to systemic inflammation. Moreover, intestinal inflammation causes the reduction in Tregs differentiation and IL-10 and TGFβ secretion and the promotion of a Th1/Th17-dependent immune reaction. Such a lack of anti-inflammatory signaling and promotion of proinflammatory mechanisms further activates the immune cells and exacerbates the inflammation of the CNS (;; Figure 2).

25Based on the available data, it can be concluded that the gut microbiota-induced vagal signaling affects the critical immune components of the microbiota-gut-brain axis and allows the vagus nerve to be seen as an integral part of the bidirectional neuroimmunoendocrine pathway (; Figure 3). Therefore, two pathways can be distinguished at the neuroanatomical level. The first direct way between the GI and the brain consists of the vagus nerve and the ANS. While the second indirect way is made by the connection between the ANS and the enteric nervous system (ENS).

26It has been proven that the vagus nerve plays a key role in the communication between the gut microbiota and the brain via anatomical signaling. Some researchers believe that the vagus nerve innervates the entire digestive tract, while others argue that it only innervates to the left colon flexion. Vagal afferent fibers are distributed in all layers of the gastrointestinal wall but do not penetrate the epithelial layer into the lumen of the intestine. The afferent endings of the vagus nerve have been divided into three subtypes: (1) afferent endings located at the apices of the intestinal villi directly under the epithelial wall; (2) afferent endings around the intestinal glands or crypts located below the crypt-villus junction; (3) afferent endings along the antral glands of the stomach (afferent endings of the antral gland) forms the end concentrations directly below the luminal epithelial wall. All of the above vagus nerve endings are both chemosensitive and mechanosensitive. Vagus nerve chemoreceptors are presumed to be involved in communication between the gut microbiota and the brain by detecting SCFAs and/or intestinal peptides (; Figure 3). Oleate (one of the SCFAs) acts on the vagal afferent fibers via cholecystokinin (CCK), while butyrate can directly activate the vagus nerve. Similarly, the LPS synthesized by the gut microbiota can interact with the TLRs located on the vagus nerve fibers at the nodose ganglion level (; Figure 3).

27The vagus nerve can also receive signals from the gastrointestinal tract indirectly, involving the ENS, which is part of the autonomic nervous system. The ENS consists mainly of enteric glial cells (EGCs), which resemble astrocytes in the CNS. The ENS is distributed throughout the intestinal wall, including the lamina propria of the mucosa. From the ganglia of the ENS, the neuron fibers go to the prevertebral ganglia and then to the spinal cord at the level of the T5-L2 and S2-S4 segments, and to the vagal nuclei. Afferent fibers of the vagus nerve and spinal cord concentrate hormonal and mechanical stimuli in the NTS and in the dorsal motor nucleus, where the signal is integrated, and next sent to the hypothalamus, as well as to the basal ganglia and brain stem nuclei. NTS signaling is mediated by proopiomelanocortin (POMC), catecholaminergic neurons, and N-methyl-D-aspartate (NMDA) glutamate receptors. The vagus nerve can also receive information from the intestinal lumen via enteroendocrine (EEC) cells, which make up one percent of all intestinal epithelial cells. It has been shown that EEC can release serotonin (5-HT), which then activates 5-HT3 receptors on the vagal afferent fibers. EEC effects on the vagus nerve may also be indirect through intestinal peptides such as CCK, glucagon-like peptide-1 (GLP-1), and peptide YY (PYY). In general, gut hormones fall into two broad categories: orexigenic, for example, ghrelin, which together with neuropeptide Y (NPY) and agouti-related peptide (AGRP) neurons, increases hunger; and anorexigenic, i.e., appetite-suppressing peptides such as GLP-1, PYY, and CCK. The vagus nerve has receptors for both anorexigenic and orexigenic intestinal peptides. Subsequently, the signal is sent to the CNS, ultimately leading to the modulation of reward regions (amygdala and nucleus accumbens) and appetite regulation (;; Figure 3).

28Evidence for the direct influence of the gut microbiota on the activation of the vagus nerve is provided by a few experimental studies. Ex vivo studies have shown that the application of Lactobacillus johnsonii to the isolated fragment of the jejunum contributed to an increase in the firing rate of the vagus nerve fibers, which was prevented by the previous subdiaphragmatic vagotomy. It was reported that duodenal injection of Lactobacillus johnsonii in male Wistar rats caused an increase in gastric activity of the vagus nerve. It was also shown that signals sent by the gut microbiota via the vagus nerve can go directly to the CNS. Male mice receiving Lactobacillus rhamnosus for 14 days showed a reduction in anxiety-like behavior and a decrease in the HPA axis activity. Whereas vagotomy in these mice abolished the anxiolytic effects induced by Lactobacillus rhamnosus. observed that chronic oral administration of Lactobacillus rhamnosus (JB-1) in healthy adult male BALB/c mice increased gamma-aminobutyric acid (GABA) expression in the cingulate cortex and decreased GABA in the hippocampus, amygdala, and locus coeruleus. Interestingly, the above effects were abolished by vagotomy. In addition, studies in healthy Sprague Dawley rats have shown that one of the bacterial metabolites, indole, can stimulate the ECC of the colon to secrete GLP-1, which in turn stimulated colonic vagal afferent activity. Recently, showed that administration of fecal supernatant from patients with active inflammatory bowel disease (IBD) on the C57/Bl6 mouse vagal afferent neurons (nodose ganglion; NG) increased their excitability, possibly due to mediators such as cysteine protease, activating protease-activated receptor 2 (PAR2) dependent signaling pathways, which leads to the inhibition of voltage-gated K+ currents.

29Moreover, available data have shown that gut microbiota can affect the nervous system of the host by altering the metabolism of neurotransmitters. Studies on adult germ-free Swiss Webster mice treated with live Bifidobacterium dentium via oral gavage showed that bacterial-produced acetate contributed not only to increased 5-HT release from the EEC, but also increased 5-HT2a receptor expression in the hippocampus and lowered the anxiety-like behaviors in the tested mice. In turn, a study revealed that another bacterial metabolite, succinate, has a protective effect on dopaminergic neurons in the substantia nigra. In addition, it was shown that oral supplementation of the three major SCFAs (acetate, propionate, and butyrate) in C57BL/6J mice undergoing psychosocial stress reduced disturbance in food-seeking behavior as well as reducing anti-depressant and anti-anxiety effects. SCFAs are presumed to affect the host nervous system in a hormone-like fashion via specific G-protein coupled receptors (GPCRs), which also include many metabotropic neurotransmitter receptors. On the other hand, emotional disorders such as chronic stress not only affect the metabolism of neurotransmitters but also have strong implications on the composition of the gut microbiota. Moreover, norepinephrine, a stress-related hormone has been shown to increase the abundance of Desulfovibrio vulgaris. A significant population increase of this bacterium is observed in patients with inflammatory bowel disease and irritable bowel syndrome.

30The neuroendocrine level primarily includes the HPA axis, which plays a key role in the stress response and is also one of the main components of the gut-brain axis. The HPA axis begins in the paraventricular nucleus of the hypothalamus, where the corticotropin releasing factor (CRF) is synthesized. In turn, CRF stimulates the pituitary gland to produce the adrenocorticotropic hormone (ACTH), which leads to the release of glucocorticoids (cortisol and corticosterone) from the adrenal cortex. In addition, CRF stimulates the locus coeruleus to synthesize catecholamines, thus leading to an increase in the noradrenergic activity of the brain. There is evidence that the gut microbiota develops in parallel with the HPA axis, moreover, they are in constant communication. Interestingly, the two-way communication between the gut microbiota and the HPA axis is increasingly emphasized (;;; Figure 4). It has been reported that the abnormal formation of the HPA axis during brain development may affect microbial colonization and visceral sensitivity. Irritable bowel syndrome studies have shown that cortisol can directly activate resident immune cells and external primary afferent fibers in the gastrointestinal tract. Moreover, both experimental and clinical researchers have demonstrated that the stress-related HPA axis response can increase intestinal permeability leading to dysbiosis (;; Figure 4).

31Significant information on the existence of interactions between the gut microbiota and the HPA axis was provided by. These researchers demonstrated that germ-free mice (GF; mice raised in the absence of microbes) were more sensitive to restraint stress than mice with normally functioning microbiota but in the absence of specific pathogens (SPF; specific pathogen-free mice). GF mice also showed decreased levels of cortical glucocorticoid receptor mRNA expression and increased levels of CRF mRNA and protein in the hypothalamus compared with SPF mice. Moreover, the same researchers observed that the increase in plasma ACTH and corticosterone levels in response to restrictive stress was significantly greater in GF mice compared with SPF mice. Probably the observed differences in stress response between GF and SPF mice could also be caused by decreased expression of POMC and encoding CRF receptor type 1 (Crhr1) genes.

32Experimental studies have also shown that numerous stressors causing an increase in the activity of the HPA axis may affect the development of dysbiosis. It has been shown that chronic s.c. injection of ACTH hormone fragment in female Wistar rats not only caused their depressive-like behavior, but also caused changes in the community of gut microbiota, namely a marked increase in Ruminococcus and Klebsiella and a reduction in the population of Akkermansia and Lactobacillus. in research carried out on rats with stress-induced hypertension (SIH) noted a reduction in HPA axis hyperactivity and blood pressure due to the administration of an antibiotic cocktail containing ampicillin, vancomycin, neomycin, and metronidazole. Catecholamines released in response to stress probably play an important role in this process. It was proven that catecholamines can stimulate the growth of gram-negative bacteria. The above data seem to confirm studies conducted in patients suffering from irritable bowel syndrome (IBS) in combination with emotional distress including anxiety and depressive symptoms. In these patients, already slightly lower concentrations of serotonin and norepinephrine corresponded to significant changes in the composition of gut microbiota. Namely, serotonin levels were positively correlated with the abundance of Proteobacteria, and norepinephrine was positively correlated with Bacteroidetes and negatively correlated with Firmicutes. Similarly, clinical data confirmed that stress and the accompanying increased cortisol blood level have a significant impact on the development of gut dysbiosis.

33In contrast, both experimental and clinical studies revealed that probiotics based on Bacillus licheniformis, Saccharomyces boulardii, Lactobacillus rhamnosus, and Bifidobacterium spp. contributed to the inhibition of stress-induced HPA axis hyperactivity, as well as alleviated depressive-like behavior and anxiety-related behavior.

34It appears that the gut microbiota can act on the HPA axis through several mechanisms (Figure 4): (1) gut dysbiosis contributes to the increased release of pro-inflammatory cytokines, including IL-1, IL-6, and TNF-α which can cross the BBB and activate the HPA axis; (2) the HPA axis can be activated by bacterial metabolites such as LPS, SCFAs, and peptidoglycans (components of bacterial cell walls); (3) by the influence of gut microbiota on the HPA axis via the vagus nerve, affecting the NTS activity of noradrenergic neurons; and (4) by changes in HPA axis activity caused by the modulation of central gene expression in the hippocampus and hypothalamus by gut microbiota.

35Currently, many researchers emphasize the possibility of using the gut-brain axis in the treatment of many neurological diseases such as autism spectrum disorder (ASD), Alzheimer’s disease (AD), and Parkinson’s disease (PD).

36Clinical studies increasingly point to a link between ASD and intestinal dysfunction. It is estimated that up to 70% of children with ASD have impaired function of the digestive tract. In the case of gut microbiota, these abnormalities concern the development of an excessive number of pathogenic bacteria such as Clostridium tetani. In turn, treating children with ASD with anti-clostridium antibiotics resulted in a decrease in typical behaviors for them. Moreover, high hopes are attached to Microbiota Transfer Therapy (MTT) in the treatment of patients with ASD.

37Currently, the role of disorders in the functioning of the gut brain-axis in the pathogenesis of neurodegenerative diseases such as AD and PD is increasingly emphasized. Damage to the intestinal barrier is hypothesized to lead to a systemic inflammatory response, which in turn impairs BBB function and promotes neuroinflammation leading to neurodegeneration and neuron damage. Consequently, damaged GBA potentiates β-amyloid deposition in AD and misfolding and aggregation of α-synuclein in PD. Similar to patients with ASD, also in the case of therapy of patients with neurodegenerative diseases, much attention is paid to the need to restore the proper composition of the gut microbiota with fecal microbiota transplantation (FMT) or probiotics. It is likely that FMT has a beneficial effect on reducing symptoms in patients with PD through neuroprotective effects against toxicity induced by the TLR4/TNF-α signaling pathway and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). So far, few clinical trials have shown the beneficial effect of FMT on improving cognition, memory, and mood, as well as gut microbiota biodiversity and SCFA production in patients with AD. However, the results of studies on the effect of probiotics on symptom improvement in patients with AD are inconclusive. showed that after 12 weeks of daily administration of a mixture of Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium bifidum, and Lactobacillus fermentum, AD patients showed a significant improvement in mini-mental state exam results. On the other hand, the administration of two different probiotic mixtures: one containing Lactobacillus fermentum, Lactobacillus plantarum, and Bifidobacterium lactis and the other containing Lactobacillus acidophilus, Bifidobacterium bifidum, and Bifidobacterium longum, did not contribute to the improvement of cognitive functions in patients with severe Alzheimer’s disease. Moreover, numerous studies indicate a well-chosen diet as a quick way to modify the composition and metabolism of the gut microbiota, reduce inflammation, and help maintain eubiosis and proper dependencies in the gut-brain axis.

38Gut microbiota, which is an integral part of the human body, is able to summon and control many of its physiological processes. The host organism also has great influence on the composition and activity of the gut microbiota. Recent studies show that the gut-brain axis is a well-established concept, indicating the bidirectional cooperation between two organs of the human body, the brain, and the gut, and in particular the microbiota residing in the intestines. Many studies have shown the engagement of different routes of communication able to transmit information between the two separate organs, such as immune, nervous, and endocrine. It was also proved that disturbances in one of the organs may affect the proper functioning of the other, also in a bidirectional manner. Unfortunately, the routes for signal transduction involved in the gut-brain communication are still not fully known as well as what roles are played by inflammation, neurotransmitters, intestinal peptides, and bacterial metabolites. In addition, a significant amount of information on the gut-brain axis activity comes from studies on animal models, which, while providing relevant information, should not be directly extrapolated to the human population. Therefore, more research is needed to elucidate the importance of gut microbiota not only for adult organisms, but also for developing ones, with the target of preventing or treating CNS diseases.

39KK and KC: methodology, resources, and writing—original draft preparation. KC and AC-J: supervision. KC: project administration. All authors conceptualization, writing—review and editing, read and agreed to the published version of the manuscript.