Effects of Probiotics on Gut Microbiota, Endocannabinoid and Immune Activation and Symptoms of Fatigue in Dancers

Professional dancers are often referred to as artistic athletes due to their high level of physical activity and competition. Training dance for over 11.5 hours a week increases the risk of injury (women - mostly bone injuries, men - mostly bruises and tendon injuries). One of the reasons for the increased susceptibility to injuries among dancers is psychological stress. The observations results indicate that dancers often decide to continue training despite feeling pain, which limits the smoothness of their movements and leads to a further increase of stress. Highly physically active groups of people are more exposed to functional dyspepsia (FD) and irritable bowel syndrome (IBS), which occurence is correlated to chronic stress. As a result of chronic stress and the activation of the hypothalamic pituitary-adrenal axis (HPA), inflammatory processes in the gastrointestinal mucosa are initiated. The presence of noradrenaline increases the adhesion of bacteria and viruses to the epithelium, which in turn modulates the immune system within the intestines and increases their inflammation. Systemically acting cortisol causes the leakage of tight junctions (TJ) cells and an increase in the permeability of the intestinal barrier. Inflammation stimulates the secretion of cortisol, which initiates the further cycle of inflammation and weakening of the intestinal barrier function. As a consequence, cytokines and inflammatory mediators released in the course of inflammation act directly on the nerve endings that transmit these afferent nerve signals to the brain, becoming an endogenous stressor.

When exogenous and endogenous stress factors overlap, the body's perception of stress increases, which ultimately leads to psychological, physiological and behavioral changes.

Pain and depression are common comorbidities of neuropsychiatric origin. One of the body's regulatory systems, which in both cases is dysfunctional, is the endocannabinoid system (ECS). It has been suggested that these diseases can be caused by intense and prolonged exercise, which disrupts the intestinal barrier, causing changes in the profile of metabolites and the function of the intestinal microbiota. Overtraining leads to an increase in the levels of cortisol, adrenaline and noradrenaline, and to the translocation of lipopolysaccharide (LPS) outside the gut, increasing the concentration of pro-inflammatory cytokines in the body. This results in dysregulation of the balance between serotonin (5-HT), dopamine (DA) and gamma-aminobutyric acid (GABA) and fatigue. These changes lead to further activation of the endocannabinoid system and, in the long-term, weakening of adaptive abilities.

The neuromodulatory properties of the cannabinoid system are manifested e.g. in short- and long-term synaptic plasticity and modulation of pain conduction. Research results indicate that the endocannabinoid system, in addition to its key role in regulating intestinal motility, also affects the secretory functions of the gastrointestinal tract and the integrity of the intestinal epithelium, which may be an alternative way to regulate the immune system and inflammation in the intestines.

Type 1 cannabinoid receptors (CB1) are located presynaptically in the cell membrane of central and peripheral nervous system neurons, and their activation inhibits the release of many neurotransmitters, i.e. acetylcholine, noradrenaline, dopamine, serotonin, glutamate and γ-aminobutyric acid. Type 2 cannabinoid receptors (CB2) are found mainly on the surface of cells of the immune system, especially B-lymphocytes, macrophages and monocytes. Their activation inhibits pro-inflammatory cytokines release and increases the release of anti-inflammatory cytokines.

Cannabinoid receptors are found in both, the immune and digestive systems. Endogenous cannabinoids modify the body's response to stress by influencing the hypothalamic pituitary-adrenal axis (HPA). Stress leads to an increased activity of endocannabinoids which, mainly through the CB1 cannabinoid receptors, lead to the inhibition of the release of corticosteroids. In addition, the activation of CB1 receptors in the gastrointestinal tract reduces the intensity of pain conduction (nociception), which is induced by activation, e.g. TRPV1 vanilloid receptors. It has been shown that inflammation within the intestinal epithelium of the gastrointestinal tract causes an increase in the neural conduction of neurons containing CB1 receptors, which contributes to a change in the proportion of these receptors in relation to the TRPV1 located mainly in the cell membrane of dorsal root ganglion afferents (DRGs). As a result, two different receptors interact and the TRPV1 receptor activity is abolished. Inflammatory mediators released from the intestinal epithelium stimulate visceral pain, while activation of CB2 receptors probably reduces their action.

There are indications that a qualitative and quantitative change in the intestinal microbiota may affect the activity of endogenous ligands and mediators of the endocannabinoid system (ECS). Although the mechanisms of regulation of the level of endocannabinoids and related bioactive lipids by selected bacteria are not fully understood, it has been shown that probiotic supplementation can induce an increase in the concentration of endogenous cannabinoids, i.e. 2-AG (2-arachidonoylglycerol), 2-OG (2-oleoylglycerol) and 2-PG (2-palmitoylglycerol). In an animal experiment, the use of monoacylglycerol lipase inhibition reduced the degradation of 2-AG, which reduced endotoxemia and systemic inflammation. In another study, the deletion of the Myd88 gene encoding the TLR (toll-like receptor) receptor protein in intestinal epithelial cells changed the composition of the intestinal microbiota, decreased the synthesis of anandamide (AEA) and increased the synthesis of anti-inflammatory endocannabinoids, such as: 2-AG, 2-PG, 2 OG. Released endocannabinoids with anti-inflammatory properties have the ability to activate so-called orphan GPR119 (G protein-coupled receptor 119) receptors, which are associated with the secretion of anti-inflammatory mediators. Moreover, it has been shown that an increase in LPS concentration induces AEA synthesis and a decrease in fatty-acid amide hydrolase (FAAH) in macrophages, as well as an increase in AEA production in peripheral lymphocytes, which may be important for the regulation of the intestinal barrier function and the level of inflammation. The results of studies carried out on a mouse model revealed the importance of the intestinal microbiota in the regulation of the expression of the NAPE-PLD gene (phospholipase D specific for n-acylphosphatidylethanolamine), i.e. an enzyme involved in the synthesis of anandamide and in the selective regulation of CB1 mRNA (messenger ribonucleic acid) expression. In an obese mouse model, it was observed that the administration of a probiotic decreased the expression level of CB1 receptor mRNA while reducing the concentration of AEA ligand and increasing the expression of FAAH mRNA. In other studies, administration of a probiotic to mice resulted in a decrease in the concentration of LPS in the blood plasma, which correlated with both the level of AEA and the expression of CB1 mRNA in the colon epithelium. In in vitro and in vivo experiments on animal models, it has been observed that CB2 receptors are activated when an imbalance in the innate immune system occurs. The NLRP3 inflammasomes are suppressed by autophagy, a mechanism that may be involved in the suppression of inflammation and the regulation of the intestinal barrier, e.g. in irritable bowel syndrome. Another factor that is a potential NLRP3 inhibitor is the intestinal microbiota metabolite, butyrate, which belongs to the short-chain fatty acids. Its action has a positive effect on the functions of the intestinal epithelial cells and the stability of the intestinal barrier. In studies conducted on germ-free mice, after the intestinal microbiota was transferred, the metabolism of endocannabinoids in the gastrointestinal tract changed. The use of probiotic therapy in mice with previously induced intestinal dysbiosis changed the activation of endocannabinoid receptors, the behavior of animals and a decrease in intestinal inflammation. Other research results indicate that inhibition of the CB1 receptor in obese mice stimulates the secretion of mucin, which is the primary source of nutrients for the development of Akkermansia muciniphila. This observation confirms the results of studies by other authors, who demonstrated the possibility of regulation of CB2 receptors by some species of intestinal bacteria and the related immune response.

The mechanisms of the transmission of inflammatory signals from the gut to the CNS (central nervous system) are not fully elucidated. The results of the research indicate, however, that the systemic inflammation accompanying depressive symptoms is associated with changes in the ecosystem of the intestinal microbiota and the production of SCFA (short-chain fatty acids) and other metabolites of a neurobiological nature. Moreover, it can also lead to changes in the synthesis and release of endocannabinoids or the metabolic pathways of tryptophan and kynurenine (KYN). So far, it has been observed that there is a link between ECS and the kynurenine pathway in neurological disorders such as epilepsy or migraine headaches, which are associated with excessive cell stimulation and excitotoxicity. The latest research results indicate the possibility of modifying the transformations of tryptophan and kynurenine by probiotic therapy.

There is limited number of human research on the role of the gut microbiota in the modulation of the ECS. The available literature lacks studies describing the relationship of changes in the intestinal microbiota and its metabolome under the influence of targeted, multi-strain probiotic therapy on the response of the endocannabinoid and immune system in people subjected to high physical and mental stress. Observations that will be made in dancers supplemented with a multi strain probiotic containing Lactobacillus helveticus Rosell-52 and Bifidobacterium longum Rosell-175 may help finding effective tools in the treatment of gastrointestinal disorders and stress related to it. A potential mechanism behind this action may be the restoration of the normal gut microbiota and the profile of its metabolites, as well as the improvement of the gut barrier and endocannabinoid function.