Rather interesting results are demonstrated by a study of germ-free mice. As it turned out, they behave much more aggressively than their normal counterparts. At the same time, returning gut microorganisms to them immediately or 6 weeks after birth demonstrated a significant reduction in aggressive behavior during experiments conducted at the 12th week of their lives. Additionally, a number of studies[1][2] demonstrate that the absence of gut microbes leads to impairments in brain development, including a decrease in serotonergic signaling and the expression of 5-HT1A receptors, which play a key role in the full functioning of the violence inhibition mechanism. In general, the gut-brain axis has a significant influence on the regulation of behavior and mood. For example, disruption of the gut microbiota caused by alcoholism leads to depressive and anxious states due to dysfunction of emotional control in the amygdala, prefrontal cortex, and hypothalamus (it should be noted that these brain regions are also key in the regulation of aggression).
Such data indicate to us not only the fact that gut microorganisms are important for the full development of the brain, but also the possibility of therapy for certain neurophysiological abnormalities, including the same dysfunction of the violence inhibitor, through the use of probiotics; researchers are already pointing to the potential of probiotics in the therapy of affective (mood) disorders. A result similar to the use of certain 5-HT1A/1B receptor agonists can be expected, i.e., the elimination of attacking aggression from behavior without affecting defensive aggression and other behaviors. At the same time, it is potentially possible to achieve a more stable and long-term result by selectively developing probiotics specifically for the therapy of this dysfunction.
Theoretically, probiotics that could be suitable for such a task can be identified. Bifidobacterium breve (M2CF22M7) and Bifidobacterium longum infantis (E41), administered to mice over a 5-week period with stress exposure experiments, improved the regulation of the secretion of the serotonin precursor 5-HTP, and also increased the level of serotonin itself in the brain. Given that they also improved the expression of the TPH1 gene, a possible similar effect on the TPH2 gene in the brain (which is important in the regulation of aggression and the functioning of the serotonergic system) exists, although this still requires confirmation. Lactobacillus casei (54-2-33), necessarily administered together with the prebiotic inulin (which is necessary to stimulate bacterial growth) for 14 days to young rats, increased the level of 5-HT1A receptor mRNA in their hippocampus. Also, a 4-week administration to young rats of a mixture of galacto-oligosaccharide and polydextrose prebiotics, as well as the prebiotic lactoferrin, which stimulated the growth of various types of lactobacilli, prevented the stress-induced decrease in 5-HT1A receptor mRNA levels.
At the moment, the topic of the influence of probiotics on the development and functioning of the brain can be called quite poorly studied. However, existing data already point to another promising direction in the development of therapy for various neurophysiological dysfunctions, including problems with violence inhibition. If bacterial strains are developed that, in an adequate (non-harmful) quantity in the gut, prove capable of significantly improving the work of the serotonergic system and certain brain regions, it will become possible to carry out therapy for violent behavior extremely effectively without the need for the constant use of corresponding medications.
