The gut-brain axis. Effect of probiotics on anxiety

The gut microbiota-flora is involved in neural development and functions, both peripherally in the enteric nervous system and centrally in the brain. It can influence the brain activity, the learning and memorizing processes and it could play an important role also in maintaining the health of the host. Gastrointestinal system and brain are not separate entities but rather are closely related. The first one is able to influence the brain by reducing anxiety, modulating the stress levels, the memory and learning process. Perturbations of these systems result in alterations in the stress-response and overall behavior. The gut micro-flora reduces the levels of ACTH compared to control mice; increases GABAB1b mRNA in the brain with increases in cortical regions (cingulate and prelimbic); reduces the expression of GABA mRNA in the hippocampus, amygdala, and locus coeruleus, in comparison with control-fed mice; reduces GABAAα2 mRNA expression in the prefrontal cortex and amygdala, but increased GABAAα2 in the hippocampus. It has also been showed an up-regulation in the expression of brain derived neurotrophic factor (BDNF) mRNA in the dentate gyrus of the hippocampus of these germ-free animals. The alterations of those receptor systems have a great impact in the behavior and control of anxiety. In germ-free mice it has been demonstrated that the absence of a conventional microbiota may result in a reduction in anxiety behavior in the elevated plus maze, a well validated model of anxiolytic action.