The microbiota-gut-brain axis: Focus on the fundamental communication pathways.

The existence of a bidirectional communication system termed the "gut-brain axis" linking the gastrointestinal tract with the brain has been recognized since the nineteenth century. In the last decades, however, it has become evident that the saprophytic gut microbial flora plays a key role in the modulation of this gut-brain connecting pathway, which is defined now as the "microbiota-gut-brain axis". The gut microbiota plays, indeed, a pivotal role in the maintenance of local, systemic and brain homeostasis. Several neuroactive molecules, hormones and metabolites act along different communication pathways, allowing the bidirectional cross-talk. Some of these molecules are common to eukaryota and prokaryota, representing a possible interkingdom communication system. This chapter provides a description of the chief endocrine, immune and neuronal pathways involved in this bidirectional communication. Research in this field opens an exciting scenario on the possibility to target specific neuronal or hormonal pathways, by means of traditional pharmacological approaches as well as using neuroactive molecule-producing probiotics, as new potential therapeutic tools, addressed to the treatment of neurogastrointestinal and/or psychiatric disorders.