The Neuromodulatory Influence of the Gut Microbiota within the Bidirectional Gut-Brain Axis

Gut-brain axis is a complex and dynamic bilateral channel of communication between the gastrointestinal tract and central nervous system via integrated neural, endocrine, immune, and metabolic. The gut microbiota has in recent years been recognized as a major modulator of this axis, and has a central role in maintaining physiological balance and neurological activity. This variety of microorganisms that colonize the gut is highly influential on the brain functions through a number of ways which include synthesis of neurotransmitters, immune response, as well as the generation of biologically active metabolites like short chain fatty acids. Neuroactive compounds produced by microorganisms such as serotonin, gamma-aminobutyric acid (GABA), and dopamine are vital in the regulation of emotional states, cognitive performance and behavior patterns. In addition, the gut microbiota is in close contact with key regulating systems, including the hypothalamic-pituitary-adrenal (HPA) axis and the vagus nerve, thus regulating the response to stress and emotional stability. The immune-mediated mechanisms also play a role in this communication by impacting neuroinflammatory events and maintaining the structural and functional integrity of the blood-brain barrier. Dysbiosis, or disruption of the normal composition, and diversity of microbiota of the gastrointestinal tract, has been strongly implicated in multiple neurological and psychiatric disorders, such as depression, anxiety, autism spectrum, Parkinson, and Alzheimer diseases. These results highlight the important role of the gut microbiome in the pathogenesis and pathophysiology of central nervous system diseases. This review will give an in-depth analysis of the neuromodulatory functions of gut microbiota in the gut-brain axis. It is, also, an overview of the current advances in microbiome studies and a description of new forms of treatment, such as probiotics, prebiotics, changes to the diet, and fecal microbiota transplantation. Greater insight into these interactions can be used to develop novel microbiome-based therapies that can be used to improve neurology and treat neuropsychiatric diseases.