Prospects of Herbal Remedies in Neuropsychiatric Diseases from the Gut–Brain Communication Perspective

The human gastrointestinal tract harbours over 100 trillion microorganisms that collectively constitute the gut microbiota. Through the gut–brain axis, it is now believed that the composition of the gut microbiota may influence brain health and its development via multiple pathways, including neural, endocrine, immune and metabolic signalling mechanisms. Human and animal studies have suggested that an imbalance in the gut microbiota, known as dysbiosis, could lead to notable changes in behavioural and cognitive functions, possibly mediated by alterations in inflammatory activities in the intestine and beyond. This is likely an incomplete account and the full complexity of the mechanistic interaction remains to be elucidated. In particular, the extent to which gut dysbiosis may contribute to the aetiology of specific psychiatric disorders as causal risk factors remains debatable. Nevertheless, the crosstalk between the gut and brain represents an opportunity for the development of new adjunctive therapeutics, such as dietary supplements and related lifestyle modification. Modulation of gut microbiota composition, such as microbiota transplantation, has already found applications in gastrointestinal, autoimmune, infectious and bone diseases. Recent suggestions of “mind-altering microbes” have aroused intense interest. Here, we summarize the rationale behind, and evidence for, herbal modifications of the gut microbiota, and address the potential prospect of herbal remedies in neuropsychiatric disorders, such as depression. We argue that relevant microbiota changes and biological mechanisms involved could explain some of the claimed benefits of herbal medicines and related supplements in treating psychiatric disturbances, and thus open up novel therapeutic perspectives.