In recent years, the term psychobiotics has been used to define a new generation of probiotics involved in the relationship between the brain-gut axis and the digestive microflora (Dinan et al., 2013).

According to the authors, psychobiotics “are live microorganisms that, when ingested in the adequate amounts, produce a health benefit in patients sufferingfrom psychiatric illness’.’ These bacteria are capable of producing and delivering neuroactive substances such as gamma-aminobutyric acid and serotonin, catecholamines, and acetylcholine, which act on the brain-gut axis. Current evidence suggests that multiple mechanisms, including endocrine and neurocrine pathways, may be involved in gut microbiota-to- brain signaling, and that the brain can alter the microbial composition and behavior via the autonomic nervous system. Little information is actually available on how these findings may be transferred to healthy humans or to disease involving the brain or the brain-gut axis. (Wall et al., 2014; Mayer et al., 2015).

The first scientific evidences show that their effects might be mediated via the vagus nerve, spinal cord, or neuroendocrine systems. Psychobiotics have been studied in a liaison psychiatric setting in patients with IBS, where positive benefits have been reported for a number of organisms such as some microorganisms belonging to Bifidobacterium infantis. The trial results show evidence of benefits in alleviating depression symptoms and in chronic fatigue syndrome. Such benefits may be related to the anti-inflammatory actions of certain psychobiotics and their capacity to reduce hypothalamic-pituitary-adrenal (HPA) axis activity. More large-scale, placebo-controlled trials are needed to provide definitive evidence of these benefits and to detect which probiotics have psychobiotic potential on some diseases such as IBS, autism, anxiety, depression, and Parkinson's disease (Cryan et al., 2013).

Moreover, studies on animals treated with probiotics have underlined a blunted HPA response. Stress increases permeability of the gut, allowing bacteria and bacterial antigens to affect the epithelial barrier and also to activate a mucosal immune response, which can modify the composition of the microbiome and lead to enhanced HPA drive. Data from patients with irritable bowel syndrome and major depression indicate that in these syndromes alteration of the HPA may be induced by increased gut permeability (Dinan et al., 2012).

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