Over the past 10 years, considerable evidence has emerged from animal studies to suggest that the gut microbiome has significant effects on brain development and behavior, with bidirectional communication between the enteric nervous system, gut and the central nervous system (CNS) (Diaz Heijtz et al, 2011; Douglas-Escobar et al, 2013; Dinan et al, 2014). The gut microbiota have been shown to: a) produce multiple neurotransmitters, including gamma-aminobutyric acid (GABA), dopamine, norepinepherine, and serotonin, and may regulate CNS levels of these neurotransmitters; b) modulate brain development through the regulation of synaptogenesis; and c) modulate the levels of stress hormones during brain development, which may affect stress response and anxiety behavior (Diaz Heijtz et al, 2011; Dinan et al, 2014; Sudo et al, 2004; O'Mahony et al, 2015). Moreover, the gut microbiota effects the production of neurotrophins, including brain-derived neurotrophic factor (BDNF), which plays a significant role in neurogenesis and synaptic plasticity (Sudo et al, 2004; Nemani et al, 2015).
The gut microbiome may also affect brain development and function through its regulation of immune system function, which is mediated, in part, through the production of short-chain fatty acids (SCFA). There are three major SCFAs: butyrate, propionate, and acetate. Butyrate is of particular interest, since it plays a key role in maintaining gut homeostasis and epithelial integrity: butyrate is the primary energy source for intestinal colonocytes; and, of the three SCFAs, butyrate appears to have the most pronounced effects on immune system function and may exert its effects directly through immune pathways and indirectly through the maintenance of the integrity of the intestinal-blood barrier (Hamer et al, 2008; Louis et al, 2010; Brahe et al, 2013; Vital et al, 2014). The intestinal-blood barrier restricts the entrance of toxins, pathogens and antigens into the blood circulation; thus, increased permeability could lead to the entrance of substances and subsequent immune response.
The multiple effects of the gut microbiome on brain development and behavior, suggest that alterations in the gut microbiome may occur in schizophrenia and play a part in the pathophysiology of the disorder. The increased prevalence of gastrointestinal disorders in schizophrenia; the association of infections, including infections with Toxoplasma gondii, which can induce intestinal inflammation, with the risk for the development of schizophrenia; and evidence of increased gut permeability provide further indirect evidence for disruption of the gut microbiome in this disorder (Dinan et al, 2014; Nemani et al, 2015; Severance et al, 2012; Severance et al, 2014). Although a number of studies have been conducted in other neuropsychiatric disorders, including autism (Parracho et al, 2005; Tomova et al, 2015), which demonstrate altered bacterial composition of the gut microbiome, there is only one published study of the microbiome in schizophrenia. Yolken and colleagues examined the oropharyngeal microbiome in people with schizophrenia, and found that there were increased levels of the bacteriophage, Lactobacillus phage phiadh, genome in the schizophrenia group, which were correlated with co-occurring immunological disorders (Yolken et al, 2015). There is one published study of gut microbiota in schizophrenia. Shen and colleagues found a significant reduction in butyrate producers in people with schizophrenia compared to healthy controls (Shen et al. Schiz Res, https://doi.org/10.1016/j.schres.2018.01.002).
The purpose of this study is to examine changes in serum butyrate levels with the prebiotic: Prebiotin (12g/day), an oligofructose-enriched inulin (OEI); the effect of OEI on the composition of the gastrointestinal microbiota in people with schizophrenia; and the relationship of the composition of the gut microbiota to various clinical, cognitive, and neuroimaging variables.
