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Secretory IgA in the saliva is essential for protection from mucosally transmitted pathogens and maintaining homeostasis at mucosal surfaces of the oral cavity. Expression of submandibular gland polymeric Ig receptor (pIgR) is essential for IgA secretion. In the present study, we investigated the influence of indigestible carbohydrates on IgA production in the salivary gland and saliva. Five-week-old rats were fed a fibre-free diet (control), or a diet with 5 % (w/w) fructo-oligosaccharide (FOS) or a combination of 2·5 % (w/w) polydextrose (PDX) and 2·5 % (w/w) lactitol for 21-d. IgA concentrations in the caecal digesta, submandibular gland tissue, and saliva in the FOS and PDX þ lactitol diet groups were significantly higher than those in the control group (P,0·05). The increase in IgA in the submandibular gland tissue was confirmed using immunohistochemical analysis. However, the IgA concentrations of serum did not differ between the FOS or PDX þ lactitol groups and the control group (P¼ 0·5). In the FOS and PDX þ lactitol groups, the pIgR mRNA ( pIgR/b-actin) expression level in the submandibular gland tissue was significantly higher than that in the control group (P,0·05). The present study suggests that indigestible carbohydrates play an important role in the increase in IgA concentrations in the submandibular gland tissue, saliva, and caecal digesta. The intestinal epithelium is the largest mucosal surface in the human body, and is constantly exposed to potentially toxic environmental antigens, pathogenic food and water-borne micro-organisms and commensal microflora (1) . Dietary approaches to influence human gut have long been used as an approach to improve host health (2) . Indigestible carbohydrates have beneficial effects in human health and nutrition (3) . Human digestive enzymes have little or no effect on carbohydrates such as fructo-oligosaccharides (FOS) and polydextrose (PDX). These substances are digested by colonic bacteria with the production of organic acids, mainly SCFA and gas (CO 2 and hydrogen). Small amounts of lactic, formic and succinic acids are also produced (4) . These health effects include immune regulation, normalization of blood glucose and insulin levels, and prevention of cancer recurrence (5) . Many indigestible carbohydrates influence aspects of intestinal function through fermentation (6) .The use of indigestible carbohydrates results in increased IgA levels in the caecal digesta and faeces of animals (7 -11) . In human subjects, IgA is the most abundant Ig isotype in the body in the absence of infection. It is synthesized mainly in the secretory form in gut-associated lymphoid tissues. Secretory IgA (sIgA) prevents pathogens and commensal bacteria from binding to the epithelial cells of the mucosa and neutralizes their toxins to maintain homeostasis at mucosal surfaces (12) . The polymeric Ig receptor (pIgR), also known as membrane secretory component, is an integral membrane protein expressed by intestinal epithelial cells. The physiological role of pIgR is to bi...

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Salivary glands produce various compounds, including brain-derived neurotrophic factor (BDNF), which serve as biomarkers of stress-related disorders. Social isolation-induced stress models a form of chronic mild stress that induces neurodegenerative changes in the brain and behavioral alterations. This study employed a rat model to determine whether social isolation stress affects BDNF levels in saliva. Male Sprague-Dawley rats were randomly allocated to social isolation stress (1 animal/cage) or control (3-4 animals/cage) groups and reared for 8 weeks. The concentration of BDNF was quantified in specific brain regions, blood, and saliva using enzyme-linked immunosorbent assay (ELISA). The levels of expression of Bdnf and tyrosine kinase B (TrkB) mRNA were quantified using reverse transcriptase-polymerase chain reaction. Behavioral alterations were analyzed using the open-field and elevated plus maze assays. The BDNF concentration was lower in the hippocampus, prefrontal cortex, blood, and saliva of the stress group than in those of the controls. Trkb expression in the hippocampus and prefrontal cortex was decreased by social isolation stress. Moreover, the social isolation stress group showed behavioral deficits in both tests. In conclusion, these findings indicate that social isolation stress may reduce the expression of BDNF protein in blood and saliva, thus providing a potentially valuable biomarker for diagnosis of stress-related disorders.