The gut-brain axis is a bidirectional information interaction system between the central nervous system (CNS) and the gastrointestinal tract, in which gut microbiota plays a key role. The gut microbiota forms a complex network with the enteric nervous system, the autonomic nervous system, and the neuroendocrine and neuroimmunity of the CNS, which is called the microbiota-gut-brain axis. Due to the close anatomical and functional interaction of the gut-liver axis, the microbiota-gut-liver-brain axis has attracted increased attention in recent years. The microbiota-gut-liver-brain axis mediates the occurrence and development of many diseases, and it offers a direction for the research of disease treatment. In this review, we mainly discuss the role of the gut microbiota in the irritable bowel syndrome, inflammatory bowel disease, functional dyspepsia, non-alcoholic fatty liver disease, alcoholic liver disease, cirrhosis and hepatic encephalopathy via the gut-liver-brain axis, and the focus is to clarify the potential mechanisms and treatment of digestive diseases based on the further understanding of the microbiota-gut- liver-brain axis.

Analysis of the intestinal microbiota and physiological parameters in mammalian infancy can reveal health status. In this study, we used a combination of molecular and immunochemical approaches to assess fecal microbiota as well as Cortisol (Cor), Triiodothyronine (T3), and immunoglobulin A (IgA) levels of young forest musk deer (FMD), from birth to one month after weaning (7 days of age-110 days of age). During development as the diet of FMD changes from consuming milk to eating plants, the richness and diversity of intestinal microbiota of young FMD increased significantly. Cor levels remained unchanged throughout early development while significantly increased after weaning, T3 and IgA initially were derived from milk during lactation, significantly decreased after weaning. Correlation network analysis showed that the community of food-oriented microbes were highly structured and that many genera were correlated. Overall, this study provides scientific insights into effective management strategies for the protection of FMD population.

Weaning is an important event for all mammals, including young forest musk deer. However, weaning stress may cause intestinal microbiota-related disorders. Therefore, high-throughput 16S rRNA gene sequencing was applied to study the dynamic changes in intestinal microbiota during pre-weaning (10 days before weaning) and post-weaning (10 days after weaning) in 15 young forest musk deer. We saw that intestinal microbiota diversity in the post-weaning period was significantly higher than that in the pre-weaning period. The most dominant bacterial phyla were similar in the two groups (Firmicutes, Bacteroidetes and Verrucomicrobia). Meanwhile, we applied Linear discriminant analysis effect size (LefSe) to identify the most differentially microbial taxa in the pre-weaning and post-weaning groups. In the post-weaning forest musk deer, the relative abundance of Actinobacteria, Spirochaetes, Ruminococcaceae_UCG-005, Treponema and Prevotella was higher than in the pre-weaning group. However, higher relative abundance of the phyla Bacteroidetes was found in the pre-weaning group compared with that in the post-weaning group. In summary, this research provides a theoretical foundation for the dynamics of young forest musk deer intestinal microbiota during the weaning transition, which may benefit in understanding the growth and health of forest musk deer.

Neurotransmitters are special molecules that serve as messengers in chemical synapses between neurons, cells, or receptors, including catecholamines, serotonin, dopamine, and other neurotransmitters, which play an important role in both human physiology and pathology. Compelling evidence has indicated that neurotransmitters have an important physiological role in various digestive diseases. They act as ligands in combination with central or peripheral receptors, and transmits signals through chemical synapses, which are involved in regulating the physiological and pathological processes of the digestive tract organs. For instance, neurotransmitters regulate blood circulation and affect intestinal movement, nutrient absorption, the gastrointestinal innate immune system, and the microbiome. In this review, we will focus on the role of neurotransmitters in the pathogenesis of digestive tract diseases to provide novel therapeutic targets for new drug development in digestive diseases.

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