Minireview: Gut Microbiota: The Neglected Endocrine Organ

Abstract

The concept that the gut microbiota serves as a virtual endocrine organ arises from a number of important observations. Evidence for a direct role arises from its metabolic capacity to produce and regulate multiple compounds that reach the circulation and act to influence the function of distal organs and systems. For example, metabolism of carbohydrates results in the production of short-chain fatty acids, such as butyrate and propionate, which provide an important source of nutrients as well as regulatory control of the host digestive system. This influence over host metabolism is also seen in the ability of the prebiotic inulin to influence production of relevant hormones such as glucagon-like peptide-1, peptide YY, ghrelin, and leptin. Moreover, the probiotic Lactobacillus rhamnosus PL60, which produces conjugated linoleic acid, has been shown to reduce body-weight gain and white adipose tissue without effects on food intake. Manipulating the microbial composition of the gastrointestinal tract modulates plasma concentrations of tryptophan, an essential amino acid and precursor to serotonin, a key neurotransmitter within both the enteric and central nervous systems. Indirectly and through as yet unknown mechanisms, the gut microbiota exerts control over the hypothalamic-pituitary-adrenal axis. This is clear from studies on animals raised in a germ-free environment, who show exaggerated responses to psychological stress, which normalizes after monocolonization by certain bacterial species including Bifidobacterium infantis. It is tempting to speculate that therapeutic targeting of the gut microbiota may be useful in treating stress-related disorders and metabolic diseases.

A dysbiotic subpopulation of alcohol-dependent subjects

“…Several pathways may be involved for the gut microbiota to affect drinking habits. The gut micobiota secretes more than 100 different metabolites, 28 including neurotransmitters such as dopamine, serotonin, noradrenaline or GABA. The pathways related to these neurotransmitters are all affected by the development of alcohol-dependence.…”

Neural Mechanisms of Exercise: Effects on Gut Miccrobiota and Depression

“…Therefore the exercise-induced brain benefits might as well be modulated through gut microbiota changes. For instance, serotonin is required for running induced benefits on neurogenesis [16,17], while gut the major source of serotonin production in the body [37]; GF mice exhibited altered serotonin signaling as well as relevant behaviors [9,14,38], suggesting that GF mice might lack the benefits of exercise on neurogenesis. Future studies are required to systemically examine such possibilities.…”

GUT-the Trojan Horse in Remote Organs’ Autoimmunity

“…In contrast, host-derived hormones increase the bacterial proliferative capacity and pathogenicity. In the gut lumen, this type of cross-talk between microorganisms and the host is presumed to be performed continually through various kinds of luminal molecules [65,66]. Dopamine, norepinephrine, nitric oxide and the inhibitory transmitter GABA are few examples of our own gut microbiota generating compound that interact with our own endocrine system and can be called the microbial endocrinology [64][65][66].…”