Melatonin alleviates weanling stress in mice: Involvement of intestinal microbiota.

Melatonin influences intestinal microbiota and the pathogenesis of various diseases. This study was conducted to explore whether melatonin alleviates weanling stress through intestinal microbiota in a weanling mouse model. Melatonin supplementation in weanling mice (provided in the drinking water at a dosage of 0.2 mg/mL for 2 weeks) significantly improved body weight gain (1.4 ± 0.03 g/day in melatonin group vs 1.2 ± 0.06 g/day in control group) and intestinal morphology (ie, villus length, crypt depth, and villus to crypt ratio), but had little effect on the proliferation or apoptosis of intestinal cells, the numbers of Paneth cells and goblet cells, as well as the expression of makers related to enterocytes (sucrase) and endocrine cells (chromogranin A and peptide YY) in the ileum. Melatonin supplementation had little effect on serum levels of amino acids or stress-related parameters (eg, SOD, TNF-α, and angiotensin I). 16S rRNA sequencing suggested that melatonin supplementation increased the richness indices of intestinal microbiota (observed species, Chao 1, and ACE) and shaped the composition of intestinal microbiota (eg, increase in the abundance of Lactobacillus [19 ± 3% in melatonin group vs 6 ± 2% in control group]), which was demonstrated using an ex vivo proliferation assay and colonic loop proliferation assay. Melatonin supplementation also significantly influenced the metabolism of intestinal microbiota, such as amino acid metabolism and drug metabolism. More importantly, in antibiotic-treated weanling mice and germ-free weanling mice, melatonin failed to affect body weight gain or intestinal morphology. Melatonin significantly reduced (by about 60%) the bacterial load in enterotoxigenic Escherichia coli (ETEC)-infected weanling mice, but had little effect on ETEC load in antibiotic-pretreated animals. In conclusion, melatonin affects body weight gain, intestinal morphology, and intestinal ETEC infection through intestinal microbiota in weanling mice. The findings highlight the importance of intestinal microbiota in mediating the various physiological functions of melatonin in the host.