A Crosstalk between Melatonin and Taste-Receptors’ Signaling Tunes Quinine-Induced Gut Hormone Secretion in Mice

Quinine consumption has been shown to reduce appetite and food intake in human and mice. Here, tested on two common mouse strains, C3H/lbg and C57BL/6J, it exerted a different effect. While quinine reduced weight gain in C3H/lbg mice, C57BL/6J were unaffected by the bitter molecule. Among the differences between the two strains, C57BL/6J present a blunted Melatonin production. In this study, we investigate if endogenous Melatonin is playing any role in the different response of C57BL/6J mice to quinine. The effect of dietary supplementation with Melatonin as well as of endogenous gastrointestinal and pineal produced Melatonin was investigated by supplementing quinine diet with pure Melatonin, L-Tryptophan or by reversion of light/dark cycle, respectively. The consumption of Melatonin reverts the phenotype and makes C57BL/6J mice sensitive to quinine. Similarly, quinine potency in C3H/ Ibg mice augments upon supplementation with exogenous Melatonin or upon increase of Melatonin endogenous levels. In vivo, as well as in in vitro cell cultures, Melatonin Receptor modulation inhibits quinine dependent secretion of Ghrelin, while potentiates quinine dependent secretion of Cholecystokinin. Acting via Melatonin Receptors, Melatonin tunes the effect of quinine, reducing and potentiating its effect in enteroendocrine cells of the upper and the lower digestive tract, respectively. Our results indicate that signaling pathways activated by Melatonin tunes the activity of Bitter Receptors located in the gastrointestinal tract.