Aim: The microbiota shows diurnal oscillations that are synchronized by the host's circadian clock and feeding rhythms. Short-chain fatty acids (SCFAs) produced by the microbiota are possible synchronizers of peripheral circadian clocks.We aimed to investigate whether faecal SCFAs show a diurnal rhythm that regulates the rhythm of SCFA receptor expression (FFAR2/3, OLFR78, HCAR2) and SCFA-induced colonic contractility. The role of the circadian clock was studied in mice lacking the core clock gene Bmal1. Methods: Mice were sacrificed at 4-hour intervals. Faecal SCFA concentrations and SCFA receptor expression were determined. The effect of increasing concentrations of a SCFA mix on electrical field-induced neural responses in colon strips was measured isometrically. Results: Diurnal fluctuations in faecal SCFA concentrations (peak 4 hours after lights on) were observed that were in phase with the rhythm of Ffar2/3 expression in the colonic muscle layer. Olfr78 expression was not diurnal and Hcar2 was not detectable. The inhibitory effect of a SCFA mix on neural contractions in colonic smooth muscle strips showed a diurnal rhythm and oscillated in phase with faecal SCFA concentrations and Ffar2/3 expression. In contrast, neither excitatory neural responses nor acetylcholine-induced smooth muscle contractions showed a diurnal rhythm. In Bmal1 −/− mice, no fluctuations in faecal SCFA levels, Ffar3 expression and neural responses to SCFAs were observed. Conclusion: Diurnal microbial SCFA levels regulate the rhythm of Ffar3 expression in the colonic myenteric plexus, which causes rhythmicity in SCFA-induced colonic motility. Deletion of Bmal1 abolishes rhythmicity of SCFA levels and their downstream effects.
K E Y W O R D Scircadian clock, colon contractility, short-chain fatty acids
| INTRODUCTIONThe circadian clock coordinates the timing of physiological processes with solar time. The circadian system consists of a master clock, located in the suprachiasmatic nuclei of the anterior hypothalamus, and several peripheral clocks in different organs that are synchronized by the master clock. 1