Short-Chain Fatty Acids Activate Myenteric Neurons and Delay Colonic Motility via Free Fatty Acid Receptors 2 and 3...

Short-chain fatty acids (SCFAs), metabolic products of the gut microbiota, are important regulators of intestinal homeostasis. SCFAs exert their effects through free fatty acid receptors (FFAR), which are located on enteroendocrine cells and enteric nerves. The actions of SCFAs include the regulation of motility. How SCFAs affect enteric neuronal activity and the regulation of colonic motility remains to be fully elucidated.

Intact preparations of the proximal colon from mice expressing a genetically encoded fluorescent Ca²⁺ reporter specifically in intrinsic primary afferent neurons (IPANs; Calb1-GCaMP6 mice) or in all enteric neurons (Wnt1-GCaMP6 mice) were used. Preparations were luminally perfused with SCFAs, with or without the FFAR2 antagonist, β-hydroxybutyrate (BHB) or the FFAR3 antagonist, GLPG0976. SCFA-induced Ca²⁺ responses were visualized by confocal microscopy. Colonic transit was determined using a bead propulsion assay.

In Calb1-GCaMP6 mice, propionate exhibited the highest and most sustained Ca²⁺ fluorescence (∆F/F₀ 288.2 ± 81.2) with the fastest time to peak (7.3 ± 1.6 min) and longest decay time (10.1 ± 3.0 min), followed by acetate, butyrate, and a mixture of the three SCFAs at physiological concentrations. Similar patterns were observed in Wnt-1-GCaMP6 mice, where propionate induced the highest Ca²⁺ fluorescence (∆F/F₀ 240.5 ± 25.7) and the longest decay time (10.2 ± 0.9 min). BHB and GLPG0976 significantly attenuated SCFA-induced Ca²⁺ fluorescence in both IPANs and myenteric neurons. SCFAs delay colonic bead propulsion, an effect abolished by both FFAR antagonists.

Acutely, luminal administration of SCFAs activates myenteric neurons and slows colonic transit through FFAR2 and FFAR3 receptors.