Serotoninergic Mechanisms of Action in the Relaxant Properties of Saccharomyces boulardii CNCM I-745 on the Intestine.

BACKGROUND: Perturbations of intestinal serotonergic neurotransmission seem to be involved in bowel dysmotility associated with irritable bowel syndrome (IBS) with diarrhea. Oral administration of probiotics is an emerging strategy to improve IBS symptoms, possibly via influencing local serotonin metabolism and neurotransmission. In the present study, we evaluated the effects of the yeast Saccharomyces boulardii CNCM I-745 (S. boulardii) on intestinal motility and serotonergic receptors. METHODS: Isolated rat ileum was contracted in a cumulative concentration way by serotonin (5-HT), various 5-HT agonists or by acetylcholine to determine their effective concentration 50% (EC(50)). Single concentrations of S. boulardii or 5-HT antagonists were added before agonists to identify the receptors targeted by S. boulardii. RESULTS: The serotonin antagonists 5-HT(1A) WAY100635, 5-HT(2A) ketanserin and 5-HT(4) GR113808 inhibited 5-HT-induced contractions in a concentration-dependent manner. S. boulardii between 0.05 and 1.5 mg/mL increased the EC(50) value of 5-HT suggesting an inhibitory effect against serotonin-induced contraction. Ileum contractions induced by the serotonin agonist 5-HT(1) carboxamidotryptamine or by the serotonin agonist 5-HT(2) alpha-methyl-5-HT were significantly reduced by S. boulardii at 1.5 mg/mL. The yeast did not affect acetylcholine-induced ileum contraction. CONCLUSION: S. boulardii CNCM I-745 possesses relaxant properties on the rat ileum involving the inhibition of 5-HT and more specifically 5-HT(1A) and 5-HT(2A/2B/2C) receptor-induced contractions. These data suggest that the attenuation of 5-HT-induced ileal contractions by S. boulardii represents a probable mechanism of action sustaining its efficacy in patients affected by IBS with diarrhea.