Keywords: MT2; melatonin receptors; 5-HT; serotonin; CCK2; cholecystokinin B; IBS; irritable bowel syndrome; IBS-C; constipation-predominant IBS; IBS-D; diarrhea-predominant IBS; EC; enterochromaffin cells; GIT; gastrointestinal; NOS; nitric oxide synthase; NF-κB;