HTR3A and irritable bowel syndrome: The three pharmacologic agents currently indicated in the USA for treatment of irritable bowel syndrome with diarrhea (IBS-D) are non-systemic antibiotic rifaximin, the mixed μ- and κ-opioid receptor agonist/δ-opioid antagonist eluxadoline, and the selective serotonin 5-HT3 antagonist alosetron [21].