HTR3A and irritable bowel syndrome: IBS-D is particularly a debilitating form of IBS as it reduces the quality of life due to the fear of pain, urgent defecation, and even incontinence.6 Serotonin or 5-hydroxytryptamine 3 (5-HT3) is an important neurotransmitter in the brain-gut axis and is involved in several functions of the GI tract, including the peristaltic reflex.