Can anyone explain to me what serotonin's part in digestion is? I've been doing some research and I don't quite understand it.For some reason when I take SSRI's I get more depressed, then I was put on Zelnorm and I just realized yesterday that it's primary function is to act upon the serotonin and it's receptors in the abdomen. I believe that this is what is causing my major, wild mood swings. I'm wishing I knew more about how these drugs affect my body and the chemical reactions that they cause because if I could, maybe I would start figuring out why I have these problems. Has anyone investigated their serotonin levels after they have been diagnosed with IBS? I'm wondering if my problem actually stems from a lack of being able to either metabolize or use serotonin properly anywhere in my body. Just a theory, but I'm open to people's ideas about it.