And indeed, Gershon declared, the gut contains every class of nerve-transmitting chemical that has been found in the brain. Take, for instance, serotonin. Epithelial cells that line the gut secrete serotonin. Serotonin then excites sensory nerves in the gut and in turn triggers gut reflexes that signal the brain about pain, nausea, and so forth. Such a copious amount of serotonin is released into the gut every day, in fact, that it would be lethal if the excess were not disposed of by certain cells in the gut lining that contain serotonin transporters.