Serotonin, illustration. Serotonin is a tissue hormone and neurotransmitter in the peripheral and central nervous system. As a neurotransmitter, serotonin is produced in the brain and influences perception, sleep, temperature regulation, sensorial function, pain perception and processing, appetite, sexual behavior as well as hormone secretion. As a tissue hormone, serotonin is involved in vasoconstriction, vasodilatation, increased platelet activity, intestinal peristalsis and hemostasis, among other things.