Serotonin, molecular model. Serotonin is a 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 and sexual behaviour as well as hormone secretion. As a hormone, serotonin is involved in vasoconstriction, vasodilatation, increased platelet activity, intestinal peristalsis and haemostasis, among other things. Atoms are represented as spheres and are colour-coded: carbon (black), hydrogen (white), nitrogen (blue) and oxygen (red).