Serotonin. Computer model of a molecule of the neurotransmitter serotonin (5-hydroxytryptamine, C10.H12.N2.O). Atoms (spheres) are colour-coded: carbon (purple), hydrogen (grey), oxygen (red) and nitrogen (yellow). Serotonin is released by blood platelets during clot formation, where it causes the constriction of blood vessels. It is an important neurotransmitter in the brain, and a lack of it has been shown to cause depression. It is this property which has led to the development of the SSRI (selective serotonin reuptake inhibitor) antidepressants, such as Prozac (fluoxetine). The hormone melatonin is made from serotonin in the brain's pineal gland.