Serotonin. Molecular graphic of a molecule of the neurotransmitter serotonin (5-hydroxytryptamine, chemical formula C10. H12. N2. O). The atoms are represented as spheres and the bonds between them as sticks. Serotonin is widely distributed in the body. It is released by blood platelets during clot formation, where it has a vasoconstrictive effect. 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 inhibitors) antidepressants, such as Prozac (fluoxetine). The hormone melatonin is made from serotonin in the brain's pineal gland.