Serotonin molecule. Computer artwork of a molecule of serotonin (5-hydroxytryptamine, C10.H12.N2.O), with a nerve synpase in the background. Serotonin is a neurotransmitter, and is also seen here as the blue balls being released across the synaptic gap. In the molecule, the atoms (spheres) are colour-coded: carbon (black), hydrogen (green), nitrogen (orange) and oxygen (red). A deficiency of serotonin in the brain has been shown to cause depression. This has led to the development of the SSRI (selective serotonin reuptake inhibitor) antidepressants, such as Prozac (fluoxetine). A nerve synapse is the junction between two nerve cells (neurons).