Serotonin molecule. Computer artwork of a molecule of serotonin (5-hydroxytryptamine, C10.H12.N2.O), with a nerve synpase. Serotonin is a neurotransmitter. 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).