Serotonin neurotransmitter. Animation of molecules of the neurotransmitter serotonin, as part of a sequence showing its effects in the brain and nervous system. Serotonin is an important neurotransmitter in the brain. 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.