Serotonin release in synapse. Animation of a nerve synapse (junction), showing the release of molecules of neurotransmitters such as serotonin. This animation is part of a sequence showing the effects of serotonin 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.