Elevated serotonin levels in synapse. Animation of a nerve synapse (junction), showing molecules of neurotransmitters such as serotonin. Here, the levels of serotonin have increased due to the use of drugs that inhibit its reuptake. 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.