The+Biology+of+Mental+Illnesses

Most scientists believe that mental illnesses result from problems with the communication between neurons in the brain (**neurotransmission**). For example, the level of the neurotransmitter serotonin is lower in individuals who have depression. This finding led to the development of certain medications for the illness. **Selective serotonin reuptake inhibitors** (SSRIs) work by reducing the amount of serotonin that is taken back into the presynaptic neuron. This leads to an increase in the amount of serotonin available in the synaptic space for binding to the receptor on the postsynaptic neuron. Changes in other neurotransmitters (in addition to serotonin) may occur in depression, thus adding to the complexity of the cause underlying the disease. Scientists believe that there may be disruptions in the neurotransmitters dopamine, glutamate, and norepinephrine in individuals who have schizophrenia. One indication that dopamine might be an important neurotransmitter in schizophrenia comes from the observation that cocaine addicts sometimes show symptoms similar to schizophrenia. Cocaine acts on dopamine-containing neurons in the brain to increase the amount of dopamine in the synapse.

http://science.education.nih.gov/Supplements/Nih5/Mental/guide/info-mental-b.htm#causes