The neurotransmitters involved in depression are dopamine (DA), serotonin (5-HT), norepinephrine (NE) and acetylcholine (ACH).
It is believed that during depressive episode, there is a functional deficiency of these neurotransmitters or hyposensitive receptors.
Antidepressant medications increase the amount of available neurotransmitters by inhibiting neurotransmitter reuptake, by inhibiting monoamina oxidase (MAO) or by blocking certain receptors.