Our knowledge of the human brain is still fairly limited, therefore we do not really know what actually happens in the brain to cause depression.
It’s likely that with most instances of clinical depression, neurotransmitter function is disrupted. Neurotransmitters are chemicals that carry signals from one part of the brain to the next. There are many neurotransmitters, serving different purposes, however three important ones that affect a person’s mood are serotonin, noradrenaline and dopamine.
In normal brain function, neurotransmitters jump from one nerve cell to the next, with the signal being as strong in the second and subsequent cells as it was in the first. However, in people who are depressed, the mood regulating neurotransmitters fail to function normally, so that the signal is either depleted or disrupted before passing to the next nerve cell.
In non-melancholic depression, it is likely that the transmission of serotonin is reduced or less active, whereas in people with melancholic and psychotic depression, the neurotransmitters noradrenaline and dopamine are more likely to be functioning abnormally.