Biochemistry of Depression

At the cellular level depression has clear biochemical roots, affecting the way nerve cells in the brain. Severely depressed people have unusual levels of the hormone cortisol, and several brain chemicals, ie: the neurotransmitters serotonin, dopamine, and norepinephrine.

The unusual levels of these chemicals may be inherited, which may be part of the reason why depression tends to run in families. Researchers hypothesize a genetic connection, but no "depression gene" has been discovered.

Some believe that depression is a learned behavior, hence it runs in families and children pick up depressed parents' negative coping skills and world view.

Without a family history, deep emotional losses may trigger the biochemical changes that cause depression. Profound trauma in early childhood -- physical or sexual abuse, a bitter divorce, the death of a parent, or other deeply disturbing experiences -- can set the emotional stage for depression later in life.

But equally often, the brain chemistry of depressed individuals slips for no apparent reason. Just as people who are never exposed to smoke can get lung cancer, people who have lived happy, well-loved lives can become seriously depressed.

(the above information is provided by 
http://www.planetrx.com, 'Health at your fingertips"  
http://depression.com/)