Research has indicated several potential causes for the low levels of serotonin, norepinephrine, or dopamine:
<ul>
<li>Molecules that help make neurotransmitters are in short supply</li>
<li>Not enough receptor sites to receive the neurotransmitter</li>
<li>Presynaptic cells are taking the neurotransmitter back up before it has a chance to reach the receptor cell</li>
<li>Too few of the molecules that build neurotransmitters (chemical precursors)</li>
<li>Too little of a specific neurotransmitter (for example, serotonin) is being produced</li>
</ul>

Causes of Low Neurotransmitter Levels

Neurotransmitters are chemical messengers in the brain. The nerve cells of the brain use neurotransmitters to communicate with each other. The messages they send are believed to play a role in mood regulation.
The three neurotransmitters implicated in depression are:
<ul>
<li>Dopamine</li>
<li>Norepinephrine</li>
<li>Serotonin.</li>
</ul>

Neurotransmitters

Norepinephrine is both a neurotransmitter and a hormone. It plays a role in the "fight or flight response" along with adrenaline. It helps send messages from one nerve cell to the next.
Changes in norepinephrine levels do not affect mood in every person. Researchers now understand that having too little norepinephrine isn't the only chemical cause of depression.

Norepinephrine

While psychotherapy is helpful for some people with depression, if there is a chemical imbalance in the brain, it may not be enough to address their symptoms. Antidepressants combined with psychotherapy proves especially effective for addressing the symptoms. 
While taking an antidepressant medication might help with the symptoms, it doesn't necessarily address the cause of the low levels. In this situation, therapy to improve stress management and reduce stress could potentially be helpful.

Current and Future Depression Treatments

Serotonin is considered to be the "feel good" chemical in the brain. In addition to helping regulate your mood, serotonin has a number of different jobs throughout the body from your gut to blood clotting to sexual function.

Serotonin

Dopamine is a neurotransmitter that creates positive feelings associated with reward or reinforcement that motivate us to continue with a task or activity. There is evidence that reduced dopamine levels can contribute to depression in some people.
Dopamine is believed to play an important role in a variety of conditions affecting the brain, including Parkinson's and schizophrenia.

Dopamine

Accepting how little we truly know about the chemistry of depression can help us maintain perspective and expectations for the medications used to treat depression. 
For people who are trying to find the right treatment, understanding the complex chemistry can be reassuring when a particular drug doesn't work for them or if they need to try more than one antidepressant.

The Chemistry of Depression

What is the chemistry of depression? How do changes in neurotransmitters, the messengers of the brain, cause symptoms and respond to medications?

Neurotransmitters

Similar ideas to Neurotransmitters

Norepinephrine

Some neuroscience terms defined

The Role of Dopamine in Our Brain's Communication