Certain brain chemicals called neurotransmitters are associated with depression, including the neurotransmitters serotonin (ser-oh-TOE-nin) and norepinephrine (nor-ep-ih-NEF-rin). Research suggests that abnormalities in neurotransmitter activity can affect mood and behavior.

Instead of inhibiting the reabsorption of certain neurotransmitters as other antidepressants do, tetracyclic antidepressants prevent neurotransmitters from binding with nerve cell receptors called alpha-2 receptors. This indirectly increases the levels of norepinephrine and serotonin in the brain. In turn, that may improve and elevate mood.

Here is the only tetracyclic antidepressant that has been approved by the Food and Drug Administration specifically to treat depression, with its generic or chemical name followed by available brand names in parentheses:

• Mirtazapine (Remeron, Remeron SolTab)

In addition to being available as a pill, mirtazapine (mir-TAZ-a-peen) is also available as a tablet that dissolves on your tongue (Remeron SolTab).

Side effects of tetracyclic antidepressants include:

• Drowsiness
• Weight gain
• Dry mouth
• Dizziness
• Lightheadedness
• Thirst
• Muscle or joint aches
• Constipation
• Increased appetite
• Increased cholesterol

Because of a potential drug interaction, don't take mirtazapine with another type of antidepressant called a monoamine oxidase inhibitor (MAOI). You may have increased drowsiness if you take mirtazapine with other medications or substances that also cause drowsiness, such as antihistamines, sedatives or alcohol.

In addition, in rare circumstances, mirtazapine can cause a potentially dangerous drop in white blood cell counts (agranulocytosis). This condition can make you vulnerable to infection. Talk to your doctor if you develop a sore throat, fever, inflammation of the mouth, flu-like symptoms or other signs of infection.

In some cases, antidepressants may be associated with worsening symptoms of depression or suicidal thoughts or behavior, particularly early in treatment or when you change your dosage. Be sure to talk to your doctor about any changes in your symptoms. You may need more careful monitoring at the beginning of treatment or upon a change in treatment, or you may need to stop the medication if your symptoms worsen.