Monoamine Oxidase Inhibitors
The monoamine oxidase inhibitors (MAOIs) were some of the first antidepressant medications developed. The neurotransmitters responsible for mood, primarily norepinephrine and serotonin, are also known as monoamines. Monoamine oxidase is an enzyme which breaks these substances down. Monoamine oxidase inhibitors, as the name implies, inhibits this enzyme, thus allowing a greater supply of these chemicals to remain available.
MAOIs have fallen out of favor as first-line antidepressants because they offer several disadvantages to patients compared to newer medications. Potentially fatal drug-drug interactions can occur with MAOIs when combined with a variety of drugs which are serotonin agonists (the "serotonin syndrome") or norepinephrine agonists.3 People on these medications must also follow strict dietary restrictions of foods rich in tyramine4 to avoid potential hypertensive (high blood pressure) crisis. A major adverse effect that occurs on MAOIs alone is hypotension (low blood pressure), which can present as fatigue and may mimic worsening of the underlying depressive syndrome. For this reason, the blood pressure should always be monitored when using these antidepressants.5
Tricyclics
Tricyclics, also known as heterocyclics, came into broad use in the 1950's. These drugs inhibit the nerve cell's ability to reuptake serotonin and norepinephrine, thus allowing a greater amount of these two substances to be available for use by nerve cells.
In addition to acting on norepinephrine and serotonin, tricyclics exhibit similar effects on histamine and acetylcholine. This is responsible for the troublesome side-effects we usually associate with these medications, such as dry mouth, blurry vision, weight gain and sedation.6
With tricyclics, a patient's medical history must be closely considered. These medications may cause orthostatic hypotension (dizziness upon standing rapid heartbeat, sometimes with palpitations; and may aggravate preexisting heart conditions. Patients with a history of seizures or head injury must also be cautious as these drugs may cause seizure.
