How Do Monoamine Oxidase Inhibitors Work?

Monoamine oxidase inhibitors are a class of drugs used to treat depression. They are also referred to as MAO Inhibitors or MAOIs. Although they are effective, they have many side effects, and can interact with many other drugs and foods. MAOIs have fallen out of favor due to the development of medications with fewer side effects and interactions.
Selegiline and rasagiline, two newer forms of MAOIs with fewer side effects, are used in the treatment of Parkinson's disease. MAOIs used for depression include phenelzine (Nardil) and tranylcypromine (Parnate).

Neurotransmitters

Signals in the nervous system are transmitted via chemicals called neurotransmitters. Neurotransmitters are released in the nerve endings and travel over the gaps, or synapses, between the nerves. Any extra neurotransmitters are either reabsorbed by the nerve endings or broken down by enzymes.
MAO Inhibitors block the action of the enzyme monoamine oxidase. There are two known types of monamine oxidase: monoamine oxidase A breaks down tyramine, norepinephrine and serotonin; monoamine oxidase B breaks down dopamine.

Depression

Drugs that increase the amount of serotonin and norepinephrine relieve the symptoms of depression. MAO Inhibitors allow more serotonin and norepinephrine to accumulate in the synapses, and allow more of these chemicals to be released.

Parkinson's Disease

Parkinson's disease is a disorder characterized by muscle rigidity and slow, jerky movements. It is caused by a decrease in the amount of the neurotransmitter dopamine in the central nervous system. Drugs that specifically inhibit monoamine oxidase B, the enzyme that breaks down dopamine, are useful in treating Parkinson's. Rasagiline (Azalect) and selegiline (Eldepryl) are MAO Inhibitors that primarily affect monoamine oxidase B.

Food Interactions

The enzyme monoamine oxidase is also responsible for breaking down tyramine, a chemical that is found in high concentrations in fermented foods, and as a result, people taking monoamine oxidase inhibitors cannot break down tyramine as effectively. This can lead to a dangerous spike in blood pressure. Foods that include tyramine include sauerkraut, liver, pickled herring, wine, aged cheeses and processed meats. People taking MAOIs should get a complete list of foods to avoid from their doctors. Since normal doses of the newer MAOIs selegiline and rasagiline mainly affect monoamine oxidase B, they are less likely to interact with foods that contain tyramine.

Drug Interactions

Many drugs interact with MAOIs. Some of these reactions can be severe. Antihistamines, cough suppressants and antidepressants can increase the level of the neurotransmitter serotonin to dangerous levels if taken with a MAOI. This is known as serotonin syndrome.
Decongestants and many asthma medications can lead to a dangerous spike in blood pressure if combined with MAOIs.
Because MAOIs react severely with so many drugs, consult with a doctor or pharmacist before taking any new medications--even over-the-counter medications and herbal products.