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MAOIs alone do not have very substantial effects apart from lowering blood pressure, increasing the likelihood of episodes of dizziness or fainting such as standing up quickly from a period of rest. Unfortunately, they are reactive with a wide variety of other drugs, ultimately leading to their role as an alternative drug in current pharmacological treatment of depression.
The deactivation of monoamine oxidase by some MAOI drugs does not only allow for a buildup of monoamines; it's also responsible for deactivating another amine called tyramine, a molecule that affects blood pressure. The deactivation of tyramine is a normal part of the digestive process of many foods, but when monoamine oxidase is inactivated by an MAOI drug, levels of tyramine begin to rise. While an increase in neurotransmitters is beneficial, an increase in tyramine is disastrous.
Tyramine is found in a variety of foods, including aged cheese, pickled herring, beer, broad pod beans, wine and chocolate. After eating these foods when MAO is inhibited, tyramine accumulates in the body and causes high blood pressure, which can lead to headaches, internal bleeding, and in some cases stroke and death.
There are actually two different types of MAO, MAO-A which is primarily responsible for the breakdown of serotonin and norepinephrine, and MAO-B which is most active in metabolizing dopamine. MAO-A is located within the intestine and normally metabolizes the tramine in foods just after it is consumber.
Any tyrosine missed by the intesting MAO-A is destroyed by MAO-B in the liver and lungs before it reaches general circulation throughout the body.
First generation MAOIs blocked both forms, but newer MAOIs have been developed to selectively block MAO-A, with minimal affects on MAO-B. These drugs are much safer, as patients do not have to be as careful with their diet. The tyrosine that gets past the inhibited MAO-A in the intestine can still be metabolized by the MAOB in the liver and lungs. These drugs are most effective if taken after eating, allowing any dietary tyrosine to be metabolized before the full effect of the MAOI occurs.
Treatment and Precautions
Before Taking MAOIs:
As a patient, a doctor will probably quiz you about a range of medical conditions before prescribing an MAO inhibitor. It's especially important to tell your doctor if you have frequent headaches or chest pain. Since a severe headache or chest pain during MAOI therapy is the primary warning sign of a serious spike in blood pressure, anyone who normally gets severe headaches might overlook an important warning sign. Your doctor will also want to know if you have diabetes mellitus (you may need to change your insulin level) or an alcohol problem, since drinking while taking MAOIs may cause serious side effects. Also tell your doctor if you have heart or blood-vessel disease, liver or kidney problems, Parkinson's disease, or an overactive thyroid.
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