Physiological Role of the Enzyme and Its Isoforms
The enzyme monoamine oxidase (MAO) plays a pivotal role in the nervous system. Its primary physiological function is the oxidative deamination of monoamines, which include norepinephrine, serotonin, and dopamine. The result of this biochemical reaction is the inactivation of these neurotransmitters.
In the body, the enzyme is represented by two main isoforms that differ in their substrate specificity:
- MAO-A — primarily inactivates norepinephrine and serotonin. Inhibition of this enzyme type leads to an increased concentration of these neurotransmitters in brain tissue, producing an antidepressant effect.
- MAO-B — primarily inactivates dopamine. Inhibition of MAO-B (e.g., by selegiline) increases dopamine levels and is used in the treatment of parkinsonism, though it has no effect on depression.
Non-Selective Irreversible Inhibitors
A classic representative of this group is nialamide. The drug acts as an irreversible and non-selective inhibitor, meaning it targets both MAO-A and MAO-B enzymes simultaneously in both the central nervous system and peripheral tissues.
- Pharmacodynamics: The therapeutic effect develops extremely slowly, with a latency period ranging from 7 to 14 days of regular administration.
- Recovery: Following drug discontinuation, MAO activity recovers very slowly (about two weeks). The reason lies in the irreversible nature of the inhibition: the body requires time to synthesize entirely new enzyme molecules.
- Effects and Clinical Use: In addition to its antidepressant action, nialamide exhibits pronounced psychostimulatory effects (it can cause excitation and eliminate drowsiness). Historically, it was prescribed for depressive states accompanied by lethargy and psychomotor retardation, as well as for angina pectoris (reducing pain perception).
- Side Effects: Due to total enzyme inhibition, the drug causes a wide range of complications. Central nervous system effects include anxiety, insomnia, tremor, and seizures. Cardiovascular effects feature lowered blood pressure and orthostatic hypotension. Hepatotoxic effects are also typical. Consequently, this drug is rarely used today.
Selective Reversible MAO-A Inhibitors
A modern and safer representative of this class is moclobemide.
- Mechanism of Action: The drug selectively and—unlike its predecessors—reversibly inhibits MAO-A activity. The biochemical result is a temporary suppression of biogenic amine metabolism (primarily serotonin and norepinephrine) and an increase in their concentration.
- Advantages: The duration of action of moclobemide is shorter than that of nialamide. While maintaining high efficacy, it possesses a significantly better safety profile, lacking pronounced toxic properties, particularly hepatotoxicity.
Drug and Dietary Interactions
The use of non-selective MAOIs requires strict monitoring due to the risk of severe complications.
- The “Cheese Effect” (Tyramine Syndrome): The development of a severe hypertensive crisis when consuming specific foods while taking nialamide. The triggering agent is tyramine. Prohibited foods include aged cheeses, bananas, soybeans, beer, and yeast extracts. Notably, selective moclobemide does not trigger this syndrome.
- Sympathomimetics: Drugs such as ephedrine and amphetamine are strictly contraindicated. When MAO is blocked, norepinephrine accumulates in nerve endings, and sympathomimetics provoke its massive release, exponentially increasing the pressor (blood pressure-raising) effect.
- Tricyclic Antidepressants: Concurrent use with MAOIs is strictly prohibited. A mandatory washout period of 2–3 weeks is required between courses of these medications.