The role of monoamine oxidase A in the metabolism and function of noradrenaline and serotonin

Monoamine oxidase (MAO) inhibitors were among the first psychotropic drugs to be discovered and introduced into clinical use either alone or in combination with serotonin (5-hydroxytryptamine) (5-HT) precursors for the treatment of depressive illness (Youdim & Finberg, 1982). The result of MAO inhibition is that brain 5-HT and noradrenaline (NA) are elevated and the pharmacological effects of indirectly acting amines of dietary origin (e.g. tyramine and phenylethylamine) are potentiated, whether or not the indirectly acting amines are substrates for MAO. Perhaps the most common effect is the hypertensive response produced by dietary tyramine in individuals medicated with MAO inhibitors (MAOIs). Since this syndrome was first recognized in patients who ingested certain cheeses which contained high levels of tyramine, the syndrome is referred to as the ‘cheese effect’. The ‘cheese effect’ occurs because these indirectly acting amines, which are also substrates for the neurotransmitter uptake system, release noradrenaline (NA) into neuronal cytoplasm where it is normally deaminated by MAO before egress from the neurone occurs.