<P> In addition to reversibility, MAOIs differ by their selectivity of the MAO enzyme subtype . Some MAOIs inhibit both MAO - A and MAO - B equally, other MAOIs have been developed to target one over the other . </P> <P> MAO - A inhibition reduces the breakdown of primarily serotonin, norepinephrine, and dopamine; selective inhibition of MAO - A allows for tyramine to be metabolised via MAO - B. Agents that act on serotonin if taken with another serotonin - enhancing agent may result in a potentially fatal interaction called serotonin syndrome or with irreversible and unselective inhibitors (such as older MAOIs), of MAO a hypertensive crisis as a result of tyramine food interactions is particularly problematic with older MAOIs . Tyramine is broken down by MAO - A and MAO - B, therefore inhibiting this action may result in its excessive build - up, so diet must be monitored for tyramine intake . </P> <P> MAO - B inhibition reduces the breakdown mainly of dopamine and phenethylamine so there are no dietary restrictions associated with this . MAO - B would also metabolize tyramine, as the only differences between dopamine, phenethylamine, and tyramine are two phenylhydroxyl groups on carbons 3 and 4 . The 4 - OH would not be a steric hindrance to MAO - B on tyramine . Two MAO - Bi drugs, selegiline and rasagiline have been approved by the FDA without dietary restrictions, except in high - dosage treatment, wherein they lose their selectivity . </P> <P> MAOIs started off due to the serendipitous discovery that iproniazid was a potent MAO inhibitor (MAOI). Originally intended for the treatment of tuberculosis, in 1952, iproniazid's antidepressant properties were discovered when researchers noted that the depressed patients given iproniazid experienced a relief of their depression . Subsequent in vitro work led to the discovery that it inhibited MAO and eventually to the monoamine theory of depression . MAOIs became widely used as antidepressants in the early 1950s . The discovery of the 2 isoenzymes of MAO has led to the development of selective MAOIs that may have a more favorable side - effect profile . </P>

Contraindications for maois include which of the following