Tricyclic Antidepressants and Monoamine Oxidase Inhibitors

Tricyclic antidepressants inhibit both the serotonin and norepinephrine reuptake transporters, producing dual monoaminergic enhancement analogous to serotonin-norepinephrine reuptake inhibitors. This is the basis for their antidepressant efficacy. The critical pharmacological difference from modern serotonin-norepinephrine reuptake inhibitors is not the primary mechanism but the extent of off-target receptor binding: tricyclic antidepressants bind muscarinic acetylcholine receptors, histamine H1 receptors, and alpha-1 adrenergic receptors with potencies that produce substantial adverse effects at every therapeutic dose.

Tricyclic antidepressants are classified as tertiary or secondary amines based on the chemical structure of their side chain, and this distinction has direct clinical consequences. Tertiary amines — amitriptyline, imipramine, clomipramine, doxepin — have stronger serotonin reuptake inhibition and substantially more anticholinergic, antihistaminic, and alpha-1 antagonist activity. They are harder to tolerate and more sedating. Secondary amines — nortriptyline, desipramine — are relatively more selective for norepinephrine reuptake, have less off-target receptor binding, and are better tolerated, particularly in elderly patients.

Several tertiary amines are metabolically converted to their secondary amine counterparts: amitriptyline is demethylated to nortriptyline, and imipramine is demethylated to desipramine. Both parent and active metabolite contribute to the clinical effect in patients taking the tertiary amine.

Three receptor systems account for most tricyclic antidepressant adverse effects. Muscarinic receptor blockade produces the anticholinergic syndrome: dry mouth, constipation, urinary retention, blurred vision, tachycardia, and confusion or delirium in elderly patients. Urinary retention is a particular concern in elderly males with prostatic enlargement. Tricyclic antidepressants are listed on the Beers Criteria as inappropriate for routine use in older adults because of anticholinergic central nervous system effects — confusion, falls, and cognitive impairment.

Histamine H1 blockade produces sedation and weight gain. The sedation is most pronounced with tertiary amines and is the basis for low-dose doxepin's Food and Drug Administration approval specifically for insomnia. Alpha-1 adrenergic blockade produces orthostatic hypotension — the most dangerous cardiovascular adverse effect during chronic tricyclic antidepressant use — because it impairs the normal vasoconstriction that compensates for standing. Orthostatic hypotension causes syncope and falls, with consequent hip fractures and head injuries in elderly patients. Nortriptyline has the least alpha-1 antagonism of the commonly used tricyclic antidepressants and is the preferred choice when a tricyclic antidepressant is clinically indicated.

Beyond depression, specific tricyclic antidepressants have established indications that remain clinically relevant. Amitriptyline and nortriptyline are first-line or second-line agents for neuropathic pain and migraine prophylaxis, through norepinephrine-mediated augmentation of descending pain inhibition. Clomipramine, with its especially potent serotonin reuptake inhibition among the tricyclic antidepressants, is the most efficacious pharmacological agent for obsessive-compulsive disorder and is reserved for cases refractory to selective serotonin reuptake inhibitors. Imipramine is approved for nocturnal enuresis in children.

Monoamine oxidase exists as two isoforms with overlapping but distinct substrate preferences. Monoamine oxidase A preferentially degrades serotonin and norepinephrine and is found in noradrenergic and serotonergic neurons, the intestinal mucosa, and the liver. Monoamine oxidase B preferentially degrades dopamine and phenylethylamine and is found in dopaminergic neurons, platelets, and glial cells. Tyramine — the dietary amine responsible for the dangerous food interaction — is metabolized predominantly by monoamine oxidase A in the gut and liver during first-pass extraction.

This isoform distribution has important clinical implications. Monoamine oxidase A inhibition is required for antidepressant effect (serotonin and norepinephrine are the relevant substrates). Monoamine oxidase A inhibition in the gut and liver is what eliminates the protective first-pass tyramine metabolism, creating the dietary tyramine interaction. Selective monoamine oxidase B inhibition at low doses — as with selegiline in Parkinson's disease — does not produce meaningful antidepressant effect and preserves enough gut monoamine oxidase A to handle normal dietary tyramine loads safely.

The classical monoamine oxidase inhibitors — phenelzine, tranylcypromine, and isocarboxazid — form irreversible covalent bonds with the monoamine oxidase enzyme, permanently inactivating it. Recovery of monoamine oxidase activity after stopping an irreversible monoamine oxidase inhibitor depends entirely on the synthesis of new enzyme, a process that takes approximately two weeks. This is the pharmacological basis for the two-week washout period required after stopping an irreversible monoamine oxidase inhibitor before starting any serotonergic agent or indirect sympathomimetic: the drug effect persists long after the drug itself has been eliminated from plasma.

Selegiline in transdermal form (Emsam) delivers drug systemically while substantially reducing first-pass inhibition of gut and hepatic monoamine oxidase A, which is what eliminates dietary tyramine. At the lowest transdermal dose, dietary restrictions are not required because systemic monoamine oxidase A inhibition is insufficient to impair gut tyramine metabolism. At higher transdermal doses, some dietary restriction is still necessary. The two-week washout requirement applies to the transdermal formulation as well when stopping before starting serotonergic drugs.

Moclobemide — a reversible monoamine oxidase A inhibitor used in Europe, Canada, and Australia but not Food and Drug Administration-approved in the United States — has a substantially reduced dietary interaction risk due to its reversible binding. Its washout before serotonergic drugs is only 24 hours.