Amphetamines interaction with norepinephrine

Many other adrenomimetic drugs, such as amphetamine, do not themselves interact with adrenoceptors, yet they produce sympathetic effects by releasing norepinephrine from neuronal storage sites (vesicles). The norepinephrine that is released by these compounds interacts with the receptors on the effector cells. These adrenomimetics are called indirectly acting adrenomimetic drugs. The effects elicited by indirectly acting drugs resemble those produced by norepinephrine. 

Amphetamine and related substances show symphaticomimetic and CNS stimulant activity. Amphetamines are indirect monoamine agonists and interact with the membrane transporters involved in neurotransmitter reuptake and vesicular storage systems. Therefore, they stimulate the release of norepinephrine, dopamine, and serotonin from presynaptic terminals in the CNS and at the peripheral level (De La Torre et al., 2004). Methamphetamine and the methylenedioxy derivatives (MDA, MDMA, MDEA, MBDB) can inhibit the activity of enzymes of dopamine or serotonin biosynthesis (De La Torre et al., 2004). 

When this enzyme is inhibited, the concentrations of norepinephrine within adrenergic neurons increase and drugs that stimulate its release can bring about an exaggerated response. Interactions between MAO inhibitors and indirectly acting sympathomimetic amines (e.g., amphetamine) develop by this mechanism. If amphetamine is administered to a patient whose stores of norepinephrine have been increased by MAO inhibition, the patient may experience severe headache, hypertension (possibly a hypertensive crisis), and cardiac arrhythmias. The serious consequences associated with these interactions contraindicate the use of these agents in combination. 

MAO inhibitors (MAOIs) These drugs (eg, phenelzine, tranylcypromine, isocarboxazid) are stmcturally related to amphetamines and are orally active. They inhibit both MAO-A (which metabolizes norepinephrine, serotonin, and tyramine) and MAO-B (which metabolizes dopamine). Tranylcypromine is the fastest in onset of effect but has a shorter duration of action (about a week) than do other MAO inhibitors (with durations of 2-3 weeks). In spite of these prolonged actions, the MAO inhibitors are given daily. These drugs are inhibitors of hepatic drug-metabolizing enzymes and cause many drug interactions. 

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