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Monoamine neurotransmitters (dopamine, serotonin, norepinephrine, histamine) are synthesized from amino acids and catabolized by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT). The balance between synthesis rate, reuptake efficiency, and degradation determines synaptic monoamine concentration. Individual differences in these enzymatic activities (influenced by genetics, diet, and aging) contribute to personality traits and vulnerability to mood disorders.
Trace the biochemical pathway from tyrosine to dopamine to trace metabolites, and from tryptophan to serotonin. Study how MAO inhibitors increase synaptic levels and understand why COMT variation affects working memory.
Monoamines are not rapidly degraded solely by reuptake; enzymatic breakdown by MAO/COMT is a major pathway. MAO inhibitors increase monoamine levels but carry dietary restrictions due to tyramine risks.
Monoamine neurotransmitters are built from amino acids you already know from biochemistry. Dopamine and norepinephrine (catecholamines) both trace back to tyrosine. The pathway runs: tyrosine → L-DOPA (via tyrosine hydroxylase, the rate-limiting step) → dopamine (via DOPA decarboxylase) → norepinephrine (via dopamine β-hydroxylase). Serotonin (a non-catecholamine monoamine) starts from tryptophan instead: tryptophan → 5-hydroxytryptophan → serotonin via analogous steps. Understanding these pathways tells you immediately where drugs can intervene — L-DOPA supplements are given in Parkinson's precisely because tyrosine hydroxylase activity has collapsed in the substantia nigra.
Once released into the synapse, monoamines face two fates: reuptake into the presynaptic terminal (the dominant route) or enzymatic degradation. The two major degrading enzymes are monoamine oxidase (MAO), located primarily in mitochondria of presynaptic terminals and astrocytes, and catechol-O-methyltransferase (COMT), located in postsynaptic neurons and glial cells. MAO oxidatively deaminates monoamines into aldehyde intermediates; COMT transfers a methyl group. Dopamine's primary breakdown products are DOPAC (via MAO) and HVA (via COMT then MAO); serotonin's main metabolite is 5-HIAA. You'll see these metabolites measured in cerebrospinal fluid as indirect proxies for neurotransmitter turnover.
The balance between synthesis, reuptake, and degradation determines synaptic monoamine concentration — and this balance is highly tunable pharmacologically. MAO inhibitors (MAOIs) block enzymatic degradation, flooding the synapse with monoamines; they're used as antidepressants but require dietary tyramine restriction because tyramine (normally degraded by MAO in the gut) can trigger hypertensive crises if it accumulates. Selective serotonin reuptake inhibitors (SSRIs) block the serotonin transporter (SERT) rather than degradation, prolonging serotonin's presence in the synapse without affecting synthesis. COMT inhibitors like entacapone are used adjunctively in Parkinson's to reduce L-DOPA breakdown in peripheral tissues.
Individual genetic variation in these enzymes creates meaningful differences in mood, cognition, and disease vulnerability. The COMT Val158Met polymorphism is one of the most studied: the Val variant degrades dopamine roughly four times faster than the Met variant. Val homozygotes have lower prefrontal dopamine levels, which impairs working memory performance but may confer resilience to certain psychotic symptoms. Met homozygotes maintain higher prefrontal dopamine, boosting working memory capacity but potentially increasing anxiety and rumination. This single nucleotide difference illustrates how the biochemical pathways you've learned aren't just abstract chemistry — they're the molecular substrate of personality differences.