Neurotransmitters in Behavior & Disease
A high-yield Step 1 walkthrough of the six key neurotransmitters — their precursors, synthesis sites, and the directional changes tested in Parkinson, Alzheimer, schizophrenia, depression, anxiety, and Huntington disease — with two classic vignettes and next-best-step management.
How the boards test neurotransmitters
Behavioral science on Step 1 attacks neurotransmitters (NTs) three ways: where each is synthesized in the brain, how it is made (precursor + rate-limiting enzyme), and the direction it shifts in psychiatric and neurodegenerative disease. Six players dominate: dopamine, norepinephrine (NE), serotonin (5-HT), acetylcholine (ACh), GABA, and glutamate. Dopamine, NE, and epinephrine are catecholamines built sequentially from tyrosine; serotonin from tryptophan; GABA from glutamate; ACh from choline + acetyl-CoA.
Vignettes rarely name the disease. They hand you the classic picture — resting tremor, progressive memory loss, hallucinations, a panic attack — then ask which NT is up or down, which brain nucleus degenerated, or which drug rebalances the system. Learn the synthesis map plus the disease-direction facts and most items collapse to one-step recall.
- Parkinson disease: ↓ dopamine (degeneration of substantia nigra pars compacta, Lewy bodies = α-synuclein) with relative ↑ ACh
- Schizophrenia: ↑ dopamine (mesolimbic pathway → positive symptoms)
- Depression: ↓ NE, ↓ serotonin, ↓ dopamine
- Anxiety: ↑ NE, ↓ GABA, ↓ serotonin
- Alzheimer disease: ↓ ACh (loss of the basal nucleus of Meynert)
- Huntington disease: ↓ GABA, ↓ ACh, ↑ dopamine
- Rate-limiting enzyme for all catecholamines = tyrosine hydroxylase (needs cofactor BH4/tetrahydrobiopterin)
- Same NT, opposite directions is a favorite trap: ACh is LOW in Alzheimer but relatively HIGH in Parkinson; dopamine is LOW in Parkinson but HIGH in schizophrenia/Huntington
Six neurotransmitters: synthesis at a glance
| NT | Precursor | Key enzyme | Synthesis site | Primary role |
|---|---|---|---|---|
| Dopamine | Tyrosine | Tyrosine hydroxylase (RLS) | Ventral tegmentum, substantia nigra pars compacta | Reward, movement |
| Norepinephrine | Tyrosine | Dopamine β-hydroxylase | Locus ceruleus (pons) | Arousal, mood |
| Serotonin (5-HT) | Tryptophan | Tryptophan hydroxylase (RLS) | Raphe nuclei | Mood, sleep, appetite |
| Acetylcholine | Choline + acetyl-CoA | Choline acetyltransferase | Basal nucleus of Meynert | Memory; parasympathetic & NMJ output |
| GABA | Glutamate | Glutamate decarboxylase (needs B6) | Nucleus accumbens | Main inhibitory NT |
| Glutamate | Glutamine | Glutaminase | CNS-wide | Main excitatory NT |
- Catecholamine chain: Tyrosine →(tyrosine hydroxylase, RLS, needs BH4)→ L-DOPA →(DOPA decarboxylase, B6)→ dopamine →(dopamine β-hydroxylase, vit C)→ NE →(PNMT, induced by cortisol)→ epinephrine (adrenal medulla)
- Serotonin: tryptophan → 5-HTP → serotonin → (pineal gland) → melatonin
- GABA: glutamate →(glutamate decarboxylase / GAD, needs vitamin B6)→ GABA. Isoniazid depletes B6 → ↓ GABA → seizures (give pyridoxine)
- Degradation: catecholamines broken down by MAO and COMT → VMA (from NE/Epi), HVA (from dopamine), metanephrines — the labs ordered for pheochromocytoma
- Levodopa is given with carbidopa (blocks peripheral DOPA decarboxylase → less nausea, more levodopa crosses into the CNS)
Stem: A 67-year-old man has a 1-year history of a 4–6 Hz resting 'pill-rolling' tremor that quiets with voluntary movement, cogwheel rigidity, bradykinesia, a stooped shuffling gait, and a masked, expressionless face.
Diagnosis: Parkinson disease — loss of dopaminergic neurons in the substantia nigra pars compacta with intracytoplasmic Lewy bodies (α-synuclein).
NT shift: ↓ dopamine with a relative ↑ ACh; the dopamine-to-ACh imbalance drives the tremor.
Next best step / concept: Restore dopaminergic tone with levodopa + carbidopa (most effective in older patients). In a younger, tremor-predominant patient, an antimuscarinic (benztropine, trihexyphenidyl) rebalances the DA:ACh ratio and preferentially targets tremor. Recognizing which NT fell — and that ACh is relatively excessive — is the tested insight.
Stem: A 74-year-old woman has 2 years of progressive short-term memory loss, getting lost in familiar neighborhoods, and word-finding difficulty. Neuro exam shows no focal deficits and a normal gait.
Diagnosis: Alzheimer disease. Cortex shows β-amyloid (senile) plaques and neurofibrillary tangles (hyperphosphorylated tau).
NT shift: ↓ ACh from degeneration of the basal nucleus of Meynert.
Next best step: Symptomatic therapy with an acetylcholinesterase inhibitor — donepezil, rivastigmine, or galantamine — to raise synaptic ACh. Add the NMDA-receptor antagonist memantine in moderate-to-severe disease to blunt glutamate excitotoxicity. Contrast with Parkinson, where you reduce ACh — the boards love pairing these two to test the same NT moving in opposite directions.
Anchor each NT to its brain nucleus with the built-in wordplay:
- seRotonin ← Raphe nuclei (R to R)
- Locus ceruleus = 'sky-blue spot' → NorEpinephrine → feeling 'wired/on edge' in anxiety
- Basal nucleus of Meynert → ACh → Memory → lost first in Alzheimer
- Substantia Nigra dies in ParkiNsoN → ↓ dopamine
- Catecholamine build order: Tyrosine → DOPA → Dopamine → NE → Epi (each step adds one modification; tyrosine hydroxylase is the rate-limiter)
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