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Neurotransmitters & Receptors: The Complete Pre-Med Guide

Neurotransmitters are the molecular language of the brain — and one of the most clinically tested topics in all of medicine. Drug mechanisms, psychiatric disorders, anesthesia, and toxicology all hinge on understanding which neurotransmitter does what, where, and through which receptor. This is your complete reference.

AI-generated content. This guide was written by MedAI's AI and is intended as a study aid. Always cross-reference with your official course materials, textbooks, and instructor guidance before your exam.

The 7 Major Neurotransmitter Systems

While there are hundreds of neuroactive molecules, 7 systems account for the overwhelming majority of MCAT, USMLE, and AP Biology exam questions. Master these before worrying about anything else.

1. Acetylcholine (ACh)

Feature	Detail
Synthesis	Choline + acetyl-CoA → ACh, catalyzed by choline acetyltransferase (ChAT)
Degradation	Acetylcholinesterase (AChE) cleaves ACh → choline + acetate in the synaptic cleft
Receptors	Nicotinic (ionotropic, Na⁺/K⁺): NMJ, autonomic ganglia, CNS. Muscarinic (metabotropic, GPCR): M1-M5; parasympathetic targets
Locations	Neuromuscular junction; all preganglionic autonomic neurons; parasympathetic postganglionic; basal forebrain (cognition)
Clinical relevance	Myasthenia gravis: anti-nicotinic antibodies at NMJ. Alzheimer's: loss of cholinergic neurons in nucleus basalis. Botulinum toxin: blocks ACh release. Neostigmine: AChE inhibitor → ↑ ACh → treats MG and reverses neuromuscular blockade

2. Dopamine (DA)

Feature	Detail
Synthesis	Tyrosine → DOPA → Dopamine (tyrosine hydroxylase is rate-limiting)
Pathways	Mesolimbic: reward/motivation. Mesocortical: executive function. Nigrostriatal: motor control. Tuberoinfundibular: prolactin regulation
Receptors	D1-D5 (all GPCRs). D1/D5 activate adenylyl cyclase (↑ cAMP). D2/D3/D4 inhibit adenylyl cyclase (↓ cAMP)
Clinical relevance	Parkinson's: degeneration of nigrostriatal dopamine neurons (substantia nigra). Schizophrenia: dopamine excess in mesolimbic pathway. Treatment: antipsychotics (D2 blockers). ADHD: dopamine transporter dysfunction. Cocaine/amphetamines: block dopamine reuptake

3. Serotonin (5-HT)

Feature	Detail
Synthesis	Tryptophan → 5-HTP → Serotonin (rate-limiting: tryptophan hydroxylase)
Location	Raphe nuclei (brainstem) project throughout CNS; 95% of total body serotonin is in gut (enterochromaffin cells)
Receptors	5-HT1–7; most are GPCRs except 5-HT3 (ionotropic, Na⁺/K⁺)
Functions	Mood, sleep, appetite, pain, gut motility, platelet aggregation
Clinical relevance	Depression: SSRIs block serotonin reuptake transporter (SERT). Serotonin syndrome (excess): hyperthermia, clonus, agitation. Carcinoid syndrome: 5-HT secreting tumor → flushing, diarrhea, right heart valvular disease

4. Norepinephrine (NE) & Epinephrine

Feature	Detail
Synthesis	Dopamine → NE (dopamine β-hydroxylase); NE → Epi (PNMT, only in adrenal medulla)
Receptors	α1: vasoconstriction, mydriasis. α2: presynaptic inhibition, ↓ insulin. β1: heart (↑HR, ↑contractility). β2: bronchodilation, vasodilation. β3: lipolysis
Location	Locus coeruleus (main NE nucleus); sympathetic postganglionic neurons; adrenal medulla (Epi)
Clinical relevance	Pheochromocytoma: adrenal medulla tumor → catecholamine excess → hypertensive crisis. β-blockers: decrease HR and BP. α1-blockers: treat hypertension and BPH. Tricyclic antidepressants: block NE/serotonin reuptake

5. GABA (γ-Aminobutyric Acid) — Main Inhibitory NT

Feature	Detail
Synthesis	Glutamate → GABA via glutamate decarboxylase (requires pyridoxal phosphate/B6)
Receptors	GABA-A (ionotropic, Cl⁻ channel): fast inhibition. GABA-B (GPCR, K⁺/Ca²⁺): slow inhibition
Effect	Hyperpolarizes neuron via Cl⁻ influx → inhibitory postsynaptic potential (IPSP)
Clinical relevance	Benzodiazepines: positive allosteric modulator of GABA-A (increase frequency of Cl⁻ channel opening). Barbiturates: increase duration of Cl⁻ channel opening — more dangerous in overdose. B6 deficiency → seizures (cannot make GABA). Huntington's: loss of GABAergic neurons in striatum

6. Glutamate — Main Excitatory NT

Feature	Detail
Receptors	AMPA (fast Na⁺ influx), NMDA (Na⁺ + Ca²⁺; requires glycine co-agonist; voltage-gated Mg²⁺ block), Kainate, mGluR (metabotropic)
Key role	NMDA receptor: LTP (long-term potentiation) — the molecular basis of memory formation
Clinical relevance	Excitotoxicity: excess glutamate → NMDA overactivation → Ca²⁺ overload → cell death (stroke, TBI). Ketamine: NMDA antagonist → dissociative anesthetic, rapid antidepressant. Memantine: NMDA antagonist → Alzheimer's treatment

7. Endogenous Opioids (Endorphins, Enkephalins, Dynorphins)

Endogenous opioids act on μ (mu), κ (kappa), and δ (delta) opioid receptors — all GPCRs. Activation inhibits adenylyl cyclase (↓ cAMP), opens K⁺ channels (hyperpolarization), and closes voltage-gated Ca²⁺ channels → decreased neurotransmitter release and reduced pain signaling.

Opioid Overdose Pharmacology

Opioids cause the classic triad: miosis (pinpoint pupils), respiratory depression, coma. Naloxone (Narcan) is a competitive μ-receptor antagonist that reverses opioid overdose — works within minutes. Know this for clinical scenarios.

Neurotransmitter Summary Table

NT	Effect	Precursor	Key Drugs	Key Disease
ACh	Excitatory (NMJ/para)	Choline	Neostigmine, atropine, succinylcholine	Myasthenia gravis, Alzheimer's
Dopamine	Variable by pathway	Tyrosine	Levodopa, haloperidol, cocaine	Parkinson's, schizophrenia
Serotonin	Mood, sleep, gut	Tryptophan	SSRIs, triptans, ondansetron	Depression, carcinoid
NE	Fight-or-flight	Tyrosine	β-blockers, TCAs, clonidine	Pheochromocytoma, ADHD
GABA	Inhibitory	Glutamate	Benzodiazepines, barbiturates	Epilepsy, anxiety
Glutamate	Excitatory	Glutamine	Ketamine, memantine	Stroke (excitotoxicity), Alzheimer's
Opioids	Pain inhibition	Pro-opiomelanocortin	Morphine, naloxone	Addiction, chronic pain