Basic Science Physiology

Neurotransmitters

Neurotransmitters

What Do You Need to Know?

  • Three NT classes — amino acids, monoamines, neuropeptides; excitatory vs inhibitory pairs
  • ACh — muscarinic vs nicotinic receptors, NMJ blockade (pre vs post), anticholinesterases
  • Catecholamine synthesis — TYR → L-DOPA → DA → NE → Epi; rate-limiting = tyrosine hydroxylase
  • Dopamine pathways — nigrostriatal, mesolimbic, mesocortical, tuberoinfundibular; D1 vs D2
  • Serotonin — raphe nuclei, 5-HT receptor subtypes, serotonin syndrome
  • Glutamate — NMDA (Ca2+, Mg2+ block, glycine co-agonist), AMPA (fast EPSPs), excitotoxicity
  • GABA — GABA-A (Cl−, 5 binding sites), GABA-B (K+, baclofen), GAD enzyme
  • Glycine — spinal inhibition, strychnine antagonist, tetanus blocks release
🚩 Don’t Miss — Test-Day Priorities
  • Rate-limiting enzymes: tyrosine hydroxylase (catecholamines: DA, NE, Epi) — tryptophan hydroxylase (5HT) — GAD (GABA, B6-dependent) — ChAT (ACh)
  • 4 DA pathways: nigrostriatal (motor → PD when lost), mesolimbic (reward/positive psychosis sx), mesocortical (executive/negative sx), tuberoinfundibular (inhibits prolactin → D2 block causes galactorrhea/amenorrhea)
  • NMDA receptor: Ca²⁺-permeable, voltage + ligand gated, Mg²⁺ block at rest, glycine co-agonist site — blocked by ketamine, memantine, PCP, dextromethorphan, ethanol — central to LTP and excitotoxicity
  • GABA-A vs GABA-B: A = ionotropic Cl⁻ channel (benzos, barbiturates, propofol, ethanol, neurosteroids); B = GPCR Gi (baclofen for spasticity, GHB)
  • Disease NT pairs: AD = ↓ACh (Meynert) — PD = ↓DA + relative ↑ACh in striatum — HD = ↓GABA (medium spiny neurons) — depression = ↓5HT/NE/DA — schizophrenia = ↑mesolimbic DA + ↓mesocortical DA
  • MAO selectivity: MAO-A breaks down 5HT + NE + DA (target in depression; tyramine crisis with cheese); MAO-B is DA-selective (selegiline/rasagiline for PD); COMT degrades catecholamines (entacapone for PD)
  • Orexin/hypocretin: lateral hypothalamus — loss → narcolepsy type 1 with cataplexy; DORAs (suvorexant, lemborexant, daridorexant) block it for insomnia
  • CGRP: migraine pathophysiology — gepants (ubrogepant, rimegepant, atogepant) and mAbs (erenumab, fremanezumab, galcanezumab, eptinezumab)
  • Pyridoxine (B6)-dependent seizures: B6 is cofactor for GAD → deficiency → ↓GABA → neonatal refractory seizures responsive to IV pyridoxine
  • Glycine = main spinal inhibitory NT: strychnine blocks → tetanic spasms; hyperekplexia from GLRA1 mutation; tetanus toxin blocks glycine release from Renshaw cells
🔍 Buzzwords & Pathognomonic FindingsReceptors / pathways · Synthesis / breakdown · Disease association
Receptors / pathways
  • NMDA receptor — Mg²⁺ block + glycine co-agonistLTP, memory, excitotoxicity; ketamine/memantine/PCP target
  • α4β2 + α7 nicotinic AChRmain CNS nicotinic subtypes; α4β2 in nicotine addiction, α7 in cognition
  • D1 family (D1, D5) Gs vs D2 family (D2, D3, D4) GiD2 block = antipsychotic effect + EPS + hyperprolactinemia
  • GABA-A pentameric Cl⁻ channelbenzos, barbiturates, propofol, ethanol, neurosteroids binding sites
  • 5HT3 ionotropic receptorarea postrema — ondansetron antiemetic target
  • H3 autoreceptorpitolisant inverse agonist for narcolepsy (boosts histamine wake drive)
Synthesis / breakdown
  • Tyrosine → L-DOPA (TH, rate-limiting) → DA (AADC) → NE (DBH) → Epi (PNMT)catecholamine pathway
  • Tryptophan → 5-HTP (Trp hydroxylase) → 5HT (AADC)serotonin synthesis
  • Glutamate → GABA (GAD, B6-dependent)main inhibitory NT synthesis — B6 deficiency causes seizures
  • Choline + acetyl-CoA → ACh (ChAT), broken down by AChEcholinesterase inhibitors (donepezil, rivastigmine, neostigmine, pyridostigmine)
  • MAO-A vs MAO-B; COMTmonoamine degradation — selegiline/rasagiline (MAO-B), entacapone/tolcapone (COMT)
  • AADC deficiencyinfantile combined DA + 5HT deficiency (treatable inborn error)
Disease / drug association
  • Nucleus basalis of Meynert ACh lossAlzheimer disease — cholinesterase inhibitors
  • Substantia nigra pars compacta DA lossParkinson disease; tuberoinfundibular block → galactorrhea
  • Strychnine blocking glycine; GLRA1 mutationtetanic spasms; hereditary hyperekplexia
  • Orexin/hypocretin neuron loss in lateral hypothalamusnarcolepsy type 1 with cataplexy; DORAs for insomnia
  • CGRP release from trigeminal afferentsmigraine — gepants and CGRP mAbs
  • Riluzole reducing glutamate releaseALS (only NT-targeted disease-modifying therapy)
  • Vigabatrin (GABA-T inhibitor); tiagabine (GAT-1 block)AEDs that boost GABA — vigabatrin for infantile spasms
Overview
CategoryExamplesKey Features
Amino acidsGlutamate, GABA, glycine, aspartateFast synaptic transmission; most abundant CNS NTs
MonoaminesDA, NE, epinephrine, 5-HT, histamineSlow modulators; small nuclei, diffuse projections
NeuropeptidesSubstance P, enkephalins, orexinCo-transmitters; slow, modulatory; G-protein coupled
  • Main excitatory (CNS): glutamate — Main inhibitory (CNS): GABA — Main inhibitory (spinal): glycine
  • NMJ + autonomic ganglia: ACh — Postganglionic sympathetic: NE (except sweat glands = ACh)

Electrolyte Concentrations

IonIntracellular (mM)Extracellular (mM)Equilibrium Potential
K+~140~4−90 mV
Na+~15~145+60 mV
Ca2+~0.0001~2+120 mV
Cl−~5–15~110−70 to −80 mV
Acetylcholine (ACh)
  • Synthesis: choline + acetyl-CoA → ACh (via ChAT); rate-limiting = choline supply
  • Degradation: AChE → choline + acetate
  • Key locations: basal forebrain (nucleus basalis of Meynert → cortex), medial septum / diagonal band → hippocampus, pedunculopontine + laterodorsal tegmental nuclei (PPT/LDT) → thalamus, brainstem — wake, REM sleep, DBS target for PD gait/freezing; NMJ, all autonomic ganglia, Renshaw cells, striatal interneurons

Cholinergic Receptors

ReceptorTypeMechanismLocation
Nicotinic (NM)Ionotropic (Na+/K+)Fast excitationNMJ
Nicotinic (NN)Ionotropic (Na+/K+)Fast excitationAutonomic ganglia, adrenal medulla, CNS
M1, M3, M5Muscarinic (Gq)IP3/DAG → excitatoryCNS, glands, smooth muscle
M2, M4Muscarinic (Gi)↓ cAMP → inhibitoryHeart (↓ HR), presynaptic, CNS
  • nAChR subunit composition: Adult muscle nAChR = (α1)2β1δε; fetal/immature/denervated = (α1)2β1δγ (γ→ε perinatal switch). MG main immunogenic region (MIR) on α1. Neuronal nicotinic = α7 homomeric (CNS) and α4β2 (most abundant; nicotine addiction).

Cholinergic Agonists and Antagonists

DrugActionUse
BethanecholMuscarinic agonistUrinary retention
PilocarpineMuscarinic agonistGlaucoma
AtropineMuscarinic antagonistBradycardia, organophosphate poisoning
BenztropineCentral muscarinic antagonistEPS, PD tremor
HexamethoniumGanglionic nicotinic blockerExperimental

Anticholinesterases

TypeExamplesUse
ReversibleEdrophonium, pyridostigmine, neostigmine, donepezil, rivastigmineMG diagnosis/treatment; Alzheimer's
IrreversibleOrganophosphates (sarin, parathion)Cholinergic crisis (SLUDGE); Tx: atropine + pralidoxime

NMJ Blockade

FeaturePresynapticPostsynaptic
Mechanism↓ ACh releaseBlock nicotinic receptor at endplate
ExamplesBotulinum toxin, Lambert-Eaton, aminoglycosides, Mg2+Non-depolarizing: curare, vecuronium; Depolarizing: succinylcholine
Rep stimDecremental; Lambert-Eaton: incremental at high rateDecremental (non-depolarizing)
Board Pearl

Nucleus basalis of Meynert degenerates early in Alzheimer's. AChE inhibitors (donepezil, rivastigmine) compensate for this cholinergic loss.

Catecholamine Synthesis
  • Tyrosine → (tyrosine hydroxylase; RATE-LIMITING) → L-DOPA → (aromatic L-amino acid decarboxylase (AADC; also called DOPA decarboxylase) — same enzyme converts 5-HTP→5-HT and L-DOPA→dopamine; carbidopa/benserazide inhibit AADC peripherally to allow L-DOPA to cross BBB in PD) → DA → (dopamine β-hydroxylase) → NE → (PNMT) → Epi
  • Degradation:
    • MAO-A (intraneuronal, mitochondrial outer membrane) — preferentially metabolizes NE, 5-HT, DA; non-selective MAOI + SSRI/SNRI/linezolid/tramadol → serotonin syndrome
    • MAO-B — preferentially metabolizes DA; predominant in glia and brain; MAO-B inhibitors (selegiline metabolized to amphetamine, rasagiline, safinamide) for PD — at PD doses, MAO-B selective avoids tyramine reaction (transdermal/high-dose loses selectivity)
    • COMT (extraneuronal)
  • Metabolites: DA → HVA; NE/Epi → VMA; 5-HT → 5-HIAA
Board Pearl

Tyrosine hydroxylase = rate-limiting step in catecholamine synthesis. Tryptophan hydroxylase = rate-limiting for serotonin. Know the metabolites: HVA (DA), VMA (NE), 5-HIAA (5-HT).

Dopamine

Four Pathways

PathwayOrigin → TargetFunctionClinical
NigrostriatalSNc → striatumMovement↓ DA = Parkinson's; D2 block = EPS
MesolimbicVTA → nucleus accumbensReward↑ DA = positive symptoms (schizophrenia)
MesocorticalVTA → prefrontal cortexCognition↓ DA = negative/cognitive symptoms
TuberoinfundibularHypothalamus → pituitaryInhibits prolactinD2 block → hyperprolactinemia

D1 vs D2 Receptors

FeatureD1-like (D1, D5)D2-like (D2, D3, D4)
MechanismGs → ↑ cAMPGi → ↓ cAMP
EffectStimulatory; activates direct pathwayInhibitory; inhibits indirect pathway
ClinicalD1 agonists: PD motor improvementD2 antagonists: antipsychotics; D2 agonists: pramipexole, ropinirole

DA in Disease — Comparison

FeatureParkinson'sSchizophreniaHuntington's
DA level↓↓ nigrostriatal↑ mesolimbic; ↓ mesocorticalRelative ↑ DA (loss of GABAergic medium spiny neurons (MSNs) in striatum; striatal cholinergic interneurons relatively spared early → chorea)
PathologyLoss of SNc neuronsMesolimbic DA hyperactivityLoss of GABAergic MSNs in caudate
MovementBradykinesia, rigidity, tremorEPS from DA blockade (iatrogenic)Chorea (early), rigidity (late)
TreatmentL-DOPA, DA agonists, MAO-B inhibitorsD2 antagonists (antipsychotics)VMAT2 inhibitors (tetrabenazine)
Board Pearl

Antipsychotic side effects map to pathway blockade: nigrostriatal → EPS; tuberoinfundibular → hyperprolactinemia; mesolimbic → therapeutic; mesocortical → worsens negative symptoms.

Norepinephrine
  • Origin: locus coeruleus (pons) → diffuse projections to cortex, limbic, cerebellum, spinal cord
  • Functions: arousal, attention, stress response, mood
  • Inactivation: reuptake (NET), MAO, COMT
ReceptorMechanismKey Effects
α1Gq → IP3/DAGVasoconstriction, mydriasis
α2Gi → ↓ cAMP↓ NE release (presynaptic); ↓ sympathetic outflow
β1Gs → ↑ cAMP↑ HR, ↑ contractility
β2Gs → ↑ cAMPBronchodilation, vasodilation
  • Clonidine, guanfacine: α2 agonists (HTN, ADHD) — Prazosin: α1 blocker (PTSD nightmares)
  • SNRIs: block NE + 5-HT reuptake — TCAs: block NE + 5-HT reuptake (plus anticholinergic effects)
Serotonin (5-HT)
  • Synthesis: tryptophan → (tryptophan hydroxylase; rate-limiting) → 5-HTP → 5-HT (via AADC)
  • Origin: raphe nuclei (midbrain to medulla) → diffuse CNS projections
  • Degradation: MAO-A → 5-HIAA

5-HT Receptor Subtypes

ReceptorTypeFunction / LocationClinical
5-HT1AGiAutoreceptor (raphe); anxiolyticBuspirone = partial agonist (anxiety)
5-HT1B/1DGiCranial vasoconstriction, ↓ CGRPTriptans = agonists (migraine)
5-HT2AGqCortical excitation, plateletsHallucinogens = agonists; atypical antipsychotics = antagonists
5-HT2CGqAppetite, mood (hypothalamus)Antagonists (olanzapine) → weight gain
5-HT3Only ionotropic 5-HT receptor (cation channel)Area postrema, vagus, GI → emesisOndansetron = antagonist (anti-emetic)

Serotonin Syndrome

Board Pearl

Drug combinations causing serotonin syndrome: SSRI + MAOI, SSRI + triptan, SSRI + tramadol, SSRI + linezolid.

  • Triad: mental status changes + autonomic instability (hyperthermia, tachycardia) + neuromuscular hyperactivity (clonus, hyperreflexia)
  • vs NMS: serotonin syndrome = rapid onset, clonus, hyperreflexia; NMS = slow (days), lead-pipe rigidity, bradyreflexia
  • Tx: stop offending agent + cyproheptadine (5-HT2A antagonist) + benzodiazepines + supportive
Clinical Pearl

The key differentiator: clonus and hyperreflexia (serotonin syndrome) vs lead-pipe rigidity and bradyreflexia (NMS). Serotonin syndrome develops within hours; NMS over days.

Glutamate
ReceptorIonKey FeaturesDrugs/Clinical
AMPANa+Fast EPSPs; majority of excitatory transmissionPerampanel (antagonist, epilepsy)
NMDACa2+, Na+Voltage + ligand gated; Mg2+ block; requires glycine co-agonist; LTP, excitotoxicityKetamine, PCP, memantine (antagonists)
KainateNa+, K+Modulates excitabilityExperimental seizure models
mGluRsMetabotropic; slow modulation. Group I (mGluR1, 5): Gq, postsynaptic; Groups II (mGluR2, 3) and III (mGluR4, 6, 7, 8): Gi, presynaptic autoreceptorsExperimental targets
  • Glutamate transporters: EAAT1 (GLAST) and EAAT2 (GLT-1) on astrocytes clear synaptic glutamate; feed the glutamate-glutamine cycle (astrocytic glutamine synthetase → glutamine → neuronal PAG → glutamate).
  • Excitotoxicity: ischemia → ↓ ATP → depolarization → glutamate flood → NMDA Ca2+ influx → neuronal death
  • Anti-NMDA receptor encephalitis: Ab vs NR1 subunit; young woman → psychiatric symptoms → seizures → orofacial dyskinesias → autonomic instability; associated with ovarian teratoma
Board Pearl

NMDA receptor is a coincidence detector: requires membrane depolarization (Mg2+ block relief) AND glutamate + glycine binding simultaneously. This is fundamental to LTP and memory.

GABA
  • Synthesis: glutamate → (GAD, requires vitamin B6) → GABA
  • Degradation: GABA transaminase; vigabatrin = irreversible inhibitor → ↑ GABA (infantile spasms; side effect: visual field constriction)
FeatureGABA-AGABA-B
TypeIonotropic (Cl− channel)Metabotropic (Gi)
EffectFast IPSPs (Cl− influx); ionotropic Cl− channel; postsynaptic in cortex/cerebellum/hippocampus (targets of GABAergic interneurons)Slow IPSPs; GPCR (Gi) → K+ open; presynaptic and postsynaptic
DrugsBZDs, barbiturates, zolpidem, alcoholBaclofen, GHB (spasticity; GHB also acts at direct GHB receptor)

GABA-A — 5 Binding Sites

SiteAgentEffect
GABAGABA, muscimolOpens Cl− channel
BenzodiazepineDiazepam, lorazepam, midazolamFrequency of Cl− channel opening (require endogenous GABA — cannot directly open channel, hence safer in overdose)
BarbituratePhenobarbital, thiopentalDuration of channel opening — no ceiling, more dangerous in overdose (can open WITHOUT GABA)
NeurosteroidAllopregnanolone; ganaxolone (FDA 2022, CDKL5-DD); brexanolone (FDA 2019, postpartum depression)Positive allosteric modulator
PicrotoxinPicrotoxinBlocks Cl− channel → convulsant
  • Huntington's: loss of GABAergic MSNs in caudate → relative DA excess → chorea
  • Anti-GAD antibodies: stiff-person syndrome, cerebellar ataxia, temporal lobe epilepsy
Board Pearl

BZDs increase FREQUENCY; barbiturates increase DURATION of Cl− channel opening. Barbiturates can open the channel without GABA → no ceiling effect → respiratory depression/death. BZDs require GABA → safer.

Glycine
  • Main inhibitory NT in spinal cord and lower brainstem
  • Ionotropic Cl− channel receptor (similar to GABA-A)
  • Renshaw cells: glycinergic AND GABAergic interneurons (recurrent inhibition of motor neurons); explains tetanus toxin disinhibition phenotype.
  • Dual role: also a mandatory co-agonist at NMDA receptors
  • Strychnine: competitive glycine receptor antagonist → muscle spasms, opisthotonus
  • Tetanus toxin: blocks glycine + GABA release (cleaves synaptobrevin) → disinhibited motor neurons → rigidity, trismus
Clinical Pearl

Both strychnine and tetanus produce spinal disinhibition but differ in mechanism: strychnine blocks the glycine receptor directly; tetanus prevents presynaptic release of glycine/GABA by cleaving VAMP/synaptobrevin.

Other Neurotransmitters

Histamine

  • Tuberomammillary nucleus (posterior hypothalamus) — only source of brain histamine
  • H1 (Gq): wakefulness; blockade → sedation — H3 (Gi): autoreceptor; pitolisant (H3 inverse agonist) approved for narcolepsy

NT–Neuropeptide Co-transmitter Pairs

NeurotransmitterCo-released NeuropeptideLocation
AChVIPParasympathetic postganglionic
NENeuropeptide YSympathetic postganglionic
DopamineCCKMesolimbic neurons
SerotoninSubstance P, TRHRaphe nuclei
GABAEnkephalin, dynorphinStriatal MSNs
GlutamateSubstance P, CGRPDorsal horn afferents

Opioid Receptors

ReceptorEndogenous LigandEffectsDrugs
Mu (μ)β-endorphin, enkephalinsAnalgesia, euphoria, respiratory depression, miosis, constipationMorphine, fentanyl; naloxone (antagonist)
Delta (δ)EnkephalinsSpinal analgesia, mood modulationAntidepressant/analgesic contribution
Kappa (κ)DynorphinAnalgesia, dysphoria, sedation, diuresisButorphanol; dysphoria limits use

Substance P

  • Pain transmission in dorsal horn (C fibers); NK1 receptor — aprepitant (NK1 antagonist) for chemotherapy-induced nausea
Board Pearl

Narcolepsy type 1 = loss of orexin-producing neurons in lateral hypothalamus (undetectable CSF orexin-A). CSF orexin/hypocretin <110 pg/mL = narcolepsy type 1 diagnosis. Suvorexant (orexin antagonist) is used for insomnia — opposite mechanism.

High-Yield Newer Agents

AgentMechanismUse / Pearl
Lasmiditan (Reyvow)5-HT1F agonistAcute migraine; no vasoconstriction (safe in CAD); CIII; can't drive for 8h
CGRP mAbs / gepantsCGRP pathway blockadeMigraine prevention/acute (atogepant, rimegepant, ubrogepant; erenumab, fremanezumab, galcanezumab, eptinezumab)
Istradefylline (Nourianz)A2A adenosine antagonistPD adjunct for OFF episodes
Pitolisant (Wakix)H3 inverse agonistNarcolepsy EDS + cataplexy
GanaxoloneNeurosteroid GABA-A PAMFDA 2022 for CDKL5 developmental encephalopathy
BrexanoloneNeurosteroid GABA-A PAM (allopregnanolone)FDA 2019 postpartum depression (IV infusion)

Quick Reference

NT Summary Table

NTRate-Limiting StepNucleusFunctionKey Disorders
Glutamate— (glutamate-glutamine cycle: astrocytic glutamine synthetase ↔ neuronal glutaminase (PAG); not rate-limiting like TH/TPH)UbiquitousMain excitatoryStroke, epilepsy, anti-NMDAR encephalitis
GABAGAD (B6)Interneurons, striatumMain CNS inhibitoryEpilepsy, Huntington's, stiff-person
GlycineSpinal cord, brainstemSpinal inhibitionStrychnine, tetanus, hyperekplexia
DATyrosine hydroxylaseSNc, VTAMovement, rewardPD, schizophrenia, Huntington's
NETyrosine hydroxylaseLocus coeruleusArousal, attentionDepression, anxiety, ADHD
5-HTTryptophan hydroxylaseRaphe nucleiMood, sleep, painDepression, migraine, serotonin syndrome
AChCholine supplyNucleus basalisMemory, NMJAlzheimer's, MG, Lambert-Eaton
HistamineHistidine decarboxylaseTMNWakefulnessNarcolepsy (pitolisant)

Board Cheat Sheet

  • Parkinson's: ↓ DA (SNc → striatum)
  • Alzheimer's: ↓ ACh (nucleus basalis)
  • Depression: ↓ NE and 5-HT
  • Schizophrenia: ↑ mesolimbic DA, ↓ mesocortical DA
  • Huntington's: Loss of GABAergic medium spiny neurons (MSNs) in striatum; striatal cholinergic interneurons relatively spared early → relative DA excess → chorea
  • MG: Ab vs postsynaptic nicotinic AChR — Lambert-Eaton: Ab vs presynaptic Ca2+ channels
  • Stiff-person: anti-GAD Ab → ↓ GABA
  • BZDs: ↑ frequency — Barbiturates: ↑ duration of Cl− opening
  • Triptans: 5-HT1B/1D agonists — Buspirone: 5-HT1A agonist
  • Anti-NMDAR encephalitis: young woman, psychiatric → dyskinesias → look for ovarian teratoma

References

  • Bhatt A. Ultimate Review for the Neurology Boards. 3rd ed. Demos Medical; 2016. Chapter 1.
  • Blumenfeld H. Neuroanatomy Through Clinical Cases. 3rd ed. Sinauer; 2021.
  • Purves D, et al. Neuroscience. 6th ed. Oxford University Press; 2018.
  • Ropper AH, et al. Adams and Victor's Principles of Neurology. 12th ed. McGraw-Hill; 2023.
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