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Dopamine - Comprehensive Overview

1. Chemistry and Basic Nature

Dopamine (3,4-dihydroxyphenylethylamine; DA) is a catecholamine neurotransmitter and the immediate metabolic precursor of norepinephrine (NE) and epinephrine (EPI). It is the predominant catecholamine in the CNS. Peripheral stores are found in the adrenal medulla, the enteric nervous system, and the GI tract plexuses. In the kidney, it is synthesized locally in proximal tubule epithelial cells and exerts paracrine diuretic and natriuretic effects.

Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 1472

2. Synthesis Pathway

All three catecholamines (DA, NE, EPI) share a common synthetic pathway starting with the dietary amino acid tyrosine:

Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine
            (TH)     (AADC)       (DBH)            (PNMT)

Key steps:

Step	Enzyme	Notes
Tyrosine → L-DOPA	Tyrosine hydroxylase (TH)	Rate-limiting step; requires tetrahydrobiopterin (BH4) cofactor
L-DOPA → Dopamine	Aromatic amino acid decarboxylase (AADC)	Cytosolic; very high activity - L-DOPA barely accumulates
Dopamine → NE	Dopamine-β-hydroxylase (DBH)	Inside vesicles only
NE → EPI	Phenylethanolamine-N-methyltransferase (PNMT)	Present only in adrenal medulla & central EPI neurons

Tyrosine hydroxylase is the rate-limiting step and the primary target for physiological regulation and pharmacological manipulation.
Dopamine neurons express only TH and AADC; they lack DBH.
BH4 cofactor synthesis depends on GTP-cyclohydrolase-1.

Clinical Relevance:

Parkinson's disease: Loss of dopaminergic neurons in the substantia nigra. Treatment with L-DOPA bypasses the rate-limiting TH step; AADC converts it efficiently to dopamine.
Carbidopa/benserazide: Peripheral AADC inhibitors (do not cross the blood-brain barrier) - given with L-DOPA to prevent peripheral conversion, reducing emesis and other peripheral side effects. Peripheral dopamine stimulates D2 receptors in the area postrema (outside BBB), causing nausea.
α-Methyltyrosine: Competitive inhibitor of TH; blocks catecholamine synthesis.
PKU (Phenylketonuria): Mutation in phenylalanine hydroxylase impairs phenylalanine metabolism, which feeds the tyrosine pool; can cause mental retardation if untreated.
Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 419

3. Storage, Release, and Reuptake

Storage:

Dopamine is actively pumped into synaptic vesicles by the Vesicular Monoamine Transporter (VMAT), which protects it from mitochondrial MAO degradation.
VMAT-2 is the neuronal form (CNS, PNS, enteric NS); VMAT-1 is expressed in chromaffin cells.
Amphetamines disrupt vesicular storage, mobilizing DA into the cytoplasm and increasing extracellular DA levels.
Reserpine blocks VMAT, depleting monoamine stores.

Reuptake:

After release, dopamine is rapidly cleared by the Dopamine Transporter (DAT), a Na⁺/Cl⁻-dependent reuptake transporter on presynaptic terminals.
Cocaine blocks DAT (and NET/SERT), increasing synaptic DA and producing euphoria/addiction.

Degradation:

Dopamine is a substrate for both MAO (monoamine oxidase) and COMT (catechol-O-methyltransferase).
The principal metabolites are:
- 3,4-Dihydroxyphenylacetic acid (DOPAC) - via MAO
- Homovanillic acid (HVA) - via MAO + COMT; the major urinary metabolite
Both are conjugated primarily to sulfate before urinary excretion.
DA is inactive orally because of first-pass MAO/COMT metabolism.
Ganong's Review of Medical Physiology, p. 635
Goodman & Gilman's, p. 1472

4. Dopamine Receptors

Five receptor subtypes, all G protein-coupled receptors (GPCRs), grouped into two families:

D1-like Family (D1, D5)

Couple to Gs → activate adenylyl cyclase → ↑ cAMP
Located postsynaptically on dopamine-receptive cells
High density in striatum, nucleus accumbens, frontal cortex
D5 expressed at low levels in frontal cortex, hypothalamus, hippocampus

D2-like Family (D2, D3, D4)

Couple to Gi → inhibit adenylyl cyclase → ↓ cAMP; also modulate voltage-gated ion channels
Expressed both postsynaptically and presynaptically (autoreceptors)
D2: Highest levels in striatum and nucleus accumbens; two splice variants:
- D2-Short (D2S) - primarily presynaptic; autoreceptor function (regulates firing rate, synthesis, release)
- D2-Long (D2L) - predominantly postsynaptic
D3: Highly restricted to limbic areas, especially nucleus accumbens shell; can function as autoreceptor
D4: Lowest expression level; found in frontal cortex, amygdala, hippocampus, hypothalamus; high affinity for clozapine (atypical antipsychotic)

Receptor	Family	G Protein	Effect on cAMP	Key Location
D1	D1-like	Gs	↑	Striatum, NAc, frontal cortex
D5	D1-like	Gs/Golf	↑	Frontal cortex, hypothalamus
D2	D2-like	Gi	↓	Striatum, NAc, pituitary
D3	D2-like	Gi	↓	Nucleus accumbens shell (limbic)
D4	D2-like	Gi	↓	Frontal cortex, amygdala

Peripheral distribution: All subtypes are found in the kidney, adrenal glands, sympathetic ganglia, GI tract, blood vessels, and heart.

Kaplan & Sadock's, pp. 428-429
Ganong's Review of Medical Physiology, p. 644

5. Major Dopaminergic Pathways in the CNS

1. Nigrostriatal System

From: Substantia nigra (midbrain) → Striatum (basal ganglia)
Function: Motor control
Disease link: Degeneration causes Parkinson's disease (resting tremor, rigidity, bradykinesia, postural instability)
Drug effects: D2 blockade by antipsychotics here causes extrapyramidal side effects (EPS) - Parkinsonism, dystonia, tardive dyskinesia

2. Mesolimbic/Mesocortical System

From: Ventral tegmental area (VTA) → Nucleus accumbens, limbic subcortical areas, frontal cortex
Function: Reward, motivation, addiction, cognition, emotion
Disease link: Implicated in schizophrenia (positive symptoms from mesolimbic hyperactivity; negative/cognitive symptoms from mesocortical hypoactivity)
Addiction: Virtually all drugs of abuse increase DA signaling in this pathway
PET scanning in healthy humans shows steady loss of DA receptors in basal ganglia with normal aging

3. Tuberoinfundibular System

From: Hypothalamus (arcuate nucleus) → Pituitary portal vessels
Function: Inhibits prolactin secretion from the pituitary
Disease link: D2 blockade by antipsychotics causes hyperprolactinemia → menstrual disturbances, galactorrhea, gynecomastia, sexual dysfunction, decreased bone mineral density
Goodman & Gilman's, p. 483
Kaplan & Sadock's, pp. 428-432

6. Pharmacological Actions (Peripheral - as a drug)

Dopamine has dose-dependent cardiovascular effects:

Low Dose (≤2 µg/kg/min)

Activates D1 receptors in renal, mesenteric, and coronary vasculature
→ Vasodilation, ↑ GFR, ↑ renal blood flow, natriuresis
Note: Clinical trials (DAD-HF II, ROSE-AHF) show no proven benefit of low-dose DA for renal protection in acute heart failure; it does not improve renal function and increases tachycardia

Intermediate Dose (2-10 µg/kg/min)

Stimulates β1-adrenergic receptors (directly and via NE release)
→ Positive inotropic effect, mild peripheral vasoconstriction
Limitation: Poor inotrope in severe heart failure because of depleted myocardial catecholamine stores

High Dose (10-20 µg/kg/min)

Activates α1-adrenergic receptors
→ Peripheral and pulmonary vasoconstriction
Risk of limb and end-organ ischemia; should be used very cautiously

Additional note: DA is ineffective orally (substrate for MAO and COMT). It can cause tachycardia and arrhythmias at initiation of therapy due to rapid NE release.

Braunwald's Heart Disease, pp. 178-186
Goodman & Gilman's, pp. 1477-1479

7. Clinical Pharmacology Summary

Drugs Increasing Dopaminergic Tone

Drug	Mechanism	Use
L-DOPA	DA precursor (bypasses TH)	Parkinson's disease
Carbidopa/Benserazide	Peripheral AADC inhibitor (with L-DOPA)	Reduce peripheral L-DOPA side effects
Bromocriptine, Pramipexole, Ropinirole	DA receptor agonists	Parkinson's, restless legs, hyperprolactinemia
Cocaine	DAT blocker	Drug of abuse
Amphetamine	VMAT disruptor + DAT reversal	ADHD (in low doses); abuse
MAO inhibitors (selegiline)	Block DA degradation	Parkinson's adjunct

Drugs Blocking Dopaminergic Tone

Drug	Mechanism	Use
Haloperidol	D2 antagonist (typical antipsychotic)	Schizophrenia
Chlorpromazine	D1 + D2 antagonist (typical)	Schizophrenia
Clozapine	D4 > D2 antagonist (atypical)	Treatment-resistant schizophrenia
Risperidone	D2 + 5HT2A antagonist (atypical)	Schizophrenia (higher hyperprolactinemia risk)
Aripiprazole	D2 partial agonist (atypical)	Schizophrenia (low prolactin elevation risk)
Metoclopramide, Promethazine	D2 antagonist	Antiemetic
Reserpine	VMAT blocker → depletes DA stores	Antihypertensive (historical); causes depression

Extrapyramidal Side Effects (EPS) from typical antipsychotics: Parkinsonism, acute dystonia, akathisia, tardive dyskinesia - all from nigrostriatal D2 blockade.

Atypical antipsychotics have reduced EPS due to additional 5HT2A antagonism and lower D2 occupancy (or partial agonism). However, D2 blockade in the tuberoinfundibular pathway causes hyperprolactinemia with most antipsychotics.

Kaplan & Sadock's, pp. 429-430

8. Dopamine in Disease - Key Associations

Disease	Dopamine Abnormality
Parkinson's disease	↓ DA in nigrostriatal pathway (loss of substantia nigra neurons)
Schizophrenia	↑ DA in mesolimbic (positive symptoms); ↓ DA in mesocortical (negative/cognitive symptoms)
Addiction/Reward	↑ DA in nucleus accumbens (all drugs of abuse)
Hyperprolactinemia	↓ tuberoinfundibular DA (or D2 blockade by drugs)
Restless legs syndrome	Impaired DA signaling in CNS
ADHD	Reduced DA (and NE) in prefrontal cortex
Depression	Reduced DA (along with NE, 5HT) in mesocortical pathways

9. Dopamine in the Kidney

Synthesized locally in proximal tubule cells from circulating L-DOPA. Local renal DA:

Acts on D1 receptors → inhibits Na⁺-H⁺ exchanger and Na⁺-K⁺-ATPase → natriuresis
Acts on D2 receptors → decreases Na⁺ transport via cAMP-dependent and independent mechanisms
Causes vasodilation of renal microvasculature

This makes DA pharmacologically relevant in low-output states with compromised renal function (e.g., cardiorenal syndrome), though robust clinical trial evidence for "renal-protective" low-dose dopamine is lacking.

Goodman & Gilman's, p. 1477

Sources:

Kaplan & Sadock's Comprehensive Textbook of Psychiatry - Life Cycle of Dopamine section (pp. 418-432)
Goodman & Gilman's Pharmacological Basis of Therapeutics - Dopamine chapter (pp. 1470-1490)
Ganong's Review of Medical Physiology, 26th ed. - Dopamine section (pp. 633-645)
Braunwald's Heart Disease - Inotropes/Vasopressors chapter (pp. 176-188)

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