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Dopaminergic Pathways in the CNS
- Nigrostriatal Substantia nigra → striatum
- Dopamine release inhibits GABAergic neurons in the striatum
- Regulates motor planning and execution
- Loss of DA neurons → Parkinson disease (bradykinesia, rigidity, tremor)
- DA receptor blockade → reversible pseudoparkinsonism (EPS)
- DA overstimulation (agonists) → dyskinesias
- Mesolimbic-Mesocortical Midbrain → limbic & cortical areas
- Modulates affect, reinforcement, cognition, and sensory perception
- Hyperdopaminergic activity → positive symptoms of schizophrenia (hallucinations, delusions)
- Hypodopaminergic activity in prefrontal cortex → negative symptoms (amotivation, flat affect)
- Drugs that enhance DA function → reinforcement; high doses → psychosis
- DA antagonists → cognitive blunting (antipsychotic effect)
- Tuberoinfundibular Hypothalamus → anterior pituitary
- Dopamine tonically inhibits prolactin secretion via D2 receptors
- DA agonists (bromocriptine) → suppress prolactin in hyperprolactinemia
- DA antagonists → prolactin elevation → gynecomastia, galactorrhea, amenorrhea
- Hyperprolactinemia → ↓ GnRH → ↓ FSH/LH → hypogonadism
- Chemoreceptor Trigger Zone (CTZ)
- Area postrema (medulla) — outside the blood-brain barrier
- DA receptor activation → emetic response
- DA agonists (apomorphine) → induce vomiting
- DA antagonists (antipsychotics, metoclopramide) → antiemetic effect
High-yield: Prolactin elevation from DA blockade inhibits GnRH, causing ↓ FSH/LH and amenorrhea — key exam link between antipsychotics and endocrine effects.
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Dopamine Receptor Families
- D1-like family (D1, D5)
- Gs-coupled → stimulates adenylyl cyclase → ↑ cAMP
- Predominantly postsynaptic
- Found in striatum, cortex, limbic system
- D2-like family (D2, D3, D4)
- Gi-coupled → inhibits adenylyl cyclase → ↓ cAMP
- Both presynaptic (autoreceptors) and postsynaptic
- D2A subtype — enriched in nigrostriatal pathway (motor control)
- D2C subtype — enriched in mesolimbic pathway (cognition, affect)
- D2 autoreceptors regulate DA synthesis and release (negative feedback)
- Receptor distribution & function
- Nigrostriatal D2 → motor coordination; blockade → EPS
- Mesolimbic D2 → reward, psychosis; blockade → antipsychotic effect
- Tuberoinfundibular D2 → prolactin inhibition; blockade → hyperprolactinemia
- CTZ D2 → emesis; blockade → antiemetic
Exam trap: Antipsychotic potency correlates with D2 receptor affinity — not all atypical agents are equally potent at D2.
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Parkinson Disease — Pathophysiology
- Core pathology: Progressive degeneration of nigrostriatal dopamine neurons
- Loss of dopaminergic input to the striatum
- Dopamine ↓ / Acetylcholine ↑ imbalance in the striatum
- Relative cholinergic overactivity drives motor symptoms
- Cardinal motor features (TRAP)
- Tremor — resting, pill-rolling, 4–6 Hz
- Rigidity — cogwheel or lead-pipe
- Akinesia / bradykinesia — slow movement, reduced amplitude
- Postural instability — late feature, falls
- Non-motor features
- Autonomic dysfunction (orthostatic hypotension, constipation)
- Neuropsychiatric (depression, anxiety, dementia)
- Sleep disorders, anosmia, fatigue
- Pharmacologic strategy:
- Restore dopaminergic tone (DA replacement or agonism)
- Reduce cholinergic overactivity (antimuscarinics)
- Balance restored via combination of ↑ DA and ↓ ACh
Clinical pearl: Tremor in Parkinson disease is typically present at rest and diminishes with voluntary movement — distinguishing it from cerebellar or essential tremor.
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Parkinson Disease — Pharmacotherapy
- Dopamine augmentation strategies
- Levodopa + carbidopa — gold standard
- Levodopa is a prodrug converted to DA by aromatic amino acid decarboxylase (AAAD)
- Carbidopa is a peripheral AAAD inhibitor — prevents peripheral conversion
- Allows more levodopa to cross the BBB; reduces peripheral side effects (nausea, hypotension)
- Side effects: dyskinesias, on-off fluctuations, psychosis, orthostatic hypotension, nausea
- On-off phenomenon — motor fluctuations due to pulsatile stimulation and disease progression
- COMT inhibitors (tolcapone, entacapone)
- COMT converts levodopa to 3-O-methyldopa (a partial DA agonist)
- Inhibition of COMT → ↑ levodopa bioavailability and half-life
- Entacapone — peripheral COMT inhibitor, used adjunctively
- Tolcapone — both peripheral and central; hepatotoxic (requires LFT monitoring)
- MAO-B inhibitors (selegiline, rasagiline)
- Selective MAO-B inhibition → ↓ dopamine metabolism in the brain
- Selegiline — initial monotherapy or adjunct to levodopa
- No tyramine interactions at standard doses (MAO-B selective)
- Side effects: dyskinesias, psychosis, insomnia (selegiline metabolized to amphetamine derivatives)
- Dopamine receptor agonists (pramipexole, ropinirole, bromocriptine)
- Direct D2/D3 receptor stimulation — no need for enzymatic conversion
- Pramipexole and ropinirole — non-ergot, first-line in early disease
- Bromocriptine — ergot derivative; used for hyperprolactinemia and acromegaly
- Side effects: dyskinesias, psychosis, impulse control disorders, somnolence
- Levodopa + carbidopa — gold standard
- Cholinergic reduction strategies
- Antimuscarinics (benztropine, trihexyphenidyl)
- Block muscarinic receptors in the striatum → reduce ACh overactivity
- More effective for tremor and rigidity than bradykinesia
- Side effects: atropine-like (dry mouth, blurred vision, urinary retention, confusion)
- Amantadine
- Antiviral agent with dual action: blocks muscarinic receptors and ↑ dopamine release
- Modest benefit for bradykinesia and rigidity
- Side effects: atropine-like effects, livedo reticularis (benign skin discoloration)
- Antimuscarinics (benztropine, trihexyphenidyl)
Exam trap: Carbidopa does not cross the blood-brain barrier — it only prevents peripheral conversion of levodopa. Central conversion still occurs.
High-yield: Levodopa-induced dyskinesias correlate with disease duration and dose — managing "on-off" fluctuations involves adjusting dosing frequency, adding COMT inhibitors, or using DA agonists.
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Antipsychotic Drugs — Schizophrenia & Beyond
- Schizophrenia — symptom domains
- Positive symptoms: delusions, hallucinations, thought disorder, paranoia
- Negative symptoms: amotivation, social withdrawal, flat affect, poverty of speech
- Cognitive symptoms: impaired attention, working memory, executive function
- Dopamine hypothesis
- Excessive dopaminergic activity in mesolimbic pathway → positive symptoms
- Reduced dopaminergic activity in mesocortical pathway → negative and cognitive symptoms
- DA agonists (amphetamines, cocaine) can precipitate or worsen psychosis
- DA antagonists (antipsychotics) reduce positive symptoms
- Serotonin (5-HT) in schizophrenia
- 5-HT2A receptor antagonism is a key feature of atypical antipsychotics
- May improve negative symptoms and reduce EPS risk
- Interplay between DA and 5-HT systems modulates cortical and limbic circuits
- Mechanisms of action
- D2 receptor blockade — primary antipsychotic effect (especially mesolimbic)
- 5-HT2A receptor blockade — improves negative symptoms; reduces EPS (atypicals)
- Additional actions: α1 blockade (hypotension), H1 blockade (sedation), M1 blockade (anticholinergic)
- Clinical uses
- Schizophrenia and schizoaffective disorder
- Bipolar disorder (mania, maintenance)
- Tourette syndrome and Huntington disease chorea
- Chemotherapy-induced emesis (antiemetic doses)
- Agitation and delirium (acute behavioral control)
Clinical pearl: Atypical antipsychotics are preferred for first-line treatment due to lower EPS risk, but each has a unique side-effect profile (metabolic, cardiac, hematologic).
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Antipsychotic Adverse Effects — By Receptor
- Dopamine (D2) blockade — EPS & endocrine
- Acute EPS
- Pseudoparkinsonism — bradykinesia, rigidity, tremor (reversible)
- Dystonia — sustained muscle contractions (tongue, neck, eyes) — acute, treat with anticholinergics
- Akathisia — intense restlessness, pacing (management: β-blockers, benzodiazepines, anticholinergics)
- Treatment: antimuscarinic drugs (benztropine, diphenhydramine) or dose reduction
- Chronic EPS — Tardive dyskinesia (TD)
- Involuntary choreoathetoid movements (tongue, face, limbs)
- Irreversible in many cases; risk ↑ with prolonged use and higher potency
- Management: switch to atypical antipsychotic, discontinue if possible; VMAT2 inhibitors (tetrabenazine) may help
- Endocrine — hyperprolactinemia
- Galactorrhea, amenorrhea, gynecomastia, sexual dysfunction
- More common with typical antipsychotics and risperidone
- Neuroleptic Malignant Syndrome (NMS)
- Life-threatening emergency: fever, muscle rigidity, autonomic instability, altered mental status
- Treatment: dantrolene (muscle relaxant) + bromocriptine (DA agonist); supportive care
- Risk highest with high-potency typical agents (haloperidol, fluphenazine)
- Acute EPS
- Muscarinic (M1) blockade
- Tachycardia, dry mouth, blurred vision, urinary retention, constipation
- Decreased seizure threshold (especially with clozapine)
- Confusion in elderly
- α1-Adrenergic blockade
- Orthostatic hypotension, dizziness, reflex tachycardia
- More prominent with low-potency typicals and some atypicals (clozapine, quetiapine)
- Histamine (H1) blockade
- Sedation, drowsiness, weight gain
- Prominent with clozapine, olanzapine, quetiapine
- Metabolic effects (atypicals)
- Weight gain, dyslipidemia, hyperglycemia/diabetes
- Highest risk: clozapine, olanzapine
- Lower risk: aripiprazole, ziprasidone
Exam trap: Tardive dyskinesia is a late-onset, potentially irreversible EPS — anticholinergic drugs do NOT treat TD and may worsen it.
Critical: NMS is a hyperthermic emergency with muscle rigidity and autonomic instability — stop the antipsychotic immediately and initiate supportive care.
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Comparison Tables
- Antiparkinsonian agents — key features
| Drug Class | Examples | Mechanism | Key Side Effects |
|---|---|---|---|
| DA precursor | Levodopa + carbidopa | AAAD conversion → DA | Dyskinesias, on-off, psychosis, hypotension |
| COMT inhibitor | Entacapone, tolcapone | ↓ levodopa metabolism | Hepatotoxicity (tolcapone), diarrhea |
| MAO-B inhibitor | Selegiline, rasagiline | ↓ DA breakdown | Insomnia, dyskinesias, psychosis |
| DA agonist | Pramipexole, ropinirole | Direct D2/D3 stimulation | Impulse control, somnolence, dyskinesias |
| Antimuscarinic | Benztropine, trihexyphenidyl | M1 blockade | Atropine-like, confusion |
| Other | Amantadine | ↑ DA release + M blockade | Livedo reticularis, atropine-like |
- Antipsychotic agents — comparative profiles
| Agent | EPS Risk | Anticholinergic | Sedation | α-Block | Distinctive Features |
|---|---|---|---|---|---|
| Chlorpromazine | ++ | + | +++ | ++ | Low potency; many side effects |
| Thioridazine | + | ++ | ++ | ++ | Cardiotoxicity (torsades), retinal deposits |
| Fluphenazine | ++++ | + | + | + | High potency; depot available |
| Haloperidol | ++++ | + | + | + | High NMS/TD risk; parenteral/depot |
| Clozapine | + | +++ | +++ | ++ | Agranulocytosis (WBC monitoring), seizures, salivation |
| Olanzapine | + | ++ | +++ | + | Weight gain, metabolic syndrome |
| Risperidone | ++ | + | + | ++ | Hyperprolactinemia; 5-HT2 antagonist |
| Aripiprazole | + | + | + | + | Partial D2 agonist; low metabolic risk |
EPS = extrapyramidal symptoms; NMS = neuroleptic malignant syndrome; TD = tardive dyskinesia
- Key clinical pearls
- Parenteral and depot formulations (fluphenazine, haloperidol) improve adherence in noncompliant patients
- Clozapine is the only agent approved for refractory schizophrenia — requires weekly WBC monitoring
- Aripiprazole is a partial D2 agonist — acts as both an agonist and antagonist depending on DA tone
- Quetiapine and ziprasidone are additional atypical options with distinct side-effect profiles
High-yield: Clozapine carries the lowest EPS risk but requires strict hematologic monitoring due to 1–2% risk of agranulocytosis.
Exam trap: Atypical antipsychotics are NOT free of EPS — they have a lower but still significant risk, especially at higher doses.