A 26-year-old man is brought to the hospital by his wife who complains that her husband has been behaving oddly for the past few hours. The patient’s wife says that she has known him for only 4 months. The wife is unable to give any past medical history. The patient’s speech is difficult to follow, and he seems very distracted. After 15 minutes, he becomes agitated and starts to bang his head on a nearby pillar. He is admitted to the psychiatric ward and is given an emergency medication, after which he calms down. In the next 2 days, he continues to become agitated at times and required 2 more doses of the same drug. On the 4th day of admission, he appears very weak, confused, and does not respond to questions appropriately. His vital signs include: temperature 40.0°C (104.0°F), blood pressure 160/95 mm Hg, and pulse 114/min. On physical examination, he is profusely diaphoretic. He is unable to stand upright or even get up from his bed. Which of the following is the mechanism of action of the drug which most likely caused this patient’s current condition?

Agonistic effect on dopamine receptors

A 44-year-old man presents to his psychiatrist for a follow-up appointment. He is currently being treated for schizophrenia. He states that he is doing well but has experienced some odd movement of his face recently. The patient's sister is with him and states that he has been more reclusive lately and holding what seems to be conversations despite nobody being in his room with him. She has not noticed improvement in his symptoms despite changes in his medications that the psychiatrist has made at the last 3 appointments. His temperature is 99.3°F (37.4°C), blood pressure is 157/88 mmHg, pulse is 90/min, respirations are 14/min, and oxygen saturation is 98% on room air. Physical exam is notable for rhythmic movements of the patient's mouth and tongue. Which of the following is a side effect of the next best step in management?

Worsening of psychotic symptoms

An investigator is studying a drug that acts on the thyroid hormone pathway. Levels of serum free T3 and T4 in healthy participants are measured before and after administration of the drug. After administration, there is a decrease in the average serum free T3 level, while the average serum free T4 level is increased compared to initial serum studies. Inhibition of which of the following is the most likely mechanism of action of this drug?

Follicular iodotyrosine deiodinase

An 18-year-old boy presents to the clinic with shortness of breath and fever for the last 2 days. He also has a cough for the same duration. He is asthmatic and uses inhaled albuterol for symptom relief when required. He used albuterol today 3 times at 10-minute intervals but has not had relief of his symptoms. On physical examination, his temperature is 38.3°C (101.0°F), pulse is 130/min, blood pressure is 116/80 mm Hg, and respirations are 28/min. Auscultation of the chest reveals bilateral crackles. Considering that he has already taken inhaled albuterol and has tachycardia, the physician nebulizes him with inhaled ipratropium bromide, which significantly improves his symptoms. Which of the following is the mechanism of action of this drug?

Inhibition of vagally-mediated contraction of bronchial smooth muscles

Inhibition of degranulation of mast cells

Inhibition of phosphodiesterase-4, leading to prevention of release of cytokines and chemokines

Inhibition of adenosine receptors in the respiratory tract

Stimulation of β2-adrenergic receptors in bronchial smooth muscle

Antipsychotic mechanisms of action — USMLE Lesson

Typical Antipsychotics - Classic Dopamine Blockers

Primary Mechanism: Block dopamine D₂ receptors in the central nervous system.
Effective against positive symptoms (hallucinations, delusions).
Potency is directly related to D₂ receptor affinity.

Adverse Effects by Pathway:
- Nigrostriatal Blockade: Causes Extrapyramidal Symptoms (EPS) like dystonia, akathisia, and parkinsonism.
- Tuberoinfundibular Blockade: Disinhibits prolactin release → ↑ Prolactin levels, causing gynecomastia and galactorrhea.

Four major dopamine pathways in the human brain

⭐ High-potency agents (e.g., Haloperidol) carry a greater risk of EPS, while low-potency agents (e.g., Chlorpromazine) cause more antihistaminic (sedation), anti-α-adrenergic (orthostasis), and anticholinergic effects.

📌 Mnemonic (High Potency): "Highly Try to Fly" → Haloperidol, Trifluoperazine, Fluphenazine.

Atypical Antipsychotics - Serotonin's Sidekicks

Dual Action: Primarily block both Dopamine $D_2$ and Serotonin $5HT_{2A}$ receptors.
- Serotonin's Role: $5HT_{2A}$ blockade is key. It ↑ dopamine release in the nigrostriatal pathway, counteracting the motor side effects of $D_2$ blockade.
- Benefit: This dual action leads to a lower risk of Extrapyramidal Symptoms (EPS) compared to typical antipsychotics.
Variable Receptor Profiles: Many atypicals also have effects at other receptors, contributing to their side-effect profiles:
- H1-antagonism: → Sedation, weight gain
- α1-antagonism: → Orthostatic hypotension

Antipsychotic drugs and dopamine pathways of the brain

⭐ Clozapine stands out for its minimal $D_2$ blockade and high affinity for $D_4$ receptors, making it highly effective for treatment-resistant schizophrenia but with a risk of agranulocytosis.

📌 Mnemonic: For common atypicals: "All Really Quiet Only Zebras Can Pass" (Aripiprazole, Risperidone, Quetiapine, Olanzapine, Ziprasidone, Clozapine, Paliperidone).

Adverse Effect Pathways - Receptor Roulette

Dopamine (D₂ Receptor) Blockade
- Nigrostriatal Pathway: Leads to Extrapyramidal Symptoms (EPS).
  - Acute Dystonia: Sudden, sustained muscle contractions.
  - Akathisia: Subjective feeling of restlessness.
  - Parkinsonism: Bradykinesia, rigidity, tremor.
  - Tardive Dyskinesia (TD): Involuntary, repetitive body movements (chronic use).
- Tuberoinfundibular Pathway: ↑ Prolactin levels.
  - Causes gynecomastia, galactorrhea, and amenorrhea.
Histamine (H₁ Receptor) Blockade
- Results in sedation and ↑ appetite leading to weight gain.
Muscarinic (M₁ Receptor) Blockade
- Anticholinergic effects: Dry mouth, blurred vision, constipation, urinary retention.
- 📌 Mnemonic: "Dry as a bone, blind as a bat, red as a beet, hot as a hare."
Alpha-adrenergic (α₁ Receptor) Blockade
- Causes orthostatic hypotension and dizziness.

⭐ Neuroleptic Malignant Syndrome (NMS): A rare but life-threatening emergency from D₂ blockade. Key features are Fever, Encephalopathy, Vitals instability, Elevated enzymes (CK), and Rigidity (“lead-pipe”).

High‑Yield Points - ⚡ Biggest Takeaways

Typical antipsychotics primarily block D2 receptors in the mesolimbic pathway to treat positive symptoms.

Atypical antipsychotics combine D2 antagonism with serotonin 5-HT2A receptor blockade, which mitigates the risk of Extrapyramidal Symptoms (EPS).

D2 blockade in the nigrostriatal pathway is responsible for EPS.

D2 blockade in the tuberoinfundibular pathway causes hyperprolactinemia.

Newer agents like aripiprazole exhibit D2 partial agonism, stabilizing dopamine output.

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