A 17-year-old male presents with altered mental status. He was recently admitted to the hospital due to a tibial fracture suffered while playing soccer. His nurse states that he is difficult to arouse. His temperature is 98.6 deg F (37 deg C), blood pressure is 130/80 mm Hg, pulse is 60/min, and respirations are 6/min. Exam is notable for pinpoint pupils and significant lethargy. Which of the following describes the mechanism of action of the drug likely causing this patient's altered mental status?

Neuronal hyperpolarization due to potassium efflux

Neuronal hyperpolarization due to sodium influx

Neuronal depolarization due to sodium efflux

Neuronal depolarization due to potassium influx

Neuronal hyperpolarization due to chloride influx

A 21-year-old man with a recent history of traumatic right femur fracture status post open reduction and internal fixation presents for follow-up. The patient says his pain is controlled with the oxycodone but he says he has been severely constipated the past 4 days. No other past medical history. Current medications are oxycodone and ibuprofen. The patient is afebrile and vital signs are within normal limits. On physical examination, surgical incision is healing well. Which of the following is correct regarding the likely role of opiates in this patient’s constipation?

Opiates increase fluid absorption from the lumen leading to hard stools

Opiates decrease the sympathetic activity of the gut wall

Opiates increase the production and secretion of pancreatic digestive enzymes

Opiates cause rapid gastrointestinal transit

Opiates activate the excitatory neural pathways in the gut

A 40-year-old woman with a recent history of carcinoma of the breast status post mastectomy and adjuvant chemotherapy one week ago presents for follow-up. She reports adequate pain control managed with the analgesic drug she was prescribed. Past medical history is significant for hepatitis C and major depressive disorder. The patient denies any history of smoking or alcohol use but says she is currently using intravenous heroin and has been for the past 10 years. However, she reports that she has been using much less heroin since she started taking the pain medication, which is confirmed by the toxicology screen. Which of the following is the primary mechanism of action of the analgesic drug she was most likely prescribed?

Pure agonist at the µ-opioid receptor

Pure antagonist at opioid receptors

Inhibits prostaglandin synthesis

Mixed agonist-antagonist at opioid receptors

Central action via blockade of serotonin reuptake

Opioid analgesics — USMLE Lesson

Opioid MOA & Receptors - The Brain's Bliss Buttons

Opioid receptor locations in CNS pain pathways

Receptors: Opioids are agonists at three main G-protein coupled receptors (GPCRs): Mu (μ), Delta (δ), and Kappa (κ).
Cellular MOA: They act on both presynaptic and postsynaptic neurons.
- ↓ Presynaptic Ca²⁺ influx → ↓ release of neurotransmitters (e.g., Substance P, glutamate).
- ↑ Postsynaptic K⁺ efflux → hyperpolarization → ↓ neuronal signaling.

Key Receptor Functions:
- μ (Mu): Analgesia, euphoria, respiratory depression, miosis, constipation. 📌 Mu = Most effects.
- κ (Kappa): Spinal analgesia, sedation, miosis.

⭐ Opioids primarily exert their powerful analgesic effects by acting on μ-receptors in the spinal cord (substantia gelatinosa) and supraspinal sites (e.g., periaqueductal gray).

Opioid Classification - The Agonist & Antagonist Cast

Class	Mechanism & Key Drugs	Clinical Notes
Full Agonists	Strong μ-receptor agonists providing maximal analgesia.	Morphine, fentanyl, methadone, meperidine. Used for severe pain; high abuse potential.
Partial Agonists	Mixed agonist-antagonist activity; act as agonists at some receptors and antagonists at others.	Buprenorphine (μ-partial agonist, κ-antagonist), nalbuphine, butorphanol. Can precipitate withdrawal in opioid-dependent patients.
Antagonists	Competitive antagonists at all opioid receptors; displace agonists to reverse effects.	Naloxone (short-acting for acute overdose), naltrexone (long-acting for relapse prevention).

Uses & Side Effects - Pain Relief and Its Perils

Opioid Analgesics: Side Effects on Major Organ Systems

Primary Uses
- Analgesia: Moderate to severe pain.
- Cough suppression (dextromethorphan, codeine).
- Diarrhea treatment (loperamide, diphenoxylate).
- Anesthesia adjunct (fentanyl).
Adverse Effects
- CNS: Sedation, euphoria, miosis (pinpoint pupils).
- Respiratory: ↓ Respiratory rate & depth.
- GI: Nausea, vomiting, constipation (no tolerance).
- CV: Hypotension, bradycardia.
- GU: Urinary retention.
- Skin: Pruritus (histamine release).
- Tolerance and physical dependence with long-term use.

⭐ Opioid Overdose Triad: Coma, respiratory depression, and pinpoint pupils (miosis). Treat with naloxone.

Overdose & Withdrawal - When The Bliss Ends

Overdose Triad: Coma, respiratory depression, miosis (pinpoint pupils).
Management: Naloxone (opioid antagonist). Caution: short half-life may require repeat doses.

⭐ Naloxone can precipitate severe, acute withdrawal in opioid-dependent individuals.

Withdrawal Symptoms: Anxiety, lacrimation, rhinorrhea, yawning, sweating, piloerection ("cold turkey"), mydriasis.
Treatment: Supportive. Clonidine (↓ autonomic hyperactivity). For long-term detox: methadone (long-acting agonist) or buprenorphine (partial agonist).

High‑Yield Points - ⚡ Biggest Takeaways

Opioids bind to μ, δ, and κ receptors, causing analgesia, euphoria, and sedation.

The classic triad of opioid toxicity is coma, pinpoint pupils (miosis), and respiratory depression.

Naloxone is a competitive antagonist used to reverse opioid overdose.

Tolerance develops to most effects, but not to miosis or constipation.

Withdrawal is managed with long-acting agents like methadone or buprenorphine.

Co-administration with other CNS depressants (e.g., benzodiazepines) risks fatal respiratory depression.

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