Naloxone mechanism — MRCPsych Paper A MCQ

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HardNeurophysiology and neurochemistryNaloxone mechanismMRCPsych Paper A

A 41-year-old man with opioid dependence is found with pinpoint pupils, respiratory depression and reduced consciousness after using heroin. Intravenous naloxone restores ventilation within 2 minutes but precipitates yawning, sweating, piloerection and agitation. Which receptor-level description of naloxone is most accurate?

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Reveal the answer and explanation

Correct answer: E — Reversible competitive antagonism at opioid receptors

Explanation lettering: B = shown as A · E = shown as B · A = shown as D · D = shown as E

D is correct. Naloxone is a high-affinity, competitive opioid-receptor antagonist. It displaces heroin-derived opioid agonists from receptor sites without clinically significant intrinsic agonist activity, rapidly reversing respiratory and central nervous system depression. Displacement of agonists also explains the abrupt withdrawal syndrome in an opioid-dependent patient. Its antagonism is not irreversible, excluding A. It is not principally a negative allosteric modulator, excluding B. Partial μ-opioid agonism describes drugs such as buprenorphine rather than naloxone, excluding C. Although ligand behaviour can vary between experimental systems, selective μ-receptor inverse agonism is not the accepted clinical or SmPC description of naloxone’s mechanism, excluding E.

Reference: Electronic Medicines Compendium. Naloxone Hydrochloride 400 micrograms/ml Solution for Injection/Infusion, Summary of Product Characteristics, section 5.1 Pharmacodynamic properties. Revised 2023. https://www.medicines.org.uk/emc/product/6589/smpc