Nitrous Oxide Mechanism — MFDS Part 1 MCQ
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Correct answer: E — NMDA receptor antagonism and activation of endogenous opioid pathways
Explanation lettering: E = shown as A · C = shown as B · D = shown as C · B = shown as D · A = shown as E
A is correct. Nitrous oxide analgesia is attributed to attenuation of glutamatergic (NMDA-mediated) nociceptive transmission together with release of endogenous opioid peptides, which recruits descending inhibitory pathways. B is the classic trap: GABA-A potentiation is linked chiefly to the anxiolytic and anaesthetic components of nitrous oxide, not the analgesic effect, and describes benzodiazepine-type action. C confuses downstream spinal noradrenergic mediation of opioid-triggered descending inhibition with direct alpha-2 agonism, which is the mechanism of clonidine/dexmedetomidine, not nitrous oxide. D (antipsychotics/antiemetics) and E (SSRIs) belong to unrelated drug classes and do not explain rapid, titratable analgesia. The mechanism remains incompletely elucidated, so these pathways should be regarded as predominant rather than exclusive.
Reference: Fernández-Guisasola/Buchanan-type mechanistic review: 'A new mechanistic approach for the treatment of chronic neuropathic pain with nitrous oxide integrated from a systems biology narrative review' (2021) — NMDA/glutamatergic attenuation plus endogenous opioid release: https://pubmed.ncbi.nlm.nih.gov/33642336/ (UK clinical context: SDCEP Conscious Sedation in Dentistry, 3rd edn 2017, reviewed 2022, https://www.sdcep.org.uk/published-guidance/conscious-sedation/)