Which general anesthetic is also an NMDA receptor antagonist used for anesthesia, analgesia, and depression management?

• A. Propofol
• B. Fentanyl
• C. Ketamine
• D. Isoflurane

Answer: (C)

Ketamine is distinct because it blocks NMDA receptors, a key part of excitatory glutamate signaling, and that action underpins its unique ability to produce anesthesia, analgesia, and rapid antidepressant effects.

For anesthesia, ketamine creates a dissociative state where the patient seems awake but feels detached from the surroundings, with analgesia and amnesia, while often preserving airway reflexes and breathing. This dissociation stems from NMDA receptor blockade in thalamocortical and limbic networks, which disrupts the integration of sensory information.

In terms of analgesia, blocking NMDA receptors dampens central sensitization and wind-up that can amplify pain signals after injury or surgery, helping reduce postoperative pain and opioid requirements.

Regarding depression management, subanesthetic doses of ketamine produce rapid antidepressant effects within hours. This is thought to involve a surge of glutamate that activates AMPA receptors, boosting synaptic plasticity through BDNF and mTOR signaling pathways—an effect not seen with typical antidepressants. Esketamine, a ketamine enantiomer, is used clinically for treatment-resistant depression.

Other options don’t fit because they work through different mechanisms: propofol and isoflurane are not NMDA antagonists, and fentanyl is an opioid analgesic without the NMDA-blocking or rapid antidepressant actions.