NMDA Receptor Antagonists & Clinical Applications

NMDA Receptor Antagonists & Clinical Applications

1. Definition & overview

• NMDA receptor antagonists are drugs that inhibit the N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptor — the principal molecular substrate of slow excitatory neurotransmission, synaptic plasticity (LTP), and glutamate-mediated excitotoxicity in the CNS (R&D 10e Ch.38, pp.1–11).
• The class is therapeutically heterogeneous: only two NMDA antagonists are in mainstream clinical use — ketamine (anaesthesia, analgesia, depression) and memantine (Alzheimer's disease) — with amantadine (Parkinson's/antiviral), nitrous oxide (N₂O, anaesthetic gas), dextromethorphan (antitussive/antidepressant component) and phencyclidine (PCP) as additional channel-blocking members of pharmacological interest (R&D 10e Ch.38, pp.23–26).
• Unifying theme: despite great hope, broad NMDA blockade has been therapeutically disappointing because glutamate is ubiquitous and multifunctional — flooding the brain with an antagonist produces unwanted hallucinatory/dissociative effects alongside benefit. The clinically successful agents are those that block the channel with kinetics permitting physiological signalling to escape (memantine) or that exploit the dissociative state itself (ketamine) (R&D 10e Ch.38, pp.26–27).
• The topic spans three therapeutic contexts united by one receptor: (1) dissociative anaesthesia — ketamine, N₂O; (2) neurodegeneration/excitotoxicity — memantine (AD), amantadine (PD, levodopa-induced dyskinesia); (3) rapid-acting antidepressants — esketamine/ketamine, dextromethorphan-bupropion (G&G 14e Ch.16, p.314).

GROUP A — Receptor biology & molecular pharmacology (the mechanistic foundation)