Which statement best describes NMDA receptor involvement for methadone versus morphine?

• A. Morphine blocks NMDA receptors; methadone does not.
• B. Neither drug affects NMDA receptors.
• C. Only high-dose methadone affects NMDA receptors.
• D. Methadone blocks NMDA receptors contributing to analgesia and neuropathic pain relief; morphine does not.

Answer: (D)

NMDA receptor antagonism can augment pain relief by blocking central sensitization, which is especially important in neuropathic pain. Methadone acts as both a mu-opioid receptor agonist and an NMDA receptor antagonist, so it can reduce pain not only through mu-opioid pathways but also by dampening NMDA receptor–mediated excitation. This dual action contributes to analgesia and can help neuropathic pain relief, as well as influence opioid tolerance and hyperalgesia. Morphine, in contrast, mostly targets mu-opioid receptors and does not block NMDA receptors, so it lacks that additional mechanism. Therefore, methadone’s NMDA receptor blockade contributing to analgesia and neuropathic pain relief, with morphine not having this effect, best describes their involvement.