Big Dynorphin Causes Pain Through NMDA Receptors, Not Opioid Receptors | Research

The takeaway

Big dynorphin (a large opioid peptide precursor) caused pain and neurological symptoms when injected spinally, working through NMDA glutamate receptors rather than opioid receptors — an opioid peptide causing pain through a non-opioid mechanism.

Opioid causes pain
Big dynorphin caused pain through NMDA glutamate receptors, NOT opioid receptors — explaining the paradox of elevated dynorphin in painful conditions

What the researchers found

Intrathecal big dynorphin at femtomole doses caused nociceptive behavior through NMDA receptor activation (blocked by MK-801) but not through opioid receptors (not blocked by naloxone), with higher doses causing reversible paralysis.

Why it matters

In chronic pain conditions where dynorphin is elevated, big dynorphin may paradoxically CAUSE pain through NMDA receptors — explaining the pain-promoting role of dynorphin in some chronic pain states.

How the study worked

Animal study in mice with intrathecal injection of big dynorphin at graded doses. Pain behavior and paralysis assessed. MK-801 (NMDA antagonist) and naloxone (opioid antagonist) used to dissect mechanisms.

What this study cannot tell us

Mouse study with intrathecal injection of a precursor peptide at extremely low doses. The physiological relevance of big dynorphin at these spinal concentrations is uncertain.

How to read the evidence

Preliminary animal evidence with clear pharmacological dissection (MK-801 blocks, naloxone doesn't) identifying an unexpected non-opioid mechanism.

When this study was published

Published in 2002. Big dynorphin's pronociceptive role through NMDA receptors has been confirmed and is now a recognized mechanism in chronic pain pathophysiology.

The bigger picture

The same opioid system that normally controls pain can paradoxically promote it. Big dynorphin's NMDA-mediated pain causation helps explain why some chronic pain patients have elevated dynorphin yet still experience pain.

Questions still open

Does elevated spinal big dynorphin contribute to chronic pain syndromes?
Could NMDA antagonists targeted at dynorphin's site of action relieve specific chronic pain types?
Is big dynorphin processing disrupted in chronic pain?

Common questions

Can the body's own opioids cause pain?

Yes. Big dynorphin, a large opioid peptide, caused pain when it accumulated in the spinal cord — but NOT through opioid receptors. It activated NMDA glutamate receptors, which cause excitotoxic pain.

Why does this matter for chronic pain?

In chronic pain, dynorphin levels rise in the spinal cord. Instead of helping with pain (through opioid receptors), big dynorphin may actually be making pain worse through NMDA receptors. This explains why opioid drugs don't help some chronic pain patients.

Read the original research

Intrathecally administered big dynorphin, a prodynorphin-derived peptide, produces nociceptive behavior through an N-methyl-D-aspartate receptor mechanism.

Brain research, 952(1), 7-14

View on PubMed ↗

Citation

Tan-No, Koichi; Esashi, Akihisa; Nakagawasai, Osamu; Niijima, Fukie; Tadano, Takeshi; Sakurada, Chikai; Sakurada, Tsukasa; Bakalkin, Georgy; Terenius, Lars; Kisara, Kensuke. (2002). Intrathecally administered big dynorphin, a prodynorphin-derived peptide, produces nociceptive behavior through an N-methyl-D-aspartate receptor mechanism.. Brain research, 952(1), 7-14.