CB1 receptor positive allosteric modulator unexpectedly antagonized opioid pain relief in morphine-withdrawn rats | Research

The takeaway

A positive allosteric modulator of CB1 receptors (GAT211) antagonized rather than enhanced opioid pain relief in the periaqueductal gray of morphine-withdrawn rats, challenging the expected cannabinoid-opioid synergy.

Pain pharmacologists, opioid researchers, scientists studying cannabinoid-opioid interactions

CB1 positive allosteric modulation antagonized rather than enhanced opioid analgesia

What the researchers found

Intra-PAG DAMGO (opioid agonist) dose-dependently reversed morphine-induced hyperalgesia. GAT211 (CB1 PAM) alone did not affect nociception. When co-administered, GAT211 antagonized DAMGO's pain-relieving effects in morphine-withdrawn rats. Electrophysiology showed GAT211 attenuated DAMGO-induced suppression of synaptic inhibition in vlPAG neurons.

Why it matters

Cannabinoid-opioid combinations are being explored for pain management. This finding that a CB1 positive allosteric modulator can antagonize opioid effects in certain contexts adds important complexity to this therapeutic strategy.

The numbers in context

DAMGO dose-dependently reversed hyperalgesia. GAT211 alone had no effect on nociception. Co-administration: GAT211 antagonized DAMGO in morphine-withdrawn rats. Electrophysiology confirmed GAT211 blocked DAMGO's synaptic effects via CB1R.

How the study worked

Rats chronically treated with morphine or saline received intra-PAG injections of DAMGO (opioid agonist), GAT211 (CB1 PAM), or both. Thermal nociception measured. Slice electrophysiology examined synaptic transmission in the ventrolateral PAG.

What this study cannot tell us

Single brain region examined (PAG); morphine-withdrawn state may not represent all clinical contexts; GAT211 is one specific CB1 PAM and results may not generalize to others.

How to read the evidence

Single animal study with both behavioral and electrophysiological data, but limited to one brain region and one CB1 PAM.

When this study was published

Published in 2020.

The bigger picture

The assumption that enhancing cannabinoid signaling always synergizes with opioids may not hold in all contexts, particularly in opioid-dependent states, with implications for pain management strategies.

Questions still open

Is this antagonism specific to the withdrawal state, or would it occur in opioid-naive contexts? Could different CB1 PAMs have different interactions with opioids?

Common questions

Doesn't combining cannabinoids with opioids usually help pain?

Some evidence suggests cannabinoid-opioid synergy for pain, but this study found the opposite in morphine-withdrawn rats: enhancing CB1 signaling via a positive allosteric modulator actually blocked opioid pain relief. This suggests the interaction may depend on opioid exposure history.

What is a positive allosteric modulator?

A PAM enhances the receptor's response to its natural ligands without directly activating it. For CB1, this means boosting the effects of endocannabinoids rather than adding an external agonist like THC. PAMs were expected to avoid some side effects of direct agonists.

Read the original research

Positive allosteric modulation of the cannabinoid type-1 receptor (CB1R) in periaqueductal gray (PAG) antagonizes anti-nociceptive and cellular effects of a mu-opioid receptor agonist in morphine-withdrawn rats.

Psychopharmacology, 237(12), 3729-3739

View on PubMed ↗Publisher’s paper ↗

Citation

Datta, Udita; Kelley, Leslie K; Middleton, Jason W; Gilpin, Nicholas W. (2020). Positive allosteric modulation of the cannabinoid type-1 receptor (CB1R) in periaqueductal gray (PAG) antagonizes anti-nociceptive and cellular effects of a mu-opioid receptor agonist in morphine-withdrawn rats.. Psychopharmacology, 237(12), 3729-3739. https://doi.org/10.1007/s00213-020-05650-5