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periaqueductal gray (PAG) antagonizes anti-nociceptive and cellular effects of a mu-opioid receptor agonist in morphine-withdrawn rats.

Abstract

Opioid drugs are a first-line treatment for severe acute pain and other chronic pain conditions, but long-term opioid drug use produces opioid-induced hyperalgesia (OIH). Co-administration of cannabinoids with opioid receptor agonists produce anti-nociceptive synergy, but cannabinoid receptor agonists may also produce undesirable side effects. Therefore, positive allosteric modulators (PAM) of cannabinoid type-1 receptors (CB1R) may provide an option reducing pain and/or enhancing the anti-hyperalgesic effects of opioids without the side effects, tolerance, and dependence observed with the use of ligands that target the orthosteric binding sites. This study tested GAT211, a PAM of cannabinoid type-1 receptors (CB1R), for its ability to enhance the anti-hyperalgesic effects of the mu-opioid receptor (MOR) agonist DAMGO in rats treated chronically with morphine (or saline) and tested during withdrawal. We tested the effects of intra-periaqueductal gray (PAG) injections of (1) DAMGO, (2) GAT211, or (3) DAMGO + GAT211 on thermal nociception in chronic morphine-treated rats that were hyperalgesic and also in saline-treated control rats. We used slice electrophysiology to test the effects of DAMGO/GAT211 bath application on synaptic transmission in the vlPAG. Intra-PAG DAMGO infusions dose-dependently reversed chronic morphine-induced hyperalgesia, but intra-PAG GAT211 did not alter nociception at the doses we tested. When co-administered into the PAG, GAT211 antagonized the anti-nociceptive effects of DAMGO in morphine-withdrawn rats. DAMGO suppressed synaptic inhibition in the vlPAG of brain slices taken from saline- and morphine-treated rats, and GAT211 attenuated DAMGO-induced suppression of synaptic inhibition in vlPAG neurons via actions at CB1R. These findings show that positive allosteric modulation of CB1R antagonizes the behavioral and cellular effects of a MOR agonist in the PAG of rats.

• Authors: Udita Datta, Leslie K Kelley, Jason W Middleton, Nicholas W Gilpin
• Journal: Psychopharmacology (Psychopharmacology (Berl)) Vol. 237 Issue 12 Pg. 3729-3739 (Dec 2020) ISSN: 1432-2072 [Electronic] Germany
• PMID: 32857187 (Publication Type: Journal Article)

Chemical References

• Analgesics, Opioid
• Cannabinoid Receptor Agonists
• Receptor, Cannabinoid, CB1
• Receptors, Opioid, mu
• Enkephalin, Ala(2)-MePhe(4)-Gly(5)-
• Morphine

Topics

• Allosteric Regulation (drug effects, physiology)
• Analgesics, Opioid (administration & dosage, adverse effects)
• Animals
• Cannabinoid Receptor Agonists (administration & dosage)
• Enkephalin, Ala(2)-MePhe(4)-Gly(5)- (administration & dosage)
• Hyperalgesia (chemically induced, drug therapy, metabolism)
• Male
• Microinjections (methods)
• Morphine (administration & dosage, adverse effects)
• Nociception (drug effects, physiology)
• Periaqueductal Gray (drug effects, metabolism)
• Rats
• Rats, Wistar
• Receptor, Cannabinoid, CB1 (agonists, antagonists & inhibitors, physiology)
• Receptors, Opioid, mu (agonists, metabolism)
• Substance Withdrawal Syndrome (drug therapy, physiopathology)