Morphine-like opiates selectively antagonize receptor-arrestin interactions

Area of Science:

• Pharmacology
• Molecular Biology
• Neuroscience

Background:

• Opioid addiction is a major public health concern.
• The addictive potential of opioids is linked to their interaction with mu (μ) and delta (δ) opioid receptors.
• Opioid receptors signal through G proteins and recruit beta-arrestin proteins, influencing cellular responses.

Purpose of the Study:

• To compare the efficacy of various opioid ligands in activating G protein signaling versus beta-arrestin 2 recruitment.
• To investigate the structural basis for differential signaling of opioid receptors.
• To understand how signaling bias relates to the addictive potential of opioids.

Main Methods:

• Bioluminescence resonance energy transfer (BRET) assays were used to measure receptor-G protein and receptor-beta-arrestin 2 interactions in intact cells.
• Cell-free BRET assays with isolated membranes assessed G protein coupling independently of arrestin.
• Pertussis toxin and guanine nucleotides were used to confirm G alpha-mediated signaling.

Main Results:

• Significant differences in efficacy were observed between G protein activation and beta-arrestin 2 recruitment across 20 opioid ligands.
• Opioid ligands exhibited varying structural requirements for arrestin efficacy compared to G protein efficacy.
• Addictive opioids, such as morphine and oxymorphone, showed a marked discrepancy, effectively activating G protein coupling while acting as antagonists or partial agonists for beta-arrestin 2 at delta and mu receptors, respectively.

Conclusions:

• Opioid ligands display distinct efficacies for G protein and beta-arrestin 2 pathways.
• Arrestin recruitment has more stringent structural requirements than G protein coupling.
• The observed signaling bias, particularly the arrestin-selective antagonism by addictive opioids, may inhibit receptor internalization and down-regulation, contributing to their addictive potential.