Biased signaling by endogenous opioid peptides

Area of Science:

• Neuroscience
• Pharmacology
• Molecular Biology

Background:

• Opioids (morphine, fentanyl) treat severe pain but cause tolerance and opioid use disorder.
• The opioid epidemic necessitates understanding opioid receptor signaling.
• Three opioid receptors (μ, δ, κ) are activated by peptides from three precursors, generating over 20 active peptides.

Purpose of the Study:

• To systematically evaluate the ligand binding and signaling properties of 22 endogenous opioid peptides at the three opioid receptors.
• To investigate the functional selectivity and significance of the diverse opioid peptide repertoire.

Main Methods:

• Systematic evaluation of 22 peptides using [35S]GTPγS binding and β-arrestin recruitment assays.
• Testing ligand binding and signaling at μ, δ, and κ opioid receptors.
• Validation in cultured cells and acute brain slice preparations.

Main Results:

• Nearly all 22 tested peptides activated all three opioid receptors.
• Many peptides demonstrated agonist-directed receptor signaling (functional selectivity).
• Shorter β-endorphin forms exhibited robust signaling, challenging existing dogma.

Conclusions:

• The endogenous opioid system is more complex than previously thought, with broad peptide-receptor interactions.
• Functional selectivity offers potential for developing targeted therapeutics.
• Findings advance understanding of opioid signaling for improved pain and addiction treatments.