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The third extracellular loop of the mu opioid receptor is important for agonist selectivity
1Department of Pharmacology, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA.
Abstract:
To investigate the interaction between the mu opioid receptor and its ligands, we compared the binding of mu-selective ligands to two mu/kappa chimeric opioid receptors and to mu and kappa receptors. The two chimeras were constructed from cloned rat mu and kappa receptors in which a segment from the middle of the third intracellular loop to the C terminus was exchanged. When this portion of the kappa receptor was replaced by that of the mu receptor, affinities of mu selective agonists, DAMGO (Tyr-D-Ala-Gly-NMePhe-Gly-ol), PL017 (Tyr-Pro-NMePhe-D-Pro-NH2), sufentanil, and morphine, were greatly increased as compared to those for the kappa receptor. Conversely, when this region of the mu receptor was substituted by that of the kappa receptor, affinities for these agonists were substantially decreased as compared with those of the mu receptor. Unlike selective agonists, the mu-selective antagonist, CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-penicillamine-Thr-NH2), displayed a low affinity for both chimeric receptors, similar to that of the kappa receptor. Thus, the region from the middle of the third intracellular loop to the C terminus of the mu receptor is important for the binding of selective agonists. Conversely, the determinants for selective binding of the antagonist CTAP reside in a more extended region of the receptor.
Insights
The mu opioid receptor
Area of Science:
- Pharmacology
- Molecular Biology
- Neuroscience
Background:
- Opioid receptors are critical targets for pain management.
- Understanding ligand-receptor interactions is key to developing selective therapeutics.
- Mu and kappa opioid receptors share structural similarities but differ in ligand binding.
Purpose of the Study:
- To elucidate the specific regions of the mu opioid receptor responsible for selective ligand binding.
- To investigate the role of the third intracellular loop and C-terminus in mu-opioid receptor interactions.
Main Methods:
- Construction and characterization of mu/kappa chimeric opioid receptors.
- Comparative binding affinity assays using mu-selective agonists and antagonists.
- Utilizing cloned rat mu and kappa receptors for chimera development.
Main Results:
- The region from the middle of the third intracellular loop to the C-terminus of the mu opioid receptor significantly enhances agonist binding affinity.
- Substitution of this region with the corresponding kappa receptor sequence decreases agonist affinity.
- Mu-selective antagonists, like CTAP, show low affinity for chimeras, indicating their binding determinants are more widespread.
Conclusions:
- The C-terminal portion including the third intracellular loop is crucial for mu-opioid receptor agonist recognition.
- Antagonist binding sites involve a broader receptor region than previously thought.
- These findings provide insights into opioid receptor pharmacology and drug design.