Investigating interactions between mu and delta opioid receptors using bifunctional peptides

• Main Author: Purington, Lauren C. S
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2011
• Subjects: Pharmacology
• ISBN: 9781267141361; 1267141360

Morphine and other opioids exert their effects by activation of opioid receptors, which belong to the G protein-coupled receptor (GPCR) superfamily. Clinically used opioids exert their effects through the mu (μ) opioid receptor although two other types of opioid receptors (delta (δ) and kappa (κ)) exist. Opioid receptors couple to Gαi/o proteins and agonists produce inhibition of adenylyl cyclase enzymes and Ca 2+ channels with activation of K+ channels. Chronic activation of μ receptors is known to produce adaptive side effects including tolerance and dependence, limiting the long-term utility of opioids as pain-relieving agents. In this thesis, I examined the ability of the δ antagonist naltrindole to prevent acute antinociceptive tolerance after a single dose of the μ agonist morphine. I demonstrate reduced μ agonist-induced antinociceptive tolerance with δ antagonist administration in vivo. Ex vivo , morphine exposure produced a decrease in high-affinity μ receptors and decreased ability of subsequent μ agonist to stimulate G protein. These effects were reversed with δ antagonist administration. Evidence for interactions between μ and δ receptors in the production of tolerance has furthered interest in novel drugs devoid of tolerance liability. Based on experiments in this thesis and previous reports, compounds displaying μ agonism for appropriate analgesia with δ antagonism to prevent tolerance development are desirable. Therefore, I synthesized bifunctional opioid peptide ligands displaying this mixed efficacy profile with equivalent binding affinities to both μ and δ receptors. Novel opioid peptides were synthesized using both tetra- and pentapeptide scaffolds. Naphthylalanine-substituted pentapeptide ligands identified a novel μ/κ agonist, δ partial agonist/antagonist peptide. Development of tetrapeptide ligands led to the characterization of KSK-103, which demonstrated μ agonist efficacy on par with the clinical standard morphine and δ antagonism at both the level of G protein stimulation and inhibition of adenylyl cyclase. In silico docking of peptides in computational models of the putative 'active' and 'inactive' conformations of the μ and δ receptors revealed steric hindrance in binding the δ 'active' conformation, potentially preventing δ agonist efficacy. These studies highlight the potential of bifunctional ligands displaying μ agonism and δ antagonism in producing analgesia without the limitations of tolerance development.