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Different Effects of Opioid Antagonists on μ-, δ-, and κ-Opioid Receptors with and without Agonist Pretreatment

Opioid receptors display basal signaling (constitutive, agonist-independent activity), which seems to be regulated by agonist exposure. Whereas agonist pretreatment desensitizes receptors to subsequent agonist stimulation, basal signaling of μ-opioid receptor (MOR) was shown to increase. Moreover, agonist pretreatment converts the neutral antagonists naloxone and naltrexone into inverse agonists, suppressing basal signaling, whereas analogs with reduced C6-position, e.g., 6β-naltrexol, remain neutral antagonists at MOR under any condition. This study compares the regulation of basal signaling of MOR, δ-(DOR), and κ-(KOR) opioid receptors after pretreatment with morphine or receptor-selective agonists, in transfected human embryonic kidney 293 cell membranes. Moreover, naloxone, naltrexone, and related antagonists were compared for binding potency and effect on basal and agonist-stimulated receptor signaling, measuring guanosine 5′-O-(3-[35S]thio)triphosphate binding. The results demonstrate basal activity for each opioid receptor, which is modulated by pretreatment with agonists. Even closely related opioid antagonists display distinct patterns of neutral and inverse effects before and after agonist pretreatment, including distinct efficacies between naloxone and naltrexone at agonist-pretreated DOR and KOR. Pretreatment with different agonists has varying effects on inverse and neutral activities of some analogs tested. These results demonstrate that antagonist efficacy is context-dependent, possibly accounting for paradoxical pharmacological effects. Activity profiles at the three opioid receptors under different conditions could lead to antagonists with optimal clinical properties in treatment of addiction and adverse opioid effects.