The molecular mechanisms of cellular tolerance to δ-opioid agonists

Varga, Eva V. (2003) The molecular mechanisms of cellular tolerance to δ-opioid agonists. Acta Biologica Hungarica, 54 (2). pp. 203-218. ISSN 0236-5383

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Abstract

Chronic treatment with δ-opioid agonists, similar to other agonist drugs, causes tolerance. Tolerance is a complex adaptation process that consists of multiple, cellular and neural-system adaptations. Cellular tolerance to δ-opioid agonists involves feedback-regulation of the function, concentration, and localization of the d-opioid receptors (receptor desensitization) as well as of intracellular effectors (functional desensitization). We are using a recombinant Chinese hamster ovary cell line expressing the human d-opioid receptors (hDOR/CHO) to investigate the molecular mechanisms of cellular tolerance We found that the structurally distinct δ-opioid agonists mediate receptor down-regulation by different mechanisms. Thus, truncation of the last 35 C-terminal amino acids of the hDOR completely abolished DPDPE, but not SNC 80-mediated receptor down-regulation. In addition, down-regulation of the wild type-, and the truncated hDORs exhibited different inhibitor sensitivity-profile. Chronic δ-opioid agonist treatment also causes functional desensitization of forskolin-stimulated cAMP formation and cAMP overshoot in the hDOR/CHO cells. We have demonstrated that chronic SNC 80 treatment also causes concurrent phosphorylation of the adenylyl cyclase (AC) VI isoenzyme hDOR/CHO cells. Both AC superactivation and AC VI phosphorylation were SNC 80 dose-dependent, naltrindole-sensitive, and exhibited similar time course-, and protein kinase inhibitor-sensitivity profile. We hypothesize that phosphorylation of AC VI plays an important role in δ-opioid agonist-mediated AC superactivation in hDOR/CHO cells.

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