Role of Protein Kinase C (PKC) in Agonist‐Induced μ‐Opioid Receptor Down‐Regulation

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Abstract
: Agonist‐induced down‐regulation of opioid receptors appears to require the phosphorylation of the receptor protein. However, the identities of the specific protein kinases that perform this task remain uncertain. Protein kinase C (PKC) has been shown to catalyze the phosphorylation of several G protein‐coupled receptors and potentiate their desensitization toward agonists. However, it is unknown whether opioid receptor agonists induce PKC activation under physiological conditions. Using cultured SH‐SY5Y neuroblastoma cells, which naturally express μ‐ and δ‐opioid receptors, we investigated whether μ‐opioid receptor agonists can activate PKC by measuring enzyme translocation to the membrane fraction. PKC translocation and opioid receptor densities were simultaneously measured by 3H‐phorbol ester and [3H]diprenorphine binding, respectively, to correlate alterations in PKC localization with changes in receptor binding sites. We observed that μ‐opioid agonists have a dual effect on membrane PKC density depending on the period of drug exposure. Exposure for 2‐6 h to [d‐Ala2, N‐Me‐Phe4, Gly‐ol]enkephalin or morphine promotes the translocation of PKC from the cytosol to the plasma membrane. Longer periods of opioid exposure (>12 h) produce a decrease in membrane‐bound PKC density to a level well below basal. A significant decrease in [3H]diprenorphine binding sites is first observed at 2 h and continues to decline through the last time point measured (48 h). The opioid receptor antagonist naloxone attenuated both opioid‐mediated PKC translocation and receptor down‐regulation. These results demonstrate that opioids are capable of activating PKC, as evidenced by enhanced translocation of the enzyme to the cell membrane, and this finding suggests that PKC may have a physiological role in opioid receptor plasticity.