Opioids are one of the most efficacious analgesics used in human. Prolonged administration of opioids, however, often causes the development of drug tolerance, thus limiting their effectiveness. To elucidate the biophysical basis on the mechanisms that may contribute to opioid tolerance, we have constructed a stably-transfected human embryonic kidney cell line, HEK 293 with mouse δ-opioid receptor gene. Using perforated whole-cell voltage clamp method, the voltage-dependent outwardly rectifying K+ currents are identified from these transformed HEK 293 cells. Sustained stimulation of these cells with [DPen², DPen5]
enkephalin, a selective agonist to δ-opioid receptor, decreased the conductance of these voltage-dependent outwardly rectifying K+ currents. As protein Kinase C (PKC) is supposed to involve in these δ-opioid receptor desensitization, effect of PMA, a protein Kinase C activator, on the receptor desensitization was examined. PMA application to desensitized HEK 293 cells shows apparently the potentiation of the δ-opioid receptor desensitization, that is, a further decrease in desensitized K+ currents. These results suggest that δ-opioid receptor desensitization may involve a PKC mediating phosphorylation-dependent mechanisms.
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