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Received for publication March 28, 2005.
Revised July 14, 2005.
Accepted for publication July 14, 2005.
Morphine has been widely accepted as the opioid agonist that sustains signaling because it does not cause receptor desensitization or internalization. This notion has led to the hypothesis that chronic morphine treatment initiates downstream adaptations that underlie tolerance and dependence. This study uses whole cell recordings from neurons in the locus coeruleus (LC) to measure the potassium current induced by morphine. The results show that morphine does cause acute desensitization. The desensitization induced by morphine was slower and smaller then that induced by [MET]5enkephalin (ME). After a brief application of a saturating concentration of ME, the current induced by morphine was smaller and desensitization was not observed. In tissue taken from morphine treated animals, the peak current induced by morphine was the same as in untreated animals but morphine-induced desensitization was facilitated. The results suggest that morphine, like other agonists, can initiate receptor desensitization to decrease signaling.
Key words:
Opioid, Gi family, Desensitization/uncoupling, GRKs, barrestins, Receptor degradation, Recycling, Drug tolerance/dependence, Opioids
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