µ-, δ-, κ-Opioid-Receptor
Springer Online Journal Archives 1860-2000
Abstract We investigated the nature of interaction of morphine-3-O-β-D-glucuronide (M3G) and morphine6-O-β-D-glucuronide (M6G) with opioid binding sites at the µ-, δ- and κ-opioid receptors (µ-OR, δ-OR and κ-OR) in cerebral membranes. Saturation binding experiments revealed a competitive interaction of M6G with all three opioid receptors. Inhibition binding experiments at the µ-OR employing combinations of morphine and M6G resulted in a rightward shift of the IC50 for morphine proportional to the M6G concentration, thus strengthening the finding of competitive interaction of M6G at the µ-opioid binding site. Data in absence and presence of M6G were included in a three-dimensional model. Compared to a model with one binding site a model with two binding sites significantly improved the fits. This might indicate that different µ-OR subtypes are involved. Hydrolysis of M6G to morphine was investigated and did not occur. Therefore the effects of M6G on binding to the μ-OR were due to M6G and not due to morphine. In contrast, M3G at the three opioid receptors was found to inhibit binding being about 300 times weaker than morphine. This effect was well explained by the amount of contaminating morphine (about 0.3%) identified by HPLC. We conclude that M6G binds to µ-, δ- and κ-OR in a competitive manner. Some of our results on the µ-OR suggest two binding sites for agonists at the μ-OR and that M6G binds to both sites. Our results suggest that the high potency of M6G as an analgesic is mediated through opioid receptors. In contrast, M3G does not interact with the µ-, δ- or κ-OR. We therefore doubt that any effect of M3G is mediated via opioid receptors.
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