Morphine-6-O-?-D-glucuronide but not morphine-3-O-?-D-glucuronide binds to ?-, ?- and ?- specific opioid binding sites in cerebral membranes

Abstract

We investigated the nature of interaction of morphine-3-O-beta-D-glucuronide (M3G) and morphine-6-O-beta-D-glucuronide (M6G) with opioid binding sites at the mu-, delta- and kappa-opioid receptors (mu-OR, delta-OR and kappa-OR) in cerebral membranes. Saturation binding experiments revealed a competitive interaction of M6G with all three opioid receptors. Inhibition binding experiments at the mu-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 mu-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 mu-OR subtypes are involved. Hydrolysis of M6G to morphine was investigated and did not occur. Therefore the effects of M6G on binding to the mu-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 mu-, delta- and kappa-OR in a competitive manner. Some of our results on the mu-OR suggest two binding sites for agonists at the mu-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 mu-, delta- or kappa-OR. We therefore doubt that any effect of M3G is mediated via opioid receptors.

Morphine-3β-d-glucuronide Suppresses Inhibitory Synaptic Transmission in Rat Substantia Gelatinosa

“…Moreover, intrathecal M3G has no effect on C-fiber- evoked responses in the superficial dorsal horn (Sullivan et al, 1989;Hewett et al, 1993) The failure of M3G to antagonize the effects of DAMGO and the lack of effect of M3G on evoked EPSCs is consistent with receptor binding studies, which show M3G has little or no affinity for the -opioid receptor (Pasternak et al, 1987;Löser et al, 1996). In addition, our findings agree with a previous electrophysiological study (Hewett et al, 1993), which indicated that M3G does not antagonize the antinociceptive actions of intrathecal morphine.…”

“…Figure 2, C and D, shows time courses of these drug effects on EPSC amplitude. These observations are consistent with binding studies that show that M3G does not bind to -opioid receptors (Pasternak et al, 1987;Lambert et al, 1993;Löser et al, 1996).…”

“…Because actions of M3G at -, ␦-, and -opioid and ORL 1 receptors (Pasternak et al, 1987;Sullivan et al, 1989;Löser et al, 1996) have now been excluded, the receptor through which M3G exerts its effect remains to be determined. Interestingly, the selective presynaptic effect of M3G on IPSCs is similar to that of the recently identified neuropeptide, nocistatin.…”

Affinities of Dihydrocodeine and its Metabolites to Opioid Receptors

“…7,8-Dihydro derivatives had slightly higher affinities when compared to their unsaturated counterparts (Chen et al 1991;Mignat et al 1995;Lö ser et al 1996). 3-O-Glucuronides had essentially no relevant affinity to opioid receptors, and the apparent displacement of the radioactive ligands observed may well result from contamination with the more active parent substances (Bartlett & Smith 1995;Löser et al 1996).…”

Quantification of Glucuronide Metabolites in Biological Matrices by LC-MS/MS

“…Furthermore, the glucuronide moiety can be bound to different nucleophilic functional groups on the parent aglycone, giving rise to different glucuronide structural isomers, which can then be selectively quantified. Differentiation among glucuronide isomers might be important as they can have profoundly different pharmacological activity, for example the morphine-6-glucuronide has a many times greater affinity towards the -opioid receptor than the morphine-3-glucuronide or the morphine-6,3-diglucuronide (Loser, Meyer et al 1996).…”