Morphine-6 beta-glucuronide and morphine-3-glucuronide, opioid receptor agonists with different potencies

Using heterologous expression in Xenopus laevis oocytes, we compared the po tencies of morphine, morphine-6 beta -glucuronide (N16G), and morphine-3-gl ucuronide (M3G) for cloned human mu- (hMOR), kappa- (hKOR), and delta -opio id receptors (hDOR). Each receptor subtype was individually co-expressed wi th heteromultimeric G-protein coupled inwardly rectifying K+ (GIRK) channel s, consisting of GIRK1 and GIRK2 subunits, and RGS4, a regulator of G-prote in signaling. The two-microelectrode voltage clamp technique was used to me asure the opioid receptor-activated GIRK1/GIRK2 channel responses. Compared with morphine, M6G had higher potency at the hMOR, lower potency at the hK OR, and similar potency at the hDOR, while M3G showed a 1000-fold lower and non-selective potency via opioid receptors. In contrast to naloxone, M3G d id not antagonize the effects of morphine at the hMOR. We also investigated whether Trp318 and His319 provide the molecular basis for mu/delta selecti vity and mu/kappa selectivity of morphinan alkaloids by mutating these resi dues to their corresponding residues in kappa- and delta -opioid receptors. A single-point mutation (W318L) on hMOR completely conferred delta -like p otency for morphine and N16G on the mutant mu -receptor. Double mutation at Trp318 and His319 positions (Trp318Y/His319Y) only partially conferred K-l ike potency for morphine and M6G; the decrease in potency for M6G was signi ficantly larger than for morphine. The results of our study show that both M6G and M3G are opioid receptor agonists with different potencies and that the potency of morphinan receptor ligands can be changed by selective mutat ions of hMOR at the Trp318 and His319 positions. (C) 2001 Elsevier Science Inc. All rights reserved.