Induction of morphine-6-glucuronide synthesis by heroin self-administration in the rat.
ABSTRACT Heroin is rapidly metabolized to morphine that in turn is transformed into morphine-3-glucuronide (M3G), an inactive metabolite at mu-opioid receptor (MOR), and morphine-6-glucuronide (M6G), a potent MOR agonist. We have found that rats that had received repeated intraperitoneal injections of heroin exhibit measurable levels of M6G (which is usually undetectable in this species).
The goal of the present study was to investigate whether M6G synthesis can be induced by intravenous (i.v.) heroin self-administration (SA).
Rats were trained to self-administer either heroin (50 μg/kg per infusion) or saline for 20 consecutive 6-h sessions and then challenged with an intraperitoneal challenge of 10 mg/kg of heroin. Plasma levels of heroin, morphine, 6-mono-acetyl morphine, M3G, and M6G were quantified 2 h after the challenge. In vitro morphine glucuronidation was studied in microsomal preparations obtained from the liver of the same rats.
Heroin SA induced the synthesis of M6G, as indicated by detectable plasma levels of M6G (89.7 ± 37.0 ng/ml vs. 7.35 ± 7.35 ng/ml after saline SA). Most important, the in vitro V (max) for M6G synthesis was correlated with plasma levels of M6G (r (2) = 0.78). Microsomal preparations from saline SA rats produced negligible amounts of M6G.
Both in vivo and in vitro data indicate that i.v. heroin SA induces the synthesis of M6G. These data are discussed in the light of previous studies conducted in heroin addicts indicating that in humans heroin enhances the synthesis of the active metabolite of heroin and morphine.
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ABSTRACT: Using heterologous expression in Xenopus laevis oocytes, we compared the potencies of morphine, morphine-6beta-glucuronide (M6G), and morphine-3-glucuronide (M3G) for cloned human mu- (hMOR), kappa- (hKOR), and delta-opioid receptors (hDOR). Each receptor subtype was individually co-expressed with heteromultimeric G-protein coupled inwardly rectifying K(+) (GIRK) channels, consisting of GIRK1 and GIRK2 subunits, and RGS4, a regulator of G-protein signaling. The two-microelectrode voltage clamp technique was used to measure the opioid receptor-activated GIRK1/GIRK2 channel responses. Compared with morphine, M6G had higher potency at the hMOR, lower potency at the hKOR, 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 did not antagonize the effects of morphine at the hMOR. We also investigated whether Trp318 and His319 provide the molecular basis for mu/delta selectivity and mu/kappa selectivity of morphinan alkaloids by mutating these residues to their corresponding residues in kappa- and delta-opioid receptors. A single-point mutation (W318L) on hMOR completely conferred delta-like potency for morphine and M6G on the mutant mu-receptor. Double mutation at Trp318 and His319 positions (Trp318Y/His319Y) only partially conferred kappa-like potency for morphine and M6G; the decrease in potency for M6G was significantly 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 mutations of hMOR at the Trp318 and His319 positions.Biochemical Pharmacology 12/2001; 62(9):1273-82. · 4.58 Impact Factor
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ABSTRACT: Although MOR-1 encodes a mu opioid receptor, its relationship to the pharmacologically defined mu receptor subtypes has been unclear. Antisense mapping now suggests that these subtypes result from alternative splicing of MOR-1. Three oligodeoxynucleotide probes targeting exon 1 and another oligodeoxynucleotide directed against the coding region of exon 4 block supraspinal morphine analgesia, a mu1 action, while five of six oligodeoxynucleotides directed against exons 2 and 3 are inactive. Inhibition of gastrointestinal transit and spinal morphine analgesia, two mu2 actions, are blocked only by the probe against exon 4 and not by those directed against exon 1. In contrast, the analgesic actions of the extraordinarily potent mu drug morphine-6 beta-glucuronide are blocked by six different antisense oligodeoxynucleotides targeting exons 2 and 3, but not by those acting on exons 1 or 4. These results suggest that the mu1 and mu2 receptor subtypes originally defined in binding and pharmacological studies result from alternative splicing of MOR-1 while morphine-6 beta-glucuronide acts through a novel, previously unidentified receptor which is yet another MOR-1 splice variant.FEBS Letters 09/1995; 369(2-3):192-6. · 3.58 Impact Factor
- Journal of Pharmacology and Experimental Therapeutics 10/1961; 133:388-99. · 3.89 Impact Factor