Nonselective coupling of the human mu-opioid receptor to multiple inhibitory G-protein isoforms.
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The human mu-opioid receptor was expressed in Saccharomyces cerevisiae. Binding of [3H]diprenorphine to yeast spheroplasts was specific and saturable (Kd = 1 nm, Bmax = 0.2-1 pmol x mg-1 of membrane proteins). Inhibition of [3H]diprenorphine binding by antagonists and agonists with varying opioid selectivities (mu, delta and kappa) occurred with the same order of potency as in mammalian tissues. Affinities of antagonists were the same with yeast spheroplasts as in reference tissues whereas those of agonists, except etorphine and buprenorphine, were 10-fold to 100-fold lower. Addition of heterotrimeric Gi,o-proteins purified from bovine brain shifted the mu-opioid receptor into a high-affinity state for agonists. Using individually purified Galpha-subunits re-associated with betagamma-dimers, we showed that alphao1, alphao2, alphai1, alphai2 and alphai3 reconstituted high-affinity agonist binding with equal efficiency. This suggests that the structural determinants of the mu-opioid receptor responsible for G-protein coupling are not able to confer a high degree of specificity towards any member of the Gi,o family. The selective effects of opioid observed in specialized tissues upon opioid stimulation may be a result of regulation of G-protein activity by cell-specific factors which should conveniently be analysed using the reconstitution assay described here. (+info)
Human mu-opioid receptor overexpressed in baculovirus system and its pharmacological characterizations.
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AIM: To overexpress human mu-opioid receptor (muOR) with characteristics similar to those of mammalian origin. METHODS: Human muOR with a tag of 6 consecutive histidines at its carboxyl terminus was expressed in recombinant baculovirus infected Sf9 insect cells. Then the pharmacological characterizations of the product were studied by receptor binding assay and cAMP assay. RESULTS: The maximal binding capacity for the [3H]diprenorphine and [3H]ohmefentanyl (Ohm) were 9.1 +/- 0.7 and 6.52 +/- 0.23 nmol/g protein, respectively. The [3H]diprenorphine or [3H] Ohm binding to the receptor expressed in Sf9 cells was strongly inhibited by alpha-selective agonists [D-Ala2, N-methyl-Phe4, glyol5] enkephalin (DAGO), Ohm, and morphine, but neither by the delta-selective agonist [D-Pen2, D-Pen5] enkephalin (DPDPE) nor by the kappa-selective agonist inverted question marktrans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] inverted question mark benzacetamide (U50488). NaCl 100 mmol.L-1 and guanosine triphosphate (GTP) 50 mumol.L-1 could reduce mu agonists Ohm and etorphine affinity binding to the expressed muOR. DAGO and Ohm effectively inhibited forskolin-stimulated cAMP accumulation. This agonist-dependent effect was blocked by opioid antagonist naloxone. CONCLUSION: The overexpression of human muOR with a tag of six consecutive histidines at its carboxyl terminus in Sf9 insect cells retained the characteristics of wild-type human muOR. (+info)
Immunosuppressive effects of intravenous self administration of dihydroetorphine on lymphocyte functions in rats.
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AIM: To study the effects of dihydroetorphine (DHE) on lymphocyte functions in rats and to further assess the abuse potential of DHE. METHODS: An intravenous self administration (SA) procedure in rats was used to determine the SA liability of DHE. Concanavalin A (Con A)-stimulated lymphocyte proliferation and lymphokine production of rat splenocytes were measured. RESULTS: DHE 178 +/- 13 micrograms established a stable and typical rat model of psychological dependence, suppressed lymphocyte proliferation (129 +/- 11 Bq) compared with control group (620 +/- 36 Bq), and inhibited the activity of interleukin-2 (IL-2) (A570 = 0.28 +/- 0.06) compared with control group (A570 = 0.51 +/- 0.03). CONCLUSION: DHE had a high abuse potential and inhibited the Con A-induced lymphocyte proliferation and interleukin-2 production in rats. (+info)
Differential G-protein activation by alkaloid and peptide opioid agonists in the human neuroblastoma cell line SK-N-BE.
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Differences in the specificity of coupling of delta-opioid receptor with G-protein have been reported in the literature. We have observed a differential desensitization of delta-opioid receptors, endogenously expressed in the neuroblastoma cell line SK-N-BE, induced by peptide and alkaloid agonists. By combining photoaffinity labelling of receptor-activated G-proteins with [alpha-(32)P]azidoanilide-GTP and an anti-sense oligodeoxynucleotide strategy, we examined whether the chemical nature of opioid agonists, alkaloid or peptide, has a critical role in determining a G(i)alpha/G(o)alpha-protein-selective activation by the human delta-opioid receptors. Etorphine, a non-selective alkaloid agonist, was shown to stimulate the incorporation of [alpha-(32)P]azidoanilide-GTP into G(i)alpha1, G(i)alpha2, G(i)alpha3 and pertussis-toxin-insensitive Galpha subunits. In contrast, [d-Pen(2),d-Pen(5)]enkephalin (DPDPE; Pen is penicillamine) and Tyr-d-Ala-Phe-Asp-Val-Val-Gly-NH(2) (deltorphin I), selective peptide agonists, mainly activated G(i)alpha2 and G(o)alpha2 subunits. The 'knock-down' of G(o)alpha2 subunits by anti-sense oligodeoxynucleotides selectively decreased the inhibition of adenylate cyclase induced by DPDPE and deltorphin I, whereas anti-sense oligodeoxynucleotides directed against G(i)alpha2 subunits only decreased the potency of etorphine in inhibiting cAMP accumulation. These results suggest that the nature of the agonist, peptide or alkaloid is critical in determining the interaction between human delta-opioid receptors and Galpha subunits. (+info)
Dihydroetorphine-induced place preference was mediated by dopamine D1 receptors in rats.
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AIM: To study the influence of dopamine (DA) receptor antagonists upon the rewarding property of dihydroetorphine (DHE). METHODS: Conditioned place preference (CPP) paradigm was used to characterize the rewarding effect of DHE. DA receptor antagonists were injected administered subcutaneously or peritoneally and microinjected into nucleus accumbens (NAcc). RESULTS: DHE (0.05, 0.5, and 5.0 micrograms.kg-1, s.c.) produced place preference (P < 0.01). Both the DA receptor antagonist haloperidol and the selective D1 receptor antagonist Sch-23390 attenuated the place preference produced by DHE (0.5 microgram.kg-1, s.c.). l-Sulpiride and spiperone, selective D2 receptor antagonists, had no such effects. CONCLUSION: The D1 (but not D2) receptors in NAcc are crucial in the mediation of the rewarding effect of DHE. (+info)
Dissociation of functional roles of dynamin in receptor-mediated endocytosis and mitogenic signal transduction.
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Dynamin plays a critical role in the membrane fission mechanism that mediates regulated endocytosis of many G protein-coupled receptors. In addition, dynamin is required for ligand-induced activation of mitogen-activated protein kinase by certain receptors, raising a general question about the role of dynamin in mitogenic signal transduction. Here we report that endocytosis of mu and delta opioid receptors is not required for efficient ligand-induced activation of mitogen-activated protein kinase. Nevertheless, mitogenic signaling mediated by these receptors is specifically dynamin-dependent. Thus a functional role of dynamin in mitogenic signaling can be dissociated from its role in receptor-mediated endocytosis, suggesting a previously unidentified and distinct role of dynamin in signal transduction by certain G protein-coupled receptors. (+info)
Functional dissociation of mu opioid receptor signaling and endocytosis: implications for the biology of opiate tolerance and addiction.
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Opiate analgesia, tolerance, and addiction are mediated by drug-induced activation of the mu opioid receptor. A fundamental question in addiction biology is why exogenous opiate drugs have a high liability for inducing tolerance and addiction while native ligands do not. Studies indicate that highly addictive opiate drugs such as morphine are deficient in their ability to induce the desensitization and endocytosis of receptors. Here, we demonstrate that this regulatory mechanism reveals an independent functional property of opiate drugs that can be distinguished from previously established agonist properties. Moreover, this property correlates with agonist propensity to promote physiological tolerance, suggesting a fundamental revision of our understanding of the role of receptor endocytosis in the biology of opiate drug action and addiction. (+info)
A previously unidentified acepromazine metabolite in humans: implications for the measurement of acepromazine in blood.
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High-performance liquid chromatography-diode-array detection results obtained during the investigation of two cases involving acepromazine prompted us to study the stability of the drug in blood. It was found that acepromazine can undergo in vitro conversion by human red blood cells to 2-(1-hydroxyethyl)promazine, a product that has been reported as a minor urinary metabolite in horse urine but not previously identified in humans. Further, our analytical findings in the two cases examined suggest that 2-(1-hydroxyethyl)promazine may be the major unconjugated metabolite of acepromazine in humans. These findings have important implications for the analytical toxicology of acepromazine. (+info)