Influence of Gz and Gi2 transducer proteins in the affinity of opioid agonists to mu receptors.
The affinity displayed by different opioids to mu receptors (ORs) was determined in mouse brain membranes incubated with antibodies directed to Galpha subunits of the guanine nucleotide-binding proteins Gi2 and Gz. Assays were conducted with 10 pm 125I-Tyr27-beta-endorphin in the presence of 300 nm N, N-diallyl-Tyr-(alpha-aminoisobutyric acid)2-Phe-Leu-OH (ICI-174 864), which prevented the binding of the iodinated neuropeptide to delta-ORs. Gpp(NH)p or the preincubation of mouse brain membranes with IgGs to Gi2alpha or Gzalpha subunits, promoted reductions in the affinity exhibited by the labelled probe. The potencies of beta-endorphin, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and [D-Pen2,5]enkephalin (DPDPE) were reduced after impairing the coupling of mu-ORs to Gi2 or Gz proteins. Morphine showed a loss of affinity towards the mu-OR after preincubation of membranes with IgGs to Gzalpha subunits. However, it retained its potency after treatment with the anti-Gi2alpha IgGs. Conversely, [D-Ala2, D-Leu5]enkephalin (DADLE) and [D-Ser2, Leu5] enkephalin-Thr6 (DSLET) showed decreased affinity to mu-ORs after treatment with anti-Gi2alpha IgGs, with no noticeable change following the use of IgGs to Gzalpha subunits. The affinity exhibited by the opioid antagonists naloxone, naltrexone, naloxonazine and [Cys2,Tyr3,Orn5, Pen7 amide]somatostatin analogue (CTOP) remained unchanged after either treatment. Therefore, the affinity exhibited by opioid agonists of mu-ORs, but not antagonists, depends on the nature of the G-protein coupled to these receptors.
Neurofarmacología, Instituto Cajal, Consejo Superior de Investigaciones Científicas, C/Dr Arce 37, 28002 Madrid, Spain. jgarzon@cajal.csic.es
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Garzón
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M
Castro
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Sánchez-Blázquez
D000818:Animals; D000906:Antibodies / Q000378:metabolism; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D051960:GTP-Binding Protein alpha Subunit, Gi2; D044385:GTP-Binding Protein alpha Subunits; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D019204:GTP-Binding Proteins / Q000276:immunology / Q000378:metabolism*; D020962:Heterotrimeric GTP-Binding Proteins; D006801:Humans; D007457:Iodine Radioisotopes; D008297:Male; D051379:Mice; D007470:Monoiodotyrosine / Q000378:metabolism; D009292:Narcotic Antagonists / Q000494:pharmacology; D009294:Narcotics / Q000819:agonists* / Q000378:metabolism / Q000494:pharmacology; D011485:Protein Binding / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000276:immunology / Q000378:metabolism*; D017465:Receptors, Opioid, delta / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000378:metabolism*; D001615:beta-Endorphin / Q000378:metabolism
D000906:Antibodies; C099387:GNAZ protein, human; D044385:GTP-Binding Protein alpha Subunits; C490964:Gnaz protein, mouse; D007457:Iodine Radioisotopes; D009292:Narcotic Antagonists; D009294:Narcotics; D011518:Proto-Oncogene Proteins; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; D001615:beta-Endorphin; D019204:GTP-Binding Proteins; C498015:GNAI2 protein, human; D051960:GTP-Binding Protein alpha Subunit, Gi2; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; C498016:Gnai2 protein, mouse; D020962:Heterotrimeric GTP-Binding Proteins; D007470:Monoiodotyrosine