title: Unmasking of magnesium-dependent high-affinity binding sites for [dAla2, dLeu5]enkephalin after pretreatment of brain membranes with guanine nucleotides.
abstract: The regulation of mu- and delta-opiate receptors by guanine nucleotides and cations was studied by examining the binding of [3H][DAla2, DLeu5]enkephalin to rat brain membranes. The binding to mu-opiate receptors could be suppressed by 1 microM [DPro4]morphiceptin, a highly specific mu-agonist, thus permitting separate assessment of delta-opiate receptor binding. GTP, GDP, and the nonhydrolyzable analogs 5'-guanylyl imidodiphosphate (Gpp[NH]p) and guanosine 5'-O-(2-thiodiphosphate) (GDP-S) effectively decreased the binding to both receptor types. This inhibitory effect was potentiated by Na+. The inhibitory effect of GTP and GDP, but not of the nonhydrolyzable analogs, was reversed by Mg2+. Pretreatment of membranes with GDP or GTP increased substantially the subsequently measured high-affinity binding, and this effect required the presence of Mg2+ in the binding assay. Similar pretreatment with GDP-S resulted in only a partial increase of binding compared to GTP or GDP, and Gpp[NH]p was relatively ineffective. Similar results were observed for both mu- and delta-receptors, although the effects on mu-receptors were quantitatively more profound. These data suggest that guanine nucleotides play a dual function in regulating opiate receptor binding in a manner dependent upon the presence of Na+ or Mg2+.
mesh_terms: D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D004492:Edetic Acid / Q000494:pharmacology; D004745:Enkephalins / Q000378:metabolism*; D006150:Guanine Nucleotides / Q000494:pharmacology*; D008274:Magnesium / Q000378:metabolism; D008297:Male; D051381:Rats; D011957:Receptors, Opioid / Q000145:classification / Q000378:metabolism*
chemical_list: D004745:Enkephalins; D006150:Guanine Nucleotides; D011957:Receptors, Opioid; D004492:Edetic Acid; D008274:Magnesium