Modulation of Binding at Opioid Receptors by Monoand Divalent Cations and by Cholinergic Compounds

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Abstract
In membrane suspensions from guinea-pig brain, NaCl, LiCl, NH4Cl and KCl, inhibit the equilibrium binding (25°C) of the selective μ-agonist [3H]-[D-Ala2,MePhe4,Gly-ol5]enkephalin, the selective δ-agonist [3H]-[D-Pen2,D-Pen5]enkephalin and the selective δ-agonist [3H]-dynorphin A (1-9). Choline chloride inhibits the binding of the μ- and δ-agonists but not of the δ-agonist; the choline derivative, methacholine, inhibits also the binding of the δ-agonist. Binding of the δ-agonist is potentiated by CaCl2, MgCl2 and MnCl2; these salts inhibit binding of the δ-agonist. As far as binding of the μ-agonist is concerned, MgCl2 and MnCl2 may potentiate or inhibit whereas CaCl2 is only inhibitory. The binding of the μ-antagonist [3H]-naloxone is potentiated by NaCl; while the threshold of inhibition by LiCl is increased there is no potentiation. In membrane suspensions of the rabbit cerebellum about 80% of the opioid binding sites are of the μ-type; the binding of the μ-agonist [3H]-[D-Ala2, MePhe4, Gly-ol5]enkephalin is inhibited by NaCl, LiCl, KCl and choline chloride whereas that of the μ-antagonists [3H]-naloxone and [3H]-(-)-bremazocine is potentiated at low concentrations but inhibited at higher concentrations of NaCl. In membranes of the guinea-pig cerebellum about 80% of the opioid binding sites are of the δ-type; they are particularly effective for assays of K-receptors when the selective K-agonist [3H]-dynorphin A (1-9) is used as ligand.