Modulation of binding at opioid receptors by mono- and divalent cations and by cholinergic compounds.

In membrane suspensions from guinea-pig brain, NaCl, LiCl, NH4Cl and KCl, inhibit the equilibrium binding (25 degrees C) of the selective mu-agonist [3H]-[D-Ala2,MePhe4,Gly-ol5]enkephalin, the selective delta-agonist [3H]-[D-Pen2,D-Pen5]enkephalin and the selective kappa-agonist [3H]-dynorphin A (1-9). Choline chloride inhibits the binding of the mu- and kappa-agonists but not of the delta-agonist; the choline derivative, methacholine, inhibits also the binding of the delta-agonist. Binding of the delta-agonist is potentiated by CaCl2, MgCl2 and MnCl2; these salts inhibit binding of the kappa-agonist. As far as binding of the mu-agonist is concerned, MgCl2 and MnCl2 may potentiate or inhibit whereas CaCl2 is only inhibitory. The binding of the mu-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 mu-type; the binding of the mu-agonist [3H]-[D-Ala2,MePhe4,Gly-ol5]enkephalin is inhibited by NaCl, LiCl, KCl and choline chloride whereas that of the mu-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 kappa-type; they are particularly effective for assays of kappa-receptors when the selective kappa-agonist [3H]-dynorphin A (1-9) is used as ligand.