Site-dependent effects of ions on mu-, delta- and kappa-opioid binding in suspensions of guinea-pig brain membranes.

NaCl, LiCl, NH4Cl and KCl inhibit the equilibrium binding of peptide agonists at each of the mu-, delta- and kappa-sites; the orders of potencies and the slopes of the dose-response curves are site-dependent. In contrast, the monovalent salt choline chloride inhibits mu- and kappa-binding but has little effect on delta-binding. The profiles of activity of MnCl2, CaCl2 and MgCl2 are also site-dependent but differ markedly from those of the monovalent salts.