Dopamine D-2 receptors in canine brain: ionic effects on [ 3H]neuroleptic binding

Abstract

In order to determine the effects of monovalent cations on the binding of 3H-neuroleptics to canine striatal dopamine D-2 receptors, a study of the effects of ions on the binding of two 3H-neuroleptics from different drug classes was undertaken. Dopamine D-2 receptors are selectively labeled in a sodium-sensitive manner by the benzamide ligand [ 3H]YM-09151-2. In the presence of 120 mM NaCl, [ 3H]YM-09151-2 had a dissociation constant of 55 pM and a maximal receptor density of 34 pmol/g of tissue. In the absence of NaCl, the dissociation constant was 440 pM with no change in the receptor density. The binding of [ 3H]YM-09151-2 (52 pM) was increased by 700% with 150 mM Na +, 440% with 500 mM Li + and 290% with 500 mM K +. The ion concentrations producing half-maximal increases in binding were 4 mM Na +, 8.5 mM Li + and 115 mM K +. The ionic strength control, N-methyl-D-glucamine, did not increase [ 3H]YM-09151-2 binding. [ 3H]Spiperone binding was much less affected by monovalent cations. Analysis of saturable binding showed that these changes were due to changes in binding affinity, independent of changes in receptor density. The sulfhydryl alkylating agent, N-ethylmaleimide, inactivated [ 3H]YM-09151-2 binding. Sodium ions at 120 mM protected approximately 40% of the susceptible sites while lithium and potassium ions (120 mM) did not. The anion exchange blocker, 4,4′-diisothiocyano-2,2′-stilbene disulfonic acid (DIDS), has been shown to be capable of blocking the effect of Na + on the binding of agonists to α 2-adrenoceptors. Pretreatment with DIDS had no effect on the ability of Na + to alter [ 3H]YM-09151-2 binding. Consistent with their actions on other benzamide ligands, monovalent cations are capable of affecting the interaction of the potent benzamide [ 3H]YM-09151-2 with the canine striatal dopamine D-2 receptor. The insensitivity to N-methyl-D-glucamine and extent of stimulation by low millimolar Na + concentrations indicate that this is not solely due to ion strength.