Effects of Monovalent and Divalent Cations on 3‐(+)[125I]Iododizocilpine Binding to the N‐Methyl‐d‐Aspartate Receptor of Rat Brain Membranes

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Abstract
We have investigated the binding of 3‐[125I]iododizocilpine ([125I]iodo‐MK‐801) to the N‐methyl‐d‐aspartate (NMDA) receptor in well‐washed rat brain membranes. [125I]Iododizocipline binding was displaced by the following: dizocilpine > thienylphencyclidine > phencyclidine > ketamine. Binding of [125I]iododizocilpine was enhanced by glutamate, glycine, and spermidine, whose actions could be reversed by CGS‐19755, 7‐chlorokynurenate, and arcaine, respectively. [125I]Iododizocilpine binding was also enhanced by a number of divalent cations, including Ba2+, Ca2+, Mg2+, Mn2+, and Sr2+, and several monovalent cations, including Na+ and K+. These cations enhanced [125I]iododizocilpine binding by an action at the polyamine site. In addition, the inhibitory effects associated with high concentrations of these cations was markedly reduced compared to those found in previous studies with [3H]dizocilpine. Analysis of the ability of spermidine, Mg2+, and Sr2+ to alter the inhibition of [125I]iododizocilpine by arcaine gave pA2 values of 5.41, 4.47, and 4.93, corresponding to EC50 concentrations of 3.9, 34.7, and 12.0 μM, respectively, suggesting that physiological concentrations of Mg2+ may occupy the polyamine site. These results demonstrate that [125I]‐iododizocilpine is a useful probe for the NMDA receptor. Moreover, its high specific activity and relative insensitivity to the inhibitory actions of divalent cations should make [125I]iododizocilpine a valuable ligand for the study of NMDA receptors in intact cellular systems.