Cations differentially affect subpopulations of l-glutamate receptors in rat synaptic plasma membranes

Several cations were examined for their ability to specifically affect one of the 3 l-glutamate (l-Glu) binding sites in rat forebrain synaptic plasma membranes (i.e. Na⁺-dependent, Cl^--dependent and Cl^--independent). Na⁺-dependent binding was potently inhibited by K⁺ and NH₄⁺ ions. Other monovalent cations testedd (Cs⁺, Li⁺, triethylammonium) had no effect on this binding site. Polyvalent cations (Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Cr³⁺) also had little effect on the Na⁺-dependent l-Glu binding site. Cl^--dependent l-Glu binding was potently inhibited by Na⁺ ions but was not affected by other monovalent ions. All of the divalent cations were potent inhibitors of both Cl^--dependent and -independent binding. The results show that these binding sites of l-Glu can be distinguished by their response to cations and suggest possible novel modes of regulation in vivo.