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 tested (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.