l-Glutamate effects on electrical potentials of synaptic plasma membrane vesicles

The electrogenic nature of the l-glutamate-stimulated Na⁺ flux was examined by measuring the distribution of the lipophilic anion [³⁵S]thiocyanate (SCN^?) into synaptic membrane vesicles that were incubated in a NaCl medium. Concentrations of l-glutamate from 10^?7 to 10^?4 M added to the incubation medium caused an enhanced intravesicular accumulation of SCN^?. Based on the SCN^? distribution in synaptic membrane vesicles it was calculated that 10 μM l-glutamate induced an average change in the membrane potential of + 13 mV. l-Glutamate enhanced both the Na⁺ and K⁺ conductance of these membranes as determined by increases in SCN^? influx. Other neuroexcitatory amino acids and amino acid analogs (d-glutamate, l-aspartate, l-cysteine sulfinate, kainate, ibotenate, quisqualate, $\text{N-}\text{methyl}\text{-}\text{d}\text{-}\text{aspartate}$, and dl-homocysteate) also increased SCN^? accumulation in synaptic membrane vesicles. These observations are indicative of the activation by l-glutamate and some of its analogs of excitatory amino acid receptor ion channel complexes in synaptic membranes.