Characterization of Na+-dependent binding sites of [3H]glutamate in synaptic membranes from rat brain.

Some biochemical characteristics of Na+-dependent binding of [3H]L-glutamic acid (Glu) were studied using crude synaptic membrane preparations from the rat brain as compared with Na+-independent binding. In vitro addition of sodium chloride (1-100 mM) exhibited a significant enhancement of [3H]Glu binding to synaptic membranes in a concentration-dependent manner independent of the incubation temperature employed (2 or 30 degrees C). In contrast, sodium acetate elicited a concentration-dependent augmentation of the binding at 2 degrees C to a significantly greater extent than that found at 30 degrees C. It was found that the binding found in the presence of 100 mM sodium acetate reached its maximal value within 10 min of incubation followed by a rapid decline up to 60 min at 30 degrees C, while gradually increasing up to 60 min at 2 degrees C. The Na+-independent basal binding was significantly activated by the alteration of incubation temperature from 2 to 30 degrees C and reached equilibrium within 10 min of incubation at both incubation temperatures. The Na+-dependent binding was more promptly attenuated by the addition of excess of non-radioactive Glu (1 mM) at 30 degrees C than that at 2 degrees C, whereas the Na+-independent binding was greatly suppressed by the addition at 2 degrees C in comparison with that at 30 degrees C. Quisqualic acid induced a considerably less-potent inhibition of the Na+-dependent binding than that of the Na+-independent binding. Neither N-methyl-D-aspartic acid nor kainic acid had such a significant effect on each binding.(ABSTRACT TRUNCATED AT 250 WORDS)