Excitatory Amino Acid Receptors and Depolarization-Induced Ca2+ Influx into Hippocampal Slices

Hippocampal brain slices were incubated with depolarizing agents or excitatory amino acids either alone or in the presence of excitatory amino acid antagonists omega-phosphonic alpha-aminocarboxylic acids--2-amino-4-phosphonobutyric acid (AP4), 2-amino-5-phosphonovaleric acid (AP5), or 2-amino-7-phosphonoheptanoic acid (AP7)--or gamma-D-glutamylaminomethylsulphonic acid (GAMS)] or a calcium-channel blocker, (S)-1-(3-methoxyphenyl)-3-methylaza-7-cyano-7-(3,4-dimethoxyphenyl )-8-methyl- nonane hydrochloride (-)-D888]. The uptake of 45Ca2+ and the efflux of glutamate or aspartate induced by veratrine or high K+ was blocked (54-76%) by AP7 (IC50 46-250 microM). AP5 and AP4 were less effective. (-)-D888 (10 microM) caused 100% block of evoked 45Ca2+ uptake. Uptake of 45Ca2+ induced by exogenous glutamate, aspartate, and N-methyl-D-aspartate (NMDA) was also inhibited by AP7, whereas GAMS completely blocked the action of kainate and partially blocked that of glutamate. The action of NMDA in stimulating 45Ca2+ uptake was Mg2+-sensitive, low Mg2+ levels in the incubation medium selectively enhancing the response. It is concluded that Ca2+ uptake evoked by excitatory amino acids is receptor-mediated, and that released excitatory amino acids are responsible for a large part of the action of veratrine and high K+ in stimulating 45Ca2+ uptake.