Glutamate Release from Guinea‐Pig Synaptosomes: Stimulation by Reuptake‐Induced Depolarization

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Abstract
Glutamate (10–100 μM) reversibly depolarizes guinea-pig cerebral cortical synaptosomes. This does not appear to be because of a conventional autoreceptor. Neither kainate at 1 mM, 100 μMn-methyl-D-aspartate (NMDA), 100 μM L-2-amino-4-phosphonobutanoate (APB), nor 100 μM quisqualate affects the Ca2+-dependent release of glutamate from suboptimally depolarized synaptosomes. However, kainate, quisqualate, and the quisqualate agonists β-N-ox-alylamino-L-alanine and α-amino-3-hydroxy-5-methylisox-azole propionate cause a slow Ca2+-independent release of glutamate from polarized synaptosomes. However, unlike kainate, quisqualate does not inhibit the acidic amino acid carrier. APB, NMDA, and the NMDA receptor-mediated neurotoxin β-N-methylamino-L-alanine do not influence Ca2+-independent release at 100 μM. The depolarization of the plasma membrane by glutamate can be mimicked by D-aspartate, can be blocked by the transport inhibitor dihydro-kainate, and is accompanied by the net uptake of acidic amino acids. L-Glutamate or D-aspartate at 100 μM increases the cytoplasmic free Ca2+ concentration. D-aspartate at 100 μM causes a Ca2+-dependent release of endogenous glutamate, superimposed on the Ca2+-independent heteroexchange with glutamate through the acidic amino acid carrier. The results suggest that the glutamatergic subpopulation of synaptosomes can be depolarized by exogenous glutamate.