Compartmentation of glutamate and aspartate within cerebral cortical synaptosomes: evidence for a non-cytoplasmic origin for the calcium-releasable pool of glutamate.

4.3% of the endogenous l-glutamate in a guinea-pig cerebral cortical synaptosome preparation can be released in a Ca(2+)-dependent manner by 60 s of KCl depolarization. There is no significant Ca(2+)-dependent release of endogenous aspartate. Although [(14)C]-l-glutamate and [(3)H]-d-aspartate equilibrate across the plasma membrane within 3 min, equilibration of [(14)C]-l-glutamate with the Ca(2+)-dependent glutamate pool is only 56% complete after 15 min. Relative to (86)Rb(+) as a cytosolic marker, 16 and 11%, respectively, of the total endogenous glutamate and aspartate are resistant to hypo-osmotic lysis. The resistant compartment is not mitochondrial and slowly incorporates added [(14)C]-l-glutamate, equilibration being 51% complete at 15 min. Accumulated [(3)H]dopamine is found in the osmotically resistant compartment, consistent with the presence of synaptic vesicles in the fraction. Most of the osmotically resistant [(3)H]dopamine and endogenous and [(14)C]glutamate can be sedimented by 10,000 g max. for 1 min, suggesting that synaptic vesicles remain largely associated with the lysed synaptosomes. Further centrifugation at 178,000 g max. for 15 min only slightly increases the yield of sedimented transmitter. The results are consistent with a vesicular storage and exocytotic release mode for l-glutamate, and indicate that aspartate is not released as a transmitter in this preparation.